Recent advances in inorganic oxide semiconductor-based S-scheme heterojunctions for photocatalytic hydrogen evolution

Abstract

In recent years, inorganic oxide semiconductors have received vast attention as photocatalysts for hydrogen (H₂) evolution. However, poor H₂ evolution activity and rapid recombination of the photoexcited charge carriers confine their practical applications. To address these constraints of pure inorganic oxide semiconductor photocatalysts, the creation of S-scheme heterojunctions has emerged as a promising alternative, which efficiently enhances optical absorption, promotes efficient charge separation, and retains relatively strong redox potential compared to traditional heterojunctions. Herein, we overview the fundamentals of some representative inorganic oxide semiconductor (tungsten oxide, titanium oxide, zinc oxide, and copper oxide)-based step (S)-scheme heterostructures, including their preparation strategies, photocatalytic H₂ performance, and charge transfer mechanisms. This review covers recent developments in the formation of these heterostructures via different synthesis strategies that modulate electronic band alignments to enhance H₂ evolution. This review also comprehensively highlights the essential role of S-scheme charge transport mechanism in promoting the migration and separation of photoinduced electron–hole (e⁻/h⁺) pairs, which in turn improves the H₂ evolution activity. Additionally, the future prospects are discussed, which provide guidance for designing efficient inorganic oxide semiconductor-based photocatalysts and the development of sustainable H₂ generation technologies.

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Ikram Ullah

Ikram Ullah is currently a Postdoctoral Research Fellow in Professor Pei Zhao research group at the School of Energy and Power Engineering at Shandong University. He obtained his PhD degree in Inorganic Chemistry from the Hefei National Research Center for Physical Sciences at the Microscale, under the supervision of Professor An-Wu Xu at the University of Science and Technology of China. His research interests include the exploration of new nanomaterials and heterostructures for photocatalytic hydrogen production from water splitting.

Muhammad Amin

Dr Muhammad Amin received his PhD in Chemical Engineering from the Prince of Songkla University, Thailand in 2019. He is a recipient of Best PhD Thesis Award. He has more than 13 years of experience in academic and administration. Currently, he is working as an Associate Professor at Department of Chemical, Petroleum and Petrochemical, Mir Chakar Khan Rind University of Technology. D. G. Khan. His research is focused on algal biorefinery, biomass and bioenergy, waste to valuable products, biomaterials, renewable materials, nanomaterials, and solar drying.

Pei Zhao

Pei Zhao is a Professor at the School of Energy and Power Engineering at Shandong University. She obtained her PhD degree from the Functional Ceramic Materials Group at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. From 2014 to 2015, she was a Postdoctoral Fellow at the Max Planck Institute for Plasma Physics, Germany. From 2015 to 2022, she held positions as a Postdoctoral Fellow, Research Associate, and Research Assistant Professor at the University of Waterloo, Canada. Her research interests include novel environmental monitoring, microfluidic technology and device development, and material design and preparation for water pollution treatment.

Ning Qin

Ning Qin is a professor at the School of Energy and Power Engineering, Shandong University. He received his PhD degree in Mechanical Engineering at the University of Waterloo, Canada in 2017. From 2018 to 2019, he was a Postdoctoral Fellow at Harvard Medical School, United States. From 2019 to 2022, he served as a Research Associate and Postdoctoral Fellow at University of Waterloo, Canada. His research interests includes nano and microscale fluid dynamics, carbon capture, storage and utilization, and microengineering for advanced diagnostics and therapeutics.

An-Wu Xu

An-Wu Xu is a Professor at the Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China. He received his PhD degree in Applied Chemistry from the same university in 1998. He has published more than 270 SCI papers, including those in prestigious journals such as J. Am. Chem. Soc., Angew. Chem., Int. Ed., Adv. Mater., ACS Nano, ACS Energy Lett., ACS Catal., and Adv. Funct. Mater. His work has been cited more than 27 000 times, with his highest single citation of more than 1300.

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1. Introduction

The growing global energy demand and eco-friendly concerns arising due to the depletion of fossil fuels have accelerated the quest for sustainable and clean energy alternatives.^1–3 Among the different possible alternatives, H₂ has garnered substantial consideration as a viable and environment-friendly energy carrier owing to its elevated energy density and carbon-free emission factors after combustion.^4–7 Nevertheless, sustainable H₂ production remains a crucial challenge. In recent years, photocatalytic water splitting-based H₂ production has emerged as one of the most significant strategies, which uses solar radiation to drive the reaction, thereby utilizing an eco-friendly and renewable energy source.^8–15 Another crucial challenge is the development of highly efficient, stable, and cheap photocatalysts for large-scale H₂ production from water splitting. Semiconductor photocatalysts, such as CdS, MoS₂, g-C₃N₄, TiO₂, WO_x, and ZnO, have received wide research focus owing to their distinct electronic characteristics that facilitate optical absorption, charge separation, and high performance.^16–21 Inorganic oxide semiconductor photocatalysts have also emerged as a favorable choice owing to their excellent chemical stability, low price, and adjustable electronic configurations.^22–24 Regardless of these benefits, traditional single-component inorganic oxide semiconductor photocatalysts suffer from several drawbacks, such as low charge separation efficiency, limited optical absorption, and quick recombination of photogenerated charges,²⁵ which limit their photocatalytic activities. These challenges in inorganic oxides are crucial compared to organic semiconductor photocatalysts, which demonstrate excellent charge migration and photocatalytic activity due to their improved optical absorption characteristics and efficient charge separation pathways.^26–28 For instance, poly(heptazine-triazine) imides exhibited significantly enhanced photocatalytic H₂ production owing to their special electronic configuration and the existence of semi-coherent interfaces.²⁹ To address the challenges faced by inorganic oxide semiconductors, researchers have focused on designing nanostructured systems that integrate inorganic oxide semiconductors with other semiconductors to augment their photocatalytic performances.^30–33 Specifically, the construction of emerging S-scheme heterojunctions introduces a new mechanism for charge separation by employing the S-scheme strategy.^34–36 These systems boost charge carrier separation by creating an internal built-in electric field that promotes electron and hole transport and consequently hinders their recombination and boosts the redox potential of the photocatalytic system. Meanwhile, the S-scheme heterojunctions facilitate the efficient utilization of photoinduced e⁻/h⁺ pairs, which greatly improves the redox potential of the photocatalytic system, making it extremely favorable for H₂ production.³⁷ Inorganic oxide semiconductor-based S-scheme heterojunctions show a synergistic enhancement in the photocatalytic performance. These heterojunctions benefit from the favorable characteristics of individual components while simultaneously addressing the challenges of typical semiconductor systems.

