Photocatalytic degradation of brilliant green and 4-nitrophenol using Ni-doped Gd(OH)3 nanorods

Gadolinium hydroxide (Gd(OH)3) was synthesized via a microwave-assisted synthesis method. Nickel ion (Ni2+) was doped into Gd(OH)3, in which 4–12% Ni-Gd(OH)3 was synthesized, to study the effect of doping. The structural, optical, and morphological properties of the synthesized materials were analyzed. The crystallite sizes of the hexagonal structure of Gd(OH)3 and Ni-Gd(OH)3, which were 17–30 nm, were obtained from x-ray diffraction analysis. The vibrational modes of Gd(OH)3 and Ni-Gd(OH)3 were confirmed using Raman and Fourier-transform infrared spectroscopies. The band gap energy was greatly influenced by Ni-doping, in which a reduction of the band gap energy from 5.00 to 3.03 eV was observed. Transmission electron microscopy images showed nanorods of Gd(OH)3 and Ni-Gd(OH)3 and the particle size increased upon doping with Ni2+. Photocatalytic degradations of brilliant green (BG) and 4-nitrophenol (4-NP) under UV light irradiation were carried out. In both experiments, 12% Ni-Gd(OH)3 showed the highest photocatalytic response in degrading BG and 4-NP, which is about 92% and 69%, respectively. Therefore, this study shows that Ni-Gd(OH)3 has the potential to degrade organic pollutants.

. Moreover, Ullah et al. synthesized Pd@Gd(OH) 3 through hydrothermal method at 180 °C for 12 h 32 .Rod-like morphology of Pd@Gd(OH) 3 was observed to be 100-200 nm in length and 30 nm in diameter.Au@Gd(OH) 3 was also successfully synthesized through the hydrothermal route 25 .The synthesized Au@Gd(OH) 3 exhibited rod-like morphology with 150 nm in length and 17 nm in diameter.
In this paper, Gd(OH) 3 and Ni-Gd(OH) 3 NRs were synthesized using a microwave-assisted synthesis method.To the best of the authors' knowledge, no report on the microwave-assisted synthesis of Gd(OH) 3 and Ni-Gd(OH) 3 has been reported.The structural, optical, and morphological properties of Gd(OH) 3 and Ni-Gd(OH) 3 NRs were investigated using various techniques.Furthermore, the photocatalytic degradation of 4-NP and brilliant green (BG) by using Gd(OH) 3 and (4, 8, and 12%) Ni-Gd(OH) 3 under UV light irradiation was studied.

Microwave-assisted synthesis of Gd(OH) 3 NRs
Gd(OH) 3 NRs were synthesized using the microwave-assisted synthesis method.In brief, 15 mL of 0.05 M of Gd(NO 3 ) 3 •6H 2 O aqueous solution was prepared in a microwave vessel.Next, 2.4 mL of 1 M NaOH was added dropwise into the solution.The pH of the solution was about 10.The vessel was then put in the microwave reactor in which the temperature was increased in a step-wise manner; from room temperature to 90 °C and finally to 180 °C.The temperature was maintained at 180 °C for 15 min at 850 W microwave power.Once the precipitate was formed, it was then centrifuged and washed three times with water before drying at 80 °C.

Microwave-assisted synthesis of Ni-Gd(OH) 3 NRs
Ni-Gd(OH) 3 NRs were synthesized using the same method as mentioned above.A 15 mL of 0.05 M aqueous Gd(NO 3 ) 3 •6H 2 O solution was prepared and a specific amount of Ni(NO 3 ) 2 •6H 2 O was added to prepare 4, 8, and 12% Ni-Gd(OH) 3 NRs.NaOH with a concentration of 1 M was then added dropwise into the solution and the pH of the solution was about 10.Subsequently, the synthesis reaction was heated in a step-wise manner to 90 °C and finally to 180 °C and maintained at 180 °C for 15 min at 850 W microwave power.The precipitate was then centrifuged and washed three times with distilled water before it was dried at 80 °C.The products were coded as 4% Ni-Gd(OH) 3 , 8% Ni-Gd(OH) 3 , and 12% Ni-Gd(OH) 3 .

