Solid solutions of gadolinium doped zinc oxide nanorods by combined microwave-ultrasonic irradiation assisted crystallization

Accepted Manuscript Solid solutions of gadolinium doped zinc oxide nanorods by combined microwaveultrasonic irradiation assisted crystallization Armin Kiani, Kamran Dastafkan, Ali Obeydavi, Mohammad Rahimi PII:

S1293-2558(17)30790-2

DOI:

10.1016/j.solidstatesciences.2017.10.002

Reference:

SSSCIE 5574

To appear in:

Solid State Sciences

Received Date: 17 August 2017 Revised Date:

26 September 2017

Accepted Date: 8 October 2017

Please cite this article as: A. Kiani, K. Dastafkan, A. Obeydavi, M. Rahimi, Solid solutions of gadolinium doped zinc oxide nanorods by combined microwave-ultrasonic irradiation assisted crystallization, Solid State Sciences (2017), doi: 10.1016/j.solidstatesciences.2017.10.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

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Solid solutions of gadolinium doped zinc oxide nanorods by combined microwave-ultrasonic irradiation assisted

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crystallization Armin Kiani a, ¶, Kamran Dastafkan b, *, ¶, Ali Obeydavi b, Mohammad Rahimi c

Research Center for Analytical Sciences, KAVA Research Institute, Tehran, Iran

b

Young researchers and Elite Club, Ahvaz Branch, Islamic Azad University, Ahvaz,

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a

Iran

Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran

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c

* Corresponding author. (K. Dastafkan).

E-mail address: [email protected]

Author contributions:

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These authors contributed equally to this work.

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¶

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ACCEPTED MANUSCRIPT ABSTRACT Nanocrystalline solid solutions consisting of un-doped and gadolinium doped zinc oxide nanorods were fabricated by a modified sol-gel process utilizing combined ultrasonic-microwave irradiations. Polyvinylpyrrolidone, diethylene glycol, and

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triethylenetetramine respectively as capping, structure directing, and complexing agents were used under ultrasound dynamic aging and microwave heating to obtain crystalline nanorods. Crystalline phase monitoring, lattice parameters and variation,

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morphology and shape, elemental analysis, functional groups, reducibility, and the

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oxidation state of emerged species were examined by PXRD, FESEM, TEM, EDX, FTIR, micro Raman, H2-TPR, and EPR techniques. Results have verified that irradiation mechanism of gelation and crystallization reduces the reaction time, augments the crystal quality, and formation of hexagonal close pack structure of Wurtzite morphology. Besides, dissolution of gadolinium within host lattice involves

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lattice deformation, unit cell distortion, and angular position variation. Structure related shape and growth along with compositional purity were observed through microscopic and spectroscopic surveys. Furthermore, TPR and EPR studies

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elucidated more detailed behavior upon exposure to the exerted irradiations and

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subsequent air-annealing including the formed oxidation states and electron trapping centers, presence of gadolinium, zinc, and oxygen disarrays and defects, as well as alteration in the host unit cell via gadolinium addition. Keywords: ZnO nanorods; Gadolinium doping; Ultrasonic processing; Microwave irradiation; Solid solution; Lattice-solid state investigation.

1. Introduction

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ACCEPTED MANUSCRIPT ZnO is among the utmost investigated nanoscale materials and multifarious fields of expertise including photodetectors, light-emitting devices, photocatalysts, photovoltaics, energy conversion and storage materials, environmental and analytical sensors, imaging and fluorescence platforms as well as antibacterial

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applications due to its versatile and surpassing electronic, piezoelectric, optical, magnetic, electrochemical, adsorptive, and catalytic properties [1-8]. That is, ZnO possesses one of the widest band gaps amongst semiconductors with energy of

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3.37 eV, potent n-type semiconductivity, peculiar optical transparency, impactful excitation binding energy of about 60 MeV at room temperature, and excellent bio-

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conjugated stability [9,10]. A significant part of these properties is due to synthetic tuning including controlling crystal/grain size, morphology, and shape through various optimizations such as precursor type, external molecular/ionic doping, static/dynamic aging, kind of nucleation and growth steps, conventional/radiation

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based heating, and post-preparation annealing. Primarily, crystallographic and microstructural characteristics such as lattice imperfection, i.e. dislocation and distortion, deviation in unit cell parameters, aspect ratio and textural growth,

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electronic states and transitions, unoccupied binding sites, and unpaired electrons

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are in direct correlation with the optimal synthetic conditions [11-14]. In this regard, effects of metal doping have been surveyed more than any other factor in nanocrystalline ZnO system with which emerging changes within host lattice like crystallite diminution/expansion, transformation of morphology and preferential orientation,

microstrain

and

tensile/compressive

stress,

along

with

accumulated/dwindled surface chemistry might occur. Herein, high doping practice may cause unintentional textural alterations that may lead to detracted control over final size and morphology. While should low limits of dopants be introduced the

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ACCEPTED MANUSCRIPT attributed functionality might lessen severely, specifically for anisotropic scrutiny [15]. Therefore, it is deemed crucial to consider the dopant concentration influence in order for achieving parallel applicability and unit cell behavior. To date, ZnO has been hosting multifarious non-metals, alkaline metals, metalloids, transition metals,

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and lanthanides. In the recent years, rare earth metals (REMs) have been increasingly incorporated as dopant in ZnO microstructure partly to augment innate characteristics e.g. surface roughness and faceting, luminescence and scintillation,

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hole conductivity, electron and carrier trapping, to abate the exciton recombination, and increment oxygen vacancy as well as partly to affect structure-dependent

and

activation,

energy

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behavior e.g. excitonic red/blue shifts, magnetic susceptibility, photonic sensitization up(down)

conversion,

and

(bio)adsorption

[16-24].

Accordingly, miscellaneous exercises from optoelectronics, spintronics, light emission, optical storage, to photocatalysis have been on track utilizing REM doped-

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ZnO nanocrystals.

A pragmatic ground for diverse experimentations on ZnO could be its compatibility with the adaptability of various synthetic procedures to variant parameters to induce

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microstructural changes and subsequent physiochemical features. Hereupon,

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numerous synthesis methods have been outlined for the textural and morphological evolution of zero, one, two, and three dimensional ZnO nanocrystals. Bottom-up soft chemical approaches such as co-precipitation [25], solution-liquid-solid (SLS) [26], template-based

assembly

[27],

(micro-)

emulsion

[28],

hydrothermal

[29],

solvothermal [30], and sol-gel [31] are of utmost significance in this regard. Figures of merits including diminished time and energy consumption, simplicity, and capability of obtaining high orders of crystallinity accompany these solution-based approaches. Sol-gel method has gained intense consideration for the preparation of

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ACCEPTED MANUSCRIPT crystalline semiconductors and metal oxide nanomaterials as it is a delicate procedure and susceptible to disparate optimizations during and post-synthesis conditions such as multiple functionality of the solution media as solvent and ligand, reaction rates of hydrolysis/condensation steps, structure director type, pH and

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temperature, stirring/aging process, chemistry of dopants, and calcination/annealing treatments [32]. So far, vast modifications have been applied to sol-gel method, however in comparison to other factors, less observation can be found in order for

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obviating some drawbacks with conventional heating and crystallization based processes like hydrothermal and solvothermal. Generally, the extent of required

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energy from these sources is not sufficient for wide-range cases to obtain the best quality in the derived sol, gel, and xerogel and to start crystallization, especially when aqueous solvent is used. Therefore, high amorphous degree entails high temperatures in the post thermal treatment that hinder the advertent and precise

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control over size, shape, and morphology [33]. Notwithstanding the advantages brought by non-hydrolytic sol-gel namely molecular ligand induction though solvation mechanism, in situ condensation, coordination effect over surface, ameliorated

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dispersion of the particles, improved kinetics and thermodynamics through better

