Coupled Semiconductor Systems for Photocatalysis. Preparation and

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J. Phys. Chem. B 1999, 103, 8236-8244

Coupled Semiconductor Systems for Photocatalysis. Preparation and Characterization of Polycrystalline Mixed WO3/WS2 Powders A. Di Paola,*,† L. Palmisano,† A. M. Venezia,‡ and V. Augugliaro† Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniVersita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy, and ICTPN-CNR, Via Ugo La Malfa 153, 90146 Palermo, Italy ReceiVed: April 8, 1999; In Final Form: June 15, 1999

Mixed WO3/WS2 powders were prepared by oxidation of WS2. The physical properties of the samples were characterized by X-ray diffraction, diffuse reflectance spectroscopy, and scanning electron microscopy; they were strongly dependent upon the time and temperature of oxidation. The chemical state and the elemental distribution of the sample surface were investigated by X-ray photoelectron spectroscopy. The photocatalytic results have showed that the aqueous suspensions of the mixed WO3/WS2 systems have significantly higher activities than pure WS2 and WO3 for the photodegradation of phenol. The enhanced performance can be related to the presence of heterojunctions WO3/WS2 on the single particles. The best results were obtained when the surface WS2 molar fraction was about 0.5.

Introduction The photocatalytic action of a semiconductor particulate system is based on the generation of electron-hole pairs which move to the surface of the particles and react with redox species at the interface. In order to obtain a high reaction rate, the recombination of the two kinds of charge carriers must be kept low. An efficient charge separation can be obtained by coupling two semiconductor particles.1-4 The improvement of efficiency is explained as the result of a vectorial transfer of electrons and holes from a semiconductor to another. Serpone et al.5-8 were the first who demonstrated that photogenerated electrons can be transferred from CdS to TiO2. More recently, the simultaneous migration of both electrons and holes has been reported.3,4 A large variety of coupled semiconductor systems has been obtained by employing suspensions of mixed polycrystalline powders9-11 or colloidal semiconductors.12-17 Composite thin films have also been investigated.18-22 The coupled semiconductors include CdS/TiO2,1,15 CdS/ZnO,1,12 CdS/ZnS,9-12 CdS/ AgI,15 Cd3P2/TiO2 and Cd3P2/ZnO,16 AgI/Ag2S,14 ZnO/ZnS,13 ZnO/ZnSe,3 SnO2/TiO2,3 SnO2/CdS,21 CdSe/TiO2,22 and Cu2S/ CdS.23 Different systems can be prepared according to the preparation procedures. Serpone et al.4 have studied the behavior of mechanical mixtures of several couples of semiconductor powders (CdS, TiO2, ZnO, WO3, Fe2O3, WS2, and SnO2). Ueno et al. found an improved activity when mixed ZnS/CdS particles were supported on Nafion,9 silica,9,10 or alumina;11 the best results were obtained when the ZnS particles were superimposed on the surface of the CdS particles.11 Parmon et al.23,24 obtained a substantial improvement of efficiency by contacting cadmium sulfide or zinc sulfide (or a mixed sulfide of these metals) with copper or silver sulfides. The colloidal semiconductor systems can be classified in two categories: (1) coupled systems, where the different particles of two semiconductors adhere to each other in so-called “sandwich structures”12-16 or (2) capped systems which have a † ‡

Universita` di Palermo. ICTPN-CNR.

core-shell geometry.3,17 Most of the colloidal systems fall into the first category. Bedja and Kamat17 demonstrated that the interfacial charge transfer in coupled and capped semiconductors is significantly different. Coupled semiconductor films are obtained by deposition of colloidal semiconductors on an optically transparent electrode.18-22 These thin films possess a highly porous structure and reveal interesting photoelectrochemical and photocatalytic properties. Recently, polycrystalline WO3/WS2 systems have been prepared by partial sulfidation of WO3.25,26 Unlike other coupled semiconductor systems, the enhanced photoactivity of these powders is not due to a mechanical mixture of two semiconductors but to the coexistence of WO3 and WS2 in the same particle. In this paper we report on the preparation and characterization of mixed WO3/WS2 powders obtained by oxidation of WS2. The purpose of the work was to elucidate the influence of the composition on the photocatalytic properties of the mixed WO3/ WS2 powders. Experimental Section Sample Preparation. Mixed WO3/WS2 catalysts were prepared by oxidation of WS2 (Aldrich). The powder (particle size < 2 µm) was placed in a cylindrical Pyrex reactor and allowed to react with oxygen or air at various temperatures and for different times. The resulting powders were subsequently cooled in helium and stored in air. Some samples were obtained by oxidizing WS2 with air in a muffle oven. The catalysts were denoted with a coding, e.g. 673/1/6, which identifies the temperature (K), the time of oxidation (h), and the amount of initial WS2 powder (g), respectively. Powder composition was gravimetrically determined by calcining the samples in air at 823 K for 3 h. The sulfur content was calculated from the weight loss, assuming WO3 as the only product of calcination. Sample Characterization. X-ray Diffraction and Specific Surface Areas Determinations. XRD patterns of the powders were obtained with a Philips powder diffractometer using Cu KR radiation and a 2θ scan rate of 2°/min. The specific surface areas were measured by the single point BET method using a Micromeritics Flow Sorb 2300 apparatus.

