Hydrothermal synthesis of nanosized ZnO powders Brief/Hydrothermal-synthesis-nanosized-Zn… · The hydrothermal synthesis of ZnO powders has been relatively well described in the

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Systematic Study of Hydrothermal Synthesis of Zinc Oxide (ZnO)

Nanosized Powders with Superior UV Absorption

Wojciech L. Suchanek Sawyer Technical Materials, LLC

35400 Lakeland Boulevard, Eastlake, OH 44095, USA

Abstract

Zinc oxide (ZnO) powders were synthesized by the hydrothermal method at 160-280ºC for 6-

24 hours, under 90-930 psi pressure, from concentrated precursors without stirring. The

precursors were formed by mixing aqueous solutions of zinc acetate, chloride, sulphate, or nitrate

with KOH (aq) under either controlled or rapid precipitation conditions. The precursor quantities

were chosen to yield zinc concentration of 0.3-4.0 m, KOH concentration of 0.6-8.0 m, and pH

between 4 and 14 after precipitation has been completed. Most of the synthesized powders

exhibited equiaxed morphology, low aggregation levels and narrow crystallite size distributions

in the range of 60-200 nm. Some of the ZnO powders consisted of nanosized equiaxed

crystallites mixed with large, mostly elongated crystals, 0.5-4.0 μm in size. Based upon this

systematic study encompassing a wide range of synthesis conditions, temperature-controlled

growth rate and Ostwald ripening were found to dominate the hydrothermal crystallization of

ZnO powders. Comparison with several commercially available ZnO powders revealed superior

purity, particle size distributions, defect structure, and ultimately UV absorption, in selected

powders synthesized hydrothermally. Results of this study enable the use of hydrothermally

made ZnO powders in sunscreens or other UV absorption applications and make their

hydrothermal production commercially feasible.

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Introduction

Zinc oxide (ZnO) is a wide-band semiconductor, with a variety of applications in photonic,

photovoltaic, and electronic devices, such as blue light emitting diodes (LEDs), lasers, UV

detectors, transistors, and solar cells 1-4. In addition, ZnO exhibits attractive piezoelectric

properties, which enable its applications as transducers, SAW, and BAW devices 5. Among other

major uses of ZnO, particularly in powder form, are pigments 6,7, photocatalysts 8,9, and UV

absorbers.

Zinc oxide is one of only two inorganic UV filters in sunscreens approved for use in Europe

and in the USA 10. Advantages of ZnO in this application include broad absorption of the UV

radiation both in UVA and UVB range, much better than in the case of TiO2 11,12, combined with

high transparency, and biocompatlibility superior to the widely used organic UV filters, such as

benzophenone or others 13,14. Excellent UV absorption of ZnO can be also utilized in cosmetics,

paints, varnishes, and plastics 15,16. Performance of ZnO in the UV absorption applications is

determined primarily by such properties of the ZnO powders as particle size, morphology,

agglomeration level, defect structure, and chemical composition (purity, dopants). The UV filters

must exhibit sufficiently low agglomeration level, low crystallite size, and high dispersability to

provide transparent layers. At the same time, they should possess high UV absorption, which is

determined by the crystallographic and chemical properties. Achieving synergy between all of

these features requires highly engineered ZnO powder thus necessitates use of advanced

synthesis techniques.

ZnO powders can be synthesized by several well-established synthesis methods, for example

oxidation of metallic zinc vapor from zinc metal (French process) or ore (American process) 17,

decomposition of zinc-containing salts 18, or spray pyrolysis 8,19. All of these methods use high

temperatures up to 1,000°C in order to crystallize the ZnO phase. In most cases, the high

synthesis temperatures lead to the formation of strong aggregates in the ZnO powders, which

subsequently have to be excessively milled resulting in poor control of particle morphology and

size distribution.

Hydrothermal synthesis is a low-temperature alternative to the methods described above and

crystallizes anhydrous materials in a variety of forms directly from aqueous solutions at elevated

temperatures and pressures, for fine powders usually up to 200-300ºC under corresponding

saturated vapor pressure of water 20-26. This technology offers several advantages over

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conventional powder synthetic methods, such as one-step synthesis without high temperature

calcination and milling, low aggregation levels and narrow crystallite size distributions, in

addition to excellent control of particle morphology and size 20-26. The purity of hydrothermally

synthesized powders usually significantly exceeds purity of the starting materials, because the

growing crystallites tend to reject impurities present in the growth environment, which are

subsequently removed from the system with together the crystallizing solution 27. In addition,

low synthesis temperatures can result in unique chemical defect structures, which cannot be

achieved at very high temperatures required by the classical synthesis methods, thus may result

in unique materials properties in particular applications.

The hydrothermal synthesis of ZnO powders has been relatively well described in the

literature 28-62. The hydrothermal synthesis conditions of ZnO powders encompass a very broad

range of temperatures (20-400°C), pressures (0-4,000 psi), durations at peak temperature (0.1

second – 21 days), types of precursors, reactants concentrations (0.001 – 1.5 M), pH (5-14),

additives, precursor preparation methods, and types of autoclaves, i.e. both batch and continuous

flow. The synthesized ZnO powders exhibit a broad variety of sizes (20 nm – tens of microns),

morphologies (equiaxed, needles/whiskers, rods, fibers, sheets, flakes plates, polyhedra,

ellipsoid), and aggregation levels as well. Unfortunately, most of these papers report only a few

experimental points, often not well characterized, i.e. without specified reactants concentration

and/or temperature and/or pH etc. While some trends can be noticed in individual papers, there is

no consistency between them, which will be discussed in detail later. Moreover, none of the

published reports on ZnO shows well documented and characterized hydrothermal synthesis of

nanosized ZnO powders from concentrated precursors, with equiaxed morphology, low

aggregation level, and properties superior to the ZnO powders made by classical techniques.

In the present work, we have synthesized a wide range of ZnO powders by the hydrothermal

technique from concentrated precursors without stirring. It is a systematic study encompassing a

wide range of synthesis conditions including temperature, reactants concentration, pH, types of

precursors and chemical additives, based upon which the growth mechanism can be elucidated.