This review highlights recent developments in the design and preparation of inorganic oxide semiconductor-based nanoheterostructures for S-scheme photocatalytic H₂ generation. We overview the basic principles of photocatalytic H₂ production and S-scheme photocatalysis, design approaches for developing efficient inorganic oxide semiconductor-based heterostructures, and their crucial role in augmenting H₂ production by the emerging S-scheme mechanism. The S-scheme mechanism assists the transport and separation of e⁻/h⁺ pairs, thus significantly improving H₂ evolution. Additionally, we unveil the challenges and future viewpoints in this field with the aim of guiding future research toward stable, efficient, and high-performance photocatalytic H₂ production technologies.

2. Fundamentals of photocatalytic H₂ evolution and S-scheme heterojunctions

2.1. Fundamentals of H₂ evolution

In a typical water splitting process, photocatalytic H₂ generation consists of four primary steps, such as light absorption, charge separation and migration, charge recombination, and the utilization of photoinduced charges in redox reactions,^38,39 as depicted in Fig. 1. Upon light illumination, the semiconductor photocatalyst captures photons with energy (hv) equivalent to or more than its bandgap energy (E_g).⁴⁰ Then, electrons (e⁻) are excited from the valence band (VB) to the conduction band (CB) and leave holes (h⁺) behind, thereby generating e⁻/h⁺ pairs. These photoexcited charges then transfer to the surface of the semiconductor, whereas e⁻ migrate to the reduction centers for reducing protons (H⁺) to produce H₂. Meanwhile, h⁺ migrate to the oxidation centers. Nevertheless, before reaching the surface, some e⁻ and h⁺ may recombine, which wastes energy and decreases the photocatalytic activity. Therefore, for efficient photocatalytic performance, it is essential that the semiconductor photocatalyst must promote efficient charge transport while hindering the recombination of photoexcited charge carriers.

Fig. 1 Main reaction mechanism of photocatalytic H₂ production over semiconductor photocatalysts.

From a thermodynamic standpoint, surface redox reactions can only proceed when the redox potentials are locate between the VB and CB of the photocatalysts.⁴¹ The electronic band configuration of semiconductor photocatalysts, particularly VB and CB positions, and the E_g determine the capability of photocatalysts to promote these reactions. Specifically, for H₂ production through water splitting, the CB of the semiconductor needs to be negative compared to the reduction position for H₂ generation (0 V vs. NHE at pH = 0). On the other hand, the VB needs to be positive than the oxidation potential (+1.23 V vs. NHE at pH = 0).⁴² As presented in Fig. 2, several semiconductors with medium band gaps, including g-C₃N₄, Cu₂O, CdS, and TaON, suit these conditions. Nevertheless, wide band gap semiconductors contain more negative CB and more positive VB positions. This greatly restricts the solar light absorption in the visible region, culminating in a low solar energy transformation efficiency.⁴³ For instance, anatase TiO₂ possesses a E_g of about 3.20 eV, which can harvest less than 5% of the total solar energy in the ultraviolet (UV) region.⁴⁴ Thus, for single component photocatalysts, it is an inherent challenge to balance the redox potential with optical absorption, making it hard to achieve both efficient light absorption and high photocatalytic performance.

Fig. 2 Typical band structures of some representative photocatalysts. Adopted with permission from ref. 45. Copyright 2022, Elsevier.

