Impact of Inorganic Hydroxides on ZnO Nanoparticle Formation and

Article pubs.acs.org/crystal

Impact of Inorganic Hydroxides on ZnO Nanoparticle Formation and Morphology Alojz Anžlovar,*,† Ksenija Kogej,‡ Zorica Crnjak Orel,† and Majda Ž igon†,§ †

National Institute of Chemistry, Hajdrihova 19, SI-1000, Ljubljana, Slovenia University of Ljubljana, Faculty of Chemistry and Chemical Technology, Aškerčeva 5, SI-1000, Ljubljana, Slovenia § Polymer Technology College, Ozare 19, SI-2380, Slovenj Gradec, Slovenia ‡

S Supporting Information *

ABSTRACT: ZnO nanoparticles (NPs) were synthesized by a seeded polyol process in di(ethylene glycol) (DEG) using zinc acetate as a precursor in the presence of inorganic hydroxides (NaOH, KOH, LiOH). The precursor was transformed into ZnO in DEG without solid intermediates as shown by Fourier transform infrared spectroscopy. Both Raman and photoluminescence spectroscopies confirmed the presence of defects in the ZnO crystal structure which generated visible light emission when excited with UV light. The molar ratio of seeds/precursor affects ZnO particle growth, and at a high ratio (10/1) growth of ZnO NPs is practically prevented, giving ZnO with a narrow particle size distribution. By decreasing the precursor concentration from 1.0 to 0.01 M, ZnO particle size decreased from 50−200 nm to 20−60 nm; however, the degree of agglomeration was increased. Type of cation plays only a minor role in the ZnO NPs formation. By increasing the hydroxide/precursor molar ratio from 1/1 to 5/1, the ZnO particle size is reduced from 50−200 nm to 20−40 nm. Therefore, the hydroxide/precursor molar ratio has a significant role in the formation of ZnO because it defines the concentration of OH− ions, which is the key factor in this process. and Spanhel and Anderson,20 comprising the dissolution of ZnO precursors (zinc perchlorate, zinc nitrate, zinc acetate dihydrate, zinc acetylacetonate monohydrate) in organic solvents (alcohols, diols, etc.) and subsequent addition of a basic aqueous solution (NaOH, NH4OH, LiOH).21 Although the authors frequently use various hydroxides as additives in solvothermal synthesis of ZnO, there have been very few systematic studies on the impact of the addition of a hydroxide on ZnO particle size and morphology.22,23 Kim et al. found that the mole ratio of hydroxide/precursor is the most influencing parameter on ZnO particle size and particle size distribution.24 Viswanatha et al. reported on how NaOH influenced the formation and growth of ZnO NPs and also gave an explanation of the mechanism.25 They conclude that the ZnO growth rate in monoalcohol media is strongly dependent on the OH− ion concentration, which leads to the formation of a passivating layer of Na+ ions on the ZnO surface, thus significantly slowing down the growth of ZnO particles and preventing particle agglomeration. It was also reported that in addition to the size of the cation, the dissociation constant of the hydroxide plays a significant role in the growth of ZnO particles, and that above the critical concentration of cations, the growth of ZnO particles is practically prevented.26

1. INTRODUCTION The synthesis and design of nanoscale and nanostructured materials have stimulated intense research activities that attempt to find new morphologies, properties, and functions. Particle size and morphology substantially affect various properties and consequently potential applications of these materials. Recently, complex nanostructures of semiconductors have attracted remarkable attention due to their promising electronic and optical properties being closely related to their structure and morphology. They are expected to become essential building blocks of various nanodevices with interesting functions in practical applications.1 ZnO is an n-type semiconductor that has a wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature and has gained wide scientific and technological interest. ZnO is a promising material for various applications such as catalysis, UV light emitting, photovoltaics, chemical sensing, UV absorbing materials, and phosphors.2 In the last 10 years, the syntheses of various ZnO nanostructures such as wires,3 rods,4 belts,5 tubes,6 coaxial cables,7 branches,8 bipods,9 towers,10 columns,11 tetrapods,12 hollow microspheres,13 helices,14 combs,15 cones,16 and mesocrystals17 have been reported. ZnO nanostructures can be synthesized by various hydrothermal or solvothermal syntheses using a variety of additives. ZnO nanoparticles (NPs) are often prepared by modifying the synthetic routes developed by Koch et al.,18 Bahnemann et al.,19 © XXXX American Chemical Society

