Influence of the Dopant Concentration on the Photocatalytic Activity: Al

Subscriber access provided by UNIV OF LETHBRIDGE

Article

Influence of the Dopant Concentration on Photocatalytic Activity: Al-doped TiO

2

Anna A. Murashkina, Petr D. Murzin, Aida V. Rudakova, Vladimir K Ryabchuk, Alexei V Emeline, and Detlef W. Bahnemann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06252 • Publication Date (Web): 28 Sep 2015 Downloaded from http://pubs.acs.org on October 9, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Influence of the Dopant Concentration on Photocatalytic Activity: Al-doped TiO2 Anna A. Murashkina,*,1 Petr D. Murzin,1 Aida V. Rudakova,1 Vladimir K. Ryabchuk,1 Alexei V. Emeline,1 Detlef W. Bahnemann*,1, 2 1

Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University,

Ulyanovskaya str. 1, Peterhof, Saint-Petersburg, 198504 Russia 2

Institut fuer Technische Chemie, Gottfried Wilhelm Leibniz Universitaet Hannover,

Callinstrasse 3, D-30167 Hannover, Germany

Keywords: titanium dioxide, aluminum-doping, band gap energy, photoactivity, phenol photodegradation.

Abstract The 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 synthetized by sol-gel method and tested in the reaction of photocatalytic degradation of phenol in aqueous suspension. XRD and Raman studies show that TiO2 samples have mixedphase rutile-anatase crystalline structure with linear increase of anatase fraction from 0.0 wt% for 0.0-Al-TiO2 up to 18 wt% for 1.1-Al-TiO2. The decrease of the particle size from ∼ 800 nm to 50 nm and the increase of 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 Al(3+) oxidation state of aluminum incorporated in TiO2 lattice. The bang gap energy Eg = (2.93 ± 0.1) eV corresponding to indirect allowed transitions does not depend on 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 Al concentration about 0.5 wt%. *Corresponding

Authors:

e-mail:

[email protected]

[email protected] (D.W.B.). ACS Paragon Plus Environment

(A.A.M.);

e-mail:


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 high transparency to visible light and high average refractive index (na = 2.5, nr = 2.7 for anatase and rutile, respectively) TiO2 powder were known as a white paint pigment and a reflecting coating for a long time.4 The pioneer publication5 by Fujishima’s and Honda’s 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 both as a white pigment and as a photocatalyst favorable. At the same time the photocatalytic (photochemical) activity of titania is a sort of antagonistic phenomenon in relation to TiO2 application as a pigment since it promotes the destruction of the binder that, finally, yields to chalking of paints and coatings.4 It is seen that 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 achive the higher photoactivity for titania-based photocatalysts. In particular, this concerns cumulative impurity/dopant levels within the TiO2-based pigments and photocatalysts. In general, the less the content of impurities in TiO2 pigment the better its optical properties, namely the reflectivity of titania powders in the visible region. Thus, the commercial TiO2 pigment technologies include the removal of the most part of impurities from the final products down to tenths of the weight percentage.4 In contrast, doping or co-doping of titania by metallic and non-metallic impurities with concentration up to few percent is supposed to be a prospective way to create photoactive materials that are called the visible light active (VLA) photocatalyts.1,9,11 Non-metal-doped titania has been extensively studied as VLA photocatalyst in the last fifteen years.1,11 Titanias doped by metals of platinum group, transition 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 (the aluminum concentration is about 1 wt%) is noticeably more active in 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 reference TiO2 Degussa P25. Navas et al. have synthesized a highly aluminium-doped TiO2 nanoparticles (AlxTi1–xO2–x/2, where x = 0.083, 0.154, 0.2) using a sol–gel method and studied their photocatalytic activity in the ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 co-authors.16 At the same time, no effect of aluminum doping on photocatalytic degradation of steric acid by the titania thin solid films (in contrast with Cu-doped titania films) has been found.17 The comparative study of the efficiencies of Al-, Sn- and Pt-doped and co-doped titanias synthesized by flame spray pyrolysis (FSP) in the methyl orange dye degradation has shown that the efficiency of Aldoped TiO2 is two-fold lower than that for undoped TiO2.18 However, activity of co-doped Al/Pt-TiO2 sample was higher than the activity of the reference Degussa P25 sample.18 According to Gesenhues’s results19 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 pigment particles, while at gradual increase of the dopant concentration it remains at the particle surface, and the photoactivity of such pigments approaches to some reduced saturation value. A possible reason for this inconsistency of results reported by different authors for the aluminum doping effect on photocatalytic activity of titania most likely is the difference in synthesis and 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 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 (rutileanatase) 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 powder contain the aluminum as a surfactant with concentrations up to 1 wt%.4 In particular, a well-known photocatalyst Evonik Aeroxide® TiO2 P25 contains22 up to 0.3 wt% of Al2O3 that corresponds to the optimal metal dopant concentration range as suggested elsewhere.20 Here we report the results of studies of the aluminum doping effect within dopant content range 0.0–1.1 wt%, on physical-chemical characteristics and photocatalytic performance of titanium dioxide photocatalyst.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

