Electrical behavior and enhanced photocatalytic activity of (Ag, Ni) co-doped ZnO nanoparticles synthesized from co-precipitation technique

Environmental problems are becoming a more and more serious problem with the development of industries. In particular, most of the dye industries are producing a lot of waste dye waters, which directly enters the soil and natural water body streams and affects the water ecosystem. These dyes create severe environmental problems by releasing toxic and potential carcinogenic substances into the aqueous phase. Compared with all other textile and industry wastages, methylene blue (MB) dye is the most common effluent and is very harmful to human because it causes increased vomiting, heart rates, cyanosis, tissue necrosis, skin diseases, and intestinal problems. For that reason, the toxic organic compound present in MB dye should be degraded using suitable techniques for preventing environmental water pollution. Compared with all other techniques, the photocatalytic process has the most significant effect on degradation of MB dye (Saleh & Djaja 2014). In recent years, the nanosized semiconductor materials (TiO₂, SnO₂, and ZnO) were effective materials for the dye degradation process (Hamrouni et al. 2014a, 2014b, 2015; Carini et al. 2015; Bellardita et al. 2017) because the band gap of the semiconductor materials has an ultraviolet (UV) region. Therefore, semiconductor nanomaterials were promoting photocatalysis upon illumination under UV radiation (SoliCasados et al. 2009; Wang et al. 2009a). Among the various semiconductor nanomaterials, ZnO NPs are one of the most promising material for electronic and optoelectronic properties due to its band gap energy (3.37 eV) and it has very large excitation binding energy (60 meV) at room temperature (Wang et al. 2007). In addition, ZnO NPs were applied in many fields, including photocatalysis (Wang et al. 2007), optical devices (Hong et al. 2003), and cosmetic products, such as sunscreens (Schulz et al. 2002). However, ZnO NPs significantly differ in chemical and physical properties from the macrosized bulk material with identical chemical composition (Senthil Kumar et al. 2017a). ZnO NPs can be produced by various techniques such as hydrothermal method, co-precipitation method, sol–gel method, biosynthesis method, and sono-chemical method (Rathnasamy et al. 2017; Senthil Kumar et al. 2017a). Among these, co-precipitation techniques have advantages over the other synthesis methods due to its inexpensive experimental costs for synthesizing the particles. In addition, the obtained particles of the shape and size were controlled easily by adjusting pH medium via co-precipitation method (Jeyachitra et al. 2018).

In the photocatalytic reactions in aqueous solutions, ZnO has many drawbacks due to their fast recombination rate of photogenerated electron-hole pair and a low quantum yield, which obstruct commercialization of the photocatalytic degradation process (Romero et al. 1999). However, there is an increasing demand for making suitable modifications in the photocatalytic activity of semiconductor materials for improving the degradation of organic dyes in water ecosystem. It has been found that the interfacial electron transfer efficiency and recombination rate of electron-hole pairs of semiconductor materials can be easily changed by different kind of surface modification methods, such as surface chelation, surface derivatization, platinization, selective metal ion, and nitrogen doping (Serpone et al. 1994). In addition, it is also found that the presence of heavy metals such as Pt, Pd, Au, and Ag on semiconducting metal oxides or zeolite-supported ZnO can enhance the degradation efficiency from the photocatalytic reactions (Ranjit et al. 2001; Shankar et al. 2006).

In recent years researchers focused on developing the two metal atoms co-doping into semiconductor materials for higher photocatalytic degradation process compared with single element doping into semiconductor material (Lin et al. 2007; Wang et al. 2009b; Yang et al. 2010; Ma et al. 2011; Subash et al. 2012; Cerrón-Calle et al. 2019). The present study focused on developing (Ag, Ni) co-doped ZnO NPs via co-precipitation technique for the improvement of photocatalytic activity. The dopant (Ag) anchored on ZnO traps the photogenerated electrons from the semiconductor material and Ni doping exhibits recombination. Due to this, the number of surface hydroxyl groups are increased for the enhancement of photocatalytic activity (Cerrón-Calle et al. 2019). The prepared ZnO NPs are doped with Ni (0.04%) and different concentrations of Ag (0.01%, 0.02% and 0.03%). MB dye was used as a model pollutant for the dye degradation process and the prepared pure and doped materials were applied to analyze the dye degradation efficiency.

