Article pubs.acs.org/JPCC
Synergistic Effect in Polyaniline-Hybrid Defective ZnO with Enhanced Photocatalytic Activity and Stability Zengxia Pei, Luyao Ding, Meiliang Lu, Zihan Fan, Sunxian Weng, Jun Hu, and Ping Liu* State Key Laboratory of Photocatalysis on Energy and Environment, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, P. R. China S Supporting Information *
ABSTRACT: Polyaniline (PANI) hybrid defective ZnO nanoparticles were synthesized by a facile chemisorption method together with a cold plasma treatment (CPT) technique. The PANI was dispersed uniformly onto the defective ZnO surface, and an intimate contact on the interface was observed. The coated PANI could act cooperatively with deliberately introduced defects (oxygen vacancy and interstitial zinc) to achieve remarkably enhanced photocatalytic activity. Moreover, the monomolecular-layered PANI could effectively stabilize defects on the surface of ZnO, which is of significance for practical application. It is hoped that the present work may provide an efficient and applicable method to develop photocatalysts with excellent performance.
1. INTRODUCTION Semiconductor-based photocatalyst has attracted considerable attention as a promising material for dealing with environmental pollution and clean energy production.1−5 Among various semiconductors used for photocatalysis, ZnO is one of the most widely investigated materials due to its abundance, low cost, and little toxicity. Many works2,6,7 reported that ZnO shows even better photocatalytic activity than TiO2. Even so, the photocatalytic performance of ZnO is still not that satisfying for practical utilization on a large scale, because recombination of photogenerated charge carriers remains a dominant barrier. Thus, it is indeed challenging and significant to prepare a ZnO-based photocatalyst with a high charge transfer rate and good separation efficiency of electron−hole pairs. In allusion to the above-mentioned disadvantages, many methods have been developed to decrease the recombination rate of charge carriers in ZnO.7−10 In particular, defect-related mediation has drawn much interest because not only are defects effective to modulate the activity of a photocatalyst but also defects are prevalent in materials.8,11,12 Despite the controversy concerning the role of defects in photocatalysis, it is generally accepted that surface defects act as trapping centers and thus facilitate the separation of photoinduced charge carriers whereas localized bulk ones usually serve as recombination sites.13,14 We have also, in our previous work,15 demonstrated that defects on the outer surface of a ZnO film can remarkably boost the photocatalytic activity in degradation and photoreduction reactions. Unfortunately, defects located on the surface are usually prone to be repaired. Interestingly, another thin layer of ZnO on the outer surface, acting as protecting shelter, can well stabilize those defects.15 However, the defects © 2014 American Chemical Society
in the subsurface exhibit inferior promoting capability to the photocatalytic activity, subject to the fact that catalytic reactions take place primarily on the surface of catalyst. Thus, the protecting layer should be as thin as possible, and a monomolecular protecting layer seems ideal to balance the trade-off relationship. Recently, combination of inorganic semiconductor with conjugated polymer for enhanced photoreactivity is an emerging area of research.7,16−18 Among many polymer materials, PANI has exhibited great potential for its rapid charge separation capability, relatively slow charge recombination rate in electron-transfer processes, and good stability.7,18 Moreover, this polymer, which can behave like a p-type semiconductor, is an excellent hole transporting material.16,19 Zhu et al.7 reported that PANI, even in its monomolecular form, can effectively suppress the photocorrosion of ZnO. Nevertheless, both surficial defects and PANI can act as active sites and charge separation centers, but knowledge about the interaction (synergistic or antagonistic) of the two is still lacking. Besides, the role of PANI molecular layer (as protecting shelter) on stabilizing defects is yet to be known. In the present work, we developed a facile route to prepare monolayer PANI hybrid, oxygen vacancy (Vo) and interstitial zinc (Zni) mediated ZnO by the chemisorption method and the CPT technique. The fabricated composite displayed remarkably enhanced photocatalytic activity in degradation of MO and 4CP, due to the synergic effect between defects and PANI. The coupling mechanism was investigated systematically. FurtherReceived: February 26, 2014 Revised: March 28, 2014 Published: April 14, 2014 9570
