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Heterojunction Ag-TiO [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Silver-titanium dioxide (Ag-TiO2) heterojunction nanopillars were fabricated by a facile one-pot method coupled with further calcinations. The results showed that Ag nanoparticles were well dispersed on the surface of TiO2 nanopillars. The heterojunction composite nanopillars were uniform, approximately 10-20 µm in length and 500-800 nm in diameter. The nanojunctions and surface plasmon resonance effect of Ag-TiO2 heterojunction nanopillars were confirmed by scanning electron microscopy, transmission electron microscopy, and UV/Vis spectroscopy. The heterojunction Ag-TiO2 nanopillars showed good photocatalytic H2 production activities under visible-light irradiation without costly platinum, owing to the good dispersion and closely contacted junctions, as well as the surface plasmon resonance effect and the porous one-dimensional structure. Keywords : heterojunctions · hydrogen · nanostructures · photocatalysis · surface plasmon resonance Introduction A promising technique of hydrogen production from solar energy by use of semiconductor photocatalysts has attracted much attention since its discovery in 1972.[1] To date, a great number of semiconductor photocatalysts, including sulfides, nitrides, metal/metallic oxides, and other composites, have been investigated.[2-6] However, further commercial development, as well as large-scale manufacture of photocatalysts, is still restricted by their low efficiency. Based on recent research, the transportation and recombination of photogenerated electron-hole pairs and the difficult utilization of visible light of solar origin are the main reasons for the low efficiency of present photocatalysts.[7-10] Therefore, enhancing charge separation, accelerating the transportation of electrons and preventing their recombination before they move to the surface active sites of the photocatalysts, and improving the light utilization play key roles in determining the solar energy to H2 conversion efficiency. One of the effective strategies for charge transportation and recombination is to load lower work function materials (metals or other compounds) on the surface of semiconductors as co-catalyst, so that junctions can be formed after loading.[11] Previous work demonstrated that the interface/junctions between the lightharvesting semiconductor and co-catalyst should be very important for improving the activity. Charge transfer and separation could be improved owing to the heterogeneous junction interface.[12,13] Metal-semiconductor heterojunction materials have attracted increasing attention as a result of their unique performance in photocatalysis.[14] Their enhancement in visible-light-driven photocatalytic performance is usually ascribed to the surface plasmon resonance properties of metal nanoparticles.[15] In particular, Ag nanoparticles have been widely investigated as an enhanced species in designing plasmonic metal-semiconductor photocatalysts, such as Ag-TiO2 composites, because TiO2 is a suitable material for designing high-performance visiblelight-driven photocatalysts coupled with Ag nanoparticles.[16] Herein, we report Ag-TiO2 heterojunction nanopillars synthesized without any surfactant by a one-pot method. Ethylene glycol (EG) plays a crucial role in the preparation, not only as solvent but also as structure-inducing agent for nanopillars. Moreover, EG favors the dispersion of Ag as well, according to previous reports.[17, 18] Although EG is used to prepare one-dimensional Ag-TiO2 nanostructures, the large quantities of surfactant (such as polyvinylpyrrolidone) are hard to remove and affect the photocatalytic performance.[19, 20] Under visible-light excitation, the "hot" photogeneration electron goes from Ag and then rapidly transports to TiO2 through the well-contacted junctions. Therefore, Ag-TiO2 heterojunction nanopillars synthesized without surfactant should have excellent photocatalytic activities. The plasma resonance of Ag could utilize more visible light, the junction could improve the charge transfer and separation, and the one-dimensional pillar structure could be propitious to charge transportation. Results and Discussion Firstly, a pillarlike metal glycolate complex was prepared by stirring a solution of silver nitrate, tetrabutyl titanate, and EG at 180 8C for 1 hour. EG has been widely used in the synthesis of metal nanoparticles because of its strong reducing power and relatively high boiling point (^ 197 8C). It also promoted oriented crystal growth, which supplied a promising way to design materials with different morphologies. Different Ag nanostructures, especially nanowire, were synthesized by using EG as well as metal oxides with hierarchical structures, such as TiO2 nanopillars and titanates.[21-23] Here, EG plays a crucial role owing to the multifunctional effect of reducing and dispersing Ag nanoparticles as well as coordination and polymerization of titanium. The typical diffraction peak at around 108 observed from the XRD pattern in Figure S1 (see the Supporting Information) gives evidence of a glycolate precursor, which was discussed in a previous report.