Tunable emission of AgIn5S8 and ZnAgIn5S8 nanocrystals: electrosynthesis, characterization and optical application

Materials Today Chemistry 16 (2020) 100238

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Tunable emission of AgIn5S8 and ZnAgIn5S8 nanocrystals: electrosynthesis, characterization and optical application F.L.N. Sousa a, D.V. Freitas a, R.R. Silva a, S.E. Silva a, A.C. Jesus b, H.S. Mansur b, W.M. Azevedo a, M. Navarro a, * a b

Universidade Federal de Pernambuco, Departamento de Química Fundamental, 50670-901, Recife, PE, Brazil Universidade Federal de Minas Gerais, Departamento de Engenharia Metalúrgica e Materiais, 31270-901, Belo Horizonte, MG, Brazil

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 September 2019 Received in revised form 25 November 2019 Accepted 11 December 2019 Available online xxx

Ternary AgIn5S8 (AIS) and quaternary ZnAgIn5S8-alloy (ZAIS) nanocrystals, stabilized by L-glutathione, were produced by a clean and eco-friendly electrochemical method, eliminating the need of reducing agents. AIS-GSH colloidal solution was obtained by constant current electrolysis (i ¼ 30 mA) in cavity cell. S2 ions (0.051 mmol) were generated into a graphite powder macroelectrode, reacting in the intermediate compartment of the cell containing Agþ/In3þ aqueous solution at different ratios (0.5, 0.28, 0.18, and 0.14), and 0.025 mmol/L1 glutathione (GSH). ZAIS-GSH NCs were synthesized in the same cavity cell containing the previously prepared AIS-GSH solution. A paired electrolysis (i ¼ 30 mA) was used for simultaneous production of Zn2þ and S2 (Zn0 sacrificial anode and graphite powder macroelectrode/S0 cathode). The electrochemical method promoted a high reproducibility and efficient luminescence in the preparations of NCs. The sizes of the AIS-GSH and ZAIS-GSH nanoparticles were determined by HRTM (3.4 and 4.0 nm, respectively), and quantum yields reaching 16% (AIS-GSH, Agþ/In3þ ¼ 0.18). The spectrophotometric characterization showed that Agþ/In3þ ratio can be used for the tuning of the AIS-GSH nanoparticle emission wavelength, which is associated to electronic defects introduced in the NCs lattice. XRD/EDS analysis of ZAIS-GSH nanoparticles point out to Zn2þ ion-exchange into the AIS-GSH lattice. XPS analysis was carried out at different etching levels of the ZAIS nanocrystals surface, making possible to identify the 2p Zn doublet signal, indicating two different Zn2þ sites in the alloy structure. Time-resolved spectroscopy measurements/decay curves were carried out to evaluate the effect of silver amount on radioactive and non-radioactive terms. Additionally, the AIS-GSH and ZAIS-GSH photoluminescence and stability were used to produce the active parts of commercial white LEDs, and modulate the colour perception from the respective emission bands. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Electrosynthesis AgIn5S8 Ternary nanocrystals Tunable emission ZnAgIn5S8 alloy

1. Introduction The production of high-performance photonic devices, associated to green synthetic routes of nanomaterials, is among the current demands of the scientific community. In the last decades, the synthesis of quantum dots (QDs) has presented advances in the structural and compositional modelling for tuning emission/absorption energy bands [1]. In this context, QDs of type II-VI made of Cd, Pb and Hg (toxic heavy metals) were extensively studied and applied in solar cells, LEDs, sensors, drug-delivery systems, and

* Corresponding author. E-mail address: [email protected] (M. Navarro). https://doi.org/10.1016/j.mtchem.2019.100238 2468-5194/© 2019 Elsevier Ltd. All rights reserved.

biological imaging [2,3]. However, the applications based on hazardous nanomaterials are limited because of the increasing environmental and biological concerns associated with the heavy metal chemical composition. Thus, this trend has generated a scientific challenge for the development of new semiconductor nanomaterials with improved eco-friendliness and greater biocompatibility, beyond new synthetical approaches [4]. The ternary nanocrystals (NCs) of type AIBIIICVI 2 (where, A ¼ Cu or Ag; B ¼ In, Al or Ga; C ¼ S, Se or Te) can be considered promising active components for the preparation of luminescent devices and development of bioprobes for biomedical applications [5e9]. The luminescent properties are associated to different crystalline phases, predicting phase diagrams and the formation of intragap states through the introduction of crystalline and electronic defects [10e13]. The large Stokes shift and an emission wavelength with a

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full width at half maximum (FWHM) in the order of 100e150 nm are a typical property of the ternary NC, (75e200 nm) [9,10]. Among the ternary systems, the AgInS2/AgIn5S8 (AIS) systems stand out for its high quantum yield values that can reach 70% [9]. Particularly, direct bandgap AgIn5S8 nanocrystal semiconductors (1.7 eV) have photoluminescence attributed not only to the scale effect, but mainly by recombination of donor and acceptor states [14,15]. AIS nanoparticles also have high emission tuning, ranging from the visible to the near-infrared, outside the region of autofluorescence of biological tissues, allowing hyperthermia treatment [4,16,17]. About the AIS synthesis, it can be prepared in aqueous medium by different methods: hot injection [6,18e20], solvothermal [21], photoetching [9], micro-waves [22] and ultrasound [23]. Recently, an electrochemical method for the generation of chalcogenide ions (X2 ¼ S2, Se2 and Te2) was developed by using an electrochemical cavity cell [24], basically composed by a graphite powder macroelectrode and an auxiliary compartment. The NCs are formed in a controlled way in the presence of stabilizer agent, to furnish MX or M2X3 QDs [24,25]. The cavity cell also allows a paired electrosynthesis, where the QD precursor ions are simultaneously generated by reduction of the elemental chalcogen and oxidation of a metal sacrificial electrode [24]. In conventional synthetic procedures, the control of the reaction parameters is operator dependent or involves more than one-step for the production of NCs. Thus, making the process reproducibility difficult. The electrochemical synthesis of semiconductor NCs allows the kinetic control by current adjustment, i.e. controlled injection of the precursors in solution. Thus, there is a greater reproducibility associated to low cost and environmentally correct assembly, according to the green chemistry principles [26e29]. In this work, a facile and eco-friendly aqueous electrochemical method was demonstrated to synthesize cubic phases of AgIn5S8 (AIS) and ZnAgIn5S8 (ZAIS) NCs, stabilized by L-glutathione (GSH). The in situ electrogeneration of S2 ion increases the control of nucleation steps. The Agþ/In3þ ratio was adjusted in order to observe the tuning of the optical properties, in the same cubic crystal phases. The Agþ/In3þ ratio and alloy formation effects over the luminescence of the synthesized NCs was accompanied by time-resolved spectroscopy, and the ZAIS interface architecture was investigated by X-ray photoelectrons spectroscopy (XPS). Regarding technological application, the AIS-GSH and ZAIS-GSH NCs were tested for the colour perception modification of white cold light-emitting diodes (WLEDs). 2. Experimental 2.1. Materials All chemicals were of reagent grade and used without further purification. Elemental sulphur (S0) powder (100 mesh, Aldrich), silver nitrate (99%, Sigma-Aldrich), indium (III) nitrate (99.9%, Sigma-Aldrich), graphite powder (particle size < 20 mm, Aldrich), Lglutathione reduced (98%, Aldrich), NaClO4 (98%, Sigma-Aldrich) were used as purchased. Solvents and reagents of analytical grade were used as received. The water was of Milli-Q grade (17 MU cm). 2.2. Electrosynthesis of AIS-GSH NCs The sulphur electroreduction methodology was previously described and applied with some modifications [25]. The cathodic compartment of the cavity cell (graphite powder macro electrode) was filled with 38.3 mg (3.2 mmol) of graphite powder mixed to 1.6 mg (0.051 mmol) of elemental sulphur powder (S0), and the mixture was pressed under P ¼ 3.2 kg/cm2 for 10 min. A sintered

