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Article

Interfacial Charge Transfer in MoS2/TiO2 Heterostructured Photocatalysts: The Impact of Crystal Facets and Defects

by
Tingcha Wei
1,
Woon Ming Lau
2,*,
Xiaoqiang An
3,* and
Xuelian Yu
4
1
Beijing Computational Science Research Center, Beijing 100193, China
2
Center for Green Innovation, School of Mathematics and Physics, University of Science & Technology Beijing, Beijing 100083, China
3
Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
4
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Molecules 2019, 24(9), 1769; https://doi.org/10.3390/molecules24091769
Submission received: 25 March 2019 / Revised: 22 April 2019 / Accepted: 23 April 2019 / Published: 7 May 2019
(This article belongs to the Special Issue Photo- and Photoelectro-Chemical Processes for Energy Applications)
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Versions Notes

Abstract

:
One of the most challenging issues in photocatalytic hydrogen evolution is to efficiently separate photocharge carriers. Although MoS2 loading could effectively improve the photoactivity of TiO2, a fundamental understanding of the charge transfer process between TiO2 and MoS2 is still lacking. Herein, TiO2 photocatalysts with different exposed facets were used to construct MoS2/TiO2 heterostructures. XPS, ESR, together with PL measurements evidenced the Type II electron transfer from MoS2 to {001}-TiO2. Differently, electron-rich characteristic of {101}-faceted TiO2 were beneficial for the direct Z-scheme recombination of electrons in TiO2 with holes in MoS2. This synergetic effect between facet engineering and oxygen vacancies resulted in more than one order of magnitude enhanced hydrogen evolution rate. This finding revealed the elevating mechanism of constructing high-performance MoS2/TiO2 heterojunction based on facet and defect engineering.

Graphical Abstract

1. Introduction

Photocatalysis based on semiconductor nanomaterials has been extensively studied during the past decades, due to its great potential to solving the worldwide energy and environmental crisis [1,2]. Due to the abundance and high chemical stability, TiO2 is one of the most widely used semiconductors for photocatalysis [3,4,5]. However, its large-scale application is seriously limited by its poor light absorption and the fast recombination of electron–hole pairs. Tremendous strategies have been used to improve the photoreactivity of TiO2, such as morphology control, microstructure modulation, co-catalyst loading and metal/nonmetal doping [6,7,8,9]. Among these strategies, heterojunction design has proved to be the most promising way to separate photo-induced charge carriers [10,11].
MoS2, a typical layered transition-metal dichalcogenide, attracted a lot of recent attention because of its narrow energy gap and large surface area [12,13,14]. Although a fundamental understanding of the heterostructured interface between TiO2 and MoS2 is still lacking, as-constructed junctions phenomenologically exhibited remarkably enhanced activity for photocatalytic hydrogen evolution. For example, several reports demonstrated that photo-generated electrons efficiently transferred from the conduction band of TiO2 to MoS2 [15,16,17,18,19]. The role of MoS2 as electron reservoir renders it a potential candidate to replace the costly noble metal cocatalysts. In contrast, several recent studies have revealed the Type II band alignment between TiO2 and MoS2. This meant that electrons moved from the conduction band of MoS2 to that of TiO2 [20,21,22]. This controversy was due to the fact that many factors can affect the charge transfer behavior around heterostructured interfaces. Up to now, the lithium-free fabrication of trigonal phase MoS2 with metallic characteristic remains challenging. Ordinarily, the tunable bandgap of hexagonal phase MoS2 (1.3–1.9 eV) unavoidably complicated the interfacial mechanism [16,23]. Moreover, the charge carrier behavior in semiconductor photocatalyst is strongly dependent on the exposed facets and structural defects. For example, {001} and {101} facets of TiO2, two most common facets of anatase phase, usually exhibit different adsorption characteristics and redox abilities during the photocatalytic reaction [24,25,26,27]. Our research further revealed the significant contribution of facet-dependent formation of oxygen vacancy defects on the interfacial behavior of photo-induced charge carriers [28]. In this regard, it is a critical point to investigate the impact of crystal facets and defects on the charge separation in MoS2-grafted faceted TiO2.
In this paper, {001} and {101}-faceted TiO2 were used to construct MoS2/TiO2 heterostrutures. The influence of exposed facets and oxygen vacancy defects on the interfacial separation of charge carriers was fundamentally investigated. Experimental characterizations indicated that {001} facets of TiO2 are apt to accept electrons from MoS2, while {101} facets are favorable for the Z-scheme recombination of electrons in TiO2 with holes in MoS2. The facet-dependent charge transfer process and the defect-enhanced charge separation pave a novel way to design high-efficiency heterostructured photocatalysts for energy and environmental applications.