[24, 25] The typical (111) plane diffraction peak of Ag at around 388 is observed clearly and becomes stronger with increasing Ag content.[26] The FTIR spectrum in Figure S2 gives further evidence of the glycolate precursor, with the absorption peaks at approximately 1650 cm^1 corresponding to C=O bands and in the range of 1030-1090 cm^1 corresponding to 1(CH2), u(C^O), and u(C^C) bands. These organic groups reveal that the precursor is a glycolate complex.[27] The thermogravimetry curve in Figure S3 shows no apparent peak and a significant weight loss is observed at temperatures over 500 8C, which indicates that the minimum crystalline temperature to obtain stable TiO2 is about 500 8C. The Ag-TiO2 heterojunction nanopillars were prepared by annealing the precursor in air. XRD patterns of the Ag-TiO2 heterojunction nanopillars calcined at 500 8C for 2 hours are shown in Figure 1. Five main Bragg diffraction peaks at 2q = 25.2-62.718 were indexed as (101), (004), (200), (105), and (211), respectively, with a pattern typical of anatase (JCPDS, No. 21-1272). In addition, the intensity of the peak at approximately 388 (the typical peak of Ag) becomes stronger in accordance with the increasing amount of Ag, which suggests the formation of Ag-TiO2 composites. The redshift in the Raman spectra in Figure S4 from 147.7 cm^1 for pure TiO2 nanopillars to 152.7 cm^1 for Ag-TiO2 nanopillars also indicates the combination of Ag and TiO2. The scanning electron microscopy (SEM) images in Figure 2 show that the Ag nanoparticles are dispersed uniformly on the surface of TiO2 nanopillars, especially in sample AT-20, for which the length is approximately 10-20 mm and the diameter is 500-800 nm. If the molar ratio of Ag is low, there are few Ag nanoparticles on the surface of the nanopillars. Otherwise, the high molar ratios of Ag can lead to aggregation of Ag particles as shown in Figure 2 F. Thus, the optimal molar ratio of TiO2 and Ag is 20 in this work. The morphology of the Ag-TiO2 heterojunction nanopillar was further studied by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), as shown in Figure 3. We find that the Ag nanoparticles are well dispersed on the surface and even in the bulk of the TiO2 nanopillar ; meanwhile, the TiO2 nanopillar is constructed from small nanoparticles with a diameter of about 30-50 nm. The diameter of the Ag nanoparticles is about 5-10 nm. The HRTEM image in Figure 3 C clearly shows lattice fringes corresponding to the (101) (d101 = 0.35 nm) crystallographic planes of anatase TiO2 and (111) (d111=0.236 nm) crystallographic planes of Ag. The HRTEM image of the heterojunction Ag-TiO2 nanopillars reveals that the material is highly crystalline, as evidenced by well-defined lattice fringes. There are also some pores around the pillars owing to the pyrolysis Ag-Ti-EG framework. The N2 adsorption-desorption isotherms in Figure S5 reveal the existence of pores and also that there is little decline of surface area on increasing the content of Ag, further indicating the efficient loading of Ag nanoparticles. The elemental composition and the chemical environment of different elements in Ag-TiO2 nanopillars were studied by Xray photoelectron spectroscopy (XPS). The results indicated that all samples contained Ag, Ti, O, and C elements. The unexpected peak for C 1s is attributed to the residual carbon from the sample, and also to the adventitious hydrocarbon from the XPS instrument itself. The high-resolution XPS spectra of Ag 3d and Ti2p core levels in the composite material are shown in Figure 4 B and C, respectively. The peaks observed at 367.7 and 373.7 eV can be ascribed to Ag 3d3/2 and Ag3d5/2 of the metallic silver, respectively. Relative to the standard Tip peak, the Ti 2p3/2 peak (458.5 eV) shifted to a lower binding energy for the Ag-TiO2 nanopillar composites (458.1 eV), thus indicating the presence of Ti3 + produced from pyrolysis of the titanium glycolate precursor. The 6.0 eV difference between the binding energy of peaks is also characteristic of metallic Ag 3d states.[28] It is clearly observed that metallic silver is present on the surface of TiO2 in Ag-TiO2 composites. The light absorption by the photocatalysts as well as migration of the light-induced electrons and holes are key factors to control a photocatalytic reaction, which is relevant to the electronic structure characteristics of the material. Figure 5 indicates the light absorption properties of Ag-TiO2 nanopillars with different contents of Ag, from ultraviolet to visible light (200-800 nm). It is shown that the absorption spectra of Ag- TiO2 nanopillars are photoresponsive in both the ultraviolet and visible-light ranges. In the ultraviolet range, a broad absorption edge situated at about 385 nm indicates the optical band gap attributed to the O2^ !Ti4+ charge-transfer interaction. In the visible range, wide absorption bands from 450 to 700 nm are observed, owing to the surface plasmon resonance effect of Ag.