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TiO2 Nanoparticle Photocatalysts Modified with Monolayer-Protected Gold Clusters Myeongsoon Lee,† Piyadarsha Amaratunga,‡ Junhyung Kim,†,‡ and Dongil Lee*,† Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry, Western Michigan UniVersity, Kalamazoo, Michigan 49008, United States ReceiVed: July 8, 2010; ReVised Manuscript ReceiVed: September 23, 2010
TiO2 nanoparticle photocatalysts modified with uniform gold nanoparticles are prepared by anchoring bifunctional glutathione-coated monolayer-protected gold clusters (MPCs) onto TiO2. In this method, the MPC loading on TiO2 can be precisely controlled in the range of 1-5 wt % without incurring size change upon anchoring. The photocatalytic degradation of Uniblue A (UBA) is carried out with thus prepared MPC-TiO2 composites, which shows, however, no enhancement upon MPC anchoring. The MPC-TiO2 composites are thermally treated to activate the catalytic activity. When the MPC-TiO2 composites are calcined at 250 °C, the glutathione ligand on gold surface is partially removed and the photocatalytic activity of the composites significantly increases, highlighting the role of gold in the photocatalytic reactions. However, when the calcination temperature is raised to 400 °C, the photocatalytic activity of the composites drastically decreased. X-ray photoelectron spectroscopy analysis of the calcined composites reveals that significant amount of oxidized sulfur remains after calcination that appears to act as a recombination center for the photogenerated electrons and holes, resulting in a drastic decrease in the photocatalytic activity. These results emphasize the important role of ligands in the use of MPCs in photocatalysis. Introduction It has been found that the photocatalytic activity of metal oxide semiconductors, such as TiO2 and ZnO, can be greatly enhanced by coating the surface with noble metal deposits.1-7 The metal deposit on the semiconductor surface acts as a sink for photogenerated electrons, promoting charge carrier separation.5-9 The metal deposit can also be a redox center for catalytic reactions, mediating the photoelectron transfer to a solutionphase redox couple.10 These metal deposits are typically prepared by chemical11 or photochemical12 reduction of metal salts on semiconductor oxides. Another preparative method includes sputtering of metal deposits, followed by annealing to improve the adhesion property to the semiconductor surface.13,14 In these methods, however, it is very difficult to control the deposit formation, typically producing polydisperse size and shape.9,15 This is the major challenge in the optimization of such photocatalysts that often exhibit critically size-dependent catalytic activities.16 We report here a new strategy to prepare welldefined nanoparticle photocatalysts using monolayer-protected gold clusters (MPCs). MPCs are stable, structurally and energetically well-defined nanoparticles that exhibit size-dependent electrochemical and optical properties.17-19 One of the most interesting properties of these MPCs is the ability to control the transfer of electrons into and out of the metallic core. The energetic and dynamics of the electronic charging of these MPCs have been described.20-24 In a previous report,25 we have demonstrated that electrons in the photoexcited TiO2 nanoparticles can readily transfer to Au MPCs in their colloidal mixtures (i.e., collisional electron transfer from TiO2 to MPCs) and the electron transfer efficiency can be controlled by the MPC core size. In the interest of applying this capability to photocatalysis, we have prepared * To whom correspondence should be addressed. E-mail: dongil@ yonsei.ac.kr. † Yonsei University. ‡ Western Michigan University.
