Aqueous Growth of Pyramidal-Shaped BiVO4 Nanowire Arrays and

Dec 10, 2009 - Jinzhan Su , Liejin Guo , Ningzhong Bao , and Craig A. Grimes. Nano Letters 2011 11 (5), 1928-1933 ... Jinzhan Su , Xinjian Feng , Jenn...
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DOI: 10.1021/cg9012125

Aqueous Growth of Pyramidal-Shaped BiVO4 Nanowire Arrays and Structural Characterization: Application to Photoelectrochemical Water Splitting

2010, Vol. 10 856–861

Jinzhan Su,†,‡ Liejin Guo,† Sorachon Yoriya,‡ and Craig A. Grimes*,‡ †

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, PR China and ‡Department of Electrical Engineering, Department of Materials Science and Engineering The Pennsylvania State University, University Park, Pennsylvania 16802 Received October 2, 2009; Revised Manuscript Received November 14, 2009

ABSTRACT: BiVO4 thin films comprised of ordered arrays of pyramidal-shaped nanowires vertically oriented to a fluorine doped tin oxide coated glass substrate have been successfully fabricated by seed-mediated growth in an aqueous BiVO4 suspension. The effect of the growth temperature, 40 to 95 °C, pH, stirring and reaction time on the resulting BiVO4 film morphology was investigated using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy with a growth mechanism proposed. We consider the effects of film morphology in application to photoelectrochemical water splitting.

Introduction Nanostructure materials have attracted considerable attention due to their high surface-to-volume ratios and sizedependent properties.1,2 One dimensional (1-D) nanocrystalline nanotube3-5 and nanowire6 arrays have been investigated for potential enhancement of electron percolation pathways as well as improved ion diffusion at the semiconductorelectrolyte interface. To that end, a great many semiconductor nanotube/wire arrays have been fabricated, and have shown enhanced efficiency for photocatalysis, photoelectrolysis, and solar cells.7-9 However, most such nanostructures have been made of large band gap semiconductors such as TiO2 or ZnO and have limited utility with respect to solar spectrum light. BiVO4, known for its ferroelastic10 and ion conductive11 properties, as well as use as a yellow pigment,12 has recently been under investigation for use in solar water photoelectrolysis13,14 because of its low band gap (2.4 eV) photocatalytic properties.15,16 One-dimensional arrays of narrow band gap semiconductors offer promising applications in solar energy harvesting, motivating us to grow BiVO4 nanowire arrays. Neves and Trindade grew microscale BiVO4 rectangles on glass substrates immersed within a chemical bath the temperature of which was slowly increased from room temperature to 85 °C.17 EDTA ligands were added to the Bi(NO3)3 and NaVO3 containing aqueous precursor to form complex [Bi(EDTA)-], avoiding spontaneous formation of BiVO4 precipitate. When the solution temperature increased Bi3þ species were gradually released forming a film comprised of BiVO4 microrectangles approximately 6 μm by 12 μm by 15 μm.17 Zhou and co-workers18 reported flower-like bundles of crystalline BiVO4 microtubes synthesized by reflux of a BiVO4 precipitate suspension. The individual microtubes possess a square cross section, approximately 800 nm by 800 nm, with lengths of 2-5 μm.18 The flower-like growth of the microtube bundles indicated that they grow from polycrystalline nuclei along favorable directions. *To whom correspondence should be addressed. E-mail: cgrimes@engr. psu.edu. pubs.acs.org/crystal