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Germany) with a typical three-electrode system. Different ZnO samples dispersed in ethanol solution were evenly spread onto FTO glass substrates and served as the working electrode. The counter and reference electrodes were Pt plate and Ag/ AgCl electrode, respectively. 0.2 M Na2SO4 (pH = 6.8) served as electrolyte. A 300 W xenon lamp was used to provide a UV light source with an optical filter (365 ± 15 nm). The amplitude of the sinusoidal wave was 10 mV, and the frequency range of the sinusoidal wave was from 1 MHz to 0.01 Hz. 2.4. Photocatalytic Activity Test. The photocatalytic activity of different ZnO samples was estimated by photodegradation of MO and 4-CP with a concentration of 3 × 10−5 M and 10−4 M, respectively. For both reactions, 40 mg of photocatalyst was added into 80 mL of each solution in a quartz tube. Four UV lamps (4 W, Philips TL/05) surrounded the tube reactor and provided illumination with a predominant wavelength at 365 nm (1 mW/cm2). Prior to irradiation, stirring in the dark for 100 min was allowed for establishment of adsorption/desorption equilibrium. Change in absorbance of the solution was used to monitor the extent of photocatalytic reaction. At given irradiation time intervals, 3 mL of the solution was taken out, centrifuged, and then quickly analyzed on the UV−vis spectrometer. The final efficiency was calculated by the equation Et (%) = (1 − Ct/C0) × 100%, where C0 and Ct stand for the initial and final concentration of reactants, respectively. The active oxidants generated in the photocatalytic process were measured by different trapping agents, isopropanol for hydroxyl radical (•OH) and ammonium oxalate for photogenerated hole. To check the role of superoxide radical, N2 was bubbled into the solution with a flow rate of 60 mL/min.
more, we found that even a monolayer of PANI could efficaciously protect the surficial defects in a prolonged UV light irradiation process. It is hoped that the present work may open a strategy for designing a high performance and high stability photocatalyst for practical applications.
2. EXPERIMENTAL SECTION 2.1. Materials. ZnO with average particle size of 30 nm was purchased from Alfa Aesar. PANI (molecular weight 5 × 104, conductivity ∼2 S/cm) was obtained from Taizhou Yongjia Trade Corp. (P. R. China). Tetrahydrofuran (THF) or ether, methyl orange (MO), 4-chlorophenol (4-CP), isopropanol, and ammonium oxalate were all obtained from Sinopharm Chemical Reagent Co. Ltd., and all of these reagents were analytically pure and used without further purification. Deionized water was used throughout the work. 2.2. Sample Preparation. The preparation process was modified from a reported method.7 Typically, 1 g of ZnO powder was spread on a glass sheet and transferred to a previously He flushed quartz vacuum chamber, followed by cold plasma discharge (300 W) treatment for 10 min under a rarefied He atmosphere. After that, the defective ZnO was added to 50 mL of 0.1 g/L PANI (THF or ether) solution, sonicated for 30 min to disperse the ZnO powder, and then stirred for 48 h. The suspension was then centrifuged and washed with water three times and transferred to an oven to dry at 80 °C for 12 h under vacuum. Hereto, a PANIhybridized defective ZnO sample was prepared and marked as Z-D-P. In contrast, hybridized ZnO according to the above method without CPT treatment was denoted as Z-P. Moreover, pure ZnO is labeled as Z while defective ZnO (Z-D) was the original ZnO treated only with CPT. 2.3. Characterization. Crystal structure identification was performed using Bruker D8 X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 40 mA. Raman scattering measurements were performed with a multichannel modular triple Raman system (Renishaw Co.) with confocal microscopy at room temperature using the 532 nm laser. A 50× microscope objective lens was used for focusing the laser beam and collection of the scattered light. Scanning electron microscope (SEM) images was obtained with an FEI Nova NanoSEM 230 field-emission scanning electron microscope. Microstructures and morphologies were investigated using TecnaiG2 F20 S-TWIN (FEI company) transmission electron microscopy (TEM) with a field emission gun at 200 kV. The nitrogen adsorption and desorption isotherms were characterized using a Micrometrics ASAP 2020 analyzer. The degassing process was conducted at 120 °C for 4 h. Diffuse reflection spectra (DRS) of the samples were recorded on a Varian Cary-500 spectrophotometer, and BaSO4 was used as a reference. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 photoelectron spectroscope (Thermo Fisher Scientific) at 1.2 × 10−9 mbar using an Al Kα X-ray beam (1486.6 eV). The XPS spectra are charge corrected to the adventitious C 1s peak at 284.6 eV. UV−vis spectra were measured on a UV−vis−NIR spectrometer (Cary-500). Total organic carbon (TOC) assays of the degraded solution after 120 min of irradiation were carried out on a TOC analyzer (TOC-VCPH, Shimadzu). The photoluminescence (PL) spectra were obtained using a Varian CaryEclipse 500 with an excitation wavelength at 277 nm. The electrochemical impedance spectra (EIS) test was conducted on a ZENNIUM electrochemical workstation (Zahner,
3. RESULTS AND DISCUSSION 3.1. Structure of PANI Hybridized Defective ZnO. Hereafter, Z represents the pristine ZnO sample, Z-P denotes the PANI coated ZnO, and Z-D stands for CPT treated defective ZnO sample, while Z-D-P stands for PANI coated defective ZnO. XRD patterns of all ZnO samples in Figure 1 exhibit no changes despite the CPT process and PANI modification. We have demonstrated that CPT is just a surface-treatment technique and it creates defects only on surficial atom layers,14,15 thus no discernible distinction is observed in the XRD patterns here due to the CPT treatment.
Figure 1. XRD patterns of different samples. 9571
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Figure 2. HRTEM images of Z (a) and Z-D-P (b) samples. (Inset) The amorphous edge of chemisorbed PANI.
The observed peaks can be well indexed to hexagonal ZnO wurtzite structure (JCPDS card No. 01-089-0511). Pure PANI usually gives a broad band at approximately 20−25°. However, no peak is detected in this region because the loading amount is too small, which also indicates that the PANI is possibly dispersed uniformly onto the surface of ZnO nanoparticles.7,20 The SEM image of the Z-D-P powders is given in Figure S1 in the Supporting Information, and only irregular aggregators are observed. To get a closer view of the hybridized structure, the as-prepared Z-D-P sample was observed by HRTEM, with the Z sample being a reference. As illustrated in Figure 2a, the Z sample shows quite ordered lattice structure in both the inside and the boundary, revealing good crystallization of pure ZnO. However, the hybridized Z-D-P in Figure 2b displays a distinct noncrystal edge, in contrast with the ZnO core. The noncrystal structured PANI layer adsorbed evenly on the surface of ZnO with a thickness around 0.56 nm. As the diameter of the benzene structure is about 0.5 nm and the PANI molecule has a fold-line structure, it is estimated that the PANI on the Z-D-P sample is approximately a monomolecular layer.7 Furthermore, considering that the BET surface area of the ZnO is 13.25 m2 g−1 (see Figure S2 in the Supporting Information), the weight ratio at which compact PANI monolayer coverage is formed on the surface of ZnO is about 0.6%.7 In the present system, the loading amount of PANI is 0.5 wt %. Thus, the surface area data are in good agreement with the above speculation since PANI can only occupy the active adsorption sites on the ZnO surface.7,20 In the following, Raman and XPS spectra were recorded to provide information about the binding state of the chemisorbed structure on the interface. Raman spectra of pure ZnO, Z-D-P, Z-P, and PANI samples are illustrated in Figure 3. Two intensive bands at 99 and 441 cm−1 are attributed to the low and high E2 modes of nonpolar optical phonons, respectively, typical of a wurtzite phase.21,22 Both of these bands are observed in Z, Z-D-P, and Z-P samples. Besides, Raman bands of the pure PANI chains in their emeraldine salt form (as main conductive structure) emerge at 1596 cm−1 (the CC and C∼C stretching vibrations of the quinonoid (Q) and/or semiquinonoid (SQ) rings respectively, where ∼ denotes a bond intermediate between a double and a single bond), 1343 cm−1 (the C∼N stretching vibration of polaronic structures), and 1176 cm−1 (the C−H bending in-plane vibration of SQ rings in polaronic structures).16,23 It is interesting to note that, as circled in the spectra of the Z-D-P and Z-P samples, the bands of PANI at 1596 and 1176 cm−1 move toward lower
Figure 3. Raman spectra of pure ZnO, hybrid Z-D-P, and Z-P samples and pure PANI.
