Optical Properties and Photocatalytic Performances of Pd Modified

Oct 6, 2009 - Buddha Deka Boruah and Abha Misra . Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc Oxide Nanorods. ACS Applied ...
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J. Phys. Chem. C 2009, 113, 18761–18767

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Optical Properties and Photocatalytic Performances of Pd Modified ZnO Samples Yonggang Chang,† Jian Xu,‡ Yunyan Zhang,*,† Shiyu Ma,† Lihui Xin,‡ Lina Zhu,‡ and Chengtian Xu† Department of Chemistry, East China Normal UniVersity, Shanghai, 200062, and Shanghai Institute of Measurement and Testing Technology, Shanghai, 201203 ReceiVed: May 30, 2009; ReVised Manuscript ReceiVed: September 12, 2009

In this paper, Pd/ZnO samples with various palladium contents were prepared through a facile one-pot hydrothermal method. The crystal structures, chemical compositions, and optical properties of the samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy, and photoluminescence (PL) spectroscopy. The results demonstrate that partial Pd loads on the surface and that the rest is doped in the lattice of ZnO. In addition, the relationship among the photoluminescence, photocatalytic performance, and electronic structure of the samples is investigated in detail. It is found that the modification of palladium not only modulates the electronic structure of ZnO but also affects the amount of surface hydroxyl. The former determines the optical properties of Pd/ZnO, leading to the shift of absorption and PL peaks, while the latter influences the photocatalytic activity of the samples. The modification of palladium at an optimal value not only promotes the separation of photogenerated electron-hole pairs but also increases the amount of the surface hydroxyl resulting in the promotion of photodegradation efficiency. 1. Introduction The application of semiconductor metal oxides (SMOs) in the production of photoelectrochemical cells, gas sensors, fieldeffect transistor, surface acoustic wave devices, and photoluminescence devices1-5 has triggered much research activity. For example, TiO2 could be used in the photocatalytic reaction and photoelectric conversion because of its exceptional optical and electronic properties, nontoxicity, low cost, and long-term stability against photocorrosion and chemical corrosion.6 ZnO has significant advantages in optoelectronic applications such as ultraviolet lasing media owing to its extreme large binding energy of the exciton (about 60 meV at room temperature), which enables the stable formation of the exciton. Transparent and semiconducting tin dioxide (SnO2) is widely used for gas sensors and photosensors because of its excellent photoelectric and chemical properties.7 However, the simplex SMO could not completely meet the increasing application needs in constructing high-performance semiconductor devices. Therefore, considerable efforts have been spent in enhancing physical and chemical properties of the SMOs, such as loading noble metals on the surface of SMOs, doping metals into the lattice of the SMOs, incorporating among the SMOs, and so on. Generally, one accepted solution is to modify the semiconductor metal oxides with noble metals, which could improve the photocatalytic, gassensing, and photoluminescence properties.8-11 As a matter of fact, the interaction between the noble metals and the SMOs is complicated because the interaction relates to the carrier concentration, defect level, and surface states of the semiconductor, electronic, optical properties, and so forth. Therefore, good understanding of the interaction will facilitate the fundamental and technical application of the SMOs modified with noble metal. * To whom correspondence should be addressed. Fax: +86 21 62232704. E-mail: [email protected]. † East China Normal University. ‡ Shanghai Institute of Measurement and Testing Technology.

