Enhanced Photo Catalytic Activity of ZnO Nano Particles Co-doped with Rare Earth Elements (Nd and Sm) Under UV Light Illumination

In the present report, synthesis of zinc oxide (ZnO) nano particles (NPs) in pure form, 1 wt% of neodymium (Nd)-doped and 1 wt% of neodymium (Nd) and samarium (Sm) co-doped ZnO NPs in doped form by using simple co-precipitation method (samples namely PZ as pure, NZ as Nd-doped ZnO and NSZ as Nd–Sm co-doped ZnO NPs). Powder X-ray diffraction patterns of all the synthesized samples expose the hexagonal crystal structure of ZnO NPs without any impurity. The various functional groups presented in the synthesized samples were analyzed by Fourier-transform infrared spectroscopy studies. From Ultraviolet (UV)–Visible Diffuse Reflectance Spectroscopy (DRS), the band gap was found to be 2.81 eV, 2.90 eV and 3.10 eV respectively for pure, Nd-doped and Nd–Sm co-doped ZnO NPs. Photoluminescence (PL) spectrum displays the broad emission at 393 nm and 450 nm for all the synthesized samples. The agglomeration of flower-like morphology of pure ZnO NPs, flake-like structure of Nd-doped and rod-like morphology of Nd–Sm co-doped ZnO NPs were examined by scanning electron microscopy. The surface chemical composition of samples was carried out with X-ray photoelectron spectroscopy. The photocatalytic activity of the prepared samples for dye degradation of acid orange 7 (AO-7) and acid red 13 (AR-13) was studied under UV light. The result revealed that, the Nd–Sm co-doped ZnO NPs found to have efficient degradation candidate materials.


Introduction
ZnO NPs is a versatile n-type semiconducting material in the II-VI group elements with a direct band gap of 3.37 eV and possess the most varied nano crystalline designs. Also, it has high exit on energy (60 MeV), hexagonal wurtzite structure, low resistivity, good transparency, high electron mobility, non-toxicity, high photo stability and also has been used a countless remarkable applications like opto-electronics, solar cell, spintronics, sensors, gas sensors, antimicrobial, photoconductive, photo catalytic, PN junction diode liquid crystal display, magnetic storage media, lithium-ion battery and laser source [1][2][3][4][5]. Various synthesis methods such as, combustion method, hydro thermal method, polyol method, co-precipitation method, sono chemical method and simple soft chemical route and sol-gel method [6] were used to synthesis the ZnO NPs [7,8]. Among them methods, Coprecipitation method is the most convenient method for the synthesis of NPs due to its easiness in operation, low cost, no need of high temperature and simplicity when compared to other reported methods [9].
The clean environmental, safe drinking water and sufficient energy are highly polluted by several dyes coming out from the various factories like leather, textiles, printing, cosmetics, hair colouring, medical laboratories, convulsions mutagenic, plastics, foods, pharmaceuticals, teratogenic and other industries. In addition, most industrials dyes are easily soluble in water and which can cause severe disorders on aquatic organisms, humans and animals (affecting the brain, liver, skin, kidneys and nervous system) due to their high toxicity, low biodegradability, stability and mutability [10,11]. In past few years, several techniques have been used for the removal of organic dye pollutants from the industrials waste water. Among them, the photo catalysis methods are great treatment for the removal of organic dye pollutants from the industrial waste water due to low cost and environmentally friendly [12]. The various researchers are reported, the semiconducting nano structured ZnO is a good effective material for activity due to low cost, excellent biocompatibility, high red ox potential, outstanding chemical and physical durability, which is used for oxidative degradation of organic pollutants in wastewater [13,14]. In addition, the activity of ZnO NPs is artificial by the fast recombination of charge carriers and comparatively lower charge separation, which decrease the efficiency of photo catalytic activity [10]. Therefore, the activity of ZnO NPs can be easily improving the efficiency by doping or co-doping technique by using metals (Transition and Rare Earth metals). Also, efforts have been made by various material scientists to develop ZnO NPs with co-doped by rare earth metals such as Eu, Sm, Gd, Tb, Er and are significantly enhance the photo catalytic degradation of organic dye pollutants from the industrials waste water. In particular, rare earth metal doping with ZnO will have been band gap energy is increases and the recombination of charge carrier is reducing which will increases of photo catalytic efficiency. The improvement of performance under visible light using Sm doped ZnO NPs was reported by Mohd Faraz et al. [15]. Divya et al. [16], also reported that the enhanced photo catalytic activity of Nd-doped ZnO NPs using methylene blue dye [16]. The improved photo catalytic activity of Gd-Nd co-doped ZnO NRs was observed for methyl blue dye was reported by Javaid Akhtar et al. [17]. The better photo catalytic activity of Er-Yb co-doped ZnO NPs was observed for the degradation of methyl orange dye was reported by Irshad Ahmadet al. [18].
Hence, the synthesis of pure ZnO (PZ), Nd-doped ZnO (NZ) and Nd-Sm co-doped ZnO NPs (NSZ NPs) using a co-precipitation and study of their activity for degradation of acid orange 7 and acid red 13 under UV light illumination. Also, characterization such as structural and optical properties are also studied and reported.

