Positron Annihilation Spectroscopy Study on Annealing Effect of CuO Nanoparticles

The microstructure and defects of CuO nanoparticles under isochronal annealing were investigated by positron annihilation spectroscopy (PAS), X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD and SEM results indicated that the average grain sizes of CuO nanoparticles grew slowly below 800 °C, and then increased rapidly with the annealing temperature from 800 to 1000 °C. Positron lifetime analysis exhibited that positrons were mainly annihilated in mono-vacancies (VCu , VO) and vacancy clusters when annealing from 200 to 800 °C. Furthermore,W-S plot of Doppler broadening spectra at different annealing temperatures found that the (W,S) points distributed on two different defect species, which suggested that V Cu − − V O + complexes were produced when the grains grew to bigger size after annealing above 800 °C, and positrons might annihilate at these complexes.


Introduction
Cupric oxide (CuO) is a well-known p-type semiconductor material with an indirect band gap of 1.0 eV 1 , which has attracted a large number of studies in polycrystalline as well as single crystal of CuO by using a variety of techniques.Especially, nano-sized CuO has raised the most prospective applications over the years, such as gas sensors 2 , catalysis 3,4 , supercapacitor 5,6 , ceramics 7 , batteries 8 , solar energy cells 9 and field emission emitters 10 .
Though CuO has numerous potential applications, the lack of control over the defects is still a big problem for practical devices.It has been found that the grain surface and interface are rich of defects such as vacancies (V Cu or V O , where the subscript refers to the missing host atom), antisite defects (Cu O or O Cu , where A B means "B" atom is replaced by "A" atom) and isolated interstitials (O i or Cu i ).However, the high formation energies of these defects lead to an unstable state at equilibrium 11 .These defects offer a scope to tune the useful material properties through proper choice of annealing environment and temperature.The negatively charged vacancy, − 2 Cu V , where the superscript refers to the charge q(−2) of the defect, is thought to be the source of CuO having p-type conductivity 12,13 .Gao et al., 14 reported that oxygen vacancies at the interface of the particles were suggested to be responsible for the ferromagnetism in the CuO nanoparticles, vacuum annealing enhanced the ferromagnetism of the CuO nanoparticles, while oxygen atmosphere annealing reduced it.Shi et al. 15 found that the tunable ferromagnetism for the Cu 2 O/Cu nanoparticles composites was attributed to Cu vacancies at the interfaces.
With a large surface to volume ratio, surface effects dominate nanoparticles properties over their respective bulk features.CuO nanoparticles exhibit unique optical, electrical, and magnetic properties based on the effects of grain size and quantum dots size as the surface contribution becomes dominant [16][17][18][19] .Positron annihilation spectroscopy (PAS) has been used as a sensitive probe to characterize surface and interface defects in nanoctystalline metals [20][21][22] and semiconductor nanoparticles [23][24][25] .This is mainly due to the fact that the size of nanoparticles is much smaller than the positron diffusion length (L + ~ 100nm) 26 .Positrons are trapped at defects such as open volumes (vacancies, vacancy clusters and voids) located at the grain surface or interface.Therefore, the positron annihilation radiation parameters provide information on the microstructure of grain surfaces or interfaces.Positron lifetime annihilation parameters can be directly correlated to the size and concentration of defects at the surfaces or interfaces.Doppler broadening parameters give information about the electron momentum distribution at the annihilation site 27 .Some authors investigated defect structure of CuO nanoparticles by positron annihilation technology [28][29][30] .They found that there existed a large number of copper-and/or oxygen-related vacancies or complexes in CuO nanoparticles, which depended on the annealing environment and temperature.However, it should be mentioned that the feature of defects in different sizes of CuO nanoparticles is far from understood and requires extensive studies in CuO nanoparticles.In this work, the generation and recovery of defects located at grain surfaces or interfaces in CuO nanoparticles under isochronal annealing were investigated by positron annihilation lifetime and Doppler broadening spectroscopy, together with traditional XRD and SEM techniques.