As one of the most important II-VI semiconductors, ZnO with a wide band gap has been extensively studied because of its intrinsic properties and potential uses in devices, such as field-effect transistors, resonators, gas sensors, solar cells, and as a catalyst.12 Particularly in recent years, the modification of ZnO with noble metals has attracted significant attention in photocatalysis. For example, Zheng et al.13 studied the relationship between the oxygen defect and the photocatalytic property of Ag/ZnO heterostructure. Wu and Tseng14 studied the influence of different diameters and densities of the nc-Au on the photocatalytic activity in Au/ZnO nanorod composites, while Zeng et al.15 synthesized Pt/ZnO nanocages and studied the photocatalytic activity. On the other hand, as a noble metal, palladium, whose ionic radius (0.080 nm) is close to that of Zn2+ (0.074 nm), has been widely used in the industry catalysis. Therefore, modifying ZnO with palladium may show a special performance. Although palladium-modified ZnO has been applied in the area of catalytic reaction,16,17 to the best of our knowledge, there are no detailed studies relating the electronic structure with the optical and photocatalytic properties of ZnO modified with palladium. In this work, we have successfully prepared ZnO modified with various contents of palladium by a one-pot hydrothermal method. Furthermore, the information of the samples was investigated using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-vis absorption spectroscopy (UV-vis), and photoluminescence (PL) spectroscopy, which was in an attempt to understand well the interaction between the palladium and ZnO. Especially, the influences of the modified Pd on the electronic structure, optical property, and surface property of ZnO have been emphasized. The photocatalytic performance of the prepared Pd/ZnO samples with various Pd contents for degradation of rhodamine B was also investigated.

10.1021/jp9050808 CCC: $40.75  2009 American Chemical Society Published on Web 10/06/2009

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2. Experimental Section 2.1. Materials. Zinc acetate dihydrate (Zn(CHCOO)2 · 2H2O), hexamethylenetetramine (C6H12N4), palladium chloride (PdCl2), and rhodamine B (RB) were purchased from Shanghai Chemical Reagent Co. All the reagents were analytical grades and were used without further purification. Deionized water was used in the experiments. 2.2. Synthesis of Pd Modified ZnO (Pd/ZnO). In a typical experiment, the materials of 4 mL of Zn(CHCOO)2 · 2H2O (0.5 mol/L), 6 mL of C6H12N4 (0.5 mol/L), and the required volume of PdCl2 solution were added into a Teflon-lined stainless steel autoclave of capacity 100 mL followed by the addition of a certain amount of water under vigorous stirring to make the total volume 80 mL. The autoclave was sealed, was heated at 393.15 K for 6 h, and finally was allowed to cool naturally to room temperature. The solid was separated from the solution by centrifugation at 5000 rpm for 5 min, and the collected solid product was washed with deionized water several times by centrifugation. After being washed, the sample was vacuumdried at 323.15 K for 24 h. In this paper, the samples are denoted as x at.% Pd/ZnO, where x indicates the percentage of Pd in products measured by an energy dispersive X-ray spectrometer (EDX). 2.3. Characterization. The phases of the samples were determined by XRD on a D/max 2500PC diffractometer with Cu KR radiation (λ ) 0.154056 nm) at 2θ ranging from 20° to 80°. EDX spectra were determined using an EDAX FALCON EDX spectrometer attached to scanning electron microscopy (SEM). The X-ray photoelectron spectroscopies (XPS) were determined using a Kratos AXIS Ultra DLD X-ray photoelectron spectrometer. Transmission electron microscopy (TEM) images of the sample were taken at 200 kV with a FEI TECNAI G2 S-TWIN TEM. The UV-vis measurements were monitored on a Shimadzu UV-2450 spectrophotometer operated at a resolution of 0.5 nm. The room-temperature photoluminescence (PL) spectra of the ZnO and Pd/ZnO were obtained using a Hitachi F-4500 spectrophotometer with a Xe lamp as the excitation source. The excitation wavelength used in the PL measurement was 325 nm. 2.4. Photocatalytic Testing. Rhodamine B (RB), a widely used dye, was employed as a representative dye pollutant to evaluate the photocatalytic activity of Pd/ZnO samples. The ultraviolet light was provided by a 300 W high pressure Hg lamp with the major emission at 365 nm, which was cooled to room temperature by a circulating water jacket. The distance between the light and the reaction beaker was maintained at 10 cm. For a typical procedure, a mixture of 50 mL of RB aqueous solution (1.0 × 10-5 mol/L) and 50 mg of Pd/ZnO samples was stirred for 30 min in a quartz beaker in the dark to reach an adsorption/desorption equilibrium for RB on the surface of Pd/ZnO. After the mixture was irradiated for a given time, 5 mL samples were withdrawn each time and were centrifuged for 5 min. The quantitative determination of RB was performed by measuring its absorption with a UV-vis spectrophotometer. The degradation efficiency was calculated using the following equation:

degradation(%) ) (C0 - C)/C0 × 100 ) (A0 - A)/A0 × 100 where C0 represents the initial concentration after the equilibrium adsorption, C represents the reaction concentration of RB, A0

