袁慧敏，吕林林，李亚萍，钱 东*

(1a.中南大学化学化工学院，1b.中南大学粉末冶金国家重点实验室，中国长沙 410083)

As one of the most important semiconductors，ZnO nanoparticles have been intensively studied during the past few years due to its wide bandgap of 3.37 eV，unique electrical and optoelectronic properties，and excellent chemical and thermal stability，which could lead to a broad range of technologically important applications.Therefore，the preparation of ZnO nanoparticles is attracting much attention，and many techniques have been developed such as sol-gel synthesis［1-3］，hydro/solvo-thermal synthesis［4］，hydrolysis［5］，chemical precipitation［6］，equilibrium gas expanding method［7］，microemulsion mediated synthesis［8］，and mechanochemical processing［9］.The sol-gel technology is of great scientific interest because of its advantages in terms of cleaner particles，better homogeneity and stoichiometry，finer particle size，and lower calcination temperature.In 1991，Spanhel and Anderson［1］proposed a convenient sol-gel procedure for preparing highly confined ZnO nanocrystals，which has encouraged many groups［10-11］to conduct further studies on the ZnO nanocrystals.

The properties of ZnO nanocrystals are highly dependent on their sizes and agglomeration situations.The synthesis of ZnO nanocrystals with controllable sizes and less agglomerations is still one of the most challenging and urgent topics.

It is well known that a facile precipitation method is an operative way to synthesize ZnO nanocrystals with different sizes.Hoyer et al［11］precipitated ZnO nanoparticles through the addition of water to the boiling colloidal ZnO solution.Meulenkamp［2］achieved this by using alkanes，e.g.，heptane，to precipitate ZnO nanoparticles.In our previous paper［12］，we found that sonicating the colloidal ZnO solution could also precipitate ZnO nanoparticles without using any solvents.Inevitably，these different methods for the precipitations of ZnO nanoparticles have effects on the sizes，polydispersities and agglomerations of ZnO nanoparticles.

Herein，we prepared ZnO nanoparticles with different sizes by diverse facile precipitation methods during a solgel synthesis procedure and investigated the influences of these precipitation methods on the sizes，polydispersities and agglomerations of ZnO nanoparticles.Meanwhile，their photocatalytic properties were also studied and a new mechanism for the formation of ZnO nanocrystals via the sol-gel route was proposed based on our previous findings.

1 Experimental

1.1 Syntheses of colloidal ZnO solution

The synthesis of colloidal ZnO solution was similar to the methods described by Meulenkamp［2］and in our previous paper［13］.A 2.86 g(13 mmol)amount of Zn(Ac)2·2H2O was placed in 130 mL of absolute ethanol，and then the mixture was heated to dissolve Zn(Ac)2·2H2O under magnetic stirring.When Zn(Ac)2·2H2O was dissolved completely，the Zn2+-containing solution obtained was diluted to 130 mL by the addition of absolute ethanol and cooled to 0℃.0.754 g of LiOH·H2O(18 mmol)was dissolved in 130 mL of absolute ethanol at room temperature under magnetic stirring.The hydroxide-containing solution was then added dropwise into the Zn2+-containing solution at 0℃ under stirring.The sol obtained was the mixture of ZnO colloid and an intermediate of hydroxy double salt Zn5(OH)8(Ac)2·2H2O(Zn-HDS).The hydroxy double salt could easily transform into ZnO phase through sonicating or heating［13］.

1.2 Precipitation of ZnO nanoparticles

Different methods were employed to treat the colloidal ZnO solution obtained until white precipitates were formed:(1)sonicating the colloidal ZnO solution by an ultrasonic cleaner at 60℃ to give sample A，(2)adding 1 mL of water to 50 mL of the colloidal ZnO solution and then boiling it to result in sample B，(3)sonicating the colloidal ZnO solution for 20 min at 0℃to ensure the transformation of the hydroxy double salt intermediate into ZnO phase and then adding heptane to it to lead to sample C，and(4)removing part of the solvent by distillation to produce sample D.

1.3 Characterization of X-ray powder diffraction(XRD)and transmission electron microscopy(TEM)

The XRD patterns of the as-precipitated ZnO nanoparticles were characterized by a Rigaku-D-Max rA 12 kW diffractometer with Cu-Kα radiation(λ =1.54056 Å)at an operation voltage and current of 40 kV and 300 mA，respectively.TEM measurements were performed on a JEM-2010 microscope operated at an acceleration voltage of 200 kV.

1.4 Photocatalytical degradation of methyl orange

A 0.20 g amount of the sample A，B，C or D was put into 150 mL of methyl orange solution with a concentration of 20 mg/L.Prior to the irradiation，the suspension was sonicated for 20 min，magnetically stirred in a dark condition for 30 min to establish an adsorption/desorption equilibrium，and then put into a self-assembly reactor.The irradiation resource of the photocatalytic reactor was four 40 W UV lamps set in parallel from the suspension surface of 20 cm with a maximum emission at ca.365 nm.Samples were taken from the reaction suspension at a 20 min interval.The upper lucid liquid obtained after centrifugal separation was analyzed by a UV─Vis spectroscopy.

2 Results and discussion

2.1 XRD analysis of samples

The XRD patterns of the as-prepared samples are shown in Fig.1.The reflections recorded can be indexed to hexagonal ZnO(JCPDS 36-1451).The peaks are overlapped，which are caused by line-broadening because of the small crystal size.Average crystal sizes for the samples A，B，C and D，estimated by XRD using the Scherrer formula for the(102)reflection，are 6.5，7.7，4.7 and 6.4 nm，respectively.