Received: February 19, 2013 Revised: June 21, 2014

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dx.doi.org/10.1021/cg401870e | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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spectral range between 400 and 4000 cm−1 with a spectral resolution of 4 cm−1 in transmittance mode using the KBr pellets technique. Raman spectra were recorded in the spectral range from 90 to 3600 cm−1 using a Witec Alpha 300 spectrometer that employed a green laser with an excitation wavelength 532 nm and resolution of 4 cm−1 at 10 mW laser power. The 13C NMR spectra of seeds solutions and reaction media were recorded on a Varian Unity Inova 300 MHz spectrometer (Varian Palo Alto, California) under the following quantitative conditions: pulse 90°, delay 2.0 s, acquisition time 2.0 s, and temperature 25 °C. Photoluminescence spectra of ZnO powders and PMMA/ZnO nanocomposites were recorded on PerkinElmer LS-55 spectrometer using excitation wavelength of 325 nm in the range from 330 to 620 nm. The morphology and size of the synthesized particles were studied by SEM and HRTEM. SEM micrographs of gold sputtered samples were taken on a Zeiss Supra 35 VP field emission electron microscope at an acceleration voltage of 12, 15, or 20 kV using mixed in-lens and secondary electron detectors at a ratio of 0.75/0.25 and a working distance between 3−6 mm. For SEM microscopy, ZnO NPs were placed on a conductive film and sputtered with Au. For scanning transmission electron microscopy (STEM) or HRTEM microscopy, ZnO NPs were dispersed in an organic solvent (e.g., ethanol) by sonication, and a drop of dispersion was transferred to a Cu grid and dried. TEM micrographs were obtained on the electron microscope JEOL 2000F operated at an acceleration voltage of 200 kV using an energy dispersive X-ray spectrometer (LINK ISIS-300 with an UTW Si−Li detector). STEM micrographs were analyzed using Image Tool software to obtain average particle sizes and particle size distributions. The sizes of ZnO NPs in DEG were measured by DLS using the 3D-DLS-SLS spectrometer (LS Instruments, Fribourg, Switzerland) equipped with a 20 mV He−Ne laser (Uniphase JDL 1145 P) operating at 632.8 nm. Scattering was measured at an angle of 90°. Samples in the scattering cells were immersed in a large diameter bath thermostated at 20 °C, and 10 measurements of 60 s were recorded for each sample and averaged afterward. In a DLS experiment, the translational diffusion coefficient D is determined, while the hydrodynamic radius Rh is calculated from D using Stokes−Einstein equation. The dynamic viscosity of the solvent (DEG) needed for this calculation was η = 30.2 mPas at 25 °C.32 Crystalline fractions of the synthesized ZnO NPs were characterized by wide-angle XRD on an XPert Pro diffractometer with a Cu anode as the X-ray source. X-ray diffractograms were measured at 25 °C in the 2Θ range from 2 to 90° with a step of 0.04° and step time of 1 s. Crystallite sizes were calculated using the Scherrer formula, and a Si wafer was used to determine the experimental broadening. Zeta potential was measured with the Zetasizer Nano ZEN 3600 instrument on the basis of electrophoretic mobility using folded capillary cell DTS 1060. ZnO NPs were dispersed in DI H2O by mixing and sonication. pH was varied by the addition of 0.1 M KOH to cross the isoelectric point.

At high concentrations of the hydroxide, the nucleation of ZnO during ZnO NPs formation is faster, producing a higher number of nuclei, and consequently ZnO particle size is reduced.27 At high concentrations of LiOH, the excess hydroxide is distributed in the form of a complex around the ZnO NPs that prevents their agglomeration.28 Tang et al. reported that the zeta potential and photoluminescence spectra of ZnO NPs, synthesized in ethanol, significantly changed by increasing the pH from 8 to 12 with the addition of a hydroxide (LiOH, NaOH, KOH, Ca(OH)2).29 Diols (polyols) are organic solvents with low toxicity, high boiling points (up to 300 °C), and low vapor pressure. They are environmentally friendly solvents of moderate polarity that can dissolve various inorganic compounds and organometallic complexes. They offer a wide temperature range in which chemical reactions can be performed, and they can interact via hydroxyl groups with the surface of ZnO particles, thus reducing the rate of their growth.30,31 In our work, we systematically studied the role of various inorganic hydroxides and their cations as well as the molar ratio of hydroxide/precursor on the ZnO morphology and particle size using seeded alcoholysis of Zn acetate dihydrate in di(ethylene glycol) (DEG) as an environmentally friendly medium. Namely, when aqueous medium is replaced with diol medium, it is of vital importance to understand the difference between ZnO formation mechanisms in both media. We focused on the influence of precursor and hydroxide concentration and type of cation as well as the influence of the molar ratio of hydroxide/precursor and of seeds/precursor on the particle size and morphology of ZnO, synthesized in DEG medium.

