How Phase Composition Influences Optoelectronic and Photocatalytic

ARTICLE pubs.acs.org/JPCC

How Phase Composition Influences Optoelectronic and Photocatalytic Properties of TiO2 Joana T. Carneiro,† Tom J. Savenije,‡ Jacob A. Moulijn,* and Guido Mul§ †

Catalysis Engineering, ChemE, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Opto-Electronic Materials, ChemE, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands § PhotoCatalytic Synthesis Group, Faculty of Science and Technology, University of Twente, Postbus 217, 7500 AE, Enschede, The Netherlands ‡

ABSTRACT: In the present study the ratio of rutile and anatase phases in sol-gel-synthesized TiO2 was varied by calcination at temperatures ranging from 500 to 900 °C. Changes in opto-electronic properties were analyzed by time-resolved microwave conductance measurements (TRMC) and evaluated by comparison of the photocatalytic activity. The presence of rutile improves the charge separation efficiency by trapping of positive charges at the rutile surface, as derived from the increased levels of conductivity and electron lifetimes. These phenomena result in a decrease in the effective hole concentration at the anatase surface, the TiO2 surface with the highest intrinsic reactivity, when in contact with rutile. Indeed, the presence of rutile results in inferior performance in the degradation of methylene blue and cyclohexane-selective photocatalytic oxidation. The negative effect of the presence of rutile can be compensated by improved morphological properties of the anatase phase, such as those present in P25. A novel structure-activity relationship is proposed and discussed.

’ INTRODUCTION The photocatalytic activity of TiO2 is dependent on the intrinsic properties, such as crystal phase, crystallinity/amount of defects, and specific surface area. 1-3 A high crystallinity of anatase, reducing the amount of electron traps which act as recombination centers for the electron-hole pairs, was shown to be a requirement for high photocatalytic activity.1 The OHgroup content at the surface also has an influence on the rates of electron-hole recombination.4,5 It should be noted that the OH concentration is related to the crystalline form and surface area.6 Recently, a structure-activity relationship was found for TiO2based photocatalysis by combining results of time-resolved microwave conductivity and photocatalytic activity measurements.1,2 Increasing the particle size and crystal quality by thermal treatment, avoiding phase transformations, increased photoactivity in methylene blue degradation, explained by extended lifetimes of photoexcited electrons and holes.7 There are several contradicting reports in the literature concerning the effect of the crystal form on catalyst performance. Anatase has been reported to be more active than rutile,8-11 but also rutile has been claimed to be most effective.12-14 Furthermore, it has been reported that titania containing small fractions of the rutile phase show enhanced photocatalytic activity compared to pure anatase, due to electron and hole transfer between the two phases, inducing enhanced lifetimes of electrons and holes.15-20 This charge separation is claimed to proceed when the two crystal phases are in intimate contact, so that the electrons formed in the rutile are trapped in the lower energy trapping sites of anatase, making the photoprocess more efficient.17,21 In materials r 2011 American Chemical Society

containing both phases such as Degussa P25, this explanation for high photocatalytic activity is generally posed. In this work we correlate changes in opto-electronic properties induced by the presence of rutile in TiO2 catalysts with performance in two different types of photocatalytic reactions: cyclohexane-selective photocatalytic oxidation (i.e., organic medium with no OH groups available) and methylene blue degradation (i.e., aqueous medium with OH groups available). The performance of Degussa P25 and Hombikat UV100 is also evaluated and discussed for comparison.