[29a] Compared with Ag nanoparticles (Figure S6), the redshift of Ag-TiO2 nanopillars may be caused by the exciton-plasmon coupling and high dielectric constant of the semiconductor.[29b] The H2 production efficiency of Ag-TiO2 heterojunction nanopillars under visible-light irradiation (780>l >400 nm) is shown in Figure 6 A. The Ag-TiO2 heterojunction nanopillars exhibit high efficiency of H2 evolution without platinum. Sample AT-20 displays the highest efficiency under visible-light irradiation, which is up to 72.1 mmolh^1, higher than that of AT-10 (38.2 mmolh^1) and AT-40 (44.2 mmolh^1). Importantly, all three photocatalysts exhibit enhanced photocatalytic performance relative to that of 1 wt % Pt-loaded TiO2 nanoparticles of type P25 (5.4 mmolh^1), which is consistent with the reported result.[30] Sample Ag-P25 with the same Ag and Ti molar ratio as AT-20 was investigated and shows an H2 production activity of 36.8 mmolh^1, lower than that of AT-20. Here, the surface plasmon resonance is believed to contribute to the dominance of the visible-light response. The apparent quantum efficiency of the AT-20 is 0.29 % at 420 nm and 0.25 % at 520 nm. The time course of H2 generation in Figure 6 B shows the samples are stable that produce H2 during the time evolution. These results illustrate the clear promotion of heterojunction Ag-TiO2 nanopillars for the following reasons : 1) the junctions between Ag and TiO2 are propitious to charge separation, which promotes the photocatalytic activity ; 2) the surface plasmon resonance effect of Ag can utilize much visible light ; and 3) the one-dimensional structure favors charge transport. This is demonstrated by electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis. The Nyquist plot of EIS in Figure 6 C displays a semicircle at high frequency, which represents the charge-transfer process, and the diameter of the semicircle reflects the charge-transfer resistance. It is clearly observed that the diameters of the semicircle for heterojunction Ag-TiO2 nanopillars both in the dark and light are much smaller than those of Ag-P25 nanoparticles, thus implying that the heterogeneous junctions as well as the one-dimensional structure can promote the electron mobility by reducing the recombination of electron-hole pairs that favors charge transportation and separation.[31] This result is supported by Mott- Schottky analysis, as shown in Figure 6 D. Both the heterojunction Ag-TiO2 nanopillars and Ag-P25 nanoparticles reveal a positive slope in the Mott-Schottky plots, as expected for the n-type semiconductor.[32] In addition, the carrier density Nd can be calculated from the slope of the Mott-Schottky plot by using Equation (1):[33] Nd ¼ ð2=e0ee0Þ½dð1=C2Þ=dV^^1 ð1Þ in which e0 is the electron charge, e is the dielectric constant, e0 is the permittivity of a vacuum, and V is the applied bias at the electrode. Importantly, the heterojunction Ag-TiO2 nanopillars show smaller slopes of the Mott-Schottky plot compared with Ag-P25 nanoparticles, which suggests a faster charge transfer, thus enhancing the photocatalytic H2 production efficiency. Conclusion Silver-titanium dioxide (Ag-TiO2) heterojunction nanopillars have been synthesized by a facile one-pot ethylene glycol (EG)-mediated route. The EG acted not only as ligand and pore-making agent of the nanopillars, but also as reducing agent for reduction and dispersion of Ag nanoparticles. Junctions between Ag and TiO2 were found in the composite nanopillar, which promoted the charge separation and favored photocatalytic performance. In addition, the surface plasmon resonance of Ag improved the light-harvesting capability. The composite nanopillars exhibited great photocatalytic H2 production efficiency owing to the well-contacted junctions and surface plasmon resonance. These heterojunction nanopillar composites could play a significant role in industrial applications and mechanistic studies of photocatalytic H2 production as well as water splitting. Experimental Section Synthesis A typical synthesis of heterojunction Ag-TiO2 nanopillars with different Ag and TiO2 ratios was performed as follows. A clear solution (60 mL) containing silver nitrate, tetrabutyl titanate, and EG was loaded in a round-bottomed flask and then heated in an oil bath. Three different molar ratios of Ti and Ag (10 :1, 20 :1, and 40 :1) were prepared. After about 30 min, a brown precipitate was clearly observed. Precipitation continued on heating to 180 8C for 1 h. Magnetic stirring was applied throughout the process. After cooling to room temperature, the resultant precipitate was washed several times with ethanol, then the precursor was obtained after drying at 60 8C for 4 h in a vacuum. The three precursors were directlyannealedat 5008C(ramp of18Cmin^1) for2hin air,anddenoted as AT-10, AT-20, and AT-40, respectively. Pure TiO2 nanopillars were prepared under the same conditions except for the silver nitrate. Ag-P25 with a Ti/Ag molar ratio of 20 :1 was prepared by using a photoreducing method in EG/ethanol solution according to a previously reported method.