Effect of Water Adsorption on Carrier Trapping Dynamics at the

Jan 25, 2016 - Experimental procedures and characterization data of TiO2 nanoparticles including XRD patterns, TEM images, diffuse reflectance UV–vi...
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Effect of Water Adsorption on Carrier Trapping Dynamics at the Surface of Anatase TiO2 Nanoparticles Kenji Shirai,† Toshiki Sugimoto,*,† Kazuya Watanabe,† Mitsutaka Haruta,‡ Hiroki Kurata,‡ and Yoshiyasu Matsumoto*,† †

Graduate School of Science, Department of Chemistry, Kyoto University, Kyoto 606-8502, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

‡

S Supporting Information *

ABSTRACT: Charge carrier trapping plays a vital role in heterogeneous photocatalytic water splitting because it strongly affects the dynamics of photogenerated charges and hence the photoconversion efficiency. Although hole trapping by water at water/photocatalyst interface is the first step of oxygen evolution in water splitting, little has been known on how water adsorbate itself is involved in hole trapping dynamics. To clarify this point, we have performed infrared transient and steady-state absorption spectroscopy of anatase TiO2 nanoparticles as a function of the number of water adsorbate layers. Here, we demonstrate that water molecules reversibly adsorbed in the first layer on TiO2 nanoparticles are capable to trap photogenerated holes, while water in the second layer hydrogen bonding to the first-layer water makes hole trapping less effective. KEYWORDS: Photocatalysis, TiO2 nanoparticles, water splitting, charge dynamics, water adsorption

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adsorbate directly interacting with TiO2 is hydrogen bonded to water molecules surrounding it, another important question is how hydrogen bonding among water molecules at the interface influences the probability of interfacial trapping and transfer of charges. These questions have remained unanswered in a plethora of transient absorption (TA) studies with anatase TiO2 nanoparticles in various environments,6−15 in vacuum, air, water, and alcohol solutions, because these measurements do not allow investigation of, in a controlled manner, the charge dynamics, the adsorption structures, and interactions of water at the interface of water−TiO2 nanoparticles as a function of the number of water layers. Transient absorption measurements in water vapor atmosphere have an advantage over those in liquid water because the number of water layers at the interface can be controlled by relative humidity; this allows us to fill the pressure gap for photogenerated charge dynamics. In this paper, we report how time profiles of TA correlate with vibrational spectra of water adsorbates on nanoparticles of anatase TiO2 as a function of the number of water layers. We show that water adsorbates strongly interacting with substrate serve as effective hole traps, but the trapping ability is reduced by hydrogen bonding with other water molecules in the second layer.

ater splitting with heterogeneous photocatalysts has been extensively studied because this provides us a promising route for effective conversion of solar energy to chemical energy. Among a number of photocatalysts, TiO2 has been a central metal oxide for mechanistic studies on the heterogeneous photocatalysis. The primary concern in the mechanistic studies is to clarify the dynamics of photogenerated carriers responsible for redox reactions. The following picture on this issue has emerged from extensive studies on single crystals and nanoparticles of TiO2.1−5 Electrons and holes created by electronic excitation across the band gap of photocatalyst relax rapidly to each of the band edges. During the relaxation and transportation processes of photogenerated carriers, a substantial portion of carriers are lost owing to electron−hole recombination, which is a major loss process in the photon energy conversion. If charge carriers generated at the surface or transported from the inner part of the nanoparticle are trapped by a surface-bound species, net charge transfer takes place for redox reactions to occur. Because the kinetics of surface reactions, particularly of the oxygen evolution from water, is substantially slow, the prolonged lifetime of trapped carriers is required without loosing redox potentials. Trapping of photogenerated holes by water at the liquid water/photocatalyst interface is the first step of the oxygen evolution in water splitting. In spite of the extensive studies in the past,1−5 it is surprising that an important problem has not been thoroughly investigated: how are photoinduced holes trapped at the interface with water? Moreover, because water © 2016 American Chemical Society

Received: November 19, 2015 Revised: January 19, 2016 Published: January 25, 2016 1323

DOI: 10.1021/acs.nanolett.5b04724 Nano Lett. 2016, 16, 1323−1327

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Nano Letters Experiments were performed with commercially available TiO2 powder (ST-01 of Ishihara Sangyo). The anatase crystal structure of the sample was confirmed by X-ray diffraction (XRD) (Figure S1). The morphology and size distributions of nanoparticles were observed with a high-resolution transmission electron microscope (TEM). As shown in Figure 1,

Figure 1. Images of anatase TiO2 nanoparticles (ST-01) taken with a high-resolution transmission electron microscope.

