ARTICLE pubs.acs.org/JPCC
ZnO/CdS Hierarchical Nanospheres for Photoelectrochemical Sensing of Cu2+ Qingming Shen,†,‡ Xiaomei Zhao,† Shiwei Zhou,† Wenhua Hou,*,† and Jun-Jie Zhu*,† †
State Key Laboratory of Analytical Chemistry for Life Science, Key Laboratory of Mesoscopic Chemistry (MOE), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China ‡ Key Laboratory for Organic Electronics and Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing 210046, P. R. China ABSTRACT: ZnO/CdS hierarchical nanospheres were prepared for photoelectrochemical selective sensing of Cu2+. Hierarchical ZnO nanospheres were first synthesized by the hydrolysis of zinc salt under ultrasound irradiation, and then CdS nanocrystals were selectively grown on the hierarchical ZnO nanospheres. The light scattering of ZnO nanospheres and the heterointerfaces between CdS and ZnO provided significant advantages for enhanced light absorption and charge separation, thus resulting in an improvement in the photocurrent intensity. A photoelectrochemical sensor was developed based on the interaction between Cu2+ and CdS. The results showed that this sensor has a good selectivity and high sensitivity for Cu2+ detection.
1. INTRODUCTION In the past three decades, numerous efforts in the fields of photocatalysis and photovoltaic cells have focused on the use of semiconductor nanomaterials.13 Among various semiconductor nanomaterials, metal-oxide semiconductors such as ZnO and TiO2 have been researched intensively as practically applicable materials in photocatalysis and photovoltaic cells, because of their suitable band gaps, stability against photocorrosion, and high photoelectric and photocatalytic activities.114 In particular, ZnO nanomaterials exhibit a few distinct advantages, including good carrier mobility, simple tailoring of the structures, facile and low-cost for large-scale manufacturing, and so on.48 Despite the success of ZnO nanomaterials in photovoltaic cells, many researchers are still looking for combinations of nanostructure and device architecture to further improve the performance of ZnO photovoltaic cells. One possible strategy to enhance the efficiency of ZnO photovoltaic cells is to construct one-dimensional ZnO nanostructures that provide a more direct electron-transport path for the rapid collection of photogenerated electrons and, therefore, reduces the charge recombination and increases the efficiency of ZnO photovoltaic cells.914 Another way to improve the efficiency is to increase the light-harvesting capability of the photoelectrode film by utilizing optical enhancement effects, which can be achieved by means of light scattering by introducing scatterers into the photoelectrode film. Cao et al. reported hierarchically structured ZnO submicrometer spheres as photoelectrodes for the enhancement of energy conversion efficiency. The polydisperse ZnO aggregates were used as efficient light scatterers, whereas the compositive nanocrystalline ZnO provided the necessary mesoporous structure and large internal surface area for the adsorption of dye molecules in the film.7,8 Bare ZnO is known to be a wide-band-gap semiconductor, which is disadvantageous for the absorption and use of the visible r 2011 American Chemical Society
range of solar energy. Thus, to use visible light and improve the performance of ZnO photovoltaic cells, types of narrow-bandgap semiconductor nanocrystals, including CdS,15,16 CdSe,17,18 PbS,19 InP,20 and InAs,21 have been introduced as photosensitizers. There are some specific advantages for using narrow-bandgap semiconductor nanocrystals as light-harvesting media. First, the quantum size effect allows nanocrystals to response to visible light by tuning particle size.22 Second, the charge injection from narrow-band-gap semiconductor into a wide-band-gap one can lead to efficient and longer charge separation by decreasing the recombination of electronhole pairs. In addition, it has been shown that semiconductor nanomaterials can generate multiple charge carriers with a single photon, which improves the efficiency of the device.23,24 Among sensitized narrow-bandgap semiconductors, CdS is a well-known semiconductor that has been used as a visible-light photosensitizer of ZnO in photovoltaic cells because of its narrow direct band gap (2.4 eV) and flat band potential at 0.66 V (pH 7.0).25 Recently, there have been a few reports on the fabrication of CdS nanoparticle/ZnO nanowire heterostructures that can efficiently separate and transfer electronhole pairs in each semiconductor material, reduce their recombination, and improve photocatalytic efficiency.2629 However, to the best of our knowledge, there is still no report on the using of ZnO/CdS hierarchical nanospheres to develop optoelectronic devices. ZnO/CdS hierarchical nanospheres can provide enhanced light scattering and the effective separation and transportation of electronhole pairs. Herein, based on these ideas, we examine the combination of the narrow-band-gap semiconductor CdS with ZnO hierarchical Received: April 26, 2011 Revised: June 24, 2011 Published: August 03, 2011 17958
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Figure 1. TEM and HRTEM images of (a,b) ZnO and (c,d) ZnO/CdS and SEM images of (e) ZnO and (f) ZnO/CdS. The insets in a and c are the TEM images at high magnification.
