Plasmonic Pd Nanoparticle- and Plasmonic Pd Nanorod-Decorated BiVO4 Electrodes with Enhanced Photoelectrochemical Water Splitting Efficiency Across Visible-NIR Region
© The Author(s). 2016
Received: 9 March 2016
Accepted: 18 May 2016
Published: 4 June 2016
The photoelectrochemical (PEC) water splitting performance of BiVO4 is partially hindered by insufficient photoresponse in the spectral region with energy below the band gap. Here, we demonstrate that the PEC water splitting efficiency of BiVO4 electrodes can be effectively enhanced by decorating Pd nanoparticles (NPs) and nanorods (NRs). The results indicate that the Pd NPs and NRs with different surface plasmon resonance (SPR) features delivered an enhanced PEC water splitting performance in the visible and near-infrared (NIR) regions, respectively. Considering that there is barely no absorption overlap between Pd nanostructures and BiVO4 and the finite-difference time domain (FDTD) simulation indicating there are substantial energetic hot electrons in the vicinity of Pd nanostructures, the enhanced PEC performance of Pd NP-decorated BiVO4 and Pd NR-decorated BiVO4 could both benefit from the hot electron injection mechanism instead of the plasmon resonance energy transfer process. Moreover, a combination of Pd NPs and NRs decorated on the BiVO4 electrodes leads to a broad-band enhancement across visible-NIR region.
Solar hydrogen generation through photoelectrochemical (PEC) water splitting offers an efficient and sustainable solution to the global energy problem [1–5]. Recently, BiVO4 has emerged as a promising material for PEC water splitting due to its photoactivity in visible light region . However, BiVO4 photoanodes suffer from rapid charge carrier recombination, slow water oxidation kinetics, and insufficient photoresponse in the spectral region with energy below the band gap of 2.4 eV , which limit its water splitting efficiency. Therefore, strategies such as doping [7–12], nanostructuring [13–18], and loading of oxygen evolution catalysts (OECs) [7, 9, 19, 20] have been adopted to improve the water splitting efficiency of BiVO4.
As reported, several dopants, such as W, Mo, and P, are reported to improve the PEC performance of BiVO4 [7–12]. Doped BiVO4 exhibits the improved carrier density, enhanced conductivity, or even increased hole diffusion length, thus resulting in enhanced PEC properties. Furthermore, the short diffusion length of photoexcited charge carriers is the main reason for the dominant electron-hole recombination in the bulk of BiVO4. To address this issue, the diffusion length for charge carriers can be shortened by nanostructuring, thereby reducing bulk recombination [13–18]. To increase the water oxidation kinetics, research efforts have been placed on the loading of the OECs on BiVO4 [7, 9, 19, 20]. Among these OECs, the Co-Pi and FeOOH catalysts can lead to a negative-shift of onset potentials of water oxidation and effectively enhance the magnitude of photocurrent [7, 9, 19]. However, through these strategies, the enhanced photoactivity of BiVO4 has only been achieved in the spectral region with energy above the band gap. To improve the PEC water splitting efficiency of BiVO4 in the spectral region with energy below the band gap or even in the near-infrared (NIR) region, whose energy accounts for 56.3 % of that of the solar spectrum , still remains a big challenge.
Recently, a new approach involving metal nanostructures in enhancing the photoactivity of TiO2 in the spectral region with energy below the band gap via plasmonic effect has received much attention [22–24]. Surface plasmon resonance (SPR) is an intrinsic property of metal nanostructures, in which the oscillation frequency is highly sensitive to their shape and size of the metal nanostructures as well as the dielectric constant of the surrounding environment [25–30]. The plasmonic metal nanostructures localize the optical energy by SPR and enhance the photoactivity of semiconductors through either near-field electromagnetic enhancement or hot electron injection [22–24]. For example, Hsu et al. reported that the performance of Au nanostructure-decorated TiO2 nanowires for PEC water splitting was enhanced across entire UV-visible region . Although an enhancement in the PEC water splitting efficiency was also observed on BiVO4 electrodes with plasmonic metal nanostructures, such as Au and Ag [31–33], there are no reports that the photoactivity of BiVO4 can be improved in the spectral region with energy below the band gap or even in the NIR region by exploiting plasmonic metal nanostructures.
This work reports that the enhancement of PEC water splitting efficiency can be effectively extended into the visible-NIR region by the combination of Pd nanoparticles (NPs) and nanorods (NRs) with BiVO4 electrodes. The mechanisms of activity enhancement both in the visible and NIR regions have been discussed.
Preparation of BiVO4 Electrodes
The BiVO4 electrodes were prepared as previously reported procedure . Solutions for electrodeposition were prepared by dissolving 10 mM Bi(NO3)3 (98 %, Alfa Aesar) in a solution of 35 mM VOSO4 (97 %, Sigma Aldrich) at pH < 0.5 with HNO3 (65 %, Acros Organics). Then 2 M CH3COONa (≥99.0 %, Alfa Aesar) was added, raising the pH to ∼5.1, which was then adjusted to pH 4.7 with a few drops of HNO3. Acetate serves to stabilize other insoluble Bi (III) ions at pH 4.7. This mildly acidic pH condition must be used because at pH lower than 2 where Bi(III) is soluble, no film can be formed while at pH higher than 5, V (IV) precipitates from solution. A three-electrode cell was used for electrodeposition, with an fluorine-doped tin oxide (FTO, 8 Ω/□, Hartford Glass Co.) coated glass substrate as working electrode, a Ag/AgCl (4 M KCl) as reference electrode and a platinum foil as counter electrode. A potentiostat (Sloartron SI 1287) was used for electrodeposition. Deposition of amorphous Bi–V–O films was carried out potentiostatically at 1.9 V vs Ag/AgCl (4 M KCl) for 5 min at 70 °C (ca. 2 mA cm−2). The as-deposited films were converted to crystalline BiVO4 and amorphous V2O5 by annealing at 500 °C for 1 h in air, and pure BiVO4 was achieved by dissolving the V2O5 in 1 M KOH under stirring for 20 min.
