RESEARCH PAPER

Synthesis of ZnO nanoparticles with tunable sizeand surface hydroxylation

Giang Van Ngo • Andre Margaillan •

Sylvie Villain • Christine Leroux •

Christine Bressy

Received: 10 July 2012 / Accepted: 20 November 2012 / Published online: 13 December 2012

� Springer Science+Business Media Dordrecht 2012

Abstract Zinc oxide (ZnO) is an important metal

oxide for hybrid inorganic–organic devices in which

the surface properties can dictate the overall charac-

teristics of the system. The particle size and the

amount of hydroxyl groups’ density at the surface are

key parameters to promote further bonding of organic

phase on metal oxide. The precipitation method was

used to successfully prepare ZnO nanoparticles at

room temperature with a wurtzite structure and a

controlled surface hydroxylation. Spherical nanopar-

ticles with diameters around 6–8 nm were synthesized

in ethanolic solutions whereas the addition of water in

the reaction mixture led to bigger particles within the

range of 20–50 nm together with a change in mor-

phology. The X-ray diffraction data revealed that a

high crystal quality of ZnO with hexagonal (wurtzite-

type) crystal structure could be obtained with increas-

ing the amount of water and the annealing tempera-

ture. Transmission electronic microscopy images

demonstrated the presence of two populations of

particles size synthesized in an ethanol/water reaction

mixture together with the presence of a zinc

dihydroxide amorphous layer surrounding the well-

crystallized grains in water solution. The amount of

physically and chemically adsorbed water on to ZnO

particles was determined through thermogravimetric

analysis. The surface hydroxylation of ZnO particles

and the hydrophilic character of the particles surface

were shown to be modulated by the solvent, the time,

and the annealing temperature of the precipitated

particles. In ethanol/water solutions, the use of a

reactive silane capping agent such a 3-(trimethoxysi-

lyl)propylmethacrylate was shown to limit the growth

of ZnO particles with diameters around 5 nm to switch

their wetting characteristics from a hydrophilic to a

hydrophobic surface.

Keywords Zinc oxide � ZnO � Precipitation method �Surface hydroxylation � TEM � Silane � XRD

Introduction

Zinc oxide (ZnO) is an important semiconductor with

associated properties such as good transparency, high

electron mobility, wide bandgap, and room tempera-

ture luminescence (Wang 2004). These properties

have already been used in emerging applications,

e.g., solar cells (Gao and Nagai 2006; Mawyin et al.

2011), gas sensors (Kim and Yong 2011; Li et al.

2010), varistors (Wu et al. 2002; Singhai et al. 1997),

catalysts (Curri et al. 2003; Kamat et al. 2002; Seung

and Yun 1997), electrical and optical devices

G. Van Ngo � A. Margaillan � C. Bressy (&)

Universite de Toulon, MAPIEM, EA 4323, 83957

La Garde, France

e-mail: christine.bressy@univ-tln.fr

S. Villain � C. Leroux

IM2NP, UMR 6242 CNRS–Universite de Toulon,

Campus de La Garde, Bat R, BP 20132,

83957 La Garde, France

123

J Nanopart Res (2013) 15:1332

DOI 10.1007/s11051-012-1332-4

(Feldmann 2003; Zheng et al. 2002; Wu and Xie

2004), transistors (Nomura et al. 2003), light-emitting

diodes (Bakin et al. 2007), and electrostatic dissipative

coatings (Kitano and Shiojiri 1997). In addition, ZnO

nanoparticles have received much attention due to a

variety of other applications such as antibacterial

treatment (Applerot et al. 2009) and biocidal pigments

in antifouling paints (Yebra et al. 2004, 2006; Singh

and Turner 2009). Particular technological interest is

the insertion of nanosized ZnO powders in organic

materials such as plastics or rubbers (Begum et al.

2008). Various synthetic ways were reported to

synthesize ZnO nanoparticles such as laser-ablation

(Scarisoreanu et al. 2005), spray pyrolysis (Tani et al.

2002), hydrothermal method (Søndergaard et al. 2011;

Ni et al. 2005), sol–gel method (Ivanova et al. 2010;

Znaidi et al. 2003a; Znaidi et al. 2003b), vapor

condensation method (Haldar et al. 2010), common

thermal evaporation method (Takahashi et al. 2000;

Zhou and Li 2005), and precipitation method (Zhou

and Li 2005; Wu et al. 2007; Briseno et al. 2010; Zhou

et al. 2007; Liewhiran et al. 2006). In recent years, the

precipitation method have been used for the synthesis

of ZnO with a wurtzite structure, and a wide range of

particle sizes and morphologies such as nanowires

(Zhou and Li 2005), nanorods (Zhou and Li 2005;

Briseno et al. 2010), and spherical nanoparticles (Zhou

and Li 2005; Wu et al. 2007; Briseno et al. 2010; Zhou

et al. 2007; Liewhiran et al. 2006). The particle

formation including nucleation and growth steps and

the particle size and morphology depend on several

parameters such as: (1) the nature of the precursor and

its concentration (Zhang and Li 2003), (2) the type of

solvent (Hu et al. 2003, 2005) and the acidity/basicity

of the mixture (Demir et al. 2006, Viswanatha et al.

