One-Step Synthesis of MoS 2 / TiSi 2 via an In Situ Photo-Assisted Reduction Method for Enhanced Photocatalytic H 2 Evolution under Simulated Sunlight Illumination

A new MoS2/TiSi2 complex catalyst was designed and synthesized by a simple one-step in situ photo-assisted reduction procedure. The structural and morphological properties of the composites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and ultraviolet-visible diffused reflectance spectroscopy (UV-vis DRS), which proved the formation of MoS2/TiSi2. MoS2/TiSi2 with optimized composition showed obviously enhanced photocatalytic activity and superior durability for water reduction to produce H2. The H2 generation rate over the MoS2/TiSi2 photocatalyst containing 3 wt % MoS2 reached 214.1 μmol·h−1·g−1 under visible light irradiation, which was ca. 5.6 times that of the pristine TiSi2. The improved photocatalytic activity of MoS2/TiSi2 could be related to the broad response spectrum, large visible light absorption, and synergies among MoS2 and TiSi2 that enhance photoexcited charge transfer and separation.


Introduction
In recent decades, semiconductor photocatalysis has received close attention owing to its potential application in the production of renewable hydrogen [1,2].Since the discovery of photoelectrocatalytic H 2 production in the TiO 2 electrode by Fujishima and Honda in 1972 [3], the semiconductor photocatalyst (Eg ≈ 3.2 eV) has been investigated extensively [4,5].However, due to its inherent shortcomings, such as its broad bandgap and low quantum efficiency [6], researchers have been making great efforts to modify TiO 2 or seek novel semiconductor photocatalysts to improve the photocatalytic activity and efficiency [7].
In recent years, the indirect-gap semiconductor MoS 2 (Eg ≈ 1.8 eV) has found many potential uses in the fields of microelectronics, lithium batteries, H 2 storage, and catalysis for hydrodesulphurization.More importantly, composites formed by modifying the primary catalysts with MoS 2 , such as MoS 2 /CdS [14], MoS 2 /grapheme [15], and MoS 2 /zinc cadmium sulfide [16], demonstrated improved activity for photocatalytic water splitting to produce hydrogen.
The conduction band (CB) of TiSi 2 (−0.43 eV) [10] is higher than that of MoS 2 (−0.1 eV) [17], and the valence band (VB) of TiSi 2 (1.07 eV) [10] is lower than that of MoS 2 (1.7 eV) [17].When TiSi 2 and MoS 2 are combined to form a composite photocatalyst, the electrons on the conduction band of TiSi 2 can be transferred to the conduction band of MoS 2 , and the holes on the valence band of MoS 2 can be transferred to the valence band of TiSi 2 .The combination of MoS 2 used as co-catalysts and TiSi 2 used as main catalysts is probably beneficial to the catalytic performance of hydrogen production.According to our own survey of the literature, MoS 2 -functionalized TiSi 2 materials have not been reported.Therefore we cover a novel hybrid consisted of MoS 2 and TiSi 2 by one-step in situ photo-reduction and its photocatalytic application for the photocatalytic H 2 evolution under visible light.
In recent years, the indirect-gap semiconductor MoS2 (Eg ≈ 1.8 eV) has found many potential uses in the fields of microelectronics, lithium batteries, H2 storage, and catalysis for hydrodesulphurization.More importantly, composites formed by modifying the primary catalysts with MoS2, such as MoS2/CdS [14], MoS2/grapheme [15], and MoS2/zinc cadmium sulfide [16], demonstrated improved activity for photocatalytic water splitting to produce hydrogen.
The conduction band (CB) of TiSi2 (−0.43 eV) [10] is higher than that of MoS2 (−0.1 eV) [17], and the valence band (VB) of TiSi2 (1.07 eV) [10] is lower than that of MoS2 (1.7 eV) [17].When TiSi2 and MoS2 are combined to form a composite photocatalyst, the electrons on the conduction band of TiSi2 can be transferred to the conduction band of MoS2, and the holes on the valence band of MoS2 can be transferred to the valence band of TiSi2.The combination of MoS2 used as co-catalysts and TiSi2 used as main catalysts is probably beneficial to the catalytic performance of hydrogen production.According to our own survey of the literature, MoS2-functionalized TiSi2 materials have not been reported.Therefore we cover a novel hybrid consisted of MoS2 and TiSi2 by one-step in situ photoreduction and its photocatalytic application for the photocatalytic H2 evolution under visible light.