nanospheres in an optoelectronic device. ZnO/CdS hierarchical nanospheres were prepared through a facile strategy. First, hierarchical ZnO nanospheres were synthesized by the hydrolysis of zinc salt under ultrasound irradiation, and then CdS nanocrystals were grown on the ZnO nanospheres by a one-step process. It was found that the light scattering of the ZnO nanospheres and the presence of the nanosized heterointerfaces between CdS nanocrystals and ZnO nanospheres provided significant advantages for enhanced light absorption and charge separation, resulting in the improvement of the photocurrent intensity. Furthermore, a sensitive photoelectrochemical sensor for the detection of Cu2+ was fabricated based on the selective interaction between Cu2+ and CdS. The ZnO/CdS hierarchical nanospheres show promising applications in the fields of sensing and photovoltaic cells.
2. EXPERIMENTS 2.1. Materials. All reagents were of analytical purity and were used without further purification. Zn(NO3)2 3 6H2O, triethanolamine (TEA), thioacetamide (TAA), and Cd(NO3)2 3 6H2O were purchased from Sinopharm Chemical Reagent Co., Ltd.; acetone and ethanol were purchased from Shanghai Second Chemical Reagent Factory of China. All aqueous solutions were prepared with ultrapure water (18 MΩ cm1), which was obtained from a Milli-Q water purification system. 2.2. Synthesis of ZnO Hierarchical Nanospheres. The synthesis of ZnO hierarchical nanospheres was performed according to the method described in our previous report with a slight modification.30 Zn(NO3)2 3 6H2O (0.45 g, 0.0015 mol) was 17959
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The Journal of Physical Chemistry C dissolved in H2O (100 mL), and then TEA (2 mL) was introduced with stirring to form a clear solution. The transparent solution was exposed to high-intensity ultrasound irradiation under ambient air for 30 min. Ultrasound irradiation was generated with a high-intensity ultrasonic probe (ultrasonic liquid processor VC-750, 20 kHz, Sonics & Materials) immersed into the solution. After the reaction, the resulting white suspension was purified by centrifugation, and the precipitate was washed with water and then dried in air. The final products were collected for characterization and further preparation. 2.3. Fabrication of ZnO/CdS-Hierarchical-NanosphereModified ITO (ITO/ZnO/CdS) Electrodes. Indium tin oxide (ITO) slices (sheet resistance 20 Ω/square) were sonicated in acetone, ethanol, and water for 15 min each. Then, 20 mg of ZnO powder was dispersed ultrasonically in 10 mL of water, and 20 μL of the resulting colloidal dispersion (2.0 mg/mL) was drop-cast onto a piece of ITO slice with a fixed area of 0.25 cm2. After drying in air, the film was sintered at 450 °C for 30 min in air to remove residual solvent and any organic compounds to improve the contact between the film and the substrate and the connection between the nanocrystallites. It was then naturally cooled to room temperature. The ZnO-nanosphere-coated ITO is referred as an ITO/ZnO electrode. The CdS nanocrystals were grown directly on the ZnO sphere surface by incubating the ITO/ZnO electrode in an aqueous solution of 10 mM Cd(NO3)2 and 10 mM TAA at room temperature for 590 min. The growth of CdS nanocrystals was indicated by the emergence of a yellow color on the electrode surface. The obtained ITO/ZnO/CdS electrode was then rinsed thoroughly with water and air-dried. 2.4. Characterization. High-resolution transmission electron microscopy (HRTEM) was performed using a JEOL-2100 highresolution transmission electron microscope with an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) was performed on a Hitachi S-4800 field-emission scanning electron microscope. X-ray powder diffraction (XRD) analysis was performed on a Philips X0 Pert X-ray diffractometer in the 2θ range from 10° to 80°, with graphite-monochromatized Cu KR radiation (λ = 0.15418 nm). UVvisible (UVvis) absorption spectra were obtained on a Shimadzu UV-3600 UV/vis spectrophotometer. Electrochemical impedance spectroscopy (EIS) was carried out on an Autolab potentiostat/galvanostat (PGSTAT30, Utrecht, The Netherlands) in 0.1 M KCl containing a redox probe of 10.0 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture at an open-circuit potential of 216 mV with an applied voltage of 5 mV over a frequency range of 0.1 Hz100 kHz. 2.5. Photoelectrochemical Measurements and Cu2+ Sensing. Photoelectrochemical measurements were performed with a homemade photoelectrochemical system. A 500 W Xe lamp equipped with a monochromator was used as the irradiation source. Photocurrent was measured on a CHI 660D electrochemical workstation. An ITO/ZnO/CdS electrode with an area of 0.25 cm2 was employed as the working electrode. A Pt wire was used as the counter electrode, and a saturated Ag/AgCl electrode was used as the reference electrode. All photocurrent measurements were performed at a constant potential of 0.2 V (vs saturated Ag/AgCl). A 0.1 M phosphate buffer solution (PBS, pH 7.0) containing 0.1 M TEA was used as the electrolyte for photocurrent measurements. The solution was deaerated with highly pure nitrogen for 15 min before experiments, and then a N2 atmosphere was kept over the solution for the entire experimental process. Each stock solution of Cu2+, Zn2+, Ni2+, Fe3+, Fe2+, Co2+, Cr3+, K+, Na+, Mg2+, Pb2+, Ca2+, Ba2+, Al3+,
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Figure 2. XRD patterns of the ITO/ZnO electrode dipped in the reaction solution for different times.
and Mn2+ ions was prepared by dissolving suitable amounts of the compounds CuSO4, AgNO3, Zn(NO3)2, CdCl2, NiCl2, Fe(NO3)3, FeSO4, CoCl2, Cr(NO3)3, KCl, NaCl, MgSO4, Pb(NO3)2, CaCl2, BaCl2, Al(NO3)3, and MnCl2 in water, respectively. The stock solution of Hg2+ was prepared by dissolving 14.8 mg of HgSO4 in 50 mL of water containing 0.3 mL of concentrated HCl. The stock solutions were further diluted whenever necessary.
3. RESULTS AND DISCUSSIONS 3.1. Characterization of ZnO/CdS Hierarchical Nanospheres. The as-prepared ZnO sample appeared to consist of
uniform hierarchical nanospheres with a diameter of ca. 120 nm, as shown in the TEM image (Figure 1a). The hierarchical character is evident and is built up by tens of primary nanoparticles with an average dimension of ∼10 nm (inset in Figure 1a). It appears that these small primary nanoparticles are interconnected to form larger secondary hierarchical architectures with recognizable boundaries or voids between the component subunits. A more detailed examination of the ZnO hierarchical nanospheres by high-resolution TEM (HRTEM, Figure 1b) shows a lower contrast between the crystallites, which reveals that nanopores separate many primary nanoparticles. The clearly marked interplanar d spacing is 0.25 nm, which corresponds to the (111) lattice planes of hexagonal ZnO. When Cd2+ reacted with TAA in the presence of ZnO template, CdS nanoparticles with a diameter of ∼5 nm were formed on the ZnO sphere surface, as shown in Figure 1c and the inset. The HRTEM image of the shells of the ZnO/CdS nanospheres (Figure 1d) shows that the shells are polycrystalline in nature. The interplanar spacing is ∼0.33 nm, which corresponds to the lattice spacing for the (111) faces of cubic CdS. To further confirm the hierarchical spherical structure of ZnO and ZnO/CdS on the ITO electrode, the samples were aslo examined by SEM. The SEM image in Figure 1e clearly shows that the obtained ZnO truly consists of hierarchical spheres and also confirms the production of large-scale and narrow size distribution with a diameter of ca. 120 nm. Figure 1f clearly shows that the resultant ZnO/CdS also has a hierarchical spherical structure. Figure 2 shows the XRD patterns of the resultant ZnO and ZnO/CdS nanospheres. The obtained ZnO sample is of wurtzite structure. The diffraction peaks in the XRD pattern (Figure 2, pattern a) are well-assigned to hexagonal-phase ZnO, as reported in JCPDS card no. 80-0075. The XRD patterns of ZnO/CdS nanospheres obtained by dipping ZnO into the mixture solution of TAA and Cd2+ for different times are shown in patterns bd. 17960
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Figure 3. (A) UVvis absorption spectra of ZnO nanospheres and ZnO nanoparticle films. (B) Schematic of light scattering occurring on a ZnO/CdS modified electrode. (C) Eelectronhole pairs generation, separation, and transfer between ZnO and CdS at a ZnO/CdS-modified electrode.
