pubs.acs.org/Langmuir © 2010 American Chemical Society
Synthesis of ZnO and Zn Nanoparticles in Microwave Plasma and Their Deposition on Glass Slides Alexander Irzh,† Isaschar Genish,† Lior Klein,† Leonid A. Solovyov,‡ and Aharon Gedanken*,† †
Departments of Chemistry and Physics and the Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel, and ‡Institute of Chemistry and Chemical Technology, Krasnoyarsk 660049, Russia Received November 29, 2009. Revised Manuscript Received March 14, 2010 This work represents a new method to synthesis of ZnO and/or Zn nanoparticles by means of microwave plasma whose electrons are the reducing agents. Glass quadratic slides sized 2.5 2.5 cm were coated by ZnO and/or Zn particles whose sizes ranged from a few micrometers to ∼20 nm. The size of the particles can be controlled by the type of the precursor and its concentration. In the current paper, the mechanism of the reactions of ZnO and/or Zn formation was proposed. Longer plasma irradiation and lower precursor concentration favor the fabrication of metallic Zn nanoparticles. The nature of the precursor’s ion (acetate, nitrate, or chloride) is also of importance in determining the composition of the product. The glass slides coated by ZnO and/or Zn nanoparticles were characterized by HR-SEM, HR-TEM, AFM, XRD, ESR, contact angle and diffuse reflectance spectroscopy (DRS).
Introduction A gaseous system wherein charged particles (electrons, ions) are important species is called plasma.1,2 Plasma is created by accelerating electrons (or ions, which are heavier) in the gas (a gain in kinetic energy) using an electric field until they collide with gas molecules. In this collision, kinetic energy is transferred to the collision partner, and if the kinetic energy of the electron is high enough, the ionization of the collision partner will take place, yielding an increase in the total amount of free charged particles. The collective behavior of these particles is known as plasma, and there is a large amount of literature that describes their chemical and physical properties.3-7 Recently, many techniques for coating substrates by inorganic nanoparticles have been developed, e.g., electroplating,8 coating by means of microwave irradiation,9,10 sonochemical coating,11,12 sputtering,13 chemical vapor deposition (CVD),14 and more. All these techniques have both advantages and disadvantages. For example, electroplating is a quick and simple method but is usable only for conductive substrates. Sonochemical coating and coating by microwaves do not require conductive substrates as electroplating does, but in these methods the reaction system needs a *To whom correspondence should be addressed. (1) Lifshitz, E. M.; Pitaevskii, L. P. Physical Kinetics; Butterworth-Heinemann Press: Woburn, MA, 1981; p 129. (2) Chen, F. F. Introduction to Plasma Physics and Controlled Fusion, 2nd ed.; Springer Press: Heidelberg, Germany, 1984. (3) Fridman, A. Plasma Chemistry; Cambridge University Press: Cambridge, UK, 2008. (4) Lieberman, M. A.; Allan, J., Lichtenberg, A. J. Principles of Plasma Discharges and Materials Processing, 2nd ed.; Wiley Press: Malden, MA, 2005. (5) Conrads, H.; Schmidt, M. Plasma Sources Sci. Technol. 2000, 9, 441. (6) Vollath, D. J. Nanopart. Res. 2008, 10, 39. (7) Denes, F. S.; Manolache, S. Prog. Polym. Sci. 2004, 29, 815. (8) Blum, W. Ind. Eng. Chem. 1927, 19, 1111. (9) Irzh, A.; Perkas, N.; Gedanken, A. Langmuir 2007, 23, 9891. (10) Irzh, A.; Gedanken, A. J. Appl. Polym. Sci. 2009, 113, 1773. (11) Kotlyar, A.; Perkas, N.; Amiryan, G.; Meyer, M.; Zimmermann, W.; Gedanken, A. J. Appl. Polym. Sci. 2007, 104, 2868. (12) Perelshtein, I.; Applerot, G.; Perkas, N.; Wehrschetz-Sigl, E.; Hasmann, A.; Guebitz, G. M.; Gedanken, A. ACS Appl. Mater. Interfaces 2009, 1, 363. (13) Boumans, P. W. J. M. Anal. Chem. 1972, 44, 1219. (14) Biefeld, R. M. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 525.
