Firkala, Tamás

Nagy, Dávidné

Szilágyi, Imre Miklós

Xianfeng, Fan

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                Available online at www.sciencedirect.com   Energy Procedia  00 (2014) 000–000 www.elsevier.com/locate/procedia  The 6th International Conference on Applied Energy – ICAE2014 Study about the morphology effect on the photo-efficiency of WO3  Dávidné Nagy1, Imre Miklós Szilágyi2,3, Tamás Firkala2, Xianfeng Fan1* 1University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh, EH9 3JL,UK; 2Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry, Szent Gellért tér 4., Budapest, H-1111, Hungary; 3Technical Analytical Research Group of the Hungarian Academy of Sciences, Szent Gellért tér 4., Budapest, H-1111, Hungary Abstract The photocatalytic activity of hexagonal (h-) WO3 nanoparticles (NPs) and nanowires (NWs) was investigated and compared with the performance of monoclinic (m-) WO3 nanoparticles in the aqueous photo-bleaching reaction of methyl orange (MO) under UV irradiation. It has been known that the m-WO3 is better photocatalyst than the h-WO3, but due to the advantageous morphology we investigated whether the h-WO3 NWs can reach the photo-efficiency of the m-WO3 NPs. The h-WO3 was successfully synthesized in NW and NP morphology using a microwave (MW) assisted hydrothermal procedure starting from Na2WO4 and thermal annealing of hexagonal ammonium tungsten bronze, (NH4)0.33-xWO3, respectively. We found that the h-WO3 NWs exhibited almost three times higher photoactivity than the h-WO3 NPs. The improved performance of the NWs can be attributed to the enlarged surface area and the good charge carrier ability of the nanowire morphology. The catalytic tests also confirmed that the morphological effect could lead to as high photoactivity in the case of h-WO3 NWs as exhibited by the m-WO3 NPs.  © 2014 The Authors. Published by Elsevier Ltd.  Selection and/or peer-review under responsibility of ICAE  Kewords: photocatalysis, WO3, hydrothermal, annealing, synthesis, morphology 1. Background and objectives In the last few decades much attention has been attracted to alternative energy resources including renewable and nuclear fusion energy due to the limitation of fossil fuel availability on the Earth. Exploring solar light is one of the highly studied possibilities for future energy as it can provide environmental friendly and renewable source of energy. Photocatalysts are able to convert solar energy into chemical energy which can be used for numerous purposes like generation of hydrogen fuel through water splitting; purification of different aqueous media; green synthesis of various chemicals. [1]  * Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: author@institute.xxx . 2 Author name / Energy Procedia 00 (2014) 000–000 TiO2 was historically the first photocatalyst and still remained one of the most efficient photocatalysts under UV light. However, the large band gap energy of TiO2 makes its application in solar photocatalysis insufficient. An option to develop solar sensitive photocatalyst is to couple TiO2 with other semiconductors, which absorb in the Vis range. As a beneficial consequence of making composites with TiO2, delayed recombination of the photo-generated charges can be provided. The electrons and holes can be effectively separated in the different semiconductor layers resulting in higher photo-efficiency. Narrow band gap semiconductors like WO3, V2O5, Bi2O3 are usually considered to form a heterojunction with TiO2. [2-4]  WO3 attracted much attention recently, owing to its broad application prospective and advantageous chemical and electrical properties. [4, 5] WO3 is a good candidate for the fabrication of solar response photocatalyst with TiO2 as it has absorption in the Vis range. [6] The photocatalytic activity depends on many factors, and numerous attempts were made in order to control the size distribution and the dimensionality of the nanocatalysts, because these are considered key parameters in affecting their photocatalytic performance. [7-10] Various morphologies were already achieved including nanotrees, nanorods, nanospheres etc. [9, 11, 12] Unexpectedly, it was recently reported that despite their high specific surface area in some cases certain WO3 particles showed lower