Controlled Sn-Doping in TiO2 Nanowire Photoanodes with Enhanced

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Controlled Sn-Doping in TiO2 Nanowire Photoanodes with Enhanced Photoelectrochemical Conversion Ming Xu,† Peimei Da,† Haoyu Wu, Dongyuan Zhao, and Gengfeng Zheng* Laboratory of Advanced Materials, Department of Chemistry, Fudan University, Shanghai, 200433, People’s Republic of China S Supporting Information *

ABSTRACT: We demonstrate for the first time the controlled Sn-doping in TiO2 nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO2 and TiO2, Sn dopants are incorporated into TiO2 NWs by a one-pot hydrothermal synthesis with different ratios of SnCl4 and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl4 hydrolysis rate. The obtained Sn-doped TiO2 (Sn/ TiO2) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO2 NWs is well controlled at a low level, that is, 1−2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO2 NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm2 at 0 V vs Ag/AgCl under 100 mW/cm2 simulated sunlight illumination up to ∼100% enhancement compared to our best pristine TiO2 NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO2 NWs in H2 further improves their photoactivity with an optimized photoconversion efficiency of ∼1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO2 NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO2 materials in a variety of solar energy driven applications. KEYWORDS: Photoelectrochemical, water splitting, TiO2, Sn doping, photocurrent, photoconversion

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is an attractive approach,17 especially for TiO2, as the small lattice mismatch between SnO2 and TiO2 leads to good structural compatibility and stability. The incorporation of Sn or SnO2 into TiO2 materials has been investigated by Sn doping in TiO2 nanocrystals to facilitate TiO2 crystal structure change or shift its band edge18−20 and growth of SnO2/TiO2 core−shell nanoparticles21 that reduces the exciton recombination. However, the effect of Sn doping has not been successfully realized for TiO2 NWs to enhance their PEC conversion. This may be rationalized as the increase of Sn doping percentage upshifts the TiO2 bandgap (as SnO2 is a ntype semiconductor with a bandgap of 3.8 eV) and creates more interfaces, which subsequently reduces both the light absorption efficiency and photogenerated charge separation. We hypothesize that controlling the Sn doping at a low level in TiO2 NWs can maintain or provide higher photon absorption efficiency than pristine (undoped) TiO2 NWs, while at the same time increasing the charge carrier density, leading to a significant photoactivity enhancement. Herein, we report the one-pot hydrothermal synthesis and controlled Sn doping in TiO2 (Sn/TiO2) NW arrays and demonstrate their

he interest of finding sustainable energy sources has been fast growing in the past decade because of the imminent depletion of global fossil fuels and increasing concern on the climate change.1−4 Since the first report of solar-driven photoelectrochemical (PEC) energy conversion in 1972,5 titanium dioxide (TiO2) has been extensively investigated and remained as one of the most promising candidates as a photoanode for water splitting, due to its high chemical stability, favorable band edge positions, strong optical absorption, and inexpensive cost.6,7 Recently, hydrothermally synthesized TiO2 nanowire (NW)8 has become a research hotspot for PEC conversion due to its large surface area, fast charge transport, and short diffusion distance for photogenerated carriers.9,10 However, as TiO2 is a well-known n-type semiconductor with a wide band gap (3.0 eV for rutile structure and 3.2 eV for anatase structure), it only absorbs in the UV regime (less than 5% over the full solar energy) and thus significantly limits its widespread applications.11 Various strategies have been explored to improve the photocatalytic efficiency of TiO2 NWs or nanotubes, such as tuning its crystallite size and structure,7,9 doping with metal or nonmetal elements to induce red shift of bandgap absorption,12,13 sensitizing with other small bandgap semiconductor materials like CdSe,14 synthesizing branched structures,15 and postgrowth hydrogen annealing.16 Doping materials with Sn or Sn4+ © 2012 American Chemical Society

Received: December 6, 2011 Revised: February 15, 2012 Published: February 24, 2012 1503

