Influence of Ni and Sr on the structural, morphological and optical properties of ZnO synthesized by sol gel

Influence of Ni and Sr on the structural, morphological and optical properties of ZnO synthesized by sol gel

Optical Materials 98 (2019) 109427 Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optm...

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Optical Materials 98 (2019) 109427

Contents lists available at ScienceDirect

Optical Materials journal homepage: http://www.elsevier.com/locate/optmat

Influence of Ni and Sr on the structural, morphological and optical properties of ZnO synthesized by sol gel ~ a-Garcia a, b, *, Y. Guerra c, R. Milani a, D.M. Oliveira a, F.R. de Souza a, R. Pen �n-Hern� E. Padro andez a, c a b c

Universidade Federal de Pernambuco, Departamento de Física, Recife, PE, Brazil Universidade Federal de Piauí, Programa de P� os-Graduaç~ ao em Ci^encia e Engenharia dos Materiais, Teresina, Pi, Brazil Universidade Federal de Pernambuco, P� os-Graduaç~ ao em Ci^encia de Materiais, Recife, PE, Brazil

A R T I C L E I N F O

A B S T R A C T

Keywords: ZnO Sol gel Rietveld refinement FTIR Photoluminescence Band-gap

ZnO nanoparticles doped and co-doped with Ni and Sr were synthesized by the sol gel method. Rietveld refinement indicated the single phase of ZnO for all the samples and suggests that the variations in structural parameters are dependent on the ions type. Fourier transform infrared studies showed the functional groups and chemical bonding of ZnO and confirmed the replacement of Zn by Ni and Sr ions. The micrographs showed particles with clusters shapes and different sizes. Photoluminescence measurements show a shift in near band edge UV emission, from 380 to 384 nm for doped and co-doped samples. Additionally, the Zn substitution by Ni and Sr ions minimizes the intrinsic defects in the ZnO structure. The optical study showed that the replacement of Zn by Ni decreases the band gap, whereas the substitution by Sr increases it. For co-doped samples, this value depends on the dopant concentration and the ions type. This work contributes to the understanding of optical properties for the ZnO nanoparticles co-doping with Ni and Sr, which may be important for future applications.

1. Introduction The modification of zinc oxide (ZnO) structure plays an important role to obtain materials with desirable properties [1–4]. Changes in the ZnO structure improve the photocatalytic activity, conductivity and optical properties. An easy way to modify the ZnO structure is by doping with different elements [1,5]. For example, ZnO nanoparticles doped with transition metal, rare earths and other elements, has drawn much attention for applications in photocatalysis, UV photodetectors, solar cells, photoluminescence, light emitting diodes, etc [5–12]. Studies on co-doped ZnO have been important to obtain p-type conductivity [13,14], as well as, to improve the electrical, magnetic and optical properties [15–17]. Al–In co-doped ZnO presents better con­ ductivity than Al single-doped ZnO [18] and Al–V co-doped ZnO is thermal stable [19]. Sharma et al. [20] studied the structural and optical properties of Co–Mn co-doped ZnO. Pascariu et al. [21] analyzed the photocatalytic activity of Ni–Co co-doped ZnO obtained by co-precipitation method. Karzazi et al. [22] studied the transparent conducting properties of Mg–Al co-doped ZnO. Das et al. [13] synthe­ sized ZnO co-doped with Al and Mg for ultraviolet photoconductive

detectors. Shi et al. [23] investigated the effect of Mg concentration on some properties of Mg–K co-doped ZnO thin films obtained by sol-gel method. The microstructure and magnetic behavior of (Mg/Ni) co-doped ZnO nanoparticles were analyzed by Boyraz et al. [24]. On the other hand, different studies have shown the effect of Ni and Sr on structural, magnetic and optical properties in ZnO nanostructures [25–33]. Ni-doped ZnO has been shown to cause changes in magnetic behavior as well as transmit red shift in the optical band gap [34,35]. The photocatalytic activities of Ni and Sr doped ZnO have attracted the attention of several researchers. For example, Jafari et al. [36] studied the photocatalytic degradation of the organophosphorus insecticide diazinon in aqueous suspensions by using Ni-doped ZnO nanorods as a photocatalyst. Zhao et al. [37] reported that the Ni-doped ZnO nanorods fabricated exhibited better photocatalytic activity than ZnO in the degradation of rhodamine B (RB). Li et al. [38] observed that Sr-doped ZnO presents high performance photocatalytic performance for Rhoda­ mine B under visible radiation. Furthermore, the Sr-doped ZnO for love wave filter applications was studied by Water et al. [39]. Gas sensing behavior of Sr-doped ZnO was studied by Vijayan et al. [40]. It is known that, some properties for specific applications can be

