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Preparation, characterisation and photocatalytic activity of La-doped ZnO nanopowders synthesised using auto-combustion.


Fabrication and characterisation of semiconducting nanostructured materials have received considerable attention over the last few years due to increasing influence on our everyday life (Suwanboon and Amornpitoksuk, 2011; Wang and Herron, 1991). Zinc oxide (ZnO) has attracted interest due to its photocatalytic ability useful for the degradation of environmental pollutants and is attractive alternative to Ti[O.sub.2] as a photoactive catalyst (Kaneva et al., 2011; Anandan et al., 2007). ZnO has exhibited superior performance with lower cost compared to Ti[O.sub.2] in degrading organic dyes in acidic and basic media. It has a hexagonal wurtzite structure with an [<<]-type conductivity (Reddy et al., 2011; Chen et al., 1998). The structural, optical, electrical and physical properties of ZnO nanoparticles and the effects of doping on these properties have been studied earlier by several researchers (Ahmad et al., 2013; Shinde etal., 2006; Ismail etal., 2001). Dramatic changes in the electrical and optical properties of ZnO and unique properties due to doping have been observed by Bouderbala et al. (2008). Due to its useful properties, ZnO has been used in photocatalytic applications (Xiao et al., 2015; Zhou et al., 2009). ZnO is an ideal catalyst in photocatalysis since it is nontoxic with holes of strong oxidising power and being inexpensive (Peng et al., 2007). When ZnO nanoparticle is illuminated with a light of appropriate wavelength, the valence band potential is positive enough to generate hydroxyl radicals at the surface, and the conduction band potential is negative enough to reduce molecular [O.sub.2]. The hydroxyl radical is a powerful oxidising agent and can attack organic pollutants present at or near the surface of the ZnO to degrade it. However, the photocatalytic efficiency at the present stage is still very low, which is mainly caused by the fast recombination of photogenerated electron-hole pairs during photocatalysis. Thus, further improving the photocatalytic efficiency is still a major challenge in the research field of photocatalysis until now. Therefore, for enhancing photocatalytic activity of ZnO, doping of metal or nonmetal ions is a useful technique (Abed et al., 2015; Elangoran et al., 2015). Metal doping induces localized states between the valence band and the conduction band. These mid-gap states could trap temporarily the photogenerated electronhole pairs, thus suppress the recombination of charge carriers (Wan et al., 2015; Zhao et al., 2015). Recent research has been focused on the doping of ZnO by lanthanide ions with 4f electron configuration. Lanthanide ions can form complexes with various Lewis bases including organic acids, amines, aldehydes, alcohols, and thiols by the interaction of the functional groups with their f-orbital (Ahmad et al., 2015). Thus, doping lanthanide ions into a ZnO matrix could provide a means to concentrate organic pollutants on the semiconductor surface and therefore, can improve the separation efficiency of photo-induced electron-hole pairs of ZnO to enhance the photoreactivity (Ahmad et al., 2015; Ciciliati et al., 2015).

In this study, pure ZnO and La-doped ZnO using the auto-combustion technique has been synthesised producing highly pure nanocrystalline powders with large surface area at relatively low temperatures (Hwang and Wu, 2004). Variations in the properties of ZnO powders were studied as a function of the lanthanum dopant concentrations.

Materials and Methods

Synthesis of undoped and La-doped ZnO nanopowders. Pure and La-doped ZnO nanopowders were synthesised using the combustion technique. Zinc nitrate (Zn[(N[O.sub.3]).sub.2].6[H.sub.2]O) and glycine (N[H.sub.2]C[H.sub.2]COOH) were used as starting reagents. The zinc nitrate served as a source of zinc and as an oxidising agent whereas glycine acted as a fuel (Ahmad et al., 2013).

Precursor formation. For pure ZnO, homogeneous mixture of glycine and zinc nitrate was obtained by mixing them together in the optimised molar ratio (Ahmad et al., 2012a) through magnetic stirring. This resulted in the formation of transparent slurry by absorbing moisture from air due to the hygroscopic nature of zinc nitrate. The slurry was then dehydrated at130[degree]C to obtain precursor using a hot plate.