Experimental 1. Material synthesis Polycrystalline x-Al-TiO2 (x, Al wt% ‒ 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1) were prepared by 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 disolved in the TTIP solution in isopropanol in accordance dihydrate

with

desired

C6H8O7.2H2O

stoichiometry

of the

doped samples.

Then

citric acid

(CADH, powder of 98%-purity, Vekton) was added as a complexing

agent with appropriate molar stoichiometry of (Al3++Ti4+)/CADH:1/3. Obtained mixtures were heated up to 200°C for 3 hours to remove the majority of organic decomposition products and then annealed at 650°C for 5 hours 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 CuKa (anode current 15 mA, accelerating voltage 30 kV) radiation in the angle range of 20°≤2θ≤80° with a scanning speed 5.0°/min. Structural data for anatase and rutile phases were taken from 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 см-1 spectral region at ambient temperature using a SENTERRA Raman spectrometer (Bruker) with 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 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 Photocatalytic activity of materials synthesized was evaluated by the decomposition of phenol under light irradiation. Before irradiation, the aqueous suspension of x-Al-TiO2 photocatalyst (1 g/L, pH = 3.0) and phenol (100 ppm; purity of 99.5 %) was pretreated with ACS Paragon Plus Environment

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ultra-sonic processor for 10 min and then magnetically stirred for 1 hour 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 1260 Infinity (Agilent Technologies) chromatograph with UV–Vis detector and C18 column (Agilent). The mobile phase was a mixture of methanol and water at volume ratio 50:50. The detection wavelength was set at 210 nm with bandwidth 4 nm.

Results and Discussion 1. Material characterization The XRD patterns of all x-Al-TiO2 samples are given in Figure 1. According to phase analysis, the 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, 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 XDR peaks is given in the Supporting Information section). It means that Al-doping inhibits the formation of rutile phase during heating treatment at 650°C. Our observation is in agreement with previous studies on influence of aluminum on the anatase-rutile phase transformation even at 750-800°C.25,26 The effect of Al-substitution in 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 (ionic radius of aluminum smaller than the ionic radius of titanium), which consequently lead to a shift of the peak in large angles region on the X-ray diffraction patterns (Inset in Fig.1).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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, 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 xAl-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 rutile phase and peaks marked by A correspond to anatase phase.

XRD data on the phase composition of 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 crosssection. Raman spectra show that in case of 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 ACS Paragon Plus Environment

Page 6 of 24

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 sample with 0.1 wt% Al. Such behavior of the band at 143 cm-1 is confirmed by appearance and increase in intensity of the band at 145 cm-1, which belongs to the Eg1 vibrational mode of anatase (Ti-Ti vibrations in octahedral chains). This findings indicate 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 apparent growth of the bands at 197 and 640 cm-1 (Eg), 398 and 515 cm-1 (B1g), and 515 cm-1 (A1g). For all xAl-TiO2 samples, there is no spectral evidence of the presence of Al-involving species vibrations

27,28

and carbon species (characteristic bands near 1342 cm-1 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/fR) from both XRD and Raman data. The relation between the ratio of intensities (IA/IR) of the strongest reflection peaks for both phases (27.40o for rutile and 25.22o for anatase, see Figure A in the Supporting Information section) was taken from X-ray diffraction patterns. This relation is given by the expression29:

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/fR) was semi-quantitatively 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 utilize linear relationship for the weight ratio of rutile to anatase:

where IA and IR are 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 I. It is seen that different methods gives a little beat different phase composition. Note, that the fraction ratios fA/fR in both cases are directly proportional to the Al ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

concentration (see Figure 3a). The dopant concentrations higher than 0.5 wt% Al remarkably reduce the anatase rutilization degree until 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 results in the unit cell deformation and increase of the lattice energy. In turn, that leads to the preferential orientation of crystal growth along (101) and (004) planes of the anatase phase (see Figure 1) and, thus, the inhibition of the rutile phase formation14,30‒32

Table I. Composition and morphological properties of synthesized x-Al-TiO2 samples. fA/ fR,*** wt% Sample

xtheor,* xexp,** wt% Al wt% Al

(XRD; Raman)

Agglomerate size (SSEM), nm

SBET, m2/g

36

0.00/100.00; 0.0-Al-TiO2

0.0

0.0

50-250 0.00/100.00

1.7 27

0.01/99.99; 0.1-Al-TiO2

0.1

0.22

50-100 2.96/97.04

4.2 26

6.67/93.33; 0.3-Al-TiO2

0.3

0.43

50-100 3.21/96.79

8.7 24

1.23/98.77; 0.5-Al-TiO2

0.5

0.59

30-80 4.30/95.70

9.5 22

10.26/89.74; 0.7-Al-TiO2

0.7

0.72

30-50 8.63/91.37

18.1 20

14.26/85.74; 0.9-Al-TiO2

0.9

0.96

30 12.00/88.00

28.0 21

18.23/81.77; 1.1-Al-TiO2 *

1.1

1.00

Crystallite size (XRD), nm

30 12.47/87.53

27.2

xtheor is the weight percentage of Al in TiO2 samples was calculated with respect to the weight

proportion of reagents. **

xexp is the weight percentage of Al in TiO2 samples was experimentally determined by EDX

spectroscopic microanalysis (see Figures Bc-Hc in the Supporting Information section).


Page 9 of 24

***

fA and fR are the mass fractions (in wt%) of the anatase (fA) and the rutile (fR) phases,

respectively, evaluated using XRD and Raman data.

b

30

a

SBET, m /g

0.20 0.15

2

fA/fR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10

20

10 0.05 0.00 0.0

0.2

0.4

0.6 0.8 x, wt% Al

1.0

1.2

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

x, wt% Al

Figure 3. Dependences of the fraction ratios fA/fR (a: squares – from XRD data, circles – from Raman data) and of the specific surface area SBET (b) on the Al concentration x(Al). fA/fR = (0.20±0.03)*x (for XRD data) and fA/fR = (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, the significance level P is 0.0003.

For all powders, it is observed that the nanoparticles form the aggregates of irregular form (Figures B-H in the Supporting Information section). Here we present the SEM images for pristine TiO2 and 1.1-Al-TiO2 samples as an example (Figure 4a and 4b). a

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 1.1-Al-TiO2 sample).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Both the crystallite and the particle size are changed with the Al-doped concentration (Table I). 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 I). Worth to note that there is a linear dependence between 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 section and in the Figures 4c. As seen, the distribution of Ti and O elements in all samples is homogenous. Aluminum mapping demonstrates a unifrom distribution of dopant as well. 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 the Figure 5. The peaks related to the binding energies (BE) of ~458.4 eV and ~464.2 eV correspond to of Ti2p3/2 and Ti2p1/2, respectively. According to the literature,14,16,35,36 Ti4+ is the dominant oxidation state of Ti in the both pristine and Al-doped samples. As depicted in Figure 5, binding energies of 74.28 eV and 118.85 eV in the spectra of 0.5-Al-TiO2 and 1.1-Al-TiO2 correspond to the peaks of Al2p and Al2s, respectively. These BE values of the Al XPS spectra indicate a stable oxidation state Al3+.14,37 The O1s peak at BE of ~529.60 eV demonstrates a symmetric shape for both pristine and doped TiO2. The O1s 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 titania dioxide with much higher dopant concentrations than in our study.13,14 According to the quantitative analysis the area ratio of the individual spectral peaks of Ti2p and O1s atoms state after appropriate


Page 10 of 24

Page 11 of 24

background subtraction, by their respective atomic sensitivity factor (ASF) are in the range of stoichiometric values for titanium dioxide.