2.2. Fundamental of S-scheme heterojunctions

The integration of one semiconductor (A) with another semiconductor (B), based on the band structures of CB and VB of these two semiconductors, typically culminates in Type-I, Type-II, Type-III, Z-scheme, and S-scheme heterojunctions.^46–52 In traditional Type-I heterojunctions (Fig. 3a), e⁻ and h⁺ build up in the B with the narrow band gap energy, which leads to a higher possibility of recombination, thereby resulting in low photocatalytic activity. In Type-II heterojunctions, e⁻ and h⁺ transfer to the corresponding low energy positions. The e⁻ transfer from the CB of B to the CB of A. Meanwhile, h⁺ migrate in the reverse direction (Fig. 3b). This charge carrier separation route promotes the photocatalytic activity; however, it decreases the redox abilities due to the accumulation of e⁻ and h⁺ in the bands with lower energy positions. In Type-III heterojunctions, e⁻ and h⁺ remain in their initial semiconductors without the transport of charge carriers (Fig. 3c). Thus, the band gaps of the two semiconductors remain apart, culminating in a broken gap. In direct Z-scheme heterojunctions, the photoexcited e⁻ in the CB of A directly recombine with the VB of B at the interface (Fig. 3d). This direct recombination facilitates the accumulation of photoexcited e⁻ in the CB of B for reduction, while h⁺ remain in the VB of A for oxidation reaction. The formation of direct Z-scheme heterojunction assists in effective charge transfer and separation, while hindering charge carrier recombination, resulting in enhanced photocatalytic activity. Nevertheless, in direct Z-scheme, it is difficult to maintain stability and firm interaction between the two semiconductors. Furthermore, the direct recombination of e⁻ and h⁺ at the interface can cause some energy loss, restricting the photocatalytic performance. Compared to these traditional heterojunctions, S-scheme systems provide a more effective charge transport and separation route for photoinduced charge carriers.^53–55 Additionally, the internal built-in electric field (IEF) in the S-scheme systems promotes charge migration via Coulomb forces and band bending. Consequently, e⁻ with weak reducing energy and h⁺ with weak oxidizing energy recombine; thus, only charge carriers with strong redox potential remain to drive the photocatalytic reactions. In S-scheme heterojunctions, the band alignment of the two semiconductors with staggered energy levels and intimate contact between the semiconductors resembles to that of Type-II. However, the main difference lies in the charge migration routes of these two systems. The typical Type-II heterojunctions are limited due to basic thermodynamic and kinetic limitations, which results in a low redox potential. The emerging S-scheme heterojunctions address these challenges by promoting more effective charge carrier separation and optimized redox performance.^56–58 As depicted in Fig. 3e, the CB and VB of A are lower than that of B. In contrast, the work function (WF) of A is greater than B. Upon the interaction of the two semiconductors, the e⁻ move from B to A, which creates an e⁻ build-up layer at the interface between A and B. This culminates in negative charge on A and positive charge on B, forming an IEF from B toward A and induces e⁻ in A to migrate into B. Afterward, the Fermi levels (E_f) of semiconductor A and B match upon interaction, which shifts the E_f of B downward and that of A upward, resulting in band bending.^59,60 This facilitates photoexcited e⁻ migration from the CB of A to the VB of B. At the same time, e⁻ in the CB of A recombine with h⁺ in the VB of B at the junction due to the presence of Coulomb forces, culminating in improved charge transport and efficient photocatalytic performance. Overall, the emerging S-scheme systems present significant benefits over Z-scheme heterojunctions by combining effective charge separation, improved redox potential, and hindered charge carrier recombination. The band bending at the interface facilitates spatial charge carrier recombination, enabling e⁻ and h⁺ to transfer to their corresponding active centers. This framework holds the strongest reducing and oxidizing abilities of the semiconductors, thus augmenting the photocatalytic performance.

Fig. 3 Band configurations and charge transport direction in (a) Type-I, (b) Type-II, (c) Type-III, (d) direct Z-scheme, and (e) S-scheme heterojunctions.

Therefore, S-scheme offers a more efficient and potential strategy for highly-efficient photocatalytic H₂ production.

3. Inorganic oxide semiconductor-based S-scheme heterojunctions for H₂ evolution

3.1. Tungsten oxide-based S-scheme heterojunctions

Tungsten oxides are regarded as versatile inorganic oxide semiconductors owing to their wide band gap energy and excellent photocatalytic characteristics.⁶¹ Therefore, tungsten oxides have received significant consideration for photocatalytic H₂ production, particularly due to their remarkable photochemical stability and the ability to improve visible-light absorption via various approaches, such as doping, typical heterojunction construction, and surface functionalization. These strategies can effectively decrease its band gap or form new electronic states, thus boosting its visible-light absorption ability, which in turn augments the H₂ production activity. Nevertheless, the activity of tungsten oxide is still restricted by its quick e⁻/h⁺ pairs recombination and low CB position, which greatly hinders its reduction potential. To tackle these challenges, many contributions have been made by researchers to design tungsten oxide-based heterostructures,^62–67 especially, the emerging S-scheme heterojunctions that accelerate charge separation and improved redox ability.^68–71 Typically, tungsten oxide is integrated with another semiconductor that hold a more negative CB potential to promote e⁻ transfer and decrease its recombination,^72–78 thus improving H₂ generation. Table 1 presents an overview of the latest tungsten oxide-based S-scheme systems.

Table 1 Photocatalytic H₂ production with tungsten oxide-based S-scheme heterojunctions

Photocatalysts	Light source (300 W Xe lamp)	Conditions	AQY (%)	H2 production (mmol g^−1 h^−1)	Ref.
WO/ZIS-CoxP	AM 1.5	TEOA	≈12 (420 nm)	45.993	79
W18O49/Mn0.45Cd0.55S10	λ ≥ 420 nm	Na2S/Na2SO3	16.5 (420 nm)	32.47	80
In2S3/WO3	λ > 400 nm	TEOA	1.7 (400 nm)	0.39	81
9% W18O49/Cd0.9 Zn0.1S	AM 1.5	Lactic acid	56.0 (370 nm)	66.3	82
W18O49/g-C3N4	λ ≥ 420 nm	TEOA, 3% Pt	9.78 (420 nm)	4.67	83
WO2.72/Zn0.5Cd0.5S-DETA	λ ≥ 420 nm	Na2SO3/Na2S·9H2O	—	3.94	84
10 wt% W18O49/CdS	AM 1.5	Lactic acid	7.5 (370 nm)	9.7	85
20% W18O49/Cd0.5Zn0.5S	λ > 200 nm	Na2S/Na2SO3	26.2 (365 nm)	147.7	86
40% CdS/W18O49	AM 1.5	Lactic acid	15.4 (370 nm)	15.4	77
Mn0.1Cd0.9S/6% WO3	420 nm	Lactic acid	13.4 (450 nm)	21.25	87

