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Synthesis of Gd/N co-doped ZnO for enhanced UV-vis and direct solar-light-driven photocatalytic degradation

The construction of a UV-Vis and direct sunlight functioning photocatalyst is a puzzling task for organic pollutant removal. Herein, we have fabricated Gd/N co-doped ZnO nanoparticles for the first-time using a simple co-precipitation method for photocatalytic degradation application. The heteroatom doping enhances the light absorption ability and acts as a photo-induced electron–hole separator by creating a trap state. Co-doped ZnO shows comparatively high photocatalytic degradation efficiency of about 87% and 93% under UV-Vis and direct solar light respectively. Moreover, the prepared photocatalyst exhibits excellent stability for the recycling process. Hence, we believe that this heteroatom co-doped ZnO photocatalyst is an auspicious material for the photocatalytic organic pollutant degradation reaction.


Introduction
Over the last few years, semiconductor-based photocatalysts for organic pollutant degradation have become an attractive research area due to being cost-effective and environmentally friendly. In a photocatalytic reaction, waste pollutants are oxidized with the aid of semiconductor nanoparticles under UV or visible-light irradiation. 1 Compared to other conventional wastewater treatment methods, photocatalysis is considered the best approach to degrade organic waste without any toxic by-products. Besides, semiconductor-based photocatalyst usage was extended to degrade the inorganic and microbial-pollutants from wastewater. 2 Among various semiconducting metal oxide photocatalysts, ZnO is a widely studied photocatalyst due to its non-toxic nature, and excellent electrical, optical, and oxidationresistant properties. 3 ZnO (n-type semiconductor) has a wide band gap of 3.4 eV with a large excitation binding energy (60 meV) value at ambient temperature. Moreover, ZnO has a higher quantum efficiency compared to TiO 2 in certain cases. 4 Though it has several advantages, the commercialization of ZnO is still affected by low photocatalytic performance, low light absorption ability, and a high rate of electron-hole recombination. To tackle these disadvantages of ZnO, several methods for example metal, non-metal doping, making the heterojunction with other semiconductors are used to improve the photocatalytic efficiency. 5 Rare earth metal ion doping is an effective way to enhance the ZnO photocatalytic efficiency by creating the impurity level in the forbidden energy gap. In addition to this, rare-earth elements can create the trap state in ZnO crystal to reduce the electron-hole recombination rate. 6 Several rare-earth metals like Gd, La, Sm, and Yt are used as doping ions with ZnO to enrich the photocatalytic degradation efficiency. Among various rare earth metals, Gd has received great attention due to partially lled f-orbital, which is benecial for improving the ZnO photocatalytic degradation process. [7][8][9][10][11][12] Further, photocatalytic degradation efficiency was improved by doping the non-metals to ZnO in addition to the metal ions. The non-metal doping preferred for narrowing the bandgap of ZnO. Combination of N 2p and O 2p states aids in narrowing the bandgap of the semiconductor and resulting in the higher absorption in the visible light region. Furthermore, the existence of an N atom in the ZnO nanoparticles can advance the charge carrier transportation, enhance the specic surface area, and constrains the charge carrier's recombination rate. [13][14][15][16] Rimzhim Gupta et al. reported that Cu and N co-doped ZnO prepared by combustion and hydrothermal methods exhibited the better photocatalytic inactivation of E. coli bacteria under UV and visible light illumination. 13 Mahesh Dhonde et al., reported the Cu and N co-doped TiO 2 photoanode shows 11.70% of power conversion efficiency in dye-sensitized solar cell application. 14 Inspired by the above-mentioned literature reports, herein we have presented the Gd and N co-doped ZnO nanoparticle for photocatalytic organic pollutant degradation under UV-Vis and direct solar light irradiation for the rst time. The prepared pure and co-doped ZnO photocatalysts were well analyzed through various analytical characterization techniques to know the crystallite, morphological, vibrational, and optical properties. The prepared Gd and N co-doped ZnO exhibit the enhanced light absorption ability and improved photocatalytic degradation efficiency. Reactive oxygen radicals for the photocatalytic process and constancy of the prepared nanoparticles were examined by elemental trapping and reutilize experiments.