Auto-ignition. The precursor was heated at 200 [degree]C to cause precursor ignition. During combustion the evolution of gases occurred with generation of heat. Heat treatment at 700 [degree]C was done for 3 h resulting in the liberation of gases and formation of dry, crystalline white ZnO powders. For the preparation of La-doped ZnO samples, lanthanum nitrate [La[(N[O.sub.3]).sub.3].6[H.sub.2]O] was added to the initial mixture of zinc nitrate and glycine. Rest of the process was similar to that used for pure ZnO. The following chemical reaction occurs during the synthesis of La-doped ZnO samples:


Characterisation. Powder X-ray diffraction (XRD) patterns of as-synthesised ZnO and La-doped ZnO samples were recorded using a Rigaku Dmax-IIIA X-ray diffractometer with Ni filtered Cu K[alpha] radiation source. The surface morphology, particle size and compositional analysis of photocatalysts were examined using a scanning electron microscope (Hitachi S-4800) with EDX and transmission electron microscope (JEOL JEM 1200EX) and X-ray photoelectron spectroscopy (XPS, VG ESCALAB MARK II) with a monochromatic Mg K[alpha] X-ray source. Specific surface areas of samples were determined using Brunauer Emmett Teller (BET) surface area analyzer (NOVA 2200e Quantachrome, USA) using nitrogen as a purge gas. UV-vis diffuse reflectance spectra (DRS) were measured in the range of 380-460 nm using a Hitachi U-4100 UV-vis pectrometer. PL emission spectra were recorded using Hitachi F-4500 fluorescence spectrophotometer. The sample excitation was made at 325 nm at room temperature.

Measurement of photocatalytic activity. Photocatalytic activities of ZnO and La-doped ZnO nanopowders were estimated by monitoring the degradation of methyl orange (MO) as a model compound in a self-assembled apparatus under natural sunlight radiation source. In this study, photocatalytic activities under natural sunlight were investigated. The peak at 464 nm was used to monitor the photocatalytic degradation of MO. For the photocatalytic experiment, 50 mg photocatalysts were suspended in MO aqueous solution (50 mL) with a concentration of 20 mg/L in a beaker. The suspension was magnetically stirred for 30 min to reach the adsorption/desorption equilibration without light exposure. This was followed by a photocatalytic reaction started by the exposure of the desired light. The temperature of the suspension was kept at about 20 [degree]C by an external coolingjacket with recycled water. After a setup exposure time, 5 mL suspension was sampled, centrifuged and the supernatant was taken out for uv-vis absorption spectrum measurement. The intensity of the main absorption peak of the MO dye was referred to as the measure of the residual dye concentration.

Analytical method. Total organic carbon (TOC) content was measured after the degradation of model dye in the presence of the photocatalysts under sunlight irradiation (Mukherjee, 2011). After photodegradation, the total organic carbon content of model dye was observed to decrease with time. The decrease of carbon content indicates the degradation of the organic dye into nontoxic decomposed compounds. Moreover, the mineralisation of substrate was measured by chemical oxygen demand (COD) reduction method. Chemical oxygen demand of textile effluent was estimated before and after the photocatalytic treatment with a standard dichromate method, using COD digester. Arsenic effluent was diluted to facilitate light penetration through solution and the initial COD of diluted effluent was 412 mg/L. The amount of catalysts was 50 mg/25 mL and an average intensity of sunlight was 1.20 x [10.sup.5] lux during the photocatalytic measurements. The percentage photodegradation efficiency ([eta]) was calculated from the following expression:

[eta] = [CODi-CODf/CODi] x 100


CODi = before treatment CODf=after treatment

All the experiments were performed under the same experimental conditions such as sunlight irradiation (between 9 am and 3 pm), constant temperature, pH and photocatalyst load etc.