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For example, Liu et al.⁷⁹ designed W₁₈O₄₉/ZnIn₂S₄ S-scheme heterojunction grafted with Co_xP (WO/ZIS-Co_xP) for solar-driven H₂ production (Fig. 4a). Briefly, the WO/ZIS-X (X shows that the mass of the WO) heterojunction was constructed by the integration of WO with ZIS precursors in glycol. Then, WO/ZIS-Co_xP was prepared by the irradiation of WO/ZIS with CoCl₂·6H₂O, and NaH₂PO₂ at λ ≥ 420 nm. As presented in Fig. 4b, WO displayed no H₂ production owing to its inappropriate conduction band position, while ZIS achieved an H₂ evolution rate of 5.523 mmol g⁻¹ h⁻¹. In comparison, the WO/ZIS nanocomposite exhibited a noticeable increase in the H₂ production rate (21.301 mmol g⁻¹ h⁻¹). Remarkably, the WO/ZIS-CoxP heterostructure shows a maximum H₂ production rate of 44.993 mmol g⁻¹ h⁻¹ upon simulated solar light, which is 8.1 and 2.1-fold higher than that of ZIS and WO/ZIS, respectively. Upon visible-light irradiation, the H₂ production rate of 1.541, 4.512, and 9.372 mmol g⁻¹ h⁻¹ was achieved with ZIS, WO/ZIS, and WO/ZIS-CoxP, respectively (Fig. 4c). Conversely, no H₂ was generated over WO and ZIS under NIR light. However, WO/ZIS and WO/ZIS-Co_xP exhibited H₂ production rate of 0.221 and 0.288 mmol g⁻¹ h⁻¹, respectively (Fig. 4d), indicating the localized surface plasmon resonance (LSPR) effect of WO. As depicted in Fig. 4e, the activity slightly declined at first, possibly owing to the consumption of TEOA, which was recovered after reloading. The simulation results (Fig. 4f and g) demonstrate that WO possesses a higher work function (WF) of 6.18 eV to that of the ZIS plane (5.12 eV), enabling e⁻ migration from ZIS to WO at the interface until the E_f reached equilibrium.⁸⁶ The S-scheme mechanism was proved by electron paramagnetic resonance (EPR) spectroscopy (Fig. 4h), whereas WO/ZIS displayed a stronger DMPO-˙O₂⁻ signal,⁸⁸ suggesting efficient charge carrier separation. Finally, a plausible S-scheme mechanism was proposed (Fig. 4i). Under UV-vis light, the e⁻ migrate from the CB of WO to the VB of ZIS, which quickly recombine with the h⁺ of ZIS. Then, the LSPR effect of WO creates hot e⁻, which are incorporated into the ZIS. At the same time, e⁻ migrate from the CB of ZIS into Co_xP to reduce H⁺ into H₂, while the h⁺ in the VB of WO oxidize TEOA. Taken together, the WO/ZIS-Co_xP S-scheme heterojunction shows significantly augmented photocatalytic H₂ evolution upon simulated solar light, visible-light, and NIR light illumination. The formation of the S-scheme mechanism, LSPR effect of WO, and the Co_xP electron trapping center synergistically promoted the charge separation and transport in a wide range of light illumination. Consequently, the designed WO/ZIS-Co_xP composite exceeded both bare ZIS and WO/ZIS in terms of photocatalytic H₂ production activity, highlighting its ability for effective solar-induced H₂ production.

Fig. 4 (a) Schematic for the synthesis of WO/ZIS-Co_xP. H₂ production rate of the samples under (b) simulated solar light, (c) visible-light, and (d) NIR illumination. (e) Photocatalytic stability test of WO/ZIS-Co_xP. Electrostatic positions of (f) W₁₈O₄₉ (010) surface and (g) ZnIn₂S₄ (001) surface. (h) EPR spectra with DMPO-˙O₂⁻ for W₁₈O₄₉, ZnIn₂S₄ and WO/ZIS. (i) S-scheme charge transfer process of WO/ZIS/Co_xP. Adopted with permission from ref. 79. Copyright 2024, Springer Nature.

Furthermore, Luan et al.⁸⁹ reported lychee-shaped W₁₈O₄₉/CdWO₄/CdS (named as WOCS) dual S-scheme heterostructures, as presented in Fig. 5a. The optimal WOCS photocatalyst with TEOA exhibited an H₂ evolution rate of 1645 μmol h⁻¹ g⁻¹ under visible-light, which is 71-fold higher than that of CdS (Fig. 5b). Conversely, pristine W₁₈O₄₉, CdWO₄, and their heterojunction displayed no performance, possibly due to the inappropriate CB position. Additionally, WOCS also surpassed W₁₈O₄₉/CdS and CdWO₄/CdS by 20 and 25-fold, respectively, suggesting firm integration among its components. The activity of WOCS first increased with increasing CdWO₄/CdS content and then declined with further increasing content (Fig. 5c). The optimal WOCS displays stable activity under continuous irradiation of six reaction cycles (Fig. 5d). Additionally, the maximum H₂ evolution rate (3222 μmol h⁻¹ g⁻¹) was obtained with Na₂S and Na₂SO₃ as e⁻ donors (Fig. 5e). Then, the performance of the samples was evaluated employing Na₂S and Na₂SO₃ (Fig. 5f). The pristine W₁₈O₄₉ exhibited no activity, while CdWO₄ showed activity due to conversion into CdS. Moreover, the H₂ evolution rate of 2150 and 3222 μmol h⁻¹ g⁻¹ were achieved over W₁₈O₄₉/CdWO₄ and WOCS, respectively. However, WOCS reacts with Na₂S/Na₂SO₃ and eventually transforms into CdS, which caused light corrosion and reduced its photocatalytic stability. As depicted in Fig. 5g, the maximum apparent quantum yield (AQY) of WOCS was measured to be 10.3% at 420 nm. Eventually, defect-transit dual S-scheme mechanism was proposed (Fig. 5h), whereas the E_f level of CdS was lower compared to those of CdWO₄ and W₁₈O₄₉ before contact, respectively. After contact, e⁻ migrate from CdS to CdWO₄ and then to W₁₈O₄₉, generating charge redistribution and an IEF that promotes charge migration and boosts the photocatalytic performance. After irradiation, the CdWO₄ band configuration defects enable e⁻ movement between the CB and defect levels. The e⁻ migrate from the CB of W₁₈O₄₉ to the VB of CdWO₄ and recombine with its h⁺. Simultaneously, e⁻ migrate from the CB of CdWO₄ and recombine with h⁺ in the VB of CdS. This culminates in e⁻ build-up in the CB of CdS and h⁺ in the VB of W₁₈O₄₉, creating a dual S-scheme mechanism that boosts H₂ evolution.