Figure 2. Transient absorption of anatase TiO2 nanoparticles at 4 μm as a function of water-vapor pressure in the range of: (a) 0.01−100 Pa and (b) 100−1000 Pa.

most of the nanoparticles have round shapes, and their facets are not perpendicular to {110}, {001}, and {101} directions (Figure S2), indicating that the nanoparticles have the facets with high Miller indices. Therefore, nanoparticles have a large density of steps in the facets. The average diameter of particles was estimated to be 5 ± 1 nm from TEM images. The electronic structure of anatase TiO2 nanoparticles can be very different from that of bulk depending on size and shape.16 Di Valentin and co-workers have calculated the energy structures of anatase TiO2 nanoparticles with two different shapes: nanospheres and faceted nanocrystals.17 Among them, the calculations regarding nanospheres are very relevant to the nanoparticles used in this study. The similarity between the nanospheres in the calculations and the nanoparticles in this study is not only in their round shapes, but also in surface modifications with water adsorbates for stabilization; water is strongly adsorbed on ST-01 nanoparticles even in vacuum as described later. The calculations have shown that there are two types of electronic states: localized states and bandlike states delocalized on several atoms. The “band gap” is developed by the bandlike states, while the localized states are in the band gap, that is, “in-gap” states. The electron densities of unoccupied and occupied in-gap states are localized in the central part and at the surface of the nanosphere, respectively. The nanoparticles in this study have a long tail of absorption band down to 2.1 eV (Figure S3). By adapting the theoretical predictions,17 we assign the tail extending to the energy much lower than that of the band edge to the transitions in which the in-gap states are involved. Because the excitation energy of pump pulses at 400 nm (3.1 eV) is slightly less than the band edge, the excitation likely creates holes mainly at the surface and electrons in the conduction band. According to TA measurements of anatase nanoparticles in the past, electrons excited in the conduction band relax to the band edge rapidly. These photogenerated electrons have a characteristic absorption band whose intensity monotonically increases with wavelength extending from near IR to midIR.12,18 Thus, at 4 μm, we can monitor them without suffering from any transient changes in the OH stretching band. Figure 2 shows that the time profiles of TA at 4 μm upon the excitation at 400 nm remarkably depend on water-vapor pressure P. The profiles are composed of fast (τ = 1−3 ps) and slow (τ = 95 ps) decay components. Interestingly, the transient absorption intensity increases with water-vapor pressure at P < 100 Pa,

whereas it decreases at P > 100 Pa. Because the absorbance at 400 nm monotonically decreases with P (Figure S3), the changes in absorbance at 400 nm caused by water adsorption are irrelevant to the peculiar water-vapor pressure dependence. There are two possible ways to explain the pressure dependence of the TA time profiles at 4 μm: water adsorption affects the trapping probabilities of (1) photogenerated electrons and (2) holes. The first-principles calculations17 on nanospherical particles have predicted that the states near the top of the valence band including in-gap states are predominantly contributed by surface OH groups. In addition, it is widely known that photogenerated holes are trapped at surface OH, forming OH radicals that are one of active oxidative species responsible for decomposition of pollutants.4 Thus, we expect that photogenerated holes are localized at the surface of a nanosphere more substantially than electrons by excitation at 400 nm. In this case, it is reasonable to assume that holes at the surface are more significantly affected by water adsorption than electrons in the nanoparticle. Because photogenerated electrons and holes are lost mainly by nongeminate recombination, the decay rates of charges depend on the densities of both electrons and holes. Thus, if holes are trapped (detrapped) by water adsorption, the density of photoinduced electrons could be increased (decreased) because of less (more) efficient recombination with electrons confined in the central part of the nanoparticle. It is not clear, however, why hole trapping and detrapping probabilities are governed by water adsorption in such a complex way. To identify the origin of the peculiar changes in TA profiles, we measured steady-state IR spectra of water adsorbed on TiO2 nanoparticles as a function of water-vapor pressure in comparison with the TA time profiles. Figure 3, panel a shows diffuse reflectance IR spectra I(P) under various water-vapor pressures. Before introducing water vapor, we have observed two prominent absorption bands around 1640 and 3100 cm−1, assignable to the bending and OH stretching bands of water adsorbed on the nanoparticle surfaces, respectively. In addition, the weak band was observed at ∼3680 cm−1; this is typical to the OH stretching band of “free” OH of surface hydroxyl or the strongly bound water adsorbate. The water adsorbates observed under the vacuum are very strongly bound to the surface; they are not completely eliminated by annealing to 700 K (Figure S4). The low peak 1324