glass (previously sonicated in 0.1 mol/L1 NaClO4 aqueous solutions and with the same cavity diameter of 1.0 cm), was placed over the cavity to avoid the dispersion of the graphite, and at the same time allowing the migration of sulphide ions (S2) formed during the electroreduction. 0.25 mol/L1 AgNO3 aqueous solution (x ¼ 120, 68, 43 or 30 mL) and 0.25 mol/L1 In(NO3)3 ethanoic solution (240 mL) were used to prepare Agþ/In3þ precursor solutions (1:2 or 0.5; 1:3.5 or 0.28; 1:5.4 or 0.18; 1:7.4 or 0.14 ratios, respectively) in 10 mL of MilliQ water. L-glutathione stabilizer (0.25 mmol) dissolved in 10 mL of MilliQ water was added to the Agþ/In3þ solution, followed by 5 mL of NaClO4 (2.0 mmol) solution, and the pH was corrected to pH 9 by dropwise of 1.0 mol/L1 NaOH solution. The Agþ/In3þ ratio was varied from 0.03 to 0.0084 mmol of Agþ, while the In3þ amount was fixed to 0.06 mmol. A stainless steel grid was used as an anode, placed in the anodic compartment containing 0.1 mol/L1 NaOH solution, and separated from the intermediate compartment by a Nafion® membrane. Electrolysis was carried out at 30 mA constant current for a period of 350 s (Q ¼ 10.5 C), under argon. The AIS-GSH formation can be identified by the solution colour changing in the intermediate compartment, which becomes yellow. The AIS-GSH seeds were heated for 10 min under reflux and stored at 4  C, under dark conditions. 2.3. Alloy and core/shell structures The preparation of ZAIS-GSH NCs was also carried out in the electrochemical cavity cell, using a Zn0 sacrificial anode. The preparation of the cathodic compartment occurred similarly to that described for the AIS-GSH preparation, using 0.05 mmol of elemental sulphur mixed to 3.2 mmol of graphite powder. In that case, the stainless steel anode was replaced by the Zn0 sacrificial anode. The electrolysis was carried out at 30 mA constant current for a total time of 350 s (Q ¼ 10.5 C), under argon. The ZAIS-GSH NCs prepared were heated under reflux for a period of 30 min, and stored at 4  C, under dark conditions. 2.4. Characterization The synthesized nanoparticles were characterized by absorption and emission spectrometry using 2 mL aliquots of each AIS-GSH or ZAIS-GSH colloidal solutions, at different Agþ/In3þ ratios (1:2 or 0.50; 1:3.5 or 0.28; 1:5.4 or 0.18; 1:7.4 or 0.14). The UVeVis absorption spectra were registered with an Agilent 8453 spectrophotometer (tungsten and deuterium lamp) in 190e1100 nm of range with 1 nm resolution. The emission spectra and timeresolved photoluminescence spectra were collected on a Fluorolog3 Horiba Jobin Yvon spectrofluorometer equipped with Hamamatsu R928P photomultiplier and a pulsed 150 W XeeHg lamp. The quantum yield of luminescence for AIS-GSH and ZAISGSH NCs was calculated using rhodamine 6G in ethanol as standard (QY ¼ 95%, lexc ¼ 488 nm) [30], and keeping the samples optical density below 0.1 to avoid photon reabsorption. For timeresolved photoluminescence measurements, the samples were prepared with the same optical density, 0.1, and excited at 339 nm. It was used Ludox® such light-scattering standard. Controlled-current electrolyses were carried out by using an Autolab PGSTAT 30 potentiostat/galvanostat and electrochemical cavity cells. For the powder x-ray diffractograms, 30 mL of NCs colloids were precipitated after addition of acetone (1:1 by volume) and the solid was washed with ethanol. The samples were dried under vacuum, macerated in an agate mortar, and the powder was stored in an inert atmosphere. X-ray diffraction (XRD) patterns were taken on a Bruker X-ray diffractometer model D8 Advance

