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Thermo- and Photo-stimulated Effects on the Optical Properties of Rutile Titania Ceramic Layers Formed on Titanium Substrates V. N. Kuznetsov,† V. K. Ryabchuk,† A. V. Emeline,† R. V. Mikhaylov,† A. V. Rudakova,† and N. Serpone*,‡ †

Fock Institute of Physics, St. Petersburg State University, St. Petersburg, Russian Federation Gruppo Fotochimico, Dipartimento di Chimica, Universita di Pavia, Via Taramelli 10, Pavia 27100, Italy

‡

S Supporting Information *

ABSTRACT: This Article reports on the thermo- and photostimulated effects on the optical properties of rutile titania ceramic layers fabricated in an air atmosphere by hightemperature calcination of (technical grade) titanium substrates. The so-formed layers peeled off spontaneously during the cooling phase back to ambient temperature to reveal a yellow-colored upper surface and a cream-colored bottom surface that was in contact with the titanium plate. The two surfaces of the layers and a powdered specimen (formed from grinding the peeled-off layers) were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, electron dispersive spectroscopy, and diffuse reflectance spectroscopy. The upper surface demonstrates a strong photochromic effect. A pronounced increase of the amplitude of the absorption bands at 2.06 eV (AB3) and 1.56 eV (AB4) seen under irradiation in the UV or visible spectral region and a strong decrease of these bands during the heating of irradiated samples to 200−230 °C were characteristics of the upper layer’s surface. A wide set of spectra resulting from the reversible absorption changes made possible the disclosure of higher-energy absorption bands at 2.91 eV (AB1) and 2.54 eV (AB2); the latter were not affected by irradiation and heating. An electronic mechanism based on known properties of intrinsic point defects of TiO2, F-type centers (two electrons trapped in oxygen vacancies) and Ti3+ centers, is proposed to account for the optical changes that occurred through the photoinduced formation, photobleaching, and thermal bleaching of the absorption bands. KEYWORDS: rutile titania, thermochromism, photochromism, ceramic TiO2 layers, optical properties, extrinsic defects

1. INTRODUCTION Despite a significant number of both experimental and theoretical studies of various defect states in TiO2, which are responsible for the absorption of light in the visible spectral range and, therefore, determining the visible-light activity of TiO2-based photocatalysts, reliable data in the area of optical spectroscopy of such defects have tended to be rather limited.1−4 During the past few years, much attention has been given to the effect of metal doping on the optical properties of TiO2 in the visible spectral region. For instance, significant similarities were found between the shapes of the absorption bands in the wavelength range 400−700 nm in the spectra of TiO2 doped with 13 different metal ions.5 The authors deduced that the absorption spectra of modified TiO2 in the visible region likely originated from defects associated with oxygen vacancies that gave rise to colored centers.6−8 Nonetheless, details of the optical properties of doped metal oxides remain poorly understood. A major problem in the analysis of absorption spectra in the region of extrinsic absorption of metal oxides, in general, and titanium dioxide, in particular, is that the spectra of both single crystals and powders tend to be poorly resolved. To the best of our knowledge, in the case of TiO2 only the studies by Sekiya et al.9,10 and Emeline and co-workers11 have demonstrated well© 2012 American Chemical Society

resolved (i.e., nonoverlapping) absorption bands in the visible spectral region. Analyses of numerous absorption spectra of reduced TiO2 (both anatase and rutile crystals, and powders) available in the literature have shown that such spectra generally consist of six strongly overlapping absorption bands, with spectral maxima (hνmax) lying in the range from ca. 2.9 to ∼0.8 eV.12 The problems inherent in the fabrication of TiO2 materials more appropriate to optical studies (i.e., materials with spectra responsive to different treatments) are of current interest. In this regard, some years ago, a novel approach was proposed for fabricating rutile ceramic layers by direct high-temperature oxidation of titanium substrates possessing a complex form.13−15 Under calcinations in air at 850 °C, the thickness of the final rutile layer reached 3−4 mm. It was found that during the cooling period a white or cream-colored oxide layer readily separated from the titanium substrates.13 This was further demonstrated recently by Nakata and co-workers16−18 who also reported a facile fabrication method for the production of rutile layers by high-temperature calcinations of Received: October 1, 2012 Revised: December 12, 2012 Published: December 13, 2012 170