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control over reaction rate and lower solution temperature, oxygen supplement for metal oxide fabrication via organic solvent [32,33], still degrees of aggregation among the grains and temperature of the calcination/annealing steps can be observed [34,35]. Even at low temperature post thermal treatments, formation and transformation of organic species and by-products are not thoroughly resolvable [33]. Consequently, physical and thermal processing of sol, gel, and xerogel involving steps such as stirring, aging, evaporation, desiccation, and compaction are essential to intensely affect the internal microstructure, homogeneity, colloidal stability,

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ACCEPTED MANUSCRIPT remaining organics disposal, and ultimate crystallinity. For sol processing, ultrasonication has developed as an alternative to static/dynamic aging i.e. mechanical/magnetic stirring and as nanofabrication method in recent years where multifarious organic and inorganic nanomaterials have been swiftly fabricated with

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ultrahigh stability and reproducible size-shape modifications [36,37]. Effectiveness of sonochemistry places reliance on the fast transfer of acoustic energy to the reactant/precursor through ultrasound wave motion and subsequent induction of

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chemical activation energy [38]. To procure this, a phenomenon called acoustic cavitation should take place by applying ultrasound waves and their following

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compression and rarefaction cycles which evince the development of bubbles within solution [39]. The bubbles outreach to a critical size posthaste then crumble leading to the inception of shock waves and releasing energy. Thereupon, intended reactions i.e. nucleation, growth, complexation, and polymerization duly proceed

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following the enlarged seed momentum and particle collision as a result of these shock waves [40]. Through collision process, successive aggregation and splintering occur leading to two main ultrasonication mechanisms; erosion and fragmentation. In

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the first case, the cleavage of disparate nucleated seeds in form of particles from the

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aggregates eventuates, while in the latter compact aggregates are detached into diminutive ones [39-41]. Various factors such as ultrasound power, frequency, continuous/pulse irradiation, and intrinsic properties of solvent and precursors are determinative of sonochemical cavitation efficiency. In this fashion, not only puissant synthetic mechanism via sonochemistry provides privileges including time and energy saving, anisotropy attainment, homogeneity and evolution of size, shape, and morphology, as well as qualified crystallization, but also potent ultrasonic agitation averts the timely and costly need of conventional stirring/aging routes. Increasing

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ACCEPTED MANUSCRIPT attempts to utilize ultrasonication for nanocrystalline ZnO preparation in the past years postulate this superiority [42-44]. However, ample and broad ranges of ultrasound power and frequency encompass these reports. While, too large magnitudes of power hamper the proper acoustic energy transfer because of

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superfluous cavitation, too high frequency hinder the cavitation by stunting the wave diffusion and spread within solution [39]. On the contrary, exerting very low amplitudes of above parameters necessitates prolong reaction time. Hence,

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moderate levels between 100-300 W and 20-40 kHz are befitting to acquire promising crystalline features.

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Increasing use of alternative heating approaches like microwave irradiation in solgel method allude merits such as abated cost and synthesis time, augmented reaction selectivity, efficient distribution of microwaves and energy within sol and gel, boosted kinetics and size/shape control, as well as electronic adjustment [45,46].

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The main trend in microwave-based sol processing arises from dielectric constant difference between solvent and reactant entailing solvent dipole rotation and reactant ionic conduction upon which the radiant heat straightly actuates through liquid and

heating”

hint

at

this

mechanism

with

which

physiochemical

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“molecular

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imparts the energy to the reactive precursors [47]. The terms, “dielectric heating” and

transformations and electronic changes e.g. lattice imperfection and extrinsic doping with ionic radius gap proceed with prompt kinetics that is unlikely achievable via conventional heating [48]. In this way, homogeneous and instantaneous rise in temperature

expedites

the

precursor

disintegration

and

bursts

into

the

“supersaturation” phase that is suitable for nucleation, growth, and final crystallization. This also implies the potential of microwave radiation for industrial production with minimum energy gradient [47-49]. Recent reports indicate the

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ACCEPTED MANUSCRIPT successful microwave-based fabrication of ZnO nanocrystals with different shapes and sizes [47-53]. A great share of lattice imperfection and deviation in unit cell parameters betide because of solid state non-stoichiometry. To preserve neutrality, charge compensation should be fulfilled through either introducing extrinsic impurity

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or intrinsic defect evolution. Usually high energy processes like electron beam and microwave radiations along with (post)thermal treatment with various air, vacuum, inert, or oxygen atmospheres accompanied by rapid thermal quenching which

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entangle the evolved defects are considered as common synthetic routes to induce non-stoichiometry [54,55]. Nevertheless, prolong thermal annealing can diffuse out

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the emerged defects and stabilize the system. Hence, the cooperation of radiation/irradiation and short annealing processes would be beneficial. In the present study, nanocrystalline gadolinium (Gd) doped ZnO nanorods in form of solid solutions were synthesized by a modified sol-gel method in which steps

including

nucleation,

sol processing,

growth,

gelation,

and

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various

crystallization occur using ultrasonic combined microwave irradiation and post air annealing. Ionic dopant with different concentrations (Gd/Zn: 1, 5, and 10 mol%) was

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incorporated in the host texture, thus microstructural and electronic analyses were

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exerted in terms of lattice imperfection, crystallize/grain size, morphology, shape, aspect ratio, elemental composition, oxidation state as well as probing surface and bulk defects. To date exiguous studies have been performed for the preparation of crystalline ZnO via combined ultrasound-microwave irradiation strategy [56-59], however to our knowledge no report is present in this scheme for fully crystalline GdZnO. To refer to un-doped and Gd doped samples, ZnO nanorods are denoted by ZO and GZO marks accompanied by the assigned dopant concentration.

2. Results and discussion 8

ACCEPTED MANUSCRIPT 2.1. Microstructural study 2.1.1. Lattice analysis The commencing microstructural resolution was done by phase examination via PXRD patterns as depicted by ZnO typical reflections in Fig. 1. The formation of

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nanocrystallites in all samples is approved by the recorded peaks at Bragg angles of 31.8692°, 34.5464°, 36.3502°, 47.6415°, 56.6688°, 62.9446°, 66.4410°, 68.0314°, and 69.1591° assigned to ZnO crystal lattice and attributed crystal planes, i.e. (100),

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(002), (101), (102), (110), (103), (200), (112), and (201), respectively denoting a

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hexagonal close pack (HCP) structure, P63mc space group, and lattice parameters of a=b=3.2498 Å and c=5.2066 Å which conform with crystalline structure in JCPDF card No. 00-036-1451. Sharp and segregated reflections in all patterns address the well-crystallinity of both ZO and GZO samples. Together with the shifts in the positions of three major peaks as shown in Fig. 2, change in the peak widths

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illustrate the dissolution of Gd species within the host lattice and the corresponding impact over internal microstructure. This alludes that in spite of notable lattice mismatch due to ionic radius discrepancy between Zn (0.74 Å) and Gd (0.94 Å)

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species, the proposed synthesis method manages to render solid solutions.

D=β

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Crystallite size (D) was calculated by the well-known Debye–Scherer formula: (1)

where k is the so-called shape factor taking a value of 0.94, λ is the incident X-ray wavelength, β is the line broadening of the recorded reflections at their half-intensity maximum (FWHM), and θ is the Bragg angle of observed diffractions. Since no characteristic Gd reflection peak was found in the patterns, the case of secondary phase is excluded and thus Gd-dopant incorporation is significant. The effect of Gd-

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ACCEPTED MANUSCRIPT doping and increasing concentration on the host ZnO phase was assessed by manifold internal microstructural parameters namely, crystallite size (D), lattice constants (a, c, V), unit cell normalization (a/a0, c/c0, V/V0), lattice microstrain (ε%), axial strain (εzz), residual stress (σ), dislocation density (δ), Zn—O bond length (L),

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and stacking fault probability (α) within ZnO texture. The very first assessment shows the enhancement of crystal size with Gd loading as dopant dissolves in the host lattice with higher ionic radius (Table 1). Lattice analysis was studied as a function of

=2

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(2)

= "=

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Gd doping through lattice geometry equations:

(3)

!

√$% & '

= 0.866,' -

(4)

where dhkl is the interplanar spacing between contiguous crystal planes in the HCP

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system and is derived via Bragg's formula (eq. 2). a and c constants could be obtained for (100) and (002) reflected planes should the first order approximation (n

,=

.

