Enhanced visible light absorption and photocatalytic activity of [KNbO₃]_1−x[BaNi_0.5Nb_0.5O_3−δ]_x synthesized by sol–gel based Pechini method

Abstract

Ba, Ni co-modified KNbO₃ nanocrystals (KBNNO) were synthesized by sol–gel based Pechini method. X-ray diffraction and Raman measurements reveals the phase transformation of KBNNO from orthorhombic to cubic phase with increasing doping ratio x. Particle sizes of KBNNO nanocrystals were measured to range from 50 nm to 200 nm by scanning electron microscopy. X-ray photoelectron spectroscopy and electron dispersive X-ray elemental analysis reveal the elemental composition of KBNNO samples. Ultraviolet-visible-near infrared optical absorption spectra of KBNNO show enhanced visible light absorption compared with pure KNbO₃ synthesized under the same conditions. Photocatalytic activity evaluation displays an enhanced visible light photocatalytic activity of KBNNO in degrading methylene blue in comparison with KNbO₃ and P25 TiO₂. Notably, KBNNO with x = 0.2 displays the greatest photoreactivity compared with KNbO₃ and P25 TiO₂. The mechanism responsible for the enhancement of visible light absorption by Ni²⁺ in KBNNO was discussed, highlighting the crucial role that Ni²⁺ ions have played in the sub-energy levels of KBNNO.

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1. Introduction

Lots of efforts have been devoted to photocatalysis research for clean energy sources and environmental protection. All sorts of photocataylsts have been developed for different photocatalysis applications, such as photocatalytic water splitting using solar energy and photodegradation of organic compounds and industrial dyes.^1,2 Semiconductor oxide photocatalysts, due to their unique photophysical properties, have gained particular attention. Binary oxides such as ZnO,³ Fe₂O₃ (ref. 4) and GeO₂ (ref. 5) and TiO₂ (ref. 6) semiconductor photocatalyts have been investigated for photocatalytic applications. Besides, ternary and other complex oxide systems have been increasingly explored as photocatalysts. Bi₂MoO₆ (ref. 7), InTaO₄ (ref. 8), Bi₂WO₆ (ref. 9), AgNbO₃ (ref. 10), BaTiO₃ (ref. 11), SrTiO₃ (ref. 12) and BiVO₄ (ref. 13) photocatalysts have been reported to show visible light photocatalytic activity. Among these various classes of materials studied, perovskites-based photocatalysts are of particular interest due to their unique photophysical properties.

Perovskites present a general formula of ABO₃, which generally shows a cubic lattice. But with the varying ionic radii and electronegativity of atoms at A and B sites, lattice distortion and structural phase transition may occur and give rise to different crystal fields, which will lead to different electronic and optical properties of perovskite materials.^14,15 This makes it favorable to design and alter the band structure as well as other optical properties of perovskites for photocatalytic applications. Researches have shown that the bandgap of PbTiO₃ can be tuned to achieve a better optical absorption by Ni doping.^16,17 And it has been demonstrated experimentally that Ni doped in TiO₂ and Ni in Ba₃NiM₂O₉ (M = Nb, Ta) can induce sub-bandgap and lead to enhanced light absorption in corresponding spectrum regions.^18,19 Pure KNbO₃ has a bandgap value about 3.2 eV in the UV-responsive region. But with suitable modifications of the band structure, the bandgap of KNbO₃ can be tuned or lowered and applied in visible light photocatalysis. Nitrogen doping in KNbO₃ has been studied for water splitting as well as organic pollutant degradation.²⁰ Ultraviolet and visible light photocatalytic activity of KNbO₃ nanowires deposited with Au nanoparticles are also reported.²¹ Perovskite solid solution of KNbO₃ with non-stoichiometry BaNi_0.5Nb_0.5O_3−δ (BNNO) synthesized by solid state reaction (SSR) at sintering temperatures over 1050 °C exhibits both ferroelectricity and lowered band gaps of 1.1–1.4 eV. It also shows an enhanced light absorption and a larger photocurrent density compared with classic ferroelectric (Pb,La)(Zr,Ti)O₃ (ref. 22). Zhou et al. also reported the structural phase transition and narrowed bandgap of KBNNO synthesized by SSR.²³ Junctionless solar cells built with KBNNO thin films with commercial Streaming Process for Electrode-less Electrochemical Deposition (SPEED) method were reported by Rezaie et al., showing band gap as low as 1.1 eV and also a photovoltaic effect.²⁴ It can be seen that KBNNO is a new material with great potential in photovoltaic and photocatalytic applications. It is worth to study the synthesis of KBNNO powders or bulks and to investigate their potential role as an attractive candidate for visible light driven photocatalyst. Compared with solid state reaction, Pechini sol–gel method can achieve better chemical stoichiometry and homogeneity. It can also lower the crystalline temperature of KBNNO, which is typically 1050 °C to 1250 °C in SSR.^22,23 Recent report showed that KBNNO can be synthesized at the temperature lower than 525 °C by sol–gel method.²⁵ Besides, Pechini sol–gel method also has the advantage of low-cost compared with the commercial technology of SPEED applied by Rezaie et al.

