Preparation and photocatalytic activity characterization of activated carbon fiber–BiVO₄ composites

Abstract

Herein, we describe the hydrothermal immobilization of BiVO₄ on activated carbon fibers (ACFs) and characterize the obtained composite by several instrumental techniques, using Reactive Black KN-B (RB5) as a model pollutant for photocatalytic performance evaluation and establishing the experimental conditions yielding maximal photocatalytic activity. The photocatalytic degradation of RB5 is well fitted by a first-order kinetic model, and the good cycling stability and durability of BiVO₄@ACFs highlight the potential applicability of the proposed composite. The enhanced photocatalytic activity of BiVO₄@ACFs compared to those of BiVO₄ and ACFs individually was mechanistically rationalized, and the suggested mechanism was verified by ultraviolet-visible spectroscopy, attenuated total reflectance Fourier-transform infrared spectroscopy, and RB5 degradation experiments. Thus, this work contributes to the development of BiVO₄@ACF composites as effective photocatalysts for environmental remediation applications.

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Introduction

The development of human society unavoidably causes environmental problems, necessitating the development of highly efficient pollution control methods such as photocatalytic decontamination. In particular, the application of photocatalysis for energy production and environmental protection (e.g., for organic pollutant degradation, hydrogen reduction, and solar cell fabrication) has recently received increased attention.^1–3 In photocatalysts, photoexcited electrons are transferred from the valence band to the conduction band, where they can freely move within the crystal.⁴

TiO₂ is a widely used traditional photocatalyst.^5,6 However, its large bandgap (3.2 eV) implies that this catalyst can only utilize ultraviolet light, thus necessitating the development of visible light-driven photocatalysts⁷ such as Bi-based semiconductors. For example, BiVO₄, first synthesized in 1963, exhibits a relatively narrow bandgap of 2.3–2.4 eV and is therefore a promising photocatalyst.⁸ The valence band of BiVO₄ mainly comprises the O 2p and V 3d orbitals, which allows photocatalytic O₂ evolution under visible light irradiation. The use of BiVO₄ to catalyze visible light-induced water splitting was first reported by Kudo et al., who demonstrated a new application of this semiconductor as a photocatalyst⁹ and inspired extensive research on its synthesis, characterization, and photocatalytic properties.¹⁰

Although photocatalysis has been widely used for the purification of dye-contaminated wastewater with partial contaminant degradation, the separation of spent photocatalysts remains challenging. Separation is typically addressed by immobilizing the active material on macroscopic supports.^11,12 For example, Wu et al. reported a novel procedure for immobilizing BiVO₄ on silica fibers, obtaining a catalyst with increased surface area and activity.¹³ Importantly, the choice of the immobilization method significantly affects the performance of the supported BiVO₄, e.g., hydrothermal synthesis commonly yields catalysts with high degrees of crystallinity.

The abundance of delocalized electrons in their conjugated π-systems makes carbon materials promising supports, including fullerenes (C₆₀), carbon nanotubes (CNTs), graphitic carbon nitride (g-C₃N₄), and activated carbon fibers (ACFs), which can potentially enhance charge separation and transport within the immobilized catalyst.^14,15 Among them all, it was observed that the construction of a semiconductor heterojunction can greatly promote photocatalytic activity.^16,17 Recently, ACFs have attracted much attention as pollutant adsorbents because of their high surface areas, diverse functional groups, narrow pore size distributions, and many raw material precursors.¹⁸ However, the adsorption of a given pollutant on ACFs is not accompanied by degradation, and ACFs have therefore been extensively explored as photocatalyst supports offering further photocatalytic performance enhancement. Lin et al. described CF-supported ordered mesoporous TiO₂, revealing that the synergistic active material–support interactions enhanced the photocatalytic activity of this composite to levels exceeding those of commercial photocatalysts.¹⁹ Furthermore, ACF-supported N-doped TiO₂ photocatalysts have also been reported.²⁰ Nevertheless, the immobilization of BiVO₄ on ACFs has been underexplored.

Herein, we investigated the hydrothermal immobilization of BiVO₄ nanoparticles on ACFs, aiming to (a) develop a simple procedure of immobilizing BiVO₄ on ACFs, (b) characterize the structure, properties, and photostability of the synthesized photocatalysts, and (c) determine the influence of the processing parameters on photocatalyst performance.