Fig. 5 (a) Schematic for the preparation of WOCS. (b) H₂ production rate over different samples and (c) WOCS with different amounts of CdWO₄/CdS using TEOA. (d) Photocatalytic stability test of the optimal WOCS with TEOA. (e) H₂ production rate of optimzed samples with different hole scavengers. (f) H₂ production rate of different samples with NaS₂-Na₂SO₃. (g) AQY with its corresponding absorption spectrum of WOCS over different monochromatic light wavelengths using TEOA. (h) S-scheme mechanism of photocatalytic H₂ production over WOCS. Adopted with ref. 89. Copyright 2023, John Wiley and Sons.

The WO/ZIS-Co_xP and WOCS S-scheme heterojunctions illustrate the key role of integrating inorganic oxide-based semiconductors with other semiconductors to facilitate charge separation and augment photocatalytic H₂ evolution. In WO/ZIS-Co_xP, H₂ production is significantly improved due to the construction of the S-scheme mechanism, LSPR effect of WO, and Co_xP electron trapping, especially under simulated solar light and visible light. Similarly, the WOCS nanocomposite leverages from a dual S-scheme charge carrier pathway between CdS, CdWO₄, and W₁₈O₄₉, which culminated in augmented H₂ evolution. However, both heterojunctions depend on various semiconductor integrations and mechanisms with a common goal promoting charge transport routes, which is vital for high photocatalytic performance. Additionally, both heterojunctions reveal the significance of design integration of appropriate materials with W₁₈O₄₉ to boost the overall photocatalytic activity through S-scheme mechanisms in solar-induced H₂ generation.

3.2. Titanium oxide-based S-scheme heterojunctions

Titanium oxides have been widely known for good photocatalytic abilities due to their strong oxidation and stable characteristics since Fujishima and Honda first discovered its capability in water splitting under UV light.⁹⁰ However, single titanium oxide photocatalysts suffer from drawbacks, including poor light absorption, low photocatalytic activity, and quick charge carrier recombination. To resolve these issues, the creation of titanium oxide-based S-scheme heterostructures has achieved substantial attention.^91–101 The emerging S-scheme systems not only improve optical absorption but also efficiently facilitate charge transport and separation, which significantly augments its redox ability and photocatalytic H₂ generation.^49,102–104 In this part, we outline the recent advancements in titanium oxide-based S-scheme composites with the focus on their preparation approaches, controlled H₂ evolution, and S-scheme mechanism that enable efficient charge separation and transfer. Table 2 displays representative titanium oxide-based S-scheme heterostructures constructed for augmented H₂ evolution, demonstrating the transition from single-component titanium oxide to advanced S-scheme systems.

Table 2 Photocatalytic H₂ production with TiO₂-based S-scheme heterojunctions

Photocatalysts	Light source (300 W Xe lamp)	Conditions	AQY (%)	H2 production	Ref.
H-TiO2/g-C3N4/Ti3C2	—	CH3OH, 0.5% Pt	16.8 (380 nm)	53.67 mmol g^−1 h^−1	105
TiO2/50%ZnIn2S4	Simulated sunlight	TEOA	0.85 (365 nm)	6.85 mmol g^−1 h^−1	106
5% NiCoSe2/TiO2	—	TEOA	—	4976 μmol g^−1 h^−1	107
20% Co9S8/TiO2	350–780 nm	TEOA	—	3982 μmol g^−1 h^−1	108
15% Co9Se8/TiO2	—	TEOA	—	8282.7 μmol h^−1 g^−1	109
TiO2/ZnS-5	Simulated solar light	Na2S·9H2O/Na2SO3	—	5503.8 μmol g^−1 h^−1	110
10% Ni3Se4/TiO2	Simulated sunlight	TEOA	—	8409.3 μmol g^−1 h^−1	111
Cu3P/TiO2	Simulated sunlight	CH3OH	—	5.83 mmol g^−1 h^−1	112
Mxene@CdS/TiO2	λ > 420 nm	Lactic acid	—	16.2 mmol h^−1 g^−1	113
35% O-ZnIn2S4/TiO2−x	λ ≥ 420 nm	Benzyl alcohol	—	2584.9 μmol g^−1 h^−1	114
TiO2/ZnIn2S4	—	TEOA, 1% Pt	10.49 (365 nm)	6.03 mmol h^−1 g^−1	115
ZnCo2S4/TiO2	Simulated sunlight	TEOA	11.5 (420 nm)	5580 μmol g^−1 h^−1	116
BP/(Ti3C2Tx@TiO2)	λ ≥ 325 nm	—	2.7 (380 nm)	564.8 μmol h^−1 g^−1	117
P-CuWO4/TiO2	350–780 nm	TEOA	—	6169.25 μmol h^−1 g^−1	118
25% Co3Se4/TiO2	—	TEOA	1.8 (365 nm)	6065 μmol g^−1 h^−1	119
40-MTO/PCN	420 nm	TEOA, 3% Pt	6.73 (450 nm)	5252.9 μmol h^−1 g^−1	120

