Influence of the Dopant Concentration on the Photocatalytic Activity: Al

Sep 28, 2015 - A set of Al-doped titania x-Al-TiO2 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 wt % Al) has been synthesized by a sol–gel method and test...
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Influence of the Dopant Concentration on the Photocatalytic Activity: Al-Doped TiO2 Anna A. Murashkina,*,† Petr D. Murzin,† Aida V. Rudakova,† Vladimir K. Ryabchuk,† Alexei V. Emeline,† and Detlef W. Bahnemann*,†,‡ †

Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Ulyanovskaya str. 1, Peterhof, Saint-Petersburg 198504, Russia ‡ Institut fuer Technische Chemie, Gottfried Wilhelm Leibniz Universitaet Hannover, Callinstrasse 3, D-30167 Hannover, Germany S Supporting Information *

ABSTRACT: A set of Al-doped titania x-Al-TiO2 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 wt % Al) has been synthesized by a sol−gel method and tested in the reaction of the photocatalytic degradation of phenol in aqueous suspension. XRD and Raman studies show that the TiO2 samples have a mixedphase rutile-anatase crystalline structure with linear increase of the anatase fraction from 0.0 wt % for 0.0-Al-TiO2 up to 18 wt % for 1.1-Al-TiO2. A decrease of the particle size from ∼800 to 50 nm and an increase of the specific surface area from 1.7 m2/ g up to 28 m2/g with increased content of Al have been observed. The data of XRD, XPS, Raman spectroscopy, and EDS techniques show rather homogeneous aluminum distribution with only the Al(3+) oxidation state of aluminum incorporated in the TiO2 lattice. The bandgap energy Eg = (2.93 ± 0.1) eV corresponding to indirect allowed transitions does not depend on the aluminum content within the 0.0−1.1 wt % Al concentration range. The photocatalytic testing of Al-doped TiO2 samples in the reaction of phenol degradation shows the existence of a maximal initial rate of phenol degradation at an Al concentration of about 0.5 wt %.

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INTRODUCTION Titanium dioxide is a well-known material that is widely used in various applications.1−3 These applications are based on specific optical and photophysical properties of titania. Due to its high transparency to visible light and its high average refractive index (na = 2.5 and nr = 2.7 for anatase and rutile, respectively), TiO2 powder was known as a white paint pigment and a reflecting coating for a long time.4 The pioneer publication5 by Fujishima and Honda opened a new promising area of application of this material as a perspective photocatalyst.1,6−9 TiO2 is also known as a photochromic material.10 In general, the red limit of fundamental absorption at the blue edge of the visible light spectrum, the thermodynamic stability and nontoxicity of titanium dioxide, as well as the low cost fabrication of dispersed titania make its application favorable both as a white pigment and as a photocatalyst. At the same time, the photocatalytic (photochemical) activity of titania is a sort of antagonistic phenomenon in relation to the TiO2 application as a pigment since it promotes the destruction of the binder that, finally, leads to chalking of paints and coatings.4 Consequently, two opposite strategies are used to improve the quality of titania powders: to decrease the photochemical activity of titania used in pigments and cosmetics and to achieve a higher photoactivity for titaniabased photocatalysts, respectively. In particular, this concerns © 2015 American Chemical Society

cumulative impurity/dopant levels within the TiO2-based pigments and photocatalysts. In general, the lower the content of impurities in TiO2 pigment is the better are its optical properties, namely, the reflectivity of titania powders in the visible region. Thus, the commercial TiO2 pigment technologies include the removal of the major part of impurities from the final products down to tenths of the weight percentage.4 In contrast, doping or codoping of titania by metallic and nonmetallic impurities with concentrations up to a few percent is supposed to be a prospective way to create photoactive materials that are called visible light active (VLA) photocatalysts.1,9,11 Nonmetal-doped titania has been extensively studied as a VLA photocatalyst in the last 15 years.1,11 Titanias doped by metals of the platinum group, transition metals, and some other metals also demonstrate enhanced photocatalytic activity (see, for example, Choi’s work12). At the same time, the photocatalytic activity enhancement of titania by aluminum addition has been poorly studied, and the results obtained by different authors are inconsistent. For instance, Liu et al.13 have found that Al-doped mesoporous titania (with the aluminum concentration being about 1 wt %) is noticeably more active in Received: June 30, 2015 Revised: September 7, 2015 Published: September 28, 2015 24695