Results and Discussion

XRD analysis. Figure 1 shows powder X-ray diffraction patterns of pure ZnO and La-doped ZnO as a function of lanthanum dopant concentrations. The physical characteristics such as crystallite size and unit cell parameters are presented in Table 1. The strong and sharp peaks existing in the XRD spectra show the presence of wurtzite structure of ZnO. The strong peaks are located at an angle (2[theta]) of 31.7[degree], 34.4[degree] and 36.2[degree] correspond to (1 0 0), (0 0 2) and (1 0 1) along with the other peaks which are found at the angles of 47.5o, 56.6[degree], 62.9[degree], 66.4[degree], 67.9[degree], 69.2[degree], 72.7[degree] and 77.0[degree] correspond to (1 0 2), (1 1 0), (1 0 3), (2 0 0), (11 2), (2 0 1),(0 0 4) and (2 0 2) planes, which all are found in correspondence to those given in JCPDS 05-0664 and previous studies (Ghouri et al., 2014; Ahmad et al. 2013). The diffraction patterns of La-doped ZnO were similar to the undoped ZnO showing that lanthanum doping in ZnO did not affect its crystal structure.This also indicates that [La.sup.3+] is uniformly dispersed on ZnO nanoparticles in the form of small [La.sub.2][O.sub.3] cluster (Anandan et al., 2007). The observed peaks for Ladoped ZnO, however, are wider with the width increasing with increasing La-content. La-doping caused a random variation in diffraction peak intensities. Intensity decreased upto2.0 at% La-content and then increased. The change in the intensity and width of the observed diffraction peaks as a function of La-content is attributed to the decrease in the crystallite size (Suwanboon and Amornpitoksuk, 2011; Flickyngerova et al., 2008; Anandan et al., 2007), which goes on changing inversely with change in La concentration. Lattice parameters 'a' and 'c' were estimated using the formula:

[[1/dhkl].sup.2] = 4/3 [[h.sup.2] + [k.sup.2] + hk/[a.sup.2]] + [1.sup.2]/[c.sup.2]


Lattice parameters and crystallite size variation as functions of La-content were calculated. The calculated values of lattice parameters 'a' and 'c' are 3.254 [Angstrom] and 5.208 [Angstrom] while for 2.0 at.% La-doped ZnO are 3.262 [Angstrom] and 5.199[Angstrom], respectively. Lattice parameters increased with increase in La-content which is obvious as ionic radius of [Zn.sup.+2] is 0.074 nm and that of [La.sup.+3] is 0.106 (Suwanboon and Amornpitoksuk, 2011) and lattice parameters should expand if ionic radius of the dopant is greater as compared to Zn ions (Dole et al., 2011). Average crystallites sizes were estimated using the full width at half maximum (FWHM) of highly intense diffraction peak in Debye-Scherrer's formula (Culity and Stock, 2001).

t = 0.9[lambda]/[beta]cos[theta]

Average crystallite size estimated for pure ZnO to be 27 nm which decreases up to 19 nm with increase in La-content. It is interesting to note that the particle size of La doped ZnO is smaller as compared with that of the pure ZnO. The decrease in the particle size of La-doped ZnO is mainly attributed to the formation of La-O-Zn on the surface of the doped samples, which hinders the growth of crystal grains (Korake et al., 2014; Jia et al., 2009; Anandan et al., 2007).

SEM, EDX and TEM. Figure 2(a-d) displays the surface morphology of pure ZnO and 2.0 at% La-doped ZnO nanopowders. SEM micrographs of pristine ZnO nanopowders exhibited clusters of tiny particles, whilst 2.0 at% La-doped ZnO nanopowders revealed flakes of ultrafine particles with voids or holes. The high porosity in the nanopowders could be attributed to the liberation of large amount of gaseous products such as [H.sub.2]O vapours, C[O.sub.2] and [N.sub.2] during combustion process (Ahmad et al., 2013b). TEM images in Fig. 2, show that, for undoped ZnO nanopowders, the particles are spherical and. larger and, while for 2.0 at% La-doped ZnO nanopowders, they are smaller and quasi spherical. Lanthanum doping thus changes the morphology of the final product. TEM investigations of all of the samples revealed that the crystallites are of nanometer size. The diameter of ZnO and 2.0 at% La-doped ZnO were found to be about 33 and 24 nm, respectively agreeing with the XRD results. The chemical compositional analysis is important for monitoring the concentration of the dopant. Figure 3 shows the EDX spectra of undoped ZnO and La-doped ZnO nanoparticles. The EDX spectrum of the La-doped ZnO displays a clear lanthanum line at 4.65 keV showing La doping. No traces of other elements were found in the spectra which confirm the purity of the samples. Also Table 2 shows the atomic percentage of Zn, La and O for undoped and La-doped ZnO samples. Here the actual concentration of La is found to be 1.76 at% for 2.0 at% La-doped ZnO sample.


XPS analysis. The nature and the co-ordination of the elements present in the La-doped ZnO were investigated by X-ray photoelectron spectroscopy (XPS). The La 3d spectrum of 2.0 at% La-doped ZnO is shown in Fig. 4.



