Toward a Physically Sound Structure−Activity Relationship of TiO2

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Toward a Physically Sound Structure-Activity Relationship of TiO2-Based Photocatalysts Joana T. Carneiro,† Tom J. Savenije,‡ Jacob A. Moulijn,† and Guido Mul*,† Catalysis Engineering, Opto-Electronic Materials, DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009

The morphological (analyzed by N2-Brunauer-Emmett-Teller, transmission electron microscopy, X-ray diffraction), surface chemical (analyzed by NH3-temperature programmed desorption), and opto-electronic properties (analyzed by time resolved microwave conductivity measurements (TRMC)) of nanoparticulate anatase TiO2 (Hombikat, Sachtleben) were modified by calcination at temperatures ranging from 200 to 800 °C. Upon calcination the primary particle size and the crystallinity increase, strongly influencing the performance in the degradation of methylene blue. It is striking that for the investigated set of samples the rate versus primary particle diameter goes through a minimum. This is explained by the equation of the kinetic rate constant, keff ) k[OH][h+]; [OH] decreases continuously as a function of increasing particle size, which is overcompensated by a strong increase in [h+] for particle sizes above ∼15 nm. The latter is the result of an increasing crystal quality of the anatase phase induced by calcination, as derived from the TRMC measurements. Implications for the rational design of improved photocatalysts are discussed. Introduction TiO2 is known as an efficient photocatalyst for several applications, in particular for the degradation of environmental pollutants present in water.1 A remarkably large number of papers have appeared in the literature, describing the activity of TiO2 of various morphologies, suppliers, and chemical modification in the decomposition of a multitude of aqueous phase pollutants and organic substrates. While the effect of process parameters on performance has been documented extensively,2 insight in the structure-activity relationship is limited. This is nicely illustrated by a recent study of Ryu and Choi,3 in which TiO2 samples obtained from various suppliers were compared in different photocatalytic reactions. Catalyst properties, such as surface area, OH-group density, crystallinity, and substrate adsorption affinity were discussed to influence the photocatalytic properties, but a clear correlation of these parameters with activity was not provided. Also other authors have suggested that the physicochemical properties, such as surface area, the crystalline phase (rutile/anatase) and quality,4-7 and amount of OH-groups at the catalyst surface4,8 determine the rate of photochemical processes at the semiconductor surface,9 but these parameters are typically discussed in a general context, without providing clear structure-activity trends. Of particular interest is the effect of particle size on photocatalytic activity of TiO2, which has been studied rather systematically.10-15 Unfortunately, the outcome of these studies is not evident. For example, Wang et al.14 found an optimum particle size of 11 nm for chloroform destruction in water, while Almquist et al.10 report an optimum effective particle size ranging from 25 to 40 nm in phenol photocatalytic oxidation. Both use a higher recombination probability of electron-hole pairs in nanoparticles due to charge proximity, to explain relatively low activity of TiO2 in the size range of 5-15 nm, whereas loss of surface * To whom correspondence should be addressed. Address: PhotoCatalytic Synthesis Group, Faculty of Science and Technology, University of Twente, Postbus 217, 7500 AE, Enschede. E-mail: [email protected]. † Catalysis Engineering. ‡ Opto-Electronic Materials.

area explains the decreasing activity above the optimized particle size. Differences in charge-carrier dynamics have also been proposed to affect the rate of surface photochemistry in other studies,7,16 in particular in relation to the promoting effect of metal nanoparticles, such as Au17 and Pt.18-21 Unfortunately, experimental evidence for changes in the opto-electronic properties for the samples under investigation is typically not provided in these studies. In this paper, we elucidate the contribution of various physicochemical and opto-electronic properties of TiO2 in determining photocatalytic activity. To this end, a high surface area, pure anatase TiO2 was calcined at different temperatures (T < 800 °C) for a limited time, ensuring the TiO2 remained in the anatase crystalline phase. The morphological changes induced by the heat treatment will be shown to have a strong influence on the catalytic activity in the methylene blue (MB) degradation, as well as on the opto-electronic properties of the materials determined by time-resolved microwave conductivity (TRMC) experiments. The effective rate constant is found to be predominantly determined by both the surface OH-group concentration, and the amount of holes available at the catalyst surface. A simple expression is proposed to describe the performance. Experimental Section Materials and Characterization. Hombikat UV 100 TiO2 was calcined at different temperatures from 200 to 800 °C with a heating rate of 10 K min-1 for 4 h in air. The materials obtained were named H, H200, H400, H500, H600, H700, and H800 according to the temperature of calcination used. Powder X-ray diffraction (XRD) was performed on a Phillips PW 1840 diffractometer equipped with a graphite monochromator using Cu-Ka radiation (λ ) 0.1541 nm). The Scherrer equation was used to calculate the crystal particle size of the samples. Transmission electron microscopy was performed using a Philips CM30UT electron microscope with a field emission gun as the source of electrons operated at 300 kV. Samples were

