Role of surface defects on visible light enabled plasmonic photocatalysis in Au–ZnO nanocatalysts†

Visible light photocatalytic activity of the plasmonic gold–zinc oxide (Au–ZnO) nanorods (NRs) is investigated with respect to the surface defects of the ZnO NRs, controlled by annealing the NRs in ambient at different temperatures. Understanding the role of surface defects on the charge transfer behaviour across a metal–semiconductor junction is vital for efficient visible light active photocatalysis. Au nanoparticles (NPs) are in situ deposited on the surface of the ZnO NRs having different surface defect densities, demonstrating efficient harvesting of visible light due to the surface plasmon absorption. The surface defects in the ZnO NRs are probed by using photoluminescence (PL) spectroscopy, X-ray photoemission spectroscopy (XPS), and photo-electro-chemical current–voltage measurements to study the photo-generated charge transfer efficiency across the Au–ZnO Schottky interface. The results show that the surface situated oxygen vacancy sites in the ZnO NRs significantly reduce the charge transfer efficiency across the Au–ZnO Schottky interfaces lowering the photocatalytic activity of the system. Reduction in the oxygen vacancy sites through annealing the ZnO NRs resulted in the enhancement of visible light enabled photocatalytic activity of the Au–ZnO plasmonic nanocatalyst, adding vital insight towards the design of efficient plasmonic photocatalysts. rsc_RA_c5ra16569e higher surface-to-volume ratios resulting in improved catalytic activity compared to their bulk counterparts. Metal oxides, for example, titanium dioxide (TiO2), zinc oxide (ZnO), etc. are commonly used as photocatalysts, and are typically active under ultra-violet (UV) light due to their wide band gap energies. In recent times, designing photocatalysts that can be activated by visible light is one of the major research areas in order to efficiently and cost-effectively utilize them using the sunlight. Activation of wide bandgap metal oxide photocatalysts under visible light irradiation can be achieved in various ways, among which doping with transition metals,4–6 creating intermediate defects,7,8 sensitizing with visible light active dyes or other semiconductors materials,9–11 narrow bandgap semiconductor coupling12–14 etc. are some of the commonly used methods. Sensitizing the metal oxide photocatalysts with noble metal NPs to harvest visible light utilizing the surface plasmon resonance (SPR) absorption of the metallic NPs, and hence the name plasmonic photocatalysis, is emerging as a new area for designing smart photocatalysts.15,16 Plasmonic photocatalysis typically involves distribution of noble metal NPs, like gold (Au), silver (Ag) etc. on the surface of semiconductor photocatalysts and thus contains metal–semiconductor junctions in the system.17,18 Upon activating with visible light, these smart metal–semiconductor plasmonic photocatalysts show several advantages over traditional photocatalysts. Among these, improved visible light harvesting due to the localized SPR absorption and enhanced photo-generated charge separation across the metal–semiconductor interface are primarily important for efficient visible light photocatalytic degradation of pollutants.16,19 Recently plasmonic Au–ZnO system has been studied extensively for enhancement of solar cell efficiencies as well as in photocatalysis, for improved visible light harvesting and reduced interfacial charge recombination at the Au–ZnO interface due to the formation of the Schottky barrier at the metal–semiconductor interface.20–22 However, the formation of the Schottky barrier at the Au–ZnO interface is highly dependent on defects on the semiconductor surfaces. For instance, it has been found earlier that depending on the concentration of surface defect states in ZnO, the Au–ZnO contact behaves either as ohmic or as a Schottky contact.23,24 Hence understanding the role of surface defects of the semiconductor material on the formation of Schottky barrier at the metal–semiconductor interface and thereby its contribution to the plasmonic photocatalysis by metal–semiconductor photocatalysts is vital. In the current study, we have modulated the surface defect states of ZnO