Preparation of Ti/SnO2-Sb2O4 photoanode by electro-deposition and dip coating for photoelectrocatalytic application

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Author: C.M. Fana,*, B. Huaa, Y. Wangb, Z.H. Lianga, X.G.Haoa, S.B.Liua, Y.P. Suna

Introduction

Pure SnO2 is a wide band gap semiconductor (Eg=3.5-3.8eV), and it is transparent as well as chemically and thermally stable. When it is doped with some special metal element, such as antimony, the doped SnO2 shows a metal-like properties and a changeable property of absorption light. Therefore it has been used as electrodes in solar cell and electrochemical cell, liquid crystal displays, gas sensor, etc[1-5]. Many reports have described the preparations and properties of antimony-doped SnO2 thin film and their application [6-8]. As so far in water treatment, SnO2 electrode is almost applied on electrocatalytic process duo to its good properties such as extremely high overpotential for oxygen-evolution, good electrical conductivity and catalytic activity, we were not able to find the investigation of its applications in photoelectrocatalytic degradation of organics in water. From the nature properties of material Sb-doped SnO2 is still a semiconductor, but as a photocatalyst, the band gap of SnO2 is larger than that of TiO2, thus a UV-light (λ=326-355nm) had to be used in order to excite the SnO2 in the photocatalytic application. On the other hand, it should be noticed that the valence band level of SnO2 is lower (or more positive) than that of TiO2 as shown in Figure1. Compared with the TiO2 the oxidizing potential of photogenerated holes of SnO2 is higher, the oxidizing capacity of these holes should be stronger, then these holes can easily oxide the H2O or OH- into •OH due to the larger potential difference between their redox potentials. The conduct band level of SnO2 is also lower than that of TiO2, maybe which is not negative enough to reduce water to hydrogen, dioxygen to superoxide, or to hydrogen peroxide, but by the help of external bias, i.e., electro-assistance photocatalysis, the oxidation-reduction reaction could be easily achieved. Therefore the external bias will plays an important role in the SnO2 photocatalytic process, even an indispensable role. For this reason, we design to introduce Sb-doped SnO2 as a novel photoanode into PEC process, the aim is to integrate the electrochemical reaction process and electro-assisted photocatalytic reaction process in one reaction system in case to get a enhanced oxidation rate for organic removal. Figure 1. The energy band structure diagram of semiconductors Antimony doped SnO2 are used for making film electrode in electrochemical treatment of organics in aqueous solution because it has a unique feature of controllable electrical conductivity. The electrical conductivity of this material is very sensitive to the relative amounts of Sb presence in the samples and the Sb oxidation states. The addition of a trivalent cation of Sb3+ to SnO2 increases the number of oxygen vacancies and generates holes, whereas Sb5+ incorporated in SnO2 creates cation vacancies and free electrons in the lattice. Hence, the conductivity of Sb-doped SnO2 samples is higher than that of an un-doped sample. Therefore the task of this paper is first to prepare a Sb-doped SnO2 thin film photoelectrode using a simple procedure of electro-deposition and dip coating, and then conduct a series of experiments to confirm the effects of Sb-doping levels, heat treatment temperature, and the structure of the thin films on the properties of this photoanode, furthermore clarify the feasibility of this novel photoanode in the photoelectrocatalytic application of organic removal via the degradation of phenol in water. Materials and methods Preparation of Ti/SnO2-Sn2O4 electrode A Sb-doped SnO2 thin film was prepared on the surface of the Ti plate substrate with the size of 70mm×10mm(effective area 6.0cm2) by electro-deposition for the sub-layer and dip-coating for the outer-layer. The concrete prepared procedure was as follows, firstly, the