Wu Et Al-2013-Crystal Research and Technology

Cryst. Res. Technol. 48, No. 3, 145–152 (2013) / DOI 10.1002/crat.201200438

Structure and optical properties of Mg-doped ZnO nanoparticlesby polyacrylamide method

Yan Wu*, Jin Yun, Linqin Wang, and Xiang Yang

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China

Received 21 October 2012, revised 30 January 2013, accepted 31 January 2013Published online 27 February 2013

Key words Mg-doped ZnO nanoparticles, polyacrylamide method, optical properties.

Mg-doped ZnO (MgxZn1-xO) nanoparticles with precise stoichiometry are synthesized through polyacrylamidepolymer method. Calcination of the polymer precursor at 650 ◦C gives particles of the homogeneous solidsolution of the MgxZn1-xO system in the composition range (x < 0.15). ZnO doping with Mg causes shrinkageof lattice parameter c. The synthesized MgxZn1-xO nanoparticles are typically with the diameter of 70–85 nm.Blue shift of band gap with the Mg-content is demonstrated, and photoluminescence (PL) from ZnO has beenfound to be tunable in a wide range from green to blue through Mg doping. The blue-related PL thereforeappeared to be caused by energetic shifts of the valence band and/or the conduction band of ZnO. MgxZn1-xOnanoparticles synthesized by polyacrylamide-gel method after modified by polyethylene glycol surfactant havea remarkable improvement of stability in the ethanol solvent, indicating that these MZO nanoparticles couldbe considered as the candidate for the application of solution–processed technologies for optoelectronics atambient temperature conditions.

C© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

ZnO, an important semiconductor material with direct band gap 3.2 eV, shows attractive potential in UV laserdiodes and light emitting diodes due to its large exciton binding energy (∼60 meV) at room temperature. ZnOnanostructures have been also extensively investigated in order to develop more efficient optoelectronic devices[1–3]. One of the mostly challenges of application of these optoelectronics is to develop a simple low-costmanufacturing method based on solution process at ambient temperature conditions [2,4–6].

Recently Mg-doped ZnO materials have attracted much attention because of their unique UV-luminescentproperties based on radiative recombination of the electron−hole pairs [7,8]. Stoichiometry and homogeneity ofcomposition are key to the potential applications of such mixed metal oxides [9]. Previous attempts at dopingnanocrystals have been fraught with problems because the synthetic schemes used to dope frequently yieldinhomogeniously doped materials. Wet chemistry synthesis route seems to meet these requirements, enablingthe preparation of metal ion doped ZnO with a high crystalline quality and precise control of stoichiometryby adjusting the different ion proportions in the solution. Fabricating doped ZnO nanoparticles in the networksof hydro-, micro-, and nano-gel systems is also considered as the most important approach due to its directapplicability in various catalyst and electronic devices [10–13]. Polyacrylamide gel process is a well-knownmethod for synthesizing various mixed metal ions oxide nanoparticles at a large scale. This polymer gel methodhas been used to prepare many different oxide nanoparticles, metallic and oxide compounds, such as NdFe10Mo2

[14] and Ce1-xBixO2-x/2 solid electrolytes [15]. With polymeric chains forming a network, metal ions are entrappedevenly within the polyacrylamide gel, which is helpful for forming uniform oxide nanoparticles [16]. Therefore,in this study, we develop a solution-processing method, i.e. polymer gel synthesis of smaller size and finerdistribution of magnesium doped zinc oxide nanoparticles in hydrogel networks and test its suspension stability

∗ Corresponding author: e-mail: [email protected]

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146 Y. Wu et al.: Structure and optical properties of Mg-doped ZnO nanoparticles by polyacrylamide method

under the treatment of polyethylene glycol (PEG) surfactant, which could be as the candidate metal oxidematerials for the future application of solution-processed optoelectronics at ambient temperature conditions.

2 Experimental

2.1 Synthesis of MZO nanoparticles The system MgxZn1-xO (MZO) was prepared as polycrystallinenanoparticles with various compositions (0 ≤ x ≤ 0.15) by applying the sol-gel technique. Mg-doped ZnOnanoparticles were synthesized by the following procedure. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O),Magnesium acetate (Mg(CH3COO)2), were used as starting precursors to prepare solution in water. The pH valueof aqueous solution was adjusted to 5 by adding HNO3. Acrylamide (AM) and N, N′-methylenebisacrylamide(MABM) were used as a monomer and as a network reagent, respectively. After mixed AM and MABM (themass ratio is 5:1 if not specially mentioned), the mixture were added to 0.2 mol/L metal salt aqueous solution.The aqueous solution was heated to solute completely, and then polymerized at 80 ◦C for 2 hours to obtain a whitetranslucent wet gel by a solicitation action of ammonium persulfate (NH4)2S2O8. The amount of (NH4)2S2O8 is0.1% of the mass amount of AM/MABM mixture. The wet gel was dried to obtain a dried gel in a drying cabinetat 120 ◦C for about 10 hours. The xerogel was calcined at 400 ◦C for 1 hour to burn the organic compounds.Finally the preheated powder was sintered at high temperature for 1 hour to form the crystallized nanoparticlesand prepared for different characterization.