DOI: 10.1021/acs.jpcc.5b06252 J. Phys. Chem. C 2015, 119, 24695−24703

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The Journal of Physical Chemistry C the test reaction of the Congo Red dye degradation than undoped titania synthesized by the same solid state method, and Al-TiO2 occurred to be even slightly more active than the reference TiO2 Degussa P25. Navas et al. have synthesized highly aluminum-doped TiO2 nanoparticles (AlxTi1−xO2−x/2, where x = 0.083, 0.154, 0.2, respectively) using a sol−gel method and studied their photocatalytic activity in the photodegradation of methylene blue. It was shown that the Al-doped samples synthesized in this study reveal higher photoactivity than the undoped ones.14,15 Similarly, the positive effect of aluminum doping on the efficiency of photocatalytic mercury removal by plasma chemically synthesized titania has been reported by Tsai and coauthors.16 At the same time, no effect of aluminum doping on the photocatalytic degradation of steric acid by thin solid titania films (in contrast to Cu-doped titania films) has been found.17 A comparative study of the efficiencies of Al-, Sn-, and Pt-doped and codoped titanias synthesized by flame spray pyrolysis (FSP) for the methyl orange dye degradation has shown that the efficiency of Aldoped TiO2 is two-fold lower than that for undoped TiO2.18 However, the photocatalytic activity of codoped Al/Pt-TiO2 samples was higher than the activity of the reference Degussa P25 sample.18 According to Gesenhues’s results,19 aluminum incorporated in TiO2 white pigments unambiguously suppresses its photocatalytic activity regardless of the Al2O3 dopant concentration in the 0−8 mol % range. At low concentration, aluminum dissolves in the bulk of the pigment particles while at gradual increase of the dopant concentration it remains at the particle surface, and the photoactivity of such pigments approaches some reduced saturation value. A possible reason for this inconsistency of results reported by different authors for the aluminum doping effect on the photocatalytic activity of titania most likely is the difference in synthesis, doping methods, and dopant concentrations. Recently, a theoretical model that describes the correlation between photocatalytic activity, doping ratio, and particle size was developed for different metal-doped TiO2.20 It was shown that the optimal metal dopant concentration for a higher active photocatalyst covers an average range from 0.01 at % to 1 at % regardless of the type of metal dopant and depending on the given particle size. The particle size strongly depends on the synthesis procedure. Besides, the formation of the two-phase (rutile−anatase) structure can occur for Al-doped TiO2 that also can affect the photocatalytic activity of TiO2.21 Interestingly, in accordance with the manufacturing technologies, the most commercial TiO2 powders contain aluminum as a surfactant with concentrations up to 1 wt %.4 In particular, the well-known photocatalyst Evonik Aeroxide TiO2 P25 contains22 up to 0.3 wt % of Al2O3 corresponding to the optimal metal dopant concentration range as suggested elsewhere.20 Herein we report the results of studies of the aluminum doping effect within the dopant content range 0.0−1.1 wt % on the physical-chemical characteristics and on the photocatalytic performance of titanium dioxide photocatalysts.

TTIP solution in isopropanol in accordance with the desired stoichiometry of the doped samples. Then citric acid dihydrate C6H8O7·2H2O (CADH, powder of 98% purity, Vekton) was added as a complexing agent with appropriate molar stoichiometry of (Al3++Ti4+)/CADH:1/3. The obtained mixtures were heated up to 200 °C for 3 h to remove the majority of organic decomposition products and then annealed at 650 °C for 5 h in the air. 2. Material Characterization. X-ray diffraction and Raman spectroscopy methods were used to analyze phase formation. A Rigaku Miniflex II diffractometer with Cu Kα (anode current 15 mA, accelerating voltage 30 kV) radiation was used in the angle range of 20° ≤ 2θ ≤ 80° with a scanning speed of 5.0°/ min. Structural data for anatase and rutile phases were taken from the ICSD database. The phase analysis by the Rietveld method was carried out using the TOPAS (Bruker AXS) software. Raman spectra were recorded in the 45−2735 cm−1 spectral region at ambient temperature using a SENTERRA Raman spectrometer (Bruker) with a resolution of 1 cm−1 (the excitation laser wavelength was 532 nm, the laser beam power was 20 mW). The microstructural formation and the distribution of elements (Ti, O, and Al) for all samples were investigated by using the Zeiss Merlin scanning electron microscope with an Oxford Instruments INCAx-act system for energy dispersive X-ray spectroscopic microanalysis. The XPS spectra were recorded using a Thermo Fisher Scientific Escalab 250Xi spectrometer. The specific surface area of all x-Al-TiO2 samples pretreated at 100 °C was measured by the BET method using the Quadrasorb SI surface area analyzer. The diffuse reflectance spectra R(λ) and absorbance spectra [A(λ) = 1 − R(λ)] were recorded in the 380−800 nm spectral range at ambient conditions using a Cary 5000 spectrophotometer equipped with a DRA 2500 external diffuse reflectance accessory. The optical-grade Spectralon was used as a reference standard. 3. Testing of the Photocatalytic Activity. The photocatalytic activity of the synthesized materials was evaluated by the decomposition of phenol under UV(A) light irradiation. Before irradiation, the aqueous suspension of the x-Al-TiO2 photocatalyst (1 g/L, pH = 3.0) and phenol (100 ppm; purity of 99.5%) was pretreated with an ultrasonic processor for 10 min and then magnetically stirred for 1 h to ensure the adsorption−desorption equilibrium. The photocatalytic degradation was carried out at ambient conditions. The light source for irradiation was a 150 W Xe-lamp (OSRAM). An optical filter was installed to cut off wavelengths below 300 nm. The light intensity was 50 mW/cm2. At certain time intervals, probes were sampled and filtered using a 0.2 μm Minisart filter to remove solid particles before analysis. The concentration of phenol was determined by HPLC analysis using the 1260 Infinity (Agilent Technologies) chromatograph with a UV−vis detector and a C18 column (Agilent). The mobile phase was a mixture of methanol and water at a volume ratio of 50:50. The detection wavelength was set at 210 nm with a bandwidth of 4 nm.