O1s Ti2p

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O2s Ti3p Ti3s Al2p Al2s

0

Ti2s C1s

200

1.1 0.5 0.1 0.0

400

600

Binding energy, eV

Figure 5. XPS spectra of pure (0.0 wt%) and Al-doped (0.1, 0.5, and 1.1 wt%) TiO2 samples. Taylor et al.34 have characterized the commercial aluminum doped titania pigments by the X-ray photoelectron spectroscopy and the time-of-flight secondary ion mass spectroscopy and demonstrated that concurrent increase in aluminum concentration occur both at the surface and in the core of the pigment particles with 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 sub-surface region changes the composition of hydroxyl-hydrated coverage of 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 х-Al-TiO2 samples 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 fundamental absorption edge of titania, and the 430 – 800 nm range which includes the visible range (extrinsic absorption region of titania).



0.5 0.256

0.254

A

0.4

0.252

0.3

0.250

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

415

420

425

λ , nm

0.2

0.1

x R

0.0 400

500

600

700

800

λ , nm

Figure 6. The 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), 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.

Fundamental absorption edge region. No significant and regular spectral shift of

fundamental absorption edge of TiO2 with variation of Al dopant concentration in the 0.0–1.1 wt% concentration range has been observed. The inset on 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 nm to 423 nm, i. e. ∆λ=4 nm (≈0.03 eV). The spectrum of undoped sample 0.0-Al-TiO2 (green curve, Figure 6) falls inside this wavelength interval. At the same time, for the rutile (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. Similar red shift (∆λ ≈ 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 doped TiO2 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 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


Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transition is considered to be rather week compared with indirect-allowed transition. Therefore, we estimated the bandgap energy values related to 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.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 II. The graphical illustration of transformed spectra of Kubelka-Munk function to determine the optical band gap energy is presented in the Supporting Information section (Figure I).

Table II. The bandgap energies (Eg) for synthesized x-Al-doped TiO2 and rutile TiO2 (R) samples. x, wt% Al

0.0

Eg, eV

2.934 ± 0.001

0.1

0.3

2.929 ± 2.922 ± 0.004 0.003

0.5

0.7

0.9

1.1

R

2.923 ± 0.003

2.922 ± 0.003

2.940 ± 0.003

2.936 ± 0.002

2.999 ± 0.001

It is seen from Table II 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 only slightly lower comparing 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 the both theoretical and experimental study of Al-doped titanias.14,44,45 According to periodical DFT method,45 the substitution of Ti cation by Al cation leads to raising of the lowest energy of the conduction band as well as to displacement of the highest level of valence band so that the 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 the significant bandgap energy reduction for studied Al-doped TiO2 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 visible spectral region

(430–800 nm) is monotonously increased with the aluminum concentration, and from 0.5 wt% Al the 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sample to sample depending on its dispersion (as mentioned above, the 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: 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 formation of two major primary intermediate products, hydroquinone (HQ) and catechol (Cat), was defined as46,47: SHQ = (dCHQ/dt) / (dCPhOH/dt) (t0) and SCat = (dCCat/dt) / (dCPhOH/dt) (t0). Evaluation of the “red” spectral limit of Al-doped TiO2 samples with the set of cut-off filters infers that no activity of Al-doped titania in visible region of spectra (λ ≥ 400 nm) is observed. Thus, Al-doping does not cause the 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 increase of the Al dopant content. The initial rate of phenol photodegradation over TiO2 Degussa P25 is also presented as a reference.


Page 14 of 24

Page 15 of 24

0.04

0.03

r0, ppm/min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02

0.01

0.00 0.1

0.3

0.5

0.7

0.9

1.1

x, wt% Al

Figure 7. The dependence of initial rate of the phenol photodegradation reaction (r0) for the xAl-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. As evident from the presented experimental data, the activity of Al-doped TiO2 in photocatalytic phenol degradation increases within Al concentration within the range 0.0–0.5 wt% reaches maximal value at Al concentration of 0.5 wt% and then significantly falls down at higher Al concentration. Note, that no correlation between the activity and specific surface area is observed since the specific surface area increases with the Al-concentration (see Table I, 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. That means that Al-doping does not change the type of the surface active centers and therefore, the selectivity of the titania surface. At the same time, the selectivity of the Al-doped TiO2 is different comparing 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%).