The synthesis procedure of Gd, N doped ZnO
Wet chemical co-precipitation method was used to prepare the pure, doped and co-doped ZnO nanoparticles. In brief, 5 mmol of zinc nitrate hexahydrate was dissolved in 50 ml of de-ionized water until the clear solution was obtained. Followed by 3 mol% of gadolinium nitrate and 0.5 g of melamine was added in the above solution. Then 2 g of NaOH (sodium hydroxide pellet) was added to the solution to form the precipitate. Followed by the precipitate was washed with ethanol and de-ionized water for numerous times and dried at 80 C for 5 h in a hot-air oven. The dried powder was calcined at 500 C for 2 h with a ramp rate of 10 C min À1 . The obtained sample was crushed with the help of an agate mortar and washed three times with ethanol to remove the residual carbon waste. Then the sample was named as Gd/N/ZnO. The similar process was followed for Gd/ZnO and N/ZnO exclude the addition of melamine and gadolinium nitrate correspondingly.

Characterization
Powder X-ray diffraction analysis of the synthesized photocatalysts was analysed through BRUKER USA D8 Advance, Davinci powder X-ray diffractometer. The surface nature of the nanoparticles was examined by FESEM (Thermoscientic Apreo S) and HRTEM (JEOL, JSM-2100plus) microscopes. The optical behaviour was studied by a Shimadzu UV3600+ spectrometer (UV-DRS) and HORIBA Fluorolog (PL). The surface elements conrmation of the nanoparticles was carried out by PHI Versaprobe III. The FTIR spectrum of the synthesized nanoparticles were examined using a SHIMADZU, IRTRACER-100 with KBr mode.

Photocatalytic degradation test
Methylene blue was chosen as a target pollutant to investigate the photocatalytic degradation efficiency of the prepared nanoparticles. Homemade, pyrex glass reactor (250 ml capacity) was used for photocatalytic degradation process. In typical, 100 mg of photocatalysts was added in 100 ml of (10 mg L À1 ) MB dye solution. The photocatalytic degradation process was carried out under 300 W halogen lamp (UV-Vis lamp) (84 319 lux) and direct sunlight (November 2020 in the period of 1 p.m to 2 p.m) (97 871 lux) irradiation. Initially, the dye solution and photocatalysts were stirred for 30 min under the dark condition to attain the adsorption-desorption equilibrium. At a regular interval of time, 3 ml of dye solution was taken out and examined by UV-Vis spectroscopy with an absorption maximum of 664 nm.

X-ray diffraction analysis
The crystalline structure of the prepared photocatalysts was studied by powder XRD analysis and the recorded patterns were displayed in Fig. 1. From the diffraction results, it was conrmed that all the samples have an analogous diffraction pattern and it is compatible with the standard JCPDS card (36-1451) and former literature result. 3 The diffraction planes (110) respectively. Gd doped ZnO shows the relatively low intense XRD peaks, which revealed the doping of Gd ions reduces the crystallinity of ZnO. Moreover, the new peak positioned at 28.63 related to the Gd 2 O 3 and this is wellmatched with the previous report. 12 There is no peak shi and new peaks were observed at N doped ZnO nanoparticle, which demonstrated N doping have not affected the ZnO crystal system. 17 The diffraction pattern of Gd and N co-doped ZnO shows lower intensity compared to pure and N doped ZnO, which revealed the Gd 3+ doping reduces the crystallinity and it is similar to Gd doped ZnO. Moreover, the diffraction peaks of Gd/N/ZnO shows the slight shi towards lower angle side compared to pure ZnO, which revealed that Gd 3+ ions are interstitially doped between the ZnO lattice planes. This interstitial doping leads to increases in the lattice space of ZnO. 12