It can be seen that cubic-phase CdS (JCPDS card no. 80-0019) coexists with hexagonal-phase ZnO in XRD patterns b and c of Figure 2. With increasing dipping time, greater numbers of CdS nanoparticles are formed on the ZnO surface, and the characteristic diffraction peaks of CdS gradually increase and thus suppress the XRD signals of ZnO (pattern d). The apparent broadening of the diffraction peaks indicates that the sample is composed of nanosized particles. Using the Scherrer formula,31 the crystallite sizes were estimated at around 9.3 and 4.8 nm for ZnO and CdS nanoparticles, respectively. From the UVvis absorption spectra of ZnO secondary nanospheres and ZnO nanoparticles (Figure 3A), an absorption peak at around 370 nm can be observed for both films, which is due to the band gap absorption of the ZnO nanomaterials. Moreover, compared with the absorption spectra of the ZnO nanoparticles, an additional broad absorption peak was observed at around 420 nm for the ZnO nanosphere film. This additional absorption peak could be due to light scattering caused by the presence of larger secondary colloidal spheres, which is in accordance with previous reports.7,8,32,33 As shown in Figure 3B, the light was scattered multiple times in the presence of the ZnO/ CdS nanospheres. The enhanced light scattering ability of the secondary nanospheres could increase the absorption path length of light, resulting in the absorption of more photons, the generation of more electronhole pairs, and an increase of the photocurrent density.7,8,32,33 In addition, as shown in Figure 3C, coupling of large-band-gap semiconductor ZnO with small-band-gap CdS nanocrystals could facilitate charge separation through the quick electron transfer from the conduction band of CdS to that of ZnO, thereby hindering the recombination of electrons and holes.2629 Because ZnO is a good electron acceptor with a high electron mobility, the introduction of ZnO can enhance the transfer of the generated electrons and effectively minimize the charge recombination.6,7 To gain further insight into the electron-transport and -recombination properties of the ITO/ZnO/CdS electrode,
Figure 4. Electrochemical impedance Nyquist plot of modified ITO electrodes: (a) ITO, (b) ITO/ZnO, (c) ITO/ZnO/CdS, and (d) ITO/ CdS.