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238

with a CuKa radiation (l ¼ 1.5418 Å) and with the 2q range from 10 to 80 and with a step of 0.01. Nanostructural characterizations of the AIS-GSH and ZAIS-GSH NCs were based on the images obtained using a Tecnai G2-20-FEI high-resolution transmission electron microscope (HRTEM) at an accelerating voltage of 200 kV. Energydispersive X-ray spectra (EDX) were collected using the HRTEM for element chemical analysis. In all of the HRTEM analyses, the samples were prepared by dropping the colloidal dispersion onto a porous carbon grid. The NC size and size-distribution data were obtained based on the HRTEM images by measuring at least 200 randomly selected nanoparticles using an image processing program (DigitalMicrograph, version 3.11, free license, Gatan). XPS analysis was performed by using Mg-Ka as the excitation source (Amicus spectrometer, Shimadzu, Japan). The samples were concentrated and dropped onto copper grids and dried under vacuum. All peak positions were corrected based on C 1s binding energy (284.6 eV). Ion bombardment with argon ions (Arþ, 2 cycles, 3s/each, emission current 50 mA, and beam voltage 0.5 kV). AIS-GSH and ZAIS-GSH NCs were analyzed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method (Thermo Fischer, Nicolet 6700) over the range of 400e4.000 cm1 using 64 scans and a 2 cm1 resolution. These samples were prepared by placing a droplet of the dispersions onto KBr powder and drying at the temperature of (40 ± 2  C) for 24 h. For DRIFTS, a (12.5 mmol/ L1) GSH solution at pH 9 was used as a reference. The dynamic light scattering (DLS) and zeta potential (z) measurements were performed in colloidal media of NCs, using a Omni instrument by applying the laser light diffusion method (Brookhaven Instruments). This instrument uses the laser Doppler electrophoresis technique (35 mW red diode laser at k ¼ 660 nm). All tests were carried out in triplicates (n ¼ 30), and the values were averaged. 2.5. Modification of the emission colour of a white LED AIS-GSH and ZAIS-GSH NCs were precipitated from 1 mL of the respective colloidal solutions by addition of 1 mL of acetone. The nanocrystals were decanted by centrifugation (6000 rpm) and the supernatant solution extracted with a Pasteur pipette. The isolated NCs were re-dispersed into 21 mL of deionized water to give a viscous solution. 7 mL of the AIS-GSH or ZAIS-GSH re-dispersed solution was directly dropped onto the commercial white LEDs (WLEDs, CTB® BFL-3528, 12.5 lm, 113 lm/W, operating with 12 V, 20 mA) and dried under reduced pressure, during 30 min. Modified WLEDs were prepared with AIS-GSH NCs obtained from Agþ/In3þ solution ratios 0.28 and 0.18, and ZAIS-GSH NCs 0.14 in order to obtain the red, yellow and green emission, respectively. The procedure reported by Kang et al. was used as reference [31].

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electrostatic repulsion, the S2 ions are expelled out the cathodic cavity, migrating to the intermediate compartment. Following the sulphur Pourbaix diagram, at pH 9, sulfide ions are protonated, giving HS (Eq. (2)) [33], which reacts with [Ag(GSH)2]5 and [In(GSH)3]6- complexes. Simultaneously, water is oxidized in the anodic compartment (stainless grid anode) separated by a Nafion membrane. S0(s) þ 2e / S2(aq)

(1)

S2(aq) þ H2O(l) % HS(aq) þ OH(aq)

(2)

Therefore, the formation of AIS-GSH seeds occurs in the presence of [Ag(GSH)2]5 and [In(GSH)3]6- complexes, and the constant addition of HS(aq) ions from electrolysis. The nucleation and growth of AIS-GSH nanoparticles are developed during the heat treatment, as presented in Eq. (3).

  5 6 D  þ 8 HS AgðGSHÞ2 ðaqÞ þ 5 InðGSHÞ3 ðaqÞ þ 8OHðaqÞ / ðaqÞ AgIn5 S8  GSHðcolloidalÞ þ 8 H2 OðlÞ (3) Zn0(s) / Zn2þ(aq) þ 2e

(4)

AgIn5S8-GSH(colloidal) þ ZnS-GSH(seed) / AgIn5S8/ZnSGSH(colloidal)

(5)

The procedure of ZnS shell deposition on the AIS-GSH surface was carried out by a paired electrolysis, i.e. reduction of elemental sulphur to S2 (Eqs. (1) and (2)) and oxidation of a Zn0 sacrificial anode to Zn2þ (Eq. (4)). It was observed that the addition of Zn2þ to the ternary nanocrystal structure could promote the formation of AgIn5S8eZnS-GSH NCs (Eq. (5)). 3.2. Structural characterization The powder X-ray diffraction patterns for the AIS-GSH and ZAISGSH NCs were recorded for Agþ/In3þ ratio ¼ 0.14 (Fig. 1). According to Ag2S/In2S3 phase diagram, the crystalline structure of AgInS ternary salt can change from cubic to orthorhombic. From 81 to 96 mol% of In3þ present in the AgInS ternary salt, the cubic phase is the most stable [37]. Thus, the AIS-GSH X-ray diffractogram showed

3. Results and discussion 3.1. Electrosynthesis of AIS-GSH and ZAIS-GSH NCs The synthesis of AIS-GSH and ZAIS-GSH NCs was performed by an electrochemical method [24,32]. S2 ions were generated by the reduction of elemental sulphur in a graphite powder macro electrode. AgNO3 and InBr3 were used as salt precursors, using GSH as a stabilizer in aqueous medium and pH 9. At these conditions, Agþ, In3þ, and HS are the stable species in solution, according to the respective Pourbaix diagrams [33]. Initially, GSH (in excess) complexes with the Agþ and In3þ ions, giving [Ag(GSH)2]5 and [In(GSH)3]6- [34e36], respectively, which were added in the intermediate compartment of the cavity cell, under argon atmosphere. Then, the electrolysis was started to prepare the AIS-GSH NCs. The elemental sulphur (S0) dispersed into the graphite powder macro electrode was reduced to generate S2 ions (Eq. (1)). Due to

Fig. 1. XRD patterns of AgIn5S8 and ZnAgIn5S8 (Agþ/In3þ ratio ¼ 0.14) NCs prepared by electrochemical method.