photo-efficiency. [13] The peculiar finding was accounted for the facilitated charge recombination which surpassed the positive effect of the enlarged surface area. It is also known that the crystalline phase and the composition of WO3 play an important role in the photo-efficiency. It was revealed that usually m-WO3 possesses better photocatalytic ability than h-WO3. [14] Considering these factors in this study we aimed to investigate the overall effect of the morphology on the photocatalytic performance of different polymorphs of WO3, with the pronounced future objective to fabricate solar active nanocomposites by coupling WO3 polymorphs with TiO2. 2. Experimental H-WO3 NPs were prepared by thermal annealing of hexagonal ammonium tungsten bronze,  (NH4)0.33-xWO3 at 470 ºC in air. [15] H-WO3 NWs were obtained in a microwave assisted hydrothermal synthesis at 160 ºC. [5] The reaction mixture was prepared from sodium tungstate dihydrate (Na2WO4∙2H2O), ammonium sulphate ((NH4)2SO4), HCl and distilled water followed by the MW process in a Synthos 3000 Anton Paar reactor. After the synthesis the nanocrystals were centrifuged, washed with water and ethanol and finally dried at 80 ºC for 24 h. For the preparation of the m-WO3 NPs, (NH4) 0.33-xWO3 was annealed at 600 ºC in air. [15]  The morphology of the as-prepared catalysts was investigated by TEM, and the images were collected by a FEI Morgagni 268D instrument. The determination of the crystalline phase of the WO3 photocatalysts was performed by recording Raman spectra. A Jobin-Yvon Labram type Raman spectrometer coupled with an Olympus BX-41 microscope was used. Nd-YAG laser with a wavelength of 532 nm was applied as a light source in the Raman spectrometer. The specific surface area was determined by applying the BET model based on the absorption isotherm of nitrogen at 77 K using a NOVA 2000E equipment (Quantachrome, USA).  The photocatalytic activity was measured in aqueous methyl orange (MO) (10 mg/350 ml) solution under UV irradiation. In a typical test 100 mg catalyst powder was suspended in the MO solution. The photo-reactor was a cylindrical glass reactor equipped with a Heraeus TQ 150 mercury immersion lamp. The solution was stirred and oxygen bubbling was provided. Room temperature was maintained by circulating cold water in the jacket of the photo-reactor. The concentration of MO was followed by a Jasco V-550 type UV-Vis spectrophotometer at 465 nm.   Author name / Energy Procedia 00 (2014) 000–000 3 3. Results and discussion The morphology of the nanostructures was confirmed by TEM (see in Fig. 1.). The images revealed that the h-WO3 NWs had diameters between 5 and 10 nm and were several hundred nm long. The h-WO3 NPs consisted of 50-70 nm particles, while the m-WO3 NPs were made up by 60-90 nm particles.  The BET specific surface area of the h-WO3 NP and NW was found to be 11 m2g-1 and 101 m2g-1, respectively. The nanowire morphology resulted in one order of magnitude higher specific surface area, compared to that of the nanoparticle. [14] The Raman spectra were in good agreement with the literature data and confirmed the hexagonal and monoclinic crystalline phase of the certain WO3 nanostructures. [14]    Fig. 1. TEM images of (a) h-WO3 NPs; (b) h-WO3 NWs; (c) m-WO3 NPs Fig. 2. MO concentrations given in percentage after 4 hour UV irradiation in the photocatalytic test reaction In the photocatalytic test we found that the h-WO3 NWs were almost three times more effective in the degradation of MO than h-WO3 NPs (see Fig. 2.). By the end of the 4 hour UV irradiation 12 % of the initial MO was decomposed by the h-WO3 NPs, while the h-WO3 NWs photo-bleached 33% of the original concentration of MO. The remarkable improvement in the photocatalytic activity could be attributed to the beneficial morphology of the h-WO3 NWs. The findings also confirmed how important the precise control is over the morphology. The nanowire structure can provide better carrier transport to the separated charges and possess higher specific surface area leading to triple efficiency. The performance of h-WO3 NWs was compared with the activity of m-WO3 NPs as well. It was concluded that by the end of the photocatalytic test the h-WO3 NWs were even more efficient (34 % MO decomposed) than the nanoparticles of the inherently better photocatalyst m-WO3 (33 % MO decomposed). This finding revealed that the h-WO3, which attracted much interest due to its unique open-tunnel structure, can also achieve similarly high photon efficiency as m-WO3.  