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TiO2 layers (see below) were selected using a set of glass cutoff and band-pass filters or otherwise a combination of such filters. The Alentsev−Fock method19 was applied for the numerical analysis of the absorption spectra and of the difference absorption spectra. The approach underlying this method is that the shape of the spectral bands, that is, the position of the spectral maxima (hνmax) and half-widths (δhν1/2)) constituting the spectra, is independent of the experimental conditions (pretreatment, excitation) that ultimately yield the spectra. Accordingly, the shape of each spectrum from the set of spectra obtained depends only on the relative intensities of the absorption bands (contributions of each band in the overall spectrum). On carrying out the analysis, no assumptions were made initially with regard to the number and the shape of the bands. This method simplifies the spectrum/spectra by eliminating one or more bands. To do this, one spectrum is multiplied by a coefficient that makes the relative intensity of the selected band equal to one in the other spectrum. As a result, the difference between these two spectra should no longer include the selected band; that is, the band’s relative intensity should now be equal to zero. When using the Alentsev−Fock method, the best results are obtained only if a single spectral band that does not overlap with others is responsible for the low-energy or high-energy tail (part) of the spectra. In such a case, the simple difference between two spectra when one spectrum is taken with suitable coefficient allows for the exclusion of one of the bands. When all of the bands overlap, the Alentsev−Fock method also provides useful information relative to the number and parameters of the spectral bands.

titanium plates. The white rutile layer so-formed on these plates peeled off spontaneously from the substrate during the cooling phase. Using the above-mentioned method, we fabricated rutile layers through calcinations of (technical grade) titanium plates and an ultrapure titanium substrate. In the case of the rutile layers so-obtained from the technical titanium substrates, we found pronounced differences in the coloration and in the diffuse reflectance spectra of different layers’ surfaces (bottom and upper), as well as a strong photochromic effect. Irradiation of the upper surface of the ceramic layer in the UV or visible spectral regions led to increase in absorption in the range of 2.5 > hν > ∼1.3 eV, while heating the irradiated samples to 200− 230 °C eliminated this photoinduced absorption. Furthermore, irradiation of the layer with photons at hν ≤ 2.27 eV also led to a decrease of the previously photoinduced absorption. A wide set of absorption spectra were obtained, which revealed photoand thermo-stimulated absorption changes in the absorption bands AB3 (2.06 eV) and AB4 (1.56 eV), whereas the bands AB1 at 2.91 eV and AB2 at 2.54 eV were not affected by such treatments.

2. EXPERIMENTAL SECTION TiO2 layers were fabricated by simple calcinations of a titanium substrate in a static open air atmosphere. The 170 × 30 × 2 mm titanium plates of technical grade were placed in a muffle furnace and calcined at T = 850 °C for 80 h or at T = 900 °C for 30 h. After being cooled to ambient temperature, the TiO2 layers broke off from the substrate surface spontaneously or by applying a slight deformation of the plate. The thickness of the so-detached layer was 150−300 μm when the calcination temperature was 850−900 °C. The density of the TiO2 layers was about 3.3−3.5 g cm−3. The chemical composition of technical Ti was analyzed by a Spark optical emission spectrometer MSA2 (“Spectral laboratory”, St. Petersburg, Russia); the material contained 0.5 at. % of aluminum as a major impurity plus other minor impurities at levels less than 0.08 at. %. Concomitantly, for comparison purposes, powdered “Degussa” P25 TiO2 was also placed in the muffle furnace together with ultrapure grade (99.98%) titanium substrates and were subsequently calcined under otherwise identical conditions. Characterization of the calcined substrates by X-ray diffraction (XRD) was performed with a diffractometer model Rigaku Miniflex II (CuKα). Raman spectra of the samples under examination were recorded at room temperature using a NXR FT-Raman module of the Nicolet 6700 FTIR spectrometer at 2 cm−1 resolution. The laser excitation line was 1064 nm. To minimize any heating effect of the laser beam, the output power was kept at the lowest possible value of 300 mW. Scanning electron microscopic and EDS analyses of the TiO2 ceramic layers were performed with a Zeiss Supra 40 VP system. UV−visible diffuse reflectance spectra (DRS) were measured using a Specord M40 spectrophotometer equipped with an integrating sphere assembly; BaSO4 was the reference standard. Difference DRS spectra {ΔR(hν)} were subsequently plotted to obtain the absorption spectra6,11 of the layers. If Rr(hν) and Rl(hν) are the experimentally recorded diffuse reflectance spectra of the rutile powder and of the disconnected layer, respectively, then the difference {Rr(hν) − Rl(hν)} corresponds to the absorption spectrum of the layer. Difference absorption spectra (i.e., ΔΔR(hν)) were also plotted to illustrate the absorption increases or decreases resulting from changes in the samples’ treatment (see the Appendix). Four 20 W fluorescent lamps were used as the light source to study the effect(s) of light illumination on the absorption spectra of the broken-off TiO2 layers. The emitted spectrum of the lamps consisted of a quasi-continuous fluorescent emission in the visible region along with the characteristic Hg lines at λ > 300 nm. The spectral regions of the active wavelengths in the photocoloration and photobleaching of

