© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 49, DECEMBER 4, 1997
LETTERS Chemically Modified Ni/TiO2 Nanocomposite Films: Charge Transfer from Photoexcited TiO2 Particles to Hexacyanoferrate Redox Centers within the Film and Unusual Photoelectrochemical Behavior Norma R. de Tacconi,* Judith Carmona, and Krishnan Rajeshwar* Department of Chemistry and Biochemistry, The UniVersity of Texas at Arlington, Arlington, Texas 76019-0065 ReceiVed: July 16, 1997; In Final Form: September 30, 1997X
The photoelectrochemical behavior of chemically modified Ni/TiO2 nanocomposite films in aqueous 0.1 M NaNO3 electrolyte is described. Two types of such films were prepared, either starting with a Ni/TiO2 nanocomposite photoelectrode that was subsequently cycled in 0.01 M K3Fe(CN)6 + 0.1 M NaNO3 (type I) or with a Ni electrode derivatized in situ with nickel hexacyanoferrate (NHF) and TiO2 (type II). The photoactivity of both types of films was compared with the parent Ni/TiO2 film in 0.1 M NaNO3. Type I electrodes exhibited minimal photoactivity until potentials into the NHF II f III redox regime were accessed (>ca. 0.40 V). Thereafter, the anodic photocurrents were significantly higher than the parent Ni/TiO2 counterpart. Type II electrodes exhibited “bipolar” photoactivity, the switch from cathodic photo- to anodic photobehavior again occurring at potentials close to the NHF II f III redox location. These observations on type I and type II nanocomposite films stand in marked contrast to the usual photocurrent-voltage behavior of n-type semiconductor electrodes. The unusual photoeffects are interpreted within the framework of a model including charge transfer from the photoexcited TiO2 particles to the NHF redox sites within the nanocomposite film.
Introduction The photoelectrochemical behavior of particulate semiconductor films has been a subject of much activity in recent years.1 We recently reported on a new family of such films derived from occlusion electrodeposition of nickel from a bath loaded with colloidal particles of the semiconductor of interest (e.g., TiO22 or CdS3). The resultant “nanocomposite” films are comprised of photoactive semiconductor islands that are dispersed in a continuous nickel matrix. The latter creates spatial separation of the photocatalyst sites at the film/solution boundary in these nanocomposites unlike in their nanocrystalline film counterparts wherein there is a contiguous distribution of such sites. Accordingly, Ni/TiO2 nanocomposite films afford “cur* To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 1, 1997.
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rent-doubling” yields of over five for the oxidation of multiequivalent redox species (e.g., formate).4 In contrast, nanocrystalline films derived from the same semiconductor (for example, by pyrolysis of TiCl4 on a Ti substrate) yield values of only ca. 1.2 under comparable conditions.2b,5 In this communication, we report unusual photoelectrochemical effects when the nickel component of the Ni/TiO2 nanocomposite is electrochemically derivatized6,7 with hexacyanoferrate. Electron transfer from the photoexcited TiO2 particles to nickel hexacyanoferrate(III) centers within the chemically modified film gives rise to improved quantum yields (relative to the parent Ni/TiO2 film) for hole transfer to the film/solution boundary. Remarkably, the direction of hole transfer from TiO2 can even be inverted under certain conditions. Thus, photocathodic currents are observed for a nickel electrode anodized © 1997 American Chemical Society
10152 J. Phys. Chem. B, Vol. 101, No. 49, 1997
Letters
SCHEME 1: Schematic Diagrams (Not to Scale) of the Two Types of Derivatized Ni/TiO2 Films in This Study