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For instance, Zhang et al. designed TiO₂/ZnIn₂S₄ heterostructures for S-scheme photocatalytic H₂ evolution (Fig. 6a).¹⁰⁶ First, TiO₂ microspheres were prepared by solvothermal and calcination methods, then followed by in situ chemical bath deposition method to construct TiO₂/ZnIn₂S₄ heterostructures. The optimal TiO₂/ZnIn₂S₄ (TZ-2) showed a greater H₂ production rate (6.85 mmol g⁻¹ h⁻¹) at simulated sunlight illumination, which is 3.9 and 171.2 times greater than those of ZnIn₂S₄ and TiO₂, respectively (Fig. 6b and c). Generally, hole scavengers serve an essential role in photocatalytic H₂ evolution reaction.¹²¹ As depicted in Fig. 6d, nearly no H₂ was observed over TZ-2 without any hole scavenger owing to charge carrier recombination.^122,123 However, significant H₂ was obtained in the presence of hole scavengers with a maximum activity over TEOA, making it the most efficient hole scavenger for TiO₂/ZnIn₂S₄. The highest AQY of TZ-2 was measured to be 0.85% at 365 nm (Fig. 6e). The AQY values show a decreasing trend with increasing monochromatic light illumination, which is consistent with optical absorption spectrum. As depicted in Fig. 6f, TZ-2 maintained H₂ production over six cycles, suggesting excellent stability associated with the firm interaction between TiO₂ and ZnIn₂S₄. Moreover, no clear structural deformation was noticed in the XRD pattern after the stability test (Fig. 6g). In the S-scheme mechanism (Fig. 6h), the e⁻ moves from ZnIn₂S₄ to TiO₂ on the interface due to the change in the E_f level,^124–126 generating an IEF and band bending configurations.^127,128 Under light illumination, the e⁻ in the CB of TiO₂ recombines with h⁺ in the VB OF ZnIn₂S₄, facilitating the reduction of e⁻ in the CB of ZnIn₂S₄ and the oxidation of h⁺ in the VB of TiO₂,¹²⁹ which is quenched by TEOA. This route promotes the tailored separation and migration of photogenerated charge carriers, retaining robust redox ability and augmenting the H₂ production activity. Overall, the constructed heterojunctions display superb photocatalytic H₂ evolution. The improved charge separation and transport enabled by the S-scheme pathway greatly boosted the H₂ production rate. This report highlights the significance of designing TiO₂-based S-scheme systems for green and sustainable H₂ generation.

Fig. 6 (a) Schematic for the synthesis of TiO₂/ZnIn₂S₄. (b) H₂ production and (c) H₂ production rate over different samples. (d) H₂ evolution rate over TZ-2 in the presence and without hole scavengers. (e) AQY with its corresponding UV-vis absorption spectrum and (f) recycling experiment of TZ-2. (g) XRD patterns before and after stability of TZ-2. (h) Proposed S-scheme charge carrier mechanism of photocatalytic H₂ production over TiO₂/ZnIn₂S₄. Adopted with permission from ref. 106. Copyright 2024, Elsevier.

3.3. Zinc oxide-based S-scheme heterojunctions

Zinc oxide (ZnO) is a broadly studied inorganic oxide semiconductor due to its high stability, environment-friendly features, and an appropriate bandgap.^130,131 However, its performance is limited by quick e⁻/h⁺ pairs recombination and a narrow light absorption range. To address these challenges, various ZnO-based heterostructures have been constructed by grafting ZnO with other materials to augment charge separation, prolong optical absorption, and boost photocatalytic activity.^132–135 Typical heterojunctions, including Type-II and Z-scheme heterostructures, provide enhanced charge carrier separation.^136–138 However, low redox abilities and poor performance restricted their use in practical applications.¹³⁹ Recently, ZnO-based S-scheme systems have manifested as a more prominent approach, creating an IEF that promotes carrier separation and maintains the robust redox potentials of the photocatalysts.^135,140 Interestingly, the S-scheme systems not only facilitate effective charge separation and transport within ZnO but also extend its optical absorption spectrum, thus significantly improving its H₂ production activity.^141–148 Table 3 demonstrates latest zinc oxide-based S-scheme heterojunctions.

