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Surface modification of ZnO nanocrystals
Applied Surface Science 253 (2007) 5473–5479 www.elsevier.com/locate/apsusc
Surface modification of ZnO nanocrystals Y.L. Wu a,b,*, A.I.Y. Tok a, F.Y.C. Boey a, X.T. Zeng b, X.H. Zhang c a
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, 638075 Singapore, Singapore c Institute of Materials Research and Engineering, 3 Research Link, 117602 Singapore, Singapore Received 11 September 2006; received in revised form 8 December 2006; accepted 15 December 2006 Available online 23 January 2007
Abstract Nano-crystalline ZnO particles were synthesized using alcoholic solutions of zinc acetate dihydrate through a colloidal process. Five types of capping agents: 3-aminopropyl trimethoxysilane (Am), tetraethyl orthosilicate (TEOS), mercaptosuccinic acid (Ms), 3-mercaptopropyl trimethoxysilane (Mp) and polyvinylpyrrolidone (Pv) were added at the first ZnO precipitation time (first PPT) to limit the particle growth. The first three capping agents effectively capped the ZnO nanoparticles and limited the growth of the particles, while the last two capping agents caused agglomeration or larger clusters in the solutions. Particles synthesized were in the size range of 10–30 nm after capping, and grew to 60 and 100 nm in 3 and 6 weeks, respectively, during storage at ambient conditions. Refluxing time was found to only affect the first PPT time. Washing by ethanol and slow drying were very important in converting Zn(OH)2 into ZnO. XRD analyses revealed single phase ZnO Wurtzite crystal structure. Photoluminescence (PL) spectra showed high-intensity in UV emission and very low intensity in the visible emission, which indicates a good surface morphology of the ZnO nanoparticles with little surface defects. Optical absorption spectra showed a blue shift by the capped ZnO due to the quantum confinement effect by the single crystal size of 5–6 nm as analysed by TEM. Capping effectiveness of each agent is discussed through possible capping mechanism and chemical reaction of each capping agent. This synthesis process is a low cost, high purity, easy to control method using only bio-compatible materials. # 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Nanocrystal; Surface; Capping agent; Photoluminescence; Colloidal
1. Introduction Semiconductor nanocrystals have attracted a great deal of attention from researchers for both their fundamental sizedependent optoelectronic properties and their wide range of applications. Zinc oxide (ZnO) is a versatile material that has achieved applications in photo-catalysts, solar cells, chemical sensors, piezoelectric transducers, transparent electrodes [1,2], electroluminescent devices and ultraviolet laser diodes [3,4]. Compared to other II–VI compounds semiconductors, ZnO has a wide band-gap of 3.37 eV and rather large exciton binding energy, which makes the exciton state stable even at room temperature. Furthermore, ZnO is an environmentally friendly material, which is desirable especially for bio-applications, such as bio-imaging and cancer detection. * Corresponding author at: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore. Tel.: +65 67938999; fax: +65 67916377. E-mail address: [email protected] (Y.L. Wu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.091
Numerous reports are available on the use of core-shell quantum dots made of CdS, CdSe and ZnS for applications on photonic and display devices, as well as bio-labeling [5–8]. These quantum dots are made by the well-developed thermochemical synthesis methods using TOPO/TOP surfactants at high temperatures (250–300 8C) in a controlled nitrogen environment [9–11]. Due to the increasing demands globally for ‘‘green’’ materials and processes, new quantum dots and synthesis processes are desired. Zn-based quantum dots are recently being investigated in a much wider scope, from materials aspects in terms of nanocrystal shapes (wire, rod, cone and spherical), lattice structures, doping and surface modifications [12–15] to optoelectronic aspects such as luminescence properties, banding energy and band gap studies [16,17]. Several wet-chemical processes have been developed to synthesize ZnO nanocrystals. For example, air oxidation and thermal decomposition of inorganic complexes in organic solvents was employed to synthesize ZnO nanocrystals [18,19]. The material purity and expensive process are the main issues of these processes. Nanocolloids or nanopowders of ZnO have
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been prepared under basic solution conditions [20–26]. Zinc salts such as Zn(ClO4)2, Zn(NO3)2, Zn(CH3COO)22H2O, or Zn(CH3COCHCOCH3)22H2O (zinc acetylacetonate) are dissolved in alcoholic or other organic solvents to which basic solutions containing NaOH, LiOH or NH4OH are added. In view of the bio-applications, chemically pure or compositionally well-defined ZnO is required. Although efforts have been made to remove alkali ions (Li+ or Na+) by washing in order to obtain pure ZnO [27,28], development of the process without addition of base is a primary requirement for many more applications. Kumar et al. [29] reported on the synthesis of ZnO from aqueous solutions of Zn(Ac)22H2O using a high-intensity ultrasonic horn under 1.5 atm of argon at room temperature. Recently, chemical synthesis of ZnO nanoparticles through hydrolysis of zinc salts in polyol media (diethylene glycol or ethylene glycol) and monool solvents, i.e. methanol, ethanol, and 2-methoxyethanol have been suggested [30,31]. The main issues include the control of high purity, particle shape and size, surface defects, and optical properties. In this study, nanocrystalline ZnO colloids were synthesized by methanol solutions of Zn(Ac)22H2O at a low temperature under an ambient conditions. This process eliminates the addition of basic or acid solutions containing foreign elements, high temperature and complex or vacuum systems. The main advantages of this process are the ability to control the ZnO purity and the use of bio-compatible capping agents, which provide triple functions: (1) to control the particle size by limiting the growth of the particles after the nucleation, (2) to provide a side group on surface which can be further conjugated to bio-cell, (3) to eliminate the surface defects which would affect the optical properties of the ZnO nanocrystals. Five types of capping agents were added at different stages of reaction and precipitation to control the particle size. Crystal and particle size and size distribution were analyzed by microscopy, X-ray diffraction and laser scattering (Zetasizer). Photoluminescence and optical absorption properties were analyzed as well.
