D. HONGXING, L. QIUPING: ENHANCED PHOTOELECTROCATALYTIC ACTIVITY OF A Pt-MODIFIED ... 769–775 ENHANCED PHOTOELECTROCATALYTIC ACTIVITY OF A Pt-MODIFIED Ti/TiO 2 /BiVO 4 PHOTOELECTRODE FOR WATER OXIDATION IZBOLJ[ANJE FOTOKATALITI^NE AKTIVNOSTI S PLATINO MODIFICIRANE Ti/TiO 2 /BiVO 4 FOTOELEKTRODE ZA OKSIDACIJO VODE Dong Hongxing, 1* Liu Qiuping 2 1 College of Electromechanical Engineering, Hangzhou Polytechnic, Hangzhou, Zhejiang, China, 2 College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang, China Prejem rokopisa – received: 2020-02-15; sprejem za objavo – accepted for publication: 2020-07-15 doi:10.17222/mit.2020.030 Pt was successfully deposited on the Ti/TiO2/BiVO4 photoelectrode surface using pulsed potential electrodeposition from an H2PtCl6 aqueous solution. The photoelectrodes were characterized with the XRD, SEM, EDS and XPS techniques. The prepared Pt-modified Ti/TiO2/BiVO4 electrode exhibited both an enhanced visible-light absorption and higher photoconversion efficiency for water oxidation under light irradiation. The photocurrent density on the Pt-modified Ti/TiO2/BiVO4 photoanode was about 0.3845 mA/cm 2 at 0.6 V (vs SCE), and the photoconversion efficiency was 8.4 times higher than that of Ti/TiO2 and 2.1 times higher than that of the Ti/TiO2/BiVO4 photoanode. Electrochemical impedance spectroscopy was performed to confirm the charge transfer at the photoelectrode surface. It provided a strategy for the design of composite films for more efficient photoelectrochemcial water-splitting applications. Keywords: water oxidation, Ti/TiO2/BiVO4 photoanode, photoelectrochemical, nanotube Avtorja ~lanka sta uspe{no, s postopkom pulzno-potencialnega elektri~nega nanosa iz vodne raztopine H2PtCl6, nanesla platino (Pt) na povr{ino Ti/TiO2/BiVO4 fotoelektrode. Izdelane fotoelektrode sta okarakterizirala z rentgensko difrakcijo (XRD), vrsti~no elektronsko mikroskopijo (SEM), energijsko disperzijsko spektroskopijo (EDS) in rentgensko fotoelektronsko spektroskopijo (XPS). Pripravljene s Pt modificirane Ti/TiO2/BiVO4 elektrode imajo izbolj{ano absorpcijo vidne svetlobe in so tudi fotokonverzijsko u~inkovitej{e pri obsevanju s svetlobo za vodno oksidacijo. Fototokovna gostota s Pt modificirane Ti/TiO2/BiVO4 fotoanode je okoli 0,3845 mA/cm 2 pri 0,6 V (v primerjavi z SCE), in fotokonverzijska u~inkovitost je bila 8,4-krat vi{ja kot pri Ti/TiO2 in 2,1-krat vi{ja kot pri Ti/TiO2/BiVO4 fotoanodah. Elektrokemijska impedan~na spektroskopija je potrdila proces prenosa naboja na povr{ini fotoelektrode, kar omogo~a novo strategijo oblikovanja kompozitnih tankih plasti (filmov) za u~inkovitej{o fotoelektrokemi~no cepitev vode (na vodik in kisik). Klju~ne besede: oksidacija vode, Ti/TiO2/BiVO4 fotoanoda, fotoelektrokemija, nanocevke 1 INTRODUCTION Photocatalytic water splitting under visible light, namely the solar water splitting, has attracted great atten- tion. 1 Many kinds of semiconductors such as TiO 2 , 2,3 WO 3 4 and so on are considered to be the most promising materials with light-absorption ability. Various TiO 2 nanostructures including nanowires, nanofilms and nanofibers have been used as the photoanodes for solar water splitting. 5–7 However, the bandgap of TiO 2 shows that TiO 2 has almost no visible-light-absorption ability. 8 There are two main ways to solve this problem. One is to dope TiO 2 with other atoms to expand the light-ab- sorption edge of TiO 2 . 9,10 The other is to combine TiO 2 with other narrow-bandgap materials to enhance the light-absorption ability. 