J. Phys. Chem. C 2007, 111, 13437-13441
13437
Efficient Hydrogen Production by a Composite CdS/Mesoporous Zirconium Titanium Phosphate Photocatalyst under Visible Light Dengwei Jing and Liejin Guo* State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, P. R. China ReceiVed: March 1, 2007; In Final Form: June 20, 2007
A novel composite CdS/meoporous zirconium titanium phosphate (ZTP) photocatalyst working under visible light was successfully prepared by a two-step thermal sulfidation procedure. The composite photocatalyst prepared by this method was found to show an activity superior to that of catalysts prepared by direct sulfidation either at room or high temperature. By choosing different Zr/Ti ratios, the conduction band of ZTP could be continuously adjusted. The optimal Zr/Ti ratio was found to be 1:3, where the energy difference between the conduction bands of CdS and ZTP can ensure a large driving force for fluent electron transfer from CdS to ZTP and the conduction band of ZTP substrate is still high enough for efficient hydrogen production. The quantum yield of this composite photocatalyst at 420 nm as measured in experiments reached 27.2%.
1. Introduction Photocatalytic hydrogen production occurring under solar illumination has attracted increasing attention for the development of fossil-fuel-independent energy systems.1-3 Taking the solar spectrum into account, it is indispensable to develop visible-light-driven photocatalysts instead of photocatalysts responding only to UV light. Unfortunately, the number of photocatalysts working under visible light irradiation is still limited.4-6 Among all of the visible-light-sensitive photocatalytic materials developed so far, CdS is one of the most active, owing to its suitable band gap (2.3 eV) that corresponds well with the spectrum of sunlight and its negative conduction band allowing for the efficient reduction of water into hydrogen.7 However, CdS is prone to photocorrosion in photocatalytic reactions where CdS itself is oxidized by photogenerated holes.8 Attempts have been made to improve the stability of the metal sulfide by, for example, embedding CdS particles in a polymer matrix, coupling CdS with another wide-band-gap semiconductor, or incorporating nanoparticles of metal sulfides into interlayer photocatalysts.9-11 However, few of these efforts have been successful in terms of increasing both the stability and the efficiency of CdS. Recently, a novel two-step route for the preparation of highly stable and active CdS photocatalyst with a special nanostep surface structure was reported by our group.12 In this method, freshly prepared CdO obtained by decomposing cadmium acetate at a certain temperature was subjected to thermal treatment in H2S atmosphere. The apparent quantum yield at 420 nm and the energy conversion efficiency for the entire visible light range over this CdS photocatalyst reached 24.1% and 6.35%, respectively. It is noted that the particle size of CdS samples prepared by the above two-step route is relative large so that it lead to a low surface area.12 This suggests that choosing substrates of high surface area to prepare highly dispersed CdS-supported photocatalyst might provide one approach that could be used to further increase the stability and activity of this CdS * To whom correspondence should be addressed. E-mail: lj-guo@ mail.xjtu.edu.cn.
