Crystallization of Disordered Nanosized ZnO Formed by Thermal

ARTICLE pubs.acs.org/crystal

Crystallization of Disordered Nanosized ZnO Formed by Thermal Decomposition of Nanocrystalline Hydrozincite S. V. Nistor,* L. C. Nistor, M. Stefan, D. Ghica, Gh. Aldica, and J. N. Barascu National Institute of Materials Physics, P.O. Box MG-7, Magurele, Ilfov, Romania

bS Supporting Information ABSTRACT: The formation and crystallization of disordered nanosized ZnO resulting from the thermal decomposition of nanocrystalline hydrozincite [Zn5(CO3)2(OH)6] has been observed and investigated during pulse annealing experiments up to 625 C in air or vacuum by electron paramagnetic resonance of trace amounts of substitutional Mn2+ impurity ions, in correlation with X-ray diffraction and transmission electron microscopy measurements. The mesoporous structure of the disordered ZnO, which initially forms in air and vacuum at 225 and 175 C, respectively, further transforms into nanocrystalline ZnO of increasing particle size and improved lattice quality at higher annealing temperatures. The crystallization process, which does not affect the concentration of the substitutional impurity ions, as well as the simultaneous presence of both disordered and crystalline phases, should be considered in further applications of the resulting nanosized ZnO.

1. INTRODUCTION The nanostructured direct wide band gap (3.37 eV) zinc oxide (ZnO) is very likely the most investigated binary semiconductor nanomaterial due to a broad range of possible high-performance applications in electronics, optics, photonics, sensing, energy, medicine, etc.1,2 Considering specific dopantcarrier magnetic exchange interactions3,4 and possible room temperature (RT) ferromagnetism,5,6 it is expected that doping with transition metal (TM) ions would result in a diluted ferromagnetic semiconductor nanomaterial for spintronic applications. Preparing ZnO nanocrystals of controlled size, morphology, and lattice perfection, homogeneously doped with TM ions, has proven to be a complex task. Besides the synthesis methods for preparing ZnO nanoparticles,79 the simpler thermal decomposition of the crystalline hydrozincite [Zn5(CO3)2(OH)6], or zinc carbonate basic (ZCB) doped with TM ions such as Mn2+ seems to lead to a promising material for such applications.10 The decomposition of hydrozincite into ZnO, which was found to take place above 220 C in air,11,12 has been investigated by thermogravimetry (TG) and differential thermal analysis (DTA), and the results have been correlated with data about the decomposition products obtained by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform IR (FTIR) spectroscopy.1315 To our knowledge, the associated structure and morphology transformations, essential in controlling the size, shape, and structure of the resulting particles, which determine the usefulness of the product, are still very little known. Here we present the results of a detailed study of the structural transformations that take place during pulse annealing up to 625 C, in air and in vacuum, of nanocrystalline hydrozincite containing an estimated few hundred parts per million of Mn2+ impurity ions (to be abbreviated as ZCB:Mn when the presence r 2011 American Chemical Society

of Mn2+ impurities is underlined). The dispersed substitutional Mn2+ ions were employed as trace paramagnetic ion probes highly sensitive to the local structural changes at the Zn2+/Mn2+ cation sites during and after the thermal decomposition of hydrozincite into ZnO, reflected in changes in the electron paramagnetic resonance (EPR) spectra. Their quantitative analysis, correlated with XRD, transmission electron microscopy (TEM), TG, and DTA data obtained from the same samples, revealed for the first time the initial formation of a strongly disordered ZnO phase. Its crystalline ordering was found to take place by further annealing at higher temperatures, with formation of ZnO nanocrystals of increasing size and improved lattice quality, a process more efficient in the ambient atmosphere than in vacuum. Our results demonstrate that EPR of the substitutional Mn2+ trace ions is a highly sensitive tool in investigating temperature-induced chemical ZCB f ZnO decomposition and structural changes in the resulting nanosized ZnO.

2. EXPERIMENTAL SECTION Commercial powdery reagent grade hydrozincite from Alfa Aesar (code 033398) has been employed in the present investigation. Although according to the chemical formula supplied by the producer [Zn5(CO3)2(OH)6xH2O] a certain amount of hydrating water was present, we could not detect it with the experimental techniques employed in this study. The XRD pattern analysis of the as received ZCB (see Figure S1 from the Supporting Information) corresponds to the Zn5(CO3)2(OH)6 compound [JCPD file no.19-1458], consisting of nanocrystals of 11 nm average size. A roughly estimated concentration of 200 ( 100 ppm Received: July 19, 2011 Revised: September 8, 2011 Published: September 12, 2011 5030

dx.doi.org/10.1021/cg2009286 | Cryst. Growth Des. 2011, 11, 5030–5038

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were prepared by crushing the investigated coarse samples, dispersing the resulting fine powder in ethanol and dropping it on holey carbon grids.