2. Results and Discussion

Facile hydrothermal reaction was used to fabricate {001}-faceted (T001) and {101}-faceted TiO2 (T101). Then, as-synthesized TiO2 was heat treated under hydrogen atmosphere to obtain defective {001}-TiO2 (Vo-T001) and {101}-TiO2 (Vo-T101). Different types of TiO2 was hybridized with commercial MoS2 by ultrasonication (Figure S1). MoS2-grafted {001}- and {101}-faceted TiO2 without and with oxygen vacancies were denoted as T001/MoS2, Vo-T001/MoS2, T101/MoS2 and Vo-T101/MoS2, respectively.
The phase structure of as-prepared samples was examined by X-ray diffraction (XRD). As shown in Figure 1, all diffraction peaks can be indexed to anatase TiO2 (JCPDS card No. 01-078-2486). The diffraction peaks at 25.31° and 37.79° are ascribed to (101) and (004) planes, respectively [26,29]. Thermal reduction exhibits neglectable influence on the crystal phase and crystallinity of TiO2. No characteristic peaks of MoS2 are discernible in the composite photocatalysts due to the relatively lower amount of MoS2 and the high dispersity.
Figure 2 shows the morphology of Vo-T001/MoS2 and Vo-T101/MoS2. Based on field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) observations, Vo-T001/MoS2 sample is composed of uniform square-shaped nanosheets with an average side size of 100 nm and thickness of ~10 nm (Figure 2a,b). Vo-T101/MoS2 sample is observed for rhombic morphology with an average apex-to-apex diameter of ~15 nm (Figure 2c,d). Figure 2e presents the high-resolution TEM image of Vo-T001/MoS2. The lattice spacing of 0.24 nm corresponds to the (004) plane of TiO2 [30]. Besides, the surface of square-shaped TiO2 is intimately coupled with layered nanosheets MoS2. The interlayer distance of 0.62 nm corresponds to the (002) planes of hexagonal MoS2. The formation of MoS2/{101}-faceted TiO2 can be further confirmed by Figure 2f.
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of component elements. Figure 3a exhibits Ti 2p XPS spectra of {001}-faceted TiO2, MoS2/TiO2 heterostructures with and without oxygen vacancies. The Ti 2p peaks shift to lower binding energies after the deposition of MoS2 onto TiO2 nanosheets. It means that Ti atoms accept electrons from MoS2, resulting in more Ti3+ in the heterostructures. The creation of oxygen vacancies in TiO2 further accelerates this process, as the shift value increased to 0.5 eV for Vo-T001/MoS2. The coupling of MoS2 with {101}-faced TiO2 results in totally different change of spectra. The shift of Ti 2p peaks to higher binding energies indicates that Ti atom was electron donor. The facet-dependent interfacial electronic structure implies the different charge transfer behavior in T001/MoS2 and T101/MoS2 heterostrutures.
The electronic structure of different samples was further studied by electron spin resonance (ESR). In Figure 3c, pristine {001}-faceted TiO2 presents a weak signal around g value of 1.98–1.99, which can be ascribed to electrons trapped by Ti3+ [30,31]. Differently, {101}-faceted TiO2 possesses a much stronger signal at 2.002 (Figure 3d). It indicates that the exposure of {101} facets is more favorable for the formation of surface oxygen vacancies [32,33]. The deposition of MoS2 onto T001 and T101 results in the significantly increased ESR signals. It means that there are strong interfacial interactions between MoS2 and TiO2. The retention of this signal in Vo-T001/MoS2 and Vo-T101/MoS2 proves the formation of defective heterostructured photocatalysts.
To evaluate the influence of MoS2 loading and defect formation on the light absorption ability of photocatalysts, UV-vis diffuse reflectance spectra were collected. As shown in Figure S3, {001}- and {101}-faceted TiO2 only present strong absorption in the UV light region. The formation of structural defects and subsequent loading of MoS2 result in the obvious visible light absorption. On the basis of Kubelka-Munk function, shown in Figure 4a,b, the corresponding band gaps of T001 and T101 are determined to be 3.06 and 2.91 eV, respectively.