* Corresponding author. Universidade Federal de Pernambuco, Departamento de Física, Recife, PE, Brazil. E-mail addresses: [email protected], [email protected] (R. Pe~ na-Garcia). https://doi.org/10.1016/j.optmat.2019.109427 Received 7 August 2019; Received in revised form 13 September 2019; Accepted 25 September 2019 Available online 30 September 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.

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Fig. 1. a) X-ray diffraction analysis. All samples have similar spectra, indicating the replacement of Zn by Ni and Sr in the ZnO structure, b) Shift of the (101) peak in all samples.

improved by properly controlling grain size, reaction parameters, den­ sity, porosity and temperature. Despite extensive work on the synthesis of ZnO, little is known about the influence of Ni and Sr co-doping on it structural and optical properties. The co-dopant ions are chosen ac­ cording to the ionic radius, valence states and coordination number [41]. Compared to the Zn, the Ni has a smaller ionic radius, whereas the Sr has a larger ionic radius. Due to these size effects, Ni and Sr co-doping may induce modifications on the structural and optical properties of ZnO. The present work aims the synthesis and characterization of ZnO codoped with Ni and Sr, focused on analyzing the influence of the ion type on the structural and optical properties. Low concentration of Ni and Sr were chosen in order to avoid phase segregation. The correlationships between the intrinsic defects and the properties are discussed and compared with the undoped ZnO sample. The sol gel method was used due to low cost and excellent chemical homogeneity [42–46].

Table 1 Values of a and c, D and band gap energy (Eg) for all samples. Samples

a [Å]

ZNS00 ZNS20 ZNS02 ZNS24 ZNS42

3.2530 3.2507 3.2534 3.2537 3.2536

c [Å] (1) (2) (1) (2) (1)

5.2091 5.2066 5.2020 5.2015 5.2111

(1) (1) (1) (2) (1)

D [nm]

Eg (eV)

54 60 43 33 31

3.258 3.194 3.243 3.236 3.218

Sr ions were included on Zn sites without changing the wurtzite struc­ ture. Through the Rietveld refinement were determined the crystallite size (D) and the lattice parameters (a and c), Table 1. For the ZNS00 sample, a and c values are 3.2530 Å and 5.2091 Å, respectively. From the results shown in Table 1 we can see that, the lattice parameter a, decreases for ZNS20 samples and then increases for ZNS02, ZNS24 and ZNS42 samples. The variations observed in a are associated with the differences in the ionic radius of Zn, Ni, and Sr, being 0.60 Å, 0.55 Å and 1.18 Å, respectively [41]. These differences provoke gradual changes in the lattice, as more Ni and Sr ions are incorporated in the ZnO structure. Moreover, the difference in atomic size causes changes in the defects density, induces stress, lattice distortion and leads to the reduction of oxygen vacancies (Vo) [46,47]. Additionally, these changes can be related to the increase in the repulsive dipole interactions on the parti­ cles surface, caused by the existence of unpaired electronic orbitals [46, 47]. Finally, according to Fig. 1b, Sr incorporation leads a slight shift in the (101) XRD peaks toward lower diffraction angles. This result pro­ vides indirect evidence that Sr is incorporated into the crystal structure, causing that the ZnO crystal lattice to expand, as reported by Yousefi et al. [25]. On the other hand, a slight change in the intensity of some peaks, pointing a variation in the mean crystallite size and loss crystallinity was also observed in Fig. 1b. For the Ni-doped sample (ZNS20) we see a small increase in peak intensity (101) compared to the other samples. Similar results were obtained for Ni-doped ZnO films by Bayram et al. [50,51]. According to the authors Ni doping plays an important role in the crystallinity of ZnO nanostructures. The estimated values of crystallite size for all samples are shown in Table 1. It is clearly seen that the value of crystallite size for ZNS00 sample is 54 nm and increases to 60 nm for ZNS20 sample. This phenomenon can be related, due to the stress and strains in the lattice caused by the Ni ions incorporation. For ZNS02, ZNS24 and ZNS42 samples, the D values are 43 nm, 33 nm and 31 nm, respectively. In this case, the inclusion of Sr ions, limits the growth of crystals, which may occur due to variation of the nucleation mechanism during the ZnO crystallization. A similar result was reported for �n et al. [52]. Xu et al. [29] ensure that the Sr Fe-doped ZnO by Beltra inclusion in the ZnO structure causes a change in the point defects density, and their distribution and these variations result in changes in the ZnO lattice, which will cause stress and as a result shift in peak

