Efficient solar water splitting using a CdS quantum dot decorated TiO2/Ag2Se photoanode
Graphical abstract
Introduction
Photoelectrochemical (PEC) water splitting offers a green way of solar energy conversion by hydrogen production [[1], [2], [3], [4], [5]]. In a typical PEC cell, photoanodes exposed to photons of suitable energy generate charge carriers that facilitate the overall water splitting [[6], [7], [8], [9]]. Generally, the band positions of semiconductors relative to the redox values of water determine the conversion efficiency of solar photons to hydrogen (STH) [10,11]. Various research groups reported high STH conversion efficiencies while ensuring cost-competitiveness. TiO2 is the most promising photoanode owing to its high photocatalytic property, good resistance against photocorrosion, and low cost [[12], [13], [14], [15]]. However, the performance of TiO2 photoelectrode isn't remarkable due to its wide bandgap nature, demanding UV-radiation. Heterojunction materials with narrow bandgap semiconductors are promising to overcome this limitation, which is expected to broaden the light absorption and facilitate charge separation, and transportation [[16], [17], [18], [19], [20]]. Various narrow bandgap semiconductors were tested in PEC water splitting, including Bi2S3, CdS, CdSe, and CuInS2 which improve the visible light absorption, transfer charge carriers to their respective electrodes, and initiate redox reactions [[21], [22], [23], [24], [25]]. Among all, TiO2/CdS heterojunction has been widely studied for PEC water splitting since the narrow (~2.4 eV) bandgap of CdS enables the efficient absorption of the UV–Vis light. Moreover, it has strong reduction abilities for hydrogen evolution reaction (HER) owing to the favorable conduction band (CB) position and facile charge transportation from CdS to TiO2 [[26], [27], [28]]. Thus, by using TiO2/CdS NTs in 0.35 M Na2SO3 + 0.24 M Na2S, Banerjee, et al. achieved high current density (9.5 mA/cm2 @ 0.5 V Vs Ag-AgCl) [29]. Similarly, Wang et al. achieved 5.7 mA/cm2 at 0 V by using the FTO/TiO2/CdS photoanode [30].
Despite these achievements, the practical STH efficiency reported for TiO2/CdS is not appreciable due to its high charge carrier recombination. Other limitations include photocorrosion through the S−2 oxidation by photogenerated holes, underscoring the importance of hole-scavengers. Recent studies show the significance of ternary junctions to achieve high STH efficiency, including TiO2/Au/CdS, and CdS/TiO2/WO3 [[31], [32], [33], [34], [35], [36], [37]]. These ternary systems improve not only the optical absorption window to increase the carrier concentration but also charge separation, transportation, and photostability. Kim et al. demonstrated the excellent performance by CdS/TiO2/WO3 photoelectrode with 1.5 mA cm−2 at 0 V vs Ag/AgCl [38]. Similarly, by using the TiO2/Au/CdS ternary composite, Li et al. achieved high-efficiency PEC water splitting, and the photocurrent density was 4.07 mA cm−2 at 0 V [39]. Subsequently, Lee has achieved 14.9 mA cm−2 at 0 V by using TiO2/CdS/CdSe electrode, which is the highest value reported [40]. Despite these attempts, ternary systems suffer from a slow electron injection rate and a high exciton recombination rate [41]. In order to improve the electron transfer rate, in our present study, Ag2Se is investigated as a sensitizer, which is owing to its ability to absorb photons in the NIR region (bandgap of 1.1–1.6 eV). Also, the favorable band positions of TiO2/CdS electrode help for the hydrogen evolution reaction towards water splitting.
We report PEC water splitting using the triadic heterojunction composite photoanode TiO2/Ag2Se/CdS. The coupling of Ag2Se with TiO2/CdS enables transport and prevent the recombination of charge carriers. Thus, the photo-generated charge carriers in the heterostructure show an extended life when corelated to CdS/TiO2. For the fabrication of the composite electrode TiO2/Ag2Se/CdS, we prepared TiO2 by the doctor blade method, Ag2Se by the hydrothermal followed by drop-casting method, and a CdS QD layer was prepared on TiO2/Ag2Se by the SILAR method. A detailed study is conducted to unveil the relationship among the PEC activity of TiO2/Ag2Se/CdS, charge carrier transport, and band alignments of the electrode for hydrogen evolution. Herein we discussed the alteration between PEC activities specific to water splitting and sulfite oxidation. These evidences can in fact vary extensively, with sulfite oxidation at the photoanode qualifying a much larger hydrogen current which is not reasonable in a sustainable process.
Section snippets
Chemicals
Extra pure silver nitrate (AgNO3), selenium powder (Se), and potassium iodide (KI) were obtained from SDFCL. TiO2 paste (18NR-T) for electrode preparation was obtained from Dyesol. Sodium sulfide hydrate (Na2S.xH2O), cadmium acetate dihydrate (Cd(CH3COO)2.2H2O), sodium sulfite (Na2SO3), sodium sulfate (Na2SO4), and polyvinylpyrrolidone (PVP), were purchased from Sigma Aldrich. Ethanol, acetone, and methanol (CH3OH) were procured from Avra. Fluorine-doped tin oxide (FTO) conductive glass with
Result and discussion
Absorption spectra of the photoactive materials such as TiO2/CdS and TiO2/Ag2Se/CdS are shown in Fig. 1a. The bandgap (EB) of TiO2, Ag2Se, and CdS was calculated using the equation EB = 1240/λ, where λ is the wavelength at the absorption edge. Fig. S1a shows that TiO2 has strong absorption in the UV region with the band-edge at 390 nm which means the bandgap is 3.15 eV. For CdS QDs, the absorption band-edge is at 520 nm (Fig. S1b) which means the bandgap is 2.4 eV, while Ag2Se has a broad
Conclusions
We have prepared the TiO2/Ag2Se/CdS triadic heterojunction photoanode and demonstrated its enhanced PEC properties. The improved light absorption by this triadic electrode increased the charge separation and transportation properties. This heterojunction photoanode showed the best PEC performance with the photocurrent density as high as 24.6 mA cm−2 at 1.23 V, corresponding to the highest solar-to-hydrogen (STH) conversion efficiency (14% at 0.43 V vs RHE) in the presence of a hole scavenger (Na
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
B. R. M. and P. S. acknowledge the research scholarship from CSIR,India. C. S. and V. B. acknowledge the financial support under a bilateral project by DST and JSPS, Japan.
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