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Confined migration of hot electrons is presented in nanorods of layered Ag/graphene/TiO2 structure for highly efficient plasmonic photocatalytic water treatment. The light-illuminating titanium dioxide (TiO2) nanorods provide a large amount of high-energy hot electrons for the generation of highly-active superoxide radical (*O2−) that leads to the degradation of organics in water. Comparison between photocatalytic processing efficiency by photocatalysts with various composite materials were presented based on the preferred propagation path of induced hot electrons that leads to generation of *O2−. The best results done by Ag/graphene/TiO2 nanorods showed that the sandwiched layer of graphene on TiO2 nanorods collects the induced hot electrons and results in high efficiency photocatalytic reaction.
© 2016 Optical Society of America
1. Introduction
Photogenerated hot electrons and hot holes in TiO2 provide excellent potential [1] for reduction of toxic organics, anti-bacterial action [2], organic dye degradation [3–5], NOx reduction [6], CO2 reduction [7], and dissociation of water [8]. However, the fast recombination rate of photogenerated hot electrons and hot holes limits the processing efficiency of TiO2. The narrow absorption band of TiO2 also limits the use of solar light as a main energy source. The high affinity of active sites on a TiO2 surface leads to problem that greatly reduces the processing efficiency. The surface of TiO2 nanoparticles tend to become occupied by products of photocatalytic reactions and resist the later coming target reactants. If the hot electrons generated on TiO2 nanoparticles can be transferred to another adjacent material and photocatalytic reaction can be continued without interruption, this problem might be directly resolved.
Recently, a nanocomposite of Ag, Au, graphene, and TiO2 [12–15] provided a simple solution for solving the problems of electron-hole recombination, narrow light absorption bands, and the high affinity to products of photocatalytic reactions to TiO2. The graphene nanosheets adhered to TiO2 nanoparticles retrieve the induced hot electrons for photocatalytic reactions [12, 14, 26] and prevent recombination with the photogenerated holes. The deposited Ag and Au nanoparticles also increase the light absorbing range through plasmonic effects. The metal/graphene/TiO2 nanocomposites provide high processing efficiency for hydrogen production [12, 13] and provide decomposition of methylene blue [14] and Bisphenol A [15]. The plasmonic effects of metallic nanoparticles [11, 16–20] provide a highly sensitive optical response for enhancing the photocatalytic reaction [4, 5, 7, 9–11, 21], and energy transformation [22, 23]. The integration of adhered graphene and the plasmonic effects from deposited metal nanoparticles gives the TiO2 high photocatalytic processing efficiency with great use of harvested energy. The referenced works [12–15] are based on TiO2 and metal nanoparticles with a size of around several nanometers, wrapped in graphene nanosheets. The materials were joined with multiple metal/TiO2, TiO2/graphene, and metal/graphene interfaces with complex and ambiguous migration path of photogenerated hot electrons. The nanocomposites that were used had a small size of around several nanometers and a small light absorbing cross-section for generating hot electrons to further enhance the processing efficiency.
In this paper, a composite fabrication process for surface decoration provides micrometer sized Ag/graphene/TiO2 (denoted as Ag-rGO-TiO2 for simplicity) nanorods for a highly efficient plasmonic photocatalytic reaction. Nanorods of TiO2 have a large light absorption cross-section that provides a large amount of photogenerated hot electrons and hot holes. The graphene and Ag nanoparticles were deposited one-by-one. Degradation of MB with various samples of TiO2 nanorods show high photocatalytic processing efficiency and revealed the confined migration path of photogenerated hot electrons.
2. Sample fabrication
A hydrothermal method [24] was used for fabricating TiO2 nanorods of ~0.1 to several μm in length without calcination at temperatures higher than 200 °C. The environmentally friendly hydrothermal method can fabricate a large quantity of crystal powder at a low cost. Exactly 1 g of TiO2 powder (P90, UniRegion Bio-Tech, Germany) was homogeneously suspended in 50 mL of 10 M sodium hydroxide (NaOH, Avantor, Sweden) after treatment with an ultrasonic cleaner. A sealed hydrothermal reaction tank with a Teflon baker inside and outer stainless steel protection shield containing the mixed solution was then baked in an oven at 200 °C for 24 h. After cooling to room temperature without stirring, a lump of sodium titanate (Na2Ti3O7) nanorods accumulated at the bottom of the reaction tank which were washed by a 0.1 M hydrochloric acid (HCl, Sharlau, Spain) solution in a clean baker in an ultrasonic cleaner. The Na2Ti3O7 wires fully transformed to TiO2 nanorods and were dissolved in a NaCl solution. The upper portion of the NaCl solution in centrifugal tube was removed after centrifuging and carefully replaced with DI water two times. The upper water in the centrifugal tube was again carefully replaced with ethanol after the high speed centrifugal process. The acquired TiO2 nanorods were transfered to a glass disk and baked to remove residual ethanol.