2007a), (3) the type of stabilizers or capping agent and

their concentrations (Guo et al. 2000), (4) the aging

time and temperature of the mixture (Viswanatha et al.

2007b), and (5) the annealing temperature of the

precipitated particles (Noack and Eychmuller 2002).

Among all the studies focused on the synthesis of ZnO

nano-objects, few of them investigated the impact of

the above parameters on the surface hydroxylation

(Asakuma et al. 2003; Traeger and Kauer 2011). The

surface of all metal and metalloid oxides is known to

be covered by varying degrees of hydroxyl groups or

ions, which play an important role in the adsorption

processes occurring at the oxide surface (Armistead

et al. 1969). In this context, nanometer inorganic

particles easily agglomerate because of their high

surface energy. On the other hand, it is difficult to

achieve homogeneous dispersion of nanoparticles in

polymer matrix. Surface modification of an inorganic

particle with an organic substance is a useful way to

reduce its surface energy, to increase its compatibility

with polymer matrixes and its dispersion, and thus to

improve the properties of the polymer/inorganic

particles nanocomposites (Posthumus et al. 2004;

Bourgeat-Lami 2004). The formation of an organic

layer on to metal oxide surfaces involves adsorption or

covalent bonding from hydroxyl groups present on the

particles surface. Organosilanes bearing a vinyl group

could be also used to modify the surface properties and

further initiate the grafting of polymer chains from the

surface (Bourgeat-Lami 2004). To enhance the graft-

ing efficiency of such organic molecules or macro-

molecules on metal oxide particles, high amounts of

hydroxyl groups on to the surface are needed (Allen

et al. 2008). A large number of papers reported the

methods used for the determination of the amount of

hydroxyl groups on to the surface of oxides such as

infrared spectroscopy (Fripiat and Uytterhoeven

1962), chemical methods (Mueller et al. (2003);

Gilpin et al. 1997) and TGA (Kellum and Smith

1967; Nagao 1971; Costa et al. 1997; Mueller et al.

2003; Jal et al. 2004). Most of the commercially

available metal oxide particles were usually annealed

at temperatures higher than 500� C to remove any

volatile compounds and stabilizers coming from the

synthetic process. Thus, low concentrations of hydro-

xyl groups were found on the particle surface (Noack

and Eychmuller 2002).

In this context, we report a simple method to

produce ZnO particles in the nanometer scale with a

large range of hydroxyl groups concentration on to the

particle surface. The synthetic pathway was based on

the transformation of precipitated zinc hydroxide to

ZnO particles under mild conditions, i.e., at room or

low temperature and under atmospheric pressure. The

effects of several parameters on the particles size and

morphology and on the concentration of hydroxyl

groups on to the surface of particles were investigated

in detail. The solvent of reaction, the aging time, the

annealing temperature were considered. A new route

to synthesize ZnO nanoparticles in using MPS as

reactive capping agent was also reported. XRD and

TEM were used to study the crystal structure and

morphology together with the distribution of particles

Page 2 of 15 J Nanopart Res (2013) 15:1332

123

size. The amount of physically and chemically

adsorbed water was determined through TGA. Both

the related concentration of hydroxyl groups and the

wetting properties of the ZnO surface was intended to

be modulated.

Experimental

Materials

Zinc acetate dihydrate (98 %) and powered NaOH

(85 % min) were received from Acros company.

Commercially, zinc oxide nanopowder (ZnO-ref) with

a density of 5.67 g/cm3, a median particles size from 50

to 70 nm and a surface area from 15 to 25 m2/g) and

MPS (98 % purity, 1.045 g/mL) were received from

Sigma-Aldrich. Absolute ethanol was analytical grade

(99 % purity) and purchased from Sigma-Aldrich.

Synthesis of ZnO nanoparticles

ZnO nanoparticles were synthesized via the basic

hydrolysis of zinc acetate dihydrate by sodium

hydroxide. The reaction can be written as depicted in

Scheme 1.