In selected ZnO powders we observed unique combination of equiaxed morphology, low

aggregation level, and narrow particle size distribution, with high chemical purity and unique

defect structure, which resulted in superior UV absorption. Results of this study enable the use of

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hydrothermally made ZnO powders in sunscreens or other UV absorption applications and make

their hydrothermal production commercially feasible.

Experimental Procedure

Preparation of precursors for ZnO hydrothermal synthesis. Hydrothermal syntheses of

4-35 g of ZnO powders were performed as follows. Appropriate weight of zinc chloride (ZnCl2,

USP grade or technical grade), zinc acetate (Zn(Ac)2⋅2H2O, USP grade), zinc sulphate

(ZnSO4⋅7H2O, USP grade), or zinc nitrate (Zn(NO3)2⋅6H2O, reagent grade, all Spectrum

Chemical, New Brunswick, NJ) was dissolved in 28.3-233.2 mL of DI water in a glass beaker.

Several drops of 37% HCl were added to prevent formation of gel-like precipitates in some

cases. In a separate glass beaker, appropriate weight of potassium hydroxide (KOH, 85%,

reagent grade, Spectrum Chemical, New Brunswick, NJ) was dissolved in 20.0-160 mL of DI

water. Both types of solutions were cooled down to room temperature. The quantities of the

starting materials were selected to yield 0.3-4.0 m ZnCl2, Zn(Ac)2, Zn(NO3)2, or ZnSO4 and 0.6-

8.0 m KOH (aq) after mixing (see Table I for details). Subsequently, the zinc salt solution and

KOH solution were mixed using two different methods, denoted hereafter “controlled

precipitation” or “rapid precipitation”. During the controlled precipitation, the Zn-salt solution

was placed on a Cimarec® digital stirring hot plate (Model No. SP131325,

Barnstead/Thermolyne, Dubuque, IO) and the KOH solution was slowly poured into it using a

pipette, under intensive stirring by Teflon-coated magnetic stirrer. The stirring rate was adjusted

depending on the viscosity of the aqueous slurry in such a way that vortexing was always visible.

During the KOH addition, formation of white precipitates of zinc hydroxide was observed.

Starting pH was 2.42-5.54, temperature slightly increased to about 30-45°C during the course of

the precipitation process. The precipitation was carried out until the desired temperature-

compensated pH of about 9-14 was achieved (see Table I for detailed pH values). The

temperature-compensated pH was being monitored with high accuracy using an Accumet®

Excel dual channel pH/ion meter (Model No. XL25, Fisher Scientific, Hampton, NH) equipped

with a universal Accumet® glass electrode and temperature probe. Prior to the measurements,

the pH meter was checked against 3 standard buffers (measurement results in brackets): 7.00

(7.03), 10.00 (10.01), and 12.43 (12.43). The total precipitation time was about 15-30 min. The

slurry was subsequently agitated for another 10 min in order to stabilize pH. In the case of

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reagent mixing by “rapid precipitation” (i.e. ZOP-15, ZOP-30), the Zn salt solution was rapidly

poured into the KOH solution and the mixture was stirred with a pipette for only 1 minute, while

the final pH was measured using pH paper. The precursor slurries were then transferred to one or

several clean 120 mL Teflon containers.

Hydrothermal synthesis of ZnO powders. The containers were closed with a screw caps,

placed in a special steel holder and put into cleaned autoclave (13”Dia x 120”H, Autoclave

Engineers, Erie, PA), together with several other small containers with loads targeting different

types of ZnO powders. About 60 L of DI water were then placed in the bottom of the autoclave.

The autoclave was sealed using a special plate closure with an O-ring. Calibrated pressure gauge

and two J-type thermocouples were attached. A few hours after closing the autoclave, the heating

cycle of the hydrothermal treatment was initiated as follows: from room temperature to 160-

280°C with heating rate of 29.0°C/hr, followed by holding at 160-280°C for 6-24 hours, with

temperature stability of a few °C, with pressure of 90-920 psi, which were saturated vapor

pressures of water at 160-280°C. After completing soaking at the maximum temperature, the

autoclave was shut down and cooled in an uncontrolled manner. The cooling to room

temperature took approximately 48 hours. After unloading, pH of the aqueous slurry was again

measured precisely using the pH meter (see Table I for detailed pH values). In some cases (i.e.

ZOP-15, ZOP-30), the pH after the synthesis was measured using pH paper. Subsequently,

content of the Teflon containers in form of aqueous slurries was placed on a 0.45 μm Supor®-

450 (Pall Gelman Laboratory, Ann Arbor, MI) membrane filter with 47 mm diameter inside a

porcelain Büchnner funnel, and vacuum filtrated using a laboratory vacuum/pressure pump

(Model No. 2522B-01, Welch Rietschle Thomas, Skokie, IL). After filtering off the initial

aqueous solution (mother liquor), the powders were washed several times using DI water while

being filtrated under vacuum. After using at least 250 mL of DI water for washing, all water was

filtered off and the remaining powder cake was placed on a hot plate and dried overnight at

120°C. Subsequently, the dried powder was collected and analyzed by different methods.

Estimated ZnO powder yield was measured only for one composition (ZOP-52) and was 91.6%.

Compositions ZOP-52 and ZOP-46 were successfully reproduced and scaled-up, in order to

manufacture kilogram quantities from a single batch.

Materials Characterization. Phase composition of all powders after hydrothermal synthesis

was characterized by X-ray diffraction using Advanced Diffraction System X1 diffractometer

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(XRD, Scintag Inc.) using Cu Kα radiation, in the 2Θ range between 10-70° with a 0.05° step

size and 0.1-0.3 s count time. The chemical identity of the powders was determined by

comparing the experimental XRD patterns to standards compiled by the Joint Committee on

Powder Diffraction and Standards (JCPDS), i.e. card # 01-1136 for ZnO (zincite).