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the diffraction peaks at 27.5 , 47.5 and 53.5 , corresponding to the set of planes (311), (440) and (620), respectively, concerning the AgIn5S8 cubic structure (JCPDS N 25e1329), where peak widening is related to the sample polycrystallinity [37,38]. After addition of Zn2þ ions to the AIS-GSH NCs, the diffraction peaks were narrowed and the diffraction peaks shifted to higher values of 2q, showing diffraction peaks at 28.5 , 47.4 and 56.3 which correspond to the plans (111), (220) and (311) of the facecentered cubic zinc blende, suggesting the formation of the ZAISGSH quaternary NC structure. Such peak displacement is attributed to the local effect caused by the insertion of Zn2þ ions in the unit cell, by ionic change, promoting a decrease in the lattice parameters [6]. The average size and composition of the AIS-GSH and ZAIS-GSH nanoparticles were determined by high-resolution transmission electron microscopy (HRTEM) coupled to energy-dispersive X-ray (EDX) analyses. Fig. 2 shows the HRTEM images of the AIS-GSH ternary NCs (Fig. 2A) and ZAIS-GSH quaternary NCs (Fig. 2B), for the 0.14 Agþ/In3þ ratio. The inset in Fig. 2A and B shows the interplanar distances for the AIS-GSH (0.319 nm) and ZAIS-GSH NCs (0.313 nm), respectively, which corresponds to the (311) plane for the cubic phase of AIS-GSH [38]. The nanoparticles show anisotropic growth and approximate bead geometry for both cases. Other HRTEM images of AIS-GSH and ZAIS-GSH are present in Figs. S1 and S2, in Supplementary Material. After addition of Zn2þ ions into the AIS-GSH structure, it was observed the nanoparticle size increasing, from 3.4 ± 0.9 nm (AIS-GSH, Fig. 2C) to 4.0 ± 0.9 nm (ZAIS-GSH, Fig. 2E).

The EDX analysis of the AIS-GSH nanoparticles allowed the identification of Ag La1 (2.983 eV), In La1 (3.286 eV) and Lb1 (3.487 eV) and S ka1 (2.309 eV) elements (Fig. 2D). Zn ka1 (8.630 eV) was additionally identified in the corresponding ZAIS-GSH sample (Fig. 2F), confirming the insertion of Zn2þ ions into the ZAIS-GSH lattice. The analysis of the contrast image shows a nonuniformity in the crystallographic positions of the ZAIS-GSH NCs (Fig. 2B), which is an indicative of crystalline planes formed by ions of different atomic numbers: Agþ (Z ¼ 47), Zn2þ (Z ¼ 30) and In3þ (Z ¼ 49), characterizing a homogeneous alloy [39]. In Fig. S3 (Supplementary Material), the infrared spectra illustrates the GSH performance as nanoparticle stabilizer and its coordination mode with the ions present in the nanoparticle/solution interface. The FTIR spectra for the AIS-GSH and ZAIS-GSH nanoparticles and GSH were recorded for all Agþ/In3þ ratio systems. For all samples, the peak at 2524 cm1 relative to the eSH group is absent, indicating the deprotonation of the group and coordination to the nanocrystal surface. The eNHR groups stretching at 1584 cm1 is present in the GSH sample, but absent in the AIS-GSH and ZAIS-GSH samples, also indicating the coordination of the eNHR group on the nanocrystal surface [22,40,41]. Dynamic light scattering (DLS) and zeta potential (z) analyses were carried out for the NCs with the best quantum yield (Agþ/In3þ ratios: 0.28, 0.18 and 0.14; Table 1) for obtaining the hydrodynamic radius (HD) and surface charge. The HD decreasing as a function of the silver amount (Table 1, entries 1 to 3) is related to the nucleation and growth mechanism of AIS NCs. In solution, lower Agþ concentrations leads to a greater number of Ag2S nuclei of minor size.

Fig. 2. TEM images for (A) AIS-GSH and (B) ZAIS-GSH nanoparticles with Agþ/In3þ rate 0.14 (Inset: interplanar distance of NCs). Normal distribution of particle size for (C) AIS-GSH and (E) ZAIS-GSH NCs. EDX spectrum for the (D) AIS-GSH and (F) ZAIS-GSH NCs.

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238 Table 1 Dynamic light scattering (DLS) and zeta potential (z) values for AIS-GSH and ZAISGSH nanoparticles at Agþ/In3þ ratios: 0.28, 0.18 and 0.14. Entry

Sample (Agþ/In3þ ratio)

DLS (nm)

1 2 3 4 5 6

AIS-GSH (0.28) AIS-GSH (0.18) AIS-GSH (0.14) ZAIS-GSH (0.28) ZAIS-GSH (0.18) ZAIS-GSH (0.14)

35.2 26.5 23.4 27.3 24.4 20.5

± ± ± ± ± ±

1.8 2.4 1.5 2.5 1.4 1.7

z (mV) 27.5 22.0 18.3 30.2 23.6 24.3

± ± ± ± ± ±

2.7 1.2 2.5 4.5 2.9 3.4

At these conditions, the unit cell propagation is controlled by In3þ, leading to a size decreasing of the final AIS NCs. By addition of Zn2þ into the NC lattice, the ZAIS HD measurements showed the same behaviour observed for AIS (entries 4 to 6), i.e., the AIS nanocrystal structure is maintained after its formation. However, it can be observed that all ZAIS HD values (entries 4 to 6) are lower than AIS HD (entries 1 to 3), when associated to the same Agþ/In3þ ratios. Thus, the modification of the nanocrystal composition by introduction of Zn2þ leads to a higher ordering of the stabilizer on the nanocrystal surface. The Zn2þ ion has a d10 configuration, and it is not a good acceptor of the charge donated by the ligand. Liu et al. determined by DFT calculations that the coordination between ZnGSH occurs preferentially with -COO- and eNH groups, which present a charge withdraw character [42]. The different modes of coordination to GSH may lead to lower stabilizer agglomeration in the first passivating layer, promoting to higher z values, as observed in Table 1. The z measurements presented values between 18 mV and 30 mV, indicating a negative charge on the nanoparticle surface and good stability. A charged surface ensures the passivation of the nanocrystal, preventing degradation processes at the nanoparticle/solution interface, which is related to the balance of the electrostatic and attractive Van der Waals processes [43]. 3.3. Optical characterization NCs obtained at different ratios of Agþ/In3þ ions were characterized by absorption spectroscopy. The UVeVis spectra for AIS-

