Effect of Noble Metal in CdS/M/TiO₂ for Photocatalytic Degradation of Methylene Blue under Visible Light

ABSTRACT

The CdS/M/TiO₂ (M = Ag, Ru, Au, Pd, and Pt) three-component nanojunction systems were constructed using a two-step photodeposition method, and evaluated for their photocatalytic activities through the degradation of methylene blue in aqueous solution under visible light irradiation. The authors found that the photocatalytic activity of CdS/M/TiO₂ (M = Ag, Ru, Au, Pd, Pt) three-component nanojunctions was superior to that of CdS/TiO₂ two-component system. Moreover, the photocatalytic activity of the three-component nanojunction system was found to be dependent significantly on the type of the noble metals. The results can be explained by the influence of charge transfer on the basis of the work functions of different noble metals.

KEYWORDS:

• CdS/M/TiO2
• heterojunction
• noble metal
• photocatalysis
• Z-scheme

INTRODUCTION

Since the discovery of photo-induced splitting of water on TiO₂ electrodes,^{[ 1 ]} water splitting and environmental clean-up become two active fields supported by heterogeneous photocatalysis.^{[ 2 , 3 ]} Environmental applications of heterogeneous photocatalysis have been considered among the most effective methods for elimination of many hazardous organic pollutants from the environment, particularly from the wastewater. Heterogeneous photocatalysis offers a “green” technology for completely decomposing such contaminants in the presence of semiconducting catalyst particles.

Among numerous semiconducting photocatalysts developed so far, TiO₂ is regarded to be the most efficient and environmentally benign, which has been most widely used for photodegradation of various pollutants.^{[ 4 ]} However, its large band gap along with a fast recombination rate of photogenerated electron-hole pairs in TiO₂ hinders its photocatalytic activity, limiting further commercialization and industrial applications. One effective approach to overcome such a hurdle involves coupling of different semiconductors with appropriate energy levels. For instance, CdS/TiO₂ heterojunction has been widely studied for effective decomposition of organic compounds^{[ 5 , 6 , 7 ]} and for photocatalytic water splitting.^{[ 8 , 9 , 10 ]} In CdS/TiO₂ heterojunction, CdS with narrow band gap acts as a visible-light sensitizer; combined with TiO₂, CdS is also responsible for effective charge separation that enables suppression of the recombination process.^{[ 11 , 12 ]}

In the past decade, Z-scheme photocatalytic systems (two-step photoexcitation) mimicking photosynthesis in a green plant have been investigated to realize a system for overall water splitting that are composed of H₂- and O₂-photocatalysts and a suitable electron mediator.^{[ 13 , 14 , 15 , 16 , 17 ]} The electron mediator in solution plays an indispensable role in shuttling the photo-generated carriers between the H₂- and O₂-photocatalysts. Recently, Kudo and co-workers succeeded in constructing a simple Z-scheme photocatalytic system driven by interparticle electron transfer without an electron mediator under visible light irradiation.¹⁸ In this (Ru/SrTiO₃:Rh)-(BiVO₄) system, the excited electrons in SrTiO₃:Rh reduce water to form H₂ on Ru co-catalyst, the holes in BiVO₄ oxidize water to form O₂ to accomplish overall water splitting, and the reversible Rh species at the surface of photocatalyst plays a pivotal role for electron transfer between particles. On the other hand, all solid-state Z-scheme CdS/Au/TiO₂ three-component nanojunction systems have been developed to show much higher photocatalytic activity than single- and two-component systems for decomposition of organic compounds.^{[ 19 , 20 ]} In such a three-component system, photo-induced electrons achieved a vectorial transfer of TiO₂ → Au → CdS through a two-step excitation of CdS, TiO₂, and Au as a mediator. Similarly, metal Ag species in the AgBr-Ag-Bi₂WO^{[ 21 ]} ₆ and AgBr-Ag-TiO²² ₂ Z-schematic nanojunction systems also acted as the electron transfer mediator, contributing to the enhancement of electron-hole separation and interfacial charge transfer. However, to the best of our knowledge, there have been few reports on the effects of different noble metals (such as Au, Ag, etc.) on the photocatalytic activity of such three-component Z-scheme systems.