The morphology and phase purity of all synthesized ZnO powders were examined using

scanning electron microscope (SEM, Model S-4500, Hitachi, Japan) at 5 kV accelerating

voltage. The powders were attached to the aluminum holders using a conductive carbon tape. All

samples were sputtered with thin conductive layers of palladium prior to the SEM examination.

Chemical moieties present on the surface of the ZnO powders were determined using X-ray

photoelectron spectroscopy (XPS) using the Phi 5000 VersaProbe Scanning ESCA Microprobe

(Ulvac-Phi, Inc., Minneapolis, MN), utilizing Al Kα radiation with energy of 1,486.6 eV. The

ZnO powders were attached to the holders using conductive carbon tape. The XPS spectra were

acquired from the surface areas with diameters of approximately 0.3 mm on each sample. Only

one spot on each sample was analyzed by this technique. In a typical XPS measurement, a 20-60

min. overview scans were performed in the binding energy range of 0-1,100 eV with 0.4 eV step.

High-resolution XPS analysis of the O1s peaks was performed in the range of 523-543 eV for

30 min, with a step of 0.1 eV. Standard software was subsequently used for peak deconvolution

using Gaussian-Lorenzian curve fits.

Selected ZnO powders and starting materials were subjected to chemical analysis at NSL

Analytical (Cleveland, OH). The powders were analyzed for the following elements (analytical

technique in brackets): Pb, Cd, Cu, Mn, Fe, and As (inductively-coupled plasma-mass

spectrometry, i.e. ICP/MS), SO4 (Parr/IC), and CO3 (wet chem.).

Particle size distributions of selected ZnO powders were measured using X-ray Disc

Centrifuge Sedimentation method (XDC, Brookhaven X-ray Disc Centrifuge Particle Size

Analyzer Ver. 3.69). In order to prepare sample for analysis, 2 g of ZnO powder was dispersed in

0.1% aqueous solution of NaHMP at pH=10. Disc speed was 1,500 rpm in all cases. Particle

density, spin fluid density, and spin fluid viscosity were 5.6 g/cm3, 0.997 g/cm3, and 0.890 cP,

respectively. Average crystallite sizes were calculated from SEM images based upon averaging

data for at least 100 randomly selected crystallites.

UV attenuation of selected ZnO powders was measured using the following methodology.

First, the test sunscreen formulation was prepared, which was a non-water resistant oil-in-water

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emulsion with the following constituents: 7.5% octyl methoxycinnamate, 3.0% Arlacel 165,

1.0% DC345 fluid, 1.5% cetearyl alcohol, 1.5% Ceteareth-20, 0.5% Trivasperse NH, 0.1%

Crodaphos N3N, 10.0% isopropyl myristate, 15.0% of Xanthan Gum (2% solution), 1.5%

propylene glycol, 0.1% tetrasodium EDTA. The formulation contained 5.0% of ZnO powder,

which is concentration that should produce in-vitro SPF number of about 15-20. The rest of the

formulation was water. Subsequently, in-vitro SPF measurements were performed using

Labsphere UV Transmittance Analyzer. The substrate used was Transpore Tape® (3M) and the

test sunscreen formulation was applied at 2 mg/cm2 (as per the FDA in-vivo methodology). The

SPF value was calculated from the difference in transmission of UV radiation (from 290 nm to

400 nm) between the substrate with and without applied sunscreen. The mean SPF values were

based on averaging 6 measurements.

Results and Discussion

Properties of ZnO powders synthesized hydrothermally. Typical synthesis conditions and

properties of selected ZnO powders prepared in this work are summarized in Tables I-III.

Powder XRD analysis revealed high level of crystallinity and 100% phase purity of the ZnO

powders shown in Fig. 1 and Tables I-III. Only XRD peaks derived from the ZnO (zincite) phase

were observed.

SEM photographs in Fig. 1 reveal two types of morphologies of the ZnO powders. Fig. 1(a)-

(c) show mostly nanosized ZnO powders consisting of very well defined, equiaxed, weakly

agglomerated, and round crystals with narrow size distributions (as shown are powders with

primary particle size ranging between 60 nm and 200 nm). Conversely, powders shown in

Fig. 1(d)-(h) exhibit significantly high level of agglomeration, aspect ratio higher than 1

(Fig. 1(d)), and clearly bi-modal crystallite size distribution, consisting of micron-sized mostly

elongated particles mixed with submicron equiaxed particles (Fig. 1(e)-(h)). It is apparent from

the pictures in Fig. 1 that the level of the aggregation and crystallite size distributions of the

hydrothermally synthesized ZnO powders could be controlled in wide ranges by changing the

processing parameters, thus the powders could be used for a wide range of applications. The

nanosized ZnO powders with narrow particle size distributions may be utilized as pigments,

photocatalysts, or UV absorbers. Such powders should also exhibit high sinterability due to the

small crystallite size and very narrow crystallite size distributions. The ZnO powders with

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bimodal size distributions could be used to fabricate textured ceramics with quasi-single crystal

like properties, if the large elongated particles could be oriented, for example by tape casting or

hot pressing.

Chemical analysis of the bulk of the ZnO powders (Table II) and XPS analysis of the surface

(will be discussed in detail in the last sections of this paper) confirmed high purity of the

hydrothermally synthesized ZnO powders. The major impurity was carbonate, detected by the

chemical analysis on the level of 400ppm-0.1%. XPS confirmed high purity of the ZnO powders

surfaces. Except for traces of chlorine detected in one case, no other major impurities were

detected either in the bulk or on the surface of the hydrothermally synthesized ZnO powders.

Chemical analysis data shown in Table II for both starting ZnCl2 (technical grade, column 5)

and hydrothermally synthesized ZnO powder (ZOP-52, column 3) additionally confirmed effect

of hydrothermal purification, i.e. synthesized crystals having higher purity than the starting

materials, because the growing crystals tend to reject impurities under hydrothermal conditions 27. Concentrations of all analyzed elements/ions shown in Table II, except for carbonate and

copper, were either reduced in the synthesized ZnO powder vs. starting ZnCl2 or at least at

comparable level. Particularly strong reduction was observed for sulphate, iron, and arsenic.