In this report, Pechini sol–gel method was employed to synthesize [KNbO₃]_1−x[BaNi_0.5Nb_0.5O_3−δ]_x (KBNNO) powders. The crystalline temperature of KBNNO was lowered to 700 °C, compared to the over 1050 °C crystalline temperature in SSR synthesis.^22,23 The perovskite crystal structures of KBNNO are confirmed by X-ray diffraction (XRD) and Raman analysis. Crystal phase transitions from orthorhombic to cubic with the increase of doping concentration are observed from the Raman spectra. Scanning electron microcopy (SEM) shows that the as synthesized KBNNO powders are composed of nano-size crystal particles. X-ray photoelectric spectroscopy (XPS) and electron dispersive X-ray (EDX) elemental analysis and reveals the chemical composition of KBNNO. Optical absorption properties KBNNO were evaluated by ultraviolet-visible-near infrared diffuse reflectance spectroscopy (UV-vis-NIR DRS). Visible and infrared broadband absorption peaks were observed in the DRS. An explanation based on crystal field theory was applied to interpret these absorption band. Photocatalytic experiment show that KBNNO with the doping ratio of x = 0.2 presents the best photocatalytic activity towards the degradation of methylene blue compared with KNbO₃ and P25 TiO₂.

2. Experimental

2.1 Synthesis of KBNNO

All chemical reagents used in this work were of analytical grade purchased from Sinopharm without further purification. Nanocrystalline [KNbO₃]_1−x[BaNi_0.5Nb_0.5O_3−δ]_x (x = 0, 0.1, 0.2, 0.3 and 0.4) perovskites were synthesized by Pechini sol–gel technique, similar to the method utilized in the synthesis of BiFeO₃ powders and thin films^26,27 and Na_0.5K_0.5NbO₃ powders.²⁸ Niobium oxide (Nb₂O₅, 99.9%) and potassium hydroxide (KOH, 97%) were mixed and calcined at 350 °C for 2 h to obtain a soluble potassium niobate (K₃NbO₄). The K₃NbO₄ was dissolved in distilled water and titrated by nitric acid (HNO₃) to form a precipitate of niobium hydroxide (Nb(OH)₅). This freshly precipitated Nb(OH)₅ was dissolved in oxalic acid and then mixed with excess ammonia hydrate (NH₃·H₂O) to gain alkalescent Nb(OH)₅ precipitate, which was chelated with citric acid to produce Nb⁵⁺–citric acid solution. And the concentration of Nb⁵⁺ ions in the precursor solution was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Then 50 mL of freshly prepared Nb⁵⁺–citric acid solution and different amounts of K₂CO₃ (99.5%), BaCO₃ (99.95%) and NiCO₃·2Ni(OH)₂·4H₂O (99.5%) were amalgamated according to the chemical stoichiometry in KBNNO samples. NH₃·H₂O was added to adjust the pH value of the solution to 7. Then ethylene glycol was added as stabilizer. The molar ratio of metal ions, citric acid and ethylene glycol was 1 : 2 : 4. The solution was heated to 80 °C with stirring for 2 h and was further heated to 110 °C for about 16 h to obtain dried gels. The dried gels were calcined at a temperature of 700 °C for 5 h to obtain the final KBNNO nanocrystals.

2.2 Characterization of KBNNO

The crystal structure of synthesized samples were characterized by X-ray diffraction (Bruker D8 X-ray diffractometer with Cu Kα radiation λ = 0.154718 nm). Morphologies of KBNNO powders were observed with a field emission scanning electron microscope (Hitachi S-4800 FESEM). EDX spectra and mapping images of KBNNO powders were obtained with a JSM F7800P field emission scanning electron microscope. Raman scattering spectra were obtained with a Raman spectrometer (inVia + Reflex Raman spectrometer with 514 nm excitation). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with an Mg/Kα source (hv = 1253.6 eV). Ultraviolet-visible-near infrared (UV-vis-NIR) optical absorption of KBNNO samples were recorded with a Cary 500 spectrophotometer equipped with integration sphere. Brunauer–Emmett–Teller (BET) specific surface areas of KBNNO nano-crystals were measured using a Quantachrome Autosorb-iQ apparatus.

