Porous nickel doped titanium dioxide nanoparticles with improved visible light photocatalytic activity

A green hydrothermal synthesis route to prepare a porous nickel doped titanium dioxide (Ni–TiO2) nanostructured photocatalyst has been developed in this research. The results show that Ni doping can greatly increase the visible light photocatalytic performance of TiO2 through the introduction of impurity bands in the band gap of TiO2. After 5 cycles of reuse, Ni–TiO2 nanoparticles still show stable photocatalytic activity for MB degradation. The Ni–TiO2 nanoparticles developed in the present study are expected to have great potential applications in wastewater treatment due to the advantages of strong visible light photocatalytic performance, a simple synthetic process and high cycle utilization performance.


Introduction
Due to the prosperity of modern industries, especially the ones dealing with plastics, paper and textile dying, a huge amount of wastewater with various kinds of effluents is discharged, resulting in a great crisis in the acquirement of fresh water. 1 Organic dye pollutants, one of the main produced effluents, can seriously disturb and destroy the ecological balance, leaving a heavy negative impact on the living, both human beings and plants. 2 To mitigate the above mentioned crisis, a great number of studies have focused on dye wastewater treatment, and various strategies have been developed, such as biodegradation, 3 chlorination, 4 electrochemical, 5 photocatalytic 6-10 and adsorption 11,12 methods. As one of the most effective methods, heterogeneous photocatalysis can greatly facilitate the oxidation of the pollutants and the by-products of hazardous organic pollutants. 12 These catalysts typically have an excellent capability to convert photon energy into chemical energy which is favorable for the decomposition of the main toxic organic contaminants. Among these catalysts, TiO 2 has been proved to be the most effective one due to its rst usage in heterogeneous photocatalysis under UV light irradiation by Fujishima and Honda in 1972. 13 Aerwards, photocatalysis with TiO 2 catalysts became a research hotspot to decay the harmful chemical effluents present in wastewater. 14,15 Aer several decades of development, anatase TiO 2 is now considered to be one of the most common photocatalysts with high photocatalytic activity. 16 TiO 2 has a favorable band gap, good chemical stability, good photostability, and high corrosion resistance. 17,18 TiO 2 is also one of the most noticeable photocatalysts with particular properties: it is a recoverable and reusable catalyst and can offer an eco-friendly and non-toxic approach for dye wastewater treatment. The photocatalytic activity of TiO 2 is based on the mechanism of the formation of electron/hole (e À /h + ) pairs under the illumination of light which can initiate chemical reactions by generating radical species on the surface of TiO 2 . 19 However, its poor efficiency in response to visible light limits its application due to the hindrance of the large band gap to catalyst efficiency under natural sunlight illumination which mostly contains visible regions. 20 Doping of TiO 2 with different transition metals (Fe, Mn, Cu, Ni, etc.) can enhance the degradation under visible light irradiation, which has been successfully applied in wastewater treatment. 21 The reducing of the band gap of the catalysts has been achieved by doping metals through different processes. Benjwal et al. reported that a Zn and Mn co-doped TiO 2 photocatalyst showed high activity and excellent adsorption properties in low concentration aqueous solutions. 22 Copper phthalocyanine (CuPc) doped TiO 2 was conrmed to be an efficient and stable photocatalyst for degradation of methylene blue from aqueous solution under solar light irradiation. 23 The doping of TiO 2 with other transition metals such as Fe, Ni and Co has also been employed in various applications. [24][25][26] However, the applications of doped TiO 2 are still limited by their high cost and relatively low stability.
To obtain highly effective TiO 2 photocatalysts, the synthesis techniques need to be well controlled. In this work, novel Ni-TiO 2 nanoparticles have been developed using a green hydrothermal-synthesis route. Different from traditional TiO 2 preparation techniques, this synthesis route is easy to be operated and could save time. Meanwhile, the novel Ni-TiO 2 nanoparticles exhibit outstanding performance on adsorption of MB dye from aqueous solutions in darkness and high photocatalytic activity towards MB dye under visible light. The catalyst also exhibits extremely high cycle performance and recyclability. The synthesis strategy presented in this work can prepare materials with outstanding properties and will show potential application in water treatment systems.

