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Polyoxometalate Complexes As Precursors to Vanadium Doped

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Polyoxometalate Complexes As Precursors to Vanadium Doped ManuscriptView metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by UCL Discovery 1 Polyoxometalate complexes as precursors to vanadium doped-molybdenum or 1 2 2 tungsten oxide thin films via aerosol-assisted CVD 3 4 3 Sapna Ponja, Sanjayan Sathasivama,b, Hywel O. Daviesc, Ivan P. Parkina and Claire J. 5 6 4 Carmalta* 7 8 9 5 *Corresponding author 10 11 12 6 aMaterials Chemistry Centre, Department of Chemistry, University College London, 13 14 7 20 Gordon Street, London WC1H 0AJ, UK 15 16 17 8 Fax: (+44) 20-7679-7463 18 19 20 9 E-mail: [email protected] 21 22 10 bBio Nano Consulting Ltd, The Gridiron Building, One St. Pancras Square, London 23 24 11 N1C 4AG, UK 25 26 27 12 Fax: (+44) 20-7396-1056 28 29 13 E-mail: [email protected] 30 31 32 14 cACAL Energy Ltd, Heath Business and Technology Park, Runcorn WA7 4QX, 33 34 15 Cheshire, England. 35 36 37 16 Abstract 38 39 17 Aerosol assisted chemical vapour deposition of substituted polyoxometalates: 40 n n 41 18 H4[PMo11VO40]; H7[PMo8V4O40]; [ Bu4N]4[PVW11O40] and [ Bu4N]5[PV2W10O40] 42 43 19 resulted in the formation of vanadium-doped metal oxide thin films. Depositions were 44 45 20 carried out at 550 oC in methanol or acetonitrile solution for the molybdenum or 46 47 21 tungsten containing POMs, respectively. The as-deposited films were X-ray 48 o 49 22 amorphous and relatively non-adherent however, on annealing in air at 600 C 50 23 decolourised translucent films which were more mechanically robust were obtained. 51 52 24 Films deposited from H4[PMo11VO40] and H7[PMo8V4O40] consisted of V-doped 53 n 54 25 MoO3 in the orthorhombic phase and films from [ Bu4N]4[PVW11O40] and 55 n 56 26 [ Bu4N]5[PV2W10O40] comprised of monoclinic V-doped WO3. All films were fully 57 58 27 characterised using XPS, EDX, SEM and UV-Vis. 59 60 61 62 63 64 65 28 Introduction 1 2 3 29 Polyoxometalates (POMs) are characterized as anionic transition metal oxygen cluster 4 30 compounds, diverse in structure,[1] physical and chemical properties, that lend 5 6 31 themselves to a wide range of applications, most notably as catalysts and conductive 7 8 32 materials, in the form of membranes and thin films.[2] POM clusters have a high 9 10 33 degree of solubility in a variety of inorganic and organic solvents and hence are 11 [2a] 12 34 termed ‘molecular metal oxides’. In solution, POMs interact electrostatically with 13 14 35 cationic species which leads to interactions between POMs and cationic ions, 15 36 molecules, complexes, polymers and positively charged solid surfaces.[2a] POMs are 16 17 37 classified into two subfamilies depending on the absence (as in heteropolyanions) or 18 19 38 presence (as in isopolyanions) of a central cation or heteroatom.[1] A number of 20 [1, 2b] 21 39 different structural types of POM clusters exist but the Keggin structure was the 22 [1] 23 40 first to be discovered. Keggin-type POMs can be represented by the formula 24 n- 25 41 [XM12O40] where X is the central heteroatom (e.g., P, Si, or B) with 4 oxygen atoms 26 42 bonded tetrahedrally to it, and M (usually Mo, W or V) is the addenda or peripheral 27 28 43 atom (the metal atoms that make up the framework). The central atom is surrounded 29 30 44 by 12 octahedrons made of MO6 and all the oxygens are shared except for the 12 31 32 45 terminal oxygens, which are attached to only one atom. 33 34 46 35 47 Substituted POMs are a modification of the Keggin structure where an additional 36 37 48 addenda atom can be incorporated, such as vanadium to give POMs of formula, 38 n- [3] 39 49 [PM12-xVxO40] . The physical and chemical properties of a POM are a function of 40 50 their chemical composition, i.e. the identity of the heteroatom and addenda atom(s). 41 42 51 POMs with Mo or W as the addenda atoms with different heteroatoms (e.g., P, V, Nb 43 44 52 or W) are easy to prepare. Changing the heteroatom and/or substituting an addenda 45 46 53 atom also provides the means to add an additional element into the metal oxide films 47 48 54 which may have a significant effect on the chemical and physical properties of the 49 55 film and lead to doped-metal oxide films. Vanadium has been used as a dopant in the 50 51 56 production of thin films for reducing the band gap and improving photocatalytic 52 53 57 properties.