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large amount of solvent, which results at the end of the reaction in chemical waste. In addition, the sonochemical method takes quite a long time, with a typical reaction time of 1-2 h. Two additional common techniques for the deposition of nanomaterials are sputtering and chemical vapor deposition (CVD). The use of CVD takes place using gaseous15,16 or volatile17 precursors. This requirement limits the variety of materials that can be fabricated using this technique. The use of aerosol-assisted chemical vapor deposition (AACVD)18 has been a major development that has enabled the use of a large variety of precursors dissolved by aerosol for coating. Unfortunately, sputtering, CVD and AACVD are relatively expensive techniques. The current paper describes extended research on the deposition of Zn and/or ZnO particles on glass surfaces by means of microwave (MW) plasma. This is performed by using an ethanolic solution of zinc salts. Our first results were described in previous papers,19,20 which focused on the deposition of various metals (Au, Ag, Pt, Pd, Co, Ni)19 on glass surfaces as well as the deposition of zinc layers on dielectric (glass and quartz) and conductive (Al and Si) surfaces by the solvent-assisted deposition in plasma (SADIP) method.20 The current paper discusses the various parameters affecting the nature of the product (precursor concentration, length of plasma radiation, and the nature of the negative ion) and suggests a possible reduction mechanism for obtaining metallic zinc from its precursors under plasma treatment. In addition, this paper describes the various morphologies of Zn and ZnO that can be formed using the solvent-assisted deposition in plasma (SADIP) method. Our method is very simple, short, and inexpensive. In addition, while at the sputtering, CVD, and AACVD the substrate and the (15) Pant, A.; Huff, M. C.; Russell, T. W. F. Ind. Eng. Chem. Res. 2001, 40, 1386. (16) Gladfelter, W. L. Chem. Mater. 1993, 5, 1372. (17) Cheon, J.; Dubois, L. H.; Girolami, G. S. Chem. Mater. 1994, 6, 2279. (18) McCain, M. N.; Schneider, S.; Salata, M. R.; Marks, T. J. Inorg. Chem. 2008, 47, 2534. (19) Genish, I.; Irzh, A.; Gedanken, A.; Anderson, A.; Zaban, A.; Klein, L. Surf. Coat. Technol. 2010, 204, 1347. (20) Irzh, A.; Genish, I.; Chen, L.; Ling, Y.-C.; Klein, L.; Gedanken, A. J. Phys. Chem. C 2009, 113, 14097.
Published on Web 03/25/2010
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Figure 1. Schematic illustration of the reaction system.
whole chamber require coating by the depositing material; at the SADIP method only the substrate is coated. Therefore, by using SADIP, the waste of the material is very limited. It also should be noted that by using CVD, the typical roughness of the produced layers is smaller than the roughness produced by SADIP (∼25 nm at the Zn layer). However, zinc production by means of CVD requires unstable and volute precursors like diethylzinc, while by means of SADIP, zinc can be produced from stable salts of nitrate, acetate, and/or chloride.
Experimental Section Reaction System. The reaction system contains a domestic microwave oven (900 W, Crystal) with a drilled hole on the upper part, a plasma chamber made from Pyrex, an argon cylinder (99.999%), a vacuum pump (Franklin Electric), and rubber and Pyrex pipes that connect the plasma chamber to the argon cylinder and to the vacuum pump. A schematic illustration of the reaction system is presented in Figure 1. The photograph of the system can be also seen in our previous paper.20 Coating Procedure. All the reagents were of the highest commercially available purity, purchased from Aldrich Co., and used without further purification. Solutions of zinc salts in ethanol in various concentrations have been prepared. The concentrations of the ethanolic solution of Zn(NO3)2 were 0.03, 0.5, and 1 M. A saturated solution of Zn(O2C2H3)2 in ethanol (∼0.03 M) and a solution of ZnCl2 (0.5 M) in ethanol were also prepared. Three drops of each solution (∼0.023 mL each drop) were dropped on a quadratic glass slide of 2.5 2.5 cm. The wet glass slide was placed in the plasma chamber shown in Figure 1. Next, the vacuum pump was switched on (∼10-2 Torr), and the system was washed by argon for about 10 s at a rate of 5 L/min. After washing the system, the argon flow was set at 300 cm3/min. The MW oven was then activated and a plasma glow discharge of argon appeared. A typical time for the plasma reaction was 60 s, although reaction times could vary from 10 to 120 s. The resulting product was found to be a glass slide coated by zinc oxide or by metallic zinc. The nature of the coating was found to depend on the zinc salt (i.e., on the anion), the zinc salt concentration in the ethanol solution, and the reaction time. The glass slides with ZnO particles were white, while the glass slides with Zn particles were black or gray. It also should be noted that all the above-mentioned reactions were also performed without an argon flow and that the product was only ZnO and not metallic zinc. A list of the reactions that were carried out in this research and the main characterizations are summarized in Table S1 of the Supporting Information. Characterization Techniques. The X-ray diffraction (XRD) patterns of the products were measured with a Bruker AXS D* advance powder X-ray diffractometer (using Cu KR radiation = 1.5418 A˚). The morphology of the glass surface coated by Zn and ZnO nanoparticles was measured with a JEOL-JSM 7000F highresolution scanning electron microscope (HR-SEM) operating at 15 kV accelerating voltage and by atomic force microscopy Langmuir 2010, 26(8), 5976–5984
Figure 2. XRD patterns of films deposited on a glass surface from an ethanol solution of zinc acetate dihydrate (0.03 M): (a) before irradiation in plasma, (b) after 20 s of irradiation, (c) after 30 s of irradiation, (d) after 50 s of irradiation, and (e) after 60 s of irradiation in plasma. (AFM) that was carried out using a Digital Instruments Nanoscope. High-resolution transmission electron microscopy (HRTEM) was measured using a JEOL JEM-2100 electron microscope. The diffuse reflection spectroscopy (DRS) spectra were recorded on a Cary 100 Scan UV-vis spectrophotometer. Contact angle measurements were done on a Rame-Hart model 100 contact angle goniometer. Measurements were taken under ambient conditions. Contact angles were determined by placing a drop (∼3 μL) of deionized-distilled water on the sample with a microsyringe. The electron spin resonance (ESR) measurements were conducted on a Bruker EPR 100d X-band spectrometer. DMPO (0.02 mol L-1) was added to 10 mg mL-1 of a ZnO aqueous suspension and was drawn by a syringe into a gas-permeable Teflon capillary (Zeus Industries, Raritan, NJ) of 0.082 cm inner diameter, 0.038 in. wall thickness, and 15 cm length. Each capillary was folded twice, inserted into a narrow quartz tube that was open at both ends, and then placed into the ESR cavity.