Synthesis of ZnO NPs
0.5 M of Zn (NO 3 ) 3 ·2H 2 O was liquified into DD water (50 ml) under magnetic stirrer at room temperature for 10 min. After that, NaOH solution was added drop by drop into Zn (NO 3 ) 3 ·2H 2 O solution until the pH attains 12 and the mixture was stirred continuously (2 h). The solution transformed to black precipitates. The precipitate was filtered and washed three times using ethanol and DD water. The collected precipitate was dried at 80 °C (hot oven). Finally, it was annealed at 250 °C for 2 h. The final product was labeled as PZ NPs.

Photo Catalytic Experiment
Two separate dyes, namely acid orange 7 (AO-7) and acid red 13 (AR-13), were used for the photo catalytic study. Two dyes were evaluated individually from 100 ml of 20 mg/L concentrations and 0.1 mg of the photo catalyst was been used as a catalyst dose. With the commercial configuration of the photo-reactor, the process is accomplished. The module comprising of a 150 W tungsten halogen lamp for the source of light with emission wavelengths ranging around 340 to 850 nm was installed between the borosilicate well with double-wall having outlet and inlet wherein the cooling liquid was circulating to optimize the temperature. The solution of 2 M sodium nitrate (NaNO 2 ) was used here as cooling liquid to screen the UV light produced from the light source (320-430 nm). The aliquots of the reaction mixture were collected at regular intervals (20 min) and characterized by a UV-Vis spectrophotometer in order to analyze the 1 3 dye molecules disintegration. Using the following equation further calculates the percentage of degradation of the dye molecules with regard to time.
where c 0 is the preliminary dye solution intensity before illumination and c is the dye solution intensity after illumination with light at time (t).  [19,20]. From the Fig. 2, when the addition of Nd 3+ and Sm 3+ dopants, the high peak intensity of (plane-101) of NZ and NSZ NPs decreases and shifted towards the lower

Structural Properties
angle side when compared with PZ NPs, which indicates Nd 3+ and Sm 3+ ions are incorporated into the ZnO crystals lattice [21]. This shift is also caused by the strain developed due to the interaction of a larger ionic radius of Nd 3+ (0.99 Å) and Sm 3+ (1.08 Å) with smaller ionic radii of Zn 2+ (0.74 Å) [18,22]. Debye Scherrer's formula can be used to estimate the average crystallite size of samples.
where, K, λ, θ, β represent the shape factor (0.9), X-ray wavelength (0.1541 nm), diffraction angle and full width half maximum respectively. The average crystallite size measured is found to be increased as 16.23 nm for PZ NPs, 23.10 nm for NZ NPs and 25.84 nm for NSZ NPs respectively. As a result, the increase in average crystalline size for the PZ and NZ NPs was observed due to the addition of dopants Nd and Nd-Sm. The vibrational properties of synthesis samples were investigated using FT-IR Tracer 100 Shimadzu spectrophotometer (vibrational scale range of 4000-400 cm −1 ). FT-IR spectra of (a) PZ, (b) NZ and (c) NSZ NPs are shown in Fig. 3. The absorption peak that emerges at 3426 cm −1 and 1613 cm −1 are attributed to the H-O-H stretching and O-H bending vibration due to surface adsorbed H 2 O groups during the studies [23]. The peaks located at 1452 cm −1 and 867 cm −1 reveal the C-H stretching vibration, which arises from alcohol used in synthesis [24]. The peak at 1051 cm −1 and 712 cm −1 are ascribed due to stretching vibrations of C-O via CO 2 absorbed in the air medium [25]. Zn-O vibrational mode is confirmed from the observed peak positioned at 561 cm −1 [25]. As a result, the peak of ZnO NPs is suppressed by adding co-dopant of Nd 3+ and Sm 3+ ions, which confirms it is successfully doped into the Zn-O lattice.

