J. Phys. Chem. C 2009, 113, 10755–10760
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Photocatalytic Activity of PbS Quantum Dot/TiO2 Nanotube Composites Chalita Ratanatawanate, Yuan Tao, and Kenneth J. Balkus, Jr.* Department of Chemistry and the Alan G. MacDiarmid NanoTech Institute, The UniVersity of Texas at Dallas, Richardson, Texas 75083-0688 ReceiVed: April 2, 2009; ReVised Manuscript ReceiVed: May 8, 2009
PbS quantum dots (PbS QDs) were attached to TiO2 nanotubes on both the inside and outside surfaces of the nanotubes by using thiolactic acid, a bifunctional linker. The PbS QDs with diameters of 4-5 nm were controlled by adjusting the concentration of thiolactic acid. The PbS QDs can be placed only inside the nanotubes by first blocking the outer surface of the TNTs with the double-chain cationic surfactant. The photocatalytic activity and stability of PbS/TiO2 nanotubes were evaluated for the photodegradation of organic dyes. The results indicate that the functionalized TiO2 nanotubes were superior catalysts for photodegradation of cationic dyes. Additionally, the quantum dots enhance the activity and expand the usable portion of the solar spectrum. Introduction About 0.7 million tons of dyes are produced annually worldwide.1 This is expected to increase as consumption of organic dyes grows with the demand for textile colorants, food additives, paints, cosmetics, and so on. It has been reported that approximately 10-15% of the dyes are lost during manufacturing processes,2-5 resulting in a seriously environmental problem. The dye-contaminated wastewater is unattractive and harmful to aquatic life.6,7 Therefore, the decolorization and degradation of organic dyes before release to the environment are important. Various techniques have been employed to remove the dye molecules from wastewater such as adsorption on porous absorbents, filtration, coagulation, ozone treatment, aerobic, and anaerobic biological treatment, and advanced oxidation processes.8-11 Unfortunately, absorption and chemical coagulation result in transformation of dyes to another phase rather than degradation of dyes and create an ongoing waste disposal problem,12 whereas in filtration, low-molar-mass dyes can pass through the filter system. Although chlorination is effective, the byproduct of chlorination are chlorinated organics that may be more toxic than the dye itself.13 Advanced oxidation processes, on the other hand, have shown great promise. The main advantage of this method is its inherent destructive nature. For example, it does not require mass transfer and can be carried out under ambient conditions. Heterogeneous semiconductor photocatalysis has generated significant interest for environmental detoxification. Among the semiconductors most studied, TiO2 (anatase) has proven to be an effective photocatalyst because of its high activity, chemical stability, low cost, and nontoxicity. Nanoscale TiO2 catalysts in the form of nanoparticles, nanorods, nanofibers, and nanotubes have also been studied.14 TiO2 nanotubes (TNT) are particularly interesting because of the unique tubular structure, size confinement in the radial direction, and a large surface-to-volume ratio, which are expected to enhance photoactivity.15 When TiO2 is irradiated by UV photons with energy greater than or equal to the band gap energy, the electron (e-)shole (h+) pairs are generated where the h+ is a powerful oxidant and e- is a strong reductant. The redox reactions of e- or h+ with O2, H2O, and OH- also * Corresponding author. E-mail:
[email protected].
result in the generation of very reactive species such as superoxide (O2•-),15 singlet oxygen (1O2),16 peroxide (H2O2),17 and hydroxyl radicals (OH•).18,19 Those reactive oxygen species (ROS) are known to be nonselective oxidizing agents for many organic pollutants. Although high-energy UV light can be used to activate the TiO2 catalysts, it is not only costly but also can be hazardous. Therefore, the renewable energy from sunlight is the best alternative. Several studies showed the potential use of sunlight for photobleaching of organic dyes. For example, Nogueira and Jardim reported the photodegradation of methylene blue, using sunlight.20 Nappolian and co-workers reported the degradation efficiency of the active blue 4-textile dye under solar irradiation.21 However, the highly efficient use of TiO2 (anatase) is limited by its wide band gap (3.2 eV), meaning that only a small fraction of solar energy (3-5%) can be utilized. To make TiO2 photocatalytically active beyond its absorption threshold of 400 nm, the modification of the TiO2 band gap is required. One approach is to combine TiO2 with a semiconductor that has a narrow band gap and an energetically high-lying conduction band. Many semiconductors have been employed to sensitize TiO2, including CdS, CdSe, PbSe, PbS, etc.22-24 Lead sulfide (PbS) is an attractive semiconductor for this approach because of its small band gap (0.41 eV) and its large exciton Bohr radius of 20 nm, which leads to extensive quantum size effects.25,26 PbS quantum dots (PbS QDs) can further improve the photocatalytic activity of TiO2 because of multiple exciton generation (MEG) and efficient spatial separation of photogeneratedcharge,preventingelectron-holerecombination.27,28 The present study focuses on the photocatalytic behavior of TiO2 nanotubes and PbS quantum dot decorated TNT catalysts for the photodegradation of both anionic and cationic dyes. TiO2 nanotubes were selected as a support because of their tubular shape, providing even higher surface area compared with the commercial nanoparticles such as Degussa P25. The decoration of PbS QDs on both the inside and outside of TNTs can be achieved by using thiolactic acid linkers.29 Furthermore, the PbS QDs can be encapsulated inside of the TNT pore by blocking the outer surface of the TNTs before growing PbS QDs. The photocatalytic activity of as synthesized TiO2 nanotubes and P25 were evaluated for the photodegradation of methylene blue, methylene green, rhodamine B, rose bengal and indigo carmine.
