Introduction

Semiconductors have been intensively investigated as efficient photocatalysts to oxidize organic pollutants into nontoxic products or deplete H2O (Karahaliloglu et al. 2014; Kandula and Jeevanandam 2014; Nogueira et al. 2014; Ramadan et al. 2013; Weng et al. 2014). Until now, semiconductors have been developed to be potential photocatalysts such as TiO2 (Zhang et al. 2015; Zhu et al. 2014; Liu et al. 2015; Lázaro-Navas et al. 2015), ZnO (Yin et al. 2014), Bi2O3 (Dang et al. 2015), CdS (Xiong et al. 2015), NaTaO3 (Su et al. 2015), Bi2WO6 (Song et al. 2015), SrTiO3 (Márquez-Herrera et al. 2014), Fe3O4 (Zhang et al. 2014a, b), Co3O4 (Elazab et al. 2014), etc. Among them, TiO2 is certainly the most widely investigated photocatalyst for its high efficiency, nontoxic nature, photochemical stability, and low cost.

Usually, the oxides such as TiO2 has a large band gap (3.2 eV) (Lu et al. 2014; Chadha et al. 2014), and therefore, only UV light can be absorbed. This limits its application in the industrial field because the UV light accounts for merely 5 % of the sunlight energy. It is necessary that the band gap of a material is narrow enough between 1.9 and 3.1 eV with an optimal value of 2.03 eV to harvest visible light (400 nm < λ < 760 nm) which accounts for ∼45 % of the solar spectrum’s energy (Murphy et al. 2006). In order to reach this goal, the photocatalytic properties have been obtained by many methods in the different modifications such as Gold-nanocage-coupled TiO2 (Chadha et al. 2014), g-C3N4-P25/TiO2 (Zhu et al. 2014), La/N-codoped TiO2 (Liu et al. 2015), Pt@SiO2@TiO2 (Zhang et al. 2015), and NiO/N-doped TiO2 (Li et al. 2015). It is well known that to extend the activity of a photoelectrode into the visible-light region, various new methods of photocatalysts have been investigated.

In this work, we try to develop a new photocatalyst, Ni-containing vanadate K2Ni(VO3)4. The research motivations are the followings. First, one of the most characteristic features is that the band-gap energy of the photocatalyst can be narrowed. Usually, Ni-3d energy band plays an important role in deciding the visible-light-induced photocatalytic activity (Yao and Ye 2006). For example, Wang et al. (2005) have reported that in M3V2O8 (M = Mg, Ni, Zn) with the same structure, Ni3V2O8 presents a smaller band gap of 2.25 eV than those of Mg3V2O8 (3.02 eV) and Zn3V2O8 (2.92 eV). This is induced by the split Ni-3d orbitals inserted between the O-2p and the V-3d orbitals in Ni3V2O8. It also has been confirmed that an occupied level was created in the center of the band gap due to the Ni-3d band splitting in the oxides such as Ni2+-doped TiO2 and SrTiO3 (Kudo et al. 2007). The results show that the Ni2+ addition shows an efficient photocatalytic performance and an excellent photo stability in photocatalytic materials such as Ni-laded TiO2 (Liu et al. 2014), Ni x Cd1−x S (Chen et al. 2015), Ni-doped InVO4 (Zhang 2014), and NiO/N-doped TiO2 (Li et al. 2015).

Second, K2M(VO3)4 (M = Ni2+, Zn2+, Mn2+) has monoclinic structure constructed by a metal–oxygen polyhedral framework. Figure 1a is an example structural view of the unit cell of K2Ni(VO3)4 along [100] direction obtained from the inorganic crystal structure database (Witzke et al. 2001; Liao et al. 1996). The metal V5+ ions (in [VO4]3−) completely occupy the fourfold Td sites. The V–O distances are changed from 1.645 to 1.781 Å. There is an infinite VO4 chain along [100] in the lattices as shown in Fig. 1b. The Ni2+ ions are located in the NiO6 octahedral sites in a tunnel along [100] displayed in Fig. 1c. The Ni–O band length ranges from 2.0683 to 2.1266 Å. There are two kinds of optically active centers in the lattices, i.e., NiO6 and VO4. Under light excitation, photoinduced electrons can be easily created from O-2p to the empty Ni-3d, or by the charge transfer (CT) of an electron from the oxygen 2p orbital to the vacant 3d orbital of V5+ (Nakajima et al. 2009). The separated excitons could react with dye molecules into nontoxic products.