Experimental

Materials

ACFs were sourced from Nantong Sutong Carbon Fiber Co., Ltd. (Jiangsu, China). Bi(NO₃)₃·5H₂O, NH₄VO₃, HNO₃ (64–69%), NH₄OH (35%), KI and ethylenediaminetetraacetic acid (EDTA) of analytical grade were purchased from Meryer Chemical (Shanghai, China). Reactive Black KN-B (RB5) was provided by Bomei Company (Anhui, China). All solutions were prepared using deionized water.

Preparation of photocatalysts

ACFs pretreated with ethanol (95%) and deionized water were dried at 70 °C for 12 h. As shown in Scheme 1, Bi(NO₃)₃·5H₂O (2.1828 g) and a certain amount of EDTA were dissolved in dilute HNO₃ (50 mL, 2 mM), and the resulting mixture was stirred at 90 °C for 30 min (200 rpm) with a magnetic stirrer to afford solution A. Solution B was obtained by dissolving NH₄VO₃ (0.5263 g) in deionized water (50 mL) at 60 °C. Subsequently, a dispersion of ACFs (1.0 g) in solution B was added to solution A, and the pH of the resulting mixture was adjusted by the slow dropwise addition of NH₄OH. The above mixture was continuously stirred for 1 h at 50 °C, poured into a Teflon-lined stainless-steel autoclave (150 mL), and maintained at 180 °C for 6 h. The produced precipitate (BiVO₄@ACFs) was washed with ethanol and deionized water several times and dried in a vacuum oven at 75 °C overnight.

Scheme 1 Preparation of BiVO₄@ACFs.

Instrumentation

X-ray diffraction (XRD; Ultima IV, Rigaku, Japan) patterns were recorded using high-intensity Cu Kα radiation (λ = 0.154 nm) in the range of 2θ = 10–80° at a scan rate of 5° min⁻¹. Sample morphologies and surface elemental compositions were probed by scanning electron microscopy (SEM; JSM6510, JEOL, Japan) coupled with energy-dispersive spectrometry (EDS) at an accelerating voltage of 10 kV, and the sample surface was coated by a thin layer of gold before SEM characterization. The chemical states of the as-prepared composites were determined by X-ray photoelectron spectroscopy (XPS; ESCALAB 250XI, Thermo Electron Corporation, USA). Thermal stabilities were determined by thermogravimetric analysis (TGA; TG209F3, NETZSCH, Germany) performed under N₂ in the temperature range of 40–800 °C at a heating rate of 10 °C min⁻¹. Ultraviolet-visible (UV-Vis) absorption spectra were recorded on a UV-Vis spectrophotometer (Cary300, Agilent, USA) in the wavelength range of 200–800 nm. Dye concentrations were monitored by (a) UV-Vis spectroscopy and (b) Fourier-transform infrared (FTIR) spectroscopy (IS10, Thermo Fisher, USA; 600–4000 cm⁻¹) using an attenuated total reflectance (ATR) accessory with a diamond crystal.

Photocatalytic activity evaluation

Photodegradation tests were conducted in a photocatalytic reactor (XPA-XL, XJ, China) equipped with a 1000 W Xe arc lamp as a simulated sunlight source and featuring a 6 cm-diameter water-cooled quartz jacket placed in a stainless steel box. BiVO₄@ACFs (0.2 g) and pure ACFs (0.2 g) were independently placed in a quartz tube containing aqueous RB5 (50 mL, 40 mg L⁻¹), and the tube was immediately placed in darkness for 30 min to establish an adsorption equilibrium. Subsequently, the solution was magnetically stirred under visible-light irradiation for 3 h, and the residual dye concentrations were determined spectrophotometrically (V-1200, MAPADA, Shanghai, China). The efficiency of RB5 degradation (R) was calculated as²¹

R(%) = (1 − A/A₀) × 100, (1)

where A₀ is the initial absorbance of the RB5 solution and A is the absorbance of the RB5 solution after irradiation.