raw Ti plate was burnished cleanly with abrasive paper, and etched with 40% NaOH solution and subsequent with 10% oxalic acid solution at about 95℃ for 4h for surface pretreatment, then fully rinsed with distilled water. Finally, this processed Ti plate substrate was then used to take on catalyst layer. For the electro-deposition of the sub-layer, a processed Ti plate as the cathode was placed in 100 ml of alcohol solution containing 1.753g SnCl4•4H2O, 0.114g SbCl3, and 12ml ethanol alcohol solution(analytical reagent) of tetrabutyl titanate(analytical reagent). A constant DC current of 0.12A was charged for 30 min for electroplating the cathode, and a Ti plate electrode was used as the anode. The sub-layer on the Ti plate was then dried in an oven at 450℃ for 2h. For the thermal deposition of the activation layer, the Ti plate with sub-layer was dipped into a coating solution that consisted of 4g SnCl4•5H2O, 0.156g SbCl3 and 1ml concentrated HCl (37%) in 9 ml h-butanol(analytical reagent), then dried for 5 min in an oven at 100℃and then sintered in a muffle furnace at 450 ℃ for 10 min. These coating, drying, annealling steps could be repeated 15 times in order to reach a desired thin film layers. Finally, the electrode was annealed at 450℃ for 1h for the complete decomposition of compounds (Sn, Sb) and the formation of the Sb-doped SnO2 thin film. Because Sb exists in states of Sb3+ and Sb5+, the mixture of two kinds of oxide is writted in Sb2O4[9], so our photoanode of Sb-doped Ti/SnO2 sometimes is writted in Ti/SnO2-Sb2O4. Preparation of gas diffusion electrode The gas diffusion electrode (or called oxygen electrode) used as cathode for PEC process was lab-made. It was prepared by coating and pressing the carbon materials of graphite, carbon black and activated carbon powder on nickel mesh, then annealled in a muffle furnace at 400 ℃ for 1 h. The thickness of cathode was about 1mm, the detailed preparing process see literature [10]. Analytical methods To study the surface morphology, scanning electron microscopy (SEM) (LEO-438VP, Japan) equipped with an energy-disperse spectrometer (EDS) analyzer was used. To determine the crystal phase composition of the Ti/SnO2-Sb2O4electrode, X-ray diffraction (XRD) (D/max-2500, Japan) measurements were carried out, the samples were measured in the step scan mode with the step of 0.02 degree and a counting time of 1s for angular range of 2θ = 15-70°. The absorption spectrum of the Sb-doped Ti/SnO2 electrode was measured by a UV-VIS spectrophotometer (Lambda Bio 40, USA). The phenol concentration was measured by UV-visible spectrophotometer (Model Cary50, USA). The total organic carbon (TOC) concentration was measured by a total organic carbon analyzer (Multi N/C 3000, Germany) to determine the total mineralization degree of the phenol compound during the advanced oxidation process. The bias added photoelectrocatalytic system was controlled using a potentiostat (Model 363, USA). Phenol degradation Figure 2 shows a schematic diagram of the batch scale experimental reactor system, which consistes of a cylindrical quartz glass reactor, a 250 W high-pressure mercury lamp (Institute of Electric Light Source, Beijing), a potentiostat (MODEL363 England) and electrodes. The UV lamp used as the side light source was positioned by the cylindrical reactor, the UV lamp was surrounded by a circulating water jacket in case the temperature of reaction solution was warmed up. The photoelectroreactor had an effective volume of 100 mL，in which the Ti/SnO2-Sb2O4 electrode was regarded as the working electrode, the gas diffusion electrode was used as the counter electrode. The two electrodes were placed apart and facing each other. A DC potentiostat was employed as the power supply with a voltage output up to 2V. During the reaction the phenol solution was irradiated by UV light and was aerated by mini-type air pump to provide air. At different irradiation time intervals, the samples of phenol solution were drawn for chemical analysis of the phenol concentration and the TOC concentration.

