Elsevier

Journal of Power Sources

Volume 492, 30 April 2021, 229667
Journal of Power Sources

Si-doped Cu2O/SiOx composites for efficient photoelectrochemical water reduction

https://doi.org/10.1016/j.jpowsour.2021.229667Get rights and content

Highlights

  • A facile method for Si doped and SiOx protected Cu2O films was developed.

  • Gradient Si doping led to downward band bending from the bulk to the surface.

  • Amorphous SiOx effectively restrained the self-redox reactions of Cu2O.

  • The photostability of the modified Cu2O has been enhanced by 3.8 times.

Abstract

Cuprous oxide (Cu2O) is an attractive photo-electrocatalyst for sustainable hydrogen production. The main issues limiting the use of Cu2O as a photocathode are the severe electron-hole recombination and photo-corrosion in aqueous electrolyte. Herein, to address the above issues, amorphous SiOx overlayers have been deposited on the surface of Cu2O as a protective cover using a facile dip-coating method. The overall PEC performances have been greatly improved, as evidenced by the increased photocurrent density (3.1 times of bare Cu2O) and improved stability (3.8 times of bare Cu2O). The structure and PEC characterizations prove that Si has been gradient-doped into the Cu2O, leading to a downward band bending from the bulk to the surface region, which promotes the charge separation and transfer efficiently. In addition, the amorphous SiOx layers serve as protection layers and prevent the Cu2O from direct contact with the electrolyte, maintaining a high photocurrent density over the stability test. Decoration with Pt catalyst has further boosted the photocurrent density to be 2.8 mA cm−2 at 0 V vs. RHE with better stability, indicating the synergistic effect of SiOx layer and Pt catalyst This work provides a facile strategy to improve the PEC activity and stability of Cu2O, which may be extended to other systems with photo-corrosion problems, such as BiVO4 or ZnO.

Introduction

In developing sustainable, ecologically benign hydrogen production technologies, solar driven photoelectrochemical (PEC) water splitting has received considerable attention [[1], [2], [3], [4]]. The key components of a PEC system are the photo-electrodes, which integrate solar energy harvesting and water electrolysis together [1,5]. Hydrogen and oxygen can be produced from photocathodes and photoanodes respectively, avoiding the necessity of gas separation [3].

The activity, stability and availability of the photo-electrode materials are critical issues limiting the practical applications of the PEC systems [5,6]. So far, a variety of n-type semiconductors such as TiO2, BiVO4 Fe2O3 and WO3 photoanode materials have been intensively studied [2,7,8]. However, limited research efforts were paid for developing efficient and stable photocathodes [[9], [10], [11]].

Cuprous oxide (Cu2O) is p-type semiconductor and is an attractive candidate for photocathodes. It has a direct band gap of ~2.0 eV with theoretical photocurrent estimated to be - 14.7 mA/cm2 [12,13]. And it possesses suitable band energy positions for water splitting, with conduction band located at 0.7 V negative of the H+/H2 redox potential [14]. Most importantly, copper element is non-toxic and naturally abundant. It can be processed by low cost and facile method, such as electro-deposition [13,15].

One of the main issue limiting the use of Cu2O as a photocathode is the severe electron-hole recombination [16,17]. The electron diffusion length of Cu2O ranges approximately from 20 to 100 nm [16]. However, the film thickness is required to be at least 1 μm to adsorb most of the sunlight [17]. Therefore, it is challenging to achieve efficient charge separation. Another issue is its photo-instability in aqueous solution, since Cu2O may be self-reduced to be metallic cupper or self-oxidized to be CuO during the PEC reactions [[18], [19], [20], [21]].

Great efforts have been made to address the first issue, such as morphology control to shorten the charge transfer path and adjust the exposed surface facets [19,[22], [23], [24]], doping elements to improve band structure and conductivity [25,26], as well as heterostructure construction to promote charge separation [24,[27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]]. Normally, different strategies need to be used in combination to achieve a considerable photocurrent density [19,34,40]. A highest photocurrent densities of 10 mA cm−2 was achieved by Gratzel group through combining the above strategies together, including fabrication Cu2O nanowire structure, p-n junction construction (Cu2O/ZnO), protective overlayer deposition (TiO2) and catalyst decoration (RuOx) [19].

However, the second issue is even more challenging. Cu2O would suffer severe photo-corrosion in aqueous solution under illumination [6,41]. As shown in Scheme S1, the redox potentials of Cu+ are positioned within the band gap of Cu2O, which may render Cu2O readily reduced to be metallic cupper or oxidized to be CuO by the photo-generated electrons and holes [18,19]. Deposition of a protective layer on the surface of Cu2O is a basic strategy, since it can isolate the Cu2O from the ions in the electrolyte and cut off the side reactions. Many attempts have been made to fabricate different protective layers (eg. Ga2O3 [42], TiO2 [13], NiOx [43], WO3 [44], carbon layer [17] polymer layer [18]) through ALD or multi-chemical synthesis approach, which required expensive equipment or complex process.