In a typical procedure, Zn(CH3COO)2.2H2O

(74 mmoL, 16.24 g) was dissolved in a beaker with

1 L absolute ethanol (E) or deionized water (W) under

magnetic stirring at room temperature for 1 h. At the

same time, powdered NaOH 85 % min (148 mmoL,

5.92 g) was dissolved in another beaker with 1.48 L of

absolute ethanol or deionized water and stirred for

30 min. Then, this basic solution was dropped into the

zinc acetate solution for 20 min. The mixture was

stirred slowly for 4 h at room temperature. Two flasks

were prepared in the same way but with different aging

times at room temperature, 1 day (24 h, 1d) and

3 days (72 h, 3d), respectively. During the aging step,

the solution was not stirred. Separated ZnO nanopar-

ticles can be obtained by centrifugation with a rotation

of 6,000 rpm. The solid product was washed three

times with ethanol and deionized water, and treated

separately under vacuum at different drying temper-

atures (25 and 120 �C) for 6 h to remove the volatile

compounds. The reaction yield of the synthesis of ZnO

from zinc acetate dihydrate was higher than 70 % for

all samples. In addition, small amounts of residual

organic compounds were found on dried powders. The

elemental analysis revealed the presence of impurities

with a percentage of carbon atoms less than 0.15 %.

Considering zinc acetate as the main residual com-

pound, a maximum value of 0.65 wt% of impurities

was estimated. Detailed conditions are presented in

Table 1. The ZnO samples were named indicating the

solvent used for the Zn(CH3COO)2.2H2O solution

(E for ethanol and W for water), the solvent used for

the NaOH solution (E for ethanol and W for water), the

aging time at 25 �C (1 or 3d) and the drying

temperature (25 or 120 �C), respectively. ZnO-E/E-

3d-120/T200, ZnO-E/E-3d-120/T400, ZnO-E/E-3d-

120/T600, and ZnO-E/E-3d-120/T800 were prepared

from the ZnO-E/E-3d-120 sample annealed at 200,

400, 600, and 800 �C under atmospheric pressure for

3 h, respectively. Two ZnO samples were prepared

using MPS as capping agent. This liquid compound

MPS (10.2 mmoL, 2.52 g) was added after 4 h of slow

stirring at room temperature and the reaction mixture

was aged for 1 and 3 days. The samples are named

ZnO-E/W-1d-MPS and ZnO-E/W-3d-MPS, respec-

tively. In addition, the sample ZnO-E/W-1d-MPS was

further pyrolyzed under air at 600 �C for 3 h to

remove the organic part of the capping agent.

Characterization of ZnO nanoparticles

X-ray diffraction patterns were recorded on a Sie-

mens-Bruker D5000 equipment working in a classical

h–2h angles coupled mode, with copper X-ray source

(k = 0.15406 nm), soller slits, a secondary mono-

chromator and a rotating sample holder. Particle sizes

and intensity values of peaks were estimated from a

Lorentzian fit of X-ray diffraction intensity profiles

using Origin� as software. ZnO particles diameter D

was assessed using the Debye–Scherrer formula as

follows (Eq. 1):

D ¼ K � kb � cosh

ð1Þ

Where K is the Scherrer constant, K = 0.94 for

Lorentzian peaks, k is the X-rays’ wavelength

+ 2NaOH Zn(OH)2Zn(CH3COO)2 2CH3COONa+

Zn(OH)2 ZnO + H2O

Scheme 1 Synthesis of ZnO nanoparticles through the precip-

itation method

J Nanopart Res (2013) 15:1332 Page 3 of 15

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(k = 0.154056 nm), b is integrated breadth of peaks

corrected from the instrumental broadening, and h is the

Bragg diffraction angle. TEM images were taken using

a Tecnai G2, operated at 200 kV (k = 0.251 pm), with

a point-to-point resolution of 0.25 nm. The powders

were dispersed in ethanol, and a few drops were put on a

holey carbon grid and dried by evaporation in air.

Images were taken from particles suspended over holes,

to avoid any misinterpretation with the amorphous

carbon film of the grid. Energy dispersive spectroscopy

(EDS) was performed, using commercial ZnO as a

standard.

Fourier transform infra-red spectroscopic (FTIR)

measurements were realized on a Thermo-Nicolet-

Nexus spectrometer. The samples were prepared in

KBr pellets with a weight content of 1 %. The

characterizations of physically and chemically

adsorbed water were performed on a TGA–DSC

instruments-Q600 from TA Instrument. The samples

were heated under nitrogen (N2) at a rate of 10 �C/min,

from 30 to 800 �C. The ZnO particles exhibit two

distinct decomposition zones during the heating

procedure. The first step removes the physically

adsorbed water (100–180 �C) (Nagao 1971). The

second step represents the weight loss associated

to the removal of chemically adsorbed water

(180–550 �C). The hydroxyl group concentration

[OH] (mmoL/g ZnO) was assessed according to

Eq. 2 assuming a complete dehydroxylation in the

second step (Mueller et al. 2003):

OH½ � ¼ 2:103

MH2O

� Weight loss ð%Þ180�550�C

ð100� Weight loss ð%Þ100�180�C

� �ð2Þ

Where MH2O is the molar mass of water (g/moL). The

hydroxyl group concentration [OH] (mmoL/gZnO) of

ZnO powders was calculated using the TGA weight

loss in step 2 of a physically water-desorbed sample.