Morphology and Structure
The XRD patterns of MoS2, TiSi2, and MoS2-modified TiSi2 composite (MoS2/TiSi2-3) are shown in Figure 1.The XRD pattern of the measured MoS2 sample demonstrates peaks located at 13.8°, 32.0°, 39.0°, 57.7°, and 59.6°, which can be indexed to the (002), (100), ( 103), (110), and (003) hexagonal crystallographic faces of molybdenum disulfide (JCPDS No. 37-1492), respectively.The XRD pattern of TiSi2 displays distinct diffraction peaks at 23.3°, 38.6°, 41.7°, 42.6°, and 49.3°, corresponding to the (111), (311), ( 040), (022), and (331) planes of face-centered orthorhombic structure of TiSi2 (JCPDS No. 35-0785).From the XRD pattern of MoS2/TiSi2, the characteristic peaks of TiSi2 and MoS2 with similar position can be detected after the reaction process, indicating that MoS2 has been synthesized by photo-reduction.The morphology of MoS2/TiSi2 was determined by SEM and TEM and demonstrated in Figure 2 and Figure S1.TiSi2 consists of grains of irregular shape with flat surfaces and sharp edges.The size of the grains is in the range of 1-10 μm.MoS2 particles of about 400 nm are well dispersed at the surface of TiSi2.The EDS results of the MoS2/TiSi2 sample (Figure S2) reveal that the composite consists of Mo, S, Ti, and Si, further proving that MoS2 has been successfully deposited at TiSi2.The morphology of MoS 2 /TiSi 2 was determined by SEM and TEM and demonstrated in Figure 2 and Figure S1.TiSi 2 consists of grains of irregular shape with flat surfaces and sharp edges.The size of the grains is in the range of 1-10 µm.MoS 2 particles of about 400 nm are well dispersed at the surface of TiSi 2 .The EDS results of the MoS 2 /TiSi 2 sample (Figure S2) reveal that the composite consists of Mo, S, Ti, and Si, further proving that MoS 2 has been successfully deposited at TiSi 2 .XPS was carried out and the corresponding results are shown in Figure 3 and Figure S3. Figure 3A shows the high-resolution Ti 2p XPS spectra of the samples.The peaks located at 453.5, 458.9, and 464.8 eV are attributed to Ti 0 p3/2, Ti 4+ p3/2, and Ti 4+ p1/2 [10,18], respectively.The peaks located at 98.6 and 102.4 eV in the pattern of TiSi2 (Figure 3B) were attributed to Si 0 p3/2 and Si 4+ p3/2 [18,19].Comparing the above peak positions with those of MoS2/TiSi2, the binding energy of Ti 4+ and Si 4+ in MoS2/TiSi2-3 is higher than in TiSi2, while the binding energy of Ti 0 and Si 0 in MoS2/TiSi2-3 is lower than in TiSi2.The peak located at 163.2 eV confirms the presence of S 2− in MoS2 (Figure 3C).The blue shift of the S 2p peak position of MoS2/TiSi2-3 can be clearly observed.The changes in binding energy of S were possibly due to the S-Ti and S-Si bond formation.The shift of the relative peak position is attributed to the interaction of TiSi2 with MoS2 [1,18,20].Figure 3D demonstrates the XPS spectra of Mo 3d for MoS2 and MoS2/TiSi2-3.The Mo 3d peak positions of MoS2/TiSi2-3 also shift to higher binding energies, indicating a low load of MoS2 and the remarkable combination effect of MoS2 and TiSi2 [14,21].