Fig. 1
figure 1

Schematic structure views of K2Ni(VO3)4 nanoparticles along [100] direction

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K2Ni(VO3)4 nanoparticle powders were developed by the modified Pechini method. The sample was investigated by the crystal phase, morphologies, UV–Vis absorption, and band-gap structure. The efficient photocatalytic activity was confirmed by the degradation of methylene blue (MB) under visible-light irradiation, which was discussed on the base of the crystal structure.

Experimental

K2Ni(VO3)4 nanoparticles were prepared by the Pechini method. The raw materials are stoichiometric amounts of potassium nitrate (KNO3), nickel nitrate (Ni(NO3)2·6H2O) and ammonium meta-vanadate (NH4VO3). As an example, the raw materials of 1.01 g KNO3, 1.49 g Ni(NO3)2·6H2O, 2.34 g NH4VO3 were first dissolved in deionized water. The nitrate solutions were complexed by citric acid with the double molar weight of the cations. The three solutions were mixed uniformly, and the mixed solution was neutralized by controlled addition of ammonium hydroxide (30 wt%). The solution was promoted by heat treatment at 80–95 °C for 1–3 h. Then, a certain amount of aqueous polyvinylalcohol (PVA) was slowly added in the solution to adjust the viscoelasticity of the solution. The mixed solution was stirred for 1–3 h to obtain a homogeneous viscous solution for the spin-coating on several clean glasses. The dried precursor thin film can be obtained by natural withering of the coated glasses. Then, the films were taken down from the glasses, and then heated at 680 °C in a muffle furnace. Finally, K2Ni(VO3)4 nanopowders can be obtained. Another isostructural sample of K2Zn(VO3)4 was also prepared in the same method to make a comparison of optical absorption and photocatalysis. The referred sample also keeps a pure crystal phase and has similar morphology to those of K2Ni(VO3)4 nanoparticles. The motivation is to elucidate the role of Ni2+ ions in the lattices which can narrow the band gap and promote the photocatalysis under near-UV light.

XRD was obtained using a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Cu Kα as an incident radiation. The SEMs were used to study the surface morphologies of the samples. Diffuse reflection spectra (DRS) were taken on a Cary 5000 UV–Vis–NIR spectrophotometer by means of BaSO4 powder as a standard reference. X-ray photoelectron spectroscopy (XPS) analyses were performed using an XPS, Kratos analytical, ESCA-3400, Shimadzu. Nitrogen adsorption and desorption isotherms were obtained on an ASAP 3020 (Micromeritics Instruments, USA), a nitrogen adsorption apparatus. All the samples were degassed at 150 °C to remove the absorbed gases prior to the nitrogen adsorption measurement. Particle sizes and size distributions of the particles were measured using a particle size analyzer (Mastersizer 3000, Malvern, UK) by dynamic light scattering (DLS) technique. The specific surface area (S BET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P 0) range of 0.05–0.3. The adsorption isotherm data were used to determine the pore-size distribution via the Barret–Joyner–Halender (BJH) method, assuming a cylindrical pore model. The adsorbed nitrogen volume at the relative pressure (P/P 0) of 0.99 was used to determine the pore volume and the average pore size.

Photocatalysis experiments were carried out in a quartz beaker filled with MB solution (100 mL and 10–40 mg/L) containing the catalyst (0.25 g/L). A 300-W Xenon lamp with a UV cutoff filter (JB450) was positioned at about 10 cm beside the photoreactor. Prior to visible-light irradiation, the suspension was dispersed by ultrasonic irradiation for 40 min in dark to favor the adsorption and desorption equilibration. The concentration of MB was analyzed by recording the absorption band maximum (665 nm) in the absorption spectra and taken as the initial concentration (C 0). During the photocatalysis, 7 mL of the suspension was extracted at an interval of 10 min, and the absorption (C) was measured after 5 min of centrifugation. The normalized temporal concentration changes (C/C 0) of MB were obtained.