Optical Properties
Optical properties of synthesized materials were explored by Hitachi UV3010-Visible Diffuse Reflectance Spectroscopy (UV -DRS). Figure 4 exposes the UV-DRS Reflectance spectra of (a) PZ, (b  [26,27]. According to UV reports of the synthesized samples, the NSZ NPs harvested high photon energy during the light illumination and it is responsible for enhanced photo catalytic activity (Fig. 5). A Photoluminescence (PL) spectrum of synthesized samples was investigated by florescence spectrometer (Hitachi F-4500) at room temperature. The PL spectra of (a) PZ, (b) NZ (c) NSZ NPs (excitation wavelength = 320 nm) are shown in Fig. 6. From the results, the two emission peaks Near Band Edge (NBE) emission (393 nm) and blue emission (450 nm) are attributed to the photo-induced electronhole recombination of free excitons on the surface of ZnO NPs [28,29]. When compared with PZ NPs, the PL emission intensity of NZ and NSZ NPs decreases with respect to addition of dopants (Nd 3+ and Sm 3+ ions) into ZnO NPs due to various defects such as interstitial oxygen, zinc and oxygen vacancy [30]. The results confirm that the synthesized material possesses potential capability to promote photo catalytic activity.

Morphological Properties
The surface morphology of the synthesized samples was investigated by scanning electron microscope (SEM-ZEISS EV018) with Energy Dispersive X-ray spectrum (EDX). SEM images of (a and b) PZ, (c and d) NZ and (e and f) NSZ NPs at the resolution of 1 μm and 200 nm (Fig. 7). Figure 7a and b reveals the agglomeration of flower-like morphology of PZ NPs, Fig. 7c and d shows the agglomerated flake-like morphology of NZ NPs and Fig. 7e and f presents the perfectly oriented rod-like morphology of NSZ NPs. From the results, the morphology of the prepared material is improved while doping. The coexistence of Nd 3+ and Sm 3+ ions in the ZnO lattice might be the reason for the improved the morphology of the synthesized NPs. Figure 8 shows the EDAX spectra of (a) PZ, (b) NZ NPs and (c) NSZ NPs. From the

