CCG Multidimensional

Mar 5, 2012 - The combined heterojunction helps to improve photocatalytic activity by increasing light .... Qui Thanh Hoai Ta , Sungho Park , Jin-Seo ...
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Photocatalytic Performance of a Ag/ZnO/CCG Multidimensional Heterostructure Prepared by a Solution-Based Method Dae-Hwang Yoo,† Tran Viet Cuong,‡ Van Hoang Luan,‡ Nguyen Tri Khoa,† Eui Jung Kim,‡ Seung Hyun Hur,*,‡ and Sung Hong Hahn*,† †

Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea School of Chemical Engineering and Bioengineering, University of Ulsan, Ulsan 680-749, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The photocatalytic performance of a multidimensional heterojunction composed of decorated Ag nanoparticles on ZnO nanorods vertically grown on a chemically converted graphene (CCG) was investigated. The combined heterojunction helps to improve photocatalytic activity by increasing light absorption, preventing photoinduced electron−hole recombination, and providing a carrier pathway for the giant π-conjugated system of CCG. A significant finding was that the low work function value of Ag (−4.74 eV) at the (111) surface makes possible the transfer of electrons from CCG to Ag. Consequently, the degree of photodegradation by Ag/ZnO/CCG is much higher than the sum of photodegradations by Ag/ZnO and ZnO/CCG samples, which indicates that the combination of metal, CCG, and semiconductor provides enhanced photocatalytic activity through the double transference of electrons. high catalytic activity has been reported.13 Ag nanoparticles and oxygen vacancy defects on the surface of ZnO nanocrystals help to suppress electron−hole recombination, thus enhancing the photocatalytic activity. In heterostructures, a different dimensional structure of the composite materials can result in a difference in the photocatalytic performance. In particular, particles with a nanosized and 0-dimensional structure are most useful for absorbing light and trapping photoinduced electrons.14,15 Graphene, with a 2-dimensional honeycomb structure, has attracted considerable attention due to its outstanding mechanical, thermal, optical, and electrical properties. Functionalized graphene-based semiconductor photocatalysts have attracted interest due to their good electron conductivity, large specific surface area, and high adsorption.16−18 Numerous attempts have been made to combine graphene with semiconductor photocatalysts to enhance photocatalytic performance.19−22 In our previous studies, it was found that graphene oxide (GO) on TiO2 film helps to enhance the photocatalytic activity of TiO2 by providing π−π stacking interactions between methylene blue (MB) and the aromatic regions of GO and by electron transfer of photoinduced electrons.23 It is well known that significant improvement in photocatalytic performance can be achieved with a vertical ZnO nanostructure because of its intrinsic high absorption efficiency. We

A. INTRODUCTION Metal oxide semiconductors, such as TiO2 and ZnO, are promising materials for the degradation of organic pollutants and the dissolution of contaminants by light irradiation due to their high chemical stability, low toxicity, high oxidation capacity, and ready availability.1−3 Of the common oxide semiconductor materials, ZnO, with a wide direct band gap of 3.37 eV,4 has been a material of considerable interest for photocatalysis applications.5 When a photon with an energy higher than the band-gap energy (E g) of the semiconductor is incident to the semiconductor, photoinduced electron−hole pairs are created; an electron in the valence band (VB) is excited into the conduction band, leaving a positive hole in the VB. Photogenerated holes and electrons play a very important role in pollutant degradation and photocatalytic disinfection. However, photoinduced electrons and holes can also recombine easily to reduce the photocatalytic activity of a semiconductor. To overcome this fast recombination process, considerable research has been carried out utilizing a combination of semiconductors and other materials, such as Fe, Pt, and Ag.6,7 Recently, there has been increased interest in fabricating block heterojunctions with compositions and/or new structures that can modulate the properties of the materials and can be applied to diverse areas, such as electronic devices8,9 and photocatalysts.10−12 The metal/semiconductor is one of the most popular types of heterostructures, which has been extensively studied because of its excellent catalytic activity. For example, a Ag/ZnO heterostructure photocatalyst with © 2012 American Chemical Society

Received: October 25, 2011 Revised: February 27, 2012 Published: March 5, 2012 7180