[34] Characterization Powder X-ray diffraction (XRD) patterns were obtained by a Bruker D8 instrument. Scanning electron microscopy (SEM) images were taken using a Hitachi S-4800 instrument operating at 15 kV. The transmission electron microscopy (TEM) experiment was performed on a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holders. Raman measurements were performed with a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. The laser beam was focused with a 50 ^ objective lens to an approximately 1 mm spot on the surface of the sample. Pyrolysis experiments on nanopillar precursors were performed by thermogravimetry (TA, Q600) under a stream of air at a heating rate of 10 8Cmin^1. The specific surface area was determined according to the Brunauer-Emmett-Teller (BET) method by using a Tristar II 3020 surface area and porosity analyzer (Micromeritics). UV-visible absorption spectra were recorded with a UV/Vis spectrophotometer (Shimadzu UV-2550). X-ray photoelectron spectroscopy (XPS) was measured using a Kratos-AXIS ULTRA DLD apparatus with an Al(Mono) X-ray source, and the binding energies were calibrated with respect to the signal for adventitious carbon (binding energy =284.6 eV). Photocatalysis Photocatalytic H2 production experiments were conducted in an online photocatalytic H2 production system (AuLight, Beijing, CELSPH2N) at ambient temperature (20 8C). The catalyst (0.1 g) was suspended in a mixture of distilled water (80 mL) and methanol (20 mL) in the reaction cell by using a magnetic stirrer. Prior to the reaction, the mixture was deaerated by evacuation to remove O2 and CO2 dissolved in the water. The reaction was performed by irradiating the mixture with visible light from a 300 W Xe lamp (AuLight, CEL-HXF300) with a UVCUT filter (AuLight, 400-780 nm). Gas evolution was observed only under photoirradiation, and was analyzed by an online gas chromatograph (SP7800, TCD, molecular sieve 5 ^, N2 carrier, Beijing Keruida Limited). The determination of the apparent quantum efficiency for hydrogen generation was performed by using the same closed circulating system under illumination of a 300 W Xe lamp with bandpass filter (420 and 520 nm) system. The light intensity was measured using a Si photodiode (oreal 91105V). The total light intensities were 4.32 mW s^1 (420 nm) and 1.87 mW s^1 (520 nm). The irradiation area was around 7 cm2. The apparent quantum efficiency (AQE) at different wavelengths was calculated by Equation (2): 2^thenumberof evolvedH2molecules the number of incident photons Photoelectrochemical tests Photocurrent measurements were performed using a three-electrode configuration, with the heterojunction Ag-TiO2 nanopillars and Ag-P25 nanoparticle films as the working electrode, saturated Ag/AgCl as the reference electrode, and platinum foil (3 ^2 cm) as the counter electrode. The working electrode films were prepared by the doctor-blade method, using a thin glass rod to roll a paste on fluorine-doped tin oxide to form a film (2^ 1 cm). The paste was prepared by stirring photocatalyst powders (0.2 g) in ethanol (0.5 mL) for at least 24 h. The films were annealed at 400 8C (ramp of 1 8C min^1) for 1 h to make them firm enough. Electrochemical impedance spectroscopy (EIS) measurements were performed in the dark and under visible-light illumination (l > 400 nm) in 0.5m Na2SO4 solution at open-circuit voltage over a frequency range from 105 to 0.05 Hz with an AC voltage at 5 mV. The Mott-Schottky plots were obtained at a fixed frequency of 1 kHz in the dark. Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (21031001), the National Natural Science Foundation of China (91122018, 21371053, 21376065), Ministry of Education of China (708029), Program for Innovative Research Team in University (IRT-1237), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20112301110002, 20112301120002), the Program for New Century Excellent Talents in University of Heilongjiang Province (1253-NCET-020), the Natural Science Foundation of Heilongjiang Province (QC2012C046), and the Heilongjiang University Excellent Youth Foundation (JCL201102). [1] A. Fujishima, K. Honda, Nature 1972, 238, 37 - 38. [2] X. Chen, S. Shen, L. Guo, S. S. Mao, Chem. Rev. 2010, 110, 6503 - 6570. [3] A. Kubacka, M. Fern^ndez-Garc^a, G. Col^n, Chem. Rev. 2012, 112, 1555 - 1614. [4] L. Jing, W. Zhou, G. Tian, H. Fu, Chem. Sov. 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Fu State Key Laboratory of Theoretical and Computational Chemistry Institute of Theoretical Chemistry Jilin University, Changchun 130023 (P. R. China) Fax: (+ 86) 451-8666-1259 E-mail : [email protected] [b] Dr. Y. Qu, Dr. W. Zhou, Dr. Z. Ren, Dr. C. Tian, Dr. J. Li, Prof. H. Fu Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education of the People's Republic of China Heilongjiang University, Harbin 150080 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402012. (c) 2014 Blackwell Publishing Ltd. |