Review on Ti₃C₂ MXene-based binary and ternary composites for photocatalytic applications

Abstract

2D layered materials have been employed as suitable co-catalysts for many photocatalytic reactions owing to their numerous advantages, such as high specific surface area, surface-active sites and suitable surface acid–base properties. Materials, such as graphene, MoS₂, black phosphorous and MXenes, have been extensively explored in this regard, with MXenes gaining tremendous interest. MXenes not only mediate the charge carrier transfer pathways when conjugated with semiconductors but also act as excellent precursors to obtain the respective metal oxides at a moderate annealing temperature. Given that titania is the gold standard photocatalytic material, initially, this review presents the design and fabrication of Ti₃C₂/TiO₂ from the viewpoint of both preparation methods and applications towards pollutant degradation and hydrogen evolution reactions. Subsequently, Ti₃C₂/TiO₂-based ternary composite comprising superior nanomaterials, such as BiOX, AgX, g-C₃N₄, metal sulphides, metal oxides, quantum dots and metal particles, are emphasized to demonstrate their advancements. Alternatively, the photocatalytic applications of C@TiO₂ obtained from the complete oxidation of Ti₃C₂ and the related ternary systems are emphasized. The charge carrier dynamics specific to the heterostructure and participation of free radicals in the pollutant degradation mechanism are highlighted at the respective stages. Finally, the associated challenges and future perceptions of Ti₃C₂-based heterostructures are emphasized for broader applications.

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Prathibha C P

Cheelangi Prasannakumar Prathibha, a native from Karnataka State, completed her postgraduate studies (2023) from Davangere University, Davangere, Karnataka. At present, she is pursuing her PhD degree at the Department of Chemistry, REVA University, Bangalore. Her research interests are associated with the synthesis of functional nanomaterials and their photocatalytic applications. In particular, the design and fabrication of Ti3C2 MXene-based semiconducting materials using a wide variety of wet-chemical approaches and evaluating them for photocatalysis are her specific interests.

Mallapur Srinivas

Srinivas Mallapur, a native from Karnataka State, received his Master's in Organic Chemistry (2013) and PhD (2019) from the Department of Chemistry, Bengaluru University, Bengaluru. He is currently serving as an Assistant Professor in the Department of Chemistry, School of Applied Sciences, REVA University, Bengaluru. His research interests are directed towards the synthesis and characterization of functional hybrid materials for electro/photocatalytic applications. Google Scholar: https://scholar.google.com/citations?user=IW8gVZMAAAAJ&hl=en.

S. Girish Kumar

S. Girish Kumar, a native of Karnataka, (Kolar District, Malur Taluk), obtained his PhD from the Department of Chemistry, Bangalore University (2012) and completed his Post-Doctoral Fellow studies at the Department of Physics, Indian Institute of Science (2015). His research interests cover the area of heterojunction photocatalysts, nanomaterial synthesis, phase transitions in TiO2 and Fenton processes for wastewater treatment. He has been recognized as a ‘Top 2% researcher’ by the survey conducted by ‘Stanford University, USA’ based on the Scopus author profiles. He is serving as an Associate Editor for ‘Chemical Papers, Springer’, Managing Guest Editor for ‘Applied Surface Science Advances, Elsevier’ and reviewer panel committee member for ‘RSC Advances, RSC’. He has reviewed more than 2200 research articles from various international journals. At present, he is working as an Assistant Professor at the Department of Chemistry, RV College of Engineering, Bengaluru. Details: https://www.webofscience.com/wos/author/record/B-8236-2012.

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1. Introduction

The exorbitant usage of fossil fuels has resulted in significant crises worldwide, including environmental pollution and energy shortage. Furthermore, fossil fuel energy resources are depleting rapidly, and thus renewable sources are the best alternative to address this issue.^1,2 AOPs based on the utilization of solar energy to trigger the degradation of pollutants and the fuel generation through redox pathways are perceived as an ideal technology to address the energy and environment crises.^3–6 Among the diverse semiconductors, TiO₂ is the most well-known photocatalyst owing to its extravagant properties, like tremendous photostability, non-toxicity, convenient preparation with diverse morphologies and suitable band edge potentials to generate free radicals in aqueous suspension.^7–9 However, it is well known that the wider applications of single phase semiconductors is limited by their demerits, such as narrow optical response, high degree of charge carrier recombination, low surface area and aggregation during reuse.¹⁰ Thus, the fabrication of composites comprised of a co-catalyst together with a semiconductor has become an indispensable research strategy to broaden the horizon of photocatalysis.^11–13 Compared to the use of bulk particulate co-catalysts, 2D materials, such as graphene,^14,15 hexagonal BN,¹⁶ transition metal dichalcogenides¹⁷ and black phosphorous,^18,19 have attracted significant attention because of their thin atomic layers, excellent electronic properties, good thermal stability, and mechanical properties as well as the superior light absorption/interaction capabilities. Among them, MXenes are promising 2D materials in many research areas, which were discovered in 2011.²⁰ The structure of MXenes resembles that of the versatile graphene and they possess good electrical conductivity and are intrinsically more hydrophilic than graphene.^14,21 MXenes are 2D transition metal carbides/nitrides, which are represented as M_n+1X_nT_x (n = 1, 2, 3), where ‘M’ is an early transition metal (Sc, Ti, V, Mo and Nb), ‘X’ is carbon/nitrogen and ‘T’ is a surface terminal groups (–OH, –O, and –F). In MXene solid solutions, the single M elements and double-M elements are out-of-plane ordered, with the transition metals occupying their outer layer. In-plane ordering, ordered vacancies, and randomly distributed vacancies have also been observed in MXenes. The stability and features of MXenes can be modified by their terminal groups (Fig. 1).²²

Fig. 1 Schematic of MXene structures (reproduced with permission from ref. 22; Copyright2023, Springer).

It has been well established that ternary carbide and nitride MAX phases have the typical formula M_n+1AX_n, where n = 1, 2, or 3, ‘M’ is an early transition metal, ‘A’ is an A-group (mostly group 13 and 14) element, and ‘X’ represents C and/or N, forming laminated structures with anisotropic properties. These are layered hexagonal (space group P63/mmc) structures, with two formula units per unit cell. The near-close-packed M-layers are interleaved with pure A-group element layers, with the X-atoms filling the octahedral sites between the former. To date, it has been widely accepted that more than 300 phases exist, which are obtained under diverse reaction conditions.^23–25 The selective etching of the MAX phase with HF results in the breaking of M–A instead of the M–X bond because of the higher bond strength of M–X.²⁶ The first MXene, Ti₃C₂, was discovered by the HF etching of Ti₃AlC₂ at RT, and since then, many MXenes including Ti₃CN_xT_x,²⁷ Nb₂C,²⁸ (Nb_0.8Ti_0.2)₄C₃T_x,²⁹ (Nb_0.8Zr_0.2)₄C₃T_x,²⁹ Mo₂TiC₂,³⁰ Mo₂ScC₂,³¹ V₄C₃T_x³² and Ta₄C₃T_x³³ have been successfully prepared. After HF etching, the obtained multi-layered MXene was delaminated with DMSO through intercalation and sonication, resulting in the formation of monolayer or few layered MXene.^34–38 Both computational and experimental studies suggest that the physicochemical properties of MXenes are strongly influenced by the composition and functionalities of their surface terminations.^39,40 When the A-site of MAX is etched, the freshly exposed and unsaturated transition metal atoms are immediately coordinated by the anions in the etchant, forming the surface termination, T_x, of M_n+1X_nT_x.⁴¹ Alternative routes such as in situ development of HF with fluoride salts (HCl and LiF)⁴² and exploring other fluorinating agents such as NaF,⁴³ KF,⁴³ (CH₃CH₂CH₂CH₂)₄N⁺F⁻,⁴² and bifluoride (NaHF₂, KHF₂ and NH₄HF₂)⁴⁴ have also been employed to synthesize MXenes. Also, organic base-driven intercalation and delamination method,⁴⁵ and alkali etching (KOH) with a minimal amount of water,⁴⁶ electrochemical etching⁴⁷ and fluorine-free molten salt etching⁴⁸ have also been developed.^49,50 The band gap of MXene was tuned with –F and –OH terminating groups, converting the metallic MXene into (Ti₃C₂(OH)₂) semiconducting nature with a small band gap of ∼0.1 eV.⁵¹ Furthermore, the terminal groups of Ti₃C₂ MXene can change its properties such as conductivity, hydrophilicity and mechanical properties.²¹ Thus, MXenes show potential in a wide range of applications such as photocatalysts,^52–54 batteries,^55–58 supercapacitors,^59–61 sensors,^62,63 cancer treatment,⁶⁴ lasers,^65–67 photodetectors,^68,69 water splitting,⁷⁰ CO₂ reduction^69,71,72 and N₂ fixation.^73–75

Among the MXenes, Ti₃C₂ is interesting given that it can serve as a reliable precursor to obtain TiO₂ photocatalytic materials through oxidation and the fraction of Ti₃C₂/TiO₂ in composites can be conveniently tailored by changing the reaction conditions.^76–78 Alternatively, the complete oxidation of Ti₃C₂ also results in the formation of C@TiO₂, which is also regarded as an active visible light photocatalyst (Fig. 2). The preparation of Ti₃C₂/TiO₂ or C@TiO₂ can be achieved through several techniques such as hydrothermal/solvothermal approach, calcination method, ultrasonic assisted oxidation, electrostatic self-assembly, and chemical vapor deposition methods (Table 1). Based on the above-mentioned consideration, the preparation and photocatalytic applications of Ti₃C₂/TiO₂ and C@TiO₂ will be discussed in this review article, which has scarcely been attempted in the literature. Furthermore, the ternary composites associated with these materials are highlighted for their advancement.

Fig. 2 Synthesis of Ti₃C₂ MXene and its oxidized composites.

Table 1 Preparation of Ti₃C₂ MXene-based composites

Photocatalyst	Precursor	Preparation method	Reaction condition	Specific surface area (m^2 g^−1)	Ref.
Ti3C2/TiO2	(NH4)2TiF6, Ti3AlC2 and H3BO3	Electrostatic self-assembly	60 °C, 24 h	—	85
Ti3C2/TiO2	Ti3C2, Ti(OBu)4, HF and ethanol	Solvent-thermal	80 °C, 24 h	—	86
TiO2/Ti3C2	Ti3C2Tx, NaOH, H2O2 and HCl	Hydrothermal followed by calcination	160 °C, 12 h and 550 °C, 4–6 h	29.50	91
TiO2/Ti3C2	P25, Ti3C2Tx and NaOH	Hydrothermal followed by calcination	180 °C, 10 h and 500 °C, 2 h	—	92
TiO2/ML-Ti3C2	Ti(OC4H9)4, C2H6O and ML-Ti3C2	Hydrothermal method		83.97	93
Si@TiO2/Ti3C2Tx	Ti3AlC2, Si powder and 50% HF	High-temperature diffusion	800 °C, 1000 °C and 1200 °C	33.00	94
N-TiO2/Ti3C2	N-TiO2 and Ti3C2	Ultrasonic treatment	4 h in an ice bath	—	95
(111) r-TiO2−x/Ti3C2	Ti3C2, HCl, NH4F and hydrazinium hydroxide	Hydrothermal followed by N2H4 reduction	200 °C, 12–28 h and 200 °C, 12 h	10.20	108
3D TiO2/Ti3C2	Ti3C2, NaOH and HCl	Hydrothermal followed by heat treatment	140 °C, 15 h and 500 °C, 8 h	—	113
In2S3/TiO2/Ti3C2Tx	In(NO3)3·xH2O, CH3CSNH2 and Ti3C2Tx	Hydrothermal	180 °C, 24 h	44.99	127
TiO2/Ti3C2Tx/AgI	TiO2/Ti3C2Tx, AgNO3 and KI.	Solvothermal	150 °C, 24 h	—	137
TiO2 (B)/Ti3C2/Ag3PO4	Na2HPO4·12H2O, AgNO3, TiO2 (B) and Ti3C2	Electrostatic self-assembly		—	138
Ti3C2/TiO2/BiOCl	Ti3C2, Hac, Bi(NO3)3 5H2O and KCl	Hydrothermal	180 °C, 24 h	8.77	143
BiOI/(001)TiO2/Ti3C2	(001) TiO2/Ti3C2, Ethylene glycol, Bi(NO3)3·5H2O and KI	Hydrothermal	160 °C, 24 h	—	144
Bi2WO6/TiO2/Ti3C2		Electrostatic self-assembly		79.40	145
NCDs/TiO2/Ti3C2Mx	TiO2/Ti3C2Mx and NCDs solution	Hydrothermal	120 °C, 2 h	16.10	154
BPQDs/Ti3C2/TiO2	Ti3C2/TiO2 and BPQDs	Solvent–heat	80 °C, 2 h	15.29	156
TiO2@ Ti3C2/GCN	Ti3C2, GCN and ethanol	Ultrasonic-assisted calcination	200 °C, 30 min	9.10	167
GCN/TiO2/Ti3C2	Melamine and TiO2/Ti3C2	Calcination	550 °C, 2 h	82.00	169
GCN/TiO2/Ti3C2	CAM precursor, Ti3C2	High-energy ball mill-assisted one-step calcination	580 °C, 3 h	—	170
GCN/Ti3C2/TiO2	Melamine and Ti3C2	Chemical vapour deposition	550 °C, 3 h	—	171
PDI/GCN/TiO2/Ti3C2	Ti3C2, GCN and PMDA powder	Calcination	325 °C, 4 h	—	173
Fe-C3N4/Ti3C2/C-TiO2	Fe-C3N4 and Ti3C2	Two-step calcination	550 °C, 2.0 h	—	174
Ti3C2/TiO2/CuInS2	InCl3·4H2O, CuCl2·2H2O, sulphur powder and Ti3C2/TiO2	Hydrothermal	180 °C, 15 h	—	186
MoxS@TiO2/Ti3C2	Na2MoO4·2H2O, CN2H4S and TiO2@Ti3C2	Solvothermal	160 °C, 12 h	—	188
d-Ti3C2/TiO2/GCN	d-Ti3C2 colloidal solution and GCN	Calcination	350 °C, 1 h	34.30	193
TiO2/Ti3C2/GCN	Ti3C2 and GCN	Calcination	450 °C, 5 h	—	195
Ti3C2/HCN	HCN and Ti3C2	Ultrasonic assisted impregnation approach	Stir for 12 h	—	196
Cu/TiO2/Ti3C2Tx	TiO2/Ti3C2Tx–z and CuAc2·H2O	Hydrothermal followed by photodeposition	500 W Xe lamp, 3 h with N2 flow	—	197
TiO2/Ti3C2/Ru	TiO2/Ti3C2, methanol and RuCl3	Photodeposition	300 W Xe lamp, 2 h	—	198
Cu2O/(001)TiO2/Ti3C2Tx	Cu(CH3COO)2, N,N-DMF and (001)TiO2/Ti3C2Tx	Solvent reduction	reflux at 150 °C, 12 h	167.20	200
PtO@Ti3C2/TiO2	Ti3C2/TiO2 and H2PtCl6	Photodeposition	300 W Xe lamp, 2 h	—	201
Ni2P/TiO2NPs/Ti3C2	TiO2, Ti3C2 and Ni2P	Ultrasonic-assisted oxidation	100 °C, 24 h	—	202

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2. Preparation and photocatalytic activity of Ti₃C₂-based binary composites for organic pollutant degradation