The spectrum exhibits two components with the binding energy values of 837.1 eV and 840.0 eV. The highest energy contribution is assigned to the bonding between lanthanum and zinc while the lowest energy contribution is attributed to the bonding in the lanthanum oxide clusters (Anandan et al., 2007). These results demonstrated that the bonding between the lanthanum and zinc has occurred and the interaction between the lanthanum oxide clusters and the ZnO surface are strongly developed.

UV-visible spectra. Optical properties of ZnO and La-doped ZnO photocatalysts with different La-content were measured by diffuse reflectance spectroscopy (DRS) at room temperature. The absorbance spectra of different samples are shown in Fig. 5. The band edge for the ZnO sample was appeared at about 387 nm while the band edges of the La-doped ZnO samples was shifted to shorter wavelength regions with higher absorption intensity after La-doping. The blue shift in wavelength is attributed to gradual increase in band gap of ZnO on La-doping. The absorbance coefficient (a) was calculated from the raw absorbance data to obtain the optical band gap (Eg). The band gap values were thus determined by the extrapolation of the linear portion of the [([alpha]hv).sup.2] curve versus the photon energy hv and are shown in Table 3. With increasing the La-content from 0.0 at% to 8.0 at%, the band gap energies increased in the range of 3.24-3.27 eV as shown in Fig. 6. The following explanation may be given; the band gap of ZnO is about 3.37 eV whereas that of [La.sub.2][O.sub.3] is about 5.5 eV (Jia et al., 2009), which is much higher than that of pure ZnO. An additional energy level could be formed above the conduction band of ZnO due to the La-doping. Furthermore, this could be attributed to the quantum size effect as well as the strong interaction between the surface oxides of Zn and La. These observations strongly suggest that the La doping significantly affects the particle size and hence the absorbance properties. Moreover, these results are in good agreement with the conclusion derived from the XRD results and a similar result has also been reported in La-doped Ti[O.sub.2] by Liqiang et al.(2004) and La-doped ZnO by Anandan et al. (2007).



Photoluminescence. Photoluminescence (PL) is a powerful tool to investigate optical properties of wide band gap semiconductors. A comparison of PL emission spectra intensity of prepared samples excited with 325 nm wavelengths ranging up to 600 nm is shown in Fig. 7. PL emission intensity decreased with La-concentration i.e. pure ZnO has highest emission intensity, which can be attributed to several defects such as oxygen and zinc vacancies or oxygen and zinc interstitials (Ismail et al., 2001). However, the addition of La ions tends to stop the reduction in the band gap resulting in lower PL emission intensity first and then increasing with increasing dopant La-content.


Photocatalytic activity. Pristine ZnO and doped ZnO are well known catalysts used in photocatalytic degradation of pollutants. Photocatalytic activities of the ZnO and La-doped ZnO samples were determined through a series of experiments carried out for degradation of methyl orange (MO) in aqueous suspension under natural sunlight irradiation. A blank experiment study of MO degradation without catalyst loading under the same condition was also observed. The results indicate that the mere photolysis can be ignored as the corresponding degradation is about 0.3% after irradiations for 2 h. Figure 8 shows a comparison of photocatalytic performances of pure ZnO and Ladoped ZnO nanoparticles. About 9% of the MO was adsorbed for ZnO sample upon stirring for 30 min in the dark and increased for La-doped ZnO in comparison to ZnO nanopowders. The pronounced adsorption is due to increase in specific surface area, increase in porosity and defects creation within band gap by lanthanum doping. Photocatalytic degradation of MO follows pseudo first-order reaction kinetics for low dye concentrations

ln([C.sub.o]/C) = [k.sub.appt]



[] = the apparent constant, used as the basic kinetic parameter for different photocatalysts; [C.sub.o] = the initial concentration of MO in aqueous solution and C = the residual concentration of MO at time t.