EXPERIMENTAL SECTION 1. Material Synthesis. Polycrystalline x-Al-TiO2 (x, Al wt % is 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1) were prepared by a sol− gel method at ambient conditions using titanium isopropoxide (TTIP, liquid of 97% purity, Sigma-Aldrich) and aluminum isopropoxide (ATIP, powder of 99.98% purity, Acros Organics) as precursors. Initially, the ATIP powder was dissolved in the

RESULTS AND DISCUSSION 1. Material Characterization. The XRD patterns of all xAl-TiO2 samples are given in Figure 1. According to the phase analysis, rutile is the only phase for undoped titania. The rutile TiO2 phase is also the dominant phase for all Al-doped samples. No formation of any aluminum-containing phase23,24 was observed for any of the synthesized samples. At the same time,

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Figure 1. X-ray diffraction patterns of x-Al-TiO2 powders with different dopant concentration (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 wt % Al). Peaks marked by R belong to the rutile phase, peaks marked by A correspond to the anatase phase. Inset: the shift of rutile phase peak of x-Al-TiO2 powders.

Figure 2. Raman spectra of pure and Al-doped TiO2 samples. The Al weight content (x) is increased with the arrow (0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 wt %). Peaks marked by R belong to the rutile phase and peaks marked by A correspond to the anatase phase.

cm−1 (B1g), and 515 cm−1 (A1g). For all x-Al-TiO2 samples, there is no spectral evidence of the presence of Al-involving species vibrations27,28 or of carbon species (characteristic bands near 1342 and 1601 cm−1 assigned to sp3- and sp2-hybridized carbon; this region is not shown in Figure 2). We evaluated the anatase/rutile phase weight ratio (fA/f R) from both XRD and Raman data. The relation between the ratio of intensities (IA/IR) of the strongest reflection peaks for both phases (27.40° for rutile and 25.22° for anatase, see Figure A in the Supporting Information) was taken from X-ray diffraction patterns. This relation is given by the expression:29 1 fA = I 1 + 1.26 IA

with increasing the Al dopant concentration, the XRD peaks corresponding to the anatase TiO2 phase have been broadening and increasing in their intensities for the doped TiO2 (the interpretation of XRD peaks is given in the Supporting Information). Apparently, Al-doping inhibits the formation of the rutile phase during heat treatment at 650 °C. This observation is in agreement with previous studies on the influence of aluminum on the anatase−rutile phase transformation even at 750−800 °C.25,26 The effect of Alsubstitution in the Ti-position can be confirmed by a small displacement of peaks in the region of large angles in the XRD data. Thus, in the process of substitution, the unit cell is reduced (the ionic radius of aluminum is smaller than the ionic radius of titanium), which consequently leads to a shift of the peak in the large angles region on the X-ray diffraction patterns (inset in Figure 1). The XRD data on the phase composition of the studied samples is confirmed by Raman spectroscopy (Figure 2). An important distinction between Raman and XRD is that the XRD signal is the result of long-range structural ordering among crystalline lattice planes, whereas the Raman spectrum results from local molecular bond vibrations, making Raman highly sensitive even for detecting nanocrystallite formations in materials with a high Raman cross section. Raman spectra show that in the case of the pristine TiO2 sample, the only phase determined is rutile: peaks at 611 cm−1 (A1g), 447 cm−1 (Eg), and 143 cm−1 (B1g). The band at 143 cm−1 associated with the B1g vibrational mode of rutile (Ti−Ti covalent interactions) is changed dramatically in its shape and intensity with the dopant concentration increase even in the spectrum for the sample with 0.1 wt % Al. Such a behavior of the band at 143 cm−1 is confirmed by the appearance and the increase in the intensity of the band at 145 cm−1, which belongs to the Eg1 vibrational mode of anatase (Ti−Ti vibrations in octahedral chains). This finding indicates that the content of the anatase phase is increased starting with the Al dopant concentration of 0.1 wt %. The conspicuous increase of the anatase phase weight percentage observed for Al-doped samples with the dopant concentration above 0.7 wt % is characterized by the apparent growth of the bands at 197 and 640 cm−1 (Eg), 398 and 515

