IRâSpectrophotoelectrochemical Characterization of Mesoporous

Letter pubs.acs.org/ac

IR−Spectrophotoelectrochemical Characterization of Mesoporous Semiconductor Films Thomas Berger* and Juan A. Anta Departamento de Sistemas Físicos, Químicos y Naturales, Á rea de Química Física, Universidad Pablo de Olavide, Ctra. Utrera, km 1, E−41013 Sevilla, Spain S Supporting Information *

ABSTRACT: A combined IR−spectroscopic and electrochemical approach for the study of photo- and bias-induced reactions at the semiconductor/electrolyte interface is presented. Information on the electronic structure of a mesoporous semiconductor nanoparticle network, concretely the density and distribution of band gap states, as well as the nature of solution species are analyzed in situ. It has been shown that under appropriate conditions the electrode potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation, i.e., application of a bias voltage or electrode exposure to photons exceeding the semiconductor band gap at open circuit. Importantly, electronic properties of the semiconductor and vibrational properties of solution species can be addressed simultaneously by IR−spectroscopy. In addition, electrochemical methods provide a means for the active manipulation (in potentiostatic measurements) or the passive tracking (during open circuit potential decay) of the quasi-Fermi level in the mesoporous film together with the possibility of electron quantification (by charge extraction experiments).

T

conductor. Indeed, both types of perturbation have been used to study the charge transfer at dye-sensitized semiconductor electrodes by the open-circuit potential decay method yielding comparable results.3 Many applications of nanocrystalline semiconductors are based on the photoinduced generation of charge carriers. Semiconductor photocatalysis relies on the chemical reactivity of electrons and holes, which are generated by the absorption of photons exceeding the band gap. In dye-sensitized solar cells, on the other hand, electron injection from an adsorbed dye molecule is exploited for energy conversion. The macroscopic performance of semiconductor films in functional devices depends on transport and recombination of the charge carriers. These processes can readily be addressed by electrochemical studies, which therefore constitute a valuable tool for studying the semiconductor/solution interface in an application-relevant environment. Both kinetic and thermodynamic information can be gained from electrochemical measurements.3 Furthermore, quantitative information can readily be extracted, e.g., by current integration or coulometry. However, whereas electrochemical approaches succeed in addressing the macroscopic properties of the semiconductor thin films, a combination with complementary analytical methods providing specific chemical information on a molecular level is desirable.4,5 In this context, a combined spectrophotoelectrochemical approach allowing for

he use of semiconductor oxides in functional devices for energy conversion, energy storage, and sensoric, optoelectronic, and photocatalytic applications is frequently associated with the deposition of an active nanocrystalline material in the form of thin mesoporous films on conducting substrates. Apart from technological exploitation, these electrodes constitute appropriate systems for the analytical study of processes at the semiconductor/solution interface as they are accessible to a whole body of electroanalytical tools. The importance of electrochemical approaches for the characterization of nanocrystalline electrodes is based on the possibility of externally addressing the quasi-Fermi level1 in the mesoporous semiconductor film and of simultaneously detecting the charge transfer associated with an external perturbation. The possibility of Fermi level control is based on the specific electrode characteristics as the small crystallite size (inhibiting significant band bending), a low level of doping, good electronic connectivity, and the presence of a surrounding equipotential surface.2 Both faradaic and capacitive processes can be addressed, being the first associated with a charge transfer from the semiconductor to solution species and the second with the reversible accumulation of charge carriers in the semiconductor or at the semiconductor/solution interface, respectively. Beyond external control as exploited, e.g., in potentiostatic and voltammetric measurements, the Fermi level in the films can be tracked, following an initial external perturbation, at open circuit in potential decay measurements.3 The Fermi level can be shifted by the application of a bias voltage or, alternatively, by exposure of the electrode to photons exceeding the band gap of the respective semi© 2012 American Chemical Society

