CVD Synthesis of Shape and Size Controlled ZnO Nanoparticles for Application as UV Filters

CVD Synthesis of Shape and Size Controlled ZnO Nanoparticles for Application as UV Filters

Revathi Bacsa1,2, Andrzej Sienkiewicz,4 Katarzyna Pierzchała4,

Jeannette Dexpert-Ghys3, László Forro4 and Philippe Serp1,2

1 CNRS LCC UPR 8241 (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, 31077 Toulouse, France

2 Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France 3Centre d'élaboration des matériaux et d'études structurales, CNRS, 29, rue Jeanne-

Marvig, BP 4347, 31055 Toulouse, France 4 Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne,

1015, Lausanne, Switzerland E-mail: [email protected],[email protected]

For the application of nanocrystalline ZnO in pharmaceuticals and cosmetics, in addition to high purity, a precise control of particle size, shape and agglomeration is necessary so that transparent films with high absorption coefficients can be obtained. In this communication we report the synthesis and structure-property correlation of ZnO nanospheres and tetrapods prepared by the gas phase oxidation of zinc metal precursor. We have evaluated the ecotoxicity of the ZnO tetrapods and spheres by studying the rate of formation of reactive oxygen species using EPR spectroscopy. We find that ZnO tetrapods form reactive oxygen species, but at a rate much lower than nanocrystalline TiO2 when irradiated with UV-A

Introduction

Due to its direct band gap of 3.34 eV, ZnO efficiently absorbs UV radiation and for this reason finds application in medicine as a bactericide and in cosmetics as a UV filter in suntan lotions (1-4). The effect of ultraviolet radiation on human skin depends on the wavelength of the UV radiation. It is generally accepted that UV-A (320-400 nm) is responsible for immediate pigmentation, UV-B (290-320 nm) is more dangerous in terms of long-term effects that can cause skin cancer, while UV-C is blocked by absorption by the ozone layer. Most creams for solar protection block UV-B effectively and stop sunburn so that the effects of UV-A radiation remain un-diagnosed. It has been established however, that UV-A can cause the formation of free radicals in the presence of organic compounds and biomolecules and causes the photoaging of human skin (5). Therefore, products that efficiently block UV-A radiation should be developed.

Both titanium dioxide and zinc oxide powders have been used as additives in cosmetics for protection against UV-A. A comparison of the UV protection efficiency and whiteness for both ZnO and TiO2 has been carried out using in vivo diffuse reflectance spectroscopy (6). It was observed that in the spectral region of 340-380 nm, ZnO offered superior UV protection and appeared less white (greater transparency in the visible spectrum) as compared to TiO2 for all concentrations tested. This result was attributed to the lower scattering efficiency of ZnO particles when compared to TiO2 for the same particle size. Hence, in addition to high purity, a precise control of the particle size, shape and agglomeration is necessary to obtain films with high transmittance in the

ECS Transactions, 25 (8) 1177-1183 (2009)10.1149/1.3207722 © The Electrochemical Society

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visible light range and high absorption coefficients for UV light. Particle sizes in the range 20-50 nm are particularly attractive for these applications in view of their small size, non toxicity and ease of dispersion in solvents. Recently, we have reported the shape controlled synthesis and structure-property correlation of nanoparticles of ZnO by chemical vapor synthesis (CVS) using Zn metal precursor (7). It was observed that due to the unique solid-vapor mechanism operating during the oxidation of zinc, spheres, or nanotetrapods of ZnO can be produced selectively.

It is well documented that nano TiO2 and ZnO are photosensitive and produce reactive oxygen species (ROS) in the presence of UV-A radiation (8). It has been observed that the bactericidal activity of ZnO is due to the production of ROS in water that cause cell damage due to oxidative stress in bacteria and that this effect was limited to nanoparticles of ZnO, micron sized powders remaining inert (9). In this context, we have compared the ROS generation under UV-A radiation in aqueous suspensions of ZnO nanoparticles of varying shape (tetrapods and spheres) prepared by chemical vapor synthesis.