described in a separate paper. For the colloidal concentration used in this study, we used 0.08–0.2 g of capping agent in 20 ml of ZnO colloidal solution. All chemicals were purchased from leading suppliers without further purification. 2.2. Analysis and measurement Field emission scanning electron microscopy (FESEM) (JEOL JSM-6340F) was used for particle shape and size analyses. A sample was prepared from each of the colloidal solution synthesized after the capping agent was added. A drop of the solution was withdrawn by a pipette and spread onto an aluminum round disc which was mounted onto the sample holder for FESEM. The coated samples were dried in an oven at 60 8C for 1 h before analyses. To determine the possible growth of the particles in the solution after capping, particle size was measured by a Malvern Zetasizer Nano ZS, which is equipped with a 4 mW, 633 nm laser. The particles in colloidal solution at the initial stage after capping were also analysed by transmission electron microscope (TEM) (JEOL 2010). White powder was prepared by washing the colloidal solution with ethanol or methanol and a small amount of water during centrifugation at 5000 rpm for 10 min. The collected precipitate was dried at 40 8C in ambient atmosphere for 3 days. Crystal structure identification and crystal size analysis were carried out by X-ray diffraction (XRD) and Rietveld simulation. Optical absorption spectra were measured with a SHIMADZU UV-3101PC UV–vis-NIR scanning spectrophotometer. Photoluminescence (PL) spectra were measured at room temperature with an accent PL mapping system (2000 rpm). The sample was excited with the 325 nm line of a He–Cd laser. The luminescence was dispersed with a monocharomator and was recorded with a CCD. 3. Results and discussion 3.1. Particle size and composition
2. Experimental procedures 2.1. Colloidal synthesis Zinc acetate dihydrate, Zn(Ac)22H2O, was dissolved in methanol in a flask under vigorous stirring in molar ratio of 0.03:4, and refluxed for 5, 6, 6.5 and 7 h, respectively, at 68 8C. The solution was then cooled down to room temperature (25 8C) and observed for the appearance of white precipitates. This is defined as the first PPT time. The selected capping agent was added to the solution at both clear stage and the first PPT time in order to study the right capping time for controlled particle size. The capping agents in this study include: 3aminopropyl trimethoxysilane (Am), 3-mercaptopropyl trimethoxysilane (Mp), mercaptosuccinic acid (Ms), polyvinyl pyrrolidone (Pv) (Mw = 10,000), hydrolyzed tetraethyl orthosilicate (TEOS). The intended amount of capping agent is to coat the surface of the ZnO particles with one or two monolayer of capping agent. To do this calculation, we need to know the surface area of the ZnO particles. Detailed calculation will be
From the FESEM images, it was found that the uncapped ZnO particles and all the capped particles were spherical in shape. The size of the initial ZnO particles was about 10–20 nm (Fig. 1(a)). Am, Ms and TEOS could cap the ZnO particles and still remained as small particles (10–30 nm) as shown in Fig. 1(b). However, the affinity of capping agent Mp–ZnO surface was poor. Mp formed agglomerates itself leaving ZnO particles un-coated as shown in Fig. 1(c). Pv is a large molecule polymer (molecular weight 10,000), although it could coat onto the ZnO particles, they formed larger clusters as shown in Fig. 1(d). All these particles could contain several crystals due to the method of preparation that allows the stacking and agglomeration of initial single crystal particles. Chemical compositions of the SEM samples were analysed by energy dispersive X-Ray (EDX). It was confirmed that the samples contained mainly ZnO with 2–5% of other elements (Si, C), which were from the capping materials. Since the particles size is too small to be identified by FESEM, the Am-capped ZnO colloidal particles at the initial stage after capping was further
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Fig. 1. FESEM images of the (a) initial ZnO particle, (b) Am capped, (c) Mp agglomerate with ZnO uncapped, and (d) Pv capped ZnO forming larger clusters. Magnification: 100,000, scale bar: 100 nm, particle size range: 10–70 nm.
analysed by TEM, as shown in Fig. 2. Image (a) shows several particles in close-to-spherical shape with particle size about 6– 10 nm, which is in line with the estimated size by FESEM. Fig. 2(b) shows the high magnification lattice image, in which the individual crystal size and lattice structure can be seen. The lattice parameter agrees with the ZnO Wurtzite hexagonal structure and the individual crystal size is 5–6 nm. This crystal size is in the quantum confinement effect range, therefore, contributes to the band edge shifting, which will be discussed in Section 3.3 of this paper. The Malvern Zetasizer was used to measure the particle size in the colloidal solution after the Am-capped ZnO was stored in an ambient environment for 3 and 6 weeks. The particle size grew to 60 nm in 3 weeks and 100 nm in 6 weeks. The size distribution remained in a narrow range as shown in Fig. 3. Comparing to the results obtained from FESEM, the Zetasizer showed slightly larger particle sizes. It must be noted that the particle size determined by the Zetasizer is the hydrodynamic diameter, which is detected by the scattered laser light and influenced by the viscosity and concentration of the solution. Clusters containing several particles may be detected as one particle by the light. Therefore, Zetasizer is only used as a tool to monitor the size changes in this study.