MoS2
Binding Energy(eV)  XPS was carried out and the corresponding results are shown in Figure 3 and Figure S3. Figure 3A shows the high-resolution Ti 2p XPS spectra of the samples.The peaks located at 453.5, 458.9, and 464.8 eV are attributed to Ti 0 p 3/2 , Ti 4+ p 3/2 , and Ti 4+ p 1/2 [10,18], respectively.The peaks located at 98.6 and 102.4 eV in the pattern of TiSi 2 (Figure 3B) were attributed to Si 0 p 3/2 and Si 4+ p 3/2 [18,19].Comparing the above peak positions with those of MoS 2 /TiSi 2 , the binding energy of Ti 4+ and Si 4+ in MoS 2 /TiSi 2 -3 is higher than in TiSi 2 , while the binding energy of Ti 0 and Si 0 in MoS 2 /TiSi 2 -3 is lower than in TiSi 2 .The peak located at 163.2 eV confirms the presence of S 2− in MoS 2 (Figure 3C).The blue shift of the S 2p peak position of MoS 2 /TiSi 2 -3 can be clearly observed.The changes in binding energy of S were possibly due to the S-Ti and S-Si bond formation.The shift of the relative peak position is attributed to the interaction of TiSi 2 with MoS 2 [1,18,20].Figure 3D demonstrates the XPS spectra of Mo 3d for MoS 2 and MoS 2 /TiSi 2 -3.The Mo 3d peak positions of MoS 2 /TiSi 2 -3 also shift to higher binding energies, indicating a low load of MoS 2 and the remarkable combination effect of MoS 2 and TiSi 2 [14,21].XPS was carried out and the corresponding results are shown in Figure 3 and Figure S3. Figure 3A shows the high-resolution Ti 2p XPS spectra of the samples.The peaks located at 453.5, 458.9, and 464.8 eV are attributed to Ti 0 p3/2, Ti 4+ p3/2, and Ti 4+ p1/2 [10,18], respectively.The peaks located at 98.6 and 102.4 eV in the pattern of TiSi2 (Figure 3B) were attributed to Si 0 p3/2 and Si 4+ p3/2 [18,19].Comparing the above peak positions with those of MoS2/TiSi2, the binding energy of Ti 4+ and Si 4+ in MoS2/TiSi2-3 is higher than in TiSi2, while the binding energy of Ti 0 and Si 0 in MoS2/TiSi2-3 is lower than in TiSi2.The peak located at 163.2 eV confirms the presence of S 2− in MoS2 (Figure 3C).The blue shift of the S 2p peak position of MoS2/TiSi2-3 can be clearly observed.The changes in binding energy of S were possibly due to the S-Ti and S-Si bond formation.The shift of the relative peak position is attributed to the interaction of TiSi2 with MoS2 [1,18,20].Figure 3D demonstrates the XPS spectra of Mo 3d for MoS2 and MoS2/TiSi2-3.The Mo 3d peak positions of MoS2/TiSi2-3 also shift to higher binding energies, indicating a low load of MoS2 and the remarkable combination effect of MoS2 and TiSi2 [14,21].

Optical and Photoelectrochemical Properties
Figure 4 exhibits the UV-vis absorption spectra of the obtained samples.The absorption of pure TiSi 2 is in the range of ca.400 up to 850 nm, where its absorption is higher than that of MoS 2 .The MoS 2 /TiSi 2 -3 sample demonstrates higher absorption intensity compared with MoS 2 and TiSi 2 , indicating that the MoS 2 /TiSi 2 composite possesses enhanced ability for harvesting visible light.