11 BiVO 4 has been identified as a typical material that can enhance the visible-light re- sponse for solar water oxidation. 12,13 Moreover, TiO 2 /BiVO 4 can enhance the photocatalytic activity be- cause of the suitable energy-level alignment between BiVO 4 and TiO 2 . 14,15 Our work was designed to develop the photocatalytic performance of TiO 2 nanotubes by coupling them with BiVO 4 . 16 The photocurrent of the TiO 2 nanotube elec- trode was increased when coupled with BiVO 4 .Itwasre - vealed that the method of the photoanode-film fabrica- tion is an important factor for the photocatalytic activity. However, the photocatalytic activity of these photoanode films also needs to be improved for practical solar water splitting. Noble metals, especially Pt nanoparticles, have attracted attention because they can be used as the cata- lysts in the interfacial charge-transfer process. 17,18 There- fore, the combination of Pt and Ti/TiO 2 /BiVO 4 photo- anodes was expected to enhance the photocatalytic performance of Ti/TiO 2 /BiVO 4 electrodes. Pt nanostruc- tures were deposited on Ti/TiO 2 /BiVO 4 photoanodes us- ing pulsed potential electrodeposition in an H 2 PtCl 6 aqueous solution and the enhanced photocatalytic ability for solar water oxidation was confirmed. Materiali in tehnologije / Materials and technology 54 (2020) 6, 769–775 769 UDK 544.6:544.52:628.16:66.094.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(6)769(2020) *Corresponding author's e-mail: donghx20004@163.com (Dong Hongxing) 2 EXPERIMENTAL PART 2.1 Preparation of the Ti/TiO 2 /BiVO4 film photoelec- trode TiO 2 nanotubes were prepared with anodization in ac- cordance with the previous findings 16 and then annealed in a furnace at 450 °C for2htoobtain a Ti/TiO 2 film. BiOI was electrodeposited on a TiO 2 substrate. 19 Bi(NO 3 ) 3 ·5H 2 O was dissolved in a 0.4 M KI solution and the pH was adjusted to 1.7 by adding HNO 3 , obtaining a 0.04 M Bi(NO 3 ) 3 solution. Then, the solution was mixed with absolute ethanol containing 0.23 M p-benzoquinone under vigorous magnetic stirring. A CHI660E electro- chemical workstation (Shanghai Chenhua Device Com- pany, China) was used for electrodeposition and the sub- sequent photocatalytical performance. Electrodeposition was performed potentiostatically at –0.15 V vs SCE at room temperature for 15 s. Then the working electrode was put into a DMSO so- lution containing 0.2 M vanadyl acetylacetonate (VO(acac) 2 ) and was heated at 450 °C for2hinair .In order to remove the excess V 2 O 5 from the Ti/TiO 2 /BiVO 4 photoanode, the sample was soaked in 1 M NaOH solu- tion for 2 h while being stirred. At last, the resulting Ti/TiO 2 /BiVO 4 photoanode was rinsed with deionized water and dried in air. 2.2 Preparation of a Pt-modified Ti/TiO2/BiVO 4 film electrode Pt nanoparticles were loaded on Ti/TiO 2 /BiVO 4 with pulsed electrodeposition in a 4 mmol/L H 2 PtCl 6 solution. The deposition process was also conducted on the CHI660E workstation in a pulsed potentiostatic regime with negative pulse at –0.5 V vs SCE for 75 ms and –0.2 V vs SCE for 200 s. The electrodepostion time was about 500 s. 2.3 Characterization of Ti/TiO2 /BiVO 4 and Pt-modified Ti/TiO 2/BiVO4 electrode The surface morphology of the photoelectrodes was observed with a field-emission scanning electron micro- scope (FESEM, Hitachi S-4700). The composition was investigated with energy-dispersive X-ray spectroscopy (EDS). The microstructure of the Pt-modified Ti/TiO 2 /BiVO 4 film was