Facile fabrication of InSe nanosheets: towards efficient visible-light-driven H₂ production by coupling with P25

Abstract

A cubic InSe nanosheet photocatalyst has been successfully synthesized via a hydrothermal process followed by a calcination treatment, and it was for the first time used for visible light photocatalytic H₂ production. Its photoactivity was further improved by coupling with P25, which was attributed mainly to the higher efficiency of charge separation and transfer in the InSe–rutile–anatase surface junctions.

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Solar energy utilization is a sustainable strategy to ultimately solve the problems of fossil fuel shortage and environmental pollution, while semiconductor based photocatalytic water splitting for hydrogen production has been considered as a promising and green route for the efficient utilization of solar energy.¹ Among various reported semiconductors, TiO₂ is the most studied semiconductor as an efficient photocatalyst due to its appropriate electronic band structure, environmental friendliness, low cost and high stability.² However, TiO₂ photocatalysis has been limited to ultraviolet (UV) light instead of the more abundant visible light due to its wide band gap (3.0–3.2 eV). Besides TiO₂, CdS, Pt nanowires/Ni(OH)₂, etc. also exhibit superior catalytic activity for H₂ evolution reaction.³ Therefore, developing visible-light-responsive photocatalysts has become one of the most important topics in photocatalytic hydrogen production research today.

As compared to their wide bandgap counterparts, metal selenides, such as CdSe, are attractive photocatalytic materials which can be widely used in visible-light-driven H₂ production due to their narrow band gap.⁴ However, toxic Cd²⁺ is left in the liquid after irradiation from the photodecomposition of CdSe.⁵ Hence, the other metal selenides like NiSe and In₂Se₃ are good candidates instead of toxic CdSe.⁶ In our previous study, porous γ-In₂Se₃ tetragonum was successfully synthesized and used for hydrogen production under UV and visible light irradiation.^6b As the photocatalytic efficiency of a single selenide photocatalyst is limited by its photocorrosion and the rapid recombination of photogenerated electrons and holes, great efforts should be devoted to the development of a rational design for the fabrication of a selenide heterostructure system in which the photogenerated carriers can be transferred to different counterparts thus promoting the photoactivity.^4a,b

As an important III–VI group semiconductor, hexagonal phase InSe with a layered structure has attractive applications in solar energy conversion due to its well-known optical and electrical properties, particularly controllable band-gap energy in the full visible spectral range. There have been several reports on hexagonal phase InSe-based electrical devices, optoelectronic devices and on their photoelectrochemical properties.⁷ Although hexagonal phase InSe have been obtained by several methods, such as chemical vapor deposition and the electrochemical technique, it is difficult to produce single phase and stoichiometric InSe due to the co-existence of several phases of the In–Se system like InSe, In₂Se₃ and In₆Se₇.⁸ In addition, hexagonal phase InSe obtained by the conventional process are general bulk materials, and a few are InSe nano-materials with a regular shape due to the harsh synthetic conditions.⁹

Besides hexagonal phase, InSe also has a cubic phase, which is quite rare both in the bulk state and in nanosized materials.¹⁰ As the optical and electrical properties are size- and shape-dependent, great effort has been focused on the phase structure and morphology-selective fabrication of this material. Therefore, it is a vital concern to develop a new and convenient method to synthesize single-phase InSe nanostructures. In particular, it is still rather challenging to synthesize cubic phase InSe nanostructures.

Recently, Qian et al. reported a surfactant assisted hydrothermal approach to prepare size and shape controlled nanostructures, such as CTAB-assisted synthesis of bundle-like Te nanorods, SDBS-assisted synthesis of Cu nanowires and Ni nanowires, SDS-assisted synthesis of nickel film and uniform PbS nanorod bundles, PVP-assisted synthesis of NiSe₂ tubular microcrystals and so on.¹¹ In this study, we firstly simulate this synthetic strategy to prepare cubic phase InSe nanosheets and InSe–TiO₂ (anatase and rutile) heterostructures. We also investigated the modification effects of TiO₂ (anatase and rutile) on the visible light-driven photocatalytic H₂ production activity of the cubic phase InSe nanosheets. Experimental results demonstrated that the as-prepared InSe–TiO₂ (anatase and rutile) photocatalyst showed drastically enhanced photocatalytic H₂ production from an aqueous ascorbic acid (AA) solution compared to that of pure InSe and InSe–TiO₂ (anatase), indicating that the synergetic effect of mix-phase TiO₂ nanoparticles (NPs) played an important role in the photocatalytic H₂ production rate.