Optical and Photoelectrochemical Properties
Figure 4 exhibits the UV-vis absorption spectra of the obtained samples.The absorption of pure TiSi2 is in the range of ca.400 up to 850 nm, where its absorption is higher than that of MoS2.The MoS2/TiSi2-3 sample demonstrates higher absorption intensity compared with MoS2 and TiSi2, indicating that the MoS2/TiSi2 composite possesses enhanced ability for harvesting visible light.The photocurrent responses for TiSi2, MoS2, and MoS2/TiSi2-3 are displayed in Figure 5A.The photocurrent density of TiSi2 or MoS2 is rather low (ca.0.22 μA•cm −2 ).The photocurrent density of MoS2/TiSi2-3 (0.59 μA•cm −2 ) is more than 2.6 times that of TiSi2 in the same irradiation conditions.The results suggest the positive synergetic effect between MoS2 and TiSi2, which leads to enhanced photoinduced chargers transfer and separation.Figure 5B shows the electrochemical impedance spectra (EIS) of the samples are shown in.The diameter of the semicircle of MoS2/TiSi2-3 plot is obviously smaller than that of MoS2 and TiSi2, which indicates effectively enhanced carrier transfer at the interface between the MoS2/TiSi2-3 electrode and the electrolyte [22].The experimental data for all the electrodes can be expressed as an equivalent circuit, displayed as shown as the inset of Figure 5B, in which CPE1 is constant phase elements connected in parallel with R2 [23,24], R2 is the resistance of solution, and the ohmic series resistance (R1) is the resistance of charge-transfer resistance at interfaces [25,26].The photocurrent responses for TiSi 2 , MoS 2 , and MoS 2 /TiSi 2 -3 are displayed in Figure 5A.The photocurrent density of TiSi 2 or MoS 2 is rather low (ca.0.22 µA•cm −2 ).The photocurrent density of MoS 2 /TiSi 2 -3 (0.59 µA•cm −2 ) is more than 2.6 times that of TiSi 2 in the same irradiation conditions.The results suggest the positive synergetic effect between MoS 2 and TiSi 2 , which leads to enhanced photoinduced chargers transfer and separation.Figure 5B shows the electrochemical impedance spectra (EIS) of the samples are shown in.The diameter of the semicircle of MoS 2 /TiSi 2 -3 plot is obviously smaller than that of MoS 2 and TiSi 2 , which indicates effectively enhanced carrier transfer at the interface between the MoS 2 /TiSi 2 -3 electrode and the electrolyte [22].The experimental data for all the electrodes can be expressed as an equivalent circuit, displayed as shown as the inset of Figure 5B, in which CPE1 is constant phase elements connected in parallel with R2 [23,24], R2 is the resistance of solution, and the ohmic series resistance (R1) is the resistance of charge-transfer resistance at interfaces [25,26].

Optical and Photoelectrochemical Properties
Figure 4 exhibits the UV-vis absorption spectra of the obtained samples.The absorption of pure TiSi2 is in the range of ca.400 up to 850 nm, where its absorption is higher than that of MoS2.The MoS2/TiSi2-3 sample demonstrates higher absorption intensity compared with MoS2 and TiSi2, indicating that the MoS2/TiSi2 composite possesses enhanced ability for harvesting visible light.The photocurrent responses for TiSi2, MoS2, and MoS2/TiSi2-3 are displayed in Figure 5A.The photocurrent density of TiSi2 or MoS2 is rather low (ca.0.22 μA•cm −2 ).The photocurrent density of MoS2/TiSi2-3 (0.59 μA•cm −2 ) is more than 2.6 times that of TiSi2 in the same irradiation conditions.The results suggest the positive synergetic effect between MoS2 and TiSi2, which leads to enhanced photoinduced chargers transfer and separation.Figure 5B shows the electrochemical impedance spectra (EIS) of the samples are shown in.The diameter of the semicircle of MoS2/TiSi2-3 plot is obviously smaller than that of MoS2 and TiSi2, which indicates effectively enhanced carrier transfer at the interface between the MoS2/TiSi2-3 electrode and the electrolyte [22].The experimental data for all the electrodes can be expressed as an equivalent circuit, displayed as shown as the inset of Figure 5B, in which CPE1 is constant phase elements connected in parallel with R2 [23,24], R2 is the resistance of solution, and the ohmic series resistance (R1) is the resistance of charge-transfer resistance at interfaces [25,26].