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GSH NCs (Fig. 3A) and ZAIS-GSH NCs (Fig. 3D) do not exhibit distinct absorption bands due to defects and polydispersity of the nanoparticles [17,44]. For the richest-silver composition (Agþ/ In3þ ¼ 0.5), AIS-GSH spectrum shows absorption values in the nearinfrared region (NIR) (l ~ 800 nm, Fig. 3A), which can be explained by the increase of the intragap states introduced by the Agþ vacancies. The reduction of the Agþ/In3þ ratio, from 0.5 to 0.14, promotes a hypsochromic shift, which can be justified by the diminishing of donor-acceptor states, due to the lower amount of Agþ ions into the NC, making the structure closer to the In2S3 lattice (Fig. 3A) [45]. The emission spectra of the AIS-GSH NCs (Fig. 3B) showed an asymmetric profile, indicative of structural defects, as observed in other works [46e49]. The variation of the Agþ/In3þ ratio allowed the tuning of the AIS-GSH emission bands, which remain located between the emission wavelength region of Ag2S-GSH (lem ¼ 815 nm) and In2S3-GSH (lem ¼ 503 nm) NCs (Fig. S4). Thus, a hypsochromic shift (701 nme564 nm) and FWHM narrowing (213 nme96 nm) can be observed when decreasing the amount of Agþ ions in the crystalline structure (Table 2, entries 1 to 4). For the AIS-GSH (Agþ/In3þ ratio ¼ 0.5) emission spectrum at 701 nm, a shoulder at 815 nm can be associated to emission of Ag2S NCs present in the colloidal solution (Fig. 3B). The same behaviour is observed for ZAIS-GSH NCs (Fig. 3E), with emission bands located between 646 nm and 548 nm, and FWHM diminishing from 171 nm to 88 nm, according to Agþ/In3þ ratio ¼ 0.5 to 0.14, respectively. The hypsochromic shift and narrowing of emission bands can be related at the diverse crystalline phases predicted in Ag2S/In2S3 pseudo-binary phases diagram, and the decrease in crystalline defects (silver vacancies and interstitial positions) promote the decrease in donor-acceptor states [37]. The tuning of the AIS-GSH and ZAIS-GSH light emission can be visualized in Fig. S5 (solutions under visible light and UV excited, l ¼ 365 nm). AIS-GSH NCs presented emission quantum yields ranging from 1.3% to 16.6% (Table 2, entries 1 to 4), reaching a maximum value of 16.6% at Agþ/In3þ ratio ¼ 0.18 (entry 3), higher than results described in other works, using similar reaction conditions in aqueous (3.0%) [50] and organic (11.0%) [46] solvents. Lower quantum yields were observed with addition of Zn2þ ions to

Fig. 3. Absorption, emission spectra and Tauc plot of (A, B, and C) AIS-GSH and (D, E, and F) ZAIS-GSH NCs.

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Table 2 Optical parameters of the synthesized AIS-GSH and ZAIS-GSH NCs. Entry

Sample (Agþ/In3þ)

Eg (eV)

lem (nm)

FWHM (nm)

QY (%)

1 2 3 4 5 6 7 8

AIS-GSH (0.5) AIS-GSH (0.28) AIS-GSH (0.18) AIS-GSH (0.14) ZAIS-GSH (0.5) ZAIS-GSH (0.28) ZAIS-GSH (0.18) ZAIS-GSH (0.14)

1.99 2.22 2.48 2.73 1.97 2.31 2.48 2.96

701 618 597 564 646 614 584 548

213 129 116 96 171 147 108 88

1.3 12.7 16.6 6.7 1.2 6.2 12.2 10.0

the AIS-GSH NCs (Table 2, entries 5 to 8). In the same way, the highest emission quantum yield observed for the ZAIS-GSH nanoparticles (12.2%) occurred at Agþ/In3þ ratio ¼ 0.18 (Table 2, entry 7). When comparing AIS-GSH (Table 2, entries 1 to 4) and ZAIS-GSH (entries 5 to 8) data, for the respective Agþ/In3þ ratios, it can be observed lower values of emission wavelength and FWHM for ZAISGSH nanoparticles. This result can be related to the addition of the Zn2þ ion into the nanocrystal structure sites, performing a homogeneous alloy among the metallic ions. Moreover, the ZnS shell addition does not induce a quantum yield increase of the ZAIS-GSH samples, except for the 0.14 Agþ/In3þ ratio (entry 8). It can be related to new intragap states that Zn2þ ions introduce via Frenkel defects [51]. Mao et al. showed that the addition of a ZnS shell to AIS-GSH promotes the reduction of the Agþ amount into the nanocrystal structure, and a respective increase of the Zn2þ amount, without the observation of bandgap changes [46]. Time resolved emission spectra were recorded to understand the role of Ag and Zn in the AIS and ZAIS photoluminescence. The method can point to recombination and transfer processes of photogenerated charge carriers. Decay curves were simulated for AIS NCs by using biexponential functions, as shown in Fig. 4A. The average lifetime is described by tave ¼ (A1t1 þ A2t2)/(A1 þ A2), where t1 and t2 are time constants, and A1 and A2 are relative magnitudes. The first term, t1, is related to recombination in surface traps and the term t2 is related to intra-gap states and recombination of donor-acceptor states [52]. The results are summarized in Table 3 (entries 1 to 4). It can be observed that the increase of silver amout (Agþ/In3þ: 0.50, 0.28, 0.18, 0.14) leads to the decrease of A1 value (5.77%, 9.53%, 12.37%, 14.96%, respectively), related to the radioactive decay. The inverse effect is observed for the term A2 (94.23%, 90.47%, 87.63%, 85.04%, respectively), which is related to recombination of donor-acceptor states. Thus, it is evidenced the direct relation between the silver amount and structural defects present in the nanocrystal lattice [47]. The AIS and ZAIS (Agþ/In3þ ¼ 0.14) decay curves are shown in Fig. 4B. The insertion of Zn2þ in the ternary AIS semiconductor

Table 3 Photoluminescence time resolved data: amplitudes (A1 and A2), time constants (t1 and t2) and average lifetime and tave for AIS and ZAIS NCs. Entry 1 2 3 4 5

Sample (Agþ/In3þ) AIS-GSH (0.50) AIS-GSH (0.28) AIS-GSH (0.18) AIS-GSH (0.14) ZAIS-GSH (0.14)

t1 (ns)

А1 (%)

t2 (ns)

А2 (%)

tave (ns)