In the present study, we constructed CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component Z-scheme nanojunction systems with different noble metals as the electron transfer mediator. These CdS/M/TiO₂ nanojunctions prepared by a two-step photodeposition method were used to carry out photocatalytic degradation of methylene blue. In particular, we investigated the effect of noble metals (Ag, Ru, Au, Pd, and Pt) on the photocatalytic activity of such Z-scheme systems.

EXPERIMENTAL

Sample Preparation

TiO₂ (P25) was acquired from Nippon Aerosil. Cd(NO₃)₂·4H₂O, Na₂S·9H₂O and ethanol were purchased from Sigma-Aldrich and sulfur powder was purchased from Allied Chemical. Aqueous solutions of noble metal compound were prepared from H₂PtCl₆·H₂O (Sigma-Aldrich), AgNO₃ (J.T. Baker), Na₂PdCl₄ (Aldrich), RuCl₃ (Strem), and HAuCl₄·4H₂O (Sigma-Aldrich). All chemicals were used as received.

CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component photocatalysts were prepared by the two-step photodeposition approach,^{[ 19 ]}as described in the Supplementary Material, Scheme 1. In a typical synthesis process, 0.1 g P25 TiO₂ was dispersed in 100 mL ethanol aqueous solution (v:v= 1:1). To this suspension, a 0.4 mg/mL (noble metal) aqueous solution (0.75 mL) of noble metal compound was added. Under stirring, the mixture was bubbled with argon for 30 min and then irradiated with a 150-W Xe lamp for 5 h to load noble metal on TiO₂ (M [0.3 wt%]/TiO₂). After the resulting M/TiO₂ ethanol/water suspension, added with sulfur powder (0.011 g) and Cd(NO₃)₂·4H₂O (0.154 g), had been bubbled with argon for 30 min, irradiation was carried out for a given period with a 150-W Xe lamp. Then the products CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) were centrifuged, washed with water and ethanol, and dried at 65°C in air. The CdS/TiO₂ photocatalyst without noble metal loading was prepared by photodeposition of CdS on P25 TiO₂ instead of M/TiO₂.

CdS/TiO₂ and CdS/Ag/TiO₂ samples as reference (CdS/TiO₂-R and CdS/Ag/TiO₂-R) were prepared by the precipitation method. P25 TiO₂ (or Ag [0.3 wt%]/TiO₂ prepared by photodeposition), 0.1 g, was dispersed in an 100-mL ethanol aqueous solution (v:v=1:1) containing Cd(NO₃)₂·4H₂O (0.154 g). Under stirring, 10 mL aqueous solution containing double excess of Na₂S was added to the mixture, and the resulting yellow suspension was stirred overnight at room temperature. The products CdS/TiO₂-R and CdS/Ag/TiO₂-R were centrifuged, washed with water and ethanol, and dried at 65°C in air.

In all these photocatalyts, the mass contents of CdS and M (versus TiO₂) were 72.2% and 0.3%, respectively.

Characterization

X-ray diffraction (XRD) patterns were obtained from a PANalytical X’pert diffractometer using Ni-filtered Cu Kα irradiation (wavelength 1.5406 Å) with the scanning step of 0.05°/s, the operation voltage and current were 45 kV and 40 mA, respectively. Ultraviolet-visible (UV-Vis) absorption spectra of the samples were determined on a Varian Cary 50 UV spectrophotometer with MgO as the reference, the scanning range was from 200 to 900 nm. Transmission electron microscopy (TEM) studies were carried out on a JEOL JEM 2010 instrument. Elemental analysis of the photocatalytst was conducted by an energy-dispersive X-ray spectrometer (EDS) attached to the transmission electron microscope.

Photocatalytic Degradation of Methylene Blue

Photocatalytic activity of the photocatalyts was determined by measuring the decomposition of methylene blue (MB) using a 150-W xenon lamp (Newport) as light source at ambient conditions.^{[ 23 , 24 ]} Light was passed through a UV cut-off filter (λ> 400 nm; Newport FS-C S/N 147) (Supplementary Material, Figure S1), and guided onto the side window of an open cuvette filled with 3 mL of MB auqeous solution (optical density ≈1.0) and a given amount of photocatalyst (60 mg/L). The suspension was stirred continuously during the whole process. The concentration of methylene blue was estimated by measuring its maximum absorbance at 664 nm with a Varian Cary 50 UV spectrophotometer after irradiation for a period of time (Supplementary Material, Figure S2).

RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) prepared by photodeposition method. Even though the noble metals (M) in CdS/M/TiO₂ were different, these heterojunctions displayed quite similar crystal structures. All samples exhibited some diffraction peaks assigned to the anatase and rutile phases, which coexisted in P25 TiO₂. In addition, there were three peaks with 2θ values of 26.5°, 44.0°, and 52.1°, corresponding to (111), (220), and (311) crystal planes of cubic CdS (space group F-43m; a= 5.818 Å; JSPDS Card No. 10-0454), respectively. The diffraction peaks assigned to noble metals could not be identified in these CdS/M/TiO₂ heterojunctions because the mass content of noble metals (0.3%, versus TiO₂) is under the detection limit for XRD analysis. The XRD patterns of CdS/TiO₂ and CdS/Ag/TiO₂ prepared by precipitation method as reference (CdS/TiO₂-R and CdS/Ag/TiO₂-R) are shown in Figure 2. They also possessed cubic CdS, anatase and rutile TiO₂ phases, similar to CdS/TiO₂ and CdS/Ag/TiO₂ prepared by the photodeposition method. However, as indicated by the weaker intensity of diffraction peaks assigned to cubic CdS phase in CdS/TiO₂-R and CdS/Ag/TiO₂-R, the cubic CdS in CdS/TiO₂ and CdS/Ag/TiO₂ prepared by the photodeposition method had better crystallinity than the samples prepared by precipitation.

FIGURE 1 XRD patterns of CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) prepared by photodeposition method. (Figure provided in color online.)

FIGURE 2 XRD patterns of CdS/TiO₂ and CdS/Ag/TiO₂ prepared by photodeposition method, and CdS/TiO₂-R and CdS/Ag/TiO₂-R prepared by precipitation method. (Figure provided in color online.)

The UV-Vis diffuse reflectance spectra for CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) are shown in Figure 3. The single TiO₂ (P25) photocatalyst showed a sharp edge, whereas the CdS/TiO₂ composite prepared by photodepostion method had two absorption edges; the main edge due to CdS is located at ∼550 nm and second one due to TiO₂ at ∼400 nm. This is in agreement with the previous report by Jang et al. that the spectra of CdS/TiO₂ composite photocatalysts prepared by precipitation method showed a combination of these two spectra.^{[ 8 , 25 ]} When compared to CdS/TiO₂, all the CdS/M/TiO₂ nanojunctions had two similar absorption edges assigned to CdS and TiO₂. However, additional absorption appeared in the range of 600–900 nm in the spectra of CdS/M/TiO₂. This could be related to the existence of noble metal particles (M=Ag, Ru, Au, Pd, Pt) in these CdS/M/TiO₂ nanojunctions, corresponding to the report by Doremus that small particles of noble metals showed an optical absorption band in visible light that resulted from collective oscillations of the free electrons in them.^{[ 26 ]} The UV-Vis diffuse reflectance spectra of CdS/TiO₂-R and CdS/Ag/TiO₂-R were also given in Figure 4, which makes a comparison between optical property of the samples prepared by photodeposition method and precipitation method. CdS/TiO₂ and CdS/Ag/TiO₂ prepared by different methods had similar spectra with two absorption edges; the increased absorption in visible region (longer than 600 nm) of the CdS/Ag/TiO₂ (or CdS/Ag/TiO₂-R) spectrum was attributable to the presence of Ag, as discussed previously.

FIGURE 3 UV-Vis diffuse reflectance spectra of TiO₂ and CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) prepared by photodeposition method. (Figure provided in color online.)

FIGURE 4 UV-Vis diffuse reflectance spectra of CdS/TiO₂ and CdS/Ag/TiO₂ prepared by photodeposition method, and CdS/TiO₂-R and CdS/Ag/TiO₂-R prepared by precipitation method. (Figure provided in color online.)