The weight percentages of carbon atoms were

assessed by elemental analysis performed at Vernai-

son, CNRS laboratory, France. The wetting behavior

of the nanoparticle surface was evaluated by measur-

ing the water contact angle using a contact angle meter

DIGIDROP from GBX Instrument. ZnO powders

were firstly pressed into compact pellets with a

pressure of about 300 bars. The volume of the

deionized water droplet was fixed as 1.0 lL. Five

measurements were done per sample.

Results and discussion

Structural and morphological characterizations

In order to compare the morphology and structure of the

various synthezised ZnO powders, we characterized

first a commercial ZnO powder. The grains were

crystallized in the wurtzite structure and the relative

intensities of the various peaks in the X-ray diffraction

Table 1 Experimental

conditions for the synthesis of

ZnO nanoparticles: solvent of

reaction, aging time and drying

temperature values

Sample Solvent of reaction Aging

time at

25 �C (day)

Drying

temperature

for 6 h (�C)

NaOH solution Zn(CH3COO)2�2H2O solution

ZnO-E/E-1d-120 Ethanol Ethanol 1 120

ZnO-E/E-3d-25 Ethanol Ethanol 3 25

ZnO-E/E-3d-120 Ethanol Ethanol 3 120

ZnO-E/W-1d-25 Water Ethanol 1 25

ZnO-E/W-1d-120 Water Ethanol 1 120

ZnO-E/W-3d-25 Water Ethanol 3 25

ZnO-E/W-3d-120 Water Ethanol 3 120

ZnO-W/W-3d-25 Water Water 3 25

ZnO-W/W-3d-120 Water Water 3 120

ZnO-E/W-1d-MPS Water Ethanol 1 25

ZnO-E/W-3d-MPS Water Ethanol 3 25

Page 4 of 15 J Nanopart Res (2013) 15:1332

123

pattern (Fig. 1a) corresponded to the expected ones for

a non-textured sample (JCDDS n�= 36–1451,

a = 3.249 A, c = 5.206 A). The shape and the size

of the commercial ZnO nanoparticles are heteroge-

neous (Fig. 1b), with a mean size around 30 nm (see

Table 2). This sample was used as a standard for the

relative oxygen composition of some ZnO powders. All

the synthesized ZnO powders were crystallized in the

wurtzite structure, whatever the starting solution, the

aging time or the drying temperature (see X-ray

diffraction pattern Figs. 2, 3, and 4). The products

consisted of pure ZnO phase and no characteristic peaks

were observed for other impurities such as Zn(OH)2

(Zhou et al. 2007) or zinc acetate (Wang et al. 2005).

However, the XRD patterns of the synthesized ZnO

particles exhibited difference in relative intensities of

some peaks and in the broadness of the peaks,

depending on the synthesis conditions. The most intense

peak of the XRD patterns was always the (101) peak,

and was taken as I0. For some samples, the intensity

ratio I002/I100, are not in agreement with the theoretical

value for randomly oriented ZnO particles, which is

I002/I100 = 0.75.

Depending on the solvent, and on the subsequent

thermal treatment, the I002/I100 ratio in XRD patterns

varied significantly (see Table 2). For particles syn-

thesized in ethanol/water mixtures, this phenomenon

diminishes significantly with increasing both the aging

time and the drying temperature (Fig. 2a, b). The

powder synthesized with ethanol (ZnO-E/E-3d-120)

has a high I002/I100 ratio of 1.19, but a thermal

treatment leads to a decrease of I002/I100, up to 0.7 for

powders annealed at high temperature (ZnO-E/E-3d-

120/T600 and ZnO-E/E-3d-120/T800). Let us remind

that the theoretical value for this ratio is 0.75, in the

case of randomly oriented ZnO spherical particles. For

powders synthesized with water as solvent, the ratio

I002/I100 is close to the value corresponding to

randomly oriented particles. The ZnO wurtzite struc-

ture is already obtained at 25 �C after 1 day, and

further aging (results not shown) or a higher drying

temperature does not influence the crystallinity of the

powders (Fig. 3a, b).

Variations in I002/I100 ratio for ZnO powders

were interpreted in the literature as a shape effect

20 25 30 35 40 45 50 55 60 65

Inte

nsit

y (a

rb. U

nits

)

(002)

(101)

(102)(110)

(a)

(100)

(103)

(b)

Fig. 1 Commercially available ZnO powders (ZnO-ref)

a X-ray diffraction patterns, b TEM image, showing grains

with irregular shape and size

Table 2 Particle sizes of ZnO nanopowders obtained from

XRD analyses, along with integrated intensities of the (100)

and (002) peaks

Sample Particle size

(from XRD

analysis) (nm)