DOI: 10.1021/acs.nanolett.5b04724 Nano Lett. 2016, 16, 1323−1327

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Figure 3. (a) Water-vapor pressure dependence of diffuse reflectance IR spectra I(P) in the pressure range of 0.01−1000 Pa. (b) Two-dimensional plot of difference spectra ΔI(P) = [I(P + ΔP) − I(P)]/Imax(P) as a function of water-vapor pressure for the bending band and (c) the stretching band. (d) Water-vapor pressure dependence of relative integrated area of the bending (green) and the OH stretching (red dotted) bands. The intensities of both bands are normalized at 0.01 Pa. The number of water layer estimated with adsorption isotherm (open circles) is also plotted. The saturation vapor pressure P0 is 5500 Pa at 34.5 °C. (e) Three components A, B, and C used in fittings of the bending band and (f) the OH stretching band, normalized by total integrated area of each peak.

frequency and the broad feature of the OH-stretching band imply that water is strongly bound through a dative O−Ti bond and a hydrogen bond to a surface oxygen atom.19,20 The OH stretching band at 3100 cm−1 might be due to interstitial hydrogens in nanoparticles; those have been know in the rutile form, showing the absorption bands around 3290 cm−1.21−23 To test this hypothesis, we exposed the nanoparticles with D2O vapor. If hydrogens were in nanoparticles, it would be hard to exchange with deuterium. However, we found that this band diminishes almost completely, and a new band grows at ∼2300 cm−1, assignable to the OD stretching band of D2O (Figure S5). Therefore, the facile hydrogen−deuterium exchange indicates that the OH stretching band at 3100 cm−1 is due to water adsorbates at surfaces. As water-vapor pressure increases, water molecules subsequently adsorb at sites with smaller adsorption energies. The changes in the spectral profiles of the bending and stretching bands as a function of P are more clearly exhibited in difference spectra: ΔI(P) = [I(P + ΔP) − I(P)]/Imax(P) (Figure 3b,c) where the intensity is normalized at each peak. Both the stretching and bending band are blue-shifted with increasing water-vapor pressure. The large red-shift of the OH-stretching band at P < 0.1 Pa is another manifestation of strong interactions between water adsorbates and TiO2 nanoparticles; the interactions are much stronger than hydrogen bonds in water layers formed at high pressures. The intensities of two bands grow differently with increasing the water-vapor. Figure 3, panel d shows integrated intensities of both the bending and OH stretching absorption bands as a function of water-vapor pressure. Both the bands grow with pressure very similarly below 10 Pa, but the intensity of OH stretching band deviates from that of bending band at P > 10 Pa. This indicates that hydrogen bonding among the adsorbates influences the oscillator strengths of OH stretching and bending bands differently. To find out which band intensity is a more reliable measure for water coverage, we estimated the coverage of water by