2.1. Integrating Ti₃C₂ with titania NPs

The combination of Ti₃C₂/TiO₂ is of prominent interest given that their preparation involves the partial oxidation of Ti₃C₂ at a moderate annealing temperature.⁷⁹ Furthermore, the distinct crystal phases of titania such as anatase, rutile and brookite and their mixed phases have been reported to be photocatalytically active for the degradation of organic pollutants.⁸⁰ It is well known that the anatase/rutile bicrystalline framework possess ‘catalytic hot spots’ at its interface owing to its appropriate band alignment, which not only avoids charge carrier recombination, but also enhances the generation of free radicals in aqueous solution.^81–83 The rutile/anatase ohmic heterojunction of TiO₂/Ti₃C₂ was synthesized via the calcination of NH₃@Ti₃C₂ at different temperatures. When the calcination temperature reached 500 °C, Ti₃C₂ was oxidized, and the NSs were encapsulated by TiO₂.⁸⁴ Due to the oxidation of some carbon layers in Ti₃C₂, the surface of the NSs was enriched with defects. The increment in the calcination temperature decreased the content of anatase, which was accompanied by an increment in the fraction of rutile phase. The Ti₃C₂/TiO₂ calcined at 500 °C had anatase and rutile fractions of 72.46% and 27.54%, respectively. As the temperature increased, the anatase TiO₂ fraction decreased, while the rutile TiO₂ fraction increased, indicating that Ti₃C₂ was first oxidized to anatase TiO₂, and then transformed into rutile TiO₂. Thus, the crystal composition can be effectively controlled by adjusting the reaction temperature. The maximum TC degradation was achieved by TiO₂/Ti₃C₂ within 3 h of light illumination and the solution pH (1, 3, 4, 7 and 9) did not exhibit any adverse effect on the degradation kinetics. However, an increase in TC concentration (10–50 mg mL⁻¹) was found to be detrimental owing to its shielding effect, which suppressed the excitation of the composite. The photogenerated hydroxyl and superoxide radicals play a crucial role in the TC degradation reaction.⁸⁴ Wang et al.⁸⁵ proposed a single-step method for the synthesis of HF-free multilayered Ti₃C₂/TiO₂ using Ti₃AlC₂, (NH₄)₂TiF₆ and H₃BO₃. The proposed reactions for the formation of TiO₂ are as follows (eqn (1)–(3)):

(NH₄)₂TiF₆ + H₂O → NH₄TiOF₃ + 2HF + NH₄F (1)

The highest MB degradation was achieved by Ti₃C₂/TiO₂ after 180 min of UV light illumination.⁸⁵ Ti₃C₂/TiO₂ was also synthesized via the solvent thermal method using Ti(OBu)₄ as the Ti source and HF, to optimize its morphology and microstructure. The close contact between Ti₃C₂ and TiO₂ promotes the transfer of the charge carriers to increase their activity. According to the DOS curves, the density of the electronic states of Ti₃C₂, F-terminated Ti₃C₂, and O-terminated Ti₃C₂ deviates from zero at the Fermi level, indicating the metallic characteristics of Ti₃C₂. The comparison of the three samples showed a reduced density of electronic states in the O-terminated Ti₃C₂ and F-terminated Ti₃C₂vs. pure Ti₃C₂, signifying that the terminal O and F atoms decreased the conductivity of Ti₃C₂. The new peaks at −6.4 eV and −4.9 eV correspond to the F and O terminal group-modified Ti₃C₂, implying an increase in stability. The photogenerated electrons flow from TiO₂ to F-terminated and O-terminated Ti₃C₂. Additionally, the work function of (001) TiO₂ was found to be 4.9 eV with an E_F of 0.4 V vs. NHE (Fig. 3). Ti₃C₂/TiO₂ exhibited an appreciable performance towards RhB degradation with excellent photostability.⁸⁶

Fig. 3 (a and b) Side and top views of the structural model, (c and d) DOS and PDOS and (e and f) calculated potential of F-terminated and O-terminated Ti₃C₂ (reproduced with permission from ref. 86; Copyright 2020, Elsevier).

Ti₃C₂/TiO₂ was synthesized via the hydrothermal oxidation of Ti₃C₂ with NaBF₄ at 180 °C for 10 h and exhibited the highest MO degradation efficiency compared to TiO₂ (Fig. 4). When the MO initial concentration increased from 30 to 50 mg L⁻¹, the degradation efficiency declined. To clarify the important effects of various species in photocatalytic reactions, radical trapping experiments were performed by adding EDTA, t-BuOH and p-BQ as hole, hydroxyl radical and superoxide radical scavengers, respectively.⁸⁷ Among them, EDTA showed a relatively low impact on the degradation of MO, while the reaction efficacy was dramatically suppressed by the addition of p-BQ and t-BuOH. This suggests that oxygenated radicals play a major role in promoting the photocatalytic reaction (Table 2). Alternatively, the degradation efficiency towards BPB and RhB was relatively equal, which highlights that the molecular structure of the dyes hardly influenced the reaction kinetics.⁸⁸

Fig. 4 (a) MO degradation, (b) effect of different MO concentrations, (c) recycling runs, (d) radical trapping tests and (e) degradation of various dyes (reproduced with permission from ref. 88; Copyright 2021, Elsevier).

Table 2 Role of free radicals in the degradation of pollutants

Photocatalyst	Pollutant	Relative contribution of free radicals for degradation	Ref.
Scavengers for trapping experiment: isopropyl alcohol = Hydroxyl radicals; para-benzoquinone and tempol = superoxide radicals; ammonium oxalate and EDTA = holes; silver nitrate and potassium bromate = electrons.
TiO2/Ti3C2	TC	Hydroxyl radicals > holes > superoxide radicals	84
TiO2/Ti3C2	MO	Superoxide radicals > hydroxyl radicals > holes	88
TiO2/Ti3C2	RhB	Hydroxyl radicals > superoxide radicals > holes	91
Si@Ti3C2/TiO2	DNP	Hydroxyl radicals ∼ superoxide radicals > holes	94
N-TiO2/Ti3C2	RhB	Superoxide radicals > hydroxyl radicals > holes	95
(111) TiO2−x/Ti3C2	MB	Superoxide radicals > holes > hydroxyl radicals	108
TiO2/Ti3C2	POME	Hydroxyl radicals > superoxide radicals > holes > hydrogen peroxide	112
TiO2/Ti3C2	RhB	Superoxide radicals > hydroxyl radicals > holes	113
In2S3/TiO2/Ti3C2Tx	MO	Superoxide radicals > hydroxyl radicals > holes	127
Ti3C2/1T & 2H-MoS2/TiO2	SMZ	Holes > superoxide radicals > hydroxyl radicals	129
Ti3C2/TiO2/Bi2S3	TC	Superoxide radicals > holes > hydroxyl radicals	130
TiO2/Ti3C2/AgI	TC	Superoxide radicals > singlet oxygen > holes	137
AgBr/TiO2/Ti3C2	TC	Superoxide radicals > hydroxyl radicals > holes	139
Ti3C2/TiO2/BiOCl	RhB	Superoxide radicals > holes > hydroxyl radicals	143
BiOI/(001)TiO2/Ti3C2	RhB	Superoxide radicals > oxygen > holes > hydroxyl radicals	144
BPQDs/Ti3C2/TiO2	MO	Holes > superoxide radicals > hydroxyl radicals	156
Ti3C2/TiO2/GCN	NO	Superoxide radicals > holes > hydroxyl radicals	168
Ti3C2/TiO2/GCN	NO	Holes > superoxide radicals > electrons > hydroxyl radicals	169
GCN/TiO2/Ti3C2	MO	Superoxide radicals > holes > hydroxyl radicals	170
GCN/TiO2/Ti3C2	RhB, TC	Holes ∼ superoxide radicals > hydroxyl radicals	171
Ti3C2@GCN/TiO2	RhB	Hydroxyl radicals > holes > superoxide radicals > electrons	172
Fe-C3N4/Ti3C2/C-TiO2	TC	Hydroxyl radicals > holes > superoxide radicals	174
C-TiO2/Bi4NbO8Cl	RhB	Holes > superoxide radicals > hydroxyl radicals	209
C/Ti3C2/(001)TiO2	TC	Hydroxyl radicals > holes > superoxide radicals	210
N-TiO2@C	Phenol	Superoxide radicals > hydroxyl radicals > holes	211
C@TiO2/GCN	TC	Superoxide radicals > holes > hydroxyl radicals	214

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Ti₃C₂/TiO₂ was synthesized via the hydrothermal treatment of Ti₃C₂ with TiSO₄ for 180 °C (Table 3). Compared with TiO₂ and Ti₃C₂ and Ti₃C₂/TiO₂ displayed a higher PCA under UV light due to the heterojunction formed between Ti₃C₂ and TiO₂.⁸⁹ Chen et al.⁹⁰ proposed the solvothermal method for the preparation of Ti₃C₂/TiO₂ at 200 °C for 24 h using different solvents such as water, ethanol and IPA as a morphology directing agent. The TiO₂ grains could nucleate and grow on both the upper and lower surfaces of Ti₃C₂T_x. The TiO₂/Ti₃C₂T_x with IPA exhibited a slightly higher light absorption intensity, good dispersion and mesoporous structure. Superior photocurrent values were observed with the sample synthesized using IPA, which can increase the charge transfer at the TiO₂/Ti₃C₂T_x interface.

Table 3 Partially oxidized MXene Ti₃C₂/TiO₂ for photocatalytic degradation

Photocatalyst	[C] mg	P	[P] mg L^−1	V (mL)	T (min)	Excitation Source	Ref.
TiO2/Ti3C2	20	TC	20	50	180	300 W Xe lamp	84
Ti3C2/TiO2	30	MB	10	30	180	UV lamp	85
Ti3C2/TiO2	20	RhB	20	50	60	300 W Xe lamp	86
Ti3C2/TiO2	35	MO	30	60	40	Solar light	88
TiO2/Ti3C2	100	MO	20	100	30	175 W Hg lamp	89
TiO2/Ti3C2Tx	50	MO	10	50	75	500 W Hg lamp	90
TiO2/Ti3C2	40	RhB	20 μM	50	60	Solar light	91
TiO2/Ti3C2	50	MB	10	10	60	UV lamp (40 W)	92
TiO2/ML-Ti3C2	50	MB, 4-CP		100	300, 150	UV light source	93
Si@TiO2/Ti3C2Tx	50	2,4-DNP	0.1 mM	50	120	Solar light	94
N-TiO2/Ti3C2	15	RhB, MB, levofloxacin and p-NP	20, 5, 20 and 25	30	25	500 W Xe lamp	95
(001) TiO2/Ti3C2	10	MO	20	200	50	300 W Hg lamp	107
(111) r-TiO2−x/Ti3C2	10	MO, MB	20	200	150	500 W Xe lamp.	108
TiO2/Ti3C2Tx		MO		100	50 at 200 °C	500 W Hg lamp	109
TiO2/Ti3C2Tx	50	MB, MO	1	50	30	500 W Hg lamp	110
TiO2/Ti3C2Tx	10	CBZ	5	10		Solar light	111
(001) TiO2/Ti3C2	34	POME		20	1440	100 W black light lamp	112
3D TiO2/Ti3C2	30	RhB	10	70	40	300 W Xe lamp	113
In2S3/TiO2/Ti3C2Tx	60	MO	20	100	60	300 W Xe lamp	127
TiO2/Ti3C2Tx/AgI	50	TC	20	50	180	350 W Xe lamp	137
TiO2 (B)/Ti3C2/Ag3PO4	20	RhB	20	100	60	Solar simulator	138
AgBr/Ti3C2/TiO2	50	TC	20	50	20	70 W metal halide lamp	139
Ti3C2/TiO2/BiOCl	50	RhB	10	50		Solar light	143
Bi2WO6/TiO2/Ti3C2		MG			40	Sunlight	145
BiOBr/TiO2/Ti3C2Tx		RhB	100		30	Visible light	146
BPQDs/Ti3C2/TiO2	50	MO	10	50	60	400 W metal halide lamp	156
Ti3C2/TiO2/CuO	100	MO	20	100	80	175 W Hg lamp	163
TiO2@Ti3C2/GCN	100	Aniline, RhB	20, 10		480 and 60	300 W Xe lamp	167
GCN/TiO2/Ti3C2	50	MO	20		120	300 W Xe lamp	170
GCN/Ti3C2/TiO2		RhB and TC	10		90	300 W Xe lamp	171
Ti3C2/TiO2@rGO	20	(Cr(VI)) and RhB	10		60	300 W Xe lamp	175
2D CdS@Ti3C2/TiO2	50	RhB, MB, SCP and phenol	20	200	60, 88, 88 and 150	Visible light	128

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A unique safflower-shaped structure composed of TiO₂/Ti₃C₂ NRs was obtained by stepwise synthesis including hydrothermal, alkalization, ion exchange and heat treatment method. The layered Ti₃C₂ became spherical during the alkalization step and ion exchange of the intermediate composite, followed by annealing (550 °C, 4–6 h) generated to safflower shape. The heterostructures processed for 4 and 6 h consisted of only anatase TiO₂ and Ti₃C₂T_x, while increasing the heat treatment time (8–10 h) resulted in a mixture of anatase and rutile. This indicates that a heat-treatment window of 4–6 h is optimal for phase purity. In fact, TiO₂/Ti₃C₂ showed higher RhB degradation within 15 min of light illumination. The higher activity is ascribed to the fast carrier generation in TiO₂, efficient suppression of carrier recombination by Ti₃C₂, and good light absorption by the porous safflower structure. The superoxide radicals had a significant effect on the photodegradation reaction compared to hydroxyl radicals and holes.⁹¹ The TiO₂ loaded Ti₃C₂ with a small interlayer spacing was synthesized via the hydrothermal method, followed by the calcination under an Ar flow. The interplanar distance of ∼0.35 nm was evidenced, corresponding to the (101) crystal plane of anatase. –OH- and/or –F-terminated Ti₃C₂ sheets have a very large surface area, high surface energy and are thermodynamically metastable. The chemical-bonded –OH groups will be lost as H₂O or O₂ during the calcination process in an Ar atmosphere, which may result in a decrease in the thickness of the Ti₃C₂ sheets. Meanwhile, the loss of –OH groups changes Ti–OH to Ti–O or uncoordinated Ti is present on some surface of sheets to increase the surface energies. Thus, the electrostatic force of attraction between Ti–O on the surface of one sheet and uncoordinated Ti on the surface of another sheet reduces the free energy, forming a new metastable structure. Therefore, the interlayer spacing of the Ti₃C₂ sheets will decrease during the calcination process in an Ar atmosphere. A small interlayer spacing facilitates the storage of holes in Ti₃C₂, resulting in spatial separation of the charge carriers. The decolorization ability of Ti₃C₂/TiO₂ for MB was relatively well-maintained after three runs with a 9% decrease in MB decolorization, indicating its good stability.⁹² The TiO₂ nanospheres crystallized and in situ grew between the Ti₃C₂ layers, which stretched the MXene sheet to form a multi-layer MXene sheet with swelling space.⁹³ The equilibrium adsorption capacity for MB demonstrates that TiO₂/ML-Ti₃C₂ has a high adsorption capacity for dye molecules (149.47 mg g⁻¹, 40 min), which is consistent with the second-order kinetic model and the Langmuir isotherm model. The surface of Ti₃C₂ has abundant functional groups, which endow the composite with both physical adsorption and chemical adsorption capabilities, and ultimately increasing the removal ability of pollutants. The light absorption intensity of TiO₂/ML-Ti₃C₂ in the visible region increased due to the characteristic absorption of ML-Ti₃C₂. TiO₂/ML-Ti₃C₂ showed the highest photocurrent intensity, indicating the improved lifetime of photogenerated carriers, probably due to the excellent conductivity of Ti₃C₂. ML-Ti₃C₂ exhibited both hydrophilic and lipophilic “parental” properties, and its contact angle to oil droplets was 55° (Fig. 5). TiO₂ is oleophobic, and the oil droplets have a contact angle of ∼130° on its surface. After loading ML-Ti₃C₂, the contact angle of TiO₂/ML-Ti₃C₂ decreased to 65° (Fig. 5). The improved lipophilicity is conducive to the adsorption of non-polar pollutants by TiO₂/ML-Ti₃C₂, also proving that the composite materials have good prospects in the field of oil–water separation and recovery.⁹³

Fig. 5 Wettability photographs of (a) water and (b–d) oil droplets on ML-Ti₃C₂, TiO₂ and TiO₂/ML-Ti₃C₂ (reproduced with permission from ref. 93; Copyright 2021, the Royal Society of Chemistry).