wavenumbers for about 3 and 8 cm−1, respectively. The red shift of the bands suggests that the conjugated structure of PANI is weakened, and an intensive interaction exists between PANI and defective ZnO.7 The intensity of the band at 1343 cm−1 is lowered, probably because the C∼N bond is distorted. The above postulation is further confirmed by results from XPS tests. As displayed in Figure 4a, the C 1s core level spectrum of the Z-D-P sample is quite asymmetric, indicating rich valent states of C atoms. The spectral curve can be well fitted into into 3 peaks, located at 284.6, 286.7, and 288.6 eV, separately. The peak with the smallest binding energy (BE) is attributed to C atoms bound only to C or H atoms.17 The peak located at 286.7 eV corresponds to C in C−N groups whereas the 288.6 eV centered peak is related to C within CN species.17 Nevertheless, according to earlier investigation,17 the BEs of the above three types of C species in pure PANI are 284.6, 285.6, and 287.8 eV, respectively. Compared with those in the pure PANI sample, the binding energy of C atoms in the hybrized structure shifts to higher BE in C−N and CN groups, which implies that the binding site of PANI on ZnO is very likely to be the benzenoid unit. Besides, both O 1s spectra of pure ZnO and the Z-D-P sample in Figure 4b can be best deconvolved into 4 curves. The main peak at some 530.1 ± 0.1 eV is assigned to oxygen within the ZnO crystal lattice (OL).24 The peak with higher binding energy at about 532.0 ± 0.1 eV is associated with hydroxyl groups (OH).25 A medium binding energy component, centered at 531.3 ± 0.2 eV, is generally 9572
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Figure 4. (a) XPS C 1s spectra of Z-D-P sample; (b) XPS O 1s spectra of Z and Z-D-P samples.
Figure 5. (a) The temporal absorption spectra of MO solution at the desired reaction time in the presence of sample Z-D-P. (b) Photocatalytic degradation of MO over the as-prepared different samples under UV (365 nm) light irradiation. (Inset) Rate constant k of different samples.
attributed to O2− ions in the oxygen deficient regions (OD) in the matrix of ZnO.15,24,26 An even higher peak, located at 532.7 ± 0.2 eV, can be ascribed to adsorbed O2 or H2O species (OAD) on the surface of samples.15,24 The ratio of OD species varied noticeably after CPT treatment, as it increased from 13% to 29% in pure ZnO and Z-D-P sample, respectively. Moreover, a more intensified asymmetry is observed in XPS Zn 2p spectra (see Figure S3 in the Supporting Information) of the Z-D-P sample in comparison with pure ZnO, which denotes that the number of Zni atoms has also increased.26,27 ESR results (see Figure S4 in the Supporting Information) further confirmed the existence of these defects. A strong signal with g = 1.96 is associated with Zni as shallow donors, while a small signal with g value at 2.003 is attributed to Vo.11,15 It is not surprising that the ZnO nanocrystal has a nonstoichiometric atomic ratio since this material is known to have abundant native defects.12 However, CPT treatment could further introduce more defects into ZnO. Also, XPS results suggest that both of the defects are preserved in ZnO after PANI coating. Based on all the above results, it can be concluded that an intimate contact does exist between PANI and defective ZnO. The PANI is in its monomolecular layer form, with the binding site probably being the benzenoid unit. Nonetheless, it has to be mentioned here that, despite the intensive interaction and