10.1021/jp9911797 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/11/1999

Coupled Semiconductor Systems for Photocatalysis Diffuse Reflectance Spectroscopy. Diffuse reflectance spectra were determined in the range 200-500 nm, by means of a Varian DMS 90 UV-vis spectrophotometer. BaSO4 was used as reference sample. Scanning Electron Microscopy. SEM was performed using a Philips 505 microscope, operating at 25 kV on specimens upon which a thin layer of gold had been evaporated. An electron microprobe used in an EDX mode was employed to obtain information on the degree of conversion of WS2 to WO3. X-ray Photoelectron Spectroscopy. The XPS spectra were recorded by means of a Perkin-Elmer PHI 5600ci spectrometer using a standard Al KR source (1486.6 eV) working at 300 W. The working pressure was around 1 × 10-6 Pa. Detailed spectra were recorded for the following regions: C 1s, O 1s, W 4f, W 4d, and S 2p, at a pass energy of 11.75 eV and with an energy step of 0.05 eV. The precision on the binding energy (BE) values was (0.15 eV. After a Shirley type background subtraction, the raw spectra were fitted using a nonlinear least-squares fitting program with Gaussian-Lorentzian peak shapes. As an internal reference for the absolute binding energies, the C 1s peak of hydrocarbon contamination set at 284.8 eV was used. The atomic composition was evaluated using PHI sensitivity factors.27 The samples were analyzed as pellets after being ground in a mortar. WS2 (Aldrich) and WO3 (Carlo Erba) were analyzed as reference compounds. PhotoreactiVity Experiments. A 1.5 L batch cylindrical Pyrex reactor was used. The photoreactor was provided with ports in its upper section for the inlet and outlet of gases, for sampling and for pH measurements. A 1000 W medium-pressure Hg lamp (Helios Italquartz) was immersed within the inner part of the photoreactor. The IR component of the incident beam as well as any radiation below 300 nm was eliminated by the circulation of cooling water through a Pyrex jacket surrounding the lamp. The photon flux transmitted (22 mW cm-2) was measured by using a radiometer (UVX Digital) leaned against the external wall of the photoreactor containing only pure water. The amount of catalyst employed was 1.5 g L-1 for all the experiments. The phenol concentration was always 20 mg L-1. The initial pH of the solution was adjusted to 4.5 by addition of H2SO4 (Carlo Erba, RPE) and the reaction mixture was magnetically stirred. Oxygen was continuously bubbled into the suspension before and during the irradiation. The temperature inside the reactor was about 313 K. Samples for analysis (10 mL) were withdrawn at fixed intervals from the reaction mixture. The catalyst was separated from the solution by filtration through a 0.45 µm cellulose acetate membrane (HA, Millipore). The samples were subsequently centrifuged for 15 min at 5000 rpm. The quantitative analysis of phenol was performed by a standard colorimetric method.28 The presence of SH- in the solution was checked by addition of lead acetate. The mineralization of the substrate was monitored by determining the total organic carbon (TOC) with a Carlo Erba TCM 480 analyzer, using samples of about 6 mL volume. The formation of the main intermediates was investigated by highperformance liquid chromatography (HPLC), using a Varian 9050 UV-vis detector fitted with a Varian Model 9010 pump and an Econosphere C18 3 µm 150 mm × 4.66 mm i.d. column (Alltech). An acetonitrile-water solution (1:1 v/v) was used as the eluting solvent. The products were identified by comparison with standards. Results Sample Preparation. Different mixed WO3/WS2 samples were prepared by varying the starting amount of WS2, and the

J. Phys. Chem. B, Vol. 103, No. 39, 1999 8237

Figure 1. X-ray diffraction patterns of WO3/WS2 samples obtained by oxidation of 3 g of WS2 at 773 K for different times: (a) 0.5 h, (b) 1.5 h, (c) 3 h.

