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Atmospheric pressure chemical vapour deposition and characterisation of crystalline InTaO4, InNbO4 and InVO4 coatings
Surface & Coatings Technology 204 (2010) 3864–3870
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Atmospheric pressure chemical vapour deposition and characterisation of crystalline InTaO4, InNbO4 and InVO4 coatings A. Abrutis a,⁎, L. Parafianovic a, V. Kazlauskiene b, V. Kubilius a, G. Sauthier c, A. Figueras c a b c
Vilnius University, Faculty of Chemistry, Naugarduko g. 24, 03225 Vilnius, Lithuania Vilnius University Institute of Material Science and Applied Research, Saulėtekio al. 9, 10222 Vilnius, Lithuania Centre d'Investigació en Nanociència i Nanotecnologia (CIN2), Campus de la UAB-Edifici Q, 08193 Bellaterra, Spain
a r t i c l e
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Article history: Received 26 December 2008 Accepted in revised form 4 May 2010 Available online 11 May 2010 Keywords: [D] oxides [C] chemical vapour deposition [X (B)] photocatalytic activity [B] crystalline structure [B] x-ray diffraction [B] photoelectron spectroscopy
a b s t r a c t The possibilities to grow crystalline complex InTaO4, InNbO4 and InVO4 coatings as well as single oxide layers In2O3, Ta2O5, Nb2O5, and VOx were investigated using aerosol assisted atmospheric pressure chemical vapour deposition technique. Indium(III) and niobium(IV) tetramethylheptanedionates, tantalum(V) tetraethoxyacethylacetonate and vanadium(III) acethylacetonate were used as precursors, monoglyme and toluene as solvents. The influence of deposition conditions and solution composition on elemental and phase compositions of layers was studied. Indium tantalate layers containing pure monoclinic InTaO4 phase were obtained ex-situ, i.e., after high-temperature (800 °C) annealing of layers grown at lower temperature (500 °C). Films containing pure orthorhombic indium vanadate or monoclinic indium niobate phase may be prepared using both in-situ (600 °C) or ex-situ (deposition at 400 °C, annealing at 800 °C) approaches. Under optimised deposition conditions and solution compositions, Ni-doped InVO4 and InTaO4 films were also deposited and their photocatalytic activity was tested. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Due to energy and environment problems, increasing attention has been paid to the splitting of water using solar energy and semiconductor photocatalyst. Recently, a novel series of photocatalysts — InMO4 (M Ta, Nb, V) and their transition metal-doped phases were developed [1–8] for water splitting under visible light and for other photocatalytic ox-red reactions. The bandgap of these new semiconductors (2.6 eV, 2,5 eV and 1.9 eV, respectively) is narrower than that of TiO2-based photocatalysts (3.0 eV for rutile) [4], so they may exhibit catalytic activity under visible light irradiation. One of the most characteristic features is that the bandgap energy of InTaO4 may be narrowed by doping with Ni (from 2.6 eV to 2.3 eV in case of 0.1 Ni-doped) or other 3d-transition elements what results in increased photocatalytic activity under visible light irradiation [2]. Among various 3d-dopants, the Ni-doped InTaO4 photocatalysts showed the best activity under visible light irradiation [3,6]. However, there are no literature data about the doping of indium niobate and vanadate for photocatalytic applications. Bulk polycrystalline/nanocrystalline form of photocatalyst prepared by solid-state reaction or sol–gel synthesis is usually applied for water splitting. In photocatalyst synthesis, the increasing of surface area by decreasing particle size is important not only for increasing of the number of active surface
⁎ Corresponding author. E-mail address: [email protected] (A. Abrutis). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.05.002
sites, but also for shortening of a distance that photogenerated electrons and holes have to migrate from the bulk to surface. In such migration process, the high crystallinity of bulk plays an important role, because various defects of crystalline structure and impurities of other phases may work as recombination sites between electrons and holes, so decreasing their mobility and life time [6]. Thin film form is also interesting for the use as photocatalyst in various ox-red reactions. In particular, visible light photocatalytic activity and superhydrophylic properties of semiconductor films, including new photocatalysts InMO4, may be applied for selfcleaning walls and anti-fogging windows [9,10]. Additionally, indium vanadate films have special electrochemical properties (as Li insertion electrode) and may be used as anodes for lithium rechargeable batteries and as electrochromic windows due to their transparency [11–14]. Sol–gel (spin or dip coating) method [11–13] and RF-sputtering [14] have been used for the preparation of InVO4 films, which were amorphous [14] or contained mixtures of monoclinic and orthorhombic phases [11,12]. Only porous sol–gel InVO4 films prepared by Zhang et al. [13] have been tested as photocatalyst in formaldehyde photodegradation process and showed some catalytic activity under visible light irradiation. Crystalline purely monoclinic InNbO4 films were recently prepared by sol–gel technique and showed photoinduced superhydrophilic property determined by contact angle measurements under light irradiation (λ N 300 nm) [10]. Attempts to grow InTaO4 and Ni-doped InTaO4 films were made using the low pressure chemical vapour deposition (LP-CVD) technique [15]. Although the films contained
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impurities phases of indium and tantalum oxides, they demonstrated photoinduced superhydrophilic property under UV light. Admixture of indium oxide has also been found in thin InTaO4 films grown by RF-sputtering [16,17]. Sol–gel deposition is rather a long ex-situ procedure, including sol–gel preparation/raw film formation/annealing stages, while LP-CVD and sputtering require rather complicated vacuum equipment. In this article we investigated the possibilities to grow un-doped and Ni-doped InMO4 (M Ta,Nb,V) layers by simple aerosol assisted atmospheric pressure CVD method (aerosol-pyrolysis, spray-pyrolysis), with main attention to exploring of both in-situ and ex-situ approaches for the preparation of highly crystalline films containing pure InMO4 phase. Photocatalytic activity of deposited films was also tested. 