3. RESULTS AND DISCUSSION

Figure 1. The experimental (Exp) X-band (left) and Q-band (right) EPR spectra of the nanocrystalline ZCB:Mn are compared with the sum (Sim) of the simulated spectra (the lower curves in the figures) of the Mn2+(A) and Mn2+(B) centers and of the agglomerated Mn2+ phase, obtained with the parameters given in Table 1. Mn2+ ions, partly dispersed as substitutional ions at Zn2+ cations sites, was detected by EPR. The isolated Mn2+ ions were employed as paramagnetic ion probes in our study. Both X- (9.5 GHz) and Q-band (34 GHz) EPR measurements were performed at room temperature (RT), in the Research center for advanced ESR/EPR techniques (CetRESav). Details about the equipment and magnetic field calibration procedures can be found in refs 16 and 17 and at http://cetresav.infim.ro/. The powdery samples were inserted into pure fused silica EPR sample tubes. The pulse annealing treatments were performed in a temperature stabilized ((1 C) furnace at temperatures that were usually increased in steps of 25 C. The sample tube was kept at each increasing set temperature for 15 min and afterward cooled to RT for EPR measurements. Thus, we could investigate in a “frozen” sequence the temperature-induced transformations. The annealing was carried out either in air or in vacuum generated by a turbo pumping station. In the latter case, the sample tube was annealed to the set temperature after reaching 1 104 mbar. Afterward it was withdrawn from the furnace, cooled to RT, filled with 99.998% purity argon gas, and transferred to the EPR spectrometer. The EPR spectral parameters of the Mn2+ centers were determined with the EasySpin v.3.1 software.18 The analysis took into consideration both forbidden hyperfine transitions and line broadening effects due to parameter value fluctuations, which were included by employing the weighted summation procedure outlined in ref 19. Structural investigations were performed with a Bruker D8 Advance XRD diffractometer, in the θθ geometry with a Cu anode. After annealing at a certain temperature, a small quantity of the ZCB powder was extracted from the EPR sample tubes and measured as a thin layer on a glass plate. The Rietveld refinement of the experimental data was performed with the Topas software from Bruker. The 2θ range of 10 to 140 was swept for higher accuracy of the refined lattice parameters values. TG/DTA measurements were performed with a SETARAM Setsys Evolution 18 instrument, in the TG-DSC thermal analyzer mode, either in synthetic air (80% N2/20% O2) or in argon gas of 99.998% purity, at a flow rate of 16 mL/min. Weighted samples of ZCB:Mn were inserted and measured in open cylindrical alumina crucibles. To approximate the pulse annealing treatments, the temperature was ramped from 50 to 400 C in 3.5 h, with a precision of 0.01 C. For the TEM investigations, a conventional JEOL 200 CX TEM instrument has been employed, while the high-resolution transmission electron microscopy (HRTEM) study was performed on an analytical high-resolution JEOL ARM 200F electron microscope. The specimens

The EPR Investigation. Typical X- and Q-band EPR spectra of the as received ZCB are displayed in Figure 1. In the better resolved Q-band spectrum, one can clearly see two sets of six lines, partly superposed, attributed to the so-called Mn2+(A) and Mn2+(B) centers. Each set of lines, of almost equal intensity and separation, corresponds to the allowed hyperfine central transitions (Ms: 1/2 T 1/2, ΔMI = 0) from isolated Mn2+ ions in a certain lattice site. The integrated intensity ratio of the two sets of lines, representing their concentration ratio, was found to be ∼3:2. In the X-band spectrum, they are partly overlapping, creating a complex spectrum with the narrower lines superposed over a broader, Gaussian line, centered at g = 2.004 with ΔB = 54 mT line width, not visible in the Q-band, attributed to aggregated Mn2+ ions, possibly as a separate crystal phase. The Mn2+(A) and Mn2+(B) centers are very likely substitutional Mn2+ impurity ions at the Zn2+ lattice cation sites. The substitution is favored by the same electrical charge (+2) and close ionic radii values (0.080 nm for Mn2+ and 0.074 nm for Zn2+). Such localization in the ZCB lattice agrees with its crystalline structure, in which the Zn2+ cations are present at lattice sites with both octahedral and tetrahedral coordination, in the 3:2 ratio.20 Therefore, one assumes that the Mn2+(A) and Mn2+(B) centers correspond to octahedrally and tetrahedrally coordinated substitutional Mn2+ ions, respectively. The EPR spectra of the Mn2+ ions are described by the following spin Hamiltonian (SH) with usual notations:21,22 1 2 H ¼ μB S 3 g 3 B þ S 3 A 3 I þ D Sz SðS þ 1Þ 3

