Interconnection between the Concentration of Dopant and Photocatalytic Activity: Al-doped TiO2

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Interconnection between the Concentration of Dopant and Photocatalytic Activity: Al-doped TiO₂

Introduction

structure titanium dioxide

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) powdered TiO2's 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

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 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 ?m 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 phase^23,24 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^oC. Our observation is in agreement with previous studies on influence of aluminum on the anatase-rutile phase transformation even at 750-800^oC.^25,26

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.

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 cross-section. Raman spectra show that in case of pristine TiO₂ sample the only phase determined is rutile: peaks at 611 cm^-1 (A_1g), 447 cm^-1 (E_g), and 143 cm^-1 (B_1g). 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 appearance of the bands at 146 cm^-1 (E_g1), 197 and 640 cm^-1 (E_g), 398 and 515 cm^-1 (B_1g), and 515 cm^-1 (A_1g) corresponding the anatase phase. The band at 143 cm^-1 associated with the B_1g vibrational mode of rutile is increased in intensity and shifted to 145 cm^-1, which belongs to the E_g vibrational mode of anatase. This also indicates that the content of the anatase phase is increased. It might be due to two simultaneous processes: the slump of the "rutile" band and the proliferation of the intense "anatase" band. For all x-Al-TiO₂ samples, there is no spectral evidence of the presence of Al-involving species vibrations.^27,28

We evaluate the anatase/rutile phase weight ratio (f_A/f_R) using the relation between the ratio of intensities (I_A/I_R) of the strongest reflection peaks for both phases (27.40^o for rutile and 25.22^o for anatase, see Figure A in the Supporting Information section). This relation is given by the expression²⁹:

where f_A is the mass fraction of the anatase phase and I_R and I_A are the intensities of the reflection of the (110) and (101) planes for the rutile and anatase phases, respectively. The estimated values of the weight fractions of both anatase and rutile phases are presented in the Table I. Note, that the fraction ratio f_A/f_R is directly proportional to the Al 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 formation.^14,30?32

Table I. Composition and morphological properties of synthesized x-Al-TiO₂ samples.

^* x_theor is the weight percentage of Al in TiO₂ samples was calculated with respect to the weight proportion of reagents.

^** x_exp 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).

^*** f_A and f_R are the mass fractions (in wt%) of the anatase (f_A) and the rutile (f_R) phases, respectively.

^**** The particle and the crystallite sizes (in ?m and nm, respectively) were evaluated from SEM images. Also the particle size (l) was calculated as following: l = 6/d_*S_BET, where d = 4.32 g/cm³ (density for rutile).

Figure 3. Dependences of the fraction ratio f_A/f_R (a) and of the specific surface area S_BET (b) on the Al concentration x(Al). f_A/f_R = (0.2 ??0.03)_*x, the correlation coefficient r is 0.87, the significance level P is 0.02. S_BET = (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).

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's (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 Ti2p_3/2 and Ti2p_1/2, 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

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).

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's 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. ??=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 ? 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 (?? ? 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's 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 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 week compared with indirect-allowed transition. Therefore, we estimated the bandgap energy values related to indirect-allowed transitions using the dependencies of [F(R)•h?]^1/2 on the photon energy (h?) 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(h?) ignoring the possible wavelength dependence of the scattering coefficient. Estimated values of bandgap energy (E_g) 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 (E_g) for synthesized x-Al-doped TiO₂ and rutile TiO₂ (R) samples.

It is seen from Table II that the bandgap energy values estimated from spectral data remain the same within the experimental error (E_g= 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 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).

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 as^46,47:

S_HQ = (dC_HQ/dt) / (dC_PhOH/dt) (t?0)

and

S_Cat = (dC_Cat/dt) / (dC_PhOH/dt) (t?0).

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 (l і 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.

Figure 7. The 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.

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%).

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 mark 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 anatase-rutile 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.

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 S_BET (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:

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.

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

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

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.

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 results^21,51 the XRD and Raman studies show that TiO₂ samples have mixed- rutile-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 results^14,15 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.

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