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Figure 1. (a) XRD patterns of the ZnO and Pd/ZnO samples with various palladium contents. (b) Exaggerated plotting of XRD patterns in the range 2θ from 30.7° to 32.6° of pure ZnO and Pd/ZnO samples.

represents the initial absorbance, and A represents the changed absorbance of the RB at the characteristic absorption wavelength of 554 nm. 3. Results and Discussion 3.1. Particle Characterization. Figure 1a shows the XRD patterns of the as-prepared samples with various Pd contents. All the diffraction peaks can be indexed to two crystal phases. The peak marked with “Regular” can be assigned to the wurtzite structure zinc oxide (JCPDS Card File No. 36-1451), while the others marked with “Italic” agree well with the face-centered cubic (fcc) palladium (JCPDS Card File No. 05-0681). No characteristic peaks from other impurities are detected in the patterns demonstrating that all of the samples have high phase purity. It suggests that the obtained products are Pd/ZnO samples. However, the significant differences observed from various samples are the diffraction peak positions of ZnO. Figure 1b is a high-resolution diffraction peak of ZnO(100). The peak positions shift to lower angles with the increase of PdCl2 content in the source precursor, which is an indication of lattice expansion of ZnO. As Pd2+ ions (0.080 nm) have a similar size to Zn2+ ions (0.074 nm), it is possible to substitute Zn2+ ions with Pd2+ ions in ZnO crystal. Furthermore, the larger size of Pd2+ ions results in the lattice expansion. Consequently, the result provides indirect evidence that partial Zn2+ is substituted by Pd2+. Therefore, partial Pd loads on the surface and the rest is possibly doped in the lattice of ZnO. To investigate the morphology and the state of Pd in the sample, TEM and high-resolution transmission electron microscopy (HRTEM) for a typical Pd/ZnO were carried out (Figure 2). A TEM image representing the general morphology of the ZnO modified with palladium is displayed in Figure 2a. As can be seen from Figure 2a, the morphology of ZnO is irregular flake, while the palladium nanoparticles with an average

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Figure 3. XPS spectra of (a) Zn2p spectrum of 4.07 at.% Pd/ZnO; (b) Pd3d spectrum of 4.07 at.%; and (c) O1s spectra of the as-prepared Pd/ ZnO samples with various Pd contents. Figure 2. TEM (a) and HRTEM (b) images of a typical Pd/ZnO sample. Insert: particle size distribution for Pd particles loaded on the surface of ZnO.

diameter of about 32 nm are loaded on the surface of ZnO. The corresponding HRTEM image for Pd/ZnO sample is shown in Figure 2b. The measured d spacing of 0.266 nm between adjacent lattice planes corresponds to the distance between two (002) crystal planes, which is slightly larger than that in pure ZnO (0.260 nm). It indicates the presence of an effective substitution of Pd2+ for Zn2+ in the lattice of ZnO. The lattice fringes with d ) 0.224 nm are clearly visible, which can be attributed to the (111) planes of palladium nanoparticles. Consequently, it provides another indirect evidence that partial Pd is incorporated into the crystal structure and that others form nanoparticles loading on the surface of ZnO. To gain more information about the obtained samples, XPS technique was employed to detect the composition of the ZnO and Pd/ZnO samples. The surface components and the chemical states of the samples with various Pd contents are shown in Figure 3. From Figure 3a, it can be seen that the peak position