Sample preparation
CuO nanoparticles with a size of about 40 nm were purchased from Beijing dekedaojin High Technology Ltd, China with a purity of 99.9%.The nanoparticle powders The microstructure and defects of CuO nanoparticles under isochronal annealing were investigated by positron annihilation spectroscopy (PAS), X-ray diffraction (XRD) and scanning electron microscope (SEM).XRD and SEM results indicated that the average grain sizes of CuO nanoparticles grew slowly below 800 °C, and then increased rapidly with the annealing temperature from 800 to 1000 °C.Positron lifetime analysis exhibited that positrons were mainly annihilated in mono-vacancies (V Cu , V O ) and vacancy clusters when annealing from 200 to 800 °C.Furthermore, W-S plot of Doppler broadening spectra at different annealing temperatures found that the (W, S) points distributed on two different defect species, which suggested that − Positron Annihilation Spectroscopy Study on Annealing Effect of CuO Nanoparticles were hand milled for 2 h in an agate mortar with a pestle and then pressed into pellets (diameter: 15 mm, thickness: 1 mm) under a static pressure of about 10 MPa for 5 min at room temperature.The annealing experiments were made over a temperature range in the stable region of the CuO phase, i.e.T < 1027 °C1, 31 .So, the samples were isochronally annealed at different temperature of 200, 400, 600, 700, 800, 900 and 1000 °C in air for a constant duration (2 h).

Experimental method
XRD analysis was performed by using Cu-K α radiation (λ=1.5406Å) on "Bruker D8" X-ray diffractometer.Si crystal was used as an external standard to correct for the instrument line broadening.Data were collected from 20° to 80° (2θ) with a step size of 0.02°.There were no monochromators in the whole XRD system.Thus there occurred Cu-K α1 and Cu-K α2 radiation at the same time, and each Bragg reflect ion took place with slightly different diffraction angles.When the XRD data were processed, the contribution of Cu-K α2 was stripped by using Jade software and only the pure Cu-K α1 radiation was left.The average particle sizes were determined by Debye-Scherrer formula.The morphology of the samples was investigated by a Hitachi S-4800 field emission scanning electron microscope (SEM).
Positron annihilation lifetime spectroscopy (PALS) was performed using a conventional fast-fast coincidence system with a temporal resolution of about 280 ps.A 22 Na positron source with intensity of about 20 μCi was used.The positron source of 22 Na was sealed between two Kapton films.Each spectrum was collected with a total count of 1×10 6 .During the measurement, the positron source was sandwiched between two identical sample pieces.The components of the positron annihilation in the source and Kapton films were carried out using Si single crystal as reference.The spectra were analyzed using an analysis software (LT, version 9) 32 based on the sum of the exponential decay curves given by the following equation: Where n(t) is that the positron is alive at time t after its birth and t i is the lifetime associated with a decay curve with its initial intensity, I i .Two lifetime components τ 1 , τ 2 , and corresponding percentage intensities, I 1 , I 2 , were resolved in the case of the CuO nanoparticles samples.Doppler broadening spectroscopy (DBS) was measured using a liquid-nitrogen cooled high-purity (HP)-Ge detector with an energy resolution of about 1.50 keV at 511 keV.The source is sandwiched between the sample pair similar to positron lifetime experiments.The DBS were characterized by shape (S) and wing (W) parameters, which are linked to the interaction of the electron-positron pair with low-momentum valance electrons and high-momentum core electrons, respectively.The S and W parameters were defined respectively as the ratio of the central region (511 ± 0.8 keV) and the wing region (511 ± 3.4 to 511 ± 6.8 keV) of the spectrum to the entire region of the spectrum.

Results and discussion
The XRD patterns for all the samples are showed in Figure 1(a).No peaks of impurity phases (Cu 2 O or Cu) are detected in these XRD patterns.All diffraction peaks as the typical monoclinic structure of CuO are observed, which show that the experimental samples are located in the CuO stable region when annealing in the temperature ranging from 200 to 1000 °C for 2 h.The whole CuO planes are used to calculate the average grain size.The average grain size of the CuO samples, as is shown in Figure 1(b), is calculated from the full width at half maximum of their diffraction curves by using Debye-Scherrer formula 33 , i.e.