2. EXPERIMENTAL SECTION 2.1. Materials. Di(ethylene glycol) (DEG) (Merck, p.a.); zinc(II) acetate dihydrate (Zn(Ac)) (Sigma-Aldrich, 99%, ACS reagent), LiOH (Merck, 98%), NaOH (Kemika, 98%), KOH (Kemika, 98%), ethanol (KeFo - technical), deionized water (DI H2O). 2.2. Procedure. The solution of ZnO seeds was prepared by dispersing the precursor Zn(Ac) and DI H2O (2 mol/1 mol Zn) in 60 mL of DEG by mixing and sonication (10 min) and by subsequent heating and mixing at 180−190 °C for 1 h. The resulting ZnO dispersion was centrifuged, and the supernatant was further used in a seeded synthesis of ZnO NPs. The concentration of Zn(Ac) during preparation of seeds was 10 times lower than its concentration in the second stage of preparation of ZnO NPs in order to reduce the influence of ZnO seeds on the concentration of zinc species in the reaction mixture. In some experiments, the concentration was equal or 3 or 10 times higher in order to study the effect of the mass ratio of seeds on the precursor. Zn(Ac), hydroxide and deionized water (2 mol/1 mol Zn) were dispersed in DEG containing ZnO seeds (supernatant to which a sufficient volume of fresh DEG was added to obtain 60 mL) by mixing and sonication (10 min). The concentration of precursor was 0.01, 0.1, and 1.0 M. The dispersion was heated for 2 h at 180−190 °C, and the precipitate (nano ZnO) was separated from the supernatant by centrifugation (4500−8000 rpm for 15−60 min depending on the particle size). The precipitate was washed three times with 100 mL of ethanol and dried in the air. 2.3. Characterization. Synthesized ZnO NPs were characterized by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS) analysis, powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) analysis. Intermediates and the obtained particles were characterized by FTIR spectroscopy using an FTIR spectrometer Spectrum One in the

3. RESULTS AND DISCUSSION ZnO NPs were synthesized by seeded alcoholysis of the precursor (Zn(II) acetate dihydrate (Zn(Ac))) in diols with the addition of various inorganic hydroxides (NaOH, KOH, LiOH). In the initial stage of experiments, various diols (EG, DEG, TEG, BD, and PD) were studied as media. By simple reaction of Zn(Ac) in the presence of the hydroxide, the product of synthesis was highly agglomerated ZnO. To avoid the problem of particle agglomeration, the synthesis with seeds was introduced.33 This approach was effective only when DEG was used as a medium indicating that this diol has exceptional properties compared to other diols. According to Dakhlaoui et al.22 who ascribed the formation ZnO nanorods in DEG to its intense adsorption to the polar surface of ZnO crystal, this phenomenon is most probably responsible for the prevention of ZnO agglomeration in DEG medium. B



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3.1. Preparation and Characterization of Seeds. Since a seeded synthesis approach was successful only in DEG, only this diol was used as a medium in further experiments. In the first step, a solution containing seeds, i.e., particle nuclei, was prepared by the partial transformation of Zn(Ac) into ZnO in DEG. After 1 h, the reaction was stopped, the solid product was separated from the medium by centrifugation, and the supernatant was used as a medium in the second step of ZnO synthesis. Seeds were characterized by DLS (Figure 1)

SEM micrographs (Figure 3, Supporting Information) showed 70−150 nm nonagglomerated ZnO particles, which confirms that this approach is successful in the DEG medium, while no significant improvement was observed in all other diols. Therefore, our study focused on the correlations between ZnO particle size, morphology, and particle size distribution and • type of hydroxide • concentration of precursor • mole ratio hydroxide/precursor • mole ratio seeds/precursor 3.3. Seeded Synthesis of ZnO NPs−Type of Hydroxide. HRTEM and SEM micrographs of ZnO, synthesized at a precursor concentration of 1.0 M, showed mixed irregular cubes and rodlike ZnO particles with sizes between 70−150 nm (Figure 2 and Figure 3, Supporting Information). Comparing