Increase of Cu concentration cannot be explained at this time. Increase in carbonate content is

most likely related to the additional presence of dissolved CO2 in water used in hydrothermal

synthesis of ZnO, as all precursor preparation and synthesis work was conducted in air.

Effects of the synthesis variables on properties of the ZnO powders. Hydrothermal

synthesis of the ZnO powders was performed in a wide range of synthesis conditions, such as

temperature/pressure, type and concentrations of the starting materials, pH, and methodology of

precursor preparation. A schematic map, which summarizes the experimental conditions of the

hydrothermal synthesis of ZnO, is shown in Fig. 2. All these synthesis factors were found to

affect in various degrees properties of the ZnO powders synthesized in this work, as described

below.

Effects of the precursor (chemical composition, concentration, pH, preparation method).

Both concentration of the starting materials and pH of the precursor were found to be very

important parameters in synthesis of ZnO, as demonstrated in Fig. 2. Formation conditions of

pure ZnO phase were primarily determined by the combination of these two parameters. When

pH of the ZnO precursor was between 4 and 7, and zinc concentration (denoted hereafter [Zn])

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between 0.0 and 2.0 m, no pure ZnO phase was observed to form, even at 250°C. In the

temperature range of 200-280°C, pH in excess of 9-10 (preferably 12-14) was required to obtain

pure ZnO phase when [Zn] was below 2.0 m. In the same temperature range, [Zn]=2.0 m at

pH=9 resulted in the formation of pure ZnO phase. However, at temperatures under 200°C, pH

of 9 did not produce pure ZnO phase even at [Zn]=2.0 m. In the investigated temperature range

(160-280°C), the optimum combination of pH and [Zn] to obtain pure-phase ZnO powders

appears to be 9-14 and 1-4 m, respectively. All findings of this systematic study were

summarized in Fig. 2, which is an experimentally generated phase diagram for the hydrothermal

synthesis of ZnO powders from concentrated precursors.

Effect of pH and [Zn] on size, morphology, and aggregation level of ZnO crystallites were

difficult to elucidate (see also Fig. 3). Nevertheless, ZnO powders with the smallest crystallites,

most narrow particle size distributions and lowest aggregation levels, were synthesized at

[Zn]=1-2 m, pH=9-12, 160-200°C for 12-24 hours. High [Zn]=4.0 m combined with rapid

precipitation resulted in bi-modal crystallite size distributions, i.e. mixtures of micron size

crystals surrounded by submicron crystallites (ZOP-30). Base concentration was typically twice

as high as [Zn] (see Table I), thus it is considered a dependent variable and its effect was not

studied.

The investigated starting materials containing zinc were aqueous solution of the following

zinc-containing salts: ZnCl2, ZnSO4⋅7H2O, Zn(NO3)2⋅6H2O, and Zn(Ac)2⋅2H2O. Only aqueous

solution of KOH as a base was investigated. It was found that zinc chloride and acetate produced

comparable phase composition, crystallite size, morphology, and aggregation level of the ZnO

powders, although the use of ZnCl2 appears to produce slightly superior results. Different grades

of ZnCl2 [USP grade, technical grade] produced more or less comparable results as well.

However, use of zinc nitrate or sulfate resulted in the formation of bi-modal crystallite size

distributions even at low synthesis temperature of 200ºC (ZOP-63, ZOP-66).

Controlled precipitation with controlled pH appears to produce better results than quick

mixing of the starting materials during “rapid precipitation”, although the second method could

also produce ZnO powders with low aggregation level and uniform particle size distribution of

the primary ZnO crystallites, bracketing the size range of 100-200 nm (ZOP-15).

It is worth mentioning that zinc concentrations in excess of 1 m guarantee high production

yields in case of commercialization, while significantly lower concentrations do not. Several

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papers report hydrothermal synthesis of equiaxed ZnO crystallites with sizes under 100 nm 29,34,43-46,53. However, all of them use diluted precursors with concentrations well below 1 m,

sometimes even as low as 0.001 m. Only Li et al. report hydrothermal synthesis of ZnO with

crystallite size of 70 nm from 1 M-zinc acetate-based precursor 35,36. However, these ZnO

powders do not have as uniform particle size distributions as ZnO powders synthesized in this

work. The results obtained in the present work at zinc concentrations of 1.0-2.0 m demonstrated

commercial potential for the hydrothermal synthesis of ZnO powders in batch autoclaves without

stirring. Subsequently, experiments ZOP-52, ZOP-46, and ZOP-43 were reproducibly scaled-up

to produce kilogram quantities of ZnO powders from a single batch.

Conditions of the of the hydrothermal synthesis. In the investigated temperature range, most

of the synthesis conditions yielded pure phase ZnO powders. However, other requirements also

had to be met in order to obtain pure ZnO phase, particularly appropriate pH of the precursor

slurry and concentrations of starting materials (see Fig. 2). As clearly demonstrated in Fig. 3,

increasing synthesis temperature facilitated formation of larger ZnO crystallites. This trend was

applied to all precursor concentrations at pH between 9.6 and 12.

The hydrothermal synthesis of ZnO powders described in this work was carried out under

saturated vapor pressures of water at the syntheses temperatures, thus the pressure is here a

temperature-dependent variable and will not be discussed separately. Time is here also a

temperature-dependent variable and its effects were not investigated in detail. However, it is

believed that formation of ZnO requires time in the order of minutes to hours in the investigated

temperature range of 160-280°C. The higher the synthesis temperature, the lower the time

required to complete the synthesis. The heating rate of 29°C/hr was used in all syntheses of ZnO

powders in this work. Heating rate may impact size, morphology, and aggregation of ZnO

crystallites, however its effects were not investigated in this work.