10.1021/jp903050h CCC: $40.75 2009 American Chemical Society Published on Web 05/26/2009
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The effect of PbS QDs was evaluated for the photodegradation of methylene blue by using the visible light >600 nm.
Ratanatawanate et al. TABLE 1: Physical Properties of Dyes
Experimental Section Materials. The Evonik Degussa P25 TiO2 (80% anatase and 20% rutile) was supplied by Evonik (Degussa Corp). Thiolactic acid ( P25. This can be explained by the influence of pH and surface area. The large difference in activity between the P25 and TiO2 nanotube catalysts may reflect the difference in surface area as well as the effect of charge on catalyst surface. Because the pHpzc of TiO2 nanotubes is lower than P25, the TNTs are negatively charged compared to the surface of P25. As a result, the cationic methylene blue and methylene green have a greater affinity for the surface of the TiO2 nanotube catalyst, resulting in a faster degradation rate compared to P25. Moreover, the surface area
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Figure 4. Plot of dye concentration versus irradiation time for the photodegradation of rhodamine B catalyzed by P25 ([), TNTs (9), PbS/TNTs (2), PbS/TNTs (in) (•), and blank without catalyst ( ×).
of the TiO2 nanotube catalyst is approximately 3 times larger than that of P25, providing more active sites and faster bleaching. In an effort to further improve the TiO2 nanotube catalysts, we decorated the TNT surface with PbS quantum dots. The PbS QDs on TNTs may result in the generation of multiple excitons and reduction of the electron-hole recombination.41,42 Brahimi et al. suggested that PbS plays an important role in promoting charge separation in a PbS/TiO2 heterojunction, yielding an increasing degradation rate. It was concluded that the best PbS/TiO2 heterojunction was obtained using 10 wt % PbS.24 Both PbS/TNT catalyst consisting of 25 wt % PbS, based on elemental analysis results.29 Figures 2 and 3 show that the dye degradation rate catalyzed by PbS/TNTs is greater than that for TNTs alone. The photocatalytic activity of PbS/TNTs (in) when the QDs are only inside the pore is lower. The growth of PbS QDs inside the TNT pore may block the access of the dye molecules to the pore, effectively reducing the number of active sites associated with the nanotubes. In addition, it may be possible to have the blocking agents remain on the outer surface of the TNTs. As a result, photocatalytic activity of PbS/TNTs (in) is lower than that of PbS/TNTs and TNTs. The photocatalytic degradation of anionic dyes such as indigo carmine and rose bengal might be expected to be lower on the TNTs because of the surface charge. Figures 5 and 6 show that the relative catalytic activities are P25 > PbS/TNTs > PbS/TNTs (in) > TNTs. The negative charge on the surface of TiO2 nanotube-based catalyst at pH 6 results in repulsion of the anionic dye, causing the lower activity. The influence of PbS QDs on improvement of photocatalytic activity of TNTs can be found in photodegradation of anion dyes. Effect of PbS QDs. The combination of PbS QDs with TNTs may help to prevent the charge recombination and expand range of light wavelengths adsorbed. Previously, we found that the PbS/TNTs (QD size ) 5.6 nm) has an exciton absorption at 1104 nm. Morgado and co-workers reported that the band gap of titania nanotube is 3.38 eV.43 That means an exciton absorption around 366 nm. To further evaluate the effect of PbS QDs in the photoactivity of TNTs beyond their absorption wavelength, the photodegradation of methylene blue was studied in a Ray-sorb flask (optical filtration at wavelength 600 nm is shown in Figure 7. Photodegradation of the organic dye occurs when photocatalyst was activated by absorbing light at the wavelength less than or equal
Ratanatawanate et al.