R

where fA is the mass fraction of the anatase phase and IR and IA are the intensities of the reflection of the (110) and (101) planes for the rutile and anatase phases, respectively. The weight ratio of rutile to anatase (fA/f R) was semiquantitatively evaluated using the ratio of the integrated Raman peak intensity of rutile at 446 cm−1 (IR) to that of anatase at 396 cm−1 (IA). These bands are due to the Ti−O vibrations in the lattice of rutile and anatase, respectively. We obtained the following linear relationship for the weight ratio of rutile to anatase: IA fA /fR = 3.64 × IR where IA and IR are the integrated Raman peak intensities of anatase at 396 cm−1 and rutile at 446 cm−1, respectively. The values of the weight fractions of anatase and rutile phases estimated by both means are presented in Table 1. It is obvious that different methods yield somewhat different phase compositions. Note that the fraction ratios fA/f R in both cases are directly proportional to the Al concentration (see Figure 3a). Dopant concentrations higher than 0.5 wt % Al remarkably reduce the anatase rutilization degree by 10−18%. This finding is consistent with Raman spectroscopic data (Figure 2). It was shown earlier that the superfluous dopant ions at high dopant concentrations are placed in interstitial positions that result in an unit cell deformation and in an increase of the lattice energy. 24697

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The Journal of Physical Chemistry C Table 1. Composition and Morphological Properties of Synthesized x-Al-TiO2 Samples sample

xtheor,a wt % Al

xexp,b wt % Al

fA/ f R,c wt % (XRD; Raman)

agglomerate size (SSEM), nm

SBET, m2/g

crystallite size (XRD), nm

0.0-Al-TiO2 0.1-Al-TiO2 0.3-Al-TiO2 0.5-Al-TiO2 0.7-Al-TiO2 0.9-Al-TiO2 1.1-Al-TiO2

0.0 0.1 0.3 0.5 0.7 0.9 1.1

0.0 0.22 0.43 0.59 0.72 0.96 1.00

0.00/100.00; 0.00/100.00 0.01/99.99; 2.96/97.04 6.67/93.33; 3.21/96.79 1.23/98.77; 4.30/95.70 10.26/89.74; 8.63/91.37 14.26/85.74; 12.00/88.00 18.23/81.77; 12.47/87.53

50−250 50−100 50−100 30−80 30−50 30 30

1.7 4.2 8.7 9.5 18.1 28.0 27.2

36 27 26 24 22 20 21

a xtheor is the weight percentage of Al in the TiO2 samples and was calculated with respect to the weight proportion of reagents. bxexp is the weight percentage of Al in the TiO2 samples and was experimentally determined by EDX spectroscopic microanalysis (see Figures Bc−Hc in the Supporting Information). cfA and f R are the mass fractions (in wt %) of the anatase ( fA) and the rutile ( f R) phases, respectively, evaluated using XRD and Raman data.

Figure 3. Dependences of the fraction ratios fA/f R (a: ■, from XRD data; ●, from Raman data) and of the specific surface area SBET (b) on the Al concentration x(Al). fA/f R = (0.20 ± 0.03)x (for XRD data) and fA/f R = (0.13 ± 0.03)x (for Raman data), the correlation coefficient r is 0.87, the significance level P is 0.02. SBET = (25 ± 3)x, the correlation coefficient r is 0.97, and the significance level P is 0.0003.