2. Experimental Indium(III), niobium(IV) and nickel(II) 2,2,6,6-tetramethyl-3,5heptanedionates [In(thd)3, Nb(thd)4, Ni(thd)2], tantalum(V) tetraethoxyacethylacetonate [Ta(OEt)4(AcAc)] and vanadium(III) acethylacetonate [V(AcAc)3] were used as precursors for single or complex oxides deposition. Single precursor or their mixtures were dissolved in monoglyme or toluene in appropriate concentration/ratio. In(thd)3 and Ni(thd)2 were synthesized in our laboratory, other precursors were purchased from Strem Chemicals. Simple aerosol generation/pyrolysis/film growth system similar to that described in [18] was used. Ultrasonic aerosol generation equipment consisted of the ultrasonic power generator (2.56 MHz) coupled with a piezotransducer. The aerosol was generated in a separate glass container and transported to the deposition chamber by N2 + O2 gas flow. Aerosol generation rate was about 2.5 ml/min. The use of volatile metal–organic precursors in the solution transported to a hot substrate in the form of an aerosol makes the deposition process similar to CVD because film grows from a mixture of precursors and solvent vapour formed near the substrate. Deposition was performed at atmospheric pressure, thus without the use of an expensive vacuum equipment typically employed in LP-CVD or physical vapour deposition techniques. Films were mainly grown on Si(100) substrates heated by resistive furnace. For films study by UV/Vis spectroscopy, some depositions were made on glass substrates (MENZEL microscope slides, softening point 720 °C). High temperature (700 °C and higher) results in the inflammation of arriving aerosol in the presence of oxygen in a carrier gas, so the studied deposition temperature was limited to 650 °C. After deposition films were cooled to room temperature. Solution of Ta(OEt)4(AcAc) in organic solvent is not stable when for a long time exposed in air (some turbidity appeared after ∼ 1 h due to hydrolysis), however the stability of this solid Ta precursor is higher than that of liquid Ta(OEt)5. No change of solution was observed during its rapid transfer (under nitrogen flow) into the glass container for aerosol generation and during the deposition process (in a total of about 10–15 min). Preparation of all solutions was made in a glove box under dry and inert atmosphere, precursors were kept in a glove box as well. High-temperature annealing of deposited films was made in a separate furnace under air or Ar flow. Film crystallinity was studied by X-ray diffraction (XRD) (Brucker D8, Cu Kα radiation). The composition of films was determined by Energy-Dispersive X-Ray Spectroscopy (EDS) coupled with scanning electron microscope (SEM, Philips, operating voltage 25 kV, collection time 200 s). X-ray photoelectron spectroscopy (XPS) measurements were performed by RIBER LAS200 using Al K α radiation (hν = 1486.6 eV). The binding energy scale was calibrated with respect to the C 1s (284.6 eV). Films surface morphology was observed by optical microscopy and SEM. Light absorbance spectra of films were measured by means of Perkin Elmer UV/VIS spectrometer Lambda 35. Thickness of the films on Si(100) substrates was
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determined by profilometry (Talystep, Taylor-Hobson) on lithographically formed steps. Photocatalytic activity of deposited films was studied by measuring the changes in photodegradation rate of organic dye methylene blue in aqueous solution (30 ml 10− 5 M) under light irradiation (125 W Hg lamp, λ ∼ 365 nm, light intensity on the film surface was ∼ 0.2 mW/cm2). Dye concentration change in the solution was measured by spectrophotometer (ThermoSpectronic, Helios Gamma). 3. Results and discussion 3.1. Growth of In, Ta, Nb and V oxides Before the deposition of complex oxide films, the growth of single oxide films was studied, using 0.01 M solutions of precursors in monoglyme and toluene. Fig. 1a shows the growth rate dependencies on temperature for In, Ta, Nb and V oxides grown from precursor solutions in monoglyme. β-dicetonate precursors show typical MOCVD growth rate vs. temperature dependencies (kinetic and diffusion controlled growth ranges), with the highest growth rate in case of V(AcAc)3 precursor. Growth rate curve for the mixed alkoxideβ-dicetonate precursor of Ta shows the lowest thermal stability of this precursor. Moreover, the typical saturation range is changed by the range of a continuous decrease of the growth rate with temperature what may be related to increasing premature decomposition of precursor at higher temperatures. Growth rate of oxide films using as solvent toluene was about 1.5 times lower, mainly due to the lower rate of aerosol generation from toluene compared to monoglyme. So in further investigations mainly monoglyme was used as precursor solvent, however toluene is suitable as well. Fig. 1b–e shows the crystallinity of oxide films grown at different temperatures. Crystalline films of cubic In2O3 may be grown in all studied temperature range. Crystallisation of Ta oxide films is most difficult among the studied oxides. Weak XRD peaks of the hexagonal δ-Ta2O5 phase appear only for films grown at 600 °C. Some lower crystallisation temperature was observed for Nb2O5 films containing already well crystallised orthorhombic phase in films deposited at 600 °C. In case of vanadium oxide growth, low deposition temperature results in crystalline films of monoclinic VO2, while at higher temperature films of orthorhombic V2O5 were obtained. 3.2. Growth of InTaO4, InNbO4 and InVO4 films The in-situ growth of InTaO4 films was unsuccessful in the temperature range 500–650 °C. Only the crystalline cubic In2O3 phase was observed in films by XRD or films became amorphous when Ta precursor quantity in the solution was increased in order to adjust the Ta/In ratio. So, a two-step ex-situ approach was investigated for the preparation of InTaO4 films. It consisted of deposition of complex film at 500 °C in the first step (growth rate is about 25 nm/min) and film annealing in air or Ar atmosphere at 800 °C in the second step. Indium excess should be used in solutions in order to obtain crystalline InTaO4 film after annealing. Usually solutions with In/Ta ∼ 1.2–1.4 (total concentration 0.02 M) resulted in films containing pure monoclinic InTaO4 phase. Fig. 2a shows a typical XRD pattern and SEM picture of InTaO4 films prepared on silicon substrates in the two-step process. Pure monoclinic phase is present in films annealed under argon, the films have uniform surface morphology. XRD patterns of films annealed in air contained a small additional unidentified peak at 2θ = 44.3°, probably corresponding to a product of film–substrate interaction at high temperature. Films of InNbO4 may be obtained by both ex-situ and in-situ processes. Films grown at 400 °C were amorphous. Annealing (800 °C, 1 h, Ar) of amorphous films grown from solutions with proper In/Nb ratio (∼ 1,1, total concentration 0.02 M) results in crystallisation of the pure monoclinic InNbO4 phase (Fig. 2b). At higher temperature (500
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Fig. 1. Growth rates of oxide films vs. temperature using different precursors (a) and XRD patterns of In (b), Ta (c), Nb (d) and V (e) oxide films grown on silicon at different temperatures. Solvent monoglyme. Deposition temperature and thickness are indicated for each film.