Figure 5. Plot of dye concentration versus irradiation time for the photodegradation of indigo carmine catalyzed by P25 ([), TNTs (9), PbS/TNTs (2), PbS/TNTs (in) (•), and blank without catalyst ( ×).
Figure 6. Plot of dye concentration versus irradiation time for the photodegradation of rose bengal catalyzed by P25 ([), TNTs (9), PbS/ TNTs (2), PbS/TNTs (in) (•), and blank without catalyst ( ×).
to its exciton absorption wavelength. After filtering the UV region, the TNTs could not catalyze the photobleaching. On the other hand, the photodegradation of methylene blue on PbS/ TNTs was observed. Since PbS QDs absorb at longer wavelength (1104 nm), there is enough energy to excite electrons for the valence band (VB) of PbS to the conduction band (CB). The electrons in its CB then transferred through the CB of TNTs to generate radical species, causing the degradation of methylene blue. It should be noted that electron transfer occurs only when both semiconductors in heterojunction are in close contact. Because our fabrication process involves the use of bifunctional linker, each PbS QD is bound to the TNT surface. Hyun et al. attached PbS QDs on TiO2 nanoparticles using 3-mercaptopropionic acids as a linker.44 They reported that the fluorescence spectra showed strong quenching that is evidence of electron transfer from the PbS QDs to the TiO2. Because the present experiments were carried out using thiolactic acid, which has a structure similar to that of 3-mercaptopropionic acids, electron transfer at the PbS/TNT heterojunction can be expected. The lowest unoccupied molecular orbital (LUMO) of PbS QDs is size-dependent. Therefore, the size of QDs plays the important role in electron injection from PbS QDs to TiO2 nanotubes. The most favorable case to have charge transfer from the PbS QDs
Photocatalytic Activity of PbS QD/TiO2 Nanotubes
Figure 7. Plot of dye concentration versus irradiation time for the photodegradation of methylene blue catalyzed TNTs (9) and PbS/TNTs (2) in the quartz reactor, TNTs (0) and PbS/TNTs (∆) in the Raysorb reactor, and the blank without catalyst (×).
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10759 However, there was no evidence of PbS oxidation according to FTIR results (not shown). This result indicates that PbS QDs are stable under the reaction conditions. The decrease in % conversion may also reflect minor losses in the amount of catalyst. Although centrifugation is reasonably effective at isolation of the nanotubes, it is possible that some tubes remain in solution. The efficiency of TNTs dropped (12%) more than that of the PbS/TNTs after 3 cycles. The photodegradation rate of the PbS/TNTs is faster than that of the TNTs. The activity of the PbS/TNTs in Figure 2 shows that the photoreaction is completed after 30 min of reaction. After recovery, the color of PbS/TNT particles remained unchanged, meaning there is no methylene blue molecules left on the surface of PbS/TNTs. In contrast, the TNT catalyzed reaction is not quite complete after 30 min (Figure 2), and the recovered TNT catalyst is blue. Thus there is some dye remaining on the surface of the TNTs which will impact the subsequent cycles. It should be noted that the methylene blue adsorbed on the TNTs will bleach after a few hours by allowing the dry recovered catalyst to sit on the benchtop exposed to room lighting only. Conclusion
Figure 8. Plot of % conversion for the photodegradation of methylene blue after 3 cycles.
in to TNTs is when the LUMO level of QDs is above the electron affinity of TNTs. Thus the efficient electron transfer from PbS QDs to TiO2 will occur only for QD size below 4.3 nm;44 this is based on TiO2 nanoparticles. The electron affinity of TiO2 nanoparticles is around -3.9 eV,45 whereas the electron affinity of TNTs is approximately -4.2 eV.46 Because the TNTs have lower LUMO level, the electron transfer from QDs to TNTs can be observed with the maximum quantum dot size up to ∼6 nm. It was reported that there is no catalytic activity when using PbS QDs only for photodegradation of benzamide or 4-hydroxybenzoic.24 Wang and co-worker also reported that there is no photodegradation of rhodamine B occurred when using PbSe QDS alone.47 It should also be noted that the maximum intensity for the mercury lamp is