3. RESULTS AND DISCUSSION 3.1. Characterization. High-temperature calcinations of technical grade titanium plates resulted in the formation of a relatively thick layer (i.e., coating) of TiO2 that broke off spontaneously from the plate upon cooling to ambient temperature. The layers and the powder that formed upon grinding the layer were subsequently characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. Analysis of the XRD spectra (see Figure S1 of the Supporting Information) demonstrates that such high-temperature calcinations yielded only the rutile phase of TiO2. Figure 1 illustrates the Raman spectra of the upper surface of the as-synthesized TiO2 layer (spectrum 1), and for comparison the Raman spectrum 2 is that of the TiO2 powder obtained by calcination of Degussa P25 titania in the muffle furnace at the same time as the titanium substrates. The Raman

Figure 1. Raman spectra of the upper surface of the as-synthesized TiO2 layer (spectrum 1) and of the powder rutile TiO2 sample obtained by calcination of Degussa P25 titania in the muffle furnace at the same time as the titanium substrates (spectrum 2) . 171

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also confirmed by X-ray photoelectron spectroscopy (XPS; see Figure S4). 3.2. Optical Spectroscopy. The diffuse reflectance spectra of the bottom surface (attached to the metal plate during the preparation) and of the upper surface of the layer are displayed in Figure 3 (spectra 1 and 2, respectively). Note that the as-

spectrum of the bottom surface of the as-synthesized TiO2 layer, which was in contact with the plate, displayed the same features as the upper surface. Moreover, all Raman spectra recorded for several ceramic layers for both the upper and the bottom surfaces and at different positions along the surfaces were identical. Two strong broad bands are evident in the Raman spectrum of the ceramic layer’s upper surface (spectrum 1) at 447 and 608 cm−1, together with a weak narrow peak at 142 cm−1, a broad complex band with maximum at 235 cm−1, and less intense shoulders at 693 and 274 cm−1. The spectrum of the powdered sample (spectrum 2) displays the same or similar spectral features. Figure S2 illustrates the Raman spectrum characteristic of TiO2 with a high content of anatase. The dominant peak at 144 cm−1 in this spectrum as compared to the spectra of Figure 1 demonstrates that the oxide layers fabricated on the Ti substrate possess only the rutile structure. An exhaustive interpretation of the Raman spectra of rutile has been reported in several studies.20−22 Clearly, two complementary methods, the XRD and Raman spectroscopy, revealed a single rutile phase in both layers and in the powders obtained from the layers. Figure 2 illustrates the SEM images of the side view of the layer (Figure 2b), the surface of the upper layer (Figure 2c),

Figure 3. Diffuse reflectance spectra of the delaminated TiO2 layers. Curve 1 is the spectrum of the bottom (attached to metal) creamcolored surface, and curve 2 is the spectrum of the upper yellow surface. Spectrum 2a was obtained after illumination of the upper surface by the full light of the fluorescent lamps and spectrum 2b after additional illumination of the same surface by fluorescent lamps at wavelengths greater than 545 nm (hν ≤ 2.27 eV). Curve 3 is the spectrum of a powder sample of rutile prepared from Degussa P25 TiO2.

prepared layer should be kept in the dark because additional absorption of the yellow upper surface can easily be induced even under weak room illumination. Indeed, additional absorption (Figure 3, spectrum 2a) occurred during illumination for a 30 min period under the full, nonfiltered light of four fluorescent lamps. Spectrum 2b shows one more interesting photoinduced phenomenon, a decrease of the previously photoinduced absorption in the region 1.5−2.6 eV under irradiation at wavelengths above 545 nm. For comparison, curve 3 of Figure 3 depicts the diffuse reflectance spectrum of the powder rutile TiO2 sample obtained by calcination of Degussa P25 titania in the muffle furnace carried out at the same temperature and time period as for the titanium substrates. We considered this spectrum suitable to calculate the absorption spectra of the upper and bottom surfaces of the TiO2 layer (see the Experimental Section). Results of the calculated spectra are displayed in Figure 4 in normalized format obtained using the ΔRmax factor. Two main features of the normalized absorption spectra of the titania layers (Figure 4, curves 1−3) are worth noting: (i) the shape of the spectra with maxima at 2.85 eV is typical of the reduction of TiO2 in air (reduction in the absence of oxygen yields blue or dark blue coloration resulting from the dominance of the absorption in the red and near-IR region), and (ii) a minor dependence of the spectral shape on the value of the absorbance [the absorption band at 2.85 eV for the yellow upper surface (curve 2 in Figure 3) exceeds that of the cream-colored bottom surface (curve 1 in Figure 3) by a factor of 1.5]. The latter means that the same defects (color centers)