Growth mechanism of the ZnO powders under hydrothermal conditions. The balance

between nucleation and growth rates determines crystallite size during the hydrothermal

crystallization 20. With increasing supersaturaion (controlled by temperature, reactants

concentration, pH), both growth rate and nucleation rate increase. Consequently, when the

growth rate dominates over the nucleation rate, crystallite sizes increase, while in the case of

nucleation rate domination, crystallite size decreases 63. The results of the present work are

summarized in Fig. 3 and show strong relationship between the synthesis temperature and the

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crystallite size, i.e. increasing crystallite size with increasing synthesis temperature on all

investigated levels of precursor concentration and pH. Such behavior is consistent with the

growth rate-dominated crystallization. Effects of the pH and concentration are difficult to

elucidate probably because the solution supersaturation during hydrothermal synthesis of ZnO

reaches maximum at different combinations of pH and [Zn] for different Zn salts 64,65. Thus the

temperature effect at constant [Zn] and pH is the only clearly visible effect during hydrothermal

synthesis of ZnO, unless a very detailed study of varying [Zn] or pH with all other parameters

kept constant could be performed.

The bi-modal crystallite size distributions (Fig. 1e-h) were observed in both “rapid

precipitation” and “controlled precipitation” cases thus the uniformity of the precursor, which is

lower in the earlier case, does not seem to have played any role in creating such large particles.

The large micron-sized ZnO particles were formed mostly from precursors prepared by “rapid

precipitation”, all of which where prepared at had pH of about 14 and also during “controlled

precipitation” at pH of about 12 but at high temperatures of 250-280ºC. Most of the bi-modal

crystallites forming conditions do not correspond to maximum supersaturation, which occurs at

pH below 14 in hydrothermal ZnO synthesis 64,65. Uniform crystallite size distributions were

obtained under intermediate conditions, i.e. [Zn]=1-2 m, pH=9-12, 160-200°C, which appear to

be close to maximum supersaturation, at least from the point of view of pH. All this is pointing

out towards Ostwald ripening effects, i.e. dissolution of fine particles and re-precipitation on

large particles, which occurs in batch systems due to supersaturation decrease 66. During the

hydrothermal synthesis of powders, the supersaturation gradually decreases as the crystallization

progresses, and at certain point small particles start dissolving, resulting in constant

supersaturation, which favors growth of only large crystallites 66. This may occur when the

synthesis time is too long with respect to the synthesis temperature or when the initial

supersaturation is already low. It seems, that the Ostwald ripening effects were responsible for

bi-modal size distributions in some batches of the hydrothermally synthesized ZnO powders.

Literature data for ZnO hydrothermal synthesis is summarized in Fig. 4. No clear relationship

between crystallite size and synthesis temperature or precursor concentration could be observed.

There are trends in individual papers 30,36,37,40, which however are not consistent with other

works. The present work is thus the first systematic report studying effects of various synthesis

parameters and their combinations in wide ranges, establishing experimentally stability phase

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diagram for ZnO under hydrothermal conditions and elucidating the synthesis temperature as the

most essential parameter in control of size of ZnO crystallites.

Comparison of the hydrothermally synthesized ZnO powders with commercial ZnO

powders. For comparison with the hydrothermally prepared powders, several commercially

available ZnO powders used as UV filters in sunscreens are shown in Fig. 5: NyasorbUV

(Nyacol Nano Technologies, Ashland, MA) synthesized by decomposition of Zn-containing

salts, ZCote, and SunSmart (both Nanophase Technologies Co., Romeoville, IL) both

synthesized by oxidation of Zn metal. It is obvious that the commercial ZnO powders exhibit

much higher agglomeration levels, non-uniformly sized crystallites, and broader particle size

distributions than the ZnO powders from this work, although all of them are highly-crystalline,

phase-pure ZnO, as revealed by powder XRD patterns shown in Fig. 6. As estimated using the

Scherrer’s equation, broad peaks of the NyasorbUV powder shown in Fig. 6a are related to the

small size of the primary crystallites rather than to the lattice disorder. Properties of

commercially available ZnO powders are also summarized in Table III.

Typical particle size distributions of selected ZnO powders synthesized hydrothermally and

the commercially available ZnO powders are compared in Fig. 7 and Table III. The particle size

distributions data are consistent with the SEM observations: ZnO powders from this work exhibit

much more narrow particle size distributions than the other commercial ZnO powders, due to

very uniform crystallite size and low aggregation levels. The differences are particularly strongly

pronounced for particle sizes greater than 150-200 nm. Nyasorb UV exhibits narrow particle size

distribution and reasonable median/mode particle sizes, but this is due to high aggregation level

of 10-30 nm primarily crystallites.

Crystallite size and aggregation levels are important features of ZnO powders designated for

UV filter applications in sunscreens. Size of primary particles should not be lower than 50 nm,

like in the case of the Nyasorb UV powder, because particles smaller than 50 nm may penetrate

through the skin pores and cause adverse health effects. High level of aggregation can result in

broader particle size distribution and larger median/average particle sizes, which may result in

whitening of the sunscreen composition applied on skin (particularly above 150 nm). Thus the

optimum size of ZnO particles for UV filter applications in sunscreens is around 50-80 nm, with

low aggregation level and high particle uniformity. It is clear from Fig. 5 and Table III that the

hydrothermally synthesized ZnO powders are most close to this requirement.

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Survey XPS spectra of both hydrothermal and commercial ZnO powders (Fig. 8) did not

reveal significant differences between the powders, although NyasorbUV had substantial

magnesium contamination and ZOP-46 had traces of chlorine adsorbed on the surface.

Otherwise, all four powders exhibited XPS spectra typical for ZnO, showing strong Zn- and O-

derived bands 67. However, high resolution XPS analysis of the O1s band of the ZnO powders

revealed significant differences between the commercial and hydrothermally synthesized ZnO

powders. As shown in Fig. 9, the hydrothermally synthesized ZnO powders had much smaller

shoulders of the O1s band, located at about1.5 eV higher binding energy than the main

component of the O1s peak, than the commercial ZnO powders synthesized by the high-

temperature methods. The low binding energy component of the O1s peak, located at 530.0 eV is

ascribed in the literature to the O2- ions in stoichiometric ZnO, while the higher binding energy

shoulder located 1.0-1.5 eV higher is derived from O2- ions in oxygen-deficient ZnO 68-71.

