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

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.


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, TiO 2 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 TiO 2 , 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].
MoS 2 , 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 TiO 2 and MoS 2 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 TiO 2 to MoS 2 [15][16][17][18][19].The role of MoS 2 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 TiO 2 and MoS 2 .This meant that electrons moved from the conduction band of MoS 2 to that of TiO 2 [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 MoS 2 with metallic characteristic remains challenging.Ordinarily, the tunable bandgap of hexagonal phase MoS 2 (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 TiO 2 , 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 MoS 2 -grafted faceted TiO 2 .
In this paper, {001} and {101}-faceted TiO 2 were used to construct MoS 2 /TiO 2 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 TiO 2 are apt to accept electrons from MoS 2 , while {101} facets are favorable for the Z-scheme recombination of electrons in TiO 2 with holes in MoS 2 .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.
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 TiO 2 (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 TiO 2 .No characteristic peaks of MoS 2 are discernible in the composite photocatalysts due to the relatively lower amount of MoS 2 and the high dispersity.
Molecules 2019, 24, x FOR PEER REVIEW 2 of 11 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 MoS2grafted 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 defectenhanced charge separation pave a novel way to design high-efficiency heterostructured photocatalysts for energy and environmental applications.
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/MoS 2 and V o -T101/MoS 2 .Based on field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) observations, Vo-T001/MoS 2 sample is composed of uniform square-shaped nanosheets with an average side size of 100 nm and thickness of ~10 nm (Figure 2a,b).V o -T101/MoS 2 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 V o -T001/MoS 2 .The lattice spacing of 0.24 nm corresponds to the (004) plane of TiO 2 [30].Besides, the surface of square-shaped TiO 2 is intimately coupled with layered nanosheets MoS 2 .The interlayer distance of 0.62 nm corresponds to the (002) planes of hexagonal MoS 2 .The formation of MoS 2 /{101}-faceted TiO 2 can be further confirmed by Figure 2f.
Molecules 2019, 24, x FOR PEER REVIEW 3 of 11 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.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 Ti 3+ [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 electronic structure of different samples was further studied by electron spin resonance (ESR).In Figure 3c, pristine {001}-faceted TiO 2 presents a weak signal around g value of 1.98-1.99,which can be ascribed to electrons trapped by Ti 3+ [30,31].Differently, {101}-faceted TiO 2 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] To evaluate the influence of MoS 2 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 TiO 2 only present strong absorption in the UV light region.The formation of structural defects and subsequent loading of MoS 2 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 V o -T001/MoS 2 and V o -T101/MoS 2 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 MoS 2 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 MoS 2 /TiO 2 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 TiO 2 exhibits moderate activity, with a hydrogen evolution rate of 20 µmol h −1 .After the deposition of MoS 2 , four-fold increased photocatalytic performance is achieved.An additional 30% enhancement is achieved after the creation of oxygen vacancies in {001}-TiO 2 .In comparison, poor photoactivity is achieved for {101}-faced TiO 2 .The formation of T101/MoS 2 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/MoS 2 possesses the highest hydrogen production rate of 61 µmol h −1 , which is about 20 and 3 times higher than pristine TiO 2 and MoS 2 /TiO 2 heterojunction.The above results indicate that the construction of MoS 2 /TiO 2 heterostructures can efficiently improve the photocatalytic performance of TiO 2 .Moreover, defect modulation is a more effective strategy to improve the photoactivity of {101}-faceted TiO 2 .
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.In order to study the synergetic effect between oxygen vacancy defects and MoS 2 , the charge carrier behavior was studied by photoluminescence (PL) measurements.Obviously, the deposition of MoS 2 and oxygen vacancy formation results in the different change of fluorescence emission of {001}and {101}-faceted TiO 2 .
In Figure 6a, the transition emission of T001/MoS 2 is much higher than pristine TiO 2 , implying the possible electron transfer from MoS 2 to TiO 2 .Differently, the formation of heterostructure and defective interface leads to the obvious PL quenching of {101}-faceted TiO 2 (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.Based on the above results, the charge transfer mechanism in MoS 2 -grafted faceted TiO 2 is illustrated in Figure 7a,b.Firstly, according to the previous reports, the purchased commercial MoS 2 was readily 2H-MoS 2 with a hexagonal structure [36,37].Theoretically, this type MoS 2 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 TiO 2 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 TiO 2 [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/MoS 2 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}-TiO 2 are more positive than {001}-TiO 2 .Therefore, more electrons should migrate to the {101} facets of TiO 2 , while holes accumulate on the {001} facets.It means that {001}-TiO 2 are prone to accept electrons from MoS 2 , i.e., the formation of Type II junction between MoS 2 and {001}-TiO 2 .As shown in Figure 7a, this charge transfer process is further facilitated by the construction of defective structure.However, the deposition of MoS 2 onto the electron-rich {101}-TiO 2 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 MoS 2 , 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.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.