Results and discussions

Phase formation and surface structure

The crystal phase of K2Ni(VO3)4 nanoparticles was investigated by XRD measurement as shown in Fig. 2. The pattern is in agreement with the standard card PDF#44-0350 in the International Centre for Diffraction Data (ICDD) database. All the diffraction peaks in XRD pattern can be well indexed on the basis of the monoclinic structure of K2Ni(VO3)4. No impurity lines can be observed.

Fig. 2
figure 2

The experimental X-ray diffraction profiles of K2Ni(VO3)4 nanoparticles

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The typical scanning electron microscopy (SEM) images of K2Ni(VO3)4 nanoparticles in two magnifications are shown in Fig. 3a, b. The sample crystallized in the irregular ball-like nanoparticles. The size distribution of the nanoparticles was determined by dynamic light scattering (DLS) technique as shown in Fig. 3c. The average size of the nanoparticles estimated by the micrograph is 67 nm. TEM image of K2Ni(VO3)4 nanoparticles is shown in Fig. 3d. It confirms that K2Ni(VO3)4 nanoparticles are well crystallized with a single-phase structure, which is in good agreement with the observed XRD pattern. The particles reveal the formation of spherical grains of nanonature (size <100 nm), which presents some aggregations. Figure 4 shows the EDS measurement to examine the elemental compositions on the sample. Several specific lines show the signals of Ni, K, V, and O elements. The average Ni/V ratio was calculated to be about 0.234, which is in agreement with the theoretical stoichiometric value in the chemical formulae of K2Ni(VO3)4.

Fig. 3
figure 3

The typical SEMs (a, b), the size distributions (c), and the TEM images (d) of K2Ni(VO3)4 nanoparticles

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Fig. 4
figure 4

The EDS spectrum of K2Ni(VO3)4 nanoparticles

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BET surface area and pore-size distribution

BET surface area and pore-size distribution of the nanoparticles were investigated. Figure 5 shows the N2 adsorption–desorption isotherm and the corresponding pore-size distribution curve of K2Ni(VO3)4. According to the IUPAC classification, the isotherm of the sample is of the typical IV pattern, which is characterized with a hysteresis loop. The high adsorption at P/P 0 approaching to 1.0 indicates the coexistence of mesopores and macropores. The specific surface area of K2Ni(VO3)4 particles was measured to be 56 m2 g−1. The pore-size distribution of the sample was quite narrow and monomodal, implying that the prepared nanoparticles are uniform. The pore-size distribution is centered on 3 nm (inset Fig. 5).

Fig. 5
figure 5

The nitrogen adsorption–desorption isotherms of K2Ni(VO3)4 nanoparticles; inset: the corresponding pore-size distribution curve

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Optical absorption and band-gap structure

The UV–Vis absorption spectrum of K2Ni(VO3)4 nanoparticles is shown in Fig. 6a. There is a very broad optical transition from 200 to 600 nm. The onset wavelength around 600 nm corresponds to an optical band gap if a single transition was assumed. The absorption band can be divided into three distinct absorption bands, i.e., I: 200–400 nm, II: 400–600 nm, and III: 600–800 nm.

Fig. 6
figure 6

a UV–Vis absorption spectrum of K2Ni(VO3)4 compared with K2Zn(VO3)4, inset showing the estimated band gap, and the band-gap structure; b the schematic band-gap structure, inset showing the digital photo of the powders

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Absorption III can be unambiguously assigned to the spin-allowed d–d transitions from the Ni2+ ions in the octahedral environment, i.e., 3 A 2g(F) → 3 T 1g(F) (Biswas et al. 2008). While bands I and II form the band-gap transitions, i.e., one is from CT transitions inside VO4 3− groups, and another is the spin-allowed d–d transitions from the Ni(II) ions in the octahedral environment of 3 A 2g(F) → 3 T 1g(P) (Biswas et al. 2008), respectively.

CT band edge in VO4 3− groups has been reported only in UV to near-UV wavelength region, not extending to blue region (Nakajima et al. 2009). Consequently, the absorption in UV region I: 200-400 nm can be assigned to the CT (O2− → V5+) in VO4 3− groups. This can be clearly understood by the comparison of the optical absorption of K2Zn(VO3)4 in Fig. 6a, which have no optical-activated cations in K and Zn ions. By the way, the absorption band II corresponds to the spin-allowed d–d transitions of the Ni2+ in octahedral in K2Ni(VO3)4. Such a typical absorption has been reported in Ni-containing photocatalysts such as Ni3V2O8 (Wang et al. 2005) and CsLaSrNb2NiO9 (Yao and Ye 2006).

