Regular ArticleBiNV bond: A hole-transfer bridge for high-efficient separation and transfer of carriers
Graphical abstract
BiNV bond was formed between CxNy and BiVO4, which served as a hole-transfer bridge to prominently accelerate holes transfer driven by the interfacial electric field. The photocurrent of CxNy/BiVO4 photoanode reached 1.5 mA/cm2 at 0.6 VRHE. The photocurrent of CxNy/BiVO4 photoanode remained 83.3% of the initial photocurrent after 10h at 1.23 VRHE.
Introduction
Nowadays, environmental problems and the energy shortage seriously restrict the sustainable progress of human society. Harvesting energy directly from sunlight is an ideal solution for energy challenges with minimal environmental impact. Photoelectrochemical (PEC) technology emerges a huge potential for acquiring, storing and transforming solar energy, particularly by splitting water into oxygen (O2) and hydrogen (H2), which provides an extraordinary solution for the energy shortage [1]. The oxygen evolution reaction (OER) involving the transfer of four electrons has higher thermodynamic requirements than the hydrogen evolution reaction (HER) involving the transfer of two electrons [2]. In consequence, OER plays a decisive role for overall water splitting. To date, plentiful metal oxides such as TiO2 [3], Fe2O3 [4], WO3 [5], and BiVO4 have been used as photoanodes to enhance OER. Bismuth vanadate (BiVO4) is considered as one of the most promising photoanodes for solar driven water splitting, which is ascribed to the proper band gap (2.4 eV) for adequate absorption of visible light, sufficiently positive valence band edge for driving OER [6], [7], [8], and stability in neutral environment [9]. Nevertheless, efficiency of pristine BiVO4 for OER is terrible due to superabundant carriers recombination, poor charges transportation and sluggish water oxidation kinetics [10]. Various methods, including doping [11], [12], [13], [14], [15], morphological engineering [16], [17], [18], heterogeneous structure construction [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], and crystal plane engineering [29], [30], have been used to overcome inherent defects of pristine BiVO4 photoanode. Unfortunately, above-mentioned means were principally committed to the intrinsic charges separation and transportation of BiVO4 photoanode, whereas the sluggish OER at the photoanode/electrolyte interfaces scarcely was availably conquered by means of the above-mentioned strategies [31]. The loading of oxygen evolution catalysts (OECs) can significantly accelerate OER kinetics via extracting holes from BiVO4 photoanode to reduce the surface recombination of photo-induced carriers. Nevertheless, the pursuit of such an ideal OEC remains challenging. For example, precious metal oxides like IrO2 and RuO2, which exhibited excellent PEC performance, while their valuableness and scarcity seriously impeded their industrial applications [32]. There is growing concern about cobalt based OECs [33], [34] since amorphous Co-Pi has been developed as an effective and inexpensive OEC. Despite excellent surface holes transfer efficiency of Co-Pi /BiVO4 photoanode, the long-term stability remained a problem since that Co-Pi is easily dissociated in the phosphate buffer solution (PBS). Furthermore, other OECs such as CoOx [35], NiOx [36], CoOx-NiO [37],Co3O4 [38], FeCoOx [39], FeOx [40], CoMoO4 [41], FeOOH [42], and FeOOH/NiOOH [43], [44] have been widely coupled with BiVO4 photoanode to suppress surface carriers recombination, thereby facilitating improving OER kinetics. Unfortunately, aforementioned OECs containing metallic elements may be toxic, which hindered their practical applications. In addition, the enormous thickness of these metal-based OECs might prolong transport distance of holes, resulting in further recombination of photo-induced carriers. Therefore, cheap nonmetallic OECs with rapid holes transfer capability could be used as an alternative to promote PEC performance of BiVO4 photoanode.
g-C3N4 has a stable graphite-like layered structure under ambient conditions. It is composed of two dimensional p-conjugated polymeric structures with s-triazine or tri-s-triazine (s-heptazine) units connected by tertiary amines [45]. Bulk g-C3N4 was rarely used as the OECs due to tough hole transport caused by large size and feeble van der Waals force. Hence, researchers fabricated OECs derived from bulk g-C3N4 by various means and compounded them with BiVO4 photoanode to improve its PEC performance. For instance, Bi et al. [46] coupled ultrathin graphitic-phase C3N4 to BiVO4 photoanode via impregnation method. Zhang et al. [45] anchored three-dimensional carbon nitride nanonetworks to BiVO4 photoanode by electrostatic self-assembly. Mat Teridi [47] et al. obtained g-C3N4 film/BiVO4 photoanode, which was prepared by spin coating g-C3N4 onto the FTO layer-by-layer and then electrodepositing BiVO4. However, for all the above photoanodes, g-C3N4 and BiVO4 photoanode were coupled through the physical methods, which caused higher interfacial holes transfer barriers, thus resulting in poor PEC performance and barren stability. In order to effectively reduce the influence of interfacial resistance, the most effective strategy might be to construct the hole-transfer bridge by means of interface effects [48], [49], [50], [51].