characterized with a transmis- sion electron microscope (TEM, Tecnai G2 F30) com- bined with an energy-dispersive X-ray spectroscopy de- tector. The composition present in the film electrode was also characterized with a small-angle diffractometric study carried out on a Rigaku D/max 2550PC X-ray au- tomatic diffractometer. The surface composition and chemical-valence status of the elements were character- ized using a Kratos AXIS Ultra DLD (Japan) X-ray pho- toelectron spectroscopy (XPS). The optical performance of the as-prepared photoanodes was evaluated using a UV-vis Lambda 750S with a wavelength ranging from 300 nm to 600 nm. The band gap of the semiconductor was calculated employing the following equation: 20 () hv hv E 2 =− g (1) Where is the absorption coefficient, v is the light frequency and E g is the band gap of the semiconductor. Electrochemical measurements were recorded in 0.2MNa 2 SO 4 by CHI660E using a platinum sheet and a saturated SCE electrode as the counter electrode and ref- erence electrode, respectively. Electrochemical imped- ance spectroscopy (EIS) was carried out under the open-circuit voltage with frequencies ranging from 10 5 Hz to 10 –2 Hz with an AC voltage amplitude of 5 mV. The potentials of the I–V curves and the PEC perfor- mance experiments were also controlled with CHI660E. A 150-W Xe lamp (25 mW/cm 2 , Beijing Trust Tech Co. Ltd) was used to provide visible light. The photoconversion efficiency is a function of the photocurrent density and can be calculated using the fol- lowing equation: 21,22 = − JE V P () RHE app light (2) where J is the photocurrent density at the measured bias, E RHE is the standard reversible potential of 1.23 V corre- sponding to the Gibbs free energy change per photon re- quired for water splitting, V app is the applied bias poten- tial vs SCE and P light is 25 mW/cm 2 in our work. 3 RESULTS AND DISCUSSION 3.1 XRD analysis The crystalline structures of the annealed Ti/TiO 2 film and Ti/TiO 2 /BiVO 4 photoanode were characterized with x-ray diffraction (XRD). The Ti/TiO 2 film showed that the peaks appear at around 20° corresponding to TiO 2 in the amorphous state due to the broad peak. From the XRD pattern, the diffraction peaks of Ti from the Ti D. HONGXING, L. QIUPING: ENHANCED PHOTOELECTROCATALYTIC ACTIVITY OF A Pt-MODIFIED ... 770 Materiali in tehnologije / Materials and technology 54 (2020) 6, 769–775 Figure 1: XRD patterns of the Ti/TiO 2 film and Ti/TiO 2 /BiVO 4 photoanode substrate and anatase TiO 2 were detected (JCPDS No. 84-1285). After the modification of the BiVO 4 layer, new peaks appear, which correspond to the crystalline state of BiVO 4 . The observed diffraction peaks are in conformity with the monoclinic scheelite structure (JCPDS 14-0688). From the XRD pattern, it is clear that BiVO 4 could be successfully modified on the Ti/TiO 2 film. Be- cause of a small amount of coatings, the intensity of the diffraction peaks of BiVO 4 is also weak. The XRD spec- tra of the Pt-modified Ti/TiO 2 /BiVO 4 photoanode is not shown here because it is almost identical with the Ti/TiO 2 /BiVO 4 photoanode. It is speculated that the rea- son for this is the fact that the doping content of Pt on the surface of the Ti/TiO 2 /BiVO 4 film is low and the dis- persion is high. 3.2 Morphology analysis Figure 2 shows SEM images of the Ti/TiO 2 film, Ti/TiO 2 /BiVO 4 film and Pt-modified Ti/TiO 2 /BiVO 4 film. TiO 2 nanotubes with a pore diameter of 80 nm are shown in Figure 2a. The BiVO 4 nanoparticles are