CoS2/TiO2 Nanocomposites for Hydrogen Production under UV Irradiation

Transition metal chalcogenides have intensively focused on photocatalytic hydrogen production for a decade due to their stronger edge and the quantum confinement effect. This work mainly focuses on synthesis and hydrogen production efficiencies of cobalt disulfide (CoS2)-embedded TiO2 nanocomposites. Materials are synthesized by using a hydrothermal approach and the hydrogen production efficiencies of pristine CoS2, TiO2 nanoparticles and CoS2/TiO2 nanocomposites are compared under UV irradiation. A higher amount of hydrogen production (2.55 mmol g−1) is obtained with 10 wt.% CoS2/TiO2 nanocomposite than pristineTiO2 nanoparticles, whereas no hydrogen production was observed with pristine CoS2 nanoparticles. This result unveils that the metal dichalcogenide–CoS2 acts as an effective co-catalyst and nanocrystalline TiO2 serves as an active site by effectively separating the photogenerated electron–hole pair. This study lays down a new approach for developing transition metal dichalcogenide materials with significant bandgaps that can effectively harness solar energy for hydrogen production.


Introduction
Depletion of fossil fuel deserves utilization of hydrogen as a renewable energy source. It could be one of the promising energy sources alternative to fossil fuels in meeting the energy demand of the current world population. Currently, the major hydrogen production is from steam-methane reforming and thermal cracking of natural gas, and coal gasification, which cannot alleviate the emission of greenhouse gases [1]. Electrolysis is another method, which is being used to produce hydrogen. Electrochemical reduction of water [2][3][4] is an ecofriendly method and exhibits high-purity (99.999%) of hydrogen [5]; in addition, this is a key to many clean energy technologies. Although there have been several methods used to produce hydrogen, photocatalytic hydrogen generation have gained much attention these days. This technique integrates solar energy collection together with water splitting, therefore, it is a more cost-effective method compared with the water electrolysis process. Unfortunately, only a small percentage of hydrogen is produced from the photocatalytic method under solar extended irradiation. Therefore, the development of new materials for sustainable hydrogen production is necessary to overcome the detrimental environmental impacts. In the past decade, different photocatalyst materials, such as TiO 2 [6][7][8], ZnO [9], CdS [10,11], WS 2 [12,13], mixed oxides [14,15], perovskites [16,17], dye and metal doped oxide materials [18,19] have been used as phototcatalysts for environmental remediation and energy production, such as water splitting applications. In particular, catalysts contain noble metals, such as Pt have also been utilized in the state-of-the-art hydrogen evolution reaction. However, large scale hydrogen production is limited with these catalysts [20][21][22]. Among the materials studied, TiO 2 has been considered as the golden standard due to its significant characteristics that include photochemical stability, low toxicity, relative affordability, and ease of preparation [23]. Although the bandgap energy of TiO 2 necessitates the use of UV irradiation, composites of TiO 2 with co-catalysts, such as transition metal chalcogenides (TMCs), enable the catalysts to absorb visible light abundant in solar irradiation [24]. In this regard, TMCs have gained much attention among the research community in the field of lithium ion batteries, solar cells and hydrogen evolution, due to their significant characteristic features that include indirect bandgaps, optoelectronic properties and stability [24]. In addition, nanodots (quantum dots)/nano structures of these metal chalcogenides show stronger edge effects, and the quantum confinement effect make them suitable to be utilized under solar simulated irradiation [24,25]. The transition metal chalcogenides can be synthesized by employing different techniques, such as one-pot wet chemical method [10], impregnation-sulfidation [11], simple microwave-assisted solvothermal process [26], ion exchange and precipitation methods [27], and hydrothermal method [28]. Several studies on bare transition metal chalcogenides (MoS 2 , NiS 2 , WS 2 , CdS and CoS) for the hydrogen evolution reaction (HER) and oxygen reduction reactions (ORR) are reported [29][30][31][32], however, most of these studies mainly focused on electrochemical water splitting. Co-catalysts, such as, reduced graphene oxides [33], metal oxides [34], dyes, for example, Porphyrin (Zn(II)-5,10,15,20-tetrakis(4-carboxyphenyl)-porphyrin (ZnTCPP)) [19], graphene [35], metals [36], and CdLa 2 S 4 nanocrystals [37] were used to enhance the rate of HER. Inorganic crystals with desired properties can be used as excellent candidates for HER. It was reported that the first-row transition metal chalcogenides (MS 2 ; where M = Fe, Co, Ni) exhibit excellent catalytic properties for HER due to their pyrite or marcasite structure, in which the metal atoms are octahedrally bonded to adjacent S atoms. Stability is an important criterion in the development of catalysts [30,[38][39][40][41][42][43][44][45]. In this regard, computational modelling was also employed on these metal chalcogenide materials to evaluate their structural properties for photocatalytic water splitting and hydrogen production [46]. In particular, MoS 2 has been utilized with other elements via adsorption or intercalation of a cation, such as Li for electrochemical water splitting [46]. In another modelling work, a comparative study was done between WS 2 and MoS 2 in electrocatalytic water splitting [12]; In an experimental study, Yuexiang Li et al. reported hydrogen production of 99 µmol h −1 with MoS 2 loaded on the composites of reduced graphene oxide and CdS, and this was found to be over 20 times higher than the bare CdS. CoS 2 doped with Mn [47] and Al [48] used for electrocatalytic HER [49].
Although all the reported work on TMC-embedded TiO 2 mainly focused on the electrochemical water splitting and computer modelling, to the best of our knowledge, no experimental study on TMC-embedded TiO 2 for heterogeneous hydrogen production over extended solar irradiation has been reported yet. This study focuses on the synthesis of CoS 2 (metal chalcogenide)-embedded TiO 2 nanocomposite, and the impact of doping CoS 2 with TiO 2 in hydrogen evolution under UV irradiation.

