Fabrication of tantalum oxyfluoride and oxynitride thin films via ammonolysis of sol–gel processed tetraethoxo (β-diketonato) tantalum (V) precursors for enhanced photocatalytic activity

In the present report, the generation of Tantalum oxyfluoride and oxynitride upon ammonolysis of the gel obtained from modified tantalum-alkoxo complexes is reported. To the best of our knowledge, this is the first report of the formation of tantalum oxyfluoride thin films via ammonolysis of the β-diketone modified tantalum-alkoxo complex [Ta(OEt)4(CF3COCH2COCH3)]m. The integration of nitrogen and fluorine in lattice sites of metal oxides leads to significant reduction in the bandgap, resulting in their activation under visible light. Moreover, in this report the effect of the modified alkoxide precursors and ammonolysis on the photophysical properties of Ta2O5 thin films have also been investigated and compared with the results obtained from films fabricated from unmodified tantalum (V) ethoxide. 1H NMR, 13C NMR and elemental analyses confirmed successful modification of tantalum (V) ethoxide to [Ta(OCH2CH3)4(CH3COCHClCOCH3)]m (1), [Ta(OCH2CH3)4(CF3COCH2COCH3]m (2) and [Ta(OCH2CH3)4 (CH3COC(CH3)2COCH3))]m (3). The fabrication of Ta2O5 thin films involved the spin casting of the gels of modified tantalum alkoxo complexes (processed by sol–gel method) on to glass substrate. X-ray photoelectron spectroscopy results show that nitrogen was incorporated into the ammonolyzed films fabricated from complex precursors (1) and (3), while the presence of fluorine as tantalum oxyfluoride was confirmed in the ammonolyzed film fabricated from complex (2) precursor. The optical characterization insinuate bandgap narrowing from 3.55 eV for undoped film prepared from tantalum (V) ethoxide to 3.47 eV for undoped film prepared from [Ta(OEt)4(CF3COCH2COCH3)]m and 3.05 eV for ammonolyzed film obtained from [Ta(OEt)4(CF3COCH2COCH3)]m precursor. Furthermore, enhanced photocatalytic efficiency of the films is demonstrated by degradation of methylene blue dye.


Introduction
The discovery of photocatalytic properties of titanium dioxide by Honda-Fujishima paved way for the development of technologies with a huge potential applicability towards environmental remediation such as purification of water via degradation of dyes and pesticides, oxidation of volatile organic compounds and CO 2 reduction on various types of photocatalytic materials [1][2][3][4][5][6]. Over the past decades, nanoparticles and thin films of TiO 2 have been examined widely for their photophysical properties establishing TiO 2 as a benchmark photocatalyst [7][8][9][10]. However, thin films of group V metal oxides (Nb 2 O 5 and Ta 2 O 5 ) are of significant technological relevance owing to their excellent photophysical properties (comparable to that of benchmark photocatalyst) and favorable chemical, thermal, mechanical properties [11]. Also, they have a number of other potential uses in advanced electronics as high-j dielectric materials or as diffusion barriers [11][12][13][14][15]. Recently, Ta 2 O 5 has been established as a robust memristive material [16][17][18][19].
Although, for the past few years the group V transition metal oxide investigations were focused on the properties and behavior of oxides of niobium. However, recent investigations showed that oxides of tantalum has exceptional photocatalytic potential and can be an alternative of TiO 2 [20][21][22][23][24][25][26][27]. Furthermore, most investigations involved Ta 2 O 5 photocatalyst in powder form, making it unfeasible to separate and recycle the photocatalyst after reaction [28][29][30]. In this perspective, exploring Ta 2 O 5 based photocatalytic thin films is of great significance [31]. Thin films in nanometer range have attracted a lot of attention due to significant changes in physico-chemical properties due to the high fraction of surface atoms in nanostructured materials, resulting in increased surface to volume ratio of these structures. Ta 2 O 5 thin films can be applied to surfaces to change the physical and chemical properties of a material. Therefore, visibly active photocatalysts like tantalum oxynitride (TaO x N y ) or tantalum oxyfluoride (TaO x F y ) coated as films on different substrates, offering high surface to volume ratio have persistently been investigated for environmental remediation technologies [25][26][27][28][29][30][31].