Si@TiO₂/Ti₃C₂T_x was synthesized in situ from Ti₃AlC₂via a combination of high-temperature Si diffusion and HF etching. The optimized Si@TiO₂/Ti₃C₂T_x with the annealing temperature of 1000 °C achieved a higher DNP degradation efficiency after 2 h of light illumination. The DNP degradation of Si@TiO₂/Ti₃C₂T_x is related to its good charge separation capability. The TiO₂ component of this heterostructure generates charge carrier pairs under light illumination. The excited electrons migrate to Ti₃C₂T_x due to the work function difference, forming a Schottky barrier at the interface and Si atoms act as a bridge at the termination groups of Ti₃C₂T_x. The separated electrons and holes interact with the H₂O molecule, thereby creating superoxide radicals and hydroxyl radicals, which can decompose DNP through the formation of several intermediates.⁹⁴ Ding et al.⁹⁵ reported the synthesis of N-TiO₂/Ti₃C₂via the ultrasonic treatment of N-TiO₂ and Ti₃C₂. N-TiO₂/Ti₃C₂ exhibited an excellent sonophotocatalytic performance, which was higher than that of TiO₂ for RhB degradation. The improved activity is attributed to the decrease in its band gap by the N 2p doping level and the improvement in the separation/transfer of photoinduced carriers by Ti₃C₂.

2.2. Integrating Ti₃C₂ with defective titania

In addition to inducing mid-gap states in the band gap, surface imperfections can modify the VB or CB of the host matrix. This is an effective method for reducing the band gap and speeding up the charge carrier movement in semiconductors to achieve high PCA driven by visible light. The interfacial electron transport and PCA are largely mediated by the OVs.^7,96 Given that the localized OV states are situated 0.75 to 1.18 eV below the CB minimum of TiO₂, the presence of OVs can increase the visible light absorption range of titania.^97,98 The carbon species, Ti³⁺ and OVs co-exist in the oxidized composite of Ti₃C₂, resulting in superior PCA.⁹⁹ The OVs in TiO₂ not only serve as active sites for the adsorption and activation of O₂, but also generate impurity levels inside the forbidden band of TiO₂.¹⁰⁰ OV-modified TiO₂/Ti₃C₂ was synthesized through a combined hydrothermal and annealing step. Typically, the hydrothermal oxidation of Ti₃C₂ with ultrapure water at 180 °C for 12 h produced TiO₂/Ti₃C₂, which upon annealing at 450 °C under a pure N₂ gas flow produced a defect-rich composite. The PCA of BPA degradation using defect-rich TiO₂/Ti₃C₂ was enhanced compared to the hydrothermally synthesized composites under similar experimental conditions. The low charge transfer resistance and high photocurrent response for the composite annealed at 2 h indicate that the annealing time plays a crucial role in tailoring the defect density. The flat-band potential (V_fb) of the hydrothermally treated and defect-rich composite extracted from the Mott–Schottky plot was −0.27 V and −0.17 V vs. SCE, which correspond to −0.03 V and +0.07 V vs. NHE, respectively. It is well known that the CB position of TiO₂ can be calculated from its V_fb potential (CB = V_fb − 0.2 V). Thus, the CB position of the as-prepared TiO₂ and TiO_2−x was determined to be −0.23 V and −0.13 V, respectively. The Schottky barrier height of defect-rich TiO₂/Ti₃C₂ is lower than that of the hydrothermally obtained composite. A decrease in the Schottky barrier is beneficial for the transfer of charge carriers, subsequently improving the separation efficiency of photoinduced charge carriers, ultimately boosting the degradation of BPA. The OVs can also promote the adsorption of contaminants and increase the interfacial reactions.¹⁰¹

Ti₃C₂/Ti³⁺-TiO₂ was synthesized via the oxidation of Ti₃C₂ with H₂O₂. The oxidation of Ti₃C₂ with H₂O₂ caused a drastic increase in SSA compared to Ti₃C₂. The formation of the mosaic structure of TiO₂ NPs embedded in the layered substrates is the main reason for the tremendous increase in the SSA. ESR confirmed the formation of a Ti³⁺ signal from the ‘g’ value of 1.95 to 2.004, which may be related to the formation of OVs. Compared with Ti₃C₂/Ti³⁺–TiO₂ (3 mL H₂O₂) and Ti₃C₂/Ti³⁺–TiO₂ (100 μL H₂O₂), Ti₃C₂/Ti³⁺–TiO₂ (50 μL H₂O₂) not only had a larger amount of Ti³⁺–TiO₂ NPs, but also possessed a mosaic heterojunction structure, which prevents the recombination of charge carriers. These two effects ensure the generation of sufficient active radicals for the degradation of acetaldehyde.¹⁰²

2.3. Integrating Ti₃C₂ with faceted and 3D titania

Exposing the highly active facets has been regarded as a powerful approach to synthesize high-performance photocatalysts.¹⁰³ The highly active (001) facets of TiO₂ facilitate the efficient photogeneration of charge carrier pairs, while the interfacial Schottky junction with 2D Ti₃C₂ enhances the charge separation by trapping holes. Ti₃C₂ acts as a hole reservoir, leading to improved charge separation. The combination of enhanced charge separation and the exposure of active facets significantly enhance the PCA.¹⁰⁴ The oxidation of the Ti-precursor with a facet-capping agent such as NaBF₄ and NH₄F enhanced the formation of high-energy facets during sequential crystal growth. Under UV irradiation, the exposed (001) facets of TiO₂ are excited to produce charge carrier pairs. The transfer of electrons is forbidden at the interfaces between the TiO₂ (001) surface and Ti₃C₂ because of the more negative Fermi level of Ti₃C₂ than the CB of TiO₂. The crystallographic facet proportions of TiO₂ can be controlled by the facet-capping agents (NaBF₄/NH₄F).^104–106 Peng et al.¹⁰⁷ reported the synthesis of 2D Ti₃C₂/TiO₂ with exposed (001) facets via the hydrothermal method with different times and temperatures. The in situ growth of TiO₂ NSs on Ti₃C₂ created an interface with minimized defects (Fig. 6). The highly active (001) facets of TiO₂ resulted in the high-efficiency photogeneration of charge carrier pairs; meanwhile, the carrier separation is substantially promoted by the hole trapping effect by the interfacial Schottky junction with 2D Ti₃C₂ acting as a reservoir of holes. By controlling the degree of oxidation through the reaction duration and temperature, the TiO₂ NSs could be homogeneously distributed around the layered Ti₃C₂ to provide improved accessibility to light and reactants. According to the width and thickness of the (001) TiO₂ NSs, the proportion of (001) facets was estimated to be about 77.5% based on a geometric model of anatase TiO₂. The intimate contact between these two phases facilitates the separation of charge carriers on the (001) surfaces, thereby improving the PCA. At the interface, Ti₃C₂ and TiO₂ conjugated seamlessly at the atomic level, benefitting from the small mismatch between the (103) facet of Ti₃C₂ and (111) facet of TiO₂, despite their different crystallographic forms. As the hydrothermal reaction proceeds, the content of TiO₂ increases, while the content of other species decreased, indicating that Ti₃C₂ is consumed and transformed to (001) TiO₂ NSs. The (001) TiO₂/Ti₃C₂ showed about a 2.3-fold higher MO degradation rate than its non-faceted counterparts, demonstrating the importance of (001) facets. The steady photovoltage of (001) TiO₂/Ti₃C₂ for different hydrothermal durations followed the same order as the transient photocurrent (12 h > 24 h > 4 h). The MO degradation reaction with scavengers indicates that hydroxyl radicals are the most important reactive species, followed by holes and superoxide radicals. The growth of TiO₂ and the interfaces between TiO₂ and Ti₃C₂ supports the growth mechanism for the heterojunction (Fig. 6a–f). The DFT studies on the initial and final optimized interface between TiO₂ and Ti₃C₂ (Fig. 6g) outlined the interface formation by the overlapping of the Ti d-orbital and O p-orbital of the Ti–O–Ti interface (Fig. 6h and i, respectively).

Fig. 6 (a–f) TEM image of TiO₂/Ti₃C₂, (g) initial and final optimized interface structures and (h and i) corresponding PDOS (reproduced with permission from ref. 107; Copyright 2016, the American Chemical Society).

Peng et al.¹⁰⁸ introduced NH₄F, followed by hydrazine hydrate treatment during the hydrothermal synthesis of (111) rutile TiO_2−x/Ti₃C₂. The crystals with adjustable ratios of (111) and (110) facets could be grown on layered Ti₃C₂ without any additional Ti sources. The unique interlayer pores in the layered Ti₃C₂ provide excellent channels for trapping holes. After treatment with hydrazine hydrate, the absorption peak shifted towards a higher wavelength with improved light absorption in the visible range. This observation showed the presence of bulk Ti³⁺ defects, which induced a continuous vacancy band of electronic states in the forbidden band of TiO_2−x. Furthermore, annealing the composite (2 h) in air made them inactive, demonstrating that Ti³⁺ doped in (111) rutile-TiO₂ enabled PCA under visible light. The facet-exposed TiO₂/Ti₃C₂T_x synthesized via the solvothermal method at different temperatures (160–220 °C) for 24 h.¹⁰⁹ The interfacial angle between the (001) and (101) facets of TiO₂ is about 68.3°, which is in good agreement with the (001) crystal plane of anatase TiO₂. By controlling the reaction time and temperature, the (001) and (101) facets of TiO₂ had uniform nucleation and growth around Ti₃C₂T_x. The in situ formation of TiO₂ on the Ti₃C₂T_x layers will increase the charge separation efficiency and improve the PCA of the exposed facets. The facet-exposed TiO₂/Ti₃C₂T_x possessed a stronger bonding interface via in situ solvothermal synthesis, which effectively promoted the separation of charge carrier pairs. The facet-exposed TiO₂/Ti₃C₂T_x synthesized at 200 °C exhibited the highest MO degradation. Its superior adsorption performance was mainly due to the high PCA of the (001) TiO₂ surface, which made it easier to adsorb pollutants.¹⁰⁹ Exposed-facet TiO₂/Ti₃C₂T_x with chemical bonding (Ti–O–Ti) was constructed via the in situ solvothermal method using different concentrations of NaBF₄ as a cosurfactant to control the oxidation of Ti₃C₂. When the reaction concentration was 1.5 mmol NaBF₄, the content of (101) and (001) TiO₂ increased significantly and its crystal size increased. The crystal planes of (101) and (001) TiO₂ were mostly square and truncated biconical octahedra. According to this comprehensive analysis, it was determined that increasing the fluoride concentration can adjust the crystal structures of TiO₂/Ti₃C₂T_x. When the concentration exceeded 1.5 mmol NaBF₄, most of the Ti₃C₂T_x layers were oxidized to form (101) and (001) TiO₂ NPs, which are prone to agglomeration. The superior degradation of MB and MO was achieved by TiO₂/Ti₃C₂T_x within 30 min of light illumination.¹¹⁰ The (001) facets of TiO₂/Ti₃C₂ synthesized via the hydrothermal method showed an appreciable degradation rate for CBZ. The zeta-potential measurements of (001)-TiO₂/Ti₃C₂ showed that the PZC value was around ∼3.1, and thus lower pH values (3.0 and 5.0) were favourable for their degradation.¹¹¹ POME degradation was performed under black light irradiation and without external oxygen bubbling to mimic the degradation process under natural ponding conditions using a faceted TiO₂/Ti₃C₂ aerogel. The hydrothermal treatment (95 °C, 6 h) of the Ti₃C₂ colloidal solution resulted in the formation of a Ti₃C₂ hydrogel. Further freeze drying of the Ti₃C₂ aerogel at −50 °C for 96 h resulted in the formation of the respective aerogel. The FESEM and TEM results showed an increase in the surface roughness and porosity of the Ti₃C₂ aerogel structure, which enhanced its contact with TiO₂, leading to the formation of interfacial heterojunctions. It was observed that significant POME degradation progressed during the first 8 h of reaction (Table 3).¹¹² Quyen et al.¹¹³ prepared unique TiO₂@Ti₃C₂ NFWs, where heat treatment played a significant role. The TiO₂/Ti₃C₂ NFWs achieved the superior degradation of RhB compared to TiO₂ and Ti₃C₂. In fact, TiO₂/Ti₃C₂ showed ∼74% removal efficiency toward RhB, which declined to ∼50% and ∼30%, with IPA and p-BQ, indicating that hydroxyl and superoxide radicals play a major role in PCA, respectively. The layered morphology of Ti₃C₂T_x completely disappeared using the simultaneous hydrothermal oxidation and ion exchange synthesis method, and another morphology was formed with nanowhiskers packed together as flowered spheres (Fig. 7). As shown in Table 2, the degradation of RhB (any pollutant) is quite complex and distinct free radicals and charge carriers mediate the degradation pathways depending on the excitation source, concentration of scavengers, band edge potential of the semiconductors, nature of the heterostructure and concentration of RhB.

Fig. 7 SEM images of (a) Ti₃AlC₂, (b) Ti₃C₂, (c) inter-Ti₃C₂ and (d) TiO₂/Ti₃C₂@500 °C (reproduced with permission from ref. 113; Copyright 2021, Elsevier).

3. Preparation and photocatalytic activity of Ti₃C₂-based ternary composites for organic pollutant degradation

Although binary composites have been reported to be active, their PCA is not satisfactory from the prospect of complete utilization of excited charge carriers. Another setback is the narrow visible light response and poor charge carrier migration pathways across their junction interface, which often result in a lower efficiency. Thus, to circumvent these problems, the design and fabrication of ternary composites comprised of narrow gap semiconductors will be interesting given that their optical response can span the complete solar spectrum. Furthermore, Ti₃C₂ can form different interfaces with other semiconductors conveniently and facilitate the migration of excited charge carriers, thereby avoiding the recombination pathways.^114–118

3.1. Integrating Ti₃C₂/TiO₂ with metal sulphides

Most metal sulphides have their absorption edge in the visible region and their CB potential is highly negative, making them suitable as catalysts for HER.^119,120 In addition, they also facilitate the production of superoxide radicals in aqueous medium. Furthermore, the preparation of metal sulphides with diverse morphologies using different sulphur precursors is feasible, which remains an added merit for the fabrication of heterostructures.^121–126 A high density of irregular In₂S₃-NPs grew on the edge and surface of Ti₃C₂T_x NSs. The In³⁺ ions can be adsorbed near the edges of multilayer hydrophilic Ti₃C₂T_x, and also diffuse into the deeper sites given that the interior of NPs has favourable activation energies for the adsorption and intercalation of In³⁺ ions. The increase in SSA is due to the presence of In₂S₃ or TiO₂ on the edge or surface of Ti₃C₂T_x. The quasi-core–shell In₂S₃/TiO₂/Ti₃C₂T_x exhibited a good TOC removal efficiency, indicating the gradual mineralization of the MO dye.¹²⁷ The 2D CdS@Ti₃C₂/TiO₂ was synthesized via facile calcination and a subsequent hydrothermal process. Ti₃C₂/TiO₂ was synthesized via calcination at 500 °C for 2 h and CdS was grown on the composite surface through hydrothermal treatment (200 °C for 6 h). The complete degradation of a wide variety of pollutants such as RhB, MB, SCP and phenol was achieved using CdS@Ti₃C₂/TiO₂. The PCA was enhanced due to the formation of a Z-scheme heterostructure, which facilitated the separation of charge carrier pairs. Moreover, the hybrids possessed higher redox ability because the electrons accumulated in the more negative CB of CdS and the holes at the positive VB of TiO₂.¹²⁸ Ti₃C₂/1T & 2H-MoS₂/TiO₂ was prepared through the combined assistance of the solvothermal method and electrostatic self-assembly approach. The concentration of SMZ was reduced only 4.48% after 110 min of light illumination, suggesting that the photolysis of SMZ is negligible. The degradation of SMZ was also less than 5% after adsorption–desorption equilibrium in the absence of light for 40 min. Almost complete degradation of SMZ was achieved by Ti₃C₂/1T & 2H-MoS₂/TiO₂ within 90 min of light illumination. The holes and superoxide radicals dominated the SMZ degradation mechanism, while the hydroxyl radicals had less impact. The mechanism of SMZ degradation by Ti₃C₂/1T & 2H-MoS₂/TiO₂ involves the following steps: (i) both TiO₂ and MoS₂ are excited to form charge carrier pairs in the respective semiconductors; (ii) electrons flow from TiO₂ to MoS₂, creating an internal electric field and facilitating charge separation at the TiO₂–MoS₂ interface; (iii) then electrons migrate from MoS₂ to Ti₃C₂, where they react with O₂ to form superoxide radicals, extending the lifespan of electrons with strong reduction abilities; (iv) the holes in the VB of TiO₂ react with OH⁻ or H₂O to form hydroxyl radicals, which along with superoxide radicals, oxidize SMZ, leading to its degradation (Fig. 8).¹²⁹

Fig. 8 Charge carrier dynamics in Ti₃C₂/1T & 2H-MoS₂/TiO₂ (reproduced with permission from ref. 129; Copyright 2024, Elsevier).