2. Experimental procedure The synthesis was performed by sol gel method, as used in previous works [1,47–49]. The chemical formula employed was (Zn1-(xþy)NixSry) O with Ni and Sr in different concentrations [(x; y) ¼ (0.00; 0.00), (0.02; 0.00), (0.00; 0.02), (0.02; 0.04) e (0.04, 0.02)]. The samples were named as: ZNS00 (for pure ZnO), ZNS20 (for Zn0.98Ni0.02O), ZNS02 (for Zn0.98Sr0.02O), ZNS24 (for Zn0.94Ni0.02Sr0.04O) and ZNS42 (for Zn0.94 Ni0.04Sr0.02O). The raw materials were Zn(NO3)2, Ni(NO3)2, Sr(NO3)2 and citric acid were weighed in stoichiometric quantities to get a solu­ tion of 0.02 mol/L in 25 ml of distilled water. All samples were obtained in the same syntheses conditions and were heat treated at 700 � C for 4 h. For doped and co-doped samples, the materials amount was adjusted along with x and y values (respecting the mixture at the atomic level). More detailed procedures about the synthesis are explained by Pe~ na-Garcia et al. [1]. The samples were studied using a Bruker X-ray diffractometer (XRD) with Cu-Kα radiation. Fourier transform infrared (FTIR) spectra were measured in the range of 4000–400 cm 1, using an IRTracer-100-SHIMADZU Spectrophotometer, with KBr pellet tech­ nique. The morphology was analyzed by Scanning Electron Microscopy (SEM) using a TESCAN MIRA3. Optical properties were measured using a Spectrofluorometer Horiba-Jobin Yvon Fluorolog-3 and at excitation wavelength of 320 nm and a High Resolution Spectrometer-HR4000CG-UV-NIR-Ocean Optics in the range of 320–500 nm. 3. Results and discussions Fig. 1a shows the XRD diffractograms of doped and co-doped ZnO samples. All samples showed the ZnO single phase, confirmed by the reference code (COD: 96-900-4182). No impurity peaks or secondary phases were observed in the diffraction pattern, confirming that Ni and 2

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doped with Ni. According to Hassan et al. [53] O–H vibrations in ZnO lie in the range from 3216 to 3644 cm 1 and depend on the configuration and number of hydrogen atoms absorbed by ZnO. In addition, the au­ thors suggest that changes in this band as a result of doping indicate microstructural variations occurring in the ZnO matrix [53] In our study, this fact can be corroborated by the difference of the ionic radius of Sr, Ni and Zn, being 1.18 Å, 0.55 Å and 0.60 Å, respectively. According to the SEM analysis, all samples present particles with cluster shapes, different sizes and agglomerates, Fig. 3 (a-e). These ag­ glomerates can appear by the formation of secondary particles, gener­ ated from the aggregations of smaller primary particles. Similar results were obtained for ZnO co-doped with other ions by different authors [20,53]. Fig. 3f, shows the Energy Dispersive Spectroscopy (EDS) spectrum for the ZNS24 sample. The peaks of the elements Zn, O, in the spectrum are the evidence of ZnO phase formation and the peaks asso­ ciated to the Ni and Sr confirms the substitution in the hexagonal wurtzite structure. Fig. 4 displays the room-temperature PL spectra for all samples. Four main peaks were observed for undoped ZnO sample, whereas for the doped and co-doped samples two peaks were observed in the UV and visible regions. For the ZNS00 sample, the intense peak in UV region at 380 nm is attributed to the direct exciton recombination arising from near band edge transition of the ZnO, through an excitonic transition between an electron and a hole of the same exciton [20]. The emission at 410 nm is related to the Vo [53] and the violet emission band at 435 nm is usually associated zinc interstitials (Zi) and Vo [54]. This emission has been attributed to radioactive defects related to trapping states in the grain boundaries and to radioactive transitions between the valence band and levels related to traps in the grain boundaries. The green emission at 505 nm is caused due to Vo and results from the recombi­ nation between electrons localized at the oxygen defect and the holes trapped in the valence band [20,53,54]. For the ZNS20, ZNS02, ZNS24 and ZNS42 samples, the emission peak in UV region is displaced towards higher wavelength to 384 nm, indicating the substitution of Zn by Ni and Sr in the ZnO structure. In addition, this variation is ascribed to the nature of induced defect states during the growth processes. The defects could affect the position of the band edge emission as well as the shape