I002/I100

ZnO-ref 27(3) 0.68

ZnO-E/E-1d-120 6(1) 0.79

ZnO-E/E-3d-25 6(1) 0.82

ZnO-E/E-3d-120 8(1) 1.19

ZnO-E/E-3d-120/T200 15(2) 1.03

ZnO-E/E-3d-120/T400 16(2) 0.96

ZnO-E/E-3d-120/T600 34(4) 0.72

ZnO-E/E-3d-120/T800 43(4) 0.69

ZnO-E/W-1d-25 24(3) 1

ZnO-E/W-1d-120 24(3) 0.95

ZnO-E/W-3d-25 24(3) 0.98

ZnO-E/W-3d-120 26(3) 0.88

ZnO-W/W-3d-25 42(4) 0.71

ZnO-W/W-3d-120 47(5) 0.68

The error into brackets were estimated, according to Balzar

et al. 2004

J Nanopart Res (2013) 15:1332 Page 5 of 15

123

(Li et al. 2008), or as a faulted wurtzite structure

(Snedeker et al. 2005). TEM observations showed no

particular shape for the ZnO particles. Furthermore,

the disappearance of this phenomenon with thermal

treatment accounts for a metastable faulted structure.

Theoretical diffraction patterns were calculated using

the CaRIne software. I002/I100 as high a 1.1 could be

obtained by varying the oxygen positional parameter

up to 0.4. This value, associated to the same intensity

repartition of diffraction peaks, was already observed

and explained by a faulted wurtzite structure, with

irregular stacking of wurtzite and zinc blende (Snede-

ker et al. 2005). Thus, one can conclude that when

using ethanol as solvent for the zinc acetate, the ZnO

particles adopt a faulted wurtzite structure.

Figure 4 shows two diffraction patterns acquired

under the same conditions. The peaks corresponding

to the powder aged 3 days are much more intense than

those due to the powder aged 1 day, showing that the

crystallinity of the powder synthesized in ethanolic

solutions increases with increasing aging time.

In addition, peaks were narrower indicating an

increase of the grain size with aging time.

TEM images show that homogenous spherical

nanoparticles were obtained in ethanolic solutions

after 1 and 3 days of aging time and dried at 120 �C

(Fig. 5). The particles shown in Fig. 5 exhibit sizes

around 10 to 15 nm.

For an ethanol/water solution, two populations of

particles were obtained, as can be seen in Fig. 6a

with quasi-spherical nanoparticles around 30–40 nm

and much bigger grains around 200 nm, with both

crystallized in the wurtzite structure (see Fig. 6b).

The biggest particles exhibited a platelet-based mor-

phology.

Using only water as solvent leads to monocrystal-

line platelets with size around 100 nm (Fig. 7a). The

shape of these platelet crystals varied from cubic to

hexagonal. These platelets are more or less embedded

in an amorphous phase (see diffraction pattern,

Fig. 7b). EDS analyses in this amorphous layer, using

20 25 30 35 40 45 50 55 60 65

Inte

nsit

y (a

rb. U

nits

)

(a)

20 25 30 35 40 45 50 55 60 65

Inte

nsit

y (a

rb. U

nits

)

(b)

Fig. 2 X-ray diffraction patterns of ZnO nanopowders synthe-

sized with ethanol for zinc acetate dihydrate and water for

NaOH. a Aged 1 day and dried at 25 �C (ZnO-E/W-1d-25),

b Aged 3 days and dried at 120 �C (ZnO-E/W-3d-120)

20 25 30 35 40 45 50 55 60 65

Inte

nsit

y (a

rb. U

nits

)

(a)

20 25 30 35 40 45 50 55 60 65

Inte

nsit

y (a

rb. U

nits

)

(b)

Fig. 3 X-ray diffraction patterns of ZnO powders synthesized

with water as solvent for both solutions a Aged 3 days and dried

at 25 �C (ZnO-W/W-3d-25), b Aged 3 days and dried at 120 �C

(ZnO-W/W-3d-120). The diffraction patterns have the same

intensity scale

Page 6 of 15 J Nanopart Res (2013) 15:1332

123

commercial ZnO as standard, showed that the relative

zinc and oxygen content corresponds to zinc hydrox-

ide Zn(OH)2.

Table 2 shows the size of the particles assessed

from XRD results. The mean diameter value for

homogeneous ZnO particles synthesized in ethanolic

solutions was less than 10 nm, and corresponds to

spherical nanoparticles observed by TEM. The mean

diameter values of the particles prepared in ethanol/

water solutions were around 24 nm. This differs from

TEM data where two particle size populations were

found. Particle sizes from XRD analysis matched with

the smallest particles observed by TEM. These results

revealed that ZnO particles synthesized in ethanol/

water solutions were composed of higher amounts of

smaller particles. Sizes ranging from 70 to 100 nm

were observed by TEM for platelet-type powders

synthesized in water solutions, the mean value

obtained from XRD being smaller, around 50 nm.