photocatalyst. In fact, a number of reports have been published regarding the synthesis and application of CdS supported on silicate-based mesoporous molecular sieves such as MCM41 and SBA15. However, none of these supports are, themselves, photoactive under light irradiation.13,14 It is well-known that the coupling of two semiconductors often leads to enhanced efficiency of photoinduced charge-carrier separation and thus high activity of the composite photocatalyst. One can thus suppose that supporting CdS on a photoactive support of high surface area might greatly enhance the activity of CdS. Recently, the synthesis of CdS supported on photoactive aluminosilicate zeolites containing Ti centers such as ETS-4 and ETS-10 has been reported.15 Although improved activity of CdS was achieved, the authors did not report the long-term stability of the photocatalyst. In this regard, it should be noted that the dissolution of SiO2 in supports such as MCM41 or ETS in strongly basic Na2S/Na2SO3 solutions, which are often employed as sacrificial agents in photocatalytic hydrogen production, is almost inevitable. Therefore, a stable silicon-free support with a high surface area is highly desired. Very recently, the preparation of mesoporous zirconium titanium mixed phosphates (ZTPs) with a range of compositions was reported.16 These materials showed both cation- and anionexchange capacity and can decompose pure water for hydrogen production under UV light irradiation. These properties make ZTPs ideal candidates as photocatalyst supports for CdS. Furthermore, the adjustable Zr/Ti ratio of this material offers the unique opportunity to control the optical properties of the support and the charge transfer within the composite photocatalyst. By optimizing the preparation parameters, significantly improved photocatalytic activity can be achieved. 2. Experimental Section 2.1. Preparation of Various CdS Samples. Mesoporous zirconium titanium phosphates (ZTPs) with various Zr/Ti ratios were prepared as described in the literature.16 In a typical synthesis, octadecyltrimethylammonium chloride (C18TMACl, Aldrich) was first dissolved in deionized water (90 g, 5 mol) by heating at 50-60 °C. Zirconium n-propoxide as the
10.1021/jp071700u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
13438 J. Phys. Chem. C, Vol. 111, No. 36, 2007 zirconium precursor was then added, followed by a dropwise addition of H3PO4. The gel immediately formed was homogenized by vigorous stirring for 30 min. Titanium isopropoxide dissolved in 2-propanol was added very slowly, and the pH of the gel was adjusted to 6.5 after 1 h of stirring. Finally, the gel was stirred at room temperature for 3 days. Mixed phosphates with a range of molar compositions were synthesized. The initial molar compositions of the gels were 1.0[ZrO2 (0-100)/TiO2 (100-0)]/(1-4)H3PO4/(1-4)C18TMA/400H2O/(0.01-0.25)NH4OH. The product was filtered off, washed with deionized water, and dried under vacuum at room temperature. Surfactants were then removed by extracting samples with HCl/ethanol solution (2 mL of 2.0 mol % HCl in 150 mL of ethanol) at room temperature for 12 h. CdS particles were incorporated into ZTP samples by ion exchange, thermal treatment, and then sulfurization, i.e., a twostep sulfidation method. In a typical procedure, 1.0 g of the prepared ZTP powders was stirred for 24 h in an aqueous solution containing Cd(NO3)2 at 353 K. The mixture was then filtered and washed thoroughly with deionized water to ensure the complete removal of Cd ions possibly located at the surface of the ZTP. The dried Cd ion-exchanged ZTP powders were then thermally treated in a quartz tube at 350 °C for 1 h in flowing air to decompose Cd(NO3)2 and then treated at 400 °C for 2 h in H2S to obtain CdS-incorporated ZTP composite photocatalysts. The final products are denoted as CdS@ZxTyP1, where x and y indicate the molar percentages of Zr and Ti, respectively. As a comparison, we also prepared CdS@ZTP by ion exchange followed directly by sulfidation at room temperature and at 400 °C, i.e., a one-step method. These products are denoted as CdS@ZxTyP-2 and CdS@ZxTyP-3, respectively. 2.2. Characterization. X-ray diffraction (XRD) patterns were obtained on a PANalytical X’pert MPD Pro X-ray diffractometer using Cu KR irradiation. Elemental analysis was conducted on a Bruker S4 PIONEER X-ray fluorescence (XRF) spectrum instrument, using a Ru target and a 4 kW maximum power. The sample morphology was observed with a JEOL JEM 2010 transmission electron microscope. N2 adsorption-desorption isotherms at 77 K were measured using a Beckman Coulter SA3100 instrument. The diffuse reflection of the samples was determined with a Hitachi U-4100 UV-vis-near-IR spectrophotometer. Photoluminescence (PL) spectra were recorded on a Hitachi F-3010 fluorescence spectrophotometer. 2.3. Evaluation of Photocatalytic Activity. Photocatalytic hydrogen evolution was performed in a side-irradiation Pyrex cell. The hydrogen evolved was analyzed by an on-line thermal conductivity detector (TCD) gas chromatograph (NaX zeolite column, nitrogen as the carrier gas). In all experiments, 200 mL of deionized water containing 0.2 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed as the sacrificial agent was added into the reaction cell. Here, the sacrificial agent was used to scavenge photogenerated holes. Nitrogen was purged through the cell before the reaction to remove oxygen. The reaction solution was replaced by a fresh one every 10 h as one run to compensate for the loss of sacrificial agent, and the whole system was evacuated and purged with nitrogen after each run. Pt as a cocatalyst for the promotion of hydrogen evolution was photodeposited in situ on the photocatalyst from the precursor H2PtCl6‚H2O. The temperature for all photocatalytic reactions was kept at 35 ( 5 °C. A 300 W Xe lamp was used as the light source, and the UV part of the light was removed by a cutoff filter (λ > 430 nm, T ) 65%). The apparent quantum yields and energy conversion efficiency in the visible light region were determined as reported previously.12
Jing and Guo
Figure 1. PL emission spectra for various CdS@Z25T75P samples. Excitation: 370 nm.