In order to reduce the interfacial hole transfer barrier, it is necessary to establish the hole-transfer bridge between g-C3N4 and BiVO4 photoanode. Herein, carbon nitride hydrogel (CNH) was obtained by hydrothermal self-assembly based on our previous work [52]. Obtained CNH contains lots of unsaturated N, which could form chemical bonds with BiVO4 photoanode during annealing. The CxNy/BiVO4 photoanode with BiNV bond was prepared by coupling CNH onto BiVO4 photoanode during annealing. Obtained CxNy/BiVO4 photoanode exhibited prominent PEC performance and stability. After careful characterizations and theoretical calculations, BiNV bond at the interface of CxNy and BiVO4 photoanode can serve as a hole-transfer bridge and prominently accelerate holes extraction driven by the interfacial electric field (IEF), thus speeding up OER kinetics. Besides, BiNV bond could reduce the dissociation of metal ions due to the change of their coordination environment, thus improving stability of CxNy/BiVO4 photoanode. This work reveals the role of the hole-transfer bridge in improving PEC performance, in which the extraction of holes through BiNV bond accelerates transfer of holes and boosts the lifetime of holes.
Section snippets
Preparation of carbon nitride hydrogel (CNH) [52]
Dicyandiamide and urea, which have a molar ratio of 4 to 1, are heated in air. The calcination temperature procedure as follows: 30–300 ℃ (8 ℃ /min); 300–500 ℃ (2 ℃/min); 500–550 ℃ (1 ℃ /min); 550–550 ℃ (4 h); 550–30 ℃ (5 ℃/min); Bulk carbon nitride (BCN) was obtained by the above method. Subsequently, 0.5 g BCN was dispersed in 20 mL deionized water, which was transferred to a 50 mL Teflon-lined reactor for hydrothermal reaction, the suspension was obtained by keeping at 180 ℃ for 5 h.
Preparation of BiVO4 photoanode [38]
First,
Structure and morphology
Bismuth oxyiodide (BiOI) nanosheets was fabricated via electrodeposition on a F doped SnO2 (FTO) substrate, and then transformed into BiVO4 through vanadium intercalation [38]. As shown in Fig. S2, BiOI presented ultrathin sheet with a two-dimensional structure. After the introduction of vanadium and calcination, BiVO4 photoanode presented a worm-like three-dimensional nanoporous structure composed of interconnected pores with an average diameter of 100–200 nm (Fig. 1a-d), which provides
Conclusions
In summary, CxNy/BiVO4 photoanode with BiNV bond was fabricated via interfacial chemical coupling. The CxNy/BiVO4 photoanode with BiNV bond exhibited an evident cathodic shift (150 mV) of the onset potential and a high photocurrent density of 2.4 mA/cm2 (1.23 VRHE), which was elevated to 228% of pristine BiVO4 photoanode. Moreover, the photocurrent density of CxNy/BiVO4 photoanode remained 83.3% of the initial photocurrent density after 10 h at 1.23 VRHE. EIS, IMPS, TPV and TR-FL
CRediT authorship contribution statement
Yuhong Wang: Methodology, Investigation, Writing - original draft. Wenjun Jiang: Supervision, Resources, Conceptualization. Wei Yao: Validation, Investigation. Zailun Liu: Formal analysis, Writing - review & editing. Zhe Liu: Software, Visualization. Yajun Wang: Validation, Investigation. Lijie Shi: Writing - review & editing. Lizhen Gao: Supervision, Conceptualization.
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.
Acknowledgements
This work was supported by the Beijing Natural Science Foundation (2204100), National Natural Science Foundation of China (22002185), National Key Research and Development Program of China (2020YFA0710304), Civil Aerospace Technology Research Project (B0108) and Qian Xuesen Youth Innovation Foundation.
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