þ EðSx 2 Sy 2 Þ þ μN gN B 3 I

ð1Þ

Here the first two terms represent the main Zeeman and hyperfine interactions of the S = 5/2 electron spin with the external magnetic field B and the I = 5/2 nuclear spin of the 55Mn (100% abundance) isotope, respectively. The next two secondorder zero-field-splitting (ZFS) terms describe the interaction of the electron spin with the local axial and rhombic crystal fields, respectively, while the last term describes the nuclear Zeeman interaction. The fourth-order cubic and axial ZFS terms, characterized by the parameters a and (a F) respectively, were not included, because their contributions to the powder-like EPR spectra are small and difficult to determine.22 Accurate SH parameters of the two Mn2+ centers responsible for the EPR spectrum of the as received ZCB have been obtained by a two-step procedure described in ref 22, using both low(X-band) and high-frequency (Q-band) experimental spectra. The resulting parameters are given in Table 1, together with the SH parameters of the Mn2+ ions in some reference Zn-based (nano)crystalline materials.9,23,24 The validity of the resulting SH parameters is proven by the good fit of the simulated and experimental X- and Q-band spectra presented in Figure 1. Table 1 also contains the individual derivative peak-to-peak line width value, ΔB, used as input data in the spectrum simulations and the standard deviation, σ(D), associated with the Gaussian distribution of the D parameter 5031

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Table 1. The Spin Hamiltonian Parameters g, A, and D, As Well As the Individual Linewidth (ΔB) and the Standard Deviation σ(D) Describing the Line Broadening of the Mn2+ Centers in the As Received and Annealed Hydrozincite (ZCB)a Center/host 2+

Mn (A)/ZCB nanocrystals Mn2+(B)/ZCB nanocrystals

g 2.0012 ( 0.0002 2.0055 ( 0.0005

A [104 cm1]

|D| [104 cm1]

84.7 ( 0.3 86 ( 0.5

ΔB [mT]/σ(D) [104 cm1]

ref

150220

ΔB(Q) = 0.5/σ(D) = 70 for

this work

150240

D = 210 104 cm1 ΔB(Q) = 0.7/σ(D) = 70 for

this work

D = 240 104 cm1 Mn2+-d/disordered ZnO

2.0012 ( 0.0001

73.5 ( 0.1

2.0012 ( 0.0002

74 ( 0.2

242b

ΔB(X/Q) = 0.2/σ(D) = 102

this work

242 ( 4, |a F| = 5.5b

ΔB(X/Q) = 0.1/σ(D) = 17

this work

(625 C, air) Mn2+-c/ZnO nanocrystals (Tann = 625 C, air)

a

Mn2+ (0.035%)/ZnO single crystal

2.0012

73.4

225, |a F| = 5.5

σ(D) = 6.7

23

Mn2+(0.017%)/ZnO thin film Mn2+(0.02%)/ZnO nanocrystals

2.0012 1.999

75.05 74.0

238.5, |a F| = 5.5 236

σ(D) = 8.3 σ(D) = 4.7

23 9

Mn2+/ZnCO3 single crystal

2.003

A = 85.9, B = 86.7

41.4, |a F|= 11.4

24

SH parameter values in some reference (nano)crystalline materials are also presented. b Included in the fitting as a fixed parameter.

Figure 2. The most significant Q-band EPR spectra of the ZCB:Mn samples annealed in air (a) and in vacuum (b) up to 625 C reflecting the ZCB f ZnO transformation and subsequent ZnO crystallization. The six hyperfine transitions of the substitutional Mn2+(A) and Mn2+(B) centers in ZCB, of the Mn2+-d center in ZnO, and the two derivative peaks from the Mn2+-c centers in ZnO are marked with vertical bars.

values, which describes the spectral line broadening due to local crystal field fluctuations and thus reflects the degree of lattice disorder. One should mention that in the absence of any resolved fine structure feature or forbidden central transitions, we could only determine a range of values for the axial D parameter of both Mn2+centers in ZCB. Dramatic changes were observed in the EPR spectra during pulse annealing experiments up to 625 C. The decay of both Mn2+(A) and Mn2+(B) centers in the 225250 C and 175225 C temperature range in air and in vacuum, respectively, is accompanied by the formation of new Mn2+-type centers characterized by spectra with smaller hyperfine splitting. Thus, in Figure 2a,b, a narrower line spectrum from the so-called Mn2+-d centers, superimposed on the much broader line spectrum from the so-called Mn2+-c centers can be observed. The latter becomes dominant and better resolved at higher annealing temperatures. The hyperfine splitting of both centers is practically constant up to the maximum annealing temperature, which means that once formed no further changes in the neighboring ligand configuration takes place.25 As will be further shown, the observed transformation of the Mn2+(A) and Mn2+(B) centers into Mn2+-d and Mn2+-c centers reflects the structural changes associated with the temperature-induced ZCB f ZnO chemical

Figure 3. The experimental (Exp) X-band (left) and Q-band (right) EPR spectra of the ZCB:Mn samples pulse annealed in air up to 625 C are compared with the sum (Sim) of the simulated spectra of the Mn2+ centers in the two different ZnO phases plus the broad line of the agglomerated Mn2+ phase. Simulations with parameters from Table 1.

