Photocatalytic H₂ generation over In₂TiO₅, Ni substituted In₂TiO₅ and NiTiO₃ – a combined theoretical and experimental study

Abstract

We report here the role of Ni substitution in modifying the crystal structure, optical absorption properties and electronic properties of indium titanate, In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x ≤ 0.4) and its consequent effect on the photocatalytic properties for H₂ generation. Rietveld refinement of observed XRD patterns of the titanates revealed that Ni²⁺ substitution has led to a decrease in lattice cell parameters and cell volume, contraction of InO₆ octahedra and consequently improved charge carrier properties. Furthermore, the conduction band maximum (CBM) was found to be a hybrid state between Ni, Ti and In orbitals in 10% Ni-doped sample, which suggests that the photo-induced charges can be better transported in the substituted samples from zigzag chains of [·Ni–O–Ti⋯In–O–Ti----]. The UV-visible diffuse reflectance spectra exhibited that the band gap of the indium titanate phase decreased sequentially with an increase in the extent of Ni substitution. The underlying cause for band gap narrowing on Ni substitution was evaluated from plane wave based DFT calculations using the GGA + U approach. The decreasing order of photocatalytic activity (as a percentage of Ni substitution) for hydrogen generation from water–methanol mixture is as follows: 10% > 5% > indium titanate > 15% > 20%. The fall in activity below indium titanate coincided with the appearance of ilmenite NiTiO₃ phase. Plane wave based DFT calculations performed on NiTiO₃ revealed that strong intermixing of Ni-3d with O-2p orbitals occurred in the valence band of NiTiO₃ and resulted in generation of a pseudo band gap of 0.3 eV at 1.4 eV below the Fermi level. This pseudo band gap might act as a hindrance and may contribute to weakening the intensity of the electronic transition due to Ni²⁺ → Ti⁴⁺ charge transfer. We propose here that an optimal concentration of 10% Ni substitution in indium titanate modifies the structural and electronic properties favorably leading to better photocatalytic activity by reducing the band gap, enhancing of the electron–hole separation and improving charge carrier properties.

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1. Introduction

The photocatalytic hydrogen generation from water using solar energy is a potentially clean and attractive method for solar energy conversion and storage. Since Honda and Fujishima first discovered titanium dioxide (TiO₂) photoassisted electrochemical splitting of water in 1972,¹ semiconductor photocatalysts have attracted much attention. Although much work has been conducted over the past three decades to exploit new photocatalysts responsive to UV and visible light irradiation, detailed research has been mainly focussed on titania (TiO₂).^2–5 Besides, multi-metal oxide systems fulfill the essential requirement of materials with better chemical stability and modified catalytic properties and this fact have been emphasized in recent years.^6–8 So, in the search for efficient photocatalysts under visible light for hydrogen generation from water, a variety of mixed oxide semiconductors based tantalates, titanates nitrides, niobates, sulphides, oxysulfides, oxynitrides etc. have been studied and have been reviewed in details in several articles.^9–11 A common structural feature of the mixed metal oxide photocatalysts is the presence of [BO₆] (B = Ti, In, Nb, Ta, etc.) octahedral moiety in the crystal structure. The compound indium titanate (In₂TiO₅) contains both InO₆ & TiO₆ octahedra and is expected to be a suitable photocatalyst for hydrogen generation from the structural point of view, but it suffers from the disadvantage that it absorbs in the UV region of the solar spectrum.¹² So it is relevant to investigate the photocatalytic properties of modified indium titanate by suitable adjustment of band gap, so that it absorbs in the visible region.

With this perspective we initiated our study by investigating the photocatalytic properties of bulk and nano In₂TiO₅ and established that In₂TiO₅ possesses a suitable electronic and crystal structure for photocatalytic hydrogen generation reaction.¹³ Wang et al.¹² first reported that indium titanate can serve as an active photocatalyst for the UV-induced photo-degradation of methyl orange dye. Besides, V-doped In₂TiO₅ were also investigated for photocatalytic splitting of water under UV-vis irradiation.¹⁴ We have also studied the effect of B-site¹⁵ (Ti⁴⁺) substitution by Fe³⁺ and Cr³⁺ and also A-site¹⁶ (In³⁺) substitution by Ni²⁺ on thermal properties and reduction behavior of In₂TiO₅. Recently, we carried out a systematic study on the relationship between the structure and the photocatalytic properties of Nd³⁺ doped In₂TiO₅.¹⁷ First principle investigations were carried out to analyze the distribution of valence states of the constituent atoms near the Fermi level of the In₂TiO₅ which revealed the suitability of In₂TiO₅ as a photocatalyst material and the role of Nd in decreasing the band gap.¹⁷ Bahers et al.¹⁸ have evaluated the ability of density functional theory (DFT) to compute the fundamental properties such as band gap, dielectric constant, the charge carrier effective masses, and the exciton binding energy, which decide the photocatalytic and photovoltaic properties of semiconductor oxides. They concluded that DFT is a reliable tool for the evaluation and prediction of these key properties. These interesting photoactivity results of hydrogen generation over pristine and rare earth (Nd³⁺) doped In₂TiO₅ prompted us to investigate the effect of transition metal like Ni²⁺ on the photocatalytic properties of In₂TiO₅. To the best of our knowledge, investigations of photocatalytic properties by experimental or first-principles electronic structure calculations on Ni-doped In₂TiO₅, have not been attempted earlier. The modified form of In₂TiO₅, In_2(1−x)Ni_2xTiO_5−δ, is expected to have absorbance in the visible region in contrast to the pristine In₂TiO₅ (band gap ∼3.02 eV in UV region) as transition metals with partly filled d-orbital (e.g. Ni²⁺, Cr³⁺, Fe³⁺) are known to add impurity levels near the conduction band of titanates thereby decreasing its band gap and increasing its photoactivity.^{9–12,14,19,20} This phenomenon along with the basis of photocatalytic hydrogen generation has been dealt in details in our chapter in the book edited by Banerjee and Tyagi.²¹ Additionally, the ionic radius of Ni²⁺ (0.69 Å) being much smaller than that of In³⁺ (0.81 Å), substituting In³⁺ by Ni²⁺ is expected to reduce the volume of the InO₆ octahedra, and thus also the cell volume in In₂TiO₅. Consequently, the metal–metal bonding is expected to be altered in the crystal lattice such that the In–In distance shortens, resulting in profound alterations of the charge carrier properties of the compounds and therefore the photocatalytic properties.²⁰ So, in this article we report the effects of A-site substitution of Ni²⁺ ion on crystal structure, optical properties and on the photocatalytic behavior of In₂TiO₅ for hydrogen generation reaction from water–methanol mixture. For this purpose, In_2(1−x)Ni_2xTiO_5−δ, (0.0 ≤ x ≤ 0.2) mixed oxide catalysts were synthesized using solid-state reaction and characterized by powder X-ray diffraction (XRD), SEM, DRUV and the photocatalytic properties were investigated for hydrogen generation reaction. To establish the contribution of secondary phase, NiTiO₃ in photocatalytic activity of In_2(1−x)Ni_2xTiO_5−δ, samples with higher Ni content (≥15%), a pure single phase sample of NiTiO₃ was also prepared, characterized and evaluated for photocatalytic H₂ generation. Plane-wave based first-principles calculations within the density functional theory (DFT) with projector augmented wave (PAW) potentials were employed using Vienna Ab initio Simulation Package (VASP) code for calculations of ground state properties of In₂TiO₅, In_1.8Ni_0.2TiO_5−δ, and NiTiO₃ to understand the role played by Ni dopant ions in modifying the electronic structure of In₂TiO₅ and photocatalytic activity. Finally, the photocatalytic H₂ yield was correlated with the structure, optical and electronic properties of the material.