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Figure 1. (a−c) Top-view SEM images of as-synthesized Sn/TiO2 NW arrays with different densities on FTO-coated glass substrates. (a) High density, (b) median density, (c) low density. Scale bars in panels a−c are 500 nm. (d) Side-view SEM image of as-synthesized Sn/TiO2 NW array with median density, as in b. Scale bar is 1 μm. (e) HR-TEM images of a typical Sn/TiO2 NW. The two d-spacing values of 0.324 and 0.248 nm are shown, correlated to (110) and (101) planes of a rutile TiO2 structure, respectively. Scale bar is 2 nm. Inset: SAED pattern showing single crystalline rutile TiO2 structure.

of the as-synthesized Sn/TiO2 NW arrays were well tuned by different synthesis conditions. For a typical reactant mixture of 15 mL of H2O, 0.25 mL of TBOT, and 100 μL SnCl4 solution (0.1 M), the addition of 9 mL hydrochloric acid (37 wt %) yielded a highly packed Sn/TiO2 NW array (Figure 1a). When the added hydrochloric acid was increased to 13 and 17 mL, the Sn/TiO2 NW array density gradually decreased (Figure 1b,c, respectively). This trend was also observed on pristine TiO2 NWs and consistent with previous reports.8 In order to optimize both NW density and surface area for PEC conversion, the acidity for median Sn/TiO2 NW density (Figure 1b) was selected for all the remaining experiments in this paper. Close examination of the image revealed that each NW indeed consisted of many thinner NWs with rough rectangular top facets. The side-view of the same NW array sample (Figure 1d) confirmed that all NWs were grown almost perpendicularly from the FTO substrate. In addition, a wide range of precursor ratios (SnCl4/TBOT = 0−50%) in the initial reactant mixture were tested in our experiment, and no noticeable morphology change was observed from the obtained Sn/TiO2 NWs, compared to pristine TiO2 NWs without Sn doping. High-resolution transmission electron microscopy (HRTEM) showed that a typical as-synthesized Sn/TiO2 NW had a clear single crystalline structure (Figure 1e and inset) with a well-resolved lattice spacing of 0.324 and 0.248 nm in accord with the d-spacing of rutile TiO2 in (110) and (101) planes, respectively. The selected area electron diffraction (SAED) pattern revealed that the Sn/TiO2 NW was a rutile structure (Figure 1e, inset). TEM characterization of a number of Sn/ TiO2 NWs showed other d-spacings of 0.30 and 0.21 nm, correlated to (200) and (210) planes of single crystalline rutile TiO2 structure (Supporting Information Figure S1). Energy dispersive X-ray spectroscopy (EDX) showed that a representative Sn/TiO2 NW sample with initial SnCl4/TBOT ratio of 12.5% had a Sn/Ti ratio of 1.1% (Supporting Information Figure S2). Although this number was close to the detection limit of EDX and thus other characterization techniques such as

application as photoanodes for PEC water splitting. The morphology and density of Sn/TiO2 NW arrays were well controlled under different amount of hydrochloric acid added and molar ratios of the initial Sn and Ti precursors in the reactant mixture. Structural characterization indicated that the as-synthesized Sn/TiO2 NWs were single crystalline rutile structures with the Sn/Ti ratio well tuned in the range of 12 h.