Figure 2. (a) Photograph showing the yellow upper surface and the bottom cream-colored surface of the TiO2 layer that broke off from the titanium plate. Scanning electron microscopy (SEM) images of the asprepared titanium dioxide by calcinations of technical grade titanium plates at high temperatures: (b) side view of the layer formed on the plate; (c) view of the upper surface of the layer; and (d) view of the bottom surface of the layer.

and the surface of the bottom layer (Figure 2d). SEM images of the crystals forming the ceramic layers revealed well-defined rutile crystals with sizes from ∼100 nm to ≥4 μm. An energy dispersive X-ray spectroscopic (EDS) analysis (Figure S3) indicates the presence of 0.5−1.0 atom % of aluminum as the principal impurity in the thermochemically fabricated rutile layers. This result accords with the data obtained by the optical emission spectroscopy analysis of technical Ti. The latter was 172

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Figure 5. Spectra resulting from additional absorption of the upper yellow surface induced by illumination at selected spectral ranges: 340 ≤ λ ≤ 390 nm (i.e., 3.65 ≥ hν ≥ 3.18 eV) for 35 min (spectrum 1a), 105 min (spectrum 1b), and 205 min (spectrum 1c); in the range 390 ≤ λ ≤ 490 nm (i.e., 3.18 ≥ hν ≥ 2.53 eV) for 30 min (spectrum 2a) and 135 min (spectrum 2b); and in the range 440 ≤ λ ≤ 690 nm (i.e., 2.81 ≥ hν ≥ 1.80 eV) for 135 min (spectrum 3). Spectrum 4 (taken with opposite sign; magnified 2-fold) demonstrates a decrease in absorption (bleaching) after additional illumination at wavelengths greater than 545 nm (i.e., hν ≤ 2.27 eV) of the layer previously illuminated with the full light of the fluorescent lamps. Note that spectrum 4 is the difference between spectra 2b and 2a of Figure 3.

Figure 4. Absorption spectra (difference diffuse reflectance spectra) normalized by ΔRmax of the bottom (curve 1) and the upper (curve 2) surfaces of the delaminated layers and the powder sample obtained from the delaminated layers (curve 3). Absorption spectra 2a and 3a were obtained after irradiating the upper surface and the powder sample by the full light of the fluorescent lamps, while spectra 4a and 4b were obtained after heating the irradiated upper surface of the layers in air at T = 230 °C for 15 and 30 min, respectively. Curve 5 represents the uncertainty in spectrum 2 (note the 20× amplification factor).

are responsible for the absorption spectra of both surfaces, and furthermore the relationship of the defect concentration is not altered significantly between the two surfaces of the layer. The invariance of the absorption spectral shape of the TiO2 layers formed thermochemically (through oxidation of the Ti plate; Figure 4, curves 1−3) is in strong contrast with the photoinduced spectral change (Figure 4, curves 2a and 3a). To the best of our knowledge, the first spectral study on photoinduced absorption increase/decrease phenomena was reported in 2007 by Emeline and co-workers.11 The data reported in Figure 4 also infer that (1) the shape of the spectra, that is, the relationship of the defect concentrations, does not depend on the surface morphology (see Figure 2c and 2d); and (2) the major factors affecting the shape of the spectra in the low-energy (longer wavelength) spectral region are photoexcitation that yields an additional number of defects (spectra 2a and 3a) and heating at relatively moderate temperatures (T < 230 °C; spectra 4a and 4b), which leads to the thermal destruction (thermo-bleaching) of the same types of defects. Figure 5 displays the increase in absorption spectra (curves 1−3) of the upper (yellow) surface of TiO2 layers induced after irradiation in air in various spectral ranges of the fluorescent lamp emission during different time periods. The photoinduced absorption becomes apparent as a prominent absorption band with maxima around 1.9−2.0 eV and a shoulder at ∼1.6 eV. The spectra photoinduced by UV (Figure 5, curves 1a−1c) and visible irradiation (Figure 5, curves 2a, 2b, 3), normalized by the ΔRmax factor, demonstrate a great similarity for each spectral region independent of the time of irradiation (see Figure S5). At the same time, Figure S5 also shows a noticeable difference in contribution of the main band and the weaker shoulder into the spectra obtained under UV and visible irradiation. This allowed us to deconvolute the spectra shown in Figure 5 into two individual absorption bands.

