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Controlled growth of hematite (α-Fe2O3) nanorod array on fluorine doped tin oxide: Synthesis and photoelectrochemical properties
Electrochemistry Communications 13 (2011) 951–954
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Controlled growth of hematite (α-Fe2O3) nanorod array on fluorine doped tin oxide: Synthesis and photoelectrochemical properties H.K. Mulmudi a, N. Mathews a,⁎, X.C. Dou a, L.F. Xi a, S.S. Pramana a, Y.M. Lam a,⁎, S.G. Mhaisalkar a, b a b
School of Materials Science and Engineering, Nanyang Technological University, Singapore Energy Research Institute @NTU (ERI@N), Nanyang Technological University, Singapore
a r t i c l e
i n f o
Article history: Received 18 May 2011 Received in revised form 7 June 2011 Accepted 7 June 2011 Available online 15 June 2011 Keywords: Hematite nanorods Electrochemical impedance spectroscopy Photoelectrochemical solar cell Minority charge carriers
a b s t r a c t Hematite nanorods were grown on fluorine doped tin oxide (FTO) substrates by hydrothermal means utilizing urea as a pH regulating agent. XRD for nanorods revealed pure hematite phase after annealing at 500 °C for 30 min with preferential orientation in the [110] direction. Electrochemical impedance spectroscopy was carried out to investigate the electrical properties. Using Mott–Schottky analysis, charge carrier density was estimated to be 5.62 × 10 19 cm − 3 in the hematite nanorod array. Among the nanostructures, the n-type hematite nanorod array showed the best conversion efficiency among the samples studied in a two electrode photoelectrochemical cell. Photocurrent measurements versus light intensity were performed to investigate the device performance and the limiting factors to the performance were attributed to the short diffusion length of minority charge carriers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synthesizing controlled arrays of aligned semiconducting metal oxide nanorods on conducting substrates have attracted a lot of attention in recent years due to their potential applications in photovoltaic device[1], light emitting diodes [2], gas sensors [3] and electrode material for electrochemical batteries[4]. One such semiconducting material is hematite (α-Fe2O3) which is an n-type semiconductor. It is abundant, low cost, highly stable, nontoxic and is gaining importance in photoelectrochemical devices due to its band gap (Eg = 2.2 eV) [5]. Nevertheless, the overall photoelectrochemical conversion ratios have been severely hampered due to two conflicting properties: a) long photon penetration depth and b) short hole (minority charge carrier) diffusion lengths. Ideally, about 118 nm of hematite film is necessary to absorb 63% of the incoming solar radiation at λ = 550 nm. Thus, majority of the photogenerated carriers are produced within a distance of 100–120 nm from the semiconductor electrolyte interface. However, due to short hole (minority carriers) diffusion lengths (2–4 nm), most of the photogenerated carriers recombine even before reaching the semiconductor electrolyte interface [6,7]. Solution based methods for growing aligned hematite nanorods on conducting substrates offer an inexpensive alternative which could be employed to reduce recombination of photogenerated carriers, increase the surface area and hence yield better photochemical conversion efficiencies [8]. Herein, we report an environmentally friendly
⁎ Corresponding authors. E-mail addresses: [email protected] (N. Mathews), [email protected] (Y.M. Lam). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.06.008
route to synthesize hematite nanorod array on FTO by hydrothermal means. Urea which is nonionic, nontoxic, inexpensive, stable, and water-soluble, was used as a pH regulating agent which hydrolyzes at elevated temperatures leading to the formation of hydroxide ions. Electrical properties of these nanostructures were investigated by Mott–Schottky analysis in photo-electrochemical solar cells. 2. Experimental section 2.1. Synthesis An aqueous solution (10 ml) of 0.2 M FeCl3 and 0–0.6 M NH2CONH2 were sealed in a 20 ml glass vial and heated at 100 °C for 24 h. FTO was placed vertically in these glass vials with the conducting edge facing the wall of the vial. After the reaction, the film formed on FTO was thoroughly rinsed in DI water and annealed at 500 °C for 30 min to get the desired phase (α-Fe2O3). The notation for the samples (molar ratio of urea/FeCl3) are F1(0), F2(0.75), F3(1.5) and F4(3.0) respectively. 2.2. Characterization and measurements Structural and phase characterizations of the as-prepared film and annealed film were done by X-ray diffraction (XRD) using Bruker AXS (D8 ADVANCE) X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). The surface morphology of annealed samples was characterized by FESEM (JEOL, JSM-7600 F, 5 kV). Optical absorption measurements films were performed using UV–visible absorption spectrometer (Shimadzu UV 2501PC).