Table 3 Zinc oxide-based S-scheme systems for H₂ generation

Heterojunctions	Light source (Xe lamp)	Conditions	AQY (%)	H2 evolution	Ref.
ZnO@ZnS	350 W Xe lamp (simulated sunlight)	Na2S/Na2SO3	27.4 (365 nm)	15.7 mmol g^−1 h^−1	141
3% Co3O4/ZnO	500 W Xe lamp (420–700 nm)	Glycerol, 0.5% Pt	—	2241 μmol g^−1 h^−1	142
5% MTM/ZnO	200 W Hg-Xe arc lamp (450 nm)	Na2S/Na2SO3	41.42	5.25 mmol g^−1 h^−1	149
ZnO/In2S3	300 W Xe lamp	TEOA	11.35 (420 nm)	2488 μmol g^−1 h^−1	144
ZnO-20% ZnBi2O4	570 W Xe lamp (simulated sunlight)	Na2S·9H2O/Na2SO3	—	3.44 mmol g^−1 h^−1	145

---

For instance, Zhou's group developed V_O,_Zn-ZnO/ZnS hollow-structured heterostructures via ion-exchange and pyrolysis (Fig. 7a).¹⁵⁰ Briefly, a specific amount of ZnO hollow microspheres and thiourea was dispersed in deionized water via stirring and then heated at 160 °C for 24 h in autoclave followed by washing, drying, and calcined at 550 °C for 4 h under Ar. ZnO and ZnS achieved an H₂ evolution rate of 0.25 and 0.75 mmol g⁻¹ h⁻¹, respectively (Fig. 7b and c), while V_O,_Zn-ZnO/ZnS nanocomposites showed enhanced performance caused by the combination of thin-shell ZnO with ZnS and the incorporation of dual vacancies. The optimal V_O,_Zn-ZnOS-2 exhibits the maximum H₂ evolution rate (160.91 mmol g⁻¹ h⁻¹), exceeding ZnS and ZnO by 214.5 and 634.6-fold, respectively. The AQY of the optimal sample were measured to be 15.4%, 7.6%, and 4.0% at 365, 400, and 420 nm, respectively (Fig. 7d). The AQY values are in alignment with the corresponding absorption spectrum of the optimal sample. Moreover, V_O,_Zn-ZnOS-2 maintained outstanding H₂ production stability over five reaction cycles, suggesting the firm interaction at the interface between ZnS and ZnO (Fig. 7e). According to the electronic band configurations of ZnO and ZnS, S-scheme charge migration mechanism was presented for V_O,_Zn-ZnO/ZnS heterostructure (Fig. 7f). The photoexcited e⁻ in ZnS reduce H₂O to yield H₂, while h⁺ in ZnO are quenched by the sacrificial agent (Na₂S and Na₂SO₃). The ZnO/ZnS S-scheme system with oxygen and zinc vacancies promotes light absorption, charge separation, and redox capacity, thus augmenting photocatalytic H₂ production.

Fig. 7 (a) Schematic for the synthesis route of V_O,Zn-ZnO/ZnS. (b) Time-dependent H₂ production (c) and H₂ evolution rate of samples. (d) Wavelength-dependent AQY with corresponding UV-vis absorption spectrum and (e) H₂ production stability test of V_O,Zn-ZnO/ZnS. (f) Proposed S-scheme charge transfer H₂ evolution under simulated sunlight illumination over V_O,Zn-ZnO/ZnS. Adopted with permission from,¹⁵⁰ Copyright 2024, Elsevier.

3.4. Copper oxide-based S-scheme heterojunctions

Copper oxides have gained wide attention as photocatalysts for H₂ generation due to their narrow bandgap, good visible-light absorption, and robust reduction capacity.¹⁵¹ However, copper oxides suffer from drawbacks such as rapid charge carriers’ recombination and relatively low photocatalytic stability.¹⁵² Recent studies indicated that the formation of copper oxide-based S-scheme systems provides a prominent solution to these issues by promoting effective charge separation.^153–159 In these systems, the IEF and band edge bending at the interface enhance charge transport while facilitating e⁻ and h⁺ transfer toward the appropriate reaction sites for augmented photocatalytic performance. This allows the full use of copper oxide reduction potential and improves its overall redox ability, thereby improving the H₂ generation activity. In this section, we highlight recent advancements in copper oxide-based S-scheme heterostructures for H₂ generation. Table 4 presents recent copper oxide-based S-scheme systems.

Table 4 Copper oxide-based S-scheme heterostructures for H₂ generation

Heterojunctions	Light source	Conditions	AQY (%)	H2 evolution	Ref.
Cd0.8Mn0.2S/Cu2O	10 W white light (visible-light)	Na2S/Na2SO3	0.79 (500 nm)	40.06 mmol g^−1 h^−1	160
GDY/NiCo2O4/Cu2O	5 W LED light	TEOA, EY dye	—	17.84 mmol g^−1 h^−1	161
CuO/pCN	300 W Xe lamp (λ > 420 nm)	TEOA	1.67	30 μmol g^−1 h^−1	162
Ti3C2-CdS-Cu2O@NC	300 W Xe lamp (420 nm)	Lactic acid	—	12 366 μmol h^−1 g^−1	163
1.5% CuO/CN	300 W Xe lamp	TEOA	2.08 (420 nm)	130.1 μmol g^−1 h^−1	164
MnCdS/Cu2O	300 W Xe lamp	Na2S/Na2SO3	18.5 (420 nm)	66.3 mmol g^−1 h^−1	165
1-Cu2O/g-C3N4	500 W Xe lamp (420 nm)	TEAO	—	480.6 μmol g^−1 h^−1	166
CuO/CdS/CoWO4	300 W Xe lamp (λ > 420 nm)	Na2S/Na2SO3	—	457.9 μmol g^−1 h^−1	167