In this communication we have evaluated the applicability of ZnO particles with different morphologies for application as UV filters. Using diffuse reflectance spectroscopy we have determined the absorption profiles in the different regions of the UV spectrum. More importantly, we have compared the ecotoxicity of aqueous suspensions of the ZnO particles by measuring the rate of generation of reactive oxygen species (ROS), such as hydroxyl (OH●) and superoxide (O2

●-) radicals, under UV-A illumination and compared with TiO2 nanoparticles having an average particle size in the similar range (P25 formulation by Degussa containing a 20%rutile+ 80%anatase mixture). The electron spin resonance (ESR) technique offers an ideal method for the study of formation of reactive oxygen species (ROS) which are photo-synthesized in the presence of light-excited nanoparticles (10). Therefore, for the detection of ROS, we applied ESR-spin trapping technique with a spin-trap, 5,5`-dimethyl-pyrroline-1-oxide (DMPO), which is known to scavenge both OH● and O2

●- radicals. Under similar experimental conditions the tetrapods generated slightly more ROS than spheres, but both revealed a much lower ROS generation capacity when compared to Degussa P25 nano-TiO2.

Experimental

The tetrapods and spheres of ZnO have been produced by CVS. The details of this process are given in reference 7. A summary of the results is given below. The synthesis was carried out in a 100 cm long and 3 cm internal diameter tubular quartz gas flow reactor heated by a horizontal three zone furnace. All experiments were carried out under atmospheric pressure. A typical CVS involved the following procedure: 7g of Zn metal powder (325 mesh size) contained in an alumina boat was introduced into the central hot zone maintained at temperatures varying from 650°C to 950°C under argon atmosphere. Dried, filtered compressed air or oxygen was used as the oxygen source. After the temperature was stabilized at the desired value, air, oxygen or air + water vapor (500-3000 sccm) was introduced at a specified point in the third zone (Tox= 400-900°C) the distance of which from the center could be varied. Zinc vapor oxidized giving rise to thick white fumes that turned to fluffy white flakes that came flying with a high velocity towards the cold end. These were collected in cold traps. The as produced powders were characterized without further treatment. The chemical composition and crystal structure were determined using X-ray powder diffractograms, where from crystallite sizes and

ECS Transactions, 25 (8) 1177-1183 (2009)




lattice parameters were calculated using standard techniques (Fullprof software using the Rietveld method). Both scanning and transmission electron microscopy were used to characterize the size and shape of ZnO. BET surface areas were calculated by measuring the N2 adsorption at liquid nitrogen temperature. Optical absorption spectra were measured for colloidal dispersions of the nanoparticles in ethanol that were prepared by dispersing 20 mg of the particles in 20 mL of ethanol followed by sonication for 5 minutes. Diffuse reflectance spectra were measured using BaSO4 as the standard. Sample preparation for ESRD of ROS

For performing ESR spin-trapping experiments of ROS, 3.2 mg of ZnO nanopowder was dispersed in 10 mL of ultrapure water (Millipore, Simplicity UV, Millipore Corp., France). The water suspension of ZnO nanoparticles was then homogenized for 10 min in an ultra-sound bath. The standard commercially available spin trap, 5,5`-dimethyl-pyrroline-1-oxide (DMPO), from Sigma was used for ESR detection of ROS (hydroxyl and superoxide radicals). Before application, DMPO was carefully purified by filtration on charcoal. The obtained stock solution of 0.5 M DMPO in ultrapure water was stored at -200C. Immediately before performing ESR measurements, the aqueous suspension of ZnO nanoparticles was mixed with the stock solution of DMPO to achieve the final spin trap concentration of 100 mM. Subsequently, the 1 mL aliquot of prepared suspensions containing DMPO and ZnO nanoparticles was transferred into a small pyrex beaker (5 mL volume, 20 mm OD and 30 mm height) and exposed to UV-A illumination (wavelength λ = 365 nm) using a UV spot-light source, LightingcureTM, model LC-8, from Hamamatsu Photonics, France. The LightingcureTM source incorporates a 200W Mercury-Xenon lamp (model L7212-01) and a quartz fiber light guide, which is optimized for high transmittance in the UV spectral region. The light guide termination was positioned at 0.5 cm distance over the open face of the beaker, thus yielding an illumination power density of 10 mW/cm2 (for the lamp power setting at 20%). The intensity of UV-A light illuminating the aqueous suspension of ZnO nanoparticles was measured with a UV-meter; model C6080, from Hamamatsu Photonic, France. To avoid sedimentation during illumination, the suspension of ZnO nanoparticles was constantly stirred with the use of a small magnetic stirrer. ESR detection of spin-trapped ROS