uniformly distributed in the voids and surface of the nanotube TiO 2 film (Figure 2b). Figure 2c presents the surface morphology of the Pt-modified Ti/TiO 2 /BiVO 4 film fabricated with pulsed potential deposition of Pt on the Ti/TiO 2 /BiVO 4 film. Figure 2d shows the EDS energy spectrum of the Pt-modified Ti/TiO 2 /BiVO 4 film. The EDS energy spec- trum shows that there are Ti and O in the film, and there is a weak Pt signal, which shows that the Pt doping amount is small and Pt is uniformly dispersed on the sur- face of the Ti/TiO 2 /BiVO 4 film. Bi and V in the energy spectrum show that BiVO 4 is loaded in the composite photoanodes. This structure gives the photoanodes a high surface area, which can greatly increase the contact be- D. HONGXING, L. QIUPING: ENHANCED PHOTOELECTROCATALYTIC ACTIVITY OF A Pt-MODIFIED ... Materiali in tehnologije / Materials and technology 54 (2020) 6, 769–775 771 Figure 3: TEM images and EDS analysis, showing the distribution of elements Pt, Bi and Ti Figure 2: SEM images and EDS analysis of the prepared samples: a) image of Ti/TiO 2 film, b) image of Ti/TiO 2 /BiVO 4 film, c) image of Pt-modified Ti/TiO 2 /BiVO 4 film, the inset image is the cross-section of the sample, d) EDS analysis of the sample and c) tween the Pt catalyst and solution, resulting in improving the photocatalytical properties of the photoanodes. The TEM images and EDS analysis of the Pt-modi- fied Ti/TiO 2 /BiVO 4 film are shown in Figure 3. The TiO 2 nanotubes and the deposits are clearly shown. The EDS analysis map shows that the elements are distributed across the nanotubes. Elements Pt, Bi and V are homo- geneously distributed on the prepared sample. 3.3 XPS analysis Figure 4 shows XPS spectra of the Pt-modified Ti/TiO 2 /BiVO 4 film. It can be found on the full spectrum (Figure 4a) that five elements, Ti, Pt, O, Bi and V, exist in the Pt-modified Ti/TiO 2 /BiVO 4 film. The strong peaks at around 460 eV were matched with the data about Ti 4+ in TiO 2 . 23 The peaks at around 160 eV were attributed to Bi 4f, and the peaks at around 520 eV were matched with V 2p. These results indicated that the valence states of Bi and V were Bi 3+ and V 5+ , respectively. 23 The peaks at around 530 eV correspond to O 1s. Figure 4b is the Pt 4f spectrum of the ternary com- plex film. It can be seen from the diagram that the peak is at 70.6 eV and 73.8 eV, which means that the valence state of Pt is 0. 24 It is proved that Pt was successfully loaded in the Ti/TiO 2 /BiVO 4 film using pulsed deposition in the form of a simple metal substance. 3.4 UV-Vis absorption spectrum analysis UV-Vis spectra of the Ti/TiO 2 , Ti/TiO 2 /BiVO 4 and Pt-modified Ti/TiO 2 /BiVO 4 film are shown in Figure 5. The absorption edge of the annealed Ti/TiO 2 film was at around 375 nm, which was in agreement with the band gapofTiO 2 (3.2 eV). 25 After being coupled with BiVO 4 , the absorption edge of Ti/TiO 2 /BiVO 4 was significantly enhanced in a visible-light region between 400 nm and 500 nm. The reason is the fact that the band gap of BiVO 4 is 2.4 eV. 26,27 According to Equation (1), the band-gap energy of Ti/TiO 2 /BiVO 4 was around 2.85 eV. The UV-Vis spectra of the Pt-modified Ti/TiO 2 /BiVO 4 film shows that the light-absorption range of the film is not an obvious red shift because Pt has no obvious effect on the light absorption. In addition, the band-gap energy of the Pt-modified Ti/TiO 2 /BiVO 4 film was 2.80 eV based on Equation (1). 