electrochemical impedance spectroscopy (EIS) of different electrodes was measured using K3Fe(CN)6/K4Fe(CN)6 as a redox probe (Figure 4). Using the ZnO-modified ITO electrode, the electron-transfer resistance decreased from 50.4 Ω (curve a) to 38.3 Ω (curve b). After CdS had been coated on on ITO/ZnO electrode, because the formed CdS nanocrystals block the electron transfer between the redox probe and electrode, the diameter of the high-frequency semicircle increased to 146.2 Ω (curve c), but was much smaller than that of the ITO/CdS electrode (curve d), 241.8 Ω. This indicates that both the interface resistance and the charge-transfer resistance were obviously decreased upon the introduction of ZnO. 3.2. Photoelectrochemical Measurements. When the ITO/ ZnO electrode was dipped into the mixture solution of TAA and Cd2+, the electrode color changed from light yellow to deep yellow as the dipping time increased. As shown in Figure 5A, the UVvis absorption spectra also confirmed this observation. As the dipping time increased, the absorption spectra exhibited a red shift from ∼370 nm (curve b, ZnO) to ∼480 nm (curves df), 17961
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Figure 5. (A) UVvis absorpotion spectra of the ITO/ZnO electrode dipped in the reaction solution for different times. (B) Photocurrent action spectrum of the ITO/ZnO/CdS electrode. (C) Photocurrent of the ITO/ZnO electrode dipped in the reaction solution for different times. (D) Timebased photocurrent response of the ITO/ZnO/CdS electrode with irradiation (480 nm) repeated every 10 s. Photocurrent spectra were recorded in PBS (0.1M, pH 7.0) that contained 0.1 M TEA.
and the absorption intensity gradually increased. This is because, as the dipping time increased, the particle size and the amount of CdS gradually increased, and thus, the absorption of CdS nanoparticles shifted to longer wavelengths due to the quantization size effect. Because the growth of CdS is determined by the reactive surface of ZnO, when the surface of ZnO is covered with growing CdS, the reaction gradually slows and eventually stops after the reactive surface of ZnO is completely consumed or covered by the CdS.29 With a further increase in dipping time from 30 to 60 min, the absorption spectra changed little (curves f and g, Figure 5A), suggesting that no more CdS nanocrystals grew on the ZnO surface. This behavior results in a uniform, dense layer of CdS that is only a few nanocrystals thick on the ZnO nanospheres. Figure 5B shows the photocurrent action spectrum of an ITO/ZnO/CdS electrode in the presence of TEA as a sacrificial electron donor. The photocurrent closely follows the absorbance spectrum of CdS nanoparticles as shown in Figure 5A, and it reaches a peak value at ∼480 nm, indicating that the photocurrent originates mainly from the excitation of the CdS nanocrystals. In the subsequent experiments, 480 nm was chosen as the optimal excitation wavelength. Based on the absorption spectra, photocurrent action spectrum, and the use of TEA as a hole scavenger, the resulting photocurrent can be attributed to the injection of the photogenerated electrons of CdS into the conduction band of ZnO and then into the ITO electrode. As the dipping time increased from 5 to 30 min, the photocurrent also gradually increased (Figure 5C), indicating that higher photocurrents can be observed when more CdS nanocrystals are on the surface of the ZnO nanospheres. However, with the reaction ceasing at 30 min, because no more CdS nanocrystals could be
formed (curves f and g, Figure 5A), the photocurrent was almost unchanged after 30 min of dipping (curves ce, Figure 5C). Figure 5D shows the photocurrent response of an ITO/ZnO/ CdS electrode upon irradiation repeated every 10 s. The irradiation process was repeated 30 times over 600 s, and the photocurrent did not show obvious change, indicating that the photocurrent response of ITO/ZnO/CdS electrode was very stable and suitable for the construction of photoelectrochemical sensors. 3.3. Photoelectrochemical Sensing of Cu2+. Given their enhanced light scattering ability, good charge separation, and good charge-carrier-transfer behavior, ZnO/CdS nanospheres have an enhanced photocurrent intensity and are an ideal candidate material for photoelectrochemical applications. The photoelectrochemical performance of ZnO/CdS has been evaluated by the sensing of Cu2+, which is an essential element for many living organisms but toxic at high concentrations.34 It has been reported that the addition of Cu2+ to CdS solution leads to the binding of Cu2+ with S2 and the reduction of Cu2+ to Cu+ under illumination. As a result, CuxS (x = 1, 2) could form on the CdS surface due to the chemical displacement of surface Cd2+ by Cu2+ and Cu+ resulting from the lower solubility of CuxS than CdS. The formation of CuxS on the CdS surface generates a lower energy level that provides an effective path for the recombination of excited electrons in the conduction band and holes in the valence band.3538 Because of the existence of electronhole recombination centers (Cu+ or CuxS) on the CdS surface, the photocurrent intensity of ZnO/CdS decreases in the Cu2+ solution (Figure 6A). Therefore, it can be concluded that the competitive binding of Cu2+ with CdS is the primary 17962
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Figure 6. (A) Photocurrent intensity of ITO/ZnO/CdS in the presence of different Cu2+ concentrations, (B) Plot of photocurrent decrease [(I0 I)/ I0] of ITO/ZnO/CdS versus log[Cu2+].