time or the temperature of reaction. During the oxidation the black color of WS2 disappeared, changing gradually from darkgray to yellow-green. The powders were more or less heterogeneous depending on the preparative conditions. If the samples were obtained by oxidizing with oxygen or with a high air flow rate, the color of the powder was never uniform, revealing clear zones rich of WO3 and dark zones rich of WS2. Heterogeneous powders were in any case obtained after long times of oxidation or at high temperatures (773 K). The homogeneity of the samples depended also on the starting amount of sulfide. The most homogeneous catalysts were obtained by oxidizing 3 g of WS2 at 673 K, with an air flow rate of 20 mL/min. The samples obtained by oxidation of WS2 at 673 K in the muffle oven were found to be strongly heterogeneous. X-ray Diffraction. The XRD diffraction pattern of commercial WS2 was typical of hexagonal tungsten disulfide.29 The basic structural unity of WS2 is a sandwich of three planes: sulfur-tungsten-sulfur. The sandwiches are loosely bound together through van der Waals forces. The diffractogram revealed the main peak at 2θ about 14.32°, which represents the (002) crystallographic plane. Small and narrow peaks were found at values of 2θ characteristic of the (004), (006), and (008) planes. All these planes are perpendicular to the c-axis. Figure 1 shows the X-ray diffraction patterns of some samples obtained by oxidation of WS2 with air at 773 K for different times. The features of the XRD spectra reveal the contemporary presence of peaks of tungsten trioxide and tungsten disulfide.

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Figure 2. X-ray diffraction patterns of: (a) WO3 obtained by complete oxidation of WS2 at 823 K for 3 h, (b) WO3 (Carlo Erba), (c) WO3/WS2 ox/673/1.5/6, and (d) WO3/WS2 sulf/673/1/10.

TABLE 1: ASTM d Spacings WO3 (monoclinic)

WO3 (triclinic)

2θ (deg)

d (Å)

I

2θ (deg)

d (Å)

I

23.18 23.64 24.44 26.74 28.70 33.38 33.68

3.835 3.762 3.642 3.342 3.109 2.684 2.661

100 95 100 50 50 75 60

34.27 41.57 47.43 48.44

2.617 2.172 1.917 1.879

90 50 50 50

55.54

1.654

40

23.14 23.66 24.40 26.88 28.90 33.60 33.69 33.95 34.06 41.93 47.30 48.42 49.90 55.14

3.840 3.760 3.650 3.320 3.090 2.667 2.660 2.640 2.632 2.154 1.922 1.880 1.827 1.660

80 65 100 6 20 30 30 25 25 16 16 16 40 6

In particular WO3 is characterized by three narrow peaks at 2θ about 23°-25°, whose intensities increase with increasing temperature or reaction time. WO3 normally crystallizes in a monoclinic phase,30 although a triclinic structure31 is stable from about 233 to about 290 K. A tetragonal form exists only above 998 K.32 In Table 1 the principal d spacings and the corresponding 2θ values of triclinic and monoclinic WO3 are reported. The d spacings of these two crystalline phases are very similar and this is in accord with the very close values of all their cell parameters. In Figure 2 the X-ray diffractogram of a powder obtained by complete oxidation of WS2 is compared with that of a commercial WO3 (Carlo Erba). Both diffractograms reveal practically identical positions of the various reflection peaks, although their relative intensities are quite different. While the peaks and the intensities found for the pure reference WO3 fit well with those of the monoclinic structure, the relative intensities of the main peaks of the oxidized sulfide are different from the values reported both for the monoclinic and the triclinic form. In the present case, indeed, the presence at 2θ between 23° and 25° of a strong and narrow peak followed by two much

smaller ones instead of three peaks of comparable intensities indicates a strong orientation of the WO3 crystals along some preferential planes,33 as for instance the (001) plane. A similar X-ray spectrum was obtained by oxidizing a different commercial WS2 powder (Strem). It is worth noting that the X-ray patterns of the powders obtained by partial oxidation of WS2 always reveal this type of oxide. On the contrary the diffractograms of the mixed samples obtained by sulfidation of WO3 show the peaks of monoclinic WO3. The spectra of an oxidized WS2 sample (labeled as ox) and that of a sulfidized WO3 one (labeled as sulf) are shown in Figure 2. Chemical Analysis and Specific Surface Areas Measurements. The composition and the specific surface areas of some samples are reported in Table 2. The areas are rather low and not much different from that of commercial WS2. As already reported for mixed powders obtained by sulfidation of WO3,25 this result can be attributed to the slight difference between the molar volumes of WS2 and WO3.34 When the reaction time and the amount of starting sulfide were held constant, the molar fraction of WS2 decreased with increasing temperature of oxidation. A higher oxide content was always found by reducing the amount of starting powder to be oxidized. At 673 K, with increasing the oxidation time, the composition of the samples reached a practically constant value. Scanning Electron Microscopy and EDX Results. The SEM micrographs did not reveal significant morphological differences among the pure WS2 and the partially or totally oxidized powders. As shown in Figure 3a, WS2 consists of an agglomeration of small and large layer crystals having small thicknesses. During the oxidation, the size and the shape of the particles did not change appreciably (Figure 3b), in agreement with the surface area measurements. The EDX results indicated a not homogeneous distribution of WS2 or WO3 on the surface of the particles, revealing the presence of zones with a high content of WS2 and zones richer in WO3. Parts c and d of Figure 3 show, respectively, the SEM

