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

Abstract

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.

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1. 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, TiO₂ has been proved to be the most effective one due to its first usage in heterogeneous photocatalysis under UV light irradiation by Fujishima and Honda in 1972.¹³ Afterwards, photocatalysis with TiO₂ catalysts became a research hotspot to decay the harmful chemical effluents present in wastewater.^14,15 After several decades of development, anatase TiO₂ is now considered to be one of the most common photocatalysts with high photocatalytic activity.¹⁶

TiO₂ has a favorable band gap, good chemical stability, good photostability, and high corrosion resistance.^17,18 TiO₂ 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 TiO₂ is based on the mechanism of the formation of electron/hole (e⁻/h⁺) pairs under the illumination of light which can initiate chemical reactions by generating radical species on the surface of TiO₂.¹⁹ 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 TiO₂ 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 TiO₂ photocatalyst showed high activity and excellent adsorption properties in low concentration aqueous solutions.²² Copper phthalocyanine (CuPc) doped TiO₂ was confirmed to be an efficient and stable photocatalyst for degradation of methylene blue from aqueous solution under solar light irradiation.²³ The doping of TiO₂ with other transition metals such as Fe, Ni and Co has also been employed in various applications.^24–26 However, the applications of doped TiO₂ are still limited by their high cost and relatively low stability.

To obtain highly effective TiO₂ photocatalysts, the synthesis techniques need to be well controlled. In this work, novel Ni–TiO₂ nanoparticles have been developed using a green hydrothermal-synthesis route. Different from traditional TiO₂ preparation techniques, this synthesis route is easy to be operated and could save time. Meanwhile, the novel Ni–TiO₂ 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.

2. Experimental

2.1 Materials

Butyl titanate ([CH₃(CH₂)₃O]₄Ti), absolute ethyl alcohol (C₂H₅OH), hydrochloric acid (HCl), ammonium hydroxide (NH₃·H₂O), nickel nitrate (Ni(NO₃)₂·6H₂O), and methylene blue (MB), were all purchased from Sinopharm Reagent Co Ltd. All the chemicals were of analytical grade and used without further purification. Deionized water was used throughout for the preparation of all the experimental solutions.

2.2 Preparation of TiO₂ and Ni–TiO₂ 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. After homogenization for 30 min, the mixed solution was transferred into a Teflon-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 final product was dried at 80 °C overnight to obtain TiO₂ powders. The synthetic steps for Ni-(3 wt%) TiO₂ nanoparticles were little different from the above. In step three, solution A and a nickel nitrate solution (0.85 mL, 1 mol L⁻¹) were added into solution B dropwise under magnetic stirring for 30 min.

2.3 Characterization

FT-IR spectra were recorded using a Shimadzu instrument (model 8400S) in the region 4000–400 cm⁻¹. The phase analysis of the as-synthesized products was carried out using X-ray diffraction (XRD, DX-2700) with Cu-Kα radiation (λ = 1.5406 Å). 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 specific 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.

2.4. Photocatalytic experiments

Degradation of MB was used as an indicator for the photocatalytic activity of the TiO₂ nanoparticles. The prepared TiO₂ nanoparticles were immersed in 10 mg L⁻¹ 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.

3. Results and discussion

3.1 FTIR spectra

Fig. 1 shows the FTIR spectra of the TiO₂ and Ni–TiO₂ nanoparticles. The strong absorption bands at 662 and 704 cm⁻¹ might be due to the Ti–O vibrations in the TiO₂ lattice. Furthermore, a broad absorption band in the region of 3000–3500 cm⁻¹ can be assigned to the surface-bound hydroxyl groups and their stretching vibration on the surface of TiO₂.²⁷ A second adsorption band at 1000–1700 cm⁻¹ is assigned to surface-adsorbed water molecules (H–O–H bending, 1633 cm⁻¹).²⁸ It can confirm a strong interaction of water molecules with the TiO₂ surface to form a number of broad OH– stretching vibrations.²² A broad intense vibration region at 1000–1200 cm⁻¹ is credited to the Ti–O–Ti vibration. Moreover, occurrence of bands between 1300–1500 cm⁻¹ for Ni–TiO₂ nanoparticles indicates the presence of a small amount of organic material in the sample.²⁹ With an increase in Ni concentration, the shift to lower wavenumbers of the Ti–O–Ti band could be attributed to the increase in powder particle size.³⁰

Fig. 1 FT-IR spectral analysis of (a) TiO₂ and (b) Ni–TiO₂ nanoparticles.