2.2 Characterization of MZO nanoparticles Thermal decomposed properties were characterized by STA409 PC simultaneous Thermogravimetric Analyzer and Differential Scanning Calorimeter (TGA-DSC), with theheating rate of 10 ◦C/min in air. The structural properties were studied by XRD spectra recorded using BrukerAXS-D8 X-ray diffractometer operated at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5406 Å). The datawere collected with a step size of 0.01◦ and a counting time of 5 s per step in the 2θ range from 10◦ to 70◦. FEIQuanta200 Environmental Scanning Electron Microscopy equipped with energy-dispersive X-ray spectroscopy(EDX) was operated at 25 kV. Ultraviolet-visible (UV-vis) absorption experiments were performed on a Lambda35, Perkin-Elmer double beam UV-vis absorption spectrometer equipped with a deuterium lamp and a tungsten-halogen lamp. The samples for this study were used in the form of powder and pure BaSO4 used as reference.The optical absorption measurements of the prepared samples were done by pressing the MZO nanopowders intocopper sample holders with the diameter of 10 mm. Room temperature photoluminescence (PL) of the sampleswere recorded on a Jasco Instruments FP-6500 fluorescence spectrophotometer with a xenon arc lamp as theexcitation source.

2.3 Evaluation of dispersion stability Different concentrations of PEG surfactant in ethanol (25 mg/ml,5 mg/ml, and 2.5 mg/ml) were prepared. Then MZO (Mg0.05Zn0.95O, 0.5 g) nanopowders were added into eachbatch of PEG solutions for ultrasonic treatment. The ultrasonic agitation time was 60 min with the bath temperature50 ◦C. Sequentially the suspension was evaporated into dried powders in the preheated oven with the temperatureof 120 ◦C for 24 h. Finally each batch of dried powders mixed with 100 ml ethanol was under ultrasonic treatmentfor 30 min to be ready for the evaluation of dispersion stability.

The apparent sedimentation stability (S) was assessed by measuring the sedimentation speed of the ZnO andPEG-ZnO nanoparticles suspension in ethanol (0.5 g in 100 ml ethanol) with the aging time. They were locatedinto a glass tube of 0.5 cm inner diameter and 20 cm length, respectively. Then the sedimentation stabilities weremonitored at a given aging time, as follows [17]:

S(%) = lt

l0× 100 (1)

where, l0 is the length of initial opaque dispersion and lt is the length of sediment part at a given time t in thetube, respectively.

3 Results and discussion

3.1 Thermogravimetric/Differential thermal analysis Figure 1 shows the TGA-DSC curves of 5% Mg-doped ZnO xerogel precursor under the synthesized condition of 5:1 mass ratio of the monomer AM and thenetwork reagent MABM. The xerogel slightly loses weight up to 5% when the temperature increasing from 50to 288 ◦C, which could correspond to the decomposition of small-molecule end-groups like those of C–N, C–H

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Cryst. Res. Technol. 48, No. 3 (2013) 147

Fig. 1 TG-DSC curves of 5% Mg-doped ZnO xerogelprecursor.

Fig. 2 The X-ray diffraction patterns of the5% Mg-doped ZnO nanopowders samples cal-cinating at different temperatures.

and C = O. When further increasing the temperature up to about 650 ◦C, the weight drops down to about 50%.During this temperature range, there are two broad exothermal peaks centered at 540 ◦C and 590 ◦C, whichcorresponds to the decomposition of carboxyl groups [18]. After 650 ◦C, the weight tends to keep constant.Thereby we conclude that the organic compounds have been completely ruled out and the crystallized oxideforms.

3.2 Structural analysis

3.2.1 The effect of calcinating temperature

Figure 2 presents four different spectra of 5% Mg-doped ZnO nanoparticles calcined at different temperatures.For as-synthesized powder calcined at 400 ◦C, there are seven broad peaks corresponding to (100), (002),(101), (102), (110), (103), and (112) peaks which corresponding to the standard ZnO pattern of hexagonal ZnO(JCPDS card No. 361451). But there are two more extra peaks marked by red arrows for the impurities of zinccarboxyl-containing compounds. When the annealing temperature increasing to 550 ◦C, all the peaks belong tothe hexagonal lattice of ZnO, and no indication of a secondary phase is found. The peaks become sharp with theannealing temperature increasing further. The lattice parameters a and c were calculated from (100) and (002)oriented XRD peaks using equations (2) and (3) [19]. The grain sizes of the crystallite were calculated from thefull width at half maximum (FWHM) of the peak by using the Debye Scherrer formula [20]. The average particlesizes (d) of MZO samples were estimated from the width of lines in the XRD spectrum using the Sherrer’sequation (4).