Next, a graphene layer was deposited on the TiO2 nanorods using a hydrothermal method [25]. Reduced graphene oxide (rGO) [15] provides graphene nanosheets of high thermal conductivity, great charge carrier mobility, and good surface contact with TiO2 and other nanoparticles. A mixed solution of 10 mL of graphene oxide (GO), 30 mL of DI water and 10 mL of ethanol were mixed in an ultrasonic cleaner for 1 h. To the mixed solution, 3 g of powder TiO2 nanorods or the purchased TiO2 nanoparticles were added, and the solution was treated in an ultrasonic cleaner for 2.5 h. After an additional 2.5 h treatment of magnetic stirring, the solution became homogeneously gray in color. The acquired solution was poured into the hydrothermal reaction tank, baked in an oven for 3 h at 150 °C, and cooled to room temperature without stirring. The solution turned dark gray in color. Finally, the composite material of TiO2 with reduced graphene oxide (rGO-TiO2) was obtained by washing with water using the centrifugal cleaning process.
Next, Ag nanoparticles were deposited on the rGO-TiO2 nanorods and TiO2 nanorods using the silver mirror method. A solution 5 mL of 15 mM silver nitrate (Sigma-Aldrich, USA) in beaker was treated in an ultrasonic cleaner for 5 min, then constantly disturbed using a magnetic stirrer. A 6 mL 2.5 M sodium hydroxide solution was added and a precipitate appeared at the bottom of the beaker. A 2 M ammonium hydroxide (NH4OH, J. T. Baker, USA) solution was added carefully, drop-by-drop, into the solution until the precipitate disappeared. The magnetic stirrer was removed and the sample substrates with various photocatalysts were immersed in the solution. Next, 5 drops of a 10% glucose solution was injected into the mixture to trigger the silver mirror reaction. After 10 min, the sample substrate was cleaned with DI water in an ultrasonic cleaner and was heated to dryness.
Various synthesized samples of photocatalysts were inspected by a scanning electron microscope (Hitachi, Japan). In Fig. 1(a), the TiO2 nanorods acquired from the hydrothermal method are several μm in length and 100 to 300 nm in diameter. The synthesized TiO2 nanorods show sharp-edged outer surface and a long preferred growing axis. The Ag nanoparticles grew homogeneously and are fixed on the surface of pure TiO2 nanorods, see Fig. 1(b), after the silver mirror reaction with a 15 mM AgNO3 solution. The size of the Ag nanoparticles is about 50 to 100 nm. The Ag nanoparticles also grew homogeneously and are fixed to the surface of rGO-TiO2 nanorods, see Fig. 1(c). The graphene covered the whole surface of TiO2 nanorods and smoothened the surface of TiO2 NPs. The Ag nanoparticles grew homogeneously and are fixed on the surface of rGO-TiO2 nanorods.
Fig. 1 (a) Pure TiO2 nanorods, and (b) Ag nanoparticles deposited on TiO2 nanorods. (c) Ag nanoparticles homogeneously deposited on rGO-TiO2 nanorods.
The synthesized photocatalysts were also characterized by X-ray diffraction (XRD, Panalytical X’Pert Pro MPD) measurements and compared to the Joint Committee on Powder Diffraction Standards (JCPDS) reference card. XRD measurements in Fig. 2(a) display the crystal information of various synthesized photocatalysts. The 25° signal reflects the typical (101) surface of TiO2 in the various synthesized photocatalysts in this paper. The signal peak from GO around 10°, a (001) surface [27], didn’t appear in the data for Ag-rGO-TiO2 and rGO-TiO2. Therefore, the layered structure of GO along a (001) surface is believed to be reconstructed as loose nano-sheets covering the entire surface of TiO2 nanoparticles. The 38° signal peak shows a typical (111) crystal surface, according to JCPDS card No. 65-2871, for the successfully deposited Ag nanoparticles.
Fig. 2 (a) XRD of Ag/TiO2, rGO/TiO2, and Ag/rGO/TiO2. (b) FTIR measurements of synthesized Ag-rGO-TiO2 nanorods and GO. (c) XPS measurements of Ti 2p scan
The FTIR (Bruker V-70 spectrophotometer) spectral measurements (Fig. 2(c)) show the C and O bonds and structural interpretation of the synthesized Ag-rGO-TiO2 photocatalyst. The valley in the transmittance spectra around 1635 cm−1 is assigned to the C-C oscillation [21, 27]. The C = O bond and C-O bond valleys at around 1750 and 1050 nm in the GO spectrum appeared in the data for Ag-rGO-TiO2. The oxygen atoms on the GO were removed during the chemical treatment with ethanol as reductant. The 2D structured graphene is believed to have higher conductivity for trapping the UV light to induce holes inside the TiO2 core of the synthesized photocatalyst, but prevent recombination with induced electrons [12].
