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J. Phys. Chem. B 2001, 105, 1026-1032
Preparation Characterization and Photocatalytic Activity of Polycrystalline ZnO/TiO2 Systems. 1. Surface and Bulk Characterization Giuseppe Marcı`,† Vincenzo Augugliaro,† Marı´a J. Lo´ pez-Mun˜ oz,‡ Cristina Martı´n,§ Leonardo Palmisano,*,† Vicente Rives,§ Mario Schiavello,† Richard J. D. Tilley,| and Anna Maria Venezia⊥ Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniVersita` degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy, Escuela Superior de Ciencias Experimentales y Tecnologı´a, Campus de Mo´ stoles, UniVersidad Rey Juan Carlos, c/Tulipa´ n, 28933 Mo´ stoles, Madrid, Spain, Departamento de Quı´mica Inorga´ nica, UniVersidad de Salamanca, 37008 Salamanca, Spain, School of Engineering DiVision of Materials Engineering, UniVersity of Wales, Cardiff, Wales, CF2 1XH U.K., and ICTPN-CNR, Via Ugo La Malfa 153, 90100 Palermo, Italy ReceiVed: September 5, 2000; In Final Form: October 28, 2000
Polycrystalline ZnO/TiO2 solids have been prepared with four different methods using home prepared TiO2 (anatase) or TiO2 (rutile) as supports and Zn(NO3)2‚6H2O or Zn(CH3COO)2‚2H2O as precursors for ZnO. The bulk and surface properties of the samples were investigated by means of TG-DTA, XRD, TEM, SEMEDAX, XPS, BET surface area determination, and porosity measurements. XRD and TEM results indicate that no significant defect structures exist in any of the samples. The ZnO crystallinity and its enrichment on the surface of TiO2 particles were dependent on the preparation method. The surface areas generally decrease by increasing the amount of ZnO except when ZnO from Zn(CH3COO)2‚2H2O was supported on TiO2 (rutile). The samples prepared from Zn(CH3COO)2‚2H2O were more porous than those prepared from Zn(NO3)2‚ 6H2O. This was confirmed by BET surface area determinations and SEM observations. XPS spectra indicate that the atomic ratio between OH- and O2- on the particles surface is similar for samples with the same ZnO content independent of the precursor used for the samples’ preparation. Moreover a much higher segregation of ZnO was found for samples obtained by using the acetate precursor.
Introduction Polycrystalline TiO2 and ZnO, both in the pure form or as a mixture, are semiconductor oxides widely used in photocatalytic reactions in liquid-solid as well as in gas-solid regimes.1-9 A photocatalytic process is based on the generation electronhole pairs by means of band-gap radiation which can give rise to redox reactions with species adsorbed on the surface of the catalysts. The allotropic anatase phase of TiO2 is more efficient than the rutile one but sometimes less efficient than ZnO.5,6 Rutile TiO2 has been shown to be negligibly photoactive for the photooxidation of many organic pollutants in aqueous medium, but its resistance to disaggregation is higher than that of anatase TiO2 and ZnO. In principle the coupling of anatase and rutile TiO2 with ZnO seems useful in order to achieve a more efficient electronhole pair separation under illumination and, consequently, a higher reaction rate. A large variety of coupled polycrystalline or colloidal semiconductor systems, in which the particles adhere to each other in so-called “sandwich structures” or present a “coreshell geometry”, have been prepared and used for many photocatalytic reactions.10-26 Typical examples of such couples * To whom correspondence should be addressed. Phone: ++39-0916567246. Fax: ++39-091-6567280. E-mail:
[email protected]. † Universita ` degli Studi di Palermo. ‡ Universidad Rey Juan Carlos. § Universidad de Salamanca. | University of Wales. ⊥ ICTPN-CNR.