The InSe nanosheets were synthesized by a facile two-step hydrothermal–calcining process (shown in Scheme 1), which is an effective method to synthesize single phase photocatalysts.¹² Firstly, a yellow In–Se-based precursor was hydrothermally synthesized by using sodium dodecylbenzyl sulfate (SDBS) as a surfactant. It was believed that the surfactant SDBS might play a role in preventing the aggregation of precursor nanoparticles in the initial stage of nanosheet growth and kinetically controlling the growth rates of various crystallographic facets.^11b Subsequently, the precursor was calcined under an N₂ atmosphere at 250 °C, then it turned into red color InSe nanosheets. The preparation of InSe–TiO₂ (anatase and rutile) heterostructures was similar to that of InSe nanosheets, and P25 was added during the hydrothermal step. The detailed synthesis process of both InSe and InSe–TiO₂ heterostructures is described in the ESI.†

Scheme 1 Formation paths of cubic InSe and InSe–TiO₂. They were used for photocatalytic H₂ production.

The phase composition of as-prepared samples was first investigated by X-ray powder diffraction (XRD). As shown in Fig. 1a (black curve), all diffraction peaks in the XRD pattern for the sample obtained in the absence of P25 match well with the cubic InSe phase, and no characteristic peaks from impurities were detected.¹⁰ When 0.015 g P25 was used, the resulting sample was made up of cubic InSe and TiO₂ (Fig. 1a, blue curve), which was named InSe–TiO₂(0.015), suggesting the fact that InSe is coupled with P25.

Fig. 1 (a) XRD patterns of as-prepared samples as well as P25; (b) SEM image and crystal structure (inset) of InSe nanosheets; SEM image of the InSe–TiO₂(0.015) heterostructure: (c) low-magnification SEM image, (d) high-magnification SEM image; (e) HRTEM image and (f) magnification of (e) for the InSe–TiO₂(0.015) heterostructure.

Fig. 1b presents a scanning electron microscopy (SEM) image of the InSe sample. Clearly, the InSe sample is composed of square-like nanosheets with side lengths in the range of 5–10 μm and a thickness of about 100 nm. When SDBS was replaced by other surfactants, such as, CTAB, SDS and PVP, or when no surfactant was used, pure InSe could still be obtained, but the morphologies of the products were irregular (Fig. S1 and S2†). The explanation for this probably indicates that these surfactants were different in the carbon chain length and the hydrophilic group, and thus they were alien in modulating the growth kinetics of InSe crystals. Fig. 1c displays the SEM image of the InSe–TiO₂(0.015) sample, from which it can be seen that this sample also exhibits a nanosheet shape. However, different from the above mentioned pure InSe sample, the nanosheets are coated by TiO₂ NPs with an average diameter of ca. 10 nm, demonstrating the successful synthesis of InSe–TiO₂ heterostructures (Fig. 1d). Obviously, the introduction of TiO₂ phase into InSe does not change their respective morphology. The high-resolution TEM (HRTEM) images (Fig. 1e and f) of the InSe–TiO₂(0.015) sample further demonstrate its microstructure. There are three types of lattice spacings, 0.357, 0.189, and 0.202 nm, which can be indexed to the (111) plane of cubic InSe, the (200) plane of anatase TiO₂, and the (210) plane of rutile TiO₂, respectively. This result confirms that the hydrothermal treatment has no influence on the phase composition of P25.

X-ray photoelectron spectroscopy (XPS) measurements were carried out to determine the surface compositions and chemical states of the as-obtained samples. As clearly disclosed by Fig. 2a, the high-resolution In 3d XPS spectra of the InSe sample show two typical In 3d_5/2 and In 3d_3/2 peaks centered at 444.6 eV and 452.2 eV, respectively, assigned to the In–Se bond.¹³ For the InSe–TiO₂(0.015) heterostructure, the In 3d_5/2 and In 3d_3/2 peaks are slightly shifted to a lower binding energy of 444.3 eV and 451.9 eV, respectively, indicating that there exists electronic coupling between InSe and TiO₂.¹⁴ The possible reason may be that the electronegativity of Ti is smaller than that of In, which causes the increase of electron density around the In atom. Therefore, the presence of titanium species in InSe–TiO₂(0.015) decreases the binding energies of In 3d. The XPS results are in good agreement with those from HRTEM described above.