Received: January 11, 2012 Accepted: March 7, 2012 Published: March 7, 2012 3053

dx.doi.org/10.1021/ac300098u | Anal. Chem. 2012, 84, 3053−3057

Analytical Chemistry

Letter

open new possibilities for the characterization of functional devices. The response of the anatase TiO2 nanoparticle network to UV exposure in the pure electrolyte is shown in Figure 2. On

the study of photoinduced processes at wet-chemically deposited electrodes was presented recently.6,7 However, due to design and stability restrictions of the spectrophotoelectrochemical (SPEC) setup, these investigations were limited to a single type of TiO2 nanowire electrode and could not be extended to more relevant systems like, e.g., those consisting of sintered nanocrystals forming a random particle network, which are frequently used in photocatalytic8 and photovoltaic applications.9 We therefore designed a modified SPEC setup, which allows for separating electrode preparation and modification from spectroelectrochemical in situ analysis. Thus, thin film preparation is not limited by the thermal, mechanic, or chemical stability of the SPEC cell. The feasibility of the new design is highlighted using a state-of-the-art thinfilm electrode prepared from a commercially available anatase TiO2 particle suspension (Ti−Nanoxide T, Solaronix) spread onto a F-doped SnO2 (FTO) transparent glass substrate (Pilkington, TEC 8) and finally sintered at 450 °C. These electrodes can be considered a standard material in photocatalytic and photovoltaic studies. The resulting sintered layers with a film thickness of ∼3 μm consist of anatase nanocrystals with a mean diameter of 20 nm and are transparent in the vis/ NIR.10 The SPEC cell consists of a glass body, which is connected to a hemispheric ZnSe prism (Figure 1). An U-

Figure 2. (a) IR spectra of an anatase TiO2 nanocrystal electrode during UV exposure (illumination time: 80 s) and after 270 s in the dark. The reference spectra were taken prior to UV exposure in the dark. (b) Open circuit potential (EOC) profile measured simultaneously. Electrolyte: N2-saturated 0.1 M HClO4 aqueous solution. Irradiance: 5 mW·cm−2.

the one hand, a weak and structureless signal, which increases monotonically toward lower wavenumbers, is observed in the IR spectrum. Furthermore, negative going bands at 3240 and 1655 cm−1 appear. The IR signals are completely reversible, when UV exposure is stopped (Figure 2a). The broad structureless IR signal in Figures 2 and 3 can be associated with the accumulation of electrons in the TiO2 film. Similar signals have previously been observed upon UV exposure on nanocrystalline TiO2 both at high vacuum conditions11−13 and in contact with an aqueous phase in the presence of hole acceptors.14 Furthermore, they were observed on TiO2 powders after thermal reduction12,13 or n-type doping by atomic hydrogen15,13 under high vacuum conditions. Only recently, we reported on the spectroscopic fingerprint in the vis/NIR and MIR of electrons accumulated upon electrochemical polarization in mesoporous anatase TiO2 electrodes comparable to those studied here.10 Accumulation at electrode potentials significantly more positive than the photocurrent onset potential resulted in a broad absorption in the vis/NIR, leveling off toward longer wavelengths (lower energies), and a broad MIR signal, monotonically increasing toward lower wavenumbers (lower energies). The appearance of the absorption signals at potentials more positive than the photocurrent onset clearly indicates a location of the corresponding electronic states in the band gap.10 The signals in the vis/NIR and the MIR were linearly correlated with each other and with the number of extracted charges. Furthermore, absorbance and extractable charge showed the same exponential dependence on electrode potential indicating that the signals in the vis/NIR and MIR are associated with an exponential distribution of band gap states.10

Figure 1. Scheme of the SPEC cell. Bottom left: TiO2 electrode with the prestructured FTO substrate. Bottom right: Cross section of the ATR crystal/electrolyte/electrode contact.

shaped perforation of the FTO substrate has to be realized prior to thin film deposition (Figure 1, bottom left). This aperture assures contact between the bulk electrolyte and the thin electrolyte layer between the hemisphere and the electrode (Figure 1, bottom right). The working electrode (TiO2 thin film on FTO) is pressed mechanically against the ZnSe hemisphere via a glass tube. The attenuated total reflection (ATR) hemisphere is placed in a reflection unit (PIKE Technologies, Veemax II) attached to a Bruker IFS 66/S FTIR spectrometer equipped with an MCT detector. Further experimental details can be found in the Supporting Information. Here, we demonstrate the feasibility of the novel IR− spectrophotoelectrochemical approach and highlight its suitability for studying both the electronic properties of the semiconductor as well as the vibrational properties of solution species during a photocatalytic reaction in an aqueous electrolyte. The advantages of the combined approach should 3054