The formed MS could also act as recombination centers for photogenerated electronhole pairs because of its lower band energy and surface defects. However, the ITO/ZnO/CdS electrode is much more sensitive to Cu2+ than to other different metal ions, because the Ksp value of CuS is much lower than that of PbS, ZnS, and so on.41 Thus, based on the sensitive and selective decrease effect of Cu2+ on the photocurrent intensity of ITO/ZnO/CdS electrode, a sensitive photoelectrochemical Cu2+ sensor was developed.
Figure 7. Photocurrent intensity of ITO/ZnO/CdS in the presence of 20 μM solutions of various metal ions.
mechanism for the decrease of the photocurrent. The effect of Cu2+ on the photocurrent at the ITO/ZnO/CdS electrode was found to be concentration-dependent. As the concentration of Cu2+ increased, the photocurrent gradually decreased, and in Figure 6B, a good linear relationship (R2 = 0.9988) can be seen between the value of the photocurrent decrease, (I0 I)/I0 and the logarithm of the concentration of Cu2+ over the range from 0.02 to 40.0 μM, which is beyond the upper guideline levels for copper in drinking water legislated by various governments [the EU standard is 2.0 ppm (30 μM)].39,40 I0 and I are the photocurrent intensities of the ITO/ZnO/CdS electrode in the absence and presence of Cu2+, respectively. The detection limit was evaluated using 3σ/S and was found to be 0.01 μM, where σ is the standard deviation of the blank signal and S is the slope of the linear calibration plot. The standard deviation for six replicate measurements of a solution containing 0.1 μM Cu2+ is 2.5%, indicating good reproducibility. Compared to the previous method, this photoeletrochemical method is simple and sensitive. In addition, it avoids the relatively costly apparatus required for the fluorescence method and the preconcentration process used in the electrochemical method.3639 To investigate the selectivity of the Cu2+ sensor, the influence of 20 μM concentrations of different metal ions (K+, Na+, Ca2+, Mg2+, Ba2+, Pb2+, Al3+, Cr3+, Cu2+, Fe3+, Mn2+, Hg2+, Fe2+, Ni2+, and Co2+) on the photocurrent intensity of ITO/ZnO/CdS electrode was examined. In Figure 7, it can be observed that Pb2+, Zn2+, Mn2+, Hg2+, Ni2+, and Co2+ could also decrease the photocurrent intensity of the ITO/ZnO/CdS electrode. This can be explained by the displacement of Cd2+ and thus the formation of MS (for M = Pb, Zn, Mn, Hg, Ni, Co) on the CdS surface.
4. CONCLUSIONS The as-prepared ZnO/CdS hierarchical nanospheres were used to fabricate photoelectrochemical sensor. Because of the light scattering of the ZnO nanospheres and many nanometersize heterointerfaces between the CdS nanocrystals and ZnO nanospheres, the light absorption and charge separation were significantly enhanced, resulting in the improvement of photocurrent intensity. A sensitive and selective photoelectrochemical Cu2+ sensor was developed based on the interaction between Cu2+ and CdS. The ZnO/CdS hierarchical nanospheres show promising applications in the fields of photoelectrochemical sensing and photovoltaic cell. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (J.-J.Z.),
[email protected] (W.H.). Fax/Tel.: +86-25-8359-7204.
’ ACKNOWLEDGMENT We are grateful for the financial support of the National Natural Science Foundation of China (Nos. 21073084 and 20821063), “973” Program (Nos. 2011CB933502, 2007CB936302), Doctoral foundation from Ministry of Education (No.20100091110023), and Fundamental Research Funds for the Central Universities (1112020504). ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnmann, D. W. Chem. Rev. 1995, 95, 69. (3) Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gr€atzel, M. J. Am. Ceram. Soc. 1997, 80, 3157. 17963
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