The reaction of tantalum (V) ethoxide, with the used β-diketone derivatives in 1:1 molar ratio, gave modified alkoxides [Ta(OEt)4(β-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 1H NMR data for compounds 1, 2 & 3 are tabulated in table 1 and 13C NMR values for these compounds are shown in table 2. The 1H NMR of compound 3 shows three sets of triplet (δ 0.84, 1.04 and 1.07) and quartet (δ 3.72, 3.89 and 3.91 ) suggesting that in solution the molecule contains OEt ligands in three types of environments in an intensity ratio of 2:1:1. Singlets at δ 1.89 and 5.12 respectively indicate the presence of methyl and methine protons of the 3,3-Dimethyl-2,4-pentanedione ligand. Similarly, 1H NMR spectra of compounds 1 & 2 also demonstrate octahedral structure for the precursor complexes. Moreover, the IR spectra of each complex exhibited bands for νC=O in the range of 1640 cm-1 to 1690 cm-1 and νc=c in the range of 1550 cm-1 to 1560 cm-1 insinuating the presence of the substituted β-diketonate ligand.
The thicknesses of the films were investigated via variable angle ellipsometry. It was found that the thicknesses of the films were ≈200 nm. Moreover, the thickness of the glass substrate was observed to be 3 mm with a refractive index of 1.52.
3.1 X-Ray photoelectron spectroscopy analysis
The chemical environment of the fabricated films was determined through XPS. All the films calcined under oxygen flow as well as ammonolyzed films NA and NC insinuated same chemical environment. However a comparison between the XPS survey spectra of ammonolyzed films NA fabricated from synthesized precursor [Ta(OCH2CH3)4(CH3COCHClCOCH3)]m (1), NB fabricated from precursor [Ta(OCH2CH3)4(CF3COCH2COCH3)]m (2) at 500 °C and film B fabricated from precursor (2) but calcined under oxygen flow is as shown in fig.1 (a). 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 1s which is used as reference.
Fig. 1 (b) illustrates the chemical state of Tantalum metal obtained for all the fabricated films. The presence of symmetric 4f peaks for Ta5+ and absence of Ta0 metal peaks, confirming the presence of tantalum as Ta2O5. Moreover, the peaks at binding energy 26.29 and 22.72 eV confirms the presence of O–Ta–O 4f7/2 and O–Ta–N 4f7/2 . However, for film NB, the peak for Ta-4f7/2 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) .
Notably, fig. 1 (c) 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 . The fluorine atoms are believed to originate from the 1,1,1-Trifluoro-2,4-pentanedionate ligand upon degradation at high temperature.
However, the analysis of chemical environment of ammonolyzed film NA (Fig. 3d) confirms the interstitial nitrogen in Ta2O5 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 Ta2O5. 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 .
The overall atomic percentage of doped nitrogen and fluorine atoms is tabulated in table 3.
3.2 Crystal structure analysis
The diffractograms of Ta2O5 thin films are shown in fig. 2. The films A, B, NA, NB and NC were amorphous. Diffraction peaks at 2θ = 23.41°, 46.59° for film D (control) were observed and respectively indexed to (-1 0 1) and (2 0 0) metastable triclinic crystal planes of H-Ta2O5 phase described by space group P*,-1. [JCPDS card no: 00-021-1198] . Interestingly, diffractogram obtained for film C exhibit diffraction peaks for hexagonal as well as tricilinic crystal structure of Ta2O5. The peaks at 2θ = 22.90°, 28.49°, 46.89°, 51.20° and 57.90° were indexed to (0 0 3), (2 0 0) (0 0 6), (2 2 0) and (2 1 1) hexagonal crystal planes of δ -Ta2O5 described by space group P6/mmm [JCPDS card no: 00-018-1304]  and peaks at 2θ = 32.38° and 40.15° were respectively indexed to (0 -1 9) and (0 -1 13) metastable triclinic crystal planes of H-Ta2O5 phase described by space group P*,-1 . Diffraction peaks corresponding to crystalline TaON at 2θ = 28.2° and 37.2° and TaOF2 at 2θ = 22.8°, 32.5°, 46.6° and 52.5° were not observed [52-55].
