Enhanced Photoelectrochemical Water Splitting of In₂S₃ Photoanodes by Surface Modulation with 2D MoS₂ Nanosheets

Abstract

Photoanodes with ample visible-light absorption and efficient photogenerated charge carrier dynamics expedite the actualization of high-efficiency photoelectrochemical water splitting (PEC-WS). Herein, we fabricated the heterojunction nanostructures of In₂S₃/MoS₂ on indium-doped tin oxide glass substrates by indium sputtering and sulfurization, followed by the metal–organic chemical vapor deposition of 2D MoS₂ nanosheets (NSs). The photocurrent density of In₂S₃/MoS₂ was substantially enhanced and higher than those of pristine In₂S₃ and MoS₂ NSs. This improvement is due to the MoS₂ NSs extending the visible-light absorption range and the type-II heterojunction enhancing the separation and transfer of photogenerated electron–hole pairs. This work offers a promising avenue toward the development of an efficient photoanode for solar-driven PEC-WS.

1. Introduction

Photoelectrochemical water splitting (PEC-WS) is propitious to produce hydrogen (H₂) to satisfy the world’s energy demands and environmental challenges since H₂ gained its importance as an ideal carbon-free energy carrier and an alternative to fossil fuels in addition to its key roles in hydrogenation, petroleum refineries, and fertilizers [1,2]. However, the slow anode oxygen evolution reaction (OER) impedes the applicability of PEC-WS on a large scale [1]. As a solution, semiconductor photoanodes have gained research attention and become popular in solar energy conversion.

In₂S₃, an n-type semiconductor, has attracted considerable attention due to its relatively narrow band gap of 2.0–2.3 eV for visible-light utilization, high photosensitivity, and chemical stability [3,4]. However, pristine In₂S₃ shows a relatively low PEC efficiency owing to its fast charge recombination inside the bulk and on the surface. Li et al. [5] reported β-In₂S₃ nanosheets (NSs) with a photocurrent density of 35.7 μA/cm². Yao et al. [6] showed a PEC performance around 15 μA/cm² by In₂S₃ NSs arrays. The formation of a heterojunction with an appropriate semiconductor can effectively minimize this drawback, resulting in improved charge separation and transfer and enhanced optical absorption.

Among the semiconductors that form favorable energy band alignments with In₂S₃, 2D-layered MoS₂ can be a promising candidate because of its tunable bandgap energy, excellent photoexcitation, good chemical stability, and earth abundance [7,8]. It also exhibits tunable bandgaps from ~1.2 eV for the indirect gap of the bulk form to ~1.9 eV for the direct gap of the monolayer and a relatively high mobility (a few hundred cm²/Vs) [8,9,10]. The photoelectrochemical (PEC) activity of 2D MoS₂ is also strongly affected by its architecture standing vertically on the substrate, which provides additional conductive channels for photoexcited carriers [11].

Information on the PEC-WS of In₂S₃ heterojunctioned with vertically-standing 2D MoS₂ NSs is limited. Singh et al. [12] reported the photocatalytic reaction of In₂S₃ functionalized with MoS₂ nanoflowers. Liu et al. [13] showed that MoS₂ nanodot-decorated In₂S₃ nanoplates can be applied for PEC but at a low photocurrent level of 1 μA cm⁻². Sun et al. [14] later successfully applied a one-pot strategy for growing In₂S₃/MoS₂ with an anodic photocurrent of 0.06 mAcm⁻² at 0.341 V vs. RHE; nevertheless, the performance still has room for improvement. In the present study, we report vertical 2D MoS₂ NSs on In₂S₃ nanoparticles (NPs) as an alternative anodic choice to OER for significantly improved PEC-WS. The heterojunction effect of In₂S₃/MoS₂ was demonstrated through systematic PEC analysis and photo-excited carrier transfer properties across In₂S₃/MoS₂.

