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Hydrothermal Synthesis of Bi12TiO20 Nanostrucutures Using Anodized TiO2 Nanotubes and Its Application in Photovoltaics Sankaran Murugesan,† York R. Smith, and Vaidyanathan (Ravi) Subramanian* Chemical & Metallurgical Engineering Department, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557
ABSTRACT A one-step synthesis method to prepare bismuth titanate nanostructures that belong to the sillenite family group of compounds (A12BO20) using TiO2 nanotubes (T-NT) is reported. Preformed TiO2 nanotubes prepared by anodization of titanium foils have been used in a hydrothermal procedure with a bismuth salt to prepare cube-like nanostructures. Surface characterization has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results indicate that these nanostructures are cube-like crystalline Bi12TiO20 (BTO) and demonstrate tight size and dimensional control. The photoelectrochemical properties of the BTO have been examined and compared with the properties of T-NT. The BTO demonstrates a higher photocurrent density compared to T-NT in the presence of both UV-vis as well as visible light illumination. Depending on the magnitude of external bias, a stable increase of at least 40 and 10% in the values of photocurrent has been noted with the BTO compared to T-NTunder UV-vis and visible light illumination, respectively. SECTION Energy Conversion and Storage
itanium dioxde (TiO2) is considered a good material for solar energy conversion due to its photostability over a wide pH range, compatibility with other lightharvesting materials, and environmental friendliness.1 TiO2 nanoparticles cast as thin films, and its composites with other materials prepared on conducting substrates have been extensively studied for many solar-based applications such as photovoltaics and photoelectrocatalysis.2-5 However, films prepared from TiO2 nanoparticles inherently have many interparticle grain boundaries which can reduce photocurrent by functioning as recombination centers for photogenerated electron-hole pairs. To counter this, recently, TiO2 in the form of hollow nanotubes (T-NTs) has been synthesized via anodic oxidation.6 The T-NTs display better photoelectrochemical properties such as reduced recombination rates between photogenerated charges compared to nanoparticles cast as films due to the absence of interparticle boundries that promotes efficient transport of photogenerated electrons.7,8 This innate property of T-NTs appears to be promising for photovoltaic6,9,10 as well as photocatalytic applications.11,12 However, one of the limitations of TiO2 is that it is only effective under the ultraviolet (UV) region of the solar spectrum. Therefore, methods to shift the absorption spectra from UV to visible light have been investigated.13-16 A few reports suggest that nitrogen- and carbon-doped T-NTs can demonstrate visible light photoactivity.16-18 Other approaches that explore the development of T-NT-based composite materials capable of absorbing visible light are required. Sillenite type
compounds (I23 space group) of the form Bi12MO20 (M=Pb, Ni, Al, Ti, Fe, Si) have gained much interest due to their light absorption capabilities and unique noncentrosymmetry crystal structure.19-23 Among the sillenite type compounds, bismuth titanate (Bi2TiO20) receives considerable attentation because of its high referective index, electro-optic coefficient, and application in photocatalysis.24-26 The Bi12TiO20 crystal consists of seven-oxygen-coordinated Bi polyhedra, which is the corner shared by other identical Bi polyhedra and with TiO4 tetrahedra. Visible light absorbance of bismuth titanates is due to the contribution of 6s electrons of Bi in the valence band along with O 2p orbitals (Supporting Information Figure S1). Various methods such as chemical solution decomposition (CSD),26 isopropanol-assisted hydrothermal synthesis,27 and coprecipitation methods28 have been used to prepare bismuth titanate of the sillenite structure (Bi12TiO20). However, many of these preparation techniques typically involve several additives and complex synthesis steps and still produce nanoparticles that require immobilization for photovoltaic applications. Coating such materials over a suitable conducting substrate may reduce their photoactivity even further. There is a need to examine alternate methods to develop
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Received Date: March 27, 2010 Accepted Date: April 26, 2010 Published on Web Date: May 05, 2010
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DOI: 10.1021/jz100404v |J. Phys. Chem. Lett. 2010, 1, 1631–1636
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Figure 1. Scanning electron micrographs (SEM) of (a) TiO2 nanotubes (T-NTs), (b) material formed after hydrothermal treatment of the T-NTs, (c) a higher-resolution image of (b), and (d) an image showing the dimensions of nanocube edges.
Figure 2. Glancing angle X-ray diffraction patterns of TiO2 nanotubes (T-NTs) and hydrothermal synthesized Bi12TiO20 nanocubes (BTO). A(hkl) indicates reflections due to the anatase phase of TiO2, Ti(hkl) indicates reflections due to the base titanium metal, and S(hkl) indicates reflections due to sillenite crystal phase Bi12TiO20. All peaks are indexed to standard JCPDS data cards.