Synthesis
(1) Titanium dioxide Nanocrystalline titanium dioxide material was prepared under hydrothermal condition using sol-gel technique. In a typical synthesis, 32.5 mL of ethanol was acidified with 0.3 mL concentrated HNO 3 (Sigma-Aldrich, Oslo, Norway) in a Teflon liner and stirred at a constant speed (300 rpm, Velp/AREC, VELP Scientifica Srl,). 6.60 mL of titanium (iv) isopropoxide was added drop wise into it with continuous stirring, Finally, 3.0 mL of water was added to the above solution. Then, it was transferred into an autoclave (AUTOCLAVE-PTFE-0100, TECINSTRO, Maharashtra, India) and kept at 180 • C for 9 hours. The final material was heated at 500 • C for 3 h.
(2) CoS 2 embedded TiO 2 nanocomposite 118.24 mg of Co(NO 3 ) 2 and 192.72 mg of Na 2 S 2 O 3 were added in to a 100 mL aqueous solution containing deionized water and ethanol in 2:1 ratio under constant stirring (300 rpm) for 30 min. Finally, required amount of TiO 2 was dispersed into above solution and the resulting mixture was hydrothermally treated at 180 • C to prepare 10 wt.% of CoS 2 embedded TiO 2 material. Similar conditions were followed in the preparation of pristine CoS 2 nanoparticles without adding titanium dioxide.

Characterization
Synthesized materials were subjected to different characterization techniques, such as Powder X-ray Diffraction (P-XRD, Ultima IV Rigaku, USA) Method, Diffuse Reflectance Spectra (DRS Cary 100 Bio UV-Visible spectrophotometer, Santa Clara, CA, USA) and Scanning Electron Microscopy (SEM, Oxford instrument, NanoAnalysis, Concord, MA, USA). P-XRD patterns were recorded on a Rigaku Ultima IV instrument (Scottsdale, AZ, USA) with Cu Kα radiation (λ = 1.5408 Å) at ambient temperature, under the following operating conditions; accelerating voltage of 40 kV; emission current of 44 mA; scanned range (2θ) between 20 • and 80 • with a step size of 0.02 • , and a scan speed of 1 • /min. DRS were acquired using a Cary 100 Bio UV-Visible spectrophotometer, and the SEM images were captured on an Oxford instrument.