Similarly, Ti₃C₂/TiO₂/Bi₂S₃ was also synthesized using a multi-stage approach, wherein Bi₂S₃ was obtained under hydrothermal conditions and later integrated with Ti₃C₂/TiO₂via a electrostatic self-assembly process. Ti₃C₂/TiO₂/Bi₂S₃ exhibited excellent PCA, achieving superior catalytic oxidation efficiency. The strong light absorption of the composite improved the band gap excitation process, thereby generating more charge carriers. In addition, the formation of a Z-scheme heterojunction between Bi₂S₃ and Ti₃C₂/TiO₂, with Ti₃C₂ serving as a mediator, effectively separated the energetic charge carriers for the photocatalytic reactions (Fig. 9).¹³⁰

Fig. 9 Possible photocatalytic mechanism of I-type, Ti₃C₂-bridged Schottky/Z-scheme, and Ti₃C₂ Schottky/Z-scheme (reproduced with permission from ref. 130; Copyright 2023, Elsevier).

3.2. Integrating Ti₃C₂/TiO₂ with Ag-based semiconductors

Ag-based semiconductors have attracted significant attention owing to their convenient preparation at RT through the simple precipitation approach, and also their suitable band edge positions to form heterojunctions with TiO₂. AgCl, AgBr, AgI, Ag₃PO₄, Ag₂O and Ag₂CO₃ have been used as co-catalysts for wastewater purification and HER.^131–136 TiO₂/Ti₃C₂T_x/AgI was synthesized via an in situ solvothermal method for 24 h at 150 °C and co-precipitation method. The agglomeration of AgI was effectively inhibited when AgI was combined with TiO₂/Ti₃C₂T_x. TiO₂/Ti₃C₂T_x/AgI demonstrated superior activity than AgI and TiO₂/Ti₃C₂T_x for TC degradation under simulated sunlight. The recombination rate of the electrons and holes was effectively reduced due to the formation of a Z-scheme heterojunction with Ti₃C₂T_x as an electron acceptor. The EPR analysis showed that superoxide radicals and singlet oxygen radicals were essential participants in the degradation process. The HPLC-MS analysis identified the intermediates in the degradation of TC by TiO₂/Ti₃C₂T_x/AgI, with TC breaking down into simpler products such as mesoxalic acid, propionic acid, and carbamic acid. The CB position of TiO₂ (−0.28 eV) is more positive than that of AgI (−0.45 eV), and the photogenerated electrons in the TiO₂ CB transfers to Ti₃C₂T_x (CB, −0.11 eV) through the Z-scheme heterojunction. Because Ti₃C₂T_x has metal properties, a Schottky barrier is formed at the interface of TiO₂/Ti₃C₂T_x, which is used as a trap to capture electrons. Thus, flowing back of the photogenerated electrons to TiO₂ is prevented, thereby effectively inhibiting the recombination of TiO₂ photogenerated electron–hole pairs. The VB holes of AgI migrate to Ti₃C₂T_x to recombine with the CB electrons of TiO₂. The VB position of TiO₂ (2.72 eV) has a more positive potential than that of AgI (2.33 eV). Therefore, it provides stronger oxidation and is more conducive to the photocatalytic oxidative degradation of TC (Fig. 10).¹³⁷

Fig. 10 Proposed charge carrier transfer pathways and generation of oxygenated species on the surface of TiO₂/Ti₃C₂T_x/AgI (reproduced with permission from ref. 137; Copyright 2022, Elsevier).

TiO₂ bronze (B)/Ti₃C₂/Ag₃PO₄ was synthesized via the electrostatic self-assembly process. Ag₃PO₄-QDs were introduced on the surface of Ti₃C₂ and TiO₂ (B) via in situ self-growth, wherein Ag₃PO₄ and Ti₃C₂ formed a Schottky junction, and Ag₃PO₄ and TiO₂ (B) formed a heterojunction. These junctions created a synergistic effect with the Schottky junction formed by Ti₃C₂ and TiO₂ (B), further promoting the rapid migration and separation of charge carrier pairs. This composite showed a higher RhB degradation efficiency than TiO₂(B), Ag₃PO₄ and TiO₂(B)/Ti₃C₂.¹³⁸ Wu et al.¹³⁹ described the preparation of AgBr/Ti₃C₂/TiO₂via the precipitation method. In a typical approach, Ti₃C₂/TiO₂ was dispersed in a solution containing AgNO₃ and KBr was added dropwise for 2 h under the stirring conditions to form the composite. The heterostructure showed enhanced PCA due to the formation of a p–n heterojunction between AgBr and Ti₃C₂/TiO₂. The excellent electronic properties of Ti₃C₂, enhanced visible light absorption capacity, low internal resistance, and reduced recombination efficiency of charge carriers additionally contributed to the overall efficiency. The PCA of AgBr/Ti₃C₂/TiO₂ was 24.5-times higher than that of Ti₃C₂/TiO₂ for the photodehydrogenation of 1,4-DHP and 1.9-times higher than that of pure AgBr for TC degradation (Fig. 11). Further, upon the addition of IPA and ammonium oxalate, the degradation of TC slightly decreased and showed little effect, respectively. In contrast, when BQ was added to the reaction system, the degradation of TC was obviously inhibited, indicating the major role of superoxide radicals in the degradation of TC (Table 2).

Fig. 11 Photocatalytic oxidation of 1,4-DHP: (A) reaction kinetics, (B) degradation efficiency, (C) linear kinetic fitting and (D) apparent rate constant (reproduced with permission from ref. 139; Copyright 2023, the Royal Society of Chemistry). [Note: AgBr/T@T-x% = AgBr/Ti₃C₂/TiO₂ with different mass percentages of Ti₃C₂/TiO₂.]

3.3. Integrating Ti₃C₂/TiO₂ with Bi-based semiconductors

Bi-based semiconductors possess typical layered structures and have an optical response spanning the UV-visible region. They can also serve as suitable substrates for the growth of another semiconductor and integrated with Ti₃C₂ to form 2D–2D composite. The Bi₂WO₆, BiOX and non-stoichiometric bismuth oxyhalides have been extensively explored in PCA.^140–142 Ti₃C₂/TiO₂/BiOCl was fabricated via the hydrothermal oxidation of Ti₃C₂ with Bi(NO₃)₃·5H₂O and KCl for one day at 180 °C. The radical trapping study revealed that the superoxide radicals play a significant role in the degradation pathways of RhB (Table 2). The degradation efficiency of RhB was 78.36%, 60.37%, 55.55% and 32.78% at a solution pH of 2, 4, 6 and 8 with simulated sunlight irradiation, respectively. Thus, the results reveal that acid pH is favourable for improving PCA.¹⁴³

Ke et al.¹⁴⁴ reported the synthesis of faceted BiOI/(001)TiO₂/Ti₃C₂ in two stages, where (001) TiO₂/Ti₃C₂ was synthesized via the hydrothermal oxidation of Ti₃C₂ with NaBF₄ at 160 °C for 12 h, followed by precipitating BiOI in ethylene glycol under the hydrothermal conditions (12 h at 160 °C). The SEM analysis showed that BiOI flowers and nanoflakes were deposited on the surface of (001) TiO₂/Ti₃C₂, especially on the (001) facets of TiO₂. BiOI/(001) TiO₂/Ti₃C₂ exhibited the highest degradation rate, which is 6.26, 1.72, and 1.35 times that of pure BiOI, BiOI/TiO₂ and BiOI/Ti₃C₂, respectively. Bi₂WO₆/TiO₂ and Bi₂WO₆/TiO₂/Ti₃C₂ synthesized via the electrostatic self-assembly route showed superior MG degradation. The highest value of k was exhibited by Bi₂WO₆/TiO₂/Ti₃C₂, which was 2.1 times higher than that of Bi₂WO₆/TiO₂. Both TiO₂ and Bi₂WO₆ are n-type semiconductors with appropriate band positions for S-scheme-driven charge separation and transfer process. Upon sunlight irradiation, both Bi₂WO₆ and TiO₂ n-type semiconductors can be excited. Under the influence of the internal electric field, the electrons from the CB of Bi₂WO₆ recombine with holes in the VB of TiO₂. Consequently, the highly reducing electrons at the CB of TiO₂ and highly oxidizing holes at the VB of Bi₂WO₆ are available on Bi₂WO₆/TiO₂, which degrade the MG dye. In Bi₂WO₆/TiO₂/Ti₃C₂, Ti₃C₂ acts as an electron acceptor and helps in the further efficient transfer of electrons from the CB of TiO₂, which subsequently enhances the PCA. The trapping studies showed the significant role of hydroxyl radicals in the MG degradation process by using IPA scavengers.¹⁴⁵ BiOBr/Ti₃C₂/TiO₂ synthesized via a one-step hydrothermal method exhibited almost complete degradation of RhB within 30 min of light illumination (Table 3). This is due to the optimized synergistic effects of BiOBr, TiO₂ and Ti₃C₂T_x. The construction of a heterojunction between BiOBr and TiO₂ makes it rational that the photo-generated holes will rapidly migrate from the VB of BiOBr to the VB of TiO₂ and the excited electrons will also transfer from the CB of TiO₂ to BiOBr, leading to the efficient separation and transfer of photo-generated electrons and holes.¹⁴⁶

3.4. Integrating Ti₃C₂/TiO₂ with QDs

QDs can be uniformly distributed on the surface of Ti₃C₂ to form the typical 0D/2D heterojunction, which not only improves their contact area but also their optical response can be easily tailored depending on their size of QDs.¹⁴⁷ Furthermore, the decoration of QDs on semiconductor surfaces is beneficial from the prospect of both light absorption properties and the high surface area of QDs, which can promote surface reactions between free radicals and pollutant molecules.^148–153 A novel 0D/2D/2D NCDs/TiO₂/Ti₃C₂T_x composite was synthesized via the hydrothermal method. NCDs/TiO₂/Ti₃C₂T_x showed HER and TC degradation activity nearly ∼4 and 9 times higher than that of pure TiO₂, respectively. Further increasing the hydrothermal duration to 48 h, the Ti₃C₂T_x layer completely collapsed and oxidized to TiO₂. The optimum content of Ti₃C₂T_x in TiO₂/Ti₃C₂T_x was found to be 42%, resulting in higher activity. NCDs/TiO₂/Ti₃C₂T_x could quickly produce a large amount of hydroxyl radicals and superoxide radicals under irradiation. The work function of Ti₃C₂F₂ increased to 4.704 eV when its surface groups were –OH and –O, while the work function of Ti₃C₂O₂ and Ti₃C₂(OH)₂ changed significantly to 1.877 and 5.936 eV, respectively. According to the UV photoelectron spectra and DFT calculation results, the work function of Ti₃C₂T_x was determined to be 4.16 eV and 3.63 eV, respectively. These values are larger than that of bare Ti₃C₂ (work function = 3.55 eV), while the work function of TiO₂ and NCDs is 3.98 and 3.09 eV, respectively.¹⁵⁴ BPQDs exhibited a high absorption coefficient, significant edge and quantum confinement effects, and easy hybridization with different materials.¹⁵⁵ BPQDs/Ti₃C₂/TiO₂ was synthesized via a combined hydrothermal method and solvent heat treatment, which involved the addition of ∼50 mg of Ti₃C₂/TiO₂ to 50 mL of BPQD solution and heated at 80 °C for 2 h. BPQDs/Ti₃C₂/TiO₂ was beneficial for the enhancement of optical absorption performance and exhibited the highest degradation efficiency. The trapping experiment revealed that the photogenerated holes and superoxide radicals play an important role in the degradation efficiency.¹⁵⁶

3.5. Integrating Ti₃C₂/TiO₂ with metal oxides and carbon materials

Non-TiO₂-based metal oxides have attracted attention owing to their easy preparation with affordable precursors and their suitable band gap as well as band edge potentials, resulting in increased light absorption properties and generation of oxygenated free radicals in aqueous medium.^157–162 Ti₃C₂/TiO₂/CuO was synthesized by annealing a mixture of cupric nitrate and Ti₃C₂ under an Ar atmosphere at 500 °C with different weight ratios of cupric nitrate. The MO quickly degraded within 80 min of light illumination with the sample having 0.01 g cupric nitrate as the catalyst. With a further increase in the cupric nitrate content, agglomeration occurred, leading to a decrease in the PCA. Upon irradiation from a UV light source, Ti₃C₂/TiO₂/CuO exhibited more effective charge carrier separation, and significantly enhanced PCA.¹⁶³ Carbon materials such as graphene and GCN have also been extensively used owing to their inherent 2D anisotropy and high surface area. Graphene can trap electrons from the excited semiconductors to prevent their recombination, while GCN is an excellent metal-free photocatalytic material with visible light absorption capacity.^164–166 TiO₂/Ti₃C₂/GCN was synthesized via an ultrasonic-assisted calcination method. The porous diameter was estimated to be about ∼3.81 nm for Ti₃C₂ and ∼25.6 nm for TiO₂/Ti₃C₂/GCN. The increased pore size is favourable for the mass transfer of both the reactants and degradation products inside the mesoporous structure. After 8 h of reaction time, TiO₂/Ti₃C₂/GCN could remove aniline, with a 5-fold increase in degradation compared to that of GCN. Furthermore, its activity for the TOC removal of aniline only decreased by 43.7% in 8 h. The GC analysis results of the final reaction mixtures revealed the presence of BQ (6.098 min) and phthalic acid (17.664 min), indicating that the ROS resulted in the poor oxidation of the intermediate products of quinones. In the case of RhB, the total degradation time was 60 min, and TiO₂/Ti₃C₂/GCN also showed the highest removal rate, which was 1.33 times higher than that of pristine GCN. The TOC removal of RhB reached 98.2% after 60 min. The synergistic effects of the n–n heterojunction and n-type Schottky heterojunction can effectively separate the photogenerated carriers in TiO₂/Ti₃C₂/GCN.¹⁶⁷ In another study, this type of composite was obtained by grinding GCN with different amounts of hydrothermally synthesized Ti₃C₂/TiO₂ at 160 °C. The SEM images confirmed that Ti₃C₂/TiO₂ was embedded in the cotton-like GCN, forming an intimate contact. GCN could effectively remove a high concentration of NO, which discharged a huge amount of NO₂. After exposure to visible light for 30 min, Ti₃C₂, Ti₃C₂/TiO₂, and GCN demonstrated the NO removal rates of 0.6%, 1.6%, and 25.2%, which released 2.28, 3.43, and 57.46 ppb of NO₂, respectively. In the case of Ti₃C₂/TiO₂/GCN, a high NO removal rate of 28.9% and a small amount of NO₂ (18.75 ppb) could be simultaneously achieved (eqn (4)–(9)).¹⁶⁸

e⁻ + O₂ → O˙₂ (4)

O˙₂ + H₂O(H⁺) + e⁻ → H₂O₂ + OH˙ (5)

TiO₂(h⁺) + H₂O(OH) → OH˙ (6)

O˙₂ + NO → NO₃⁻ (7)

OH˙/h⁺ + NO → NO₂ (8)

OH˙/h⁺ + NO → NO₃⁻ (9)

A GCN/TiO₂/Ti₃C₂ S-scheme heterojunction was synthesized via the calcination of melamine with partially oxidized TiO₂/Ti₃C₂ at different temperatures under an N₂ flow. The NSs connected with each other, forming a foam-like structure, which increased the absorption of light, and also enlarged the reaction interface between NO and the composite surface. Considering that the VB edge of GCN (1.76 V) is higher than the redox potential of H₂O/˙OH (2.37 V) and OH/˙OH (1.99 V), the holes in the VB of GCN could not oxidize H₂O or –OH to ˙OH. Thus, the holes could not be transferred from the VB of TiO₂/Ti₃C₂ to the VB of GCN, leading to the accumulation of holes. Based on the above-mentioned experimental results, the NO degradation on GCN/TiO₂/Ti₃C₂ can be ascribed to the S-scheme heterojunction mechanism. An obvious inhibitory effect was observed for KI compared to p-BQ and K₂Cr₂O₇, while the presence of TBA was the least inhibitory. The order of free radical contribution for the photocatalytic degradation mechanism follows the order of holes > superoxide radicals > electrons > hydroxyl radicals (Table 2).¹⁶⁹

The ball milling (1060 rpm) of the GCN precursors and MXene and subsequent annealing (580 °C, 3 h) under an air atmosphere resulted in the formation of GCN/TiO₂/Ti₃C₂. It was suggested that Ti₃C₂ as a carrier, loaded with GCN and in situ oxidized TiO₂ could further enlarge the catalytic reaction area of the composite and facilitate the degradation reaction. GCN/TiO₂/Ti₃C₂ showed the best MO degradation rate, which was 3.62 times that of g-C₃N₄ and 6.60 times as that of Ti₃C₂.¹⁷⁰ Diao et al.¹⁷¹ synthesized GCN/Ti₃C₂/TiO₂ nanotube arrays on Ti mesh via anodization, chemical vapor deposition method and in situ growth of TiO₂ by calcination at 550 °C for 3 h. The ternary structure exhibited the highest RhB and TC degradation efficiency with the illumination of light for 90 and 180 min, respectively. The active species trapping experiment showed that the holes and superoxide radicals were the main oxidative radicals for the degradation. The electrons and holes migrated in the opposite direction between TiO₂ and GCN to improve the charge carrier separate rates (Fig. 12).