FTIR analysis
The vibrational behaviour of the functional groups which are presented in the prepared nanoparticles were inspected by FTIR analysis. The transmittance FTIR spectra of the pure and codoped ZnO nanoparticles were shown in Fig. 2. Pure ZnO showed the FTIR peak positioned at 420 cm À1 related to the lattice oxygen and metal (Zn-O) bond. The FTIR spectrum associated to the metal-oxygen bond in Gd doped ZnO was broaden compared to pure ZnO. This result demonstrated the latticedistortion in metal-oxygen bonding due to the presence of Gd ion  doping. 12 N doped ZnO has the FTIR peaks positioned at 1385 and 1513 cm À1 attributed to stretching vibration of Zn-N and N-Zn-O bond respectively. This result provides the evidence for incorporation of N atom in the ZnO crystal system. 18 The broaden peak appeared at frequency values of 3100-3600 cm À1 attributed to stretching vibrational peak of surface adsorbed O-H molecules. The co-doped ZnO FTIR spectrum consists of all vibrational peaks related to Gd/ZnO and N/ZnO nanoparticles. From the FTIR spectrum, it can be concluded that all elements are presented in the prepared nanoparticles have chemically interacted with ZnO nanoparticle.

UV-Vis-DRS and PL analysis
Optical absorption behaviour of the synthesized photocatalysts was studied with the help of UV-Vis-DRS spectrum and is demonstrated in Fig. 3. Bare ZnO shows the absorption wavelength of 396 nm with energy-gap of 3.13 eV. The absorption wavelength of Gd doped ZnO shows a slight blue shi compared to pure ZnO. The Gd/ZnO nanoparticle exhibits the cut-off wavelength of 392 nm with energy-gap of 3.16 eV. This blue shi revealed the reduction of crystallization and quantum connement effect. The higher electronic population during the Gd doping in ZnO leads to the quantum connement effect and blue shi in optical absorption behaviour. 12,16 The UV-Vis-DRS absorption spectrum of N doped ZnO shows the redshi with a cut-off wavelength of 406 nm. This redshi credited to electronic state alteration in ZnO crystal system owing to the existence of N atom. Comparatively high 2p p-electrons in N atom creating the sub-energy level in ZnO between the conduction and valance band. This leads to visible light absorption at N doped ZnO nanoparticle. 19 The calculated bandgap energy of N/ ZnO was 3.05 eV. The co-doped ZnO nanoparticle shows the absorption wavelength of 401 nm with a bandgap of 3.09 eV. Hence, one could conclude that Gd and N doping can change the electronic level and boost the light absorption capability of co-doped ZnO nanoparticle.
Charge carriers recombination rate of the pure and doped ZnO nanoparticles during the photon illumination was analysed by photoluminescence spectrum with an excitation wavelength of 320 nm at room temperature and the outcomes were shown in Fig. 4. The PL spectrum demonstrated that the higher intensity attributed to fast charge carriers recombination, whereas the low intensity reveals the slow electron-hole recombination. The emission spectrum positioned at 388 nm attributed to charge carriers recombination at ZnO nanoparticle. 19 Compared to pure, Gd, and N doped ZnO, the Gd/N/ZnO nanoparticle exhibits lower emission peak intensity. This result implies that the Gd and N doping help to suppress the electron-hole recombination and it is benecial to enhance the photocatalytic performance.

