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