In comparison, the band gap of Vo-T001/MoS2 and Vo-T101/MoS2 heterostructures are determined to be 2.87 and 2.74 eV, respectively. The influence of junction formation on the energy levels was investigated by valance band XPS. As shown in Figure 4c, all T001-based samples exhibit similar valence band positions, i.e. the negligible change of electronic structure. In contrast, the coupling of T101 with MoS2 leads to the 0.31 eV down-shift of valence band, while the shift value decrease to 0.11 eV after the introduction of oxygen vacancies in the heterostructures (Figure 4d). All these results demonstrate the facet-dependent electronic structure in MoS2/TiO2 heterostructures, which should exhibit impact on the behavior of interfacial charge separation.
Photoactivity of different photocatalysts was thereafter evaluated by hydrogen evolution reactions. As show in Figure 5a, {001}-faceted TiO2 exhibits moderate activity, with a hydrogen evolution rate of 20 μmol h−1. After the deposition of MoS2, four-fold increased photocatalytic performance is achieved. An additional 30% enhancement is achieved after the creation of oxygen vacancies in {001}-TiO2. In comparison, poor photoactivity is achieved for {101}-faced TiO2. The formation of T101/MoS2 improves the hydrogen evolution rate from 3 μmol h−1 to 22 μmol h−1. The formation of defective structure presents significant impact on the photocatalytic performance. Vo-T101/MoS2 possesses the highest hydrogen production rate of 61 μmol h−1, which is about 20 and 3 times higher than pristine TiO2 and MoS2/TiO2 heterojunction. The above results indicate that the construction of MoS2/TiO2 heterostructures can efficiently improve the photocatalytic performance of TiO2. Moreover, defect modulation is a more effective strategy to improve the photoactivity of {101}-faceted TiO2.
In order to study the synergetic effect between oxygen vacancy defects and MoS2, the charge carrier behavior was studied by photoluminescence (PL) measurements. Obviously, the deposition of MoS2 and oxygen vacancy formation results in the different change of fluorescence emission of {001}- and {101}-faceted TiO2.
In Figure 6a, the transition emission of T001/MoS2 is much higher than pristine TiO2, implying the possible electron transfer from MoS2 to TiO2. Differently, the formation of heterostructure and defective interface leads to the obvious PL quenching of {101}-faceted TiO2 (Figure 6b) [21,34,35]. The improved separation of charge carriers can be further evidenced by the reduced radius of semi-circle in the Nyquist plots in Figure S2.
Based on the above results, the charge transfer mechanism in MoS2-grafted faceted TiO2 is illustrated in Figure 7a,b. Firstly, according to the previous reports, the purchased commercial MoS2 was readily 2H-MoS2 with a hexagonal structure [36,37]. Theoretically, this type MoS2 in the faceted heterostructures should not be simply considered as co-catalysts for hydrogen generation. The different coordination environment of component atoms in {001}- and {101}-faceted TiO2 further complicate this case. It should be noted that the amount of 5-fold-coodinated Ti on {001} facets (100%) is much larger than {101} facets (50%) of TiO2 [38]. With the creation of oxygen vacancies, undercoodinated Ti4c on the surface of {001} facets can expectedly provide sufficient reactive sites for photocatalysis. However, defect modulation seemed to be more effective to improve the performance of T101/MoS2 heterostructures. It indicated that this improvement was resulted from the interfacial process of charge carriers, rather than intrinsic electronic structures. Based on the UV and valence-bond XPS measurements, both the conduction band and valence band of {101}-TiO2 are more positive than {001}-TiO2. Therefore, more electrons should migrate to the {101} facets of TiO2, while holes accumulate on the {001} facets. It means that {001}-TiO2 are prone to accept electrons from MoS2, i.e., the formation of Type II junction between MoS2 and {001}-TiO2. As shown in Figure 7a, this charge transfer process is further facilitated by the construction of defective structure. However, the deposition of MoS2 onto the electron-rich {101}-TiO2 results in remarkable down-shift of band gap energy levels. Its much lower conduction band is favorable for the direct recombination of these electrons with holes in the valence band of MoS2, analogous to the Z-scheme mechanism [39]. When oxygen vacancies are introduced into this system, the mid-gap defect states can act as electron mediator to facilitate this charge transfer. The synergetic effect between oxygen vacancies and crystal facets leads to the more than an order of magnitude increase in hydrogen production activity.