’ EXPERIMENTAL SECTION Photocatalyst Preparation. The samples were prepared by a modified sol-gel method based on thermal hydrolysis.22 Titanium(IV) butoxide (97%, Sigma-Aldrich) was added dropwise to a solution of 0.1 M HNO3. The temperature of the solution was subsequently increased to 80 °C in a period of 36 min, according to the following steps: 12 min at 40 °C, 12 min at 60 °C, and 12 min at 80 °C. The mixture was stirred for 8 h at this temperature and aged for 24 h. The pH of the mixture was around 1 during the entire procedure. Calcination at different temperatures (10 K min-1, 4 h) of the sol leads to materials with different anatase/rutile ratios. The materials are denoted as SG500, SG600, SG700, SG800, and SG900, according to the calcination temperature applied. For reference, two commercial Received: October 25, 2010 Revised: December 10, 2010 Published: January 7, 2011 2211

dx.doi.org/10.1021/jp110190a | J. Phys. Chem. C 2011, 115, 2211–2217

The Journal of Physical Chemistry C

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TiO2 catalysts were included in the study: Hombikat UV100 (Sachtleben), pure anatase and P25 (Degussa), a mixture of 20% rutile and 80% anatase, referred to as H and P25, respectively. The H sample was calcined at different temperatures ranging from 200 and 800 °C (10 K min-1, 4 h), and the materials obtained were named H200, H600, H700, and H800, according to the calcination temperature used. Characterization. The Brunauer-Emmett-Teller (BET) surface area of the samples was obtained by N2 physisorption at 77 K in a Quantachrome Autosorb 6B apparatus. Before the measurements, the samples were pretreated in vacuum at 110 °C for 16 h and subsequently cooled to liquid N2 temperature. The X-ray diffraction (XRD) patterns were recorded in a Philips PW1840 X-ray diffractometer using Cu KR radiation at a scanning rate of 2θ = 0.01° s-1 and used to identify the crystal phase and their corresponding crystallite size. The relative abundance of anatase to rutile in the samples was calculated using the following equation23 XR ¼

1:26I R I A þ 1:26I R

ð1Þ

where XR is the rutile fraction and IR and IA are the strongest intensities of the rutile (110) and anatase (101) diffraction angles, respectively. The crystal sizes (D) of anatase and rutile were determined by the Scherrer equation D ¼

Kλ β cos θ

ð2Þ

where λ is the wavelength of the Ni-filtered Cu KR radiation used (λ = 0.1541 nm), β the full width at half-maximum of the diffraction line considered, K a shape factor (0.9), and θ the angle of diffraction. Transmission electron microscopy was performed using a Philips CM30UT electron microscope with a FEG (field emission gun) as the source of electrons operated at 300 kV. Samples were mounted on a Quantifoil microgrid carbon polymer supported on a copper grid. IR absorption spectra of the samples were recorded using a Bruker IFS66 spectrometer with a DTGS detector and Spectratech Diffuse Reflectance Accessory equipped with a hightemperature cell. The KBr spectrum at 120 °C under 20 mL min-1 He flow was used as background. Water was removed from the catalyst surface to facilitate characterization of the OH-group composition by recording spectra at 120 °C after 1 h equilibration in He flow (20 mL min-1). All spectra were recorded by collecting 128 scans with a 4 cm-1 resolution. Temperature-programmed desorption of ammonia (NH3-TPD) was carried out on a Micrometrics TPR/TPD 2900 apparatus equipped with a thermal conductivity detector (TCD). Approximately 25 mg of TiO2 was flushed with helium at 773 K for 1 h (at a heating rate of 10 K min-1), except for the sample activated at 398 K, which was pretreated at this temperature in the ammonia TPD setup. After pretreatment, the sample was rapidly cooled to 373 K and loaded with ammonia, applying a flow of 30 mL/min for about 60 min, after which a helium flow of 30 mL min-1 was applied to remove weakly adsorbed NH3. A linear temperature program was started (373-873 at 10 K min-1 ), and the desorbed amount of ammonia was analyzed by the TCD. The