29.7 28.5 30.1 22.4 25.3

5.77 9.53 12.37 14.96 5.65

303.5 287.3 248.2 193.3 250.8

94.23 90.47 87.63 85.04 94.35

287.3 262.6 221.2 167.7 238.3

promotes a faster non-radioactive process (Table 3, entries 4 and 5) from 85.04% to 94.35%. This result justifies the lower QY observed for ZAIS NCs (Table 2), indicating the replacement of Agþ and In3þ by Zn2þ ions, and generation of new donor-acceptor states. The average lifetime increases from 167.7 ns (AIS, Table 3, entry 4) to 238.3 ns (ZAIS, entry 5) as result of reabsorption and reemission processes [8,22,53]. Considering Pearson's theory of acids and bases, Agþ can be considered as soft acid and In3þ as hard acid [54]. Thus, these ions have intrinsic differences in their reactivity, which are related to their polarizability and the covalence factor of the ionic bond. The S2 anion is known to be a soft base and the Zn2þ is classified as a frontier acid/base [12,54]. Due to the charge/radius ratio, the Zn2þ has a high diffusion coefficient and, therefore, it can diffuse in the AIS-GSH network to assume positions of most polarizable cations (Agþ), without the formation of a core-shell structure. The hypothesis of the addition of Zn2þ ions into the AIS-GSH nanostructure follows the solid-state reaction model, where the Zn2þ moves toward the centre of the AIS-GSH NCs, and the Agþ ions diffuse in the opposite direction via formation of Frenkel defects, toward the nanocrystal surface [46]. The bandgaps of the NCs were obtained by extrapolating the AIS-GSH (Fig. 3C) and ZAIS-GSH (Fig. 3F) Tauc curve [55], presenting bandgap values ranging from 1.97 to 2.96 eV, depending on the Agþ/In3þ ratio. To higher levels of Agþ, the nanoparticle system presents lower bandgap values, due to the high number of electronic defects associated with Agþ gaps, inserting a p-type doping effect [56], The bandgap value of 1.97 eV observed for AIS-GSH NCs (prepared in Agþ/In3þ aqueous solution at 0.5 ratio) is higher than the bandgap described for the cubic phase of AgIn5S8 bulk (1.80 eV) [38,45,57]. For the ZAIS-GSH NCs, the shift to higher bandgap values is also observed after the addition of Zn2þ, which occurs due to the reduction of the Agþ ratio into the nanocrystal lattice. Based on the experimental evidences of AIS-GSH and ZAIS-GSH optical and structural characterizations, it is possible to suggest a mechanism for Zn2þ insertion into the AIS-GSH ternary structure, giving new crystalline defects. As a function of the Agþ/In3þ ratio, there will be a preferential substitution and defect formation, by replacing positions of Agþ or In3þ, Eq. (6). For high Agþ/In3þ ratios,

Fig. 4. (A) Photoluminescence decay curves for AIS NCs (Agþ/In3þ ¼ 0.5, 0.28, 0.18, 0.14). (B) Photoluminescence decay curves for ZAIS and AIS (Agþ/In3þ ¼ 0.14).

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238

7

Thus, the low quantum yield of ZAIS-GSH (Table 2) can be justified by Eq. (6), since the Zn2þ diffused in the AIS-GSH lattice, inserts new electronic defects. This approach also justifies the absence of a well-defined ZnS shell region in the HRTEM images, indicating the formation of a homogeneous alloy structure. 3.4. Growth mechanism of NCs

Fig. 5. UVeVis absorption spectra recorded during AIS-GSH electrochemical synthesis (t ¼ 0 se350 s).

the Zn2þ cations will preferentially occupy Agþ positions, injecting a p-type electronic defect. For low Agþ/In3þ ratios, Zn2þ also occupies an In3þ position, the substitution inserts a free electron in the system, creating an n-type electronic defect [39]. AgIn5 S8 structure

0

2 Zn2þ ðaqÞ ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!ZnIn3þ , þ ZnAg þ

(6)

Ag 3d

378

376

374

n Ag2S-GSH(seed) / Ag2nSn-GSH(colloidal)

372

370

368

366

In 3d

458 456 454 452 450 448 446 444 442 440

364

Intensity (cps)

Intensity (cps)

Zn 2p3/2

C

2p3/2 (163.2)

2p1/2 (162.0)

164

162

160

Binding energy (eV)

B

Binding energy (eV)

S 2p

166

3d5/2

3d3/2

Binding energy (eV)

168

(8)

The addition of In3þ ions (Eq. (9)) to the Ag2nSn-GSH(colloidal) nuclei occurs through Frenkel defects, occupying Agþ positions and generating crystalline defects (Inþþ ). The nanoparticle growth ocAgþ curs according to theoretical estimates, where the crystalline defect e formation ðV Ag Þ is energetically favoured [39,61]. Agþ ions remaining in the crystalline structure may aid the propagation of

A

3d3/2

380

2 [Ag(GSH)2]5-(aq) þ HS(aq) þ OH(aq) / Ag2S-GSH(seed) þ H2O(l)(7)

Intensity (cps)

Intensity (cps)

3d5/2

The mechanism proposed for the growth of the NCs can be based on the polarizability of Agþ and In3þ, and solubility product constant (KSP). The HS(aq) is a polarizable (soft) Pearson's base and had the tendency to form stable compounds with a polarizable (soft) Pearson's acid like an Agþ(aq) [54]. Although the solubility product constant of the In2S3 salt (Ksp ¼ 7.0  1073) is lower than the Ag2S respective value (Ksp ¼ 7.0  1050), the solubility of In3þ ion is higher (s ¼ 3.4  1015 mol/L1) than Agþ ion (s ¼ 2.7  1017 mol/L1), in presence of S2 ions [7,37,58e60]. Eq. (7) presents the nucleation step for Ag2S-GSH monomer, followed by the colloid formation (Eq. (8)):

158

156

1030

1028

1026

D

1022.6

AIS ZAIS Surface ZAIS Inner st 1 etching ZAIS Inner nd 2 etching

1024

1022

1020

Binding energy (eV)

Fig. 6. XPS high-resolution scans of (A) Ag 3d, (B) In 3d and (C) S 2p peaks of AIS NCs. (D) Zn 2p peaks of ZAIS NCs.