the existence of defects, the band gap of ZnO does not change as no shift of band edge absorption is observed in DRS spectra (see Figure S5 in the Supporting Information). Even so, the hybrid structure is expected to improve the photocatalytic activity, which will be discussed below. 3.2. Enhanced Photocatalytic Activity by Defects and PANI. For clarifying the roles of PANI and defects, we compared the photocatalytic activity of different samples in degradation of MO. Figure 5a shows the typical temporal absorption spectra of MO solution in the presence of a Z-D-P sample. As shown in Figure 5b, the photodegradation process undergoes pseudo-first-order kinetics, thus the value of the rate constant equals the corresponding slope of the linear fitting line.7 MO, as a typical widely used azo dye, is quite stable under UV light irradiation without catalyst. Also, pure PANI shows a quite low degradation rate (see Figure S6 in the Supporting Information). While ZnO could eliminate MO, both defect and PANI modification can enhance the photocatalytic activity. However, the Z-D-P sample exhibits remarkably improved performance among all photocatalysts, and its rate of photodegradation is 2.5 times higher than that of pure ZnO. The histogram in the inset graph tells that, in comparison to pristine ZnO, the net improvements for Z-P, Z-D, and Z-D-P are 32%, 74%, and 150%, respectively. Considering that ZnO 9573
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Figure 6. (a) Photodegradation of 4-CP over different samples under UV (365 nm) light irradiation. (Inset) Rate constant k of different samples. (b) TOC removal of different samples on degradation of 4-CP (for 120 min) under UV light irradiation.
has been reported by many works2,6,7 to be a rather good photocatalytic material under UV light and that the value of the rate constant for Z-D-P exceeds the simple accumulation of those of Z-D and Z-P samples, it is reasonable to deduce that there exists a synergic effect between PANI and the defects. While the treatment of azo dye wastewater remains a difficult problem at present, phenols are even more toxic, stable, and widespread in effluents.28 Moreover, to avoid indirect oxidation via a sensitization effect like that of MO and truly reflect the performance of our samples, 4-CP was further used as another target pollutant, and the results are displayed in Figure 6a. Promisingly, PANI hybrized defective ZnO still shows excellent photodegradation activity and a synergistic effect is also observed. The rate constant is more clearly distinguished in the inset histogram. For the Z-D-P sample, its k value exhibits 318% improvement, which is much bigger that the other modified ZnO (79% for Z-P, 141% for Z-D). In addition, this efficiency is also verified by TOC results in Figure 6b. The mineralization capacity of different samples follows the same sequence, namely, Z-D-P > Z-D > Z-P > Z. Nearly 82% of organic carbon species are removed within 2 h. 3.3. Promotion of Interface Charge Separation. The large exciton binding energy (60 meV) endows ZnO materials with excellent photoluminescent property, especially for those with defects.29,30 Here, to check the role of the hybrized PANI on defective ZnO, room temperature PL spectra were studied, and the results are shown in Figure 7. A sharp emission peak at 421 nm is attributed to the transition from shallow donors (mainly Zni) to the valence band.30,31 Another intensive peak at ∼490 nm is still disputable, but the most popular argument is the transition related with oxygen defects on the surface of the photocatalyst.32,33 Two medium peaks, at 446 and 460 nm, are generally associated with grain boundary defects and Zni−VZn complex, respectively.32,34 These observations conform well with the XPS and ESR results, namely, the dominant defects in all defective samples are Vo and Zni. After the CPT process, PL intensity of both defective ZnO samples (Z-D and Z-D-P) increases, which further confirms the introduction of defects. However, after PANI coating, the emission intensity of the Z-P and Z-D-P samples shows an obvious decrease, in contrast to pristine/defective ZnO. The quench of fluorescence is supposed to stem from the separation of charge carriers on
Figure 7. Room temperature PL spectra of different samples (excitation wavelength, 277 nm).