Sol-gel process has been widely applied in the fields of ceramics, polymer chemistry, organic and inorganic chemistry, nanotechnology and played a crucial role in preparation of novel organic-inorganic hybrid materials OIHMs [32][33][34]. The mechanism of sol-gel can simply be defined as generation of oxide networks through progressive condensation reactions of molecular precursors in liquid medium [33,35]. In general, sol-gel process can be categorized into two methods: the inorganic method and the organic method.
The inorganic method involves the creation of networks in continuous liquid phase through metal oxide colloidal suspensions known as sols and then gelation. Whereas, widely used organic approach involves: (i) the preparation of a solution of metal alkoxide precursor M (OR) n in an alcohol or other low molecular weight organic solvent which is miscible with water. Here M represents a network forming central metal atom such as Ti, Si, Nb, Ta, Al, Fe, B etc. and R is typically an alkyl group with general formula C n H 2n?1 ; (ii) hydrolysis of the homogenous solution to form sol; (iii) condensation and polymerization of sol to form chains and particles; (iv) growth of particles followed by agglomeration of polymer structure followed by the formation of network resulting in thickening of solution known as gel; (v) drying and (vi) thermal treatment. Traditional Ta 2 O 5 thin films fabrications via sol-gel processing have used tantalum alkoxides as most common precursors due to their high propensity towards water. Hydrolysis followed by condensation of tantalum alkoxides result in the formation of oxopolymers finally generating an oxide network upon calcination. The higher electrophilicity and coordination expansion of central tantalum atom due to the presence of high number of facile alkoxy groups make them highly susceptible to hydrolysis. Since, slower hydrolysis of the precursor in the solgel process is the most influential factor for the formation of homogenous materials therefore controlled hydrolysis has been achieved through increased steric hindrance offered to the substitution of alkoxy groups by hydroxyl groups via increasing the alkyl chain length of the alkoxy group or using alkoxy precursor with secondary or tertiary alkyl group [36,37]. Since, complex reactions are involved in lengthening of the alkyl chain of the alkoxy group, therefore substitution of some alkoxy groups by nucleophilic reagents like b-diketones, organic acids and glycols has been widely used to control the rate of hydrolysis of highly reactive metal alkoxides [36,[38][39][40][41][42][43]. Upon substitution, some of the coordination sites of the central metal atom are occupied by these ligands through less hydrophilic bonds or they impose other physical impedances such as steric effect to efficiently hinder the substitution of alkoxy group by hydroxyl group. Although, these ligands do not completely prevent the hydrolysis of the sites occupied by remaining alkoxy groups but effectively reduce the rate of hydrolysis.
More importantly, the photocatalytic efficiency of Ta 2 O 5 has limited utilization due to its high bandgap energy profile (3.9 eV), rendering it active only under ultraviolet region of electromagnetic spectrum which constitutes only a small fraction (\ 5%) of the total spectrum [44]. To enhance the photocatalytic efficiency of Ta 2 O 5 , several novel materials were developed by doping with metal atoms (Mo, W) or nonmetal atoms (N, C) and fabricating various mixed oxide as improved photocatalysts [20][21][22][23][24][25][26][27][28][29]. Moreover, the recombination of photogenerated electron-hole pairs is a major drawback, because it reduces the quantum efficiency of photocatalytic activity as it has faster kinetics than surface redox reactions [45].
Nevertheless, in order to effectively exploit of solar energy and indoor artificial illuminations, the development of visible-light active Ta 2 O 5 thin films via a simple and time-saving approach at moderate temperature has become imperative [46]. To the best of our knowledge, this is the first report of the fabrication and application of tantalum oxyfluoride thin films via ammonolysis of modified tantalum-alkoxo complex [Ta(OEt) 4 (CF 3 COCH 2 COCH 3 )] m . Furthermore, a comparative investigation of the effect of modified precursors and ammonolysis on structural, optical and photocatalytic properties of sol-gel derived tantalum pentaoxide thin films systematically prepared from b-diketones [3-Chloro-2,4-pentanedione, 3,3-Dimethyl-2,4-pentanedione,1,1,1-Trifluoro-2,4-pentanedione] substituted tantalum (V) ethoxide precursors has also been discussed.