The effects of synthesis parameters such as refluxing time of 5, 6, 6.5 and 7 h at fixed refluxing temperature of 68 8C were studied. It was found that the refluxing time only affected the first PPT time. Longer refluxing time shortened the first PPT time. Five-hour refluxing took 7 days for the 1st PPT, 6 h refluxing took 16 and 7 h refluxing took 1.5 h for the first PPT. No effect was observed on the initial particle size and capping behavior. But the particle growth was much faster by 7 h refluxing if no capping agent was added. All the FESEM analyses were conducted in the initial stage after capping. 3.2. Crystal structure White powders were obtained from the colloidal solutions refluxed for 5, 6, 6.5 and 7 h (without capping), and analyzed under XRD. The powder preparation procedure was very critical to obtain pure ZnO, while no trend was observed for different refluxing time. Fig. 4 shows four XRD patterns: (a) powder collected by washing with ethanol, centrifuged and dried at 60 8C for 1 day, (b) powder collected by slow drying at 40 8C for 3 days, (c) powder collected by washing with methanol and a small amount of water, centrifuged and dried in oven at 65 8C for 1 day, (d) powder by direct evaporation in an
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Fig. 4. XRD patterns of powders collected by (a) washing by ethanol and drying at 60 8C for 1 day, (b) slow drying at 40 8C for 3 days, no washing, (c) washing by methanol and water and drying at 65 8C for 1 day and (d) direct evaporation at 75 8C for 5 h. ZnO main peaks are labelled with (Ł), Zn(OH)2 main peaks are labelled with (*).
Fig. 2. HRTEM image of Am-capped ZnO particles at the initial stage after capping, showing spherical shape with particles size ranging from 6 to 10 nm (a), and individual crystal size of 5–6 nm with Wurtzite ZnO structure (b).
oven at 75 8C for 5 h. The sharp peaks indicate that the products were well crystallized. The five peaks in the 2u 30–608 range of pattern (a) are the characteristic peaks of ZnO polycrystalline hexagonal Wurtzite structure, indicating single phase ZnO (P63 mc) was obtained by washing with ethanol and slow drying at 65 8C. The peaks between 2u 108 and 208 in pattern (b) and (c) are recognized as the a-form of Zn(OH)2, which was also reported by Hosono et al. [31], which indicates the incomplete conversion of Zn(OH)2 into ZnO without washing or washing with methanol and water and fast drying at higher temperature. The several peaks between 2u 108 and 308 in
Fig. 3. Particles size distribution measured by Zetasizer Nano ZS on the Amcapped ZnO colloidal solution after stored for 3 weeks. About 80% of the particles are between 40 and 70 nm.
pattern (d) are identified as a mixture of ZnAc22H2O and Zn(OH)2, which indicates that direct evaporation of methanol at higher temperature of 75 8C is not a suitable treatment to achieve pure ZnO nanocrystals. Hosono et al. [31] reported that washing by methanol was very important. Our results suggest that washing by methanol or water could not achieve pure ZnO. Washing with ethanol and slow drying at lower temperature are important for preparing pure ZnO (uncapped) particles. Patterns (a) and (b) were further analyzed by Rietveld refinement. Quantitative analyses revealed that (a) consists of single phase ZnO in crystal size of 15 nm, (b) consists of 75%wt. ZnO in crystal size of 53 nm and 25%wt Zn(OH)2 in crystal size of 8.8 nm. The crystal size obtained in white powder is bigger than the capped crystals. This is due to the fact that the powders were collected after some crystal growth without capping. Therefore, capping is important to control the crystal growth. 3.3. Photoluminescence and absorption Fig. 5 displays the photoluminescence spectra of the ZnO nanoparticles. A laser source was used to excite the samples at 325 nm. The original ZnO nanocrystal without capping layer showed the highest emission, while the capped crystals showed gradually decreasing intensity by Am, Pv and Ms in a sequence. For all the samples, a blue-green emission peak (486 nm) and a UV emission peak (377 nm) were observed in the PL spectra. The emission at 377 nm corresponds to the band-gap of ZnO material. This UV emission is assigned to the recombination of bound excitons of ZnO. The blue-green emission mechanism in ZnO has been extensively investigated [32]. Single ionized oxygen vacancy results in green emission of ZnO material because of recombination of a photo-generated hole with a single ionized electron in the valence band [20]. All the spectra in this study showed high UV emission and low blue-green emission. The high UV to visible emission ratio indicates a good crystal quality of the nanoparticles, i.e. a low density of
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the colour of the solution and the sediment formed at the bottom of the container. 3.4. Effectiveness of capping agent
surface defects. Dangling bonds and defect states at the surface of ZnO nanoparticles are limited. It is noticed that there is no shift of UVemission peak position by the capping agents, which indicates that the capping layers did not result in size changes or increased surface defect. Fig. 6 shows the optical absorption spectra of uncapped, Am-capped and TEOS-capped ZnO colloidal solutions measured after they were stored for 7 weeks. The uncapped ZnO showed band-edge at 380 nm, while both of the capped colloids showed band-edge at 350 nm. The wavelength of 380 nm corresponds to the bulk band-edge of 3.26 eV for ZnO. The absorbance at wavelength of 350 nm indicates a blue shift, which should be due to the quantum confinement effect from the small individual crystal size of 5–6 nm as found in TEM analyses. Therefore, it can be concluded that Am-capping and TEOS-capping effectively controlled the growth of individual crystals as well as particles. While the uncapped ZnO grew quickly during storage, which also can be seen from
