One-Step Synthesis of MoS₂/TiSi₂ via an In Situ Photo-Assisted Reduction Method for Enhanced Photocatalytic H₂ Evolution under Simulated Sunlight Illumination

Abstract

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.

1. 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 H₂ production in the TiO₂ 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 TiO₂ or seek novel semiconductor photocatalysts to improve the photocatalytic activity and efficiency [7].

A novel compound material, titanium disilicide (TiSi₂) has been attracting interest because of its thermal stability, good electrical conductivities, single electron tunneling characteristics, and eminent light absorption ability from near-UV to visible light (e.g., ~3.4–1.5 eV) [8,9]. Ritterskamp et al. first used TiSi₂ as a photocatalyst for water splitting [10]. After that, a series of TiSi₂-based compounds or composites, including TiSi₂–SiC [11], Ti₅Si₃ [12], TiO₂/TiSi₂ [7,8], were synthesized. RuO₂/TiSi₂/RGO hybrid and WS₂/TiSi₂ composite were also successfully synthesized by our group [1,13]. These novel materials showed improved photoelectrochemical properties and enhanced photocatalytic activity.

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 photo-reduction and its photocatalytic application for the photocatalytic H₂ evolution under visible light.

2. Results and Discussion

2.1. 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.

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 Ti⁰ p_3/2, Ti⁴⁺ p_3/2, and Ti⁴⁺ p_1/2 [10,18], respectively. The peaks located at 98.6 and 102.4 eV in the pattern of TiSi₂ (Figure 3B) were attributed to Si⁰ p_3/2 and Si⁴⁺ p_3/2 [18,19]. Comparing the above peak positions with those of MoS₂/TiSi₂, the binding energy of Ti⁴⁺ and Si⁴⁺ in MoS₂/TiSi₂-3 is higher than in TiSi₂, while the binding energy of Ti⁰ and Si⁰ in MoS₂/TiSi₂-3 is lower than in TiSi₂. The peak located at 163.2 eV confirms the presence of S²⁻ 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].

2.2. 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].

2.3. 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.

2.4. 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):

[MoS₄]²⁻ + 2e → MoS₂ + 2S²⁻.

(1)

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.

3. Experimental

3.1. 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.

3.2. 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.

3.3. 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 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).

3.4. 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 cm². During the photoelectrochemical measurements, the electrodes were immersed in 50 mL of 0.5 M Na₂SO₄ 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 1–10⁵ Hz with an AC perturbation signal of 5 mV.

3.5. 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 cm². 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.

4. Conclusions

A robust and effective MoS₂/TiSi₂ photocatalyst has been successfully fabricated through the convenient in situ photo-reduction method. The characterization results of prepared catalysts reveal that the MoS₂ was evenly distributed at TiSi₂ forming heterojunctions benefiting photoexcited charges transfer and separation. The hydrogen production rate by the optimized MoS₂/TiSi₂ catalyst reaches 214.1 μmol·h⁻¹·g⁻¹ by visible light illumination, which is significantly superior to that of TiSi₂ and MoS₂. The characteristics possessed by MoS₂/TiSi₂, such as broad spectral response, enhanced absorption capability and the synergistic effect between MoS₂ and TiSi₂, are owed to high photocatalytic performance of the catalyst. This work demonstrates the feasibility of increasing H₂ evolution activity of TiSi₂-based catalysts by combining TiSi₂ with MoS₂.