3. Results and Discussion 3.1. Comparison of Different Preparation Methods. As a comparison, Z25T75P was chosen as the support for the preparation of CdS@ZTP also by ion exchange followed directly by sulfidation at room temperature or at 400 °C for 3 h (denoted as CdS@ZTP-2 and CdS@ZTP-3, respectively). Photocatalytic hydrogen production rates for the three photocatalysts with 2 wt % Pt loading were recorded. The results showed that the activities of CdS@ZTP-2 and CdS@ZTP-3 are 117 and 74 µmol/h, respectively, much lower than that of CdS@ZTP-1 prepared by the two-step sulfidation method, which amounts to a remarkable 453 µmol/h. Photoluminescence (PL) measurements were thus carried out to reveal the difference in chargetransfer properties for CdS@ZTP prepared by these three methods, with pure CdS prepared by two-step thermal sulfidation as the reference. As shown in Figure 1, an intense emission peak centered at ca. 734 nm can be observed for all of the CdSbased samples, and the PL peak intensities increase in the order CdS@ZTP-1 < CdS@ZTP-2 < CdS@ZTP-3. As has been reported, the peak at 734 nm could be related to electron-hole recombination within the semiconductor.17 The most significant quenching of PL for CdS@ZTP-1 indicates the efficient charge transfer within this sample. In addition to the good crystallinity, this efficient charge transfer can be ascribed to the additional step of thermal treatment in air in the preparation of CdS@ZTP1, which is assumed to result in the formation of a -Cd-OZTP intermediate. The interface oxygen atoms in the -CdO-ZTP intermediate can then be replaced in situ by sulfur atoms during the thermal sulfidation process. The strong covalent linkage, and therefore the close contact that is indispensable for efficient charge transfer between CdS and ZTP, is thus expected to be achieved by such a two-step procedure. Detailed characterization and investigation of the underlying mechanism behind the above-described transformation is in progress in our group and will be presented later. In the following sections, the two-step thermal sulfidation method is studied, and our attention is focused on the characterization and optimization of CdS@ZTP-1 photocatalysts with various Zr/Ti ratios to investigate the effects of the optical properties of ZTP on the photoactivity of the composite photocatalyst. 3.2. Characterization of Various CdS@ZTP-1 Samples. As a control experiment, all of the as-prepared mesoporous ZTP materials were thermally treated first in air at 673 K for 1 h and then in H2S atmosphere for 2 h. Elemental analysis by X-ray
Hydrogen Production by a Composite CdS/ZTP Photocatalyst
J. Phys. Chem. C, Vol. 111, No. 36, 2007 13439
Figure 2. Low-angle XRD patterns for Z25T75P (a) before and (b) after thermal treatment in air at 673 K for 1 h and then in H2S atmosphere for 2 h.
Figure 3. UV-vis spectra of ZTP samples with various Zr/Ti ratios thermally treated at 673 K in air for 1 h and then in H2S atmosphere for 2 h.