Figure 2. The schematic diagram of photoreaction systems Results and discussion XRD analysis of Sb-doped SnO2 film

Figure 3. XRD pattern of the Ti/SnO2-Sb2O4 electrode The typical XRD pattern of the Sb-doped SnO2 electrode calcined at 450℃ is shown in Figure 3. This pattern shows clearly that the surface of electrode was mainly SnO2 crystal with rutile structure, it is in good agreement with X-ray powder data file of ASTM card. Meanwhile, the peaks for antimony oxides were difficult to be observed, this may be related to the similar phase's lattice structure of antimony oxide with SnO2 or less content of antimony in mixtures. Wang Yuheng et al[11] studied the structural and photoluminescence characters of SnO2-Sb(4wt%) films, they found neither Sb2O3 nor Sb2O5 phase was detected in XRD measurement, and thought that Sb3+ and Sb5+ replace Sn4+ in the rutile lattice. Gržeta et al [12] studied on Sb-doped SnO2 nanocrystallites by XRD and revealed that antimony in SnO2 samples exist in both Sb3+ and Sb5+ oxidation states, with as the dominant species. Phenol degradation by PC, EC and PEC processes In order to clarify the role of photocatalysis, electrocatalysis as well as the combination of photocatalysis and electrocatalysis in the photoelectrocatalytic reaction, the photocatalytic, electrocatalytic and photoelectrocatalytic processes were carried out, and the results of the changes of phenol and TOC degradation with reactive time are shown in Figure 4. During all the experiments, the Ti/SnO2-Sb2O4 electrode and a gas diffusion electrode were immersed in the phenol solution, and air continuously was supplied to the reaction system from the bottom of reactor using a mini-type air pump. The degradation rate Χ in Figure 4 was obtained by evaluating the ratio of the phenol concentration change C0−Ct at time t and the initial phenol concentration C0 in the solution at t=0, using the formula of X= (C0-Ct)/C0.

Figure 4. The comparison of phenol removal at different degradation processes For the photocatalytic degradation process, the Ti/SnO2-Sb2O4 electrode and a gas diffusion electrode were immersed in the phenol solution without connecting to potentiostat, the phenol degradation rate and TOC mineralization rate were respectively 65% and 55% after UV irradiation for 3.0h, respectively. The experiment results demonstrate that the 365nm light can excite the thin film of Sb-doped SnO2. For electrocatalytic process, the Ti/SnO2-Sb2O4 electrode was used as the anode and a gas diffusion electrode was used as the cathode in absence of UV illumination, the results show that 80 % of phenol was degraded and 68 % of organic substance was mineralized on Sb-doped Ti/ SnO2 under 2.0 V cell voltages after 3.0 h reaction, which means that the Sb-doped Ti/SnO2 electrode has obvious electrocatalysis for phenol removal. For photoelectrocatalytic process, the Ti/SnO2-Sb2O4 electrode was used as the anode and a gas diffusion electrode was used as the cathode with 2.0 V cell voltages in presence of UV illumination, the phenol degradation rate and TOC mineralization rate were respectively 100% and 85% after 3.0 h reaction, it indicates that photoelectrocatalytic reaction could achieve a much better performance of degradation than photocatalysis and electrocatalysis. These results also confirm that the combined technology of electrochemistry and electro-assisted photocatalysis could significantly enhance not only the degradation efficiency of phenol but also TOC reduction. This existed synergic effect in the photoelectrocatalytic reactor may be attributed the capturing effect of anode bias for the photogenerated electrons and the direct or indirect electrocatalytic oxidation of objective organic pollutants. Effect of different heat treatment temperature Figure 5 shows the photoelectrocatalytic activity of the Ti/SnO2-Sb2O4 electrodes proceeded by different heat treatment temperature of 400℃ to 500℃. It can be seen that the degradation performance order of Sb doped SnO2 film was in the following: 450℃> 500℃>400℃, and the maximum rate was exhibited whether phenol degradation or mineralization at 450℃. It should be ascribed to the changeable electrical conductivity and photocatalytic activity due to the changeable Sb3+ and Sb5+ oxidation states with the changeable annealing temperature for the electrode.