X-Ray Photoelectron Spectroscopy (XPS)
The carrier transporting property of the semiconductors (NZ and NSZ NPs) are closely related with their chemical states, vacancies and defects [31,32]. In order to investigate these characteristics XPS were carried out for NSZ NPs is shown in Fig. 9a-d. Figure 9a represents the core spectrum of Zn 2p of NSZ NPs. From Fig. 9a it can be observed that Zn 2p is deconvoluted and fitted with three symmetrical signals of Zn 2p 3/2 (~ 1045 eV) and Zn 2p 1/2 (~ 1020 eV) states due to spin coupling. The Zn 2p 3/2 states attributed to the hydroxyl group linkage of Zn 2+ on the ZnO surface, while Zn 2p 1/2 corresponds to the atomic linkage of zinc with oxygen species [33,34]. (Fig. 9a) with slight shift in binding energy, the shift in binding energy may be attributed to the decrease in electron density of ZnO upon co-doping with Nd-Sm. Further, the shift in binding energy can also be related with diffusive process with respect to the number of atoms in chemical states. Therefore, the shift observed in NSZ NPs was due to more diffused atoms over the surface of the sample as compared with bare ZnO [35]. Figure 9b represents the O1s XPS spectrum of NSZ NPs. The O 1 s spectrum (Fig. 9b) could be possibly deconvoluted and fitted with three different states viz.: (i) adsorption of OH − or aqueous species over the surface of ZnO at higher binding energy; (ii) oxygen vacancies/deficiency at medium binding energy and (iii) Zn atoms surrounding the O 2− ions in the wurtzite structure at the lower binding energy. Among these different states the OH − possess very low signal as compared with the oxygen deficiencies signal implying higher possibility for the conversion of OH − into O − species at higher temperature (during post treatment of synthesis). During the elevation of temperature, there is higher possibility for filling of oxygen in the vacancy state at the outside wurtzite structure of ZnO. Moreover, the higher concentration of oxygen vacancy may also induce the carrier concentration of the materials NSZ NPs an enhancement in the Fermi energy level [36,37]. Figure 9c and d shows the XPS peaks observed for Nd 3d and Sm 3d of the Nd-Sm (1 wt%) co-doped ZnO NPs, two distinguished peaks ~ 983 eV (Nd 3d 5/2 ) and ~ 1006 eV (Nd 3d 5/2 ) and one distinguished peak ~ 1084 eV (Sm 3d) for Nd-Sm (1 wt%) co-doped ZnO NPs respectively [38]. The survey spectrum recorded for (a) NZ NPs and (b) NSZ NPs is shown in Fig. 10. The observed peaks are in good agreement with earlier reports.

Photo Catalytic Activity
The photo catalytic reaction encompassing the heterogeneous nano photo catalyst generally takes place underneath the fundamental aspect that the incident photon with equal energy to that of the material's band gap energy is consumed by the valence band (VB) electrons of that substances when light hits the semiconductor materials and excited to the conductive band (CB). Consequently, in the photo catalyst substance, electron-hole isolation is formed. Such CB electrons are then interfered in the dye solution with reactive oxygen species and generate superoxide anions(O •− 2 ) . H 2 O in the reaction mixture interact with the holes in the VB, which then it produces hydroxide radical (•OH). Such formed superoxide anions ( O •− 2 ) and (•OH) hydroxide radicals then interact with molecules and mineralize the dye from toxic to non-toxic. The advanced oxidation processes (AOP's) are commonly referred to such set chemical reactions as mentioned above.
The descriptions of photo catalytic experimental setup are already mentioned in this manuscript in section-2 (experimental method). Two distinct organic textile dyes, such as Acid Orange 7 (AO-7) and Acid Red 13 (AR-13), were taken in proper proportions in separate beakers to analyze the photo catalytic ability of the synthesized Nano particles. From all the three synthesized materials, 0.1 mg of catalyst was taken and combined individually with the dye solutions. To maintain absorption/desorption stability between the dye solution and the catalyst, the dye solutions with catalyst dosage are gently stirred to blend well enough and held in the dark for 1 h. Afterwards, to activate a photo 1 3 catalytic process, the dye solutions are moved to the photoreactor framework. The lamp was continually illuminated, and to ensure optimum interaction between the nano catalyst and dye molecules, the catalyst loaded dye solution was mixed softly and steadily. To evaluate the breakdown of the dye molecules, the aliquots of the dye solutions are taken and analyzed with a UV-spectrophotometer. In Figs. 11a-c and 12a-c accordingly, the decomposition UV spectrum of PZ, NZ and NSZ NPs loaded AO-7 and AR-13 dyes has been shown. The absorption maximum intensity peaks obtained for AO-7 and AR -13 at 486 nm and 550 nm, respectively, from the spectrum and are reduced considerably as the reaction time increases. Hence, these factors prove the decay of dye molecules along with time. For each interval of time, the C/C 0 was computed and a plot was sketched between successive C/C 0 values to corresponding reaction time periods for all synthesized materials PZ, NZ and NSZ NPs to evaluate the proportion of the dye molecules destruction and is being illustrated in Fig. 13a, b for AO 7 and AR 13 dyes, respectively. The obtained degradation efficiency (%) verses reaction time (T) is shown in Fig. 14a and b. The performance of degradation PZ, NZ and NSZ NPs is measured as 52%, 75%, 82% for AO-7 dye and 50%, 67%, 80% for AR-13 dye around 120 min. The maximum destruction of AO-7 and AR-13 dye respectively reaches 82% and 80% with UV radiation by NSZ photo catalyst at 120 min. According to the materials, the degradation effectiveness extends from PZ to NZ to NSZ NPs. From the results, the photon induced recombination of charge carrier is suppressed by addition of Nd 3+ and Sm 3+ ions into ZnO NPs, resulting in NSZ degradation efficiency increase.