Experimental
Tantalum (V) ethoxide and b-diketone derivatives were purchased from Sigma Aldrich and used with no further purification. All reactions were carried out under a dry argon atmosphere using standard Schlenk techniques. All solvents were dried and degassed prior to use.

Fabrication of Ta 2 O 5 thin films
Ta 2 O 5 sol was prepared by dissolving the precursor alkoxide [1][2][3] (1.0 mmol) in ethanol (3 ml) with constant stirring for 3 h. 1 ml of deionized water was added to the resultant clear sol under vigorous stirring to obtain gel. The resultant homogenous gel was spin casted onto glass substrate at 1500 rpm for 60 s. The spin casting procedure was repeated to obtain uniform films. Two sets of the films were prepared from precursors 1, 2 and 3. One set of films (A), (B) and (C) were calcined under oxygen (40 ml/min) at 500°C for 1 h and the other set was subjected to ammonolysis (40 ml/min) under identical conditions and re-designated as NA, NB and NC, respectively. Ta 2 O 5 thin film prepared as control and designated as D was prepared from Ta 2 O 5 sol. This sol was prepared by dissolving the parent tantalum (V) ethoxide (1.0 mmol) in ethanol (3 ml) with constant stirring for 3 h. 1 ml of deionized water was added to the resultant clear sol under vigorous stirring to obtain gel. The resultant homogenous gel was spin casted onto glass substrate at 1500 rpm for 60 s. The spin casting procedure was repeated to obtain uniform film and was calcined under oxygen (40 ml/ min) at 500°C for 1 h.

Characterization
1 H NMR and 13 C NMR spectra were recorded using a Bruker Advance II 400 MHz NMR spectrometer and are referenced to TMS. IR spectra were recorded using Perkin-Elmer System 2000 FT-IR spectrometer. The crystal structures of the Ta 2 O 5 films were determined by X-ray diffractometer (Rigaku Smartlab, Japan) operated at 40 kV, 30 mA. Cu Ka (k = 1.541 78 Å ) was used at X-Ray source. Topographical features of the films were investigated with AFM (Agilent technologies). The chemical environment of films was investigated by X-ray photoelectron spectrometer (PHI 5000 Versa Probe II), using Al Ka source energy (1486.6 eV) and spot size of 200 lm with power of 50 W and 15 kV. The optical constants and thickness of the films were characterized using variable angle spectroscopic ellipsometer (VASE) (J.A. Woollam Co., Inc) at wavelengths ranging from 300 to 1000 nm at an angle of 70°. The optical transmittance was recorded using UV-Vis spectrophotometer (Shimadzu 3600?) along a range of wavelengths from 300 to 1400 nm. Absorption coefficient (a) was calculated using the equation: where d and T respectively represents the thickness and transmittance of films [39].

Photocatalytic degradation of methylene blue
The photocatalytic activities of the films were evaluated by the degradation of methylene blue (MB) over a period of 480 min at ambient temperature and pressure. The Ta 2 O 5 film was immersed in 50 ml of MB solution (4 mg/l). Prior to photocatalytic reaction the solution was kept in the dark for 30 min to establish the adsorption-desorption equilibrium. The reactor was a laboratory-built system comprising a 300 W high-pressure mercury UV tube emitting near UV radiation with a peak wavelength of 365 nm. The distance between the light source and film was kept constant to achieve a constant light intensity on the film surface. The photocatalytic degradation was monitored by measuring the absorbance of the solution with a UV-Vis spectrophotometer. Factors like dye concentration, pH, temperature. were kept constant in all the experiments.