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Figure 1. XRD patterns of the five materials studied; anatase diffraction lines are denoted with A. TEM micrographs of the materials H800, H600, and H400 are shown in the inset.

mounted on a Quantifoil microgrid carbon polymer supported on a copper grid. 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 sample was pretreated at 125 °C and rapidly cooled to 100 °C and loaded with ammonia applying a flow of 30 mL min-1 for about 1 h. A helium flow of 30 mL min-1 was applied to remove weakly adsorbed NH3. A linear temperature program was started (100-600 °C at 10 K min-1) and the desorbed amount of ammonia was analyzed by TCD. The TPD spectra were used to analyze the amount of hydroxyl groups present on the surface of the catalyst sample. Nitrogen adsorption and desorption isotherms were recorded on a QuantaChrome Autosorb-6B at 77 K. Samples were previously evacuated at 350 °C for 16 h (at a ramp rate of 10 K min-1). The Barrett-Joyner-Halenda (BJH) model was used to calculate the pore size (in this case interparticle space) distributionfromtheadsorptionbranch,andtheBrunauer-Emmett-Teller (BET) method was used to calculate the surface area (SBET). TRMC Measurements. For the time-resolved microwave conductivity (TRMC) measurements a porous film of the catalysts was deposited by solvent (water) evaporation onto a 1 mm thick quartz plate. Because of the similarity between some samples a representative set is extensively discussed, H, H400, H500, H600, and H800. 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 nitrogen laser pulse at 337 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,(∆P(t)/P), is caused by a change of the conductance induced by the laser pulse, ∆G(t), which correlates with the product of the charge carrier formation and the sum of electron and hole mobilities, η∑µi. A full description of the microwave circuit and the data analysis is given elsewhere.22 The timeresolved 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 or recombination of charge carriers. The general decay shape is characterized by the halftime,τ1/2, defined as the period involved to reduce to half of its maximum value. Because of their higher mobility, electrons contribute far more to the photoconductance in TiO2

nanoparticles22 than positive charges and, therefore, ∑µi is assumed to be close to the electron mobility, µe. MB Degradation. Photocatalytic activity measurements were carried out in a combinatorial way. A home-build multireactor assembly was used, as described elsewhere.23 Flasks of 250 mL with pyrex glass covers were used as reactor vessels, in which the suspensions were agitated by a high-performance multiposition magnetic stirrer (IKA, RT10) with an equal stirring rate of 600 rpm. The 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 purged. The 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 (0.02 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 Avaspec-1024-UV/vis) was used to measure the absorbance spectra of the reaction mixture between 400-1000 nm at different reaction periods. From the changes in absorbance the apparent reaction rate constant (kapp) was determined.23 The correlation coefficients (R2) for obtaining kapp from experimental data were never below 0.970. Results Characterization. Figure 1 shows the XRD patterns of selected samples H, H200, H400, H600, and H800, respectively. These patterns show that all five samples consist of pure anatase. Even after calcination at 800 °C, XRD does not detect a phase transformation to rutile. Sharper and more intense diffraction lines are observed as the calcination temperature is increased, revealing a higher degree of crystallinity. The particle size of the crystals was calculated with the Scherrer equation and was correlated to the particle size observed in the TEM pictures shown in the inset of Figure 1. The TEM results also show that specific crystal planes, become apparent, however the crystal structure of these planes is difficult to assess on the basis of these micrographs. The results are summarized in Table 1. For the H800 material TEM pictures showed a mean particle size of 35 ( 10 nm and the value obtained by the Scherrer’s equation equals to ∼34 nm (Table 1). The values for H600 and H400 derived from the TEM pictures are 17 ( 7 and 8 ( 5 nm, respectively, again in good agreement with the calculated values on the basis of the XRD profiles.