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H.K. Mulmudi et al. / Electrochemistry Communications 13 (2011) 951–954
Electrochemical impedance spectroscopy (EIS) measurements were carried out using an automated potentiostat (Solartronanalytical, 1470E) coupled with a frequency response analyzer (Solartron-analytical, 1255B) in a three-electrode electrochemical system with 0.1 M NaOH electrolyte. Platinum mesh and saturated calomel electrode (SCE) were employed as counter and reference electrodes respectively. Fabrication of photoelectrochemical cell (PECC) was done by sandwiching α-Fe2O3/FTO electrode and platinized FTO as a counter electrode with a 25 μm thick hot-melt spacer (Surlyn, Dupont). An iodine/iodide based electrolyte (Z960) was introduced between the sandwiched electrodes. 3. Results and discussion The chemical reactions leading to the formation of α-Fe2O3 can be understood from the following equations: NH2 CONH2 + 3H2 O → 2NH4 OH + CO2
ð1Þ
FeCl3 + 3NH4 OH → FeOOH + 3NH4 Cl + H2 O
ð2Þ
Δ
2FeOOH → Fe2 O3 + H2 O
ð3Þ
Fig. 1 shows the top view FESEM images of samples F1–F4, annealed at 500 °C for 30 min. It was clearly observed that the morphology of the products formed on FTO depends on the concentration of urea (pH regulating agent). In the absence of urea there was no significant precipitation of any product on FTO (F1, see Fig. 1(a)). F2 and F3 samples show nanorod morphology, however F3 is clearly superior and possess very good alignment with respect to FTO (inset, Fig. 1(c)). As the molar ratio of urea/FeCl3 increased from 0.75 to 3, the morphology of FeOOH precipitates on FTO changed due to the increase in the release of hydroxide ions in the system which is clearly evident from the FESEM images (F2–F4, see Fig. 1(b)–(d)). The
thicknesses of the precipitates formed on FTO depend on the amount of urea and were found to be 170 nm (F2), 900 nm (F3) and 1.5 μm (F4) (measured by a surface profiler (Alpha-Step IQ)). Following Eqs. (1) and (2), 1.5 mol of urea is required to react with 1 mol of iron chloride stoichiometrically assuming completion of reaction. Experimentally, when the molar ratio of urea/FeCl3 was 1.5, nucleation and growth of nanorod array of FeOOH indeed happens on FTO. The above observations imply that the optimum molar ratio between urea/FeCl3 is 1.5 to promote heterogonous nucleation and growth of nanorod array on FTO. Reactions carried out at different temperatures such as 50 °C, 80 °C and 150 °C at a urea/FeCl3molar ratio of 1.5 did not yield a good array of nanorods. Fig. 2(a) shows the XRD pattern of all the samples, which reveal α-Fe2O3 phase after annealing at 500 °C for 30 min (ICDD PDF No. 99-100-0141). No characteristic peaks for other impurities such as γ-Fe2O3 and Fe3O4 were observed. Compared to the commercially available powder diffraction pattern (Fig. 2(a) (v)), the (110) diffraction peak in F2, F3 are prominent compared to other peaks such as (012), (113) and (300), which could indicate that the α-Fe2O3 nanorods are highly oriented with respect to the substrate. Similar preferential growth orientation along [110] has also been reported for nanostructured α-Fe2O3 deposited using APCVD[7]. The electrical conductivity of hematite is known to be highly anisotropic and its lattice can be represented as an alternation of iron bilayers and oxygen layers parallel to the (001) basal plane in hexagonal coordinates[7]. Electrons move by hopping through Fe II/Fe III valence change within the iron bilayers, while electron exchange between neighboring bilayers is spin forbidden (Hund's rule)[7]. This leads to a strongly anisotropic conductivity of hematite, which is up to 4 orders of magnitude higher within the (001) basal plane (e.g., in [110] direction) than orthogonal to them[7]. The superior quality of the F3 nanostructures can be exploited in electrical transport devices including photoelectrochemical cells. A bandgap of 2.03 eV was estimated for both the samples F2 and F3 through Tauc analysis by a linear fit to the experimental (αhν) 1/2versus
Fig. 1. Top view FESEM images of samples (a) F1, (b) F2, (c) F3, inset shows cross-section image of hematite nanorod array and (d) F4.