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lifetime.16 It has to be taken furthermore into account that products of the hole transfer can act as scavengers for photogenerated electrons thus providing an additional recombination mechanism.17 A low quantum yield of water photooxidation associated with high recombination and thus short electron lifetime causes the Fermi level to shift only moderately upon illumination (Figure 2b). As a consequence, only a weak signal of accumulated electrons is observed in the IR spectrum (Figure 2a). It has to be mentioned at this point that in all experiments the electrolyte was purged by N2 in order to minimize the concentration of dissolved oxygen. It is well-known that some organic compounds act as efficient hole scavengers, thus significantly increasing the lifetime of photogenerated electrons.16,17 The response of the electrode to UV exposure after addition of formic acid (2 M) to the electrolyte is shown in Figure 3. Again, the monotonic, broad IR signal evolves with illumination time and is superimposed by negative going bands at 1120, 1220, 1400, 1655, 1720, and 3240 cm−1 (Figure 3a). In addition, a positive going band at 2340 cm−1 appears. The bands at 1220 cm−1 (ν(C−OH)), 1400 cm−1, and 1720 cm−1 (ν(CO)) are associated with formic acid in solution.18 It was shown that, even at pH 1, that is, in the absence of solution formate, formic acid adsorbs onto TiO2 in the deprotonated form.18 However, no formate bands are observed in the IR spectra due to sensitivity limitations. The negative band at 1120 cm−1 can be attributed to ClO4− and the bands at 1655 cm−1 and 3240 cm−1 to water bending and stretching modes, respectively.10 The intensity decrease of the electrolyte bands probably results from a modification of the electrolyte structure in the mesopores upon electron accumulation as observed in a recent study.10 The appearance of the positive going band at 2340 cm−1 is associated with the photooxidation of formic acid and can be attributed to dissolved CO2 accumulating in the mesoporous structure upon illumination. A stationary CO2 concentration in the mesoporous structure, which is determined by the balance of CO2 formation and diffusion out of the pores, is rapidly established. The fact that electrolyte species, reaction educts, and reaction products as well as accumulated electrons can be detected simultaneously at the electrode/electrolyte interface highlights the suitability of IR spectroscopy for the study of photoinduced reactions. The build-up of the IR signals with illumination time is fast, reaching saturation after 30 s (Figure 3b). Importantly, the final absorbance is about 2 orders of magnitude higher than in the absence of formic acid. In addition, signal decay after discontinuation of UV exposure is very slow. The high concentration of accumulated electrons is a consequence of the preferential transfer of photogenerated holes to the organic donor, which takes place mainly via a direct, inelastic transfer mechanism.19,20 As a consequence, the lifetime of photogenerated electrons increases significantly. The photooxidation of formic acid to CO2 at the TiO2 surface involves the following reaction steps

Figure 3. (a) IR spectra of an anatase TiO2 nanocrystal electrode during UV exposure (1−5), after 1000 s in the dark (6), and after polarization to EAg/AgCl = 0.4 V (7). The reference spectra were taken prior to UV exposure at open circuit in the dark. (b) Temporal evolution of the monotonic signal (at 2000 cm−1). (c) Open circuit potential (EOC) profile measured simultaneously. (d) Semi-logarithmic plot of absorbance (as taken from b) vs electrode potential (as taken from c). This profile was alternatively determined after application of an external bias (EAg/AgCl = −0.48 V, t = 600 s) in the dark. The straight line with a slope of −10 is added as a guide to the eye. Electrolyte: N2-saturated 2 M HCOOH/0.1 M HClO4 aqueous solution. Irradiance: 5 mW·cm−2.

The appearance of the IR signals is accompanied by a simultaneous and reversible change of the open circuit potential (EOC, Figure 2b). As photogenerated holes get preferentially transferred to solution species, electrons accumulate in the TiO2 film upon UV exposure in aqueous solution. Electron accumulation is associated with a change of EOC, which corresponds to a shift of the Fermi level in the nanocrystalline electrode to more negative potentials. Hence, measurements of EOC are directly correlated to shifts of the Fermi level if the semiconductor bands are pinned. A four-hole mechanism has been evidenced for the oxygen production by water photooxidation. As a consequence, the quantum yield of oxygen generation critically depends on hole

HCOO−ads + h+ → HCOO·ads

(1)

HCOO·ads + h+ → CO2 + H+aq

(2)

HCOO·ads → CO2 + H+aq + e−CB

(3)

The first oxidation step involving photogenerated holes (eq 1) may be followed by a second hole transfer (eq 2) or, 3055