Fig. 2 (a) High-resolution XPS spectra of In 3d from InSe and InSe–TiO₂(0.015); (b) UV-vis absorption spectra and photographs of InSe, InSe–TiO₂(0.015) and P25.

UV-visible absorption spectra for as-prepared samples as well as P25 are shown in Fig. 2b. Obviously, the InSe sample can absorb visible light with a band gap of 1.5 eV, while P25 exhibits only UV absorption with a band gap of 3.0 eV. Compared with pristine InSe and P25, the InSe–TiO₂(0.015) heterostructure exhibits in-between absorption with a band gap of 1.6 eV, which may result from the synergistic effect between the host InSe nanosheets and the guest P25 NPs in the InSe–TiO₂ heterostructure. Furthermore, the absorption of InSe–TiO₂ samples in the visible light region decreases with the increase of P25 dosage (Fig. S4b†).

In view of the excellent visible light absorption of InSe and InSe–TiO₂(0.015), we further studied their photocatalytic H₂ production under visible light irradiation (λ > 420 nm, Fig. S3†). It is widely believed that an appropriate electron donor used in the solution/semiconductor suspension contributes to high photocatalytic activity.¹⁵ Hence, in our experiments, ascorbic acid (AA) was used as a sacrificial reagent. Fig. 3a displays the photocatalytic H₂ production rates of different InSe–TiO₂ (anatase and rutile) heterostructures and other reference samples under visible light irradiation. As expected, the InSe–TiO₂ heterostructures exhibited better photocatalytic activity than the InSe nanosheets, P25 and the InSe–TiO₂ mixture. Among all InSe–TiO₂ (anatase and rutile) heterostructures, the InSe–TiO₂(0.015) sample has the maximum H₂ production rate of 1.1 mmol (g catalyst h)⁻¹. In this regard, the photocatalytic activity of InSe–TiO₂(0.015) exceeds that of pure InSe nanosheets by a factor of 4.1, which is significantly higher than that of most metal selenides and TiO₂ photocatalysts.^4,6,16 However, a further increase in TiO₂ content leads to a deterioration in photocatalytic activity (Fig. 3a and S4a†). It is reasonable because excessive TiO₂ modification led to the shielding of the InSe nanosheet surface, thus rapidly decreasing its visible light harvesting ability, as proved in Fig. S4b and S5b.† Moreover, to further confirm that co-existing anatase and rutile TiO₂ particles as well as the heterojunctions formed between TiO₂ and InSe have an important role in the enhanced photocatalytic performance, we also studied the time-dependent photocatalytic hydrogen evolution of the InSe–TiO₂ mixture and InSe–TiO₂(A) samples (Fig. 3a and S6†). Obviously, the H₂ production rates of both samples are much lower than those of InSe–TiO₂(0.015) heterostructures. In addition, the H₂ production rate (1.1 mmol (g h)⁻¹) of the InSe–TiO₂(0.015) sample is higher than that of most of the reported catalysts (Table S1†).

Fig. 3 (a) Photocatalytic H₂ production rates for different samples in 0.05 M ascorbic acid aqueous solution and with 2.5% Pt loaded under visible light irradiation; nitrogen adsorption–desorption isotherms (b), contact angles (c) and photocurrent responses (d) of InSe nanosheets and InSe–TiO₂(0.015) heterostructures; (e) GC traces of the headspace over long time photocatalytic reactions of the InSe–TiO₂(0.015) heterostructure in a 0.075 M ascorbic acid aqueous solution under visible light irradiation.