H.K. Mulmudi et al. / Electrochemistry Communications 13 (2011) 951–954
953
[9]. The depletion width for the nanorod array (sample F3) was found to be 13 nm and using Gartner–Butler model, the maximum anodic photocurrent at 550 nm was found to be about ≈0.4 mA/cm 2. Photovoltaic characteristics (J–V plots) of PECCs for samples F2, F3 and F4 are shown in Fig. 3(a). Inset in Fig. 3(a) shows the IPCE for the nanorod array which corresponds well with the absorption spectrum of Fe2O3. The maximum conversion efficiency η = 0.05% was obtained using F3 (Voc = 0.24 V, Jsc = 0.523 mA/cm 2, FF = 0.39). The conversion efficiency of F2 was 0.01% (Voc = 0.21, Jsc = 0.187 mA/cm 2, FF = 0.35) while the performance of sample F4 was negligible (10 − 4%) compared to samples F2 and F3. The varying efficiencies of the three samples can be explained by optical and morphological considerations. Compared to sample F4, samples F2 and F3 both have a preferred orientation along [110] direction (Fig. 2(a)). As the conductivity of hematite in [110] direction is highest, the photocurrents in F2 and F3 are expected to be high, corresponding to our results. Although samples F2 (170 nm) and F3 (900 nm) both have nanorod structure, the thicker sample F3 allows complete absorption of the incident light, contributing to their higher currents. Though sample F4 (1.5 μm) is thicker than sample F3, morphologically sample F3 has a clear edge over sample F4 due to higher exposed area at the semiconductor/electrolyte interface. This coupled with the unfavorable crystallographic orientation, limits the efficiency of F4. To investigate the reason behind such low efficiencies, photocurrent measurements at varying light intensities were performed.
Fig. 2. (a) XRD patterns of: (i) FTO, (ii) F2, (iii) F3, (iv) F4 and (v) commercially available Fe2O3 (hematite phase). (b) Mott–Schottky plot of space charge capacity for samples F2 (inset),F3 and F4 in 0.1 M NaOH (R2 is the coefficient of determination for the linear fits).
hν plot. This agrees well with previously reported values of 1.9 to 2.2 eV [9]. EIS measurements were conducted to investigate the type of majority carriers utilizing Mott–Schottky analysis (signal level maintained at 10 mV, frequency was scanned from 100 kHz to 100 Hz). Fig. 2(b) shows the Mott–Schottky plots of samples F2, F3 and F4 measured in the threeelectrode electrochemical cell. Data from the high frequency region (10 kHz–30 kHz)[9], where the influence of capacitance due to double layer at the electrolyte and surface states are minimal, was used to determine the space charge capacitance (Csc). The resultant capacitance values were fit to the Mott–Schottky equation: 2
ðA=Csc Þ = ð2 = qεεo ND ÞðE−EFB −kT = qÞ where A is the surface area of the electrode, ε is the dielectric constant of iron oxide = 32 [11], εo is the permittivity of the vacuum, ND is the donor carrier concentration, V is the applied potential, k is Boltzmann's constant, and T is the temperature. The positive slope from the Mott– Schottky plots of Fig. 2(b) clearly indicates the formation of n-type iron oxide in all the samples. Donor densities of samples F2, F3 and F4 were found to be 4.47 × 10 18 cm − 3, 5.62 × 10 19 cm − 3, and 8.78 × 10 19 cm − 3 respectively which are in agreement with the reported values for nanostructured hematite films [9]. Clearly, sample F3 has 1 order more charge carrier concentration compared to sample F2. The estimated higher donor density in sample F3 suggest that this sample could have better electrical properties (lower resistivity) compared to sample F2
Fig. 3. (a) J–V plot characteristic of samples F2, F3 and F4, Inset: IPCE curve for nanorod array (F3). (b) Photocurrent versus light intensity for PECCs fabricated from samples F2, F3 and F4.