√$/012344 .

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- = /012

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= 1) for the first diffraction be in effect through the following equations:

44

(5)

(6)

The corresponding amounts can be regarded in Table 1 comparably with those of bulk ZnO JCPDF 00-036-1451. The exhibited variation is in close relation with alteration in other parameters like D and ε. In the case of dopant substitution and attainment of solid solution, lattice and unit cell parameters vary while considering the ionic radii they would diminish or enlarge. Peak positions change at the Bragg’s angles and dependent on the originated lattice microstrain’s status, i.e. compressive 10

ACCEPTED MANUSCRIPT or tensile, shift toward higher or lower angles, respectively. Last but not least, anisotropy rises along unit cell directions producing polarization in the growth of nuclei [60]. In this regard, aspect ratio evolution is a benefit and was examined either by changes in lattice parameters, specifically atomic packing factor (c/a) and by

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FESEM magnification. Here, axial and lateral growth along c- and a-axis correspond to length (l) and width (w) along [00l] and [hk0] directions, respectively. Compared to bulk ZnO (JCPDF 00-036-1451) c- and a- subsequently l- and w- values decrease

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for both un-doped and doped samples. Also, a descending trend can be observed for unit cell volume, though the amounts are bolstered for GZO10 with respect to other

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synthesized samples. Hence, the highest axial and lateral growth as well as the most cell expansion belongs to GZO10. On the whole, c/a value lessens with Gd addition, but augments with further concentration to some extent. Accordingly, it is expected that ZO and GZO10 nanorods have the higher lengths. This hints at the more

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likelihood of nanorod shaping in un-doped ZnO and more possibility in Gd-enriched ZnO samples. Trivial deviations from the host ZO atomic packing factor (1.601) allocate the lattice stability with Gd addition and HCP constitution of the Wurtzite

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morphology. Damping and accelerating trends for axial-lateral growth along with unit

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cell expansion are illustrated by unit cell normalization with reference to those parameters of standard JCPDF ZnO in Fig. 3 distinctly affirming the sensitivity of the method and applied irradiation approach toward doping. It is clear that the most and least deviation exist in GZO5 and GZO10 lattices, respectively. Table 1 also reveals that GZO1 possesses the lowest atomic packing factor. The observed low crystal fraction and atomic packing density (c/a) intimate the increased line broadening (β) at major reflections due to microstrain as well as enhanced dislocation in GZO5 lattice. Besides, the lowest anisotropy and distortion/dislocation are concluded for

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ACCEPTED MANUSCRIPT GZO1 and GZO10, respectively. The amounts of normalized lattice parameters are brought in Table 1. Plausible explanation for the suppressed crystal lattice parameters in GZO5 could be the more drastic configuration between structure directing agents (PVP, DEG, and TETA) and Gd species in the peripheral layer of

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nanocrystals at nucleation stage. Consecutively partly growth inhibition effectuates its crystal planes and average size. Herein concerning the dopant effect, these structure directors accomplish an essential role in the final crystalline structure

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through affecting the crystallite size and axial/lateral growth. As Table 2 and position shifts in Fig. 2 adduce, the effect of Gd-doping on the host lattice displays the

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appearance of forces including lattice distortion (ε, εzz and σ) and dislocation (δ). The average lattice microstrain (ε) emerging due to distorted sides per primitive cell of HCP crystal is evinced by Stokes−Wilson (S-W) equation [61]: 7

5 = [89%12].100

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(7)

where K is the crystalline assigned constant and usually equals 4. The average dislocation density (δ) expressing the extent of dislocated sides per unit volume of

(8)

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δ=;

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the crystal lattice is measured by Williamson-Smallman calculation [62]:

With respect to crystal size, maximum degree of δ and ε belongs to ZO and GZO5 samples, respectively. Hence the combined size-strain effects cooperate in the overall line broadening. The contribution of microstrain in both peak shifts and line broadening demonstrates that the induced ε in the understudy system is concurrently uniform and non-uniform. In the first case, mainly uniform compressive strain is applied to the host grains making smaller d-spacing values and right-shifts to higher angles that is maximized in GZO5. Less ε in GZO10 implicates rather tensile strain 12

ACCEPTED MANUSCRIPT regarding ZO sample as indicated by left-shift to lower positions (Fig. 2). In the latter case, some of the host grains are bent and exposed simultaneously to compression and tension causing interplanar spacings fairly smaller and larger than equilibrium spacing, respectively. This exclusively leads to line broadening especially in the

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reflections of GZO5. However, diminished β could be seen for GZO10 owing to microstrain relaxation. Gd addition also results in δ increment for GZO5 then decrease for GZO10 inversely proportional to the variation of crystallite size and unit

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cell in these samples. On the other hand, applied forces specifically along c-axis including strain (εzz) and stress (σ) impose the most impacts over crystal growth and

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quality. εzz in HCP system is defined as the measure of length variation along c-axis under either tension or compression in comparison with primary (unstrained) length and can be derived using the biaxial strain model [63]: &>&4 &4

? . 100

(9)

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5<< = =

where c and c0 embodies lattice constants of the synthesized and bulk ZnO, respectively. In this regard, compressive (tensile) strains would emanate for the

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samples with detracted (enlarged) c amounts. εzz could be linearly connected to residual stress (σ) within lattice via Hooke's law which can be measured by the

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biaxial stress model hereupon [63]: A = B. 5<< = C

' 3 >[

D 33

' 3

3

E]

FC

&>&4 &4

F

(10)

where E is the Young’s modulus of elasticity and Cij stands for the elastic stiffness constants of bulk ZnO (C11=209.7, C12=121.1, C13=105.1, and C33= 210.9 GPa). In general, stress could emerge due to two sources of structural evolution; (1) interpenetration of impurities, strain provoked defects, interplanar spacing, and inelastic unit cell deformation that instigate intrinsic stress, and (2) thermal treatment, 13

ACCEPTED MANUSCRIPT doping, appending extra phases (in the case of nanocomposites), and lattice mismatch which beget extrinsic stress [64]. As outlined in Table 2, variation in εzz and σ is consistent with that of c constant and the maximum amounts are inflicted over the GZO5 c-axis in which the decrease respecting the reference c0-axis brings

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compression forces into effect. Moreover, c-axis relaxation at higher Gd concentration implies that larger crystals and lower unit cell variation in GZO10 impede the influence of dopant in line broadening and unite cell deformation.

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Although on the whole, unexpanded extents of lattice imperfection factors in Table 1 point out to the low amounts of defects and good crystallinity state. This in turn

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certifies the adequate internal energy at the interface of grain boundaries as well as free surface energy procured by the irradiation process which leads to voids/defects compensation, active specie and grain boundary mobility amplification, as well as atomic packing density and HCP structure augmentation. Consecutively grain growth

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eventuates as a result of diffusion of grain boundaries and coalescence of fine grains. Following the engendered imperfection forces, Zn—O bond length (L)

[65]: [

%

$

+ D' − JE' - ' ]

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G=

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fluctuations were evaluated by the following equations and demonstrated in Table 2

%

J = $& + 0.25

(11) (12)

where u is the position parameter signifying the atomic locality and supplanting by the adjunct atoms in HCP structure which is intensely yet inversely commensurate with atomic packing factor (c/a). Originated polarization in crystal growth and along unit cell directions effectuate the tetrahedral angle distortion and hence consistent intervals for tetrahedral facets eventuates by this reverse trend [65]. With reference

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ACCEPTED MANUSCRIPT to JCPDF bulk ZnO, through Gd addition c/a and consecutively u factors affect GZO1 the most. Simultaneously regarding lattice constants variations, L value rises for GZO10 (Table 2). This could be viewed by the improved lattice constants and crystal quality, especially along c-axis, via sufficient energy provided by irradiation-

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based processing and heating which lessen the position parameter. However, unit cell volume also expands denoting that higher dopant concentration appends more space within host unit cell boosting the growth along a-axis as well. Therefore, bond

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length augments due to both axial and lateral growths. Apropos of alterations in the lattice parameters and subsequent induced imperfection, quantification of stacking