of Zn 2p3/2 in Pd/ZnO sample is about 1021.6 eV indicating that Zn is in the formal Zn2+ valence state.18-21 The Pd3d and O1s peaks are somewhat asymmetric indicating that at least two kinds of palladium and oxygen species exist in the near surface region of Pd/ZnO samples. The XPS spectra of Pd3d and O1s have been fitted by multiple Gaussians (Figure 3b and 3c). The fitted double peaks of Pd 3d5/2 located at 335.2 and 337.0 eV are assigned to the Pd0 and Pd2+, respectively.22-24 It indicates that partial Pd loads on the surface of ZnO and that the rest dopes in the lattice of ZnO, which is in agreement with the results of XRD and TEM. As shown in Figure 3c, the XPS spectra of O1s from Pd/ZnO are composed of three peaks. The peak centered at 530.1 eV is closely associated with the lattice oxygen (OL) of ZnO,25-28 the peak at about 532.0 eV is attributed to the oxygen of surface hydroxyl (OH),18,29 while the peak position at 533.5 eV is due to the chemisorbed oxygen (Oa).30,31 As has already been proven, for a photocatalytic oxidation process, surface hydroxyls of the photocatalyst play a significant role.25,28,32-34 Their presence on the surface cannot only easily capture holes to form active · OH radicals13,27 but

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TABLE 1: Calculation Results for O1s XPS Spectra of Pd/ZnO with Various Pd Contents lattice oxygen (OL)

hydroxy oxygen (OH)

adsorbed oxygen (Oa)

Pd at.%

B.E.a (eV)

R i% b

B.E. (eV)

R i%

B.E. (eV)

Ri%

0.00 1.91 3.01 4.07 4.71 5.11

530.61 530.30 530.30 529.95 530.20 530.10

32.66 15.10 9.30 1.85 10.34 11.04

532.31 532.12 531.90 531.84 531.82 531.84

64.89 79.70 85.66 93.31 83.77 83.10

534.30 533.58 533.50 533.30 533.10 533.30

2.45 5.23 5.04 4.84 5.88 5.86

a B.E., binding energy. b The percent of the individual oxygen species calculated from the peak area.

Figure 4. UV-vis diffuse reflectance spectra of ZnO and Pd/ZnO samples with various Pd contents. Insert: Peak position around 375 nm of Pd/ZnO samples as a function of the modifying atomic percentage of Pd (at.%).

also can enhance O2 adsorption to trap electrons and to produce more · OH radicals.21,35 According to the rules of qualitative and quantitative analysis of XPS data, the binding energy and the corresponding molar percentage of O species from ZnO and Pd/ZnO have been carried out, and the results are summarized in Table 1. It can be seen that the molar percentage of OH increases with the increase of Pd content. However, when the content of Pd exceeds 4.07 at.%, the percentage of OH decreases with the further increase of Pd content. Therefore, it can be concluded that the modification of Pd with various contents can modulate the percentage of surface hydroxyl of ZnO, which will affect the photoreaction activity. 3.2. Optical Properties of Pd/ZnO Samples. The UV-vis absorption measurement is a common tool to reveal the energy structures and optical properties of semiconductors. UV-vis absorption spectra of the ZnO and Pd/ZnO samples were measured at room temperature. The UV-vis diffuse reflectance spectra of the as-prepared Pd/ZnO samples are shown in Figure 4. It can be seen that all the absorption spectra of the samples have a broad absorption band in the UV region with a shoulder peak located at about 375 nm, which is characteristic of the ZnO wide band semiconductor material. However, the peak position of Pd/ZnO shifts from 379.6 to 374.5 nm first and then shifts to 378.6 nm with the increase of Pd content (Figure 4 insert). The UV-vis absorption shoulder peak of Pd/ZnO exhibits a blue shift compared with that of pure ZnO, which is due to the wider band gap of ZnO. As is known, when the size of nanocrystals is comparable or smaller than the exciton Bohr radius, the quantum confinement effect could be induced. It would cause the shift of the energy levels of the conduction and valence bands and would give rise to a blue shift in the transition energy.36,37 However, the diffraction peak of Pd/ZnO samples is somewhat sharp (Figure 1a) indicating that the size

Figure 5. Photoluminescence (PL) spectra of ZnO and Pd/ZnO samples with various Pd contents.