Photocatalytic Activity
The photocatalytic activity for H 2 production of the samples are shown in Figure 6 and Figure S4.MoS 2 alone shows little photocatalytic activity.The hydrogen production rate for TiSi 2 is 38.4 µmol•h −1 •g −1 .The hydrogen production rate for the MoS 2 /TiSi 2 -1 sample increases to 119.6 µmol•h −1 •g −1 .The photocatalytic activity of the MoS 2 /TiSi 2 augments with the increasing of the quantity of MoS 2 in MoS 2 /TiSi 2 .The hydrogen production rate of the MoS 2 /TiSi 2 -3 sample reaches the maximum value 214.1 µmol•h −1 •g −1 , which is approximately 5.6 times that of TiSi 2 .The fact that composite catalysts exhibit much enhanced activity for H 2 evolution can be attributed to the synergistic effect between TiSi 2 andMoS 2 .Through the heterojunction formed between MoS 2 and TiSi 2 , the photogenerated electrons in TiSi 2 easily transfer to the conduction band of MoS 2 , which inhibits the recombination of e − -h + , thus improving the photocatalytic activity.
The results of the stability are shown in Figure 6.In the total 25 h recycle tests, the photocatalytic activity for H 2 production of MoS 2 /TiSi 2 -3 was almost invariable and the average value was 214 µmol•h −1 •g −1 , indicating that the composite prepared by the in situ photo-assisted reduction of MoS 2 on TiSi 2 possesses good photocatalytic stability.

Photocatalytic Activity
The photocatalytic activity for H2 production of the samples are shown in Figure 6 and Figure S4.MoS2 alone shows little photocatalytic activity.The hydrogen production rate for TiSi2 is 38.4 μmol•h −1 •g −1 .The hydrogen production rate for the MoS2/TiSi2-1 sample increases to 119.6 μmol•h −1 •g −1 .The photocatalytic activity of the MoS2/TiSi2 augments with the increasing of the quantity of MoS2 in MoS2/TiSi2.The hydrogen production rate of the MoS2/TiSi2-3 sample reaches the maximum value 214.1 μmol•h −1 •g −1 , which is approximately 5.6 times that of TiSi2.The fact that composite catalysts exhibit much enhanced activity for H2 evolution can be attributed to the synergistic effect between TiSi2 andMoS2.Through the heterojunction formed between MoS2 and TiSi2, the photogenerated electrons in TiSi2 easily transfer to the conduction band of MoS2, which inhibits the recombination of e --h + , thus improving the photocatalytic activity.
The results of the stability are shown in Figure 6.In the total 25 h recycle tests, the photocatalytic activity for H2 production of MoS2/TiSi2-3 was almost invariable and the average value was 214 μmol•h −1 •g −1 , indicating that the composite prepared by the in situ photo-assisted reduction of MoS2 on TiSi2 possesses good photocatalytic stability.Because of the wide-bandgap energy (3.4 ~ 1.5 eV) for TiSi2, TiSi2 can easily absorb the photons under visible light irradiation to generate plenty of electrons and holes in its conduction and valence band, respectively [8].Well-combined MoS2/TiSi2 heterojunctions can prompt the transfer of photogenerated chargers between MoS2 and TiSi2, and provide a pathway for charges transfer simultaneously (Scheme S1).Since the CB of TiSi2 (−0.43 eV) is more negative than that of MoS2 (−0.1 eV), the photogenerated electrons in the CB of TiSi2 were easy to transfer through the heterojunction interface between MoS2 and TiSi2 to the CB of MoS2 particles deposited on the surface of TiSi2.Simultaneously, the holes transfer from the higher VB of MoS2 (1.7 eV) to the VB of TiSi2 (1.07 eV).The shorter charge transfer route effectively restrains the recombination process of the electron-hole pairs.H + is reduced to hydrogen atom, to form hydrogen by the electrons in the CB of MoS2, while the holes on the TiSi2 surface are rapidly scavenged by H2O and OH-, generating •OH to oxidize sacrificial agents.