X-ray photoelectron spectroscopy (XPS) were needed, this result clearly confirmed the successful incorporation of Sn atoms into TiO2 NW structures. This low Sn percentage was attributed to the high acidity in our synthesis, which was designed to control the hydrolysis rate of SnCl4 at a slow level. X-ray diffraction (XRD) spectroscopy further confirmed that the Sn/TiO2 NWs had a rutile TiO2 crystal structure, which is the same as pristine TiO2 NWs (Figure 2a, upper curve). The diffraction peaks of a FTO substrate were also observed and plotted for comparison (Figure 2a, lower curve). All the diffraction peaks from Sn/TiO2 and TiO2 NW samples were in good accord with the tetragonal rutile phase (JCPDS No. 211276). Moreover, it can be seen that rutile TiO2 NWs had similar diffraction pattern and d-spacing values as the FTO substrate, indicating the similarity of lattice structures between TiO2 and SnO2. The surface composition and chemical states of assynthesized Sn/TiO2 NW arrays were further characterized by XPS (Figure 2b−d). Two representative Sn/TiO2 NW samples with initial SnCl4/TBOT ratios of 12.5 and 50% (designated as Sn/TiO2-12 and Sn/TiO2-50, respectively) were plotted together for comparison. The XPS peaks at ca. 458.3, 464.1, and 529.7 eV were attributed to Ti 2p3/2, Ti 2p1/2, and O 1s,22 and the two peaks at 486.5 and 495.6 eV corresponded to Sn 3d5/2 and Sn 3d3/2, respectively, confirming that the main dopant form was Sn4+.23 A very low level of Sn signal could also be seen in the pristine TiO2 nanowire sample, which was originated from the FTO substrate (Supporting Information Figure S3). In addition, quantitative analysis showed that the actual Sn/Ti ratio was ∼1.6% for Sn/TiO2-12 NWs and 2.2% for Sn/TiO2-50 NWs, respectively. As Sn has a larger electronegativity than Ti, the substitution of Sn for Ti in the lattice leads to the shift of binding energy to a higher value.24

This expectation was confirmed by the peak shifts of ca. 0.25− 0.30 eV in both Ti peaks and O peak to the higher binding energy direction from Sn/TiO2-12 to Sn/TiO2-50 (Figure 2b,d). To study the photocatalytic activity for solar energy driven water splitting, the pristine TiO2 NWs and Sn/TiO2 NWs with different Sn doping ratios were fabricated as photoanodes with a similar area of ∼2 cm2. A standard 3-electrode electrochemical cell with a Pt wire as the counter electrode and an Ag/AgCl reference electrode was used for all the PEC measurements in our experiments, and 1 M KOH was used for the electrolyte solution. The reversible hydrogen electrode (RHE) potential can be converted from the Ag/AgCl reference electrode as RHE = V(Ag/AgCl) + 1.0 V.12,16 The photocurrent was measured under simulated sunlight illumination at 100 mW/ cm2 from a 150 W xenon lamp coupled with an AM 1.5G filter (see the Experimental Section in Supporting Information). As shown in Figure 3a, in a potential window between −1 and 0.4 V versus Ag/AgCl, the photocurrent density of the pristine TiO2 NW array was 0.65 and 0.98 mA/cm2 at −0.4 and 0 V versus Ag/AgCl, respectively. Compared with two selected Sn/ TiO2 NW photoanodes with different Sn doping ratios (mole ratio of precursors in the reactant mixture, SnCl4/TBOT = 6.25 and 12.5%, designated as Sn/TiO2-6 and Sn/TiO2-12, respectively), the photocurrent increased from 0.65 to 1.25 and 1.63 mA/cm2 at −0.4 V versus Ag/AgCl (0.6 V vs RHE), and increased from 0.98 to 1.40 and 1.85 mA/cm2 at 0 V versus Ag/AgCl (1.0 V vs RHE), respectively. To further reveal the effect of Sn doping on the TiO2 NW photoactivity, a series of photocurrent measurements have been carried out on Sn/TiO2 NW samples with the initial SnCl4/TBOT ratio from 0 to 50%. Figure 3b showed that the photocurrent ratio between Sn/TiO2 NWs and pristine TiO2 NWs at 0 V vs Ag/AgCl first increased 1506