4. Conclusions In this study we successfully synthesized h-WO3 both with NW and NP morphology along with  m-WO3 with NP nanostructure, either by a MW assisted hydrothermal synthesis route or by annealing (NH4) 0.33-xWO3. The crystal phase of the nanostructures was confirmed by Raman spectroscopy while the microstructures were investigated by TEM. The morphological effect on the photo-efficiency of the as-prepared h- and m-WO3 nanostructures was elucidated. The photoactivity of the h-WO3 NWs was higher almost three times higher compared to the performance of the h-WO3 NPs. The significant improvement in the photo-efficiency clearly indicated the positive effect of the high aspect ratio of the NWs, which 4 Author name / Energy Procedia 00 (2014) 000–000 could provide more enhanced carrier transport in the photocatalytic reaction than the NP nanostructure. It was also studied whether the h-WO3 NWs can exhibit as high photoactivity as the m-WO3 NPs, which have been otherwise considered to be better photocatalysts. It was found that by the end of the degradation test the h-WO3 NWs surpassed even the performance of m-WO3 NPs to some extent.   Acknowledgement D. N. thanks to the University of Edinburgh for the Principle Career Development Scholarship. I. M. S. thanks for a János Bolyai Research Fellowship of the Hungarian Academy of Sciences. An OTKA-PD-109129 grant is gratefully acknowledged. The help of Dr. Eszter Drotár, Dr. Attila L. Tóth, Dr. Ágnes Szegedi (Research Centre for Natural Science, Hungarian Academy of Sciences) and Dr. Krisztina László (Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics) in parts of the experimental work is acknowledged.  References [1] Mills A, Le Hunte S. An overview of semiconductor photocatalysis. J Photoch photobio A 1997;108:1-35. [2] Bian Z et al. Self-Assembly of Active Bi2O3/TiO2 Visible Photocatalyst with Ordered Mesoporous Structure and Highly Crystallized Anatase. J Phys Chem C 2008;112:6258-6262. [3] Wang Y et al. Visible light photocatalysis of V2O5/TiO2 nanoheterostructures prepared via electrospinning. Mater Lett 2012;75:95-98. [4] Szilágyi IM et al. Photocatalytic Properties of WO3/TiO2 Core/Shell Nanofibers prepared by Electrospinning and Atomic Layer Deposition. Chem Vap Deposition 2013;19:149-155. [5] Arutanti O et al. Controllable crystallite and particle sizes of WO3 particles prepared by a spray-pyrolysis method and their photocatalytic activity. AIChE J 2014;60:41-49. [6] Phuruangrat A et al. Synthesis of hexagonal WO3 nanowires by microwave-assisted hydrothermal method and their electrocatalytic activities for hydrogen evolution reaction. J Mater Chem 2010;20:1683-1690. [7] Xu Z et al. Preparation of platinum-loaded cubic tungsten oxide: A highly efficient visible light-driven photocatalyst. Mater Lett 2011;65:1252-1256. [8] Han X et al. Controlling the morphologies of WO3 particles and tuning the gas sensing properties. New J Chem 2012;36:2205-2208. [9] Zhang J et al. Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation. Sol Energy Mater Sol Cells 2011;95:2107-2112. [10] Wang X et al. Hydrothermal synthesis of WO3·0.5H2O microtubes with excellent photocatalytic properties. Appl Surf Sci 2013;282:826-831. [11] Van Tong P et al. Diameter controlled synthesis of tungsten oxide nanorod bundles for highly sensitive NO2 gas sensors. Sensor Actuat B-Chem 2013;183:372-380. [12] Li J et al. Synthesis of monoclinic WO3 nanosphere hydrogen gasochromic film via a sol–gel approach using PS-b-PAA diblock copolymer as template. Solid State Sci 2010;12:1393-1398. [13] Amano F, Ishinaga E, Yamakata A. Effect of Particle Size on the Photocatalytic Activity of WO3 Particles for Water Oxidation. J Phys Chem C 2013;117:22584-22590. [14] Szilágyi IM et al. WO3 photocatalysts: Influence of structure and composition. J Catal 2012;294:119-127. [15] Szilágyi IM et al. Stability and Controlled Composition of Hexagonal WO3. Chemistry of Materials 2008;20:4116-4125.  

Study about the morphology effect on the photo-efficiency of WO3

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