specific surface area and the aluminum content (see Figure 3b). The observation of the reduction of both the particle growth and the particle agglomeration with the Al dopant concentration has been reported earlier by several research groups.4,25,26,28,33,34 It was suggested that the presence of dopant on the surface of TiO2 particles limits the growth of their grains.32,33 Large scale EDX mapping results for all x-Al-TiO2 samples are given in the Figures Bd−Hd in the Supporting Information and in Figure 4c. As seen, the distribution of Ti and O ions in all samples is homogeneous. Aluminum mapping demonstrates a uniform distribution of the dopant as well. The absence of characteristic peaks of Al phases in the XRD patterns (Figure 1) indicates that aluminum is incorporated into the rutile lattice. To validate the incorporation of Al into the titanium dioxide lattice, the XPS spectra of pure and doped TiO2 (0.1-Al-TiO2, 0.5-Al-TiO2, and 1.1-Al-TiO2) were also studied. The survey spectra are presented in Figure 5. The peaks related to the binding energies (BE) of ∼458.4 eV and ∼464.2 eV correspond to Ti 2p3/2 and Ti 2p1/2, respectively. In accordance with the literature,14,16,35,36 Ti4+ is the dominant oxidation state of Ti in both pristine and Al-doped samples. As depicted in Figure 5, binding energies of 74.28 and 118.85 eV in the spectra of 0.5Al-TiO2 and 1.1-Al-TiO2 correspond to the peaks of Al 2p and Al 2s, respectively. These BE values of the Al XPS spectra indicate the stable oxidation state Al3+.14,37 The O 1s peak at BE of ∼529.60 eV demonstrates a symmetric shape for both pristine and doped TiO2. The O 1s XPS spectra do not manifest any evidence of the presence of more than one type of oxygen atoms as have been observed for Al-doped titanium dioxide with much higher dopant concentrations than in our study.13,14 According to the quantitative

In turn, this leads to the preferential orientation of the crystal growth along the (101) and (004) planes of the anatase phase (see Figure 1) and, thus, the inhibition of the rutile phase formation.14,30−32 For all powders, it is observed that the nanoparticles form the aggregates of irregular form (Figures B−H in the Supporting Information). Here, we present the SEM images for pristine TiO2 and 1.1-Al-TiO2 samples as an example (Figure 4, panels a and b).

Figure 4. SEM images of (a) pure (0.0 wt %) and (b) Al-doped (1.1 wt %) TiO2 samples (insertion shows the Al mapping for the 1.1-AlTiO2 sample).

Both, the crystallite and the particle size are changed with the Al-doping concentration (Table 1). The general tendency through the Al-concentration set is a decrease of particle growth and an increase of the uniformity of grains (a decrease of aggregation) with increasing of the dopant concentration. In turn, the values of specific surface area of doped TiO2 samples are significantly increased with Al content from 1.7 m2/g for undoped titania to 27 m2/g for the 1.1-Al-TiO2 (see Table 1). It is worth noting that there is a linear dependence between 24698

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synthesized with increasing Al concentration. The absorbance spectrum of the rutile TiO2 sample treated at 1000 °C is given as a reference. In general, the spectra obtained can be tentatively divided into two regions: the 380−430 nm range, the region of the fundamental absorption edge of titania, and the 430−800 nm range which includes the visible range (extrinsic absorption region of titania). Fundamental Absorption Edge Region. No significant and regular spectral shift of the fundamental absorption edge of TiO2 upon variation of the Al dopant concentration in the 0.0− 1.1 wt % concentration range has been observed. The inset in Figure 6 presents spectra in the absorbance threshold region. As can be seen, for all Al-doped TiO2 samples, the spectra at the absorbance level of 0.25 are within the wavelength interval from 419 to 423 nm [i.e., Δλ = 4 nm (≈0.03 eV)]. The spectrum of undoped sample 0.0-Al-TiO2 (green curve, Figure 6) also falls inside this wavelength interval. At the same time, for the rutile sample (black curve, Figure 6), the absorbance value of 0.25 corresponds to λ of ∼414.5 nm, which is about 5− 9 nm red-shifted compared to the absorption edges for undoped and doped samples. A similar red shift (Δλ ≈ 10 nm) of the absorption edge for Al-doped mesoporous TiO2 in relation to TiO2 Degussa P25 has been found elsewhere.13 Also, such red absorption edge shifts have been reported for TiO2 doped by some other metals.18 Other authors have observed a blue bandgap energy shift (a few hundredth of eV).38 In turn, Egerton has demonstrated the influence of particle size on absorption and scattering of light by small TiO2 particles and, therefore, on diffuse reflectance spectra.39 In our case, apparent shifts of absorption edge for doped TiO2 are suggested to be mostly a result of different dispersion of the TiO2 samples.14 In the case of rutile, as the dominant phase in our samples, the fundamental absorption edge is formed by direct-forbidden and indirect-allowed transitions.40−42 The direct-forbidden transition is considered to be rather weak compared with the indirect-allowed transition. Therefore, we estimated the bandgap energy values related to the indirect-allowed transitions using the dependencies of [F(R) × hν]1/2 on the photon energy (hν) for all samples including the reference rutile sample (ref 43 and references herein). Here F(R) = (1 − R)2/2R is the Kubelka−Munk function calculated from experimental diffuse reflectance (DR) spectra R(hν), ignoring the possible wavelength dependence of the scattering coefficient. Estimated values of bandgap energy (Eg) are given in the Table 2. The graphical illustration of transformed spectra of the Kubelka−Munk function to determine the optical band gap energy is presented in the Figure I. It is seen from Table 2 that the bandgap energy values estimated from spectral data remain the same within the experimental error (Eg= 2.93 ± 0.01 eV) for all x-Al-TiO2 samples and are only slightly lower compared to the reference rutile sample. The insufficient effect of the aluminum doping (within 0.0−1.1 wt % concentration range) on the bandgap energy is in good agreement with the results of both the theoretical and the experimental study of Al-doped titanias.14,44,45 In accordance with the periodical DFT method,45 the substitution of a Ti cation by an Al cation leads to a raising