As discussed in Section 3.1 of this paper, Am, Ms and TEOS could cap the ZnO particles and slow down the particle growth. However, we noticed more particle growth in TEOS and Ms added solutions. Am capped ZnO colloidal showed the best stability. Mp formed agglomerates itself leaving ZnO particles un-coated. White precipitants and sediments were observed in this solution. Pv formed larger clusters with ZnO particles, and increased the viscosity of the solution. We discuss here the possible capping mechanisms and the chemical reactions that could lead to these phenomena. As we know, Am and Mp are both alkoxy silane with a –NH2 and –SH side group, respectively. When added with a controlled amount, the alkoxyl groups are partially hydrolysed by the available water molecules in the solution and one group would react with the OH group on the ZnO surface to form a capping monolayer, with the unreacted side group protruding on surface making the surface stable as shown in Fig. 7(a). This mechanism seemed valid for Am only. The –SH side group in Mp seems more reactive with water than the alkoxyl groups, therefore, the Mp formed precipitants itself before joining with ZnO particles. TEOS is an alkoxy silane with four alkoxyl groups. In the same machanism, a group would be hydrolysed and reacted with the OH group on the ZnO surface to form a capping monolayer. Since the other three alkoxyl groups could be hydrolysed and joined with the side groups of another TEOS capped molecule (see Fig. 7(b)), the chance of forming a thicker or non-uniform capping layer is high. The amount of TEOS added into the solution must be well calculated and controlled, which is not easily done, as the particle size and surface area need to be measured accurately. We noticed that the TEOS capped particle size grew to 500 nm in 2 weeks. Ms contains two OH side groups on each side and one SH group in the middle. The OH groups have more affinity to ZnO
Fig. 6. Optical absorption spectra of ZnO colloids of (from top to bottom) uncapped, Mp, Pv, TEOS, Am and Ms-capped, after stored in ambient condition for 7 weeks.
Fig. 7. Capping mechanisms of Am (a) TEOS (b) and Ms (c) showing Am has better stability, TEOS and Ms have the possibility of joining two particles together.
Fig. 5. Photoluminescence spectra of ZnO raw particles and capped by Am, Pv and Ms (from top to bottom) respectively, showing high emission at 377 nm and low emission at green wavelength.
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surface and immedietly forms a capping layer. Comparing to the other four types of capping agents, Ms is the smallest in molecular size, which could form a very thin cappping layer. However, if the other OH group on capped particles meets with another similar particle, they may condense to form a joined particle (see Fig. 7(c)). We noticed the particle size grew to 320 nm in 2 weeks, which was slower then TEOS capped ZnO particles (500 nm). Pv is one of the stabilizing moieties. Although the vinyl group could cap onto ZnO particles, the repulsion seemed not strong enough in this methanol solution. In addition, the molecular size is too large in respect to the ZnO particle size. Therefore, Pv formed larger clusters with ZnO, which is not preferred for our application. 4. Conclusions Nano-crystalline ZnO colloids have been successfully synthesized from methanol and zinc acetate dihydrate at 68 8C in ambient condition. Three types of capping agents: 3aminopropyl trimethoxysilane (Am), tetraethyl orthosilicate (TEOS) and mercaptosuccinic acid (Ms) effectively capped the ZnO particles at the first precipitation time, so that small particle size (10–30 nm) and good surface quality were achieved. Two other capping agents: 3-mercaptopropyl trimethoxysilane (Mp) and polyvinylpyrrolidone (Pv) caused agglomeration or larger cluster in the solutions (up to 70 nm), therefore, are not suitable as a capping agent in this chemical synthesis process. The size of the single crystal ZnO is 5–6 nm, therefore, a blue shift in absorption edge was obtained due the quantum confinement effect. XRD analyses proved the polycrystalline hexagonal Wurtzite structure of the ZnO. PL spectra showed high UVemission and low visible emission, indicating good quality of particles with little surface defects. Optical absorption spectra confirmed the capping effectiveness by the selected capping agents, as the capped ZnO colloids absorbed at shorter wavelength than the uncapped ZnO indicating much smaller crystal size. Acknowledgements The authors would like to acknowledge the financial support by the Agency for Science, Technology and Research (A*STAR) of Singapore to this project. The authors would like to thank Associate Professor T.J. White from the School of Materials Science & Engineering, NTU for his advice on the Rietveld refinement analyses. References [1] P. Dura´n, F. Capel, J. Tartaj, C. Moure, A strategic two-stage lowtemperature thermal processing leading to fully dense and fine-grained doped-ZnO varistors, Adv. Mater. 14 (2002) 137–141. [2] C. Bauer, G. Boschloo, E. Mukhtar, A. Hagfeldt, Electron injection and recombination in Ru(dcbpy)2(NCS)2 sensitized nanostructured ZnO, J. Phys. Chem. B 105 (2001) 5585–5588. [3] X.T. Zhang, L.G. Liu, L.G. Zhang, Y.M. Zhang, M. Lu, Z. Shen, et al., Structure and optically pumped lasing from nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films, J. Appl. Phys. 92 (2002) 3293–3298.