In addition, to investigate the adsorption kinetics and suggest a possible degradation mechanism, the rate constants of photocatalytic RB5 degradation (k) were obtained from the following equation:²²

ln(C₀/C) = kt, (2)

where C is the concentration of RB5 at time t, C₀ is the adsorption/desorption equilibrium concentration of RB5 at the reaction onset, and C/C₀ is the ratio of the above concentrations determined by absorbance measurements at 598 nm (C/C₀ = A/A₀).

Results and discussion

Crystal phase composition

The phase purities and crystallinities of the samples are examined by XRD analysis (Fig. 1). Notably, the XRD pattern of ACFs (Fig. 1a) exhibits no peaks except for a broad signal at ∼25°, which is attributed to the carbon structure.²³ Fig. 1b shows the XRD pattern of the BiVO₄@ACFs, revealing the diffraction peaks of the (110), (011), (121), (040), (161) planes for BiVO₄@ACFs at 2θ = 18.9°, 19.3°, 29.3°, 30.9°, and 53.6°; these are indexed to monoclinic scheelite-type BiVO₄ (JCPDS no. 14-0688) and indicate that the surfaces of the ACFs are largely covered by BiVO₄.^24,25

Fig. 1 XRD patterns of (a) ACFs and (b) BiVO₄@ACFs.

Surface morphology and textural properties

The morphologies and microstructures of the as-obtained products were investigated by SEM (Fig. 2). Compared to pristine the ACFs, which are smooth and feature long grooves on their surfaces (Fig. 2a), thick and sheet-like immobilized particles are observed for BiVO₄@ACFs (Fig. 2c). In addition, Fig. 2b and d show the EDS spectra and the corresponding elemental compositions of the spots marked A (Fig. 2a) and B (Fig. 2c). The obtained results indicate that the BiVO₄@ACFs contain C, O, Bi, and V (Bi : V ≈ 1 : 1 mol mol⁻¹), confirming the presence of BiVO₄.²⁶ Moreover, the insights provided by XRD analysis suggest that the BiVO₄ nanoparticles are grown in situ on the surfaces of the ACFs.

Fig. 2 SEM images of (a) ACFs and (c) BiVO₄@ACFs; (b) and (d) EDS spectra corresponding to points A and B, respectively.

XPS analysis of as-prepared samples

Subsequently, the BiVO₄@ACFs was characterized by XPS to gain further insight into its chemical composition, metal chemical states, and structure. Fig. 3a shows the survey spectrum of BiVO₄@ACFs, which resembles that of pristine ACFs and indicates the presence of C, N, and O. However, weak V 2p peaks at 510–525 eV and a Bi signal at 156–168 eV are additionally observed in the former case. The Bi 4f spectrum (Fig. 3b) of BiVO₄@ACFs displays two intense peaks at 159.58 and 164.93 eV, which are ascribed to the Bi 4f_5/2 and Bi 4f_7/2 transitions of Bi³⁺ ions.²⁷ Fig. 3c shows the corresponding V 2p spectrum, showing peaks at 515.78 (V 2p_5/2) and 523.83 eV (V 2p_3/2) that correspond to surface V⁴⁺ and V⁵⁺ species, respectively.^28,29

Fig. 3 XPS spectra of ACFs before (black) and after BiVO₄ deposition (red). (a) Survey spectra, (b) Bi 4f spectra, (c) V 2p spectra, and (d) O 1s spectra.

The O 1s spectrum (Fig. 3d) is deconvoluted into three peaks at 530.38, 531.41, and 532.67 eV, indicating the presence of at least three types of O species. Based on the results of previous studies, the high-intensity peak at 532.67 eV is attributed to surface-adsorbed O,³⁰ while the shoulder peak at 531.41 eV reflects the presence of C=O or C–O moieties.^31,32 Finally, the signal at 530.38 eV is characteristic of lattice O in monoclinic BiVO₄.³³ Thus, the results of XPS analysis confirm the successful formation of a C–BiVO₄ heterojunction.