Synthesis of Pd NPs and NRs
A simple but very efficient method that uses CO as reducing agent to synthesize Pd NPs and NRs has been developed and will be reported elsewhere. In brief, before the synthesis, 80 mM/L NaCl (≥99.0 %, Sigma Aldrich) and 40 mM/L PdCl2 (≥99.9 %, Sigma Aldrich) was dissolved in 50 mL CH3OH (≥99.9 %, Sigma Aldrich) to form Na2PdCl4 for preparation. Then, a flask was added 210-mg polyvinylpyrrolidone (K 30, average Mw 40,000, Sigma Aldrich) and 100 mL of 3.876 mM Na2PdCl4. To produce Pd NRs or NPs, 0.2- or 4.0-mL ultra-purity water (Millipore Milli-Q purification system, resistivity >18 MΩ cm) was added respectively. High-purity N2 with a flow rate of 200 mL/min was used for deaeration of the solution for 1 h under intense stirring. Then, high-purity CO with a flow rate of 200 mL/min was introduced to the flask under gentle stirring for 10 min. After that, a common balloon linked to the flask was blown up with CO to keep the reducing atmosphere for 4 h. During the whole synthesis process, the reaction system was keep at 30 °C.
Preparation of Pd Nanostructure-Decorated BiVO4 Electrodes
Pd nanostructure-decorated BiVO4 electrodes were prepared using an electrophoretic deposition process. A field of 15 V cm−1 was applied to deposit Pd nanostructures using a FTO counter electrode held at positive potentials relative to the working electrode (BiVO4 electrode). For the preparation process of Pd NP-decorated BiVO4 (NP-BiVO4) and Pd NR-decorated BiVO4 (NR-BiVO4) electrodes, the depositing time are 3 and 1 min, respectively. To prepare Pd NP- and NR-decorated BiVO4 (NP-NR-BiVO4) electrodes, Pd NRs were firstly deposited onto the surface of BiVO4 for 30 s and then Pd NPs were deposited for 90 s. After this deposition process, Pd nanostructure-decorated BiVO4 electrodes were rinsed by distilled water and then dried under atmospheric environment for 2 h.
X-ray diffraction (XRD) was carried out using a Bruker AXS D8 Advance powder X-ray diffractometer with a Cu Kα (λ = 1.5418 Å) radiation source to confirm the purity and crystallinity of the prepared BiVO4 electrodes. UV-vis spectra were obtained using a Cary 5000 UV-vis-NIR spectrophotometer in diffuse reflectance mode. Scanning electron microscopy (SEM) images were collected with a field-emission SEM (Hitachi Situation-4800). Elemental compositions were determined by Energy Dispersive X-ray Spectroscopy (EDX) using the EDX detector on the Hitachi Situation-4800. Transmission electron microscopy (TEM) images were obtained with a JEOL 2100F at an accelerating voltage of 200 kV.
Results and Discussion
Figure 3c shows LSVs of bare BiVO4 and NP-BiVO4 electrodes collected in the potential range from 0.2 to 1.4 V under dark condition. There is a decrease in current on NP-BiVO4 electrode in the potential ranges between 1.1 and 1.4 V, which could rule out the catalytic effect of Pd NPs on water splitting. Such a current decrease could be ascribed to the fact that the coverage of Pd NPs on the surface of BiVO4 reduces the interfacial surface area between BiVO4 and the electrolyte, and thereby hindering the water oxidation.
The photoactivity of Pd NPs and NRs decorated BiVO4 electrodes for PEC water splitting can be effectively enhanced across the visible-NIR region. The enhanced photoactivity in both visible and NIR regions was ascribed to the hot electron injection upon SPR excitation of Pd NPs and NRs, respectively. The present work aimed at enhancing the photoactivity of BiVO4 electrodes and at extending from the visible to the NIR region inspired us to design other plasmonic metal nanostructure-decorated semiconductor photoelectrodes for more effective utilization of the solar spectrum.
EDX, energy dispersive X-ray spectroscopy; FDTD, finite-difference time domain; LSVs, linear scanning voltammograms; NIR, near-infrared; NP-BiVO4, Pd NPs decorated BiVO4; NP-NR-BiVO4, Pd NPs and NRs decorated BiVO4; NPs, nanoparticles; NR-BiVO4, Pd NRs decorated BiVO4; NRs, nanorods; OECs, oxygen evolution catalysts; PEC, photoelectrochemical; PRET, plasmon resonance energy transfer; SEM, scanning electron microscopy; SPR, surface plasmon resonance; TEM, transmission electron microscopy; XRD, X-ray diffraction.
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