TPD spectra were used to determine the amount of hydroxyl groups (COH in mmol gcat-1) present on each catalytic material. Thermogravimetric analysis (TGA) of the catalysts was carried out on a TGA/SDTA851e thermobalance (MettlerToledo). The sample powders were heated in He from 298 to 1173 K at heating rate of 10 K min-1. The TGA spectra were use to determine the temperature-dependent mass loss of the materials, giving an indication of the strong interaction of some molecules with the catalytic surface. Optoelectronic Measurements. For the time-resolved microwave conductance (TRMC) measurements a porous film of the catalysts was deposited by solvent (water) evaporation onto a 1 mm thick quartz plate. The thickness of the films, as determined by SEM, was approximately 27 μm. The TRMC technique is based on the measurement of the normalized change of microwave power reflected by a sample after illumination by a Coherent Infinity Nd:YAG laser pulse at 300 nm. The intensity of the laser pulse was varied using a set of metallic neutral density filters. The normalized change in microwave power reflected by the sample, ΔP(t)/P, is caused by a change of the conductance induced by the laser pulse, ΔG(t), which correlates with the product of the charge carrier formation efficiency and the sum of electron and hole mobilities, ηΣμi. The response time of the cavity, where the sample is place, is 18 ns. A full description of the microwave circuit and data analysis are given elsewhere.24 The time-resolved microwave signal obtained after the laser pulse can be characterized by two stages. Up to approximately 30 ns, the signal is dictated by the instrumental response time. After this initial stage, the signal decays due to trapping and/or recombination of charge carriers. As the signal decay is not exponential, the general decay shape is characterized by the halftime, τ1/2, defined as the period involved to reduce to one-half of its maximum value. Due to their higher mobility, electrons contribute far more to the photoconductance in TiO2 nanoparticles24 than positive charges, and therefore, Σμi is assumed to be close to the electron mobility, μe. Methylene Blue Degradation. Methylene blue (MB), Merck (97%), was used without further treatment. Photocatalytic activity measurements were carried out in a combinatorial way. A homebuilt multireactor assembly was used and is described elsewhere.25 Flasks of 250 mL with Pyrex glass covers were used as reactor vessels, in which the suspensions were agitated by high-performance multiposition magnetic stirrers (IKA, RT10) with an equal stirring rate of 600 rpm. UV irradiation was provided by 8 blacklight tubes (18 W, Philips) located 20 cm above the liquid level. During the reaction, the reactor housing was continuously aerated. Temperature was controlled to be at 305 ( 2 K by water flow through the cooling coil at the back of the reactor housing. The TiO2 powder was mixed for 2 h with 100 mL of methylene blue solution (0.03 mmol L-1) in the dark to ensure equilibrium adsorption. The samples were filtered through 0.45 mm PTFE Millipore membrane filters to remove suspended titania agglomerates. A UV-vis spectrophotometer (Avantes Avaspec1024-UV/vis) was used to measure the absorbance spectra of the reaction mixture between 400 and 1000 nm at different reaction periods. From the changes in absorbance the apparent first-order reaction rate constant (kapp) was determined. For all the materials three independent equivalency runs were performed, and the relative error between the highest reaction rate constant and the lowest was within 5%. Cyclohexane-Selective Photocatalytic Oxidation. To evaluate catalyst performance in the selective oxidation of cyclohexane, reactions were carried out in a top illumination reactor 2212

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Table 1. Morphological Properties Calculated from the XRD Patterns, Surface Area, and OH-Group Concentration of the Materials Studied, Including the Commercially Obtained H and P25 dp (nm) (A)

dp (nm) (R)

XR (%)

SBET (m2 g-1)

COH (mmol gcat-1)

SG500

7

0

0

139

0.44

SG600

10

0

0

96

0.30

SG700

21

19

7

74

0.17

SG800

34

34

13

15

0.05

SG900

51

40

58

2

0.01

P25 H

19 7

40 0

28 0

51 337

0.36 1.10

Figure 1. Crystal characterization. (a) XRD patterns of the materials in the study with peak assignments: A denotes anatase and R rutile. (b) TEM micrographs of the materials SG500, SG700, and SG800.