Fabrication of {001}-Faceted TiO 2
To fabricate {001}-faceted TiO 2 (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 TiO 2 powders were obtained, which were annealed at 400 • C for 2 h in a muffle furnace.

Fabrication of {101}-Faceted TiO 2
A two-step hydrothermal method was used to fabricate TiO 2 with exposed {101} facets (T101) [40].In a typical procedure, 1 g of P25 TiO 2 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 TiO 2 nanoparticles were centrifuged and dried overnight.Then the powders were annealing at 400 • C for 2 h in a muffle furnace.

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

Fabrication of MoS 2 /TiO 2 Heterostructures
The fabrication procedures of MoS 2 /TiO 2 heterostructure were illustrated in Scheme 1.Typically, commercial MoS 2 (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 TiO 2 and a certain amount of MoS 2 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 MoS 2 /TiO 2 composites.The weight ratio of MoS 2 in the composites was kept at 1%.All samples were prepared with the same method, using TiO 2 with different exposed facets and electronic structures.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 highspeed 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.

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.

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).

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 H 2 was analyzed by an online gas chromatograph equipped with a column of 5 Å molecular sieves (GC-7806, Shiweipuxin, Beijing, China).

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 Na 2 SO 4 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.

Conclusions
In summary, we have revealed that the mechanism of interfacial charge transfer in MoS 2 /TiO 2 heterostructures was highly dependent with the exposed facets of TiO 2 .Due to the electron-rich characteristic and the subsequent band alignment, {101} facets of TiO 2 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 MoS 2 /{001}-TiO 2 , while more than one order of magnitude increased activity was achieved for Z-scheme MoS 2 /{101}-TiO 2 .Our strategy presents a paradigm for the rational design of heterostructured photocatalysts with controlled charge transfer pathways toward photocatalytic solar energy conversion.

Conflicts of Interest:
The authors declare no conflict of interest.

Figure 1 .
Figure 1.XRD patterns of perfect and defective faceted TiO2 before and after the deposition of MoS2.

Figure 1 .
Figure 1.XRD patterns of perfect and defective faceted TiO 2 before and after the deposition of MoS 2 .

Figure 2 .
Figure 2. SEM image (a) and TEM image (b) of V o -T001/MoS 2 .SEM image (c) and TEM image (d) of V o -T101/MoS 2 .HR-TEM images of V o -T001/MoS 2 (e) and V o -T101/MoS 2 (f).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 TiO 2 , MoS 2 /TiO 2 heterostructures with and without oxygen vacancies.The Ti 2p peaks shift to lower binding energies after the deposition of MoS 2 onto TiO 2 nanosheets.It means that Ti atoms accept electrons from MoS 2 , resulting in more Ti 3+ in the heterostructures.The creation of oxygen vacancies in TiO 2 further accelerates this process, as the shift value increased to 0.5 eV for V o -T001/MoS 2 .The coupling of MoS 2 with {101}-faced TiO 2 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/MoS 2 and T101/MoS 2 heterostrutures.

Figure 3 .
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.
. 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 FigureS3, {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 Figure4a,b, the corresponding band gaps of T001 and T101 are determined to be 3.06 and 2.91 eV, respectively.

Figure 3 .
Figure 3. Ti 2p XPS spectra of {001}-faceted TiO 2 (a) and {101}-faceted TiO 2 (b) before and after the deposition of MoS 2 with and without defect.ESR pattern of 001-faceted TiO 2 (c) and 101-faceted TiO 2 (d) before and after the deposition of MoS 2 with and without defect.
. The deposition of MoS 2 onto T001 and T101 results in the significantly increased ESR signals.It means that there are strong interfacial interactions between MoS 2 and TiO 2 .The retention of this signal in V o -T001/MoS 2 and V o -T101/MoS 2 proves the formation of defective heterostructured photocatalysts.

Figure 5 .
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 .
Figure 5. Photocatalytic activity of H 2 evolution under UV light irradiation of (a) {001}-faceted TiO 2 before and after the deposition of MoS 2 with and without defect and (b) {101}-faceted TiO 2 before and after the deposition of MoS 2 with and without defect.