are ZnO/ZnS,14 ZnO/ZnSe,15 CdS/ZnS,10-13 CdS/TiO2,16 SnO2/ TiO2,15 and WO3/WS2.25,26 The increase of the lifetime of the photoproduced pairs, due to hole and electron transfer between the two coupled semiconductors, is invoked in many cases as the key factor for the improvement of the photoactivity. Nevertheless, it should be considered that photoactivity also strongly depends on bulk and surface physicochemical properties of the photocatalysts, such as the kind of phases, the surface hydroxylation, the porosity, the surface area, the adsorption capacity, the distribution of the supported photoactive component, and the surface acid-base properties. Consequently, useful information related to the photoactivity of the photocatalysts can be obtained by thorough characterization with the classical tools used in thermal catalysis. In this work polycrystalline powders were prepared by supporting ZnO from Zn(NO3)2‚6H2O or from Zn(CH3COO)2‚ 2H2O on TiO2 (anatase) or TiO2 (rutile). The preparations were carried out at moderate temperature in order to minimize the occurrence of agglomeration and sintering, usually responsible for a significant decrease of surface area. The precursors and the mixed polycrystalline solids were characterized by thermogravimetry and differential thermal analyses (TG-DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with an electron microprobe used in an energy-dispersing mode (EDAX), surface area determination with the Brunauer, Emmett, and Teller method (BET), porosity measurements, and X-ray photoelectron spectroscopy (XPS). In part 227 the results of further characterization of the same solids and their photocatalytic activity
10.1021/jp003172r CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
Polycrystalline ZnO/TiO2 Systems. 1 tested by carrying out a probe photoreaction, i.e., the oxidation of 4-nitrophenol in aqueous medium, will be reported. The knowledge of the photoactivity of these materials, in addition to their bulk and surface properties, appears useful to assess their behavior also in other kinds of photoreactions carried out in the gas-solid regime and work is in progress in this direction. Experimental Section Photocatalyst Preparation. The preparation of the bare TiO2 (anatase) sample was carried out by adding dropwise an aqueous solution of ammonia at room temperature to an aqueous solution of TiCl3 (Carlo Erba, RPE). The resulting solid was separated after 24 h by filtration and washed thoroughly with distilled water until the disappearance of Cl- ions from the liquid (tested as AgCl). Subsequently, the solid was dried at 373 K for 24 h and then calcined in air for 48 h at 823 K. An aliquot of this sample was subsequently heated in air for 24 h at 1073 K to induce the anatase to rutile transition phase. The codes used for these samples are TA and TR where T indicates TiO2, A anatase, R. The TA and TR samples were used as supports for preparing ZnO/TiO2 catalysts by a wet impregnation method. Aqueous solutions containing the required amounts of Zn2+ ions ex Zn(NO3)2‚6H2O (Carlo Erba, RPE) were used. After standing for 24 h at room temperature, without stirring, the slurries were dried at 373 K for 48 h. The resulting solids were heated in air at 773 K for 24 h; the nominal amounts of zinc, expressed as mole percentages of Zn2+/(Zn2+ + Ti4+), were equal to 0.1, 0.5, 2.0, 5.0, 10.0, 50, 60, and 67%. Repeated impregnation-drying treatments were carried out for the preparation of the samples with higher amounts of ZnO (50, 60, and 67%); for these samples only one heating treatment at 773 K for 24 h was performed. Two additional sets of ZnO/TiO2 samples were prepared by impregnating TA and TR with aqueous solutions containing the required amounts of Zn2+ ions ex Zn(CH3COO)2‚2H2O (Fluka Chemika). After standing for 24 h at room temperature, the slurries were subsequently dried in air at 373 K for 48 h. Then, the resulting solids were heated at 423 K for 24 h in a continuous Pyrex reactor of cylindrical shape; a flow of wet nitrogen allowed to hydrolyze zinc acetate and to eliminate the acetic acid formed. Finally, the powders were heated for 24 h in the same reactor at 673 K in the presence of flowing wet oxygen. The resulting solids contained nominal mole percentages of zinc in the 0.1-50% range. It is worth noting that for the preparation of the samples with the highest amount of ZnO (25 and 50%), repeated impregnation steps, followed by drying treatments, were carried out. The codes used for the samples are TAZnNXX, TRZnNXX, TAZnAXX, and TRZnAXX, indicating the kind of support (TA or TR), the nitrate or acetate