(TIR) described elsewhere.5 The catalysts were dried for 1 h at 120 °C to remove adsorbed water and impurities. In a typical experiment 100 mL of cyclohexane containing 1 g L-1 of catalyst was used (slurry system). The solution was illuminated from the top of the reactor through a Pyrex window that cut off the highly energetic UV radiation.26 A high-pressure mercury lamp of 50 W was used (HBO50W from ZEISS). The light intensity of the lamp used in the wavelength absorption range of TiO 2 (275-388 nm) is 55 mW cm-2. Air presaturated with cyclohexane, dried over Molsieve 3 Å, (Acros Organics), was bubbled through the TiO2 suspension at a rate of 30 mL min-1. During the reaction liquid was withdrawn and analyzed by GC. Organic compounds were quantitatively analyzed twice using a gas chromatograph with a flame ionization detector (Chrompack, CPwax52CB). Hexadecane was used as an internal standard.

’ RESULTS Catalyst Characterization. In Figure 1a the XRD patterns of the synthesized materials are shown, with the corresponding crystal peak assignation. The TEM pictures from three materials are given in Figure 1b, where the crystal sizes are depicted. From this figure is not possible to distinguish between the rutile and the anatase crystals, but it is clear that in the case of SG700 the average particle size is large with the coexistence of large and small particles. In Table 1 an overview of the values determined from the XRD patterns for crystal size and rutile percentage are given together with the surface area (SBET) and OH-group concentration (COH) determined by NH3-TPD. In samples SG500 and SG600 only anatase and some brookite ( SG600 > SG700 > SG800 > SG900. Since TGA profiles correlate nicely with NH3-TPD values we can use them as an estimation of the total OH-group concentration.2 The amount of OH groups of SG500, the highest surface area synthesized material in this study, is approximately one-half of the value of the commercial pure anatase material, H, with the same particle size of ∼7 nm, see Table 1. IR spectra of the samples are shown in Figure 3 for the region between 4000 and 2800 cm-1 where the absorptions assigned to O-H stretching modes appear. After heating up the samples from room temperature to 120 °C in He, the amount of adsorbed water on the catalyst surface decreases, allowing a better evaluation of the nature of the different types of OH groups present. The complicated spectral signature is the result of OH being 2213



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Table 2. Optoelectronic Properties ((ηΣμi)max and τ1/2) and Methylene Blue Degradation Reaction Rate Constant of the Materials Studied (ηΣμi)maxa (10-3 cm2 V-1 s-1)

a

τ1/2 (ns)

kapp (min-1)

SG500

0.3

59

SG600

0.8

62

0.021 0.021

SG700

2.0

81

0.0090

SG800

5.6

125

0.0048

SG900

3.0

100

0.0030

I0 = 1.05 1014 photons cm-2 pulse-1.

Figure 3. High-wavenumber region of the IR spectra, with the respective peak positions indicated for the different types of OH groups.

Figure 5. Dependence of the maximum conductivity, ηΣμi, as determined by the TRMC transients, on the laser pulse intensity, I0.

Figure 4. Incident normalized conductance transients obtained for the samples studied at the same laser pulse intensity, 164 μJ cm-2, at 300 nm.

present on different defect sites as well as the result of contributions of both rutile and anatase phases.27 The bands at 3720 cm-1 assigned to an isolated anatase OH vibration and those at 3630 and 3670 cm-1 assigned to bridging OH groups correspond to OH stretching modes in anatase.28-31 The absorption bands for O-H stretching modes representing rutile are located at 3650 and at 3410 cm-1, respectively.27 For the band at 3410 cm-1 an assignment has been proposed to water molecules strongly adsorbed on TiO2 (rutile) via interactions with coordinatively unsatured Ti4þ surface cations.27 The latter bands are present in the spectra of the SG700, SG800, and SG900 catalysts. The band at 3410 cm-1 is shifted to lower wavenumbers for the SG900 material, to 3270 cm-1. The band at 3270 cm-1 present in the SG900 material corresponds to chemisorbed water at the TiO2 surface.32 This band is more clearly resolved in SG900 because of the lower contribution of the broad water band centered at 3375 cm-1. Optoelectronic Measurements. In Figure 4 the photoconductance transients studied at a laser pulse intensity of 164 μJ cm-2 are shown. The maximum conductance value of the materials increases up to a calcination temperature of 800 °C and then decreases following the order SG800 > SG900 > SG700 > SG600 > SG500. The same order is observed for the rate of decay of the