Results obtained for the spectrum induced by UV irradiation of the TiO2 layer are shown in Figure 6, curve 1. In the first instance, we averaged the normalized absorption spectra photoinduced under irradiation in the UV and visible light spectral region (see Figure S5) and calculated the difference between these (averaged) spectra {Δ(ΔRnorm (hν))}. The result is shown in the inset of Figure 6 (curve 4). This spectrum was successfully approximated by a Gaussian-type curve (curve

Figure 6. Normalized averaged absorption spectrum photoinduced in the rutile TiO2 layers by UV irradiation (curve 1) and approximation of spectrum 1 by the sum of two Gaussian-type absorption bands AB3 and AB4 (curve 2). Shown for comparison, curve 3 is the absorption band photoinduced in N-doped powder TiO2 (ref 11). The inset shows the difference between the normalized averaged spectra (spectrum 4) reported in Figure S5 and its approximation by a Gaussian-type band (spectrum 5) with maximum at 1.56 eV. 173

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5; inset of Figure 6) with hνmax = 1.56 eV and δhν1/2 = 0.57 eV; that is, spectrum 4 consists of a single absorption band denoted AB4. Second, the photoinduced absorption spectrum 1 in Figure 6 could be approximated by two Gaussian bands using random fitting (see Figure 6). Quite surprisingly, the Gaussian band with hνmax = 2.06 eV and δhν1/2 = 0.6 eV (denoted AB3) used for approximating the photoinduced spectrum is in good accord with the absorption band photoinduced in powdered Ndoped TiO2 specimens by irradiation in vacuum at λ = 436 nm reported by Emeline and co-workers10 and assigned to intrinsic defects (see below). We deduce that the high degree of agreement between the data reported by these authors and the absorption band AB3, obtained in the present study, provides unambiguous evidence that the defects responsible for the observed absorption bands are the intrinsic defects present in titanium dioxide. The goodness of the experimental and computational analyses revealing the photoinduced absorption bands AB3 and AB4 allowed us to answer the question whether these bands are also included in the absorption spectra of the thermochemically fabricated rutile TiO2 layers delaminated from the plate (Figure 4, curves 1−3). Clearly, the Alensev− Fock method provided a methodology to calculate the differences between the layer’s spectra and the spectra of the photoinduced absorption (Figure 5) multiplied by a coefficient randomly selected to minimize the amplitude of the absorption at energies hν ≈ 2.0 eV. Different coefficients were used to analyze spectra 1−3 in Figure 4. Curves 1−3 in Figure 7 clearly demonstrate the appropriateness of the Alensev−Fock method used for the separation of the high-energy portion (short-wavelength) of the layer’s absorption spectra. Indeed, the absorption near 2.0 eV in the initial normalized spectra is about 0.5 (Figure 4, curves 1 and 2), while the absorption in curves 1−3 in Figure 7 is nearly zero

at this energy. Spectra 1−3 in Figure 7 are well-known. For example, similar absorption spectra were obtained after reduction of P25 TiO2 by heating in air at temperature 110− 180 °C in the presence of various organic polymers.6 UV irradiation of the TiO2/polymers composites with the light emitted from the fluorescent lamps also gave the same absorption spectra.7 Figure 7 also illustrates the approximation of spectra 1−3 by the sum of two Gaussian-type absorption bands AB1 with spectral maximum at 2.91 eV and band AB2 at 2.54 eV. In the study reported by Kuznetsov and Serpone, these absorption bands were attributed to the presence of F centers in TiO2 (equivalent to bulk oxygen vacancies that trapped two electrons).12 As is evident from Figure 5, the amplitudes of the photoinduced absorption spectra reach a stationary value after irradiation for a time period that exceeded ∼100 min. This allowed us to estimate quantum efficiencies (Q) for the photoinduced formation of bands AB3 and AB4 at different spectral regions of the light emitted by the fluorescent lamps. The irradiance in each region (defined in caption of Figure 5) was measured using a thermocouple sensor. In our experiments, the irradiance ranged from 50 μW cm−2 (UV region) to 270 μW cm−2 (“green” region). To estimate Q, the area of the stationary absorption spectrum induced in a selected spectral region was normalized by the value of the irradiance appropriate to the region. The dependence of the relative quantum efficiency (in arbitrary units) on the spectral region of irradiation is shown in the inset of Figure 7; Q for the UV region was taken equal to 1. The result obtained indicates that the TiO2 rutile layers are visible-light-active materials. The photoactivity became apparent in the appearance of defects (color centers) responsible for bands AB3 and AB4. The inset of Figure 7 clearly shows that Q in the range 3.18 ≥ hν ≥ 2.53 eV (“blue” region), that is, in the region of light absorption into band AB1 and partly into AB2, exceeds that for irradiation in the UV region, whereas for irradiation in the range 2.80 ≥ hν ≥ 1.80 eV (“green” region), that is, irradiation into band AB2 that partially overlaps with band AB1, the quantum efficiency in the formation of bands AB3 and AB4 has a relative value of 0.25. Taken in general, the spectral dependence of Q is in agreement with the earlier reported spectral dependencies of the quantum yields for the photostimulated adsorption of the electron-acceptor molecule oxygen (i.e., photoreduction of O2) and electron-donor hydrogen and methane molecules (i.e., photooxidation of H2 and CH4) on the surface of different TiO2 powder specimens.23 In the region hν < 3.3 eV, these dependencies display a spectral maximum at ∼3.0 eV and a tail up to 2.6−2.8 eV. In some cases, a weak shoulder at 2.5−2.6 eV is also well-resolved. A pronounced thermal effect of the colored rutile layers was also evidenced in the optical spectra when the samples were heat-treated in an air atmosphere at moderate temperatures (100 < T < 230 °C). Figure 8 shows a decrease of the photoinduced absorption in the range hν < 2.5 eV after heating at 200 °C (compare curves 2 and 3). The inset of Figure 8 shows that only the absorption bands AB3 and AB4 are responsible for the photo- and thermo-stimulated changes; the bands AB1 and AB2 are not affected. Note that under heating band AB3 decreased by a factor of 3 and the absorption in this region became less than that of the as-prepared layer (compare curves 1 and 3). The effect of annealing on the initial