The initial degradation rate ([r.sub.o]=[][C.sub.o]) of 20 mg/L MO with different photocatalysts was studied and the results are presented in Table 3. The photodegradation rate of MO follows the ascending order; 1.0 at% La-doped ZnO > 2.0 at% La-doped ZnO > 4.0 at% La-doped ZnO >8.0 at% La-doped ZnO > ZnO > commercial ZnO. The degradation rate under sunlight for 1.0 at% La-doped ZnO photocatalyst was found superior to other La-doped ZnO and ZnO samples. The specific surface areas were seen to increase gradually with the increasing lanthanum content (Table 3) enhancing the adsorption performance. The photocatalytic efficiency of La-doped ZnO mainly originates from the formation of La-O-Zn bonds on ZnO surface. Photocatalytic degradation rate of MO is observed to be lower under La-doped ZnO. Table 3 shows that the degradation rate of MO enhanced with La-content up to 1.0 at% La-content and then decreased with La-loading leading to the conclusion that 1.0 at% doping concentration is the optimum loading to ZnO that is more efficient in separating photoinduced electron-hole pairs which in turn increases photoactivity of ZnO. The high photocatalytic activity at1.0 at% La-doped ZnO can be explained as follows: lanthanum ions in ZnO can result in the formation of a space charge layer on the surface of ZnO which acts as a barrier for recombination of photoinduced electron-hole pairs resulting in an increase in the photocatalytic activity. This increase was observed upto 1.0 at% La-doping in the present study. The decrease in photocatalytic activity of ZnO beyond 1.0 at% La-doping isjustified as: The incorporation of lanthanum beyond 1.0 at% is supposed to likely result in the formation of more La-O-Zn bond on ZnO surface due to which surface charge region is negatively affected causing a decrease in its efficiency to separate the photoinduced electron-hole pairs and instead of acting as traps sites for photoinduced electronhole pairs, it acts as recombination centre (Jia et al., 2009). This results in decrease of photocatalytic activity of ZnO.

A simple mechanism to understand the enhancement of photocatalytic activity of La-doped ZnO photocatalysts is the loading of lanthanum metals on the ZnO surface, which can accelerate the transport of photogenerated electrons to the outer systems. The transfer of electrons to the deposited metal causes these to become negatively charged (Behnajady et al., 2009). In the photocatalytic systems, lanthanum deposited on the ZnO surface increased the photocatalytic activity by accelerating the transfer of electrons to dissolved oxygen molecules. Superoxide anion radical was prepared as a result of oxygen reduction by transfer of trapped electrons from lanthanum metal to oxygen. Consequently, the recombination of the photo-generated carriers were effectively suppressed leading to an increase in the photo-oxidation efficiency. The photocatalyst 1.0 at% La-doped ZnO exhibited superior catalytic performance, but further increase in the concentration of lanthanum would decrease photocatalytic activity because too much doping of lanthanum is likely to serve as recombination centre than as trap sites for the charge transfer at the interface. The total organic carbon (TOC) content was measured after the degradation of methyl orange dye in the presence of the photocatalysts under sunlight irradiation. After photodegradation the total organic carbon content of MO was observed to decrease with time and the results are presented in Fig. 9. TOCo and TOCt are the initial concentration and the reaction concentration of MO, respectively. The experimental data are fitted using the pseudo-first-order kinetic equation.


During dye production and textile manufacturing processes an enormous amount of waste water containing dyestuffs with intensive colour and toxicity is introduced into the aquatic system. An effluent of this type has been considered for this photocatalytic degradation study using the same experimental conditions discussed in the experimental section. COD removal efficiencies of the effluent by ZnO, La-doped ZnO nanopowders photocatalysts under the sunlight irradiation are shown in Fig. 10. COD reduction confirms the destruction of the organic molecules in the effluents along with colour removal. The photo-stability of l.0 at% La-doped ZnO sample as catalyst under sunlight was also studied (Fig. 11). The photocatalytic activity of the sample did not decrease significantly after five successive cycles of degradation tests, indicating that the photocatalyst was fairly stable under the conditions used in this study.