Materials
Butyl titanate ([CH 3 (CH 2 ) 3 O] 4 Ti), absolute ethyl alcohol (C 2 H 5 OH), hydrochloric acid (HCl), ammonium hydroxide (NH 3 $H 2 O), nickel nitrate (Ni(NO 3 ) 2 $6H 2 O), and methylene blue (MB), were all purchased from Sinopharm Reagent Co Ltd. All the chemicals were of analytical grade and used without further purication. Deionized water was used throughout for the preparation of all the experimental solutions.

Preparation of TiO 2 and Ni-TiO 2 nanoparticles
Tetrabutyl titanate (10 mL) and absolute ethyl alcohol (10 mL) were mixed to obtain solution A. Absolute ethyl alcohol (20 mL) and deionized water (100 mL) were mixed to obtain solution B. Solution A was then added into solution B dropwise under magnetic stirring for 30 min. Then, the pH value was adjusted to 9 by ammonium hydroxide. Aer homogenization for 30 min, the mixed solution was transferred into a Teon-lined autoclave for crystallization at 140 C for 4 h. The resulting product was washed with ethyl alcohol and deionized water 3 times each. Then the nanoparticles were separated from the liquid phase by centrifugation to remove the remaining compounds. The nal product was dried at 80 C overnight to obtain TiO 2 powders. The synthetic steps for Ni-(3 wt%) TiO 2 nanoparticles were little different from the above. In step three, solution A and a nickel nitrate solution (0.85 mL, 1 mol L À1 ) were added into solution B dropwise under magnetic stirring for 30 min.

Characterization
FT-IR spectra were recorded using a Shimadzu instrument (model 8400S) in the region 4000-400 cm À1 . The phase analysis of the as-synthesized products was carried out using X-ray diffraction (XRD, DX-2700) with Cu-Ka radiation (l ¼ 1.5406 A). UV-vis-NIR absorption spectra of the samples were recorded using a UV-1800 spectrophotometer (Shimadzu). SEM images were obtained using a S-4800 instrument (Hitachi, Japan). The specic surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was obtained using the Barrett-Joyner-Halenda (BJH) model using a Micromeritics ASAP 2020 adsorption analyzer.

Photocatalytic experiments
Degradation of MB was used as an indicator for the photocatalytic activity of the TiO 2 nanoparticles. The prepared TiO 2 nanoparticles were immersed in 10 mg L À1 MB solution and were allowed to completely equilibrate with MB for 20 min in darkness. Then the system was irradiated by simulated solar light (Xe lamp, 300 W) or UV light. 10 mL of solution was taken and analysed at different reaction times (every 15 min) using a UV-1800 spectrophotometer.

Results and discussion
3.1 FTIR spectra Fig. 1 shows the FTIR spectra of the TiO 2 and Ni-TiO 2 nanoparticles. The strong absorption bands at 662 and 704 cm À1 might be due to the Ti-O vibrations in the TiO 2 lattice. Furthermore, a broad absorption band in the region of 3000-3500 cm À1 can be assigned to the surface-bound hydroxyl groups and their stretching vibration on the surface of TiO 2 . 27 A second adsorption band at 1000-1700 cm À1 is assigned to surface-adsorbed water molecules (H-O-H bending, 1633 cm À1 ). 28 It can conrm a strong interaction of water molecules with the TiO 2 surface to form a number of broad OHstretching vibrations. 22 A broad intense vibration region at 1000-1200 cm À1 is credited to the Ti-O-Ti vibration. Moreover, occurrence of bands between 1300-1500 cm À1 for Ni-TiO 2 nanoparticles indicates the presence of a small amount of organic material in the sample. 29 With an increase in Ni concentration, the shi to lower wavenumbers of the Ti-O-Ti band could be attributed to the increase in powder particle size. 30