Photocatalytic degradation of brilliant green and 4-nitrophenol
Photocatalytic degradation of BG dye and 4-NP using Gd(OH) 3 and 4, 8, and 12% Ni-Gd(OH) 3 NRs under UV light irradiation were investigated.In brief, 10 mg of Gd(OH) 3 and 4, 8, and 12% Ni-Gd(OH) 3 NRs were mixed with 50 mL of the respective pollutants: 10 ppm of BG dye or 4-NP solutions.The sample mixture was sonicated for 3 min and stirred in the dark for another 3 min.Then, the reaction solutions were continuously stirred and irradiated with UV light (300 W) for 5 h.The absorbance of the BG or 4-NP solution at λ max of 620 and 316 nm, respectively, was taken every 1 h to observe the photocatalysis progress for a total of five hours.The percentage of photocatalytic BG dye or 4-NP degradation was obtained using the following equation (Eq.1): where A blank is the absorbance of BG or 4-NP only and A sample is the absorbance of BG or 4-NP after photocatalytic degradation reaction with the respective catalyst.

Results and discussion
X-ray diffraction and Fourier Transform infrared spectroscopy XRD analysis of Gd(OH) 3 and Ni-Gd(OH) 3 NRs was conducted and the XRD patterns are presented in Fig. 1a.Pure Gd(OH) 3 showed a hexagonal phase with a space group of P6 3 /m and the peaks at 2θ = 16.22,28.07, 29.53, 32.75, and 37.68° are associated with (010), (110), (011), (020), and (111) planes, respectively (JCPDS 98-020-0093) 33 .No additional peaks were observed after doping with Ni 2+ ions, suggesting the successful incorporation of Ni 2+ into the Gd(OH) 3 lattice.Figure 1b shows the zoom-in view of the (010) plane of Gd(OH) 3 and Ni-Gd(OH) 3 .No significant shift was observed with Ni-doping which suggests that the Ni-doping did not change the Gd(OH) 3 lattice.However, the peak intensity was seen to decrease.This might be due to the reduction in the crystallinity of Gd(OH) 3 34 .The effect of Ni doping on the structural properties of Gd(OH) 3 was studied by the estimation of the average crystallite sizes of Gd(OH) 3 and Ni-Gd(OH) 3 using the Debye-Scherrer's equation (Eq.2) and their average lattice strains were also calculated (Eq. 3) 7,35 : where λ represents the wavelength of the x-ray, θ indicates Bragg's angle, and β is the full width at half maximum of the characteristic peaks.The calculated average crystallite size of Gd(OH) 3 was found to be 30.08nm (Table 1).When 4% Ni was doped into Gd(OH) 3 , the average crystallite size was reduced to 20.84 nm.Further doping showed similar crystallite size (21.59nm) for 8% Ni-Gd(OH) 3 .However, 12% Ni-Gd(OH) 3 showed smaller crystallite size of 17.34 nm.However, the lattice parameters and cell volumes of Gd(OH) 3 and Ni-Gd(OH) 3 show insignificant change.Nonetheless, the average lattice strain increased with Ni-doping from 0.0012 to 0.0021.The Ni-doping has mainly influenced the average crystallite size and the lattice strain.However, the Ni-doping was observed to maintain the Gd(OH) 3 lattice without significantly affecting the lattice parameter and the cell volume.

Cell volume (Å 3 ) Average lattice strain (ε) a c
Gd(OH) 3  The O-H vibration band and the stretching and bending of O-H vibration were observed at ~ 1600 cm -1 and ~ 3500 cm -1 , respectively 37 .Raman spectra of Gd(OH) 3 , 4%, 8%, and 12% Ni-Gd(OH) 3 are shown in Fig. 2b.Pure Gd(OH) 3 showed three main Raman peaks assigned to A g translatory, E 2g translatory, and E 1g liberation modes which are located at 308.85, 387.34, and 490.52 cm -1 , respectively 37 .One should note that, 4A g , 2E 1g , and 5E 2g are known to be Raman active for hexagonal phase Gd(OH) 3 with P6 3 /m space group 37 .The Raman peak intensity was decreased when 4% Ni was doped into Gd(OH) 3 .Expectedly, when more Nidoping was incorporated the Raman peak intensity decreased further.This suggests that with more Ni-doping, the distortion of the lattice periodicity and long-range translational crystal symmetry caused by the induced defects occurred in the crystal lattice 38 .