3.5 Photoelectrocatalytic performance Figure 6 gives photoelectrochemical performance plots for the Ti/TiO 2 film, Ti/TiO 2 /BiVO 4 film and Pt-modified Ti/TiO 2 /BiVO 4 film under a 150 W Xe-lamp illumination. The photocurrent density of the Ti/TiO 2 film was 0.0308 mA/cm 2 at 0.6 V (vs SCE). These re- sults clearly indicated that the modification of the Ti/TiO 2 film with BiVO 4 effectively reduced the recom- bination of electrons and holes generated in the Ti/TiO 2 /BiVO 4 film due to the formation of a hetero- junction. 28 The photocurrent density of the Pt-modified Ti/TiO 2 /BiVO 4 film was 0.3845 mA/cm 2 at 0.6 V (vs SCE), which was almost 2.3 times higher than that of the Ti/TiO 2 /BiVO 4 film. The dramatic enhancement of the photocurrent response of the Pt-modified Ti/TiO 2 /BiVO 4 D. HONGXING, L. QIUPING: ENHANCED PHOTOELECTROCATALYTIC ACTIVITY OF A Pt-MODIFIED ... 772 Materiali in tehnologije / Materials and technology 54 (2020) 6, 769–775 Figure 5: UV-Vis spectra of the three kinds of prepared samples Figure 4: XPS spectrum of the Pt-modified Ti/TiO 2 /BiVO 4 films: a) the binding energy is from 0 eV to 800 eV, b) the magnified spectrum at around 60–80 eV film was an indication that Pt may have positively modu- lated the surface kinetics of the photoelectrode during solar water splitting. 29 The i-t curves of the prepared samples under a con- stant bias of 0.6V vs SCE are shown in Figure 6b. The photocurrent densities of all the photoanodes dropped rapidly to nearly zero when the simulated sunlight was turned off, and the photocurrent density quickly recov- ered when the light was turned on. The rapid rising and falling of the photocurrent density indicated a fast charge-carrier transport in the Pt-modified Ti/TiO 2 / BiVO 4 photoelectrode, which was attributed to a high penetration path for ion diffusion facilitated by Pt. 29 The transient photocurrent response generated in the Pt-mod- ified Ti/TiO 2 /BiVO 4 photoelectrode was nearly 3 times greater than that of the Ti/TiO 2 /BiVO 4 photoanode. The photoconversion efficiency was calculated using Equa- tion (2) and the results are shown in Figure 6c. The Pt-modified Ti/TiO 2 /BiVO 4 photoanode also achieved the maximum conversion efficiency of 1.26 % at 0.4 V vs SCE, which was about 2.1 times higher than that of the Ti/TiO 2 /BiVO 4 photoanode (0.6 % at a bias of 0.2 V vs SCE) and 8.4 times greater than that of the Ti/TiO 2 photoanode (0.15 % at a bias of 0.01 V vs SCE). The im- provement in the photoconversion efficiency was mainly attributed to the enhanced bulk-charge separation and more efficient surface-charge transfer in the Pt-modified Ti/TiO 2 /BiVO 4 photoelectrode due to the weak effect of Pt on the light absorption. EIS measurement was used to characterize the photoelectrochemical interfacial reaction of the three kinds of prepared samples for water splitting under light irradiation. Figure 7 shows the Nyquist plots for the Ti/TiO 2 , Ti/TiO 2 /BiVO 4 and Pt-modified Ti/TiO 2 /BiVO 4 photoelectrodes in a 0.2 M Na 2 SO 4 aqueous solution at 0.1 V vs SCE. The Nyquist plots of all the samples were fitted to a simple equivalent circuit (the inset in Fig- ure 7). The proposed equivalent circuit consists of the charge-transfer resistance (Rct), which is in parallel with the constant-phase element (CPE). And the solution re- sistance (Rs) is also shown here. The diameter of a semicircle usually equals the Rct parameter, which relates to the efficiency of the charge transfer at the electrode interface. 