3950
Ind. Eng. Chem. Res. 1998, 37, 3950-3955
Active Cadmium Sulfide Photocatalysts for Hydrogen Production from Water Manjit K. Arora, A. S. K. Sinha,* and S. N. Upadhyay Department of Chemical Engineering & Technology, Banaras Hindu University, Varanasi 221 005, India
Cadmium sulfide photocatalysts for H2 production from water using visible light were prepared by different techniques, i.e., liquid-liquid, gas-liquid, and solid-liquid reactions. The crystallite size and distribution, band gap, and stoichiometric composition of cadmium sulfide and consequently the activity were affected by the preparation technique. The activity of cadmium sulfide showed a strong correlation with its semiconducting (n- or p-type) behavior. Cadmium sulfide that was prepared by gas-solid reaction had excess interstitial cadmium ions in the lattice, which had been attributed to its n-type semiconductor behavior and superior activity. The n-type behavior of cadmium sulfide has been correlated to its stoichiometry. Introduction In the past few decades, considerable attention has been focused on the utilization of solar energy for the production of hydrogen from water by photocatalysis. A large volume of work has been reported in the literature where band-gap irradiation of semiconductors in contact with suitable electrolytes results in reduction of H2O to H2 (Darwent and Porter, 1981; Matsumura et al., 1983; Buhler et al., 1984; Sabate et al., 1990; Borrell et al., 1992; De et al., 1996). Among a large number of semiconductor catalysts reported in the literature, CdS, owing to its small band gap (∼2.4 eV) and sufficiently negative flat band position (-0.66 V at pH 7), is suitable for the reaction. The literature shows that the activity of CdS depends on its preparation technique. The common preparation technique adopted by earlier workers, as reported in the literature, is precipitation from aqueous solution of cadmium salt either by reacting with an aqueous solution of Na2S or by bubbling H2S gas. The precipitation frequently follows heat treatment to remove electron traps from the precipitated CdS. However, there is no investigation reported in the literature elucidating the nature and mechanism of the formation of these traps on CdS particulate. Barbeni et al. (1985) precipitated CdS by adding aqueous solution of Na2S to aqueous solution of Cd2+ (NO3- or SO42-) under two different conditions. In one condition, excess S2- was added, whereas in the other 5 times excess Cd2+ was used. It was observed that CdS prepared with excess Cd2+ had much superior activity. Though the authors did not present any characterization study, they attributed changes in crystal and electronic structure and surface modification of CdS for the above observation. Borrell et al. (1992) prepared CdS samples from aqueous solution of Cd(NO3)2 and CdSO4 by either reacting with Na2S solution or bubbling H2S gas. The authors observed that thermal treatment of the precipitated CdS resulted in better activity. They visualized that the thermal treatment led to better crystallinity of the CdS, which removed electron traps and helped in effective charge separation because of greater band bending. * Corresponding author. Phone: 091-0542-317192 (off.), 091-0542-317179 (res.). Fax: 091-0542-316925.