The dependence of crystallization of Ta2O5 on temperature, pressure and method of preparation is experimentally well established. Therefore, on the basis of these factors the crystallographic phases of Ta2O5 are categorized into three groups: (a) Low-temperature, (b) High-temperature and (c) High pressure phases. Phases L, T, TT, β, and δ 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 δ and H- phase is exhibited in fig. 2 .
For thin films, investigations by Fukomoto et.al have shown  that hexagonal structure of δ-Ta2O5 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 Ta2O5 is heated to a temperature is greater than 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 . Interestingly, at a temperature of 500 °C (Low-temperature range) metastable triclinic crystal structure of H-Ta2O5 phase described by space group P*,-1 was observed in the films. This observation may highlight the dependence of crystallization of Ta2O5 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(OCH2CH3)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(OCH2CH3)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. Concequentially, an extensive network is generated which increases the rate of nucleation and growth. This in turn lowers the crystallization temperature for Ta2O5.
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 [40-43].
The average crystallite sizes of the tantala nanoparticles on films were calculated by Scherrer equation and tabulated in table 4.
3.3 Topographical features
AFM micrographs of the films are illustrated in fig. 3. The images show non-compact morphologies of the deposited Ta2O5 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 Ta2O5 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.
3.4 Optical Characterisation
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 λ max and most importantly decrease in the band gap of F-doped Ta2O5 film from reported value of 3.9 eV to 3.05 eV [59, 60].
Investigations focused on the band gap engineering have shown that the conduction band of pure Ta2O5 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 band gap of 3.9 eV . In general, when an oxygen vacant site is generated in the lattice of Ta2O5 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 conduction band thereby, reducing the overall band gap energy of Ta2O5 nanocrystallites .
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 Ta2O5. When oxygen is replaced with lighter elements such as nitrogen, carbon or boron, the valance band of Ta2O5 will possibly be depopulated by one, two or three electrons (depending on dopant respectively). At the same time, there is generation of intra band gap states which results in the increase of negative potential of valance band of Ta2O5 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 band gap energy of all films was quantitatively determined from Tauc equation corresponding to direct gap n-type semiconductors via plotting (αhʋ)2 versus photon energy (Eg) (fig. 5) . The band gap of Ta2O5 on film D was calculated to be 3.55 eV, which was 0.35 eV less than the reported band gap of pure Ta2O5 nanoparticles in powder form, showing the efficiency of films over powdered nanomaterials as photocatalysts . Moreover, lowering of band gap was observed upon substituting one ethoxide ligand with substituted-2,4-pentanedione. The observed band gap values of film A, B and C are 3.48 eV, 3.47 eV and 3.52 eV respectively. This lowering of band gap may be attributed to the fact that substitution of ethoxide ligands with differen β-diketones 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 Ta2O5 upon calcination at 500 °C [36, 54]. From the observed band gap 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 Ta2O5 nanocrystallites on film C fabricated from complex 3 is more crystalline leading to high band gap as compared to films A and B.
Interestingly, significant narrowing of band gap 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 Ta2O5 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 band gap value of Ta2O5 depreciates from 3.55 eV 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.
3.5 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. Fig. 6 shows the percent degradation [(C/C0) X 100, where C = (C0 − Ct) and C0 is the initial pollutant concentration after equilibrium while, Ct is the pollutant concentration after t minutes of irradiation] [63-65]. After UV irradiation on all films dipped in MB dye solution for 480 minutes (8 hrs.), 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  lead to the development of new generation of non-metals doped 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 [66-69]. Although investigation of Ta2O5 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 band gap studies discussed in section 3.4 that large band gap can mitigate the photocatalytic performance of Ta2O5 as it causes inactivity in near UV-visible region. Nevertheless, nitrogen and fluorine doping have been successful in narrowing the band gap of these materials. The higher photocatalytic activities of N-doped films NA and NC may be attributed to smaller band gaps 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 section 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 π electrons. Consequentially, two defect bands for π bonding system of NO is generated i.e. π bonding and π* antibonding NO state. While, the π NO lies below the valance band minimum but the π* NO is 0.64 eV above the valance band maximum which could be responsible of reduction in the band gap leading to an increased photocatalytic efficiency .