of the prepared ZnO is larger than its exciton Bohr radius. Therefore, the possibility for the blue shift caused by quantum confinement effect could be neglected.38 It is noticeable that the carrier concentration also plays an important role for the determination of band gap, which can be explained by the wellknown Burstein-Moss (BM) effect.39-42 The BM effect is an important phenomenon in n-type semiconductors which can cause the enlargement of the band gap in the absorption and photoluminescence spectra. When the carrier concentration in semiconductors is high enough, the Fermi level will move into the conduction band because of the filling of the conduction band by electrons. Under the condition of the high carrier concentration, the BM shift (∆EBM) of the band edge absorption in the n-type semiconductor is given as ∆EBM ) [1 + (m*)/ e 2/3 2 2/3 (h )/(8m*)n - 4KT], where h is Planck’s constant, (m*)][(3/π) h e K is the Boltzmann constant, T is the absolute temperature, n is the electron carrier concentration in the conduction band, and me* and mh* are the effective masses of electron and hole, respectively. Thus, the energy of the band gap increases with the carrier density increasing. It is reported that the work functions of ZnO and palladium are 5.2-5.3 and 5.12 eV,43,44 respectively. Thus, the Fermi energy level of ZnO is lower than that of palladium because of its larger work function (Figure 6a). Thus, when the Pd and ZnO attach together, the electrons will flow from palladium to ZnO to establish a constant Fermi energy level, and the transferred electrons will accumulate on the equilibrated Fermi level near the bottom of the conduction band of ZnO (Figure 6b). For pure ZnO, when the energy hV of a photon is equal to or higher than the band gap of the semiconductor, an electron in the valence band (VB) can be excited to the conduction band (CB) with the simultaneous generation of a hole in the VB. Then, the photogenerate electrons stored near the conduction band edge could recombine with the hole via radiative process (Figure 7a). For palladium-modified ZnO, the proposed charged separation process is similar to that of the pure ZnO. Nevertheless, we have to think about the surface plasmon resonance (SPR) effect of the noble metal45 in here. Namely, under the irradiation of incident light having a suitable wavelength, the high-density electrons of the noble metal form an electron cloud and oscillate, which is ascribed to the SPR effect of the noble metal. According to previous reports,46,47 the SPR peak of small Pd nanoparticles below 10 nm in size has no resonance between 300 and 1500 nm, while it is found that the SPR peak of Pd nanoparticles could be shifted to the visible region by increasing their sizes to >25 nm. Xiong et al.46,47 found that the 25 and 50 nm Pd nanocubes displayed SPR peaks at 330 and 390 nm, respectively. According to the result of the size distribution of Pd nanoparticles loaded on the surface of ZnO, the average size is about 32 nm, which is larger than 25 nm. Therefore, when the palladium nanoparticles are

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Figure 6. (a) The band structures of Pd and ZnO junction. (b) The band structure of Pd and ZnO with uniform Fermi level. Because of the larger work function of ZnO, the Fermi energy level of ZnO (Efs) is lower than that of Pd (Efm) resulting in the transfer of electrons from Pd to ZnO until the two systems attain equilibrium and form a new Fermi energy (Ef).

irradiated by the 325 nm light, an electron cloud and oscillate are formed because of the SPR effect. Simultaneously, the increased electron density leads to the lift of Fermi energy level of palladium, which is higher than that of ZnO. Then, the electrons of the palladium transfer to the ZnO side leading to the accumulation of the electron carrier (Figure 7b). When the carrier concentration in ZnO is high enough, the Fermi level moves to the bottom of or above the conduction band (Figure 7c) resulting in the optical band gap of ZnO being widened. Consequently, the absorption can only occur between the valence bands and about or above the Fermi level leading to the blue shift of the absorption peak of Pd/ZnO. However, the peak positions of Pd/ZnO shift from 374.5 to 378.6 nm with further increase of Pd content. Usually, the red shift of the absorption peak may be due to the formation of band tailing in the band gap.48-50 According to the results of XRD, TEM, and XPS, partial Zn2+ ions are substituted by Pd2+ ions, which may result in the formation of an impurity level between VB and CB when the Pd content reaches a certain amount. This impurity level may lie near the bottom of the conduction band leading to the decrease of the band gap. Therefore, the red shift of the absorption shoulder peak should be due to the decrease of the band gap.51,52 Photoluminescence (PL) spectra are powerful tools which are widely used to investigate the effect of chemical doping and deposition on the luminescent properties of ZnO. Figure 5 presents the room-temperature PL spectra of pure ZnO and Pd/ ZnO samples. A broad-band emission of pure ZnO around 530 nm was observed, which is normally attributed to the recombination of the photoexcited holes with the electrons occupying the singly ionized oxygen vacancies.53 It is interesting to find that this broad-band emission decreases sharply after the addition of PdCl2 to the reaction system. The result indicates that PdCl2 could help to reduce oxygen vacancies during the crystalline process of ZnO.