Optimization of experimental conditions

For more sensitive detection of BiVO₄ in the actual test, the conditions including the concentration of the precursor solution of BiVO₄ were optimized. As shown in Fig. 4, the above concentration is varied from 0.06 to 0.3 M, and the maximum RB5 degradation efficiency is observed at 0.1 M. During the synthesis of BiVO₄, the addition of EDTA caused the formation of Bi³⁺–EDTA⁻ chelates via electrostatic interactions.³⁴ At low Bi³⁺ concentrations, the gradual release of free Bi³⁺ from these complexes induced the formation of small amounts of monoclinic BiVO₄.³⁵ As the concentrations of Bi³⁺ and VO₄⁻ increased, the inhibitory effect of EDTA⁻ decreased, which caused the fast growth of particles with large sizes.³⁶ As the above concentrations continued to increase, the size of the BiVO₄ particles became progressively larger, reflecting the decreasing influence of chelation by EDTA, which corresponded to decreased photocatalytic activity.

Fig. 4 RB5 degradation profiles determined for different concentrations of precursor solution of BiVO₄: (a) pristine ACFs, (b) 0.06 M, (c) 0.1 M, (d) 0.15 M, and (e) 0.3 M.

The above assumption was further supported by TGA results. As shown in Fig. 5, the pristine ACFs exhibit good thermal stability,³⁷ whereas the thermograms of BiVO₄@ACFs recorded at temperatures from ambient to 800 °C show at least two weight losses. The first weight loss (<150 °C, ∼1%) is attributed to the removal of absorbed water,³⁸ and the second one (150–480 °C, ∼7%) is ascribed to the combustion of EDTA.³⁹

Fig. 5 TGA curves obtained for different concentrations of precursor solution of BiVO₄: (a) pristine ACFs, (b) 0.06 M, (c) 0.1 M, (d) 0.15 M, and (e) 0.3 M.

Moreover, when the concentration of precursor solution of BiVO₄ reaches 0.3 M, BiVO₄ loading on the ACFs begins to decrease. According to the (e)-differential thermogravimetric (DTG) curve in Fig. 5, this is explained by the weak inhibitory effect of high BiVO₄ concentration on the growth of BiVO₄@ACFs. Moreover, a further change in BiVO₄ crystallites is observed as the temperature reached 700 °C.⁴⁰ Therefore, the concentration of precursor solution of BiVO₄ in subsequent experiments is set to 0.1 M.

The presence of EDTA, a typical chelator, significantly affects the morphologies and microstructures of BiVO₄, because the ligand can be adsorbed on the surface of BiVO₄ nanoparticles and can thereby reduce their interfacial energy, with the possible mechanism of the above influence provided below.⁴¹

Bi³⁺ + EDTA⁻ → Bi³⁺–EDTA, (3)

Bi³⁺–EDTA + VO₄⁻ + H₂O → BiVO₄ + EDTA⁻ + 2H⁺. (4)

Fig. 6 shows the RB5 degradation profiles recorded in the presence of BiVO₄@ACFs samples prepared using different amounts of EDTA. Notably, the fastest photocatalytic degradation is observed for the EDTA loading of 4.0 g. These data indicate that smaller amounts of BiVO₄ are coated on ACFs as EDTA decreases, because EDTA balances the growth rates of different crystalline facets.³⁹ In other words, high EDTA concentrations promote the formation of BiVO₄@ACFs samples with irregular rod-like morphologies that show enhanced photodegradation performance.⁴² However, when the EDTA loading is further increased to 5.0 g, the photocatalytic activity decreases, because of the negative effects of such high concentrations on the anisotropic growth of single-crystalline BiVO₄.⁴³

Fig. 6 RB5 degradation profiles determined for different amounts of added EDTA.

Fig. 7a shows the morphology variation of ACF-supported BiVO₄ particles with pH, revealing that higher pH values promotes the lamellar crystallization of BiVO₄, which decreases the number of BiVO₄ particles.⁴⁴ At pH 1, the protonation of EDTA and its precipitation decreases the chelation ability of the ligand, thus allowing the growth of large and highly crystalline BiVO₄ particles (Fig. 7a(S1)), which corresponds to low photocatalytic activity.⁴⁵ Samples prepared at pH 3 and 5 exhibit irregular rod-like morphologies (Fig. 7a(S2 and S3), respectively) and show good photocatalytic activities (Fig. 7b). At these pH values, the increased extent of branching decreases the BiVO₄ particle size, which is ascribed to the accelerated hydrolysis of bismuth nitrate and the extensive precipitation of the poorly water-soluble BiONO₃.⁴⁶ As the pH further increases to 7.0, large numbers of pellet-like particles are produced (Fig. 7a(S4)), attributed to the crystal growth rate exceeding the nucleation rate.⁴⁷ However, this observation is also explained by the reduced crystal surface area and the weakened chelation ability of EDTA.^48,49

Fig. 7 Morphologies (a) and photocatalytic performances (b) of BiVO₄@ACFs prepared at different pH.