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H.K. Mulmudi et al. / Electrochemistry Communications 13 (2011) 951–954
Fig. 3(b) shows the photocurrent versus light intensity plot for samples F2, F3 and F4. The linear behavior qualitatively demonstrates that the interfacial electrochemical processes between iron oxide and electrolyte proceed smoothly [10]. This indicates that charge carrier transport to the electrolyte interface is the limiting factor. Due to the poor absorption of Fe2O3 at higher wavelengths, the optical absorption and hence carrier generation takes place further from the nanostructure surface. In order for these carriers to contribute to the cell efficiency, they have to make their way to the electrolyte surface. Due to the poor hole (minority carriers) diffusion lengths in Fe2O3 [12], many of these holes recombine before reaching the electrolyte, thus reducing the current density. 4. Conclusion We have demonstrated growth of oriented iron oxide nanorod arrays on FTO through hydrothermal synthesis. Aligned nanorods of 900 nm length have been grown at 100 °C for 24 h at molar ratio between urea/FeCl3 of 1.5. XRD analysis of 500 °C annealed samples showed a pure hematite phase. The n-type behavior was demonstrated by Mott–Schottky analysis and the donor density for the nanorods was estimated to be 5.62× 1019 cm− 3. The hematite nanorod array shows a conversion of 0.05% in a two-electrode photoelectrochemical cell with iodine/iodide based electrolyte. In comparison to other nanostructures, the nanorods showed better conversion efficiencies due to favorable [110] crystallographic orientation. Light intensity measurements revealed that the poor conversion efficiency of hematite nanorods can be attributed to the short diffusion length of minority charge carriers.
Acknowledgment We thank Robert Bosch (SEA) Pte Ltd and National Research Foundation, Singapore for providing financial support. We also thank Dr. Jinesh K.B., Mak Wai Fatt, Dharani S and K.R.G Karthik for valuable discussions.
References [1] N. Mathews, B. Varghese, C. Sun, V. Thavasi, B.P. Andreasson, C.H. Sow, S. Ramakrishna, S.G. Mhaisalkar, Nanoscale 2 (2010) 1984–1998. [2] H. Guo, J. Zhou, Z. Lin, Electrochemistry Communications 10 (2008) 146–150. [3] L. Liao, H.B. Lu, J.C. Li, H. He, D.F. Wang, D.J. Fu, C. Liu, W.F. Zhang, The Journal of Physical Chemistry C 111 (2007) 1900–1903. [4] J. Liu, Y. Li, H. Fan, Z. Zhu, J. Jiang, R. Ding, Y. Hu, X. Huang, Chemistry of Materials 22 (2009) 212–217. [5] X. Wen, S. Wang, Y. Ding, Z.L. Wang, S. Yang, The Journal of Physical Chemistry B 109 (2004) 215–220. [6] J. Brillet, M. Grätzel, K. Sivula, Nano Letters 10 (2010) 4155–4160. [7] A. Kay, I. Cesar, M. Grätzel, Journal of the American Chemical Society 128 (2006) 15714–15721. [8] U. Bjoerksten, J. Moser, M. Graetzel, Chemistry of Materials 6 (1994) 858–863. [9] S. Saremi-Yarahmadi, K.G.U. Wijayantha, A.A. Tahir, B. Vaidhyanathan, The Journal of Physical Chemistry C 113 (2009) 4768–4778. [10] M.A. Butler, D.S. Ginley, Journal of Materials Science 15 (1980) 1–19. [11] J.A. Glasscock, P.R.F. Barnes, I.C. Plumb, A. Bendavid, P.J. Martin, Thin Solid Films 516 (2008) 1716–1724. [12] K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, M. Grätzel, Journal of the American Chemical Society 132 (2010) 7436–7444.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Controlled growth of hematite (α-Fe2O3) nanorod array on fluorine doped tin oxide: Synthesis and photoelectrochemical properties H.K. Mulmudi a, N. Mathews a,⁎, X.C. Dou a, L.F. Xi a, S.S. Pramana a, Y.M. Lam a,⁎, S.G. Mhaisalkar a, b a b
School of Materials Science and Engineering, Nanyang Technological University, Singapore Energy Research Institute @NTU (ERI@N), Nanyang Technological University, Singapore
a r t i c l e
i n f o
Article history: Received 18 May 2011 Received in revised form 7 June 2011 Accepted 7 June 2011 Available online 15 June 2011 Keywords: Hematite nanorods Electrochemical impedance spectroscopy Photoelectrochemical solar cell Minority charge carriers