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alternatively, by electron injection from the reaction intermediate to the TiO2 conduction band (eq 3). Thus, in addition to efficient hole scavenging by formic acid, electron accumulation in the film gets further enhanced by electron injection. The irreversible character of formic acid oxidation, lacking the accumulation of reaction products acting as efficient electron acceptors,17 causes thus the accumulation of electrons in the semiconductor film to be significantly enhanced as compared to the pure electrolyte (Figure 2). In line with IR data, more negative EOC values are observed under UV exposure in the presence of formic acid and the open circuit potential decay is slow (Figure 3c). As a consequence, the Fermi level in the semiconductor is expected to be equilibrated throughout the whole mesoporous film at each potential during relaxation. A combination of IR and EOC data may thus yield the energetic distribution of IR-active states if it is supposed that the band edges of the semiconductor do not move upon a displacement of the Fermi level (band pinning).10 The semilogarithmic representation of the absorbance (A) versus open circuit potential (EOC/V) gives a straight line with a slope of approximately −10 (Figure 3d) evidencing the presence of an exponential distribution of band gap states. Such an exponential distribution has previously been observed on mesoporous anatase TiO2 electrodes2,3,10,21−26 and was associated with an intrinsic (chemical) film capacitance2 C = Ca exp[−α·e·E /(kB·T )] + const

(4)

where Ca is a preexponential factor, e is the elementary charge, kB is the Boltzmann constant, T is the absolute temperature, and E is the electrode potential. An α-value of 0.26 can be estimated from the slope in Figure 3d in good agreement with previous results.10 It was shown recently that the Fermi level and thus the population of the IR-active band gap states in a mesoporous anatase TiO2 film can be controlled externally by the application of a bias voltage.10 Electron accumulation within the mesoporous film upon external polarization is coupled to H+ uptake (adsorption/intercalation) from the electrolyte27 Ti IV O2 + e− + H+ ↔ Ti III(O)(OH)

Figure 4. (a) IR spectra of an anatase TiO2 nanocrystal electrode at EAg/AgCl = −0.48 V upon UV exposure at open circuit and external polarization in the dark, respectively. The reference spectra were taken prior to UV exposure/polarization at open circuit in the dark. Charge extraction at EAg/AgCl = 0.4 V (b) after UV exposure (30 s) at open circuit and (c) after polarization (600 s) at −0.48 V. (d) Representation of the two types of perturbation together with the electronic properties deduced. Electrolyte: N2-saturated 2 M HCOOH/0.1 M HClO4 aqueous solution. Irradiance: 5 mW·cm−2.

(5)

of the corresponding potential in the dark, respectively (Figure 4c). It turns out that in both cases ∼350 electrons get accumulated per particle if a thin film porosity of 0.5 is assumed. The chronocoulometric profile (Figure 4c) indicates that not all of the current injected to the electrode during cathodic polarization is extracted upon anodic back polarization, as significant faradaic losses due to electron transfer to solution species (as residual oxygen) take place. These faradaic currents can also be seen in the cyclic voltammogram in Figure S1 (Supporting Information) where they cause a small deviation from symmetry of cathodic and anodic currents. The conclusions reached by the combined IR−spectroscopic and electrochemical study of the photocatalytic degradation of formic acid on anatase TiO2 electrodes are schematized in Figure 4d. The new SPEC setup has been used to determine the distribution and density of electronic states in the semiconductor. For this purpose, the decay of IR absorbance and open circuit potential were tracked simultaneously after an external perturbation by a voltage bias or by UV exposure. An exponential distribution of IR-active band gap states was evidenced. Importantly, it has been shown that, in the absence of high concentrations of electron scavengers, the electrode

Importantly, the same profile is obtained in the semilogarithmic A versus EOC plot, if the semiconductor Fermi level is shifted not by UV exposure but by the application of an external bias in the dark (Figure 3d). This highlights on the one hand the good potentiostatic control over the hole film thickness and points on the other hand to the fact that, in the absence of efficient electron scavengers, equilibration of the Fermi level takes place throughout the film at each potential during EOC relaxation. As may be seen from Figure 4a, almost identical IR spectra are obtained at a given potential (EAg/AgCl = −0.48 V) independent of the perturbation method (i.e., UV exposure versus bias application). The only significant difference consists in the presence of the IR band at 2340 cm−1 during UV exposure, which is associated with the product (CO2) of the photocatalytic oxidation reaction. Finally, we took advantage of the charge extraction method, which is a straight-forward electrochemical tool for quantitatively addressing electron accumulation in mesoporous electrodes.28 For this purpose, an external bias (EAg/AgCl = 0.4 V), which is sufficiently positive to extract the majority of electrons (Figure S1, Supporting Information),10 was applied after the initial charge accumulation by UV exposure at open circuit (Figure 4b) or by external application 3056