Figure 7. High-energy (short-wavelength) portion of the normalized absorption spectra of the cream-colored bottom surface (curve 1), of the yellow-colored upper surface (curve 2), and of the irradiated yellow upper surface (curve 3); curve 4 is the convolution (i.e., the sum) of two Gaussian-type absorption bands AB1 (hνmax = 2.90, δhν1/2 = 0.35 eV) and AB2 (hνmax = 2.54, δhν1/2 = 0.54 eV). The inset shows the relative quantum efficiencies (Q) of the photostimulated growth of bands AB3 and AB4 that resulted from irradiation of the three spectral regions selected using appropriate glass filters (note that the efficiency in the UV region was set equal to 1). 174

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of both conduction band electrons and valence band holes upon irradiation into the absorption bands AB1 and AB2, as well as the recovery of the light absorbing centers during irradiation. The first photophysical event is the optically activated electronic transition from the ground state F to the excited state F* (reaction 1). The excited F* center transforms spontaneously into the F+ center and a conduction band electron (reaction 2a) if the F* level lies within the conduction band. If the level of the excited F* center is located lower than the bottom of the conduction band, a thermally activated (with ΔE) electron transition from the F* state to the conduction band should occur (reaction 2b). In either case, hole formation supposes capture of an electron by the F+ center from the valence band (reaction 3). Reaction 4 describes the synchronous formation of an electron−hole pair. If the F centers are positioned within the bulk of the TiO2 particles wherein no interaction of F+ centers with molecules is possible, reactions 1−4 would ensure the photogeneration of both conduction band electrons and valence band holes under visible light irradiation, as well as the stability of these processes during the photoexcitation events.

Figure 8. Absorption spectra of TiO2 layers nondetached from the Ti plate: curve 1 is the spectrum of the as-prepared yellow upper surface; spectrum 2 was obtained after irradiating the upper layer surface by the full light emitted by the fluorescent lamps for 40 min; and curve 3 is the spectrum obtained after heating the irradiated sample at T = 200 °C in air for 20 min. Curve 4 in the inset represents the difference between spectra 2 and 1 (i.e., 2 minus 1), while spectrum 5 is the difference between spectra 2 and 3 (i.e., 2 minus 3).

absorption spectra of colored layers is also demonstrated in spectra 4a and 4b of Figure 4 when compared to curves 1−3. 3.3. Electronic Features of the Photochromic Effect. The rutile layers and the powder specimen investigated herein displayed poorly resolved absorption spectra in the visible region, rather typical of both reduced and anion-/cation-doped TiO2 specimens (see, for instance, Figure 4). However, the rutile layers fabricated on the titanium substrates demonstrated a wide set of different absorption spectra, which allowed us to show experimentally and through numerical analyses four absorption bands AB1−AB4. The attribution of bands AB1 and AB2 to F centers in TiO2 results from consistent analyses of the origin of absorption bands in the visible region of reduced and anion-/cation-doped (visible-light-active) TiO 2 materials.6,7,23,24 Absorption spectra of reduced single crystals and powders of TiO2 available in the literature were analyzed by Kuznetsov and Serpone in an earlier study.12 Consequent physical processes occurring on optical excitation of F centers were also examined. The proposed Scheme 1 and reactions 1−4 involving photoexcitation of F centers in TiO2 infer the photogeneration