Fig. 12 Proposed charge carrier transfer mechanisms in GCN/Ti₃C₂/TNTAs (reproduced with permission from ref. 171; Copyright 2020, Elsevier). [Note: TNTAs = TiO₂ nanotube arrays.]

Ti₃C₂/TiO₂/GCN synthesized via direct electrostatic self-assembly during the hydrothermal process displayed the highest degradation for RhB. The GCN/TiO₂ heterojunction and Schottky barrier between TiO₂ and Ti₃C₂ boosted the transfer and separation of photogenerated carriers.¹⁷² In another report, PDI/GCN/TiO₂/Ti₃C₂ was synthesized via the calcination method. A certain amount of Ti₃C₂ NSs was mixed uniformly in GCN and PDI powder and calcined to 365 °C in air. The delocalization of photoelectrons induced by the π–π interaction in PDI/GCN and the intimate contact and staggered band configurations between PDI/GCN and TiO₂/Ti₃C₂ resulted in an S-scheme heterojunction. The degradation of ATZ by the composite was enhanced under visible light and the TOC analysis showed that the mineralization of ATZ reached 46.08% and further enhanced with an increase in the reaction time for the PDI/GCN/TiO₂/Ti₃C₂/PMS/vis system. The PMS and visible light in the system synergistically promoted the degradation of ATZ. It was assumed that a type-II or S-scheme heterojunction was likely formed between PDI/GCN and TiO₂/Ti₃C₂ (Fig. 13). Based on the reactive species analysis through the trapping experiments and EPR analysis, the generation and transportation of the reactive species were proposed (eqn (10)–(20)).¹⁷³

Photocatalyst + hν → h⁺ + e⁻ (10)

HSO₅⁻ + h⁺ → SO₅⁻ + H⁺ (11)

2SO₅⁻ + h⁺ → 2SO₄²⁻ + ¹O₂ (12)

H₂O + h⁺ → H⁺ + OH (13)

H₂O + SO₅²⁻ → SO₄²⁻ + H₂O₂ (14)

e⁻ + HSO₅⁻ → SO₄˙⁻ + OH⁻ (15)

SO₄˙⁻ + OH⁻ → SO₄²⁻ + OH˙ (16)

e⁻ + O₂ → O₂˙⁻ (17)

e⁻ + O₂˙⁻ + 2H+ → H₂O₂ (18)

O₂˙⁻ + OH˙ → O₂¹ + OH⁻ (19)

Fig. 13 Schematic of type-II and S-scheme heterojunction mechanism of PDI/GCN/TiO₂/Ti₃C₂/PMS/vis system (reproduced with permission from ref. 173; Copyright 2022, Elsevier).

Guan et al.¹⁷⁴ prepared Fe-C₃N₄/Ti₃C₂/C-TiO₂ by annealing melamine, FeSO₄ and Ti₃C₂ at 550 °C for 2 h in an N₂ atmosphere. The highest TC degradation was achieved within 60 min of light illumination using this composite. According to the Fresnel effect, the rough light-receiving layer possesses a small surface reflection coefficient, leading to a higher utilization efficiency and better photothermal efficiency. The degradation performance in the photo-Fenton reaction was improved with the help of the outstanding photothermal effects. The generated thermal energy could accelerate the activation of H₂O₂, adsorption of TC, and desorption of products. Alternatively, “hot electrons” were produced, which could promote the separation of charge carrier pairs. The possible charge transfer mechanism for Fe-C₃N₄/Ti₃C₂/C-TiO₂ was described (Fig. 14a). According to the work function (top of Fig. 14b), the E_F follows the order of E_F (Fe-C₃N₄) > E_F (Ti₃C₂) > E_F (C-TiO₂), suggesting that electrons can spontaneously flow from the high E_F to the low E_F, resulting in the bending of the band edge (bottom of Fig. 14b). Two internal electrical-fields are built at the two interfaces during the formation of the heterojunction, and electrons migrate from Fe-C₃N₄ to C-TiO₂ through Ti₃C₂ until they reach the same E_F level. Under visible-light (Fig. 14a), the electrons in the VB of Fe-C₃N₄ and C-TiO₂ are excited to their CB. Driven by the internal electrical-fields, the electrons in C-TiO₂ are prone to migrating to Fe-C₃N₄via Ti₃C₂ as a bridge, and then recombine with the holes in the VB of Fe-C₃N₄, leading to the formation of a Z-scheme. The projected density of states (PDOS) and partial charge densities are shown in Fig. 14c, where the left side represents the charge density of the occupied states (−0.80–0.58 eV) and the right side corresponds to the unoccupied states (0.03–0.25 eV). The results show that the electrons in the occupied states were nearly localized around the C-TiO₂ side with the character of the C 2p and Ti 3d orbital, while the unoccupied states were mainly composed of the Fe 3d orbital. Under visible-light, in the case of Fe-C₃N₄/C-TiO₂, the electrons of C 2p and Ti 3d in the C-TiO₂ side were mainly excited into the Fe 3d orbital (right top of Fig. 14c). Thus, the electrons mainly localized on Fe-C₃N₄ and the holes accumulated on C-TiO₂ during photocatalysis. The partial charge density calculation indicated that the charge density around the doped-Fe atoms largely increases (Fig. 14c).

Fig. 14 (a) Possible mechanism of photo-Fenton reaction, (b) formation of Z-scheme heterojunction before and after contact and (c) PDOS and partial charge densities of Fe-C₃N₄/C-TiO₂ (reproduced with permission from ref. 174; Copyright 2022, Elsevier).

The rGO-wrapped Ti₃C₂/TiO₂ NWs prepared via the hydrothermal method at 130 °C for 3 h exhibited a reasonable performance towards the reduction of Cr(VI). This composite showed a high photocurrent response, which may be ascribed to the enhanced light absorption capability and effective inhibition of charge carrier recombination. Furthermore, the EIS of Ti₃C₂/TiO₂@rGO displayed the shortest arc radius compared with the other samples, indicating the easiest charge transmission. The above-mentioned results implied that the Schottky barrier between Ti₃C₂ and TiO₂ and the introduction of rGO led to a higher transfer rate. Simultaneous Cr(VI) and RhB removal experiments were carried out on a mixture solution of Cr(VI) and RhB at natural pH. After 60 min of illumination, the reduction ratio of Cr(VI) for Ti₃C₂/TiO₂@rGO reached the maximum, which was higher than that in the single solution. The higher reduction efficiency of Cr(VI) in the mixture solution is likely due to the fact that RhB consumed the holes, resulting in a decrease in the charge carrier recombination. The photocatalytic degradation of RhB by Ti₃C₂/TiO₂@rGO was slightly lower than that in the single solution. This is because the electrons were completely involved in the reduction of Cr(VI), and as a result the generation of superoxide radicals was ceased, which only left holes for the degradation of RhB. The mechanism for the simultaneous reduction of Cr(VI) and degradation of RhB over Ti₃C₂/TiO₂@rGO is shown in Fig. 15.¹⁷⁵

Fig. 15 (a) Mott–Schottky plots, (b) energy band positions and (c) proposed mechanism for the simultaneous removal of Cr(VI) and RhB over (Ti₃C₂/TiO₂)@rGO (reproduced with permission from ref. 175; Copyright 2021, Elsevier). [Note: (Ti₃C₂/TiO₂)@rGO-15 = (Ti₃C₂/TiO₂)@rGO prepared with 15 mg of GO.]

4. Preparation and photocatalytic activity of Ti₃C₂/TiO₂ for hydrogen evolution reaction

4.1. Integrating Ti₃C₂ with titania NPs

Ti₃C₂T_x is a feasible co-catalyst for TiO₂, which is synthesized via the sonication of Ti₃C₂T_x with commercial TiO₂ under an Ar atmosphere. The HER rate of monolayer Ti₃C₂T_x/TiO₂ is higher than that of the pure TiO₂ and its multilayer counterpart. The enhanced activity is due to the superior electrical conductivity of monolayer Ti₃C₂T_x and charge-carrier separation at the Ti₃C₂T_x/TiO₂ interface. In fact, the surface area, light absorption capacity and larger contact interface between the monolayer Ti₃C₂T_x/TiO₂ resulted in a superior performance.¹⁷⁶ Zhuang et al.¹⁷⁷ reported the fabrication of 1D/2D TiO₂/Ti₃C₂via the electrostatic self-assembly technique using TiO₂ NFs and Ti₃C₂ NSs. The zeta potential was used to verify the possibility of electrostatic self-assembly between Ti₃C₂ NSs and TiO₂ NFs. The zeta potential of Ti₃C₂ and TiO₂ was measured to be −25.1 and 1.7 mV, respectively. The maximum HER was achieved by TiO₂/Ti₃C₂ (Table 4 and Fig. 16a and b), which was 3.8 times that of pure TiO₂ NFs. This improvement is due to the heterogenous interface between TiO₂-NFs and Ti₃C₂ NSs and it retained its activity even after 5 cycles (Fig. 16c).

Fig. 16 (a) Photocatalytic HER activity, (b) HER rate of TiO₂ and TiO₂/Ti₃C₂, and (c) recycling photocatalytic HER of TiO₂/Ti₃C₂ (reproduced with permission from ref. 177; Copyright 2019, Elsevier). [Note: TT1, TT2, TT3 and TT5 stands for TiO₂/Ti₃C₂ with different Ti₃C₂ contents.]

Table 4 Partially oxidized Ti₃C₂/TiO₂ MXene composites for photocatalytic hydrogen evolution

Photocatalyst	Sacrificial agent	HER rate (μmol h^−1 g^−1)		Stability (h)/No. of cycles	AQE (%)	Light source	Ref.
		Base Photocatalyst	Resultant photocatalyst
TiO2/Ti3C2	TEOA and H2PtCl6·6H2O		784.00	15/5	5.62	300 W Xe lamp	84
BPQDs/Ti3C2/TiO2	Acetone and TEOA	60.30	684.50	30/6	—	300 W Xe lamp	156
1D/2D TiO2/Ti3C2	Methanol and H2PtCl6·6H2O	1831.00 (TiO2 NFs)	6979.00	15/5	—	300 W Xe lamp	177
Ti3C2/TiO2	Methanol		783.11		—	300 W Xe lamp	178
Ti3C2/TiO2@MoS2	Acetone and TEOA	73.70 (TiO2)	6425.29	3 cycles	4.61	300 W Xe lamp	180
1T-WS2@TiO2/Ti3C2	Acetone and TEOA	67.80 (TiO2)	3409.80	24/3	2.46	300 W Xe lamp	181
TiO2/Ti3C2-CoSx	Methanol	140.00 (pristine TiO2)	950.00	15/5	—	UV-vis light	182
Ti3C2/TiO2/1T-MoS2	Acetone and TEOA	74.00 (TiO2)	9738.00	24/3	6.86	300 W Xe lamp	183
Ti3C2/TiO2/ZnIn2S4	Na2S·9H2O and Na2SO3		1185.80	16/4	—	300 W Xe lamp	184
Ti3C2/TiO2/CuInS2	Methanol		356.27	105/5	—	300 W Xe lamp	186
Ti3C2(TiO2)@CdS/MoS2		91.60 (CdS)	8470.00	100/20	29.70	Visible light	187
MoxS@TiO2/Ti3C2	Acetone and TEOA	54.30 (TiO2)	10505.8	24	7.53	300 W Xe arc lamp	188
d-Ti3C2/TiO2/GCN	TEOA and Pt	133.30 (GCN)	324.20	12/3	4.16	300 W Xe lamp	193
TiO2/Ti3C2/GCN	TEOA		1150	16/4	—	300 W Xe lamp	195
TiC/HCN	Methanol	132.50 (HCN)	310	16/3	—	35 W HID lamp	196
Cu/TiO2/Ti3C2Tx	Methanol	325.00	764	25/5	—	300 W Xe lamp	197
TiO2/Ti3C2/Ru	Methanol		235.30	25/5	14.33%	300 W Xe lamp	198
Cu2O/(001)TiO2/Ti3C2Tx	Methanol	251.00	1496.53	30/5	—	300 W Xe lamp	200
PtO@Ti3C2/TiO2	Methanol		∼2540.00	25/5	4.20	PLS-SXE-300 W lamp	201
TiO2/Ti3C2–TiC/Ce/Zr-UiO-66-NH2	Methanol		570.00	6/3	—		203

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A rutile/anatase Ohmic heterojunction of TiO₂/Ti₃C₂ was synthesized via the calcination of NH₃@Ti₃C₂ at different temperatures. When the calcination temperature reached 500 °C, Ti₃C₂ was oxidized, and the NSs were encapsulated by TiO₂. Due to the oxidation of some carbon layers in Ti₃C₂, the surface of the NSs exhibited the substantial presence of defects. TiO₂/Ti₃C₂ exhibited an excellent HER performance, demonstrating that it is an efficient and durable photocatalytic material for HER.⁸⁴ Ti₃C₂/TiO₂ NFWs were synthesized via in situ growth by simultaneous oxidation and alkalization, followed by ion exchange and calcination of Ti₃C₂ at different temperatures (300 °C, 400 °C and 500 °C) for 3 h. With an optimized calcination temperature of 500 °C, the Ti₃C₂/TiO₂ NFWs showed a remarkable enhancement in the HER and oxygen evolution reaction compared with that of pure TiO₂. Ti₃C₂/TiO₂ possessed a 3D porous NFW-like structure, higher porosity, stronger adsorption capacity and higher SSA (172.229 m² g⁻¹), greatly enhancing its catalytic activity. By varying the calcination temperature for the preparation of Ti₃C₂/TiO₂ NFWs, an optimum HER rate was obtained on Ti₃C₂/TiO₂ without co-catalysts. Therefore, Ti₃C₂ is a good alternative to the noble metal Pt as a co-catalyst, providing active sites for PCA.¹⁷⁸

Ti₃C₂/TiO₂ retained the 2D multilayer structure of Ti₃C₂, and TiO₂ exhibited a truncated octahedron bipyramid structure with exposed (001) facets under the participation of fluorine ions (Fig. 17). The residual Ti₃C₂ acted as a co-catalyst to enhance the HER activity by capturing electrons from TiO₂ because of its electron reservoir feature and suitable Fermi level. The (101)–(001) surface heterojunction of the truncated octahedron bipyramid TiO₂ further accelerated the separation of photogenerated carriers. As a result, Ti₃C₂/TiO₂ with calcined F-terminated Ti₃C₂ exhibited significant HER activity compared to the OH-terminated Ti₃C₂. Fig. 17a and b show the SEM images of the multilayered structure of Ti₃C₂ (thickness ∼5–15 nm). A uniformly truncated octahedron bipyramid TiO₂ was formed on the surface of Ti₃C₂ for Ti₃C₂/TiO₂ with F-terminated Ti₃C₂ (Fig. 17c and d). Ti₃C₂/TiO₂ with OH-terminated Ti₃C₂ showed the presence of TiO₂-NPs on the layered structures of Ti₃C₂ (Fig. 17e and f).¹⁷⁹

Fig. 17 SEM images of (a and b) Ti₃C₂, (c and d) Ti₃C₂/TiO₂ with F-terminated Ti₃C₂ and (e and f) Ti₃C₂/TiO₂ with OH-terminated Ti₃C₂ (reproduced with permission from ref. 179; Copyright 2019, the Royal Society of Chemistry).