All the chemicals are analytical grade and purchased from Merck. For the synthesis of pure ZnO and (Ag, Ni) co-doped ZnO NPs, the materials used are zinc sulfate heptahydrate (ZnSO₄·7H₂O) as a zinc source material, silver nitrate (AgNO₃) and nickel sulfate heptahydrate (NiSO₄·7H₂O) used as a co-dopant source material for ZnO NPs, and sodium hydrogen carbonate (NaHCO₃) material as a reducing agent.

In this experiment, 0.1 M aqueous solution of NaHCO₃ was added slowly into 0.1 M ZnSO₄·7H₂O solution and stirred continuously until the formation of white precipitate. The white precipitate was washed with double distilled water and dried at 150 °C for 1 h. In the drying process, Zn(OH)₂ was converted slowly into ZnO NPs. Finally, the obtained pure ZnO NPs was used for further analysis.

The (Ag, Ni) co-doped ZnO NPs were synthesized via co-precipitation approach and 2.57 g of nickel sulfate (NiSO₄·7H₂O) and 0.50 to 1.5 g of silver nitrate (Ag(NO₃)₂) were slowly added into 0.1 M ZnSO₄ solution to obtain co-doped ZnO NPs. NaHCO₃ solution was added slowly into the prepared solution and stirred vigorously for 5 h. After the stirring process, the obtained solution was sonicated at 1 h, centrifuged, and washed several times with water and ethanol. Finally, the Ni (0.04%) and Ag (0.01, 0.02, and 0.03%) co-doped ZnO NPs was calcinated at 450 °C for 1 h and used for further analysis.

The prepared pure and (Ag, Ni) co-doped ZnO NPs were characterized by X-ray diffraction PANalytical X-ray diffractometer with a copper Kα (λ = 1.541 Å) radiation. The surface morphology and composition of the synthesized samples were investigated using a Hitachi S-4100 scanning electron microscope. The optical absorption spectra of ZnO nanoparticles were recorded using a UV–VIS spectrophotometer (JASCO V-570). The FTIR spectra of nanoparticles in the KBr pellets were recorded using Perkin–Elmer spectrometer. The recording was performed in the wavenumber range 4,000–400 cm⁻¹ with a resolution of 2 cm⁻¹. The high resolution transmission electron microscopy (HRTEM) analysis was carried out using Tecnai G2 20 instrument.

The powder X-ray diffraction pattern (PXRD) has been used to investigate the phase of the synthesized ZnO and Ag–Ni co-doped ZnO NPs. The PXRD pattern of ZnO and Ag–Ni (0.01%, 0.02%, 0.03%) co-doped ZnO NPs are shown in Figure 1. The position of all obtained diffraction peaks observed in ZnO and Ag–Ni co-doped ZnO NPs are well matched with standard JCPDS: 36–145 report and it is polycrystalline in nature (Senthil Kumar et al. 2018). The PXRD pattern reveals that all synthesized materials have a hexagonal wurtzite structure with XRD peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1) planes. The intensity of the (1 0 1) peak linearly increases with increase of Ag–Ni dopants in ZnO, which indicates the crystalline quality of the samples. The increase of peak intensity is due to the difference in ionic radius of Ni²⁺ (0.55 Å), Ag⁺ (∼0.115 Å), and Zn²⁺ (0.60 Å). The high intensity peak is also shifted toward higher angle side by the addition of Ag–Ni dopant in ZnO, which is due to the incorporation of Ag⁺ and Ni²⁺ ions into the ZnO lattice and is shown in Figure 2. This report is well matched with the previous reports (Thota et al. 2006; Ekambaram et al. 2007; Sankara Reddy et al. 2014). XRD pattern reveals that Ni-Ag co-doping does not disturb the hexagonal wurtzite structure of parent ZnO (Bougrine et al. 2003). The presence of Ag was not observed in the XRD pattern and it may be due to the lesser amount of dopant concentrations, although Ag was detected by energy dispersive spectroscopy (EDAX).