Presence of oxygen vacancies with effective positive charge can also explain shift of the O1s

peaks towards higher energies in both commercial ZnO powders (Fig. 9c-d). The third

component of the O1s peak with the highest binding energy is derived from adsorbed or

chemisorbed O2 or OH- groups on the ZnO surface 68-71. In all investigated powders, the Zn 2p3/2

peak was highly symmetrical and positioned around 1,022 eV, which should be assigned to

theZn2+ ions in ZnO. These high-resolution XPS data indicate significantly lower level of oxygen

deficiency, i.e. higher stoichiometry, in hydrothermal ZnO powders than in the commercial ZnO

powders, which were synthesized by the high-temperature methods. It is not surprising, as the

hydrothermal synthesis of ZnO takes place under strongly oxidizing conditions. All these XPS

data demonstrate superior defect structure and higher purity of the hydrothermally synthesized

ZnO powders over the commercial powders synthesized at high temperature.

UV absorption comparison. UV absorbance of selected ZnO powders (both this work and

commercial) is shown in Fig. 10 and Table III. The ZnO powder from this work exhibits high

SPF factor of 23, high critical wavelength of 372 nm, and high UVA ratio of 0.49. The

combination of all three parameters is superior to the corresponding properties of commercial

ZnO powders. SPF number reflects mostly UVB radiation protection, whereas the critical

wavelength corresponds to UVA protection. High critical wavelength is particularly important,

because the long UVA radiation is responsible for occurrence of melanoma and premature aging

of the skin, while the UVB radiation causes erythema (sunburn) and non-malignant skin

11/7/2008

14

cancer11,12. While the ZnO powders synthesized in this work were tested as UV filters in

sunscreens, other potential UV filters applications are possible, such as in paints, plastics, etc.

One of possible explanations of superior UV absorption of the hydrothermally synthesized

ZnO powders could be different defect structures and/or impurity contents, which was

demonstrated using the XPS analysis. However, despite of finding empirical correlation in the

present work, such conclusion appears to be speculative without further, more detailed

understanding of the UV attenuation mechanism in ZnO.

Summary

Zinc oxide (ZnO) powders were hydrothermally synthesized from highly concentrated

precursors without stirring, demonstrating good potential for commercial production. The

hydrothermal method was found to be useful to produce a range of phase-pure ZnO powders

with varying level of agglomeration and crystallite size distribution, including nanosized ZnO

powders with very narrow crystallite size distributions and a range of bimodal size distributions.

Systematic study of the hydrothermal crystallization of ZnO in a very broad range of synthesis

conditions allowed identifying the growth rate and Ostwald ripening as the dominant factors

governing size of the ZnO crystallites and the synthesis temperature as the most important size

controlling parameter. The hydrothermally synthesized ZnO powders exhibited much more

uniform crystallite size distributions and significantly higher level of lattice perfection than the

commercially available ZnO powders synthesized by classical high-temperature methods. Their

UV absorption was superior to the commercial ZnO particularly in the UVA range, making them

attractive as UV filters in sunscreens or other applications.

Acknowledgments. The author is grateful to Mark Polster for support and constant interest

in the present work, Paul Lawson for managing the autoclave operations, Santino Westfield and

Anita Colegrove (all Sawyer Technical Materials, LLC) for technical assistance, Dr. Wayne

Jennings (Case Western Reserve University, Cleveland, OH) for help with the XPS

measurements and data analysis, and Dr. David Fairhurst (Dynamic Cosmetics, Inc., Newton,

PA) for conducting particle size analysis, UV absorption tests, and discussions.

11/7/2008

15

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Table I. Synthesis conditions and properties of hydrothermally synthesized ZnO powders (selected examples) Experiment No.

Zn Salt [Zn] (m)

[KOH](m)

pH* Synthesis Conditions

Phase Composition

Powder size, and morphology by SEM

ZOP-43 ZnCl2 (USP grade)

1.0 2.0 12.14 /12.65

Controlled precept.: 200°C (225 psi), 12 hours

100% ZnO (zincite)

Equiaxed, low aggregation, narrow crystallite size distribution (60-150 nm)


2.0 4.0 9.65 /11.21


100% ZnO (zincite)



1.0 2.0 12.53 /12.25


100% ZnO (zincite)


ZOP-58 Zn(Ac)2⋅2H2O (USP grade)

1.0 2.0 12.39 /12.81


100% ZnO (zincite)


ZOP-60 Zn(Ac)2⋅2H2O (USP grade)

1.0 2.0 12 Controlled precept.: 200°C (225 psi), 12 hours

100% ZnO (zincite)



2.0 4.0 14/8-9 (by paper)

Rapid precept.: 250°C (577 psi), 7 hours

100% ZnO (zincite)



4.0 8.0 14 (by paper)

Rapid precept.: 250°C (577 psi), 7 hours

100% ZnO (zincite)

Low aggregation, 2 fractions: equiaxed (100-200 nm) and large equiaxed/elongated (0.5-2.0 µm)


2.0 4.0 12.15/ 12.87


100% ZnO (zincite)

Low aggregation, 2 fractions: equiaxed (100-200 nm) and large equiaxed/elongated (0.5-4 µm)

ZOP-63 Zn(NO3)2⋅6H2O (reagent grade)

1.0 2.0 12.05/ 12.18


100% ZnO (zincite)

Low aggregation, 2 fractions: equiaxed (60-120 nm) and elongated (0.5-1.0 µm)

ZOP-66 Zn(NO3)2⋅6H2O (reagent grade)

2.0 4.0 12.12/ 12.56


100% ZnO (zincite)

High aggregation, 2 fractions: elongated (300-500 nm) and equiaxed (60-120 nm)

* temperature-compensated pH values before/after hydrothermal synthesis

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Table II. Chemical analysis of selected ZnO powders synthesized hydrothermally and selected starting materials. Element/ion

USP requirements

for ZnO

ZnO from technical grade

ZnCl2 (ZOP-52-TG)

ZnO from USP grade ZnCl2

(ZOP-54)

Technical grade ZnCl2 as starting

materials

Pb

<4.6 ppm 0.2 ppm 0.14 ppm 0.16 ppm

As

<0.23 ppm 0.008 ppm 0.16 ppm 0.83ppm

Cd

<4.4 ppm 0.75 ppm 0.006 ppm 0.46ppm

Cu

<4.0 ppm 1.2 ppm 1.6 ppm 0.42 ppm

Mn

<3.9 ppm 4.9 ppm 4.7 ppm 4.0 ppm

Fe

<3.5 ppm <5 ppm 3.0 ppm 7.2 ppm

SO4

<10 ppm <10 ppm 4 ppm 32 ppm

CO3

- 0.10% 440 ppm 280 ppm

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Table III. Particle size distributions and UV absorption properties of selected ZnO powders, synthesized in this work by the hydrothermal method and commercially available powders.