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Figure 3. Scanning electron micrographs of (a) WS2, (b) WO3/WS2 ox/673/1.5/6, (c) WO3, and (d) WO3/WS2 sulf/673/1/10.

TABLE 2: Specific Surface Area Measurements and WS2 Molar Fractions Determined by Chemical Analysis sample

specific surface area (m2 g-1)

chem anal WS2 molar fraction

WS2 WO3 WO3a ox/673/0.5/3 ox/673/1/3 ox/673/1.5/3 ox/673/3/3 ox/673/6/3 ox/673/0.5/6 ox/673/1/6 ox/673/1.5/6 ox/673/2/6 ox/673/3/6 ox/673/4/6 ox/773/0.5/3 ox/773/1.5/3 ox/773/3/3 ox/773/4/3

1.7 2.4 1.9 2.0 2.2 2.0 2.4 2.2 2.3 2.2 2.4 2.2 2.0 2.0 2.4 2.1 2.0 1.9

1.00 0.00 0.00 0.80 0.75 0.73 0.60 0.57 0.87 0.83 0.79 0.75 0.66 0.63 0.68 0.40 0.35 0.21

a

Obtained from WS2 (Aldrich) calcined at 823 K for 3 h.

micrographs of WO3 and of a powder obtained by partial sulfidation of WO3. The comparison between the micrographs

of the two mixed samples reveals that the powders present morphologies rather different and each of them keeps the features of the tungsten chalcogenide from which it derives. In particular, the samples obtained by sulfidation consist of aggregates of particles having irregular shapes but quite similar sizes. Diffuse Reflectance Spectroscopy. Figure 4 shows diffuse reflectance spectra of (a) commercial WO3, (b) WO3 obtained by calcination at 823 K, (c) a mixed WO3/WS2 powder, and (d) commercial WS2. The spectra of the mixed samples are more or less similar to those of the pure components depending on the extent of surface conversion of the starting chalcogenide. In particular, the spectrum of the sample ox/673/1.5/3 indicates an enrichment of WO3 on the surface despite the high content of WS2 revealed by the chemical analysis (see Table 2). The comparison between the reflectance spectrum of commercial WO3 and that of the powder obtained by complete oxidation of WS2 reveals that, although the two spectra are quite similar, the linear part of the curve of the “home-prepared” WO3 is slightly shifted to longer wavelengths. Such shift reflects a lower band gap value of this solid and is in agreement with other experimental findings indicating small variations in the

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Di Paola et al. TABLE 3: Binding Energies (eV), and Full Width at Half-Maximum (eV)a samples WS2 (Aldrich)

W 4f7/2

33.1 (0.9) WO3 (Carlo Erba) 35.7 (0.9) WO3 (home 36.1 prepared)b (1.3) ox/673/1.5/6c 33.0, 36.5 (0.9) (1.4) ox/673/8/6c 32.9, 35.3 (0.9) (1.6) ox/673/2/3c 33.0, 36.5 (0.9) (1.5) ox/673/6/3c 33.0, 36.3 (0.9) (1.4) sulf/573/1/6d 33.0, 36.3 (0.9) (1.2) sulf/673/1/10d 33.0, 36.4 (0.9) (1.4) sulf/873/1/10d 33.0, 36.4 (1.0) (1.5)

W 4d5/2

S 2p3/2

245.4 (4.2) 247.7 (4.0) 248.0 (4.3) 245.0, 248.4 (4.0) (4.3) 245.1, 248.2 (4.2) ((4.5) 245.2, 248.6 (4.4) (4.5) 245.1, 248.3 (4.3) (4.5) 245.6, 248.4 (4.4) (4.4) 245.2, 248.6 (4.4) (4.4) 245.4, 248.6 (4.5) (4.5)

162.7 (0.9)

162.7 (0.9) 162.3 (2.3) 162.7 (0.9) 162.3 (2.3) 162.7 (1.0) 162.6 (0.9) 162.7 (1.0)