Results and discussion
The reaction of tantalum (V) ethoxide, with the used b-diketone derivatives in 1:1 molar ratio, gave modified alkoxides [Ta(OEt) 4 (b-diketonate)] m (1-3) in good yields where m is the unknown nuclearity (the exact nuclearities of the complexes have not been determined in the present work). The 1 H NMR data for compounds 1, 2 & 3 are tabulated in Table 1 and 13 C NMR values for these compounds are shown in   (2) but calcined under oxygen flow is as shown in Fig. 1a. The spectra confirm the presence of Ta, O, N, F and C elements. The peak at binding energy of 286.0 eV is for C 1 s which is used as reference. Figure 1b illustrates the chemical state of Tantalum metal obtained for all the fabricated films. The presence of symmetric 4f peaks for Ta [47]. However, for film NB, the peak for Ta-4f 7/2 is shifted to high energies and appears at 29.21 eV. This shift may be attributed to high fluorine anion activity indicating a fluorinated oxide phase of tantalum (O-Ta-F) [48]. Notably, Fig. 1c shows the chemical environment of F 1s core electrons present in film NB with peak at 689.08 eV, confirming the substitutional fluorine atoms which occupied the oxygen sites in the lattice forming O-Ta-F bonds [49]. The fluorine atoms are believed to originate from the 1,1,1-Trifluoro-2,4pentanedionate ligand upon degradation at high temperature.
However, the analysis of chemical environment of ammonolyzed film NA (Fig. 3d) confirms the interstitial nitrogen in Ta 2 O 5 with a sharp peak at binding energy 402.00 eV and a weak intensity peak at binding energy 397.73 eV insinuating the binding of 1s electron of substitutional N atom in the environment as O-Ta-N in lattice of N-doped Ta 2 O 5 . Similar environment was observed for film NC. The XPS spectra observed above for undoped and N-doped thin films are also in concordance with our previous report [30].
The overall atomic percentage of doped nitrogen and fluorine atoms is tabulated in Table 3.
The dependence of crystallization of Ta 2 O 5 on temperature, pressure and method of preparation is experimentally well established. Therefore, on the basis of these factors the crystallographic phases of Ta 2 O 5 are categorized into three groups: (a) Low temperature, (b) High temperature and (c) High pressure phases. Phases L, T, TT, b, and d are assigned to Low temperature category with temperature ranging from 470°to 1360°C while at high temperature (T [ 1360°C) H-phase is most stable. The difference in structure for d and H-phase is exhibited in Fig. 2 [56].
For thin films, investigations by Fukomoto et al. have shown [57] that hexagonal structure of d-Ta 2 O 5 described by space group P6/mmm as most stable. This structure is characterized by value of lattice constants a = 7.24 Å ; b = 7.24 Å and c = 11.61 Å . In this structure the Ta atoms are located at Wyckoff positions 1a and 3f, while the O atoms are placed at the Wyckoff positions 1b, 3g, and 6l.
According to reports, H-phase is formed when Ta 2 O 5 is heated to a temperature is [ 1360°C. When it is cooled to room temperature metastable triclinic crystal structures can be formed. The triclinic symmetry is a distorted tetragonal structure and has a fractional occupancy of 75% in one Wyckoff position corresponding to oxygen [58]. Interestingly, at a temperature of 500°C (Low temperature range) phase described by space group P*, -1 was observed in the films. This observation may highlight the dependence of crystallization of Ta 2 O 5 on the method of preparation and growth. Therefore, the XRD patterns further substantiates the existence of structure property relationship between the precursor and the final metal oxide film in the sol-gel process. The observed patterns obtained for films A, B prepared by complexes 1 and 2, respectively, may be attributed to comparatively reduced rate of hydrolysis resulting in the formation of small oligomeric units leading to the elevation of crystallization process to a higher temperature. Moreover, peaks identifying stable hexagonal phase (film C) upon modification of Ta(OCH 2 CH 3 ) 5 to complex 3 is believed to be the an intermediate state for phase transformation from metastable triclinic phase and may be attributed result of faster rate of hydrolysis as compared to complexes 1, 2 and Ta(OCH 2 CH 3 ) 5 . The rate of hydrolysis is key factor in determining the structure of material. In general, tantalum alkoxides undergoes hydrolysis rapidly followed by slower polycondensation, resulting in excessive cross-linking of oligomers. Consequentially, an extensive network is generated which increases the rate of nucleation and growth. This in turn lowers the crystallization temperature for Ta 2 O 5 .