Fig. 2. FTIR spectra of ZNS00, ZNS20, ZNS02, ZNS24 and ZNS42 samples.

position. The FTIR analysis showed similar spectra for all samples, Fig. 2. The wide band at 460 cm 1 is attributed to an antisymmetric stretching vi­ bration of Zn–O bond in tetrahedral coordination. The band in 860 cm 1 correspond to the vibration in CO23 [1,20,30] and at 1400 and –O 1629 cm 1 are related to the antisymmetric and symmetric C– stretching modes. The peak at 2348 cm 1 is characteristic of the CO2 molecules in air and two bands located at 2851 and 2922 cm 1 are due to C–H bond bending and stretching, respectively [20,30]. These vibrational bands represent the existence of absorbed groups on the surface of nanocrystals. The band in the range of 3250–3718 cm 1 is attributed to the stretching vibration of hydroxyl groups, showing the presence of H2O on the nanoparticles surface [1,20,30]. Looking to Fig. 2, the samples containing Sr show a change in the typical bands of groups C ¼ O and O–H, compared to the pure ZnO and

Fig. 3. SEM images for all samples: a) ZNS00, b) ZNS20, c) ZNS02, d) ZNS24, e) ZNS42 and f) EDS for the ZNS24 sample. The samples present particles with hexagonal shapes, different sizes and agglomerates. 3

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emission band at a centered peak at 530 nm. This band is attributed to the Vo and may be arise from the replacement of Zn ions by Sr ions, because Sr addition leads to an increase singly ionized oxygen vacancy states and interstitials states within the band gap of ZnO [25,56]. These Vo traps a hole and creates a doubly ionized oxygen vacancy as recombination centers [56]. For co-doped samples (ZNS24 and ZNS42), the red emission peak in the visible range was shifted toward higher wavelength range, with a broad emission band and a centered peak at 602 nm, which appears due to the singly occupied Vo or Zi. Additionally, the intensity of the red emission was diminished remarkably for co-doped samples, indicating that there is an energy competition be­ tween Ni, Sr and Zn ions in the energy bands that leads to a variation in the number of intrinsic defects (oxygen vacancy concentration) within the hexagonal structure of ZnO. On the other hand, the optical band gap region was studied by diffuse reflectance (DR) measurements, Fig. 5a. As observed, the optical in­ tensity of reflectance depends of the ions type (Ni or Sr). The variation for doped and co-doped samples compared with the pure ZnO confirms the replacement of Zn by Ni and Sr in the hexagonal structure. The absorption edge exhibits a continuous red shift for doped and co-doped samples, which may be due to the formation of shallow levels inside the band gap by doping. Besides that, the shifting for the higher wavelength results in decreasing the band gap for ZnO nanoparticles doped and codoped with Ni and Sr. The optical band gap (Eg) was estimated by using the Tauc model [31], Fig. 5b and the values are in Table 1. For ZNS00 sample Eg ¼ 3.258 eV, which is below the value reported to the ZnO bulk (3.31 eV) [57]. The difference is associated with Vo or/and defects at the ZnO nanocrystals surface, corroborating the PL results. As shown in Table 1, Eg values for ZNS20, ZNS02, ZNS24 and ZNS42 samples, are smaller compared to the sample ZNS00. This implies that the Eg values for ZnO, depends on the dopant concentration and could thus be tuned by changing their constituent stoichiometry. The decrease in Eg (3.194 eV) for the ZNS20 sample may be explained by the sp-d exchange interactions between the electron band and the localized d electrons of the Ni and Sr ions [1,20]. The s-d and p-d exchange interactions lead to a variation of the conduction band, as well as the valence band edges, resulting in the Eg diminution [1,21,58,59]. For the ZNS02 sample the Eg ¼ 3.243 eV. Ac­ cording to the PL analysis, the ZNS02 sample, presents a broad band emission, associated to the Vo. Moreover, this slight increase occurs because the Fermi level merges in the conduction band with the increase in carrier concentration [1,10,58,59]. For ZNS24 and ZNS42 samples, Eg ¼ 3.236 eV and Eg ¼ 3.218 eV, respectively. This result confirms that, there is an energy competition between Ni and Sr ions and that Eg values will depend on the content of Ni or Sr. Furthermore, the Ni and Sr incorporation modify the structure, which can lead to an excess of free electrons, which may incorporate new electronic levels within the ZnO band-gap. Finally, modifications in Eg have been observed by different