To visualize the hybridization of CdS and Ag/TiO₂ in CdS/Ag/TiO₂ prepared by the photodeposition and precipitation method, CdS/Ag/TiO₂ and CdS/Ag/TiO₂-R were investigated by TEM. The representative TEM image of the CdS/Ag/TiO₂-R sample is shown in Figure 5A, displaying separated phases of TiO₂ and CdS, as confirmed by EDS element analysis (Supplementary Material, Figure S3a, b). The Ag signal could not be identified in the EDS spectra, because of the very small amount of Ag in CdS/Ag/TiO₂-R sample. In contrast, the obvious separation of CdS and TiO₂ phases does not occur in CdS/Ag/TiO₂ sample, as shown in Figure 5B. Moreover, the EDS spectra of CdS/Ag/TiO₂ (Supplementary Material, Figure S3c) indicate homogenous dispersion of CdS. Thus, CdS is supposed to deposit on the surface of Ag/TiO₂ particles and cover the Ag sites during the photodeposition process (the formation of CdS/Ag/TiO₂ Z-scheme nanojunction), as deduced from the previous studies that Ag acted as the reduction sites for photocatalytic reduction of S to S²⁻ ions.^{[ 19 , 20 ]} Photocatalytic activity of the CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) nanojunctions was tested by visible-light photodecomposition of methylene blue. Figure 6 shows the normalized optical density change of methlyene blue at ∼664 nm under visible light irradiation (>400 nm) as catalyzed by CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) as a function of time. We found that the pure TiO₂ (P25), which is a well-known UV-active photocatalyst, had a very low visible-light photocatalytic activity. This is because only a small part of light passed through the UV cut-off filter could be absorbed by TiO₂ and utilized for photocatalytic decomposition of methylene blue (Supplementary Material, Figure S1). CdS/TiO₂ two-component nanojunction showed higher photocatalytic activity than pure TiO₂ (P25), due to the photosensitization of CdS in visible light region and higher charge separation in a CdS/TiO₂ nanojunction system.²⁷ Compared to pure TiO₂ and the CdS/TiO₂ two-component nanojunction, all the CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component nanojunctions exhibited higher visible-light photocatalytic performance. The charge transfer mechanism, which is similar to Z-scheme in green plants’ photosynthesis, has been proposed by other researchers.^{[ 19 , 20 , 21 ]} The vectorial photo-generated electron transfer of TiO₂→ M (noble metal) → CdS in three-component nanojunction systems greatly improves the separation rate of photo-induced charges, resulting in higher photocatalytic activity. On the other hand, the photocatalytic activity of the CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component nanojunctions showed a significant dependence on the type of noble metal, and the photocatalytic activity decreased in the order of Ag > Ru ≈ Au > Pd > Pt.

FIGURE 5 TEM images of (A) CdS/Ag/TiO₂-R prepared by precipitation method, and (B) CdS/Ag/TiO₂ prepared by photodeposition method.

FIGURE 6 Photodecomposition of methylene blue catalyzed by CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) nanojunctions prepared by photodeposition method, under the irradiation of visible light (>400 nm) for up to 90 min. (Figure provided in color online.)

Based on the energy band diagram, the charge transfer process in CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component nanojunction systems is illustrated in Figure 7. Considering the noble metal acting as a mediator in the vectorial electron-transfer process, their Fermi energy levels are supposed to affect the interfacial charge transfer and thus the charge separation in the CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component nanojunctions. In general, the minimum energy needed to move an electron from the Fermi energy level into vacuum is defined as the work function.²⁸ According to the different work functions of these noble metals,²⁹ we obtain the height of Fermi energy levels in the reducing order of Ag > Ru > Au > Pd > Pt, as depicted in Figure 7. This trend is in good correlation with the photocatalytic activity of CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) nanojunctions depending on the type noble metals, i.e., the higher the Fermi level of the noble metal, the better the photocatalytic activity of CdS/M/TiO₂.

As the charge transfer mechanism proposed in Figure 7, photo-generated electrons in the conduction band of TiO₂ easily flow into noble metal through the Schottky barrier (electron transfer I: TiO₂→ M), and the holes left in the valence band of TiO₂ are available for oxidation reaction. Simultaneously, the photo-generated holes in the valence band of CdS also easily flow into the noble metal to recombine with the stored electrons (electron transfer II: M → CdS), because of the higher Fermi energy levels of noble metal than the valence band level of CdS, and the electrons left in the conduction band of CdS are available for reduction reaction. Thus, the resulted vectorial photo-generated electrons transfer of TiO₂→ M (noble metal) → CdS realizes the complete separation of photo-generated holes in the valence band of TiO₂ (VB-hole [TiO₂]) and electrons in the conduction band of CdS (CB-electron [CdS]). Therefore, we can deduce that the higher Fermi levels of noble metals in CdS/M/TiO₂ nanojunctions make the electron transfer II (M → CdS) much quicker, because of the larger difference between the Fermi level of noble metal and the valence band level of CdS. This will lead to more efficient charge separation between VB-hole (TiO₂) and CB-electron (CdS), which contributes to the better photocatalytic activity of CdS/M/TiO₂, depending on the noble metals (Ag > Ru ≈ Au > Pd > Pt).