Transmission electron microscopy
TEM images of Gd(OH) 3 , 4%, 8%, and 12% Ni-Gd(OH) 3 are shown in Fig. 3a 1 -d 1 , and all synthesized materials displayed rod-like morphology.This indicates that the addition of Ni-doping has no major influence on the  www.nature.com/scientificreports/morphology.However, the particle size was influenced by the Ni doping.Gd(OH) 3 showed an average length of 38 nm and a diameter of 12 nm (Table 2).When 4% Ni was incorporated, the particle length was increased to 76 nm and the diameter was increased to 26 nm.This suggests that Ni-doping has some influence on the particle size of Gd(OH) 3 as reported by Kumar et al. 39 However, further increase in Ni doping has no more influence on the particle lengths.The average particle lengths for 8% Ni-Gd(OH) 3 and 12% Ni-Gd(OH) 3 were 76 nm and 75 nm and the average diameters were 13 and 14 nm, respectively.This might be due to the hindering of crystal growth at a certain level of doping, in which, in this case, 12% Ni might hinder the crystal growth 40 .

UV-vis diffuse reflectance spectroscopy
The band gap energy of Gd(OH) 3 and Ni-Gd(OH) 3 was estimated from the Tauc plot (Fig. 4) that was obtained from the Kubelka-Munk equation (Eq.4).
where R is the measured absolute reflectance of the samples.The band gap can be obtained from the plots of [F(R)hv] 1/2 versus hv.
The band gap energy of pure Gd(OH) 3 is 5.00 eV which is in good agreement with literature 41 .Interestingly, the band gap energy decreased to 4.37 eV in the case of 4% Ni-Gd(OH) 3 .This suggests that the Ni-doping has affected the optical band gap energy of Gd(OH) 3 .Further increase in the percentage of Ni-doping has led to a decrease in the band gap energy even further.Therefore, 8% Ni-Gd(OH) 3 and 12% Ni-Gd(OH) 3 showed band gap energies of 3.75 and 3.03 eV.The band gap energies of all samples were tabulated in Table 2. ( 4)  www.nature.com/scientificreports/

X-ray photoelectron spectroscopy
The chemical state and the electronic structure of the elements in Gd(OH) 3 and Ni-Gd(OH) 3 were analyzed using XPS. Figure 5a shows the survey scan spectra of the synthesized materials confirming the presence of Gd 4d, Ni 2p, and O 1s.The Gd 4d core level peak is shown in Fig. 5b.Two major peaks at approximately 140.3 and 146.1 eV were observed, corresponding to Gd 3+ 4d 3/2 and Gd 3+ 4d 5/2 , respectively 32 .No peak shift was observed for all the synthesized materials.Figure 5c shows the XPS spectra of two prominent Ni 2p peaks of 4% Ni-Gd(OH) 3 , 8% Ni-Gd(OH) 3 and 12% Ni-Gd(OH) 3 NRs.The two prominent peaks at 853.9 and 871.6 eV correspond to the Ni 2p 3/2 and Ni 2p 1/2 , respectively 42 .Furthermore, the peak intensity was increased with more Ni-doping.The peak positions of 878.www.nature.com/scientificreports/ The XPS spectrum of O 1s can be seen in Fig. 5d, in which all the samples exhibit one major peak.The peak at approximately 529.0 eV indicates the OH -anion 44 .Slight shifts and changes in the peak position and intensity were observed for Ni-Gd(OH) 3 .The typical C 1s peaks at 282.9 eV were observed in the spectra (Fig. 5e).XPS intensities were observed to vary with the incorporation of Ni 2+ ions.The atomic concentrations of C 1s, O 1s, Gd 4d, and Ni 2p are listed in Table 3.