2.2 Size distribution and agglomeration situation of samples

The TEM images and corresponding size histograms of the samples A，B，C and D are presented in Figs.2-5，respectively，and the size distributions analyzed from the normal curves and agglomeration situations are listed in Tab.1.

Fig.1 XRD patterns of the asprepared samples A-D

Fig.2 TEM image and particle size distribution for the sample A precipitated by sonicating the colloidal ZnO solution

Fig.3 TEM image and particle size distribution for the sample B precipitated by adding water to the colloidal ZnO solution and then boiling it

Fig.4 TEM image and particle size distribution for the sample C precipitated by the addition of heptane to the colloidal ZnO solution

Fig.5 TEM image and particle size distribution for the sample D precipitated by evaporating part solvent of the colloidal ZnO solution

Tab.1 Size distributions and agglomeration situations of samples A，B，C and D analyzed from their TEM images

Mean diameters of the samples A，B，C and D determined by TEM are 6.2，7.4，4.6 and 6.6 nm，respectively，which are in good agreement with the average crystal sizes estimated by XRD.It is well known that the sonochemical synthesis has become a routine method for preparing a wide variety of nanostructured materials［12，14］，which is based on the acoustic cavitation resulting from the continuous formation，growth and implosive collapse of bubbles in a liquid.The sample A was precipitated by sonicating the colloidal ZnO solution at 60℃，and the ZnO nanoparticles obtained had a rather narrow size distribution with a particle diameter of 6.2 ±1.5 nm，standard deviation of about 8%，and less agglomeration.The sample B was precipitated through adding 1 mL of water to 50 mL of the colloidal ZnO solution and then boiling it，which gave a particle size of 7.4 ±2.6 nm，standard deviation of ca.13%，and serious agglomeration.This may be because that water and heating could accelerate the particle growth，and water could increase the particle agglomeration，which are also confirmed by the samples C and D.The sample C，which was precipitated by adding heptane to the colloidal ZnO solution at 0℃，had a particle size of 4.6 ±1.5 nm，standard deviation of around 12%，and medium agglomeration.However，the TEM image of the sample C is blurred，which could be attributed to the less crystallinity resulting from the low temperature treatment of the colloidal ZnO solution.The polydispersity，mean diameter and agglomeration degree of the sample D decreased in comparison with the sample B probably due to the fact that no water was added to the colloidal ZnO solution produced during the sol-gel synthesis of ZnO nanoparticles.

2.3 Mechanism for the formation of ZnO nanocrystals

There are some controversies about the mechanism for the formation of ZnO nanocrystals during the sol-gel synthesis［13］.In Spanhel and Anderson's procedure for the preparation of ZnO，they mentioned an organometallic Zn precursor，containing acetic acid derivatives，produced by refluxing an ethanolic Zn(Ac)2·2H2O solution before the addition of LiOH · H2O［1］.Later，Spanhel et al［15］attributed the organometallic Zn precursor to Zn10O4(Ac)12.However，the precursor was identified to be Zn4O(Ac)6by Briois et al［16］.In our previous publications［12-13］，we reported that a hydroxy double salt Zn5(OH)8(Ac)2·2H2O(Zn-HDS)intermediate is formed and could directly transform into a ZnO phase in an acetate-containing solution during the present sol-gel synthesis of ZnO nanocrystals described above.Therefore，the mechanism for the formation of ZnO nanocrystals in the acetatecontaining solution could roughly be described as:

In Eq.1 the precursor with probable formula of Zn4O(Ac)6is formed by the pre-hydrolysis of the ethanolic Zn(Ac)2·2H2O solution.The Zn-HDS intermediate is present after the addition of LiOH·H2O into the ethanolic Zn4O(Ac)6precursor solution in Eq.2.In Eq.3，the Zn-HDS intermediate transforms into ZnO particles by further hydrolysis and the ZnO phase can also transform back to the Zn-HDS phase through the dissolution/reprecipitation of ZnO nanoparticles，and a neutralization reaction between acetic acid(HAc)and LiOH exists in Eq.4.

Fig.6 Photocatalytic degradation of methyl orange in the presence of the samples A-D

2.4 Photocatalytic activities of samples

Fig.6 exhibits the photocatalytic degradation of methyl orange in the presence of the samples A，B，C and D.After irradiating for 120 min，the degradation rates of methyl orange are 89.6%，64.6%，91.1%and 78.0%for the samples A，B，C and D，respectively.The photocatalytic activities of samples A and C are comparable，being better than the sample D，while the sample B is the worst.The samples A and C have higher photocatalytic activities may be due to their smaller particle sizes and less agglomerations，while the introduction of water and boiling for the sample B leading to the particle growth and serious agglomeration may account for the lowest photocatalytic activity.

3 Conclusions

ZnO nanoparticles with different sizes less than 10 nm have been successfully synthesized by diverse facile precipitation methods via a sol-gel route.Different precipitation methods have evident effects on the sizes，polydispersities and agglomerations of ZnO nanoparticles.ZnO nanoparticles，precipitated by sonicating the colloidal ZnO solution at 60℃ (sample A)，adding water to the colloidal ZnO solution and then boiling it(sample B)，adding heptane to the colloidal ZnO solution(sample C)，and evaporating part solvent of the colloidal ZnO solution(sample D)，have particle diameters of 6.2 ±1.5，7.4 ±2.6，4.6 ±1.5 and 6.6 ±1.9 nm with standard deviations of about 8%，13%，12%and 10%，respectively.The agglomeration situation for the sample A is the least，and that for the sample B is the most serious.In the photocatalytic degradation of methyl orange，the samples A and C have the comparably better activities，which can be attributed to their smaller particle sizes and less agglomerations，while the introduction of water and boiling for the sample B leading to the particle growth and serious agglomeration may account for its lowest photocatalytic activity.

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