Photocatalytic degradation performance of as-synthesized samples and a possible mechanistic explanation

Five initial RB5 concentrations (40, 50, 60, 80, and 100 mg L⁻¹) are used to determine the effect of the above parameter on the degradation rate (Fig. 8a). The photodegradation efficiency is increased with decreasing initial RB5 concentration, which is mainly attributed to the reduction of light penetration efficiency (i.e., the decreased amount of photons arriving at the catalyst surface) at high initial concentrations.⁵⁰ Moreover, as the initial RB5 concentration decreases, the slope of the ln(C₀/C) vs. t plot becomes steeper (Fig. 8b), which indicates the existence of a certain linear correlation between these parameters at different initial RB5 concentrations and implies that the photocatalytic degradation of RB5 is a first-order reaction.⁵¹

Fig. 8 (a) RB5 degradation profiles recorded at different initial RB5 concentrations and (b) first-order kinetics describing the adsorption of RB5 on BiVO₄@ACFs.

Sample durability was evaluated by using a given specimen for multiple degradation runs. The recycling test was performed three times and all samples were washed with deionized water and ethanol and subsequently dried at 75 °C for 2 h.

Fig. 9a shows that the photocatalytic activity of BiVO₄@ACFs remains stable in the first two cycles and exceeds that of pristine ACFs at all times. In the third cycle, 65% of RB5 is removed by 2 h photolysis, which confirms that the BiVO₄@ACFs sample is less affected than ACFs during oxidation. On the contrary, the RB5 removal efficiency of only 20% is obtained by ACFs in the third cycle. Therefore, it is concluded that dye molecules and intermediates compete with each other for the limited adsorption and catalytic sites on the catalyst surface, which decreases the photodegradation efficiency.⁵² In addition, the morphologies and phases of the used BiVO₄@ACFs were further investigated. As shown in Fig. 9b, the used BiVO₄@ACFs exhibits almost no difference in SEM images and XRD patterns compared with those obtained before photocatalytic reaction, and the used BiVO₄ remains firmly attached to the surface of ACFs, rather than easily exfoliated from the mechanical stirring of the RB5 solutions for long durations. Moreover, the spent catalysts are easily separated from the reaction system.²⁴

Fig. 9 (a) Performances of tested samples in three consecutive RB5 photodegradation runs; (b) morphologies and XRD patterns of BiVO₄@ACFs after RB5 degradation.

Fig. 10a compares the UV-Vis diffuse reflectance spectra of ACFs, BiVO₄@ACFs, and BiVO₄, revealing that all samples except for BiVO₄ exhibit strong absorption in the visible-light region (400–800 nm), as indirectly confirmed by the black hue of the ACFs.⁵³ These results clearly demonstrate that the intense absorption of visible light by BiVO₄@ACFs provide greater photocatalytic activity than shown by pristine ACFs and BiVO₄ individually.

Fig. 10 (a) UV-Vis diffuse reflectance spectra of ACFs, BiVO₄@ACFs, and BiVO₄; (b) time-dependent UV-Vis spectrum of RB5; (c) ATR-FTIR spectra of pristine and decomposed RB5; (d) RB5 degradation kinetics of different samples; (e) schematic representing the origin of the enhanced photocatalytic activity of BiVO₄@ACFs; (f) effects of scavengers on photodegradation of 40 mg L⁻¹ RB5.