transformation. The decomposition of ZCB into ZnO is confirmed by the SH parameter values of the newly formed Mn2+-c and Mn2+-d centers. The corresponding values were determined22 from the best-resolved X- and Q-band EPR spectra of the samples pulse annealed in air up to 625 C (Figure 3), which did allow for the Mn2+-c centers the accurate determination of not only the g- and A-parameters, but also of the crystal field D-parameter, using the additional noncentral fine structure transitions (MS: (1/2 T (3/2, ΔMI = 0) present in the broad line spectrum as satellite lines to the six central hyperfine transitions. In the case of the narrow line Mn2+-d spectrum, which did not exhibit the noncentral fine structure lines, a very good line shape simulation of the experimental spectra could be obtained with the same D-value as for the Mn2+-c center but with a much larger σ(D)-parameter. The almost identical SH parameter values of the two centers (Table 1), very close to those reported for substitutional Mn2+ ions in ZnO single crystals, thin films, and nanocrystals9,23 but with different individual line width ΔB and line broadening parameters σ(D), strongly suggest that both 5032



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Figure 4. The sequence of Q-band EPR spectra recorded in the ZCB f ZnO transformation range, annealed (a) in air and (b) in vacuum. The two lowest field EPR lines of the Mn2+(A), Mn2+(B), and Mn2+-d centers, as well as both derivative line peak positions of the Mn2+-c centers, are marked with vertical bars.

centers represent Mn2+ ions localized in ZnO phases differing only by the degree of host lattice disorder. Thus, the broad line spectrum of the Mn2+-c centers, which can be simulated with σ(D) values comparable to those found in ZnO crystals and thin films,9,23 is due to Mn2+ ions localized in ZnO nanocrystals. Meanwhile, the narrow line spectrum of the Mn2+-d centers, described by very large σ(D) parameter values, reflecting extremely large fluctuations in the local crystal field, comparable with those found in amorphous materials,19 is attributed to Mn2+ ions localized in a highly disordered ZnO phase. The presence of such an effect has been previously demonstrated by Kliava19 in the case of the EPR spectra of Mn2+ ions in glasses or amorphous solids. Our results also demonstrate that the isolated Mn2+ ions substitutionally localized at the two types of inequivalent Zn2+ sites in ZCB maintain after the transformation their substitutional localization at the tetrahedrally coordinated Zn2+ sites in both disordered and nanocrystalline ZnO. One should also mention that the broad, featureless EPR line attributed to aggregated Mn2+ ions (see Figure 1a) is also present with the same intensity and parameters in the spectrum of the resulting ZnO (Figure 3), indicating the presence of a thermally stable phase, very likely a manganese oxide. This seems to be the most probable compound, based on both high stability of the agglomerated phase to the high-temperature annealing in both air and vacuum and the strong affinity of the manganese for oxygen. To explore in detail the formation and interdependence between the two resulting ZnO phases and the starting ZCB phase, we performed additional pulse annealing experiments in which the standard treatment in steps of 25 C for 15 min each was replaced in the transformation/decomposition range by a treatment in smaller 10 C annealing steps of 10 min each. The resulting EPR spectra recorded in the decomposition temperature range, in air and in vacuum, are displayed in Figure 4a,b, respectively. They show that although by annealing in air the formation of the Mn2+-d centers begins at Tann = 225 C, the bulk of the transformation takes place mainly in the very narrow 245255 C temperature range, where both Mn2+-d and Mn2+-c centers are produced. In the vacuum-annealed samples, the transformation begins at Tann = 175 C, also with the initial formation of Mn2+-d centers, the Mn2+-c centers being formed later and in a much smaller fraction. Based on these results, one concludes that in both cases the decomposition of the ZCB is a two step process, which begins with the formation of disordered ZnO, reflected in the initial

Figure 5. Variation of the σ(D) broadening parameter for Mn2+ ions in the ZnO nanocrystals (Mn2+-c centers) as a function of the annealing temperature in air (circles) and in vacuum (triangles). Inset: variation of the average size of the ZnO crystallites determined from XRD data as a function of the annealing temperature. The continuous lines are guide for the eye.