signal expressed as the half-life time, Table 2. The signals extend far into the microseconds regime, time scales relevant for photocatalysis. To facilitate comparison of the photoconductance signals obtained at various laser pulse intensities for different samples, the maximum of the conductance (ΔG)max, corresponding to the maximum values in Figure 4, can be converted to the product ηΣμi.4 Figure 5 shows the ηΣμi values as a function of the laser intensity. The SG500 sample has the lowest ηΣμi values, and the signal decreases with the intensity. The other samples exhibit a maximum. The signal intensity increases in the order SG500 < SG600 < SG700 < SG900 < SG800. For the SG700, SG800, and SG900 samples the dependence of ηΣμi as a function of the laser intensity resembles the values observed previously for other calcined titania nanoparticles with similar diameters24,33 and can be interpreted as follows. The increase with the laser pulse intensity of the ηΣμi values is related to the saturation of electron traps in the TiO2 nanoparticles. On increasing the light intensity of the laser pulse up to the maximum value increasingly more traps become populated, yielding a gradually higher ratio of mobile electrons vs trapped electrons, explaining the increase in ηΣμi. At higher intensities subnanosecond second-order electron hole recombination occurs, resulting in lowering of the ηΣμi values. The lower ηΣμi values in SG500 and SG600 can be explained by enhanced scattering (i.e., confinement) of the electrons at the surface of smaller nanoparticles, which hinders their motion, or by a more effective screening of the microwaves as suggested before.34,35 Photocatalytic Activity. First, the catalysts were tested in the aqueous phase methylene blue degradation reaction. The concentration decay with time is depicted in Figure 6 in the commonly applied normalized form (i.e., divided by the initial 2214



Figure 6. Methylene blue photocatalytic degradation time profiles for the materials studied.

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Figure 8. Cyclohexanone and cyclohexanol production in the selective oxidation of cyclohexane at reaction time t = 300 min as a function of anatase particle size dp (A) for the synthetized materials: SG500, SG600, SG700, SG800, and SG900.

converge to a plateau at early stages in the reaction (∼ 20 min).36 The cyclohexanone and cyclohexanol amounts produced over the catalysts at 5 h reaction time with increasing particle size and rutile content are depicted in Figure 8. For all the samples a high ketone/ alcohol ratio was obtained, as expected.36 The cyclohexanone production remains constant in the range of (anatase) particle sizes from 7 to ∼10 nm, when rutile is not present, in agreement to what was reported before.2 When an increasing amount of rutile is present, indicated in the figure, productions approach zero. Cyclohexanol formation follows a similar trend, although an increase is apparent in the range of (anatase) particle sizes from 7 to ∼10 nm. Figure 7. Correlation between the apparent first-order rate constant of methylene blue degradation with the anatase particle diameter, dp (A); the materials of pure anatase with different particle sizes, reported elsewhere,1 are also represented together with the mixed-phase TiO2, P25.