NRs by annealing the NRs in air at different temperatures and the role of the surface defects of ZnO NRs have been explored in defining the plasmonic photocatalytic performance of Au–ZnO NRs under visible light irradiation. The state of the surface defects in ZnO NRs annealed at different temperatures were explored by photoluminescence (PL) spectroscopy and X-ray photoemission spectroscopy (XPS), whereas the effect of surface defect induced band bending on the formation of Schottky junction upon Au NPs deposition was probed by valence band (VB) XPS measurements. The photo-generated charge transfer and charge separation efficiency across the Au–ZnO Schottky junctions were also explored and discussed from the photo-electro-chemical current–voltage (I–V) measurements of the Au–ZnO NRs. Experimental Synthesis of ZnO NRs ZnO NRs were grown on glass substrates (3 × 1 cm2) by using microwave assisted hydrothermal process.8 Initially the glass substrates were cleaned subsequently in soap water, acetone, ethanol and de-ionized (DI) water in an ultrasonic water bath. A ZnO seed layer was then deposited on the cleaned glass substrates by spraying 10 ml of 5 mM aqueous solution of zinc acetate at 350 °C. The seeds serve as nucleation sites and enable ZnO NRs to grow preferentially along the C-axis of the wurtzite structure during the hydrothermal growth.25 The seeded substrates were then immersed in a beaker containing an aqueous solution of 20 mM zinc nitrate and 20 mM hexamethylenetetramine. The growth of the NRs was carried out in a commercial microwave oven with microwave output power of 180 W. The hydrothermal growth process of the ZnO NRs consists of two steps: 40 minutes of growth under microwave irradiation and 20 minutes of cooling in the atmosphere. The entire growth process of the ZnO NRs consists of 5 repeated cycles, where the growth solution was replenished in between each cycle in order to maintain a constant growth rate of the NRs during the hydrothermal process. Finally the substrates were retracted from the chemical bath, rinsed thoroughly with DI water followed by drying in an oven at 85 °C for 1 hour and stored until further use. Deposition of Au NPs on ZnO NRs Au NPs were deposited in situ on the surface of ZnO NRs by photocatalytic reduction of chloroauric acid (HAuCl4·3H2O). In a typical deposition process, ZnO NRs were immersed in a 0.1 mM aqueous solution of HAuCl4·3H2O, followed by irradiation with ultraviolet (UV) light (18 W) for 10 minutes. During the process ZnO NRs absorb the high energy UV light and generate electron–hole (e–h) pairs, which lead to the formation of highly reactive radicals with the subsequent reduction of AuCl4 ions to metallic Au on the surface of the NRs. After UV irradiation, the Au–ZnO NRs coated glass substrates were rinsed with copious amount of DI water and dried in air at 85 °C. Photocatalytic tests Photocatalysis tests were conducted using an aqueous solution of methylene blue (MB) as a test contaminant with a tungsten-halogen lamp (500 W) as the visible light source. A 10 μM solution of MB was prepared in DI water and placed in poly(methyl methacrylate) (PMMA) cuvettes. A glass substrate containing the ZnO or Au–ZnO NRs was then placed inside the cuvette with the catalyst surface facing the light source. In order to avoid UV and infrared radiation (heat) from the light source a glass vessel (10 cm thick) containing water was placed in between the tungsten-halogen lamp and the cuvettes. The distance between the light source and the cuvette was then adjusted so that the light intensity measured by a rsc_RA_c5ra16569e pyranometer (Iso-Tech ISM 410) on the cuvette position is 100 mW cm−2. As a control, a bare glass substrate of similar size was placed in a cuvette containing the MB solution. Optical absorption spectra of the MB solution were then recorded after different light exposure durations in order to monitor the rate of photocatalytic degradation of the test contaminant. Prior to the photocatalytic degradation, the samples were kept in dark for 1 hour in the MB solution to reach adsorption equilibrium. The photocatalytic degradation of MB was estimated from the reduction in absorption intensity of MB at a fixed wavelength λmax = 665 nm and plotted as