Formation Mechanism of the MoS2/TiSi2 Photocatalyst
Scheme 1 shows the proposed formation mechanism of the MoS2/TiSi2 photocatalyst.The electrons are generated at the VB of TiSi2 are excited by visible-light, which in turn reduce [MoS4] 2− to MoS2 [16], as shown in Equation ( 1): Because of the wide-bandgap energy (3.4~1.5 eV) for TiSi 2 , TiSi 2 can easily absorb the photons under visible light irradiation to generate plenty of electrons and holes in its conduction and valence band, respectively [8].Well-combined MoS 2 /TiSi 2 heterojunctions can prompt the transfer of photogenerated chargers between MoS 2 and TiSi 2 , and provide a pathway for charges transfer simultaneously (Scheme S1).Since the CB of TiSi 2 (−0.43 eV) is more negative than that of MoS 2 (−0.1 eV), the photogenerated electrons in the CB of TiSi 2 were easy to transfer through the heterojunction interface between MoS 2 and TiSi 2 to the CB of MoS 2 particles deposited on the surface of TiSi 2 .Simultaneously, the holes transfer from the higher VB of MoS 2 (1.7 eV) to the VB of TiSi 2 (1.07 eV).The shorter charge transfer route effectively restrains the recombination process of the electron-hole pairs.H + is reduced to hydrogen atom, to form hydrogen by the electrons in the CB of MoS 2 , while the holes on the TiSi 2 surface are rapidly scavenged by H 2 O and OH-, generating •OH to oxidize sacrificial agents.

Formation Mechanism of the MoS 2 /TiSi 2 Photocatalyst
Scheme 1 shows the proposed formation mechanism of the MoS 2 /TiSi 2 photocatalyst.The electrons are generated at the VB of TiSi 2 are excited by visible-light, which in turn reduce [MoS 4 ] 2− to MoS 2 [16], as shown in Equation ( 1): The holes (h) of TiSi 2 are scavenged by lactic acid.MoS 2 particles supported on the surface of TiSi 2 not only form a heterojunction with TiSi 2 but also provide effective active sites to improve hydrogen production.
The holes (h) of TiSi2 are scavenged by lactic acid.MoS2 particles supported on the surface of TiSi2 not only form a heterojunction with TiSi2 but also provide effective active sites to improve hydrogen production.
Scheme 1. Illustration of the preparation of MoS2/TiSi2 via an in situ photo-assisted reduction method.

Materials
Titanium disilicide was obtained from Alfa Aesar and all other chemicals (analytical purity) were obtained from J&K Scientific Limited (Beijing, China).All of the reagents and chemicals were utilized as received without further purification.

Synthesis
Fifty milliliters of 10 vol % lactic acid (LA) solution was added into to a 60 mL three-necked round-bottom flask with a quartz window, 50 mg of TiSi2 and a certain amount of (NH4)2MoS4 were dispersed in the lactic acid solution.The mixed solution was ventilated via bubbling argon with 30 min, then the mixture was irradiated by 150 W Xe lamp through the quartz window with a cutoff filter at 420 nm.After 60 min irradiation, the suspension was centrifuged and the solid was washed by ethanol first and then through deionized water.The obtained solid was dried under vacuum under 50 °C overnight, obtaining MoS2 modified TiSi2(MoS2/TiSi2-x), where x represents the mass percentage of MoS2 in the MoS2/TiSi2 composite.For comparison, MoS2 was obtained in the same process without adding TiSi2.