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fault probability (α) within HCP lattice facilitates the conception of understudy microstructure. α describes the spatial arrangement and planar imperfection that transpire via non-sequential stacking of an atomic plane with respect to sequential stacking of other planes and can be measured by the following equation [66]: ∆D'2E

(13)

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'M

L = CNO√$F . C QRS 2 F

where θ is the angular position of the atomic planes and ∆(2θ) is the corresponding

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shift in the preferred (hkl) orientations. Inscribed results in Table 3 clearly express that GZO5 and GZO10 demonstrate the most and least levels of α owing to the

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mentioned extents of lattice deformation forces in their systems. Regarding the overall slight deformation, i.e. distortion and dislocation, the extent of changes in α is not yet substantial concordant with gentle peak shifts. This suggests that while radiation crystallization and Gd doping certainly affect the host lattice, the conditions also maintain its stability. 2.1.2. Nanorod analysis

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ACCEPTED MANUSCRIPT Size and shape of the nanorods were surveyed by FESEM and TEM micrographs. The homogeneity of the grown nanorods illustrated in Fig. 4 not only shows the crystal orientation and achieved aspect ratio via in situ metal-polymer complexationirradiation strategy, but also suggests the preservation of uniformity through doping

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procedure. Utilized PVP, DEG, and TETA are capable to form interconnected covalent, coordinate, and intermolecular bonding and provide considerably large but yet sufficiently entangled network with limited curvatures for directing the needed

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growth space [67]. Incorporation amongst charge balancing behavior within the coordination spheres of metal nuclei and the surface/crosslinking activity of these

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agents induces the apparent one-dimensional growth. Thus, there exists a synergism between organic structure directing and irradiation approach in which the binding and unbinding dynamics are improved by ultrasonic and microwave processing, respectively. Furthermore, a trend of oriented assembly could be

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observed in the FESEM magnification of ZO to GZO10 where in the latter a flower architecture is organized due to Ostwald Ripening effect generated by the rearrangement of nanorods to attenuate interfacial free energy [68]. Also, precursor

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reactivity with microwave beams varies with concentration as displayed in FESEM

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images where low amounts of added metal nuclei (Gd) in GZO1 results in smaller rods. In contrast to this path, dense population of nuclei leads to longer rods in GZO10. This conspicuous shape evolution is energetically favored for larger nanocrystals and is consistent with the bolstered lattice parameters and crystal quality in PXRD results for GZO10. Though GZO5 possesses smaller crystallite size and lattice parameters mainly due to larger lattice microstrain and unit cell strain, higher c/a ratio leads to more growth along c-axis and ultimately nanorod length as compared to GZO1. On the other hand, the elongated nanorods belong to undoped

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ACCEPTED MANUSCRIPT ZO owing to its maximum c/a and aspect ratios. Upon exposure to ultrasound waves and microwave beams, both length and width are affected as a prerequisite for aspect ratio variation. Shorter and nascent GZO1 nanorods along with longer ZO in FESEM and TEM images respectively meet minimal and optimal c/a ratios thereof.

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The average aspect ratio (R) was assessed by length to width ratio (L/W) and its variation with Gd addition as well as correspondence with atomic packing factor (c/a) are delineated in Fig. 5. Self-assembly and flower formation processes in doped

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systems clearly indicate the decrease in aspect ratio fractions approved by attenuated atomic packing factors with relatively small evolution of length with

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respect to pure ZO. Although the anisotropy trend in shape was bolstered with higher content of dopant. The synergy between the turning point in the aspect ratio decline and minimum c/a ratio in low Gd concentration as well as the appearance of highest strain/stress levels in moderate Gd concentration are consistent with lattice

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imperfection and microstructural change upon introducing Gd ions with significant ionic radius discrepancy. As for high Gd concentration, crystalline features improve and ultimately result in more elongated nanorods. Hereupon, anisotropic size and

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shape evolution are entangled with bold roles of internal microstructure and organic

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structure directing agents with which respectively unit cell variation and molecular adsorption onto crystal planes affect the growth in [00l] and [hk0] directions. TEM micrographs are depicted in Fig. 6 and 7 verifying the nanometric dimensions, rod shape, increasing rearrangement of nanorods with Gd content, and flower assembly in GZO10. Analyzing TEM images shows that the undertaken ultrasound-microwave irradiation based crystallization strategy produced nanorods with lengths between about 0.5 to 2 µm and nanoscaled widths as displayed for GZO10 in Fig. 7. The order of lengths is proportionate to R and c/a factors where

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ACCEPTED MANUSCRIPT ultralong and small nanorods belong to ZO and GZO1, respectively. The experimental reactions evinced the critical roles of PVP, DEG, TETA, pH value, time and temperature, and more importantly ultrasound/micro waves in the attainment of

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nanorod solid solutions with high quality. 2.1.3. Elemental analysis

To demonstrate the quality, composition, and distribution of Gd content in ZnO

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nanorods texture, EDX and FTIR spectra were acquired. Fig. 8 represents elemental structure of the grown nanorods as EDX spectra where zinc and oxygen content is

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clearly decreased after Gd doping. Characteristic O and Zn peaks testify the synthetic method. Peaks around 1.2 and 6.1 keV are assigned to typical Gd X-ray emissions certifying the solution of appended dopant. Lack of any extra emissions reveals the purity and quality of the synthesized samples. To emphasize the

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quantitative distribution of Gd doping in the host texture, two distinct EDX mensurations were conducted onto separate sites of all samples. Regarding the synthesis process uncertainties, Table 4 verifies the nominal values (weight

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percentage) of Gd content in the samples. The chemical nature of pure and doped ZnO samples was characterized by room temperature FTIR spectroscopy giving

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more details on the influence of Gd doping and the proposed synthesis approach. As plotted in Fig. 9, the peaks between 449 to 454 and 483 to 487 cm-1 evince the formation of Zn–O stretching vibration bond in transmission mode. These bands conforming to Wurtzite configuration of zinc oxide became resolved in all samples except GZO5 and the slight changes in their positions stipulate the Zn–O–Zn network perturbation through Gd doping. The best differentiation occurs between the peaks at 451 and 486 cm-1 for GZO10 as it exhibits the lowest lattice distortion/dislocation, but the most band shifts occur for GZO5 with because of the 18

ACCEPTED MANUSCRIPT opposite. The intensity and integrated area also changed regarding the charge compensation with the doping procedure where the maximum Zn–O bond stretching vibrations were recorded for ZO and GZO10 with the highest values of unit cell parameters. In addition, no significant and characteristic absorption band attributed

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to the formation of gadolinium oxide (Gd2O3) is recorded in this region stipulating the solution and nucleation of gadolinium ions in ZnO matrix. Observed band peaks at around 874 and 876 cm-1 arise from O═C═O chain vibrations. Absorption bands

SC

between 1178 and 1270 cm-1 represent C═C vibrations which were altered in position upon doping process. Absorption bands around 1399 to 1400, 1453 to 1460,

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and 1616 to 1630 cm-1 are respectively attributed to C═O asymmetric stretching vibration, CH2 bending vibration, and C═O symmetric stretching derived from cross linking/capping agents. Fringe bending vibrations at 2341 and 2342 cm-1 are assigned to O–C–O while fringe symmetrical and unsymmetrical stretching vibrations

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between 2850 and 2924 cm-1 are allocated to CH2 molecules (C–H mode). Both of these bending and stretching peaks at this region were drastically attenuated by radiation-thermal treatment. Broad but rather weak absorption band around 3436

EP

and 3438 cm-1 showing the stretching and bending vibration of few hydroxyl groups

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(polymeric O–H mode) depict sensible decrease in resolution from ZO to GZO10. Absorption assignments are in consistent with literature. 2.2. Solid state and electronic survey 2.2.1. Raman scattering To address the effect of Gd dopant on the vibrational attitudes of host ZnO, room temperature first-order Raman spectra of pristine ZO and GZO nanorods with visible excitation are depicted in Fig. 10 in which all symmetry-allowed modes brought forth