the interface of PANI and ZnO. Such emission quenching also implies that an interfacial charge-transfer process had occurred.35 On the other hand, photochemical impedance spectrum provides one of the most powerful methods to investigate the interfacial charge carrier behaviors.7 Here, to further validate the improved charge separation efficiency by synergistic effect between PANI and surface defects, EIS tests are conducted and the results are shown in Figure 8. The radius of the arc on the Nynquist plot reflects reaction rate occurring at the surface of different sample electrodes.7,36 It is observed that both PANI and defects can decrease the radius of the plot arc, while a combination of the two gives rise to a much smaller radius on FTO/Z-D-P electrode. Smaller arc radius means that a more effective separation of photoexcited e−−h+ pairs and faster interfacial charge transfer have occurred.36 Therefore, the EIS results demonstrate that the present conductive polymer and defects could work cooperatively to facilitate charge carriers’ transfer and separation. In other words, owing to the synergic effect, defects and PANI could fulfill a promoting effect toward photocatalytic activity, while the number of annihilated charge carriers is decreased significantly at the same time. 3.4. Discussion on Photocatalytic Mechanism. Determination of active species in photocatalytic reactions, which can 9574
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decrease is observed when isopropanol is added into the solution, which implies that the •OH species contributed to the degradation to some extent, but not as a decisive species. When N2 is bubbled in, it is surprising to see an accelerated decoloration rate according to the main absorption peak at ∼465 nm. However, after carefully checking the light absorption evolving spectra, we found that a new peak centered around 246 nm emerged (see Figure S7 in the Supporting Information). The corresponding species is attributed to hydrazine derivatives as a reducing product.17,39 Thus, O2 is indispensable for a complete degradation in the present system. We deduce that the situation is similar in decomposition of 4CP. Based on all the above results and discussion, we can depict a probable mechanism of the synergistic effect between PANI and defects accounting for an enhanced photocatalytic activity. The details are illustrated in Figure 10. In general, surficial Vos can act as trapping sites of photogenerated electron as well as active sites,11 both of which are beneficial for photocatalytic activity. Besides, Zni (Zni+, Zni0) and Vo (Vo+, Vo0) are electron donors and are considered to enhance the donor density.15 PANI has high mobility of charge carriers and excellent transporting capacity of holes. Moreover, this conjugated polymer can theoretically be photoexcited and inject electrons into the conduction band of ZnO,7,16 though PANI shows poor photocatalytic activity by itself (see Figure S6 in the Supporting Information). The HOMO and LUMO of PANI match well with the energy band of ZnO as shown in Figure 10a. When irradiated by appropriate light, the photogenerated holes transfer from ZnO toward the PANI monolayer while electrons from both ZnO and PANI can be effectively trapped by Vos on the surface ZnO, and a good separation of charge carriers can be achieved consequently. These separated fundamental initiators of photocatalytic reaction will then be transferred quickly and effectively across the photocatalyst−solution interface and execute reaction. Additionally, the increased donor density can improve charge transport and shift Fermi level toward the conduction band, which can facilitate the charge separation at the semiconductor/electrolyte interface and eventually promote the photocatalytic efficiency.40,41 In the reaction process, holes act as the main oxidative species, and O 2 •− and • OH also contribute to the photocatalytic degradation. An atomic schematic diagram depicting the charge separation process caused by the synergistic effect between PANI and defects is illustrated in Figure 10b.
Figure 8. EIS Nynquist plots of the as-prepared photocatalysts under UV (365 ± 15 nm) light irradiation.
be achieved by adding different scavengers, is important to help elucidate the reaction mechanism. In the present system, isopropanol is used as hydroxyl radical scavenger15,37 and ammonium oxalate acts as photogenerated hole trapping agent.38 N2 is bubbled into the solution to check the role of superoxide radical. In Figure 9, the photocatalytic activity for
Figure 9. Effects of different scavenger addition on the photocatalytic degradation of MO for the Z-D-P sample.
degradation of MO is dramatically reduced with the addition of ammonium oxalate, indicating that photoexcited holes play a dominant role in the decomposition process. A medium
Figure 10. (a) Proposed photocatalytic mechanism according to the band alignment. (b) Atomic schematic diagram illustrating the synergistic effect between defects and PANI. 9575
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Figure 11. (a) Photocatalytic avtivity of the Z-D-P sample in degradation of MO over cycle tests and prolonged UV light irradiation. (b) HRTEM image reviewing the morphology of the Z-D-P sample after prolonged UV light irradiation.