Photocatalytic Hydrogen Evolution
The photocatalytic experiments were carried out for pristine TiO 2 , CoS 2 and CoS 2 /TiO 2 . Catalysts were suspended in a solution containing 1.5 mL of H 2 O and 0.5 mL of methanol as a scavenging agent. The suspension was degassed for 30 minutes with high-purity argon prior to irradiation. The suspensions were continuously stirred throughout the course of the experiment. A 300 W Xenon lamp (Oriel light source, Xenon arc lamp, Newport 1000W, CA, USA) with an appropriate filter was used as the source of UV radiation. The amount of H 2 produced was measured by gas chromatography (SRI 8610 C, SRI instruments, Torrance, CA, USA) equipped with a molecular sieve column and a TCD (Thermal Conductivity detector), and the amount of hydrogen produced was quantified by using a calibration curve prepared previously.

Characterization of Materials
The powder XRD patterns of the pristine CoS 2 , TiO 2 , and 10 wt.% CoS 2 /TiO 2 nanocomposites are shown in Figure 1 Scanning electron microscopic images of blank CoS2 (a,b), TiO2 (c,d) and 10 wt.% CoS2/TiO2 (e,f) are illustrated in Figure 2. It can clearly be seen from Figure 2a, b that the bare CoS2 shows an aggregation, which is surrounded by flake like structures. An irregular 3D block-like structure covered with spongy like particles was attained for TiO2 nanocomposite and is shown in Figure 2c, and the zoom in image (Figure 2d) clearly illustrates the aggregated particles which have spongylike structures. The mixed composite, CoS2/TiO2 also exhibits the aggregation, in which the zoom in image (Figure 2f) shows the hexagonal rod like structure decorated with spongy like materials. [51]  Scanning electron microscopic images of blank CoS 2 (a,b), TiO 2 (c,d) and 10 wt.% CoS 2 /TiO 2 (e,f) are illustrated in Figure 2. It can clearly be seen from Figure 2a,b that the bare CoS 2 shows an aggregation, which is surrounded by flake like structures. An irregular 3D block-like structure covered with spongy like particles was attained for TiO 2 nanocomposite and is shown in Figure 2c, and the zoom in image (Figure 2d) clearly illustrates the aggregated particles which have spongy-like structures. The mixed composite, CoS 2 /TiO 2 also exhibits the aggregation, in which the zoom in image (Figure 2f) shows the hexagonal rod like structure decorated with spongy like materials [51].
The bandgap energies for the pristine CoS 2 , TiO 2 and 10 wt.% CoS 2 /TiO 2 nanocomposite materials corresponding to the absorbance spectra of powder samples (Figure 3a), were estimated by using the Tauc plot (Figure 3b), which was transformed via the Kubelka-Munk function [52], [F(R ∞ )E] n vs. E, when n = 0.5, for a direct allowed transition (K = F(R ∞ )). Estimates derived from the Tauc plots by extrapolating the steep portion of the plot in Figure 3b to the x-axis suggest that the bandgaps of the pristine CoS 2 (2.5 eV), TiO 2 (3.2 eV) and CoS 2 embedded TiO 2 nanocomposite (3.4 eV) materials lie in the range between 2.4 and 3.4 eV. The bandgap energies for the pristine CoS2, TiO2 and 10 wt.% CoS2/TiO2 nanocomposite materials corresponding to the absorbance spectra of powder samples (Figure 3a), were estimated by using the Tauc plot (Figure 3b), which was transformed via the Kubelka-Munk function [52], F R E vs. E, when n = 0.5, for a direct allowed transition K F R . Estimates derived from the Tauc plots by extrapolating the steep portion of the plot in Figure 3b to the x-axis suggest that the bandgaps of the pristine CoS2 (2.5 eV), TiO2 (3.2 eV) and CoS2 embedded TiO2 nanocomposite (3.4 eV) materials lie in the range between 2.4 and 3.4 eV.