formation of Mn2+-d centers. The delayed formation of the crystallized ZnO phase, reflected in the presence of the broad line Mn2+-c spectrum, strongly suggests a second crystallization step of the disordered ZnO phase, a process that becomes significant for the samples annealed in air only in the final 10 C of the transformation range, that is, just below Tann = 255 C (Figure 4a). For the samples annealed in vacuum, the crystallization of the disordered ZnO is less significant even at the end of the ZCB f ZnO transformation at 235 C (Figure 4b). In this case, the presence of the broad line Mn2+-c spectrum from crystalline ZnO is barely visible as a broadening effect in the wings of the narrow lines from the Mn2+-d centers in disordered ZnO. At even higher annealing temperatures, there is a continuous increase in the relative intensity of the broad line Mn2+-c spectrum vs the narrow line Mn2+-d spectrum (see Figures 2a,b), explained by a further crystallization of the disordered ZnO. Simultaneously a narrowing of the broad line spectrum of the Mn2+-c centers in the nanocrystalline ZnO takes place. The effect is observed in the EPR spectra of the samples annealed in air at Tann g 300 C (Figure 2a) as an increased separation of the overlapping narrow and broad line spectra accompanied by the occurrence, for Tann g 500 C, of the previously mentioned satellite fine structure component lines on both sides of the six central hyperfine lines (Figure 3). In the samples annealed in vacuum, the crystallization of the disordered ZnO is less effective (Figure 2b), the broader component lines of the Mn2+-c centers becoming visible only at Tann > 300 C. The line broadening of the Mn2+-c center spectrum with annealing temperature increase is reflected in a monotonous decrease of the σ(D) broadening parameter determined from the quantitative analysis of the EPR spectra (Figure 5). The decrease 5033



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Table 2. The Results of Rietveld Refinement of the XRD Patterns for the as Received and Annealed ZCB pulse annealing

average

conditions

crystalline

lattice

crystallite

of ZCB

phase

parameters

size (nm)

hydrozincite a = 13.76 ( 0.04 Å

as received

11

b = 6.34 ( 0.02 Å c = 5.38 ( 0.02 Å β = 95.6 o ( 0.5 up to 255 C in air

ZnO

a = 3.250 ( 0.005 Å

8

c = 5.21 ( 0.01 Å Up to 400 C in air up to 625 C in air

ZnO

a = 3.249 ( 0.003 Å

15

ZnO

c = 5.209 ( 0.005 Å a = 3.249 ( 0.002 Å

39

c = 5.206 ( 0.005 Å up to 235 C in vacuum ZnO

a = 3.25 ( 0.005 Å

6


a = 3.251 ( 0.003 Å

13


a = 3.250 ( 0.003 Å

23

c = 5.205 ( 0.005 Å

Figure 6. The XRD diffractogram of a nanocrystalline ZCB sample: (a) as received, (b) pulse-annealed in air and in vacuum up to 255 and 235 C, respectively, and (c) up to 625 C.

of σ(D), which is a measure of the local crystal field fluctuations at the Mn2+ impurity ions,17,22 reflects an increase in the ZnO lattice order. Two effects are expected to contribute: the annealing of the lattice defects and a reduced influence of the ZnO nanocrystal surface, that is, the increase in the crystallites size, the last being confirmed by the results of XRD determinations at a few annealing temperatures (inset Figure 5). One should mention that in the case of the narrow line Mn2+-d spectrum from Mn2+ ions in disordered ZnO, the variation of the σ(D) parameter with the annealing temperature was found to be much smaller, within the experimental error limit. Thermal Analysis (TA) Measurements. According to the resulting traces of the nonisothermal TG/DTA measurements (Figure S2 from Supporting Information), the decomposition of ZCB takes place both in argon and in air in the 220255 C temperature range, with a maximum around 238 C, in general agreement with previously reported data obtained from other sorts of ZCB in similar conditions.13,14 The temperature decomposition range obtained from the TA measurements is practically identical with the 225255 C range determined from the EPR investigation, confirming the validity of our EPR data analysis. XRD Investigation. XRD patterns from ZCB samples pulse annealed in air and in vacuum, previously examined by EPR, have been also recorded. Figure 6 presents the patterns of the as received ZCB together with those obtained after pulse annealing at temperatures where significant changes in the EPR spectra associated with the formation of nanosized ZnO were observed. The results of the Rietveld analysis of the recorded diffractograms are presented in Table 2. They confirm the conclusions of EPR data analysis concerning the complete decomposition of the ZCB into ZnO by pulse annealing in air and vacuum up to 255 and 235 C,

respectively. One should mention that both ZCB and the resulting nanocrystalline ZnO exhibit a certain preferential Æ021æ and Æ002æ orientation, respectively, reflected in specific XRD patterns. This texture effect has been taken into account in the Rietveld refinement of the resulting patterns. By examining the data from Table 2, one notices the smaller average crystallite size of the crystalline ZnO measured immediately after the decomposition of the ZCB, that is, 8 and 6 nm by annealing in air and vacuum, respectively, as compared with the value of 11 nm measured for the as received ZCB. This aspect will be discussed later. One also notices the increase in the average crystallite size after annealing at higher temperatures, reflecting the corresponding increase in the average diameter of the ZnO crystallites, more pronounced by annealing in air. TEM Investigations. Structure and Morphology of Annealed Hydrozincite. Preliminary TEM investigations revealed that, even for the lowest beam current and shortest irradiation time, the structure observed in the electron diffraction (ED) pattern of the as received ZCB surprisingly corresponded to ZnO with a very low crystallinity. The result was explained by the practically instant decomposition of the ZCB under exposure to the electron beam. Further TEM investigations were performed on a ZCB sample pulse annealed in vacuum up to 235 C, extracted from the EPR sample tube. According to both EPR (Figure 4b) and XRD (Figure 6b) results, in this sample the ZCB f ZnO transformation was completed. Therefore the TEM study could be performed at RT, since ZnO is not “beam sensitive”. Figure 7 shows TEM images (a, b) and electron diffraction patterns (c, d) revealing the morphology and structure of this sample. According to Figure 7a,c, the compound that has resulted in this first stage of transformation is a mesoporous ZnO of low crystallinity with very small nanoparticles and pores, but still keeping a plate- and needle-like general morphology of the initial ZCB, as could be observed in the very first few seconds of its TEM examination at the lowest beam current. At a higher magnification, Figure 7b shows a rather uniform distribution in dimensions for the low-crystallinity ZnO nanoparticles 5034