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with the SnCl4/TBOT ratio and reached a maximum of 176 ± 30% for the Sn/TiO2-12 NW samples (that is, SnCl4/TBOT = 12.5 and a measured Sn/Ti ratio of 1.6%). This value corresponded to up to ∼100% increase of the photocurrent compared to the pristine TiO2 NW arrays. With higher Sn doping, the measured photocurrent decreased and eventually dropped to a similar or lower level of pristine TiO2 NWs. This result confirms our hypothesis that doping of Sn into TiO2 NWs in a low level can effectively enhance TiO2 photocatalytic activity, while an excess of Sn incorporation leads to a decrease of the photoconversion efficiency, which may result from a phase separation and upshift of TiO2 bandgap. To quantitatively evaluate the efficiency of PEC hydrogen generation from different NW array samples, the photoconversion efficiency is calculated based on the equation,25 η = I (1.23 − V)/Jlight, where V is the applied voltage versus reversible hydrogen electrode (RHE), I is the photocurrent density at the measured potential, and Jlight is the irradiance intensity of 100 mW/cm2 (AM 1.5G). The Sn/TiO2-12 NWs (with the initial SnCl4/TBOT = 12.5) achieved the highest efficiency of ∼1.05% at a low bias of −0.44 V versus Ag/AgCl (0.56 V vs RHE), and the Sn/TiO2-6 (with the initial SnCl4/ TBOT = 6.25) achieved ∼0.80% at a voltage of −0.47 V versus Ag/AgCl (0.53 V vs RHE), as shown in Figure 3c. Compared to the photoconversion efficiency of ∼0.4% measured from pristine TiO2 NW, our results show the Sn doping can significantly increase the maximum photocurrent at a lower saturation potential, thus leading to a much improved photoconversion efficiency. Moreover, suggested by a recent paper that hydrogen annealing can effectively increase the charge carrier density of TiO2 NWs,16 the Sn/TiO2 NW photoanodes were further annealed under H2 at 450 °C. As shown in Figure 3d, the photocurrent for the Sn/TiO2-12 NWs increased from 1.62 to 1.91 mA/cm2 with an additional 15− 20% of photoconversion efficiency increase to ∼1.2% at −0.4 V versus Ag/AgCl (Figure 3c). Although this percentage was not as substantial as the percentage increase reported for pristine TiO2 NWs annealing in H2,16 it indicated that subsequent hydrogen annealing can also be applied to our as-synthesized Sn-TiO2 NWs to further enhance the photocurrent. In addition to the photocurrent density obtained under the full solar spectrum, information of photoactivity at different wavelength region, independent from the light sources and filters used, is important for photoactivity evaluation. The IPCE measurement was carried out on these fabricated photoanodes to characterize their photoconversion efficiencies. Figure 4a showed the IPCE results obtained from photoanodes comprised of pristine TiO2 NWs, Sn/TiO2-12 NWs, and H2treated Sn/TiO2-12 NWs, respectively. Compared to the pristine TiO2 NWs (black curve) with an IPCE value of 10− 20% around 300−425 nm, the Sn-doped TiO2 NW sample (red curve) showed significant increase of IPCE to 25−48% in the same wavelength range. This result indicated that our Sn/TiO2 NWs could effectively use UV light for PEC water splitting in which the separation and transportation of UV-excited charge carriers were efficient. The H2-treated Sn/TiO2-12 NWs (blue curve) had a largest IPCE in 300−380 nm range, which quickly dropped and became comparable or even smaller than that of the unannealed Sn/TiO2 NWs in 380−425 nm. As the solar energy density is low in the UV range and increases significantly around 400 nm and longer wavelengths, this resulted in ca. 15−20% increase of maximum photocurrent due to the H2 annealing on the Sn/TiO2 NWs. In addition, the