Figure 7. (a) Photoluminescence process for pure ZnO. (b) Photoluminescence process for Pd/ZnO. (c) The shifting of Fermi level of Pd/ ZnO.

It is well-known that the UV emission from ZnO is attributed to the radiative recombination of a hole in the valence band and an electron in the conduction band (excitonic emission) (Figure 7a). However, no obvious UV emission from pure ZnO is observed in Figure 5. A distinct UV peak located at about 390 nm emerges from the 1.91 at.% Pd/ZnO. In addition, the peak position shifts to a shorter wavelength with the content of Pd increasing from 1.91 to 4.07 at.%. Nevertheless, a shoulder UV peak emerges along with the further increase of Pd content to 4.71 at.%. Moreover, the shoulder peak from 5.11 at.% Pd/ ZnO broadens further. The emergence of UV emission from Pd/ZnO samples may be due to the reduction of defects after the addition of PdCl2 during the crystalline process of ZnO. On the other hand, a noble metal doping in or loading on a semiconductor may enhance the UV emission of the semiconductor owing to the excitons formed at the interface between Pd and ZnO grains.54,55 The blue shift of the UV emission peaks with the increase of Pd content and is considered to be associated with the variation of band gap energy of ZnO. As mentioned above, under the excitation of a UV light, the optical band gap of ZnO is widened because of the Burstein-Moss effect. Therefore, the photoelectrons will take up higher-energy levels at the bottom of or above

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Figure 8. Photogenerated electron transfer in ZnO modified with palladium during catalytic process.

the conduction band leading to a blue shift of the radioactive recombination of the excitons. As shown in Figure 5, the peaks of the Pd/ZnO samples are broadened with the further increase of Pd content, which could be interpreted by the formation of band tailing in the band gap. In other words, a continuous energy band is formed between the lifted Fermi level and the impurity level. Therefore, a continuous emission is brought because of the formation of the energy band. 3.3. Photocatalytic Properties and Mechanism of Pd/ZnO. To better understand how the palladium could influence the photocatalytic activity of ZnO, it is essential to understand the photocatalytic mechanism of the semiconductor. It is generally accepted that when the energy hV of a photon is equal to or higher than the band gap of the semiconductor, an electron (e-) in the VB can be excited to the CB with the simultaneous generation of a hole (h+) in the VB. Then, the photoelectron can be easily trapped by electronic acceptors like adsorbed O2 to further produce a superoxide radical anion ( · O2-), O2 + e-f · O2-, which is a highly reactive particle and which is able to oxidize organic materials, and the photoinduced holes can be trapped by surface hydroxyl to ultimately produce hydroxyl radicals ( · OH).56 These highly reactive hydroxyl radicals then attack organic compounds present at or near the surface of the semiconductor. Both the holes and the hydroxyl radicals are very powerful oxidants, which can be used to oxidize most organic contaminants. However, the photoinduced electrons and holes can also recombine to decrease the available photocatalytic efficiency. Thus, it is necessary that the electrons promoted to the conduction band should be removed rapidly from the semiconductor to prevent recombination with the holes and to allow the mechanism to continue. To reach this purpose, an efficient way is to deposit noble metals8,57-61 (such as gold, silver, platinum, and palladium) on the surface of the semiconductor because of the effect of the noble metal modifier on the interfacial charge transfer processes.62 Certainly, a primary understanding of photoinduced interactions as well as the interfacial charge transfer processes in metal-modified semiconductors is important to explain the exact role of metal in semiconductor photocatalysis. After the Pd/ZnO samples are dispersed in the solution with an organic pollutant, the surface electrons on Pd should eventually transfer to the dye in the dark. However, when these catalysts are radiated by UV light with photon energy higher than or equal to the band gap of ZnO, electrons in the valence band can be excited to the CB with simultaneous generation of the same amount of holes in the VB. As presented in Figure 8, the energy level of the bottom of the CB is higher than the new Fermi energy level of the Pd/ZnO, and so the photogenerated electrons in the conduction band of the ZnO can be transferred to Pd because of a Schottky barrier formed at the metal semiconductor interface, while the holes can remain on the