2. Experimental

Mixed oxides with nominal composition, In_2(1−x)Ni_2xTiO_5−δ, for 0.0 ≤ 2x ≤ 0.4, were synthesized through ceramic route (similar to our earlier procedure^16,17) using pre-dried In₂O₃, TiO₂ and NiO (99.99% purity) as starting materials and mixing them in appropriate stoichiometry as depicted by following equation:

(1 − x)In₂O₃ + TiO₂ + 2xNiO → In_2(1−x)Ni_2xTiO_5−δ (1)

The pellets (∼2 g) of homogeneous mixtures were calcined at 900 °C for 24 h, 1000 °C for 24 h and finally at 1250 °C for 12 h in air, with intermittent grindings so as to ensure the uniformity and the completion of the reaction.

For evaluating the contribution of secondary phases in the photocatalytic activity of In_2(1−x)Ni_2xTiO_5−δ, a sample consisting of single phased NiTiO₃ was also synthesized by solid state route. Pre-dried NiO and TiO₂ in the molar ratio of 1 : 1 were weighed, grinded for 40 minutes and mechanical mixture was subjected to a heat treatment of 700 °C for 83 h and further heated to 1000 °C for 78 h in air. Sample was grinded at intermediate levels and reaction pathway was monitored by recording powder XRD.

The powder XRD patterns were recorded on a Philips diffractometer (model PW 1710), equipped with a graphite monochromator and Ni-filtered Cu-K_α radiation.

Structural analysis has been done by using Rietveld refinement program Fullprof-2005.²² First of all, the background parameters and scale factor were adjusted. The background was fitted with sixth order polynomial. The diffraction peak profile was fitted with pseudo-Voigt profile function and then the FWHM parameters were adjusted. No absorption parameter was considered during refinement. Subsequently, individual thermal parameters were refined. Finally, positional parameters were refined. A Quantachrome Autosorb-1 analyzer was employed for measurement of N₂-BET surface area by recording the nitrogen adsorption isotherms.

For microstructural examination under a scanning electron microscope, (SEM) a thin layer of gold (100 Å) was coated on the calcined and sintered pellets at 1250 °C by thermal evaporation in a vacuum coating unit. A measured quantity of gold wire (99.99% pure) was wrapped on a tungsten wire which in turn was heated to evaporate the gold wire. The images of gold coated samples were recorded on scanning electron microscope Mirero, Korea, model-AIS2100.

Band gap measurements of all semiconductor oxide samples was estimated by recording their Diffuse reflectance UV-visible spectra using spectrophotometer of JASCO model V-530, Japan, scanned in range of 200–1000 nm at the scanning speed of 200 nm min⁻¹.

The photocatalytic activities of the samples for hydrogen generation from water–methanol mixture were carried out under irradiation of a medium-pressure mercury lamp (Hg, Ace Glass Inc., 450 W) placed in an outer irradiation-type quartz cell surrounded with water circulation jacket to absorb IR irradiation. The said lamp exhibits very broad range emission spectra having maxima at both UV and the visible range with the UV part being only 16% of the whole spectra.^14,17 The activity measurements were carried out in a rectangular quartz cell (2.1 × 2.1 × 8 cm³) equipped with a sampling and evacuation ports, at room temperature, placed horizontally in a chamber close to a water-cooled medium pressure mercury vapor lamp (400 W). The catalyst (0.1 g) was suspended in distilled water (10 ml) and methanol (5 ml). The reaction mixture was evacuated and irradiated under the medium pressure mercury lamp. After irradiation, the amount of hydrogen evolved was analysed by gas chromatograph (TCD, molecular sieve, 5 m length and Ar carrier). The number of photons falling on the reaction cell or flux of the light was determined by light flux meter. The flux observed under sunlight-type lamp after 2 h, in horizontal geometry was 19 × 10⁴ lux or 278.2 W m⁻². The apparent quantum efficiency of the samples was determined using calculations mentioned in our earlier reports.¹⁷