Results and Discussion XRD Results. In order to study the nature of the nanolayers produced on the glass substrates, X-ray diffraction (XRD) measurements were carried out. It was observed that the crystal structure of the deposited material is dependent on the reaction time and on the anion coupled to the zinc ion. First, we investigated the formation of ZnO and Zn nanoparticles on glass surfaces obtained when a saturated zinc acetate solution in ethanol (∼0.03 M) was irradiated as a function of time. Figure 2 represents the XRD measurements of the deposited material on glass after various reaction times. If microwave plasma was not employed, a recrystallized zinc acetate dihydrate was found on the surface of glass. Zinc acetate dihydrate has two strong peaks at 2θ = 22.3° and 25.1° (ICDD PDF-2 file: 00-033-1464). After irradiating the sample for 20 s in plasma, the zinc acetate dihydrate lost its hydration molecules, and an anhydrous zinc acetate was detected on the glass surface. The anhydrous zinc acetate film deposited on the glass surface shows a preferential growth along the (100) plane and reveals diffraction peaks at 2θ = 6°, 11.9°, 23.7°, 36°, and 48.7° assigned to the (100), (200), (400), (600), and (800) reflections, respectively (Acta Crystallogr. E 2006, m3291). An XRD pattern of anhydrous zinc acetate illustrating peaks at smaller diffraction angles (not shown in Figure 2) is presented in the Supporting Information (Figure S1). DOI: 10.1021/la904499s
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Figure 3. XRD patterns of films deposited on a glass surface from an ethanol solution of zinc nitrate dihydrate (0.03 M): (a) before irradiation in plasma (obtained material is Zn(OH)(NO3)(H2O)), (b) after 10 s of irradiation (obtained material is Zn(OH)(NO3)(H2O)), (c) after 20 s of irradiation (obtained material is Zn(OH)(NO3)), (d) after 30 s of irradiation (main phase is ZnO), and (e) after 60 s of irradiation in plasma (main phase is Zn).
When the anhydrous zinc acetate was treated by plasma for 30 s, it was thermally decomposed to yield hexagonal ZnO crystals, which did not grow with any preferred orientation (ICDD PDF-2 file: 01-075-0576). In addition to ZnO diffraction peaks, we could observe diffraction peaks attributed to the unreacted anhydrous zinc acetate at 23.7° (plane (400)) and 36° (plane (600)). After 50 s of irradiation in plasma, all the anhydrous zinc acetate decomposed to ZnO, and diffraction peaks of metallic zinc started to appear. This phase of metallic zinc appears as a result of the reduction of Zn2þ ion in the ZnO crystal. After 60 s of irradiation in plasma, ZnO was almost completely reduced by plasma electrons to metallic zinc with a hexagonal structure (ICDD PDF-2 file: 00-001-1238). The mechanism of the ZnO reduction to metallic zinc will be explained later. The same experiments were repeated using zinc nitrate as the precursor. The solution of zinc nitrate in ethanol was 0.03 M, identical to the zinc acetate solution. Using XRD, we followed the composition of the films obtained on the glass slides from the ethanol solution of zinc nitrate. After the dissolution of zinc nitrate in ethanol and its recrystallization by evaporating the ethanol under vacuum, the zinc nitrate was transformed to zinc hydroxide nitrate hydrate, Zn(OH)(NO3)(H2O) (ICDD PDF-2 file: 01-084-1907). The starting material in the plasma reaction is therefore Zn(OH)(NO3)(H2O) rather than zinc nitrate hydrate. After 20 s, the Zn(OH)(NO3)(H2O) was distorted and its diffraction peaks changed significantly. In addition, the diffraction peaks after 20 s became broader compared to the diffraction peaks observed after 10 s. All these changes may be explained by the loss of hydration molecules from zinc hydroxide nitrate hydrate and the formation of the anhydrous salt. After 30 s in plasma, almost all the zinc hydroxide nitrate was thermally decomposed in plasma to pure ZnO. When the reaction time was 60 s, all the ZnO was reduced by the electrons in the plasma to metallic zinc. A layer of the zinc is formed on the surface of the glass slide. All above-mentioned reactions can be seen in Figure 3. From these experiments it became clear that the SADIP reaction has two steps. The first is the thermal decomposition 5978 DOI: 10.1021/la904499s
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Figure 4. XRD pattern of zinc that was evaporated and condensed on a glass slide from a solution (0.5 M) of zinc chloride after irradiation of 120 s in plasma.