However, for films NA, NB & NC the observed diffraction patterns were consistent with the previous reports insinuating that incorporation of non-metals such as C, N or F may inhibit the process of crystallization by reducing the mobility rate of tantalum and oxygen atoms and consequentially, reducing the rate of nucleation [40][41][42][43].
The average crystallite sizes of the tantala nanoparticles on films were calculated by Scherrer equation and tabulated in Table 4.

Topographical features
AFM micrographs of the films are illustrated in Fig. 3. The images show non-compact morphologies of the deposited Ta 2 O 5 thin films and the absence of cracks. RMS surface roughness of the films is tabulated in Table 4. The micrographs clearly show that surface roughness of films increases as a function of crystallite size of Ta 2 O 5 nanocrystallites. However, the films fabricated from the gels of complexes 1, 2 and 3 were found to be smoother than the films fabricated from the parent alkoxide. It is believed that nucleophilic substitution of ethoxy group of tantalum (V) ethoxide precursor by substituted 2, 4-pantanedione derivatives, reduced the rate of hydrolysis and thus extent of cross-linking resulting in easier relaxation of polymer strands. Consequentially the films are stable, continuous and smoother. Also the RMS roughness of ammonia-calcined films was observed to significantly lower than that of oxygen calcined films.
The lowering of surface roughness of films on treatment with ammonia at 500°C may once again be attributed to the incorporation of nitrogen leading to the elevation of crystallization process to a higher temperature and hence smoother surface.

Optical characterization
The optical transmittance obtained for all the films is shown in Fig. 4. The spectra show strong transmission features at 360 nm and 390 nm with transmittance extending in the visible region in the range of 60-80% for all the films. Films A, B, C and D exhibited higher transmittance (75-80%) than ammonolyzed films NA, NB and NC (50-65%). Notably, red shift in the wavelength of transmission edge was observed on incorporation of nitrogen and fluorine. Moreover, the decrease in transmittance and red shift was greater upon fluorine incorporation. Since fluorine doping has not been extensively studied in tantala films, thus the observed transmittance can be due to several possibilities. The most relevant possibility is that a minimum energy state is achieved when the fluorine atom replaces the Oxygen atom and forms a bond with a tantalum atom. Thus, the O atom is displaced to an interstitial site generating a defect state in the mid-gap region near to the conduction band. This may be attributed to the fact that F atom accepts one electron which is originally localized at the O site, while the remaining electron stays in the Ta region with the state just below the conduction band minimum. The remaining electrons can easily be transferred away due to its high energy state and leave a positively charged F-doped center leading to significant decrease in transmittance and shift of k max and most importantly decrease in the bandgap of F-doped Ta 2 O 5 film from reported value of 3.9-3.05 eV [59,60].
Investigations focused on the bandgap engineering have shown that the conduction band of pure Ta 2 O 5 is composed of Ta 5d orbital with a negative potential of -7.3 eV and the valance band is made up of O 2p orbital having a negative potential of -3.4 eV, hence resulting in a wide bandgap of 3.9 eV [44]. In general, when an oxygen vacant site is generated in the lattice of Ta 2 O 5 due to removal of oxygen atom, two electrons localize on the two adjacent Ta atoms leading to the reduction of Ta ?5 to Ta ?4 . Consequentially, donor states are formed which are 0.75-1.18 eV below the  However, the replacement of oxygen atom with an anion in the lattice site may affect the electronic edges or introduce impurity states between the valance band and conduction band of Ta 2 O 5 . When oxygen is replaced with lighter elements such as nitrogen, carbon or boron, the valance band of Ta 2 O 5 will possibly be depopulated by one, two or three electrons (depending on dopant, respectively). At the same time, there is generation of intra bandgap states which results in the increase of negative potential of valance band of Ta 2 O 5 and absorption in near UV or visible region. However, if we keep the irradiation at 365 nm as the case in this report, the population of electrons will increase significantly in the conduction band and simultaneously the number of holes generated will also increase leading to an enhanced photocatalytic performance.