Reaction Mechanism for the Degradation Process
It is generally believed that the rate limiting factors of photocatalytic mechanism of semiconductors are electron hole pair rate and electron transport from ZnO surface to adsorbed O 2 molecules [18]. When ZnO is modified by Nd-Sm co-doping, the excited electrons in the CB can be captured by dopants, which acts as electron capture centre and significantly suppresses the electron hole recombination rate.  and AR-13) molecules adsorbed on the surface to degrade the pollutants [18]. The formation of [ •OH ] in the presence of NSZ NPs could be understood as follows It is verified that the reaction mechanism follows the pseudo first-order kinetics for the degradation phenomenon and the reaction rate for the degradation process was determined through the Langmuir-Hinshelwood relationship. A plot between ln (C/C 0 ) with their corresponding reaction period is displayed in Fig. 15a and b for AO 7 and AR 13 dyes separately.
It was concluded from all of the above results obtained that the decomposition percentage was enhanced by providing Sm 3+ element into the nanostructures of ZnO. Further the 1wt% of Nd-Sm co-doped ZnO NPs for both AO-7 and AR-13 dyes received the maximum percentage of deterioration. With the addition in the Nd 3+ and Sm 3+ ions into the ZnO crystal lattice the optical properties of the material is altered heavily. The optical absorption range of the material is blue shifted which is directly proportional to the Nd 3+ /Sm 3+ concentrations, and this is confirmed from the DRS spectrum. Hence, this increase in the optical band gap value will help the photo-generated electrons to stay in the conduction band for a while and thus the ln c c 0 = kt photo catalytic behaviour of the material is enhanced. Also, the impurity Sm and Nd atoms will create the intermediate energy levels in the material, so that the photo-excited electrons will be trapped at these sub energy levels and thus it reduces the electron-hole recombination rate which is confirmed by PL spectrum. This phenomenon is also one of the reasons for the enhancement in photo catalytic activity of the synthesized materials. The degradation mechanism is schematically represented in Fig. 16. The calculated photo catalytic parameters for all the produced materials were comparatively listed in the Table 1.

Conclusions
Pure (PZ), Nd-doped ZnO (NZ) and Nd-Sm co-doped ZnO (NSZ) NPs (1 wt%) were successfully synthesized by using a simple co-precipitation process for photocatalytic application. The hexagonal crystal structure of all synthesized samples without any impurities was confirmed by PXRD pattern. From the Fourier-transform infrared spectroscopy (FTIR) studies, the various functional groups are present in the synthesized samples was identified. From the DRS spectra, the energy band gap increase from 2.81 to 3.10 eV were observed by the addition of dopants (Nd-Sm). The decrease in intensity at near band edge and blue emission was observed in PL spectrum due to the addition of dopant Nd and Nd-Sm as co-dopant with ZnO NPs. From SEM images, the addition of dopants has modified the surface morphology of PZ NPs. The surface chemical composition of samples was carried out with X-ray photoelectron spectroscopy (XPS). Photocatalytic activity of the synthesis samples for dye degradation of acid orange 7 (AO-7) and acid red 13 (AR-13) was studied under UV light. The good degradation efficiency of acid orange 7 (AO-7) is 52, 75 and 82% and acid red 13 (AR-13) is 50, 67 and 80% for PZ, NZ and NSZ NPs was observed. The NSZ NPs has shown higher degradation efficiency as compared to PZ and NZ NPs. Under UV light illumination, the NSZ NPs achieved 82% and 80% degradation of AO-7 and AR-13 dye respectively after 120 min of irradiation. Also, the apparent rate constant and reaction kinetics are investigated. Since, the NSZ NPs is highly bio-compatible and in expensive, it may be a good option for dye degradation purposes.