in a K3Fe(CN)6 solution containing suspended TiO2 particles. Such currents, arising from photoinduced majority carrier (electron) transfer to the solution phase, are not commonly observed from n-type semiconductor/electrolyte interfaces (see below). Experimental Section The materials and instrumentation for voltammetry, laser Raman spectroscopy, and photoelectrochemical measurements have been described, as are procedures for growing Ni/TiO2 nanocomposite films.2,3,8 Electrochemical derivatization of these films generally followed protocols previously described for nickel surfaces.6,7 Two types of derivatized Ni/TiO2 films were grown: Type I films were obtained by voltammetric cycling of Ni/TiO2 nanocomposite film in 0.01 M K3Fe(CN)6 + 0.1 M NaNO3. Type II films were potentiostatically grown (0.7 V, 5 min) on a nickel film surface in a 0.01 M K3Fe(CN)6 solution loaded with 0.8 M TiO2 (Degussa, P25) particles. The derivatized films in both cases had a nominal geometric area of 1.8 mm2, although only ca. 10% of the film surface contained the photoactive TiO2 particles.2 Scheme 1 contain diagrams of the two types of films that are consistent with the differences in their preparation history and subsequent photoelectrochemical behavior. The derivatized layers in both types of films were ca. 5 nm thick (as assessed by integration of the voltammetry waves, see Figure 1b below), although the layer in type I films was somewhat thinner (see Scheme 1). All photoelectrochemical characterizations of these films were performed in 0.1 M NaNO3 electrolyte. All potentials below are quoted with respect to the Ag/AgCl/3 M KCl reference; all measurements were performed at the laboratory ambient temperature. Results and Discussion Figure 1a contains a cyclic voltammogram for a Ni/TiO2 nanocomposite film in a 0.01 M K3Fe(CN)6 + 0.1 M NaNO3 solution. Aside from the waves due to the Fe(CN)63-/4- redox couple in solution, a pair of waves at higher potentials diagnoses the gradual formation of a surface-confined nickel hexacyanoferrate (NHF) film.6,7 Removal of the derivatized Ni/TiO2 electrode followed by copious washing in distilled water and transfer to a 0.1 M NaNO3 solution yield the set of cyclic voltammograms in Figure 1b. The voltammetric behavior as a function of potential scan rate is generally in accord with the
Figure 1. (a) Cyclic voltammogram of Ni/TiO2 film in 0.01 M K3Fe(CN)6 + 0.1 M NaNO3. Scan rate: 20 mV/s. (b) Voltammograms of derivatized Ni/TiO2 (type I) film in 0.1 M NaNO3 at 20, 40, 60, and 80 mV/s. (c) Comparison of dark voltammetry (scan rate 20 mV/s) and the corresponding chopped irradiated profile during a positivegoing scan for type I film in 0.1 M NaNO3.
surface-confined nature of the NHF redox centers; these data for the Ni/TiO2 electrodes are also in broad agreement with those presented by earlier authors for nickel surfaces.6,7 Additionally, Raman spectra acquired in situ show two bands at 2102 and 2141 cm-1 in the cyanide stretching region. These transform to a broad composite band at 2184 cm-1 when the NHF film is oxidized in 0.1 M NaNO3. These vibration frequencies have been previously assigned to Fe-CtN-Ni bridge structures.8 The above data taken as a whole show that the presence of TiO2 particles on the nickel surface does not significantly alter the reactivity of the latter toward K3Fe(CN)6. Importantly, the electrochemical behavior of NHF also appears to be unperturbed by the presence of TiO2 in the nanocomposite film. Figure 1c contains voltammetric data in 0.1 M NaNO3 for the derivatized Ni/TiO2 obtained as above (these are hereafter designated as “type I” films) both in the dark and under interrupted irradiation of the derivatized films with the full output of a 75 W Xe arc lamp (incident light flux: ca. 2.2 mW/ cm2). Only the positive-going potential sweep is shown in the latter case. Interestingly, the photocurrent transients have a spiked shape at potentials lower than those corresponding to the NHF II f III redox regime. The photocurrents rise sharply thereafter, and they also exhibit a plateaulike appearance finally attaining (light flux limited) saturation at potentials near 1.0 V (vs Ag/AgCl). The return sweep under interrupted illumination
Letters
Figure 2. Photocurrent-potential curves under chopped light irradiation for (a) Ni/TiO2 film, (b) Ni/TiO2 film potentiodynamically derivatized with NHF, and (c) Ni film potentiostatically derivatized in NHF + TiO2. The three photocurrent profiles were obtained in 0.1 M NaNO3. The scans in all the cases were in the positive-going direction at a rate of 2 mV/s.