measuring the adsorption isotherm of water. The surface area of nanoparticles was estimated to be 259 m 2 /g from N 2 adsorption isotherm. Assuming that one monolayer (ML) of water corresponds to 4.3 mmol/g, the coverages of water adsorbate are estimated, as plotted in Figure 3, panel d. Because the adsorption isotherm is in good agreement with the Pdependence of bending band intensity at P ≥ 10 Pa, the intensity of bending band is rather a good measure for the coverage of adsorbed water. Having established the correlation between water coverage and the intensity of bending band, we need to decompose the bending band into a couple of components correlating to specific adsorption states to decipher how water molecules adsorb at different adsorption sites with increasing water-vapor pressure. However, it is difficult to do so in the case of bending band because the band is narrower and the frequency shifts are substantially smaller than those of the stretching band. Namely, the OH stretching band is more sensitive to the adsorbate− substrate interactions as well as hydrogen bonds in water layers than the bending band. Thus, we first decomposed the OH stretching band into three components A, B, and C as depicted in Figure 3, panel f. While component A was obtained by subtracting the spectrum at P = 0.01 Pa from that at P = 0.1 Pa, component C was obtained by subtracting the spectrum at P = 1000 Pa from that at P = 1500 Pa. By keeping the spectral features of A and C fixed, we fitted all the absorption spectra at various pressures with one more component (component B) whose spectral profile was obtained by fitting (Figure S6). Then we could decompose the bending band similarly into three components (Figure 3e), assuming that the intensity of each component follow the same pressure dependence of the corresponding component in the stretching band. Consequently, the decomposition of the bending band into the three components, A, B, and C, has been accomplished, and it provides the coverage of each adsorption state as a function of water-vapor pressure as shown in Figure 4. 1325

DOI: 10.1021/acs.nanolett.5b04724 Nano Lett. 2016, 16, 1323−1327

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Figure 4. Water-vapor pressure dependence of coverage of the three components derived from the decompositions of the bending band. Comparison is made with the pressure dependence of the intensity of the slower decay component in TA profiles at λ = 4 μm.

The spectral analysis of the vibrational bands of water provides a clue to the peculiar water-vapor pressure dependence of TA time profiles in Figure 2. In Figure 4, the watervapor pressure dependence of the slower decay component of TA is compared with those of coverages of three components. The coverage of A shows the remarkably similar pressure dependence with that of transient absorption intensity; both curves increase until about 100 Pa and decrease above 100 Pa. Consequently, water adsorbates of component A are responsible for hole trapping. The pressure dependence of IR spectra indicates that component A is contributed by two adsorption states, although they are spectrally barely distinguishable: one is molecules irreversibly preadsorbed (A0), while the other is reversibly adsorbed (A1). Assuming that the coverage of water obtained with the adsorption isotherm follows the pressure dependence of bending band intensity below 10 Pa, we estimated the coverage of A0 to be 0.44 ML by extrapolating the Pdependence of water coverage to the vacuum. The temperature dependence of IR spectra (Figure S4) indicates that the coverage of strongly bound water (A0, 0.35 ML) dominates over that of surface OH (0.09 ML). Figure 4 indicates that water molecules adsorb predominantly in the adsorption state corresponding to A at low watervapor pressure, and water adsorbates in B grow with increasing pressure. Thus, the first two layers follow Langmuir isotherm and grow in a layer-by-layer mode (Figure S7). However, at P > 1000 Pa, multilayers are rapidly formed, following BET isotherm. The analysis of the pressure dependence of adsorbates under adsorption−desorption equilibrium with Langmuir and BET isotherm allows us to estimate the adsorption energy Ea of each component (Table S1): 100− 190, 60−71, 52, and 46 kJ mol−1 for A0, A1, B, and C, respectively. Thus, we propose the following water adsorption model as shown in Figure 5: (1) At P ≤ 0.01 Pa (Figure 5b), the surfaces of nanoparticles are preadsorbed by strongly bound water (A0); (2) at 0.01 < P < 10 Pa (Figure 5c), the first layer is filled with another adsorption component A1, showing the absorption band similar to that of A0; (3) at 10 < P < 100 Pa (Figure 5d), water in the second layer (B) is formed on A0; (4) at 100 < P < 300 Pa (Figure 5e), the coverage of B increases with further increasing pressure. Upon adsorption of B on A1, we assume that A1 is converted to an adsorption state revealing the spectral band shape of B at this adsorption stage, whereas A0 is not affected by adsorption of water. The conversion reduces the coverage of A1 and hence hole trapping ability. (5) At 300 < P < 1000 Pa (Figure 5f), while adsorption in the second layer and conversion from A1 to B continue, the

Figure 5. (a) Water-vapor pressure dependence of coverages of components A0, A1, and B. (b−g) Schematic representation of the water adsorption model used in analysis. A0, orange; A1, magenta; B, green; C, blue. (h) Overlayer effect showing that A1 water is elevated away from the surface by hydrogen bonding with second-layer water, converting A1 to B. O of water, color coded similarly in panels b−g; H, white; O in titania, red; Ti, yellow.