Published on Web 12/10/2009

In this study we report on the seed-layer mediated growth of pyramidal-shaped BiVO4 nanowire arrays in aqueous solution; to the best of our knowledge, no 1-D vertically oriented BiVO4 arrays have previously been reported. Structure and photoelectrochemical properties of the nanostructured arrays are investigated in detail, with a possible growth mechanism suggested. Experimental Section BiVO4 nanostructures were grown on fluorine-doped tin oxide (FTO) coated glass slides. FTO coated glass (TEC-15 15Ω/sq., 1.25 cm  2.5 cm) were cleaned by sonication in acetone, water, and 2propanol, then blown dry with a stream of nitrogen. A 100-nm-thick BiVO4 seed layer was then deposited on the FTO coated glass by spincoating a solution onto the sample at 4000 rpm and firing at 400 °C for 5 h in air; no change in conductivity of the FTO coating was observed. The spin-coating precursor was made as follows: 0.005 mol of Bi(NO3)3, 0.005 mol of NH4VO3 and 0.01 mol of citric acid were dissolved into 15 mL of 23.3% HNO3 aqueous solution. Then 0.08 g of polyvinyl alcohol and 0.25 mL of acetic acid were added into 1 mL of this blue-colored solution. The BiVO4 coated FTO glass slides were then immersed in a BiVO4 solution, FTO side up, within an Erlenmeyer flask that was then placed within a temperature controlled water bath and connected to a condenser as a reflux system. To prepare the BiVO4 suspension, 0.002 mol of Bi(NO3)3 and 0.002 mol of NH4VO3 were dissolved in 50 mL of 14% HNO3 aqueous solution, then 14.3 g of NaHCO3 was added under vigorous stirring. At a pH of 6.5, a suspension of yellow BiVO4 precipitate was formed. The solution was refluxed under magnetic stirring at different temperatures for 6 h after which the BiVO4 coated FTO glass slides were removed from solution, rinsed with deionized water, and dried under flowing nitrogen. The samples were thermally annealed at 500 °C for 0.5 h in air. Figure 1 shows field emission scanning electron microscope (FESEM, JEOL JSM-6700) images of BiVO4 nanostructures deposited at different temperatures (no post annealing). At 40 °C, Figure 1a, arrays of nanowires 50-100 nm diameter with a slight conical tapering were grown. At 60 °C, Figure 1b, nanowires showing a distinct pyramidal topology are obtained, with a base width of 500 nm that tapers to a tip of some 25 nm. The insets of Figure 1a,b show cross-sectional views of mechanically fractured samples indicating, in general, perpendicular growth from the substrate. When the temperature was elevated to 80 °C, Figure 1c, pyramidal arrays with less r 2009 American Chemical Society

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Figure 1. Field emission scanning electron microscope (FESEM) images of samples deposited at (a) 40 °C, (b) 60 °C, (c) 80 °C, (d) 95 °C. Insets of (a) and (b) show cross sectional views. sharply defined edges and tips were obtained. Figure 1d shows the topology of a 95 °C grown film; the resulting nanostructures are granular in nature. BiVO4 is known to exist in three polymorphs: zircon structure with tetragonal system (z-t), and scheelite structure with monoclinic (s-m) and tetragonal (s-t) systems.19 The XRD patterns of BiVO4 (s-m) and BiVO4 (s-t) are quite similar, but BiVO4 (s-m) can be distinguished by the existence of a 2θ peak at 15° and peak splitting at 18.5°, 35°, and 46°.15 Figure 2 shows the glancing angle X-ray diffraction (GAXRD; X’Pert Pro MPD, Philips, Netherlands) patterns of samples deposited at different temperatures using Cu KR (0.15406 nm) radiation. Since the GAXRD patterns of all samples exhibit a peak at 15°, they can be confirmed as monoclinic structure (space group I2/b) with cell constants of a = 5.196 A˚, b = 5.094 A˚, and c = 11.704 A˚ (JCPDS 751867). Peaks at 18.5°, 35°, and 46° do not clearly show peak splitting which can be a result of peak broadening due to their small grain sizes. Figure 3 shows FESEM images of 40 and 60 °C deposited samples after annealing at 500 °C for 0.5 h. The 40 °C synthesized nanowires were extensively damaged, agglomerating into clusters 150-400 nm in diameter. The nanopyramid arrays lose their faceting transforming into cones. We believe the changes seen in the surface topology are to due surface-energy minimization.20 The GAXRD patterns (not shown) of 40 and 60 °C deposited samples showed no noticeable change after being annealed at 500 °C for 30 min, while the conductivity of the FTO layer remained the same. The BiVO4 nanowire morphology and microstructure were investigated by transmission electron microscopy (TEM, Philips (FEI) EM420T, tungsten emitter, operated at 120 kV). Figure 4a shows a TEM image of the tip of an unannealed nanowire deposited at 40 °C which appears comprised of 5-10 nm diameter nanoparticles. Figure 4, panels b and c show, respectively, a TEM image of the tip of a 60 °C deposited nanopyramid unannealed and 500 °C annealed. The 60 °C deposited nanopyramid appears largely crystalline while dotted with 2-5 nm diameter particles. The corresponding selected area electron diffraction (SAED) pattern (the inset of Figure 4c) taken

Figure 2. X-ray diffraction patterns of seed layer and samples prepared at 40, 60, 80, and 95 °C. from the annealed nanopyramid of Figure 4c reveals the nanopyramid to be single crystalline. The d-spacings measured from SAED (zone axis [010]) are 5.82 A˚ and 4.72 A˚, which well agree with the lattice spacings of (002) and (101) monoclinic BiVO4. The growth direction is [001]. The seed layer is polycrystalline, as determined by XRD, having different c axis orientations for different grains. The BiVO4 nanowires, which grow along the c axis, will grow in different directions hence as seen in the FESEM images some of the pyramids are tilted from each other. The growth of a tilted wire/pyramid can be blocked by an adjacent perpendicular structure. Only vertically oriented structures can grow freely, resulting in well-aligned vertically oriented nanowire/pyramid arrays.