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Surface modification of ZnO nanocrystals Y.L. Wu a,b,*, A.I.Y. Tok a, F.Y.C. Boey a, X.T. Zeng b, X.H. Zhang c a
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, 638075 Singapore, Singapore c Institute of Materials Research and Engineering, 3 Research Link, 117602 Singapore, Singapore Received 11 September 2006; received in revised form 8 December 2006; accepted 15 December 2006 Available online 23 January 2007
Abstract Nano-crystalline ZnO particles were synthesized using alcoholic solutions of zinc acetate dihydrate through a colloidal process. Five types of capping agents: 3-aminopropyl trimethoxysilane (Am), tetraethyl orthosilicate (TEOS), mercaptosuccinic acid (Ms), 3-mercaptopropyl trimethoxysilane (Mp) and polyvinylpyrrolidone (Pv) were added at the first ZnO precipitation time (first PPT) to limit the particle growth. The first three capping agents effectively capped the ZnO nanoparticles and limited the growth of the particles, while the last two capping agents caused agglomeration or larger clusters in the solutions. Particles synthesized were in the size range of 10–30 nm after capping, and grew to 60 and 100 nm in 3 and 6 weeks, respectively, during storage at ambient conditions. Refluxing time was found to only affect the first PPT time. Washing by ethanol and slow drying were very important in converting Zn(OH)2 into ZnO. XRD analyses revealed single phase ZnO Wurtzite crystal structure. Photoluminescence (PL) spectra showed high-intensity in UV emission and very low intensity in the visible emission, which indicates a good surface morphology of the ZnO nanoparticles with little surface defects. Optical absorption spectra showed a blue shift by the capped ZnO due to the quantum confinement effect by the single crystal size of 5–6 nm as analysed by TEM. Capping effectiveness of each agent is discussed through possible capping mechanism and chemical reaction of each capping agent. This synthesis process is a low cost, high purity, easy to control method using only bio-compatible materials. # 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Nanocrystal; Surface; Capping agent; Photoluminescence; Colloidal
1. Introduction Semiconductor nanocrystals have attracted a great deal of attention from researchers for both their fundamental sizedependent optoelectronic properties and their wide range of applications. Zinc oxide (ZnO) is a versatile material that has achieved applications in photo-catalysts, solar cells, chemical sensors, piezoelectric transducers, transparent electrodes [1,2], electroluminescent devices and ultraviolet laser diodes [3,4]. Compared to other II–VI compounds semiconductors, ZnO has a wide band-gap of 3.37 eV and rather large exciton binding energy, which makes the exciton state stable even at room temperature. Furthermore, ZnO is an environmentally friendly material, which is desirable especially for bio-applications, such as bio-imaging and cancer detection. * Corresponding author at: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore. Tel.: +65 67938999; fax: +65 67916377. E-mail address: [email protected] (Y.L. Wu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.091
Numerous reports are available on the use of core-shell quantum dots made of CdS, CdSe and ZnS for applications on photonic and display devices, as well as bio-labeling [5–8]. These quantum dots are made by the well-developed thermochemical synthesis methods using TOPO/TOP surfactants at high temperatures (250–300 8C) in a controlled nitrogen environment [9–11]. Due to the increasing demands globally for ‘‘green’’ materials and processes, new quantum dots and synthesis processes are desired. Zn-based quantum dots are recently being investigated in a much wider scope, from materials aspects in terms of nanocrystal shapes (wire, rod, cone and spherical), lattice structures, doping and surface modifications [12–15] to optoelectronic aspects such as luminescence properties, banding energy and band gap studies [16,17]. Several wet-chemical processes have been developed to synthesize ZnO nanocrystals. For example, air oxidation and thermal decomposition of inorganic complexes in organic solvents was employed to synthesize ZnO nanocrystals [18,19]. The material purity and expensive process are the main issues of these processes. Nanocolloids or nanopowders of ZnO have
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been prepared under basic solution conditions [20–26]. Zinc salts such as Zn(ClO4)2, Zn(NO3)2, Zn(CH3COO)22H2O, or Zn(CH3COCHCOCH3)22H2O (zinc acetylacetonate) are dissolved in alcoholic or other organic solvents to which basic solutions containing NaOH, LiOH or NH4OH are added. In view of the bio-applications, chemically pure or compositionally well-defined ZnO is required. Although efforts have been made to remove alkali ions (Li+ or Na+) by washing in order to obtain pure ZnO [27,28], development of the process without addition of base is a primary requirement for many more applications. Kumar et al. [29] reported on the synthesis of ZnO from aqueous solutions of Zn(Ac)22H2O using a high-intensity ultrasonic horn under 1.5 atm of argon at room temperature. Recently, chemical synthesis of ZnO nanoparticles through hydrolysis of zinc salts in polyol media (diethylene glycol or ethylene glycol) and monool solvents, i.e. methanol, ethanol, and 2-methoxyethanol have been suggested [30,31]. The main issues include the control of high purity, particle shape and size, surface defects, and optical properties. In this study, nanocrystalline ZnO colloids were synthesized by methanol solutions of Zn(Ac)22H2O at a low temperature under an ambient conditions. This process eliminates the addition of basic or acid solutions containing foreign elements, high temperature and complex or vacuum systems. The main advantages of this process are the ability to control the ZnO purity and the use of bio-compatible capping agents, which provide triple functions: (1) to control the particle size by limiting the growth of the particles after the nucleation, (2) to provide a side group on surface which can be further conjugated to bio-cell, (3) to eliminate the surface defects which would affect the optical properties of the ZnO nanocrystals. Five types of capping agents were added at different stages of reaction and precipitation to control the particle size. Crystal and particle size and size distribution were analyzed by microscopy, X-ray diffraction and laser scattering (Zetasizer). Photoluminescence and optical absorption properties were analyzed as well.