Figure 1. Hydrodynamic radii (Rh) of seeds as a function of the Zn(Ac) concentration.

and NMR spectroscopy (Figure 1, Supporting Information) DLS measurements of supernatants obtained after 1 h of reaction showed that seeds were nanostructures with hydrodynamic radii (Rh) between 80 and 100 nm (Figure 1). 13C NMR spectrum of the supernatant showed that at high Zn(Ac) concentrations (0.1 and especially 1.0 M), a transformation reaction proceeded (Figure 1, Supporting Information). NMR signals of Zn(Ac) (22.4 and at 178.0 ppm)34 are not present in the spectrum, indicating that all the precursor is transformed into the reaction intermediate. 3.2. Seeded Synthesis of ZnO NPs−Reaction Mechanism. 13C NMR spectrum of the reaction medium (Figure 2, Supporting Information) shows, besides signals of DEG (61.64 and 73.07 ppm), characteristic signals of diethylene glycol diacetate (DEG(Ac)) (21.12, 64.41, 69.43, and 172.34 ppm) as well as characteristic signals of Na acetate (Na(Ac)) (23.85 and 179.71 ppm). This indicates that reaction proceeds according to the following reaction scheme (eqs 1, 2, and 3):34

Figure 2. HR-TEM micrographs of ZnO particles synthesized by the addition of various hydroxides (conc. of Zn(Ac) = 1.0 M, conc. of hydroxide = 1.0 M): (a) LiOH, (b) NaOH, (c) KOH, (d) electron diffraction KOH.

the particle sizes of ZnO synthesized by using different hydroxides (LiOH, NaOH and KOH), it can be seen that for NaOH and KOH, the type of cation or its size has a rather small influence on ZnO particle size and morphology (Figure 3, Supporting Information, Table 1).35 The exception is LiOH, which produced smaller ZnO NPs with sizes between 40−100

4Zn(CH3COO)2 + HO‐R‐OH

Table 1. ZnO Particle Size and Crystallite Size As a Function of the Precursor Concentration and Type of the Hydroxidea

↔ Zn4O(CH3COO)6 + CH3COO‐R‐OOCCH3 + H 2O (1)

Zn4O(CH3COO)6 → 3Zn(CH3COO)2 + ZnO

(2)

hydroxide

conc of precursor (mol/L)

LiOH NaOH KOH LiOH NaOH KOH LiOH NaOH KOH

0.01 0.01 0.01 0.1 0.1 0.1 1.0 1.0 1.0

Zn(CH3COO)2 + 2NaOH → ZnO + 2CH3COONa + H 2O

(3)

DEG reacts with acetate ion forming diethylene glycol diacetate indicating that it is, besides being a medium, also a reactant in the transformation of Zn(Ac) into ZnO. The transformation reaction of Zn(Ac) into ZnO in DEG is therefore a reaction of alcoholysis. NaOH reacts with acetate ion, which is released during the reaction, forming a Na acetate.

a

C

average particle size (nm)

crystallite size 2Θ = 36.7° (nm)

yield (%)

± ± ± ± ± ± ± ± ±

14.1 12.4 10.0 16.2 11.5 11.1 42.2 23.5 34.3

23.5 21.4 22.8 37.6 36.8 38.2 58.6 55.5 57.6

26.3 34.5 39.6 29.1 37.7 41.6 56.8 66.0 65.9

0.7 0.7 1.1 1.4 1.7 1.3 2.6 2.8 3.7

Molar ratio precursor/hydroxide = 1/1. dx.doi.org/10.1021/cg401870e | Cryst. Growth Des. XXXX, XXX, XXX−XXX


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Figure 3. (a) FTIR spectra and (b) Raman spectra of ZnO NPs synthesized by the addition of various hydroxides (conc of Zn(Ac) = 1.0 M, conc of hydroxide = 1.0 M): (A) NaOH, (B) KOH, (C) LiOH.