Figure 9. Photodegradation of rhodamine B by Pd/ZnO with various Pd contents under UV irradiation for 20 min. Inset: the degradation of Pd/ZnO and the surface hydroxyl content of Pd/ZnO as a function of Pd content.

semiconductor surfaces.35,63 Therefore, Pd, acting as electron sinks, reduces the recombination of photoinduced electrons and holes and prolongs the lifetime of the electron-hole pairs. Subsequently, the electrons can be captured by the adsorbed O2, and the holes can be trapped by the surface hydroxyl, both resulting in the formation of hydroxyl radical species. Owing to the little selectivity and high oxidative capacity, the hydroxyl radical species can attack dye molecules and can oxidize most of the pollutants.35 RB is adopted as a representative organic pollutant to evaluate the photocatalytic performance of the as-synthesized Pd/ZnO samples. The photocatalytic activity of the obtained ZnO modified with palladium was evaluated under the illumination of UV light. As shown in Figure 9, the different contents of palladium enhance the degradation rate to a different degree. The degradation rate increases with the increase of Pd content up to 4.07 at.%. Then, the degradation rate decreases with the further increase of Pd content. Therefore, the optimal value of palladium is 4.07 at.%. When the content of the palladium is lower than the optimal value, the improved activity is mainly due to the better charge separation than that of pure ZnO.64 On the other hand, the incorporation of noble metal onto the ZnO surface may increase the rate of electron transfer to dissolved oxygen.65 However, palladium particles may also act as charge carrier recombination centers after the content of the palladium exceeds the optimal value, which is caused by the electrostatic attraction of negatively charged palladium and positively charged holes.66 Actually, in this work, the different photocatalytic activity not only attributes to the above interpretation but also relates to the changes of surface hydroxyl content of Pd/ZnO samples caused by the modification of Pd. As mentioned above, surface hydroxyls of the photocatalyst play a significant role in a photocatalytic oxidation process. Therefore, the surface hydroxyl content of Pd/ZnO samples may have a significant impact on the photocatalytic performance. The inset of Figure 9 shows the degradation and the surface hydroxyl content of Pd/ZnO samples as a function of Pd content. It can be seen that the photocatalytic performance of Pd/ZnO samples is in line with the surface hydroxyl content. The more the surface hydroxyl content becomes, the more efficient the photocatalyst becomes. This result is consistent with the works of Boonstra and Mutsaers and of Pelizzetti and Minero.32,33 Consequently, according to the XPS characterization (Figure 3c and Table 1) and the result of the photodegradation (Figure 9), it is reasonable to infer that various contents of surface hydroxyl on the Pd/ZnO samples caused by different Pd modification also result in a difference in the photocatalytic activity.