a

b

0.4

Selectivity

0.4

Selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

0.2

0.2

0.0

0.0 0.1

0.3

0.5

0.7

0.9

1.1

x, wt% Al

0.1

0.3

0.5

0.7

0.9

1.1

x, wt% Al

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 hydroquinone (a) and catechol (b) in the photocatalytic degradation of phenol over TiO2 Degussa P25. Taking into account that the Al-doping results in the alteration of the anatase-rutile ratio in the samples, the enhancement of photocatalytic activity of mixed-phase titania can be expalined by the increase of the number of rutile-anatase contacts with increasing of anatase fraction in titania structure.48,and

references therein

However, the enhancement of photocatalytic

activity of mixed phase titania due to existence of rutile-anatase heterostructure has been challenged.49 Besides, the possible effect of the alteration of the anatase-rutile ratio on photocatalytic activity of Al-doped TiO2 contradicts with the unchangeable selectivity of the doped samples (see Table I, Figure 3a). Indeed, one may expect that alteration of the anataserutile ratio should be followed by alteration of the major type of the surface active centers, which are different for anatase and rutile. In turn, that should result in alteration of the surface selectivity. However, 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 optimal dopant concentration for mixed-phase Al-doped TiO2 one should suppose an increase of the photocarriers recombination rate with growth of dopant Al concentration.


Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In general, photocatalytic activity of solids depends on the concentration of photogenerated charge carriers, which in turn, is dictated 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 the same for all Al-doped titania samples. In contrast, the rate of the carrier recombination under moderate photoexcitation is determined mostly by charge carrier recombination via defects.50 In order to explain the existence of a maximum in the dependence of the initial reaction rate on the Al concentration (Figure 7) one may hypothesize a rather specific dependence of the apparent recombination constant (with minimum at weight percentage of Al of 0.5 wt%) on aluminum concentration. Recently, a model explaining the existence of 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.20 The model is based on two criteria: i) the concentration of dopant should be sufficiently low since defects favor carriers recombination at high concentration when tunneling recombination dominates due to formation of “defect clusters”, and ii) each particle from powdered photocatalyst should not be “empty”, i.e. the photocatalyst particle should include a dopant. Note, that 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 specific surface area SBET (Table I)) reside in the existence domain (x – l) of 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 the excess of the negative charge that is compensated by formation of intrinsic defects such as anion vacancies.

2.

The higher Al concentration the higher concentration of the compensating intrinsic defects.

3.

The latter defects play the dual role of the surface active centers and the recombination centers (in the bulk).

4.

At high dopant concentration the formation of the defect associates occurs. Thus the Al-doping results in interplay between the formation of the new surface active

centers and new recombination centers. Therefore, dependence of the activity on the Al concentration is going through the maximum whereas the selectivity of the surface remains the same. Only at higher Al content (> 0.9 wt%) the formation of the defect associates changes the surface selectivity since in this case, the type of the surface active centers becomes different.

Concluding remarks ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The set of Al-doped titanias x-Al-TiO2 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 wt% Al) has been synthesized by sol-gel method and tested in the model reaction of photocatalytic degradation of phenol in aqueous suspensions. In accordance with the previous experimental and theoretical results21,51 the XRD and Raman studies show that TiO2 samples have mixedrutile-anatase crystal structure with linear increasing of anatase fraction from zero at Al concentration 0.0 wt% up to 18 % at Al concentration 1.1 wt%. Also, the well-known decrease of particle size and increase of specific surface area of Al-doped samples, from ∼ 800 nm up to 50 nm and from 1.7 m2/g up to 27 m2/g respectively, as well as an increase in the uniformity of grains, i.e. a decrease in aggregation with an increase of the dopant concentration have been observed.4 Combination of XRD, XPS, Raman spectroscopy, and EDS technique demonstrates the homogeneous distribution of aluminum in Al-doped samples. No evidence of Al2O3 formation neither in the bulk nor at the surface of titania particles has been observed. According to the data 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 co-authors, and the results of DFT calculations44 for Al3+ incorporation into Ti4+ position in TiO2 lattice resulting in formation of anion vacancies. The photocatalytic testing of Al-doped TiO2 samples in the reaction of phenol degradation in aqueous suspension shows the existence of maximal initial rate of phenol degradation at aluminum dopant concentration near 0.5 wt%. The particle size and aluminum concentration of Al-doped samples correspond well to the domain of the optimal particle size and metal dopant concentration predicted by Bloh and co-authors.20 Experimental dependencies of the Al-doped TiO2 selectivity toward formation of hydroquinone and catechol suggested that Al-doping results in formation of the same type of the surface active centers and affects their concentration only, that in turn, causes the alteration of the photocatalyst activity with maximal activity corresponding to Al-content 0.5 wt%.