2.3 Evaluation of photocatalytic activity of KBNNO

Photocatalytic activities of the as-synthesized samples were evaluated by degradation of methylene blue (MB) under visible light irradiation (500 W Xe lamp, Jiguang Special Appliances, with a λ > 400 nm filter), compared with the photocatalytic activity of commercial P25 TiO₂. The experiments were performed at room temperature as the following procedures: 50 mg of photocatalyst was ultrasonically dispersed in 50 mL of methylene blue solution (10 mg L⁻¹, pH = 7) and magnetically stirred for 60 min in dark to achieve an adsorption–desorption equilibrium. Then the suspension was sampled at an interval of 30 min. About 2 mL of the suspension was taken out and analyzed by a UV-vis spectrometer (Perkin-Elmer Lambda 750S) with deionized water as reference sample. The absorption of MB in dark by KBNNO was measured similarly in order to exclude the effect of absorption on photo-degradation of methylene blue. Each sample was tested five times repeatedly to obtain the standard deviations, which were presented as error bars in the methylene blue photocatalytic degradation curves and absorption curves. Degradation kinetics of MB under visible light irradiation has also been discussed.

3. Results and discussion

3.1 XRD analysis

Fig. 1(a) shows the XRD patterns of pure KNbO₃ and KBNNO samples with different x values. Phase composition and crystallinity properties of as-prepared samples can be observed from these patterns. The XRD patterns of KNbO₃ can be indexed to the orthorhombic crystal structure of Amm2 space group. While patterns of KBNNO with different BNNO concentration can be identified as cubic perovskites with space group of Pm3m. Besides, a composition dependent phase evolution behavior in KBNNO system can be observed. As shown in Fig. 1(b), with the increase of BNNO doping, (110) and (001) peaks merge into a single peak (001). Same behavior can be observed in Fig. 1(c) for (220) and (002) peaks, which combine to form (200) peak. This can be ascribed to the phase transition from orthorhombic to cubic phase of KBNNO. Moreover, it can be observed that with increasing BNNO content, the intensities of XRD patterns for KBNNO samples decrease as shown in Fig. 1(b) and (c). The (001) peak for x = 0.4 sample disappears in the XRD pattern. These may indicate that BNNO doping also affects the crystallinity of KBNNO nanocrystals.

Fig. 1 (a) X-ray diffraction patterns of KNbO₃ (x = 0) and KBNNO powders (x = 0.1–0.4); (b) magnified patterns at around 2θ = 22°; (c) magnified patterns at around 2θ = 45°.

3.2 Raman analysis

Raman scattering is an effective probe for resolving the phase structure due to its sensitivity to very small distortions of the crystal lattice. Fig. 2 shows the Raman scattering spectra of KNbO₃ and KBNNO samples. Peaks presented in Fig. 2 for KNbO₃ (x = 0) are all associated with KNbO₃ in crystalline form. Similar modes were observed in KNbO₃ samples in ref. 29 and 30. In general, orthorhombic KNbO₃ exhibit Raman-active optical modes of 4A₁ + 4B₁ + 3B₂ + A₂, which can be separated into translational modes of an isolated cation (K⁺) and internal modes of the NbO₆ octahedra. Regarding the equilateral octahedron symmetry (O_h) of a free NbO₆, v₁ mode near 610 cm⁻¹ represents a double-degenerate symmetric O–Nb–O stretching vibration, and v₅ mode near 280 cm⁻¹ represents a triply degenerate symmetric O–Nb–O bending vibration. And the sharp dip at 190 cm⁻¹ Raman spectrum of KNbO₃ are believed to be a fingerprint for the occurrence of long-range polar order. For Raman spectra of KBNNO samples, with the increase of BNNO content, modes related to A–O vibrations appeared at low wavenumber region, suggesting the existence in the lattice of nm-sized areas (clusters) that are rich in either K⁺ or Ba²⁺ cations.²³ While modes emerged at ∼860 cm⁻¹ is associated to the stretching of BO₆ octahedra due to the presence of Ni²⁺. The fade away of v₂ mode at ∼580 cm⁻¹ and relative intensity decrease of v₁ + v₅ mode at ∼860 cm⁻¹ indicate the phase transitions from orthorhombic to cubic phase.^30–32

Fig. 2 Raman scattering spectra of KNbO₃ (x = 0) and KBNNO (x = 0.1–0.4).