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ensure that particle growth was complete. The molar ratios of [PVP]/[AgNO3] and [AgCl]/[AgNO3] were selected to be 15 and 1, respectively. The final product was expected to produce a large amount of PVP-coated nanoparticles concomitant with a small amount of Ag nanowires (NWs). Thus, it was necessary to separate the Ag nanoparticles from the Ag NWs. After allowing the solution to cool overnight, it was divided into two portions, the top portion consisting primarily of nanoparticle aggregates and the sediment portion containing both selfassembled NWs and nanoparticle aggregates. The solutions were centrifuged at 2000 rpm for 30 min to isolate the nanoparticles, which remained in the sediment. The Ag nanoparticles were transferred to water by first washing with acetone to remove the excess PVP and EG, centrifugation at 5000 rpm for 5 min, and redispersal of the residue in water. This process was repeated three times. The synthesized Ag nanoparticles were coated on the ZnO/CCG sample by spraying (2 mL), and the material was annealed at 150 °C to completely remove the solvent. The process for preparing the heterojunction structure is shown in Scheme 1.

postulated that using chemically converted graphene (CCG), as a buffer layer, would aid the growth of ZnO nanorod (NR) arrays in the vertical direction.24 Considering the individual effects of 1-dimensional NRs, 0dimensional nanoparticles, and 2-dimensional graphene on the enhancement of photocatalytic activity, it was of interest to combine these different dimensional materials into a single structure to determine if additive enhancement of semiconductor photocatalytic activity could be realized. In this research, we constructed a heterostructure from materials of different dimensions, a 0-dimensional Ag nanoparticle, 1-dimensional ZnO NR, and 2-dimensional CCG, by a fully solution-based process. ZnO NRs were grown on CCG by a hydrothermal method, and prepared Ag nanoparticles were sprayed onto the ZnO/CCG. The structures and morphologies of the constructed materials were analyzed by Raman, XRD, SEM, and TEM/EDS. Photocatalytic activity was assessed by analyzing the decomposition of MB solution.

B. EXPERIMENTAL METHODS 1. Preparation of Chemically Converted Graphene (CCG). Graphene oxide was prepared by the modified Hummer method.25 A 2 g portion of natural flake graphite and 2 g of NaNO3 were mixed in 96% H2SO4 at 0 °C. The mixture was continuously stirred while 12 g of KMnO4 was gradually added, and the mixture was stirred at 0 °C for 90 min and then kept at 35 °C for 2 h. Deionized (DI) water (80 mL) was slowly added dropwise to the reaction, and then the mixture was further diluted with 200 mL of DI water. A 10 mL portion of 30% H2O2 was added, and the reaction was stirred for an additional 10 min. Centrifugation at 5000 rpm for 15 min was used to remove impurities, and subsequent washing of the suspension with DI water and centrifugation at 7000 rpm for 15 min was continued until the supernatant reached pH 7. Colloidal graphene oxide was obtained by further centrifugation at 9000 rpm for 15 min. A 1 mL portion of graphene oxide was diluted with 100 mL of DI water. This solution was kept in an ultrasonic bath for 5 min. A 2 mL portion of hydrazine monohydrate was added, and the mixture was stirred for 2 h to obtain the CCG dispersion. 2. Synthesis of the ZnO NR Structure. The CCG dispersion (2 mL) was sprayed onto a 2 × 2 cm2 quartz plate to form the CCG film. Herein, the CCG film played the role of a buffer layer to facilitate the vertical growth of ZnO NR arrays. To prepare the ZnO seed layer, the CCG film was wetted with an ethanolic solution of 5 mM zinc acetate dehydrate, rinsed with ethanol after 10 s, and then dried in a stream of argon. The CCG film coated with a thin layer of zinc acetate was heated to 350 °C in air for 30 min to form the ZnO seed layer. The ZnO NRs were grown by placing the substrate on a stainless steel holder in a 100 mL aqueous solution of 16 mM zinc nitrate hexahydrate and 25 mM methanamine, and heating the solution at 90 °C in an oven for 2 h. The substrates covered with ZnO NRs were rinsed with water and then dried in an argon stream. 3. Preparation of Silver Nanoparticles. Ag nanoparticles were synthesized in water solution by the polyol method as follows. A mixture of 0.66 g of poly(vinylpyrrolidone) (PVP) and 20 mL of ethylene glycol (EG) was heated until thermally stabilized at 170 °C in a flask. Silver seeds were prepared by adding 0.055 g of silver chloride (AgCl) to the flask. After 3 min, 0.066 g of silver nitrate (AgNO3) as a Ag precursor was slowly added, and the flask was further heated for 40 min to