30 As it can be seen in Figure 8, the Pt-modified Ti/TiO 2 /BiVO 4 photoelectrode generated the smallest semicircle, indicating a low D. HONGXING, L. QIUPING: ENHANCED PHOTOELECTROCATALYTIC ACTIVITY OF A Pt-MODIFIED ... Materiali in tehnologije / Materials and technology 54 (2020) 6, 769–775 773 Figure 7: EIS curves measured in a 0.2 M Na 2 SO 4 aqueous solution at 0.1 V vs. SCE under light irradiation Figure 6: Photoelectrochemical measurements of photoelectrodes in a 0.2M Na 2 SO 4 solution under 150 W (25mW/cm 2 ) Xe-lamp illumina- tion: a) LSV curve under simulated sunlight illumination, b) i-t curves measured at 0.6 V vs SCE, c) calculated photoconversion efficiency of the prepared samples charge-transfer impedance. However, the charge-transfer impedance was high for the Ti/TiO 2 and Ti/TiO 2 /BiVO 4 photoanodes. A reduction in the charge-transfer resis- tance of the Pt-modified Ti/TiO 2 /BiVO 4 film causes a higher carrier-separation efficiency. Moreover, Pt at the BiVO 4 surface also enhances the charge transfer at the interface of the photoelectrodes and electrolyte. 31 Based on the analysis, a schematic diagram of the charge-transfer processes on the Pt-modified Ti/TiO 2 / BiVO 4 photoelectrode for water splitting under light irra- diation is shown in Figure 8. Photogenerated electrons can easily transfer from the conduction band of BiVO 4 to that of TiO 2 because of the matching of the conduc- tion-band energy levels. Moreover, TiO 2 nanotubes act as the barrier for the hole of BiVO 4 to get to the Ti substrate and thus decrease the recombination of electrons and holes, as described in a previous study. 16 The hole-trans- fer rate at the interface between the electrolyte and photoanode was highly improved by the presence of no- ble-metal Pt particles. The improvement of the photo- catalytical property could be explained with the fact that the Pt nanoparticles act as a catalyst, enhancing the transfer rate of electrons from the electrolyte to BiVO 4 and the transfer rate of the holes from BiVO 4 to electro- lyte. 32 Therefore, holes can reach the BiVO 4 surface eas- ily without any recombination, which results in efficient photocatalytical properties. The photosensitive electrons were separated with the assistance of the TiO 2 nanotube layer from the holes. The holes could reach the BiVO 4 /electrolyte interface to oxi- dize water to form oxygen. In the whole process, the Pt nanoparticles at the BiVO 4 surface act as the catalyst, improving the oxidation reaction with the holes because of the suppression of charge recombination. Conse- quently, the Pt-modified Ti/TiO 2 /BiVO 4 photoelectrodes reported in the present work provide an improved effi- ciency of the photocatalytic system. It may be useful for other photocatalytical applications such as the degrada- tion of an organic pollutant in an aqueous solution. 4 CONCLUSIONS Pt-modified Ti/TiO 2 /BiVO 4 films were successfully fabricated with the pulsed-potential-electrodeposition technique. In a composite photoanode system, TiO 2 , BiVO 4 and Pt perform their own functions required for a high photocatalytical efficiency of water splitting. A sig- nificant enhancement in both the photocurrent and en- ergy-conversion efficiency for water oxidation is ob- served after the deposition of a small amount of Pt nanoparticles on the Ti/TiO 2 /BiVO 4 photoelectrode, sug- gesting that charge separation was improved by BiVO 4 and the water-oxidation kinetics was accelerated due to the Pt catalysts in the composite system. 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