3.3 XPS analysis

To demonstrate that Ba and Ni are successfully doped in KBNNO, XPS measurement of sample with x = 0.4 was carried out. Signals related to K, Ba, Ni, Nb, O and C elements are observed in the survey scan of Fig. 3(a). No other obvious impurity elements can be found. The binding energies values are measured for the K 2p, Ba 3d, Ni 2p, Nb 3d and O 1s lines. Fig. 3(b) shows two peaks located at 291.9 eV and 294.7 eV, which can be attributed to K 2p_3/2 and K 2p_1/2 states respectively. It indicates that potassium is in the K⁺ state. The peak at 778.6 eV can be assigned to Ba 3d_5/2 signal, which is characteristic signal of Ba²⁺ and matches well with previous report. Ni 2p XPS peak in Fig. 3(d) shows two components, at 855.3 eV and 861.2 eV, which can be referred as the main peak (855.3 eV) and the satellite peak (861.2 eV). The peak at 855.3 eV has been associated with the presence of Ni³⁺ ions, Ni²⁺–OH species, Ni²⁺ vacancies, or as a result of a non-local screening mechanism. The satellite peak as 861.2 eV involves a charge transfer ligand–metal. These peaks indicate the existence of Ni element in the sample. Fig. 3(e) shows the Nb 3d core-level spectra. The two peaks located at 206.4 eV and 209.1 eV can be assigned to Nb 3d_5/2 and Nb 3d_3/2. O 1s spectra in Fig. 3(f) can be fitted into two bands. The major component at 529.3 eV is typical for the Nb₂O₅, whereas the second minor band at higher binding energy 531.0 eV is mainly related to the O atoms bonded to the Ni atoms which may arise from some Ni²⁺ replacing Nb⁵⁺ in the KNbO₃ lattice. Based on the XRD, Raman and XPS results, it is reasonable to conclude that the as-prepared samples are Ba, Ni co-modified KNbO₃.

Fig. 3 XPS of KBNNO (x = 0.4): (a) survey scan, (b) K 2p, (c) Ba 3d, (d) Ni 2p, (e) Nb 3d and (f) O 1s.

3.4 Morphology analysis and elemental analysis

SEM images KBNNO and also typical EDX elemental analysis of KBNNO with x = 0.2 are shown in Fig. 4 and 5 respectively. It can be seen that the samples are composed of cubic nano-scale crystal particles with particle size ranging from 50 to 200 nm depending on different doping ratio. And a tendency of shrinkage of particle size can also be observed from the SEM results. This may be attributed to particle growth inhibition caused by Ba²⁺ substitution at A-site of KNbO₃ (ref. 32). The EDX spectrum in Fig. 5 of KBNNO with x = 0.2 confirmed the presence of expected elements in the KBNNO powder products. Atomic percentage of K, Nb, Ba and Ni in KBNNO sample with x = 0.2 was calculated to be 9.75 ± 0.08%, 13.13 ± 0.08%, 2.63 ± 0.06%, 1.14 ± 0.14%, which is close to theoretical ratio of 0.8 : 0.9 : 0.2 : 0.1 of K, Nb, Ba and Ni ions in the sol–gel precursor of KBNNO with x = 0.2. EDX spectra and mapping images of all KBNNO samples are presented in Fig. S1 and S2 in the ESI† document. Table S1† shows the elemental ratio of all KBNNO samples obtained from EDX spectra. It can be concluded that all elements are uniformly distributed in KBNNO samples in good accordance with the theoretical ratios of KBNNO with different x value.

Fig. 4 SEM images of KNbO₃ (a), KBNNO (b)–(d) and (e) with different x value (x = 0.1–0.4).

Fig. 5 Typical EDX elemental spectrum of KBNNO with x = 0.2.