19

ACCEPTED MANUSCRIPT significant bands with slight differing Raman shifts. As HCP microstructure possesses P63mc symmetry group (C6ν4 space group) with two tetrahedral coordinating unities within Bravias Wurtzite lattice and two-point based primitive unit cell, twelve overall phonon modes exist comprising of nine optical and three acoustic

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modes [69]. At central point (Γ) in the Brillouin zone of reciprocal space, the optical phonon modes predicted according to group theory involve A1, 2 B1, E1, and 2 E2 modes. Among them, A1, E1, and E2 modes are Raman active the first two of which

SC

are also infrared active (polar) and capable of diverging into longitudinal optical (LO) and transverse optical (TO) modules, thus instigating vibrations along z axis and x-y

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plane, respectively. Discrepancy in the frequency of these components arises from macroscopic electric fields associated with LO phonons [70]. E1 (LO) mode is usually assigned to various intrinsic and synthetic states such as excitons and lattice defects [69]. On the other hand, whilst B1 modes are both Raman and infrared inactive

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represented as silent modes, active but non-polar E2 modes are effectuated by the vibrations of oxygen and zinc species within lattice and induce sub-modes of E2high and E2low, respectively [71]. All samples exhibit Raman active modes involving first

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order E2low, A1TO, E1TO, E2high, A1LO, E1LO and 2(A1, E1)LO phonons at 102, 379, 411, 438, 560, 578, and 1153 cm-1 and second order E2high - E2low multi-phonon

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resonance at 332 cm-1, respectively typical for well-crystalline ZnO HCP unit cells. In UV ) very bulk crystals the long wavelength limit exists that is only small wave vectors (T close to the center of the Brillouin zone (k=0) are allowed for phonon dispersion and to bring forth physical momentum and vibrations of unit cells in phase. Hence, only phonons in this region could be detected in the first order Raman scattering (momentum conservation rule) [72]. However, upon downsizing to nano-dimensions, this selection rule can be mitigated, thus the phonon dispersion and respective

20

ACCEPTED MANUSCRIPT scattering on the periphery of the zone center could be regarded. Accordingly, changes in terms of band shift and broadening for these single phonons might be detected due to unit cell perturbation [73]. In this regard, indicative E2high vibration exhibits the extent of lattice deformation under the terms microstrain and stress with

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the most Raman shift and declined intensity in GZO5 denoting the incorporation of Gd3+ ions in ZnO lattice. Moreover, lattice defects including zinc vacancies, zinc interstitials, and oxygen vacancies or even free carriers are responsible in the

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occurrence of polar modes such as A1LO and E1LO [74]. Upon doping, E1LO mode also red shifted towards lower wavenumbers whilst it appears that its intensity increases

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at the cost of E2high mode implying the generation of surface and bulk defects, although the overall dominance of E2high over LO phonon is significant still, signifying the Wurtzite crystal quality. Fig. 11 demonstrates the correspondence between the level of Gd dopant in the host structure and E1LO to E2high ratio that is maximized for

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GZO10 pointing out either to the overall higher number of host and guest defects and lower unit cell deviation in this sample. Although low values of strain and stress diminish the extent of native Zn- and O-related defects, they also cause less

EP

variation in the vibrations of these species and E2high mode thereof. Simultaneously,

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E1LO augments slightly due to higher number of Gd-related defect centers. While steep slopes from ZO to GZO5 display the augmenting defect distribution both on the surface and in the bulk, the slight slope for GZO10 points to the minimum extent of defects due to ZnO lattice, though still the denser population of Gd species and their corresponding defects influence the host unit cell. Characteristic E2high - E2low band along with broad weak peak at 544 cm-1 are attributed to the second-order vibrations due to zone-boundary phonons (E2M) at the face center (point M) of Brillouin zone and E2high + E2low, respectively [69,70]. Two commentaries could be

21

ACCEPTED MANUSCRIPT discerned from the recorded E2 attributed bands for both pristine and doped samples; firstly, the prominent intensities hint at extended vibration over the surface area of the grown and rearranged nanorods, especially in form of flowers. As PXRD profiles affirm, owing to the presence of orientation and anisotropy, the nucleation

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damping and growth suppression of nanocrystals are negligible, thus the synthesized nanorods tend to vibrate along the length. As elemental spectra approve, large portions of samples surface consist of unsaturated reactive atoms

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duly competent to augment the vibration degree. Especially, distinguished E2M resonance entails noticeable multi-phonon scattering and consecutively quantum

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confinement because of dense nanoentities over specific surface area [75]. Secondly, the observed blue shifts in non-polar E2 modes in comparison with those of zone-center optical phonons in bulk ZnO (101 and 437 cm-1) point out to the compressive strain in the grown lattices [76]. Apropos of LO phonons, should the

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exciting photon be perpendicular to c-axis of ZnO lattice, no LO sub-modes could be recorded. On the other hand, should the phonon vibration polarization be parallel or perpendicular to the c-axis and crystal/grain surface, A1 (LO) and E1 (LO) related

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bands could be observed, respectively in the middle region. Here the grown

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anisotropic nanorods have naturally the c-axis orientation which accordingly leads to relatively distinguished/sharper A1 (LO) peaks. However, setting the incident photon geometry parallel to the c-axis and the fact that nanorods are haphazardly dispersed within powder samples (the occurrence of crossed polarization), both LO sub-modes are detected. Furthermore, for the synthesized systems, 2LO (A1, E1) intensities are remarkable which augment and red shift as Gd content was raised. This also imparts nano-dimension (1D) of the prepared nanorods. The detected bands at 661 and

22

ACCEPTED MANUSCRIPT 1102 cm-1 are attributed to the TA + LO mode and the acoustic overtone with A1, E2 symmetries, respectively. 2.2.2. H2-TPR study

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Figure 12 outlines the surface activity and reducibility status of nanorods through H2-TPR profiles. Usually typical reduction activities for ZnO are recorded as two peaks; one peak around 300-315°C due to O–H group reduction in the adsorbed

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hydroxylated species and moister on the surface and relatively broader and yet sharper band between 400-480°C associated with partial reduction of active and

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adsorbed surface oxygen species [77]. However, in our case no such significant reducibility could be observed in these regions indicative of the purity and high quality of irradiation procured crystals except a very weak peak around 346°C. Lack of any secondary peak related to Gd or Gd2O3 compounds in other regions is a

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symptom of the formation solid solution. This fact along with the absence of substantial surface adsorbed OH peak which prevents the close contact with Gd ions, barricade the configuration of a secondary phase in the form of an extrinsic

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oxide and the formation of nanocomposite. Instead, intense and broad bands centered at 871, 851, 848, and 853°C for ZO, GZO1, GZO5, and GZO10 samples,

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respectively, represent the bulk reduction of Gd-ZnO systems. The recorded shifts as well as overall enhanced intensity of the doped systems compared to pristine ZnO hint at the electronic interplay between the introduced dopant and internal host structure. On the other hand, the extent of H2 consumption did not vary substantially among the doped systems again indicating that no extrinsic species evolved. The reduction of ZnO is thermodynamically feasible at high temperatures (higher than 650°C) [78], so the observations in Fig. 12 correlate TPR peak positions to crystallites size and shape. As ZnO nuclei rapidly crystalize via microwaves, the 23

ACCEPTED MANUSCRIPT surface to volume fraction declines. At the same time, elongated nanorods especially ZO and GZO10 as well as rearrangement process to form nanoflowers involve agglomeration which tighten the reduction owing to lower accessible surface and bulk layers exposed to the flowing H2 gas. Therefore, higher temperatures are

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needed. Parallel to Gd molar ratio, two shoulders appeared in the main peak. The overlapping of the accompanied shoulders with the strong main reduction peak at high temperatures presupposes a spillover mechanism in which hydrogen atoms

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marginally adsorbed on surface species migrate to the bulk areas and reduce them. In this context, the homogeneous solution of Gd ions facilitates their contact with Zn–

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O species through bridging oxygen mechanism and ultimately Zn centers, amplifies the spillover mechanism, and consequently slightly reduces the bulk reduction temperature. The left shoulder describes the reduction of active surface oxygen attributed to Zn centers and the chemisorption of hydrogen atoms (HZn–OH).