The bandgap energy of all films was quantitatively determined from Tauc equation corresponding to direct gap n-type semiconductors via plotting (ah m) 2 versus photon energy (E g ) (Fig. 5) [62]. The bandgap of Ta 2 O 5 on film D was calculated to be 3.55 eV, which was 0.35 eV less than the reported bandgap of pure Ta 2 O 5 nanoparticles in powder form, showing the efficiency of films over powdered nanomaterials as photocatalysts [30]. Moreover, lowering of bandgap was observed upon substituting one ethoxide ligand with substituted-2,4-pentanedione. The observed bandgap values of film A, B and C are 3.48 eV, 3.47 eV and 3.52 eV, respectively. This lowering of bandgap may be attributed to the fact that substitution of ethoxide ligands with different bdiketones increased the steric hinderence offered to the substitution of hydroxyl group leading to reduced rate of hydrolysis to an extent that only small oligomeric units are formed. It is reasonable to conceptualize that this will lead to elimination of defects in Ta 2 O 5 upon calcination at 500°C [36,54]. From the observed bandgap values, it is believed that the rate of hydrolysis in complex 3 is faster that of 1 and 2 resulting in the formation of small oligomeric units, forming short cross-linked matrix in the gel of precursor 3 during polycondensation reaction. As a result Ta 2 O 5 nanocrystallites on film C fabricated from complex 3 is more crystalline leading to high bandgap as compared to films A and B.
Interestingly, significant narrowing of bandgap as observed for ammonolyzed films NA, NB and NC (3.31 eV, 3.05 eV and 3.37 eV, respectively). This substantiates the increase of negative potential of valance band in Ta 2 O 5 due to the mixing of O 2p orbitals with the 2p orbitals of interstitial nitrogen in films NA, NC and fluorine in film NB. As already discussed earlier, upon fluorine doping the bandgap value of Ta 2 O 5 depreciates from 3.55 to 3.05 eV, which implies a change from inactive to active in the near UV and visible region thereby making it lucrative for solar energy conversion applications.

Photocatalytic performance under UV irradiation
The photocatalytic activity of the fabricated films was assessed by the degradation of methylene blue dye (MB) under UV light irradiation. Figure 6 shows the percent degradation [(C/C 0 ) 9 100, where C = (C 0--C t ) and C 0 is the initial pollutant concentration after equilibrium while C t is the pollutant concentration after t minutes of irradiation] [63][64][65]. After UV irradiation on all films dipped in MB dye solution for 480 min (8 h), film NB degraded methylene blue up to 87.27% followed by NA and NC (77.28% and 74.21%, respectively). However, films (A, B, C and D) calcined under oxygen flux degraded only 67.83%, 69.76%, 58.87% and 62.01%, respectively. The pioneering work of Asahi et al. [64] lead to the development of new generation of non-metal-doped Fig. 5 Plot of (ah m) 2 v/s photon energy (E g ) for Ta 2 O 5 thin films A, NA fabricated from complex 1; B, NB from complex 2; C, NC from complex 3 and D from unmodified tantalum (V) ethoxide and calcined under oxygen/ammonia gas flow for 500°C for 1 h semiconductor photocatalysts. It is well established that for a material to be an efficient photocatalyst, it is required to have large surface area and/or high crystallinity so that increased number of active sites and accelerated charge separation can be achieved. Moreover, reduction ability of the excited photoelectrons is also an important factor which affects the photocatalytic performance of the material [66][67][68][69]. Although investigation of Ta 2 O 5 as photocatalyst is still in its early stages but recent investigations show that since tantalum-based oxides have relatively negative conduction band which enables them to have strong reduction ability. This unique property makes these tantalum-based oxides a promising candidate for efficient photocatalysis.
