Controlled fabrication of Sn/TiO2 nanorods for photoelectrochemical water splitting
© Sun et al.; licensee Springer. 2013
Received: 2 September 2013
Accepted: 27 October 2013
Published: 5 November 2013
In this work, we investigate the controlled fabrication of Sn-doped TiO2 nanorods (Sn/TiO2 NRs) for photoelectrochemical water splitting. Sn is incorporated into the rutile TiO2 nanorods with Sn/Ti molar ratios ranging from 0% to 3% by a simple solvothermal synthesis method. The obtained Sn/TiO2 NRs are single crystalline with a rutile structure. The concentration of Sn in the final nanorods can be well controlled by adjusting the molar ratio of the precursors. Photoelectrochemical experiments are conducted to explore the photocatalytic activity of Sn/TiO2 NRs with different doping levels. Under the illumination of solar simulator with the light intensity of 100 mW/cm2, our measurements reveal that the photocurrent increases with increasing doping level and reaches the maximum value of 1.01 mA/cm2 at −0.4 V versus Ag/AgCl, which corresponds to up to about 50% enhancement compared with the pristine TiO2 NRs. The Mott-Schottky plots indicate that incorporation of Sn into TiO2 nanorod can significantly increase the charge carrier density, leading to enhanced conductivity of the nanorod. Furthermore, we demonstrate that Sn/TiO2 NRs can be a promising candidate for photoanode in photoelectrochemical water splitting because of their excellent chemical stability.
Growing global energy demand and increasing concern for climate change have aroused the interest in new technologies to harness energy from renewable sources while decreasing dependence on fossil fuels [1, 2]. One of the most attractive approaches is to produce hydrogen by solar water splitting, because of the high energy density of hydrogen and zero harmful byproduct after combustion as a fuel [3, 4]. Since the first report of the photoelectrochemical water splitting using n-type TiO2 in 1972 , TiO2 has drawn more and more attentions in this field and is regarded as one of the most promising materials as photoanode for solar water splitting, considering its high chemical stability, low cost, and nontoxicity [6, 7].
Early efforts in using TiO2 material for solar water splitting were mainly focused on the nanoparticle-based thin films for their large surface area-to-volume ratios. However, the high charge carrier recombination and low electron mobility at the grain boundary limit the performance of the films [8, 9]. Recently, researches shifted to the one-dimensional nanostructure including nanorods [10–12], nanotubes [13–15], and nanowires [16, 17]. Various fabrication processes were developed for the synthesis of TiO2 nanorods, nanowires, or nanotubes, such as catalyst-assisted vapor–liquid-solid (VLS) , hydrothermal process , electrochemical anodization [18, 19], etc. However, TiO2 is a wide band gap semiconductor, only absorbing UV-light, which suppresses its further applications. Considerable efforts have been devoted to improve the photon absorption and photocatalytic activity of TiO2 nanostructures, including synthesizing branched structures , exposing its active surface , hydrogen annealing process [22, 23], and sensitizing with other small band gap semiconductor materials such as PbS , CdSe , and CuInS .
Doping with other elements to tune the band gap of TiO2 is another efficient method to improve the photocatalytic activity. N, Ta, Nb, W, and C have been successfully incorporated into TiO2 photoanode and been demonstrated with enhanced photoconversion efficiency [26–29]. Besides, the SnO2/TiO2 composite fibers have also emerged and showed well photocatalytic property [30, 31]. Based on these researches, we expect that the incorporation of Sn into TiO2 would be an attractive approach since the small lattice mismatch between TiO2 and SnO2 is beneficial for the structural compatibility and stability. Meanwhile, the doping would significantly increase the density of charge carriers and lead to a substantial enhancement of photocatalytic activity. In this work, we successfully realized the controlled incorporation of Sn into TiO2 nanorods by a simple solvothermal synthesis method and investigated the role of Sn doping for enhanced photocatalytic activity in photoelectrochemical water splitting.
In our experiments, a transparent conductive fluorine-doped tin oxide (FTO) glass was ultrasonically cleaned in acetone and ethanol for 10 min, respectively, and then rinsed with deionized (DI) water. Twenty-five milliliters DI water was mixed with 25 mL concentrated hydrochloric acid (37%) in a Teflon-lined stainless steel autoclave. The mixture was stirred for several minutes before adding of 0.8 mL tetrabutyl titanate (TBOT). Then, 0–80 μL of Tin tetrachloride (SnCl4) solution was added for the synthesis of Sn-doped TiO2 nanorods (Sn/TiO2 NRs), followed by another several minutes stirring. Subsequently, the clean FTO substrate was placed into the Teflon-liner. The synthesis process was conducted in an electric oven, and the reaction temperature and time were 180°C and 6 h, respectively, for most of the experiments. After that, the autoclave was cooled, and the FTO substrate was taken out and rinsed with DI water. Finally, the sample was annealed at 450°C in quartz tube furnace (Thermo Scientific, Waltham, MA, USA) for 2 h in the air to remove the organic reactant and enhance the crystallization of the nanorods. For the synthesis of pristine TiO2 nanorods, the process was all the same, except for the elimination of the Sn precursor. The white nanorods film was detached from the FTO substrate with a blade and then added into ethanol followed by sonication for about 20 min. After that, two drops of the ultrasonically dispersed solution were dropped onto the copper grid and dried by heating in the ambient air for examination. To distinguish the samples with different doping levels, the Sn/TiO2 NRs were marked in the form of Sn/TiO2-a%, where a% is the molar ratio of SnCl4/TBOT.