First principle calculations

Plane-wave based first-principles calculations within the density functional theory (DFT) with projector augmented wave (PAW)²³ potentials were employed using Vienna Ab initio Simulation Package (VASP) code²⁴ for calculations of ground state properties of three structures, viz., In₂TiO₅ (orthorhombic, Pnma), NiTiO₃ (rhombohedral, R-3) and a supercell (In, Ni)₂TiO₅ containing 18.75 at% of Ni. The PAW potentials used [3p4s3d] state of Ti atom, [2s2p] state of O atom, [5s5p] state of In atom and [3p3d4s] state of Ni atom as valence states. Generalized gradient approximation (GGA) for exchange and correlations potential as parameterized by J. P. Perdew, K. Burke and M. Enzerhof (PBE)²⁵ were used. For Ni-doped In₂TiO₅ and NiTiO₃, we employed spin-polarized calculations using GGA + U_eff approach, where U_eff is the strong intra-atomic (on-site) Hubbard term which was taken to be 3.5 eV for both the cases. The expansion of electronic wave functions in plane waves was set to a kinetic energy cut-off (E_cutoff) of 450 eV for all the structures. The Brillouin-zone was sampled using Monkhorst–Pack k-point mesh.²⁶ For each structure, optimization was carried out with respect to a k-point mesh and E_cutoff to ensure convergence of the total energy to within a precision better than 1 meV per atom. The converged k-point meshes used were 6 × 12 × 3 (36 k-points in the irreducible Brillouin zone (IBZ)), 4 × 8 × 8 (36 k-points in the IBZ) and 9 × 9 × 3 (44 k-points in the IBZ) for ITO, Ni-doped ITO and NiTiO₃, respectively. Methfessel–Paxton technique²⁷ was used for free energy calculations with a modest smearing of 0.1 eV, which resulted in a very small entropy term (<0.1 meV per atom) in all the cases. The structural relaxations (b/a, c/a ratio and atomic positions) were performed for each structure using the conjugate gradient algorithm until the residual forces on the atom were less than 0.01 eV Å⁻¹ and stresses in the equilibrium geometry were less than 5 × 10⁻² GPa. The total electronic energy and density of states (DOS) calculations were performed using the tetrahedron method with Blöchl corrections.²⁸

3. Results

Table 1 lists the abbreviations and phases identified from the powder XRD patterns of all the In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x ≤ 0.4) samples prepared by the conventional solid state route.¹⁶ Henceforth, in this article, the samples will be designated by their abbreviations. Fig. 1 show the powder XRD patterns of In₂TiO₅ and corresponding patterns observed due to substitution of Ni²⁺ in place of In³⁺. The XRD pattern of ITO matches well with that of orthorhombic In₂TiO₅ (JCPDS card no. 82-0326, space group Pnma) oxide. The XRD patterns of ITN05 (In_1.95Ni_0.05TiO_5−δ) and ITN1 (In_1.9Ni_0.1TiO_5−δ) compositions closely matches with the XRD patterns of unsubstituted indium titanate indicating that lower extent of Ni substitution results in single phase materials due to formation of solid solution of Ni with the lattice of In₂TiO₅. However, peaks are shifted slightly towards higher 2θ values as compared to those of the pristine samples, which are attributed to the decrease in lattice spacing due to substitution of smaller Ni²⁺ ions in place of larger In³⁺ ions. On further Ni substitution i.e. samples having Ni content 2x ≥ 0.2 in In_2(1−x)Ni_2xTiO_5−δ a very low intensity line (marked with #) was observed which corresponds to the 100% peak of NiO (JCPDS card no. 47-1049). Thus, for these compositions in addition to parent phase, NiO phase segregated in very small proportion. Further, for samples having higher Ni content i.e. 2x ≥ 0.3, a third phase of NiTiO₃ (JCPDS card no. 33-0960, peaks marked with *) was also observed in addition to the NiO and ITO (Fig. 1). XRD pattern of single phased NiTiO₃ sample prepared by solid state route calcined at 1000 °C for 78 h was also recorded and shown in Fig. 1. It matched with rhombohedral NiTiO₃ phase (JCPDS Card no. 33-0960). Rutile TiO₂ and NiO phase were also observed as impurities.

Table 1 Identification of phases in In_2(1−x)Ni_2xTiO_5−δ, (0.0 ≤ 2x ≤ 0.4) samples, their abbreviations, lattice tolerance and band gap

Sample no.	Nominal composition	Ni content (2x)	Abbreviation ITN (2x)	Phase identification by XRD	Tolerance factor t	Band gap (indirect)	Surface area (m^2 g^−1)
1	In2TiO5	0.00	ITO	Single phase, In2TiO5	0.751	3.02	4.4
2	In1.95Ni0.05TiO5−δ	0.05	ITN05	Single phase, In2TiO5	0.75	3.00	4.6
3	In1.9Ni0.1TiO5−δ	0.1	ITN1	Single phase, In2TiO5	0.749	2.96	4.8
4	In1.8Ni0.2TiO5−δ	0.2	ITN2	In2TiO5 & NiO	0.747	2.8	4.9
5	In1.7Ni0.3TiO5−δ	0.3	ITN3	In2TiO5, NiO & NiTiO3	0.745	2.75	4.5
6	In1.6Ni0.4TiO5−δ	0.4	ITN4	In2TiO5, NiO & NiTiO3	0.743	2.67	3.8
7	NiTiO3			NiTiO3, NiO & TiO2			3.6

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Fig. 1 XRD patterns of In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x ≤ 0.4) samples. For comparison, single phased NiTiO₃ was synthesized under similar conditions and XRD was recorded. # indicates peaks due to unreacted NiO phase and * indicates the peaks arising from NiTiO₃ phase.

To determine the crystal structure under investigation and to confirm the site of dopant cation Ni²⁺ in the crystal lattice Rietveld refinement of the diffraction patterns were carried out. It is pertinent to mention here that the presence of a small amount of these secondary phases in the samples, as mentioned above, was taken into consideration for Rietveld analysis. The Rietveld refined profiles of X-ray diffraction data of various In_2(1−x)Ni_2xTiO_5−δ samples as a function of doping are shown in Fig. 2. The substituted samples were found to be isostructural with the parent indium titanate phase and Ni²⁺ was found to substitute only at one of the 4c In³⁺ site. This was confirmed on the basis of minimization of Rietveld parameters. The typical variation in cell parameters and cell volume as a function of nickel content in In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x ≤ 0.4) samples, as deduced from the Rietveld analysis of corresponding powder XRD reflections, is shown in Fig. 3. As seen in Fig. 3d, the cell volume decreases as a function of nickel content (2x) as long as 2x < 0.3, with almost a linear decrease upto 2x < 0.2. Thereafter, the cell expands when Ni content 2x > 0.3. The lattice parameters a, b and c also shows a similar trend. Due to incorporation of smaller cation Ni²⁺ by substitution of a larger cation In³⁺ and consequent formation of single phase In₂TiO₅ (except for a negligible NiO phase for 2x = 0.2), there is a linear decrease in the values of cell parameters and cell volume initially upto 2x ≤ 0.2. But, in compounds with Ni doping content greater than 2x ≥ 0.3, the impurity phase of NiTiO₃ also appears and increases its volume fraction significantly with an increase in Ni-content. The offset from linearity and then an increase in lattice parameters can be explained from the segregation of the impurity phases. Although, the occupancy of Ni²⁺ increases with doping, the parent lattice has a maximum capacity to substitute a large cation by a smaller cation. When the maximum capacity for such substitution exceeds, secondary phases appear keeping the parent crystal phase as the major phase. Such, segregation of secondary phases and its growth, due to substitution of In³⁺ by Ni²⁺ in oxide lattices was also observed in InTaO₄ by Zou et al.²⁰

