Photo-electrochemical stability of copper oxide photocathodes deposited by reactive high power impulse magnetron sputtering
Graphical abstract
Introduction
Solar water splitting is considered a very promising technology for producing hydrogen as a source of renewable energy. A great amount of materials in the form of thin films and nanostructures have been investigated as photoanodes and photocathodes. Currently, suitable materials are greatly researched for the fabrication of a stable photocathode with a suitable band gap and good semiconductor properties in order to obtain maximum efficiency of such devices, i.e. photoelectrochemical (PEC) cells. If we consider photocathodes, promising candidates are for example p-type semiconductors involving Cu2O, CuO, CuFeO2, and InP [[1], [2], [3], [4]]. Cu2O is a nonstoichiometric abundant semiconductor with a direct band gap of Eg = 2 eV and a high absorption coefficient, which usually has a p-type conductivity [5,6]. It is known that Cu2O and CuO films are not stable due to photocorrosion in the electrolyte if they work like photocathode [2,4]. Unfortunately, as it has been stated in for example [7], the potential levels for self-reduction from Cu2O to Cu and self-oxidation of Cu2O to CuO are within the band gap. These facts make the reduction and oxidation energetically possible and thus can limit the stability of Cu2O material during the photoelectrochemical measurement and hydrogen generation process. This represents a serious problem of this material and hence further protection technics such as deposition of suitable barrier layers are needed as it was reported in [2,4]. Cu2O films or nanostructures are usually further coated by very thin films as Al:ZnO and TiO2 [4], which improves stability and electrochemical performance. CuO is another highly promising semiconductor material, having the optical band gap Eg = 1.5 eV, a high absorption coefficient and the p-type conductivity. This material was optimized as a stable photoanode for solar water splitting by adding Au − Pd nanostructures as co-catalyst and with a modified stoichiometry of CuO with an excess of oxygen [8]. Further research focused on the production of heterostructures based on CuO/Cu2O [9], showing considerably improved photocathode PEC performance and its stability. Other work also focused on the preparation of CuO and CuO/Cu2O by reactive magnetron sputtering [10,11]. Kunti et al [11] focused on the comparison of structural and physical properties of CuO/Cu2O thin films deposited by the DC magnetron and by reactive high power impulse magnetron sputtering (r-HIPIMS). However, the PEC performance of these films as photocathodes for solar water splitting was not studied in this work. All ip values in our text are in absolute values for a clear understanding of increasing/decreasing of the photocurrent.
Composite Cu2O/CuO thin film photocathodes can be deposited by electrodeposition of Cu on ITO coated glass and then by its anodic oxidation as reported in [12]. Films with a high PEC and relatively good stability were achieved for the Cu2O/CuO layer. Top CuO layer work as a protective layer, and simultaneously as a recombination inhibitor for electrons. The photocurrent density achieved for Cu2O/CuO layer was ip = 1.54 mA cm-2 at a potential 0 V vs. RHE, at a standard AM 1.5 (100 mW cm-2) illumination.
Cu2O and CuO photocathode films can be also prepared by a sol-gel method as reported in [13]. The best achieved photocurrent densities with a standard AM 1.5 (100 mW cm-2) illumination were ip = 0.28 mA cm−2 for Cu2O films and ip = 0.35 mA cm-2 at 0.05 V vs. RHE for CuO films. When NiOx photocatalyst was deposited on Cu2O films, the photocurrent density was increased on ip = 0.47 mA cm−2 at 0.05 V vs. RHE. Although both Cu2O and CuO films exhibited a certain decrease in the photocurrent, after some time the CuO films were more stable than the Cu2O films.
r-HIPIMS is a very promising technology for the preparation of various semiconducting oxide thin films such as e.g. TiO2, Fe2O3, ZnO, and WO3 [[14], [15], [16], [17], [18], [19], [20], [21], [22]]. In this technology, a large fraction of sputtered particles is ionized and there is a very high plasma density during the HIPIMS pulse. An intensive ion bombardment of the substrate exists during the pulse, but the average heating flux on the substrate is relatively low. These phenomena can improve the properties of deposited films such as higher crystallinity, better adhesion, etc. and high material density can be achieved.
In this work, we focused on the deposition of copper oxide thin films on glass substrates, with SnO2:F conducting layer by r-HIPIMS, using argon and oxygen gas as the working and reactive gases, respectively. Deposited copper oxide films were verified as the p-type semiconductors by photoelectrochemical measurement, showing photocatalytic activity in the cathodic region. Optimal r-HIPIMS deposition conditions for achieving the best film parameters were investigated.
Section snippets
Experimental
Copper oxide thin films were deposited on glass substrates with fluorine doped tin oxide (FTO) conductive electrodes by using r-HIPIMS. The plasma reactor chamber, with a volume of 30 l, was continuously evacuated by the turbomolecular (500 l s-1) as a secondary pump and by the rotary vane pump (25 m3 h−1) as a primary pump on the base pressure p = 1∙10-4Pa. Argon and oxygen were fed into the reactor chamber via gas flow controllers. A planar unbalanced magnetron with a circular magnetron
Results and discussion
Fig. 1a and b shows the voltage and current waveforms of applied pulsed magnetron discharge. It can be concluded from Fig. 1a, b and Table 1, that in order to reach a sufficient high pulse discharge current Ip> 30 A on the magnetron cathode, a relatively high pulse discharge voltage Ucathode≈ 900 V must be applied. This value of Ip ≈ 30 A corresponds in our case to the pulse discharge current density on the magnetron cathode of jD≈ 1.6 A cm-2. Such value of the obtained jD should be high enough
Conclusion
Copper oxide semiconductor thin films were deposited by r-HIPIMS on FTO glass. As-deposited copper oxide films had various crystalline structures, containing Cu2O and CuO phases. The crystalline structure of films was further improved after the postdeposition annealing at the temperature 550 °C for 10 min. Deposited copper oxide films behaved as p-type semiconductors, yielding relatively high photocurrents in the cathodic region during the PEC measurement. The influence of the oxygen flow on
Acknowledgments
This work was supported by the Grant Agency of the Czech Republic17-20008S and by the Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 - CZ.02.1.01/0.0/0.0/16_019/0000760).
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