5. Preparation and photocatalytic activity of Ti₃C₂-based ternary composites for hydrogen evolution

5.1. Integrating Ti₃C₂/TiO₂ with metal sulphides

TiO₂ NSs were in situ grown on highly conductive Ti₃C₂, and then MoS₂ NSs were deposited on the (101) facets of the TiO₂ NSs with mainly exposed high-active (001) facets through a two-step hydrothermal method. With an optimized MoS₂ loading amount, Ti₃C₂/TiO₂@MoS₂ showed a remarkable enhancement in the HER compared with Ti₃C₂/TiO₂ and TiO₂ NSs. The promoted activity is due to the in situ growth of TiO₂, creating interfacial contact with Ti₃C₂, and the surface heterojunction with facet-exposed TiO₂ and MoS₂. Ti₃C₂/TiO₂@MoS₂ showed a higher HER rate than TiO₂ and Ti₃C₂/TiO₂. The dual heterojunction with facet-exposed TiO₂ and MoS₂ promoted the activity and the high-active-exposed (001) facets of TiO₂ NSs facilitate the activation of water molecules and the photocatalytic reduction.¹⁸⁰ Li et al.¹⁸¹ obtained 1T-WS₂@TiO₂/Ti₃C₂via the hydrothermal method using WCl₆, thioacetamide and Ti₃C₂/TiO₂ in DMF (200 °C, 24 h). The in situ-grown TiO₂ and uniform distribution of 1T-WS₂ on the surface of Ti₃C₂/TiO₂ resulted in a distinctive structure, and the content of 1T phase reached 73%. This composite exhibited excellent HER, which was 50-times higher than that of TiO₂ because of the presence of conductive Ti₃C₂ and 1T-WS₂ phases. TiO₂/Ti₃C₂/CoS_x was synthesized using the solvothermal method, where TiO₂ nanocrystals were confined within the ZIF-67-templated porous CoS_x, supported by the conductive Ti₃C₂ surface. A smooth round black morphology was observed for TiO₂ with a diameter of ∼900 nm (Fig. 18a and b). TiO₂-CoS_x exhibited the presence of well-dispersed NPs due to the presence of ZIF-derived porous CoS_x. Small NPs a with course block morphology of TiO₂-CoS_x were embedded on Ti₃C₂ NSs (Fig. 18c). The agglomeration of TiO₂ occurred, while evenly dispersed TiO₂ NPs with a diameter of less than 20 nm were observed when CoS_x was added (Fig. 18d and e). The phase structure of TiO₂/Ti₃C₂/CoS_x was confirmed by the TEM and HRTEM images (Fig. 18f and g), respectively. The addition of only 1 at% ZIF-67-templated CoS_x led to a dramatic improvement in the PCA. The HER rates of TiO₂-CoS_x increased with an increase in the content of CoS_x. When the molar ratio of CoS_x reached 1 at%, the HER achieved the optimal rate, which was 2.8-times larger than that of the pristine TiO₂. A further increase in the amount of CoS_x resulted in a decrease in the HER rate, while the performance was still higher than that of the pristine TiO₂. Similarly, TiO₂/Ti₃C₂ exhibited the optimal PCA when the amount of Ti₃C₂ was 0.5 wt%, which should be attributed to the high conductivity of Ti₃C₂. When the content of black Ti₃C₂ was further increased, the colour of the sample became darker, which reduced the absorption of light by TiO₂, resulting in a decrease in the PCA.¹⁸²

Fig. 18 SEM and TEM images of (a and d) TiO₂, (b and e) TiO₂-CoS_x, (c and f) TiO₂/Ti₃C₂-CoS_x and (g) HRTEM image (reproduced with permission from ref. 182; Copyright 2019, the Royal Society of Chemistry).

1T-MoS₂ nanopatch/Ti₃C₂/TiO₂ NSs were synthesized via the hydrothermal method using MoO₃ and KSCN with Ti₃C₂/TiO₂ (200 °C, 18 h). The 1T-MoS₂ nanopatches were evenly distributed on the surface of Ti₃C₂/TiO₂ (Fig. 19). The recombination of charge carrier pairs in TiO₂ could be suppressed by transferring electrons to the Ti₃C₂ and 1T-MoS₂ surfaces. The unique 2D structure with double metallic co-catalysts Ti₃C₂ and 1T phase MoS₂ nanopatches promoted the catalytic activity. Notably, after the assembly of 1T-MoS₂ nanopatches, significantly improved PCA was observed, with the highest HER rate occurring for Ti₃C₂/TiO₂/1T-MoS₂.¹⁸³

Fig. 19 SEM images of (a) Ti₃C₂, (b) bulk MoS₂, (c) Ti₃C₂/TiO₂ and (d) 1T-MoS₂/Ti₃C₂/TiO₂ NSs (reproduced with permission from ref. 183; Copyright 2019, the Royal Society of Chemistry).

Hierarchical Ti₃C₂/TiO₂/ZnIn₂S₄ was constructed via a two-step hydrothermal method, wherein the in situ growth of TiO₂ on the surface of Ti₃C₂ occurred upon oxidation. Ti₃C₂/TiO₂/ZnIn₂S₄ exhibited a superior HER rate, which is higher than that of Ti₃C₂/TiO₂ and ZnIn₂S₄. The outstanding light harvesting by ZnIn₂S₄ and Ti₃C₂, sufficient active sites of Ti₃C₂, and intimate interfacial contact contributed to the activity. The Schottky junction between ZnIn₂S₄ and Ti₃C₂ and the type-II heterojunction between TiO₂ and ZnIn₂S₄ improved the photocurrent density. The transferred photogenerated electrons on Ti₃C₂ NSs could produce H₂ in two ways, as follows: (i) electrons directly reduced H₂O and (ii) electrons reduced the H₂O molecules captured by -OH. Then, the accumulated holes at VB would be consumed by the sacrificial agent.¹⁸⁴

Li et al.¹⁸⁵ proposed the formation of Ti₃CN/TiO₂/CdS spheres via the calcination method. A certain amount of CdS and Ti₃CN was added to a beaker containing alcohol, and then stirred at 50 °C until the complete evaporation of alcohol. The mixture was calcined at 200 °C under an air atmosphere for 2 h. The smooth surface of Ti₃CN became rough and Ti₃CN/TiO₂ was formed because Ti₃C₂ was in situ converted to TiO₂ (Fig. 20). This tight assembly between CdS and Ti₃C₂ can provide effective interfacial photoinduced charge diffusion under visible light irradiation. The unique nanometre ball structure of Ti₃CN/TiO₂/CdS could improve the stability without the loss of surface area. Thus, Ti₃CN/TiO₂/CdS exhibited enhanced PCA, achieving an excellent HER rate and cycling stability compared to CdS and Ti₃CN/TiO₂.

Fig. 20 SEM images of (a) Ti₃CN, (b and c) Ti₃CN/TiO₂/CdS (18h20C) and (d) Ti₃CN/TiO₂/CdS (21h20C) (reproduced with permission from ref. 185; Copyright 2019, the Royal Society of Chemistry). [Note: 18h20C and 21h20C according to the corrosion time of Ti₃AlCN and the amount of CdS used.]

Ti₃C₂/TiO₂/CuInS₂ was synthesized via the hydrothermal oxidation of InCl₃/CuCl₂ solution, Ti₃C₂/TiO₂ and sulphur powder at 180 °C for 15 h. The Ti₃C₂/TiO₂/CuInS₂-integrated Schottky/step-scheme heterojunction delivered increased HER, which was ∼69 and 636 times higher than that of Ti₃C₂/TiO₂ and CuInS₂, respectively. This result indicates that the charge separation and transfer can be effectively manipulated by rational specific construction design.¹⁸⁶ Ti₃C₂(TiO₂)@CdS/MoS₂ was synthesized via hydrothermal treatment at different times of 3, 9 and 12 h. The transfer direction of electrons and holes was achieved via the rational conjunction of Ti₃C₂ and MoS₂. Furthermore, a high HER rate was reached in pure water without any electron sacrificial agents. Through combination with the scope of a type-II junction between CdS and MoS₂, the new Z-scheme between CdS and TiO₂ transformed from Ti₃C₂ to setup a multi-step separation of charge carrier pairs. This process prolongs the lifetime of charge carriers and enables them to reach the active sites to initiate a redox reaction. Pure CdS exhibited poor PCA due to its inherent photo-corrosion, while loading a certain amount of Ti₃C₂ NSs (CdS@Ti₃C₂) or MoS₂ NSs (CdS/MoS₂) could improve the PCA, which was 17 and 32 times higher than that of CdS, respectively.¹⁸⁷ Li et al.¹⁸⁸ reported the in situ growth of TiO₂ as well as the molybdenum vacancy-rich MoS₂ constructed on the surface of Ti₃C₂via a two-step hydrothermal method. Thus, Mo_xS@TiO₂/Ti₃C₂ with a unique structure and Mo vacancies and double co-catalysts (Ti₃C₂ and MoS₂) was achieved. The layered Ti₃C₂ provides a Ti source for the growth of TiO₂ NSs inserted across the layered Ti₃C₂ to form TiO₂/Ti₃C₂. Further, Mo_xS@TiO₂/Ti₃C₂ was obtained by using NaBH₄ as the reducing agent to reduce part of Mo⁴⁺ and form molybdenum vacancies in MoS₂. The presence of molybdenum vacancies caused the atoms around the vacancies to leave their equilibrium position and make small movements toward the vacancy. The XPS spectrum of Mo 3d in Mo_xS@TiO₂/Ti₃C₂ presented six peaks, where besides the peaks of Mo⁴⁺ in 1T and 2H MoS₂, the two peaks at 229.8 eV and 232.9 eV are ascribed to Mo³⁺ and Mo²⁺ ions, respectively. The existence of Mo³⁺ and Mo²⁺ ions proved that some of the Mo⁴⁺ ions were reduced and molybdenum vacancies were produced. The highest HER rate was exhibited by Mo_xS@TiO₂/Ti₃C₂, which was nearly 193 and 6 times higher than that of pure TiO₂ NSs and MoS₂@TiO₂/Ti₃C₂, respectively (Table 4 and Fig. 21). The presence of molybdenum vacancies can suppress carrier recombination, which is beneficial for the reaction. The existence of molybdenum vacancies and co-catalysts (Ti₃C₂ and MoS₂) make Mo_xS@TiO₂/Ti₃C₂ possess excellent HER property.

Fig. 21 (a) Photocatalytic HER and (b) HER rate of Mo_xS@TiO₂/Ti₃C₂ in aqueous acetone solution with TEOA as a scavenger (reproduced with permission from ref. 188; Copyright 2022, Elsevier). [Note: 140 °C, 160 °C and 180 °C: hydrothermal oxidation temperature.]

The solvothermal treatment of Ti₃C₂, ammonium molybdate and thiourea at 180 °C for 18 h produced Ti₃C₂/TiO₂/MoS₂. The maximum HER rate was achieved by this composite, which was 4.3 times higher than that of TiO₂. The dual heterostructure system of MoS₂ and Ti₃C₂, interacting with TiO₂via Ti–O–Mo and C–O–Ti electron transfer channels, which enhanced HER. The electrons in the CB of TiO₂ quickly transferred to MoS₂ or Ti₃C₂, where they reduce protons to produce hydrogen at the active sites. The dual co-catalysts (MoS₂ and Ti₃C₂) store electrons in a stable, charge-separated state due to their capacitive properties, preventing their recombination and maximizing their utilization. This dual electron transfer mechanism significantly boosted the HER efficiency (Fig. 22).¹⁸⁹

Fig. 22 Schematic of the photocatalytic HER over Ti₃C₂/TiO₂/MoS₂ (reproduced with permission from ref. 189; Copyright 2021, John Wiley and Sons).

1D CdS nanorod/2D TiO₂/Ti₃C₂ was synthesized via the hydrothermal method. Initially, a homogeneous solution of CdCl₂ and thiourea was prepared with ethylenediamine and preformed TiO₂/Ti₃C₂ was dispersed in the above-mentioned solution, which was followed by hydrothermal treatment (180 °C, 24 h). After the incorporation of CdS into TiO₂/Ti₃C₂, the HER was significantly improved and the highest HER rate was achieved by CdS/TiO₂/Ti₃C₂.¹⁹⁰ ZnCdS, a ternary chalcogenide semiconductor, has potential for use in optoelectronic applications in solar energy-driven devices.¹⁹¹ ZnCdS/TiO₂/Na-MXene was prepared via the hydrothermal method. A certain amount of Na₂S was added to a Ti₃C₂ aqueous solution, followed by the addition of a solution of Zn(CH₃CO₂)₂·2H₂O and Cd(CH₃CO₂)₂·2H₂O. The mixture was stirred at RT for 12 h, and then heated to 160 °C for hydrothermal reaction and kept for 24 h to obtain ZnCdS/TiO₂/Na-MXene. The highest HER rate was achieved by the optimal composite ZnCdS/TiO₂/Na-MXene under visible light irradiation, which was 3.3 times that of ZnCdS, and the schematic diagram of HER by ZnCdS/TiO₂/Na-MXene was proposed (Fig. 23).¹⁹²

Fig. 23 Photocatalytic mechanism of HER by ZnCdS/TiO₂/Na-MXene (reproduced with permission from ref. 192; Copyright 2022, Elsevier).

5.2. Integrating Ti₃C₂/TiO₂ with GCN

Ti₃C₂(d-Ti₃C₂)/TiO₂/GCN was synthesized via the simple heat treatment of a mixture of GCN and delaminated Ti₃C₂. The sample with a GCN/d-Ti₃C₂ mass ratio of 4 : 1 calcined at 350 °C demonstrated the best HER after 4 h reaction. The controlled calcination to regulate the formation of d-Ti₃C₂/TiO₂/GCN, excellent visible light absorption ability and enlarged SSA accounted for its superior HER performance.¹⁹³ The delaminated Ti₃C₂ layered NSs promoted the charge-carrier separation efficiency by decreasing the traveling distance and increasing the visible light absorption. The highest CO evolution and HER were achieved over GCN/TiO₂/Ti₃C₂, which was 5.17 and 9.85 fold higher compared to TiO₂, respectively. The enhanced photoactivity was attributed to the better AQY during the bireforming of the methane process under visible light. The stability analysis further confirmed the high stability and durability of GCN/TiO₂/Ti₃C₂ in multiple cycles because of the presence of Ti₃C₂ NSs. This provides new pathways to construct low-cost and noble metal-free structured composites for stimulating the photocatalytic bireforming of methane under visible light, which can be employed in solar energy applications.¹⁹⁴ TiO₂/Ti₃C₂/GCN was synthesized via the calcination of GCN and Ti₃C₂ at 450 °C for 5 h. The dual heterojunction provided a unique morphology crystal structure and improved the absorption of light, resulting in superior activity towards HER, which almost 8 times higher as that of pure GCN.¹⁹⁵ The Ti₃C₂/hierarchical GCN (HCN) was synthesized using a single-step ultrasonic-assisted impregnation approach. The HER rate of Ti₃C₂-HCN was 2.34 fold higher than that of HCN after 2 h of irradiation time. This promoted HER was due to the faster transfer of electrons from HCN to Ti₃C₂ due to the higher conductivity and formation of heterojunction between HCN and TiO₂ with their synergistic effects. By varying the etchant time for the preparation of Ti₃C₂, the highest HER was obtained over the optimized Ti₃C₂ without the use of any other semiconductor photocatalyst.¹⁹⁶ (HCN: the hierarchical structure was fabricated by heating melamine as the raw material in the presence of ammonia gas by a thermal polymerization process, resulting in the formation of a 3D structure.)