The morphology and lattice structure of the nanorods were examined by the field-emission scanning electron microscopy (FESEM, JSM-7600 F, JEOL, Akishimashi, Tokyo, Japan) and field-emission transmission electron microscopy (FTEM, Tecnai G2 F30, FEI, Hillsboro, OR, USA). The energy-dispersive X-ray spectroscopy (EDX) combined with FSEM and FTEM was employed to detect the element composition of Sn/TiO2 NRs. To further determine the crystal structure and possible phase changes after introducing Sn doping, the crystal structure was examined with X-ray diffraction (XRD, PW3040/60, PANalytical, Almelo, The Netherlands). Moreover, X-ray photoelectron spectroscopy (XPS, VG Multilab 2000 X, Thermo Electron Corp., Waltham, MA, USA) was employed to determine the chemical composition and states of the nanorods. The binding energy of the C 1 s (284.6 eV) was used for the energy calibration, as estimated for an ordinary surface contamination of samples handled under ambient conditions.
To measure the performance of photoelectrochemical (PEC) water splitting, the exposed FTO was covered with a layer of silver paste and connected to Cu wires with solder. The silver paste, solder, edge and some part of the film were sealed with polydimethylsiloxane (PDMS) or epoxy, in which only a well-defined area about 1 cm2 of the white film was exposed to the electrolyte. A glass vessel filled with 400 mL 1 M KOH was used as the PEC cell, and a class AAA solar simulator (Oriel 94043A, Newport Corporation, Irvine, CA, USA) with the light intensity of 100 mW/cm2 was used as light source. The photocurrent and electrochemical impedance spectra were collected by electrochemical station (AUTOLAB PGSTAT302N, Metrohm Autolab, Utrecht, The Netherlands). Line sweep voltammograms were obtained at the scan rate of 20 mV/S. A Pt slice acting as the counter electrode and a standard Ag/AgCl reference electrode (containing saturated KCl solution) were used for the PEC measurements. The water splitting process in PEC cell was schematically illustrated in (Additional file 1: Figure S1).
Results and discussion
As shown in Figure 6c, the pristine TiO2 NRs achieve the efficiency of 0.48%, while the Sn/TiO2-0.5% NRs and Sn/TiO2-1% NRs achieve the efficiencies of 0.59% and 0.69% at about −0.53 V versus Ag/AgCl, about 23% and 44% enhancement, respectively. The photocatalytic properties of TiO2 and Sn/TiO2-1% nanorods with different morphology were depicted in (Additional file 1: Figure S5), which further supports our choice of the reaction conditions for median nanorods density. These results suggest that appropriate incorporation of Sn atoms can significantly enhance the photocatalytic activity of TiO2 NRs and lead to substantial increase of the photocurrent density and photoconversion efficiency.
The time-dependent measurements also have been carried out on the three samples, as shown in Figure 6d. With repeated on/off cycles of illumination from the solar simulator, the three samples display highly stable photocurrent densities of 0.71, 0.86 and 1.01 mA/cm2 at −0.4 V versus Ag/AgCl, respectively. These measurements have been repeated in several months, and there is no noticeable change happened. This indicates that the Sn/TiO2 NRs possess highly chemical and structural stability for PEC water splitting, which is another critical factor to evaluate their potentials as the photoanode material.
As oxygen vacancy serving as electron donor has been accepted generally as the main cause for the n-type conductivity of TiO2, we expect that the incorporation of Sn atoms may lead to the increase of oxygen vacancy which is responsible for the enhanced photocatalytic activity. Besides, other reported effects may also be at work. For instance, the formation of mixed-cation composition (Sn x Ti1−xO2) at the interface and associated modulation of electronic properties may facilitate the exciton generation and separation . The potential difference of TiO2 and SnO2 may promote the photoelectron migration from TiO2 to SnO2 conducting band with decreasing combination, allowing both of the photogenerated electrons and holes to participate in the overall photocatalytic reaction . However, the photocurrent of Sn/TiO2-3% NRs is lower to the pristine TiO2. This may be rationalized as the overly high Sn doping level upshifting the TiO2 band gap and creating much more interfaces, which substantially reduces the light absorption efficiency and impedes the photogenerated charge separation.
In summary, we have successfully realized the controlled incorporation of Sn into TiO2 NRs to enhance the photocatalytic activity for PEC water splitting. Sn concentration is well controlled by adjusting the precursor molar ratio. We studied the crystal structure of the obtained Sn/TiO2 NRs, which is the same as the pristine TiO2 NRs. The PEC measurements reveal that the photocurrent reaches the maximum value of 1.01 mA/cm2 at −0.4 V versus Ag/AgCl with a Sn/Ti molar ratio of about 1%, which corresponds to up to about 50% enhancement compared to the pristine TiO2 NRs. The Mott-Schottky plots indicate that the incorporation of Sn into TiO2 NRs can significantly increase the charge carrier density, hence improving the conductivity of TiO2 NRs and leading to the increase of photocurrent. Besides, the Sn/TiO2 NRs exhibit excellent chemical stability which further promotes them to be a promising candidate for photoanode in photoelectrochemical water splitting devices. With the enhanced conductivity, we believe the Sn/TiO2 NRs can also serves as substitution for pure TiO2 structures in other optoelectronic applications including photocatalysis, photodetectors, solar cells, etc.
The authors are grateful for the financial supports by the National Natural Science Foundation of China (grant nos. 51175210 and 51222508).
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