FIGURE 7 Energy band diagram and charge transfer mechanism in CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component nanojunction systems. (Figure provided in color online.)

In order to illustrate the effect of vectorial electrons transfer of TiO₂→ M → CdS on the improved photocatalytic activity of CdS/M/TiO₂ three-component nanojunctions, CdS/TiO₂ two-component nanojunction (CdS/TiO₂-R) and CdS/Ag/TiO₂ three-component nanojunction (CdS/Ag/TiO₂-R) were also prepared for photodegradation of methylene blue by precipitation method. As shown in Figure 8, CdS/TiO₂ (photodeposition method) and CdS/TiO₂-R (precipitation method) displayed almost the same photocatalytic efficiency for methylene blue degradation under visible light irradiation up to 90 min. This is due to the similar hybridization of CdS with TiO₂ and charge transfer in the CdS/TiO₂ two-component nanojucntions (Figure 9a), even though prepared by different methods. CdS/Ag/TiO₂-R obtained via photodeposition of Ag on TiO₂ and subsequent precipitation of CdS on TiO₂ showed higher photoactivity than CdS/TiO₂ two-component nanojunctions, as the Ag metal deposited on TiO₂ would capture the electrons from the conduction band of TiO₂ and accelerate charge separation (Figure 9b). Compared to CdS/Ag/TiO₂-R, the CdS/Ag/TiO₂ three-component nanojunction with different hybridization of CdS with Ag/TiO₂, in which Ag nanoparticles deposited on TiO₂ was covered by CdS via photodeposition (Figure 9c), showed much higher photocatalytic activity.

FIGURE 8 Photodecomposition of methylene blue catalyzed by photocatalysts prepared by photodeposition method (CdS/TiO₂ and CdS/Ag/TiO₂), and by precipitation method (CdS/TiO₂-R and CdS/Ag/TiO₂-R), under the irradiation of visible light (>400 nm) for up to 90 min. (Figure provided in color online.)

FIGURE 9 Schematic illustration of charge transfer in nanojunction systems. (a) CdS/TiO₂ two-component nanojunctions prepared by photodeposition and precipitation (CdS/TiO₂ and CdS/TiO₂-R); (b) CdS/Ag/TiO₂ three-component nanojunction prepared by precipitation method (CdS/Ag/TiO₂-R); (c) CdS/Ag/TiO₂ three-component nanojunction prepared by photodeposition method (CdS/Ag/TiO₂). (Figure provided in color online.)

CONCLUSIONS

Z-scheme CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) three-component nanojunction systems have been successfully constructed via a two-step photodeposition method. Compared to CdS/TiO₂ two-component nanojunction system, the CdS/M/TiO₂ Z-scheme systems exhibited higher photocatalytic activity for methylene blue degradation under visible light irradiation. This enhancement is mainly due to the vectorial electrons transfer of TiO₂→ M → CdS, which greatly improves the separation of photo-induced charges. Moreover, the noble metals (M=Ag, Ru, Au, Pd, Pt) make a great effect on the photocatalytic activities of CdS/M/TiO₂ nanojunction systems, depending on the work functions of the noble metals. The photocatalytic activity of CdS/M/TiO₂ (M=Ag, Ru, Au, Pd, Pt) nanojunction systems decreased in the order of Ag > Ru ≈ Au > Pd > Pt, corresponding to the height of Fermi energy levels in the reducing order of Ag > Ru > Au > Pd > Pt.

Acknowledgments

The authors acknowledge the support by the National Basic Research Program of China (No. 2009CB220000), Natural Science Foundation of China (No. 50821064 and No. 90610022), and the U.S. Department of Energy. One of the authors (S.S.) also thanks the support from the China Scholarship Council.

[Supplementary materials are available for this article. Go to the publisher's online edition of the International Journal of Green Nanotechnology: Materials Science and Engineering to view the free supplementary file.]