Figure 7. TEM images (a, b) and ED patterns (c, d) of a ZCB sample pulse annealed in vacuum up to 235 C, which was previously investigated by EPR. The indexing of the two ED patterns is given in Figure S3 from Supporting Information.

and pores in the range of 1.53 nm. One should also mention that the ZnO nanoparticles agglomerated in needles form a textured (mosaic) structure, as revealed by the ED pattern in Figure 7d from a region containing several needles. Further TEM examinations of the ZCB samples pulse annealed to higher temperatures revealed an increase in the ZnO nanoparticles dimensions and an improved crystallinity, as well as the loss of the original plate- and needle-like morphology of the ZnO nanoparticle agglomerates. Thus, the sample pulse annealed up to 300 C in air, that is, 50 C higher than the temperature at which the ZCB f ZnO transformation is completed, contains agglomerates of larger ZnO crystallites in the 815 nm range, shown in the TEM image (Figure 8a) and the ED pattern (Figure 8b). The TEM images obtained at a higher magnification (Figure 8d, as well as the detail shown in Figure 8c) also exhibit ZnO crystallites covered by a shell of a less crystallized material, very likely the remaining poorly crystallized/disordered ZnO, which, according to the EPR study, was formed in the first stage of the ZCB f ZnO transformation. Figure 9 is a high-resolution TEM (HRTEM) image of the ZCB sample pulse annealed up to 300 C in air showing a thin part of an agglomerate where the ZnO crystallites are less superposed. As in Figure 8, it also reveals the presence of the poorly crystallized ZnO phase as an outer layer of the well crystallized ZnO particles, present both in the agglomerate and forming the pore edge. TEM results of a ZCB sample pulse annealed in air up to 625 C are presented in Figure 10. The resulting images (Figure 10a,b) reveal that most of the ZnO crystallites already grew larger, in the dimension range of 1585 nm. However, there are some regions (indicated by black arrows in the TEM images) containing agglomerates of smaller, 24 nm sized crystallites. ED patterns

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Figure 8. TEM images (a, c, d) and the corresponding ED pattern (b) of the ZCB sample pulse annealed in air up to 300 C.

Figure 9. HRTEM image of the ZCB sample pulse annealed in air up to 300 C.

selected from the regions presented in Figure 10a,b are shown in Figure 10c,d, respectively. They can be indexed with the ZnO structure. They confirm a further increase in both size and crystallinity of the ZnO nanocrystals, as well as the presence of smaller crystallites (Figure 10d), very likely resulting from a late crystallization of the remaining disordered ZnO. Discussion of the Results. According to the EPR investigation, the decay of the two substitutional Mn2+(A) and Mn2+(B) centers during annealing of nanocrystalline ZCB in air and vacuum from 5035



Figure 10. TEM images (a, b) and corresponding ED patterns (c, d) of the ZCB sample pulse annealed up to 625 C in air. Both ED patterns can be indexed with the ZnO structure.

225 and 175 C up to 255 and 235 C, respectively, accompanied by the initial production of the Mn2+-d centers, followed by the formation of the Mn2+-c centers, reflects the temperatureinduced changes in the surrounding crystal lattice host of the substitutional Mn2+ ions, consisting in the initial transformation of nanocrystalline ZCB into disordered ZnO. Further on, with increasing annealing temperature, the disordered ZnO begins to crystallize into nanocrystalline ZnO with increasing average particle size and improved lattice ordering. A rough estimation of the relative concentration of the disordered and crystalline ZnO was obtained by a double integration of the simulated spectra of the Mn2+-d and Mn2+-c centers reproducing the experimental spectrum recorded at a temperature where the ZCB f ZnO transformation was complete and determining the ratio of the resulting values. Thus, for samples pulse annealed in air up to 300 C, the disordered and nanocrystalline ZnO fractions are practically equal, while upon annealing up to 625 C, the nanocrystalline ZnO fraction increases to about 80% of the total amount of ZnO, with an estimated (10% error. The crystallization of the disordered ZnO