/ cos
Where D is the particle size, K is the constant shape factor (taken as 0.89), λ is taken as the wavelength of X-ray radiation, β is the full width at half maximum (FWHM) of the XRD peak, and θ is the Bragg angle.The calculated average grain size of the as-pressured sample is about 36.8 ± 3 nm, which is consistent with the information provided by the vendors.The size of the CuO nanoparticles increases slowly with increasing annealing temperature in annealing temperature from 200 to 800 °C.Similar observations have also been reported by earlier literature 16 .
When the samples are annealed from 800 to 1000 °C, the grain size increases from 43 to 60 nm accordingly.SEM images of CuO samples annealed at different temperatures are depicted in Figure 2. The testing part is the cross section of the CuO pellets.It can be seen that the CuO morphologies are not significantly changed below 800 °C.The CuO nanoparticles annealed at 800 °C began to aggregate into bigger particles, leading to an increase of grain size.These particles have a greater tendency to form clusters or aggregates when increasing the annealing temperature.However, due to limited resolution of SEM, the size of the CuO nanoparticles that assemble into micro-size cannot be estimated from the observed images, these grains have a very large average size (100-2500 nm) compared to that obtained by XRD analysis (40-60 nm).The crystallization of CuO nanoparticles is evidently improved with increasing the annealing temperature.This indicates that the grains are clusters of crystallites, consistent with results obtained previously 16,19 .
In order to further analyze the generation and recovery of defects located at grain surfaces or interfaces in CuO nanoparticles under isochronal annealing, we performed positron lifetime and Doppler broadening measurements.Although the observed annihilation spectra in these samples appear to be two components, the simultaneous decrease in I 2 and τ 1 rules out the two-state trapping model which show a decrease in I 2 is accompanied by an increase in the τ 1 value 34 .Therefore, the short lifetime τ 1 cannot be exclusively attributed to positrons annihilated in the bulk of CuO.So, the short lifetime is associated with trapping at the bulk or mono-vacancy defects, and the long lifetime is assigned to trapped positrons in large vacancy clusters at the grain interfaces 29 .As seen in Figure 3, when the CuO nanoparticles is annealed at 200 °C, an increase in τ 1 , τ 2 and I 1 while a decrease in I 2 has been observed.The defect states of the as-pressured sample (at room temperature) might be different from that of the sample sintered at 200 °C.When the CuO nanoparticles was annealed at 200 °C, the supply of small thermal energy conduced intragrain Cu and O vacancies to migrate towards the grain surfaces or interfaces.A rearrangement of nanoparticles would lead to the formation of more two-grain interfaces accompanied by a reduction in the number of three-grain junctions in the as-pressured sample without annealing.We observe that a small increase of τ 1 , around 185 ps, is associated with positron trapping at vacancy-type defects in the two-grain interfaces, τ 2 increases from 308 to 360 ps due to positrons trapping at larger-size vacancy clusters in three-grain junctions 28 , while the decrease of I 2 may be due to a reduction in the number of three-grain junctions.The bulk lifetime of about 169 ps in CuO was fixed according to the previous PALS studies 29,35 .The value of τ 1 (~180 ps) is between the lifetime of positron annihilation in a bulk (169 ps) and a Cu mono-vacancy (230 ps) 29 .Hence, the value of τ 1 is attributed to a mixed contribution from the annihilation of trapped positrons in CuO bulk and mono-vacancies 28 .In the case of sample annealing at 1000 °C, with the largest average grain size 59.3 nm, I 2 decreases up to 2.2% and τ 1 results in a value of 165.8 ps which closes to the lifetime of positrons trapped at the CuO bulk.This likely indicates that positrons are mainly annihilated in the CuO bulk and V Cu are absent in the CuO nanoparticles annealing at 1000 °C.
It is well-known that positrons were mainly trapped at neutrally or negatively charged (anion) vacancies and rarely trapped at positively charged (cation) vacancies since the probability of the positron annihilation with 1s electrons of oxygen was small owing to a strong Coulomb repulsion of the nucleus.The positron density concentrating near Cu-O bonds 36 and the shortest distance (1.95 Å) between Cu and O ions in a monoclinic cell of CuO were identified 37 .Therefore the effects of positron trapping at positively charged vacancies O in the whole range of the Fermi level in both n-type and p-type CuO in the O-rich environment, respectively.Meanwhile, the formation energy of Cu vacancies is lower than other type's defects, and the most stable defects, Cu vacancies, were also identified in CuO.Other vacancy-type defects found in CuO are V O , but since the formation energy to V O is around 3.5 eV, which is much larger than that of V Cu (less than 1.0 eV), V O may also be trapped positrons.The results of theoretical calculation and experiment in cuprate superconductors have proved that the dominant positron traps were presented by oxygen vacancy defects in the Cu-O planes [38][39][40] .It was also reported that open volumes which were mainly identified as anion vacancy defects existed in the grain interfaces in the CuO nanoceramics as well as the oxygen vacancy defects led to a decrease in the degree of Cu-O bond covalency studied by PAS 30 .Therefore, in present study, we consider that the effects of positron trapping at positively charged vacancies (V O ) cannot be neglected.Positrons may be annihilated in both V Cu and V O when the annealing temperatures varies from 200 -800 o C.This will be analyzed in the following discussion of W-S plot.
The S and W parameters index the fraction of positron annihilating with the valence electrons and core electrons, respectively.The reduction in positron annihilation with core electrons is much more as compared to the valence electrons leading to the S parameter is higher than W parameter.Consequently, the data from DBS provides further insight on open volume defects (vacancies, vacancy clusters and voids).The S parameter reveals that the information of the size and concentration of defects in the different annihilation sites of solids 41 and can be correlated to the positron average lifetime (τ a ), which is calculated by the following equation: The S parameter and τ a as a function of annealing temperature are shown in Figure 4.It is seen that both S parameter and τ a show a similar trend.The S parameter and τ a decrease slightly with the annealing temperature from 200 to 800 °C and decrease rapidly above 800 °C.It can be concluded that the total concentration of interfacial defects reduces with the increase of grain size.Similar results were observed in ZnO nanoparticles studied by Sharma et al. 25 .
Further discussion is from the set of S and W data plotted in the W-S plane which reveals the information about the overall defect types, as is shown in Figure .temperature varies from 200 to 800 °C, τ 2 , with a value of about 350 ps, is essentially constant and larger than the lifetime of Cu mono-vacancies (230 ps).It can be understood that small-size Cu vacancies assemble and form the large-size Cu vacancy clusters at the grain interfaces.In addition, the effects of positron trapping at positively charged vacancies (V O ) cannot be also neglected when annealing below 800 °C.So, the first straight line is attributed to positrons annihilating in mono-vacancies (both V Cu and V O ) and vacancy clusters at grains in CuO nanoparticles.When annealing above 800 °C, both τ 2 and I 2 begin to decrease rapidly while I 1 increases up to 97.8%.In this case, since the two trapping model is not suitable for explaining the behavior of positron annihilation parameters, a new complex structure of defects may be formed.Cruz et al. 43