3. Materials and Methods

3.1. Fabrication of {001}-Faceted TiO2

To fabricate {001}-faceted TiO2 (T001), 1.5 mL of hydrofluoric acid was added into 12.5 mL of titanium (IV) butoxide under vigorous stirring. After 30 min, the solution was transferred into a Teflon autoclave with a capacity of 50 mL. A hydrothermal reaction was carried out at 200 °C for 24 h. When the autoclave was cooled to room temperature, the precipitants were separated by high-speed centrifugation. To remove the residual fluorine ions, the powders were soaked in 0.1 M NaOH solution for 12 h. After the fully rinsing and drying, white-colored TiO2 powders were obtained, which were annealed at 400 °C for 2 h in a muffle furnace.

3.2. Fabrication of {101}-Faceted TiO2

A two-step hydrothermal method was used to fabricate TiO2 with exposed {101} facets (T101) [40]. In a typical procedure, 1 g of P25 TiO2 nanoparticles was hydrothermally treated with 50 mL 17 M of KOH solution in a Teflon autoclave at 110 °C for 24 h. The resulting precipitates were washed and neutralized using DI water and acetic acid aqueous solution, respectively. Then the powders dried in oven for 12 h. The dried titanate powders 750 mg were further dispersed into 15 mL of ultrapure water under strong stirring. Another hydrothermal reaction was carried out at 170 °C for 24 h. Finally, the white TiO2 nanoparticles were centrifuged and dried overnight. Then the powders were annealing at 400 °C for 2 h in a muffle furnace.

3.3. Synthesis of Nonstoichiometric TiO2 Nanostructures

A thermal reduction strategy was used to fabricate defective TiO2. Typically, as-synthesized TiO2 were placed in the middle of a horizontal tube furnace. The powders were heat treated at 400 °C for 2 h in a H2/Ar flow (20 mL/min).

3.4. Fabrication of MoS2/TiO2 Heterostructures

The fabrication procedures of MoS2/TiO2 heterostructure were illustrated in Scheme 1. Typically, commercial MoS2 (Nanjing XFNANO Materials Tech Co., Ltd, Nanjing, China) were added into deionized water and sonicated for 8 h using ultrasonic cell disruptor (Biosafer 900-92, Nanjing Safer Biotech Co., Ltd., Nanjing, China). Then, 100 mg TiO2 and a certain amount of MoS2 dispersion were mixed in 50 mL of deionized water. After one hour’s ultrasonication and two hours’ stirring, the precipitation was filtrated, washed and dried to achieve MoS2/TiO2 composites. The weight ratio of MoS2 in the composites was kept at 1%. All samples were prepared with the same method, using TiO2 with different exposed facets and electronic structures. MoS2-grafted {001}- and {101}-faceted TiO2 without and with oxygen vacancies were denoted as T001/MoS2, Vo-T001/MoS2, T101/MoS2 and Vo-T101/MoS2, respectively.