Supporting Information.

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 band gap values for x-Al-TiO2 samples.

Corresponding Authors:

E-mail: A.A.M., [email protected]; D. W. B., [email protected].


Page 18 of 24

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgments. The present study was performed within the Project “Establishment of the

Laboratory “Photoactive Nanocomposite Materials” No. 14.Z50.31.0016 supported by a Megagrant of the Government of the Russian Federation. This work was partially supported by a Grant from the Russian Foundation for Basic Research (12-03-00456-а) and a Grant of President of the Russian Federation (МК-2233.2014.3). We are also grateful to the RC “Nanophotonics”, RC “Nanotechnology”, RC “Chemical Analysis and Materials Research Centre”, RC “X-ray Diffraction Studies”, 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).

References:

(1) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 7, 2891–2959. (2) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919–9986. (3) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC Publishing: Tokyo, 1999. (4) Braun, J. H.; Baidins, A.; Marganski, N. E. TiO2 Pigment Technology: A Review. Progr. Org. Coat. 1992, 20, 105–138. (5) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38. (6) Serpone, N.; Emeline, A. V.; Horikoshi, S.; Kuznetsov, V. N.; Ryabchuk, V. K. On the Genesis of Heterogeneous Photocatalysis: A Brief Historical Perspective in the Period 1910 to the mid-1980s. Photochem. Photobiol. Sci. 2012, 11, 1121–1150. (7) Nakata, K.; Ochiai, T.; Murakami, T.; Fujishima, A. Photoenergy Conversion with TiO2 Photocatalysis: New Materials and Recent Applications. Electrochim. Acta 2012, 84, 103–111. (8) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66, 185–297. (9) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 516−582. (10) He, T.; Yao, J. Photochromism in Composite and Hybrid Materials Based on TransitionMetal Oxides and Polyoxometalates. Progr. Mater. Sci. 2006, 51, 810−879.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(11) Pelaez, M.; Nolan, N. T.; Pillai, S. C.; Seery, M. K.; Falaras, P.; Kontos, A. G.; Dunlop, P. S. M.; Hamilton, J. W. J.; Byrne, J. A.; O’Sheaf, K.; et al. A Review on the Visible Light Active Titanium Dioxide Photocatalysts for Environmental Applications. Appl. Cat. B: Env. 2012, 125, 331–349. (12) Choi, J. ; Park, H.; Hoffmann, M. R. Effects of Single Metal-Ion Doping on the VisibleLight Photoreactivity of TiO2. J. Phys. Chem. C. 2010, 114, 783–792. (13) Liu, S.; Liu, G.; Feng, Q. Al-doped TiO2 Mesoporous Materials: Synthesis and Photodegradation Properties. J. Porous Mater. 2010, 17, 197–206. (14) de los Santos, D. M.; Aguilar, T.; Sánchez-Coronilla, A.; Navas, J.; Hernandes, N. C.; Alcántara, R.; Fernándes-Lorenzo C.; Martin-Calleja J. Electronic and Structural Properties of Highly Aluminum Ion Doped TiO2 Nanoparticles: A Combined Experimental and Theoretical Study. Chem. Phys. Chem. 2014, 15, 2267–2280. (15) Navas, J.; Iglesias-Arnaiz, J. M.; Fernández-Lorenzo, C.; Alcántara, R.; Blanco, G.; de los Santos, D. M.; Sánchez-Coronilla, A.; Martín-Calleja, J. Synthesis and Characterization of Gel-Derived, Highly Al-Doped TiO2(Al xTi1–xO2–x/2; x = 0.083, 0.154, 0.2) Nanoparticles: Improving the Photocatalytic Activity. Sci. Adv. Mater. 2014, 6, 2134–2145. (16) Tsai, C.-Y.; Kuo, T.-H.; His, H.-C. Fabrication of Al-Doped TiO2 Visible-Light Photocatalyst for Low-Concentration Mercury Removal, Int. J. Photoenergy 2012, 2012, 874509. (17) Maeda, M.; Yamada, T. Photocatalytic Activity of Metal-doped Titanium Oxide Films Prepared by Sol-Gel Process. J. Phys.: Conf. Ser. 2007, 61, 755–759. (18) Paulauskas, I. E.; Modeshia, A. T. T.; El-Mossalamy, E. H.; Abdullah, Y.; Obaid, A. Y.; Sulaiman, N.; Basahel, S. N.; Al-Ghamdi, A. A.; Sartain, F. K. Photocatalytic Activity of Doped and Undoped Titanium Dioxide Nanoparticles Synthesised by Flame Spray Pyrolysis. Platinum Metals Rev. 2013, 57, 32–43. (19) Gesenhues, U. Al-doped TiO2 Pigments: Influence of Doping on the Photocatalytic Degradation of Alkyd Resins. J. Photochem. Photobiol. A: Chem. 2001, 139, 243–251. (20) Bloh, J. Z.; Dillert, R.; Bahnemann, D. W. Designing Optimal Metal-Doped Photocatalysts: Correlation between Photocatalytic Activity, Doping Ratio, and Particle Size. J. Phys. Chem. C 2012, 116, 25558−25562. (21) Rodrıguez-Talavera, R.; Vargas, S.; Arroyo-Murillo, R.; Montiel-Campos, R.; HaroPoniatowski, E. Modification of the Phase Transition Temperatures in Titania Doped with Various Cations. J. Mater. Res. 1997, 12, 439−443. (22) Aeroxide® TiO2 P25, Product information, http://www.novochem.ro/letoltes/aeroxide%20tio2%20p25%20en.pdf