described in a separate paper. For the colloidal concentration used in this study, we used 0.08–0.2 g of capping agent in 20 ml of ZnO colloidal solution. All chemicals were purchased from leading suppliers without further purification. 2.2. Analysis and measurement Field emission scanning electron microscopy (FESEM) (JEOL JSM-6340F) was used for particle shape and size analyses. A sample was prepared from each of the colloidal solution synthesized after the capping agent was added. A drop of the solution was withdrawn by a pipette and spread onto an aluminum round disc which was mounted onto the sample holder for FESEM. The coated samples were dried in an oven at 60 8C for 1 h before analyses. To determine the possible growth of the particles in the solution after capping, particle size was measured by a Malvern Zetasizer Nano ZS, which is equipped with a 4 mW, 633 nm laser. The particles in colloidal solution at the initial stage after capping were also analysed by transmission electron microscope (TEM) (JEOL 2010). White powder was prepared by washing the colloidal solution with ethanol or methanol and a small amount of water during centrifugation at 5000 rpm for 10 min. The collected precipitate was dried at 40 8C in ambient atmosphere for 3 days. Crystal structure identification and crystal size analysis were carried out by X-ray diffraction (XRD) and Rietveld simulation. Optical absorption spectra were measured with a SHIMADZU UV-3101PC UV–vis-NIR scanning spectrophotometer. Photoluminescence (PL) spectra were measured at room temperature with an accent PL mapping system (2000 rpm). The sample was excited with the 325 nm line of a He–Cd laser. The luminescence was dispersed with a monocharomator and was recorded with a CCD. 3. Results and discussion 3.1. Particle size and composition
2. Experimental procedures 2.1. Colloidal synthesis Zinc acetate dihydrate, Zn(Ac)22H2O, was dissolved in methanol in a flask under vigorous stirring in molar ratio of 0.03:4, and refluxed for 5, 6, 6.5 and 7 h, respectively, at 68 8C. The solution was then cooled down to room temperature (25 8C) and observed for the appearance of white precipitates. This is defined as the first PPT time. The selected capping agent was added to the solution at both clear stage and the first PPT time in order to study the right capping time for controlled particle size. The capping agents in this study include: 3aminopropyl trimethoxysilane (Am), 3-mercaptopropyl trimethoxysilane (Mp), mercaptosuccinic acid (Ms), polyvinyl pyrrolidone (Pv) (Mw = 10,000), hydrolyzed tetraethyl orthosilicate (TEOS). The intended amount of capping agent is to coat the surface of the ZnO particles with one or two monolayer of capping agent. To do this calculation, we need to know the surface area of the ZnO particles. Detailed calculation will be
From the FESEM images, it was found that the uncapped ZnO particles and all the capped particles were spherical in shape. The size of the initial ZnO particles was about 10–20 nm (Fig. 1(a)). Am, Ms and TEOS could cap the ZnO particles and still remained as small particles (10–30 nm) as shown in Fig. 1(b). However, the affinity of capping agent Mp–ZnO surface was poor. Mp formed agglomerates itself leaving ZnO particles un-coated as shown in Fig. 1(c). Pv is a large molecule polymer (molecular weight 10,000), although it could coat onto the ZnO particles, they formed larger clusters as shown in Fig. 1(d). All these particles could contain several crystals due to the method of preparation that allows the stacking and agglomeration of initial single crystal particles. Chemical compositions of the SEM samples were analysed by energy dispersive X-Ray (EDX). It was confirmed that the samples contained mainly ZnO with 2–5% of other elements (Si, C), which were from the capping materials. Since the particles size is too small to be identified by FESEM, the Am-capped ZnO colloidal particles at the initial stage after capping was further
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Fig. 1. FESEM images of the (a) initial ZnO particle, (b) Am capped, (c) Mp agglomerate with ZnO uncapped, and (d) Pv capped ZnO forming larger clusters. Magnification: 100,000, scale bar: 100 nm, particle size range: 10–70 nm.
analysed by TEM, as shown in Fig. 2. Image (a) shows several particles in close-to-spherical shape with particle size about 6– 10 nm, which is in line with the estimated size by FESEM. Fig. 2(b) shows the high magnification lattice image, in which the individual crystal size and lattice structure can be seen. The lattice parameter agrees with the ZnO Wurtzite hexagonal structure and the individual crystal size is 5–6 nm. This crystal size is in the quantum confinement effect range, therefore, contributes to the band edge shifting, which will be discussed in Section 3.3 of this paper. The Malvern Zetasizer was used to measure the particle size in the colloidal solution after the Am-capped ZnO was stored in an ambient environment for 3 and 6 weeks. The particle size grew to 60 nm in 3 weeks and 100 nm in 6 weeks. The size distribution remained in a narrow range as shown in Fig. 3. Comparing to the results obtained from FESEM, the Zetasizer showed slightly larger particle sizes. It must be noted that the particle size determined by the Zetasizer is the hydrodynamic diameter, which is detected by the scattered laser light and influenced by the viscosity and concentration of the solution. Clusters containing several particles may be detected as one particle by the light. Therefore, Zetasizer is only used as a tool to monitor the size changes in this study.