FTIR is a powerful technique used to analyze the chemical bonding and the elemental constituents present in the samples. Generally, the chemical composition, morphology, and structure of the material establish the absorption peaks and peak position of the spectra. The FTIR spectra of pure and Ni-Ag (0.03%) co-doped ZnO NPs is shown in Figure 3. From the figure, the observed peak at 3,456 cm⁻¹ is the O–H bending vibration, and the small observed band at 1,633 cm⁻¹ is the H–OH stretching vibration. These two peaks are representing the presence of water content in the synthesized material (Cerrón-Calle et al. 2019). A small band at 2,359 cm⁻¹ is representing the presence of CO₂ in air (Cerrón-Calle et al. 2019). The narrow band at 1,122 cm⁻¹ indicates the sulfate group existing in the precursor material (Jeyachitra et al. 2018). The two peaks at 444 cm⁻¹ and 455 cm⁻¹ are representing the metal oxide (M-O) stretching mode of Zn–O (Senthil Kumar et al. 2017b). In addition, the absorption of co-doped (Ag, Ni) peak at 444 cm⁻¹ shifts toward the lower wavenumber side than the pure ZnO, which clearly representing the change in the bond length by the substitution (or) replacement of (Ag, Ni) co-doped ions instead of Zn ions (Cerrón-Calle et al. 2019). The observed FTIR spectra at (Ag, Ni) co-doped ZnO NPs clearly indicates that there is no additional bands observed for (Ag, Ni) co-dopants, which indicates that there is no additional chemical bonding presence between (Ag, Ni) co-dopants and pure ZnO (Cerrón-Calle et al. 2019). The obtained result is in agreement with the XRD results reported in Figure 1.

Figure 4(a)–4(d) show the surface morphology of pure and different concentrations of Ag–Ni co-doped ZnO NPs. The Figure 4(b) clearly shows the variation of surface morphology with doping of Ag (0.01%) on pure ZnO NPs. From the figure, the Ag (0.03%)–Ni co-doped ZnO NPs look spherical in shape and the obtained result is well matched with the HRTEM analysis. The average size of the synthesized particles was found to be 25–70 nm. The figure clearly shows that the crystallinity enhances with the doping concentration. The obtained shape and size of the field emission scanning electron microscopy (FESEM) images are in good agreement with the XRD results. The EDAX analysis of pure and Ag (0.03%)–Ni co-doped ZnO NPs is shown in Figure 4(e) and 4(f). From the EDAX spectrum, there is no additional peak present in the synthesized sample. Pure ZnO sample shows the presence of Zn and O. Similarly, Ag–Ni co-doped ZnO NPs shows the presence of Zn, Ag, Ni, and O. The EDAX spectrum reveals that the obtained synthesized samples are pure. The different magnification of the HRTEM image of Ag (0.03%)–Ni co-doped ZnO NPs is shown in Figure 5(a)–(d). The figure clearly shows that all the obtained particles are well agglomerated, uniformly distributed, and spherical in shape. The average size of the synthesized particles was found to be 25–50 nm and is shown in Figure 5(b) and 5(c). The obtained shape and size of the HRTEM images are in good agreement with the FESEM analysis.