3.5. Characterizations

X-ray diffraction (XRD) was carried out on X’Pert Pro MPD (PANalytical B.V., Almelo, the Netherlands). X-ray photoelectron spectroscopy (XPS) and valence band was performed on an ESCALAB MKII spectrometer (VG Scientific Ltd., London, UK) with an Mg Kα excitation source. The morphology of catalysts were observed by field emission scanning electron microscope (FE-SEM, SIGMA, Jena, Germany) and high-resolution transmission electron microscope (HR-TEM, JEOL-2100F, JEOL, Tokyo, Japan). UV-vis diffusion reflectance spectra (DRS) were acquired on an UV−vis−NIR spectrophotometer (Cary 5000, Varian, Palo Alto, CA, USA). The steady-state fluorescence and the time-resolved fluorescence spectra were measured on a fluorescence spectrometer (FLS-980, Edinburgh Instruments Ltd., Edinburgh, UK). Electron spin resonance (ESR) analysis was carried out using a Bruker E500 spectrometer (Bruker BioSpin Gmbh, Karlsruhe, Germany).

3.6. Photocatalytic Experiments

Photocatalytic hydrogen evolution reaction was carried out in a closed gas-circulation system. Typically, 15 mg of different photocatalysts were dispersed into 100 mL aqueous solution containing 10 mL of methanol as hole scavenger. A 300 W Xe lamp (CEL-HXF300, Ceaulight, Beijing, China) was used as the light source. The evoluted H2 was analyzed by an online gas chromatograph equipped with a column of 5 Å molecular sieves (GC-7806, Shiweipuxin, Beijing, China).

3.7. Electrochemical Measurements

The photoelectrochemical properties were investigated in a three-electrode cell using an electrochemical workstation (CHI800D, CH Instruments, Shanghai, China). The catalyst loaded FTO glass, Pt wire and Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. 0.5 M Na2SO4 solution was used as electrolyte. The electrical impedance spectroscopy (EIS) was measured at an applied potential of 0 V vs. Ag/AgCl. The Mott-Schottky curves were collected at the frequency of 1000 Hz.

4. Conclusions

In summary, we have revealed that the mechanism of interfacial charge transfer in MoS2/TiO2 heterostructures was highly dependent with the exposed facets of TiO2. Due to the electron-rich characteristic and the subsequent band alignment, {101} facets of TiO2 were more favorable for the direct Z-scheme charge transfer. The introduction of oxygen vacancies into Type II heterojunction resulted in the 5-fold increased hydrogen evolution rate over MoS2/{001}-TiO2, while more than one order of magnitude increased activity was achieved for Z-scheme MoS2/{101}-TiO2. Our strategy presents a paradigm for the rational design of heterostructured photocatalysts with controlled charge transfer pathways toward photocatalytic solar energy conversion.

Supplementary Materials

The following are available online: Figure S1: TEM image of commercial MoS2 nanosheets, Figure S2: UV-vis diffuse reflectance spectra of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect, Figure S3: EIS plots of {001}-faceted TiO2 before and after the deposition of MoS2 with and without defect.

Author Contributions

Conceptualization, W.M.L. and X.A.; Methodology, X.Y.; Formal analysis, T.W.; Investigation, W.M.L. and X.A.; Resources, X.Y.; Data curation, T.W.; Writing—original draft preparation, T.W.; Writing—review and editing, X.A.; Supervision, X.A.; Project administration, X.A.; Funding acquisition, X.A.