Characterization
X-Ray diffraction (XRD) measurement was performed under a Philips diffractometer (X'Pert-Pro MRD, Amsterdam, Netherland) with a Ni-filtered Cu Kα source (λ = 0.15418 nm) in the 2θ scanning range from 10° to 90°.Scanning electron microscopy (SEM) was taken on SEM Hitachi S-4800 (Hitachi High-Tech, Tokyo, Japan).The energy-dispersive X-ray (EDX) was carried out in a KEVEX X-ray energy detector (KEVEX, Newark, NJ, America).The transmission electron microscopy (TEM) was carried out by TECNAI-G2 electron microscope (FEI, Hillsboro, OR, America) using 200 kV accelerating voltage.UV-vis absorption spectra (DRS) were performed on a Hitachi UV-3010 spectrophotometer (Hitachi High-Tech, Tokyo, Japan) with BaSO4 as white standard.X-ray photoelectron spectroscopy (XPS) was determined on an AXIS Ultra DLD system (Kratos Analytical Inc., Manchester, UK) used a monochromatic Al Kα radiation and C 1s peak (285.5 eV) was as a reference to calibrate all of the XPS spectra using XPS Peak software (Version 4.1, Raymund W.M. Kwok, Hongkong, China).

Photoelectrochemical Measurements
Scheme 1. Illustration of the preparation of MoS 2 /TiSi 2 via an in situ photo-assisted reduction method.

Materials
Titanium disilicide was obtained from Alfa Aesar and all other chemicals (analytical purity) were obtained from J&K Scientific Limited (Beijing, China).All of the reagents and chemicals were utilized as received without further purification.

Synthesis
Fifty milliliters of 10 vol % lactic acid (LA) solution was added into to a 60 mL three-necked round-bottom flask with a quartz window, 50 mg of TiSi 2 and a certain amount of (NH 4 ) 2 MoS 4 were dispersed in the lactic acid solution.The mixed solution was ventilated via bubbling argon with 30 min, then the mixture was irradiated by 150 W Xe lamp through the quartz window with a cutoff filter at 420 nm.After 60 min irradiation, the suspension was centrifuged and the solid was washed by ethanol first and then through deionized water.The obtained solid was dried under vacuum under 50 • C overnight, obtaining MoS 2 modified TiSi 2 (MoS 2 /TiSi 2 -x), where x represents the mass percentage of MoS 2 in the MoS 2 /TiSi 2 composite.For comparison, MoS 2 was obtained in the same process without adding TiSi 2 .

Characterization
X-Ray diffraction (XRD) measurement was performed under a Philips diffractometer (X'Pert-Pro MRD, Amsterdam, The Netherland) with a Ni-filtered Cu Kα source (λ = 0.15418 nm) in the 2θ scanning range from 10 • to 90 • .Scanning electron microscopy (SEM) was taken on SEM Hitachi S-4800 (Hitachi High-Tech, Tokyo, Japan).The energy-dispersive X-ray (EDX) was carried out in a KEVEX X-ray energy detector (KEVEX, Newark, NJ, USA).The transmission electron microscopy (TEM) was carried out by TECNAI-G2 electron microscope (FEI, Hillsboro, OR, USA) using 200 kV accelerating voltage.UV-vis absorption spectra (DRS) were performed on a Hitachi UV-3010 spectrophotometer (Hitachi High-Tech, Tokyo, Japan) with BaSO 4 as white standard.X-ray photoelectron spectroscopy (XPS) was determined on an AXIS Ultra DLD system (Kratos Analytical Inc., Manchester, UK) used a monochromatic Al Kα radiation and C 1s peak (285.5 eV) was as a reference to calibrate all of the XPS spectra using XPS Peak software (Version 4.1, Raymund W.M. Kwok, Hongkong, China).