---

For example, Fan et al. constructed MnCdS/Cu₂O (MCSCO) heterojunctions via a hydrothermal method, as depicted in Fig. 8a.¹⁶⁵ First, MnCdS was prepared by dispersing Mn(CH₃COO)₂·4H₂O, Cd(CH₃COO)·2H₂O, and thioacetamide in deionized water, followed by heating at 180 °C for 24 h. Similarly, for MCSCO, different amounts of Cu₂O were dissolved in the MnCdS precursor solution during stirring. Photocatalytic H₂ generation was evaluated under visible-light illumination. Fig. 8b displays H₂ production as a function of time. CdS and MnCdS displayed H₂ production rates of 2.2 and 19.4 mmol g⁻¹ h⁻¹, respectively (Fig. 8c), while the optimal MCSCO exhibited an H₂ generation rate of 66.3 mmol g⁻¹ h⁻¹, which is 54.2 and 3.4-fold greater than those of Cu₂O and MCS, respectively. Interestingly, the MCSCO heterostructures sustained 95% of its inherent H₂ evolution activity after 6 cycles of 30 h (Fig. 8d). The S-scheme charge transport mechanism over MCSCO is illustrated in Fig. 8e. The constructed MnCdS/Cu₂O interface enabled an S-scheme charge-migration pathway, whereas the IEF promoted charge recombination, maintaining the MnCdS CB for H⁺ reduction. Overall, the designed MnCdS/Cu₂O displayed boosted light absorption and reduction ability, thus achieving a maximum H₂ production activity. This study also demonstrates that the integration of Cu₂O as a potential inorganic oxide semiconductor with other semiconductors substantially promotes efficient charge separation and migration, leading to outstanding photocatalytic activity.

Fig. 8 (a) Schematic for the synthesis of MCSCO. (b) Time-dependent H₂ evolution and (c) H₂ evolution rate of the samples. (d) Stability and (e) S-scheme charge carrier migration and separation mechanism of the MCSCO heterojunction. Adopted with permission from ref. 165. Copyright 2023, Elsevier.

4. Conclusion and perspectives

In summary, inorganic oxide semiconductor-based heterostructures employing S-scheme charge transfer pathways introduce a promising development in photocatalytic H₂ evolution. This review broadly covers the fundamental principles, preparation approaches, and S-scheme charge transport mechanism of inorganic oxide-based heterojunctions that augment photocatalytic H₂ generation. The construction of inorganic oxide semiconductor-based S-scheme heterojunctions efficiently promotes charge migration, augments optical absorption, and regulates band structures, culminating in significantly high H₂ production rates. Irrespective of these developments, several critical challenges must be addressed to further optimizing the photocatalytic activity of these systems, which are as follows:

(i) Formation of advanced heterostructures

The investigation of new and advanced hybrid nanocomposites that combine inorganic oxide semiconductors with multiple semiconductor materials are essential for tuning the band configurations and augmenting the photocatalytic activity. Through the integration of various semiconductors, researchers can tune the electronic and light absorption characteristics, which leads to more efficient charge separation and decreased recombination rates. For example, integrating a large bandgap inorganic oxide semiconductor with a narrow bandgap semiconductor can prolong the spectral absorption to use a broader range of solar energy. Furthermore, this strategy can augment the stability of the photocatalysts. Thus, these heterostructures have the ability to significantly boost the photocatalytic activity, contributing to more effective H₂ evolution and sustainable energy technologies.

(ii) Green synthesis strategies

The exploration of environment-friendly and green synthesis strategies is crucial for reducing the environmental impact and synthesis costs of photocatalysts. For instance, biomimetic techniques that utilize renewable resources and natural processes can decrease the dependence on hazardous materials. Additionally, green synthesis can promote the creation of special inorganic oxide semiconductor-based heterostructures that improve the overall photocatalytic performance.

(iii) Long-term stability

The augmentation of long-term stability of inorganic oxide semiconductors is vital for large-scale applications. This can be achieved via the optimization of material composition and constructing robust nanoheterostructure photocatalysts that are both effective and stable for sustainable H₂ evolution in practical applications.

(iv) Augmentation of light absorption and surface area

It is crucial to optimize the optical absorption within a broader region by the construction of nanoheterostructures with tunable bandgap characteristics and elevated specific surface areas. The tuning of these properties can enhance the photocatalytic performance and enable excellent absorption of sunlight for boosted H₂ generation while hindering charge recombination and augmenting the overall photocatalytic activity.

(v) Coupling with non-noble metal cocatalysts

The integration of transition metal-based materials and noble metal cocatalysts can further promote charge transport characteristic in S-scheme systems, where these cocatalysts can serve as reaction active centers for H₂ generation.

(vi) Development of scalable strategies

For large-scale applications, it is vital to design scalable, simple, and cheap modification technologies for these S-scheme systems. For instance, hydrothermal preparation, electrodeposition, and spray annealing can offer a route for large yield while retaining the desired photocatalytic characteristics.

(vii) Advanced characterization for the confirmation of the S-scheme mechanism

Advanced characterization, such as transient absorption spectroscopy and in situ characterization, are crucial to further easily understand the charge transfer mechanisms in S-scheme heterojunctions for finding the main factors that affect the photocatalytic activity. This will further help in designing more efficient heterojunction photocatalysts.

Overall, optimizing the construction of inorganic oxide semiconductor-based S-scheme systems possesses great potential for improving photocatalytic H₂ production and promoting sustainable energy development. Further efforts in this area will be essential for achieving the practical implementation of effective photocatalysts in different energy-related applications.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors confirm no conflict of interest.

Acknowledgements

The authors gratefully acknowledge the special funding support from China/Shandong University International Postdoctoral Exchange Program, the Shandong Provincial Natural Science Foundation (ZR2024ME103), the Shandong Excellent Young Scientists Fund Program (OVERSEAS) (2023HWYQ-021, 2022HWYQ-011), the National Natural Science Foundation of China (22271266), the USTC-Yanchang Petroleum New Energy Joint Research Project (2022ZKD-02), and the Fundamental Research Funds for the Central Universities (YD2340002001).

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