3.5. Influence of PANI on the Microenvironment of Defects. As is known to all, surface defects are readily repaired by many oxidative species, and sometimes they are even sensitive to dissolved oxygen.14,42 Recent works43,44 reported that the disordered outer sphere can effectively stabilize defects in defective TiO2 that has a core/shell structure. We have also demonstrated that a thin layer of the same photocatalyst, as protecting shell, can well stabilize defects located in the subsurface.14,15 In the present system, we are quite interested about the influence of PANI on the microenvironment of defects in a prolonged UV irradiation process. However, the well-known photocorrosion phenomenon may also cause remarkable deactivation of ZnO in photocatalytic reactions. Fortunately, PANI can efficiently suppress the photocorrosion of ZnO, even in its monomolecular layer form.7 Therefore, the photocatalytic reactivity of PANI-hybridized defective ZnO may reasonably reflect the effect of defects. To inspect the durability of defects, cycling tests with prolonged UV (365 nm) light irradiation are conducted in degradation of MO. Hopefully, as can be seen in Figure 11a, the Z-D-P sample shows stable activity in first three cycles with 100 min period each. Even after 40 h irradiation under UV light, the sample still exhibits excellent stability as 97% MO is degraded within another 100 min. Then we further checked the morphology of the hybrid structure. In Figure 11b, it is observed that the crystallinity of ZnO did not show obvious change and the polymer layer was well preserved, revealing the good stability of the PANI hybrid defective ZnO. Thus, it can be concluded that the coated PANI, in its monolayer form, can capably stabilize interfacial defects and inhibit the photocorrosion of ZnO, both of which are of special significance for practical utilization.
the coated PANI monolayer can effectively stabilize defects on the surface and inhibit the photocorrosion of ZnO, which are of significance for practical applications.
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ASSOCIATED CONTENT
S Supporting Information *
SEM image, ESR spectra, BET and DRS results, XPS Zn 2p spectra, and other characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-591-8377-9239. Tel:+86-591-8377-9239. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is supported by National Natural Science Foundation of China (21173046, 21033003, 21273035, J1103303), National Basic Research Program of China (973 Program: 2013CB632405).
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REFERENCES
(1) Wang, J.; Liu, P.; Fu, X.; Li, Z.; Han, W.; Wang, X. Relationship between Oxygen Defects and the Photocatalytic Property of ZnO Nanocrystals in Nafion Membranes. Langmuir 2009, 25, 1218−1223. (2) Li, Y.; Xie, W.; Hu, X.; Shen, G.; Zhou, X.; Xiang, Y.; Zhao, X.; Fang, P. Comparison of Dye Photodegradation and its Coupling with Light-to-Electricity Conversion over TiO2 and ZnO. Langmuir 2010, 26, 591−597. (3) Akyol, A.; Yatmaz, H. C.; Bayramoglu, M. Photocatalytic Decolorization of Remazol Red RR in Aqueous ZnO Suspensions. Appl. Catal. B: Environ. 2004, 54, 19−24. (4) Steinfeld, A. Solar Hydrogen Production via a Two-Step WaterSplitting Thermochemical Cycle Based on Zn/ZnO Redox Reactions. Int. J. Hydrogen Energy 2002, 27, 611−619. (5) Deng, D.; Martin, S. T.; Ramanathan, S. Synthesis and Characterization of One-Dimensional Flat ZnO Nanotower Arrays as High-Efficiency Adsorbents for the Photocatalytic Remediation of Water Pollutants. Nanoscale 2010, 2, 2685−2691.
4. CONCLUSIONS In summary, we have successfully prepared a PANI-hybrid defective ZnO photocatalyst by a facile chemisorption method and the CPT technique. The PANI monomolecular layer has an intimate interaction with the defective ZnO surface. This interface hybrid structure can boost the photocatalytic activity in degradation of MO and 4-CP, with enhanced separation of charge carriers and improved charge transfer being the main reasons. A synergistic effect between PANI and defects including Vo and Zni is proposed and demonstrated. Moreover, 9576
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dx.doi.org/10.1021/jp5020143 | J. Phys. Chem. C 2014, 118, 9570−9577