FESEM and TEM analysis
The apparent morphologies of the synthesized photocatalysts were recognized by FESEM analysis and the recorded micrographs were displayed in Fig. 5(a-d). From FESEM images, it could be veried that the prepared pure and doped ZnO nanoparticles have the spherical like morphology with extremely agglomerated particles. The surface morphology of the Gd, N and Gd/N doped ZnO remains constant as that of pure ZnO nanoparticle. This result revealed that the doping atom does not affect the morphology of the particles. The existence of all atoms and the distribution of the elements of Gd/N/ZnO nanoparticle was investigated by EDS analysis and the images are depicted in Fig. 6. The elemental distribution and EDS spectrum show the presence of Zn, Gd, N and O atoms in the synthesised Gd/N co-doped ZnO nanoparticle and elements are uniformly distributed on the photocatalyst surface.
In additional, the morphology and particle size of the Gd/N/ ZnO nanoparticle was studied by TEM spectroscopy and the images are exposed in Fig. 7(a and b). TEM image of the Gd/N/ ZnO nanoparticle shows the various sizes (30-60 nm) of the agglomerated particle, which is well-matched with the FESEM micrographs. The high-resolution TEM micrograph of the Gd and N co-doped ZnO nanoparticle was displayed in Fig. 7(c). The lattice plane distance of 0.238 nm and 0.282 nm related to the diffraction plane (101) and (110) respectively. The SAED pattern of Gd/N/ZnO nanoparticle was shown in Fig. 7(d).

XPS analysis
The chemical structure and oxidation value of the elements existing in pure and Gd/N doped ZnO nanoparticles were examined by X-ray photoelectron spectroscopy and the results    13,20 The HRXPS spectrum of Gd 4d revealed the two characteristic peaks positioned at 140.78 eV and 143.88 eV related to the Gd 4d 5/2 and Gd 4d 3/2 respectively. The presence of these two deconvoluted peaks demonstrated that the Gd 3+ oxidation state in prepared nanoparticle. 21 From the high resolution, N 1s XPS spectrum (Fig. 8(c)