XPS measurements (Thermo Scientific Theta Probe, UK) were used to characterize the electron bands and surface elements of the synthesized photocatalysts. Clear peaks at 458.9 eV and 464.5 eV, see Fig. 2(d), are assigned to the Ti(2p3/2) and Ti(2p1/2), respectively, of Ti4+. The measurements did not reveal peaks at 465.8 eV and 460.2 eV [29] attributable to Ti-C bonding. Therefore, the processing temperature used in this work did not result in a bond between the TiO2 and carbon in the graphene. Our fabrication process did not use temperatures higher than 200 °C; therefore, no Ti-C bonds are clearly observed in the XPS measurements. The Ti-C bond requires a 400 °C fabrication temperature. The energy of hot electrons should not transfer to Ti-C bonds, but directly result in generation of *O2− radicals in the surrounding water.
3. Photocatalytic reaction
The degradation of methyl blue (MB) (C37H27N3Na2O9S3) presented the plasmonic photocatalytic processing efficiency of the various synthesized materials. The 360 W xenon lamp (Blue Sky Tech Co., Ltd, Taiwan) illuminated the slurry bed reactor containing 30 mL of 0.01 mM MB and 3.11 g of the synthesized photocatalysis, with magnetic stirring. The absorbance of the 600 nm light by the solution linearly depends on concentration of MB that is stable under UV light illumination. The ratio (C/C0) is the reserved concentration of MB (C) compared to the original concentration (C0) of MB. The photocatalytic degradation of MB can be presented with the variation of the ratio C/C0.
In the 10 min experiments with low-concentrated 8 ppm MB, the Ag-rGO-TiO2 nanorods showed the greatest processing efficiency from the entire sample; see Fig. 3(a). The Ag nanoparticles deposited on the TiO2 nanorods provide a higher processing efficiency of photocatalytic reaction than the pure TiO2 nanorods. The generated hot electrons, *e¯, in TiO2 nanoparticles under UV light illumination lead to the generation of active *O2− from O2 in water for degradation of organics [10]. The generated hot electrons are also believed to propagate from the metal nanoparticles to the graphene layer fixed on TiO2 nanoparticles. The *OH generated from H2O by reserved h+ also degrade the organics in water, see Fig. 4. The deposited graphene layer has lower energy level than that of the Fermi level of Ag and the excited states energy of TiO2 and makes the graphene layer presented higher affinity to photogenerated hot electrons in the Ag-rGO-TiO2 nano-composite [12].
Fig. 3 (a) The variation of the ratio (C/C0) of MB concentration (8 ppm originally) for various synthesized photocatalysts. The ratio of the reserved concentration of MB (C) compared to original concentration (C0) is greatly reduced in 10 min processes. (b) The ratio (C/C0) of the reserved concentration of MB (16 ppm originally) compared to original concentration is greatly reduced in 50 min processes.
Fig. 4 Schematic figures of the (hot) electrons, i.e. *e¯, in the transmission cycle. The right-upper and right -lower is the original MB solution containing 3.11 g of Ag-rGO-TiO2 nanorods and the same solution after 10 min of illumination, respectively.
The synthesized rGO-TiO2 nanorods showed a higher efficiency than the pure TiO2 nanorods. The covered layer of rGO can collect the generated hot electrons [12, 14, 26] and leads to the efficient degradation of MB. The Ag-graphene-TiO2 nanorods/nanoparticles provide a higher processing efficiency than that of Ag-TiO2. The subsequently deposited second layer of rGO on TiO2 nanorods attracts the induced hot electrons to generate active *O2−
In the experiments with high-concentrated 16 ppm MB, desorption of the products formed after the photocatalytic reaction resulted in declined processing efficiency, as shown in Fig. 4(b). The Ag-rGO-TiO2 nanorods still presented the greatest processing rate in the 50 min experiments. The experiments with 16 ppm dissolved MB are ~5-7 times slower than that with 8 ppm MB for the various synthesized photocatalysts. The results also presented that the additional graphene layer and deposited Ag NPs have a smaller affinity for the products than the TiO2 nanorods, resulting in higher processing efficiency for the photocatalytic reaction.
For some specific applications, such as semiconductors or metal oxides, the confined migration of photogenerated hot electrons can enhance light harvesting and energy conversion efficiency, e.g. solar cells. The results presented in this paper provide an interesting method for achieving these properties. The understanding of confined migration of hot electrons will lead to potential developments in solar energy related technologies and applications.
5. Conclusion
In conclusion, Ag-rGO-TiO2 nanorods were synthesized to supply a larger volume of TiO2 material and more induced hot electrons for photocatalytic reactions under light illumination. The sandwiched layer of rGO confined the migration of photogenerated hot *e¯ from the TiO2 nanorods and the deposited Ag nanoparticles to the graphene layer, which further enhanced the photocatalytic degradation process. The additional graphene layer and deposited Ag NPs have a smaller affinity for the products than the TiO2 nanorods that also enhance the photocatalytic water treatment.
Acknowledgments
The authors are grateful to the Ministry of Science and Technology of Taiwan, Republic of China for supporting this research with contract number MOST 103-2112-M-019-003-MY3.
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