Fig. 4. Room temperature PL spectra for ZNS00, ZNS20, ZNS02, ZNS24 and ZNS42 samples. The spectra show different behavior depending on the doping ions type.

of the PL spectrum. Similar shift in UV emission for Sr-doped ZnO were obtained by Yousefi et al. [25]. Moreover, note that the UV emission peak intensity for the samples containing Ni became stronger. According to Cai et al. [55] the increase in UV emission peak intensity for Ni-doped ZnO, can be assigned to a lower non-radiative recombination processes, being able to contribute to higher photocatalytic oxidation activity under UV-light. For ZNS24 and ZNS42 samples, the UV emission peak intensity is larger, compared to the other samples. In this case, due to co-doping, there is an increase in the energy levels located at the bottom of the conduction band and the excitons acquire higher energy levels, and the radioactive recombination of these excitons will cause an enlargement in the peak emission of UV [55]. For the ZNS20 sample only a broad band is observed at 602 nm which is related to interstitial oxygen (Oi). For this sample, we see that some bands associated to the defects disappear, which indicates that, the Ni incorporation in the host ZnO matrix locally reduced the structural defects. For the ZNS02 sample, the emission peak in the visible range was shifted toward minor wavelength range, with a broad green

Fig. 5. a) Optical diffuse reflectance as a function of wavenumber. b) The (αhν)2 vs. hν plots and the Eg estimation for all samples. The Eg value varies depending on the ions type and suggests an energy competition between Ni, Sr and Zn ions in the ZnO structure. 4

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authors for doped and co-doped ZnO nanoparticles [1,10,20,58,59]. [14]

4. Conclusions

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In summary, we successfully prepared ZnO nanoparticles co-doped with Ni and Sr ions using sol gel method. Doped and co-doped sam­ ples studied by Rietveld method show wurtzite structure and exhibit a slight shift in the intensity and the angle position compared with pure ZnO. SEM images showed particles with hexagonal shapes, different sizes and agglomerates. PL measurements showed a shift in near band edge UV emission for doped and co-doped samples compared to the pure sample. The Ni and Sr inclusion in the ZnO structure minimizes the defects number associated to the oxygen vacancies. The optical prop­ erties study showed that the replacement of Zn by Ni decreases the bandgap, whereas the replacement by Sr increases Eg. For co-doped samples, this value depends on the dopant concentration and dopant ions type. More comprehensive studies can be performed using electronic para­ magnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS).

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgments The authors are grateful the Brazilian Agencies: FACEPE, FINEP, CAPES and CNPq. To the laboratories: Polímeros n~ ao Convencionais (PNC-DF-UFPE), Terras Raras (BSTR-DQF-UFPE) and Compostos Híbri­ �ides (CHICO-DQF-UFPE). dos, Interface e Colo

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