Therefore, both the size and the morphology seem

to be affected by the solvent of reaction. In our case, no

nanorods or nanowires were formed. It is worth noting

that this result is in agreement with the fact that a

relatively low zinc acetate dihydrate concentration

around 0.03 M was used. Pacholski et al. (2002)

reported that ZnO nanorods were mainly formed at a

zinc acetate dihydrate concentration around 0.1 M. In

addition, water considerably accelerates the growth

of the colloidal ZnO particles as the solubility of

precursor salts is favored (Meulenkamp 1998).

The growth of the nanocrystal is controlled both by

the diffusion of Zn2? ions and the rate at which the

reactions take place at the surface. The difference in

solubility leads to differences in the mass transport and

the surface equilibrium of addition and removal of

Zn2? (Viswanatha et al. 2007b). In other way, the

genesis of particles by the formation of tiny crystalline

nuclei is enhanced in non-aqueous solutions leading to

a higher number of nuclei and smaller particles.

Fig. 4 X-ray diffraction patterns of ZnO powders synthesized

with ethanol solvent. (a) Aged 1 day (ZnO-E/E-1d-120) and

(b) aged 3 days (ZnO-E/E-3d-120). Drying temperature = 120 �C

Fig. 5 TEM images of ZnO nanoparticles synthesized with

ethanol solvent (aged 3 days and dried at 120 �C (ZnO-E/E-3d-

120)) a Low magnification image b HREM image, showing the

well-crystallized particles

J Nanopart Res (2013) 15:1332 Page 7 of 15

123

The size of the particles was shown to increase with

increasing annealing temperature (Noack and Eychmul-

ler 2002). The size of ZnO particles synthesized in

ethanol/water solutions doubled above 400 �C (Fig. 8).

According to Ostwald ripening, the increase in the

particle size is due to the transfer of materials from the

smaller particles to the larger once as a result of potential

energy difference between small and large particles. In

addition, the crystal structure evolved from a faulted

wurtzite structure to a perfect one with the awaited

intensity values of (100) and (002) peaks (Table 2).

Concentration of hydroxyl groups onto the surface

of ZnO nanoparticles

Most metal oxides chemisorb water molecules to form

surface hydroxyl groups on which further water

Fig. 6 TEM images of ZnO nanoparticles synthesized with

ethanol/water solvent (aged 3 days and dried at 120 �C, ZnO-E/

W-3d-120) a Low magnification image b HREM image,

showing the well-crystallized particles

Fig. 7 TEM images and electron diffraction of ZnO synthe-

sized with water as solvent and aged 3 days, drying temperature

25 �C (ZnO-W/W-3d-25). a TEM image showing a platelet-like

grain, covered by an amorphous phase. b Electron diffraction

pattern of a corresponding to a [010] zone axis of the wurtzite

structure

Page 8 of 15 J Nanopart Res (2013) 15:1332

123

molecules are adsorbed physically through hydrogen

bonding. The temperature, at which the removal of the

chemisorbed water begins, depends on the nature of

the oxide: e.g., 200 �C in the case of ZnO and about

100 �C in the case of TiO2 (Nagao 1971). The amount

of the physisorbed and chemisorbed water released

from ZnO nanoparticles was measured from TGA

data, considering the mass losses found within tem-

perature ranges from 100–180 �C and 180–550 �C,

respectively (Fig. 9). Table 3 shows that small

amounts of physisorbed and chemisorbed water were

found on the surface of the commercially available

ZnO sample, as it was previously annealed at high

temperature (Noack and Eychmuller 2002; Nagao

1971). The amount of physisorbed and chemisorbed

water was higher for all ZnO powders synthesized

than ZnO-ref. The amount of physisorbed water is

being more affected by the drying temperature when

water was used as solvent. In addition, the amount of

chemisorbed water content decreased with increasing

aging time from 1 day to 3 days and drying temper-

ature from 25 to 120 �C, whatever the solvent used.

The content of hydroxyl groups available onto the

surface of particles was calculated from the removal of

the chemisorbed water molecules using Eq. 2. The

values varied from 3 to 14 mmoL/g. Higher amounts

were obtained for 3 days of aging in ethanol/water

solutions. Assuming that a hydroxyl group is formed

on a surface zinc atom, crystallographically the

amount of chemisorbed water on the (100) plane

gives 5.93 H2O molecules/100 A2, i.e., 11.86 OH/nm2

(Nagao et al. 1978). In addition, the number of

hydroxyl groups was reported about 7.5 OH/nm2 in

close-packed surface hydroxyl groups bonded to the

ZnO surface (Nagao, 1971). The number of hydroxyl

groups per nm2 was estimated around 14(3) and 14(4)

for ZnO-E/E-1d-120 and ZnO-E/E-3d-120 samples,

which exhibited spherical nanoparticles with a specific

surface of 180(10) m2/g and 130(20) m2/g, respec-

tively. Thus, the calculated concentrations of chemi-

sorbed water were higher than those expected for a

fully hydroxylated ZnO surface. The weight loss in the

180–550 �C region is mainly due to loss of water

molecules, which are produced because of condensa-

tion of hydroxyl groups. However, within this tem-

perature range, the presence of water in some first

layer water hydrogen bonded to the hydroxyl groups

could not be ruled out (Jal et al. 2004). Mueller et al.