F + hνAB → F*

(1)

F* → F+ + eCB

(2a)

F* + ΔE → F+ + eCB

(2b)

F+ + (O2 −)VB → F + (O−)VB

(3)

F* + (O2 −)VB → F + (O−)VB + eCB

(4) 12

Reactions 1−4 were proposed in an earlier study to explain the principal regularities of photoadsorption/photodesorption and photocatalytic reactions occurring on the TiO2 surface. These reactions seem quite appropriate to describe the first stages of the photochromic phenomenon in the rutile layers. Indeed, irradiation in the UV and visible regions does not alter the amplitudes of absorption of the bands at 2.91 eV (AB1) and 2.54 eV (AB2), see Figures 4, 5, and 8, but does increase the amplitudes of the absorption bands AB3 (2.06 eV) and AB4 (1.56 eV); see Figures 5, 6, and S5. In accord with the interpretation proposed by Emeline and co-workers,11 band AB3 (the principal band of the two photoinduced absorption bands in the rutile layers) is attributable to Ti3+ defects. Thus, an increase in the absorption band AB3 of the Ti3+ centers results from the capture of electrons (eCB trapping) photogenerated through excitation of the F centers by specific Ti4+ ions (possibly Ti4+ interstitials); see reaction 5. Because the behavior of the amplitude of band AB4 on irradiation and heating is analogous to that of band AB3, this band is likewise attributed to Ti3+-type defects. In the absorption spectra of reduced and neutron-irradiated, and then partly oxidized rutile crystals, a poorly resolved shoulder at 1.5−1.7 eV was observed.12

Scheme 1. Illustration of the Relative Positions of the Energy Levels of the Color Centers F and Ti3+ within the Band Gap of TiO2 Rutilea

Ti4 + + eCB → Ti 3 + (AB3, AB4 bands)

(eCB trapping) (5)

Ti 3 + + hνAB3/AB4 → Ti4 + + eCB heat

a

Ti 3 + ⎯⎯⎯→ Ti4 + + eCB

See text for explanation. 175

(photoionization)

(thermal ionization)

(6) (7)

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(O−)tr ⎯⎯⎯→ (O−)VB

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(h+ detrapping)

Ti 3 + + (O−)VB → Ti4 +

(e−tr /h+ recombination)

numerical fitting. Both upper and lower photoresponsive rutile layers, together with the powdered specimen, have proven to be promising materials for investigating different electronic properties of the intrinsic point defects of TiO2, as well as the mechanistic details of processes in which such defects are significant participants. Most important, the analyses of the spectra of the rutile TiO2 ceramic layers have furthered our understanding of the origin(s) of the extended visible absorption spectra of modified metal oxides.

(8a) (8b)

In accord with the electronic description of the photochromic effect, photobleaching of bands AB3 and AB4 under irradiation at hν ≤ 2.27 eV (Figure 5, curve 4; i.e., irradiation into the absorption bands) could be explained by the photoionization of the Ti3+ centers (reaction 6). Two electronic processes are considered to explain the thermal bleaching of bands AB3 and AB4. The first proposes the thermal ionization of Ti3+ centers (reaction 7), while the second involves the thermal detrapping of photogenerated holes (O−)tr (reaction 8a) and the subsequent recombination of Ti3+ centers (trapped electrons) with free holes in the valence band (O−)VB (reaction 8b). At present, we are unable to ascertain the validity of these reactions. Nevertherless, the photo- and thermo-stimulated changes of the optical properties of the rutile layers formed on titanium substrates find a consistent explanation from reactions 1−8. As noted earlier (see the Introduction), the formation of intrinsic defects in cation-doped metal oxides, including TiO2, remains poorly understood. We propose that rutile titania became uncontrollably doped during the thermochemical growth of the layers. The presence of 0.5 at. % aluminum as the major impurity in both the technical Ti substrate and the rutile layers confirms this assertion. At the same time, we found no manifestation of these impurities in the absorption spectra or in the photochromic phenomenon. The main finding of our study is that only the intrinsic defects of titania (F and Ti3+ centers) are responsible for the photo- and thermo-stimulated effects. The goodness of the description of various optical properties of the rutile layers in terms of intrinsic point defects (F and Ti3+ centers) characteristic of TiO2 in the reduced state would implicitly suggest a simple explanation for the origin of these centers in our samples. Yet that is not the case here, because a pronounced response of the optical spectra on light irradiation or heating was absent for the white rutile layers fabricated on the surfaces of ultrapure titanium substrates and the powdered P25 TiO2 specimen. Consequently, only impurities of different metals characteristic of major types of technical titanium substrates (in this case, aluminum) must be responsible for the formation of oxygen vacancies (F-type centers) during the growth of the rutile layers. Thus, we propose that the origin of the oxygen vacancies in the ceramic rutile layers is analogous to that typically found in cation-doped titania. Specifically, metalion dopants that possess valence states different from Ti4+ may induce the generation of oxygen vacancies during the synthesis.5 Specially focused studies are needed to clarify the details of this proposed mechanism.