precursor salt of ZnO (ZnN or ZnA), and the nominal molar percentage (XX) of zinc ions. Two samples (TAZnA25 and TRZnA25) were prepared by performing the final heating treatment at 773 instead of at 673 K. Two pure ZnO samples were prepared: the first one by decomposition of Zn(NO3)2‚6H2O carried out in air at 773 K for 24 h and the second one by following a procedure similar to that used for preparing the TAZnA and TRZnA sets by using Zn(CH3COO)2‚2H2O in the absence of TiO2 (anatase) and TiO2 (rutile). These samples will be denoted in the following as ZnO ex N and ZnO ex A, respectively. Thermogravimetry and Differential Thermal Analyses (TG-DTA). Differential thermal analysis (DTA) and thermo-
J. Phys. Chem. B, Vol. 105, No. 5, 2001 1027 gravimetric analysis (TG) were measured on Perkin-Elmer DTA7 and TGA7 instruments, respectively. The analyses were carried out in flowing (30 mL‚min-1) air or N2 (L’Air Liquide). Powder X-ray Diffraction (XRD). X-ray powder diffraction analysis of all of the samples was carried out at room temperature by a Philips PW 1130 generator and PW 1050 goniometer using Ni-filtered Cu KR radiation. Additional information for phase analysis was obtained with a highresolution Guinier-Hagg focusing camera and strictly monochromatic Cu KR1 radiation. KCl was added to each sample as an internal standard for lattice parameter determination. Transmission Electron Microscopy (TEM). Transmission electron microscopic studies were performed with a JEOL 200 CX microscope operated at 200 kV and fitted with a top entry goniometer stage capable of (10° tilt. The samples were prepared by crushing a small amount of material in an agate mortar under butan-1-ol and allowing a drop of the resulting suspension to dry on a holey carbon support film covering a standard Cu grid. Only those crystal fragments projected over holes in the support film were examined in detail. Scanning Electron Microscopy Observation (SEM) and Energy Dispersive X-ray Analysis (EDAX). Scanning electron microscopy measurements were performed using a 505 Philips microscope operating at 25 kV on specimens upon which a thin layer of gold or carbon had been evaporated. An electron microprobe used in an EDAX mode was employed to obtain quantitative information on the amount and distribution of ZnO in the samples. Further samples were examined on a Hitachi S570 scanning electron microscope fitted with an EDAX analysis facility. Specific Surface Areas (BET) and Porosity Measurements. The specific surface areas were measured by the single-point BET method28 using a Flow Sorb 2300 apparatus (Micromeritics International Corp.). Porosity was monitored from the adsorption-desorption isotherms of nitrogen recorded at 77 K by using a conventional, high-vacuum system (residual pressure ca. 10-4 Pa) provided with a McLeod gauge, silicon oil diffusion pump and a MKS Baratron pressure transducer. X-ray Photoelectron Spectroscopy. XPS measurements were performed with a Kratos ES 300 ESCA instrument working in fixed analyzer transmission (FAT) mode using a pass energy of 40 eV. Spectra were generated by Al KR X-rays (hν ) 1486.6 eV, 150 W). The spectral region of Ti 2p, Zn 2p, and O 1s were collected. TA, TR, ZnO ex A and ZnO ex N were analyzed as reference compounds. The constant line width of the peaks during sample analysis confirmed the absence of differential charging. Referencing all the binding energies to the C 1s core level energy, from adventitious carbon, previously found at 285.0 eV, eliminated the uniform sample charging. The experimental spectra were resolved into Lorentzian-Gaussian components after subtraction of a linear background, using a nonlinear least-squares fitting routine. The reproducibility of the binding energy values obtained from the fitting is estimated as (0.2 eV. The quantitative analysis of the sample was performed using the routine software of the XPS instrument with appropriate sensitivity factors. During the analysis, the pressure was on the order of 10-9 Torr (1 Torr ) 133.3 N m-2). The powder samples were pressed into a copper grid and then mounted onto the tip of the sample holder rod. The absence of any signal from the copper grid was checked. Results and Discussion TG-DTA. Thermogravimetry and differential thermal analyses were carried out by using the precursors (i.e., the materials
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Figure 1. DTA curves for the sample TAZnA10-precursor recorded in the presence of air (a) and N2 (b).
Figure 3. X-ray diffractograms for TA (a), TAZnN10 (b), TAZnA50 (c), TAZnN50 (d), ZnO ex A (e), and ZnO ex N (f) samples.