However, as it is obvious from the bandgap studies discussed in Sect. 3.4 that large bandgap can mitigate the photocatalytic performance of Ta 2 O 5 as it causes inactivity in near UV-visible region. Nevertheless, nitrogen and fluorine doping have been successful in narrowing the bandgap of these materials. The higher photocatalytic activities of N-doped films NA and NC may be attributed to smaller bandgaps and existence of increased defects as well as oxygen vacancies. In these films, substitutional as well as interstitial nitrogen is present as confirmed by XPS analysis in Sect. 3.2. This interstitial nitrogen atom relatively has higher electronegativity and thus binds to only one lattice oxygen, thereby, shifting the oxygen atom out of that plane and towards the interstitial cavity forming a NO fragment which binds to Ta through its p electrons. Consequentially, two defect bands for p bonding system of NO is generated, i.e. p bonding and p* antibonding NO state. While the p NO lies below the valance band minimum but the p* NO is 0.64 eV above the valance band maximum which could be responsible of reduction in the bandgap leading to an increased photocatalytic efficiency [70].
On the other hand, in case of F-doped film NB the exceptional photocatalytic performance may be possibly attributed to the increased charge separation efficiencies. Although the 2p state of fluorine has one unpaired electron as compared to oxygen which ideally should be transferred to the empty tantalum state but this electron is found trapped at one of the 5d states which is about 0.75-1.18 eV below the conduction band, leading to the generation of defect band state upon chemical reduction of Ta ?5 to Ta ?4 [39,69]. Figure 7 exhibits the changes in concentration of MB as a function of irradiation time for the all films. The slope of obtained from straight lines of the plot ln C 0 /C vs irradiation time represents the rate of reaction (k) and high value of R 2 insinuate that the degradation reaction follow pseudo first-order kinetics according to Langmuir-Hinshelwood model [71]. The rate constants (k) and R 2 values are tabulated in Table 5.
Thus, reduction of the rate of recombination can be done using electron-hole scavengers. In the reductive degradation process, the rate of the oxidative half reaction concerning positively charge hole in the   [72]. Whereas, in the oxidative destruction of a target organic substrate, a hole scavenger should be present in the same environment so as to consume the generated hole in the valance band [73,74]. The most frequently used electron scavenger is molecular oxygen which leads to formation selective oxygen species and thus participates in the process of oxo-functionalization of hydrocarbons. On combining with photoelectrons on the surface of Ta [72][73][74].
Since h ? , ÁOH and ÁO 2 radicals are highly reactive oxidizing species which effect the degradation of methylene blue in the order h ? [ ÁOH [ ÁO 2- [75]. Therefore, photo-induced reactions in presence of a catalyst occur via absorption of a photon (having energy equal to or greater than E g of the catalyst) leading to a charge separation due to the promotion of an electron from the VB to CB of semiconductor catalyst. This results in generation of a hole (h ? ) in VB. This activated electron combine with an oxidant producing a reduced product and also h ? combine with a reductant producing an oxidized product. The oxidant may be a dye molecule or electron acceptors such as O 2 absorbed on the catalyst surface or dissolved in water generating a superoxide radical anion ÁO 2and similarly, h ? can combine with dye molecule to form R ? or react with OHor H 2 O oxidizing them into ÁOH radicals. The resulting ÁO 2and ÁOH radicals can oxidize most azo dyes to the mineral end-products. Accordingly, the reactions occurring at semiconductor surface causing degradation of dyes can be expressed as: h þ þ OH À ! ÁOH ð5Þ The enhanced performances of the films calcined under ammonia environment is believed to be a consequence of bandgap narrowing upon incorporation of nitrogen and fluorine in Ta 2 O 5 lattice leading to increased number photogenerated electrons and holes created by the excitation of electrons and decreased rate of e -/h ? pair recombination leading to enhanced degradation of MB at the surface of Ta 2 O 5 nanostructures film [73][74][75]. Thus the observed photodegradation of MB establishes that photocatalytic activity of tantala films strongly depends on the bandgap of the catalyst, which in this case had been engineered via chelating the parent alkoxide with nucleophilic ß-diketone ligands to control the rate of hydrolysis and importantly the formation of tantalum oxyfluoride and oxynitride nanocrysallites upon ammonolysis.