Hydrogen Evolution
The amount of hydrogen evolved in the presence of UV irradiation is tabulated and compared with what is reported in the literature in table 1. Transition metal chalcogenides, including MoS2, NiS, SnS2, WS2, and CdS, have been extensively explored for photocatalytic water splitting [11,24,36,49], since they are usually inexpensive, stable, and easily prepared on a large scale for practical applications. To improve the efficiency of TMCs on hydrogen evolution elemental doping, heterojunctions, and nano structuring have been explored. In this regard, metal chalcogenides have  The bandgap energies for the pristine CoS2, TiO2 and 10 wt.% CoS2/TiO2 nanocomposite materials corresponding to the absorbance spectra of powder samples (Figure 3a), were estimated by using the Tauc plot (Figure 3b), which was transformed via the Kubelka-Munk function [52], F R E vs. E, when n = 0.5, for a direct allowed transition K F R . Estimates derived from the Tauc plots by extrapolating the steep portion of the plot in Figure 3b to the x-axis suggest that the bandgaps of the pristine CoS2 (2.5 eV), TiO2 (3.2 eV) and CoS2 embedded TiO2 nanocomposite (3.4 eV) materials lie in the range between 2.4 and 3.4 eV.

Hydrogen Evolution
The amount of hydrogen evolved in the presence of UV irradiation is tabulated and compared with what is reported in the literature in table 1. Transition metal chalcogenides, including MoS2, NiS, SnS2, WS2, and CdS, have been extensively explored for photocatalytic water splitting [11,24,36,49], since they are usually inexpensive, stable, and easily prepared on a large scale for practical applications. To improve the efficiency of TMCs on hydrogen evolution elemental doping, heterojunctions, and nano structuring have been explored. In this regard, metal chalcogenides have

Hydrogen Evolution
The amount of hydrogen evolved in the presence of UV irradiation is tabulated and compared with what is reported in the literature in Table 1. Transition metal chalcogenides, including MoS 2 , NiS, SnS 2 , WS 2 , and CdS, have been extensively explored for photocatalytic water splitting [11,24,36,49], since they are usually inexpensive, stable, and easily prepared on a large scale for practical applications. To improve the efficiency of TMCs on hydrogen evolution elemental doping, heterojunctions, and nano structuring have been explored. In this regard, metal chalcogenides have been doped with other components, such as reduced graphene oxide, graphene, dyes, and TiO 2 using different experimental conditions.  [24]. In another study, Youngjun Yuan et al. used ZnTCPP-MoS 2 /TiO 2 material for HER (Hydrogen Evolution Reaction), where 0.10 mmol h −1 of hydrogen evolved with 1.00 wt.% of MoS 2 on TiO 2 material [19]. In line with these studies, the results from our study on metal chalcogenide-TiO 2 nanocomposites showed that 10 wt.% CoS 2 embedded on TiO 2 nanocomposites synthesized by hydrothermal method was the excellent candidate for the photocatalytic HER with better hydrogen evolution rate of 2.55 mmol g −1 . Pristine CoS 2 alone showed no activity towards hydrogen production even after 4 hours of irradiation, whereas the TiO 2 materials exhibited 1.88 mmolg −1 of hydrogen under the same experimental conditions. The reason for this observation can be correlated with the bandgap energies of the materials. The CoS 2 /TiO 2 materials with highest activity exhibit bandgap of 3.4 eV, whereas the pristine TiO 2 exhibits bandgap of 3.2 eV. Under UV irradiation, the electron-hole pair formed on the nanocomposites was effectively separated due to the bandgap >3 eV. However, in the case of pristine CoS 2 (Bandgap of 2.5 eV), it can be concluded that the faster recombination rate of photogenerated electrons and holes hinder the formation of hydrogen effectively, and, thus, there is no activity observed with this catalyst, but CoS 2 nanoparticles act as a co-catalyst in the nanocomposite of CoS 2 /TiO 2 materials to enhance the hydrogen production by exciting more electrons to the surface of titanium dioxide.

Conclusions
Pristine CoS 2 and TiO 2 , and CoS 2 /TiO 2 nanocomposites were successfully synthesized via a hydrothermal method using titanium(iv)isopropoxide, Co(NO 3 ) 2 and Na 2 S 2 O 3 as precursors. The mixed COS 2 /TiO 2 nanocomposite exhibits a high hydrogen production value of 2.55 mmol g −1 , whereas the pristine CoS 2 material was found to be inactive due to its very low bandgap energy. The TiO 2 material shows an intermediate hydrogen production of 1.88 mmol g −1 . In summary, the hydrogen production seems to depend on the band gap energy of the catalysts, and the CoS 2 may assist to effectively separate the electron-hole pair forms in the mixed nanocomposite, and thus, results in a higher value of hydrogen production.