Structure-Activity Relationship of TiO2-Based Photocatalysts

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TABLE 1: Surface Area, Surface Hydroxyl Group Concentration Obtained by NH3-TPD, Particle Diameter, Number of Hydroxyl Groups Per Particle, Product of Charge Carrier Formation and the Sum of Electron and Hole Mobilities (η∑µi), Halftime (τ1/2) at a Laser Pulse Intensity of 15.6 mJ cm-2 and 337 nm Wavelength, and kapp of the Materials Studied SBET 2

-1

(m g ) H H200 H400 H500 H550 H600 H650 H700 H750 H800 a

337 306 167 114 90 73 51 37 27 18

[OH] (mmol

dp

gcat-1)

1.15 0.92 0.69 0.50 0.41 0.34 0.25 0.18 0.13 0.08

-1

(nm)

(10 # particle )

7 9 13 15 16 19 23 26 29 34

0.32 0.49 1.26 1.80 2.15 3.05 4.80 6.09 7.74 9.78

Values for pulse intensity, I0, of 4 × 10 photons cm 12

ηΣµia

OHpp 3

-2

-3

(10

τ1/2

kapp

(µs)

(min-1)

0.7

0.1

0.7 6.8

0.3 1.0

13.8

>4.7

9.1

>4.7

0.032 0.026 0.022 0.018 0.028 0.071 0.081 0.086 0.086 0.083

2

cm V

-1

-1

s )

-1

pulse .

Figure 2. Number of OH-groups per particle, OHpp, and particle diameter, dp, plotted as a function of calcination temperature.

Table 1 also summarizes the most important morphological characteristics of the studied materials. The OH-group (OH) concentration, which can be assigned to the hydroxyl groups (OHads) and water adsorbed at the catalyst surface (determined by the NH3-TPD experiments), and the surface area are shown to decrease as a function of increasing temperature. The number of OH-groups per particle, OHpp, was calculated using the OHgroup concentration and the particle diameter according to the following equation, eq 1

OHpp(# particle-1) )

SBET AN [OH] p Av

Figure 3. TRMC transients obtained for the samples studied at the same laser pulse intensity, 15.6 µJ · cm-2 at 337 nm.

(1)

where SBET is the surface area of the material (m2 g-1), [OH] is the OH-group concentration determined by NH3-TPD (mol g-1), Ap is the surface area of the particle determined using the particle diameter (dp) and assuming spherical particles, and NAv is Avogadro’s number. These results clearly show that the number of OHpp increases as a function of increasing calcination temperature. Figure 2 graphically shows the trends in particle diameter (dp), and number of OH groups per particle (OHpp). The increase in particle diameter is exponential with an exponential factor of ∼0.0022. The number of OH-groups per particle follows a similar trend. TRMC measurements. In Figure 3 the conductance transients studied at a laser pulse intensity of 15.6 µJ cm-2 are shown. The maximum conductance value of Hombikat increases up to a calcination temperature of 500 °C and then decreases for higher calcination temperatures. The trend is as follows: H500 > H600 > H800 > H400 > H. Moreover, heat treatment results in a much slower decay of the signal, which translates

Figure 4. Dependence of the ηΣµ for the samples studied upon excitation at 337 nm on the average number of photons absorbed per nanoparticle 〈nep〉.

in higher half-life times. The signals extent far into the µs regime, time scales relevant for photocatalysis. The values of the two parameters determined by the TRMC measurements, (i) the maximum conductance value, ηΣµ and (ii) the half-life of the excess charge carriers, τ1/2, are summarized in Table 1. Figure 4 shows the ηΣµi values as a function of the average number of excitations per particle.22 The latter values were obtained by using a penetration depth of the incident light of

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Figure 6. Scheme summarizing morphological changes and electron diffusion in the materials (A) H, (B) H500, and (C) H800.

Figure 5. (a) Apparent reaction rate constants obtained for TiO2 samples of variable particle diameter; (b) OH-group concentration and maximum conductance values determined for TiO2 samples of variable particle diameter.

16.1 nm calculated from the absorption coefficient24 and optical density values at the used wavelength, as well as assuming a homogeneous excitation profile. A constant film porosity of 0.6 was assumed for all the samples.22 The pristine sample clearly shows the lowest ηΣµi value, which increases after calcination. For H and H400, a clear maximum for the ηΣµi value can be observed at 0.1 and at 1 photon pulse-1 particle-1, respectively. For the H500, H600, and H800 samples the ηΣµi values are significantly larger, in particular at low number of excitations per particle and moreover rather constant. MB degradation. The reaction rate constants calculated from the first order decay of the methylene blue concentration, are presented as a function of the average particle diameter of the applied catalysts in Figure 5a. Up to a particle diameter of ∼15 nm (obtained after calcination at 500 °C), the reaction rate constant decreases. After a minimum activity at these particle sizes, the activity strongly increases before leveling off. Figure 5b shows the trend in surface OH concentration (Table 1) as a function of particle diameter. An exponential decay is apparent. In Figure 5b also the maximum conductance values derived from the TRMC data (Table 1) are plotted as a function of particle size, assumed to be an indication for the quasi quantative trend in hole concentration, [h+]. The trend in [h+] is in good agreement with the strong increase in the value of the reaction rate constant (Figure 5a), observed for particle sizes in the range of 15-20 nm (Figure 5a). Discussion Characterization of the Samples. Heat treatment of Hombikat UV100, a pure anatase and high surface area TiO2, leads to significant morphological changes. Nanosized crystals become larger and it is expected that the crystal quality increases, that