5.3. Integrating Ti₃C₂/TiO₂ with other co-catalysts

Cu/TiO₂/Ti₃C₂T_x was prepared by photo-depositing copper nanodots using CuAc₂·H₂O precursor on the surface of TiO₂/Ti₃C₂T_x. The suspension was illuminated under a 500 W Xe lamp for 3 h with an N₂ gas flow. The TiO₂/Ti₃C₂T_x without Cu loading displayed very low activity for HER after 5 h of irradiation, while the activity for HER was significantly boosted by 10 folds for same duration in the presence of copper species. Further increasing the amount of Cu resulted in a decrease in HER, given that the formation of large Cu₂O NPs reduced the active sites at the Cu–TiO₂ interface. The optimal Cu/TiO₂/Ti₃C₂T_x demonstrated a stable performance even after five cycles.¹⁹⁷ Liu et al.¹⁹⁸ synthesized TiO₂/Ti₃C₂/Ru via the hydrothermal process at 180 °C with different reaction times (10–36 h). The in situ growth of TiO₂ NSs on the Ti₃C₂/Ru surface ensured the separation of the semiconductor and co-catalyst (TiO₂/Ti₃C₂/Ru), resulting in more effective charge segregation and migration than that achieved by the traditionally prepared Ru–TiO₂/Ti₃C₂. The average HER rate of TiO₂/Ti₃C₂/Ru reached the maximum value and the highest AQY was obtained at 350 nm. TiO₂/Ti₃C₂/Pt displayed a lower HER rate than that of TiO₂/Ti₃C₂/Ru, implying that Ti₃C₂/Pt is not an ideal HER cocatalyst. The most positive Fermi level of Ti₃C₂ with –O terminations represents the ability to accept electrons, and thus the electrons on TiO₂ were transferred to Ti₃C₂ (Fig. 24a). The Fermi level of Ti₃C₂ was more negative than that of Ru, triggering the transfer of photogenerated electrons from the former to the latter (Fig. 24b). Given that Ru is a highly efficient co-catalyst, the electron transfer was accompanied by a sharp increase in the photocatalytic HER, marking the end of the induction period.¹⁹⁹ In contrast, TiO₂/Ti₃C₂/Ru did not require an induction period for the HER, given that the more positive Fermi level of the Ti₃C₂/Ru structure allowed the direct transfer of the photogenerated electrons to the Ru active center (Fig. 24c).

Fig. 24 (a) Charge transfer in the photocatalytic induction period, (b) HER and (c) charge transfer of TiO₂/Ti₃C₂/Ru in the photocatalysis (reproduced with permission from ref. 198; Copyright 2020, the American Chemical Society).

Cu₂O/(001)TiO₂/Ti₃C₂T_x was synthesized via a wet-chemistry reduction method using DMF. DMF acted as the reducing agent during the deposition of Cu₂O, where Cu²⁺ was reduced in the presence of DMF, formic acid, and amine, leading to the formation of Cu₂O (eqn (21)–(23)).

4HCON(CH₃)₂ + H₂O → HCOOH + NH(CH₃)₂ (21)

Cu²⁺ + H₂O + NH(CH₃)₂ → CuO_(s) + 2NH₂(CH₃)₂⁺ (22)

2CuO_(s) + HCOOH → Cu₂O_(s) + H₂O + CO₂ (23)

The SSA of the photocatalyst increased with the loading of Cu₂O, which directly influenced the PCA. When the surface coverage of the photocatalyst by Cu₂O was low, Ti₃C₂T_x acted as a hole reservoir. Cu₂O was in situ reduced to metallic copper by excited electrons. Then, the reversed movement of carriers enabled the spatial separation of photogenerated charge carrier pairs and afforded relatively high HER. When the coverage of Cu₂O on (001)TiO₂/Ti₃C₂T_x was too high at high loading amounts, Ti₃C₂T_x failed to act as a hole trapping agent. Under the stated circumstances, the reaction follows the p–n junction mechanism, leading to low HER (Fig. 25a). TiO₂/Ti₃C₂T_x showed improved PCA than Cu₂O and Ti₃C₂T_x due to increase in H₂ active sites. The higher HER performance of Cu₂O/(001)TiO₂/Ti₃C₂T_x-5% than that of Cu₂O/(001)TiO₂/Ti₃C₂T_x-25% is consistent with the photocurrent response measurements. However, the loading amount increased to 45% and 65%, the HER rate decreased to a lower yield. Because of the considerable exposure of TiO₂, excited charge carrier pairs were mainly produced on TiO₂ NSs with the illumination of light. Both Cu₂O and TiO₂ NSs could be excited to produce photogenerated charge carrier pairs upon light illumination (Fig. 25b).²⁰⁰

Fig. 25 Photocatalytic mechanism and charge-transfer at the interface of Cu₂O/TiO₂/Ti₃C₂T_x with (a) low and (b) high Cu₂O coverage on TiO₂/Ti₃C₂T_x (reproduced with permission from ref. 200; Copyright 2019, Elsevier).

PtO@Ti₃C₂/TiO₂ was synthesized via the hydrothermal and photo-deposition method. The in situ growth of TiO₂ upon the oxidation of Ti₃C₂ with NaBF₄ resulted in TiO₂/Ti₃C₂ and further PtO deposition was achieved using a 300 W Xe lamp under vacuum condition for 2 h. As the hydrothermal time was extended, the content of TiO₂ gradually increased until Ti₃C₂ was completely oxidized at the expense of its visible light adsorption capacity. Especially, after the deposition of PtO nanodots, PtO@Ti₃C₂/TiO₂ displayed stronger optical absorption, which was caused by the absorption of PtO in visible light region. The HER efficiency significantly improved after the deposition of PtO nanodots and the HER rate of Ti₃C₂/TiO₂ synthesized at different reaction times followed the order of Ti₃C₂/TiO₂-12 h > Ti₃C₂/TiO₂-20 h > Ti₃C₂/TiO₂-4 h. The cycling stability of PtO@Ti₃C₂/TiO₂ confirmed its excellent stability given that similar HER efficiency was observed after five cycles. This indicates that the in situ growth of TiO₂ and Ti₃C₂ with PtO deposition resulted in the formation of a stable heterojunction structure.²⁰¹ Tahir et al.²⁰² reported that the HER rate of Ni₂P/Ti₃C₂-loaded TiO₂ was higher compared to TiO₂/Ni₂P, TiO₂/Ti₃C₂, TiO₂, and Ti₃C₂. The composite performance was tested for HER with different feed types, such as pure water, methanol, ethanol, and glycerol. Evidently, glycerol gave the highest HER rate, followed by methanol, ethanol and pure water. A novel BPQDs/Ti₃C₂/TiO₂ was synthesized via the hydrothermal method and solvent heat treatment. The BPQDs/Ti₃C₂/TiO₂ synthesized at 120 °C showed an enhanced HER rate, which was higher than that of Ti₃C₂/TiO₂, and BPQDs/Ti₃C₂. The highest HER rate was attributed to the construction of a heterojunction between BPQDs and suitable ratio of in situ growth of anatase TiO₂ NPs and Ti₃C₂ NSs at the temperature of 120 °C.¹⁵⁶ TiO₂/Ti₃C₂–TiC/Ce/Zr-UiO-66-NH₂ Z-scheme composites were synthesized via the solvothermal method (Fig. 26). An accordion-like morphology with a smooth surface texture of Ti₃C₂ and TiO₂/Ti₃C₂-TiC was observed (Fig. 27a) and the metastable thermodynamic state of the marginal Ti atoms in the accordion layer of Ti₃C₂ was also evident (Fig. 27b). The FESEM image of TiO₂/Ti₃C₂, TiC/Ce/Zr-UiO-66-NH₂, in which the Ce/Zr-UiO-66-NH₂ bimetallic MOF was decorated on the surface of TiO₂/Ti₃C₂−TiC is shown in Fig. 27c. The Ti₃C₂-TiC NSs acted as a solid-state electron mediator, constructing an electron-shuttling route between Ce/Zr-UiO-66-NH₂ and TiO₂, and thus extending the lifetime of the charge carriers. Initially, the photocatalytic rate increased, and then further decreased with an increase in the loading of Ce/Zr-UiO-66-NH₂ given that it can hinder the absorption of incident light. In addition, excessive Ce/Zr-UiO-66-NH₂ could act as new recombination centers for the charge carriers.²⁰³ Ce-based MOFs are ideal due to their easy availability and flexible Ce³⁺/Ce⁴⁺ oxidation states, but their low thermal stability and exciton segregation are major drawbacks. These issues can be addressed by incorporating Ce ions into the stable, visible-light-responsive UiO-66-NH₂ Zr-based MOF framework, improving both their stability and PCA.²⁰⁴ Bimetallic MOFs have attracted significant attention owing to their improved structure, porosity, active site, adsorption, selectivity, and stability.²⁰⁵

Fig. 26 Preparation of TiO₂/Ti₃C₂-TiC/Ce/Zr-UiO-66-NH₂ (reproduced with permission from ref. 203; Copyright 2024, the Royal Society of Chemistry). [Note: TiO₂/Ti₃C₂-TiC/CZNH = TiO₂/Ti₃C₂ TiC/Ce/Zr-UiO-66-NH₂.]

Fig. 27 FESEM images of (a) Ti₃C₂-TiC, (b) TiO₂/Ti₃C₂-TiC, (c) TiO₂/Ti₃C₂-TiC/Ce/Zr-UiO-66-NH₂ and (d–f) HRTEM images of TiO₂/Ti₃C₂-TiC/Ce/Zr-UiO-66-NH₂ (reproduced with permission from ref. 203; Copyright 2024, the Royal Society of Chemistry).

6. C@TiO₂ derived from Ti₃C₂ and their photocatalytic applications

The complete oxidation of the Ti₃C₂ MXene layer results in the formation of TiO₂@C composites. The carbon became very thin compared to the 2D Ti₃C₂ and the TiO₂ crystallized inside the carbon layers, increasing the light-harvesting ability, which is promising for photocatalytic applications. Furthermore, the carbon layer can hinder the recombination of charge carriers, allowing them to participate in the desired redox reactions.

6.1 Preparation and photocatalytic activity of C@TiO₂ composites for organic pollutant degradation

TiO₂@C NSs were fabricated via the high-energy ball milling of 2D Ti₂CT_x. A certain amount of Ti₂CT_x was put in an agate jar, and then milled with agate balls (weight ratio of 1 : 55) in an air atmosphere (1.5 h, 200 rpm). The as-milled powders were cold-pressed under a pressure of about 1 GPa to form small pellets. The high energy ball milling destroyed the multi stacked structure, leading to the formation of free-standing NSs with a much smaller SSA. The milled samples consisted of monodispersed NPs and showed different morphological structures. Clearly, the enhanced PCA of the as-prepared TiO₂@C was 1.25 times higher than that of P25. As mentioned above, the as-prepared TiO₂@C NSs exhibited a structure of uniform and close immobilization of TiO₂ NPs on the disordered graphitic carbon NSs. Owing to its high electric conductivity, disordered graphitic carbon could serve as an electron acceptor. The formation of the O–Ti–C bond in TiO₂@C NSs promoted the photo-excited electron transfer from the CB of anatase TiO₂ to carbon NSs. The relatively small size of the TiO₂ could provide more active sites and trap more reactive species, improving the PCA. Secondly, the existence of Ti³⁺ ions, which could provide donor sites and/or electron traps in TiO₂. The photoexcitation could activate the electrons in these sites to CB, thus inducing a higher density of charge carriers in the photocatalyst. The activated Ti³⁺ states could trap electrons, therefore suppressing the recombination of photogenerated charge carrier pairs. Many defects exist in the TiO₂ lattice, and among the defects, the OVs could narrow the band gap of TiO₂, which is beneficial to decrease the probability of charge carrier recombination.²⁰⁶

The ternary composite of completely oxidized Ti₃C₂-based C@TiO₂ exhibited superior activity, which was attributed to the presence of a carbon layer and heterojunction, enhancing the easy electron transport process.²⁰⁷ TiO₂/C/BiVO₄ exhibited an excellent PCA for the degradation of RhB, which was about four times higher than that of TiO₂/BiVO₄ under visible-light. The Ti₃C₂-derived 2D carbon layers served as a bridge to enhance the interaction between TiO₂/C and BiVO₄.²⁰⁸ Jiang et al.²⁰⁹ integrated C-TiO₂ with plate-like Bi₄NbO₈Cl via a flux method. Under the same synthetic conditions, C-TiO₂ was obtained via the oxidation of Ti₃C₂, and the NPs aggregated with a size of several nanometers. The improved charge separation caused C-TiO₂/Bi₄NbO₈Cl to exhibit a superior performance towards RhB removal assisted by photoinduced holes. Moreover, this composite could be expanded to deal with other pollutants, such as MO, CIP and 2,4-DNP, showing promise for efficient water purification. C/Ti₃C₂/(001)TiO₂ HMs were synthesized via a combined self-assembly and hydrothermal method. The uniformly mixed aqueous solution of PS and Ti₃C₂ produced a Ti₃C₂/PS microsphere. Further self-assembled DMC Ti₃C₂/PS microsphere was prepared by using DMC, followed by calcination at 350 °C for 1 h under an N₂ atmosphere to obtain C/Ti₃C₂ HMs. Finally, a mixture of C/Ti₃C₂ HMs and NaBF₄ was hydrothermally treated at 200 °C for 24 h to obtain the composite (Fig. 28).²¹⁰

Fig. 28 Schematic for the preparation of C/Ti₃C₂/(001)TiO₂-HMs (reproduced with permission from ref. 210; Copyright 2023, Elsevier).