Scheme 1. Synthesis of the Ag/ZnO/CCG Heterojunction Structure

4. Analyses of Material Properties. Raman spectra were measured for the ZnO, NR, and CCG layer. To verify the structures of samples, X-ray diffraction (XRD, Philips PW3710) was carried out in the 2θ mode. A scanning electron microscope (SEM, Hitachi S-4200) and transmission electron microscope (TEM, JEOL JEM-2100F) with an energydispersive spectrometer (EDS) were used to examine the morphologies and the elemental makeup of the Ag/ZnO/CCG composites. Photocatalytic properties of the prepared films were examined by measuring the photodecomposition of 1 × 10−5 mol/L aqueous methylene blue (C16H18N3S-Cl-3H2O) solution. Samples were placed with MB solution in a tubular quartz reactor. To characterize the photocatalytic properties in the UV region, the solution was stirred while being irradiated by four surrounding UV lamps of 20 W (wavelength = 352 nm). After the dark adsorption in 10 min, the measurement was done. Photodegradation of the MB solution was investigated by measuring the absorption spectra of the solution using a UV− vis spectrophotometer (HP 8453) at λmax = 664 nm. Photocatalytic activity of Ag/ZnO/CCG was compared with that of ZnO NR, ZnO/CCG, and Ag/ZnO samples.

C. RESULTS AND DISCUSSION Figure 1 shows Raman spectra of ZnO NRs grown on CCG that include three peaks at approximately 1340, 1590, and 2680 cm−1. The G band peak at around 1590 cm−1 is characteristic for graphite sheets and indicates the presence of sp2 bonds in the structure. The D band at around 1340 cm−1 can be attributed to the presence of defects within the hexagonal graphite structure. The sharpness and relatively higher intensity of the G band peak compared with that of the D band imply that CCG is well-ordered. The peak around 2680 cm−1 7181

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(111) direction for synthesized Ag nanoparticles, which is an important property related to the enhanced photocatalytic activity of our Ag/ZnO/CCG sample that will be discussed below. Figure 3a shows SEM images of the Ag/ZnO/CCG sample. The inset in the upper-left corner in Figure 3a shows cross-

Figure 1. Raman spectra of ZnO grown on CCG.

corresponds to the 2D band, which is related to the second scattering of the two D mode phonons. The inset in Figure 1 shows two Raman peaks for ZnO at 438 cm−1 and at 383 cm−1; the former is related to the E2 high frequency phonon mode, which indicates the crystal quality, and the latter corresponds to A1 symmetry in the TO mode. From the XRD pattern of the Ag/ZnO/CCG sample, shown in Figure 2, it can be seen that there are two sets of diffraction

Figure 2. XRD patterns of Ag/ZnO/CCG and Ag/ZnO.

peaks, one for ZnO and the other for Ag, which indicate that the as-synthesized samples are well-constructed. Diffraction peaks for CCG are not observed, which may be related to the low amount and low diffraction intensity of CCG. Peaks for Ag in Figure 2, pattern a, are consistent with a face-centered cubic (fcc) structure. Peaks for ZnO correspond to the wurtzite structure and show that the ZnO NRs grew in the (001) direction with high orientation. By comparison, ZnO NRs grown on a ZnO seed layer without CCG is shown in Figure 2, pattern b. The XRD pattern of the Ag−ZnO sample did not differ from that of the Ag/ZnO/CCG sample. The intensity of the (002) peak of ZnO from the Ag/ZnO/CCG sample, however, was higher than that of ZnO from the Ag/ZnO sample. This result established that graphene helps the ZnO NRs to grow with high orientation, as was reported in our previous study.24 The XRD data showed a main peak in the