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(16) Tang, J.; Durrant, J. R.; Klug, D. R. J. Am. Chem. Soc. 2008, 130, 13885. (17) Solarska, R.; Rutkowska, I.; Morand, R.; Augustynski, J. Electrochim. Acta 2006, 51, 2230. (18) Berger, T.; Delgado, J. M.; Lana-Villarreal, T.; Rodes, A.; Gómez, R. Langmuir 2008, 24, 14035. (19) Mora-Seró, I.; Lana-Villarreal, T.; Bisquert, J.; Pitarch, A.; Gómez, R.; Salvador, P. J. Phys. Chem. B 2005, 109, 3371. (20) Lana-Villarreal, T.; Gómez, R.; González, M.; Salvador, P. J. Phys. Chem. B 2004, 108, 20278. (21) Schlichthörl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (22) van de Lagemaat, J.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (23) Peter, L. M.; Duffy, N. W.; Wang, R. L.; Wijayantha, K. G. U. J. Electroanal. Chem. 2002, 524−525, 127. (24) Bailes, M.; Cameron, P. J.; Lobato, K.; Peter, L. M. J. Phys. Chem. B 2005, 109, 15429. (25) Boschloo, G.; Hagfeldt, A. J. Phys. Chem. B 2005, 109, 12093. (26) Jankulovska, M.; Berger, T.; Lana-Villarreal, T.; Gómez, R. Electrochim. Acta 2012, 62, 172. (27) Lyon, L. A.; Hupp, J. T. J. Phys. Chem. 1999, 103, 4623. (28) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. Electrochem. Commun. 2000, 2, 658.

potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation. In addition, concentration changes of solution species (formic acid, ClO4−, H2O) and reaction products (CO2) have been detected by IR spectroscopy in the course of the photocatalytic reaction. The SPEC setup thus allows one to gain complementary information by combining electrochemistry with electronic and vibrational spectroscopy in situ. This advantage makes the combined approach a powerful tool of relevance for all those technologies relying on photo- or biasinduced processes at the semiconductor/electrolyte interface such as sensoric, optoelectronic, photocatalytic, and photovoltaic applications.

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ASSOCIATED CONTENT

S Supporting Information *

Further experimental details and the cyclic voltammogram of the film. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 95434 9315. Fax: +34 95434 9814. E-mail: tberger@ upo.es. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Spanish Ministry of Science and Innovation (MICINN) through the projects HOPE CSD2007−00007 (Consolider−Ingenio 2010) and P09-FQM-04938 and the Ramón y Cajal program as well as by the Junta de Andaluciá through projects P07−FQM−02595, P07−FQM−02600, and P09−FQM−04938.

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REFERENCES

(1) Strictly speaking, the nonequilibrium replacement of the Fermi level is called the quasi-Fermi level. However, for the sake of brevity, the distinction between Fermi level and quasi-Fermi level will be omitted in the following. (2) Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Bisquert, J. J. Phys. Chem. B 2003, 107, 758. (3) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Seró, I. J. Am. Chem. Soc. 2004, 126, 13550. (4) Grassian, V. H. J. Phys. Chem. C 2008, 112, 18303. (5) Bürgi, T.; Baiker, A. Adv. Catal. 2006, 50, 227. (6) Berger, T.; Rodes, A.; Gómez, R. Chem. Commun. 2010, 46, 2992. (7) Berger, T.; Rodes, A.; Gómez, R. Phys. Chem. Chem. Phys. 2010, 12, 10503. (8) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (9) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595. (10) Berger, T.; Anta, J. A.; Morales, V. J. Phys. Chem. C 2012, DOI: 10.1021/jp212436b. (11) Szczepankiewicz, S.; Moss, J. A.; Hoffmann, M. R. J. Phys. Chem. B 2002, 106, 2922. (12) Berger, T.; Sterrer, M.; Diwald, O.; Knözinger, E.; Panayotov, D.; Thompson, T. L.; Yates, J. T. J. Phys. Chem. B 2005, 109, 6061. (13) Panayotov, D. A.; Burrows, S. P.; Morris, J. R. J. Phys. Chem. C 2012, 116, 4535. (14) Warren, D. S.; McQuillan, A. J. J. Phys. Chem. B 2004, 108, 19373. (15) Panayotov, D. A.; Yates, J. T. Chem. Phys. Lett. 2007, 436, 204. 3057


IRâSpectrophotoelectrochemical Characterization of Mesoporous

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