a b s t r a c t Hematite nanorods were grown on fluorine doped tin oxide (FTO) substrates by hydrothermal means utilizing urea as a pH regulating agent. XRD for nanorods revealed pure hematite phase after annealing at 500 °C for 30 min with preferential orientation in the [110] direction. Electrochemical impedance spectroscopy was carried out to investigate the electrical properties. Using Mott–Schottky analysis, charge carrier density was estimated to be 5.62 × 10 19 cm − 3 in the hematite nanorod array. Among the nanostructures, the n-type hematite nanorod array showed the best conversion efficiency among the samples studied in a two electrode photoelectrochemical cell. Photocurrent measurements versus light intensity were performed to investigate the device performance and the limiting factors to the performance were attributed to the short diffusion length of minority charge carriers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synthesizing controlled arrays of aligned semiconducting metal oxide nanorods on conducting substrates have attracted a lot of attention in recent years due to their potential applications in photovoltaic device[1], light emitting diodes [2], gas sensors [3] and electrode material for electrochemical batteries[4]. One such semiconducting material is hematite (α-Fe2O3) which is an n-type semiconductor. It is abundant, low cost, highly stable, nontoxic and is gaining importance in photoelectrochemical devices due to its band gap (Eg = 2.2 eV) [5]. Nevertheless, the overall photoelectrochemical conversion ratios have been severely hampered due to two conflicting properties: a) long photon penetration depth and b) short hole (minority charge carrier) diffusion lengths. Ideally, about 118 nm of hematite film is necessary to absorb 63% of the incoming solar radiation at λ = 550 nm. Thus, majority of the photogenerated carriers are produced within a distance of 100–120 nm from the semiconductor electrolyte interface. However, due to short hole (minority carriers) diffusion lengths (2–4 nm), most of the photogenerated carriers recombine even before reaching the semiconductor electrolyte interface [6,7]. Solution based methods for growing aligned hematite nanorods on conducting substrates offer an inexpensive alternative which could be employed to reduce recombination of photogenerated carriers, increase the surface area and hence yield better photochemical conversion efficiencies [8]. Herein, we report an environmentally friendly
⁎ Corresponding authors. E-mail addresses: [email protected] (N. Mathews), [email protected] (Y.M. Lam). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.06.008
route to synthesize hematite nanorod array on FTO by hydrothermal means. Urea which is nonionic, nontoxic, inexpensive, stable, and water-soluble, was used as a pH regulating agent which hydrolyzes at elevated temperatures leading to the formation of hydroxide ions. Electrical properties of these nanostructures were investigated by Mott–Schottky analysis in photo-electrochemical solar cells. 2. Experimental section 2.1. Synthesis An aqueous solution (10 ml) of 0.2 M FeCl3 and 0–0.6 M NH2CONH2 were sealed in a 20 ml glass vial and heated at 100 °C for 24 h. FTO was placed vertically in these glass vials with the conducting edge facing the wall of the vial. After the reaction, the film formed on FTO was thoroughly rinsed in DI water and annealed at 500 °C for 30 min to get the desired phase (α-Fe2O3). The notation for the samples (molar ratio of urea/FeCl3) are F1(0), F2(0.75), F3(1.5) and F4(3.0) respectively. 2.2. Characterization and measurements Structural and phase characterizations of the as-prepared film and annealed film were done by X-ray diffraction (XRD) using Bruker AXS (D8 ADVANCE) X-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). The surface morphology of annealed samples was characterized by FESEM (JEOL, JSM-7600 F, 5 kV). Optical absorption measurements films were performed using UV–visible absorption spectrometer (Shimadzu UV 2501PC).