of zinc salt (nitrate or acetate) to ZnO. The second step is the reduction of ZnO by plasma electrons to metallic zinc. Because the reaction is carried out under argon atmosphere, the only source of oxygen in ZnO has to be from the oxygens contained in the anions of the Zn salt. It was therefore of great interest to use a zinc salt that does not contain oxygen, such as zinc chloride. Our assumption was that zinc chloride will be directly reduced to metallic zinc without going through ZnO. After 30 s in plasma, the zinc chloride (0.5 M) did not react at all. After 60 s the zinc chloride became slightly gray, indicating the formation of metallic zinc, although most of ZnCl2 did not react and remained untouched on the glass. The zinc chloride is known as a very hygroscopic material which causes the substantial absorption of water on the layer. Therefore, even after 60 s under plasma ZnCl2 was still very wet (even deliquescent), so XRD measurements of this layer showed only amorphous materials. When the reaction time was increased to 120 s in order to reduce all the zinc chloride, we could observe that after about 90 s the temperature inside the plasma chamber became so high that all the metallic zinc evaporated from the substrate to the chamber walls, and no material remained on the glass slide. In order to collect the evaporated product, a few small glass slides were placed on the walls of the chamber, and the evaporated product was collected on these slides. Figure 4 shows the XRD of the layer that was deposited on the small glass slide as a result of vaporization. Figure 4 also shows the pure and highly crystalline layer of hexagonal Zn that was deposited on the glass (ICDD PDF-2 file: 00-001-1238). The dependence of the nature of the plasma product on the salt concentration was also investigated. It was discovered that using a higher concentrations of zinc nitrate leads to slower reactions, as compared with the reactions of low concentrations of this salt. It was found that while metallic zinc is formed after 60 s of reaction time from a solution of 0.03 M of zinc nitrate, it takes 5 min for its detection when the concentration is raised to 0.5 M. The 5 min reaction is performed by five consecutive reactions of 1 min each with a break of 30 s between the 1 min reactions. These breaks were needed in order to cool the plasma system; otherwise, the Pyrex plasma chamber would crack. This difference between reduction times of ZnO formed from diluted solution of zinc Langmuir 2010, 26(8), 5976–5984
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nitrate and concentrated solution of this salt can be explained in three possible ways. The first explanation is that ZnO particles formed from concentrated solution are large aggregates as can be seen in Figure 7b, while ZnO particles formed from a diluted solution are much smaller. To reduce large ZnO particles, larger amounts of electrons have to react with ZnO particles. However, a large amount of electrons cannot be adsorbed to the surface at very short period of time because “extra” electrons will be repelled by previously absorbed electrons. Therefore, reduction of large particles that need absorption of large amount of electrons requires longer periods of time. The second explanation is that when large amounts of ZnO undergo reduction, large amounts of oxygen ions are released to the plasma chamber. This oxygen can oxidize back the just reduced zinc metal and this way to disturb further reduction and slow it down. Finally, the third possible explanation is that these oxygen ions that are expelled from the ZnO surface to the plasma chamber increase the pressure inside the plasma chamber, and thus the collision frequency between electrons and oxygen ions increases. As shown in ref 6, in the RF system the increase of collision frequency (pressure) causes finally the decrease of the electron energy. Therefore, when the concentration of zinc salt is large, many oxygen ions that are released to the plasma chamber lower the plasma energy and eventually the efficiency of plasma electrons to reduce zinc ions. The general conclusion regarding the particle sizes that can be obtained by applying the Scherrer formula (D = 0.94λ/β1/2 cos θ, where D is the diameter of the crystallites, λ is source wavelength, β1/2 peak full width at half-maximum, and θ is peak position) to the various XRD patterns is that the crystallites of ZnO are always smaller than the crystallites of metallic zinc. For example, the size of ZnO crystallites obtained from a 0.03 M zinc acetate solution was estimated at ∼10 nm, but the zinc crystallites obtained after 50 s of this reaction were around 24 nm. This size increased after 60 s reaction to 34 nm. These values are very close to the size of zinc nanoparticles obtained by SEM measurements. While SEM and XRD concur regarding the size of the products of 0.03 M zinc acetate, significant differences were detected between these two characterization methods for the product obtained from 0.5 M solution of zinc nitrate. The Scherrer formula gave a 27 nm size for these particles, while according to the HRSEM image (Figure 7b) large (4 ( 0.6 μm) aggregates of ZnO are formed. Since the Scherrer formula calculates the size of the individual crystallites, this means that the micrometer-sized ZnO structures are aggregates composed of nanosized crystallites calculated by the Scherrer formula (27 nm). (Additional data for the Scherrer formula can be found in Table S1 in the Supporting Information.) Diffuse Reflection Spectroscopy (DRS). To investigate the optical properties of the deposited zinc and/or ZnO, DRS spectra were measured. DRS is a measurement of the reflectance of deposited layers and is sensitive to the detection of surface plasmons. By using the Kubelka-Munk transformation, it is possible to translate reflectance to the absorbance.21-25 Using DRS allows us to follow the formation of ZnO and metallic zinc on the surface of samples prepared under different reaction times. Zinc has a plasmon absorption peak in the UV region (242 nm).26 Since the glass substrate blocks the UV region, we also performed (21) Samuels, A. C.; Zhu, C.; Williams, B. R.; Ben-David, A.; Ronald, W.; Miles, R. W.; Hulet, M. Anal. Chem. 2006, 78, 408. (22) Bekhterev, A. N.; Zolotarev, V. M. Diamond Relat. Mater. 2007, 16, 2093. (23) Ershovaa, N. I.; Ivanov, V. M. Anal. Chim. Acta 1998, 364, 235. (24) Murphy, A. B. J. Phys. D: Appl. Phys. 2006, 39, 3571. (25) Loyalka, S. K.; Riggs, C. A. Appl. Spectrosc. 1995, 49, 1107. (26) Zeng, H.; Cai, W.; Li, Y.; Hu, J.; Liu, P. J. Phys. Chem. B 2005, 109, 18260.