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is somehow slower in vacuum. Indeed, the nanocrystalline ZnO fraction was found to be only 45% of the total ZnO after pulse annealing at 325 C, while at the maximum annealing temperature of 625 C, it represented about 60% of the total ZnO. The conclusions of the EPR investigation are fully supported by the results of the TA, XRD, and TEM data. Thus, the thermal decomposition of ZCB into ZnO by pulse annealing in air at 255 C was confirmed by both TG-DTA (Figure S2 from Supporting Information) and XRD measurements (Figure 6 and Table 2), while the transformation by pulse annealing in vacuum up to 235 C was confirmed by XRD (Figure 6 and Table 2) and TEM measurements (Figure 7). According to the XRD measurements, the resulting ZnO consists of crystallites of 6 nm average size. However, the TEM investigations point to the formation of a mesoporous ZnO structure consisting of an assembly of disordered nanoparticles with sizes from 1.5 to 3 nm. The apparent disagreement between the nanoparticles size determined by XRD and TEM, observed for the samples annealed at relatively low temperatures, just above the transformation temperature, can be explained by the presence of two ZnO nanoparticle populations with different sizes. As shown in ref 26, in such cases the Rietveld analysis leads to an overestimation of the average nanoparticles size. This effect is expected to be less significant for samples annealed at higher temperatures, where the concentration of smaller size ZnO nanoparticles diminishes, in agreement with the experimental observations. The formation of the disordered ZnO phase containing a large number of spherical voids is very likely due to the multiple chemical bond breaking process accompanied by the energetic release of CO2 and H2O gases, which is expected to take place in a rather narrow temperature range. This process explains the observed lower decomposition temperature of ZCB, as well as the slower recrystallization of the resulting disordered ZnO by thermal annealing in vacuum. As previously stated, EPR has shown that an increase in the fraction of nanocrystalline ZnO accompanied by a corresponding decrease in the amount of disordered ZnO nanoparticles occurs upon further heating. The (HR)TEM examination of the samples pulse annealed at 300 and 625 C in air (Figures 810) confirmed the presence of larger ZnO nanoparticles with improved crystal quality. Thus, for the sample pulse annealed at 300 C in air, the TEM images show agglomerates of ZnO nanocrystallites in the 815 nm range (Figure 8a). Images obtained at higher magnification (Figure 8d and a detail shown in Figure 8c) and at high resolution (Figure 9) also revealed the ZnO nanocrystals to be covered with a shell of poorly crystallized material, very likely the remaining disordered ZnO formed in the first stage of the ZCB f ZnO transformation. This observation confirms the presence of an ongoing crystallization process of the disordered ZnO, observed by EPR, but not by XRD. The TEM examination of the samples pulse annealed up to 625 C in air (Figure 10) revealed a further increase in the size of the ZnO nanocrystallites, as well as the presence of regions with agglomerates of smaller, 24 nm sized nanoparticles resulting from the late crystallization of the remaining disordered ZnO. This agrees with the results of the quantitative analysis of the EPR spectra recorded from samples pulse annealed up to 625 C both in air and in vacuum, which has shown that the nanocrystalline ZnO produced by the thermal decomposition of ZCB is accompanied by a rather large fraction of disordered ZnO, of about 20% and 40% of the total ZnO amount, respectively. 5036


Crystal Growth & Design The decrease of the broadening parameter of the EPR spectrum of the Mn2+-c centers in the resulting ZnO nanocrystals with the annealing temperature increase was also observed. The effect, reflecting a narrower local crystal field distribution at the impurity ions, is attributed to both annealing of the intrinsic defects present in the initial ZnO nanoparticles and a reduced surface influence due to their size increase (see Figure 5 and inset). A similar EPR line narrowing process with annealing temperature increase was also reported in cubic ZnS:Mn2+ nanocrystals.27,28 One also notices in Figure 5 that the σ(D) parameter values of the Mn2+ ions in crystalline ZnO are larger for samples annealed in vacuum, a difference reflecting the higher degree of lattice disorder. The higher disorder in the vacuum annealed ZCB samples can explain the observed slower crystallization of the ZnO observed in such a case. Finally, one should mention that, according to the EPR measurements, the concentration of the substitutional Mn2+ dispersed in the thermally produced ZnO is similar to their concentration in the ZCB nanocrystals, indicating that no significant segregation process of the isolated Mn2+ ions takes place at the ZCB f ZnO transformation and during the further crystallization of the ZnO nanoparticles. It means that the difficult task of uniformly doping nanocrystalline ZnO with such cation impurities can be replaced by the less difficult task of doping the precursor ZCB. Our investigation also shows that in doping ZCB with TM ions one should avoid the formation of separate phases of thermally stable TM oxides with perturbing magnetic properties, which we identified in the investigated ZCB and resulting ZnO.