Fig. 2 Rietveld refined profiles of X-ray diffraction data of In_2(1−x)Ni_2xTiO_5−δ samples, where 2x = (a) 0.0, (b) 0.05, (c) 0.1, (d) 0.2 (e) 0.3 and (f) 0.4. The dots represent the observed data, while the solid line through dots is the calculated profile, and vertical tics represent Bragg reflections for the phase. The difference pattern is also shown below the vertical tics in each case.

Fig. 3 The variation of lattice parameters with Ni content (2x) in In_2(1−x)Ni_2xTiO_5−δ samples.

It is known²⁰ that the structural stability of oxides consisting of octahedra such as ABO₃, can be estimated by calculating the tolerance factor defined as t = (r_A + r_O)/√2(r_B + r_O) where, r_A, r_B and r_O are the radii of the respective ions and t is unity for an ideal cubic lattice, while greater the deviation in t from unity, more the distortion in structure. In this case, r_A is a maximum for the undoped ITO compound and as a consequence, as shown in Table 1, the t value decreases with increasing Ni content 2x in In_2(1−x)Ni_2xTiO_5−δ, the t value becoming smallest for ITN4, suggesting that ITO lattice is destabilized by smaller cation Ni²⁺ at In site. Rietveld refinement of the powder XRD reflections (Fig. 1–3, Table 1), clearly establish the structural changes arising out on nickel ion incorporation into lattice of ITO i.e. lattice contraction and secondary phase separation. This structural modification arising out of Ni ion substitution has a significant effect on photoactivity which is elaborated in “Discussion” section.

All samples are highly crystalline with large crystallite size (>100 nm) and very low N₂-BET surface area (3–5 m² g⁻¹) as mentioned in Table 1. Fig. 4 shows the SEM images of the undoped ITO and doped samples. The ITO showed faceted structure with clear grain boundaries and Ni doping resulted in loss of faceted shape as we also observed earlier on Nd doping.¹⁷ The Ni doped samples also showed distinct grain boundaries but the particles exhibited non-uniform shape and size. All the particles were in the micrometer range.

Fig. 4 SEM images of the indium titanate and Ni substituted samples.

Fig. 5 illustrates the UV-visible absorption spectra of In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x ≤ 0.4), showing that the light absorption properties of these compounds are characteristic of photocatalysts able to respond to UV-visible light. Curve in black is the UV-vis absorption spectrum of indium titanate. It is evident from Fig. 5, that with the substitution of Ni, there is a progressive red shift in the absorption pattern of indium titanate. The indirect optical band gap of Ni substituted materials were estimated from plots of (αhν)^1/2vs. the photon energy, hν and is listed in Table 1. One of the most characteristic features is that the bandgap is narrowed with Ni doping. There are two indirect band gap in indium titanate one is manifested (E_g = 3.02 eV) in its optical spectra whereas other (E_g = 1.6 eV) is observed as shoulder.^12,14,17 On Ni substitution the band gap decreased to ∼2.8 eV in the 10% doped sample ITN2, while the most extreme sample ITN4 exhibited a band gap of ∼2.6 eV. As nickel content increases, a shoulder at 455 nm and a broad band at 735 nm were observed in ITN3 and ITN4 samples. These coincide with the reported absorption bands of NiTiO₃ at 450 nm due to Ni²⁺ → Ti⁴⁺ charge transfer (CT)²⁹ and a broad visible light absorption at 740 nm.^30,31

Fig. 5 DRUV spectra of the indium titanate samples (along with NiTiO₃) which shows a progressive red shift in absorption with an increase in Ni content. Shoulders at 455 nm and 735 nm corresponds to NiTiO₃ phase.

Detailed electronic structure as deduced from density of states calculations for ITO, ITN2 and NiTiO₃, using the DFT calculations will be shown later and explains that these optical transitions are arising from its electronic structure.

Results of photocatalytic activity of In₂TiO₅ and Ni substituted indium titanates, In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x ≤ 0.4) samples for hydrogen generation from water in presence of methanol as a sacrificial agent was evaluated. The hydrogen generated during different time courses for all the samples is shown in Fig. 6. The hydrogen yield increased linearly with time. Pristine indium titanate is able to generate hydrogen photocatalytically from water in presence of sacrificial agent methanol as reported in earlier publications.^12,17 From the present study it is evident that Ni doping modifies the photoactivity to a great extent. On Ni-doping the photoactivity for hydrogen generation increases initially and the reaches a maximum at 10% doping and the again falls with the highest Ni-content sample of 20% doping showing lower activity than the base indium titanate itself. For comparison, photocatalytic activity of single phased NiTiO₃ sample is also shown. A pure sample of NiO was also tested under identical conditions and was found to be a poor photocatalyst. Hydrogen yield of 2.5 μmol after 6 h of irradiation over 0.1 g of NiO was observed.

Fig. 6 The time dependent photocatalytic H₂-yield using different Ni doped indium titanates and single phased NiTiO₃ sample. Reaction conditions: 0.1 g catalyst, 10 ml distilled water, 5 ml methanol. Light source; UV-visible medium-pressure mercury lamp (Hg, Ace Glass Inc., 450 W) surrounded with water circulation jacket to absorb IR irradiation.