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 eV to 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].
Fig.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 C0/C vs irradiation time represents the rate of reaction (k) and high value of R2 insinuate that the degradation reaction follow pseudo- first order kinetics according to Langmuir– Hinshelwood model . The rate constants (k) and R2 values are tabulated in table 5.
Thus, reduction of the rate of recombination can be done by using electron-hole scavengers. In the reductive degradation process, the rate of the oxidative half reaction concerning positively charge hole in the valance band is closely related to the effective removal of the partner species, i.e. photoelectrons present in the conduction band, by suitable electron scavengers. This electron scavenger should be present in the solution and available at the Ta2O5 films and solution interface . 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 Ta2O5 the O2 molecules gives exceptionally reacting species such as the superoxide radical O2-• and the singlet oxygen 1O2 leading to the generation of strong oxidants like H2O2 and O3 [72-74].
Since h+, •OH and •O2 radicals are highly reactive oxidizing species which effect the degradation of methylene blue in the order h+ > •OH > •O2- . Therefore, photo-induced reactions in presence of a catalyst occur via absorption of a photon (having energy equal to or greater than Eg 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 O2 absorbed on the catalyst surface or dissolved in water generating a superoxide radical anion •O2−and similarly, h+ can combine with dye molecule to form R+ or react with OH− or H2O oxidizing them into •OH radicals. The resulting •O2−and •OH 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:
Ta2O5/TaOxNy/ TaOxFy + hν →Ta2O5/TaOxNy/TaOxFy (e−+ h+) (1)
Ta2O5/TaOxNy/TaOxFy (e−+ h+) → hν (2)
h+ + H2O → H+ + •OH (3)
h+ + OH− → •OH (4)
e− + O2 → •O2− (5)
2 •OOH → O2 + H2O2 (7)
H2O2+ e− → •OH + OH− (8)
MB + •OH → CO2 + H2O (9)
The enhanced performances of the films calcined under ammonia environment is believed to be a consequence of band gap narrowing upon incorporation of nitrogen and fluorine in Ta2O5 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 Ta2O5 nanostructures film [73-75]. Thus the observed photodegradation of MB establishes that photocatalytic activity of tantala films strongly depends on the band gap 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 Ta2O5 as photocatalyst for degradation of organic pollutants present in water is in its early stages but recent reports [76-80] indicate that with proper band gap engineering it has the potential to be an alternative to TiO2. E. Mendoza-Mendoza et.al  prepared Ta2O5 nanoparticles via mechanical milling of TiCl5 and LiOH/KOH reagents followed by a subsequent calcination step. It exhibited the in-situ generation of LiCl/KCl and the partial crystallization of Ta2O5 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. A.A. Ismail et.al  synthesized mesoporous sulfur (S)-doped Ta2O5 nanocomposites via sol-gel reaction of tantalum chloride and thiourea in the presence of a F127 triblock copolymer as structure directing agent. These hybrids gels were calcined at 700 °C for 4 h to obtain mesoporous S-Ta2O5 nanocomposites. They found an excellent photocatalytic activity of about 92% for the photodegradation of methylene blue, after three hours illumination under visible light. Similarly, Xiaomin Shi et.al  reported the synthesis of novel 3D N-doping Ta2O5 nanoflowers (NFs) at different annealing temperatures ranging from 700 °C to 850 °C under a NH3 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, Jing Li et.al  reported an improved photocatalytic performance of microspherical Ta2O5 powders prepared by flame assisted hydrolysis of tantalum ethoxide. Recently, G. Nagaraju et.al  reported the preparation of Ta2O5 nanoparticles by the ultrasonic-assisted method and 96 % degradation of MB after 2 h under visible light irradiation.