TABLE 1: N2 Absorption Results and Band Gaps for Various ZTP Samples
TABLE 2: N2 Absorption and Elemental Analysis Results for Various CdS@ZTP
before calcination sample Z100T0P Z75T25P Z50T50P Z25T75P Z0T100P
BET pore BET pore band gap (m2/g) diameter (nm) (m2/g) diameter (nm) (eV) 237 214 187 259 217
2.7 2.6 2.8 2.2 2.3
sample
CdS contenta (wt %)
particle sizeb (nm)
surface area
CdS@Z100T0P CdS@Z75T25P CdS@Z50T50P CdS@Z25T75P CdS@Z0T100P
4.62 4.63 4.87 4.82 4.79
6.7 6.9 8.1 7.9 8.3
87 83 74 78 71
after calcination
112 122 107 131 117
6.7 5.9 6.3 7.1 6.8
3.64 3.57 3.45 3.41 3.32
fluorescence (XRF) showed that the compositions of the ZTP materials were essentially unchanged by the treatment, indicating their chemical stability against thermal sulfidation treatments. The N2 absorption results summarized in Table 1 show that all of the ZTP samples experienced a distinct reduction in surface area and an increase in pore diameter as a result of the thermal treatment. However, as shown by the representative low-angle XRD patterns in Figure 2, after the initial thermal treatment in air at 673 K for 1 h and then in H2S atmosphere for 2 h, the single and broad peak characteristic of ZTP, indicating the existence of mesoporous phases, remained, although the intensity of the peak decreased. The single XRD pattern indicates that the mesoporous walls of ZTP are somewhat less ordered. UVvisible spectra for the thus-treated ZTP materials are shown in Figure 3. As can be seen in this figure, the absorption onsets shifted continuously from longer to shorter wavelength as the Zr content in the ZTP samples increased. According to Kubelka-Munk theory, the band gaps of ZTP samples were calculated to be in the range of 3.32-3.64 eV as listed in Table 1. Considering that the valance band of ZTP, which consists mainly of the O 2p orbital, has a relatively fixed energy position as the Zr/Ti ratio varies, the continuous shift of the absorption onsets indicates that the conduction band of ZTP semiconductors can be precisely controlled by changing the Zr/Ti ratio in ZTP. For CdS@ZTP samples prepared by the two-step thermal sulfidation method (CdS@ZxTyP-1), elemental analysis by XRF, as summarized in Table 2, confirms that the loading of CdS is determined mainly by the initial amount of Cd(NO3)2 employed in the ion-exchange process, regardless of the Zr/Ti ratio of the ZTP. This is not surprising considering that cation-exchange property of ZTP is due only to its defective P-OH group.16 Figure 4 further shows the XRD patterns for various CdS@ZTP-1
a
Obtained by XRF. b Calculated by the Scherrer equation.
Figure 4. Wide-angle XRD patterns of CdS@ZTP-1 samples with various Zr/Ti ratios.
samples with a 5% CdS loading. No XRD patterns other than that associated with hexagonal CdS can be found for any of these samples. The bulk of the ZTP supports are still in an amorphous state after the 400 °C thermal treatment. The intense XRD peaks for CdS crystallinites within the ZTP supports reveal their good crystallinity. Calculations by the Scherrer equation, as summarized in Table 2, revealed that the particles of CdS within these supports have similar sizes, all below 10 nm. It should be noted that, because of the strong background resulting from the amorphous ZTP, the calculated particle size of CdS might not truly reflect the real case, as is often expected for the Scherrer equation. For this reason, the UV-vis spectra of
13440 J. Phys. Chem. C, Vol. 111, No. 36, 2007
Figure 5. UV-vis spectra of various CdS@ZTP-1 samples and pure CdS obtained by thermal sulfidation.
Figure 6. TEM image of CdS@Z25T75P-1.