is less defects or grain boundaries are present in the material. On the basis of the XRD patterns of Figure 1, extensive conversion of the anatase to rutile phase can be excluded. In agreement with the XRD results, a visible Raman spectrum of sample H800 recorded in our laboratory (not shown), does not contain any features of the rutile phase. The thermal stability of anatase has also been reported previously for other TiO2 catalysts.25 Furthermore, in a study of Li and co-workers26 it was demonstrated by a combination of UV-Raman and visible Raman spectroscopy that the anatase to rutile phase transformation is initiated in the core of particles, whereas the anatase phase in the surface region can remain at relatively higher temperatures. So, in the following discussion we assume the contribution of a rutile phase, or phenomena related to mixed phase compositions, to photocatalytic activity to be negligible. The morphological transformations induced by calcination are summarized in Figure 6. In this figure the higher amount of OH-groups per particle, OHpp, as a function of increasing calcination temperature is emphasized. This is important for further discussion of the results. Finally we would like to state that large changes in band gap energies and associated light absorption properties in the range of particle sizes investigated here, are not to be expected.16 Opto-Electronic Properties. Upon UV excitation of TiO2 nanoparticles, electron-hole pairs are formed. These charge carriers can, on a subnanosecond time scale,27-29 recombine or get immobilized on a trapping site (T). Since the TRMC measurements are limited by the response time of the cavity, 16 ns, these subnanosecond phenomena cannot be time-resolved. As shown in Figure 4 the dependencies of the ηΣµi values as a function of the number of excitations per particle for the H and H400 sample are similar and comparable to the values reported previously.22,30 The increase of the ηΣµi values up to a certain number of excitations per particle is related to the saturation of these electron traps in the TiO2 nanoparticles. At higher intensities subnano second electron-hole recombination occurs resulting again in lowering of the ηΣµi values. The maximum ηΣµi values for the H sample are somewhat lower than found for the H400 material, which is explained by enhanced scattering (i.e., confinement) of the electrons by the catalyst surface of smaller nanoparticles, which hinders their motion, and by a more effective screening of the microwaves.31,32 The shift of the maximum ηΣµi between the H and H400 samples is explained by the fact that the number of traps per particle in the H400 catalyst is higher. This is related to the fact that by calcination at 400 °C, the particle size increases with limited accompanying improved crystallinity. Therefore, an increasing number of trapping sites per particle can be expected. Most strikingly, for the H500 sample an order of magnitude increase in the ηΣµi values is observed at low number of excitations per particle. This is explained by the smaller number

Structure-Activity Relationship of TiO2-Based Photocatalysts

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of electron traps present in the core of the nanoparticle, which also explains the relatively constant value of the number of mobile electrons as a function of the number of excitations per particle. When the calcination temperature is further increased the samples behave as H500 with minor differences in maximum conductivity values. The important message is that the TRMC data indicate that the samples with particle sizes up to an average diameter of ∼15 nm are likely to contain a large concentration of trapping sites relative to the samples consisting of sizes above this value of ∼15 nm, as schematically shown in Figure 6. The conductance transients until 5 µs are shown in Figure 3 and it is clear that the noncalcined sample has the fastest decay with a τ1/2 of 0.1 µs. The decay is caused by the fact that conduction band electrons get predominantly trapped,33 or alternatively recombine with holes to the ground state. This implies that for the H sample a relatively small concentration of holes reach the photocatalytically relevant surface sites. The longest decay times were found for the H600 and H800 samples implying that a relatively large fraction of photogenerated holes can reach the surface. Since also the fraction of surface OHgroups per particle is higher for bigger particles, Figure 5, the holes have a high probability to react with surface hydroxyl groups to form hydroxyl radicals.4,21 This process lowers the electron-hole recombination flux, and thus enhances the lifetime of the conduction band electrons, which is in agreement with the increasingly longer lifetimes of mobile electrons observed for larger crystallite sizes. Toward a Structure Activity Relationship. In the present study, a direct measure of the opto-electronic properties (hole concentration) and surface OH-group concentration, discussed previously, can be used to explain the observed minimum in photocatalytic activity (see Figure 5a) as a function of particle size, as follows. Although surface chemistry is complex in photocatalytic processes, hydroxyl radicals are often considered to be the primary oxidizing species in photocatalytic oxidation reactions.34-38 Principally they are formed by reaction of holes with OH-groups. The apparent rate constant of methylene blue degradation can then generally be described by eq 2