SCHEME 1: Schematic Illustration (Not to Scale) of Photoexcitation, Photoelectron Transfer, and Photocatalytic Reduction of UBA at an MPC-TiO2 Composite
MPC-TiO2 composites by attaching uniform sized Au MPCs to TiO2 nanoparticles using a bifunctional linker, glutathione (Scheme 1). Band-gap illumination of TiO2 colloids in deaerated CH3OH-H2O excites electrons to the conduction band while holes are scavenged by CH3OH. The photoelectrons in this circumstance have relatively long-lifetime and thus can participate in the catalytic reduction of a substrate such as Uniblue A (UBA). When MPCs are attached to the TiO2 surface, these photoelectrons readily transfer to the gold cores and are stored on them.25 It has been found that electron transfer from the photoexcited TiO2 to Au MPC is thermodynamically favorable and it proceeds until the Fermi levels of TiO2 and MPC are equilibrated.4,6,25 The electrons stored on the gold core are also anticipated to participate in the catalytic reactions provided their redox potential matches with that of the substrate in the solution. In this article, we report the preparation and photocatalytic activity of a new class of photocatalyst based on TiO2 nanoparticles modified with uniform Au MPCs. MPC anchoring is
10.1021/jp106337k 2010 American Chemical Society Published on Web 10/08/2010
MPC-Modified TiO2 Photocatalysts achieved via the bifunctional glutathione ligand and the MPC loading in MPC-TiO2 composites can be precisely controlled by experimental conditions. The photocatalytic activity of thus prepared MPC-TiO2 composite shows, however, no enhancement with MPC modification. Thermal treatment is employed to activate the catalytic activity of MPC-TiO2 composites. The photocatalytic activity of the MPC-TiO2 composites significantly increases upon calcination at 250 °C. By contrast, it drastically decreases after calcination at 400 °C. XPS analyses of the MPC-TiO2 composites show that significant amount of oxidized sulfur remains after calcination at 400 °C that appears to act as a recombination center for the photogenerated electrons and holes, resulting in a drastic decrease in the photocatalytic activity. Although there have recently been a few reports on supported MPC catalysts,26-28 the use of MPC-modified metal oxides in photocatalysis is very rare.29 This report provides the first results revealing the important role of ligands in the use of MPCs in supported photocatalysts. Experimental Section Chemicals. L-glutathione reduced (GSH, >99%), sodium borohydride (NaBH4, 99%), hydrogen tetrachloroaurate trihydrate (HAuCl4 · 3H2O, ACS reagent grade), Uniblue A (UBA) were purchased from Sigma-Aldrich. Methanol (99.9%) was purchased from Carlo Erba. Water was purified using a Millipore Milli-Q system (18.2 MΩ · cm). TiO2 nanoparticles (Aeroxide P25, Evonik Degussa Corp.) were purchased from Acros Organics. All chemicals were used as received without further purification. Synthesis of Glutathione-Coated MPCs. Glutathione-coated MPCs were synthesized according to a literature procedure30 with some modifications. In a typical reaction, 0.9219 g (3.0 mmol) of glutathione was dissolved in 40 mL of distilled water, and 0.3938 g (1.0 mmol) of HAuCl4 · 3H2O was dissolved in 80 mL of methanol. The two solutions were mixed and stirred for 45 min to generate a cloudy, white solution. To this solution, 0.3738 g (10 mmol) of NaBH4 in 10 mL of water was added with vigorous stirring at room temperature. The color of the solution was changed to dark brown immediately, indicative of MPC formation. After additional stirring for 90 min at room temperature, the solution was rotary evaporated to near dryness to produce a black MPC product. The MPC product is a mixture of various sized MPCs in the range of 1-3 nm and each size was solvent fractionated using water-methanol mixture. The isolated MPCs used in this work were quite monodisperse in size with an average core diameter of 2.0 ( 0.2 nm (Figure S1, Supporting Information). Preparation and Characterization of MPC-TiO2 Composites. MPC-TiO2 composites were prepared by anchoring glutathione-MPCs onto TiO2 nanoparticles. A commercially available TiO2 (Aeroxide P25) nanoparticle was employed as a semiconductor oxide nanoparticle. It is a mixture of anatase and rutile and the primary particle size is ca. 20 nm. Anchoring of MPCs onto TiO2 was carried out by stirring the mixture of aqueous MPC and TiO2 solution for 3 h. After the anchoring reaction, MPC-TiO2 composites were centrifuged and dried. To prepare calcined MPC-TiO2 composites, the MPC-TiO2 composites were thermally treated at 250 or 400 °C in air in a muffle furnace (heating rate ) 8.3 °C/min, holding time ) 30 min). Glutathione-coated TiO2 nanoparticles were prepared by stirring TiO2 in an aqueous glutathione solution that contains equivalent amount of glutathione to 4 wt % MPC-TiO2. Transmission electron microscopy (TEM) images of MPC-TiO2 composites were obtained with JEOL 2100F