various CdS@ZTP samples were recorded. As shown in Figure 5, all of the CdS@ZTP samples showed obviously blue-shifted absorption onsets compared to bulk CdS also prepared by the two-step thermal sulfidation method. The blue shift of the absorption onset indicates the existence of certain quantum confinement effects that suggest that the CdS particles in CdS@ZTP are of nanoscale sizes. It is also noted that the absorption onsets for all of the CdS@ZTP samples were located in similar ranges of wavelengths around 500 nm, revealing again that they have similar particle sizes. The absorption onset at 500 nm also ensures the strong optical absorption of the prepared materials in the visible light range, even though the CdS particles are nanoscale in sizes. The N2 absorption results in Table 2 indicate that the surface areas of CdS@ZTP-1 samples with various Zr/Ti ratios are similar and within the range of 70-90 g/m2. Figure 6 shows a transmission electron microscopy (TEM) image of the representative 5% CdS-loaded CdS@Z25T75P-1 sample. As can be seen, highly crystalline CdS nanoparticles of ca. 5 nm in size are evenly distributed within the amorphous ZTP matrix, as indicated by the fringes of crystal planes in the black circles, in good agreement with the XRD results. 3.3. Photocatalytic Properties of Various CdS@ZTP Samples. Photocatalytic hydrogen production over 5% CdSloaded CdS@ZTP-1 samples of various Zr/Ti ratios is given in Figure 7. As shown in the figure, the hydrogen production rate
Jing and Guo
Figure 7. (Left) Photocatalytic hydrogen production and (right) absolute extent of PL quenching for CdS@ZTP-1 samples with various Zr/Ti ratios.
initially increases and then undergoes a fast decrease as the Ti content increases from 0 to 1. The composite photocatalyst with a Zr/Ti ratio of 1:3 (CdS@Z25T75P-1) showed the highest activity toward hydrogen production. Its quantum yield at 420 nm was determined to be 27.2%. PL spectra for these samples were recorded, and they all showed certain quenching of fluorescence compared to pure CdS, indicating the occurrence of charge transfer from CdS to ZTP. The absolute extent of PL quenching for all of the CdS@ZTP-1 samples was calculated by comparing their absorption intensity with that of pure CdS at 734 nm. The results are also included in Figure 7. As expected, the strongest quenching of PL was obtained for CdS@Z0T100P-1, within which ZTP support has a lowest conduction band and the largest energy difference between CdS and ZTP support, providing enough driving force for charge transfer between two components. However, as shown in Figure 7, the photocatalytic hydrogen production of CdS@Z0T100P-1 is much lower than that of CdS@Z25T75P-1, which has a higher conduction band, as revealed by Figure 3. It is also well-known that, for efficient photocatalytic hydrogen production, the photocatalyst should have a sufficiently high (or negative) conduction band position. Therefore, in addition to efficient charge transfer within the composite CdS@ZTP-1 photocatalyst, the ZTP should also have high enough conduction to meet the requirements for efficient hydrogen production. It is thus inferred that, for CdS@Z25T75P-1 with a Zr/Ti ratio of 1:3, the conduction band of ZTP might be at an optimal position where the energy difference between the conduction bands of CdS and ZTP can ensure a sufficient driving force for smooth electron transfer from CdS to ZTP, whereas the conduction band of the ZTP substrate is still negative enough for efficient hydrogen production. Charge transfer within the composite photocatalyst is schematically illustrated in Figure 8, where the conduction band positions of various ZTP samples (represented by five colored lines) are approximately determined by their absorption onsets recorded in Figure 3. It should be noted that, for a photocatalyst to be commercially viable, the stability of the photocatalyst is a key issue. To test the stability of our prepared composite photocatalyst, photocatalytic hydrogen production over 5 wt % CdS-loaded CdS@ZTP-1 sample with a Zr/Ti ratio of 1:3 was carried out for an extended period of 100 h with periodic flushing of nitrogen and replacement of the sacrificial agent every 10 h. No noticeable decrease of activity was found for this photocatalyst after 100 h of reaction, indicating its ideal stability (results not shown). It is inferred that in this case, the
Hydrogen Production by a Composite CdS/ZTP Photocatalyst
J. Phys. Chem. C, Vol. 111, No. 36, 2007 13441 0.01 g. It is surprising that the quantum efficiency of pure CdS is only 2.2% when 0.01 g of photocatalyst was used. This again demonstrates the important role of ZTP in significantly improving the activity of CdS. 4. Conclusion
Figure 8. Schematic illustration of charge-separation mechanism in CdS@ZTP.