The C/Ti₃C₂/(001)TiO₂-HMs exhibited the highest TC degradation within 160 min of light illumination, which was higher than that of TiO₂ and (001)TiO₂/Ti₃C₂. The efficiency of the C/Ti₃C₂/(001)TiO₂ HMs is due to the OVs on the (001) TiO₂, which improved the active sites and light absorption. The degradation of TC in real wastewater samples (lake water) was slightly higher than that in deionized water. The reason for this could be that the pH of the lake water was slightly acidic (pH 6.1), and the adsorption capacity increased due to the strong electrostatic interaction between TC and C/Ti₃C₂/(001)TiO₂ HMs. During the experiment, the pH of the solution did not change significantly. These results confirmed that C/Ti₃C₂/(001)TiO₂ HMs were efficient materials for the treatment of TC-contaminated wastewater.²¹⁰

2D layered nitrogen-doped carbon-supported titanium dioxide (N-TiO₂@C) was synthesized via a one-step in situ fabrication mode from 2D layered Ti₃C₂ as a carbon skeleton and homologous titanium source. Based on the negatively charged and easily oxidized property of Ti₃C₂, it was assembled with a nitrogen-containing cationic compound via electrostatic interactions, and then in situ transformed into N-TiO₂@C under a CO₂ atmosphere at 550 °C. Compared with N-TiO₂@C, the hydrothermally synthesized C/TiO₂ has a low phenol degradation rate in 3 h. The uniformly distributed N-doped TiO₂ in the N-doped 2D carbon matrix provided many visible light absorption catalytic active sites. In contrast, N-TiO₂@C exhibited a higher degradation rate of phenol within 3 h of light illumination (Table 5). The obtained composites of porous 2D layered N-TiO₂@C with high stability, outstanding electron transfer performance and excellent visible-light PCA exhibited high efficiency for phenol degradation and apparent rate constant, k.²¹¹ Nitrogen doping is considered one of the most effective methods for enhancing the visible light absorption of TiO₂ by reducing its band gap. This modification significantly improves the PCA of TiO₂. The incorporation of nitrogen alters the electronic properties of TiO₂, which affects its light absorption range and the redox capabilities of its charge carriers.^212,213

Table 5 Photocatalytic degradation performance of completely oxidized MXene C@TiO₂-based composites

Photocatalyst	[C] mg mL^−1	Pollutant	[P] mg L^−1	V (mL)	Reaction time (min)	Light source	Ref.
TiO2@C	10	MB	20	50	360	500 W Hg lamp	206
TiO2/C/BiVO4	60	RhB	10	60	180	500 W Xe lamp	208
C-TiO2/Bi4NbO8	40	RhB	10	40	180	300 W Xe lamp	209
C/Ti3C2/(001)TiO2	20	TC	20	50	120	350 W Xe lamp	210
N-TiO2@C-CHN	50	Phenol	20	50		Xe lamp	211
TiO2/g-C3N4	60	RhB, TC, CIP and BPA		20	50, 80, 60 and 70	300 W Xe lamp	214

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Highly photoactive Z-scheme graphene layer-anchored TiO₂/g-C₃N₄ was synthesized using Ti₃C₂ through one-step in situ calcination at 550 °C for 4 h. This composite showed superior degradation efficiencies towards TC, CIP, BPA and RhB (Table 5). After irradiation for 80, 60, 70 and 50 min, TC, CIP, BPA and RhB degradation over the graphene layer-anchored TiO₂/g-C₃N₄ was 3.42, 5.06, 4.63 and 4.42 fold higher than that of TiO₂/g-C₃N₄, respectively. The TOC removal of TC, CIP, BPA and RhB was 66.3%, 41.8%, 63.6% and 92.1%, respectively, implying that the mineralization of typical antibiotics, endocrine disruptors and dyestuff over the composite surface is favourable.²¹⁴

6.2 Preparation and photocatalytic activity of C@TiO₂ for hydrogen evolution reaction

2D layered C/TiO₂ was synthesized via the CO₂ oxidation of Ti₃C₂ at 700 °C for 1 h. The carbon atoms in Ti₃C₂ were converted into 2D layered carbon (1/2 layer) and Ti was oxidized to TiO₂ NSs, resulting in the formation of a reduced layer structure with high transparency and delivering less light intensity decay. The 2D carbon layers provided fast charge transport pathways, resulting in efficient PCA. This result indicated that high CO₂ flux favoured the generation of larger NPs and high temperature could modulate the morphology transformation from NSs to NPs. When the calcination temperature reached 800 °C, the NSs were transformed into NPs, leading to a decrease in SSA and light utilization efficiency. The carbon layers formed on the surface of TiO₂ upon oxidation of Ti₃C₂ with optimized temperature and CO₂ gas flux enhanced the HER activity of TiO₂. The effects of carbon on the HER can be due to the following: (i) 2D layered carbon materials have high electrical conductivity, which can quickly transfer electrons; (ii) the thickness of carbon layer is much lower than that of Ti₃C₂, which can reduce the light intensity decay; and (iii) the layered structure can provide a pathway for the diffusion of produced H₂.²¹⁵ Carbon-doped TiO₂ was synthesized via the hydrothermal treatment of a Ti₃C₂ suspension at 160 °C for 9 h. The FESEM analysis confirmed the multangular flower-like morphology composed of some nanorods tightly aggregated with a diameter of 35 ± 10 nm (Fig. 29). The carbon-doped TiO₂ had a lower surface area compared to TiO₂, but still exhibited a higher performance. This can be attributed to two main reasons. Firstly, the C doping could enhance the light absorption range by shifting the VB up. Secondly, the C-doping extended the lifespan of the photogenerated carriers by inhibiting the carrier recombination rate through charge sensitization. The effective in situ carbon doping of TiO₂ could greatly improve the lifetime of photogenerated carriers and widen the absorption range of light for PCA. The steady HER rate of carbon-doped TiO₂ was 9.7 times higher than that of commercial P25.²¹⁶

Fig. 29 (a) SEM, (b) TEM, (c) HAADF-STEM with Ti, O and C element distribution and (d) HRTEM (inset: SAED pattern) images of carbon-doped TiO₂ (reproduced with permission from ref. 216; Copyright 2018, the American Chemical Society).

N-Ti₃C₂T_x obtained via ammonia nitriding-pretreatment maintained almost a similar layered structure to Ti₃C₂T_x. Carbon vacancy defects in Ti₃C₂T_x were engineered through nitrogen-remedying, without replacing the carbon atoms by N atoms. The introduction of N into MXene not only maintained its 2D skeleton, but also increased its interlayer spacing by 5.1 Å, which is favourable for the intercalation of TiO₂. TiO₂/C and NPT-TiO₂/C were synthesized via the CO₂ oxidation of Ti₃C₂T_x and N-Ti₃C₂T_x. Compared with TiO₂/C, the light absorption of NPT-TiO₂/C was significantly enhanced from the UV range to the visible-light range. One of the main reasons for this is its orderly porous structure, which resulted in multiple reflections and sufficient contact between incident light and NPT-TiO₂/C. In addition, through the above-mentioned Mott–Schottky plots, the carrier densities of TiO₂/C and NPT-TiO₂/C were calculated to be 9.21 × 10¹⁷ and 39.49 × 10¹⁷ cm⁻³, respectively, indicating that NPT-TiO₂/C has a remarkably improved carrier density compared with the disordered TiO₂/C. The orderly superstructure dramatically accelerated the electron transfer and effectively suppressed the recombination of charge carriers compared with disordered TiO₂/C. NPT-TiO₂/C under visible light and simulated sunlight without any co-catalyst exhibited a greater HER rate and maintained robust stability after recycling for 240 h under visible-light illumination.²¹⁷ Liu et al.²¹⁸ synthesized 2D C/TiO₂via boric acid-induced hydrothermal method with Ti₃C₂. The cooperative effect of the conductive layer and the electron trap OVs accelerated the migration of electrons and the heterogeneous interface formed between the TiO₂ NPs and the conductive layer, boosting the HER performance on B–C/TiO₂ (Table 6).

Table 6 Photocatalytic hydrogen evolution of completely oxidized MXene C@TiO₂-based composites

Photocatalyst	Sacrificial agent	HER rate (μmol g^−1 h^−1)		Stability (h)/No. of cycles	AQE (%)	Light source	Ref.
		Base photocatalyst	Resultant Photocatalyst
2D-C/TiO2			24.00	24/8	1.98 at 400 nm	Visible light	215
B–C/TiO2	Methanol and Pt	0 (Ti3C2)	3622.00			300 W Xe lamp	218
LDC-S-TiO2/C	Methanol		333.00	35/7	7.36 at 400 nm	300 W Xe lamp	219
C-TiO2/GCN	TEOA and Pt	174.00 (GCN) and 58.00 (C-TiO2)	1409.00	10/3	—	300 W Xe lamp	220
TiO2-C/CdS	Lactic acid	210.00 (P-CdS) and 230.00 (Ti/CdS)	1480.00	25/5	—	300 W Xe lamp	221
NPT-TiO2/C		3.10 and 51.90 (TiO2/C)	87.20 and 425.60	240/40	1.61 at 420 nm	Visible light and Sunlight	217

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A laminated junction composed of defect-controlled and sulphur-doped TiO₂ with a carbon substrate (LDC-S-TiO₂/C) was synthesized via sulphur reactant impregnation and adhering on the surface of Ti₃C₂ by a melt-diffusion process. Then, L-S-TiO₂/C was fabricated via the mild CO₂ oxidation of S-Ti₃C₂. Finally, LDC-S-TiO₂/C was synthesized via the air oxidation of L-S-TiO₂/C. The SSA of L-S-TiO₂/C reached the maximum owing to the introduction of micropores by the sulphur doping process (Table 6). The enhanced SSA and pore size distribution indicated that LDC-S-TiO₂/C can expose more S-doped TiO₂ for light absorption and favour the HER compared with L-TiO₂/C and L-S-TiO₂/C. The enhancement of HER of L-S-TiO₂/C is attributed to its broaden absorption wavelength. After air oxidation, the HER of LDC-S-TiO₂/C reached the maximum value, likely owing to the reduced defect concentration, enlarged SSA and reduced excitation energy. Upon sulphur doping and air oxidation, the PCA highly improved by a factor of nearly 50. The HER rate of LDC-S-TiO₂/C under UV-vis light irradiation and visible light irradiation was 17 and 48 times that of C/TiO₂ and 5.2 and 8.9 times that of a composite of commercial P25 and S-doped graphene, respectively.²¹⁹ C-TiO₂/GCN was prepared via a one-step facile heat treatment with the optimized mass ratio of Ti₃C₂ to GCN. When the mass ratio of Ti₃C₂ to GCN was 10 wt%, C-TiO₂/GCN showed the highest HER activity. The possible mechanism assumed that the achieved intimate heterojunction between the Ti₃C₂-derived C-doped TiO₂ and GCN can efficiently facilitate the transfer of photogenerated charges and inhibit the recombination of electrons and holes.²²⁰ Anatase TiO₂-C/CdS exhibited an excellent HER rate, which was 6.4 and 7.0 times higher than that of pristine Ti₃C₂T_x/CdS and P-CdS under visible-light illumination, respectively. The OVs in the anatase TiO₂-C co-catalyst were found to serve as electron trapping sites, facilitating the transfer of photoexcited electrons for enhanced PCA. Anatase TiO₂-C/CdS was synthesized via gamma-ray radiation-induced reduction and subsequent alkaline treatment at RT. During the preparation of this composite, a solution of Ti₃C₂T_x, CdCl₂, and Na₂S₂O₃·5H₂O was bubbled with N₂ gas for 20 min, followed by gamma-ray irradiation at a dose of approximately 300 kGy using a ⁶⁰Co source under ambient conditions to form a Ti₃C₂T_x/CdS colloidal solution. Subsequently, anatase TiO₂-C anchored on CdS was prepared through alkaline treatment.²²¹

7. Conclusion and future perspectives

Since their discovery in 2011, MXenes have emerged as versatile materials and alternative substrates and co-catalysts to the existing 2D materials for efficient photocatalytic applications. Given their immense potential to serve as precursors for the growth of robust titania photocatalysts, intense research has been carried out on binary and ternary MXene composites to circumvent the difficulties of single component photocatalysts. Owing to their large SSA, MXenes offer a diverse opportunity for the formation of surface junction photocatalysts to improve the broad light absorption and charge isolation and enhance the lifetime of photo-generated charge carriers. Oxidized Ti₃C₂/TiO₂ and C@TiO₂ have shown great promise for photocatalytic reactions. The in situ growth of TiO₂ on the layered structure of Ti₃C₂, together with the formation of Schottky barriers and heterojunctions between TiO₂/Ti₃C₂ and semiconductors significantly enhances the photocatalytic efficiency. Ti₃C₂ is considered the best co-catalyst for photocatalysis owing to its elemental abundance, highly conductive nature, and different surface terminations. Further advancements in the fabrication of oxidized Ti₃C₂-based ternary heterostructures comprised of other photocatalytic materials such as metal sulphides, metal oxides, Ag/Bi-based semiconducting materials, carbon nitrides, and quantum dots are specifically targeted for their excellent performances towards pollutant degradation and HER (Fig. 30). However, tailoring the ratio of Ti₃C₂ in hybrid structures is very important given at a lower fraction may be ineffective to separate charge carriers, while a higher content might hamper the light absorption properties. This situation is further complicated in the ternary systems given that integrated semiconductors also participate in the band gap excitation processes. The optimization of oxidizing Ti₃C₂ strongly depends on the fluoride precursors (etching processes) and annealing ambience/time. The research attempts focusing on the synthesis of anatase phase from Ti₃C₂ are very narrow and other biphasic counterparts such as anatase/rutile, anatase/brookite and rutile/brookite are scarcely investigated. This aspect is very important given that mixed-phase titania exhibits higher activity compared to its pure phases owing to synergistic effects. Furthermore, research on the synthesis of faceted and defect-rich titania should also be considered for the synthesis of efficient hybrid materials. The combination of solvo(hydro)thermal and annealing may be useful to address this challenging issues. Many researchers have used Ti precursors to develop binary and ternary heterostructures of TiO₂ with Ti₃C₂. However, the oxidation of Ti₃C₂ naturally results in the formation of TiO₂, eliminating the need for external additives. In addition, the morphology of binary and ternary Ti₃C₂ composites is crucial, given that it directly influences the surface density and the availability of active sites. We anticipate that in/ex situ analysis and DFT simulations will provide a better understanding of the charge transfer mechanisms in MXene-based composites. Additional information such as charge carrier density distribution, interfacial electric field coupling, Schottky junction, band bending, and direction of electron transfer and participation of free radicals should be thoroughly investigated to facilitate the fabrication of ideal heterojunctions based on Ti₃C₂ MXenes.

Fig. 30 Preparation methods, properties, and applications of Ti₃C₂ MXene-based composites in photocatalysis.

Abbreviations

AOPs	Advanced oxidation processes
AO	Ammonium oxalate
AQY	Apparent quantum yield
ATZ	Atrazine
BQ	Benzoquinone
BPA	Bisphenol A
BPQDs	Black phosphorous quantum dots
BPB	Bromophenol blue
CBZ	Carbamazepine
CTAB	Cetyltrimethyl ammonium bromide
CIP	Ciprofloxacin
CB	Conduction band
DFT	Density functional theory
DOS	Density of states
DMF	Dimethylformamide
DMSO	Dimethyl sulfoxide
DNP	2,4-Dinitrophenol
EIS	Electrochemical impedance Spectroscopy
EPR	Electron paramagnetic resonance
ESR	Electron spin resonance
GCN	Graphitic carbon nitride (g-C3N4)
HMs	Hollow microspheres
HER	Hydrogen evolution reaction
IPA	Isopropanol
LDC	Laminated junction composed of defect controlled
DMC	Methacryloxyethyl trimethyl ammonium chloride
MB	Methylene blue
MG	Methyl green
MO	Methyl orange
NCDs	N-doped carbon dots
NFs	Nanofibers
NFWs	Nanoflowers
NPs	Nanoparticles
NRs	Nanorods
NSs	Nanosheets
NPT	Nitriding-pretreatment
NHE	Normal hydrogen electrode
OVs	Oxygen vacancies
POME	Palm oil mill effluent
PMS	Peroxymonosulfate
PCA	Photocatalytic activity
PL	Photoluminescence
PZC	Point of zero charge
PS	Polystyrene
PVP	Polyvinylpyrrolidone
PDI	Pyromellitic diimide
QDs	Quantum dots
ROS	Reactive oxygen species
RhB	Rhodamine B
RT	Room temperature
SCE	Saturated calomel electrode
SEM	Scanning electron microscopy
SAED	Selected area electron diffraction
SSA	Specific surface area
SCP	Sulfachloropyridazine
SMZ	Sulfamethazine
TBA	tert-Butanol
TC	Tetracycline hydrochloride
TEM	Transmission electron microscopy
TOC	Total organic carbon
VB	Valence band
XPS	X-ray photoelectron microscopy

Data availability

All the data (provided in the tables) were extracted from cited articles.

Articles are cited appropriately throughout the manuscript.

No new data have been generated.

Conflicts of interest

All the authors declare no conflict of interest

Acknowledgements

The authors PCP and SM would like to thank the Department of Chemistry, School of Applied Sciences, REVA University, Bengaluru (RU/R&D/SEED/CHE/2023/20).

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