Colloidal Methods for 0D Metal Oxide Nanoparticle Synthesis

15.2.1 Colloidal Nanoparticles

This section will highlight the investigations of metal-oxide semiconductor (SC) nanoparticles produced by colloidal techniques. In the course of producing colloidal nanoparticles, researchers have focused on controlling porosity, nanoparticle size, size distribution and assembly properties [62 64]. Three major metal-oxide nanomaterials will be discussed, TiO2, WO3 and ZnO. While this is in no way a complete collection of all available nanoparticle systems, it will demonstrate the general techniques currently exploited on a well-rounded basis. The colloidal synthesis of TiO2, WO3 and ZnO nanoparticles spans several decades, and includes, but is not exclusive to, the utilization of sol-gel, hydrothermal, solvothermal, sonochemical and template-directed techniques. Metal-oxide nanomaterial synthesis has been expansive and its applications vary greatly, including sensors, photo- and electrochromics, photocatalysis, photocatalytic degradation, PVand PEC cells [65 67].

15.2.2 TiO2 Sol-Gel Synthesis

Sol-gel methods for the synthesis of TiO2 nanoparticles are based on the hydrolysis and polymerization of titanium precursors [68 74], typically metal alkoxides, inorganic metal salts and metal organic compounds. Hydrolysis of titanium alkoxides under acidic conditions allows for the formation of Ti-O-Ti bond formation, and growth of polymerized chains leading to nanoparticle growth. The rate at which hydrolysis proceeds is dependent on water content, temperature, pH and the concentration of the titanium precursor in solution. Low, medium and high water contents lead to three regimes of Ti hydrolysis: (i) at low water content, the formation of close-packing three-dimensional polymeric skeletons of Ti O Ti chains; (ii) medium water content favors the formation of Ti(OH)4 and (iii) high water content leads to Ti OH formation and loosely packed first-order particles [75]. Study of the growth kinetics of TiO2 nanoparticles with titanium(IV) isopropoxide as the precursor has found particle growth based on temperature to be in agreement with the Lifshitz Slyozov Wagner model for Ostwald ripening [71]; Ostwald ripening being the mechanism that describes the growth process of crystals, in which smaller particles of higher solubility become the precursors for the growth of larger particles (also known as "defocusing") [76]. Knowledge of growth mechanisms via sol-gel techniques has allowed nanocrystalline TiO2 porosity and nanoparticle size to be used in PV applications, as discussed below.

A comparative PV study of three TiO2 nanoparticle systems sensitized by poly(3-undecyl-2,2'-bithiophene) or P3UBT, utilized a common method for sol-gel-prepared nanoparticles [25]. In the sol-gel synthesis, 250 ml of high purity Milli-Q H2O (18 MO) was added to 10 ml of 100% pure ethanol. The solution was acidified to pH 1 2 with concentrated HNO3, and then, under inert atmosphere conditions (nitrogen glove box), 750 ml of titanium isopropoxide was added under vigorous stirring. The solution was then allowed to stir for three days in the inert atmosphere conditions. Overall nanoparticle diameter was found to be 40 60 nm in this particular synthetic route. 100 ml of the TiO2 sol-gel solution was then spin-coated at 1000 rpm onto a conducting substrate, annealed at 450 °C, and later fashioned into functioning PV cells. The above synthesis is using control of hydrolysis and polymerization as its main nanocrystal driving force. Other sol-gel routes include the use of active ionic species such as tetramethylammonium hydroxide (TMAOH) to control growth.

In a systematic examination of TiO2 nanocrystal growth, a sol-gel protocol utilizing the organic base TMAOH, H2O, ethanol and titanium isopropoxide was used [68]. Highly crystalline anatase nanocrystals were produced of varying size, shape and organization, based on ratios of the aforementioned precursors. This base-catalyzed reaction had the dual purpose of promoting hydrolysis and polycondensation reactions of the titanium precursor, but also stabilized TiO2 polyanionic cores through the organic cationic base. With high titanium concentrations, the separation of hydrolysis and condensation reactions (formation of Ti O Ti) is difficult to differentiate [68]. The growth control is understood by the ratio of Ti/TMAOH, and the relative control of crystal growth velocities in the [101] and [001]

Hydrolysis Titanium Isopropoxide

Figure 15.3 TEM images of TiO2 nanocrystal superlattices produced with tetramethylammounium hydroxide (TMAOH) and titanium isopropoxide. (Reprinted with permission from A. Chemseddine and T. Moritz, Nanostructuring titania: Control over nanocrystal structure, size, shape, and organization European Journal of Inorganic Chemistry, (2), 235 245, 1999. © 1999 Wiley VCH Verlag GmbH & Co. KGaA.)

Figure 15.3 TEM images of TiO2 nanocrystal superlattices produced with tetramethylammounium hydroxide (TMAOH) and titanium isopropoxide. (Reprinted with permission from A. Chemseddine and T. Moritz, Nanostructuring titania: Control over nanocrystal structure, size, shape, and organization European Journal of Inorganic Chemistry, (2), 235 245, 1999. © 1999 Wiley VCH Verlag GmbH & Co. KGaA.)

directions. At low TMAOH concentrations, [101] was favored and at high concentrations [001] was stabilized by TME + cations on the nanocrystal surface. This stabilization allowed for isotropic growth that lead to high monodispersity and self-assembly, as seen in Figure 15.3.