Figure 5. XPS spectra of pure (0.0 wt %) and Al-doped (0.1, 0.5, and 1.1 wt %) TiO2 samples.

analysis, the area ratio of the individual spectral peaks of Ti 2p and O 1s atoms state after appropriate background subtraction by their respective atomic sensitivity factor (ASF) is in the range of stoichiometric values for titanium dioxide. Taylor et al.34 have characterized the commercial aluminumdoped titania pigments by X-ray photoelectron spectroscopy and by time-of-flight secondary ion mass spectroscopy and demonstrated that a concurrent increase in aluminum concentration occurs both at the surface and in the core of the pigment particles upon the increase of the total aluminum content. It is remarkable that the surface was approaching aluminum saturation at a total concentration of Al2O3 up to 1.18 wt % (that is 0.624 wt % Al), while the aluminum concentration in the core was continuously increasing. The presence of aluminum at the subsurface region changes the composition of hydroxyl-hydrated coverage of the nanoparticles and also affects the surface area of samples (see Figure 3b).4 2. Absorption Spectra and Bandgap Energy. Figure 6 shows the absorbance spectra of the x-Al-TiO2 samples

Figure 6. Absorbance spectra of x-Al-TiO2 samples synthesized [x = 0.0 (green), 0.1 (red), 0.3 (blue), 0.5 (purple), 0.7 (olive), 0.9 (orange), and 1.1 (pink) wt % Al] and the rutile TiO2 sample (R, black) as a reference. A = 1 − R, where R is the diffuse reflectance coefficient of the samples. The inset shows an enlarged view of spectra in the absorbance threshold region.

Table 2. Bandgap Energies (Eg) for Synthesized x-Al-Doped TiO2 and Rutile TiO2 (R) Samples x, wt % Al Eg, eV

0.0 2.934 ± 0.001

0.1 2.929 ± 0.004

0.3 2.922 ± 0.003

0.5 2.923 ± 0.003 24699

0.7 2.922 ± 0.003

0.9 2.940 ± 0.003

1.1 2.936 ± 0.002

R 2.999 ± 0.001

DOI: 10.1021/acs.jpcc.5b06252 J. Phys. Chem. C 2015, 119, 24695−24703

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The Journal of Physical Chemistry C of the lowest energy of the conduction band as well as to a displacement of the highest level of the valence band so that a bandgap narrowing takes place. Taking into account the dopant “concentration” used in calculations as 1−2 Al atoms per supercell Ti16O32, the results obtained by Shirley et al.45 should be considered as heavy-doped titanias. Thus, the absence of a significant bandgap energy reduction for all Al-doped TiO2 studied here can be considered as an indirect evidence for the oxygen vacancy formation caused by aluminum doping of TiO2. Extrinsic Absorption Region. The absorbance of Al-doped titanias in the visible spectral region (430−800 nm) is monotonously increased with the aluminum concentration, and from 0.5 wt % Al, a tendency to saturation of absorbance is observed (Figure 6). The most probable explanation of such an observation is an alteration of the light scattering which is varied from sample to sample depending on its dispersion (as mentioned above, a reduction of the particle size and of the particle agglomeration with the Al dopant concentration is observed). 3. Testing of Photocatalytic Activity. To explore the effect of Al-doping on the photocatalytic behavior of TiO2, we have evaluated the dependencies of two major photocatalytic parameters, activity and selectivity, in the reaction of phenol photodegradation. To evaluate the photocatalytic activity of the samples, we used the initial rate of phenol photodegradation estimated from the kinetic approximation:

Figure 7. Dependence of initial rate of the phenol photodegradation reaction (r0) for the x-Al-TiO2 samples on the weight percentage of aluminum dopant (x). Red dashed line marks the initial rate of the phenol photodegradation over TiO2 Degussa P25.