Phase analysis and morphology
XRD patterns of TiO 2 and Ni-TiO 2 nanoparticles are shown in Fig. 2A Fig. 2B shows the spherically shaped Ni doped TiO 2 nanostructures. Compared with the pure TiO 2 image (Fig. 2C), Ni-TiO 2 shows homogeneous nanoparticles with sizes of 20-30 nm. This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 1352-1357 | 1353 Paper Nanoscale Advances

UV-vis spectral analysis
The electronic structure of the samples that furnishes the optical properties (e.g., absorption and band gap) through the irradiating light intensity was determined by UV-vis spectral analysis, 17 as shown in Fig. 3. In the absorption spectra, it is noticeable that the optical absorption edge of the pure TiO 2 is at 400 nm. The band gap of TiO 2 is 3.21 eV which is favorable to produce electron-hole pairs under the UV light irradiation. However, the pure TiO 2 can not degrade dyes under visible light.
Compared to pure TiO 2 , Ni-TiO 2 nanoparticles exhibit a broad absorption covering the range as shown in Fig. 3b, which can be ascribed to the doping energy levels caused by the doped Ni in the band gap of TiO 2 .

Nitrogen adsorption-desorption isotherm of Ni-TiO 2
The nitrogen adsorption-desorption isotherm and BJH pore size distribution curve (inset) of Ni-TiO 2 are shown in Fig. 4, which displays a type-IV isotherm with a specic surface area of 124.02 m 2 g À1 . This implies that the pores within the materials are mainly within the mesoporous range. The pore size distribution   1354 | Nanoscale Adv., 2020, 2, 1352-1357 This journal is © The Royal Society of Chemistry 2020 Nanoscale Advances Paper is calculated using the BJH method (desorption curve). 31 The pore-size distribution of Ni-TiO 2 shows that the pore diameters distribution (Fig. 4 inset) has a peak at about 9 nm, indicating that Ni-TiO 2 has a mesoporous structure. These small pores can enhance photocatalytic activities by favoring the adsorption of small dye molecules on the active surface.

Photocatalytic performance stability
The stability is also important for the practical application of the photocatalyst. Therefore, the cyclic stability of Ni-TiO 2 nanoparticles was investigated by monitoring the catalytic activity during successive cycles of degradation. As shown in Fig. 6, aer a ve cycles test, the Ni-TiO 2 nanoparticles exhibit a very stable photocatalytic performance without any signicant deactivation, thereby demonstrating high stability aer multiple reuse cycles.

Photocatalytic mechanism of Ni-TiO 2
The plausible mechanism of the photocatalytic activity of the synthesized Ni-TiO 2 nanoparticles can be explained by the energy band gap structure of TiO 2 shown in Fig. 7. The direct excitation of an electron from the valence band (VB) to the conduction band (CB) in the presence of visible light is not possible due to the broad band gap (3.21 eV) of pure TiO 2 . Through the incorporation of Ni ions into the TiO 2 lattice, the band gap of TiO 2 decreases due to the formation of impurity levels below the CB in the band gap, then the electrons can transfer from the VB to these energy levels. These electrons travel to the surface and are adsorbed by O 2 and produce cO 2   This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 1352-1357 | 1355 Paper Nanoscale Advances ions, which can further convert to the strong redox species cOH ions. 32 These redox ions are responsible for the degradation of the surface adsorbed hazardous MB. 22

Conclusions
Ni-TiO 2 nanoparticles were synthesized by a green hydrothermal-synthesis route and characterized in detail. The activities of the synthesized nanoparticles were studied through MB photocatalytic degradation. The results demonstrate that Ni doping can greatly increase the visible light photocatalytic performance of TiO 2 through the introduction of impurity bands in the band gap of TiO 2 . Aer 5 cycles of reuse, Ni-TiO 2 nanoparticles still show stable photocatalytic activity for MB degradation. Thus the Ni-TiO 2 nanoparticles developed in the present study are expected to have great potential applications in wastewater treatment due to the advantages of strong visible light photocatalytic performance, a simple synthetic process and high cycle utilization performance.

Conflicts of interest
There are no conicts to declare.