Applications Photocatalytic degradation of BG dye
The photocatalytic degradation of BG using Gd(OH) 3 and Ni-Gd(OH) 3 was studied under the irradiation of UV light for 5 h (Fig. 6).The progress of the photocatalytic activity was monitored every hour by measuring the absorbance of the treated BG solution at λ max = 620 nm.The experiment was conducted in triplicates and the average percentage of the photocatalysis is shown in Fig. 6a.
At the 1st and 2nd hour, Gd(OH) 3 showed no photocatalytic response but there was a slight response at the 3rd and 4th hour.It was finally able to degrade about 14.81 ± 1.96% at the 5th hour of UV light irradiation.4% Ni-Gd(OH) 3 showed a slightly better photocatalytic response even though it was only able to degrade less than 30% from 1st to 4th hour.It finally degraded about 37.82 ± 0.51% of BG at the 5th hour.The photocatalytic response was seen increasing with more Ni-doping, as can be observed from 8% Ni-Gd(OH) 3 .At the 1st hour, 8% Ni-Gd(OH) 3 degraded about 18.35 ± 0.39% and showed about 61.39 ± 0.47% degradation at the 5th hour.Moreover, the percentage photocatalytic degradation of BG was observed to be higher for 12% Ni-Gd(OH) 3 , which showed about 50.20 ± 2.28% at the 1st hour and 92.14 ± 0.29% at the 5th hour.This suggests that, when more Ni 2+ was doped, the photocatalytic degradation of BG increased.Based on their band gap energies, more Ni doping resulted in the reduction of the band gap energy (i.e., 5.00-3.30eV).The doping might help in creating defect states in the band gap which enables UV light absorption and thus inhibits rapid charge carrier recombination 45 .Table 4 shows the average percentage of the photocatalytic degradation of BG using Gd(OH) 3 and 4, 8, and 12% Ni-Gd(OH) 3 NRs.

Photocatalytic degradation of 4-nitrophenol
Photocatalytic degradation of 4-NP was also investigated using Gd(OH) 3 and Ni-Gd(OH) 3 NRs under the irradiation of UV light (Fig. 7a).Similarly, at every hour, the absorbance of the treated 4-NP solution was taken and measured to observe the photocatalytic response.The readings were taken three times to ensure repeatability.Under the UV light irradiation, the photocatalytic response of Gd(OH) 3 throughout the experiment was expected.It showed the lowest response amongst the samples which was about 62.74 ± 2.44% of the photocatalytic degradation of 4-NP.This might be due to its wide band gap (5.00 eV).When 4% Ni was doped, the photocatalytic response was increased slightly (63.69 ± 1.99%), suggesting the effect of Ni 2+ doping.Moreover, the reduction in the band gap energy from 5.00 eV to 4.37 eV might cause a slight enhancement in the separation of the photogenerated electrons (e -) and holes (h + ).Further increase in the percentage of Ni 2+ doping has shown better photocatalytic degradation of 4-NP.In the case of 8% Ni-Gd(OH) 3 , the overall photocatalytic response was about 67.67 ± 0.95% whereas 12% Ni-Gd(OH) 3 showed about 69.32 ± 1.25% of 4-NP degradation (Table 5).The band gap energies of both materials decreased with more Ni 2+ doping, which were about 3.75 and 3.03 eV, respectively.Therefore, the efficient photocatalytic response might be due to their lower band gap energies.www.nature.com/scientificreports/ The kinetic study was conducted based on the photocatalytic activity of Gd(OH) 3 and Ni-Gd(OH) 3 NRs against 4-NP (Eq.5). Figure 7b presents the pseudo-first-order reactions of the photocatalytic degradation of BG activity of Gd(OH) 3 and Ni-Gd(OH) 3 NRs.The rate constants of Gd(OH) 3 , 4% Ni-Gd(OH) 3 , 8% Ni-Gd(OH) 3 , and 12% Ni-Gd(OH) 3 NRs were estimated to be 0.2078, 0.2189, 0.2374, and 0.2490 h -1 , respectively.It was observed that, with Ni-doping, the reaction rates were increased, and 12% Ni-Gd(OH) 3 NRs showed the highest reaction rate constant.
Therefore, based on both photocatalysis experiments, 12% Ni-Gd(OH) 3 showed the highest photocatalytic degradation of BG and 4-NP.The difference in the effectiveness of 12% Ni-Gd(OH) 3 in both cases might be due to the nature of the two compounds.Moreover, the narrow band gap energy of 12% Ni-Gd(OH) 3 (3.03eV) might absorb UV light more efficiently and can photogenerate e -/h + pairs effectively.One study reported on the catalytic photodegradation of Congo red using lanthanide hydroxides (Ln = Nd, Sm, Eu, Gd, Tb, and Dy) 46 .The removal efficiencies of Ln(OH) 3 are more than 90% after 1800 min.However, this current study shows improved photocatalytic efficiency where the photocatalytic degradation of BG using Ni-Gd(OH) 3 showed a degradation of BG over 90% in 5 h.This might be also due to the recombination of the photogenerated e -/h + pairs being hindered.The photogenerated e -/h + pairs would react with adsorb O 2 and H 2 O to form O 2 •-and OH • radicals, respectively.In general, these radicals are responsible for the degradation of pollutants.However, based on the literature, O 2 •-radicals are mainly responsible for the degradation of dyes, whereas OH • radicals are responsible for the degradation of 4-NP [47][48][49] .Therefore, the difference in the response of 12% Ni-Gd(OH) 3 in the degradation of BG and 4-NP might be due to these radicals.The role of O 2 •-radicals might be more prominent during the photocatalytic degradation of BG and in the case of the photocatalytic degradation of 4-NP, OH • radicals might be the more reactive species.