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APPENDIX The energy conservation law requires the following conservation balance: (A1)

A+R+T=1

where A is the fraction of light absorbed by the system (in our case by our sample), R is the fraction of light reflected (measured by diffuse reflectance spectroscopy, DRS), and T is the fraction of light transmitted. Because our ceramic layer sample is not transparent, T = 0, so that (A2)

A=1−R

Consequently, the alteration in the absorption properties of the sample caused by any treatment is then given by: ΔA = A 2 − A1 = (1 − R 2) − (1 − R1) = R1 − R 2 = ΔR (A3)

where ΔR is the alteration in the reflectance spectra caused by the treatment. Thus, calculating the difference between the reflectance spectra before and after the treatment, one obtains the changes in the sample absorption caused by the treatment. A major “treatment” in the present study is considered to be the formation of the rutile layer that displays extrinsic absorption bands as compared to the white pure rutile sample (formed from a sample of the Evonik P25 TiO2 under otherwise identical heating conditions). Consequently, the white pure rutile sample was taken as a reference sample and its absorption spectrum as the reference spectrum, AR. Accordingly, the additional absorption in our sample caused by our treatment as compared to the reference sample is given by: ΔA tr = A str − AR = RR − R str = ΔR tr

(A4)

where Astr is the absorption of our ceramic treated sample. Therefore, ΔAtr is the difference in the absorption spectra between our sample after the treatment and the reference rutile sample. We also denote Δ(ΔA) = ΔA tr1 − ΔA tr2

(A5)

as the difference between two difference absorption spectra of our sample in two different states (for example, before and after UV irradiation). It is then easy to show mathematically that eq A6 corresponds to the difference of absorption spectra of our tested sample before and after the treatment (irradiation, heating) because both difference absorption spectra of the sample are obtained with respect to the same reference rutile sample with absorption AR. Thus, ΔA means that the difference spectrum was obtained relative to the reference sample, and Δ(ΔA) means that the difference spectrum was obtained as a difference between two spectra of the tested sample corresponding to two different states of the sample.

4. CONCLUDING REMARKS Opaque rutile layers photoresponsive to irradiation in both UV and visible region were fabricated by simple high-temperature calcinations of titanium substrates in a static air atmosphere. Reversible changes of the amplitude of the absorption bands at 2.06 eV (AB3) and at 1.56 eV (AB4) occurred under light irradiation and under heating, as a consequence of which a wide set of absorption spectra made possible the disclosure of the short-wavelength (high-energy) portions of the spectra. Two absorption bands (AB1) with spectral maxima at 2.91 and 2.54 eV (band AB2) constituting these portions were uncovered by

Δ(ΔA) = A tr1 − A tr2 176

(A6)

dx.doi.org/10.1021/cm3031736 | Chem. Mater. 2013, 25, 170−177

Chemistry of Materials

Article

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It should be emphasized that this procedure is applicable only for extrinsic absorption, that is, for the bands in the spectral region hν < Eg where different point defects (color centers) display absorption bands.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction pattern of the powder obtained from the ceramic layer, the Raman spectrum of a nontreated Degussa P25 titania sample, energy dispersive X-ray spectrum (EDS) of the ceramic layer, the XPS spectrum illustrating the Al(2p) impurity in fabricated TiO2 rutile layers, and the normalized absorption spectra photoinduced in the upper yellow surface of the rutile layer by UV and visible light irradiation (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant from the Russian Foundation for Basic Research (no. 10-03-00638-a). N.S. thanks Prof. Albini of the University of Pavia (Italy) for his continued kind hospitality in his laboratory. We also thank Dr. Vladislav Gurzhiy and Prof. Olga Frank-Kamenetskaya of SaintPetersburg State University for a fruitful collaboration. We are grateful to the Technological Resources Center “Nanofabrication of Photoactive Materials − Nanophotonics”, the Technological Resources Center “Geomodel”, the Interdisciplinary Resource Center for Nanotechnology, and the X-ray Diffraction Center of Saint-Petersburg State University for helpful assistance. Finally, we also wish to express our thanks to Viacheslav Lovcjus and Michael Alexandrov from the “Spectral Laboratory” for the emission spectral analysis performed on the Ti substrate.

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REFERENCES

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