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Fig. 3 The X-ray diffraction patterns of theZnO powder samples synthesized with differentmolar ratio of the monomer and the networkreagent.

a = λ√3 sin θ

(2)

c = λ

sin θ(3)

d = 0.9λ

βcosθ(4)

where λ is the X-ray wavelength (1.5406 Å), β is the width of the line at the half-maximum intensity, and θ is thehalf of the diffraction angle. The d was calculated for the highest three intense peaks of ZnO spectrum which are(101), (002), and (100). The average calculated particle size results are 39 ± 3 nm, 54 ± 3 nm, 72 ± 3 nm, and76 ± 3 nm for the samples calcinating at 400 ◦C, 550 ◦C, 650 ◦C, and 750 ◦C, respectively.

3.2.2 The effect of the mass ratio of monomer and network reagent

We introduced the AM-MABM copolymer in order to investigate the factors affecting the nanoparticles size bycontrolling the crystalline growth process. The mass ratio R of monomer and the network reagent varies from3:1 to 7:1. The calcinating temperature is 650 ◦C and the calcinating time is 1 hour. figure 3 presents their X-raydiffraction patterns, which demonstrate that all of them have formed the crystallized wurtzite ZnO structure.Based on the Sherrer’s equation, the particle sizes are 61 ± 3 nm, 72 ± 3 nm, 72 ± 3 nm, 77 ± 3 nm and 100 ±3 nm for the ratio of 3:1, 4:1, 5:1, 6:1, and 7:1 respectively. It is obvious that the nanoparticles size increasesdramatically when the ratio of the monomer and the network reagent in the polymer synthesis is large than 6:1.The mass ratio of 6:1 could be the crucial point to change the network into different structure because that therelative volumes of the network structures vary depending on the crosslinker concentration [16,21].

3.2.3 The effect of the various Mg dopant concentrations

The full X-ray diffractions of the powder samples with varying Mg doping rate (x = 0–0.15) at 650 ◦C for30 min in the air are depicted in figure 4a. Up to x < 0.15, all the peaks belong to the hexagonal lattice of ZnO,and no indication of a secondary phase is found. For x ≥ 0.15 two phases ZnO and MgO were identified in thediffraction patterns. Therefore, the solubility limit of Mg2+ in ZnO was determined to be lower than x = 0.15in this work. This value apparently higher than that of about 4.25% by the solvothermal method [22], but lowerthan those of 33% with pulse laser deposition [23], 16.5% by metaorganic chemical vapor deposition [24]. Whileit is close to those of about 15% reported with rheological phase reaction precursor route [1] and 10% whenusing a solution chemistry method [25]. The composition and percentage of Mg in doped ZnO samples were



Table 1 Calculated lattice parameters of MZO samples calcined at 650 ◦C. Estimated standard deviations for the leastsignificant digit are given in parentheses.

Sample a (Å) c (Å) c/a Particle size (nm)

ZnO 3.250(6) 5.206(3) 1.601(7) 77 ± 3Mg0.02Zn0.98O 3.260(6) 5.205(6) 1.601(4) 82 ± 3Mg0.05Zn0.95O 3.250(7) 5.205(2) 1.601(2) 84 ± 3Mg0.10Zn0.90O 3.250(8) 5.203(7) 1.600(7) 72 ± 3Mg0.125Zn0.975O 3.251(3) 5.202(2) 1.600(2) 77 ± 3Mg0.15Zn0.85O 3.251(9) 5.205(4) 1.600(8) 81 ± 3

Fig. 4 The full (a) and enlarged (b) X-ray diffraction patterns of the powder samples MgxZn1-xO for x = 0.01,0.05, 0.1, 0.125 and 0.15, running upwards.

confirmed by energy dispersive X-ray (EDX) spectroscopy. The substitution of Mg2+ in ZnO was confirmed bylattice parameters as a function of the dopant content of Mg. The lattice parameters are important to determinewhether Mg (II) have been doped into the lattice of ZnO nanoparticles or not. Variation of the lattice parametersa, c, c/a ratio and particle sizes for samples calcined at 650 ◦C are presented in table 1. The crystal sizes of thesesamples are around 70–85 nm. The lattice parameter c of the MZO clearly shrinks with the dopant level up to12.5% as shown in figure 4b, which is in agreement with the data obtained from poly(acrylic acid) based method[10,23] and epitaxially grown films on sapphire by a laser deposition method [23]. However, when the dopantconcentration further increases to 15%, both the parameter c and the c/a ratio start increasing. These results revealthat the limitation concentration of Mg dopant is less than 15%, which is consistent with the results of XRDpattern of Mg0.15Zn0.85O sample presented in figure 4.