The rate of photocatalytic hydrogen generation as a function of Ni ion concentration is shown in Fig. 7. It is evident from Fig. 7 that an optimal dosage of 10% Ni²⁺ ion doping, ITN2 resulted in significant enhancement in rate of photocatalytic H₂ yield (270 μmol g⁻¹ h⁻¹) while, In₂TiO₅ have shown 138 μmol g⁻¹ h⁻¹ of H₂ generation rate. The decreasing order of catalytic activity is as follows: ITN2 > ITN1 > ITO > ITN3 > ITN4. Deterioration in H₂ yield over ITN3 and ITN4 was observed which can be attributed to presence of NiTiO₃ phase. To evaluate the contribution of NiTiO₃ phase it was synthesized by solid state route and its photocatalytic activity was determined under UV-visible irradiation as shown in Fig. 6. NiTiO₃ phase was found to be photocatalytically active and yielded H₂ at a rate of 18 μmol g⁻¹ h⁻¹. It was observed that rate of hydrogen generation over NiTiO₃ is ∼7.5 times and ∼15 times less than parent oxide ITO and ITN2 respectively. This phenomenon is in agreement with literature reports, viz. Melian et al. have demonstrated that NiTiO₃ phase which appeared at higher calcinations temperature was detrimental to the photoactivity of highly efficient Ni/N modified TiO₂ samples.^32,33 The apparent quantum efficiency (η) calculated for different samples are depicted in Fig. 8. Maximum A.Q.E of 1.68% is obtained with ITN2 among all samples which is more than two fold when compared to undoped semiconductor ITO whose A.Q.E. was found to be 0.86%.

Fig. 7 The rate of photocatalytic hydrogen generation in μmol g⁻¹ h⁻¹ from water–methanol mixture as a function of Ni ion content.

Fig. 8 Plot of the apparent quantum efficiency (η) for the undoped and doped indium titanate samples and NiTiO₃ for comparison.

The electronic structure of ITO, ITN2 and NiTiO₃ were calculated using VASP code in order to elucidate the distribution of valence states of Ti, In, O atoms near the Fermi level and to understand the role of Ni ions in modifying the band structure of indium titanate and improving the photocatalytic properties. In our earlier communication,¹⁷ isovalent doping of rare earth, neodymium ions (Nd³⁺) in ITO was investigated with an objective to suppress the formation of nonstoichiometric defects which would otherwise work as nonradiative recombination centers between photogenerated electrons and holes. Here, we have attempted to investigate the role of aliovalent substitution by transition metal ions, Ni²⁺ ions at A-site in In₂TiO₅ lattice. Since orbital energies of Ni, (transition metal d⁸, valence orbitals are 3d and 4s) and Nd (f block, valence orbitals, 4f, 6s and 5d) are very different and both belongs to different class of elements, the implications of the individual substitution in indium titanate was found to be markedly different. Fig. 9, 11 and 13 show calculated band structures along a few high-symmetry directions while Fig. 10, 12 and 14 show site- and l-projected partial density of states (DOS) of ITO, ITN2 and NiTiO₃ respectively. The band structure of parent oxide, indium titanate has been discussed in details in our earlier publication by TBLMTO.¹⁷ Here we have calculated the band structure by VASP code and the findings are in good agreement with our earlier results. In brief, indium titanate was found to have two band gaps (Fig. 9), one direct band gap between the LUMO and HOMO at 1.6 eV and another indirect band gap of 3.2 eV (close to experimental 3.02 eV). Indium 5s states are of large dispersion with rather small DOS (Fig. 10), thus indirect band gap of 1.6 eV is not flat band to flat band transition, hence is less probable. Evidently, this transition manifests itself as a shoulder in the experimental optical spectrum.¹² Band structure calculations on unsubstituted ITO also revealed the large dispersion of In-5s states in conduction band. This model is further used in the present study to understand the performance of Ni-doped indium titanate photocatalysts.

Fig. 9 Band structure of ITO along high-symmetry Γ(0,0,0), X(1/4,0,0), Y(0,1/2,0), Z(0,0,1/8), S(1/4,1/2,0), R(1/4,1/2,1/8), T(0,1/2,1/8), and U(1/4,0,1/8) directions.

Fig. 10 Site- and angular momentum-projected partial density of states for ITO showing the valence band to be mainly composed of O-p, Ti-d, and In-p,d states. We got almost similar results in our earlier calculations.¹⁷

For ITN2, a 1 × 2 × 1 supercell of ITO having 18.75 atom% of Ni substituting indium sites, was considered. Fig. 11 shows the band structure of ITN2 plotted along high symmetry directions, viz., Γ(0,0,0), B(1/4,0,0) and F(0,1/2,0). As can be seen from Fig. 11 the band structure plot, the flat bands in the valence as well as conduction bands are found between Γ- and B-points and close to the F-point. The minimum direct band gap of ∼1.3 eV is observed along the Γ-point between a flat band in the valence band and that in the conduction band; while the maximum indirect band gap is ∼2.4 eV. The transition from highest occupied states at Γ point to the unoccupied states at B or F, exhibits an indirect gap of ∼1.6 and ∼2.4 eV respectively. The multi-peaked structure of the total DOS is indicative of strong hybridization and layered structure as evident from Fig. 12. The strong mixing of Ni-3d with In-4d states, close to the Fermi energy and with In-5p, in the middle part of the valence band was observed. The lower energy part of the conduction band shows weaker hybridization of In-5s and Ti-3d states as compared to that in ITO, while the O-2p states in the valence and the conduction band remain mostly unchanged on Ni-doping. We observed a rise in Fermi energy level by 0.42 eV and also a decrease in energy of the conduction band from our calculations by GGA + U method. This gives an illustration of how a change in functionality can be achieved by engineering the electronic structure through modification of structure and composition of a material.

Fig. 11 Band structure of ITN2 along high-symmetry Γ(0,0,0), B(1/4,0,0) and F(0,1/2,0) directions.

Fig. 12 Site- and angular momentum-projected partial density of states for ITN2 supercell, SC1, showing the effect of Ni-d,s states on the band gap near the Fermi energy and also the modifications in the contributions of other orbitals towards valence and conduction bands.