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18367 operating at 200 kV. The TEM samples were prepared by drop casting of an MPC-TiO2 suspension on a 400 mesh Formvar/ carbon-coated copper grid (01814-F, Ted Pella). Thermogravimetric analysis was performed with Scinco STA-1500. X-ray photoelectron spectroscopy (XPS) experiments were carried out on Sigma Probe (Thermo VG, UK) spectrometer using monochromatic Al KR X-ray source (1486.6 eV). Binding energies (BE) were referenced to the C 1s BE of glutathione at 284.5 eV. Peak position and integrated intensities were obtained by curve fitting using Thermo VG Scientific: Avantage version 2.1. Photocatalysis Experiments. For photocatalysis experiments, an MPC-TiO2 suspension (15 mg/L) in 1:1 (v/v) CH3OH:H2O was purged with high-purity Ar for 30 min to remove dissolved oxygen. Deaerated UBA solution was added to the MPC-TiO2 suspension just before the photolysis experiment. The initial UBA concentration (C0) in the MPC-TiO2 suspension was 25 µM. Photolysis experiments were performed with a 300 W xenon lamp. Light was filtered through a UV cutoff filter (λ > 320 nm, UV-32, HOYA). Absorbance spectra of the reaction mixtures were recorded in situ during photolysis at 3 min interval with a USB 4000 Fiber Optic Spectrometer (Ocean Optics). The absorbance at each time was averaged out of at least three independent experiments. Results and Discussion Knowing that carboxylic acids readily bind to TiO2 surface via ester-type or carboxylate-type linkage,31,32 glutathioneprotected MPCs were reacted with bare TiO2 to produce MPC-TiO2 composites. Parts a and b of Figures S2 of the Supporting Information show TEM images of the reaction mixtures sampled after 20 min and 3 h, respectively. Whereas a large fraction of MPCs remain unbound at 20 min, most of MPCs appear to be bound to TiO2 after 3 h. Accordingly, the anchoring reaction of MPCs onto TiO2 was carried out for 3 h. In this method, the MPC loading on TiO2 can be conveniently controlled by simply varying the initial concentration of MPCs in the mixture. TEM images of thus prepared MPC-TiO2 composites with MPC loading of 1, 2, and 3 wt % relative to TiO2 are shown in parts a, b, and c respectively of Figure 1, which clearly show that the MPC loading can be indeed controlled by controlling MPC concentration in the mixture. Whereas the MPC loading can be precisely controlled at lower loadings, it is less controllable at a higher loading than 5 wt %. For example, the actual loading for the 9 wt % MPC-TiO2 composite was found to be 6.4 wt % and there are a large fraction of unbound MPCs present in the supernatant after carrying out the anchoring reaction for 3 h.33 Nonetheless, the average MPC core size is maintained at its initial size of 2.0 nm throughout the binding process regardless the loading %, highlighting the advantage of this approach. The photocatalytic activities of the MPC-TiO2 composites were evaluated by monitoring the photocatalytic degradation of a model dye, UBA, under UV irradiation. To gain better understanding of the photocatalysis mechanism by TiO2 photocatalysts, we first compare the photocatalytic activities of TiO2 suspended in H2O in aerated and deaerated conditions. In aerated H2O (part a of Figure 2), TiO2 exhibits essentially no catalytic activity for UBA degradation. Even after purging the solution with Ar to remove the dissolved oxygen, TiO2 only exhibits very limited catalytic activity as shown in part b of Figure 2. This implies that the electron-hole charge separation is not sufficient in water and thus the photocatalytic activity for the UBA degradation is limited even when oxygen is removed. When the experiment is conducted in deaerated CH3OH-H2O,
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Figure 1. TEM images of (a) 1 wt %, (b) 2 wt %, and (c) 3 wt % MPC-TiO2 composites. MPC cores are darker than the TiO2 particles due to the higher electron scattering cross section of Au. Inserts are histograms of Au core diameters. The average core diameters of the attached MPCs are 2.0 nm for all of the composites, unchanged after binding to TiO2.