(2003) demonstrated that the thermogravimetric anal-

ysis was a simple and fast determination method of

OH surface density of silica and titania nanoparticles

using a calibration factor estimated by comparing

TGA and titration methods. A calibration factor of

0.62 value was calculated for nanostructured silica and

was used and validated for titania powders. Consid-

ering this calibration factor, the present concentration

of hydroxyl groups’ values varied from 2 to 9 mmoL/g.

Thus, the number of hydroxyl groups per nm2 could be

estimated around 9(2) and 9(3) for ZnO-E/E-1d-120

and ZnO-E/E-3d-120 samples.

Figure 10 shows that both the physisorbed and

chemisorbed water significantly decreased with

increasing the annealing temperature up to 400 �C.

The water content was thus similar to the reference one

for an annealing temperature above 400 �C. During

this treatment, any residual hydroxyls groups or

0

5

10

15

20

25

30

35

40

45

50

0 200 400 600 800

Annealing temperature (°C)

Par

ticl

e si

ze (

nm)

Fig. 8 Evolution of particle sizes (from XRD results) from

ZnO-E/E-3d-120 sample annealed at 200 �C (ZnO-E/E-3d-120/

T200), 400 �C (ZnO-E/E-3d-120/T400), 600 �C (ZnO-E/E-3d-

120/T600) and 800 �C (ZnO-E/E-3d-120/T800)

88

92

96

100

50 150 250 350 450 550

Temperature (°C)

Wei

ght

loss

(%

)

0.00

0.05

0.10

0.15

Der

iv. W

eigh

t (%

/°C

)

Fig. 9 TG (—–) and DTG (– –) diagram of ZnO-E/W-3d-120

at heating rate of 10 �C/min in inert atmosphere

J Nanopart Res (2013) 15:1332 Page 9 of 15

123

chemisorbed water were eliminated increasing the

particle size as was previously mentioned (Noack and

Eychmuller 2002).

The presence of hydroxyl groups on to the surface

could be correlated with the hydrophilic character of

the metal oxide surface. Contact angle measurement

(surface wettability) of the ZnO surface was used in

this work to evaluate or estimate how the surface could

be wet by water. To avoid deviation coming from

additional water adsorption during the storage, some

samples were dried at 120 �C for 12 h before evalu-

ating the water contact angle. No difference was

obtained before and after the drying step. The decrease

of the water contact angle values with increasing the

hydrophilic character of the ZnO surface was depicted

in Fig. 11. The static water contact angle of the

synthesized nanoparticles ranged from 17� to 21�.

Previous investigations reported also water con-

tact angle values ranging from 0� up to 35�

(Irzh et al. 2010; Guo et al. 2007; Ambade et al.

2009; Tang et al. 2007; Tang et al. 2008; Zhang et al.

2006). A higher value around 27� was obtained with

the commercial ZnO sample, which contained a lower

amount of hydroxyl groups onto the particle surface

than the synthesized ones. In addition, the water

droplets were imbibed in the pellets for longer droplet

contact time ([1 s) due to the presence of micron-

sized voids in ZnO pellets, which enables adsorption

of the liquid droplet completely within the void space.

Synthesis of ZnO nanoparticles using a silane

capping agent

3-(trimethoxysilyl)propylmethacrylate (MPS) was

used to control the growth of particles by reacting

with the zinc dihydroxyde intermediate reactant or

the hydroxyl groups available on the surface of

nanoparticles through hydrolysis of its alkoxy

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 200 400 600 800

Annealing temperature (°C)

Wat

er (

mm

oL/g

)

Fig. 10 Evolution of the concentration of water sorbed onto

ZnO particles with temperature. (filled triangle) Physisorbed,

(filled diamond) Chemisorbed, (filled square) Total water (from

25 to 800 �C)

Table 3 Water adsorption on

ZnO nanoparticle samples—

values of the physisorbed and

chemisorbed water (mmoL/g)

Sample Weight loss (%)

(100–180 �C)

Weight loss (%)

(180–550 �C)

H2O

physisorbed&nbsp

(mmoL/g)

H2O

chemisorbed

(mmoL/g)

ZnO-ref 0.050(2) 0.22(1) 0.030(1) 0.12(6)

ZnO-E/E-1d-120 0.9(1) 3.7(2) 0.52(3) 2.0(1)

ZnO-E/E-3d-120 0.9(1) 2.7(2) 0.53(3) 1.5(1)