Optical Properties of Pd Modified ZnO Samples Therefore, both the noble metal and the surface structures influence the photocatalytic activity of Pd/ZnO samples. Further work is needed to investigate these two influencing factors individually. Conclusion In summary, ZnO modified with various contents of palladium have been successfully synthesized through a one-pot hydrothermal method, which represents a facile, quick, and economical method. The obtained samples were characterized by XRD, XPS, TEM, UV-vis, and PL. The relationship among the structure, luminescence, and photocatalytic properties is investigated in detail. It indicates that the carrier concentration of Pd/ZnO increases with the increase of Pd content bringing the lift of Fermi level, which leads to the blue shift of the absorption peak and the UV emission peak. With the further increase of palladium, a continuous energy band is formed because of the formation of an impurity level resulting in the red shift of the absorption peak and a width broadening of the UV emission peak. On the other hand, the modification with palladium increases the surface hydroxyl contents of ZnO, facilitating the trapping of the photoinduced electrons and holes to form more active hydroxyl radicals, and then enhances the photocatalytic efficiency of ZnO. This work may help to provide hints for developing and designing semiconductor materials with excellent performance. Acknowledgment. This work was financially supported by the Shanghai Municipal Bureau of Quality and Technical Supervision Foundation of China (Grant I00RJ0711) and by the National Key Basic Research Special Foundation (Grant 2006CB932500). References and Notes (1) Gratzel, M. Nature 2001, 414, 338. (2) Yatsui, T.; Sangu, S.; Kawazoe, T.; Ohtsu, M.; An, S. J.; Yoo, J.; Yi, G. C. Appl. Phys. Lett. 2007, 90, 223110. (3) Gopel, W.; Schierbaum, K. D. Sens. Actuators, B 1995, B26, 1. (4) Nomura, K.; Ohta, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. Science 2003, 300, 1269. (5) Gorla, C. R.; Emanetoglu, N. W.; Liang, S.; Mayo, W. E.; Lu, Y.; Wraback, M.; Shen, H. J. Appl. Phys. 1999, 85, 2595. (6) Jing, L. Q.; Xin, B. F.; Yuan, F. L.; Xue, L. P.; Wang, B. Q.; Fu, H. G. J. Phys. Chem. B 2006, 110, 17860. (7) Yamazoe, N. Sens. Actuators, B 1991, 5, 7. (8) Arabatzis, I. M.; Stergiopoulos, T.; Bernard, M. C.; Labou, D.; Neophytides, S. G.; Falaras, P. Appl. Catal., B 2003, 42, 187. (9) Lai, C. W.; An, J.; Ong, H. C. Appl. Phys. Lett. 2005, 86, 251105. (10) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Nano Lett. 2005, 5, 667. (11) Zeng, H.; Cai, W.; Liu, P.; Xu, X.; Zhou, H.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. (12) Srikant, V.; Clarke, D. R. J. Appl. Phys. 1998, 83, 5447. (13) Zheng, Y.; Zheng, L.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K. Inorg. Chem. 2007, 46, 6980. (14) Wu, J.-J.; Tseng, C.-H. Appl. Catal., B 2006, 66, 51. (15) Zeng, H.; Liu, P.; Cai, W.; Yang, S.; Xu, X. J. Phys. Chem. C 2008, 112, 19620. (16) Semagina, N.; Grasemann, M.; Xanthopoulos, N.; Renken, A.; Kiwi-Minsker, L. J. Catal. 2007, 251, 213. (17) Liu, S.; Takahashi, K.; Eguchi, H.; Uematsu, K. Catal. Today 2007, 129, 287. (18) Jing, L.; Xu, Z.; Shang, J.; Sun, X.; Cai, W.; Guo, H. Mater. Sci. Eng., A 2002, 332, 356. (19) Peng, W.; Qu, S.; Cong, G.; Wang, Z. Cryst. Growth Des. 2006, 6, 1518. (20) Ramgir, N. S.; Late, D. J.; Bhise, A. B.; More, M. A.; Mulla, I. S.; Joag, D. S.; Vijayamohanan, K. J. Phys. Chem. B 2006, 110, 18236. (21) Lu, W.; Gao, S.; Wang, J. J. Phys. Chem. C 2008, 112, 16792. (22) Safonova, O. V.; Rumyantseva, M. N.; Kozlov, R. I.; Labeau, M.; Delabouglise, G.; Ryabova, L. I.; Gaskov, A. M. Mater. Sci. Eng., B 2000, 77, 159.

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