3.3 Optical properties Figure 5 depicts the absorption spectra of all the prepared ZnO and Mg doped ZnOsamples with the crystal sizes around 70–85 nm. Mg doping shifts the absorption onset to blue (360–338 nm)of Mg doping levels from 0 to 15%, indicating an increase of the band gap. Moreover, with the increase in Mgconcentration, the band edge shifts toward the lower wavelength (higher energy) side. Higher doping levels resultin more pronounced shifts. The indicated band gaps are similar to that which has been observed on other formsof ZnO doped with Mg [24,27]. The systematic shift with doping levels suggests that the introduction of dopantions, rather than size effects, bring about these changes [20,28]. Further, a number of previous experiments andtheoretical investigations have suggested that size effects have little or no influence on the band structure of ZnOnanocrystals with diameters greater than 7.0 nm [28,29] .The indicated band gap shifts are similar to those resultswhich have been observed on other forms of ZnO doped with Mg and Cd [24,27,30]. We therefore attributethe origin of the increase of band gap to the influence of dopant ions. Further proof follows from the emissioncharacteristics of the nanoparticles.

The room temperature PL spectra of Mg-doped ZnO nanoparticles excited at 325 nm are shown in figure 6.All samples show a broad green band with a blue-shift by increasing Mg doping concentration compared tothe emission peak of undoped sample. The green emission involves transition from the band edge (or shallow



Fig. 5 UV-vis spectra of Mg-doped ZnO nanoparticles.

Fig. 6 Room temperature photoluminescencespectra of Mg-doped ZnO nanoparticles.

level close to band) to a defect level. This emission could be due to oxygen interstitials. Also, with increasingdoping concentration, red shift of emission band decreased, which could arise from variation of surface levelsand defects levels [28]. The presence of the blue-shifting is remarkable because it suggests that the nanocrystalsdo not produce extra defects due to the introduction of the Mg ions [27]. The band edge emission shifts to theblue with increasing levels of Mg doping, reflecting the change in the exciton energy seen in the absorptionspectra.

3.4 Stability of the MZO nanoparticles modified by PEG surfactant All the MZO particles have thesimilar sedimentation stability. Hereby we take Mg0.05Zn0.95O as an example. The sedimentation stabilities ofthe bare MZO (Mg0.05Zn0.95O) nanoparticles and various concentrations of PEG modified MZO nanoparticlesdispersed in ethanol were assessed and shown in figure 7. All these nanoparticles are in the diameter range of70–85 nm, as shown in table 1, and also they are confirmed by SEM images (not shown here). The results showa remarkable difference. The first nanosuspension (with bare MZO) precipitated at a higher rate. The bare MZOnanoparticles were aggregated together because of the high surface polarity. Conventional inorganic particleshave a tendency to separate easily in solvents, because of their high density and low compatibility [31,32]. Aftermodified with PEG surfactant, the precipitation rate of the suspension decreases dramatically. The aggregationof the inorganic nanoparticles is prevented. The stability increases with the concentration of the surfactant PEG.MZO nanoparticles modified with 25 mg/ml PEG maintain their initial stability of above 85% for more than 22 h,which is qualified for the application of solution-processed technologies [17,33].



Fig. 7 Apparent sedimentation stability ofMZO modified with PEG in ethanol solvent.

4 Conclusions

We have successfully synthesized nanoparticles of Mg-doped ZnO (MgxZn1-xO, x < 0.15) with the diameter rangeof 70–85 nm by using polyacrylamide polymer method. The variation of the lattice parameters and decreaseof c/a ratio of MZO samples suggest that Mg2+ has doped into ZnO lattice and the dopant concentration isless than 15%. SEM observation shows that all MgxZn1-xO samples are an assembly of ellipsoidal nanoparticleswith tiny grain size of 60–80 nm. Introduction of Mg2+ does not change the particle size and particle shape.UV-vis absorption results indicate that the Mg doping shifts the absorption onset towards lower wavelengths(370–350 nm) with increasing of Mg doping levels from 0 to 12.5%, indicating an increase of the bandgap. Theincrease of band gap is attributed to the influence of dopant ions. Photoluminescence measurements confirmthese results. MZO nanoparticles modified by PEG suspension in ethanol solvent has a remarkable improvementof stability in ethanol solvent maintaining the initial stability of above 85% for more than 22 hours. It indicatesthat these nanocrystals could be considered as the candidate for the application of MZO nanoparticles in thesolution-processing technologies for optoelectronics.

Acknowledgments We thank the Fundamental Research Founds for National Universities, China University of Geosciences(Wuhan) for financial support.

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Wu Et Al-2013-Crystal Research and Technology