2. Materials and Methods

In₂S₃ was synthesized on indium-doped tin oxide (ITO) glass substrates via sputtering at 30 W power under a pressure of 3 mTorr for 40 s, followed by sulfurization under a H₂S flow rate of 200 standard cubic centimeters per minute (SCCM) at 300 °C for 30 min under a pressure of 10 Torr. MoS₂ NSs were then decorated on the In₂S₃/ITO and bare ITO substrates at 300 °C for 8 min under a pressure of 1 Torr by using a metal–organic chemical vapor deposition (MOCVD) system with Mo (CO)₆ and H₂S gas (5 vol. % in balance N₂) as Mo and S precursors, respectively. Mo (CO)₆ was vaporized at 20 °C and delivered into a quartz tube using Ar gas of 20 SCCM. The flow rate of H₂S gas was 65 SCCM.

The morphology of the samples was characterized via scanning electron microscopy (SEM, Hitachi S-4800). Their crystal structures were investigated by micro-Raman spectroscopy using an excitation band of 532 nm and a charge coupled device detector. Their optical property was characterized by UV–visible (UV–Vis) spectroscopy (Shimadzu UV-2600). PEC cells were fabricated on 1 × 2 cm² ITO glass substrates. PEC characterization was performed using a three-electrode system with a Pt wire mesh as the working electrode and Ag/AgCl as the reference electrode. The electrolyte solution comprised 0.3 M KH₂PO₄ with KOH. The light source was a 150 W Xe arc lamp that delivers 100 mW/cm² simulated AM 1.5 G irradiation. PEC measurements, including linear sweep voltammograms (LSVs) recorded using a sourcemeter (Keithley 2400), and electrochemical impedance spectroscopy (EIS) were conducted using an electrochemical analyzer (potentiostat/galvanostat 263A) in a three-electrode reactor. EIS analysis was performed at a bias of 0.6 V while varying the ac frequency from 100 kHz to 100 mHz. The IPCE of the electrode structure was measured using a grating monochromator within the excitation wavelength range of 300–800 nm. The hydrogen gas products were analyzed using a YL 6500 gas chromatograph (Young In Chromass, Republic of Korea) equipped with a flame ionization detector and a thermal conductivity detector.

3. Results

Figure 1a–c exhibit the top- and tilted-view SEM images of In₂S₃, 2D MoS₂, and In₂S₃/MoS₂. In₂S₃ possessed a layer of NPs on the ITO substrate with the thickness of ~50 nm (Figure 1a). This particle network resembled a uniform structure that acted as a seed layer for MoS₂ growth. Vertically standing MoS₂ NSs were uniformly generated on the ITO substrate (Figure 1b) and In₂S₃ (Figure 1c). The morphological characteristics of MoS₂ on the entire surface of In₂S₃ appeared as vertically aligned NSs with a height of ~180 nm that developed by controlling the concentration ratio of Mo⁴⁺ to S²⁻ during the MOCVD reaction [11]. The adequate S²⁻ environment encouraged the growth of vertically-standing MoS₂ NSs on In₂S₃.

The crystal structures of the samples (pristine In₂S₃, pristine MoS₂, and In₂S₃/MoS₂) were investigated by Raman spectroscopy. Our previous study revealed that the MoS₂ NSs are few-layer 2D structures [2,11], which was also confirmed by Raman spectra (Figure 1d). In₂S₃ exhibited Raman peaks around 255 and 297 cm^–1, corresponding to β-In₂S₃ [15], and two typical peaks of 2D-layered MoS₂, corresponding to E¹_2g and A_1g modes [16] for the in-plane vibration of S and Mo atoms and the out-of-plane vibration of S atoms, respectively. This finding indicates the successful growth of MoS₂ NSs on In₂S₃.

The optical properties evaluated by the UV–Vis absorbance analyses were strongly influenced by the presence of 2D MoS₂ as shown in Figure 2a. Pristine MoS₂ NSs exhibited an absorption edge of ~750 nm and two prominent absorption peaks at ~610 and ~665 nm, known as B and A excitons, respectively, which are correlated with direct excitonic transitions at the Κ point of the Brillouin zone [17]. Compared with pristine In₂S₃, the In₂S₃/MoS₂ heterostructure showed improved absorbance attributed to the enhanced surface scattering of MoS₂ 2D morphology. This result suggests a substantial improvement in the light absorption of the heterostructure with the decoration of MoS₂ NSs. The optical bandgap energies (Figure 2b) calculated according to the Tauc equation [18] were 2.12 (In₂S₃), 1.77 (MoS₂), and 1.78 eV (In₂S₃/MoS₂) as estimated from the intercept of the linear portion of the Tauc plot. The similar bandgaps of MoS₂ and In₂S₃/MoS₂ amplified the ability of MoS₂ for light absorption.