Although the investigation of Ta 2 O 5 as photocatalyst for degradation of organic pollutants present in water is in its early stages but recent reports [76][77][78][79][80] indicate that with proper bandgap engineering it has the potential to be an alternative to TiO 2 . Mendoza-Mendoza et al. [76] prepared Ta 2 O 5 nanoparticles via mechanical milling of TiCl 5 and LiOH/KOH reagents followed by a subsequent calcination step. It exhibited the in situ generation of LiCl/KCl and the partial crystallization of Ta 2 O 5 at low temperature. These nanoparticles showed enhanced degradation rate of MB attaining a degradation percentage of about 81% within 180 min under UV-vis irradiation. Ismail et al. [77] synthesized mesoporous sulfur (S)-  [78] reported the synthesis of novel 3D N-doping Ta 2 O 5 nanoflowers (NFs) at different annealing temperatures ranging from 700 to 850°C under a NH 3 flux. The nanoflowers prepared at 750°C were reported to demonstrate high surface adsorption and enhanced photocatalytic performance via degradation of Methylene Blue (MB). Moreover, Li et al. [79] reported an improved photocatalytic performance of microspherical Ta 2 O 5powders prepared by flame-assisted hydrolysis of tantalum ethoxide. Recently, Nagaraju et al. [80] reported the preparation of Ta 2 O 5 nanoparticles by the ultrasonic-assisted method and 96% degradation of MB after 2 h under visible light irradiation.

Conclusion
Tantalum pentoxide, oxynitride and oxyfluorides thin films were fabricated using tetraethoxo (b-diketonato) tantalum (V) complexes synthesized in the present work. All the films were prepared in duplicate, one set of films was calcined at 500°C under oxygen flow for 1 h while the other set of films was subjected to ammonolysis at the same temperature for same duration. Notably, upon comparative evaluation of all the films a structure-property relationship was discerned. It was observed that substituted 2,4-pantanedione derivatives, reduced the rate of hydrolysis and thus extent of cross-linking leading to easier relaxation of polymer strands in sol-gel process. Consequentially, small oligomeric units are generated due to which phase transformation as well as elevation of crystallization temperature is observed for Ta 2 O 5 crystallites.
Interestingly, the X-ray photoelectron spectroscopy results exhibited the presence of flourine as tantalum oxyfluoride in ammonolyzed film fabricated from [Ta(OEt) 4 (CF 3 COCH 2 COCH 3 )] m precursor, whereas, nitrogen was incorporated into the films fabricated from [Ta(OCH 2 CH 3 ) 4 (CH 3 COCHClCOCH 3 )] m and [Ta(OCH 2 CH 3 ) 4 (CH 3 COC(CH 3 ) 2 COCH 3 )] m precursors which were ammonolyzed. Moreover, chemical environment of the films prepared from same precursors but calcined under oxygen flow at 500°C for 1 h insinuated the formation of tantalum pentoxide only. Significant narrowing of bandgap energy was observed from 3.55 eV for film D prepared from unmodified tantalaum (V) ethoxide to 3.05 eV for film NB (tantalum oxyfluoride) prepared from [Ta(OEt) 4 (CF 3 COCH 2 COCH 3 )] m . Consequentially, film NB exhibited enhanced photocatalytic performance and photodecomposed 87.27% of methylene blue under UV irradiation for a period of 480 min. The enhanced photocatalytic performance of Ta 2 O x F y is attributed mainly to the incorporation of fluorine which facilitated lowering of bandgap energy and reduction in the rate of recombination of the photogenerated electrons and holes. Compared to their counterpart films A and C calcined under oxygen flow which degraded 67.83% and 58.87% of methylene blue dye, the ammonolyzed films NA and NC demonstrated enhanced photocatalytic performance by degrading 77.28% and 74.21% methylene blue respectively under UV irradiation for a period of 480 min.
This study establishes the generation of Tanatalum oxyfloride and oxynitride nanocrystallites upon ammolysis of the films fabricated from the gels of synthesized tetraethoxo (b-diketonato) tantalum (V) precursors. Moreover, the findings reveal lowering of bandgap energy of Ta 2 O 5 via formations of Ta 2 O x F y and Ta 2 O x N y rendering it activity in visible light. Hence, these films can become potential candidates for possible applications in industrial effluent treatment and as photo-electrodes in photo-electrochemical studies.