3.2 Phase analysis and morphology

XRD patterns of TiO₂ and Ni–TiO₂ nanoparticles are shown in Fig. 2A. The Ni–TiO₂ sample exhibits peaks at 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69°, 70.31°, and 75.03°, corresponding to the anatase phase (JCPDF 21-1272), with no other phases. In addition, peaks corresponding to Ni oxides are not detected. These results further indicate that Ni ions have been successfully doped into TiO₂ nanoparticles.²Fig. 2B shows the spherically shaped Ni doped TiO₂ nanostructures. Compared with the pure TiO₂ image (Fig. 2C), Ni–TiO₂ shows homogeneous nanoparticles with sizes of 20–30 nm.

Fig. 2 (A) XRD patterns of (a) TiO₂ and (b) Ni–TiO₂ nanoparticles; (B) morphology of Ni–TiO₂; (C) morphology of TiO₂.

3.3 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 TiO₂ is at 400 nm. The band gap of TiO₂ is 3.21 eV which is favorable to produce electron–hole pairs under the UV light irradiation. However, the pure TiO₂ can not degrade dyes under visible light. Compared to pure TiO₂, Ni–TiO₂ 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 TiO₂.

Fig. 3 UV-visible absorption spectra of (a) TiO₂ and (b) Ni–TiO₂ samples.

3.4 Nitrogen adsorption–desorption isotherm of Ni–TiO₂

The nitrogen adsorption–desorption isotherm and BJH pore size distribution curve (inset) of Ni–TiO₂ are shown in Fig. 4, which displays a type-IV isotherm with a specific surface area of 124.02 m² g⁻¹. This implies that the pores within the materials are mainly within the mesoporous range. The pore size distribution is calculated using the BJH method (desorption curve).³¹ The pore-size distribution of Ni–TiO₂ shows that the pore diameters distribution (Fig. 4 inset) has a peak at about 9 nm, indicating that Ni–TiO₂ has a mesoporous structure. These small pores can enhance photocatalytic activities by favoring the adsorption of small dye molecules on the active surface.

Fig. 4 Nitrogen adsorption–desorption isotherms and pore size distribution curve of Ni–TiO₂.

3.5 MB decomposition capacity under solar and UV light

Fig. 5 depicts the photocatalytic degradation of MB using TiO₂ and Ni–TiO₂ nanoparticles under UV and solar light irradiation as a function of time with an initial MB concentration of 10 mg L⁻¹. The degradation efficiencies of Ni–TiO₂ nanoparticles under solar and UV after 60 min irradiation are found to be 92.7% and 96.3%, respectively. However, the corresponding degradation efficiencies are only 85.9% and 27.7% for pure TiO₂. It can be found that Ni doping can increase the visible light degradation performance greatly. The TiO₂ catalyst shows very weak photocatalytic performance for MB degradation under solar light irradiation. After Ni doping, the visible light photocatalytic performance increases to a similar level compared with that under UV light. The result demonstrates that Ni doping can improve the photocatalytic activity of TiO₂ nanoparticles under visible light.

Fig. 5 Photodegradation of MB dye using TiO₂ and Ni–TiO₂ nanoparticles under UV and solar light irradiation.

3.6 Photocatalytic performance stability

The stability is also important for the practical application of the photocatalyst. Therefore, the cyclic stability of Ni–TiO₂ nanoparticles was investigated by monitoring the catalytic activity during successive cycles of degradation. As shown in Fig. 6, after a five cycles test, the Ni–TiO₂ nanoparticles exhibit a very stable photocatalytic performance without any significant deactivation, thereby demonstrating high stability after multiple reuse cycles.

Fig. 6 Photocatalytic stability test of Ni–TiO₂ nanoparticles.

3.7 Photocatalytic mechanism of Ni–TiO₂

The plausible mechanism of the photocatalytic activity of the synthesized Ni–TiO₂ nanoparticles can be explained by the energy band gap structure of TiO₂ 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 TiO₂. Through the incorporation of Ni ions into the TiO₂ lattice, the band gap of TiO₂ 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 O₂ and produce ˙O₂ ions, which can further convert to the strong redox species ˙OH ions.³² These redox ions are responsible for the degradation of the surface adsorbed hazardous MB.²²

Fig. 7 Possible mechanism of the MB degradation by Ni–TiO₂ nanoparticles.

4. Conclusions

Ni–TiO₂ 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 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. Thus 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.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the support from National Natural Science Foundation of China (51478406, 51508293), 973 Program (2015CB6555100), 111 Program and Qingdao Applied Research Project (17-1-1-87-jch).

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Footnote

† These authors contributed equally.

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