methylene blue concentration after adsorption equilibrium in the dark). The values of kapp (min-1) calculated from these trends are depicted in Figure 7. The kapp values of SG500 and SG600 are similar to but somewhat lower than the values found for the materials H and H200, with an anatase particle size of 7 and 10 nm, respectively.1 The slight difference between these values can be attributed to the different surface area, which is higher for the H and H200 materials and dependent on the synthesis procedure. At higher anatase particle sizes the kapp values of the samples containing rutile decrease with increasing rutile percentage, as indicated in Figure 7, in the following order SG700 > SG800 > SG900. On the contrary, when rutile is not present, the activity of the pure anatase TiO2 increases with the particle size until a plateau is reached, H600 < H700 ≈ H800. The P25 material apparent first-order rate constant is also indicated in Figure 7 at the corresponding anatase particle size ∼20 nm. The materials were also tested in the cyclohexane-selective photocatalytic oxidation, representing an organic phase reaction. In this reaction the reactivity of the catalyst is strongly affected by the amount of OH groups initially present at the catalyst surface.5 The main products of this reaction are cyclohexanone and cyclohexanol, and it is well known that the catalyst in this reaction suffers from deactivation because of the formation of stable surface species such as carbonates and carboxylates. As a consequence, the production patterns of cyclohexanone and cyclohexanol start to

’ DISCUSSION Material Characterization. Titania, with variable rutile percentage, was prepared by calcination at different temperatures of a sol-gel parent material. The heat treatment increased particle size and rutile percentage of the materials as shown in Table 1. The surface water and hydroxyl groups quantity of the different catalysts decreased accordingly, due to the loss of surface area, Table 1. The decreasing amount is similar for the weakly (Tr = 72 °C) and strongly (Tr = 220 °C) adsorbed water. From the IR spectra presented in Figure 4, it is clear that the absorption bands assigned to the OH stretching modes in rutile are present in samples SG700, SG800, and SG900, confirming the presence of this crystalline phase in these materials. The intensity of these peaks becomes lower with increasing pretreatment temperature, due to the induced loss of surface area. The rutile-anatase composition and interaction between phases within TiO2 particles is extremely important to understand to allow interpretation of photoelectronic phenomena. Morphological changes were investigated recently by the combination of UV and visible Raman techniques. Zhang et al. presented a model for phase transformation of TiO2 with increasing calcination temperature.37,38 Their studies showed that the interfaces of contacting anatase particles provide the nucleation sites for transformation to the rutile phase. The main conclusion of their work was that the phase transformation temperature of anatase to rutile at the surface of these agglomerates occurs at higher temperatures since they are not in contact with other particles where the phase transformation takes place.37,38 The surface phase is very important to know because it is responsible for the TiO2 photoactivity since not only do photoinduced reactions take place on the surface9 but 2215


The Journal of Physical Chemistry C also electrons and holes migrate through the surface region. Although we used only XRD to characterize the crystal phase of our materials (i.e., mostly characterization of the agglomerates forming the bulk phase), the strong effect of surface rutile phase can be discussed in view of the activity values found in the following section. Interpretation of the results here presented are, thus, based on the model that rutile-anatase configuration consists of individual particles of either anatase or rutile and do not coexist in an individual particle, i.e., there is a defined boundary between the two phases. Structure-Activity Relationship. Time-resolved conductance measurements on P25 reveal that this material has a higher photoconductivity and longer electron lifetimes than other commercial samples, such as H.16 Interestingly, P25 has higher activity values in wastewater degradation type reactions, such as methylene blue degradation, when compared to H, with kapp values of ∼0.03 min-1 for H and ∼0.05 min-1 for P25. The reason for this better performance is normally attributed to the presence of the rutile phase in P25.16,39 Furthermore, it is concluded that the combination of both phases, rather then rutile itself, enhances charge separation, since a rutile single crystal has much lower electron lifetimes when compared to anatase, predicting a decrease, but not so strong in activity.40 Previous work done in our group showed that for pure anatase materials, the photoconductivity and electron lifetimes increase with increasing particle size.1 These materials properties correlated with higher activities in methylene blue degradation for anatase particle size larger than 15 nm. The results obtained in this work lead us to a different interpretation regarding the role of the rutile-anatase ratio in photocatalysis. On comparing kapp of P25, indicated in Figure 7, with the one of anatase particles with ∼20 nm the value of the latter is substantially larger. The presently found activity values strongly suggest that the presence of rutile decreases the activity of the material: for the same anatase particle size, the activity of the calcined H samples, which do not contain rutile phase, is higher than the analogous SG samples that do contain the rutile phase. The results are in agreement with a study of Ding et al., who showed that compared to rutile, anatase contains more adsorbed water and hydroxyl groups, explaining its higher activity for phenol oxidation in water.6 Furthermore, a correlation between the number of a specific anatase Ti-OH group, yielding an IR absorption at 3635 cm-1, and the methylene blue degradation rate was determined in a previous study, also confirming the predominant role of anatase in the photocatalytic performance of TiO2 (27). TRMC measurements, performed on the SG materials, show that upon increasingly more rutile phase, up to at least 13%, the maximum photoconductivity and electron lifetime values increase (Table 2). This is in agreement with the increase of the particle size with calcination temperature. The crystal quality of the SG materials is rather similar, since the maximum in the ηΣμi vs laser intensity occurs at almost the same laser pulse intensity for all the samples. The relatively low ηΣμi values for the SG900 material are attributed to the predominance of the rutile phase in this material, being more than 50%, with relatively low electron mobility. From the decay it is apparent that the electron hole pairs formed in rutile particles recombine more rapidly than in anatase. This is in agreement with a study by Colbeau-Justin et al., who also studied the influence of the composition (anatase, rutile) on charge carrier lifetimes in TiO2 powders by timeresolved microwave conductance.40 In their work very short electron lifetimes were found for rutile particles when compared with anatase, explained by hole trapping by the surface OH