hydroxide has no effect on the formation of the defects in the crystal structure of ZnO. Photoluminescence spectra (PL) spectra of ZnO NPs, synthesized by using various hydroxides (Figure 4), were

nm (Figure 3, Supporting Information, Table 1). Similar observations have been reported in the literature, with an explanation that smaller ZnO particle size is the result of decreased positive charge on the particle surface due to the presence of Li+ ions.36 Uekawa et al. reported that the mechanism and kinetics of particle growth is changed when Li+ ions are present in the system compared to those which contain Na+, K+, and Cs+ ions.37 IR spectroscopic characterization (Figure 3a) confirmed the formation of ZnO by the presence of a characteristic ZnO absorption band at 410−430 cm−1. Shifting the ZnO absorption band to higher frequencies indicates the formation of extended ZnO structures, which confirms the observation from SEM microscopy. The presence of additional absorption bands at 1630 and 1415 cm−1 corresponds to the strong asymmetric stretching mode of CO and C−O bonds, originating from the tetranuclear oxo zinc acetate cluster, Zn4O(CH3COO)6,34 while the broad absorption band at 3450 cm−1 belongs to stretching modes of OH groups.38−41 Raman spectroscopy is complementary to IR spectroscopy since it gives information about chemical composition of the sample as well as about the crystal structure and its defects. Raman spectra of synthesized ZnO samples (Figure 3b) show intense absorption bands at 442 and 101 cm−1 corresponding to E2 (high) and E2 (low) frequency phonons associated with vibration of Zn and oxygen atoms in wurtzite crystal structure, which indicates that NPs are ZnO with a high degree of crystallinity indeed.42 A sharp peak of low intensity at 337 cm−1 corresponds to optical phonon overtone with A1 symmetry, while a less pronounced peak at 389 cm−1 is attributed to A1(TO) optical mode.41 Low intensity peak at 579 cm−1 corresponds to A1(LO) mode and is associated with structural defects like oxygen deficiency.43,44 Broad peaks at 666 cm−1 and at 1147 cm−1, respectively, should be the result of multiphonon scattering process.42 The peak at 1105 cm−1 is the acoustic combination of A1 and E2 optical modes.45 High intensities of signals at 442 and 101 cm−1 as compared to intensities of other peaks indicate a high degree of crystallinity of synthesized ZnO.44 The peak at 579 cm−1 in the Raman spectra (Figure 3b) indicates the presence of the defects in the ZnO crystal structure. The comparison of Raman spectra of ZnO samples, prepared with different hydroxides, shows no significant differences thus leading to the conclusion that the type of

Figure 4. Photoluminescence spectra of ZnO NPs synthesized by the addition of various hydroxides (conc. of Zn(Ac) = 1.0 M, conc. of hydroxide = 1.0 M): (A) NaOH, (B) KOH, (C) LiOH.

measured as well. The spectra consisted of near band edge emission at 385−395 nm, which can be attributed to the wide band gap of ZnO.46−48 In addition to the UV peak, there are numerous emission peaks in the visible region at 423, 448, 461, 485, and 529 nm. These emissions are related to various intrinsic defects in the ZnO crystal lattice, such as Zn or O vacancies, interstitial Zn or O atoms, and oxygen antisites.49 They are attributed to electron transitions from various energy levels of the defects and are schematically explained by Mishra et al.50 The most intense peaks are violet emission at 423 nm (2.93 eV), attributed to electron transition from shallow donor level of neutral Zn interstitial to the top level of valence band49 and blue emission at 485 nm (2.56 eV), which is ascribed to radiative electron transition from shallow donor level of Zn interstitial to an acceptor level of Zn vacancy.50 The mechanisms of blue emissions at 448 (2.77 eV) and 461 nm (2.69 eV) have not been clarified yet.50 Green emission at 529 nm (2.34 eV) is caused either by donor−acceptor recombinaD