and 600 °C), the pure monoclinic InNbO4 phase may be formed in-situ during the film growth (Fig. 2b) when In/Nb ratio in the solution is ∼ 0,85 and ∼ 0,7, respectively. Film growth rate is about 35 nm/min (at 500 °C) and 45 nm/min (at 600 °C). SEM showed uniform surface morphology of films prepared under different conditions (insets in Fig. 2b). Films of InVO4 may also be obtained using ex-situ and in-situ approaches like InNbO4 films. Annealing at 800 °C (1 h in argon) of amorphous films grown at 400 °C results in crystallisation of the pure orthorhombic InVO4 phase in film (Fig. 2c) when the solution composition is adjusted. Usually, the In/V ratio in the solution in the
range 0.6–0.8 (total concentration 0.02 M) leads to films containing pure orthorhombic InVO4. Using in-situ process, complex oxide films start to crystallise at a deposition temperature of 500 °C. However, the textured monoclinic InVO4 phase (instead of orthorhombic) crystallyses in films at this temperature (Fig. 2c). Pure monoclinic phase may be obtained from solutions with some In excess (In/V = 1.1–1.2). The pure orthorhombic phase crystallyses in film at higher deposition temperature (600 °C) when using solutions containing rather large In excess, In/V = 1.2–1.5. Solutions with In/V ratios outside these ranges result in crystallisation of additional In or V oxide phases together with InVO4. Growth rates of InVO4 films are about 20, 30 and 45 nm/min
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Fig. 2. XRD patterns and SEM pictures of InTaO4 (a), InNbO4 (b) and InVO4 (c) films prepared on silicon in single step (in-situ deposition) or two-step (ex-situ, deposition + annealing) processes. Solvent monoglyme. Deposition temperature and thickness are indicated for each film.
at 400, 500 and 600 °C, respectively. Observed different crystallisation behaviour of InVO4 films (monoclinic or orthorhombic phase) at different deposition temperature is in agreement with results reported previously for InVO4 powders or sol–gel prepared films [11,19]. Obtained results demonstrate that the pure InVO4 phase in films may be formed using solutions in which the In/V ratio may vary in limited range. EDS analysis showed that the In/V ratio in these films varied from stoichiometric towards the deficit of In (up to In/V ∼ 0.8). So, the films containing the pure InVO4 phase may be deficient in In and this suggests that InVO4 structure in film tolerates some In deficit. Deviation from stoichiometry was also found in pure InTaO4 films (up to In/Ta ∼ 0.9) and was not observed in case of InNbO4 films.
3.3. Growth and characterisation of Ni-doped InVO4 and InTaO4 films Ni-doped InVO4 films were grown in-situ at 600 °C. A series of films with different Ni content was prepared and studied. 400–500 nm thick films were grown using solutions with constant In and V concentrations (0.012 M In and 0.008 M V) and increasing concentration of Ni (from 0 to 0.0075 M). Deposited films were studied by EDS (Fig. 3a) and XRD (Fig. 3b). As shown in Fig. 3a, the increase of Ni quantity in the solution results in the increase of Ni doping level and decrease of In quantity in film, while V content in film remains almost constant. Such change of films composition allows suggestion that In is replaced by Ni in vanadate structure. XRD study confirms that orthorhombic In1 − xNixVO4 structure remains unchanged by Ni
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Preparation of Ni-doped InTaO4 films by ex-situ process is more complicated than in-situ high-temperature growth of Ni-doped InVO4 films. In1 − xNixVO4 films may be grown by adding Ni precursor to the solution with the In/V ratio optimum for un-doped InVO4 films, what shows some self-organised growth of Ni-doped orthorhombic InVO4 phase. In case of Ni-doped InTaO4 films, the adding of Ni precursor to the solution with the optimum (for InTaO4) In/Ta ratio resulted in large deviation of film composition from stoichiometric (In + Ni)/Ta ratio and in formation of mixtures of various phases after annealing (InTaO4, In2O3, Ni-doped In2O3, Ta2O5, NiO, depending on added Ni quantity). Only accurate adjusting of In/Ta ratio in solutions for each Ni quantity allowed obtaining the near stoichiometric (In + Ni)/Ta ratio (Fig. 4a) and the pure monoclinic In1 − xNixTaO4 phase in films doped by ∼ 4 at.% (x ∼ 0.08) and ∼ 8 at.% (x ∼ 0.16) Ni (Fig. 4b).
Fig. 3. Composition of Ni-doped InVO4 films grown at 600 °C in relation with Ni quantity in the solution (a) and XRD patterns (b) of Ni-doped films. Atomic % was calculated as element quantity ratio to the total Ni, In, and V quantity. Insets in (b) correspond to enlarged parts of XRD patterns (2θ = 32–34°) demonstrating the shift of the (112) reflection position.
doping up to ∼ 25% or x ∼ 0.5 (Fig. 3b). Substitution of larger In3+ (0.092 nm) by smaller Ni2+ (0.078 nm) should reduce the volume of the InO6 octahedra, and hence the cell volume of Ni-doped InVO4. Some shift of diffraction peaks to higher angles may be observed when Ni quantity in films increases (insets in Fig. 3b) what may be attributed to the real decrease of the cell volume and thus to the occurring In/Ni substitution in our films. The further increase of Ni quantity in solutions and consequently in films (30−40 at.% Ni in film) resulted in the apparition in XRD patterns of admixture of NiO together with orthorhombic Ni-doped InVO4 (not shown in Fig. 3). Structural change in heavy Ni-doped InVO4 films was confirmed by XPS study of film containing ∼ 40 at.% of Ni. Like InTaO4 films, Ni-doped InTaO4 films were prepared by ex-situ process, i.e. deposition at 500 °C and annealing at 800 °C (1 h in Ar).
Fig. 4. Composition of Ni-doped InTaO4 films in relation with the In/Ta ratio and Ni quantity in the solution (a), and XRD patterns of films doped with different Ni quantities (b). Films are grown at 500 °C and annealed at 800 °C (1 h in Ar). Peaks of the monoclinic InTaO4 phase are marked by asterisk.
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Attempts to introduce higher Ni quantity (∼18 at.%, x ∼ 0.36) into the film resulted in the apparition of a small NiO or NiTa2O6 peak in the XRD pattern (Fig. 4b). Formation of an admixture of NiTa2O6 phase was also observed in In1 − xNixTaO4 powder when x N 0,2 [2], what shows the instability of the heavy Ni-doped In1 − xNixTaO4 structure. For comparison, orthorhombic In1 − xNixVO4 structure in our films was stable up to x ∼ 0.5. In the case of InNbO4 films, we did not succeed to obtain pure Nidoped monoclinic InNbO4 phase using both ex-situ and in-situ preparation approaches. The addition of Ni precursor into solution resulted in formation of mixtures of different phases in films. Optical absorbance spectra were measured for 500 nm thick undoped and Ni-doped (25 at.%) InVO4 films grown on glass substrates. Fig. 5a shows that light absorbance for Ni-doped films is clearly shifted into Vis light region what demonstrates the change of band structure by Ni doping. This offers the possibility to use a wider spectral region of light irradiation and suggests higher photocatalytic activity of Ni-doped InVO4 films compared to un-doped films. Preliminary study of the photocatalytic activity of Ni-doped InVO4 films was made observing the photodegradation process under light of organic dye methylene blue in presence of vanadate film. Samples of films on Si substrate of size about 1 cm2 were immersed in aqueous solution with dye concentration 10− 5 M and irradiated from monochromatic light source (125 W Hg lamp, λ ∼ 365 nm, light intensity on film surface was ∼0.2 mW/cm2). Decrease of dye concentration in time
Fig. 5. Optical absorbance spectra of un-doped and Ni-doped InVO4 films on glass substrates (glass absorption is eliminated) (a) and photodegradation curves of methylene blue under light (λ ∼ 365 nm) in the presence of the InVO4 film on silicon doped with different quantities of Ni (b). Films thickness is 500 nm. C/C0 is the ratio of actual and initial dye concentrations in the solution. Photodegradation curve of the dye without photocatalyst film is also included.