A novel composite CdS/meoporous ZTP photocatalyst working under visible light was successfully prepared by a two-step thermal sulfidation procedure. The composite photocatalyst prepared by this method was found to show an activity superior to that of a photocatalyst prepared by direct sulfidation at room or high temperature. By choosing different Zr/Ti ratios, the ZTP conduction band could be continuously adjusted, and an optimal ZTP conduction band position was found where the energy difference between the conduction bands of CdS and ZTP ensured a large driving force for fluent electron transfer from CdS to ZTP and the conduction band of the ZTP support was still high enough for efficient hydrogen production. In summary, the first example of coupling a narrow-band-gap semiconductor with a photoactive support whose conduction band can be controlled to form a special composite system was demonstrated in this work. By adjusting the conduction band of the photoactive support to achieve an optimal optical match between the sensitizer and the support, highly efficient charge separation and hydrogen production could be attained over the designed composite photocatalyst. Moreover, it should be noted that, in our study, the ZTP in the composite photocatalyst is still in an amorphous state. If a support with a controllable conduction band is used that has a crystalline framework, the activity of the composite photocatalyst is expected to be further enhanced. Acknowledgment. The authors gratefully acknowledge financial support from the National Basic Research Program of China (No. 2003CB214500) and the National Natural Science Foundation of China (No. 50521604, 90610022). References and Notes
Figure 9. Quantum efficiencies for photocatalytic hydrogen production of various photocatalysts.
mesoporous ZTP serves as a stable host to protect the loaded CdS particles from photocorrosion and photoinduced detachment, ensuring the long-term stability of the prepared composite photocatalyst, as has also been demonstrated by our previous study on WS2 sensitized mesoporous TiO2.18 The photocatalytic hydrogen production of CdS@Z25T75P-1 with a Zr/Ti ratio of 1:3 was also compared with that of pure CdS prepared by two-step thermal sulfidation, after being loaded with 2 wt% Pt. For easy comparison, the quantum efficiencies of the tested photocatalysts were calculated according to their hydrogen production results and are summarized in Figure 9. As can be seen in this figure, CdS@Z25T75P-1 showed an obviously higher activity than pure CdS when the same amount (0.2 g) of photocatalyst was used in both cases. Considering that the content of CdS in CdS@Z25T75P-1 is 5 wt %, the true amount of CdS employed in the photocatalytic test should be
(1) Domen, K.; Kudo, A.; Ohnishi, T. J. Catal. 1986, 102, 92. (2) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (3) Zou, Z.; Ye, J.; Arakawa, H. J. Phys. Chem. B 2002, 106, 517. (4) Kudo, A.; Sikizawa, M. Chem. Lett. 1999, 58, 241. (5) Kudo, A.; Tsuji, I.; Kato, H. Chem. Commun. 2002, 1958. (6) Lei, Z. B.; Ma, G. J.; Liu, M. Y.; You, W. S.; Yan, H. J.; Wu, G. P.; Takata, T.; Hara, M.; Domen, K.; Li, C. J. Catal. 2006, 237, 322. (7) Ashokkumar, M. Int. J. Hydrogen Energy 1998, 23, 427. (8) Meissner, D.; Memming, R.; Kastening, B. J. Phys. Chem. 1988, 92, 3476. (9) Mau, A.; Huang, C. B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. (10) Fujii, H.; Ohtaki, M.; Eguchi, K. J. Mol. Catal. A 1998, 129, 61. (11) Guan, G. Q.; Kida, T.; Kusakabe, K.; Kimura, K.; Fang, X. M.; Ma, T. L.; Abe, E.; Yoshida, A. Chem. Phys. Lett. 2004, 385, 319. (12) Jing, D. W.; Guo, L. J. J. Phys. Chem. B 2006, 110, 11139. (13) Hirai, T.; Okubo, H.; Komasawa, I. J. Colloid Interface Sci. 2001, 235, 358. (14) Hirai, T.; Bando, Y. J. Colloid Interface Sci. 2005, 288, 513. (15) Guan, G.; Kida, T.; Kusakabe, K.; Kimura, K.; Abe, E.; Yoshida, A. Appl. Catal. 2005, 295, 71. (16) Kapoor, M. P.; Inagaki, S.; Yoshida, H. J. Phys. Chem. B 2005, 109, 9231. (17) Shangguan, W. F.; Yoshida, A. J. Phys. Chem. B 2002, 106, 12227. (18) Jing, D. W.; Guo, L. J. Catal. Commun. 2007, 8, 795.