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Figure 3. FESEM images of 500 °C 30 min annealed samples deposited at (a) 40 °C, and (b) 60 °C. Insets on the left show top view images at higher magnification, while insets on right show cross sectional views.

Figure 4. Transmission electron microscope (TEM) images of samples deposited at (a) 40 °C, (b) 60 °C, and (c) 60 °C then annealed at 500 °C for 30 min. Inset of (c) shows selected area electron diffraction (SAED) pattern. The pH of the reflux solution is critical for inducing BiVO4 film growth. The starting pH of the precursor was adjusted to 6.5 by adding an appropriate amount of NaHCO3. As the precursor heats during deposition carbonic acid forms and dissociates into CO2 and water. As CO2 escapes from the solution18 the precursor pH increases from 6.5 to approximately 8 and the BiVO4 nanostructures grow on the seed layer. Starting precursor pH values lower than 6 or higher than 7 saw no growth of BiVO4 film on the substrate, and no noticeable change to the spin-coated BiVO4 seed layer. For a precursor pH lower than 6, bright yellow particles in the BiVO4 suspension aggregate and settle to the bottom of the flask even with magnetic stirring. FESEM imaging (not shown) indicates that the precipitate is of a microrod structure, with XRD indicating that the microrods are well crystallized with tetragonal structure. We note that without stirring precipitate would settle to the bottom of Erlenmeyer flask, with growth of a compact layer on the substrate. With the substrate vertically oriented in the Erlenmeyer flask, the lower portion of the substrate immersed in the precipitate showed growth of a compact BiVO4 layer, with no film growth on the substrate above the precipitate. To study the film growth process FESEM images were taken for films obtained over different growth periods. Figure 5a is a FESEM image of the seed layer. Figure 5, panels b and c show, respectively, top-view images of samples grown at 60 °C for 30 and 60 min. After 30 min growth square islands, approximately 100 nm by 100 nm, grew from the seed layer; a more clearly defined pyramidal nanowire array is seen in Figure 5c. After 6 h of growth, Figure 5d, pyramidal arrays of square cross section with sharp tips were grown, clearly indicating a layer-by-layer growth process. No film growth was observed without the seed layer. Crystal grain formation could arise from decomposition of the amorphous nanoparticles (precipitate) to individual crystal grains, or from

dissolution-recrystallization of the nanoparticles. During deposition the crystal grains attach to the seed layer with vanadate (VO3-) and bismuth (Bi3þ) ion incorporation from the precursor. High pH growth of BiVO4 onto the seed layer is prevented due to the low concentration of VO3- and Bi3þ in the precursor.21 It was reported that BiVO4 possesses a layered structure of Bi-V-O units stacked parallel to the c axis.19 The binding plane should be the crystal face perpendicular to c axis; thus growth occurs along the c axis to form one-dimensional nanostructure arrays in agreement with the XRD pattern that is dominated by the (004) peak. As illustrated in Figure 6, at 40 °C the precipitate is decomposed to 5-10 nm crystal grains that stack onto the seed layer along the c axis forming nanowire arrays. At 60 °C, as a result of the higher temperature, smaller crystal grains are achieved with an increase in the vanadate ion and bismuthyl concentrations in the solution due to a temperature induced change of pH. At higher temperatures, for example, 80 °C, the VO3- and Bi3þ concentrations are higher, while the pH changes more rapidly due to greater CO2 evolution. At 95 °C, the BiVO4 growth was so rapid that there was little opportunity for c axis alignment onto the seed layer. As shown in Figure 2, the XRD pattern of the 95 °C deposited film is similar to a powder diffraction pattern, with the main peak at 28.8°, indicating that the c-axis of the crystal grains is almost randomly oriented. The film thickness increases with deposition time at a rate influenced by temperature, pH, and stirring; however, further studies are needed to understand the inter-relationships of the length determining factors. We note that stirring is important for forming the 1-D structures, as unbonded crystal grains are flushed away avoiding growth of a densely packed film. Without stirring, see Figure 7a, the crystal grains form a dense packing layer on the substrate; Figure 7b is a cross-sectional depiction of a dense film grown without stirring during the deposition. Photoelectrochemical Properties. The photoelectrochemical properties of the samples were characterized using a three electrode

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Figure 5. FESEM top-view images of samples grown at 60 °C for durations of (a) 0 min (seed layer); (b) 30 min; (c) 60 min; and (d) 6 h.