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(photodegradation) was examined by measuring the absorbance of solution samples by spectrophotometer. As shown in Fig. 5b, InVO4 films without Ni and films doped by 3 at.% Ni do not exhibit photocatalytic activity in dye photodegradation reaction. The curves of dye degradation in the presence of these films almost coincide with the “blank” curve measured in the absence of the InVO4 film. Some low photocatalytic activity may be observed for film with ∼ 12 at.% Ni. The film doped by ∼ 25% of Ni already demonstrated photocatalytic activity which may be considered as interesting taking into account very small quantity/surface of photocatalyst in film. For this film about 17% conversion of dye was observed after 5 h of irradiation by light (self-degradation of dye was 8%). Zhang et al. [13] previously tested the photocatalytic activity of porous InVO4 films in photodegradation reaction of formaldehyde (λ N 400 nm, 31 mW cm− 2, film area 4 cm2) and obtained similar photodegradation rate as in our work. However, direct comparison of these results is impossible due to different studied materials, film area and light intensity. In contrast to Ni-doped InVO4 films, no clear photocatalytic activity has been found for Ni-doped InTaO4 films in photodegradation reaction of methylene blue. Un-doped InTaO4 films also did not exhibited any photocatalytic activity. This fact suggests conclusion, that obtaining of highly crystalline films containing pure un-doped or Ni-doped indium tantalate phase is not sufficient condition for films photocatalytic activity. Many other factors may be important for photocatalytic activity of films, such as film morphology, thickness, crystallite size and surface area/roughness, necessity to use of cocatalysts for film surface activation. Such factors should be taken into attention in further attempts to prepare indium tantalate films of high photocatalytic activity. The same factors may be important and for the increase of photocatalytic activity of indium vanadate films. An un-doped and two Ni-doped (∼ 25 and ∼40 at.%) InVO4 films were studied by XPS. Positions of In, V and Ni peaks in XPS spectra for studied films are presented in Fig. 6. XPS spectra were measured from the film surface and after surface etching by Ar ions (3 keV, 15 min). In 3d5/2 peak position in all samples is almost independent of Ni quantity and varies in the narrow range of binding energies (444.2–444.4 eV) in both surface and film depth spectra. These values correspond to the In3+ state in crystalline InVO4 [12,13]. The increase of Ni doping level in films changes the vanadium 2p3/2 peak position in XPS spectra of film surface in the range 517,0–516.9–516.3 eV. Such bonding energy values show the presence of dominating V5+ oxidation state in the first two films [12,13]. The shift of peak position to lower energy suggests the presence of a mixed V5+/V4+ state in the heavy Ni-doped film. The last film (with 40 at.% Ni) is grown from the vapour rich in Ni precursor, so a competitive introduction of Ni ions into both In and V positions of indium vanadate lattice is not excluded. After surface bombardment by Ar ions, the V 2p3/2 peak becomes broader and extended to lower energies what shows the further reduction of V oxidation state up to the apparition of V3+ and V2+ states. Concerning the Ni XPS spectrum, the position of the Ni 2p3/2 peak (856.2 eV) for film with lower doping level corresponds to the Ni3+ oxidation state [19] of ions replacing In3+ positions in the InVO4 lattice. The shift of Ni peak to lower energies (855.6 eV) in case of heavy Ni-doped film already shows the presence of mixed Ni3+/Ni2+ state. An admixture of NiO was observed in the XRD pattern of this film. The surface bombardment by Ar ions resulted in the apparition of a spectrum shoulder corresponding to the Ni0 state. The shift of V 2p3/2 and Ni 2p3/2 peaks to lower energies after bombardment with Ar ions may be related with the presence of lower oxidation states in the depth of asgrown film, however, it may also be explained by reduction of V and Ni oxidation states caused by bombardment [20,21]. XPS composition analysis revealed 12–15 at.% (surface) and 2–3 at.% (film depth) of carbon what demonstrate the low level of pollution by carbon in indium vanadate films grown from carbon rich environment (precursor and solvent vapour at atmospheric pressure). In summary,
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Fig. 6. XPS spectra of un-doped and Ni-doped InVO4 films measured on surface (black curves) and after bombardment with 3 keV Ar ions for 15 min (grey curves). Indicated Ni quantity in films is calculated as Ni/(Ni + In + V).
XPS results correlate with XRD data showing that the In1 − xNixVO4 structure remains stable at Ni doping level ∼ 25 at.% (x ∼ 0.5). Larger Ni doping resulted in structural changes in films observed by XRD (additional phases) and caused changes in the XPS spectrum of films. 4. Conclusions InTaO4, InNbO4 and InVO4 layers as well as single oxide films In2O3, Ta2O5, Nb2O5, and VOx were grown using aerosol assisted atmospheric pressure chemical vapour deposition technique. Indium(III), niobium (IV) and Ni(II) tetramethylheptanedionates, tantalum(V) tetraethoxyacethylacetonate and vanadium(III) acethylacetonate were used as precursors. Monoclinic InTaO4 phase in films may be obtained only ex-situ, i.e., after high-temperature (800 °C) annealing of layers grown at lower temperature (500 °C), while films of orthorhombic indium vanadate or monoclinic indium niobate may be prepared using both in-situ (600 °C) or ex-situ (deposition at 400 °C, annealing at 800 °C) approaches. Ni-doped films of orthorhombic InVO4 may be easily grown under optimised conditions. The growth of Ni-doped InTaO4 films is more complicated — not high Ni doping level is possible (up to ∼8 at.%) and solution composition has to be well adjusted in order to obtain pure doped monoclinic phase in films. The attempts to dope InNbO4 films by Ni were not successful. InVO4 films doped by ∼ 25 at.% Ni showed photocatalytic activity in the reaction of photodegradation of dye methylene blue solutions under light irradiation (λ = 365 nm). Acknowledgements This work was partially supported by the Lithuanian State Science and Studies Foundation. A.A. is grateful to Eniko Gyorgy for valuable comments.