O 1s

530.1, 531.3 (1.1) 530.9, 532.2 (1.5) 531.4, 532.6 (1.5) 530.4, 531.9 (2.0) (2.0) 531.6, 532.6 (1.5) 530.4, 531.5 (1.5) (1.5) 531.3, 532.7 (1.5) 531.5, 532.9 (1.7) 531.6, 532.5 (1.8)

a The precision of the values is (0.1 eV. b Obtained from WS2 (Aldrich) calcined at 823 K for 3 h. c Obtained from WS2 (Aldrich) oxidized at 673 K for different times. d Obtained from WO3 (Carlo Erba) sulfidized with H2S.25

Figure 4. Diffuse reflectance spectra of various samples: (a) WO3 (Carlo Erba), (b) WO3 obtained by complete oxidation of WS2 at 823 K for 3 h, (c) WO3/WS2 ox/673/1.5/3, (d) WS2.

Eg optical values of WO3 oxide films depending on the preparation procedure.35 X-ray Photoelectron Spectroscopy. In Table 3 the binding energies of the main photoelectron peaks are listed for the reference compounds and for some mixed samples prepared by oxidation of WS2 or by sulfidation of WO3. The binding energies of the oxide obtained by complete oxidation of commercial WS2, although slightly higher than those relative to the commercial WO3, are still indicative of the chemical state W6+.36,37 The corresponding O 1s spectrum presents two peaks, one at low energy attributable to oxide and one at high energy due to hydroxide forms. The difference in binding energies between this sample and the commercial oxide may be related to the differences found between the corresponding X-ray diffraction patterns and reflectance spectra. The mixed samples contain the two chemical states of tungsten, W4+ as WS2 and W6+ as WO3, characterized by multiple components of the W 4f spectrum. A typical spectrum of a mixed sample along with those relative to commercial WS2 and WO3 obtained by complete oxidation of WS2 is shown in Figure 5. The chemical states are the same indipendently on the precursor (WO3 or WS2). The relative intensity of the two components changes according to the surface atomic composition. The extent of oxidation or sulfidation depends on several factor as temperature, time, and starting amount of powder. Table 4 reports the chemical composition of some mixed samples, in terms of molar fraction of WS2. The discrepancies

observed between the two sets of values indicate that the surface composition determined by XPS measurements is different from the overall composition obtained by chemical analysis. It is worth noting that significant differences among the surface values can be found also for samples with similar overall composition. The particle dimensions of the various samples have been obtained by the Sherrer analysis of the X-ray diffraction peaks. The sizes of the precursor particles affect the dimensions of the final product, in agreement with the SEM analysis. In particular, the oxidized samples are characterized by particles of different sizes as the starting WS2 powder. Photocatalytic Experiments. The photooxidation of phenol was chosen as a probe reaction in order to study the photoactivity of the WO3/WS2 samples.25 It is worth noting that, in the absence of illumination, the adsorption of phenol was at most 4% both for the pure tungsten chalcogenides and for the mixed powders. Figure 6 illustrates the photocatalyzed disappearance of phenol in the presence of different samples. The results indicate that the powder prepared by complete oxidation of WS2 is much more efficient than commercial WS2 and WO3. As shown in Figure 6, the mixed WO3/WS2 samples reveal an enhanced photoactivity compared with that of WO3 or WS2. The efficiency of the powders, as well as their composition, strongly depends on the preparation conditions of the samples. Generally, the photoactivity increases with increasing the time of the oxidation, reaches a maximum value, and afterwards remains practically constant. These results are consistent with the values of chemical composition reported in Table 2, indicating that the oxidation process slows down with time and then it stops. The mixed samples obtained at 773 K are significantly less active than those obtained at 673 K. Although these powders are strongly heterogeneous, their efficiency is higher than that of the home-prepared WO3. Figure 7 shows the results of phenol oxidation in the presence of different mixed powders having about the same molar composition (xWS2 ) 0.7). The mechanical mixture revealed a

Coupled Semiconductor Systems for Photocatalysis

J. Phys. Chem. B, Vol. 103, No. 39, 1999 8241

Figure 6. Photocatalyzed conversion of phenol in the presence of various samples: (0) WS2, (4) WO3 (Carlo Erba), (O) WO3 from totally oxidized WS2, (9) ox/673/0.5/6, (2) ox/673/1/6, (b) ox/673/1.5/6.

Figure 7. Photocatalyzed conversion of phenol with samples having the same bulk composition (xWS2 ) 0.7): (2) mechanical mixture of WO3 and WS2, (9) WS2 oxidized in an oven, (b) WS2 oxidized with O2, (1) WS2 oxidized with air.

Figure 5. XPS spectrum of the W 4f transition of various samples: (a) WO3 from totally oxidized WS2, (b) WS2 (Aldrich), (c) WO3/WS2 ox/673/1.5/6.