4. CONCLUSION Detailed information concerning the structural changes that take place in nanocrystalline ZCB by annealing in air and in vacuum up to 625 C has been obtained for the first time from the analysis of the EPR spectra of a low concentration of substitutional Mn2+ impurity ions used as local atomic probes, in correlation with XRD and TEM results. Our investigation shows that the decomposition of the nanocrystalline ZCB consisting of crystallites of 11 nm average size, which begins at 225 C in air and at 175 C in vacuum, is a two step process, with the initial formation of a disordered mesoporous ZnO phase, which further crystallizes with the temperature increase. The increase in the size and crystal lattice quality of the resulting ZnO nanocrystals by pulse annealing at higher temperatures could be employed in controlling the quality of the resulting ZnO nanomaterial. The presence of both disordered and crystalline ZnO phases produced by the thermal decomposition of the ZCB, even at the highest investigated annealing temperature, should be also taken into consideration in any further applications of the ZnO nanomaterial prepared by such a procedure. Our investigation also suggests a new alternative for obtaining nanostructured ZnO of controlled size and crystal quality by the thermal decomposition of ZCB in vacuum. The presence of dispersed Mn2+ impurities in both starting ZCB nanomaterial and the resulting nanosized ZnO confirms that the thermal decomposition of doped ZCB could be a method of obtaining nanocrystalline ZnO with uniformly dispersed dopant concentrations. Our investigation also demonstrates the ability of the EPR spectroscopy to monitor chemical and structural transformations

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in (nano)crystalline materials using trace amounts of transition metal ions as atomic local probes. The presently reported spectral parameters of the substitutional Mn2+ centers in ZCB can be also employed as reference data in further EPR investigations concerning the formation of ZCB by the atmospheric corrosion or during the preparation of various ZnS/ZnO nanostructures.2931

’ ASSOCIATED CONTENT

bS

Supporting Information. XRD diffractogram (both experimental and simulation with parameters from Table 2) of the as received ZCB, the TG-DTA plot of the as received ZCB sample obtained under argon gas flow, and the indexed ED patterns of ZCB pulse annealed in vacuum up to 235 C. This information is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: snistor@infim.ro. Phone: + 40 21 3690185.

’ ACKNOWLEDGMENT This work was supported by CNCSIS-UEFISCSU, project number PN-II-IDEI-523/2008 ,and the Romanian Ministry of Education, Research, Youth and Sport - ANCS, Core Program contract PN09-45. The technical support of D. Zernescu is gratefully acknowledged. ’ REFERENCES (1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (2) Beaulac, R.; Ochsenbein, S. T.; Gamelin, D. R. Colloidal Transition-Metal Doped Quantum Dots. In Nanocrystals Quantum Dots, 2nd ed.; Klimov, V. I., Ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2010; Chapter 11. (3) Furduyana, J. K.; Kossut, J. Diluted Magnetic Semiconductors. In Semiconductors and Semimetals; Willardson, R. K., Ber, A. C., Eds.; Academic Press: New York, 1988; Vol. 25 (4) Norberg, N. S.; Dalpian, G. M.; Chelikovski, J. R.; Gamelin, D. R. Nano Lett. 2006, 6, 2887. (5) Dietl, T.; Ohno, H.; Matsukura, F.; Ciebert, J.; Ferrand, D. Science 2000, 287, 1019. (6) Neal, J. R.; Behan, A. J.; Ibrahim, R. M.; Blythe, H. J.; Ziese, M.; Fox, A. M.; Gehring, G. A. Phys. Rev. Lett. 2006, 96, No. 197208. (7) Hu, X.; Masuda, Y.; Ohji, T.; Kato, K. Cryst. Growth Des. 2010, 10, 626. (8) Cao, X.; Lan, X.; Zhao, C.; Shen, W.; Yao, D.; Gao, W. J. Cryst. Growth 2007, 306, 225. (9) Norberg, N. S.; Kittilstved, K. R.; Amonete, J. E.; Kukkadapu, R. K.; Schwartz, D. A.; Gamelin, D. R. J. Am. Chem. Soc. 2004, 126, 9387. (10) Mickovic, Z.; Alexander, D. T. L.; Sienkiewicz, A.; Mionic, M.; Foro, L.; Magrez, A. Cryst Growth Des. 2010, 10, 4437. (11) Castellano, M.; Matijevic, E. Chem. Mater. 1989, 1, 78. (12) Sawada, Y.; Murakami, M.; Nishide, T. Thermochim. Acta 1996, 273, 95. (13) Kanari, N.; Mishra, D.; Gaballah, I.; Dupre, B. Thermochim. Acta 2004, 410, 93and references therein. (14) Liu, Y.; Zhao, J.; Zhang, H.; Zhu, Y.; Wang, Z. Thermochim. Acta 2004, 414, 121. (15) Sigoli, F. A.; Davolos, M. R.; Jafelicci, M. J., Jr. Alloys Compd. 1997, 262263, 292. (16) Stefan, M.; Nistor, S. V.; Ghica, D.; Mateescu, C. D.; Nikl, M.; Kucherkova, R. Phys. Rev. B 2011, 83, No. 045301. 5037



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Crystallization of Disordered Nanosized ZnO Formed by Thermal

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