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Figure 5. DRS spectra of deposited material after 40 and 60 s. DRS spectra of reference ZnO are presented in the inset to the figure. The starting solution was zinc acetate (0.03 M). F(R) = (1 - R)2/2R, where R is reflectance.
the same reactions on quartz plates of 2.5 2.5 cm instead of on glass substrates. All other reaction parameters remained unchanged. First, the DRS spectrum was measured for a sample that had been already characterized as ZnO. The sample was prepared by irradiating a 0.5 M solution of zinc nitrate with plasma for 60 s. The inset in Figure 5 exhibits two DRS peaks at 361 nm (sharp peak) and at 250-346 nm (broad peak) of ZnO. These absorption bands have already been reported previously as typical absorptions of ZnO.10 Figure 5 also presents two products that were obtained from the irradiation of 0.03 M zinc acetate after 40 and 60 s. As mentioned above, the irradiation of 60 s creates more Zn, and therefore the absorption of the Zn plasmon band should be higher. Indeed, in addition to the two peaks of ZnO that are still detected, an additional weak peak is observed at 230 nm. This peak is assigned to the metallic Zn plasmon. This assignment is well supported by the absorption spectrum of the product obtained after 60 s under plasma irradiation. A dramatic increase in the absorption intensity of the Zn plasmon is observed accompanied by a red shift of the peak to 248 nm, as compared with the 230 nm peak observed for the 40 s sample. It is wellknown that plasmon absorption can undergo blue shift as the size of metallic nanoparticle decreases and vice versa undergo red shift when the size is increased.27 Since after 40 s zinc nanoparticles are at their early nucleation stage, and therefore should be very small, its plasmon absorption is blue-shifted. When the reaction time is 60 s, the size of zinc nanoparticles increase to ∼34 nm, and therefore the plasmon red-shifts to 248 nm. This location of the Zn plasmon is in good agreement with the results of Zeng et al.26 This result also concurs with the XRD results (Figure 2e), showing that after 60 s zinc is the main product with only little residues of ZnO. The DRS results further substantiate our twostep mechanism suggested above. Morphological Studies. Morphologies of the Zn and ZnO films that were deposited on glass surfaces were studied by HRSEM and AFM. Figure 6a shows an HRSEM picture of the product after 20 s of 0.03 M zinc acetate. The film is not composed of distinct particles, but rather of irregular structures, (27) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212.
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Figure 6. HRSEM of glass with zinc acetate after reaction of (a) 20, (b) 30, (c) 50, and (d) 60 s. All images are at magnification of 100 000. The concentration of the solution was 0.03 M.
and many uncoated spots on the glass are seen. According to the XRD measurements, this film is composed of anhydrous zinc acetate. After 30 s in a plasma atmosphere, the thermal decomposition of the anhydrous zinc acetate film occurs, and ZnO nanoparticles with an average size of 21 ( 5.8 nm are formed on a glass surface (Figure 6b). The ZnO obtained after irradiation for 50 s undergo further reduction to receive particles with an average size of 22 ( 6.3 nm, as can be seen in Figure 6c. Thus, the early stage of the decomposition of the ZnO does not lead to any change in size, and thus anhydrous zinc acetate and ZnO have the same dimensions. After 60 s in plasma, a homogeneous layer of zinc nanoparticles with an average size of 34 ( 16 nm was formed on the glass surface (Figure 6d). It is interesting to note that average particle size changes upon the reduction of ZnO to Zn and increases from 21 ( 5.8 nm to 34 ( 16 nm, respectively. This can be explained by the partial melting of Zn nanoparticles in the hot plasma atmosphere. The new Zn particles are on the average composed of two sintered ZnO particles that have lost their oxygens, and therefore, the new particles are smaller than 2 21 nm. We have also used AFM measurements to find out the thickness of the zinc layer. For this reason the layer was protected by a photoresist (microposit S1813), and the sample was then treated by concentrated nitric acid. In this process the unprotected zinc atoms dissolved in the acid, after which the photoresist was removed by acetone. The AFM performed on the edge of the zinc layer found its thickness to be ∼50 nm (see Figure S2 in the Supporting Information). The product of the 0.03 M zinc nitrate solution is depicted in Figure 7a. The Zn nanoparticles similar to those presented in Figure 6d are spherical, and their average size is 35 ( 18 nm. It is therefore concluded that there are no morphological changes when Zn products fabricated from different zinc salts are compared. On the other hand, the ZnO formed after 60 s from the 1 M 5980 DOI: 10.1021/la904499s