C(t ) = C0 exp( −kt )

then dC /dt(t → 0) = −kC0

where k is an apparent quasi-first-order rate constant. The selectivity of Al-doped TiO2 toward the formation of two major primary intermediate products, i.e., hydroquinone (HQ) and catechol (Cat), was defined as46,47

Figure 8. Dependencies of the initial selectivity toward formation of (a) hydroquinone and (b) catechol in the photocatalytic degradation of phenol over synthesized x-Al-TiO2 samples on the weight percentage of aluminum dopant (x). Red dashed lines marks the value of initial selectivity toward formation of (a) hydroquinone and (b) catechol in the photocatalytic degradation of phenol over TiO2 Degussa P25.

SHQ = (dC HQ /dt )/(dC PhOH/dt )(t → 0)

and SCat = (dCCat /dt )/(dC PhOH/dt )(t → 0)

Evaluation of the “red” spectral limit of Al-doped TiO2 samples with the set of cutoff filters infers that no activity of Al-doped titania in the visible spectral region (λ ≥ 400 nm) is observed. Thus, Al-doping does not cause a spectral sensitization toward visible light, and UV-light only is responsible for the photoactivation of Al-doped titania. Figure 7 shows the alteration of the activity (initial rate of phenol photodegradation) of Al-doped TiO2 with an increase of the Al dopant content. The initial rate of phenol photodegradation over TiO2 Degussa P25 is also presented as a reference. As evident from the presented experimental data, the activity of Al-doped TiO2 for the photocatalytic phenol degradation increases for Al concentrations within the range of 0.0−0.5 wt %, and reaches a maximal value at an Al concentration of 0.5 wt %, and then significantly decreases at higher Al concentrations. Note that no correlation between the activity and the specific surface area is observed since the specific surface area increases with the Al-concentration (see Table 1 and Figure 3b). Figure 8 demonstrates the dependencies of the selectivity of Al-doped TiO2 on the Al concentration. The obtained data infers that in the Al concentration range 0.1 wt % < x < 1.1 wt

%, the selectivity toward formation of both major intermediate products, HQ and Cat, remains nearly constant. This means that Al-doping does not change the type of surface active centers and, therefore, the selectivity of the titania surface. At the same time, the selectivity of the Al-doped TiO2 is different compared to the pristine (0.0 wt % Al) and highly doped (1.1 wt % Al) titanias. Thus, the type of the surface active centers of pristine and highly doped titania differs from Al-doped samples with medium Al content (0.1 wt % < x < 1.1 wt %). Taking into account that the Al-doping results in the alteration of the anatase−rutile ratio in the samples, the enhancement of the photocatalytic activity of mixed-phase titania can be explained by the increase of the number of rutile−anatase contacts with an increase of the anatase fraction in the titania structure (ref 48 and references therein). However, the enhancement of the photocatalytic activity of mixed phase titania due to the existence of rutile−anatase heterostructures has been challenged.49 Besides, the possible effect of the alteration of the anatase−rutile ratio on the photocatalytic activity of Al-doped TiO2 is in clear contradiction to the unchanged selectivity of the doped samples (see 24700

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The Journal of Physical Chemistry C Table 1, Figure 3a). Indeed, one may expect that an alteration of the anatase−rutile ratio should be followed by analteration of the major type of the surface active centers, which are different for anatase and rutile. In turn, this should result in an alteration of the surface selectivity. However, our experimental data on the surface selectivity do not support this assumption. Interestingly, this optimal aluminum concentration is close to that for Evonik Aeroxide TiO2 P25, which is widely used as a reference photocatalyst. At the same time, in order to explain the existence of an optimal dopant concentration for mixedphase Al-doped TiO2, one should suppose an increase of the photocarriers recombination rate upon an increase of the Al dopant concentration. In general, photocatalytic activity of solids depends on the concentration of photogenerated charge carriers, which, in turn, is determined by the rates of their photogeneration and recombination. Since the photocatalytic activity of the Al-doped samples is observed in the spectral range of the fundamental absorption only, the rate of photocarrier generation might be considered to be the same for all Al-doped titania samples. In contrast, the rate of the carrier recombination under moderate photoexcitation is determined mostly by the charge carrier recombination via defects.50 In order to explain the existence of a maximum in the dependency of the initial reaction rate on the Al concentration (Figure 7), one may hypothesize a rather specific dependence of the apparent recombination constant (with a minimum at a weight percentage of Al of 0.5 wt %) on the aluminum concentration. Recently, a model explaining the existence of an optimal dopant concentration for transition metal doped titania and establishing the relation between concentration of dopants, titania particle size, and photocatalytic activity has been developed by Bloh et al. The model is based on two criteria: (i) the concentration of dopants should be sufficiently low since defects favor carrier recombination at high concentration when tunneling recombination dominates due to the formation of “defect clusters” and (ii) none of the particles from a powdered photocatalyst should be “empty” (i.e., each photocatalyst particle should include at least one dopant). Note that the presently studied Al-doped samples [aluminum concentration range (x) is 0.1−1.1 wt %, particle size range (l) is 250−30 nm or 800−50 nm, when estimated according to the specific surface area SBET (Table 1) reside in the existence domain (x − l) of the so-defined optimal metal dopant concentrations.20 Thus, we propose the following explanation of the obtained results: (1) the substitution of Ti4+ sites with Al3+ states due to Al-doping results in an excess of the negative charge that is compensated by the formation of intrinsic defects such as anion vacancies. (2) The higher the Al concentration is, the higher will be the concentration of the compensating intrinsic defects. (3) The latter defects play the dual role of being surface active centers and recombination centers (in the bulk) simultaneously. (4) At high dopant concentration, the formation of defect clusters occurs. Thus, the Al-doping results in an interplay between the formation of the new surface active centers and new recombination centers. Therefore, the dependence of the activity on the Al concentration is going through a maximum, whereas the selectivity of the surface remains the same. Only at higher Al content (>0.9 wt %) can the formation of defect clusters change the surface selectivity since, in this case, the type of the surface active centers becomes different.