The effects of synthesis parameters such as refluxing time of 5, 6, 6.5 and 7 h at fixed refluxing temperature of 68 8C were studied. It was found that the refluxing time only affected the first PPT time. Longer refluxing time shortened the first PPT time. Five-hour refluxing took 7 days for the 1st PPT, 6 h refluxing took 16 and 7 h refluxing took 1.5 h for the first PPT. No effect was observed on the initial particle size and capping behavior. But the particle growth was much faster by 7 h refluxing if no capping agent was added. All the FESEM analyses were conducted in the initial stage after capping. 3.2. Crystal structure White powders were obtained from the colloidal solutions refluxed for 5, 6, 6.5 and 7 h (without capping), and analyzed under XRD. The powder preparation procedure was very critical to obtain pure ZnO, while no trend was observed for different refluxing time. Fig. 4 shows four XRD patterns: (a) powder collected by washing with ethanol, centrifuged and dried at 60 8C for 1 day, (b) powder collected by slow drying at 40 8C for 3 days, (c) powder collected by washing with methanol and a small amount of water, centrifuged and dried in oven at 65 8C for 1 day, (d) powder by direct evaporation in an
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Fig. 4. XRD patterns of powders collected by (a) washing by ethanol and drying at 60 8C for 1 day, (b) slow drying at 40 8C for 3 days, no washing, (c) washing by methanol and water and drying at 65 8C for 1 day and (d) direct evaporation at 75 8C for 5 h. ZnO main peaks are labelled with (Ł), Zn(OH)2 main peaks are labelled with (*).
Fig. 2. HRTEM image of Am-capped ZnO particles at the initial stage after capping, showing spherical shape with particles size ranging from 6 to 10 nm (a), and individual crystal size of 5–6 nm with Wurtzite ZnO structure (b).
oven at 75 8C for 5 h. The sharp peaks indicate that the products were well crystallized. The five peaks in the 2u 30–608 range of pattern (a) are the characteristic peaks of ZnO polycrystalline hexagonal Wurtzite structure, indicating single phase ZnO (P63 mc) was obtained by washing with ethanol and slow drying at 65 8C. The peaks between 2u 108 and 208 in pattern (b) and (c) are recognized as the a-form of Zn(OH)2, which was also reported by Hosono et al. [31], which indicates the incomplete conversion of Zn(OH)2 into ZnO without washing or washing with methanol and water and fast drying at higher temperature. The several peaks between 2u 108 and 308 in
Fig. 3. Particles size distribution measured by Zetasizer Nano ZS on the Amcapped ZnO colloidal solution after stored for 3 weeks. About 80% of the particles are between 40 and 70 nm.
pattern (d) are identified as a mixture of ZnAc22H2O and Zn(OH)2, which indicates that direct evaporation of methanol at higher temperature of 75 8C is not a suitable treatment to achieve pure ZnO nanocrystals. Hosono et al. [31] reported that washing by methanol was very important. Our results suggest that washing by methanol or water could not achieve pure ZnO. Washing with ethanol and slow drying at lower temperature are important for preparing pure ZnO (uncapped) particles. Patterns (a) and (b) were further analyzed by Rietveld refinement. Quantitative analyses revealed that (a) consists of single phase ZnO in crystal size of 15 nm, (b) consists of 75%wt. ZnO in crystal size of 53 nm and 25%wt Zn(OH)2 in crystal size of 8.8 nm. The crystal size obtained in white powder is bigger than the capped crystals. This is due to the fact that the powders were collected after some crystal growth without capping. Therefore, capping is important to control the crystal growth. 3.3. Photoluminescence and absorption Fig. 5 displays the photoluminescence spectra of the ZnO nanoparticles. A laser source was used to excite the samples at 325 nm. The original ZnO nanocrystal without capping layer showed the highest emission, while the capped crystals showed gradually decreasing intensity by Am, Pv and Ms in a sequence. For all the samples, a blue-green emission peak (486 nm) and a UV emission peak (377 nm) were observed in the PL spectra. The emission at 377 nm corresponds to the band-gap of ZnO material. This UV emission is assigned to the recombination of bound excitons of ZnO. The blue-green emission mechanism in ZnO has been extensively investigated [32]. Single ionized oxygen vacancy results in green emission of ZnO material because of recombination of a photo-generated hole with a single ionized electron in the valence band [20]. All the spectra in this study showed high UV emission and low blue-green emission. The high UV to visible emission ratio indicates a good crystal quality of the nanoparticles, i.e. a low density of
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the colour of the solution and the sediment formed at the bottom of the container. 3.4. Effectiveness of capping agent
surface defects. Dangling bonds and defect states at the surface of ZnO nanoparticles are limited. It is noticed that there is no shift of UVemission peak position by the capping agents, which indicates that the capping layers did not result in size changes or increased surface defect. Fig. 6 shows the optical absorption spectra of uncapped, Am-capped and TEOS-capped ZnO colloidal solutions measured after they were stored for 7 weeks. The uncapped ZnO showed band-edge at 380 nm, while both of the capped colloids showed band-edge at 350 nm. The wavelength of 380 nm corresponds to the bulk band-edge of 3.26 eV for ZnO. The absorbance at wavelength of 350 nm indicates a blue shift, which should be due to the quantum confinement effect from the small individual crystal size of 5–6 nm as found in TEM analyses. Therefore, it can be concluded that Am-capping and TEOS-capping effectively controlled the growth of individual crystals as well as particles. While the uncapped ZnO grew quickly during storage, which also can be seen from