[4] 54 55 58 56 59 Thin films of Mo oxides have been made previously using dual-source precursors, 57 58 60 such as molybdenum hexacarbonyl, [Mo(CO)6] and oxygen, and from single-source 59 [5] 60 61 precursors such as molybdenum pentacarbonyl 1-methylbutylisonitrile. 61 62 63 64 65 62 Molybdenum(VI) oxide exists in two basic crystal structures with different molecular 1 [5] 2 63 vibrational and optical properties: α-MoO3 with orthorhombic symmetry and 3 [6] 4 64 metastable monoclinic β-MoO3; MoO2 also has a monoclinic structure. 5 65 Molybdenum trioxide (MoO3), the technologically more significant form of the oxide, 6 [6] 7 66 exists in the orthorhombic phase and has a double layered structure. MoO3 is a well- 8 9 67 known catalyst often used in the oxidative dehydrogenation of methanol to an 10 [7] 11 68 aldehyde and has also been shown to act as an excellent antimicrobial coating, 12 [8] 13 69 forming an acidic environment that retards bacterial growth and proliferation. 14 70 Applications of MoO also extend to organic electronic devices due to its low 15 3 16 71 absorption in the visible spectrum and high compatibility with other materials. MoO3 17 18 72 has also been used as a material to reduce energy barriers for charge carrier injection, 19 20 73 extraction or transport between semiconductor and organic layers.[9] 21 22 74 23 75 Like MoO3, tungsten trioxide (WO3) has many applications as an interface layer in 24 [9-10] 25 76 electronic devices. Principally, it reduces the barrier for charge injection between, 26 [10a] 27 77 for example, a tin doped indium oxide (ITO) layer and a polymer layer. WO3 also 28 [11] 29 78 has applications in gas sensing, photocatalysis and photochromism. Films 30 79 containing WO3 have been deposited by evaporation, sputtering, electrochemical 31 [12] 32 80 techniques and by CVD. WO3 undergoes a number of phase transitions during 33 34 81 annealing and cooling:[11] monoclinic II (< -43 oC) triclinic (-43 – 17 oC) 35 o o o 36 82 monoclinic I (17 – 330 C) orthorhombic (330-740 C) tetragonal (> 740 C). 37 38 83 The monoclinic I (hereon referred to as ‘monoclinic’) is the most stable phase at room 39 84 temperature and usually remains so even after annealing. 40 41 85 42 43 86 CVD has increasingly become the preferred method for producing metal oxide films 44 87 mainly because of the higher deposition rates,[13] uniform coverage, good 45 46 88 reproducibility, and highly dense and pure films.[14] Aerosol-assisted CVD is a variant 47 48 89 that has additional advantages over conventional CVD which includes that the 49 [15] 50 90 precursors do not have to be highly volatile or thermally stable. This opens up the 51 52 91 possibility of using precursors that would not have been suitable for conventional 53 92 CVD such as POMs. 54 55 93 56 [6, 16] 57 94 Keggin-type POMs have been deposited via AACVD however, this paper, to our 58 95 knowledge, is the first study on the deposition of binary metal oxide thin films: 59 60 96 vanadium doped MoOx (x = 2-3) and WO3 using single-source precursors called 61 62 63 64 65 97 substituted Keggin-type POMs via AACVD. All POMs and films were characterized 1 2 98 spectroscopically. 3 4 99 5 100 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 101 Experimental 1 102 2 103 The reagents were purchased from Sigma Aldrich (99.9% purity unless stated 3 4 104 otherwise) and used without further refinement. The identity of the POMs was 5 6 105 confirmed by 31P NMR and FT-IR which were consistent with XPS and EDX. POMs 7 [3, 17] 8 106 (1- 4) were prepared by literature procedures. 9 10 107 11 108 Preparation of H4[PMo11VO40] (1) 12 109 13 110 POM (1) was synthesised following a method given by ACAL Energy Ltd which was 14 [17] 15 111 scaled down to suit to the quantities required in the present study. V2O5 (0.45 g, 16 17 112 2.47 mmol) and MoO3 (7.92 g, 55.02 mmol) were suspended in distilled water (50 18 19 113 mL) with moderate stirring. 85% H3PO4 (0.57 g, 5.82 mmol) was added to the 20 21 114 mixture followed by additional distilled water (45 mL). The pale yellow mixture was 22 115 heated at reflux (120 oC). After two days, a drop of H O was added and the mixture 23 2 2 24 116 was left at reflux for a further five days, resulting in a clear orange/red solution.
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