Immediately after subsequent exposures to UV-A, aliquots of ca. 7 mL of illuminated suspensions were transferred into 0.7 mm ID and 0.87 mm OD quartz capillary tubes from VitroCom, NJ, USA (sample height of 25 mm) and sealed on both ends with Cha-SealTM tube sealing compound (Medex International, Inc., USA). ESR experiments were carried out at room temperature using an ESP300E spectrometer (Bruker BioSpin GmbH), equipped with a standard rectangular mode TE102 cavity. Routinely, for each experimental point, five-scan field-swept ESR spectra were recorded. The typical instrumental settings were: microwave frequency 9.38 GHz, microwave power 2.0 mW, sweep width 120 G, modulation frequency 100 kHz, modulation amplitude 0.5 G, receiver gain 4 × 104, time constant 20.48 ms, conversion time 40.96 ms, and time per single scan 41.9 s.

Results

The morphology of the ZnO produced depends both on the temperature of zinc

and the temperature of oxidation. It is found that lower zinc temperatures in the range of





650-750°C with oxidation temperatures in the range 500-650 °C produced nanocrystals of ZnO that have spherical or a facetted spherical shape (Figure1A). At these temperatures, the evaporation of zinc is a slow process and a fine off-white to pale yellow powder is deposited on the walls of the collector. The time of reaction for 7g of Zn is one hour and the yield is around 10-13% of the theoretical value assuming complete oxidation of Zn. The spheres disperse in alcohol easily and form stable suspensions. X-ray powder diffractograms of the powders show the crystalline phase of wurtzite (JCPDS 36-1451) with an average crystal size of 20 nm (7). Transmission electron micrographs show that the spheres are single crystals in the size range of 8-40 nm. In a histogram of particle size measurements made from 100 randomly selected particles, only 10% of the particles had less than 20 nm size. More than 60% of the particles had sizes in the range 20-30 nm and the rest were in the 30-40 nm range. The BET surface area of these powders was in the range of 10-12 m2/g.

Figure 1 Scanning electron microscopy images for A) ZnO spheres/facetted spheres and B) tetrapods prepared by chemical vapor synthesis.

When the zinc temperature is raised above 800°C and the oxidation temperature is in the range 750-900°C, tetrapods and multipods of ZnO are produced. Fine tetrapods of ZnO (diameter 10-30 nm and length ranging from 100 nm - 2 µm) are selectively produced when the zinc temperature and the oxidation temperature are maintained at 900°C and water vapor is added to air (7). The selectivity is further improved by changing the residence time of the zinc vapor by varying the position of the air inlet. It is found that best shape control was achieved when the air inlet was at a distance of 8 cm from the center of the boat containing the Zn powder precursor. X-ray powder diffractograms show that the tetrapods have wurtzite structure with no preferred orientation. BET surface areas of the tetrapods are in the range 20-28 m2/g. A typical SEM image of the tetrapods is shown in the figure 1B. High resolution TEM images of these samples show that the tetrapods are made up from four individual or two twin nanorods formed by self assembly (7). The tetrapods also form transparent suspensions in alcohol but these suspensions are less stable than those for the spheres and usually settle down after a few hours. Diffuse reflectance spectra

The application of ZnO as a pigment and as an additive for sun tan lotions is due to its high absorption in the UV region. Optical absorption spectra of colloidal

A B





suspensions (not shown) confirm that ZnO nanoparticles produced by CVS have high absorption coefficients in the UV region (7).