absorption of all these samples dropped almost to zero in the wavelength region over 430 nm, consistent with their UV−vis spectra (PG2000 Pro, Idea Optics Co., China) and the estimated bandgap energy ∼3.2 eV (Supporting Information Figure S4). Although some literatures reported that Sn doping could add interfacial states in TiO2 bandgap and thus reduce its photon absorption energy,26 this phenomenon was not observed in our experiment. The IPCE measurements were in good accord with the photocurrent measurement from the full solar spectrum. The electrochemical impedance measurement was further performed to investigate the role of Sn doping for the photoactivity increase of TiO2 NWs. The Mott−Schottky plot calculated from the electrochemical impedance measurement can provide a qualitative comparison on different samples with similar material composition, morphology, and device geometry.27 All the three NW samples (pristine TiO2, Sn-TiO212, H2-treated Sn-TiO2-12) were measured with a positive slope in the Mott−Schottky plot (Figure 4b), as TiO2 is a wellknown n-type semiconductor.28 Compared to pristine TiO2 NWs, the other two NW samples showed much smaller slopes in the Mott−Schottky plots, indicating significantly higher charge carrier densities. Oxygen vacancy has generally been regarded as the main electron donor for TiO2 materials,29 and the photocurrent enhancement by hydrogen treatment was previously suggested to increase the oxygen vacancies in the TiO2 structures during hydrogenation.16 As similar photocurrent enhancement and Mott−Schottky plots have been measured in our experiment by Sn doping, we expect that a similar effect of oxygen vacancy increase exists in our synthesized Sn/TiO2 NWs, although other sources of electron carriers can also contribute the improved photoactivity in the UV region. For instance, it has been reported that the potential difference between TiO2 and SnO2 allows photoelectrons to easily migrate from the TiO2 surfaces to the SnO2 conduction band, which can result in a decrease in the radiation combination of photoinduced electrons on the TiO2 surfaces.23 In addition, the formation of mixed-cation composition (SnxTi1−xO2) at the interface and associated modulation of electronic properties also facilitates the exciton generation and separation through TiO2 interface,21 which can also increase the measured photocurrent. Finally, the chemical and structural stability during PEC water splitting is another important factor to evaluate for the potential PEC materials. Time-dependent measurement carried out on a representative Sn/TiO2 (Sn/TiO2-12) NW array displayed a highly stable photocurrent density of ∼1.8 mA/cm2 versus 0 V Ag/AgCl with repeated on/off cycles correlated to the simulated solar light (Figure 4c). Extended measurement time under continuous solar illumination for over 12 h showed that this Sn/TiO2-12 NW array maintained a similar photocurrent density (Figure 4d) and had not shown any noticeable structural degradation or photocurrent decrease after a few weeks of repeated tests. These experiments suggested the excellent stability and potential long-term solar conversion application for our Sn/TiO2 NWs. In summary, we have demonstrated that the controlled doping of Sn into TiO2 NWs can significantly enhance the PEC conversion efficiency. By tuning the acidity and precursor ratios, a low level of Sn was incorporated into single crystal TiO2 NW structures by a hydrothermal synthesis with the actual doping ratio of Sn/Ti controlled below 2%. Compared to pristine TiO2 NW photoanodes, the photocurrent of Sn-doped 1507

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TiO2 NWs increased substantially with up to ∼100% of photocurrent increase. The highest photoconversion efficiency obtained from our Sn-doped TiO2 NWs was ∼1.05%, much better than pristine TiO2 NWs without Sn doping, and subsequent H2 annealing further enhanced the photoconversion efficiency of Sn/TiO2 NWs to ∼1.2%. The IPCE measurement revealed that the increase of photocurrent was mainly due to the contribution from UV region. The electrochemical impedance measurement proved that the photocurrent increase resulted from the increased charge carrier densities. We believe that this reported controlled Sn doping method can be well suitable for sensitizing other TiO2 materials. As the Sn/TiO2 NW photoanodes are convenient to fabricate and highly stable, they can serve as a good substitution for TiO2 in a variety of solar energy driven applications including PEC water splitting,30 photocatalysis,31 and solar cells.32

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ASSOCIATED CONTENT

* Supporting Information S

Details of experimental procedures, additional TEM image, EDX, XPS, and UV−vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

M.X. and P.D. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.Z. thanks the following funding agencies for supporting this work: the National Science Foundation of China (21071033), the Program for New Century Excellent Talents in University (NCET-10-0357), the Shanghai Pujiang Program (10PJ1401000), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Fudan University Startup.

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