Nickel substitution beyond 10% has resulted in segregation of NiTiO₃ phase. The decrease in photocatalytic activity of ITN3 and ITN4 samples coincided with appearance of this new phase. Thus, to investigate the electronic properties and factors responsible for lowering the activity, the band calculations using VASP code were performed on single phase NiTiO₃. The band structure and DOS plots of NiTiO₃ are plotted in Fig. 13 and 14 respectively. Band structure of NiTiO₃ are plotted along high symmetry directions, Γ(0,0,0), L(0,1/2,0), Z(1/2,1/2,1/2) and F(1/2,1/2,0). The flat bands in the valence as well as conduction bands are found between Γ- and Z- and close to the F-points. The minimum direct band gap of ∼1.7 eV is observed along the Γ-point; while the maximum direct band gap of ∼2.4 eV at Z-point is observed between a flat band in the valence band and that in the conduction band (Fig. 13). The site- and angular momentum density of states (DOS) plots shown in Fig. 14 exhibits mixing of O-2p and Ni-3d states in the valence band unlike ITN2 sample. It also exhibits a tiny pseudo band gap of about 0.3 eV at −1.4 eV as shown in inset of Fig. 14. This band gap separates the bonding and antibonding states in the valence band indicating strong mixing of Ni-3d and O-2p states attributed to the covalent character of Ni–O bonds. The lower energy part of the conduction band exhibits strong mixing of Ti-3d and O-2p states. Above calculations also reveals that NiTiO₃ is a wider band gap material as compared to ITN2 sample.

Fig. 13 Band structure of ilmenite NiTiO₃ along high-symmetry Γ(0,0,0), L(0,1/2,0), Z(1/2,1/2,1/2) and F(1/2,1/2,0) directions.

Fig. 14 Site- and angular momentum-projected partial density of states for NiTiO₃ showing the valence band to be mainly composed of O-p and Ti-d. It can be seen from DOS for Ni-d that there is existence of a pseudo band gap of ∼0.3 eV at −1.4 eV (enlarged view shown in inset).

4. Discussion

The detailed investigation of crystal structure by Rietveld refinement of the observed XRD patterns, optical absorption properties by DRUV-visible spectra, electronic properties by band calculations using first principles study and photocatalytic activity evaluation for H₂ evolution over In_2(1−x)Ni_2xTiO_5−δ (0.0 ≤ 2x≤ 0.4) compounds enabled us to emphasize the role of Ni substitution in ITO, in modifying the structural, optical and electronic properties and its consequent effect on photoactivity.

As already discussed in our previous reports^17,21 and by others^12,14 the undoped oxide, orthorhombic In₂TiO₅, has a favourable crystal structure for the photocatalytic generation of hydrogen by virtue of its structure and electronic properties. A comparison of the Ti–O bond distance/angles in octahedral [TiO₆], the density and crystal packing factor of In₂TiO₅ and TiO₂ is shown in Table 2. It reveals that the coordination environment of Ti in In₂TiO₅ is more open and flexible than that in TiO₂ – by virtue of the more distorted TiO₆ octahedral units and a lesser value of the crystal packing factor. It is considered from structure–activity correlations that photocatalysts with more open structures are more catalytically active.^{2,12,13,21,34,35} The polyhedral arrangement of In₂TiO₅ as calculated from our Rietveld refinement results, is shown in Fig. 15, with the yellow octahedral being that of [InO₆] and the green octahedral that of [TiO₆]. The [InO₆] octahedral units are edge shared and forms infinite chains while the [InO₆] and [TiO₆] octahedral units are corner shared. The presence of such octahedral InO₆ polyhedra in the crystal structure has been found to have a positive effect on the photocatalytic properties in some compounds like InNbO₄,³³ InTaO₄ ^20,36,37 or even AgInW₂O₈ ³⁸ and In₆WO₁₂.³⁹ This is because the polyhedral units which are either edge or corner shared forming infinite chains, helps in charge transfer to the surface which is an important requirement for a proper photocatalyst.^9,20 Further, our band calculations on In₂TiO₅ revealed large dispersion of In-5s states in the conduction band, and the presence of an indirect optical band gap. Upon photoabsorption in the semiconducting indium titanate the electron goes to the largely dispersed In-5s states and is transferred to the surface, via chains of InO₆ octahedra, where reduction occurs resulting in hydrogen production. The presence of the optically indirect transition restricts the electron–hole recombination to a certain extent. Thus, both the crystal structure and band structure are in favour of photon energy storage to assist the photocatalytic activity of In₂TiO₅.¹⁷

Table 2 Comparison of the octahedra [TiO₆], density and crystal packing factor of In₂TiO₅ and TiO₂

Name of the compound	Bond distance Ti–O (Å)	Bond angle O–Ti–O (°)	Crystal packing factor (%)
In2TiO5 (our results)	1.8087, 1.8230 (×2), 2.0323, 2.0386, 2.1853	73.53–179.52	68.0
Anatase TiO2 (ref. 2)	1.964 (×2), 1.937 (×4)	77.64–179.98	70.2
Rutile TiO2 (ref. 2)	1.988 (×2), 1.944 (×4)	80.86–180.00	76.6

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Fig. 15 The (A) structure of In₂TiO₅ as derived from the Rietveld refinement of the XRD pattern, also showing the unit cell. The (B) structure showing the polyhedral arrangement, with the yellow octahedral being that of InO₆ while the green octahedral that of TiO₆.

Effect of Ni²⁺ substitution on photocatalytic activity of indium titanate depends on the extent to which Ni is incorporated in the photocatalyst. An increase in photocatalytic activity for hydrogen generation upto 10% Ni²⁺ ion substitution in ITO lattice was observed which can be attributed to a combination of structural, optical and electronic factors, and is discussed in detail below.