Figure 7(a)–7(c) show the time dependent UV–Vis absorption spectra of MB (pH 2, 4, 6) dye removal using pure ZnO nanocrystals at different light irradiation time. It is observed that pure ZnO nanocrystals shows very low (16–40%) degradation when irradiated with UV light for 1–6 hours for all pH values. The synthesized Ag–Ni co-doped ZnO NPs were used as a catalyst to decolorize the various pH (2, 4, and 6) values of MB dye at different light irradiation time intervals and is given in Figure 8(a)–8(i). From the figure, it can be observed that the maximum absorption peak of MB (663 nm) dye decreases with increase of light irradiation time. Ag (0.01%)–Ni co-doped ZnO NPs shows maximum degradation of 60% for higher pH values of MB dye solution. When the dopant concentration is increases to 0.03% the degradation efficiency of 66% was achieved for higher pH values. When pH increases the decolourization of the MB dye solution increases up to 0.02% of Ag–Ni co-doped ZnO NPs and then decreases for 0.03% of Ag–Ni co-doped ZnO nanocrystals. For the increase of pH, the percentage of decolourization also increases with increase of light irradiation time. The pH 4 shows maximum decolourization of MB dye solution (77%) for the 0.02% Ni-Ag co-doped ZnO nanocrystals. When the dopant concentration was increased to 0.03% the decolourization decreases strongly, which is shown in Figure 9(b). It may due to the incorporation of Ag into the ZnO lattice and it increases the surface area for enhanced adsorption of dye (Dinesh et al. 2012). In addition, enhanced photocatalytic activity of Ag–Ni co-doped ZnO NPs is because Ag⁺ and Ni²⁺ ions adhered to the surface of ZnO. This acts as an electron sink to increase the separation of photo-generated electron-hole pairs and inhibit their recombination process (Rathnasamy et al. 2017). This process clearly indicates that the electrons are directly transferred from semiconductor (ZnO) to silver nanoparticles, which may happen due to their arrangements of electronic band structure of the noble metal and semiconductor material (Dinesh et al. 2014). From these results, we can observe that the surface of Ag–Ni co-doped ZnO NPs plays a major role in the degradation of organic dye compounds.

On the basis of these above experimental results, a possible mechanism of charge separation and photo-catalytic dye degradation of Ag–Ni co-doped ZnO nanoparticles is proposed and is illustrated in Figure 10. When heterogeneous semiconductor is irradiated by UV light, a valence band electron (VB) goes to the conduction band (CB), and makes a hole in the valence band. Due to the reaction of adsorbed O₂ and H₂O with the above produced electrons and holes, reactive oxygen species such as superoxide anion (O₂⁻) and hydroxyl radicals are formed (Türkyılmaz et al. 2017). The reactive oxygen species are strong oxidative species and can oxidize dye and other organics absorbed on the active sites of the photocatalysts into CO₂ and H₂O and mineral acids. The recombination of electron-holes reduces the photo-catalytic degradation of heterogeneous semiconductors. However, the presence of Ag and Ni holds the electron from CB of ZnO, suppressing the electron-hole recombination (Subash et al. 2013; Türkyılmaz et al. 2017). Moreover, Ag acts as a sink of the electrons from CB of ZnO and, after that, Ag is oxidized by the absorption of oxygen to produce reactive oxygen species. The reactive oxygen species is then further reduced to form the hydroxyl radical (•OH). On the other hand, the holes combine with water to form the hydroxyl (•OH) radical and these hydroxyl (•OH) radicals break down the dye molecules to degrade the dye. Ni doping also overwhelms the recombination of electron and positive holes by electron trapping. It is shown that the photo-catalytic degradation of (Ag–Ni) co-doped ZnO photocatalyst is higher than that of pure ZnO. The trapping nature of Ag and Ni produced more superoxide radical anions, and at the same time the VB holes of ZnO react with water to produce highly reactive hydroxyl radicals (•OH). The superoxide radical anion and hydroxyl radical are used for degradation of dye.

The pure ZnO and Ag–Ni co-doped ZnO NPs were successfully synthesized by inexpensive co-precipitation method. The prepared nanoparticles are characterized by XRD, SEM, and HRTEM analysis. The effect of various concentrations of Ag on structural and optical properties was analyzed. XRD result shows that the average crystallite size decreases with an increase of dopant concentration. The HRTEM image clearly confirms the formation of nanoparticles in the range of 30 nm. All the bending and stretching functional groups in the synthesized materials were identified from the FTIR analysis. In the MB dye degradation process, the Ag (0.02%)–Ni co-doped ZnO NPs exhibits higher photocatalytic activity of 77%. Ag and Ni achieved this by adhering to the surface of ZnO due to their large surface area and increased oxygen vacancy defects.