Figure 2. DTA curves of the TRZnN10-precursor (a) and TAZnN10precursor (b) recorded in the presence of N2.
dried for 48 h at 373 K) of the TAZnA10, TAZnN10, TRZnA10, and TRZnN10 samples in the presence of air or N2. The DTA curves recorded for the TAZnA10-precursor sample, both in air and in N2, are shown in Figure 1. The first endothermic peak, with minimum just below 373 K, indicates the removal of physisorbed water molecules; its position and intensity are independent of the flowing gases, indicating that this is not an oxidizing-dependent process. A similar consideration is valid for the sharp and weak endothermic effect at ca. 513 K, possibly due to the removal of strongly adsorbed water. The endothermic effect observed at about 573 K in the presence of N2 is not detected in the presence of air. In this last case a strong exothermic effect, centered around 623 K, is recorded. The exothermic effect is undoubtedly due to combustion of the acetate anions, a process that cannot take place in the presence of N2 where the anions should simply be pyrolyzed. The endothermic effect observed in the presence of N2, is masked in air possibly because of the appreciable intensity of the exothermic effect. The broad exothermic effect close to 1073 K should be related to the anatase to rutile transition phase. Similar patterns were recorded for the TRZnA10-precursor sample, thus indicating that the crystallographic phase of the support has only (if any) minor effects on the decomposition pattern of the ZnO precursor. The DTA patterns of the precursors of TAZnN10 and TRZnN10 samples are reported in Figure 2. The curves are more complex than those recorded for the precursors of the TAZnA10
Figure 4. X-ray diffractograms for TR (a), TRZnN10 (b), TRZnA50 (c), and TRZnN50 (d) samples.
and TRZnA10 samples. Weight loss starts almost at room temperature, suggesting the existence of relative large amounts of weakly adsorbed water molecules. This difference should be due to a complex decomposition mechanism for the nitrate anion. The broad, weak, exothermic feature close to 1073 K, above ascribed to the anatase to rutile transition phase, is recorded for the anatase supported sample, but not for the rutile supported one. Total weight loss for these samples was 14%, which is between the values expected from the presence of hexahydrated (21%) and nonhydrated (12%) Zn nitrate. XRD. In Figure 3 the diffractograms of TA, of two selected samples of the TAZnN set, of the TAZnA50 sample, and of ZnO ex N and ZnO ex A are reported. The lines recorded can be ascribed to TiO2 (anatase)29 with a very slight presence of
Polycrystalline ZnO/TiO2 Systems. 1
J. Phys. Chem. B, Vol. 105, No. 5, 2001 1029
Figure 5. Scanning electron micrographs of ZnO ex N (a), ZnO ex A (b), TA (c), and TR (d).
TiO2 (rutile)30 and ZnO (zincite).31 In Figure 4 the diffractograms of TR and of two selected samples of the TRZnN set and of the TRZnA50 sample are reported. Diffractograms of samples with a ZnO content less than 5% are not reported because they showed only the parent phase, either rutile or anatase. In the samples containing 5 and 10% ZnO, the main lines due to ZnO were recorded in addition to those of TA or TR. It can be observed that for the TAZnN samples reported in Figure 3, the intensity of the peaks due to zincite increases by increasing the amount of ZnO. The relative intensity of the peaks due to anatase decreases, but that of rutile does not increase accordingly; actually, the peak due to rutile at ca. 2θ ) 27.4° seems to be constant, or even disappears as the amount of ZnO is increased. This result indicates that no anatase to rutile phase transition is taking place. Moreover, for TRZnN (see Figure 4), TAZnA, and TRZnA samples, the intensities of the lines of the parent TiO2 (rutile) or TiO2 (anatase) decrease by increasing the amount of ZnO simply because the amount of TiO2 is decreasing with respect to that of ZnO. The samples of the TAZnA and TRZnA sets seem to be less crystalline in comparison to those of the TAZnN and TRZnN sets. A similar consideration applies for ZnO ex A and ZnO ex N: the second sample appears to be more crystalline. This insight, confirmed by SEM observations, can be mainly related to the lower heating temperature (673 instead of 773 K) used for preparing the TAZnA and TRZnA sets. Indeed, an appreciable increase of crystallinity can be noticed from the observation of the diffractograms of some selected TAZnA and TRZnA samples heated at 773 K instead of 673 K. Finally the