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Atmospheric pressure chemical vapour deposition and characterisation of crystalline InTaO4, InNbO4 and InVO4 coatings A. Abrutis a,⁎, L. Parafianovic a, V. Kazlauskiene b, V. Kubilius a, G. Sauthier c, A. Figueras c a b c
Vilnius University, Faculty of Chemistry, Naugarduko g. 24, 03225 Vilnius, Lithuania Vilnius University Institute of Material Science and Applied Research, Saulėtekio al. 9, 10222 Vilnius, Lithuania Centre d'Investigació en Nanociència i Nanotecnologia (CIN2), Campus de la UAB-Edifici Q, 08193 Bellaterra, Spain
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Article history: Received 26 December 2008 Accepted in revised form 4 May 2010 Available online 11 May 2010 Keywords: [D] oxides [C] chemical vapour deposition [X (B)] photocatalytic activity [B] crystalline structure [B] x-ray diffraction [B] photoelectron spectroscopy
a b s t r a c t The possibilities to grow crystalline complex InTaO4, InNbO4 and InVO4 coatings as well as single oxide layers In2O3, Ta2O5, Nb2O5, and VOx were investigated using aerosol assisted atmospheric pressure chemical vapour deposition technique. Indium(III) and niobium(IV) tetramethylheptanedionates, tantalum(V) tetraethoxyacethylacetonate and vanadium(III) acethylacetonate were used as precursors, monoglyme and toluene as solvents. The influence of deposition conditions and solution composition on elemental and phase compositions of layers was studied. Indium tantalate layers containing pure monoclinic InTaO4 phase were obtained ex-situ, i.e., after high-temperature (800 °C) annealing of layers grown at lower temperature (500 °C). Films containing pure orthorhombic indium vanadate or monoclinic indium niobate phase may be prepared using both in-situ (600 °C) or ex-situ (deposition at 400 °C, annealing at 800 °C) approaches. Under optimised deposition conditions and solution compositions, Ni-doped InVO4 and InTaO4 films were also deposited and their photocatalytic activity was tested. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Due to energy and environment problems, increasing attention has been paid to the splitting of water using solar energy and semiconductor photocatalyst. Recently, a novel series of photocatalysts — InMO4 (M Ta, Nb, V) and their transition metal-doped phases were developed [1–8] for water splitting under visible light and for other photocatalytic ox-red reactions. The bandgap of these new semiconductors (2.6 eV, 2,5 eV and 1.9 eV, respectively) is narrower than that of TiO2-based photocatalysts (3.0 eV for rutile) [4], so they may exhibit catalytic activity under visible light irradiation. One of the most characteristic features is that the bandgap energy of InTaO4 may be narrowed by doping with Ni (from 2.6 eV to 2.3 eV in case of 0.1 Ni-doped) or other 3d-transition elements what results in increased photocatalytic activity under visible light irradiation [2]. Among various 3d-dopants, the Ni-doped InTaO4 photocatalysts showed the best activity under visible light irradiation [3,6]. However, there are no literature data about the doping of indium niobate and vanadate for photocatalytic applications. Bulk polycrystalline/nanocrystalline form of photocatalyst prepared by solid-state reaction or sol–gel synthesis is usually applied for water splitting. In photocatalyst synthesis, the increasing of surface area by decreasing particle size is important not only for increasing of the number of active surface
⁎ Corresponding author. E-mail address: [email protected] (A. Abrutis). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.05.002
sites, but also for shortening of a distance that photogenerated electrons and holes have to migrate from the bulk to surface. In such migration process, the high crystallinity of bulk plays an important role, because various defects of crystalline structure and impurities of other phases may work as recombination sites between electrons and holes, so decreasing their mobility and life time [6]. Thin film form is also interesting for the use as photocatalyst in various ox-red reactions. In particular, visible light photocatalytic activity and superhydrophylic properties of semiconductor films, including new photocatalysts InMO4, may be applied for selfcleaning walls and anti-fogging windows [9,10]. Additionally, indium vanadate films have special electrochemical properties (as Li insertion electrode) and may be used as anodes for lithium rechargeable batteries and as electrochromic windows due to their transparency [11–14]. Sol–gel (spin or dip coating) method [11–13] and RF-sputtering [14] have been used for the preparation of InVO4 films, which were amorphous [14] or contained mixtures of monoclinic and orthorhombic phases [11,12]. Only porous sol–gel InVO4 films prepared by Zhang et al. [13] have been tested as photocatalyst in formaldehyde photodegradation process and showed some catalytic activity under visible light irradiation. Crystalline purely monoclinic InNbO4 films were recently prepared by sol–gel technique and showed photoinduced superhydrophilic property determined by contact angle measurements under light irradiation (λ N 300 nm) [10]. Attempts to grow InTaO4 and Ni-doped InTaO4 films were made using the low pressure chemical vapour deposition (LP-CVD) technique [15]. Although the films contained
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impurities phases of indium and tantalum oxides, they demonstrated photoinduced superhydrophilic property under UV light. Admixture of indium oxide has also been found in thin InTaO4 films grown by RF-sputtering [16,17]. Sol–gel deposition is rather a long ex-situ procedure, including sol–gel preparation/raw film formation/annealing stages, while LP-CVD and sputtering require rather complicated vacuum equipment. In this article we investigated the possibilities to grow un-doped and Ni-doped InMO4 (M Ta,Nb,V) layers by simple aerosol assisted atmospheric pressure CVD method (aerosol-pyrolysis, spray-pyrolysis), with main attention to exploring of both in-situ and ex-situ approaches for the preparation of highly crystalline films containing pure InMO4 phase. Photocatalytic activity of deposited films was also tested. 