(not shown) is similar in that the plateaulike transients give away to spiked and much lower photocurrent signals as the potential is swept past the NHF III f II voltammetric wave (Figure 1c). Figure 2a,b provides a direct comparison of the photoelectrochemical behavior of a Ni/TiO2 film before (Figure 2a) and after (Figure 2b) electrochemical derivatization with NHF. Again chopped illumination was employed in both cases. This comparison underlines two points: (a) The photoactivity of the derivatized Ni/TiO2 is appreciable only after the NHF II f III redox transition (cf. Figure 1) unlike in the parent nanocomposite case. (b) The photocurrent in the saturation region for the derivatized film exceeds that in the parent Ni/TiO2 case. This behavior of the type I film can be rationalized as follows by first noting that the observed photocurrents are anodic in polarity; i.e., the minority carriers (holes) are driven to the film/ solution interface, and the majority carriers move to the film bulk under the imposed electric field gradient. Thus in the parent Ni/TiO2 case, the nickel matrix acts as an efficient sink for the photogenerated electrons from TiO2.2 On the other hand, in the derivatized case, the NHF surrounding the TiO2 islands does not have this capacity with the iron redox centers being confined to the +2 state. The consequence is facile e--h+ pair recombination as diagnosed by the spiked nature of the photocurrent transients at potentials below ca. 0.40 V (i.e., the onset of the NHF II f III transition). Stated in different terms, the Fe (II/III) redox centers appear to mediate carrier recombination, i.e., the Fe(III) sites that are generated by the
J. Phys. Chem. B, Vol. 101, No. 49, 1997 10153 photogenerated holes are re-reduced in the dark (back to Fe(II)). This gives rise to the “reverse” spikes in the dark in Figure 2b. At higher potentials, the (oxidized) NHF redox centers can now drain the photogenerated electrons from the adjacent TiO2 sites before these electrons recombine with the holes. The net result is an improved quantum yield and higher photocurrents for the derivatized films relative to the parent Ni/TiO2 case (cf. Figure 2a,b). It is worth noting that the spikes in the photocurrent envelope at potentials greater than ca. 0.40 V in Figure 2a are now completely absent in Figure 2b. In both cases, we presume that the photogenerated holes oxidize either the surface hydroxyl groups on the TiO2 particles or water itself. Type II films prepared by derivatizing a nickel electrode in situ with both NHF and TiO2 (see Experimental Section) exhibited more unusual photoelectrochemical behavior in 0.1 M NaNO3. Figure 2c contains linear sweep voltammetric data under chopped illumination of the derivatized film. The photocurrent transients are now cathodic in polarity and gradually decay in amplitude as the potential is swept to the NHF II f III redox regime. Thereafter the transients revert to the usual anodic polarity and the “normal” S-shape envelope typical of n-type photoelectrodes.2 At potentials below the NHF II f III redox transition, the photogenerated holes from TiO2 are more efficiently transferred to the (reduced) NHF sites than to the film/solution boundary as in the type I case (Figure 2b). We presume that the photogenerated electrons from TiO2 now move to this boundary and reduce incipient electron acceptor agents such as residual O2 present in solution. This carrier transit behavior is presumably facilitated by the absence of Ni/ TiO2 contacts in the type II films unlike in their type I counterparts (Scheme 1). As the electric field gradient is increased and potentials in the NHF II f III redox regime are accessed, electron transfer ensues from TiO2 to the (oxidized) NHF sites, and holes are driven to the film/solution boundary as before. Interestingly, note that the saturation anodic photocurrent for the type II film is not “sensitized” (relative to the parent Ni/TiO2 film) like