3.5 UV-vis-NIR diffuse reflectance spectra

UV-NIR absorption spectra of KBNNO with different doping ratio are shown in Fig. 6. The optical absorption of KNbO₃ contains only UV light absorption, which can be easily determined to be the band to band charge transfer absorption between O 2p to Nb 4d orbitals.¹⁸ The band gap of KNbO₃ was direct and estimated to be 3.25 eV, which well matches previous report.²¹ Compared with KNbO₃, UV light absorption bands of KBNNO powders have slight red-shifts, which makes their band gaps slightly lower than that of KNbO₃. As shown in Fig. 6(b), the obtained band gap values for KBNNO were about 3.1 eV. Besides, three minor but board absorption bands in the visible and near infrared range can be found in the spectra. The first absorption peak centers at 2.75 eV (about 450 nm). This peak overlaps with the edge of the intrinsic band gap absorption. The second absorption peak centers at 1.72 eV (about 720 nm) with a shoulder peak at 1.93 eV (about 640 nm). The third peak is an infrared absorption peak at 1.07 eV (about 1150 nm).

Fig. 6 (a) UV-vis-NIR absorption spectra of KNbO₃ (x = 0) and KBNNO (x = 0.1–0.4); (b) plots of (αhv)² versus hv for the absorption spectra (inset).

These broadband absorptions can be ascribe to the spin-allowed d–d transitions existing in many transitional metal compounds.^19,33–36 Electronic states involved in these transitions can be qualitative described using crystal field theory.³⁶ The optical absorptions of transitional metal oxides are, in general, determined by the d level orbitals of transitional metal and 2p level of ligand O atom. The direct charge transfer transition between metal and ligand atoms give rise to the intrinsic conduction band to valence band absorption. While the d–d transitions in the metal with d_n electronic configuration cause the multiple minor absorptions in a wide wavelength range. For pure KNbO₃, the conduction band and valence band are O 2p states and Nb 4d states respectively. The charge transfer transition from O 2p to Nb 4d states caused the strong UV band gap absorption. For KBNNO, the band gap absorption is still determined by the O 2p to Nb 4d transition. The slight red shifts of absorption edge may due to the interaction between the lowest d electrons states and O 2p states, which shift up the conduction band a bit and thus lowered the band gap. And Ni²⁺ ion has the electronic configuration of [Ar] 3d8s0. The d electron states of Ni²⁺ ions will split in the crystal field of octahedron NiO₆. The unusual absorption spectrum in the visible and near infrared range can be ascribed to three spin allowed transitions.³⁶ The broadband near infrared centered at 1.07 eV (1150 nm) could be attributed to the ³A_2g(³F) → ³T_2g(³F) transition. And the band centered at 1.72 eV (720 nm) could assigned to the ³A_2g(³F) → ³T_1g(³F) transition. The shoulder absorption centered at 1.93 eV (640 nm) was due to the ³A_2g(³F) → ¹E_1g(¹D) transition. According to theoretical analysis, the double maximum is a consequence of interacting electronic states with potential energy surfaces crossing in the Franck–Condon region of the absorption, which make the spin forbidden transition possible.³³ These two bands absorb red light and give the sample a color of light green, which is the complementary color of red, as presented in previous report of Grinberg.²² The broadband absorption near ultraviolet centered at 2.75 eV (450 nm) was caused by ³A_2g(³F) → ³T_1g(³P) transition. The Racah parameter B and crystal field strength parameter D_q of octahedral Ni²⁺ in KBNNO estimated from absorption spectra were B = 716 cm⁻¹ and D_q = 853 cm⁻¹ according to methods described in ref. 35 and 36.

3.6 BET specific surface area and photocatalytic activity

Nitrogen absorption–desorption experiment were conducted to measure the BET specific surface areas of KBNNO nano-crystals with different x values. The nitrogen absorption–desorption curves are shown in Fig. S3 in the ESI† document. BET specific surface areas of KBNNO samples are shown in Table 1. The x = 0 KNbO₃ sample shows a low BET surface area compared with other Ba, Ni co-modified samples. This may be ascribed to its well crystallinity and large particle size. The increase of BET specific surface areas of KBNNO nano-crystals with x may be caused by the shrinkage of grain sizes due to doping, which is consistent with the results of XRD and SEM.