Bi2TiO20 on a conducting substrate. In this work, a lowtemperature hydrothermal method has been employed to transform T-NTs formed over Ti foils by anodization to Bi12TiO20 sillenite nanstructures (BTO). To the best of our knowledge, no report examines the applicability of preformed anodized T-NTs as one of the sources for synthesis of Bi12TiO20. The formation of BTO has been confirmed by different surface analysis tools, and its photoelectrochemical activity has been compared to T-NTs. Preliminary results indicate that the application of the hydrothermally synthesized BTO using T-NT has a potential in photovoltaic devices. Surface Characterization. The SEM image of the T-NTs is shown in Figure 1a. Uniformly sized hollow nanotubes with a diameter of 50 nm can be observed on the anodized surface. Other aspects of the T-NT prepared by anodization have been discussed in earlier reports.12,29-32 After hydrothermal treatment in the presence of a bismuth salt solution, the surface of the T-NTs is almost completely covered with the product formed by the treatment. Fairly identical and uniformly distributed structures that are completely different from the hollow T-NTs can be obsereved on the surface. Figure 1b and c shows the SEM images of the treated surface at different magnifications. A closer anlaysis of a representative area shows that the treated surface has cube-like columnar structures with edges of ∼90 nm, as shown in Figure 1d. The phase and purity of samples before and after hydrothermal treatment were determined by glancing angle X-ray diffraction patterns (GXRD). The GXRD diffraction patterns with basal planes (hkl values) corresponding to the crystalline phases are identified for each sample and marked in Figure 2. For the T-NTs, the major peaks in the diffraction pattern account for the anatase form of the oxide (JCPDS No. 211272, labeled A) and the titanium metal (JCPDS JCPDS No. 441294, labeled Ti). Similar observations for T-NTs were reported in related work.7,11 The nanocubes formed after hydrothermal treatment were noted to match with the cubic
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lattice parameters for Bi12TiO20 crystals (a=b=c=10.174 Å). Standard data cards (JCPDS No. 34-0097, labeled S) were used to identify the phase of the material. A 100% intensity peak of the (310) plane at 2θ=27.70° and the (400) plane at 2θ = 35.26° has been identified to Bi12TiO20 (BTO). Similar XRD peaks were observed by other groups in Bi12TiO20 synthesized using different methods.33-35 Some of the remaining peaks for BTO can be noted to correspond with the Ti and/or anatase phase peaks. This suggests that a very thin layer of the crystalline BTO is formed on the T-NT surface as a result of the hydrothermal treatment. We speculate that the hydroxide groups present in the nanotubes react with highconentration Bi salt (0.1 M solution) under hydrothermal conditions (pressure and temperature), causing the dissolution of the T-NTs, and to the formation of the sillenite structure. Further investigation on the role of TiO2 nanotubular precursors, its morphology, and the hydrothermal conditions on the formation of Bi12TiO20 sillenite structures is under investigation. TEM can be used to counter check the SEM and XRD results. It was used to examine the physical features of the nanocubes and identify the material. A representative TEM of the nanocubes is shown in Figure 3a. The image shows that the geometry of the nanostructures formed on the treated T-NT surface is a cube with a dimension of ∼90 nm. Highresolution TEM (HRTEM) of the samples was also obtained to determine the crystalline properties of the material. Figure 3b shows the HRTEM, and its inset shows the selected-area electron diffraction pattern (SAED) of one of the nanocubes. Very distinct and clear fringes can be noted from the SAED pattern, and the material is identified as crystalline Bi12TiO20. The TEM, HRTEM, and SAED analyses thus compliment the GXRD and SEM results and confirm the crystalline nature of the nanostructures formed by the hydrothermal treatment.
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DOI: 10.1021/jz100404v |J. Phys. Chem. Lett. 2010, 1, 1631–1636
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Figure 3. (a) High-resolution transmission electron micrograph (HRTEM) image of BTO, (b) fast Fourier transformation-HRTEM image of BTO with the inset of a selected-area electron diffraction (SAED) pattern confirming that the material is crystalline BTO. Figure 4. The current-voltage plot of (a,c) T-NT and (b,d) BTO in the (a,b) absence and (c,d) presence of UV-vis illumination. The electrolyte used is an aqueous 1 M KOH solution.
Photoelectrochemical Properties. The optical properties of the synthesized materials were compared using diffuse reflectance measurements. This information is provided in Supporting Information Figure S2. It can be noted that the BTO shows a higher absorbance in the presence of visible light compared to the T-NT, while the BTO and T-NT show almost similar responses under UV illumination. To further examine if the BTO indeed demonstrates visible light response, we also performed incident photon to current conversion effeciency (IPCE) measurements. The results are reported in Supporting Information Figure S3. The photoelectrochemical response of the BTO matches well with the absorbance spectra of the BTO. The absorbance and IPCE measurements thus indicate that the BTO films demonstrate visible light photoelectrochemical activity. The photoelectrochemical properties of the anodized T-NT are well-documented in the literature.36-39 In this section, we present the comparative analysis of the photoelectrochemical responses of the BTO and T-NT. All measurements were performed in 1 M aqueous KOH as the electrolyte in the absence and presence of illumination using a benchtop solar simulator. The photocurrent was recorded as a function of applied potential. The potential was varied between -1.0 and þ0.2 V with a scan rate of 10 mV s-1 and Ag/AgCl as the reference electrode. The photocurrent was normalized to the geometrical illuminated surface area of the electrodes. This normalized photocurrent versus voltage plot is shown in Figure 4. It is noted that the photocurrent with BTO is higher than the photocurent with T-NT under positive bias. For example, the maximum photocurrent density for T-NTs is 1.45 mA cm-2 at 0.2 V. BTO showed a photocurrent of 2.4 mA cm-2 at 0.2 V, indicating a nearly 60% increase in photocurrent density. It is intersting to note that at potentials lower than ∼-0.5 V, the photocurrent with TiO2 is higher than that with BTO. The zero current potential for BTO is less negative than that for TiO2. The difference in the location of the zero current potential for the two electrodes can be attributed to the conduction band position of the BTO being higher than that of the T-NT and/or the extent of the defect sites in the photoelectrodes. Further analysis using Mott-Schottky plots will be needed to understand this phenomena in greater detail. Control experiments were performed in the absence of any illumination. The dark scans (Figure 4a and b) display a
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Figure 5. Potentiostatic plot at 0.0 V versus Ag/AgCl of (a) T-NT under visible light irradiation, (b) BTO under visible light irradaiation, (c) T-NTunder UV-vis illumination, and (d) BTO under UVvis illumination. The electrolyte used is an aqueous 1 M KOH solution.
photocurrent density of