solution of zinc nitrate has a unique structure (see Figure 7b). These “bagel”-shaped structured are 6.8 ( 1 μm in size, and the diameter of the central hole is 2.9 ( 0.6 μm. On the basis of the Scherrer formula of the corresponding XRD results, we believe that the bagel is composed of small ZnO nanoparticles. As mentioned above, the Zn particles obtained from the irradiation in the plasma of zinc chloride were collected on the pieces of glass attached to the walls of the Pyrex chamber. A homogeneous layer of zinc microparticles was formed on the surface of the glass. The average size of these particles was 1.8 ( 0.6 μm, and they had a polyhedron (mostly hexagonally shaped) structure (Figure S2). This is reasonable since zinc has an hexagonal crystal structure. HRTEM Studies. In order to receive better understanding of the structure of the Zn-ZnO composite, HRTEM measurements were carried out. The HRTEM sample was prepared by removing the zinc layer from the glass surface. It was done by sonication bath treatment of Zn-coated glass in the 2-propanol for 30 min. The Zn removing process was conducted under argon atmosphere, and after the HRTEM sample was prepared it was stored under N2 atmosphere until the HRTEM measurement were taken. This way the exposure of Zn particles to the air was minimized. However, it should be remarked that the sample was exposed to the air for several minutes before inserting it inside the HRTEM. Figure 8a shows image of Zn and ZnO nanoparticles. Two types of particles can be seen. The big particles with sizes of 30-40 nm and small particles with sizes of 10-20 nm. Electron diffraction (Figure 8b) of this area exhibits two crystalline phases: ZnO (point 1 at the ring of plane 100 and point 2 at the ring of plane 101) and Zn (point 2 at the ring of plane 002 and points 3, 4 at the rings of planes 100, 101, respectively; Zn and ZnO share point 2). When the dark field was applied to point 4 (plane 101 of Zn), the larger particles was seen while the smaller disappeared. This means that the larger particles are zinc while the smaller are Langmuir 2010, 26(8), 5976–5984
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Figure 7. HRSEM of glass coated by a product of the 60 s reaction of a zinc nitrate ethanol solution of (a) 0.03 and (b) 1 M. (c) Depicts the enlarged image of (b). In (a) the main product is Zn nanoparticles (∼35 nm average size); (b) and (c) depict ZnO structures with ∼6.8 μm size.
Figure 8. (a) HRTEM image of zinc and ZnO nanoparticles. In the inset to (a) is shown Zn nanoparticle in the dark field mode (b) Electron diffraction of (a).
ZnO. This result matches well XRD measurements and the results obtained from Scherrer formula. In addition, some of the zinc particles seemed to be less reduced in the core part (inset to Figure 8a). This result may show that the reduction goes from the edge of ZnO particle inside as illustrated in Scheme 2. Figure 9 exhibits Zn nanoparticle under magnification of 1 200 000. The fringes of atomic plane 101 (d-spacing = 0.209 nm) of zinc can be clearly seen. In addition, it can be observed that Zn particle are coated by 2-3 nm layer of ZnO, and the fringes of plane 100 (d-spacing = 0.280 nm) also can be clearly seen. This ZnO layer was obtained when the sample was exposed to air. Contact Angle and ESR Studies. The contact angles of coated surfaces were measured using a goniometer. It was found that these surfaces can act reversibly and switch from a hydrophilic Langmuir 2010, 26(8), 5976–5984
surface to a hydrophobic surface. The production of surfaces that can be switched from a hydrophilic to a hydrophobic nature and vice versa was already studied in previous work.28-30 On very rough surfaces, air can be trapped in the small “pockets” when a drop of water is placed on the surface. Since air is a hydrophobic medium,30 the drop of water will be rejected from the surface, reflecting its hydrophobic nature. When the rough hydrophobic surface of ZnO was irradiated by UV light, many defects were formed on the surface of ZnO. These defects resulted from the (28) Papadopoulou, E. L.; Barberoglou, M.; Zorba, V.; Manousaki, A.; Pagkozidis, A.; Stratakis, E.; Fotakis, C. J. Phys. Chem. C 2009, 113, 2891. (29) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (30) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 5659.
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Figure 10. ESR spectra of hydrophilic and hydrophobic ZnO Figure 9. HRTEM image of the Zn nanoparticle with thin ZnO
particles.
layer on its edge.