in the type I case discussed earlier. This trend is consistent with the carrier transit model for type I vs type II film behavior presented above. The difference in the photoelectrochemical behavior of the two types of derivatized films is reproducible and underlines the sensitivity of the microenvironment within the nanocomposite films to their preparation history. The “bipolar” nature of the photocurrents from type II films is particularly noteworthly. Anomalous majority carrier derived photocurrents have been observed for both n-9,10 and p-type11 semiconductor electrodes in photoelectrochemical cells. However, these observations have altogether different origin related to carrier tunneling,11 surface states,9 and/or mechanical damage10 of the semiconductor surface. More recent work on semiconductor/ liquid contacts under high-level injection conditions has shown that photocurrents can be either anodic or cathodic in sign depending on the charge carrier collection properties of the rear (nonliquid) side of the semiconductor electrode.12 Perhaps most closely related to the present finding of “bipolar” photoelectrode behavior are the studies on nanocrystalline CdS and CdSe films wherein both “n-” as well as “p-”type photoactivity was observed depending on the electrolyte and the electrode-etching treatment.13 The present work demonstrates the following: (a) Quantum yields in a particulate or nanocrystalline photoelectrochemical systems can be improved by intimately contacting the semiconductor particles with a molecular redox system in a “nanocomposite” film configuration. In this regard, the chemically
10154 J. Phys. Chem. B, Vol. 101, No. 49, 1997 modified Ni/TiO2 film provides a useful framework for exploring charge-transfer processes at semiconductor/molecular solid contacts. (b) Carrier transit to the semiconductor/liquid boundary can be modulated by controlling the microenvironment within nanocomposite photoelectrodes. Even bipolar behavior can be obtained for a given photoelectrode in the same electrolyte. Further studies and characterization of these nanocomposite materials are in progress. Acknowledgment. This work was supported in part by a grant from the U.S. Department of Energy, Office of Basic Energy Sciences. References and Notes (1) For reviews, see: (a) Kamat, P. V., Meisel, D., Eds. Semiconductor NanoclusterssPhysical, Chemical and Catalytic Aspects; Elsevier: Amsterdam, 1997. (b) Pelizzetti, E., Ed. Fine Particles Science and Technology; Kluwer: Dordrecht, 1996. (c) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. (Washington, D.C.) 1995, 95, 49. (2) (a) Zhou, M.; Lin, W.-Y.; de Tacconi, N. R.; Rajeshwar, K. J. Electroanal. Chem. 1996, 402, 221. (b) Zhou, M.; de Tacconi, N. R.; Rajeshwar, K. J. Electroanal. Chem. 1997, 421, 111.
Letters (3) de Tacconi, N. R.; Wenren, H.; Rajeshwar, K. J. Electrochem. Soc. 1997, 144, 3159. (4) de Tacconi, N. R.; Wenren, H.; McChesney, D.; Rajeshwar, K., submitted. (5) For example: Roy Morrison, S. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum: New York and London, 1980; pp 209-219. (6) (a) Bocarsly, A. B.; Sinha, S. J. Electroanal. Chem. 1982, 137, 157. (b) Bocarsly, A. B.; Sinha, S. J. Electroanal. Chem. 1982, 140, 167. (c) Sinha, S.; Humphrey, B. D.; Bocarsly, A. B. Inorg. Chem. 1984, 23, 203. (7) Schneemeyer, L. F.; Spengler, S. E.; Murphy, D. W. Inorg. Chem. 1985, 24, 3044. (8) de Tacconi, N. R.; Myung, N.; Rajeshwar, K. J. Phys. Chem. 1995, 99, 6103. (9) (a) Morisaki, H.; Hariya, M.; Yazawa, K. Appl. Phys. Lett. 1977, 30, 7. (b) Minoura, H.; Tsuiki, M. Chem. Lett. 1978, 205. (10) (a) Vainas, B.; Hodes, G.; DuBow, J. J. Electroanal. Chem. 1981, 130, 391. (b) Mu¨ller, N.; Hodes, G.; Vainas, B. J. Electroanal. Chem. 1984, 172, 155. (11) Gissler, W. J. Electrochem. Soc. 1980, 127, 1713. (12) Tan, N. X.; Kenyon, C. N.; Kru¨ger, O.; Lewis, N. S. J. Phys. Chem. B 1997, 101, 2830. (13) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136.