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tion or by the electron transition from conduction band to oxygen antisites.49 PL spectra are similar to those reported by Sheini et al.46 Comparing the PL spectra of ZnO, prepared with various inorganic hydroxides, no significant differences were observed, leading to the conclusion that the type of hydroxide has no influence on the formation of defects in the crystal structure of ZnO. PL spectra (Figure 4) show significant visible emission due to abundant crystal defects giving thus prepared ZnO potential application in various areas.49 XRD diffractograms (Figure 4, Supporting Information) showed a characteristic diffraction pattern of hexagonal wurtzite structure of ZnO (JCPDS card no. 01-079-0205). The widths of diffraction maxima are very narrow, indicating large crystallite sizes. Crystallite sizes were calculated using the Scherrer equation, and the results are summarized in Table 1. The results show that in the presence of LiOH ZnO with larger crystallite size is formed, and it is independent of the precursor concentration compared to NaOH or KOH. The diffraction pattern, obtained by Selected area electron diffraction analysis during HRTEM microscopy, confirms that synthesized ZnO is polycrystalline (Figure 2d). Yields of reactions were highly dependent on the precursor concentration (Table 1). 3.4. Seeded Synthesis of ZnO NPs−Precursor Concentration. By reducing the precursor concentration, the particle size decreases, but also more intense particle aggregation is observed (Figure 5, Supporting Information). This may be a consequence of slower particle growth due to lower concentration of the precursor. FTIR spectra of ZnO NPs, synthesized at different precursor concentrations, are shown in Figure 6, Supporting Information. Besides the ZnO absorption band at 430 cm−1, there is also an intense absorption band of OH bond, corresponding to OH groups most probably located at the surface of ZnO particles, between 3200 and 3600 cm−1. The fact that OH band is more intense when smaller particles are formed (lower precursor concentration) confirms this assumption. XRD diffractogram patterns (Figure 7, Supporting Information) are characteristic for crystalline ZnO, while the widths of diffraction peaks indicate that by increasing the precursor concentration the crystallite size is also increased (Table 1). 3.5. Seeded Synthesis of ZnO NPs−Molar Ratio of Hydroxide to Precursor. By increasing the molar ratio of the hydroxide to precursor from 1:1 to 2:1 to 3:1 and to 5:1, particle size is significantly reduced in all three hydroxides used (Figure 8, Supporting Information, Table 2), while the crystallite size is not changed, which confirms previously reported results.25,26,28,39 It is reported that after exceeding a certain critical concentration of the hydroxide, particle growth is practically stopped.24,26,29 In our case, the particle size was significantly reduced when the hydroxide/precursor molar ratio was increased from 2/1 to 3/1 (Table 2), meaning that critical concentration of a hydroxide is between molar ratios 2/1 and 3/1. 3.6. Seeded Synthesis of ZnO NPs−Course of Reaction. The course of reaction and ZnO NPs formation were followed for a 0.1 M solution of Zn(Ac) and KOH by SEM microscopy and FTIR spectroscopy after 10, 20, 30, 45, 70, and 90 min. SEM micrographs show that particles were formed after 30 min of reaction and did not change significantly with an additional reaction time of 90 min (Figure 9, Supporting Information). FTIR spectra (Figure 10, Supporting Information) confirm that the formation of ZnO in seeded polyol process is completed in 30 min, which means that an

Table 2. ZnO Particle Size, Zeta Potential, and Isoelectric Point (pI) as a Function of the Hydroxide/Precursor Molar Ratioa hydroxide KOH

NaOH

LiOH

a

molar ratio hydroxide/precursor 1/1 2/1 3/1 5/1 1/1 2/1 3/1 5/1 1/1 2/1 3/1 5/1

average particle size (nm)

Zeta potential (mV)

pI

± ± ± ± ± ± ± ± ± ± ± ±

21.9 32.1 33.0 32.0 34.1 47.1 28.1 25.4 32.1 32.1 25.9 28.2

9.1 9.1 9.5 9.3 9.7 9.6 9.2 9.1 9.3 9.1 9.3 9.3

65.9 46.4 28.6 24.9 66.0 45.7 37.5 30.8 56.8 47.5 28.4 27.6

3.7 1.2 1.7 0.5 2.8 2.1 1.3 0.9 2.6 3.5 1.5 0.7

Precursor concentration = 1.0 mol/L.

optimal time, sufficient to finish the seeded hydrolysis of Zn(Ac) into ZnO in DEG, is between 30 and 60 min. 3.7. Seeded Synthesis of ZnO NPs−Molar Ratio of Seeds to Precursor. The influence of the molar ratio of ZnO seeds to Zn(Ac) precursor is also important regarding ZnO morphology and particle size. In most experiments, the ratio of seeds/Zn(Ac) was 1/10. However, the ratio was also increased to 1/1 and 10/1, while Zn(Ac) concentration was 0.1 M and NaOH or KOH were added in the molar ratio hydroxide/ precursor = 1/1. SEM micrographs (Figure 5) show that at a