Ct/Co versus the time of light exposure, where Ct represents the concentration of MB at time t and Co represents the initial concentration of MB. Characterization Surface morphologies of the ZnO and Au–ZnO NRs on glass substrates were characterized by field emission scanning electron microscopy (FESEM; Model: JEOL JSM-7600F) operated at 20 kV. X-ray diffraction (XRD) pattern of the samples were obtained by using a Rigaku MiniFlex600 X-ray diffractometer (Cu Kα radiation, wavelength = 1.54 Å). Optical absorption and photoluminescence (PL) spectra of the samples were recorded by using a Perkin Elmer Lamda 25 UV/Vis spectrometer and Perkin Elmer LS55 fluorescence spectrometer respectively. X-ray photoemission spectroscopy (XPS; Omicron Nanotechnology, Germany) with a monochromatic Al Kα radiation (energy = 1486.6 eV) working at 15 kV was used to study the surface states of the ZnO NRs. The obtained XPS spectra were calibrated with respect to the C 1s feature at 284.6 eV. During the XPS measurements, ZnO samples were flooded with electrons to avoid surface charging during XPS measurements. Photocurrent measurement in Au–ZnO NRs In order to study the charge transfer and charge separation efficiency across the Au–ZnO interface with respect to the surface defects of ZnO NRs, Au–ZnO NRs with different surface defects were synthesized on conducting fluorine doped tin dioxide (FTO) substrates as described above. The Au–ZnO NRs on FTO were then used as photoelectrodes and another FTO substrate coated with platinum (Pt) NPs was used as counter electrode to fabricate solar cells by placing the counter electrode facing the photoelectrode. Pt NPs were deposited on the counter electrode by thermal decomposition of 5 mM platinum chloride (H2PtCl6·H2O) solution prepared in isopropanol at 385 °C for 15 minutes. A single layer of Surlyn 1702 sealant film (50 μm thick) from Dupont, Australia was used as spacer between the two electrodes and the cell was sealed using heat and pressure at the same time. The inter electrode space was then filled with the redox I/I3 liquid electrolyte, which is composed of lithium iodide (LiI, 0.5 M), iodine (I2, 0.05 M) and 4-tert-butylpyridine (TBP, 0.5 M) dissolved in acetonitrile (ACN). The electrolyte was filled by using capillary force through two small holes (φ = 1 mm) drilled on the counter electrode side, which were then sealed by using another piece of Surlyn sealant film. The active area of all the solar cells used in this study was maintained at 0.25 cm2. The current–voltage (I–V) characteristics of the Au–ZnO solar cells were measured at calibrated 1 sun illumination (AM 1.5G) with intensity equalling to 100 mW cm−2. A Keithley 617 programmable electrometer was programmed with Labview software to act as both voltage supplier and current sensing unit to acquire the I–V characteristics. The fill factor (FF) of the Au–ZnO solar cells was calculated from their I–V curves by using the following equation: where, ISC is the current produced by the solar cell under short circuit condition, VOC is the open circuit voltage of the solar cell and Imax and Vmax represent the current and voltage values respectively at the maximum power point (Pmax) which can be delivered by the solar cell to an external load. Results and discussions Photocatalytic degradation of MB using Au–ZnO NRs Morphological characterization by FESEM, as shown in Fig. 1(a), indicates the formation of arrays of ZnO NRs, along with a fairly uniform deposition of Au NPs on the surface of the NRs. The ZnO NRs possess the characteristic hexagonal cross section with diameters ranging from 50 to 100 nm. The inset in Fig. 1(a) shows that the NRs growing perpendicular to the glass substrate that are nearly uniform in length (∼4 μm) with a preferential growth direction along the polar facets in the [0002] direction of the ZnO hexagonal wurtzite crystal. Fig. 1(b) shows a typical TEM micrograph of Au–ZnO NRs showing the particle size distribution of the Au NPs. It can be observed that the shape of the as synthesized Au NPs is almost spherical showing a wide distribution of diameters ranging from 7 to 23, which is expected since no capping agents were used during the synthesis of the Au NPs to control their sizes. rsc_RA_c5ra16569e