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groups in the anatase, increasing the electron lifetime. Since experimental investigations on well-defined anatase TiO2 surfaces are relatively scarce because single anatase TiO2 crystals of sufficiently large size are difficult to grow,41 the mechanism for the presence or formation of more OH groups on the surface of anatase is unfortunately far from understood. It is striking that for the SG series the activity in methylene blue degradation decreases with increasing rutile content, although the crystal quality increases in the same order, predicting an increase in activity as observed previously.1 The above observations can be explained by comparison with P25. Since P25, although containing a higher rutile percentage of 20%, has a higher catalytic activity than all the materials derived from sol-gel TiO2 containing rutile, it is known that the crystal quality of P25 is much higher than the one presented by SG700 and SG800. This is confirmed by higher conductivity and longer electron lifetimes reported for this material compared to the SG series.16 Considering that there are well-defined phases of rutile and anatase particles, electronic contact between both phases leads to efficient separation of the electron-hole pair. Since this separation however does not enhance photocatalytic activity, as concluded from the present work, the following model is evoked, which has been proposed previously for n-type semiconductor materials.42 The valence band edge of rutile is higher in energy than the one of anatase.43 Therefore, on charge carrier generation by UV excitation in anatase the hole can migrate toward the rutile phase, resulting in longer lifetimes of the conduction band electrons residing in the anatase phase. In the models generally accepted it is assumed that the charge separation induced by the rutile leads to a more efficient photocatalytic process since the hole can oxidize surface OH groups on the rutile particle and the electron can reduce O2 in the anatase material. However, the amount of OH groups in the rutile particle is far less as compared to anatase as proposed before.6,44 Accordingly, although the presence of rutile increases the charge carrier separation efficiency, this enhancement is not favorable for aqueous liquid phase photocatalytic reactions. Hence, the photocatalytic efficiency, expressed in kapp, of a material containing rutile is expected to decrease since the OH-group density is lower and the electronhole pairs formed upon excitation in the rutile phase are not being used to initiate the surface photoprocesses. This comes into agreement with the dependency of the reaction rate constant with the hole and hydroxyl group concentration at the catalyst surface previously reported.1 Rutile in Cyclohexane Photo-oxidation. Cyclohexaneselective photocatalytic oxidation is a reaction with a different nature than wastewater degradation-type reactions as already discussed.2 From the activity results shown in Figure 8 for the sol-gel materials prepared, it is clear that the presence of rutile is highly detrimental. Not only does cyclohexanone production decrease but also cyclohexanol, when this crystal phase is present. The decrease of the rate of cyclohexanone production is not surprising and confirms that holes diverted from the anatase phase to the rutile phase are inactive in photo-oxidation reactions. From previous results it was concluded that an increase in anatase particle size correlates with an increased cyclohexanol production.2 This was also observed when comparing the pure anatase samples in this study, SG500 and SG600, where cyclohexanol increases with increasing anatase particle size. However, the anatase particle size increases for other materials, SG700, SG800, and SG900, but cyclohexanol formation reduces. This observation is in line with the model proposed here, where the 2216