TABLE 4: Chemical Composition of the WO3/WS2 Systems as Molar Fraction of WS2a samples

WS2/(WS2 + WO3) (chem anal)

WS2/(WS2 + WO3) (XPS)

ox/673/1.5/6 ox/673/8/6 ox/673/2/3 ox/673/6/3 sulf/573/1/6 sulf/673/1/10 sulf/873/1/10

0.79 0.68 0.68 0.57 0.33 0.32 0.77

0.47 0.12 0.42 0.05 0.16 0.47 0.65

a

The accuracy of the values is estimated at (10%.

very low photoactivity while more than 80% of phenol was degraded after 8 h of irradiation of the samples obtained by oxidation of WS2 at 673 K. It is worth noting that the highest efficiency was exhibited by the sample which appeared more homogeneous, i.e., that obtained by oxidation with air. A correlation can be found between the surface composition and the photoactivity of the powders. As shown in Figure 8, the samples ox/673/1.5/6 and ox/673/2/3 which have comparable surface composition values but different overall chemical content

Figure 8. Photocatalyzed conversion of phenol: (9) ox/673/1.5/6, (b) ox/673/2/3, (2) ox/673/8/6.

(see Table 4) exhibit similar catalytic activities. On the contrary, the sample ox/673/8/6 which has the same chemical composition of the sample ox/673/2/3 is revealed to be less active. This means that the actual efficiency of the mixed WO3/WS2 catalysts depends on the relative amounts of WO3 and WS2 present on the surface of the particles: in particular, the maximum of photoactivity is obtained when the surface contain about 50% of WS2. The same behavior was observed for mixed powders obtained by sulfidation of WO3,25 although these catalysts are generally less efficient than the oxidized samples. Figure 9 shows that

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Figure 10. Schematic diagram representing the charge transfer process in a WO3/WS2 system.

Figure 9. Photocatalyzed conversion of phenol: (2) ox/673/1.5/6, (b) sulf/673/1/10.

the rates of phenol photooxidation for the most active powders of the two series (ox/673/1.5/6 and sulf/673/1/10) are practically coincident. Such behavior can be ascribed to the same surface composition of the two samples (x ) 0.47). The difference between the overall WS2 composition of the two powders, x ) 0.79 and x ) 0.33, respectively, is in accord with an oxidation reaction taking place at the surface of the particles while the sulfidation process is likely to occur from inside out with the sulfiding gas first diffusing and then reacting. No appreciable release of HS- was found in the solution after 8 h of illumination of the suspensions of mixed WO3/WS2 powders. Total Organic Carbon Analysis. The TOC analysis has confirmed a scarce mineralization of phenol in the presence of the mixed chalcogenide powders.25 The largest extents of mineralization were obtained by using the samples that were photocatalytically more efficient. In particular, after 8 h of irradiation, about 75% of mineralization was reached with the sample 673/1.5/6. It should be noted that the organic carbon concentration disappeared more slowly in the presence of the powders obtained by sulfidation of WO3. HPLC Analysis. Different intermediate species have been found depending on the WS2 content of the samples. Catechol, resorcinol, benzoquinone, and muconic acid (trans,trans-1,3butadiene-1,4-dicarboxylic acid) were detected when the powders contained low or medium amounts of WS2. Instead, the samples with a high WS2 content, i.e., not much oxidized, were characterized by a prevalent presence of muconic acid and the absence of dihydroxybenzenes. Discussion The photochemical characterization of mixed WO3/WS2 powders obtained by oxidation of WS2 has shown that these systems are more active than the pure tungsten chalcogenides. The rate of phenol photooxidation depends on the relative amounts of WO3 and WS2 present in the various samples and is strongly influenced by the homogeneity of the powders. Although the reflectance spectra seem to indicate that the WO3/WS2 samples absorb more photons than WS2 in the UV region, the dramatically enhanced rate of disappearance of phenol cannot be justified by a simple improved light absorption. As shown in a previous work,25 the increased photoefficiency of the mixed WO3/WS2 systems can be attributed to the contemporaneous presence of WO3 and WS2 in the same particle. The coupling of two semiconductors possessing different energy levels for their corresponding conduction and valence bands allows the vectorial displacement of holes and

electrons from one semiconductor to another and retards the recombination of the electron-hole pairs.4 Upon optical excitation, photogenerated electrons accumulate at the lower-lying conduction band of WO3 while holes accumulate at the valence band of WS2 (Figure 10). The improved charge separation enhances the efficiency of the interfacial charge-transfer process to the adsorbed substrate. The major reactions that occur38-40 can be summarized as follows:

WO3/WS2 + hν f WO3(e- + h+)/WS2(e- + h+) (1) WO3(e- + h+)/WS2(e- + h+) f WO3(e- + e-)/WS2(h++ h+) f WO3(e)/WS2(h) (2) WO3(e) + O2 f HO2• + OH-

(3)

HO2• + H2O f H2O2 + OH•

(4)

WO3(e) + H2O f OH- + OH•

(5)

WS2(h) + H2O f OH• + H+

(6)

WS2(h) + OH- f OH•

(7)

OH• + phenol f intermediates f CO2 + H2O

(8)

The absence of HS- in the solution seems to indicate that the mixed WO3/WS2 powders are quite stable toward the photocorrosion. The WO3/WS2 ratio of the powders can be tailored by varying suitably the preparation parameters, although it is very difficult to find the actual composition that maximizes the photoactivity of the samples. Indeed this is feasible for a mechanical mixture of the two chalcogenides but it is not an easy task for mixed powders obtained by an incomplete gas-solid reaction. On the other hand, chemical analysis gives information on the overall composition of the samples but not on the effective composition of the single particles which depends not only on the experimental conditions of preparation but also on their shape and size. A correlation can be found between surface composition and photoactivity of the samples since the efficiency of a powder depends on the number of active sites present on the particle surface, i.e., on the number of micro heterojunctions WO3/WS2 which behave like “photochemical diodes”.41 Figure 11 illustrates schematically the evolution of the surface of a particle with time. After short times of oxidation (Figure 11a), the conversion of sulfide into oxide is limited and the surface can be imagined consisting of few “islands” of WO3 surrounded by a “sea” of WS2. The mixed system is more photoactive than

Coupled Semiconductor Systems for Photocatalysis

Figure 11. Evolution of the surface of a particle during the oxidation.

pure WS2 because the presence of few heterojunctions is sufficient to enhance the photooxidation rate of phenol. The low photoactivity of a sample can be ascribed to a small surface content of WS2 or WO3. The size and number of islands of WO3 increase with time and this involves an increase of the number of heterojunctions WO3/WS2 which are spread on the surface like as the spots on a leopard skin (Figure 11b). By further increasing the oxidation time, the transformation of WO3 into WS2 proceeds and the surface of the particle can be represented by islands of WS2 surrounded by a sea of WO3 (Figure 11c). While the amount of WO3 increases, the efficiency of the powders will gradually decrease because the number of heterojunctions diminishes. As confirmed by the XPS analysis, the maximum of photoactivity is obtained when the surface WS2 molar fraction ranges between 0.4 and 0.5. The SEM micrographs have shown that WS2 is characterized not only by a layer structure where one of the three dimensions is much thinner than the other ones but also by significant differences in the size of the particles. This implies that, in the same reaction time, a higher conversion of the smaller WS2 particles to WO3 will occur and the number of heterojunctions WO3/WS2 present in the various particles will be different. For this reason it is very difficult to optimize the experimental conditions for the preparation of a powder which exhibits a maximum efficiency. The comparison between powders with similar WS2 content has shown that the more homogeneous a sample is, the higher its efficiency. This result is easily justifiable since a heterogeneous powder consists of a mechanical mixture of WO3, unreacted WS2, and partially oxidized sulfide. Of course, the relative ratios of these components change according to the experimental conditions of oxidation. The good photoactivity of the mixed powders requires an intimate contact between WO3 and WS2.11 Such a contact is permanently realized in the particles containing WS2 partially oxidized to WO3 but is only temporary in a mechanical mixture of these two semiconductors. A heterogeneous powder results efficient when the number of mixed WO3/WS2 particles is higher than that of the much less active particles of WS2 and WO3. The most efficient samples were obtained at the lower temperatures and after short times of oxidation. This can be explained by considering that with increasing temperature or time, the oxidation proceeds more significantly producing more heterogeneous powders. Air is preferable as oxidant because the presence of pure O2 favors a higher conversion to WO3. The chemical analysis data have shown that the oxidation at 673 K produces samples with a fixed overall concentration regardless of the time of the treament. The fact that the surface oxide is more abundant than the bulk oxide suggests that the oxidation is likely to occur from the outside of the particles of WS2. This would explain the slowing down of the process and eventually its end when a sort of passivation layer is formed.