They have further reported that for one preparation of CdS after heat treatment, increase in crystallite size was negligible but activity improved. They concluded that this increase in activity might be due to creation of specific defects on the semiconductor that increased the majority charge carrier. In this context, it is relevant to indicate that Wolkenstein (1960) has reported that for crystallite size less than ∼1000 Å the bending becomes insignificant. Therefore, the above proposition of Borrell et al. (1992) does not appear very sound. On the other hand, smaller crystallites yield greater dispersion and much larger interfacial area for the reaction, leading to better activity per unit mass of CdS. Therefore, it appears that greater band bending due to better crystallinity cannot be taken as the criterion for better activity. Rather, since the reduction of H2O to H2 is mediated by electrons, the enhanced activity should largely result from removal of electron traps and creation of specific defects that increase the majority charge carrier (electrons). In the present work, CdS photocatalysts were prepared by different techniques, viz., liquid-liquid, gassolid, and gas-liquid reactions. The crystallite size, band gap, and semiconducting property (n- or p-type) and activity of CdS depend on the preparation technique. The photocatalytic activity of CdS is observed to be strongly influenced by its semiconducting property. The mechanism of CdS developing n- or p-type semiconducting properties has been discussed. Experimental Section Catalyst Preparation. CdS photocatalysts were prepared by three different techniques as described below. All the reagents used for preparation of catalysts were of analytical reagent grade. Catalyst 1. CdS was precipitated by bubbling pure H2S gas in a dilute aqueous solution (0.1 M) of 3CdSO4‚ 8H2O at room temperature. The solution was stirred continuously with a magnetic stirrer. To ensure complete reaction, the bubbling of H2S gas was continued for an additional 30 min from the time when no precipitate formation was observed visually. The resulting precipitate of CdS was washed free of SO42- ions and dried at 373 K in an air oven for 12 h. Catalyst 2 was prepared by passing pure H2S gas for 2 h over granules (∼3 mm) of 3CdSO4‚8H2O in a
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Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3951
fixed-bed tubular reactor at 473 K. Pure H2S gas, prepared in the laboratory, was passed at a very low flow rate that was not measured. The resulting CdS was washed with distilled water to remove any unreacted CdSO4 and finally dried at 373 K in an air oven. It is appropriate to mention that preliminary studies carried out earlier had shown that the reaction at 473 K becomes complete in less than 30 min. Catalyst 3 was prepared by slow addition of the requisite volume (10% in excess of the stoichiometry) of 0.1 M aqueous solution of Na2S to an aqueous solution of 3CdSO4‚8H2O of the same concentration which was kept stirred by a magnetic stirrer during the reaction. The resulting precipitate of CdS was washed free of Na+ and SO42- ions and dried at 373 K in an air oven for 12 h. Activity Measurement. The photocatalytic activity of various catalysts for hydrogen production from water was studied in a batch reactor. The photoreactor was a 250 mL flat-bottomed flask; one of its sides was also made flat to permit the entry of light though a plane wall. The reactor had a provision for measurement and control of pH and temperature. A 150 W Philips tungsten-halogen lamp was used as the light source. The choice of the light source was based on the availability as well as on the spectral characteristics of the emitted light. No UV or IR filter was used because the spectrum of the light used showed a very negligible emission in the UV range and use of a Pyrex glass reactor further prevented this radiation from reaching the catalyst. IR radiation can only be absorbed in the form of heat and could increase the temperature of the solution. During experimentation, the temperature could be maintained at the desired value; therefore, an IR filter was also not used. Two grams of catalyst of size 200 mesh was suspended with a magnetic stirrer in 250 mL of aqueous solution of concentration 0.01 and 0.004 M with respect to Na2S and Na2SO3, respectively. The pH was maintained at 8.6 during the experimentation by adding requisite quantities of NaOH and acetic acid. The temperature was maintained at 333 K and the solution was deaerated by sparging nitrogen for 2 h prior to irradiation. The gas evolved was collected by water displacement technique and analyzed by an on-line gas chromatograph using a 5 Å molecular sieve column and thermal conductivity detector using nitrogen gas as carrier. Before entering the chromatograph, the evolved gas was passed through a cold trap to remove entrained moisture. Comparison of the retention time of the only peak that appeared on the chromatogram with standard confirmed that the gas was only hydrogen. The reproducibility of data was tested in 3 runs for each catalyst. No significant deviation was observed and the data presented represent the average value. Intrinsic activity of a photocatalyst is largely independent of pH and temperature, since negligible changes in electronic band structure of a semiconductor are expected. Therefore, the activities of various catalysts were compared at constant pH and temperature. The optimum value of pH depends on the electrolytes present in the solution. In the present study, with a mixture of sodium sulfide and sulfite as electrolytes, the optimum pH was observed to be 8.6 and the activities of all the photocatalysts have been compared at this pH. Further, the intensity of light was also not varied in
Table 1. Activity of Catalysts for Hydrogen Production
a
catalyst
hydrogen evolved in 2 h (mL at NTP.g-1 CdS)
1 2 3 1a 2a 3a
0.8 1.8 0.6 1.4 1.9 1.3
Catalyst heated at 1073 K in argon for 5 h.