In comparison to other strategies, doping element to improve the PEC properties of Cu2O was less discussed. A series of heteroatoms (eg, Bi, Zn, Mn, Si and Ca) [26,[45], [46], [47], [48]] have been doped into Cu2O films by electro-deposition, magnetron sputtering or multi-chemical synthesis method. However, only a few studies focused on the doping effect toward the PEC water reduction [25,26,49]. Doping with Eu was found to increase the photocurrent density, which was attributed to the passivation of non-radiative recombination centers by the increased grain size and Eu containing secondary phase [49]. Recently, it was reported that Ni2+ could be doped into Cu2O lattice and acted as electron traps and accepted electrons from Cu and O, which promoted the charge separation and transfer in the bulk of Ni doped Cu2O photocathodes effectively, leading to increased photocurrent density (1.3 times of bare Cu2O) and stability (7.81 times of bare Cu2O) [25].

Herein, for the first time, amorphous SiOx protective layers were deposited on the surface of Cu2O through simple dip-coating method. Interestingly,it was found that gradient Si doping and protective layers deposition were achieved in one step synthesis with improved PEC performance, as evidenced by the increased photocurrent density (3.1 times of bare Cu2O) and improved stability (3.8 times of bare Cu2O). A series of structural and PEC characterizations suggest that, Si has been doped as p-type dopant by forming silicate (xCu2Oy SiO2), which adjusted the band structure and decreased charge transport resistance. Time-resolved photoluminescence (TRPL) verified the improved charge separation of Cu2O/SiOx evidenced by much smaller time constant for fast and slow decay components. On the other hand, the amorphous SiOx layers have been uniformly deposited on the surface of Cu2O, which improved the photo-corrosion instability effectively. In addition, it has been reported that the faster charge reaction kinetics will enhance the photocurrent density and photo-corrosion stability [[50], [51], [52]]. In order to check whether a catalyst can be further coupled with the SiOx layers, Pt, as an efficient co-catalyst, was deposited by electro-deposition on the surface of sample Cu2O/SiOx. Further decoration with Pt catalyst boosted the photocurrent density to 2.8 mA cm−2 at 0 V vs. RHE with better stability, indicating a synergistic effect of SiOx layer and Pt catalyst.

Section snippets

Materials

Copper sulfate (CuSO4˙5H2O), lactic acid (CH3COOH), sodium hydroxide (NaOH), titanium chloride solution (TiCl3), (3-Aminopropyl) trimethoxysilane (APTMS), Chloroplatinic acid hexahydrate (H2PtCl6˙6H2O) and sodium sulfate (Na2SO4) were purchased from Aladdin (AR grade), fluorine-doped tin oxide (FTO, 14 Ω/sq) was supplied by Advanced Election Technology Co., Ltd (Liaoning, China).

Preparation of bare Cu2O

The bare Cu2O films were prepared by electro-deposition with transparent FTO glass (F:SnO2, 1 cm╳3 cm) as the

Physicochemical characterization

Surface morphology, crystalline structure and elemental composition were examined by scanning electron microscope (JEOL/JSM7500) and high resolution transmission electron microscope (TECNAI G2 Spirit TWIN03061887). X-Ray photoelectron spectroscopy (XPS) was employed on an ESCALAB-MKII spectrometer system (VG Co., UK) to acquire the element valence information. X-ray Diffraction (XRD) patterns were recorded by Rigaku D/MAX-2500 powder diffractometer using Cu-Kα radiation (λ = 0.15418 nm). The

PEC measurements

In order to observe the dark and light currents simultaneously, the current-voltage (J-V) plots of bare and modified Cu2O samples were recorded under chopped illumination as shown in Fig. 1a. To ruling out the annealing effect, the bare Cu2O were annealing in N2 in the same condition without APTMS treatment, which is referred to as Cu2O–N2. It can be seen that the bare Cu2O produced a photocurrent of −0.9 mA cm−2 at 0 V vs. RHE in 1 M Na2SO4 electrolyte at pH = 7. After 600 °C annealing in N2,

Conclusions

In summary, this work found that the self-reduction in the bulk and self-oxidation in the surface led to the photo-corrosion in the PEC hydrogen evolution process. To address this issue, an amorphous SiOx layers were deposited on the surface of Cu2O through simple dip-coating method. Gradient Si doping and protective layers deposition were achieved in one step synthesis with greatly improved PEC performances as evidenced by the increased photocurrent density (3.1 times of bare Cu2O) and

CRediT authorship contribution statement

Wenwen Li: contributed to the main experimental work including photocathodes preparation, characterization, data processing and manuscript preparation. Hongyan Wang: analyzed the results and contributed to the mechanism explanation and manuscript revision. Zhe Sun: contributed to the data analysis and mechanism explanation. Quanping Wu: contributed to the data analysis and mechanism explanation. Song Xue: analyzed the results and contributed to the mechanism explanation and manuscript revision.

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.

Acknowledgment

The authors would like to thank Dr. Zhao Jian and Zhou Yiwei (Tianjin University of Technology) for the help and guidance in Faradaic Efficiency measurement. We gratefully acknowledge financial support from the National Natural Science Foundation of China (No.21503147, No. 21671148, No. 21576215).

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