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H.K. Mulmudi et al. / Electrochemistry Communications 13 (2011) 951–954
Electrochemical impedance spectroscopy (EIS) measurements were carried out using an automated potentiostat (Solartronanalytical, 1470E) coupled with a frequency response analyzer (Solartron-analytical, 1255B) in a three-electrode electrochemical system with 0.1 M NaOH electrolyte. Platinum mesh and saturated calomel electrode (SCE) were employed as counter and reference electrodes respectively. Fabrication of photoelectrochemical cell (PECC) was done by sandwiching α-Fe2O3/FTO electrode and platinized FTO as a counter electrode with a 25 μm thick hot-melt spacer (Surlyn, Dupont). An iodine/iodide based electrolyte (Z960) was introduced between the sandwiched electrodes. 3. Results and discussion The chemical reactions leading to the formation of α-Fe2O3 can be understood from the following equations: NH2 CONH2 + 3H2 O → 2NH4 OH + CO2
ð1Þ
FeCl3 + 3NH4 OH → FeOOH + 3NH4 Cl + H2 O
ð2Þ
Δ
2FeOOH → Fe2 O3 + H2 O
ð3Þ
Fig. 1 shows the top view FESEM images of samples F1–F4, annealed at 500 °C for 30 min. It was clearly observed that the morphology of the products formed on FTO depends on the concentration of urea (pH regulating agent). In the absence of urea there was no significant precipitation of any product on FTO (F1, see Fig. 1(a)). F2 and F3 samples show nanorod morphology, however F3 is clearly superior and possess very good alignment with respect to FTO (inset, Fig. 1(c)). As the molar ratio of urea/FeCl3 increased from 0.75 to 3, the morphology of FeOOH precipitates on FTO changed due to the increase in the release of hydroxide ions in the system which is clearly evident from the FESEM images (F2–F4, see Fig. 1(b)–(d)). The
thicknesses of the precipitates formed on FTO depend on the amount of urea and were found to be 170 nm (F2), 900 nm (F3) and 1.5 μm (F4) (measured by a surface profiler (Alpha-Step IQ)). Following Eqs. (1) and (2), 1.5 mol of urea is required to react with 1 mol of iron chloride stoichiometrically assuming completion of reaction. Experimentally, when the molar ratio of urea/FeCl3 was 1.5, nucleation and growth of nanorod array of FeOOH indeed happens on FTO. The above observations imply that the optimum molar ratio between urea/FeCl3 is 1.5 to promote heterogonous nucleation and growth of nanorod array on FTO. Reactions carried out at different temperatures such as 50 °C, 80 °C and 150 °C at a urea/FeCl3molar ratio of 1.5 did not yield a good array of nanorods. Fig. 2(a) shows the XRD pattern of all the samples, which reveal α-Fe2O3 phase after annealing at 500 °C for 30 min (ICDD PDF No. 99-100-0141). No characteristic peaks for other impurities such as γ-Fe2O3 and Fe3O4 were observed. Compared to the commercially available powder diffraction pattern (Fig. 2(a) (v)), the (110) diffraction peak in F2, F3 are prominent compared to other peaks such as (012), (113) and (300), which could indicate that the α-Fe2O3 nanorods are highly oriented with respect to the substrate. Similar preferential growth orientation along [110] has also been reported for nanostructured α-Fe2O3 deposited using APCVD[7]. The electrical conductivity of hematite is known to be highly anisotropic and its lattice can be represented as an alternation of iron bilayers and oxygen layers parallel to the (001) basal plane in hexagonal coordinates[7]. Electrons move by hopping through Fe II/Fe III valence change within the iron bilayers, while electron exchange between neighboring bilayers is spin forbidden (Hund's rule)[7]. This leads to a strongly anisotropic conductivity of hematite, which is up to 4 orders of magnitude higher within the (001) basal plane (e.g., in [110] direction) than orthogonal to them[7]. The superior quality of the F3 nanostructures can be exploited in electrical transport devices including photoelectrochemical cells. A bandgap of 2.03 eV was estimated for both the samples F2 and F3 through Tauc analysis by a linear fit to the experimental (αhν) 1/2versus
Fig. 1. Top view FESEM images of samples (a) F1, (b) F2, (c) F3, inset shows cross-section image of hematite nanorod array and (d) F4.
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953
[9]. The depletion width for the nanorod array (sample F3) was found to be 13 nm and using Gartner–Butler model, the maximum anodic photocurrent at 550 nm was found to be about ≈0.4 mA/cm 2. Photovoltaic characteristics (J–V plots) of PECCs for samples F2, F3 and F4 are shown in Fig. 3(a). Inset in Fig. 3(a) shows the IPCE for the nanorod array which corresponds well with the absorption spectrum of Fe2O3. The maximum conversion efficiency η = 0.05% was obtained using F3 (Voc = 0.24 V, Jsc = 0.523 mA/cm 2, FF = 0.39). The conversion efficiency of F2 was 0.01% (Voc = 0.21, Jsc = 0.187 mA/cm 2, FF = 0.35) while the performance of sample F4 was negligible (10 − 4%) compared to samples F2 and F3. The varying efficiencies of the three samples can be explained by optical and morphological considerations. Compared to sample F4, samples F2 and F3 both have a preferred orientation along [110] direction (Fig. 2(a)). As the conductivity of hematite in [110] direction is highest, the photocurrents in F2 and F3 are expected to be high, corresponding to our results. Although samples F2 (170 nm) and F3 (900 nm) both have nanorod structure, the thicker sample F3 allows complete absorption of the incident light, contributing to their higher currents. Though sample F4 (1.5 μm) is thicker than sample F3, morphologically sample F3 has a clear edge over sample F4 due to higher exposed area at the semiconductor/electrolyte interface. This coupled with the unfavorable crystallographic orientation, limits the efficiency of F4. To investigate the reason behind such low efficiencies, photocurrent measurements at varying light intensities were performed.