the present study. However, experiments were conducted by varying the amount of catalysts in the reactor and it was observed that the intensity of light was not a limiting factor. Characterization Studies. X-ray diffraction (XRD) analyses of catalysts were carried out using a Philips PW-1710 X-ray powder diffractometer equipped with a graphite monochromator and Cu KR target of radiation wavelength 1.542 Å. A Shimadzu 160A UV-vis spectrophotometer was used to study the absorbance characteristics of the catalysts. Powdered samples were made into a paste with Nujol and mounted as a thin film on a strip of chromatography paper. A similar strip with only a film of Nujol was used as a reference. Thermogravimetric analysis of catalysts was performed with a Stanton RedCroft STA 700 analyzer. The total loss in the weight and rate of loss were obtained. Temperature-programmed oxidation of catalysts was carried out on a Micromeritics Pulse Chemisorb 2705 with temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). Oxygen in argon (5.0%) was used for oxidation. To determine the stoichiometry of the catalysts, seven independent stock solutions of each catalyst were prepared by dissolving 100.0 mg of the catalyst, predried at 393 K for 6 h, in a minimum volume of hot concentrated nitric acid. The solution was made up to 1000 mL by adding distilled water. Aliquots of the solution were further diluted to prepare a solution of concentration equivalent to 2.0 mg/L of CdS. The concentration of cadmium ion in each solution was determined by an atomic absorption spectrometer and the atomic ratio of Cd/S was established by back calculations. Results and Discussion Activity. Activity of the catalysts measured as milliliters of H2 evolved (at NTP) in 2 hours of irradiation (given in Table 1) are seen to vary widely. it is seen from the table that catalyst 2, which was prepared by the solid-gas reaction, showed a much superior activity in comparison to others. It is also observed that the heat treatment of the catalysts at 1073 K in argon gas for 5 h resulted in improvement in activity in all the catalysts. The improvement is significant for catalysts 1 and 3 where hydrogen evolution nearly doubled. Further, even after heat treatment the superior activity of catalyst 2, in comparison to catalysts 1 and 3, remains. X-ray Diffraction Studies. The X-ray diffraction patterns of the catalysts are shown in Figure 1. The peak positions were located accurately and the d values were calculated by applying Bragg’s equation. A comparison of calculated d values with the standard data showed that all the peaks correspond to the hexagonal phase of cadmium sulfide. Crystallite size distribution and mean crystallite size of the catalysts were estimated
3952 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 2. Unit Cell Parameters and Mean Crystallite Size of Catalyst unit cell parameters
Figure 1. XRD pattern of various catalysts (unsupported). [h,k,l] and d values are shown.