Figure 5. The effect of heat treatment temperature of Ti/SnO2-Sb2O4 electrode on the phenol removal A few reports revealed that Sb3+ and Sb5+ dopants replace Sn4+ lattice site can acts as acceptor and donor, respectively [13-15]. Fayat et al [16] gave more detail depiction about the Sb ion substitution of Sn ion. The defect formed by Sb3+ ion replacing Sn4+ lattice site is equal to binding a vacancy flabbily around a negative which forms acceptor level, and the defect formed by the replacement of Sn4+ lattice site with Sb5+ ion is equal to a monovalent cation bound flabbily by an electron which forms the donor level. Based on XPS technique Terrier et al [17] studied 500℃ heated antimony-doped SnO2 samples preparated by sol-gel method and found that antimony exists in both Sb3+ and Sb5+ oxidation states. Our XPS study [18] on antimony-doped SnO2 film heated at 450℃ also revealed the same results, and the ratio of Sb2O3/Sb2O5 in SnO2 film is 54%/44%, leading an improved conductivity. Jayakumar et al[19] studied the metallic nature of Sn1-xSbxO2±δ under different annealing tempertature by NMR, the results clearly indicated that the interaction of antimony structural units with tin structural units takes place only above 400℃, this partial replacement of Sn4+ by Sb5+/Sb3+ generates an extra free electron/hole, resulting in the improved conductivity of the sample. The scanning electron micrographs(SEM) of Ti/SnO2-Sb2O4 electrode with different calcined temperature of 400, 450, 500 and 600℃ are shown in Figure 6. For all temperatures, the films of the electrode have a cracked mud-like shape of surfaces, which may be significantly related to the difference in the coefficients of thermal expansion of these substrates. The electrode prepared at 400℃(Figure 6a) has a comparative flat structure and smaller interspace. However, with the temperature increasing, the surface of electrode becomes more and more rough and uneven (Figure 6b, Figure 6c, and Figure 6d). When the temperature is 450℃, a complex morphology with a mixture of mud-flat cracking and agglomerates was equably distributed over the electrode surface. Because the coatings have been grown layer-by-layer as described in the experimental parts, their inner structure is likely to be composed of agglomerates [20]. These small fractionlets can exactly improved specific surface area of the electrode, and obtain more active adsorption center, which is favorite to catalytic reaction. But when the temperature is above 450℃, the crack in the surface of Sb-doped SnO2 films get deeper and broader gradually, at 600℃ the mud-liked small fractionlet collapsed and the rod-shaped grains as well as smaller grains agglomerates, holes and cavities started appearing on the surface, the Sb-doped SnO2 films become quite rough enough. This rough holes structure can't prevent the diffusion of oxygen into the Ti subatrate and the formation of titanium oxide during use process [21, 22], especially in contact with corrosive medium, and then affects the performance of the electrode. In comparison the 450℃ is an appropriate annealing temperature to prepare the Ti/SnO2-Sb2O4 electrode, which is in agreement with degradation results. Figure 6. The SEM images of the Sb-doped SnO2 thin films prepared at different temperatures of 400℃(a), 450℃(b), 500℃(c) and 600℃(d) The Ti/SnO2-Sb2O4 electrodes was also studied by EDS using SEM, the molar ratios of Sn and Sb atoms from SEM, which was focused on the electrode's surface are shown in Table 1. Table1 indicates that the analytical results from EDS are in reasonable agreement with the nominal mole composition of Sn and Sb atoms (100:6), that is the molar concentrations of starting materials are basically maintained in the final coatings, but the analytic composition of the electrode surface is not completely the same as that of the starting materials due to the loss of Sn and Sb through heat-treatment and the instrumental error. Table 1. Composition of the Ti/SnO2-Sb2O4 electrodes EDS value（mole %） Samples Sn Sb O Sb/Sn 400℃ 74.45 5.81 19.74 7.8 450℃ 81.03 5.24 13.73 6.5 500℃ 77.64 4.57 17.79 5.9 600℃ 69.52 5.28 25.52 7.6 Effect of different Sb-doping levels The doping level of Sb in SnO2 is an important factor on the performance of photoanode. A series of Sb-doped SnO2 thin film electrodes were prepared by first electro-deposition and then thermal decomposing with the same preparation procedure but at different Sb-doping level of 1.5%, 6.0%, 8.0%. The influences of the Sb/Sn mol ratio on the degradation phenol in the PEC process are shown in Figure 7. This set of 3 tests was performed under the following same degradation experimental conditions which were: C0=20 mg/L, V=100 ml, T=25C, pH0=6.5, bias=2.0 V, I=9.2 mW•cm-2. The results presented in Fig.7 revealed that when the Sb-doping level was less than 6%, the PEC degradation rate increased rapidly with the increase of Sb amount in SnO2. But when the Sb-doping level was more than 6%, the degradation rate decreased with the increase of Sb amount in SnO2. The Sb-doped SnO2 film photoanode prepared at 6% Sb/Sn mol ratio achieved the highest PEC removal rate in phenol degradation. These results indicate that Sb doped in SnO2 have an optimum dopant value for improving the performance of this kind of electrode, this is in agreement with other reports [23] Figure7. The comparison of phenol removal under different Sb-doping levels