From the structural point of view, the substitution of smaller Ni²⁺ cation in place of the larger In³⁺ cation in the ITO lattice resulted in the decrease in In–In distance which is expected to decrease the volume of InO₆ octahedra, and since they participate in charge transfer, rate of electron transfer to the surface must have enhanced as compared to pristine indium titanate. Thus, the photoactivity increased on substitution. Optically, there was a decrease in band gap of indium titanate on Ni-doping. The smaller band gap as seen from DRUV result facilitated the photoinduced electron excitation from the valence band to the conduction band in the doped oxide semiconductor, thus increasing the photocatalytic activity of the material. The electronic structure calculations revealed the underlying reasons for this decrease in band gap and also disclosed the benefits obtained in charge carrier properties of indium titanate on Ni doping. The In-4d and Ni-3d states in the valence band showed a sharp peak near the Fermi level representing their localized nature (smaller dispersion resulting in flat bands) as seen in Fig. 12. Thus, the strong optical transitions were due to flat bands from valence band to flat bands in conduction band (Fig. 11) and these are found along Γ to B or Γ to F point. These transitions are responsible for improved optical properties of ITN2 because of enhanced probabilities arising from these flat bands to flat band transitions. Further, the strong intermixing of Ni-3d states with In-4d and In-5p have increased the energy of valence band. Thus valence band is raised towards the conduction band by 0.42 eV in ITN2 sample as compared to ITO sample. Besides, a decrease in energy of the conduction band was also observed. Taking into account all these factors, the modified band structure of ITN2 is shown in Fig. 16. Narrowing in band gap on Ni substitution is attributed to rise in Fermi energy level by 0.42 eV, and decrease in energy of conduction band. Thus, a net lowering of band gap (Fig. 16) resulted on Ni doping. Whereas, in our earlier study¹⁷ we found Nd to be one of the elements that was able to make a valence-band position higher than O-2p orbitals considerably and resulted in narrowing the band gap in ITO by 0.82 eV. Regarding charge carrier properties, the density of states calculations showed strong hybridization of Ni-3d and In-4d states, and also Ni-3d and In-4p mixing. The conduction band maximum (CBM) was found to be a hybrid state between Ni, Ti and In orbitals. That is, the photo-induced charges can be transported from zigzag chains of [·Ni–O–Ti⋯In–O–Ti----]. The hybrid state in CBM and the substitution – induced specific connections among the local structures enhanced the mobility for the transportation and separation of electron–hole pairs. All the above factors helped in increase in photoactivity of indium titanate on Ni doping. One interesting fact was that the formation of secondary phase NiO was seen in ITN2 sample but it was not detrimental to the photocatalytic activity. NiO homogeneously distributed on the photocatalyst surface act as an electron trap and is suitable for photoreduction as reported by other authors.^9,20,40,41

Fig. 16 Schematics of modification of band structure of indium titanate semiconductor photocatalyst on Ni doping at In site corresponding to ITN2 sample. Schematic of band structure for single phased NiTiO₃ is also shown for comparison.

Further, 15–20% substitution of Ni resulted in segregation of another phase NiTiO₃. As the appearance of this phase coincided with deterioration of photocatalytic activity, it may be responsible for lowering of photoactivity of indium titanate, a phenomenon also observed by other authors.^32,33 The ilmenite structure of NiTiO₃, consists of a pseudo close-packed-hexagonal array of O²⁻ ions with two thirds of the octahedral sites of any basal plane occupied by like cations in an ordered hexagonal net.⁴³ It is reported that traditional synthetic procedures yields large NiTiO₃ particles with low surface areas and pore volumes due to inherent problems such as high reaction temperature and heterogeneous solid phase reactions.^42,43 Co-formation of symmetric rhombohedral NiTiO₃ phase along with flexible orthorhombic Ni-doped In₂TiO₅ imparts unfavourable photocatalytic properties to ITN3 and ITN4 particles. From the electronic properties and band calculations of NiTiO_3, (Fig. 13 and 14), it is revealed that NiTiO₃ is a wider band gap material as compared to ITN2 sample. Electronic transitions from O²⁻ (2p⁶, valence band edge) → Ti⁴⁺ (conduction band) takes place under UV light illumination. Under visible light, Ni²⁺ → Ti⁴⁺ transitions takes place, but the geometry of the NiTiO₃ ilmenite, makes the oscillator strength for the Ni²⁺ → Ti⁴⁺ charge transfer too weak to have an acceptable photoresponse to visible light. In NiTiO₃ strong intermixing of Ni-3d with O-2p orbitals in valence band takes place (Fig. 14) unlike ITN2 where O-2p states in the valence and the conduction band remain mostly unchanged on Ni-doping in indium titanate lattice (Fig. 12). The pseudo band gap of 0.3 eV generated in the valence band of NiTiO₃ at −1.4 eV (Fig. 14) is outcome of intermixing of orbitals due to strong covalent character of Ni–O bonds. This pseudo band gap might act as hindrance and may contribute in weakening the intensity of electronic transition due to Ni²⁺ → Ti⁴⁺ charge transfer. Therefore, presence of NiTiO₃ phase in small amounts also affects the photoactivity adversely in ITN3 and ITN4. Other reports by Salvador et al.⁴⁴ have also shown that NiTiO₃ is not a good candidate for water photoelectrolysis in a photoelectrochemical cell, while Melian et al.³² has shown that the appearance of NiTiO₃ phase was detrimental to the photoactivity of highly efficient Ni/N modified TiO₂ samples. The combination of a proper band gap and crystal structure was present in the ITN2 sample for having the highest photocatalytic activity among these semiconductor oxides.

5. Conclusion

In the present communication, we have emphasized the suitability of indium titanate, In₂TiO₅ as a photocatalyst and investigated the role of Ni²⁺ ion substitution at A-site in modifying its crystal structure, optical and electronic properties leading to modified photocatalytic activities for hydrogen generation. From our investigations, we conclude that the optimal dosage of 10% Ni substitution in indium titanate has favourably tailored the crystal structure, optical and electronic properties for having the highest photocatalytic activity among these semiconductor oxides. This is noteworthy information needed in order to design new visible light-driven photocatalysts.

Acknowledgements

The authors acknowledge Materials Chemistry Section, Chemistry Division, BARC for recording the SEM images. Research grant received from Department of Science and Technology (DST) New Delhi (No. DST/TSG/SH/2011/106) is gratefully acknowledged.