overlayer of water (C) starts to be formed; (6) at P > 1000 Pa (Figure 5g), multilayer grows, following a BET isotherm. Note that this adsorption model is basically identical to the threelayer model of water on ST-01 nanoparticles proposed in a 1H NMR study24 except for the conversion in the adsorption state of water in the first layer through the interaction with water in the second layer, that is, from A1 to B. As shown earlier, the TEM images show that the nanoparticle surfaces are mostly curved and highly stepped ones. Thus, they contain substantial steps and ridges on the surfaces. Moreover, the nanoparticles likely have substantial defect sites with less coordinated Ti cations. Water is adsorbed strongly at these sites either molecularly or dissociatively.25 Previous theoretical works have predicted that the step edges decorated with OH groups provide sites where water is strongly adsorbed with Ea ≈ 100 kJ mol−1.26 Even on terraces, water is strongly adsorbed at the five-coordinated Ti (Ti5c) site adjacent to a subsurface oxygen defect.27 Furthermore, the facet with (001) termination also provides dissociative and molecular adsorption sites with Ea = 130−150 kJ mol−1.19 These strongly bound water are the candidates for A0. The large adsorption energy of A0 stems from two factors: a dative bond through oxygen of water to a less coordinated Ti cation and a hydrogen bond between hydrogen of water and a surface oxygen atom.19,20,28 Although the adsorption energy of A1 is smaller than that of A0, the similar spectral features suggest that basically the same interactions with the surface are responsible for the adsorption. In this adsorption scheme, STM observations and theoretical calculations have shown that water adsorption alters the surface electron charge distribution:20,29 electron density is depleted at Ti5c and accumulated along the hydrogen bond between the hydrogen of water and the surface oxygen. Therefore, it is highly probable that the excess electron density in the hydrogen bond is the origin of trapping of the hole generated at 400 nm excitation. 1326

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Without any water adsorbates in the overlayer, the adsorption structure of first-layer water is mainly governed by the water−substrate interactions described above. As water adsorbs on the first-layer water, a hydrogen bond network grows among water adsorbates. Because the adsorption energy of A1 is close to that of B, the adsorption structure of A1 can be altered as shown in Figure 5, panel h. This kind of rearrangement has been found in the first-principles molecular dynamics simulations;28 water molecules hydrogen-bonded to a bridge oxygen are elevated away from the surface because hydrogen bonds to the second-layer water relieve the stress in the first layer. Thus, we expect that the similar overlayer effect takes place upon the adsorption of second-layer water; the hydrogen bonding between A1 and B converts A1 to B, reducing the hole trapping capability of A1. In summary, we have investigated the effect of water adsorption on charge dynamics in nanoparticles of anatase TiO2 by measuring transient IR absorption time profiles and steady-state spectra as a function of the number of water layer. Water molecules in the first layer strongly interacting with TiO2 surface are capable of effectively trapping photogenerated holes at the surface. Because the adsorption of second-layer water weakens the interaction between first-layer water and TiO2 surface, the hole trapping capability of water in the first layer is reduced. The discussion in this paper is based on the assignment: photogenerated holes are at the surface of a nanoparticle, and electrons are in its interior based on the theoretical finding.17 Even if the situation is reversed, our conclusion regarding the water adsorbates effects on charge trapping capability holds. These findings clearly indicate that not only the inherent property of photocatalyst itself, but also water adsorption and hydrogen bonding among the water molecules surrounding catalysts are important factors for the dynamics of photogenerated charges and, hence, its photocatalytic activity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04724. Experimental procedures and characterization data of TiO2 nanoparticles including XRD patterns, TEM images, diffuse reflectance UV−vis spectra, temperature dependence of IR spectra, pressure dependence of intensity of spectral components, and analysis of adsorption energies of water (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +81 (0)75 753 4047. Fax: +81 (0)75 753 4050. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grants-in-Aid for Scientific Research (A) (No. 25248006) and for Young Scientists (No. 26810006) from the Japanese Society for the Promotion of Sciences. We thank H. Kitagawa and K. Otsubo for their help with the water isotherm measurements. 1327

DOI: 10.1021/acs.nanolett.5b04724 Nano Lett. 2016, 16, 1323−1327