Figure 6b shows the schematic band structure of K2Ni(VO3)4, with reference to the reported results in Ni2+-containing oxides. The absorption bands I and II form the band-gap components with the valence band (VB) of (Ni 3d + O 2p) to the conduction band (CB) of (V 3d + Ni 3d-b1g). For Ni2+ with the electronic configurations 3d8 in the photocatalysts, the split Ni 3d-t2g orbitals were fully occupied, while the split Ni 2d-eg orbitals should be partially occupied. Yao and Ye (2006) have suggested that Ni 2d-eg orbitals can further split into two parts for the distortion of the Ni–O octahedrons (Yao and Ye 2006), and the lower energy band a1g orbitals were assumed to be fully occupied to have better consistency with the photophysical and photocatalytic properties of the photocatalysts as shown in Fig. 6b. The absorption band II: 2.75 eV and band III: 1.6 eV were assumed to form Ni 3d-b2 g and Ni 3d-a1 g orbitals, respectively, by the further splitting of Ni 3d orbitals in the octahedral field. As shown in inset Fig. 6a, K2Ni(VO3)4 particles are deep yellow. The color should be generally attributed to crystal-field transitions along with some contributions from the allowed d–d transitions of the Ni2+ in octahedral environment.

The band-gap energy E g was determined by the Wood-Tauc theory based on the relation of \(\alpha h\upsilon \propto \left( {h\upsilon - E_{g} } \right)^{k}\), where α is the absorbance, h is the Planck constant, ν is the frequency, and k is a constant associated to the different types of electronic transitions (k = 1/2, 2, 3/2, or 3 for directly allowed, indirectly allowed, directly forbidden, or indirectly forbidden transitions, respectively). The best linear relation of K2Ni(VO3)4 nanoparticles was obtained for k value of 2 shown in inset Fig. 6a, indicating this is an indirectly allowed electronic transition. The band gap of K2Ni(VO3)4 particles is calculated to be about 2.08 eV, which is significantly narrower than the reported vanadates such as Ni3V2O8 (2.25 eV) and Zn3V2O8 (2.92 eV) (Wang et al. 2005). This also demonstrates that the substitution of Ni2+ can significantly reduce the band gap of a compound, which is beneficial to the improvement of the photocatalytic activity.

Theoretically, the positions for valence band and conduction band are determined by the Eqs. (1) and (2) (Ohko et al. 1997; Butler and Ginley 1978):

$$E_{\text{VB}} = X - E^{\text{e}} + 0.5\,E_{\text{g}}$$
(1)
$$E_{\text{CB}} = X - E^{\text{e}} + 0.5\,E_{\text{g}}$$
(2)

E g is the energy of the band gap, X is the absolute electronegativity of the semiconductor (geometric mean of the absolute electronegativity of the constituent atoms). E e is defined as the energy of free electrons on the hydrogen scale (∼4.5 eV vs SHE). Here the experimental E g is 2.08 eV for K2Ni(VO3)4 nanoparticles. The corresponding CB and VB levels of K2Ni(VO3)4 nanoparticles are calculated to be −0.002 and 2.078 eV versus SHE, respectively. The result shows that K2Ni(VO3)4 nanoparticles is difficult for photocatalytic hydrogen generation.

XPS spectra

It is well known that V and Ni elements have multiple valences in compounds. Binding energy X-ray photoelectron spectroscopic (XPS) measurement can provide useful information on the oxidation states of different elements in the lattices of materials. Figure 7a displays the typical XPS survey spectra of K2Ni(VO3)4 nanoparticles. It is observed that the XPS peaks corresponding to K, Ni, V and O were identified in the sample.