2. Experimental Indium(III), niobium(IV) and nickel(II) 2,2,6,6-tetramethyl-3,5heptanedionates [In(thd)3, Nb(thd)4, Ni(thd)2], tantalum(V) tetraethoxyacethylacetonate [Ta(OEt)4(AcAc)] and vanadium(III) acethylacetonate [V(AcAc)3] were used as precursors for single or complex oxides deposition. Single precursor or their mixtures were dissolved in monoglyme or toluene in appropriate concentration/ratio. In(thd)3 and Ni(thd)2 were synthesized in our laboratory, other precursors were purchased from Strem Chemicals. Simple aerosol generation/pyrolysis/film growth system similar to that described in [18] was used. Ultrasonic aerosol generation equipment consisted of the ultrasonic power generator (2.56 MHz) coupled with a piezotransducer. The aerosol was generated in a separate glass container and transported to the deposition chamber by N2 + O2 gas flow. Aerosol generation rate was about 2.5 ml/min. The use of volatile metal–organic precursors in the solution transported to a hot substrate in the form of an aerosol makes the deposition process similar to CVD because film grows from a mixture of precursors and solvent vapour formed near the substrate. Deposition was performed at atmospheric pressure, thus without the use of an expensive vacuum equipment typically employed in LP-CVD or physical vapour deposition techniques. Films were mainly grown on Si(100) substrates heated by resistive furnace. For films study by UV/Vis spectroscopy, some depositions were made on glass substrates (MENZEL microscope slides, softening point 720 °C). High temperature (700 °C and higher) results in the inflammation of arriving aerosol in the presence of oxygen in a carrier gas, so the studied deposition temperature was limited to 650 °C. After deposition films were cooled to room temperature. Solution of Ta(OEt)4(AcAc) in organic solvent is not stable when for a long time exposed in air (some turbidity appeared after ∼ 1 h due to hydrolysis), however the stability of this solid Ta precursor is higher than that of liquid Ta(OEt)5. No change of solution was observed during its rapid transfer (under nitrogen flow) into the glass container for aerosol generation and during the deposition process (in a total of about 10–15 min). Preparation of all solutions was made in a glove box under dry and inert atmosphere, precursors were kept in a glove box as well. High-temperature annealing of deposited films was made in a separate furnace under air or Ar flow. Film crystallinity was studied by X-ray diffraction (XRD) (Brucker D8, Cu Kα radiation). The composition of films was determined by Energy-Dispersive X-Ray Spectroscopy (EDS) coupled with scanning electron microscope (SEM, Philips, operating voltage 25 kV, collection time 200 s). X-ray photoelectron spectroscopy (XPS) measurements were performed by RIBER LAS200 using Al K α radiation (hν = 1486.6 eV). The binding energy scale was calibrated with respect to the C 1s (284.6 eV). Films surface morphology was observed by optical microscopy and SEM. Light absorbance spectra of films were measured by means of Perkin Elmer UV/VIS spectrometer Lambda 35. Thickness of the films on Si(100) substrates was
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determined by profilometry (Talystep, Taylor-Hobson) on lithographically formed steps. Photocatalytic activity of deposited films was studied by measuring the changes in photodegradation rate of organic dye methylene blue in aqueous solution (30 ml 10− 5 M) under light irradiation (125 W Hg lamp, λ ∼ 365 nm, light intensity on the film surface was ∼ 0.2 mW/cm2). Dye concentration change in the solution was measured by spectrophotometer (ThermoSpectronic, Helios Gamma). 3. Results and discussion 3.1. Growth of In, Ta, Nb and V oxides Before the deposition of complex oxide films, the growth of single oxide films was studied, using 0.01 M solutions of precursors in monoglyme and toluene. Fig. 1a shows the growth rate dependencies on temperature for In, Ta, Nb and V oxides grown from precursor solutions in monoglyme. β-dicetonate precursors show typical MOCVD growth rate vs. temperature dependencies (kinetic and diffusion controlled growth ranges), with the highest growth rate in case of V(AcAc)3 precursor. Growth rate curve for the mixed alkoxideβ-dicetonate precursor of Ta shows the lowest thermal stability of this precursor. Moreover, the typical saturation range is changed by the range of a continuous decrease of the growth rate with temperature what may be related to increasing premature decomposition of precursor at higher temperatures. Growth rate of oxide films using as solvent toluene was about 1.5 times lower, mainly due to the lower rate of aerosol generation from toluene compared to monoglyme. So in further investigations mainly monoglyme was used as precursor solvent, however toluene is suitable as well. Fig. 1b–e shows the crystallinity of oxide films grown at different temperatures. Crystalline films of cubic In2O3 may be grown in all studied temperature range. Crystallisation of Ta oxide films is most difficult among the studied oxides. Weak XRD peaks of the hexagonal δ-Ta2O5 phase appear only for films grown at 600 °C. Some lower crystallisation temperature was observed for Nb2O5 films containing already well crystallised orthorhombic phase in films deposited at 600 °C. In case of vanadium oxide growth, low deposition temperature results in crystalline films of monoclinic VO2, while at higher temperature films of orthorhombic V2O5 were obtained. 3.2. Growth of InTaO4, InNbO4 and InVO4 films The in-situ growth of InTaO4 films was unsuccessful in the temperature range 500–650 °C. Only the crystalline cubic In2O3 phase was observed in films by XRD or films became amorphous when Ta precursor quantity in the solution was increased in order to adjust the Ta/In ratio. So, a two-step ex-situ approach was investigated for the preparation of InTaO4 films. It consisted of deposition of complex film at 500 °C in the first step (growth rate is about 25 nm/min) and film annealing in air or Ar atmosphere at 800 °C in the second step. Indium excess should be used in solutions in order to obtain crystalline InTaO4 film after annealing. Usually solutions with In/Ta ∼ 1.2–1.4 (total concentration 0.02 M) resulted in films containing pure monoclinic InTaO4 phase. Fig. 2a shows a typical XRD pattern and SEM picture of InTaO4 films prepared on silicon substrates in the two-step process. Pure monoclinic phase is present in films annealed under argon, the films have uniform surface morphology. XRD patterns of films annealed in air contained a small additional unidentified peak at 2θ = 44.3°, probably corresponding to a product of film–substrate interaction at high temperature. Films of InNbO4 may be obtained by both ex-situ and in-situ processes. Films grown at 400 °C were amorphous. Annealing (800 °C, 1 h, Ar) of amorphous films grown from solutions with proper In/Nb ratio (∼ 1,1, total concentration 0.02 M) results in crystallisation of the pure monoclinic InNbO4 phase (Fig. 2b). At higher temperature (500
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Fig. 1. Growth rates of oxide films vs. temperature using different precursors (a) and XRD patterns of In (b), Ta (c), Nb (d) and V (e) oxide films grown on silicon at different temperatures. Solvent monoglyme. Deposition temperature and thickness are indicated for each film.