Figure 5. SEM micrographs of ZnO particles synthesized at different seeds/precursor molar ratios: (a) 1/1 (NaOH), (b) 10/1 (NaOH), (c) 1/1 (KOH), (d) 10/1 (KOH), (conc. of Zn(Ac) = 0.1 M, molar ratio hydroxide/precursor = 1/1).

mole ratio of 1/1, spherical ZnO particles with broad particle size distribution (50−200 nm) were formed (Figure 5a,c), while when mole ratio was 10/1, well-defined spherical ZnO particles with narrow particle size distribution (80−140 nm) were obtained (Figure 5b,d). Therefore, the molar ratio of seeds/precursor significantly affects ZnO morphology and particle size distribution. Particle size and particle size distribution of ZnO NPs as a function of the reaction time and molar ratio of seeds to E



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Figure 6. ZnO particle size distribution, obtained by image analysis of STEM micrographs, as a function of molar ratio seeds/precursor and of the reaction time: (A) 1/1, (B) 3/1, (C) 10/1 (conc. of Zn(Ac) = 0.1 M, molar ratio hydroxide (NaOH)/precursor = 1/1).

Figure 7. EDS spectra of ZnO synthesized in the presence of hydroxides (conc. = 1 M): (a) NaOH, (b) KOH.

precursor were studied at a precursor concentration of 0.1 M (NaOH added) by image analysis of STEM micrographs (Figure 6 and Figure 11, Supporting Information) and by DLS (Figure 12, Supporting Information). Results showed that at a high ratio of seeds to precursor (10/1), the particle size was almost constant, and the width of particle size was narrow and constant for the first 40 min of reaction after first observation of turbidity. Because of a rather low concentration of precursor,

practically all particles were initially formed from seeds, while there was very little precursor for the particle growth or for the formation of new particles initiated from precursor. With a longer reaction time (75 min), particle size and particle size distribution increased most probably due to Ostwald ripening. By reducing the ratio of seeds to precursor to 3/1, particle size started to grow immediately at a high rate, giving also a wider size distribution because there was enough precursor for F



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intermediates can be observed by FTIR spectroscopy (Figure 10, Supporting Information). This is a major difference between the ZnO formation in polyols and ZnO synthesis in aqueous medium, where various solid reaction intermediates have been identified.51 Therefore, increasing the concentration of the inorganic hydroxide has an impact on the formation of ZnO in the initial stage and produces more nucleation centers (Zn2+ and OH− associates), consequently reducing the growth of ZnO particles.27 Nevertheless, the essential species in this process are OH− ions, while cations play a minor role.

particle growth. When seeds/precursor ratio was 1/1, immediate growth of ZnO NPs was observed, while particle size distribution was very broad (from 80 to 230 nm). High concentration of the precursor promotes particle growth and also the formation of new particles even at final stage of the reaction. These results indicate that starting ZnO NPs synthesis from seeds effectively reduces particle growth and Ostwald ripening only at a high seeds/precursor molar ratio (10/1), but the effect was limited to approximately 30−40 min of reaction, while later also in this case some particle growth occurs most probably due to Ostwald ripening. Nevertheless, by introducing the seeds at a high seeds/precursor molar ratio (10/1), particle size and the width of particle size distribution can be effectively reduced and controlled. 3.8. Seeded Synthesis of ZnO NPs−Mechanism of ZnO Particle Formation. By HRTEM−EDS analysis, we investigated the presence of Na+ or K+ ions on the ZnO surface. The resulting EDS spectra (Figure 7) do not confirm the presence of Na+ or K+ on the ZnO surface. This means that the quantity of cations on the ZnO surface is extremely low, even at high hydroxide concentration. Since this method is not highly sensitive, it is possible that a mono atomic layer is formed on the ZnO surface that cannot be detected by the EDS. An additional X-ray photoelectron spectroscopy (XPS) analysis of the ZnO surface confirmed the absence of Li+, Na+, and K+ ions, thus proving that the protective layer of cations is not responsible for the reduction of ZnO particle growth in DEG medium, and thus they do not act as capping agents. Concerning the explanation of the effects of adding a hydroxide on the particle morphology, and according to our knowledge, nobody has proved the existence of a protective cation layer by any of the analytical methods.25−29 In our case, the zeta potential on the surface of ZnO particles, dispersed in deionized water, was measured to determine the charge on the particle surface (Table 2). The results show that unmodified ZnO particles have a positively charged surface (28.1−47.1 mV), while isoelectric point (pI) is at a pH between 9.1 and 9.7 as observed also by other researchers.29,41 This means that Zn2+ ions predominate on the particle surface, and Na+, K+, or Li+ cations are not attached to the ZnO surface due to the repulsion force. Therefore, we conclude that the layer of cations is not present on the ZnO surface when diols are used as a medium. Zn2+ ions on the ZnO surface are coordinated with OH− anions and acetate anions as shown by the FTIR spectra (Figure 3a, 6, Supporting Information and 10, Supporting Information); however OH− ions prevail. Therefore, the ZnO surface is covered predominantly with OH− groups.9,30,31,36,37,51,52 Molecular modeling of the ZnO particle formation in ethanolic medium showed that during the initial stage of this process Zn2+ and OH− associates are formed from the solution, followed by their growth and accompanied by a proton transfer reaction resulting in the formation of H2O and O2− ion.52 Even at very low concentrations of O2− ions in these associates, their structures are reordered from octahedral into tetrahedral (wurtzite) structures.52 Although OH− anions are the major anion species on the aggregate surface, structures containing more than 150 ions show a core without any OH− groups, meaning that the core is composed exclusively of Zn2+ and O2− ions that together form zinc oxide.52 Therefore, ZnO is formed at a very early stage of the reaction. From a macroscopic point of view, this means that it is formed directly from the polyol solution of Zn 2+ ions, and no solid