Fig. 7
figure 7

The typical XPS survey spectra (a), and the XPS high-resolution spectra of Ni 2p (b), V 2p (c), and O 1 s (d) measured in K2Ni(VO3)4 nanoparticles

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The Ni-2p3/2 XPS spectrum (Fig. 7b) shows the characteristic satellite peak with the binding energy (BE) at 864.2 eV. Such a satellite peak has been reported in Ni-containing oxides such as NiO (Carley et al. 1999) and LiNi0.5Mn0.5O2 (Manikandan et al. 2011). The reason for such satellite peak is explained as due to the multiple splitting in the energy levels of the Ni-oxides (Carley et al. 1999). The V-2p XPS curve of the K2Ni(VO3)4 nanoparticles is presented in Fig. 7c. The V-2P3/2 and V-2P1/2 peaks of the spinel appear at 517.2 and 524.5 eV, respectively. The values were close to those for LiVO3 (Kumagai et al. 1996) and LiNiVO4 (Prakash et al. 2013). The dominated signal at 517.2 eV was confirmed from V5+ ions by V oxidation at the crystal surface in K2Ni(VO3)4. The XPS curve of V 2p (Fig. 7d) shows a slight asymmetric profile. This could be related to the possible effects of defect oxygen and adsorbed oxygen on the surfaces. For example, Zhang et al. (2014a, b) have demonstrated that there are rich induced V o (oxygen vacancy) defects in the surfaces of BiVO4. Recently, Rossell et al. (2015) have reported the detailed existence of VO in BiVO4: the O-related vacancy defects are in the 5-nm deepness of the surface.

Photocatalytic activity

The photocatalytic activity of K2Ni(VO3)4 nanoparticles prepared by the Pechini method was tested by the degradation of a MB solution. Figure 8 shows the changes in UV/vis absorption spectra of MB-K2Ni(VO3)4 solution under visible-light irradiation. The intensity of the peak at 665 nm significantly decreased with the increasing irradiation time. This peak decreased after illumination, suggesting that the solution had been decolorized. Besides, the spectra keep the same profiles indicating that there were no new intermediates in the degradation process.

Fig. 8
figure 8

The changes in UV–Vis absorption spectra of MB-K2Ni(VO3)4 solution under visible-light irradiation

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Figure 9 shows the photocatalytic degradation curve of MB by K2Ni(VO3)4 nanoparticles compared with references of P25 photocatalyst and K2Zn(VO3)4 under the same test condition; The inset is the degradation kinetics curve by means of plotting ln(C 0/C) versus irradiation time. The reference of K2Zn(VO3)4 shows a small quantity of dye degraded under visible-light irradiation. The MB degradation rate of P25 is less than 40 %. In contrast, the degradation of the MB shows a fast speed in the presence of K2Ni(VO3)4, which decreases to 20 % in 2 h. Moreover, according to the raw materials and preparation processes, the costs of K2Ni(VO3)4 obtained in this work are lower than that for TiO2.

Fig. 9
figure 9

Photocatalytic degradation curve of MB by K2Ni(VO3)4 compared with references of P25 photocatalyst and K2Zn(VO3)4 under the same test conditions. Inset shows the degradation kinetics by means of plotting ln(C 0/C) vs irradiation time

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The kinetic constant is determined from the pseudo-first-order reaction rate equation of ln(C 0/C t ) = kt, where C 0 is the initial concentration of MB, C t is the concentration of MB at time t, and k is a kinetic constant. The inset in Fig. 9 shows the plot of ln(C 0/C t ) versus irradiation time. The good linear fit indicates that the kinetics of the degradation reaction is controlled dominantly by a pseudo-first-order reaction. The kinetic constant of MB degradation by K2Ni(VO3)4 is calculated to be 2.05 × 10−2 min−1.

It is a fact that the photoinduced electrons in the lattices have several possible fates; the first is a radiative transition of recombination with a hole creating a luminescence in UV–Vis–IR wavelength region; the second one is nonradiative transition, which results in the heat generated by the transferring of energy to phonons in the lattices; the third one is the electrons, which could be trapped by any possible defects in the lattices; and the last one is the photoinduced electrons taking part in the photocatalytic process.