Figure 2. Temporal evolution of absorption spectra obtained during photolysis of UBA with TiO2 in (a) aerated H2O, (b) deaerated H2O, (c) aerated 1:1(v/v) CH3OH-H2O, and (d) deaerated 1:1(v/v) CH3OH-H2O.
the photocatalytic activity increased dramatically as shown in part d of Figure 2. Before irradiation, UBA shows blue color with two specific absorption peaks at 587 and 625 nm. These peaks gradually decrease and new absorption band appears in the range of 300-500 nm as the photolysis proceeds. This pattern of change is consistent with two electron reduction of UBA as expected for TiO2 catalyst in hole scavenging solvent (methanol in this work) that blocks the oxidative pathway.34-36 It has been found that methanol is a very effective hole scavenger; it is known to eliminate the trapped holes within 300 ps,37 resulting in accumulation of long-lived photoelectrons in the conduction band of TiO2. Thus, the photolysis results shown in part d of Figure 2 indicate that UBA is effectively degraded by the long-lived photoelectrons, supporting the reductive pathway proposed in Scheme 1. A similar pattern of change in absorption spectra is observed for MPC-TiO2 composites (not shown), suggesting the same reduction mechanism is in play in the photodegradation of UBA with both bare TiO2 and MPC-modified TiO2 photocatalysts in deaerated CH3OH-H2O media. The photocatalytic activity in CH3OHH2O becomes, however, negligible in the presence of oxygen
(part c of Figure 2). This reflects the fact that dissolved oxygen can competitively eliminate the photoelectrons before they react with UBA. Because UBA displays strong absorption in the range 500-700 nm, one may speculate that UBA is degraded by direct light absorption or dye-sensitization pathways.38,39 As can be seen in Figure 3, there is no change in UBA absorption upon illumination in the absence of TiO2 nanoparticles, ruling out the former possibility. The possibility of dye degradation on TiO2 (i.e., dye-sensitization) was also examined. By using an additional long-pass filter (>455 nm), the photoexcitation of TiO2 is effectively blocked while UBA absorption is undisturbed. However, the photocatalytic results in Figure 3 show that photodegradation of UBA only occurs when TiO2 is photoexcited, confirming that TiO2 plays the major role in the photocatalytic degradation of UBA. The photodegradation of UBA with bare TiO2, 1 wt %, 3 wt %, and 4 wt % MPC-TiO2 are compared in Figure 4. All nanoparticle suspensions were irradiated equally and the absorbance at 587 nm was recorded at 3 min interval. As can be seen in part A of Figure 4, UBA dye molecules were completely
MPC-Modified TiO2 Photocatalysts
Figure 3. Change in UBA concentration (C) relative to the initial concentration (C0) during photocatalysis in deaerated 1:1 CH3OH-H2O in the absence (a) and presence of TiO2 (b), and in the presence of TiO2 using a 455 nm long-pass filter (c). The UBA concentration was estimated from the absorbance at 587 nm.