Figure 6. Schematic illustration of the growth of pyramidal-shaped nanowire array at 40 °C, and nanopyramid arrays at 60 °C. photochemical cell with the BiVO4 samples as the working photoelectrode, saturated Ag/AgCl reference electrode, and platinum foil counter electrode. 0.5 M Na2SO4 solution was used as the electrolyte.22 A scanning potentiostat (CH Instruments, model CHI 600C) was used to scan the potential at a rate of 50 mV/s and measure the current. The measurements were carried out in conventional threeelectrode cell in a quartz beaker at room temperature. A Spectra Physics Simulator with an illumination intensity of 1 sun (AM 1.5, 100 mW/cm2) with a filter to remove light of wavelength below 400 nm was used as the light source. A PHIR CE power meter was used to calibrate the input power before the photoelectrochemical measurements. Figure 8 shows the linear sweep voltammetry of samples illuminated through the FTO coated glass substrate; the nanowire

arrays films deposited at 40 °C are about 0.6 μm thick, and the films deposited at 60, 80, 95 °C are approximately 3 μm thick. The nanowire array film deposited at 40 °C achieved higher photocurrent values than the nanopyramid array films, presumably due to their higher geometric surface area. The 500 °C annealed 60 °C synthesized nanopyramid array films demonstrate the highest photocurrent amplitude, 0.4 mA/cm2 at 1 V vs Ag/AgCl, as its crystallization defects which facilitate recombination were reduced while its nanoarray structure is retained after the 500 °C anneal. The photocurrents measured for the 500 °C annealed 40 °C deposited films were very noisy; as shown in the FESEM image, 500 °C annealing of the 40 °C deposited nanowire arrays resulted in nanowire agglomeration into large clusters, which may result in

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layer. Stirring is important for formation of the nanostructured array topology,; otherwise, a compact BiVO4 layer was grown. We believe such a simple and mild synthetic route may be extended to fabricate one-dimensional architectures of other layered-structure materials. Acknowledgment. Support of this work by DE-FG0206ER15772 is gratefully acknowledged. J.S. was supported by a scholarship grant from the China Scholarship Council. All experiments were performed at The Pennsylvania State University.

References Figure 7. (a) Depiction of growth kinetics of compact BiVO4 layer achieved with no solution stirring, and (b) cross sectional FESEM image of compact BiVO4 layer obtained without solution stirring.

Figure 8. Linear sweep voltammetry of films deposited at different temperatures: the 60 °C synthesized sample was also annealed 500 °C, 30 min. Measured in 0.5 M Na2SO4 aqueous solution, under AM 1.5 illumination passing through the fluorine-doped tin oxide (FTO) coated glass substrate. the FTO substrate being in contact with the electrolyte. As shown in Figure 1D, nanopyramid arrays grown at 95 °C possess large amounts of surface defects which may be responsible for the reduced photocurrent values. When illuminated through the electrolyte, the photocurrents for the samples deposited at different temperatures were in the same rank order but significantly reduced to values, ranging from 1/5 to 1/10 of those measured with illumination through the FTO glass substrate, a behavior typical of highly porous electrodes.23

Conclusions We have synthesized nanowire/nanopyramid BiVO4 arrays via a facile surfactant-free seed-mediated growth in aqueous solution. The morphologies of the nanostructured arrays are strongly dependent on deposition temperature. At a reaction temperature of 40 °C, polycrystalline BiVO4 nanowire arrays are grown, while at 60 °C nanopyramid arrays are grown. Starting from the seed layer, the growth process is layer by layer along the c axis (001). After a 500 °C 30 min anneal, the nanopyramids turn into single crystal cones, while the nanowires agglomerate into clusters. Before annealing the nanowire array films show higher photocurrents than the nanopyramid array films, but after annealing the nanopyramid array films demonstrate photocurrents of greater magnitude than the unannealed nanowire arrays. In addition to temperature, an appropriate starting pH value of the precursor was found to be critical for the BiVO4 growth on the seed

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