Vanadium, as a kind of efficient luminescent materials, has been developed in the previous years due to its various applications for lighting and display. The luminescence has been understood by the charge transfer (CT) transition in VO4 with T d symmetry (Nakajima et al. 2009). The molecular orbits of VO4 are expressed as the ground 1 A 1 state and excited 1 T 1, 1 T 2, 3 T 1, and 3 T 2 states (Fig. 10). The absorption transitions (1 A 1 → 1 T 1,1 T 2) are allowed (Ex1, Ex2), while the luminescence process (3 T 1,3 T 2 → 1 A 1) is forbidden in the ideal T d symmetry due to the spin-selection rule. If the structure of VO4 in a host is distorted from the idealized tetrahedron, the (3 T 2 and 3 T 1) → 1 A 1 transitions are allowed by the spin–orbit interaction, leading to two emission bands, Em1 and Em2, respectively (seen in Fig. 10) (Nakajima et al. 2009). In the lattices of K2Ni(VO3)4, the VO4 tetrahedron is heavily distorted and deviated from ideal tetrahedron T d symmetry (Fig. 1). The VO4 is connected by the corner with each other forming a chain along [100]. Consequently, the absorption transitions (1 A 1 → 1 T 1, 1 T 2) are allowed, while the luminescence process (3 T 1, 3 T 2 → 1 A 1) should not be forbidden (Nakajima et al. 2009).

Fig. 10
figure 10

Luminescence spectra of K2Ni(VO3)4 excited by the excitation with 355-nm YAD:Nd laser. A reference emission of K2Zn(VO3)4 was compared under the same test conditions. Inset shows the emission processes in VO4 tetrahedron with T d symmetry in vanadates

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Actually there is a very weak luminescence in K2Ni(VO3)4 under excitation with a UV lamp. As shown in Fig. 10, K2Ni(VO3)4 excited at 355 nm had a broad emission bands with the maximum wavelength at 650 nm. It is generally acknowledged that the higher fluorescence intensity means more recombinations of electron–hole pairs and lower photocatalytic activities (Hoffmann et al. 1995). As shown in Fig. 10, K2Zn(VO3)4 has the broad emission bands with the maximum wavelength at 535 nm attributed to the ligand–metal CT bands (2p orbital of oxygen ion → 3d orbital of vanadium ion) localized within the tetrahedrally coordinated [VO4]3− group. Compared with K2Zn(VO3)4, the emission spectrum of K2Ni(VO3)4 (650 nm) shows a great red-shift. This is corresponding to the narrower band gap. It is evident that the luminescence intensity of K2Ni(VO3)4 is much lower than that of K2Zn(VO3)4 suggesting a much lower recombination rate of photogenerated charge carriers in K2Ni(VO3)4. This indicates that Ni2+ component in the lattices can effectively inhibit the recombination of excited electrons and holes.

It is commonly accepted that structure-induced dipole moments in distorted metal–oxygen polyhedra in the tunnel structural compounds are beneficial for the separation of hole–electron pairs, enhancing photocatalytic activities (Lin et al. 2006). The lattice of K2Ni(VO3)4 presents channel structure formed by distorted NiO6 along [100] as shown in Fig. 1b. The infinite chain’s corner shares NiO6 octahedra. Such a tunnel structure possesses a spatially open construction, leading to higher momentary polarizing fields that can work as accelerators for electron–hole separation. In such a tunnel, the presence of dipole moments heavily distorted NiO6 octahedra resulting in an efficient photoexcitation, charge separation, and migration. This could enable the photoexcited electron–hole effectively to delocalize, enhancing the photocatalytic activity. It can be understood that a light radiation creates electron–hole pairs, i.e., exciton, in K2Ni(VO3)4. Actually it can be observed that there is no luminescence in K2Ni(VO3)4 indicating the weak recombination of the excitons. This could be suggested as due to a long lifetime of the excitons and more chances for electron–hole separations, which further react with dye molecules to oxidize the dye pollutant into nontoxic products.

Conclusions

A new visible-light-driven photocatalyst, K2Ni(VO3)4 nanoparticles, was first developed by the modified Pechini method. K2Ni(VO3)4 has a narrowed band gap of 2.08 eV characterized by an indirectly allowed electronic transition. The methylene blue dye can be efficiently degraded under visible-light irradiation in the presence of K2Ni(VO3)4 nanoparticles. The photocatalytic ability is related to the structural features such as the regular VO4 tetrahedron, the activated optical centers of NiO6 octahedra with NiO6 tunnel structure. The luminescence quenching in K2Ni(VO3)4 could provide a low opportunity for the recombination of electron–hole pairs in the lattices. Accordingly, K2Ni(VO3)4 possesses the improved the photocatalytic activity. The obtained nanoparticles could be expected to have a potential application in environment protection technology.