Photocatalytic analysis
To determine the photocatalytic behaviour of the prepared pure and doped ZnO nanoparticles was analysed by MB dye degradation under the UV-Vis and direct solar light irradiation. The obtained degradation results and their kinetic values are displayed in Fig. 9. The dye solution was magnetically stirred in the dark condition to reach the adsorption-desorption between the surface of the photocatalyst and dye solution. From the degradation result, it could be observed that aer the light irradiation the Mb dye gets degraded. This outcome conrmed the removal of MB is chiey due to the photocatalytic degradation. Likewise, the photolysis process conrmed the constancy of the MB dye molecule in the absence of light radiance. Under the UV-Vis light irradiation, the MB dye degraded up to 22% using pure ZnO nanoparticle, which is comparatively lower than Gd/ZnO (47%) and N/ZnO (63%) photocatalysts. This outcome conrmed that the photocatalytic degradation effectiveness of the ZnO photocatalyst was considerably upgraded by Gd and N doping. The calculated MB degradation efficiency of Gd/N/ZnO nanoparticle was 87% aer 60 min of UV-Vis light irradiation. The improved degradation effectiveness of the doped nanoparticles compared to pure ZnO credited to active light absorption performance and destruction of photo-induced charge carriers recombination. To govern the reaction rate constant for MB dye degradation under UV-Vis light radiation, pseudo-rst-order kinetic relation was used and the results are exposed in Fig. 9(b). The pseudo 1 st order kinetic relation is as follow [23][24][25][26] Ln(C/C 0 ) ¼ Àkt where C 0 and C refer to the dye concentration at initial and irradiation time. The term 'k' refers to the kinetic rate constant.
The calculated rate constant for MB dye degradation under UV-Vis light irradiation was 0.004, 0.01, 0.016 and 0.034 min À1 for pure, Gd, N and Gd/N co-doped ZnO nanoparticles respectively.
Further, the MB dye degradation was performed under direct sunlight irradiation using prepared ZnO nanoparticles and the obtained results are shown in Fig. 9(c). The recorded degradation efficiency using pure ZnO nanoparticle was 31% aer 60 min of light irradiation. The maximum MB degradation values of 56%, 71%, and 93% obtained using Gd/ZnO, N/ZnO, and Gd/N/ZnO respectively. Compared to UV-Vis light, the direct sunlight can degrade the MB more effectively. This is mainly attributed to the relatively high reaction temperature and highintensity light. 27 The calculated kinetic rate constant for MB dye degradation using Gd/N/ZnO was 0.027 min À1 , which is 3.3 and 2 times higher than the Gd/ZnO and N/ZnO respectively. Hence, it can be concluded that the synthesized photocatalytsts were valuable for the real application to waste water contaminant degradation under the illumination of sunlight.
In order to determine the reactive oxygen species for the MB degradation, the elemental trapping experiment was performed in the UV-Vis light irradiation and the results are displayed in Fig. 10. To conrm the production of holes (h + ), superoxide (O 2 À ) radical, and hydroxyl radical, EDTA (ethylenediaminetetraacetic acid disodium), BQ (benzoquinone) and IPA (isopropanol) were used. From elemental trapping results (Fig. 10), it could be detected that the addition of BQ greatly reduces the MB degradation efficiency and IPA plays a secondary role in the photocatalytic reaction. This result veried the superoxide and hydroxyl radicals are the main active element for photocatalytic degradation reaction. The addition of EDTA does not affect the degradation reaction. From the above-mentioned trapping test, we have suggested the possible reaction mechanism for MB dye degradation reaction under photon irradiation. The conduction and valance band potential of the prepared Gd and N co-doped ZnO nanoparticle was calculated using the following relation 27-31 where, c, E e and E g signify the electronegativity, electron energy in hydrogen scale (4.5 eV), and energygap of the photocatalyst. The calculated CB and VB values are À0.2 V (in V vs. NHE) and 2.89 V for Gd/N/ZnO nanoparticle.  The possible reaction mechanism of Gd/N/ZnO was shown in Fig. 11(a). When the photons are illuminated on the surface of the nanoparticles the valance band (VB) electrons get excited and travelled to the conduction band (CB). These electrons are trapped by the sub-energy level created by the Gd and N doping to ZnO crystal system. The doping of heteroatom can alter the electronic arrangement of the photocatalyst and it eliminates the limitation of the narrowed light usage. The metal atom doping generates the sub-energy level under the CB, whereas N doping produces the sub-energy level above the VB. These subenergy levels help to inhibit the excited electron-hole recombination. In this way, the formed conduction band electrons and valance band holes initiates the reduction and oxidation reactions respectively. The produced superoxide radicals by the reduction reaction combine with hydroxyl radicals and it generated the H 2 O 2 , which further produces the OHc radicals. The produced hydroxyl radicals can degrade the dye molecule. The probable reaction mechanism for the MB dye degradation process  Fig. 11(b). Five consecutive recycles were completed for MB dye degradation using Gd/N/ZnO nanoparticle. The photocatalyst was recovered by ultra-centrifuge process and it was washed with acetone and de-ionized water for three times. Then the recovered sample was dried at 60 C for 1 h to use the further cycles. For each recycling process, some amount of fresh photocatalyst was added to compensate for the loss ($15 wt%) during the recycling process. From the reusing experiment, it can be concluded that the Gd/ N/ZnO nanoparticle have the high stability and it is valuable for the organic pollutant degradation reaction. Furthermore, the produced photocatalysts reveals the outstanding photocatalytic degradation efficiency compared to the previous results and the comparison table was shown in Table 1.

Conclusion
In summary, the facile wet-chemical co-precipitation method was used to prepare the Gd and N co-doped ZnO nanoparticle. The foreign ion doping enhances the light absorption ability and suppresses the charge carrier's recombination, which is conrmed by UV-Vis-DRS and PL analysis respectively. The codoped ZnO nanoparticle exhibited better photocatalytic degradation efficiency under UV-Vis and direct solar light irradiation. The superoxide and hydroxyl radicals play a key role in the degradation reaction. Besides, the prepared photocatalyst  showed excellent recycling stability, which is benecial for the practical application. Hence, the presented reports demonstrated that Gd/N/ZnO could be promising photocatalyst for environmental remediation application.

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