Table 1 BET specific surface areas of KBNNO nano-crystals with different x values

Sample	BET specific surface area (m^2 g^−1)
x = 0	4.394
x = 0.1	8.617
x = 0.2	31.411
x = 0.3	57.418
x = 0.4	54.352

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The methylene blue degradation experiments in water under visible light irradiation were conducted to evaluate the photocatalytic activity of KBNNO powders, in comparison with P25 TiO₂ powder. Fig. 7(a) shows the change of concentration of methylene blue versus time in dark and under visible light irradiation. Fig. 7(b) shows the change of concentration of methylene blue versus time in dark due to absorption. It can be seen from the absorption curves in Fig. 7(b) that, for KBNNO samples, absorption–desorption equilibrium had been reached after the 60 min of stirring in dark. In Fig. 7(a), after the Xe lamp was turned on, an obvious rate of change of concentration of methylene blue can be observed compared with the absorption curves. This may indicate that visible light photocatalytic degradation of methylene blue had happened. While for P25 TiO₂, the change of concentration curve of methylene with visible light irradiation and without visible light irradiation showed a similar pattern. This might indicate that the P25 used in our experiments shows little visible light photocatalytic activity towards the degradation of methylene blue under visible light irradiation. It can be seen among the five powders tested that KBNNO with x = 0.2 shows the highest photocatalytic activity.

Fig. 7 (a) Photodegradation of methylene blue under visible light irradiation by KBNNO (x = 0, 0.1, 0.2, 0.3 and 0.4) and P25 TiO₂ (b) absorption of methylene blue in dark.

In order to further compare quantitatively the photocatalytic reactivity of KBNNO samples, the pseudo-first-order reaction constant was calculated from the photodegradation curves using the following equation: −ln(C/C₀) = kt as shown in Fig. S4,† where k is the pseudo-first-order rate constant.³⁷ The k value of KBNNO samples with x = 0 to x = 0.4 were calculated to be 0.00293 ± 0.000238 min⁻¹, 0.00563 ± 0.000635 min⁻¹, 0.0144 ± 0.000530 min⁻¹, 0.00547 ± 0.000237 min⁻¹ and 0.00457 ± 0.000255 min⁻¹ respectively. This enhanced visible light photocatalytic performance of KBNNO with x = 0.1 to 0.4 compared with KNbO₃ may be attributed to visible light absorption induced by the sub-energy levels of Ni²⁺ ions in the crystal field of KBNNO.

Based on the above results, we could conclude that, with Pechini sol–gel method, Ba and Ni ions have been successfully incorporated into the lattice of KNbO₃ with the perovskite crystal structure of KNbO₃ intact. Visible and infrared optical absorptions of KNbO₃ were enhanced by Ni²⁺ ions doping. However, as we can see from the UV-vis-NIR spectra, no significant change of the intrinsic band gap absorption was observed. Though Grinberg and Zhou have reported a minimum band gap of 1.1 eV for KBNNO solid solutions, we may conclude that they might have considered the ³A_g(3F) → ³T_1g(³F) (720 nm) absorption band as the intrinsic absorption band. The band gap values obtained from the ³A_2g(³F) → ³T_1g(³F) (720 nm) absorption bands range exactly from 1.1 eV to 1.5 eV depending on the doping ratio in KBNNO. It is these visible absorption bands which contributed to the enhanced photocatalytic performance of KBNNO.

4. Conclusions

In conclusion, [KNbO₃]_1−x[BaNi_0.5Nb_0.5O_3−δ]_x perovskites synthesized with Pechini sol–gel method were investigated. XRD and Raman analysis show that the KBNNO system undergoes a phase transition from orthorhombic to cubic phase. Broad band visible and infrared light absorption bands were observed in KBNNO. These absorption bands can be attributed to the three intermediate energy levels introduced by Ni²⁺ ions in the crystal field of KBNNO. Band gap value of KBNNO powders was measured to be about 3.1 eV, which is 0.15 eV smaller than that of KNbO₃. We found that Ba, Ni modification only slightly affects the band gap value of KBNNO. Previous reports of band gap value of 1.1–1.5 eV of KBNNO may misinterpreted the 720 nm absorption band of KBNNO as the intrinsic band gap absorption. These Ni induced intermediate energy levels helped to enhance the overall optical absorption of KBNNO and made the visible light photocatalytic performance improved compared with P25 TiO₂ and KNbO₃ synthesized under same condition. Photocatalytic experiments in the degradation of methylene blue showed that KBNNO with x = 0.2 exhibited the best performance compared with KNbO₃ and commercial P25 TiO₂ powders. As far as we know, our report is one of the few reports on the chemical synthesis and photocatalytic performance evaluation of KBNNO powders. The analysis of Ni doping effects on visible light absorption properties of KBNNO may be extended to other oxides.

Acknowledgements

The authors would like to thank Prof. Deyan Sun, Dr Yang Yang, Dr Yuting Zheng and Dr Jinzhong Zhang for insightful discussions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15288k

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