catalyst
a (Å)
c (Å)
c/a
crystallite size (Å)
1 2 3
4.135 4.132 4.132
6.710 6.743 6.712
1.623 1.632 1.624
61 97 62
of crystallites having maximum density because the distribution for each catalyst, and particularly for the catalyst 1, is not symmetrical but exhibits a wide distribution in the higher size range. The crystallites in catalysts 1 and 3 are smaller than that in catalyst 2; still, this catalyst exhibited much superior activity. Further, the heat treatment of catalysts that increases the crystallite size of CdS (Borrell et al., 1992) also resulted in higher activity. Since, as discussed earlier, for particulates the band bending is insignificant (Wolkenstein, 1960), the increased activity as a result of efficient charge separation due to greater band bending resulting from better crystallinity on heat treatment cannot be attributed for increased activity. On the other hand, the increase in crystallite size means loss in interfacial area for the reaction. Thus, it may be concluded that the activity of catalysts is much more dependent on other properties. The unit cell parameters of CdS for each catalyst were calculated (Table 2). A comparison with the standard values (a ) 4.136 Å and c ) 6.713 Å) shows that the calculated values of catalysts 1 and 3 match with the standard. However, the c parameter of catalyst 2 is larger and also the c/a ratio does not match with the standard value of 1.623, which indicates a larger unit cell in catalyst 2. The increase in lattice constant is a result of excess cadmium located interstitially in the lattice of CdS. The increase in lattice constant of CdO with interstitial cadmium has been discussed by Solymosi (1976). The same appears to be the case for catalyst 2. Optical Absorption Studies. The absorbance spectra of various catalysts were recorded for wavelengths between 450 and 700 nm. The absorbance increases sharply below 550 nm and gradually tapers to a plateau, indicating valence to conduction band transition below 550 nm. The band gap of CdS was calculated by using a relationship for direct allowed transition (Bube, 1974):
R ) k(hν - Eg)1/2/hν
Figure 2. Crystallite size distribution curve of catalysts.
by a line broadening technique. The highest intensity [110] peak of CdS was expanded into a Fourier series and the cosine series was analyzed by the WarrenAverbatch technique (Wagner, 1966). A highly crystalline sample of CdS (CdS heated in argon at 1073 K for 12 h) was used as reference. Asymmetry and shift in the peaks were observed to be negligible; therefore, strain in the catalysts was considered to be negligible (Wagner, 1966). The crystallite size distributions are shown in Figure 2 and the mean crystallite sizes are reported in Table 2. It is observed that the crystallite size as well as the distribution is affected by the preparation technique of the catalysts. Further, in each catalyst, the mean crystallite size is larger than the size
(1)
where R is the absorption coefficient calculated from observed absorbance and hν and Eg are the energy of the incident photon and the band gap of the semiconductor, respectively. Plots of (Rhν)2 versus hν (Figure 3) yielded straight lines, which confirmed direct bandgap irradiation. Band gap for each catalyst, i.e., the intercept on the x-coordinate, has been given in Table 3. The band gap of each catalyst match with the value of ∼2.4 eV reported in the literature. It is observed from the table that catalyst 2, which showed the highest activity, has the least band gap (2.32 eV). The band gap of catalyst 3 is slightly higher but a significant increase is observed in catalyst 1. It is relevant to discuss here that the band gap of a semiconductor is influenced by the crystallite size. The shift in the band gap is a result of the quantization effects that arise from the confinement of the charge carriers in semiconductors with potential wells of small dimensions. A number of earlier workers (Brus, 1983,
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3953 Table 5. Weight Loss and Cd/S Atomic Ratio after Heat Treatment in Argon at 1073 K
Figure 3. Plot for the band gap of the catalysts. Table 3. Band Gap of Catalysts catalysts
band gap (ev)
1 2 3
2.399 2.322 2.338
Table 4. Results of Atomic Absorption Spectroscopy Cd/S atomic ratio for sample catalyst
1
2
3
4
5
6
7
average
1 2 3
0.90 1.04 0.88
0.93 1.10 0.88
0.98 1.14 0.90
0.90 1.09 0.88
0.93 1.08 0.90
0.88 1.14 0.88
0.90 1.14 0.93
0.92 1.10 0.89
1984; Rossetti et al., 1984, 1985a,b; Nozik et al., 1985; Avudhai and Kutty, 1987) state that when the particle size becomes smaller than the diameter of the 1s exciton in the bulk crystalline material, the fundamental band edge shifts toward the blue. This diameter has been calculated to be around 60 Å for CdS (Rossetti et al., 1984). Though the catalysts studied in the present study show crystallite size distribution, a significant portion of the crystallites in catalysts 1 and 3 are less than 60 Å. Therefore, the increase in the band gap of catalysts 1 and 3 with respect to that of catalyst 2 may be attributed to the smaller crystallite sizes (