Conclusions
The microstructure and defects of CuO nanoparticles under isochronal annealing were studied by XRD, SEM and PAS.XRD results indicated that the average grain sizes of CuO nanoparticles varied from 40 to 60 nm.The crystallite of CuO nanoparticles grew slowly below 800 °C, and then nanoparticles increased with the annealing temperature up to 1000 °C.SEM images also indicated that the CuO nanoparticles annealed at 800 °C have began to aggregate into bigger particles, which were consistent with the growth of CuO nanoparticles seen in XRD patterns.PAS revealed that both S parameter and τ a decreased slightly with the annealing temperature from 200 to 800 °C and decreased rapidly above 800 °C.The total concentration of interfacial defects in CuO nanoparticles reduced with the increase of grain size.PALS exhibited that the short lifetime, τ 1 , which was attributed to a mixed contribution from the annihilation of trapped positrons in the bulk and mono-vacancies (V Cu , V O ), and the long lifetime, τ 2 , which was assigned to trapped positrons in large vacancy clusters below 800 °C.W-S plot of DBS found that the (W, S) points distributed on two different straight lines, which suggested that − + − Cu O V V complexes were produced with the increase of grain size after annealing above 800 °C, positrons might annihilate at these complexes.

Figure 1 .
Figure 1.XRD patterns (a) and average grain size (b) of the CuO nanoparticles as a function of annealing temperature.

Figure 3 (
a) and Figure 3(b) show the positron lifetime parameters τ 1 , τ 2 , and their corresponding percentage intensities, I 1 , I 2 as a function of annealing temperature in the case of the CuO nanoparticles samples.No meaningful change in two lifetimes (τ 1 and τ 2 ) and their corresponding percentage intensities (I 1 and I 2 ) are observed when the annealing temperature increases from 200 to 800 °C.

Figure 4 .
Figure 4. S parameter (a) and positron average lifetime (τ a ) (b) as a function of annealing temperature.

VV
identified that oxygen vacancies existed in Cu 2 O single crystals annealing in air atmosphere at 923 °C in the stable region of the CuO phase as well as a lifetime of 320 ± 40 ps was attributed to positrons trapped at complexes.In present experiment, the value of τ 2 is close to the lifetime of the − complexes at the grain interfaces may be formed due to fast diffusion of V Cu and V O when annealing above 800 °C, together with aggregates and growth of CuO nanoparticles.It can be concluded that when annealing temperature below 800 °C (first straight line segment of Figure5), positrons are mainly annihilated in mono-vacancies (V Cu , V O ) and vacancy clusters, while some positrons may annihilate in − 800 °C (second straight line segment of Figure5).Further work is needed to study the different annealing environment of CuO nanoparticles.