Conclusions
Gd(OH) 3 and Ni-Gd(OH) 3 NRs were successfully synthesized using the microwave-assisted synthesis method.The properties of the synthesized materials such as the structural, optical, and morphological properties, were analyzed using different instruments.The hexagonal structure of Gd(OH) 3 and Ni-Gd(OH) 3 with average crystallite sizes between 17 and 30 nm was obtained from XRD.The presence of the vibrational bands of Gd(OH) 3 and Ni-Gd(OH) 3 was confirmed by Raman and FT-IR spectroscopies.The band gap energy of Gd(OH) 3 and Ni-Gd(OH) 3 were reduced with Ni-doping in which the values decreased from 5.00 to 3.03 eV.TEM images showed nanorod-shaped Gd(OH) 3 and Ni-Gd(OH) 3 with increased particle size when doping with Ni 2+ .Photocatalytic degradations of 4-NP and BG under UV light irradiation were carried out and 12% Ni-Gd(OH) 3 showed the highest photocatalytic response which is about 92% and 69%, respectively.Therefore, Ni-Gd(OH) 3 is a promising material for the degradation of organic pollutants.

Figure 2 (
Figure2(a) shows the FT-IR spectra of Gd(OH) 3 and Ni-Gd(OH) 3 NRs.All samples showed a bending vibration of Gd-O-H in the range of 600-750 cm -1 , which confirmed the synthesis of Gd(OH) 3 and Ni-Gd(OH) 3 .The common symmetric and asymmetric stretching of O-C-O in Gd(OH) 3 can be observed in the range of 1370-1530 cm -136 .The O-H vibration band and the stretching and bending of O-H vibration were observed at ~ 1600 cm -1 and ~ 3500 cm -1 , respectively37 .Raman spectra of Gd(OH) 3 , 4%, 8%, and 12% Ni-Gd(OH) 3 are shown in Fig.2b.Pure Gd(OH) 3 showed three main Raman peaks assigned to A g translatory, E 2g translatory, and E 1g liberation modes which are located at 308.85, 387.34, and 490.52 cm -1 , respectively37 .One should note that, 4A g , 2E 1g , and 5E 2g are known to be Raman active for hexagonal phase Gd(OH) 3 with P6 3 /m space group37 .The Raman peak intensity was decreased when 4% Ni was doped into Gd(OH) 3 .Expectedly, when more Nidoping was incorporated the Raman peak intensity decreased further.This suggests that with more Ni-doping, the distortion of the lattice periodicity and long-range translational crystal symmetry caused by the induced defects occurred in the crystal lattice38 .

Table 2 .
Average particle sizes from TEM and the band gap energy of Gd(OH) 3 , and Ni-Gd(OH) 3 NRs (*L = length and D = diameter).

Table 4 .
The average percentage of photocatalytic activities of Gd(OH) 3 and 4, 8, and 12% Ni-Gd(OH) 3 NRs for BG degradation under UV light irradiation.