and 600 °C), the pure monoclinic InNbO4 phase may be formed in-situ during the film growth (Fig. 2b) when In/Nb ratio in the solution is ∼ 0,85 and ∼ 0,7, respectively. Film growth rate is about 35 nm/min (at 500 °C) and 45 nm/min (at 600 °C). SEM showed uniform surface morphology of films prepared under different conditions (insets in Fig. 2b). Films of InVO4 may also be obtained using ex-situ and in-situ approaches like InNbO4 films. Annealing at 800 °C (1 h in argon) of amorphous films grown at 400 °C results in crystallisation of the pure orthorhombic InVO4 phase in film (Fig. 2c) when the solution composition is adjusted. Usually, the In/V ratio in the solution in the
range 0.6–0.8 (total concentration 0.02 M) leads to films containing pure orthorhombic InVO4. Using in-situ process, complex oxide films start to crystallise at a deposition temperature of 500 °C. However, the textured monoclinic InVO4 phase (instead of orthorhombic) crystallyses in films at this temperature (Fig. 2c). Pure monoclinic phase may be obtained from solutions with some In excess (In/V = 1.1–1.2). The pure orthorhombic phase crystallyses in film at higher deposition temperature (600 °C) when using solutions containing rather large In excess, In/V = 1.2–1.5. Solutions with In/V ratios outside these ranges result in crystallisation of additional In or V oxide phases together with InVO4. Growth rates of InVO4 films are about 20, 30 and 45 nm/min
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Fig. 2. XRD patterns and SEM pictures of InTaO4 (a), InNbO4 (b) and InVO4 (c) films prepared on silicon in single step (in-situ deposition) or two-step (ex-situ, deposition + annealing) processes. Solvent monoglyme. Deposition temperature and thickness are indicated for each film.
at 400, 500 and 600 °C, respectively. Observed different crystallisation behaviour of InVO4 films (monoclinic or orthorhombic phase) at different deposition temperature is in agreement with results reported previously for InVO4 powders or sol–gel prepared films [11,19]. Obtained results demonstrate that the pure InVO4 phase in films may be formed using solutions in which the In/V ratio may vary in limited range. EDS analysis showed that the In/V ratio in these films varied from stoichiometric towards the deficit of In (up to In/V ∼ 0.8). So, the films containing the pure InVO4 phase may be deficient in In and this suggests that InVO4 structure in film tolerates some In deficit. Deviation from stoichiometry was also found in pure InTaO4 films (up to In/Ta ∼ 0.9) and was not observed in case of InNbO4 films.
3.3. Growth and characterisation of Ni-doped InVO4 and InTaO4 films Ni-doped InVO4 films were grown in-situ at 600 °C. A series of films with different Ni content was prepared and studied. 400–500 nm thick films were grown using solutions with constant In and V concentrations (0.012 M In and 0.008 M V) and increasing concentration of Ni (from 0 to 0.0075 M). Deposited films were studied by EDS (Fig. 3a) and XRD (Fig. 3b). As shown in Fig. 3a, the increase of Ni quantity in the solution results in the increase of Ni doping level and decrease of In quantity in film, while V content in film remains almost constant. Such change of films composition allows suggestion that In is replaced by Ni in vanadate structure. XRD study confirms that orthorhombic In1 − xNixVO4 structure remains unchanged by Ni
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Preparation of Ni-doped InTaO4 films by ex-situ process is more complicated than in-situ high-temperature growth of Ni-doped InVO4 films. In1 − xNixVO4 films may be grown by adding Ni precursor to the solution with the In/V ratio optimum for un-doped InVO4 films, what shows some self-organised growth of Ni-doped orthorhombic InVO4 phase. In case of Ni-doped InTaO4 films, the adding of Ni precursor to the solution with the optimum (for InTaO4) In/Ta ratio resulted in large deviation of film composition from stoichiometric (In + Ni)/Ta ratio and in formation of mixtures of various phases after annealing (InTaO4, In2O3, Ni-doped In2O3, Ta2O5, NiO, depending on added Ni quantity). Only accurate adjusting of In/Ta ratio in solutions for each Ni quantity allowed obtaining the near stoichiometric (In + Ni)/Ta ratio (Fig. 4a) and the pure monoclinic In1 − xNixTaO4 phase in films doped by ∼ 4 at.% (x ∼ 0.08) and ∼ 8 at.% (x ∼ 0.16) Ni (Fig. 4b).
Fig. 3. Composition of Ni-doped InVO4 films grown at 600 °C in relation with Ni quantity in the solution (a) and XRD patterns (b) of Ni-doped films. Atomic % was calculated as element quantity ratio to the total Ni, In, and V quantity. Insets in (b) correspond to enlarged parts of XRD patterns (2θ = 32–34°) demonstrating the shift of the (112) reflection position.
doping up to ∼ 25% or x ∼ 0.5 (Fig. 3b). Substitution of larger In3+ (0.092 nm) by smaller Ni2+ (0.078 nm) should reduce the volume of the InO6 octahedra, and hence the cell volume of Ni-doped InVO4. Some shift of diffraction peaks to higher angles may be observed when Ni quantity in films increases (insets in Fig. 3b) what may be attributed to the real decrease of the cell volume and thus to the occurring In/Ni substitution in our films. The further increase of Ni quantity in solutions and consequently in films (30−40 at.% Ni in film) resulted in the apparition in XRD patterns of admixture of NiO together with orthorhombic Ni-doped InVO4 (not shown in Fig. 3). Structural change in heavy Ni-doped InVO4 films was confirmed by XPS study of film containing ∼ 40 at.% of Ni. Like InTaO4 films, Ni-doped InTaO4 films were prepared by ex-situ process, i.e. deposition at 500 °C and annealing at 800 °C (1 h in Ar).
Fig. 4. Composition of Ni-doped InTaO4 films in relation with the In/Ta ratio and Ni quantity in the solution (a), and XRD patterns of films doped with different Ni quantities (b). Films are grown at 500 °C and annealed at 800 °C (1 h in Ar). Peaks of the monoclinic InTaO4 phase are marked by asterisk.
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Attempts to introduce higher Ni quantity (∼18 at.%, x ∼ 0.36) into the film resulted in the apparition of a small NiO or NiTa2O6 peak in the XRD pattern (Fig. 4b). Formation of an admixture of NiTa2O6 phase was also observed in In1 − xNixTaO4 powder when x N 0,2 [2], what shows the instability of the heavy Ni-doped In1 − xNixTaO4 structure. For comparison, orthorhombic In1 − xNixVO4 structure in our films was stable up to x ∼ 0.5. In the case of InNbO4 films, we did not succeed to obtain pure Nidoped monoclinic InNbO4 phase using both ex-situ and in-situ preparation approaches. The addition of Ni precursor into solution resulted in formation of mixtures of different phases in films. Optical absorbance spectra were measured for 500 nm thick undoped and Ni-doped (25 at.%) InVO4 films grown on glass substrates. Fig. 5a shows that light absorbance for Ni-doped films is clearly shifted into Vis light region what demonstrates the change of band structure by Ni doping. This offers the possibility to use a wider spectral region of light irradiation and suggests higher photocatalytic activity of Ni-doped InVO4 films compared to un-doped films. Preliminary study of the photocatalytic activity of Ni-doped InVO4 films was made observing the photodegradation process under light of organic dye methylene blue in presence of vanadate film. Samples of films on Si substrate of size about 1 cm2 were immersed in aqueous solution with dye concentration 10− 5 M and irradiated from monochromatic light source (125 W Hg lamp, λ ∼ 365 nm, light intensity on film surface was ∼0.2 mW/cm2). Decrease of dye concentration in time
Fig. 5. Optical absorbance spectra of un-doped and Ni-doped InVO4 films on glass substrates (glass absorption is eliminated) (a) and photodegradation curves of methylene blue under light (λ ∼ 365 nm) in the presence of the InVO4 film on silicon doped with different quantities of Ni (b). Films thickness is 500 nm. C/C0 is the ratio of actual and initial dye concentrations in the solution. Photodegradation curve of the dye without photocatalyst film is also included.