J. Phys. Chem. B, Vol. 103, No. 39, 1999 8243 The photoreactivity results obtained by using sulfidized samples show that the two preparation methods (sulfidation of WO3 and oxidation of WS2) produce similar catalysts. Regardless of the overall composition, an optimum surface WS2 content (0.4-0.5) corresponding to the best performance of the samples can be reached starting either way from the oxide and from the sulfide. Both oxidized and sulfidized samples with a high WS2 content induce a fast photooxidation of the dihydroxybenzenes. This is confirmed by the presence of only muconic acid and benzoquinone as main intermediates. On the contrary, cathecol, resorcinol, quinone, and muconic acid are detected when the sulfidized samples contain low amounts of WS225 and the same intermediates are observed by using oxidized samples with high amounts of WO3. The powders obtained by oxidation of WS2 are generally more active than those obtained by sulfidation of WO3. Even if the more efficient samples of the two sets of catalysts exhibit nearly identical rates of phenol degradation, a higher extent of mineralization is reached by employing the oxidized powder. The differences in photoactivity may be related to morphological and structural differences between the WO3 phases present in the particles of the two mixed WO3/WS2 systems. As revealed by the XRD spectra, WO3 obtained by oxidation of WS2 is strongly oriented along the (001) and (002) planes. These planes are typical of the hexagonal structure of WS2 and represent van der Waals planes consisting, respectively, of parallel monolayers of sulfur and tungsten atoms. The specific surface areas of the oxidized samples are not much different from that of WS2, indicating that the volume of the solid changes very little during the oxidation, as confirmed also by the SEM micrographs. This means that the substitution of the atoms of sulfur with those of oxygen does not cause a significant distortion of the WS2 lattice and consequently the formed oxide substantially keeps the same initial structure of the sulfide. The surface of the WO3 phase will be mainly constituted by oxygen atoms which can establish an acid-base adsorption equilibrium with the phenol molecules dissolved in solution. On the contrary, in the samples obtained by sulfidation of the commercial oxide, monoclinic WO3 is present, which has no preferential crystallographic planes. In this case the surface will consist of both oxygen and tungsten atoms and the formation of active sites involving oxygen atoms will be not particularly favored. The higher activity of the powders obtained by oxidation might be then explained by hypothesizing a more significant interaction between the organic substrate and the oriented WO3 phase present on the surface of these samples. Moreover, it is worth mentioning that according to the diffuse reflectance spectra of commercial WO3 and of the powder obtained by a complete oxidation of WS2, the two different crystalline structures have slightly different electronic properties. The lower Eg value of the home-prepared oxide probably implies that the position of its energetic bands is more favorable for the vectorial displacement of electrons and holes between the particles of WO3 and WS2. Conclusions Mixed WO3/WS2 systems were prepared by oxidation of WS2. The samples were characterized by bulk and surface techniques and tested as catalysts for the photodegradation of phenol. The mixed powders are more photoactive than the pure tungsten chalcogenides and their efficiency strongly depends on the relative amounts of WO3 and WS2 in the various samples. A

8244 J. Phys. Chem. B, Vol. 103, No. 39, 1999 tentative explanation for the different photocatalytic behavior of the powders is provided, taking into account the number of microheterojunctions WO3/WS2 present on the single particles. The best photocatalytic results are obtained when the surface WS2 is about 0.5. The lower photoefficiency of the mixed WO3/WS2 powders obtained by sulfidation of WO3 is attributed to morphological and structural differences between the WO3 phases contained in the two systems. Acknowledgment. This work was financially supported by the Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (Roma). The valuable contribution of S. Fonti is gratefully acknowledged. References and Notes (1) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 6632. (2) Zamaraev, K. I.; Parmon, V. N. In Photochemical ConVersion and Storage of Solar Energy; Pelizzetti, E., Schiavello, M., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 1991; p 393. (3) Kamat, P. V.; Patrick, B. J. Phys. Chem. 1992, 96, 6829. (4) Serpone, N.; Maruthamuthu, P.; Pichat, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol. A: Chem. 1995, 85, 247. (5) Serpone, N.; Borgarello, E.; Graetzel, M. J. Chem. Soc., Chem. Commun. 1984, 342. (6) Serpone, N.; Borgarello, E.; Pelizzetti, E.; Barbeni, M. Chim. Ind. (Milano) 1985, 67, 318. (7) Serpone, N.; Borgarello, E.; Pelizzetti, E. J. Electrochem. Soc. 1988, 135, 2760. (8) Pichat, P.; Borgarello, E.; Disdier, J.; Hermann, M.; Pelizzetti, E.; Serpone, N. J. Chem. Soc., Faraday Trans. 1 1988, 84, 261. (9) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 732. (10) Ueno, A.; Kakuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 3828. (11) Kobayashi, J.; Kitaguchi, K.; Tanaka, H.; Tsuiki, H.; Ueno, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1395. (12) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834. (13) Rabani, J. J. Phys. Chem. 1989, 93, 7707. (14) Henglein, A.; Gutierrez, M.; Weller, H.; Fojtik, A.; Jirkovsky, J. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 593.

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