Figure 8. UV-Vis spectra of SnO2 electrodes of different doping levels In order to support the conclusion of PEC degradation, we measured the UV-Vis spectra of different Sb-doping SnO2 films at the 450℃ annealing temperature. The UV-Vis spectra in the wavelength range of 250-500nm are shown in Fig.8. From the Figure 8, it can be seen that the Sb-doping SnO2 films with 6% Sb doping level has a little stronger absorbency in the range of UV and visible light than that of other doping level SnO2 films, which means the presence of suitable antimony indeed changes the absorption performance of SnO2. We tentatively ascribed the increased absorption to the electron transition between the donor level and the acceptor level formed by Sb ions, leading the electrical conductivity increased and at last leading the degradation increased. Effect of different photoanodes Figure 9 shows the changes of phenol degradation with reaction time by means of photoelectrochemistry using different photoanods of Ti/SnO2-Sb2O4 electrode, Ti/TiO2 electrode and the Ti/SnO2 electrode under the same experimental conditions. The Ti/SnO2-Sb2O4 electrode exhibits a better photoelectrocatalytic activity than that of the Ti/TiO2 electrode and the Ti/SnO2 electrode, the phenol in water was almost completely degraded over a 2h time in case of the Ti/SnO2-Sb2O4 electrode as anode due to the doping of Sb is beneficial to reduce resistance and enhance the conductivity of the Ti/SnO2-Sb2O4 electrode.

Figure 9. The comparison of phenol removal by Ti/SnO2, Ti/TiO2 and Ti/SnO2-Sb2O4 electrode Conclusions From above researches we can conclude that the electrode of Sb-doped SnO2 thin film on titanium substrate is a novel and a practical photoanode for the application of organic removal from water by PEC technique. The nature of Sb-doped SnO2 material makes it feasibility as photoelectrode due to its structure of band gap and metal-like property. This research results involves important aspects that can be applied to the development of high performance photoanode. The performances of the photoanode are strongly dependent on the preparing conditions, particularly the Sb-doping level in SnO2 and the annealing temperature of photoanode. Under the 6mol % Sb-doping level, 450℃ annealing temperature as well as 2.0V bias, the PEC degradation of phenol reached favorite removal efficiency. As for the effect of Sb ion valence on PEC activity are currently in progress.

Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant No. 20476070) and Natural Science Foundation of Shanxi Province (20041020).

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