Full Length ArticleSurface modification of CuInS2 photocathodes with ruthenium co-catalysts for efficient solar water splitting
Graphical abstract
Introduction
The production, storage, and utilization of hydrogen have become one of the essential technologies as a result of the emergence of a hydrogen energy-based economy [1]. In the near future, hydrogen should be produced from non-carbon source with zero carbon dioxide emissions in order to achieve carbon neutrality [2]. Therefore, hydrogen generation from water using sustainable energy sources like solar light has received much interest. The photoelectrochemical (PEC) cell, which converts light energy to chemical energy, can be an ideal candidate for light-driven H2 production, but its present technology level is still too low to be industrialized because of poor solar to hydrogen (STH) conversion efficiency [3]. Therefore, various strategies in the material development such as energy level tuning of semiconductors, cocatalysts, doping, or passive layer deposition have been applied to PEC cells to improve the STH efficiency of PEC devices [4], [5], [6], [7].
In the PEC process, the behavior of semiconductor/liquid junctions is the most important principle of PEC cells. Photo-excited electron-hole pairs can be separated by built-in potential from depletion regions at liquid/semiconductor interfaces, and then the electron-hole pair drives a redox reaction in the electrolyte with chemical species. Thus, how to efficiently separate electron-hole pairs and how to spur redox reactions are key elements for improving the overall efficiency of PEC cells. For example, various p-n heterojunction structures were introduced to semiconductor photoelectrodes in the previous works [8], [9], [10]. The energy level difference between semiconductors induces a thermodynamic driving force to have efficient charge transfer or transportation. In other cases, a buried junction can be formed at the semiconductor surface by heteroatom doping for efficient charge separation [11], [12]. However, although a heterostructure improves charge separation at semiconductor/semiconductor interfaces, photoelectrodes without cocatalysts have poor catalytic activities for the hydrogen evolution reaction (HER) [13]. To increase the reaction kinetics of HER, the surface of heterojunction photoelectrodes is modified with various co-catalytic particles or layers. Nevertheless, although the modification with cocatalysts leads to the rise of the interface between cocatalyst-heterojunctions, the charge transfer in cocatalyst/semiconductors has not been investigated intensively compared to the case of semiconductor/semiconductor interfaces. Lin et al. [14] revealed that the cocatalyst/semiconductor junctions could be an 'adaptive' or 'buried' junction which would control the open circuit potential (Voc) of photoelectrodes. This implies that even the best electrocatalyst may not be suited for the co-catalytic modification of photoelectrodes when the charge transfer at the cocatalyst/semiconductor interface is taken into account. Therefore, the finding of a suitable cocatalyst/semiconductor combination and the investigation of its charge transfer behavior are important to improve the efficiency of cocatalyst-modified photoelectrodes.
In this work, a CuInS2/a-MoSx p-n junction was investigated for a model photoelectrode because p-type CuInS2 has a suitable bandgap and a band energy level for PEC H2 evolution. It is well known that the Voc of p-type semiconductors can be improved when it forms a p-n junction with n-type semiconductors such as CdS, TiO2, TaOx, MoS2, or MoSx [15], [16], [17], [18]. Recently, Zhao et al. [19] introduced a p-n CuInS2/Ni:a-MoSx photoelectrode with improved onset potential for HER. Our group also recently reported a p-CuInS2/n-CdS/n-(Ta,Mo)x(O,S)y heterojunction photoelectrode with enhanced charge transportation and catalytic activities for HER [20]. Moreover, both CuInS2 and a-MoSx can be economically synthesized via a solution process under non-vacuum conditions. However, the electrocatalytic activity of MoSx or related material (alloy or others) is still not enough for efficient H2 evolution compared to Pt metals [6]. Therefore, the a-MoSx surface is modified by noble metal cocatalysts, such as Pt, because Pt is well known as one of the best electrocatalysts for HER. In most previous reports, the photocathode for HER has been decorated with Pt catalysts. Recently, Ru (or RuOx) has been deposited on some photocathode materials (InP, Sb2Se3, Cu2O, (ZnSe)0.85(CuInGaSe2)0.15 etc.) as HER cocatalysts [21], [22], [23], [24], [25], but the origin of the difference for the Ru and Pt modification of photocathodes has not been fully disclosed.
In this research, CuInS2/a-MoSx is modified with Ru cocatalysts, and its PEC activities, an energy structure, and electrocatalytic activities are compared to Pt-modified photocathodes. Interestingly, CuInS2/a-MoSx photoelectrodes modified with Ru show similar PEC HER activities and higher half-cell applied bias photon to current conversion efficiency (HC-ABPE) when compared to Pt-modified photoelectrodes, although Pt has much better electrocatalytic HER activities than Ru. This unexpected high activity of a-MoSx/Ru composites on photocathodes is firstly introduced in this work for PEC HER application. Even though the Ru cocatalyst shows similar photoactivity to the Pt cocatalyst at 0 V vs. RHE, the higher HC-ABPE of CuInS2/MoSx/Ru photocathodes at anodic potential regions allows higher water splitting efficiency when combined with a solar cell or a photoanode. Physicochemical and (photo)electrochemical characterization were carried out to identify the properties of a-MoSx/Ru interfaces and reveal the origin of the high activity. Consequently, the CuInS2/a-MoSx/Ru has a more suitable energy structure for electron transfer/transportation through the barrier height at interfaces with the improved lifetime of electrons due to the higher work function of Ru. It also leads to a fast electron transient time and then, the high PEC activity for the CuInS2/a-MoSx/Ru photocathode.
Section snippets
Preparation of CuInS2 thin film photocathodes
CuInS2 thin films on Mo coated soda-lime glass were prepared according to the procedures reported previously [20]. In brief, Cu and In layers were subsequently deposited on the Mo coated soda-lime glass by an electrodeposition method. And then, to sulfurize it, the Cu-In layer was annealed in the 1 % H2S (N2 balance) gas atmosphere for 40 min at 520 °C. The final film was etched in 0.5 M KCN for 1 min.
Modification of CuInS2 surface with a-MoSx, Pt, or Ru
The surface modification of CuInS2 was carried out by electrodeposition with a three-electrode
Effects of surface modification on CuInS2 photocathodes
First, the bare CuInS2 and noble metal (Pt or Ru)-modified CuInS2 photocathodes were characterized. Fig. 1(a) shows the SEM image of a CuInS2 thin film. The CuInS2 film was confirmed to consist of hundreds of nanometer grains. After the electrodeposition of Pt or Ru, the metal nanoparticles of less than tens of nanometers are on the CuInS2 grain surface (Fig. 1(b) and 1(c)). The XRD was also measured for each photocathode (Fig. 4(a)). Peaks (27.98°, 32.16°, 32.48°, 37.25°, 46.36°, 54.66°,
Conclusion
The surface of CuInS2 was modified with a-MoSx, Pt, and Ru for PEC H2 evolution. The trend of PEC HER activities of surface-modified CuInS2 (MoSx/Ru MoSx/Pt MoSx Pt Ru) is not consistent with the trend of electrochemical HER activities of metal-modified layers (Pt MoSx/Pt Ru MoSx MoSx/Ru). The light absorption effect of CuInS2 surface layers or any synergetic improved electrochemical activity among MoSx, Pt, and Ru did not contribute to the inconsistency between PEC and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This study was supported by the institutional program of the Korea Institute of Science and Technology (KIST-2E31831). This work was supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2015M3D3A1A01064899). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R1I1A1A01073326).
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These authors have equal contribution.