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Introduction of the appended Gd ions increments the surface defect quantity and causes oxygen vacancies, hence the onset temperature for this shoulder shifts to the right for doped samples until GZO5 which possesses the utmost lattice/unit cell

EP

deformation. While, the extent of strain and stress is minimum in GZO10, thus

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surface reduction is facilitated and the onset shifts back. Simultaneously, the onset temperature ascribed to the right shoulder describing the more convoluted bulk nucleated Gd centers reduction slightly shifts toward higher values proportionate to Gd molar content. Regarding the complete Gd ionic solution in ZnO matrix and induced lattice deformation, the presence of both Zn–O and interconnected Gd species by bridging oxygen atoms as illustrated by TPR peaks are plausible and is attributed to the generated dopant to matrix interaction (DMI). Although not reinforced by OH species, surface assisted and internally afforded DMI effect is

24

ACCEPTED MANUSCRIPT assumed to show off multifarious properties e.g. electronic variation, charge compensation, and much likely highlights the progression of an atomic ensemble effect suitable for adsorptive and catalytic purposes.

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2.2.3. EPR study Solid state, surface, and defect chemistry were further pursued by EPR spectra and investigation of chemical states of the understudy nanocrystals. The very first

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domain in defect evaluation is the extent of lattice deformation and formation energy of the emerged defect species. PXRD results give a quantitative measure of the

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former and by assessing the differences in lattice relaxation determine the defect status and EPR signal viability. Regarding the theoretical studies, high formation energy (Ef) cancels the defect concentration according to the following formula [79]: - = WXYZ[

>[ \ ]^

]

(14)

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where N is the number of sites on the surface and in bulk per unit volume in which defects can emerge, kB is the Boltzmann constant, and T is temperature. Although PXRD and Raman experiments confirmed the induction of volume and vibration

EP

changes upon doping, the contributions of the formation volume and entropy are

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comparatively small with respect to the domains of energy. Hence, Ef is the major parameter for defect scrutiny. First principle calculations predict the following native defects in nanocrystalline ZnO with multiplicity in their charge states [80]: oxygen vacancy (VO), zinc vacancy (VZn), zinc interstitial (Zni), oxygen interstitial (Oi), zinc antisite (ZnO), and oxygen antisite (OZn). Dependent on the position of Fermi level, formation energy and stability vary within band gap. Theory has also determined that Ef values for VO, Zni, Oi, ZnO, and OZn are simply too high to get dense population over surface and bulk. The first two have been presumed widespread and major

25

ACCEPTED MANUSCRIPT species in the literature. However, since in most cases the prepared ZnO materials have indeed n-type conductivity, it is postulated that high Ef values lower the population of VO and Zni in the as-prepared samples [79]. Nevertheless, extreme conditions

including

photo-illumination,

radiation,

and

irradiation

procure

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thermodynamic feasibility, facilitate the provision of Ef, and subsequently enhance EPR sensitivity of these defects. Another factor is the required energy for defect mobility (Em) or otherwise defined as migration energy barrier (Eb) through lattice

SC

which should be supplied at a least temperature of 266.85°C [81]. Both VO and Zni cause displacements along c-axis in the Wurtzite system and lead to fluctuations in

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the interactions with neighbor Zn and O atoms. Yet to be EPR sensitive, VO+ and Zni+ species must be stable over the surface and in the bulk, respectively. On one hand, VO is a surface and Zni is a core defect, on the other hand, Ef and Em values are inversely proportionate to these centers. While VO possesses lower Ef, Zni has the

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lowest Em among all native defects and can be mobile at room temperature. The energy barrier could be resolved at least at 370°C for VO2+ state which is far more stable than VO+. Theoretical evaluations have stipulated that Zni cannot be detected

EP

as isolated species but as Zni-related complexes [81]. Accordingly, under non-

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stoichiometric situations, deep donor VO and shallow donor Zni could occur at surface and in bulk, though not densely, at attributed g-factors of 1.99 and 1.96 in low and high magnetic fields, respectively. Furthermore, due to low temperature air annealing, the energy barrier for the diffusion of external oxygen atoms from air into ZnO matrix is high leading to higher probability of VO centers than Zni. High field 1.96 line is sometimes presumed as an overlap of Zni resonance with that of hydrogen interstitial (Hi), also a shallow donor which replace oxygen atoms and form an intercoordinated configuration [82]. However, giving H2-TPR profiles the occurrence

26

ACCEPTED MANUSCRIPT of hydrogen atoms is inadequate even at surface. Hence, in our case Hi resonance would be impotent. Meanwhile, a more plausible native defect is VZn which has deep acceptor nature in the vicinity of valence band and suits for promoting n-type conductivity since its Ef diminishes by rising the Fermi level during the synthesis

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procedure. VZn, as a compensating center, is usually resonances with a g-factor range of 2.002-2.056 [79]. As for the rest of defects, theory determines that Oi, ZnO, and OZn species have too much Ef to take part in ZnO n-type conductivity [54].

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Regarding the respectively high and low Ef/mobility of Zni and VZn as well as the vicinity of VO- and VZn-related g-factors and their overlapping, once generated in the

M AN U

lattice, Zn atom diffusion and migration mechanism should be conducted by VZn centers. Herein, due to above observations, irradiation-assisted generation, Znicomplexation, and VZn-assisted Zn-diffusion throughout the lattice are plausible mechanisms for ZnO-based defect chemistry.

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The occurrence and evolution ZnO native and Gd ions related defects are depicted in Fig. 13 as a function of Gd concentration. Under microwave environment, temperature and consecutively crystallization rate rises. Owing to fast quenching

EP

time afterwards, nanorod formation is fast as well, so the evolved centers would

AC C

entangle in the lattice quickly. Thus fast crystallization eventuates in the reinforced number of defects. In this context, nucleation and growth thermodynamics drift from the equilibrium state. Therefore, native and guest defects could emanate through non-equilibrium conditions. As illustrated in EPR spectra, low field resonances with g-factor ranges of 2.0022-2.0225 and 1.9961-1.9984 are assigned to VZn and VO, while high field signal at 1.9615 shows Zni resonance. Superimposed paramagnetic resonances of VZn and VO also raise the charge compensating effect making them a pair center and emphasizing the Schottky defect type [54]. Correspondent to unit cell

27

ACCEPTED MANUSCRIPT variations, EPR spectra testify that with Gd addition until GZO5, VZn is the dominant center consistent with its low Ef, while for GZO10 a transition of deep level state is shown and Zni become predominant, in agreement with its low Em. For GZO10, highest lattice relaxation is marked by the lowest values of ε and σ. Consecutively, Ef

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for other VO states, i.e. VO0 and VO2+ is lessened compared with unstable and unpaired VO+ and thus VO-related signal diffuses out for this sample. The same case could be true for Zni species, but noting the lowest Em among all native defects, Zni+

SC

is more sustainable than VO+. As for ZO, GZO1, and GZO5 lattice distortion is increasing and the generated VO+ centers would not easily transform into VO0/VO2+

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because of larger energy barriers due to differential lattice relaxations surrounding these center with various charge states as verified in literature [81]. X-band EPR signals due to Gd ions mainly appear around g-factors of 2, 2.8, and 5.7 [83]. Noting the observed remarkably intense and broad paramagnetic

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resonances centered at about g=2 and g=2.8 which arise from either dipole-dipole interaction between dissolved Gd3+ ions due to high spin state and bridged via oxygen atoms or their reciprocal interplay with those of zinc and oxygen, a

EP

superimposed band of Gd-related lines (8S7/2, electron spin S=7/2) is formed. Three-

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valent lanthanide and actinide ions often give rapid relaxation and excitonic recombination period which includes the line broadening and is unrectified at room temperature [83]. Along with Gd concentration, the contributing electron spins enhance and broadening gets more intensive so that either the transition due to the resonance at g=5.7 is absent and also Gd band slightly shifts to higher magnetic field where VO is diffused out and Zni is dominant in GZO10. On the other hand, upon Gd introduction the superimposition in the low magnetic field becomes to some extent split. Being sensitive to unit cell variations as well as Gd centers origination are the

28

ACCEPTED MANUSCRIPT two main reasons for this multi-resonances. Though no anisotropy of symmetry would transpire and no complete splitting occurs owing to the homogeneous distribution of Gd ions especially over surface. Should oxygen species be nonbridging, the negative charge density increases. This means that Oi centers including

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Oi- and Oi2- centers should be present, approach Gd clusters at surface and bulk, and reduce their charge state. However, EPR spectra depict no signal due to Oi centers (because of very high Ef). Besides, VZn is more populated and regarding the

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charge compensating effect, oxygen atoms must be bridging. Alternatively, the synthesis protocol along with PXRD and H2-TPR analyses have made it clear that

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Gd ions were homogeneously doped into the host’s matrix.