and co-workers10 in the sense that the minimum activity at around 15 nm was not clearly resolved. In their study, the apparent photoactivity increases sharply as a function of increasing particle size until ∼25-30 nm, followed by a relatively small decay in rate above ∼40 nm. The optimum particle size for different materials ranging from ∼25-40 nm in the study of Almquist and Biswas, is more or less confirmed in the present study. We assume that above the largest particle sizes (∼34 nm) the performance will decrease in agreement with the data reported.10 The absence of the minimum in the data by Almquist, might be related to the model they used to predict the data. In fact, we can speculate that the trend observed here would also fit the data reported in Almquist,10 if surface OHgroup amounts and opto-electronic properties are considered to be similar to the values reported in our study. Besides hydroxyl radicals, also singlet oxygen has been proposed as an important oxidizing species in photocatalytic reactions over TiO2. It was demonstrated in a recent paper by Daimon and Nosaka40 that the quantum yield of singlet oxygen formation increases as a function of particle size in the range of 15-20 nm, similar to the size range where the conductance values largely increase as shown in Figure 5b. The formation rate of singlet oxygen is also expected to be depending on the [h+], since the reaction proposed for the generation of singlet oxygen is40

kapp ) k[h+][OH]

(2)

Up to a calcination temperature of 400 °C, there is no large increase in hole concentration, while the (total) number of surface OH-groups decreases (mmol g-1, Figure 5b). In these calculations, we assume that k is constant for all the samples despite the different crystalline quality. Apparently, the product [h+][OH] decreases for particles increasing in size from 7-15 nm, explaining the experimentally observed decrease in kapp. While upon calcination at even higher temperatures the total amount of OH-groups further reduces, it is concluded that this reducing amount is overcompensated for by a rapid increase in the availability of holes at the surface, [h+], in agreement with the TRMC measurements, demonstrating that the amount of holes that can reach the particle surface and oxidize the OHgroups is increasing.17,39 A threshold that categorizes the materials into two groups seems to exist: (i) the 7-15 nm range, where relatively poor opto-electronic bulk properties result in a strong dependency of the rate on the total OH-group concentration (a function of surface area); and (ii) the 15-35 nm range, in which the increasing crystal quality and favorable opto-electronic characteristics lead to a significantly larger surface [h+] concentration, and consequently a lower dependency of the rate on the concentration of surface OH-groups. The photocatalytic activity trend shown in Figure 5a is not in full agreement with the previously reported trend by Almquist

•

+ 1 O2 + h f O2

(3)

The superoxide anion radicals are formed by activation of surface oxygen by photoexcited electrons. Although we cannot exclude a role of singlet oxygen in the oxidation mechanism of methylene blue and in fact singlet oxygen formation might be a mechanism for reaction of the photoexcited electrons, the initially decreasing trend in reaction rate constant and apparent correlation with the surface OH-group concentration suggest that the role of OH-radicals is dominant. As a final note, the results of the present study suggest that a combination of a high OH-group concentration (available for small crystallites) in combination with a high crystallinity should provide for highly active catalysts. Perhaps the TiO2 nanotubes and other recently reported nanostructures with high activity confirm these requirements. Conclusions Calcination of pure anatase TiO2 not only leads to an increase in particle size, but also to higher crystallinity, that is particles with less defects and a lower overall surface OH concentration (mmol g-1). These transformations cause different photocatalytic activity of the materials in methylene blue degradation. The relation between particle size and activity was shown to be dominated by two different parameters: the concentration of OHgroups initially present at the catalyst surface, and the availability of holes. The reaction rate is dominated in the TiO2 particle size range of 7-15 nm by the concentration of OH-groups, while above 15 nm the significantly higher concentration of holes reaching the TiO2 surface, overcompensates for the decreasing concentration of OH-groups. Acknowledgment. We would like to acknowledge the X-ray facilities of the Faculty 3ME of the Delft University of Technology for the X-ray analyses. Indra Puspitasari of DCT/ NCHREM, Delft University of Technology, is acknowledged for performing the TEM measurements. S.T.W. (VIDI Project DPC.7065) is gratefully acknowledged for financial support.

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