The time-dependent UV-Vis spectrum of RB5 (40 mg L⁻¹) recorded under visible light irradiation in the presence of BiVO₄@ACFs (0.2 g) (Fig. 10b) reveals the presence of two main absorption peaks. The first peak is located in the visible region (598 nm) and reflects the presence of the azo (–N=N–) chromophore, whereas the second is located in the UV region (310 nm) and reflects the presence of naphthalene rings.⁵⁴ Both peaks lose intensity and eventually disappear with increased irradiation time, corresponding to the degradation of RB5.⁵⁵

Structural and elemental analyses of RB5 before and after irradiation were performed using ATR-FTIR spectroscopy. The spectrum of pristine RB5 (Fig. 10c) features a peak at 3436 cm⁻¹ assigned to O–H stretching of alcohols and N–H stretching of amines. Peaks at 2927 and 2854 cm⁻¹ are assigned to CH₃ asymmetric and symmetric vibrations and to CH₂ asymmetric vibrational stretching, respectively. The typical peak at 1492 cm⁻¹ is assigned to aromatic ring-bonded –N=N– moieties, and that at 1122 cm⁻¹ is attributed to sulfoxide (S=O) groups.⁵⁶ In contrast, the ATR-FTIR spectrum of RB5 after photocatalytic degradation (Fig. 10c) shows no peak at 1492 cm⁻¹, indicating the breakage of the azo bond, and the appearance of the peak at 1589 cm⁻¹ is thought to reflect the formation of quinine-like structures.⁵⁷

To determine the extent of C–BiVO₄ coupling in the synthesized heterostructures, we explored the photocatalytic activities of ACFs, BiVO₄ nanoparticles, and BiVO₄@ACFs for the visible light-driven degradation of RB5. Fig. 10d shows the results of kinetic data fitting obtained for the photo-degradation of RB5 solution (40 mg L⁻¹) under visible light illumination, revealing that the k value (0.01796 min⁻¹) of BiVO₄@ACFs is much higher than those of ACFs (0.0039 min⁻¹) and BiVO₄ (0.0008 min⁻¹).

The formation mechanism of BiVO₄@ACFs is represented in Fig. 10e. In the above composite, the highly conductive ACFs act as both charge carriers and electronic transmission media,⁵⁸ allowing fast electron transfer from ACFs to BiVO₄ and highly efficient dye degradation upon visible-light irradiation.⁵⁹ Moreover, water adsorbed on the sample surface reacts with the photogenerated holes (h_vb⁺) to afford ·OH_ads radicals as strong oxidizing agents.⁶⁰ Hence, the azo bond of RB5 is easily attacked and cleaved by the above radicals, causing dye degradation, as summarized in eqn (5)–(7).⁶¹

Photocatalyst + hν → h_vb⁺ + e_cb⁻, (5)

h_vb⁺ + OH⁻_ads → ·OH_ads, (6)

h_vb⁺/·OH_ads + dye → degraded products, (7)

I⁻ + h_vb⁺ →I· + e⁻, (8)

I⁻ + ·OH_ads → I· + OH⁻. (9)

The proposed mechanism was further verified by KI, which reacts with h_vb⁺ and ·OH_ads, as summarized in eqn (8) and (9). As shown in Fig. 10f, when 20 mM KI is used as a diagnostic tool, an adsorption peak appears at 227 nm, and the peak located in the visible region (310 nm) does not disappear. This result displays that the naphthalene rings are mainly cleaved by reaction with h_vb⁺ and ·OH_ads.

Conclusions

Herein, we established a facile procedure for immobilizing BiVO₄ on ACFs and investigated the photocatalytic activities of the obtained composites. To improve the above activities and meet the requirements of practical applications, the effects of parameters such as the concentrations of the precursor solutions of BiVO₄ and EDTA and the initial pH on sample structure and morphology were studied in detail. Based on the results of a series of measurements and performance tests, the above parameters were concluded to have important effects on the morphologies of the as-prepared samples.

Moreover, the photocatalytic decolorization of RB5 was shown to be a first-order reaction. Compared to BiVO₄ and ACFs, BiVO₄@ACFs exhibited improved visible-light adsorption properties as well as increased stability and charge-separation efficiency, thus featuring enhanced RB5 degradation activity. This activity increase was mainly attributed to the formation of a heterojunction electric field between BiVO₄ and ACFs, and a possible mechanism was proposed. Thus, we demonstrated that BiVO₄@ACFs is a promising carbon-based material for photocatalysis and environmental applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the Nature Science Foundation of the Jiangsu Higher Education Institutions of China (16KJB540002). It is also funded by Nantong applied research program (GY12016031). The authors are also thankful for the support of the Fundamental Research Funds for the Nantong Universities (No. 17ZH158).

Notes and references

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