Photoelectrochemical Measurements
All photoelectrochemical measurements were carried out in a three-electrode system connected with a CHI660D (CH Instruments Inc., Shanghai, China) electrochemical workstation.An indium tin oxide (ITO) glass covered by the sample was employed as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum wire as the counter electrode.The working electrode was obtained first through dispersing 0.2 mg of the sample in 1 mL of a solution composed of 0.4 mL of ethanol, 0.4 mL of ethylene glycol and 0.2 mL of chloroform, after grinding and sonication, the slurry was then dropped onto a clean ITO glass and dried in vacuum at 45 • C. The sample area on the ITO glass was ca.1.0 cm 2 .During the photoelectrochemical measurements, the electrodes were immersed in 50 mL of 0.5 M Na 2 SO 4 solution containing 5 mL lactic acid and the ITO glass with catalyst was irradiated by a GY-10 xenon lamp (150 W) (TIAN JIN TUO PU, Tianjin, China).Electrochemical impedance spectroscopy (EIS) was recorded under the frequency range 1-10 5 Hz with an AC perturbation signal of 5 mV.

Photocatalytic Reaction for Hydrogen Evolution
20 mg of as-prepared photocatalyst was added in 50 mL of 10 vol % lactic acid (LA) solution.The suspension was added into a 60 mL three-necked flask with quartz window.The area of the effective optical channel is ca. 3 cm 2 .The reaction mixture was vigorously stirred and degassed via Ar with 30 min.Then the mixture was irradiated by 150 W Xe lamp through the quartz window with a cut-off filter at 420 nm.The lamp was positioned ca. 10 cm away from the optical entry window of the reactor.The distance between the lamp and the quartz window was ca. 10 cm.The gas chromatograph GC1650 (Ke Xiao Instruments Co., Ltd., Hangzhou, China) with a thermal conductivity detector (molecular sieve 5 A column, Ar carrier) was used to detected the amount of hydrogen production.

Conclusions
A robust and effective MoS 2 /TiSi 2 photocatalyst has been successfully fabricated through the convenient in situ photo-reduction method.The characterization results of prepared catalysts reveal that the MoS 2 was evenly distributed at TiSi 2 forming heterojunctions benefiting photoexcited charges transfer and separation.The hydrogen production rate by the optimized MoS 2 /TiSi 2 catalyst reaches 214.1 µmol•h −1 •g −1 by visible light illumination, which is significantly superior to that of TiSi 2 and MoS 2 .The characteristics possessed by MoS 2 /TiSi 2 , such as broad spectral response, enhanced absorption capability and the synergistic effect between MoS 2 and TiSi 2 , are owed to high photocatalytic performance of the catalyst.This work demonstrates the feasibility of increasing H 2 evolution activity of TiSi 2 -based catalysts by combining TiSi 2 with MoS 2 .

Figure 4 .
Figure 4.The UV-vis absorption spectra of the obtained samples.

Figure 5 .Figure 4 .
Figure 5. (A) The photocurrent responses of samples under 150 W xenon lamp.The electrolyte was 60 mL of 0.5 M Na2SO4 solution containing 6 mL lactic acid; (B) The electrochemical impedance spectra (EIS) for the samples in 60 mL of 0.5 M Na2SO4 solution containing 6 mL lactic acid.

Figure 4 .
Figure 4.The UV-vis absorption spectra of the obtained samples.

Figure 5 .
Figure 5. (A) The photocurrent responses of samples under 150 W xenon lamp.The electrolyte was 60 mL of 0.5 M Na2SO4 solution containing 6 mL lactic acid; (B) The electrochemical impedance spectra (EIS) for the samples in 60 mL of 0.5 M Na2SO4 solution containing 6 mL lactic acid.

Figure 5 .
Figure 5. (A) The photocurrent responses of samples under 150 W xenon lamp.The electrolyte was 60 mL of 0.5 M Na 2 SO 4 solution containing 6 mL lactic acid; (B) The electrochemical impedance spectra (EIS) for the samples in 60 mL of 0.5 M Na 2 SO 4 solution containing 6 mL lactic acid.

Figure 6 .
Figure 6.(A) The amount of H2 evolved over the samples and (B) cycling measurement.

Figure 6 .
Figure 6.(A) The amount of H 2 evolved over the samples and (B) cycling measurement.