removal of oxygen ions leaving a positively charged ZnO crystal. This has led to the attraction of polar molecules such as water to the irradiated ZnO (and therefore “defect-rich”) surface, with ZnO becoming hydrophilic. When the irradiated hydrophilic surface was heated in a dark air atmosphere, the oxygen returned to the vacancies and the hydrophobicity of the ZnO surface was recovered. The contact angle of uncoated glass treated by plasma for 1 min was almost 0°. The measured contact angle of a freshly prepared ZnO layer prepared from the plasma treatment of 1 M zinc nitrate for 60 s was 0°. This is explained as being the result of the ZnO becoming highly loaded with defects as the result of the long plasma treatment. Under these conditions a superhydrophilic surface is formed. When this ZnO-coated glass was heated in the dark under an ambient atmosphere for 3 h at 150 °C, the contact angle increased significantly to 119°; i.e., a hydrophobic surface was formed. After another round of 30 s plasma treatment, the contact angle returned to 0°. The images of water drops on the hydrophilic and hydrophobic ZnO surfaces are shown in Figure 10. In parallel to the contact angle measurements, and as another expression of the production of defects under plasma conditions, we have investigated the ability of ZnO particles to react with water and produce hydroxyl radicals (•OH). ZnO is known as a metal oxide that is able to react with water at the sites of surface defects.31 These hydroxyl radicals are the most reactive oxygen radicals known and react very quickly with almost every type of molecule found in living cells. Thus, “defect-rich” ZnO can serve as an antibacterial agent.31 The existence of OH• radicals was measured by an ESR technique using 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. ESR is a magnetic resonance technique, based on the interaction of unpaired electron spins with an external magnetic field. Every electron has a magnetic moment and spin quantum number, s = 1/2, with magnetic components ms = þ1/2 and ms = -1/2. In the presence of an external magnetic field with strength B0, the electron’s magnetic moment aligns itself either parallel (ms = -1/2) or antiparallel (ms = -1/2) to the field, each (31) Applerot, G.; Lipovsky, A.; Dror, R.; Perkas, N.; Nitzan, Y.; Lubart, R.; Gedanken, A. Adv. Funct. Mater. 2009, 19, 1.
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Scheme 1. Mechanism of the Thermal Decomposition of Zinc Precursors and the Reduction to Metallic Zinca
a The receiving of metallic zinc from ZnO can be performed through the reduction path.
alignment having a specific energy. The parallel alignment corresponds to the lower energy state, and the separation between it and the upper state is ΔE = geμBB0, where ge is the electron’s g-factor (ge = 2.0023) and μB is Bohr magnetron (μB = 9.2740 10-24 J T-1). Unpaired electrons can move between the two energy levels by either absorbing or emitting electromagnetic radiation. Therefore, by applying MW field with energy hν the resonance can be achieved when hν = geμBB0, where h is Planck’s constant and ν is the microwave frequency. The amount of OH• radicals was measured for both the hydrophilic and hydrophobic surfaces. To perform the ESR measurements, the ZnO deposited on the glass surface was scratched off and the measurement was performed on the ZnO powder. Hydrophilic ZnO was easily dispersed in water, and four ESR derivative peaks of OH• radicals are clearly seen in Figure 10. Langmuir 2010, 26(8), 5976–5984
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Article Scheme 2. Schematic Mechanism of ZnO Reduction to Zn by Plasma Electrons
It was very difficult to disperse hydrophobic ZnO in water. To disperse homogeneously hydrophobic ZnO, it was required to introduce the ZnO nanoparticles in water and sonicate the water-ZnO solution for 1 h. At the end of the sonication the suspension was inserted to the Teflon capillary for ESR measurements. ESR yielded a signal that was 2.7 times stronger for the hydrophilic sample, as compared with hydrophobic ZnO. This result proves our assumption that hydrophobic ZnO reacts with water less efficiently than hydrophilic ZnO does. Proposed Mechanism of the Reactions. The proposed mechanism is based on the experimental results described above. In the first step argon atoms are ionized to argon cations, and the free electrons collide with other free electrons in the plasma and are accelerated in the microwave electric field. As will be shown here, these electrons play a very important role in the reaction since they are the reducing agent of the metallic ions. Zinc nitrate or zinc acetate is thermally decomposed under the Ar microwave-induced plasma to ZnO. This decomposition also forms some gaseous products in addition to ZnO. Zinc nitrate is known to release gases such as NO2 and O2 when it is heated.32,33 If the zinc salt is a hydrate, H2O will also be released. Anhydrous zinc acetate most probably releases acetone and CO2 (see Scheme 1).34 The additional exposure of ZnO to plasma causes the formation of metallic zinc (see Scheme 1). When the concentration of the zinc precursor is low (∼0.03 M), the metallic zinc layer is formed only after 50-60 s of plasma exposure. The measured temperature of the plasma chamber immediately after the reaction was 230-250 °C. This temperature is much lower than the thermal decomposition temperature of ZnO to zinc and oxygen.35 Therefore, we exclude thermal decomposition as the mechanism for the decomposition of ZnO to metallic zinc. (32) Studenikin, S. A.; Golego, N.; Cocivera, M. J. Appl. Phys. 1998, 83, 2104. (33) Chen, J.; Feng, Z.; Ying, P.; Li, M.; Han, B.; Li, C. Phys. Chem. Chem. Phys. 2004, 6, 4473. (34) Duan, Y.; Li, J.; Yang, X.; Hu, L.; Wang, Z.; Liu, Y.; Wang, C. J. Anal. Appl. Pyrolysis 2008, 83, 1. (35) Keunecke, M.; Meier, A.; Palumbo, R. Chem. Eng. Sci. 2004, 59, 2695.