The Journal of Physical Chemistry C hole diffuses to the rutile and is lost for catalytic action. If the increase in photoconductance would have the same origin as the one reported for larger anatase particles,1 then it would be expected that cyclohexanol formation would also increase, which is not the case. In this way the reaction initial step, cyclohexyl radical formation, is negatively affected and cyclohexanol and cyclohexanone formation drops drastically.45,46

’ CONCLUSIONS The thermally induced formation of rutile was shown to be detrimental to the photocatalytic performance, in methylene blue degradation and cyclohexane-selective photocatalytic oxidation, of the TiO2 materials showed in this study. Although the presence of rutile improves charge separation, the effective hole concentration at the anatase surface is reduced. In this way the photocatalytic process cannot initiate, and the overall result is activity loss. The relatively high photocatalytic performance of P25 is attributed to the anatase particle size and crystal quality rather than to the presence of rutile.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We would like to acknowledge the X-ray facilities of the 3ME Department, Faculty of Applied Sciences, of the Delft University of Technology for the X-ray analyses. STW (VIDI Project DPC.7065) is gratefully acknowledged for financial support. ’ REFERENCES (1) Carneiro, J. T.; Savenije, T. J.; Moulijn, J. A.; Mul, G. J. Phys. Chem. C 2010, 114 (1), 327–332. (2) Carneiro, J. T.; Almeida, A. R.; Moulijn, J. A.; Mul, G. Phys. Chem. Chem. Phys. 2010, 12 (11), 2744–2750. (3) Herrmann, J. M. Catal. Today 1999, 53 (1), 115–129. (4) Carneiro, J. T.; Savenije, T. J.; Mul, G. Phys. Chem. Chem. Phys. 2009, 11 (15), 2708–2714. (5) Carneiro, J. T.; Yang, C. C.; Moma, J. A.; Moulijn, J. A.; Mul, G. Catal. Lett. 2009, 129 (1-2), 12–19. (6) Ding, Z.; Lu, G. Q.; Greenfield, P. F. J. Phys. Chem. B 2000, 104 (19), 4815–4820. (7) Fujishima, A.; Zhang, X. T.; Tryk, D. Surf. Sci. Rep. 2008, 63 (12), 515–582. (8) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95 (1), 69–96. (9) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95 (3), 735–758. (10) Nagaveni, K.; Sivalingam, G.; Hedge, M. S.; Madras, G. Appl. Catal., B 2004, 48 (2), 83–93. (11) Sclafani, A.; Herrmann, J. M. J. Phys. Chem. 1996, 100 (32), 13655–13661. (12) Habibi, M. H.; Vosooghian, H. J. Photochem Photobiol., A 2005, 174 (1), 45–52. (13) Mills, A.; LeHunte, S. J. Photochem Photobiol., A 1997, 108 (1), 1–35. (14) Watson, S. S.; Beydoun, D.; Scott, J. A.; Amal, R. Chem. Eng. J. 2003, 95 (1-3), 213–220. (15) Datye, A. K.; Riegel, G.; Bolton, J. R.; Huang, M.; Prairie, M. R. J. Solid State Chem. 1995, 115 (1), 236–239. (16) Emilio, C. A.; Litter, M. I.; Kunst, M.; Bouchard, M.; Colbeau-Justin, C. Langmuir 2006, 22 (8), 3606–3613. (17) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107 (19), 4545–4549.

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