degraded within 20 min and the photocatalysis rates are very similar for all photocatalysts tested regardless MPC loading. This is surprising as one would expect higher charge separation and thus higher photocatalytic activity with higher MPC loading. It should be noted here, however, that MPCs anchored on TiO2 are not gold nanoparticles alone but are heavily coated with a long organic ligand (glutathione) that would impose a kinetic barrier for electron transfer from TiO2 to gold core. The presence of the coating ligand would also prevent the direct reduction of UBA at gold surface proposed in Scheme 1. In supported gold catalysts employing MPCs, the catalytic activity has often been activated by removing the protecting ligands by heating the composites.27-29,40-42 The MPC-TiO2 composites were thermally treated at 250 °C to remove the protecting ligands and thus to activate the catalytic activity. Thermogravimetric analysis of glutathione-protected MPCs (Figure S3, Supporting Information) indicates that glutathione ligands are partially decomposed at 250 °C. TEM image of the calcined composites (part a of Figure S4, Supporting Information) shows that the average core size of MPCs on TiO2 is 2.2 nm, largely unchanged after calcinations at 250 °C. However, as can be seen in part B of Figure 4, the catalytic activities of MPC-TiO2 composites are considerably enhanced after calcination at 250 °C. For 1 wt % MPC loading, the catalytic activity is slightly enhanced compared to bare TiO2. When the MPC loading increases, the catalytic activity increases considerably; the UBA degradation is less than 50% after 6 min photolysis with bare TiO2, which increases to 72% and 82% with 3 and 4 wt % MPC loading, respectively. The enhanced catalytic activity reflects the fact that more photoelectrons are generated in TiO2-MPC composites for UBA reduction after calcination at 250 °C. TiO2 itself does not show any change in catalytic activity after calcination at 250 °C (not shown) and MPC alone does not exhibit any catalytic activity upon photoexcitation (line e in part A of Figure 4). Thus, it can be concluded that the enhanced catalytic activity is caused by the enhanced charge separation in the calcined composites. The presence of gold on TiO2 can enhance the charge separation by accepting electrons from TiO2. The stored electrons on gold can additionally participate in the reduction of UBA, resulting in the enhanced catalytic activity. The origin of the enhanced charge separation after calcination is unclear, but it is likely that the electron transfer from TiO2 to the gold core is significantly facilitated after the partial removal of the ligands. In addition, it is expected that the gold surface is more open for the direct reduction of UBA after the partial removal of the glutathione ligands. The photostability was examined with 4 wt % MPC-TiO2 composites calcined at 250 °C. Shown in Figure 5 is the
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18369 comparison of the initial activity and the activity after 6 cycles of photoexcitation (30 min) and rest (30 min), which shows that the activity is only slightly decreased compared to the initial activity. As shown in part b of Figure S4 of the Supporting Information, the average core size (2.3 nm) is unchanged after repeated photoexcitation of the composites, demonstrating the photostability of the calcined composites. This is a special advantage over conventional metal-coated semiconductor oxide whose photocatalytic activity tends to vary drastically due to the change in metal deposit size during photocatalytic reactions. Having observed that the photocatalytic activity of MPCTiO2 composites is considerably enhanced by thermal treatment, the calcination temperature was raised to 400 °C to completely remove the ligands on the gold surface. Because it was found that the photocatalytic activity of TiO2 considerably deteriorates after thermally treated at 500 °C, the highest calcination temperature was limited to 400 °C at which the catalytic activity is unchanged (line d in part A of Figure 6). The catalytic activities of 4 wt % MPC-TiO2 composites calcined at 250 and 400 °C are compared in part A of Figure 6. Surprisingly, the catalytic activity of MPC-TiO2 composites calcined at 400 °C decreased dramatically; the degradation of UBA at 6 min is only 16% and hardly increases with time. TEM image of the composite in part b of Figure S5 of the Supporting Information shows that the average gold core diameter (3.3 nm) is considerably increased and that there are some detached gold particles observed after calcination at 400 °C, indicating significant loss of the protecting ligands. To understand the decreased catalytic activity after calcination at 400 °C, we have carried out XPS measurements of the MPC-TiO2 composites. In Figure 7, the sulfur S 2p3/2 BE value (161.9 eV) for 4 wt % MPC-TiO2 composite corresponds to bound thiolate which is in agreement with previous reports.43-45 When this composite is calcined at 250 °C, two more S 2p3/2 peaks appear at 163.5 and 168.1 eV. The former can be assigned to unbound thiol or disulfides whereas the latter corresponds to oxidized sulfur species.44,46 When temperature is raised to 400 °C, peak