As discussed in Section 3.1 of this paper, Am, Ms and TEOS could cap the ZnO particles and slow down the particle growth. However, we noticed more particle growth in TEOS and Ms added solutions. Am capped ZnO colloidal showed the best stability. Mp formed agglomerates itself leaving ZnO particles un-coated. White precipitants and sediments were observed in this solution. Pv formed larger clusters with ZnO particles, and increased the viscosity of the solution. We discuss here the possible capping mechanisms and the chemical reactions that could lead to these phenomena. As we know, Am and Mp are both alkoxy silane with a –NH2 and –SH side group, respectively. When added with a controlled amount, the alkoxyl groups are partially hydrolysed by the available water molecules in the solution and one group would react with the OH group on the ZnO surface to form a capping monolayer, with the unreacted side group protruding on surface making the surface stable as shown in Fig. 7(a). This mechanism seemed valid for Am only. The –SH side group in Mp seems more reactive with water than the alkoxyl groups, therefore, the Mp formed precipitants itself before joining with ZnO particles. TEOS is an alkoxy silane with four alkoxyl groups. In the same machanism, a group would be hydrolysed and reacted with the OH group on the ZnO surface to form a capping monolayer. Since the other three alkoxyl groups could be hydrolysed and joined with the side groups of another TEOS capped molecule (see Fig. 7(b)), the chance of forming a thicker or non-uniform capping layer is high. The amount of TEOS added into the solution must be well calculated and controlled, which is not easily done, as the particle size and surface area need to be measured accurately. We noticed that the TEOS capped particle size grew to 500 nm in 2 weeks. Ms contains two OH side groups on each side and one SH group in the middle. The OH groups have more affinity to ZnO
Fig. 6. Optical absorption spectra of ZnO colloids of (from top to bottom) uncapped, Mp, Pv, TEOS, Am and Ms-capped, after stored in ambient condition for 7 weeks.
Fig. 7. Capping mechanisms of Am (a) TEOS (b) and Ms (c) showing Am has better stability, TEOS and Ms have the possibility of joining two particles together.
Fig. 5. Photoluminescence spectra of ZnO raw particles and capped by Am, Pv and Ms (from top to bottom) respectively, showing high emission at 377 nm and low emission at green wavelength.
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surface and immedietly forms a capping layer. Comparing to the other four types of capping agents, Ms is the smallest in molecular size, which could form a very thin cappping layer. However, if the other OH group on capped particles meets with another similar particle, they may condense to form a joined particle (see Fig. 7(c)). We noticed the particle size grew to 320 nm in 2 weeks, which was slower then TEOS capped ZnO particles (500 nm). Pv is one of the stabilizing moieties. Although the vinyl group could cap onto ZnO particles, the repulsion seemed not strong enough in this methanol solution. In addition, the molecular size is too large in respect to the ZnO particle size. Therefore, Pv formed larger clusters with ZnO, which is not preferred for our application. 4. Conclusions Nano-crystalline ZnO colloids have been successfully synthesized from methanol and zinc acetate dihydrate at 68 8C in ambient condition. Three types of capping agents: 3aminopropyl trimethoxysilane (Am), tetraethyl orthosilicate (TEOS) and mercaptosuccinic acid (Ms) effectively capped the ZnO particles at the first precipitation time, so that small particle size (10–30 nm) and good surface quality were achieved. Two other capping agents: 3-mercaptopropyl trimethoxysilane (Mp) and polyvinylpyrrolidone (Pv) caused agglomeration or larger cluster in the solutions (up to 70 nm), therefore, are not suitable as a capping agent in this chemical synthesis process. The size of the single crystal ZnO is 5–6 nm, therefore, a blue shift in absorption edge was obtained due the quantum confinement effect. XRD analyses proved the polycrystalline hexagonal Wurtzite structure of the ZnO. PL spectra showed high UVemission and low visible emission, indicating good quality of particles with little surface defects. Optical absorption spectra confirmed the capping effectiveness by the selected capping agents, as the capped ZnO colloids absorbed at shorter wavelength than the uncapped ZnO indicating much smaller crystal size. Acknowledgements The authors would like to acknowledge the financial support by the Agency for Science, Technology and Research (A*STAR) of Singapore to this project. The authors would like to thank Associate Professor T.J. White from the School of Materials Science & Engineering, NTU for his advice on the Rietveld refinement analyses. References [1] P. Dura´n, F. Capel, J. Tartaj, C. Moure, A strategic two-stage lowtemperature thermal processing leading to fully dense and fine-grained doped-ZnO varistors, Adv. Mater. 14 (2002) 137–141. [2] C. Bauer, G. Boschloo, E. Mukhtar, A. Hagfeldt, Electron injection and recombination in Ru(dcbpy)2(NCS)2 sensitized nanostructured ZnO, J. Phys. Chem. B 105 (2001) 5585–5588. [3] X.T. Zhang, L.G. Liu, L.G. Zhang, Y.M. Zhang, M. Lu, Z. Shen, et al., Structure and optically pumped lasing from nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films, J. Appl. Phys. 92 (2002) 3293–3298.
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