Particle Size Distribution

UV absorption in-vitro Powder ID Synthesis Conditions

Phase Purity

Morphology/ particle size by SEM

D10 (nm)

D50 (nm)

D90 (nm)

Mean (nm)

Mode (nm)

Mean SPF

UVA ratio

Critical wave-length (nm)

ZOP-46 (present work)

Hydrothermal 100% Equiaxed (60-150nm) primary particles, narrow psd, low aggregation

68 94 154 104 72 23.0 0.49 372



74 106 160 113 100 - - -



83 118 184 128 107 - - -

ZCote (commercial, Nanophase)

Oxidation of Zn

100% Both equiaxed & elongated (60-250nm) primary particles, broad psd, moderate aggregation

78 116 204 132 97 19.7 0.47 373

SunSmart (commercial, Nanophase)

Oxidation of Zn

100% Equiaxed (30-250nm) primary particles, broad psd, highly aggregated

84 123 257 152 85 - - -

NyasorbUV (commercial, Nyacol)

Decomposition of Zn salts

100% Equiaxed (10-30nm) primary particles, uniform psd, highly aggregated

52 71 137 89 58 23.6 0.44 369

“psd” stands for particle size distribution

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Figure 1. SEM photographs of hydrothermally synthesized ZnO powders: (a) ZOP-43, (b) ZOP-46, (c) ZOP-60, (d) ZOP-51, (e) and (g) ZOP-66, (f) and (h) ZOP-30. Synthesis conditions details are given in Table I. Note varying level of aggregation and crystallite size distributions from (a) through (h).

300 nm

(a)

300 nm300 nm

(a)

(c)

300 nm

(c)

300 nm300 nm

(b)

300 nm

(b)

300 nm300 nm

(d)

300 nm

(d)

300 nm300 nm

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22

Figure 1 (cont.)

(f)

500 nm

(f)

500 nm500 nm

(e)

500 nm

(e)

500 nm500 nm

(g)

1 μm

(g)

1 μm1 μm

(h)

1 μm

(h)

1 μm1 μm

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Figure 2. Experimentally generated phase diagram for the hydrothermal synthesis of ZnO powders from concentrated precursors. Open symbols represent experimental points with no ZnO formation; solid symbols represent experimental points with formation of ZnO (mostly 100% pure phase zincite). Dotted area is the pH vs. concentration region where the ZnO precursor was precipitated under stirring under controlled pH conditions (i.e. precipitation was carried out slowly to reach desired pH value). Dashed areas are pH vs. concentration regions where the base was added rapidly without pH control (i.e. pH was determined by the quantity of added base). The dashed line separates region of pure phase ZnO formation (on the right) from the region where no pure phase ZnO could be obtained (on the left).

3 4 5 6 7 8 9 10 11 12 13 140.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0 Rapid precipitation

pH of the precursor before synthesis

[Zn]

, [Zn

]=[K

OH

], [Z

n]=0

.5[K

OH

]

no ZnO at 160 C (24h) ZnO at 160 C (24h) no ZnO at 180 C (23h) ZnO at 180 C (23h) no ZnO at 200 C (12-14h) ZnO at 200 C (12-14h) no ZnO at 250 C (6-7h) ZnO at 250 C (6-7h) no ZnO at 280 C (6h) ZnO at 280 C (6h)

Controlledprecipitation

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Figure 3. Average crystallite size of ZnO powders as function of the temperature of their hydrothermal synthesis for various sources of Zn and various concentration of Zn in the precursor. All experimental points correspond to controlled precipitation conditions, most of them with targeted pH of 12.0 (real pH variation is between 12.05-12.40), some with pH of 9.6.

160 180 200 220 240 260 280

100

200

300

400

500

600

700

800

145015001550

Ave

rage

par

ticle

siz

e (n

m)

Temperature (oC)

pH=12 in all cases 0.3m-ZnCl2 1.0m-ZnCl2 2.0m-ZnCl2 1.0m-Zn(NO3)2 2.0m-Zn(NO3)2

pH=9.6, 2.0m-ZnCl2

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(a) (b) (c) (d)

Figure 4. Literature data analysis of hydrothermal synthesis of ZnO without organic additives. (a) and (b) show effects of the concentration of Zn at various synthesis temperatures on ZnO crystallite size; (c) and (d) show effects of the synthesis temperature at various concentration levels of Zn on average ZnO crystallite size. Graphs (a) and (c) emphasizes lower particles sizes than (b) and (d), respectively. The experimental points encompass a broad variety of other synthesis variables, such as pH and type of Zn-containing salts, mineralizers (bases), if any, and their concentrations.

50 100 150 200 250 300 350 400 450

0

2000

4000

6000

8000

10000

12000

Temperature (oC)

Ave

rage

cry

stal

lite

size

(nm

)

[Zn]<=0.1 M [Zn]=0.5 M [Zn]=1.0 M

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Concentration of Zn salt (M)

T=400 C T=120 C T=160 C T=200 C T=220 C

Aver

age

crys

talli

te s

ize

(nm

)


0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

2000

4000

6000

8000

10000

12000


T=400 C T=120 C T=160 C T=200 C T=220 C

Ave

rage

cry

stal

lite

size

(nm

)


50 100 150 200 250 300 350 400 4500

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Temperature (oC)

Aver

age

crys

talli

te s

ize

(nm

)

[Zn]<=0.1 M [Zn]=0.5 M [Zn]=1.0 M

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Figure 5. SEM photographs revealing morphologies of selected ZnO powders: (a) powder from this work (ZOP-52) synthesized under hydrothermal conditions, (b) commercially available powder NyasorbUV (manufactured by Nyacol Nano Technologies, Ashland, MA), (c) commercially available powder SunSmart (manufactured by Nanophase Technologies Co., Romeoville, IL), and commercially available powder ZCote (Nanophase). Magnifications are the same in all cases.