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(photodegradation) was examined by measuring the absorbance of solution samples by spectrophotometer. As shown in Fig. 5b, InVO4 films without Ni and films doped by 3 at.% Ni do not exhibit photocatalytic activity in dye photodegradation reaction. The curves of dye degradation in the presence of these films almost coincide with the “blank” curve measured in the absence of the InVO4 film. Some low photocatalytic activity may be observed for film with ∼ 12 at.% Ni. The film doped by ∼ 25% of Ni already demonstrated photocatalytic activity which may be considered as interesting taking into account very small quantity/surface of photocatalyst in film. For this film about 17% conversion of dye was observed after 5 h of irradiation by light (self-degradation of dye was 8%). Zhang et al. [13] previously tested the photocatalytic activity of porous InVO4 films in photodegradation reaction of formaldehyde (λ N 400 nm, 31 mW cm− 2, film area 4 cm2) and obtained similar photodegradation rate as in our work. However, direct comparison of these results is impossible due to different studied materials, film area and light intensity. In contrast to Ni-doped InVO4 films, no clear photocatalytic activity has been found for Ni-doped InTaO4 films in photodegradation reaction of methylene blue. Un-doped InTaO4 films also did not exhibited any photocatalytic activity. This fact suggests conclusion, that obtaining of highly crystalline films containing pure un-doped or Ni-doped indium tantalate phase is not sufficient condition for films photocatalytic activity. Many other factors may be important for photocatalytic activity of films, such as film morphology, thickness, crystallite size and surface area/roughness, necessity to use of cocatalysts for film surface activation. Such factors should be taken into attention in further attempts to prepare indium tantalate films of high photocatalytic activity. The same factors may be important and for the increase of photocatalytic activity of indium vanadate films. An un-doped and two Ni-doped (∼ 25 and ∼40 at.%) InVO4 films were studied by XPS. Positions of In, V and Ni peaks in XPS spectra for studied films are presented in Fig. 6. XPS spectra were measured from the film surface and after surface etching by Ar ions (3 keV, 15 min). In 3d5/2 peak position in all samples is almost independent of Ni quantity and varies in the narrow range of binding energies (444.2–444.4 eV) in both surface and film depth spectra. These values correspond to the In3+ state in crystalline InVO4 [12,13]. The increase of Ni doping level in films changes the vanadium 2p3/2 peak position in XPS spectra of film surface in the range 517,0–516.9–516.3 eV. Such bonding energy values show the presence of dominating V5+ oxidation state in the first two films [12,13]. The shift of peak position to lower energy suggests the presence of a mixed V5+/V4+ state in the heavy Ni-doped film. The last film (with 40 at.% Ni) is grown from the vapour rich in Ni precursor, so a competitive introduction of Ni ions into both In and V positions of indium vanadate lattice is not excluded. After surface bombardment by Ar ions, the V 2p3/2 peak becomes broader and extended to lower energies what shows the further reduction of V oxidation state up to the apparition of V3+ and V2+ states. Concerning the Ni XPS spectrum, the position of the Ni 2p3/2 peak (856.2 eV) for film with lower doping level corresponds to the Ni3+ oxidation state [19] of ions replacing In3+ positions in the InVO4 lattice. The shift of Ni peak to lower energies (855.6 eV) in case of heavy Ni-doped film already shows the presence of mixed Ni3+/Ni2+ state. An admixture of NiO was observed in the XRD pattern of this film. The surface bombardment by Ar ions resulted in the apparition of a spectrum shoulder corresponding to the Ni0 state. The shift of V 2p3/2 and Ni 2p3/2 peaks to lower energies after bombardment with Ar ions may be related with the presence of lower oxidation states in the depth of asgrown film, however, it may also be explained by reduction of V and Ni oxidation states caused by bombardment [20,21]. XPS composition analysis revealed 12–15 at.% (surface) and 2–3 at.% (film depth) of carbon what demonstrate the low level of pollution by carbon in indium vanadate films grown from carbon rich environment (precursor and solvent vapour at atmospheric pressure). In summary,
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Fig. 6. XPS spectra of un-doped and Ni-doped InVO4 films measured on surface (black curves) and after bombardment with 3 keV Ar ions for 15 min (grey curves). Indicated Ni quantity in films is calculated as Ni/(Ni + In + V).
XPS results correlate with XRD data showing that the In1 − xNixVO4 structure remains stable at Ni doping level ∼ 25 at.% (x ∼ 0.5). Larger Ni doping resulted in structural changes in films observed by XRD (additional phases) and caused changes in the XPS spectrum of films. 4. Conclusions InTaO4, InNbO4 and InVO4 layers as well as single oxide films In2O3, Ta2O5, Nb2O5, and VOx were grown using aerosol assisted atmospheric pressure chemical vapour deposition technique. Indium(III), niobium (IV) and Ni(II) tetramethylheptanedionates, tantalum(V) tetraethoxyacethylacetonate and vanadium(III) acethylacetonate were used as precursors. Monoclinic InTaO4 phase in films may be obtained only ex-situ, i.e., after high-temperature (800 °C) annealing of layers grown at lower temperature (500 °C), while films of orthorhombic indium vanadate or monoclinic indium niobate may be prepared using both in-situ (600 °C) or ex-situ (deposition at 400 °C, annealing at 800 °C) approaches. Ni-doped films of orthorhombic InVO4 may be easily grown under optimised conditions. The growth of Ni-doped InTaO4 films is more complicated — not high Ni doping level is possible (up to ∼8 at.%) and solution composition has to be well adjusted in order to obtain pure doped monoclinic phase in films. The attempts to dope InNbO4 films by Ni were not successful. InVO4 films doped by ∼ 25 at.% Ni showed photocatalytic activity in the reaction of photodegradation of dye methylene blue solutions under light irradiation (λ = 365 nm). Acknowledgements This work was partially supported by the Lithuanian State Science and Studies Foundation. A.A. is grateful to Eniko Gyorgy for valuable comments.
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