XRD measurements indicate that TAZnN50-67 and TRZnN5067 samples consist only of a mixture of anatase or rutile and ZnO. There is no noticeable change in the lattice parameters of the anatase or rutile across the phase range investigated. TEM. The samples of all the series consisted of small crystallites, those of the TAZn series being much smaller than those of the TRZn series, reflecting what was observed for the bare supports. No significant defect structures were observed in the sets of samples and no microstructure. The TEM technique, in accord with XRD, does not reveal defect structures, thus indicating that ZnO has not entered into the TiO2 anatase or rutile structures. Because of the fact that the presence of ZnO has been confirmed by XRD and that of Zn by EDAX analyses (see below), it seems likely that ZnO exists as surface species. SEM and EDAX Analyses. In Figure 5 micrographs of ZnO ex N, ZnO ex A, TA, and TR are shown. It can be observed that ZnO ex N is much more crystalline than ZnO ex A, but both samples consist of crystallite aggregates that appear to be smaller for the second sample. As far as the bare TiO2 supports are concerned, it can be observed that aggregates of various size are present for both allotropic phases. A more significant sintering is evident for TR, due to the higher temperature used for its preparation, and several debris can be noticed on the surface of the biggest particles. In Figure 6 micrographs of TRZnN50, TRZnA50, TAZnN67, and TAZnA50 are shown. No micrographs of TRZnN0.1-10, TRZnA0.5-25, TAZnN0.1-10, and TAZnA0.1-10 samples are shown because no significant differences between these samples and the corresponding supports can be noticed. Significant differences,
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Figure 6. Scanning electron micrographs of TRZnN50 (a), TRZnA50 (b), TAZnN67 (c), and TAZnA50 (d) samples.
TABLE 1: Specific Surface Areas (SSA) of the Photocatalysts samples
SSA (m2‚g-1)
samples
SSA (m2‚g-1)
TA TAZnN0.10 TAZnN0.50 TAZnN2.00 TAZnN5.00 TAZnN10.0 TAZnN50.0 TAZnN60.0 TAZnN67.0 TAZnA0.10 TAZnA10.0 TAZnA25.0 TAZnA25.0a TAZnA50.0
59 56 55 54 53 46 22 12 11 57 48 40 31 28
TR TRZnN0.10 TRZnN0.50 TRZnN2.00 TRZnN5.00 TRZnN10.0 TRZnN50.0 TRZnN60.0 TRZnN67.0 TRZnA0.50 TRZnA1.00 TRZnA10.0 TRZnA25.0 TRZnA25.0a TRZnA50.0 ZnO ex A ZnO ex N
6 6 5 5 5 4 3 2 2 6 6 7 7 6.5 10 3 NDb
a Samples prepared by performing the final heating treatment at 773 K. b Not determined because very small.
on the contrary, can be observed (Figure 6) between the TRZnN50 and TRZnA50 samples. Well-dispersed microcrystals, forming a sort of covering layer, are present on the surface of the particles of the second sample whereas bigger crystallites (presumably ZnO) are easily noticeable on the surface of the particles of TRZnN50 sample. This could be due to the different preparation methods used, and the higher value found for the specific surface area of TRZnA50 (see Table 1) is in accord with this insight. Similar considerations can be done by comparing TAZnN67
and TAZnA50. It is worth noticing, moreover, that the particles of the TAZnN67 sample present a spongelike surface. EDAX analyses carried out for all of the samples showed a large scattering of the zinc amount figures among different particles of the same sample and also among different locations of the same particle, although the average values were close to the nominal ones. For instance, the analyses carried out for the TAZnN50.0 sample ranged between 23.1 and 83.5 mol % of zinc, while those for the TRZnN50.0 sample ranged between 45.6 and 93 mol. %. It is well-known that the wet impregnation method generally does not produce homogeneous supported particles.32 BET Surface Areas and Porosity Measurements. The BET specific surface area values (SSA) of all of the samples are reported in Table 1. It can be observed that there is no significant difference between the surface areas of the TAZnN0.1-10.0 samples and the TA sample. The surface areas of the samples containing higher quantities of zinc (TAZnN50-67) are much lower and this is understandable because the value of the ZnO ex N sample is very small (