Figure 3. (a) SEM image of the Ag/ZnO/CCG sample. The inset in the upper-left corner is a cross-sectional SEM image, and the inset in the right corner is a TEM image of Ag nanoparticles on a ZnO NR. (b) EDS of the Ag nanoparticle and ZnO NR.

sectional SEM image of the sample. The length of ZnO NRs is about 700 nm. The ZnO NRs grew vertically with high orientation in a hexagonal arrangement, which is in agreement with the XRD results. The inset in the upper-right corner in Figure 3a, which shows TEM images of the samples, indicates that the nanoparticles are attached to the ZnO NRs. The size of 7182

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surface properties, such as oxygen vacancies and interface of the Ag/ZnO heterostructure.26 Ag can trap electrons in photoinduced hole−electron pairs in the semiconductor, and the remaining photoinduced holes (hind+) produce hydroxyl radicals (•OH), an extremely strong oxidant for the partial or complete mineralization of organic chemicals.27 In the Ag/ZnO sample, the energy level of the conduction band is higher than the new Fermi energy level; thus, photoinduced electrons (eind−) could transfer from ZnO NRs to Ag. Electronic acceptors, sucha as adsorbed O2, can easily trap photoinduced electrons to produce superoxide radicals (•O2−), which would contribute to the enhancement of photocatalytic activity for the Ag/ZnO nanocomposite. In the Ag/ZnO/CCG sample, enhancement of photocatalytic activity is due to both the Ag and the CCG components, as mentioned above. The major reaction steps in this photocatalytic degradation mechanism with UV-light irradiation are summarized by the following equations:

the attached nanoparticles ranges from approximately 10 to 20 nm. The elemental composition of the nanoparticles was analyzed by EDS spectra. For the red-squared areas in the inset of Figure 3a, peaks associated with Ag, O, and Zn were observed in the EDS spectra, as shown in Figure 3b, which confirms that the ZnO NRs are decorated by Ag nanoparticles. The photocatalytic activity of Ag/ZnO/CCG samples was studied by the photodegradation of methylene blue under UV light as a model reaction, and the results were compared with those of ZnO NR, ZnO/CCG, and Ag/ZnO samples, shown in Figure 4. The baseline in Figure 4 represents the MB solution

Ag → Ag+ + e− e− + O2 → •O2−

ZnO + hν → eind− + h ind+ eind− + Ag+ → Ag

eind− + CCG → CCG−, CCG− + O2 → CCG + O2− h ind+ + OH− → •OH

After the Ag/ZnO/CCG sample is immersed in the solution with MB, the surface electrons on Ag eventually transfer to the MB in the dark. Therefore, Ag particles are in the state of Ag+, initially. Under the UV irradiation, electrons in the valence band of ZnO are excited to the conduction band and finally transferred to Ag+. An interesting result from the photocatalytic activity measurements of the samples used in this study was that the degree of photodegradation with Ag/ZnO/CCG was 22% higher than that with ZnO, which is significantly higher than the sum of the photodegradations with Ag/ZnO (5%) and ZnO/CCG (12%). This additional photodegradation suggests that another mechanism for enhancement of photocatalytic activity is operating. Mechanisms related to electron transfer in the photodegradation process are shown in Figure 5. For the Ag/ZnO/CCG heterostructured sample, there are two electron

Figure 4. Photodegradation of methylene blue by ZnO, ZnO/CCG, Ag/ZnO, and Ag/ZnO/CCG samples under UV light irradiation.