Page 20 of 24

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Sharma, P. K.; Jilavi, M. H.; Burgard, D.; Nass, R.; Schmidt, H. Hydrothermal Synthesis of Nanosize α-Al2O3 from Seeded Aluminum Hydroxide. J. Am. Ceram. Soc. 1998, 81, 2732–2734. (24) Zhou, R.-S.; Snyder, R. L. Structures and Transformation Mechanisms of the η, γ and θ Transition Aluminas. Acta Cryst. 1991, B47, 617–630. (25) Grzmil, B.; Rabe, M.; Kic, B.; Lubkowski, K. Influence of Phosphate, Potassium, Lithium, and Aluminium on the Anatase-Rutile Phase Transformation. Ind. Eng. Chem. Res. 2007, 46, 1018–1024. (26) Kumar, S.; Verma, N. K.; Singla, M. L. Study on Reflectivity and Photostability of Aldoped TiO2 Nanoparticles and Their Reflectors. J. Mater. Res. 2013, 28, 521–528. (27) Weber, W. H. Raman Applications in Catalysts for Exhaust-Gas Treatment: In Raman Scattering in Materials Science; Willes H. Weber Roberto Merlin (Eds.), Springer: NewYork, 2000, 2036–237. (28) Xu, L.; Garrett, M. P.; Hu, B. Doping Effects on Internally Coupled Seebeck Coefficient, Electrical and Thermal Conductivities in Aluminum-doped TiO2. J. Phys. Chem. C 2012, 116, 13020–13025. (29) Spurr, R. A.; Myers, H. Quantitative Analysis of Anatase-Rutile Mixtures with an X-Ray Diffractometer. Anal. Chem. 1957, 29, 760–762. (30) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced Reactivity of Titanium Dioxide. Progr. Solid State Chem. 2004, 32, 33–177. (31) Gesenhues, U.; Rentschler, T. Crystal Growth and Defect Structure of Al3+-doped Rutile. J. Solid State Chem. 1999, 143, 210–218. (32) Periyat, P.; Baiju, K. V.; Mukundan, P.; Pillai, P. K.; Warrier, K. G. K. Aqueous Colloidal Sol–Gel Route to Synthesize Nanosized Ceria-doped Titania Having High Surface Area and Increased Anatase Phase Stability. J. Sol-Gel Sci. Technol. 2007, 43, 299–304. (33) Gesenhues, U. Doping of TiO2 Pigments by Al3+. Solid State Ionics 1997, 101-103, 1171– 1180. (34) Taylor, M. L.; Morris, G. E.; Smart, R. St. C. Influence of Aluminum Doping on Titania Pigment Structural and Dispersion Properties. J. Coll. Interf. Sci. 2003, 262, 81–88. (35) Biesinger, M. C.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887–898. (36) Akhtar, H. D.; Pratsinis, S. E.; Mastrangelo, S. V. R. Vapor Phase Synthesis of Al-doped Titania Powders. J. Mater. Res. 1994, 9, 1241–1249. (37) Crist, B. V. Handbooks of Monochromatic XPS Spectra. Volume 2 – Commercially Pure