Funding

This work was supported by the National Natural Science Foundation of China (51578531, 51538013).

Conflicts of Interest

The authors declare no conflict of interest.

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Sample Availability: Samples of the TiO2 catalysts are available from the authors.
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Figure 1. XRD patterns of perfect and defective faceted TiO2 before and after the deposition of MoS2.
Figure 1. XRD patterns of perfect and defective faceted TiO2 before and after the deposition of MoS2.
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Figure 2. SEM image (a) and TEM image (b) of Vo-T001/MoS2. SEM image (c) and TEM image (d) of Vo-T101/MoS2. HR-TEM images of Vo-T001/MoS2 (e) and Vo-T101/MoS2 (f).
Figure 2. SEM image (a) and TEM image (b) of Vo-T001/MoS2. SEM image (c) and TEM image (d) of Vo-T101/MoS2. HR-TEM images of Vo-T001/MoS2 (e) and Vo-T101/MoS2 (f).
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Figure 3. Ti 2p XPS spectra of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect. ESR pattern of 001-faceted TiO2 (c) and 101-faceted TiO2 (d) before and after the deposition of MoS2 with and without defect.
Figure 3. Ti 2p XPS spectra of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect. ESR pattern of 001-faceted TiO2 (c) and 101-faceted TiO2 (d) before and after the deposition of MoS2 with and without defect.
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Figure 4. Tauc plots of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect. VB XPS of {001}-faceted TiO2 (c) and {101}-faceted TiO2 (d) before and after the deposition of MoS2 with and without defects.
Figure 4. Tauc plots of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect. VB XPS of {001}-faceted TiO2 (c) and {101}-faceted TiO2 (d) before and after the deposition of MoS2 with and without defects.
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Figure 5. Photocatalytic activity of H2 evolution under UV light irradiation of (a) {001}-faceted TiO2 before and after the deposition of MoS2 with and without defect and (b) {101}-faceted TiO2 before and after the deposition of MoS2 with and without defect.
Figure 5. Photocatalytic activity of H2 evolution under UV light irradiation of (a) {001}-faceted TiO2 before and after the deposition of MoS2 with and without defect and (b) {101}-faceted TiO2 before and after the deposition of MoS2 with and without defect.
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Figure 6. PL emission spectra of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect.
Figure 6. PL emission spectra of {001}-faceted TiO2 (a) and {101}-faceted TiO2 (b) before and after the deposition of MoS2 with and without defect.
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Figure 7. Scheme representation of charge separation in heterojunction for (a) Vo-T001/MoS2 and (b) Vo-T101/MoS2.
Figure 7. Scheme representation of charge separation in heterojunction for (a) Vo-T001/MoS2 and (b) Vo-T101/MoS2.
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Scheme 1. Fabrication of MoS2/TiO2 heterostructures.
Scheme 1. Fabrication of MoS2/TiO2 heterostructures.
Molecules 24 01769 sch001

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MDPI and ACS Style

Wei, T.; Lau, W.M.; An, X.; Yu, X. Interfacial Charge Transfer in MoS2/TiO2 Heterostructured Photocatalysts: The Impact of Crystal Facets and Defects. Molecules 2019, 24, 1769. https://doi.org/10.3390/molecules24091769

AMA Style

Wei T, Lau WM, An X, Yu X. Interfacial Charge Transfer in MoS2/TiO2 Heterostructured Photocatalysts: The Impact of Crystal Facets and Defects. Molecules. 2019; 24(9):1769. https://doi.org/10.3390/molecules24091769

Chicago/Turabian Style

Wei, Tingcha, Woon Ming Lau, Xiaoqiang An, and Xuelian Yu. 2019. "Interfacial Charge Transfer in MoS2/TiO2 Heterostructured Photocatalysts: The Impact of Crystal Facets and Defects" Molecules 24, no. 9: 1769. https://doi.org/10.3390/molecules24091769

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