The PEC performance was evaluated by LSVs under simulation with AM 1.5 G illumination as depicted in Figure 3a. Compared with dark current curves, all the samples exhibited photocurrent attributed to the PEC reaction. The photocurrent density of pristine In₂S₃ was 0.097 mA/cm², and that of In₂S₃ heterojunctioned with MoS₂ was significantly improved up to 1.28 mA/cm² at 1.23 V vs. RHE, which was higher than that of pristine MoS₂ (0.85 mA/cm² at 0.93 V vs. RHE). The enhanced PEC properties can be attributed to the effective electron–hole separation and transfer through the heterojunction.

Figure 3b shows the photoconversion efficiencies (η) of the samples estimated using the following equation [19]:

η = J(E_o − V_app)/P_light,

where J is the photocurrent density (mA/cm²) at the applied potential, E_o is the standard reversible potential (1.23 V), V_app is the applied potential, and P_light is the power density of illumination.

In₂S₃/MoS₂ showed an η of 0.75% at 1.23 V vs. RHE which was substantially higher than that of pristine In₂S₃ (~0.1%). Figure 3c,d show the Nyquist plots of the EIS fitted using a simplified Randles circuit (inset in Figure 3d). In₂S₃/MoS₂ exhibited smaller EIS semicircles, indicating a lower charge transfer resistance (R_ct) of 1727 Ω under illumination than the pristine samples (16,350 Ω and 2308 Ω for In₂S₃ and MoS₂, respectively). This result suggests that the heterojunction significantly improved the charge transfer efficiency.

A thorough study was performed using IPCE and H₂ evolution to understand how the heterojunction enhanced the PEC performance. In₂S₃/MoS₂ exhibited a peak value at ~440 nm and significant IPCE enhancement in the 600–750 nm region (Figure 4a), which was affected by the surface modulation with 2D MoS₂ NSs. Hydrogen evolution from the dark cathode (Pt) was measured at 0.5 V versus Ag/AgCl using a three-electrode configuration for 30 min. The amount of produced H₂ was significantly increased by the In₂S₃/MoS₂ heterojunction as shown in Figure 4b, suggesting that the photocurrent was attributed to the WS. In₂S₃/MoS₂ formed a staggered heterojunction (Figure 4c) [2,20], which was effective in separating and subsequently transferring photogenerated electrons and holes to the cathode (Pt electrode) through In₂S₃ and onto the anode (MoS₂), leading to a boosted PEC performance.

Figure 5a shows the photocurrent density–time (J-t) curves of all of the photoanodes over 30 min. The photocurrent of In₂S₃/MoS₂ stabilized after an initial decay period of ~400 s, which was similar to that of pristine MoS₂. The initial photocurrent decay was attributed to recombination of the photogenerated holes with electrons [11]. After PEC reaction, the peak positions of Raman and UV–Vis absorption spectra of In₂S₃/MoS₂ did not change, indicating no significant structural change. However, the full width at half maximum of Raman peaks slightly increased after reaction. Our recent study showed that MoS₂ NSs are susceptible to subtle morphological changes due to the decomposition of MoS₂, mainly the loss of S elements during PEC reaction [11].

4. Conclusions

In this study, 2D MoS₂ NSs were vertically grown on a layer of In₂S₃ NPs using MOCVD. In₂S₃/MoS₂ exhibited up to more than 13 times and 1.5 times higher photocurrent densities than pristine In₂S₃ and pristine MoS₂, respectively, because of the extended visible-light absorption range and the efficient separation and transportation of the photogenerated carriers by the type-II heterojunction. The formation of a heterojunction with MoS₂ NSs led to the maximum photoconversion efficiency of In₂S₃/MoS₂ up to 0.75% at 1.23 V vs. RHE. This work suggests that the In₂S₃/MoS₂ heterojunction is one of the feasible photoanodes for efficient PEC-WS.