Compared to pure InSe nanosheets, the efficient visible-light-driven photocatalytic activity of InSe–TiO₂(0.015) heterostructures can be attributed to the following three factors: (1) as shown in Fig. 3b, the InSe–TiO₂(0.015) heterostructure has a larger specific surface area (0.9 m² g⁻¹ for InSe and 6.8 m² g⁻¹ for the InSe–TiO₂(0.015) heterostructure), which can offer more active adsorption sites for photocatalytic reaction and thus favors an enhanced H₂ production rate.¹⁷ (2) The surface hydrophilicity of InSe nanosheets is evidently improved by modification with TiO₂ NPs, which is confirmed by the fact that the contact angle (CA) of InSe–TiO₂(0.015) is smaller than that of InSe nanosheets (Fig. 3c). This can result in better interfacial charge separation and consequent water reduction efficiency.¹⁸ (3) The junctions formed among InSe, rutile and anatase (Fig. 1e and f) favor the fast charge separation at the interface due to the proper band alignment.¹⁹ The higher separation and transfer efficiency of charges in InSe–TiO₂(0.015) has been further confirmed by the photocurrent response test (Fig. 3d).

To optimize the photocatalytic reaction conditions, the effect of the concentrations of ascorbic acid and Pt on the H₂ evolution rate catalyzed by InSe–TiO₂(0.015) was examined. As shown in Fig. S7 and S8,† when the concentration of ascorbic acid was 0.075 mol L⁻¹ and with 2.5% Pt loaded, the H₂ production rate reached the highest value of 1.5 mmol (g catalyst h)⁻¹ with apparent quantum efficiency of 25.5% at 450 nm. Moreover, the InSe–TiO₂(0.015) catalyst can go through a continuous long-time photoreaction under the optimized conditions, without evident changes in the H₂ evolution rate (Fig. 3e). The total mole of produced H₂ (1.03 mmol) significantly exceeded that of the photocatalyst (0.23 mmol), confirming the photocatalytic behavior of InSe–TiO₂(0.015).

A schematic illustration of the charge separation and transfer in the InSe–TiO₂ heterostructure is shown in Fig. 4. Driven by visible light, electrons (e⁻) are excited from the VB to the CB of the InSe semiconductor and then likely transfer in three ways (Fig. 4a): (1) to the active sites on the InSe nanosheets; (2) to Pt deposited on the surface of InSe nanosheets; (3) to Pt located on the TiO₂ nanoparticles. In our study, there exists intimate contact among InSe, rutile and anatase in the InSe–TiO₂ heterostructure. The interface among the three phases can act as a rapid separation site for the photogenerated electrons and holes due to the difference in the energy levels of their conduction bands (CB) and valence bands (VB). As shown in Fig. 4b, the heterojunction and phase junction formed between InSe and rutile, and rutile and anatase favor the transfer of photoexcited electrons from InSe to rutile, and then to anatase, thereby effectively suppressing the recombination of photoexcited electrons and holes.

Fig. 4 (a) Schematic illustration of the charge separation and transfer in the InSe–TiO₂ heterostructure; (b) the conduction mechanism of photoexcited electrons and holes under visible light irradiation on the InSe–rutile–anatase surface heterojunction and phase junction.

In summary, a novel SDBS-assisted two-step method was used to prepare unusual cubic phase InSe nanosheets. The resulting InSe nanosheets were firstly used as catalysts for H₂ production under visible light irradiation. More importantly, P25 was introduced into these new photocatalysts and highly efficient InSe–TiO₂ (anatase and rutile) photocatalysts were produced. It could be found that the InSe–TiO₂(0.015) heterostructure exhibits the highest H₂ generation rate, which was found to increase by 4.1 times compared to that of pure InSe. The enhanced activity was mainly attributed to the interfacial transfer of photogenerated electrons from InSe to rutile, and then to anatase, leading to the effective charge separation and transfer on this semiconductor, which were evidenced by HRTEM, contact angle and photocurrent analysis. Under the optimized conditions, the apparent quantum efficiency was 25.5% at 450 nm with 2.5 wt% Pt as a cocatalyst. These findings may offer a promising route to obtain high performance nanohybrids for photocatalytic applications.

Acknowledgements

This work was supported by the Natural Science Foundation of China (NSFC, no. 21173021, 21231002, 21276026, 21471016 and 21271023), 973 Program (2014CB932103), and the 111 Project (B07012).

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Footnote

† Electronic supplementary information (ESI) available: The experimental details, results of SEM and photocatalytic H₂ production rates. See DOI: 10.1039/c5qi00075k

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