300 nm

(a)

300 nm300 nm

(a)

(c)

300 nm

(c)

300 nm300 nm

(b)

300 nm

(b)

300 nm300 nm

(d)

300 nm

(d)

300 nm300 nm

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Figure 6. Powder X-ray diffraction patterns of selected ZnO powders: (a) commercially available powder NyasorbUV (manufactured by Nyacol Nano Technologies, Ashland, MA), (b) commercially available powder SunSmart (manufactured by Nanophase Technologies Co., Romeoville, IL), (c) commercially available powder ZCote (Nanophase), and powders from this work synthesized under hydrothermal conditions (d) ZOP-52, and (e) ZOP-46.

10 20 30 40 50 60 700

1000

2000

3000

4000

5000

6000

(201

)(112

)(2

00)(1

03)(110

)

(102

)

(100

)(0

02)

(d)

(c)

(b)

(e)

Inte

nsity

(arb

itrar

y un

its)

2Θ (deg)

(a)

(101)

11/7/2008

28

Figure 7. Particle size distributions of selected ZnO powders: (a) ZOP-43, (b) ZOP-46, (c) ZOP-52, (d) commercial powder NyasorbUV (Nyacol), (e) commercial powder SunSmart (Nanophase), and commercial powder ZCote (Nanophase).

(a)

0.00 0.10 0.20 0.30 0.40

Diameter (μm)

0

20

40

60

80

100

Diff

eren

tial N

umbe

r Dist

ribut

ion

(a)

0.00 0.10 0.20 0.30 0.40

Diameter (μm)

0

20

40

60

80

100

Diff

eren

tial N

umbe

r Dist

ribut

ion (b)

0.00 0.10 0.20 0.30 0.40

Diameter (μm)

0

20

40

60

80

100

Diff

eren

tial N

umbe

r Dist

ribut

ion

(b)

0.00 0.10 0.20 0.30 0.40

Diameter (μm)

0

20

40

60

80

100

Diff

eren

tial N

umbe

r Dist

ribut

ion

(c)

0.00 0.20 0.40 0.60 0.80

Diameter (μm)

0

20

40

60

80

100

Diff

eren

tial N

umbe

r Dist

ribut

ion

1.00

(c)

0.00 0.20 0.40 0.60 0.80

Diameter (μm)

0

20

40

60

80

100

Diff

eren

tial N

umbe

r Dist

ribut

ion

1.00

(d)

0.00 0.10 0.20 0.30 0.40

Diameter (μm)

0

20

40

60

80

100D

iffer

entia

l Num

ber D

istrib

utio

n

0.50

(f)(e)(d)

0.00 0.10 0.20 0.30 0.40

Diameter (μm)

0

20

40

60

80

100D

iffer

entia

l Num

ber D

istrib

utio

n

0.50

(f)(e)

11/7/2008

29

Figure 8. XPS spectra of selected ZnO powders: (a) ZOP-52 synthesized hydrothermally in this work, (b) ZOP-46 synthesized hydrothermally in this work, (c) commercial powder ZCote (Nanophase), and (d) commercial powder NyasorbUV (Nyacol). Unmarked bands are Auger peaks derived from Zn.

1000 800 600 400 200 00

500

1000

1500

2000

2500

3000

Mg

KLL

Mg

KLL

Cl2

p

Cl2

s

(d)

(c)

(b)

Zn L

MM

2

Zn L

MM

Zn L

MM

1

Zn3s

Zn2p

1

Zn3p

3

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)Zn

3d

C1s

Zn2p

3O

KLL O

1s

(a)

11/7/2008

30

Figure 9. High-resolution XPS spectra with Gaussian-Lorenzian curve fits, showing details of the O1s peak for selected ZnO powders: (a) ZOP-46 synthesized hydrothermally in this work, (b) ZOP-52 synthesized hydrothermally in this work, (c) commercial powder NyasorbUV (Nyacol), and (d) commercial powder ZCote (Nanophase). The spectra were charge-corrected with respect to the carbon C1s peak at 284.5 eV.

526528530 532534536 538

5000

6000

7000

8000

9000

10000

11000

12000

13000

NyasorbUV

Binding Energy (eV)

c/s

526528530 532534536538

4000

5000

6000

7000

8000

9000

10000

ZCote

Binding Energy (eV)

c/s

526528530 532534536538

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4x 10

4 ZOP-46

Binding Energy (eV)

c/s

5285305325345360.5

1

1.5

2x 10

4 ZOP-52

Binding Energy (eV)

c/s

526538

(c) (d)

(a) (b)

11/7/2008

31

Figure 10. UV absorbance spectra of selected ZnO powders: (a) ZOP-46 synthesized hydrothermally in this work, (b) commercial powder NyasorbUV (Nyacol), and (c) commercial powder ZCote (Nanophase).

(b)

250 316.6 383.3 450

Wavelength (nm)

0.0953

0.9355

1.7756

Abs

orba

nce

(b)

250 316.6 383.3 450

Wavelength (nm)

0.0953

0.9355

1.7756

Abs

orba

nce

(c)

250 316.6 383.3 450

Wavelength (nm)

0.1440

0.8735

1.6031

Abs

orba

nce

(c)

250 316.6 383.3 450

Wavelength (nm)

0.1440

0.8735

1.6031

Abs

orba

nce

(a)

250 316.6 383.3 450

Wavelength (nm)

0.1485

0.9095

1.6706

Abs

orba

nce

(a)

250 316.6 383.3 450

Wavelength (nm)

0.1485

0.9095

1.6706

Abs

orba

nce

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