Binary Oxides; XPS International LLC: Mountain View, 2005.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38) Pal, M.; Pal, U.; Gracia, J. M.; Jiménez, Y.; Pérez-Rodríguez, F. Effects of Crystallization and Dopant Concentration on the Emission Behavior of TiO2:Eu Nanophosphors. Nanoscale Res. Let. 2012, 7, 1–12. (39) Egerton, T. A. UV-Absorption – The Primary Process in Photocatalysis and Some Practical Consequences. Molecules 2014, 19, 18192–18214. (40) Pascual, J.; Camassel, J.; Mathieu, H. Fine Structure in the Intrinsic Absorption Edge of TiO2. Phys. Rev. B 1978, 18, 5606–5614. (41) Daude, N.; Gout, C.; Jouanin, C. Electronic Band Structure of Titanium Dioxide. Phys. Rev. B 1977, 15, 3229–3235. (42) Kernazhitsky, L.; Shymanovska, V.; Gavrilko, T.; Naumov, V.; Kshnyakin, V. Optical Absorption of Polydisperse TiO2: Effect of Surface Doping by Transition Metal Cations. Ukr. J. Phys. Opt. 2013, 14, 15–23. (43) Murphy, A. B. Band-Gap Determination from Diffuse Reflectance Measurements of Semiconductor Films, and Application to Photoelectrochemical Water-Splitting. Solar Energy Materials & Solar Cells 2007, 91, 1326–1337. (44) Islam, M. N.; Bredow, T.; Gerson, A. Electronic Properties of Oxygen-Deficient and Aluminum-doped Rutile TiO2 from First Principles. Phys. Rev. B 2007, 76, 045217. (45) Shirley, R.; Inderwildi, O. R.; Fraft, M. Electronic and Optical Properties of Aluminumdoped Anatase and Rutile TiO2 from Ab Initio Calculations. Phys. Rev. B 2010, 81, 075111. (46) Emeline, A.; Salinaro, A.; Serpone, N. Spectral Dependence and Wavelength Selectivity in Heterogeneous Photocatalysis. I. Experimental Evidence from the Photocatalyzed Transformation of Phenols. J. Phys. Chem. B 2000, 104, 11202–11210. (47) Serpone, N.; Emeline, A. V. Modelling Heterogeneous Photocatalysis by Metal-Oxide Nanostructured Semiconductor and Insulator Materials: Factors That Affect the Activity and Selectivity of Photocatalysts. Res. Chem. Intermed. 2005, 31, 391–432. (48) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. Explaining the Enhanced Photocatalytic Activity of Degussa P25 Mixed-Phase TiO2 Using EPR. J. Phys. Chem. B 2003, 107, 4545–4549. (49) Ohtani, B.; Preto-Mahaney, O.; Li, D.; Abe, R. What is Degussa (Evonik) P25? Crystalline Composition Analysis, Reconstruction from Isolated Pure Particles and Photocatalytic Activity Test. J. Photochem. Photobiol.: Chem. 2010, 216, 179–182. (50) Emeline, A. V.; Ryabchuk, V. K.; Serpone, N. Dogmas and Misconceptions in Heterogeneous Photocatalysis. Some Enlightened Reflections. J. Phys. Chem. B 2005, 109, 18515–18521. (51) Hanaor, D. A. H.; Assadi, M. N. H.; Li, S.; Yu, A.; Sorrell, C. C. Ab Initio Study of Phase Stability in Doped TiO2. Computational Mechanics 2012, 50, 185–194. ACS Paragon Plus Environment

Page 22 of 24

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract Graphic



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 24 of 24

Influence of the Dopant Concentration on the Photocatalytic Activity: Al

Recommend Documents