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CONCLUDING REMARKS

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ASSOCIATED CONTENT

A set of Al-doped titanias x-Al-TiO2 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.1 wt % Al) has been synthesized by a sol−gel method and tested in the model reaction of the photocatalytic degradation of phenol in aqueous suspensions. In accordance with previous experimental and theoretical results,21,51 the XRD and Raman studies show that the thus prepared TiO2 samples have mixed- rutile−anatase crystal structure with a linear increase of the anatase fraction from zero at an Al concentration of 0.0 wt % up to 18% at an Al concentration of 1.1 wt %. Also, the well-known decrease of the particle size and increase of the specific surface area of the Al-doped samples is seen from ∼800 nm up to 50 nm and from 1.7 up to 27 m2/ g, respectively, as well as an increase in the uniformity of the grains (i.e., a decrease in aggregation with an increase of the dopant concentration has been observed).4 The combination of XRD, XPS, Raman spectroscopy, and EDS technique demonstrates the homogeneous distribution of aluminum in the Al-doped samples. No evidence of Al2O3 formation neither in the bulk nor at the surface of the titania particles has been observed. In accordance with the data obtained by diffuse reflectance spectroscopy, Al-doping does not affect the bandgap energy of doped titania. The latter is in good agreement with recent experimental results14,15 obtained by Navas and coauthors, as well as with the results of DFT calculations44 for the Al3+ incorporation into the Ti4+ position in the TiO2 lattice, resulting in the formation of anion vacancies. The photocatalytic testing of Al-doped TiO2 samples employing the reaction of phenol degradation in aqueous suspension shows the existence of a maximal initial rate of phenol degradation at an aluminum dopant concentration near 0.5 wt %. The particle size and the aluminum concentration of the Al-doped samples correspond well to the domain of optimal particle size and metal dopant concentration predicted by Bloh and coauthors.20 Experimental dependencies of the Al-doped TiO2 selectivity toward the formation of hydroquinone and catechol suggest that Al-doping results in the formation of the same type of surface active centers and affects their concentration only, that in turn causes the alteration of the photocatalyst activity with its maximal activity corresponding to an Al content of 0.5 wt %.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06252. XRD data, SEM images, EDX spectra, and element (Ti, O, Al) mapping analysis for undoped and all Al-doped TiO2. The graphical illustration of the determination of the indirect-allowed bandgap values for x-Al-TiO2 samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 24701

DOI: 10.1021/acs.jpcc.5b06252 J. Phys. Chem. C 2015, 119, 24695−24703

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ACKNOWLEDGMENTS The present study was performed within the Project “Establishment of the Laboratory “Photoactive Nanocomposite Materials” no. 14.Z50.31.0016 supported by a Mega-grant of the Government of the Russian Federation. The work carried out in Hannover (Germany) was supported by the Global Research Laboratory (GRL) Program (NRF-2014K1A1A2041044) funded by the Korea government (MSIP) through NSF. This work was partially supported by a grant from the Russian Foundation for Basic Research (12-03-00456-a) and a grant from the President of the Russian Federation (MK2233.2014.3). We are also grateful to the RC “Nanophotonics”, RC “Nanotechnology”, RC “Chemical Analysis and Materials Research Centre”, RC “X-ray Diffraction Studies”, and RC “Optical and Laser Materials Research” of the Research Park at the Saint-Petersburg State University for helpful assistance in conducting the synthesis and the characterization of the samples. A.A.M. thanks the Saint-Petersburg State University for support within the University Postdoctoral program (no. 11.50.1595.2013).

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