Fluorine-doped CuBi2O4 nanorod arrays for enhanced photoelectrochemical oxygen reduction reaction toward H2O2 production
Graphical abstract
Introduction
H2O2 is not only a kind of useful chemicals but also potential carbon-free energy carriers [1]. As a green oxidizing agent, it can be widely applied in chemical synthesis, pulp bleaching, waste water treatment, and disinfection [2]. When it is used as both an oxidant and a reductant, the maximum output potential theoretically achievable is 1.09 V in the one-compartment fuel cell, which is comparable to those of conventional H2 fuel cell (1.23) [3]. Moreover, in contrast to H2, H2O2 is freely soluble in water and easy to store and transport. It is anticipated that the global H2O2 demand will reach 6 million tons by 2024 [4]. At present, more than 95% H2O2 is synthesized through a well-established anthraquinone oxidation process [5]. However, this method involves high energy consumption, low efficiency, a large amount of wastewater discharge, and usage of expensive Pd-based catalysts [6]. Therefore, it is very necessary to develop green and mild techniques for the production of H2O2 [7].
PEC ORR for H2O2 production represents a kind of sustainable techniques because only sunlight, water, air, and electricity that can be obtained from renewable energy are required [8]. However, the related studies are still at its infancy due to the lack of appropriate photocathodes [9]. As a p-type ternary oxide semiconductor, CuBi2O4 is very promising for PEC ORR because of its many advantages [10]. Firstly, it is composed of low-cost and environmentally friendly Cu, Bi, and O elements. Secondly, it has a relatively narrow band gap (1.5–1.8 eV), corresponding to theoretically maximum photocurrent density of 19.7–29.0 mA cm−2 under AM1.5 solar illumination [11]. Thirdly, its conduction band position is more negative than reduction potential of two-electron O2 toward H2O2 (0.68 V versus standard hydrogen electrode). On the other hand, its positive flat-band potential (∼1.3 V vs reversible hydrogen electrode) can decrease ORR overpotential [12], which will provide decent photocurrent for ORR at the applied potential. Fourthly, it have been demonstrated that CuBi2O4 photocathode could show good stability for PEC ORR [13]. However, short carrier diffusion length (10–60 nm), low separation and transfer efficiency of carriers, and poor catalytic product selectivity of the CuBi2O4 photocathode limit its performance of PEC ORR toward H2O2 production [14], [15], [16].
A proper concentration of oxygen vacancy can efficiently inhibit the recombination of photogenerated electrons and holes and reduce band gap of semiconductor [17]. The electronegativity of F− ion (4.0) is obviously larger than that of O2− one (3.5). The doping of F− ion will weaken the metal − oxygen bonds in oxide semiconductors and release O2− ions to form oxygen vacancy [18]. In addition, the product selectivity of ORR may be improved because electron density of metal active sites will be changed after the introduction of F− ions. Up to now, several kinds of cations such as Gd3+, Ag+, and Co2+ have been used to enhance PEC activity of CBO [15], [19], [20], whereas its anionic doing and related PEC activity have never been reported.
Considering the influence of F− ion doping on the performance of CBO photocathode for PEC ORR toward H2O2 production and nanostructured photocathodes facilitating the transport and transfer of carriers, here we synthesized CBO nanorod array and F-CBO nanorod array photocathodes through a seed-mediated hydrothermal route and impregnation method, respectively. The influence of sizes of CBO nanorods on their PEC activity was investigated. The roles of F− ion doping in enhancing PEC ORR toward H2O2 production performance of CBO were revealed based on experimental results and DFT calculations. Our work demonstrates a new strategy for enhancing performance of oxide semicouductors for PEC ORR toward H2O2 production, namely, the combination of morphology controlling and F− ion doping engineering.
Section snippets
Materials
Sodium bismuth ethylenediaminetetraacetate (C10H12BiN2NaO8, >98%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, AR), copper sulfate pentahydrate (CuSO4·5H2O, ACS), copper nitrate trihydrate (Cu(NO3)2·3H2O, AR), sodium fluoride (NaF, ≥99%), sodium hydroxide (NaOH, AR), iron sulfate heptahydrate (FeSO4·7H2O, AR), sodium dodecylbenzenesulfonate (SDBS, C18H29NaO3S, AR), hexadecyl trimethyl ammonium bromide (CTAB, C19H42BrN, AR), polyvinylpyrrolidone (PVP, (C6H9NO)n, k29 − k32), polyethylene glycol
Results and discussion
The hole diffusion length of CBO was measured to be 10–60 nm [11]. Therefore, it is necessary to select appropriate surfactant to control the sizes of CBO nanorods synthesized through seed-mediated hydrothermal route in order to obtain high photocurrent density. As shown in Fig. 1a, the average transverse diameter of CBO nanorods obtained without use of surfactant is larger than 300 nm. After anionic surfactant SDBS was introduced into the reaction system, the mean diameter of CBO obviously
Conclusions
We have synthesized CBO nanorod with different transverse diameter arrays through a seed-mediated hydrothermal route. CBO nanorod with smaller transverse diameter arrays showed higher PEC activity due to facile transport of photogenerated carriers from the bulk to surface of the photoelectrode. On this basis, it was found that anonic fluorine doing could further enhance both PEC ORR toward H2O2 activity and selectivity of CBO. The experimental results combined with theoretic calculations
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.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 21673160 and 12075154) and startup funds of Shaoxing University.
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These authors contributed equally to this work.