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Figure 2 Diffuse reflectance spectra for ZnO spheres and tetrapods compared to that for TiO2 measured at 25°C. The absorption here denotes 100-R where R in % is the reflectance of the sample calibrated against barium sulfate powder whose reflectance is taken as 100% over the whole spectral range. The inset shows the spectral region of 340-420 nm.

The diffuse reflectance spectra of ZnO tetrapods and spheres along with that of

TiO2 (Degussa P25) are shown in Figure 2. The spectra in Figure 2 show that both ZnO tetrapods and TiO2 particles absorb strongly in the UV with a sharp increase in absorption for ZnO at the band gap energy. The spheres have a broad absorption spectrum that extends well below the band gap. The extended absorption of the spheres in the visible region of the spectrum arises from intraband states created by the presence of surface defects thereby giving it a pale yellow color (7). The ZnO tetrapods and TiO2 have high reflectivity in the visible region. In the spectral region of 340-380 nm, ZnO tetrapods and spheres have a much stronger absorption (25% higher) than TiO2 whereas in the region 390-420 nm, TiO2 has a higher absorption (Figure inset). Thus, ZnO shows a higher sensitivity in the UV-A region when compared to titanium dioxide. ESR spectra of ZnO spheres and tetrapods under UV-A

The ESR experiments of ROS spin-trapping with DMPO clearly indicate that both ZnO pigments investigated in this study generated the reactive oxygen species in aqueous

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suspensions illuminated with UV-A light. However, as can be seen in Figure 3, in the presence of both ZnO tetrapods and spheres the intensities of the characteristic signals of the spin adduct DMPO-OH (1:2:2:1, g = 2.0056, aN = 14.9 G, aH = 14.9 G) were markedly smaller than those yielded by the photocatalyst of reference, P25 Degussa by a factor of ~30 and that of pure rutile (not shown in Figure 3) by a factor of 5. Among the two ZnO pigments, ZnO tetrapods revealed a slightly higher photocatalytic activity than ZnO spheres. This difference may arise due to the higher absorption coefficient for the tetrapods when compared to spheres (7).

Figure 3 Evolution of the ESR signal intensity of DMPO-OH spin adduct as a function of illumination time. (a) Comparison of the DMPO-OH signal intensity growth observed for two ZnO samples and a photocatalyst of reference, P25 Degussa. Inset: typical ESR spectrum of DMPO-OH spin adduct acquired in this experiment. (b) Close-up view on the evolution of the ESR intensity of DMPO-OH as a function of illumination time for two ZnO samples. Symbols: TiO2 P25 Degussa (black stars), tetrapods (black squares), ZnO spheres (open circles).

Conclusions

High purity nanocrystalline spheres and tetrapods of ZnO have been selectively

synthesized in high yield by chemical vapor synthesis using zinc metal precursor. ZnO tetrapods show greater absorption of UV-A radiation when compared to TiO2. ZnO spheres absorb both UV and visible radiation and are pale yellow in color due to the presence of surface defects. Both ZnO tetrapods and spheres generate ROS in aqueous suspensions when irradiated with UV-A. However, the concentration of these species is lower than that for nanocrystalline TiO2, P25 formulation by Degussa, by more than one order of magnitude. Hence, nanocrystalline ZnO in the size range 20-50 nm can be considered an ideal material for application as filter for UV-A radiation.

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Acknowledgments The authors thank S. Le Blond Du Plouy (TEMSCAN, Toulouse University) for the Scanning Electron Microscopy images and D. Neumeyer for BET surface area measurements and X-ray diffractograms. Authors RB and PS gratefully acknowledge financial support from Agence Nationale de Recherche, France (RNMP05-PRONANOX). K.P., L.F., and A.S acknowledge the support of the Swiss National Science Foundation, project No. 205320–112164, ‘‘Biomolecules under stress: ESR in vitro study’’.

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CVD Synthesis of Shape and Size Controlled ZnO Nanoparticles for Application as UV Filters

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