3. Conclusions

Solid solutions of gadolinium doped ZnO nanorods were attained by a modified

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sol-gel method through which a combined ultrasound-microwave irradiation approach following post thermal annealing were used for synthesis and crystallization. By varying the molar content of dopant, behavior of the host matrix

EP

was examined in terms of microstructural and solid state changes where lattice, grain, elemental, and electronic analyses provided a profound and systematic

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survey. PXRD revealed conclusive results on phase analysis, crystallite size and morphology, lattice distortion and dislocation, as well as unit cell parameter variation where maximal and minimal influence was exerted on the samples with moderate (GZO5) and high (GZO10) Gd concentration, respectively. Electron microscopies indicated the shape evolution from detached nanorods in pristine ZnO to flower assembly in GZO10. EDX and FTIR confirmed the purity and composition of the prepared samples. Micro Raman, H2-TPR, and EPR spectra evaluated the electronic

29

ACCEPTED MANUSCRIPT structure via vibrational modes and shifts, chemical states, and surface/bulk defects. All electronic transitions were in complete agreement with crystalline alterations where maximum stain-stress in GZO5 led to upmost shifts in E2high and E1LO modes and E1LO/E2high fraction corresponding to ZnO vibrations and native defects, bulk ZnO

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and surface oxygen reduction peaks and dopant to matrix interaction (DMI) effect, as well as the utmost predominance of plausible native defects, zinc and oxygen vacancies in this sample. The compliance between crystalline microstructural and

SC

electronic features entails three superimposition attributed to VO-VZn, Gd3+-Gd3+, and VO/VZn-Gd3+ centers in EPR signals and shows the defect evolution under external

Supplementary information

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doping and solid solution conditions.

Experimental section including explications on materials, synthesis procedure, and

Funding

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instrumentation is available in the online version of the paper.

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This research did not receive any specific grant from funding agencies in the public,

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commercial, or not-for-profit sectors.

Conflicts of interest There are no conflicts of interest to declare.

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luminescence of ZnO:Er3+-Yb3+@Gd2O3 nanostructures, RSC Adv. 6 (2016)

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89305-89312.

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Table and figure captions Table 1 Variation of lattice parameters in pristine and Gd doped ZnO samples. Table 2 Distortion and dislocation forces exerted on ZnO microstructure as function of Gd doping and corresponding Zn—O bond variations.

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Table 3 Values of stacking fault probability in ZnO crystal lattice after Gd doping. Table 4 Normal dopant content and nominal values of zinc, oxygen, and gadolinium concentrations. Fig. 1. PXRD patterns of the pristine and Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, and (d) GZO10.

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Fig. 2. Dopant induced shifts in the position of three major PXRD reflections; (a) (100), (b) (002), and (c) (101).

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Fig. 3. Normalization of the host ZnO unit cell parameters in the pristine and Gd-doped samples and the assigned variations along (a) width, (b) length, and (c) volume with doping process. Fig. 4. Representative FESEM images of the pristine and Gd-doped samples with different magnifications; (a, e) ZO (b, f) GZO1, (c, g) GZO5, and (d, h) GZO10. Fig. 5. Variation of (a) the average aspect ratio (R) and (b) atomic packing factor (c/a) in the

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synthesized pristine and Gd-doped nanorods along with (c) their mutual correspondence. Fig. 6. Representative bright and dark field TEM images of the pristine and Gd-doped samples; (a, b) ZO (c, d) GZO1, (e, f) GZO5, and (g, h) GZO10. Fig. 7. Bright field TEM images of GZO10 nanorods evincing (a) average nanorod diameter and (b)

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flower assembly.

Fig. 8. Distribution of Zn, O, and Gd atoms in the pristine and Gd-doped samples via EDX spectra; (a)

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ZO (b) GZO1, (c) GZO5, and (d) GZO10.

Fig. 9. FTIR spectra of the pristine and Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, and (d) GZO10.

Fig. 10. Vibrational modes and Raman shifts in the pristine and Gd-doped samples via Raman spectra; (a) ZO (b) GZO1, (c) GZO5, (d) GZO10. (♠) First order and (♣) second order optical and (♦) optical/acoustic overtone phonon modes. LO

Fig. 11. Differentiation of E1 /E2

high

fraction and the corresponding variation with Gd doping.

Fig. 12. H2-TPR profiles and the assigned reducibility behavior of the pristine and Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, (d) GZO10.

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Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, (d) GZO10.

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ACCEPTED MANUSCRIPT Table 1 Variation of lattice parameters in pristine and Gd doped ZnO samples. Average crystallite

Atomic packing

Unit cell

factor

volume

Lattice constants

GZO1 GZO5 GZO10

c (Å)

c/a

V (Å )

a/a0

c/c0

V/V0

–

3.2498

5.2066

1.6021

47.6200

̶

̶

̶

3.2422

5.1915

1.6012

47.2587

0.9976

0.9970

0.9924

3.2425

5.1865

1.5995

47.2234

0.9977

0.9961

0.9916

3.2408

5.1853

1.6000

47.1632

0.9972

0.9959

0.9904

3.2461

5.1957

1.6006

47.4129

0.9988

0.9980

0.9956

33.50 39.10 37.60 44.46

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a

a (Å)

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ZO

a

3

D (nm)

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Bulk ZnO

Normalized lattice parameters

size

Sample

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ACCEPTED MANUSCRIPT Table 2 Distortion and dislocation forces exerted on ZnO microstructure as function of Gd doping and corresponding Zn—O bond variations. Unit cell strain

Residual stress

(ε, %)

(εzz)

ZO

0.3150

- 0.2919

+ 0.6621

GZO1

0.3020

- 0.3880

+ 0.8799

GZO5

0.3233

- 0.4091

+ 0.9278

GZO10

0.2780

- 0.2113

+ 0.4792

Sample b

Zn—O bond length

(_̅, 10 nm )

(L, Å)

8.9107

1.2345

6.5410

1.2336

7.0733

1.2330

5.0590

1.2362

-4

-2

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Strain data based on (002) plane at 2θ of 34.5464°. Negative sign points out to the compressive strain. Stress data based on (002) plane at 2θ of 34.5464°. Positive sign points out to the tensile stress.

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b

(σ, GPa)

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a

a

dislocation density

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Lattice microstrain

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ACCEPTED MANUSCRIPT Table 3 Values of stacking fault probability in ZnO crystal lattice after Gd doping. Stacking fault probability (α)

Crystal plane (100)

(002)

(101)

(102)

(110)

(103)

(200)

(112)

(201)

ZO

0.0015

0.0015

0.0013

0.0010

0.0005

0.0006

0.0002

0.0016

0.0003

GZO1

0.0013

0.0018

0.0013

0.0010

0.0007

0.0003

0.0007

0.0016

0.0003

GZO5

0.0019

0.0023

0.0014

0.0010

0.0008

0.0008

0.0008

0.0016

0.0003

GZO10

0.0007

0.0010

0.0007

0.0002

0.0005

0.0002

0.0007

0.0020

0.0001

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ACCEPTED MANUSCRIPT Table 4 Normal dopant content and nominal values of zinc, oxygen, and gadolinium concentrations. Gd concentration (Mole %)

Nominal concentration (Wt. %)

Sample Zn

O

Gd

0.00

71.98

28.02

0.00

GZO1

1.00

41.31

57.58

1.11

GZO5

5.00

28.73

66.83

4.44

GZO10

10.00

34.06

54.85

11.10

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Fig. 1. PXRD patterns of the pristine and Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, and (d)

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Fig. 2. Dopant induced shifts in the position of three major PXRD reflections; (a) (100), (b) (002), and (c) (101).