Pure ZnO and La-doped ZnO nanoparticles were synthesised via fast and facile conventional combustion method and characterised using a variety of characterisation techniques. From XRD patterns all synthesised samples had a hexagonal wurtzite structure showing that La-doping has no structural effect on ZnO. It was found that [La.sup.3+] is uniformly dispersed on ZnO nanoparticles in the form of small [La.sub.2][O.sub.3] cluster. The XRD and UV-vis results revealed that the particle size of La-doped ZnO is much smaller as compared to that of pure ZnO and decreases with increasing La loading. Highly porous surface of La-doped ZnO was observed by SEM, which is critical for enhancing the adsorption and photocatalytic activity. The photocatalytic activity of La-doped ZnO for the degradation of MO was studied and the results are compared with ZnO and commercial ZnO. It was observed that the rate of degradation of MO over La-doped ZnO increases with increasing La loading up to 1.0 at% and then decreases. The TOC results demonstrated that La-doped ZnO requires shorter irradiation time for the complete mineralization of MO than pure ZnO. The relative photonic efficiencies and the photocatalytic activity of the 1.0 at% La-doped ZnO are much higher as compared to those of pure ZnO and commercial ZnO. It is concluded that, small particle size, separation of charge carriers, highly porous surface and larger specific surface area of La-doped ZnO are the major constituents for its enhanced photocatalytic activity in the present study.


Mukhtar Ahmad is thankful to Higher Education Commission (HEC) of Pakistan for providing financial assistance through Indigenous and IRSIP Scholarship Programme and the Zhejiang University of China for providing opportunity to work in its laboratories.


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Mukhtar Ahmad (ab) *, Eijaz Ahmed (a), Muhammad Ikram (a), Zhanglian Hong (b), Abdul Hafeez (c), Khalid Nadeem Riaz (d), Fezza Zafar (a), Niaz Ahmed Niaz (a) and Waqar Ahmed (ef)

(a) Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan

(b) State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China

(c) Science and Technology Division, University of Education, Lahore, Pakistan

(d) Department of Physics, University of Gujrat, Gujrat 50700, Pakistan

(e) Institute of Nanotechnology and Bioengineering, University of Central Lancashire, School of Medicine, Preston PR1 2HE, United Kingdom

(f) Dentistry and School of Pharmacy, Preston PR1 2HE, United Kingdom

(received August 18, 2014; revised August 9, 2015; accepted August 18, 2015)

* Author for correspondence; E-mail:
Table 1. Crystallite size and unit cell parameters of
pure ZnO and La-doped ZnO sample.

                                      Lattice parameters

Photocatalysts          Crystallite   'a'([Angstrom])   'c'([Angstrom])
                        size (nm)

Pure ZnO                27            3.254             5.208
1.0 at.% La-doped ZnO   24            3.258             5.203
2.0 at.% La-doped ZnO   22            3.262             5.199
4.0 at.% La-doped ZnO   23            3.263             5.195
8.0 at.% La-doped ZnO   19            3.265             5.186

Table 2. Chemical composition (atomic % of ZnO and La-doped ZnO

Element    Zn (at%)   O (at%)   La (at%)   Total (at%)

ZnO        50.68      49.32     -          100
2.0 at%    48.73      49.51     1.76       100

Table 3. Band gap energy, BET surface area of ZnO
and La-doped ZnO samples and apparent kinetic values
for the degradation MO model dye under sunlight

Photocatalyst            Band       BET
                         gap (eV)   surface area

ZnO                      3.24       33.4
1.0 at.% La-doped ZnO    3.25       37.3
2.0 at.% La-doped ZnO    3.25       39.5
4.0 at.% La-doped ZnO    3.26       42.3
8.0 at.% La-doped ZnO    3.27       45.1

                         For sunlight irradiation

Photocatalyst            [r.sub.0] (mg/L/min)   [t.sub.1/2] (min)

ZnO                      0.038                  217
1.0 at.% La-doped ZnO    0.162                  86
2.0 at.% La-doped ZnO    0.132                  104
4.0 at.% La-doped ZnO    0.106                  130
8.0 at.% La-doped ZnO    0.092                  149

Fig. 10. COD  values  of  the  treated  effluent  and
percentage photodegradation efficiency (h)
for different photocatalysts under sunlight

Photocatalysts          Efficiency (%)

ZnO (com.)                   49
ZnO                          56
1.0 at.% La-doped ZnO        70
2.0 at.% La-doped ZnO        65
4.0 at.% La-doped ZnO        62
8.0 at.% La-doped ZnO        58
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Author:Ahmad, Mukhtar; Ahmed, Eijaz; Ikram, Muhammad; Hong, Zhanglian; Hafeez, Abdul; Riaz, Khalid Nadeem;
Publication:Pakistan Journal of Scientific and Industrial Research Series A: Physical Sciences
Article Type:Report
Date:Jan 1, 2016
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