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In addition, we observed that the time required for metal ions to be reduced in plasma is strongly dependent on the reduction potential of the reduced ion. For example, precursors of metals with a high positive reduction potential, such as gold, silver, platinum, and palladium, are reduced in 30 s. It takes 60 s to reduce zinc ions, while the reduction of Li ions having a reduction potential of -3.04 V does not occur even after 2 min (the comparison was made with 0.03 M solutions). It also should be noted that the reduction of metal ions depends on the intensity and the amplitude of the applied electromagnetic field. In ref 19 we show that RF plasma (12 MHz/18 W) was able to reduce only the Au, Ag, Pt, and Pd ions that have large positive reduction potential. These results led us to the conclusion that the formation of metals from the metal precursors is via the direct reduction of the metal ions in the solid precursor by the free electrons of the plasma. The process of the reduction of metal ions by plasma electrons has hardly been investigated previously. In 2006, Liu et al. published the first paper where metal precursors of Au, Ag, Pt, and Pd were reduced directly by plasma electrons.36 No mechanism was offered for this reduction. In 2008, Cheng37 also investigated the reduction of metal precursors by plasma electrons and proposed a mechanism for the process. Herein, we extend and improve this proposed mechanism based on our ability to reduce more metallic ions, especially those ions which Cheng failed to reduce. This ability gave us a better insight and understanding of the process. When the plasma is activated, all surfaces, including solid zinc precursors, absorb electrons from plasma, forming a sheath that denies additional electrons to be attached to the surface. On the other hand, this sheath attracts positively charged species (Arþ) that hit the zinc precursors with high energy. These collisions, together with the collective collisions inside plasma, are responsible for the rise in temperature inside the plasma chamber, and the zinc precursors thermally decompose to form ZnO. The ZnO (36) Zou, J.-J.; Zhang, Y.-P.; Liu, C.-J. Langmuir 2006, 22, 11388. (37) Cheng, D.-G. Catal. Surv. Asia 2008, 12, 145.
DOI: 10.1021/la904499s
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further absorbs electrons from the plasma and are therefore hit by Arþ. The charged surface of ZnO is labeled in Scheme 1 as [ZnO]n-. The Arþ continues to hit the charged ZnO surface, causing a rise in the surface temperature and simultaneously in the temperature of the surface electrons and their energy. These energized electrons reduce the Zn2þ ions in the ZnO lattice and expel O2- to the plasma environment. It should be also noted that Arþ ions can cause a kind of sputtering of the ZnO and further Zn atoms onto the walls of the chamber and that after few sequential reactions might be coated by layers of ZnO or Zn. This sputtering process occurs as a result of collisions between Arþ and ZnO or/ and Zn surface atoms. In addition to surface heating, the electric field of the MW can further enhance the kinetic energy of the surface electrons and “move” them to the Zn2þ, in this way causing their reduction. This mechanism explains the reduction of the outer surface ZnO molecules. However, the size of the ZnO is 22 nm, and the surface molecules would constitute only ∼5% of the molecules (see calculation in the Supporting Information). The rest are inside the nanoparticle. We argue that the Zn produced on the surface are further charged by the plasma and will be labeled as Znm- in Scheme 1. Moreover, we propose that under the influence of the electric field of the MW these electrons can migrate from surface deeper (because of the zinc high conductivity) into the particle and reduce the ZnO inside (Scheme 2). When the plasma is stopped, all the electrons on the surface randomize all over the zinc particle and eventually the surface remains uncharged (see “plasma quenching” step in Scheme 1). We can realize that the surface of zinc is uncharged at the end of the reaction by measuring the contact angle. Indeed, the water contact angle of the obtained zinc surface was 80°, indicating that the produced surface is not charged (the charged surface should be superhydrophilic).
Conclusions Although two earlier reports36,37 described the use of plasma electrons to reduce metallic ions, neither of them was capable of
5984 DOI: 10.1021/la904499s
reducing a metallic ion whose standard reduction potential was as low as -0.7 V. Moreover, no paper has ever reported on the reduction of any ion having a negative standard reduction potential. This article has successfully accomplished the reduction of Zn2þ ions to metallic zinc. In the current paper we present a new method for coating glass slides by ZnO and Zn nano- and microsized particles. Various zinc precursors were dissolved in ethanol, and drops of this solution were placed on glass and were homogeneously spread. When microwaves were operated, they activated plasma inside the Pyrex chamber. This plasma thermally decomposed zinc nitrate and zinc acetate to ZnO. In a second stage, ZnO was reduced to metallic zinc by the electrons of plasma. Zinc chloride was directly reduced to the metallic zinc in this process. A zinc acetate solution (0.03 M) led to a homogeneous metallic zinc nanolayer with ∼34 nm sized nanoparticles. A zinc nitrate solution (0.03 M) also led to a metallic zinc nanolayer with a similar particle size (∼35 nm), although but the layer was less homogeneous. When the concentration of a zinc nitrate solution was increased, the preferred product after 1 min under plasma was pure ZnO and not Zn. Finally, the mechanism of reducing the metal precursors by plasma electrons was proposed. Our method can be easily extended to other metals. Acknowledgment. This study was conducted within the framework of the Integrated Project NAPOLYDE, funded by the sixth program of the EUROPEAN Commission, Contract NMP2-CT2005-515846. Supporting Information Available: Summarizing table of the performed reactions and their main results; extended XRD pattern of anhydrous zinc acetate formed on the glass surface; AFM image of the edge of the zinc layer; additional HRSEM images of glass coated by Zn particles. This material is available free of charge via the Internet at http://pubs.acs.org.
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