4. CONCLUSIONS ZnO nanoparticles with particle sizes between 50 and 150 nm, as confirmed by DLS, were synthesized by seeded alcoholysis in DEG using Zn(Ac) as a precursor and inorganic hydroxide (NaOH, KOH, or LiOH). Seeded alcoholysis produced ZnO nanoparticles only in DEG, while in other diols it was not effective. Seeds are nanostructures with Rh between 80 and 100 nm as shown by DLS measurements. NMR spectroscopy of reaction medium showed that during the reaction DEG is acetylated, indicating that the reaction is an alcoholysis. The alcoholysis of Zn(Ac) in DEG proceeded to ZnO without solid intermediates, which was an important difference as compared to hydrothermal synthesis of ZnO. Decreasing the precursor concentration, ZnO particle size was significantly reduced; however, NPs agglomeration was more intense. By increasing the molar ratio hydroxide/precursor, the particle size is significantly reduced. Raman spectroscopy detected the presence of defects in the ZnO crystal structure which generate visible light emissions at 423, 448, 461, 485, and 529 nm, as shown by photoluminescence spectroscopy. The molar ratio of seeds/precursor significantly influences the morphology, particle size, and particle size distribution of ZnO NPs. At a high ratio (10/1), 100 nm sized ZnO NPs were obtained, and the growth of ZnO NPs was almost stopped indicating that all the particles originated from seeds and no new particles were formed, as well as that Ostwald ripening was prevented. At lower ratios (3/1 and 1/1), particle growth was observed at the initial part of the reaction and ZnO NPs with wider particle size distribution were formed. The presence of Na+, K+, and Li+ ions on the ZnO surface was not detected by HRTEM-EDS and XPS, confirming that ZnO NPs were without the protective layer of cations. Accordingly, the type and size of cation have a minor influence on the particle size and morphology. It is the OH− anion that is the key factor in the initial stage of ZnO particle formation because with increased concentration of OH− anions the number of nucleation centers also increases and thus reduces the size of ZnO particles.

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary figures (Figures 1−12, Supporting Information). This material is available free of charge via the Internet at: http://pubs.acs.org/.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support given by the Ministry of Education, Science and Sport of the Republic of Slovenia, through Research Programmes P2-0145 and P2030. The authors would also like to thank Ivo Jerman, Ph.D., and Jože Grdadolnik, Ph.D., of the National Institute of Chemistry for Raman spectroscopy measurements, Igor Djerdj, Ph.D., of the Rudjer Bošković Institute, Zagreb, Croatia, for HR TEM microscopy, Joži Zabret, B.Sc., and Janez Kecelj, B.Sc., of the Centre of Excellence PoliMaT, for the measurements of zeta potential and Alenka Vesel, Ph.D., and Miran Mozetič, Ph.D., of the Institute Jožef Stefan for XPS measurements.

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