ZnO-E/W-1d-25 3.6(2) 3.7(2) 2.0(1) 2.0(1)

ZnO-E/W-1d-120 0.84(4) 4.7(3) 0.47(2) 2.6(1)

ZnO-E/W-3d-25 0.90(5) 11.2(6) 0.50(3) 6.2(3)

ZnO-E/W-3d-120 1.4(1) 9.5(5) 0.76(4) 5.3(3)

ZnO-W/W-3d-25 3.5(2) 4.1(2) 1.9(1) 2.3(1)

ZnO-W/W-3d-120 0.90(5) 3.5(2) 0.5(2) 2.0(1)

15.0

17.0

19.0

21.0

23.0

25.0

27.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Total water content (mmoL/g)

Con

tact

ang

le (

°)

Fig. 11 Evolution of water contact angles onto ZnO nanopar-

ticles with sorbed water

Page 10 of 15 J Nanopart Res (2013) 15:1332

123

groups (Kotecha et al. 2006; Scheme 2). Up to now,

capping agents such as 3-aminopropyl trimethoxysi-

lane, tetraethyl orthosilicate, and mercaptosuccinic

acid were reported to be added at the first ZnO

precipitation time to limit the particle growth with

size ranging from 10 to 30 nm (Guo et al. 2000).

Here, MPS was used as a reactive coupling agent,

bearing a vinyl group able to further radical poly-

merized with vinylic monomers to prepare nano-

composites. An ethanol/water reaction mixture was

used to enable faster hydrolysis and condensation

reactions in contrast to the use of ethanolic solutions

(Pantoja et al. 2011). TGA thermograms (not shown)

showed a mass loss assigned to the organic part

of the absorbed MPS molecules of 32 and 41 wt%

for ZnO-E/W-1d-MPS and ZnO-E/W-3d-MPS,

respectively.

Figure 12a shows the X-ray diffractogram of ZnO

samples synthesized with MPS. Broad peaks related to

the amorphous phase of the MPS condensate coming

from concomitant reactions between the self-reaction

of hydrolyzed methoxy silane functions of MPS in

basic solutions and the reaction of such hydrolyzed

groups with hydroxyl groups onto the surface of

nanoparticles (Ngo et al. 2009). Figure 12b reveals

that wurtzite ZnO was indeed produced by pyrolysis

the MPS-ZnO nanoparticles at 600 �C. The addition of

MPS at the early stage of the precipitation of ZnO

particles thus enabled the formation of nanoparticles

with sizes around 5 nm even with an ethanol/water

reaction mixture.

In addition to the control of the growth of the

particles, the capping agent provided high water

contact angle values which indicated a change in

ZnO surface wettability (Fig. 13). The ZnO surface

became more hydrophobic with an increase of the

water contact angle up to 50�. The contact angle for

water on ZnO-E/E-1d-120 was about 20.5(6)�, which

is much smaller than the value of 75(2)� for ZnO-E/W-

1d-MPS and 83(4)� for ZnO-E/W-3d-MPS. Further-

more, the drop shape formed on the hybrid pellets was

stable with contact time.

+H2O

- CH3OH

R = CH2 O CO

C CH23CH3

SiR

OCH3H3COOCH3

SiR

OHHOOH

OH

OH

HO OH

HO

HO OHOH

+

O

OH

O OH

HOO OH

OSiR

O

O Si OR

SiO

O

R Si OR

SiR

OHHOOH

+H2O SiR

- CH3OH

Si

Si Si- H2O

SiR

OCH3H3COOCH3

R

R

OO

O

O

O

OO

RO

Scheme 2 Possible parallel

and concomitant reactions

of trimethoxysilane in the

presence of ZnO particles

J Nanopart Res (2013) 15:1332 Page 11 of 15

123

Conclusion

In summary, we have demonstrated that the precipita-

tion method is a simple route to prepare ZnO nanopar-

ticles with a wurtzite structure at room temperature

together with a fully hydroxylated surface. The ratio of

ethanol to water was shown to play an important role in

the formation of ZnO nanoparticles. The smallest ZnO

nanoparticles with size around 6 nm were obtained in an

ethanolic solution with a number of hydroxyl groups

around 9(2) OH/nm2. Biggest ZnO platelets with around

50 nm length were obtained in water solution. The

amount of hydroxyl groups onto the surface and the

hydrophilic character of the surface of ZnO particles

were shown to be also modulated by the type of solvent

and the annealing temperature. In ethanol/water solu-

tions, the MPS capping agent was able to control the

particle size of ZnO powders by limiting the growth of

particles after the nucleation step. It was also found that

the ZnO surfaces can be also switched from a hydro-

philic surface to a hydrophobic one through MPS

modification. This way of capping agent-mediated ZnO

synthesis could be a promising route for further grafting

polymer chains in hybrid nanocomposites.

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Inte

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