References

1. A. Fujishima and K. Honda, Nature, 1972, 238, 37.
2. A. L. Linsebigler, G. Lu and J. T. Yates, Jr, Chem. Rev., 1995, 95, 735.
3. T. Tachikawa, M. Fujitsuka and T. Majima, J. Phys. Chem. B, 2007, 111(14), 5259.
4. M. Kitano, M. Matsuoka, M. Ueshima and M. Anpo, Appl. Catal., A, 2007, 325, 1.
5. M. Ni, M. K. H. Leung, D. Y. C. Leung and K. Sumathy, Renewable Sustainable Energy Rev., 2007, 11, 401.
6. G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal., B, 1998, 18, 1.
7. M. Woodhouse and B. A. Parkinson, Chem. Soc. Rev., 2009, 38, 197.
8. A. M. Banerjee, A. R. Shirole, M. R. Pai, A. K. Tripathi, S. R. Bharadwaj, D. Das and P. K. Sinha, Appl. Catal., B, 2012, 127, 36; A. M. Banerjee, M. R. Pai, S. S. Meena, A. K. Tripathi and S. R. Bharadwaj, Int. J. Hydrogen Energy, 2011, 36, 4768.
9. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278.
10. K. Maeda and K. Domen, J. Phys. Chem. C, 2007, 111, 7851.
11. X. Chen, S. Shen, L. Guo and S. S. Mao, Chem. Rev., 2010, 110(11), 6503.
12. W. D. Wang, F. Q. Huang, C. M. Liu, X. P. Lin and J. L. Shi, J. Mater. Sci. Eng. B, 2007, 139, 74.
13. M. R. Pai, A. Singhal, A. M. Banerjee, R. Tiwari, G. K. Dey, A. K. Tyagi and S. R. Bharadwaj, J. Nanosci. Nanotechnol., 2012, 12, 1957.
14. P. Shah, D. S. Bhange, A. S. Deshpande, M. S. Kulkarni and N. M. Gupta, Mater. Chem. Phys., 2009, 117, 399.
15. M. R. Pai, A. M. Banerjee, S. R. Bharadwaj and S. K. Kulshreshtha, J. Mater. Res., 2007, 22, 1787.
16. A. M. Banerjee, M. R. Pai, Jagannath and S. R. Bharadwaj, Thermochim. Acta, 2011, 516, 40.
17. M. R. Pai, J. Majeed, A. M. Banerjee, A. Arya, S. Bhattacharya, R. Rao and S. R. Bharadwaj, J. Phys. Chem. C, 2012, 116, 1458.
18. T. L. Bahers, M. Rerat and P. Sautet, J. Phys. Chem. C, 2014, 118, 5997.
19. D. W. Hwang, H. G. Kim, J. S. Lee, J. Kim, W. Li and S. H. Oh, J. Phys. Chem. B, 2005, 109, 2093.
20. Z. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627.
21. M. R. Pai, A. M. Banerjee, A. K. Tripathi and S. R. Bharadwaj, Fundamentals and Appplications of the Photoctalytic Water Splitting Reaction, in Functional Materials: Preparations, Processing and Applications, ed. S. Banerjee andA. K. Tyagi, Elsevier Insights, USA, 2012, ch. 14, pp. 579–606.
22. J. Rodrıguez-Carvajal, WinPLOTR, Laboratorie Leon Brillouin (CEA-CNRS), April 2005(LLB-LCSIM).
23. P. E. Blöchl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953.
24. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169.
25. J. P. Perdew, K. Burke and M. Enzerhof, Phys. Rev. Lett., 1996, 77, 3865.
26. H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Condens. Matter Mater. Phys., 1979, 13, 5188.
27. M. Methfessel and A. T. Paxton, Phys. Rev. B: Condens. Matter Mater. Phys., 1989, 40, 3616.
28. P. E. Blöchl, O. Jepsen and O. K. Andersen, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 49, 16223.
29. Y. Qu, W. Zhou, Z. Ren, S. Du, X. Meng, G. Tian, K. Pan, G. Wang and H. Fu, J. Mater. Chem., 2012, 22, 16471.
30. Yi-J. Lin, Y.-H. Chang, W.-D. Yang and B.-S. Tsai, J. Non-Cryst. Solids, 2006, 352, 789–794.
31. T.-D. Nguyen-Phan, C. Nguyen-Huy and E. W. Shin, Mater. Lett., 2014, 131, 217–221.
32. E. P. Melian, M. N. Suarez, T. Jardiel, J. M. Dona Rodriguez, A. C. Caballero, J. Arana, D. G. Calatayud and O. González Díaz, Appl. Catal., B, 2014, 152–153, 192.
33. I. Ganesh, A. K. Gupta, P. P. Kumar, P. S. C. Sekhar, K. Radha, G. Padmanabham and G. Sundararajan, Sci. World J., 2012, 2012, 1–16.
34. D. W. Kim, N. Enomoto, Z. Nakagawa and K. Kawamura, J. Am. Ceram. Soc., 1996, 79, 1099.
35. X. P. Lin, F. Q. Huang, W. D. Wang, Y. M. Wang, Y. J. Xia and J. L. Shi, Appl. Catal., A, 2006, 313, 218.
36. Z. Zou, J. Ye and H. Arakawa, Chem. Phys. Lett., 2000, 332, 271.
37. H. C. Chen, H. C. Chou, J. C. S. Wu and H. Y. Lin, J. Mater. Res., 2008, 23(5), 1364.
38. J. Tang, Z. Zou and J. Ye, J. Phys. Chem. B, 2003, 107, 14265.
39. H. Zhang, X. Chen, Z. Li, L. Liu, T. Yu and Z. Zou, J. Phys.: Condens. Matter, 2007, 19, 376213.
40. W. Wang, S. Liu, L. Nie, B. Cheng and J. Yu, Phys. Chem. Chem. Phys., 2013, 15, 12033–12039.
41. T. Sreethawong, Y. Suzuki and S. Yoshikawa, Int. J. Hydrogen Energy, 2005, 30, 1053.
42. Y. M. Chiang, D. Birnie and W. D. Kingery, Physical Ceramics: Principles of Ceramic Science and Engineering, John Wiley & Sons Inc, New York, 1996.
43. Y. Ni, X. Wang and J. Hong, Mater. Res. Bull., 2009, 44, 1797–1801.
44. P. Salvador, C. Gutierrez and J. B. Goodenough, J. Appl. Phys., 1982, 53, 7003.

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