1018

8

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238

the unit cell or can give rise to new monomers of Ag2S-GSH. The GSH acts as a ligand for the complexation or stabilization phase of the NCs [20]. Thus, the GSH-stabilized Ag2n-xInxSnþx seeds are formed (Eq. (10)), which under reflux produces the Ag2n-xInxSnþxGSH NCs (AIS-GSH). Ag2nSn-GSH(colloidal) þ x[In(GSH)3]6-(aq) þ xHS(aq) þ xOH(aq) # Ag2n-xInxSnþx-GSH(colloidal) þ x[Ag(GSH)2]5-(aq) þ xGSH3(aq) þ xH2O(l) (9) D

Ag2nx Inx Snþx  GSHðcolloidalÞ / Ag2x Inx Snþx  GSHðnanocristalÞ (10) Fig. 5 shows the absorption spectra recorded during the AISGSH electrochemical synthesis. An aqueous solution of AgNO3, InBr3, GSH and electrolyte, at pH 9, was used as blank to observe the nucleation step. The continuous profile of absorption, relative to donor-acceptor states, is typically observed. The nucleation of the nanoparticles is mediated by a cation exchange step, following the solid-state reaction model. It is also indicative of the initial formation of Ag2S and nanoparticle growth mediated by the insertion of In3þ in the structure. The results observed in AIS-GSH X-ray diffraction (Fig. 1) and emission spectra (Fig. 3B) indicates the formation of AgIn5S8 crystalline phase for (Agþ/In3þ ratio ¼ 0.14). After the preparation of Ag2n-xInxSnþx-GSH NCs, a new electrochemical procedure was carried out to promote the deposition of a

ZnS shell on the AIS-GSH surface. In a paired electrolysis, the elemental sulphur was again reduced into the cathodic compartment of the cavity cell producing HS ion precursor (Eq. (2)), while a Zn sacrificial anode was oxidized to produce Zn2þ ions in the intermediate compartment (Eq. (4)), containing the AIS-GSH colloidal aqueous solution and GSH, giving the complex [Zn(GSH)2]4- (Eq. (11)). Zn2þ(aq) þ 2 GSH3(aq) / [Zn(GSH)2]4-(aq)

(11)

As the ZnS NCs presents a greater solubility (Ksp ¼ 1.8  1025) when compared with Ag2S and In2S3 salts, thus, it is reasonable the ZnS shell formation, giving the core-shell structure (Eq. (12)) Ag2nxInxSnþx/ZnS-GSH(colloidal) (AIS/ZnS-GSH). Ag2n-xInxSnþx-GSH(colloidal) þ [Zn(GSH)2]4-(aq) þ HS(aq) þ OH(aq) / Ag2n-xInxSnþx/ZnS-GSH(colloidal) þ H2O(l) (12) However, it was observed the diffusion of Zn2þ ions into the AISGSH crystalline structure. The substitution of Agþ and In3þ positions by Zn2þ ions causes crystalline defects in the network and formation of new intragap states (Eq. (13)) [39]. Thus, a ZnAgIn5S8 alloy was produced, in addition to a thin ZnS shell, generating the core/shell structure (Eq. (14)). Ag2n-xInxSnþx-GSH(colloidal) þ y[Zn(GSH)2]4-(aq) / Zny(Ag2n-xInx)56(13) ySnþx-GSH(colloidal) þ y[Ag(GSH)2] (aq) þ y[In(GSH)3] (aq)

Fig. 7. Mechanism proposed for the AIS-GSH and ZAIS-GSH NCs formation. In agreement to equations (1)e(12). VAgþ and VIn3þ correspond to ion vacances in the nanocrystal lattice.

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238

Zny(Ag2n-xInx)-ySnþxGSH(colloidal) þ [Zn(GSH)2]4-(aq) þ HS(aq) þ OH(aq) / Zny(Ag2n3 (14) xInx)-ySnþx/ZnS-GSH(nanocrystal) þ GSH (aq) þ H2O(l) The preparation of core-shell or alloy structures follows mechanisms correlated with ion-exchange and diffusion phenomena, respectively. Under the reaction conditions there is a natural tendency to Zn2þ diffusion into the AIS-GSH lattice and formation of a homogeneous alloy (emission hipsochromic shift). At high Zn2þ concentration, the ion-exchange mechanism also occurs, promoting the formation an alloy/core-shell intermediate state [39,59,62]. XPS spectra were obtained directly at the AIS surface and, in order to get the distribution of the elements underneath the surface layer, ion bombardment with argon ions (Arþ, 2 cycles, 3 s/each, emission current 50 mA, and beam voltage 0.5 kV) were performed before further analysis. The typical XPS spectra of the AIS NCs at the surface and at inner layers are showed in Fig. 6. It was not detected significant difference between the analysis at surface and after etching. First, it can be observed that the three chemical elements

9

of the ternary AgeIneS system were found, as expected for the AIS core. As depicted in Fig. 6A, the doublet at Ag 3d region (Ag 3d5/2 and Ag 3d3/2) is broadened allowing decomposing each peak into two Gaussian peaks. The value of the binding energy of 3d5/2 equal to 368.5 eV was compatible with Ag in alloys, and the signal at 369.1 eV can be associated to Ag species interacting with carboxylates from the GSH stabilizer [63]. Each group of spin-orbit components (Ag 3d5/2 and Ag 3d3/2) was separated by a binding energy interval of approximately 6.0 eV, characteristic of silver. The two strong peaks are observed in Fig. 6B, at 445.3 eV (In 3d5/2) and 452.9 eV (In 3d3/2) and suggest the presence of In3þ species in the sample [63]. The difference between the binding energies of In 3d5/ 2 and In 3d3/2 was 7.6 eV, in agreement with literature [63]. The S 2p3/2 peak and S 2p1/2 peaks (Fig. 6C) were found at 162.0 eV and 163.1 eV, respectively, (D ¼ 1.1 eV), which can be assigned to sulphur in metal sulfides (M-S) [63,64]. XPS analysis on ZAIS surface and after etching (Fig. 6D) indicated the presence of zinc element. The spectrum at the NC surface revealed the presence of the peak at 1022.6 eV assigned to Zn 2p3/2 term [63,64]. After etching, the 2p3/2 signal was maintained (i.e. 1st

Fig. 8. (A) CIE chromaticity diagram and (B) photograph of modified LEDs: WLED (no coverage), RLED (AIS-GSH 0.28), YLED (AIS-GSH 0.18) and GLED (ZAIS-GSH 0.14) with the application of 20 mA. (C) Emission spectra of the WLED, RLED, YLED and GLED.