Fig. 2. (a) XRD patterns of: (i) FTO, (ii) F2, (iii) F3, (iv) F4 and (v) commercially available Fe2O3 (hematite phase). (b) Mott–Schottky plot of space charge capacity for samples F2 (inset),F3 and F4 in 0.1 M NaOH (R2 is the coefficient of determination for the linear fits).
hν plot. This agrees well with previously reported values of 1.9 to 2.2 eV [9]. EIS measurements were conducted to investigate the type of majority carriers utilizing Mott–Schottky analysis (signal level maintained at 10 mV, frequency was scanned from 100 kHz to 100 Hz). Fig. 2(b) shows the Mott–Schottky plots of samples F2, F3 and F4 measured in the threeelectrode electrochemical cell. Data from the high frequency region (10 kHz–30 kHz)[9], where the influence of capacitance due to double layer at the electrolyte and surface states are minimal, was used to determine the space charge capacitance (Csc). The resultant capacitance values were fit to the Mott–Schottky equation: 2
ðA=Csc Þ = ð2 = qεεo ND ÞðE−EFB −kT = qÞ where A is the surface area of the electrode, ε is the dielectric constant of iron oxide = 32 [11], εo is the permittivity of the vacuum, ND is the donor carrier concentration, V is the applied potential, k is Boltzmann's constant, and T is the temperature. The positive slope from the Mott– Schottky plots of Fig. 2(b) clearly indicates the formation of n-type iron oxide in all the samples. Donor densities of samples F2, F3 and F4 were found to be 4.47 × 10 18 cm − 3, 5.62 × 10 19 cm − 3, and 8.78 × 10 19 cm − 3 respectively which are in agreement with the reported values for nanostructured hematite films [9]. Clearly, sample F3 has 1 order more charge carrier concentration compared to sample F2. The estimated higher donor density in sample F3 suggest that this sample could have better electrical properties (lower resistivity) compared to sample F2
Fig. 3. (a) J–V plot characteristic of samples F2, F3 and F4, Inset: IPCE curve for nanorod array (F3). (b) Photocurrent versus light intensity for PECCs fabricated from samples F2, F3 and F4.
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H.K. Mulmudi et al. / Electrochemistry Communications 13 (2011) 951–954
Fig. 3(b) shows the photocurrent versus light intensity plot for samples F2, F3 and F4. The linear behavior qualitatively demonstrates that the interfacial electrochemical processes between iron oxide and electrolyte proceed smoothly [10]. This indicates that charge carrier transport to the electrolyte interface is the limiting factor. Due to the poor absorption of Fe2O3 at higher wavelengths, the optical absorption and hence carrier generation takes place further from the nanostructure surface. In order for these carriers to contribute to the cell efficiency, they have to make their way to the electrolyte surface. Due to the poor hole (minority carriers) diffusion lengths in Fe2O3 [12], many of these holes recombine before reaching the electrolyte, thus reducing the current density. 4. Conclusion We have demonstrated growth of oriented iron oxide nanorod arrays on FTO through hydrothermal synthesis. Aligned nanorods of 900 nm length have been grown at 100 °C for 24 h at molar ratio between urea/FeCl3 of 1.5. XRD analysis of 500 °C annealed samples showed a pure hematite phase. The n-type behavior was demonstrated by Mott–Schottky analysis and the donor density for the nanorods was estimated to be 5.62× 1019 cm− 3. The hematite nanorod array shows a conversion of 0.05% in a two-electrode photoelectrochemical cell with iodine/iodide based electrolyte. In comparison to other nanostructures, the nanorods showed better conversion efficiencies due to favorable [110] crystallographic orientation. Light intensity measurements revealed that the poor conversion efficiency of hematite nanorods can be attributed to the short diffusion length of minority charge carriers.
Acknowledgment We thank Robert Bosch (SEA) Pte Ltd and National Research Foundation, Singapore for providing financial support. We also thank Dr. Jinesh K.B., Mak Wai Fatt, Dharani S and K.R.G Karthik for valuable discussions.
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