without photocatalyst, and the small change in the MB concentration with UV-light irradiation indicates that the MB solution did not self-decompose. For MB solutions in the presence of the ZnO NR, ZnO/CCG, Ag/ZnO, and Ag/ZnO/ CCG photocatalysts, MB decomposed by about 70, 75, 82, and 92% after 4 h of UV irradiation, respectively. ZnO NRs with Ag and CCG exhibited higher photocatalytic activity than that of pure ZnO NRs. This result demonstrates that CCG and Ag enhance the photocatalytic activity of the ZnO NRs. In the case of CCG, adsorption of dyes is increased by π−π stacking interactions with the aromatic regions of CCG. Hence, CCG showed better adsorption of MB than ZnO mainly due to its large π-conjugated system and two-dimensional planar structure, which led to faster dye photodegradation,23 that is, an enhancement of photocatalytic activity with the ZnO/CCG. Charge separation and transport is another important role of CCG. For the ZnO/CCG material, electron−hole pairs are generated and photoinduced electrons are excited to the conduction band of the ZnO NRs with UV irradiation. This transfer mechanism can be explained by the energy band positions of ZnO and CCG. We measured the work function of CCG by ultraviolet photoelectron spectroscopy (UPS) to be about −4.5 eV, which is below the conduction band of ZnO (−4.1 eV), and the excited electrons in ZnO transfer from the conduction band of ZnO to CCG. This process separates electrons and suppresses hole and electron pair recombination, another mechanism for enhancement of photocatalytic activity. Related to the effect of Ag on the photocatalytic activity of the Ag/ZnO heterostructure, Yang et al. reported that an increase in photocatalytic activity is caused by changes of

Figure 5. Electron transfer mechanism of photoinduced electrons for the Ag/ZnO/CCG sample. 7183

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ACKNOWLEDGMENTS This work was supported by the Priority Research Centers Program (2009-0093818) and Basic Science Research Program (2011-0003237) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST).

transfer routes: (i) the transfer of electrons to Ag from ZnO and (ii) the transfer of electrons to CCG. Electrons transferred to CCG have a capacity to transfer again from CCG to Ag, resulting in the observed extra photodegradation effect. The extra photodegradation of the Ag/ZnO/CCG sample is caused by the difference of the work function between Ag and CCG. In the Ag/ZnO/CCG sample, some of the Ag particles are attached to ZnO and others are attached to CCG. In general, the work function of a material will have different values in different crystal directions, as does Ag. A common value of the work function for a Ag polycrystal is about −4.26 eV. As seen in the XRD results (Figure 2), however, Ag particles prepared for this study were located mainly in the (111) direction, and the work function of Ag at this surface is about −4.74 eV, which is lower than that of CCG (∼−4.5 eV).28 Hence, photoinduced electron transfer to CCG further transfers to Ag from CCG, as shown in Figure 5. This transfer mechanism is expected to significantly suppress recombination of photoinduced electrons and holes, thus resulting in extra photodegradation by the Ag/ ZnO/CCG sample.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4, which include SEM, AFM, XPS, UV−vis absorbance, and PL data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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D. CONCLUSIONS A multidimensional heterostructure composed of a 0-dimensional Ag nanoparticle, 1-dimensional ZnO NR, and 2dimensional CCG was prepared in the form of a Ag/ZnO/ CCG composite, and its structural and photocatalytic properties were studied. From the XRD results, ZnO on CCG showed growth with higher orientation than its growth without CCG. Ag particles were synthesized mainly in the (111) direction, and they provided enhancement of photodegradation for the Ag/ ZnO/CCG sample. The enhancement of photocatalytic activity by Ag/ZnO/CCG is due to both the Ag and the CCG components, principally through the trapping of photoinduced electrons, but also via adsorption with the large π-conjugated system of CCG and modification of surface properties by Ag. Extra photodegradation by the Ag/ZnO/CCG sample was significantly higher than the sum of the photodegradations by Ag/ZnO and ZnO/CCG. Electrons transferred to CCG undergo a second transfer to Ag attached to CCG because the work function of Ag in the (111) crystal surface is lower than that of CCG. This factor strongly suppresses recombination of photoinduced electrons and holes, thus providing extra photodegradation capability. Our results suggests that the incorporation of Ag and CCG into semiconductors can lead to greatly enhanced photocatalytic activity by the double transfer of electrons from the semiconductor to CCG and from CCG to Ag in addition to basic electron transfers by Ag and CCG.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 52 259 2330. Fax: +82 52 259 1693. E-mail: shhur@ ulsan.ac.kr (S.H.Hur), [email protected] (S.H.Hahn). Notes

The authors declare no competing financial interest. 7184

dx.doi.org/10.1021/jp210216w | J. Phys. Chem. C 2012, 116, 7180−7184