10

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238

to 2nd etching curves) with the appearance/enhancement of other peak of higher binding energy 1024.6 eV, which can be assigned to the Zn-alloying process into the AIS nanocrystal core (quaternary nanosystem) usually caused by the changes on Zn environments/ species (e.g. alloying, defects, crystal lattice strains, etc.) [64]. Fig. 7 presents details about the proposed mechanism for nucleation and growth of AIS-GSH and ZAIS-GSH nanocrystals based on our experimental results. The in situ formation of the species in an aqueous medium, in extremely controlled conditions, is the great differential of the electrochemical synthesis strategy, which guarantees reproducibility and efficiency of the synthetic procedure. The solvent dielectric constant is fundamental for the crystallization step, because phenomenologically it promotes separation of charges, leading to a phase control for the nanocrystal growth. In addition to Pearson's theory of acids and bases, Klopman quantified the hardness and softness of the species and showed the importance of the dielectric constant of the medium in the interaction between the ions [65]. Thus, the reaction for nanocrystal formation, under the water phase diagram conditions, did not favor the crystallization, once the dielectric constant of water (ε ¼ 80.1 at 25  C) promotes the charge separation, thus, the heating (ε ¼ 55.3 at 100  C) is used in major cases to help the growth of the nanocrystals. Table S1 (Supplementary Material) presents an evaluation about AIS-GSH nanocrystal synthetic methods found in the literature and the electrosynthetic procedure described in this work. At the synthetic level, the electrochemical methodology has the advantage of the direct use of electrons for the reduction of chalcogen precursors, avoiding toxic reducing agents. In addition, it was possible to obtain the NCs in powder, furnishing around 30 mg of AIS-GSH powder for each 30 mL of AIS-GSH solution. About the optical properties, a satisfactory quantum yield (up to 16.6%) was obtained, which is close to those reported for aqueous syntheses, using GSH as stabilizer (15%) or co-stabilizer (20%) [19,49]. 3.5. Tuning the colour emission of WLED The modification of the emission colour perception is an interesting application for the AIS-GSH and ZAIS-GSH NCs. The modification can be carried out by modifying the surface of commercial white LEDs without resin coating [31]. In order to obtain red, yellow or green LEDs, we performed the deposition of the synthesized nanoparticles on the surface of cold white LEDs, which promotes a light colour. The colour coordinates were obtained using the software SpectraLux 2.0, according to Commission Internationale de
l'Eclairage CIE 1931 [66]. Fig. 8A shows the colour coordinate of the WLED at 0.2084, 0.1787 (a). When coated with AIS-GSH 0.28 its coordinate was shifted to 0.5129, 0.4256 (b), giving the RLED device, with red colour perception. Fig. 8B shows the modified LEDs covered with nanoparticles, under 20 mA electric current. The electrosynthesized AIS-GSH and ZAIS-GSH NCs have proved to be efficient and have the potential to cover the optically active parts of the LEDs, bringing with them greater economic viability. When performing the same procedure, covering the white LEDs with AISGSH 0.18 and ZAIS-GSH 0.14, the coordinates were shifted to 0.4198, 0.4721 (c) and 0.3675, 0.4645 (d), respectively, giving the YLED and GLED devices with a yellow and pale green colour, respectively. Fig. 8C shows the emission spectra of the WLED and modified LEDs: RLED, YLED, GLED. The band around 430 nm related to blue emission of LED was used to excite the NCs. The lem ¼ 544 nm for GLED, 572 nm for YLED and 532/594 nm for RLED, promoted the changes in the colour coordinate.

4. Conclusions The electrochemical procedure for the nanocrystal synthesis in aqueous medium was successful, giving stable AIS-GSH and ZAISGSH nanoparticles. The mechanism follows steps of Agþ, In3þ and GSH coordination and in situ electrochemical generation of S2 ions. The formation of the nanocrystals starts from Ag2S-GSH seeds, followed by In3þ insertion into the crystal lattice during the nucleation and growth processes. The ion exchange into the AISGSH nanocrystal lattice can be controlled by the Agþ/In3þ ratio in solution, furnishing an optical bandgap tuning with appreciable quantum yield values. The electrochemical preparation of the core/shell (AIS/ZnS-GSH) NCs was unsuccessful because the Agþ/ In3þ/Zn2þ ion exchange into the AIS-GSH lattice, producing a ZAISGSH alloy. The AIS and ZAIS elemental composition was investigated by XPS, evidencing the proposed nanocrystal growth mechanism and the structure of the nanocrystals. Time-resolved spectroscopy allowed to identify that higher Agþ/In3þ ratio increases the number of AIS structural defects. Similarly, the Zn2þ insertion to AIS NCs promotes the same effect, diminishing the photoluminescence QY in both cases. Thus, the electrochemical method can be considered as a new clean procedure for the preparation of optically active ternary and quaternary nanocrystals, associating efficiency and reproducibility to the principles of the green chemistry. Credit author statement Felipe L. N. Sousa e Investigation, Methodology, Validation, Writing e Original Draft. Denilson V. Freitas e Conceptualization, Methodology, Investigation, Supervision, Writing e Original Draft. Richardson R. Silva e Investigation. Stterferson E. Silva e Investigation. A.C. Jesus e Investigation, Formal analysis. Herman S. Mansur e Funding acquisition, Resources, Supervision. Walter M. Azevedo e Resources, Supervision. Marcelo Navarro e Funding acquisition, Project administration, Supervision and Writing e Review & Editing. Acknowledgements This work was supported by the following Brazilian research agencies: CNPq (UNIVERSAL-401541/2016-9; UNIVERSAL-457537/ 2014-0; UNIVERSAL-421312/2018-1; PQ2-306132/2017-6; PQ1B306306/2014-0; PQ1A-303893/2018-4; PIBIC-2014/2015), FACEPE (APQ-0549-1.06/17/INCT 2014; APQ-0443.1.06/15), CAPES (PROEX-433/2010; PNPD; PROINFRA2010e2014); FAPEMIG (PPM00760-16; BCN-TEC 30030/12; UNIVERSAL-APQ-00291-18); and FINEP (CTINFRA-PROINFRA 2008/2009/2010/2011/2012/2013/ 2014). F.L.N.S. acknowledge the scholarship provided by CNPq (164652/2017-5). D.V.F. acknowledges the scholarship provided by FACEPE (BFP-0051.1.06/18). The authors also thank the staff at the Center of Nanoscience, Nanotechnology and InnovationCeNano2I/CEMUCASI/UFMG, Center of Microscopy/UFMG and Centro de Tecnologias do Nordeste (CETENE) for the characterization analyses. BSTR Laboratory/UFPE for photoluminescence spectra. The authors thanks BSc. Gian Duarte (CETENE), Prof.
n (Departamento de Física - UFPE) by the help to Eduardo Padro improve the XRD measurements, Isadora C. Carvalho (UFMG) by
Galembeck (DQF - UFPE) by DLS and z XPS analysis, Prof Andre measurements.

F.L.N. Sousa et al. / Materials Today Chemistry 16 (2020) 100238

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtchem.2019.100238.

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