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Fig. 3. Normalization of the host ZnO unit cell parameters in the pristine and Gd-doped samples and the assigned variations along (a) width, (b) length, and (c) volume with doping process.

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Fig. 4. Representative FESEM images of the pristine and Gd-doped samples with different

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Fig. 5. Variation of (a) the average aspect ratio (R) and (b) atomic packing factor (c/a) in the

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Fig. 6. Representative bright and dark field TEM images of the pristine and Gd-doped samples; (a, b)

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Fig. 7. Bright field TEM images of GZO10 nanorods evincing (a) average nanorod diameter and (b)

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Fig. 8. Distribution of Zn, O, and Gd atoms in the pristine and Gd-doped samples via EDX spectra; (a) ZO (b) GZO1, (c) GZO5, and (d) GZO10.

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Fig. 9. FTIR spectra of the pristine and Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, and (d)

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Fig. 10. Vibrational modes and Raman shifts in the pristine and Gd-doped samples via Raman spectra; (a) ZO (b) GZO1, (c) GZO5, (d) GZO10. (♠) First order and (♣) second order optical and (♦) optical/acoustic overtone phonon modes.

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Fig. 12. H2-TPR profiles and the assigned reducibility behavior of the pristine and Gd-doped samples;

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(a) ZO (b) GZO1, (c) GZO5, (d) GZO10.

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Fig. 13. EPR spectra representing the evolved native and emerged guest defects in the pristine and

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Gd-doped samples; (a) ZO (b) GZO1, (c) GZO5, (d) GZO10.

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Highlights •

Pristine and gadolinium-doped ZnO nanorods were synthesized in form of solid

Combined ultrasound-microwave irradiation along with post-air annealing were utilized to develop a sol-gel method.

•

The irradiation approach was based for the rapid crystallization of un-doped and gadolinium-doped ZnO nanorods.

Alterations in the microstructure, texture, electronic structure, and solid state of

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•

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solutions.

the nanorods were surveyed as a function of gadolinium doping. Significant interactive relation was observed between the vibrational, reduction,

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Supplementary Information

combined

microwave-ultrasonic

crystallization

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Solid solutions of gadolinium doped zinc oxide nanorods by irradiation

assisted

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Armin Kiani, Kamran Dastafkan*, Ali Obeydavi and Mohammad Rahimi * Corresponding author. Kamran Dastafkan Tel.: +98 9113520407.

Experimental section Materials.

Zinc

and

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E-mail: [email protected]

gadolinium

nitrate

hexahydrate

[Zn(NO3)2.6H2O,

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Gd2(NO3)3.6H2O], polyvinylpyrrolidone (PVP, MW ≈ 40,000), diethylene glycol (DEG), teriethylenetetraamine (TETA), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich. All chemicals were used as received and were of analytical grade.

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Deionized water (DIW) was used for the preparation of all the solutions. Synthesis procedure. Zinc (0.1 M) and gadolinium precursors were dissolved in 50

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mL DIW in round-bottom flasks according to Gd/Zn molar ratios of 1, 5, and 10%. Adequate amounts of PVP (0.15 g) and DEG (4 mL) as structure directors were admixed suddenly and dropwise, respectively, then the derived solutions were continuously irradiated by a middling-intensity probe sonicator (100 W cm-2, 20 kHz) with an immersing Ti-horn at 85°C for 30 min. Initially after attainment of fine stable sols, complexing agent TETA (1 mL, 1.12 M) was slowly added to the solution to fulfill the gelation step. Subsequently, the large network comprised of chained hydroxide and

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oxide species underwent the supplemental hydrolysis and polycondensation processes through simultaneous NaOH (0.01 M) titration for 10 min until the pH of 9. When a milky state stabilized, hot reflux was conducted on the gels in an ultrasonic bath for 30 min.

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After that they were transferred to microwave vessels and irradiated at 150°C with the power of 700 W for 7 min. The precipitates were segregated from the liquid phase through centrifugation at 4000 rpm and the pale yellow xerogels were harvested

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thereupon which were then cooled down to room temperature, washed with DIW and absolute ethanol four times, and dried in vacuum for 2 h. The products were then

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transferred into crucibles and treated with post-annealing in an electrical furnace at 400°C for 2 h to fulfill zinc oxidation, crystallization, and remove the organic residuals. Instrumentation. Microwave irradiation-based heating treatment was carried out using a Milestone Ethos SEL microwave processor equipped with a HPR rotor

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accommodating 10 closed stainless steel reactors housed with 5-layer plasma applied PTFE coating, a thermocouple sensor containing fiber optic system for automatic temperature control (ATC-FO) for direct and automatic temperature control in the

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reactor with high (±1°C) precision, quench protection system (QPS) detector for organic/inorganic solvents providing pressure, temperature, and time control via a

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software microprocessor with which continuous and real-time monitoring of the chemical operation is rendered through adjusting the applied power and frequency. This results in reproducible synthetic conditions. The output power was set at 700 W with a wavelength in the domain of 2.5 GHz. Ultrasonic waves were generated using a replaceable Ti-probe sonicator (Qsonica Q700) providing full amplitude control and realtime temperature monitoring. The output ultrasonic wave frequency and power were set

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to 20 kHz and 100 W cm-2. Crystallographic features of the synthesized nanorods were determined by powder X-ray diffraction (PXRD) patterns recorded at room temperature using a Philips X’pert Pro X-ray diffractometer equipped with a copper target,

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monochromatized Kα radiation source, and a wavelength of 1.54056 Å. The tube current and functional voltage during X-ray incident were set to 30 mA and 40 kV, respectively. Intensity data were collected over the angular range of 30°-70° in 2θ with a step size of

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0.015°. The growth, size, morphology, and shape of the prepared nanorods were probed through magnification by field emission scanning electron microscopy (FESEM,

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Hitachi-S-4160) and transmission electron microscopy (TEM, Philips CM30) of which the latter equipped with a high resolution CCD camera and an accelerating voltage of 150–200 kV. Prior to SEM and TEM microscopy, ZnO samples were coated on a gold film and a Cu-carbon grid, respectively. The average aspect ratio values of the samples

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were assessed by determining the average length and width of about 15-20 nanorods in FESEM micrographs using ImageJ software. Elemental and chemical composition was investigated by energy-dispersive X-ray (EDX, Philips XL30) and Fourier transform

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infrared (FTIR, 2000 PerkinElmer) spectroscopies. FTIR spectra were scanned in the wavelength range of 400 to 4000 cm-1 using KBr pellets. Room temperature laser

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Raman Microspectroscopy was performed in backscattering configuration using a high performance dispersive Raman spectrometer (Thermo Scientific, Nicolet Almega XR) and excitation by the 532 nm line of a continuous-wave Nd:YAG laser. Temperature programmed reduction with hydrogen (H2-TPR) was carried out on 30 mg of the sample powders in a U-shaped quartz reactor of a NanoSORD NS91 system (Sensiran co., Iran). Prior to each TPR measurement, the powders were degassed under 10 sccm Ar

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flow at 300˚C for 1 h, then cooled down to room temperature in the same atmosphere. The reduction process was performed under 10 sccm flow of 5.0% H2/Ar mixture thereof whilst the temperature was raised to 950˚C with a heating rate of 10˚C min-1. Radical

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species and chemical state assessment was implemented by electron spin resonance spectroscopy using an X-band EPR spectrometer (Bruker, EMS 104, υ ≈ 9.7 GHz) with the microwave power of 2.50 mW, modulation amplitude of 6.37 G, sweep width of 100

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G, receiver gain of 20 dB, and scan range of 3440–3520 G at room temperature.