15.2.3 TiO2 Hydrothermal Synthesis

Hydrothermal synthesis is an approach that is typically performed in a Teflon-lined steel vessel or similar container that can be sealed tight and placed in ovens at elevated temperatures. The sealed vessels allow for aqueous solutions to be heated past their boiling point, and exceedingly high vapor pressures to be attained within the reaction vessel. Control over the temperature of the vessel and solution volume are variables which allow for the modification of the internal pressure of individual reactions. Hydrothermal techniques for TiO2 nanoparticle synthesis have been widely applied [77 85]. In the preparation of photocatalytically active TiO2 nanoparticles, a similar approach of using an ethanol/water mixture, titanium isopropoxide and acidification with HNO3 in a hydrothermal vessel was utilized [79]. The modification of ethanol-to-water ratio and the concentration of titanium isopropoxide allowed the synthesis of TiO2 nanoparticles ranging in size from 7 25 nm. Thin films were produced from particle suspensions of 7 nm, 15 nm, 25 nm and commercially available Degussa P-25, and field emission scanning electron microscopy (FESEM) images are seen in Figure 15.4. Ethanol-to-water ratio comparisons found that increasing ratios from 1:8 to 8:1 produced continually smaller particle diameters, ranging from 25 to 7 nm. Increasing the titanium isopropoxide ratio from 0.125 1.0 M also showed a similar trend of decreasing particle size from 25 to 7 nm. All solutions were hydrothermally reacted at 240 °C for four hours in a glass-lined hydrothermal bomb. Hydrothermal routes with the use

Biomass Orr

Figure 15.4 FESEM images of TiO2 thin films produced by 7nm, 15 nm, 25 nm and Degussa P 25 commercial powder in a hydrothermal synthesis route. (Reprinted with permission from S.Y. Chae, M.K, Park, S.K. Lee, T.Y. Kim, S.K. Kim and W.I. Lee, Preparation of size controlled TiO2 nanoparticles and derivation of optically transparent photocatalytic films, Chemistry of Materials, 15, 3326 3331, 2003. © 2003 American Chemical Society.)

Figure 15.4 FESEM images of TiO2 thin films produced by 7nm, 15 nm, 25 nm and Degussa P 25 commercial powder in a hydrothermal synthesis route. (Reprinted with permission from S.Y. Chae, M.K, Park, S.K. Lee, T.Y. Kim, S.K. Kim and W.I. Lee, Preparation of size controlled TiO2 nanoparticles and derivation of optically transparent photocatalytic films, Chemistry of Materials, 15, 3326 3331, 2003. © 2003 American Chemical Society.)

of titanium butoxide, and tetraalkylammounium hydroxide has also been explored extensively [83]. For example, TiO2 nanoparticles synthesized using this approach were all found to be a pure anatase phase after the heat treatments. Chain length of the organic base (methyl, ethyl, butyl) had an effect on the morphology, based on concentration. A transition from spherical to rod-like was observed as the concentration increased with tetramethylammou-nium hydroxide. Increasing tetraethylammonium hydroxide, in contrast, caused a transition from spherical to astrerisk-like, and various concentrations of tetrabutylammonium hydroxide did not change the spherical morphology.

15.2.4 TiO2 Solvothermal and Sonochemical Synthesis

Solvothermal reaction techniques are a close relative of hydrothermal techniques, but include the use of nonaqueous solvents. Organic solvents have the advantage of higher boiling points, and, in turn, the size and morphology in organic solvents increases control in comparison to hydrothermal routes. Consequently, the crystallinity of the materials is also increased. The solvothermal method has been effectively used to generate TiO2 nanoparticles of narrow size

Figure 15.5 TEM images of various metal oxide nanoparticles produced by the liquid solid solution (LSL) route. (Reprinted with permission from X. Wang, J. Zhuang, Q. Peng and Y.D. Li, A general strategy for nanocrystal synthesis, Nature, 437(7055), 121 124, 2005. © 2005 Nature Publishing Group.)

distribution and dispersity [86 89]. A universal approach to a number of nanomaterials includes the exploitation of the liquid solid solution (LSS) process. The general protocol includes a metal linoleate phase (solid), an ethanol linoleic acid liquid phase (liquid) and a water ethanol solution (solution) at various temperatures for growth in an autoclave vessel [86]. In a typical synthesis, 0.5 g TiCl3 in 20 ml of H2O, 1.6 g sodium linoleate, 10 ml ethanol and 2 ml linoleic acid is added to an autoclave vessel and heated from 80 200 °C for 10 hours. The synthesized particles can then be collected and dispensed in a number of organic solvents, including toluene or chloroform. This particular technique was demonstrated to work for metals (Ag, Au, Rh, Ir), semiconductors (Ag2S, PbS, ZnSe, CdSe) and metal oxides (Fe3O4, BaTiO3, CoFe2O4, TiO2) [86]. The metal oxides were monodisperse nanoparticles and representative TEM images can be seen in Figure 15.5.

A more traditional solvothermal route is carried out in the production of TiO2 nanoparticles with anhydrous toluene as the solvent, with titanium isopropoxide as the titanium precursor [88]. In an inert atmosphere glove box, different ratios of titanium isopropoxide and anhydrous toluene ranging from 5:100 40: 100 (Ti:Toluene) were mixed into solutions, vigorously stirred for three hours, and placed into a stainless-steel autoclave vessel with Teflon lining. The vessel was then heated to 250 °C for three hours, in which the precursors decomposed into TiO2 and the resulting solvent. The TiO2 nanoparticles were found to be anatase when the Ti:Toluene ratio was 10: 100 30: 100 and amorphous at 5 : 100 and 40: 100. For the crystalline products, the TiO2 nanoparticles had a median diameter of 10 nm, 14 nm and 16 nm for Ti:Toluene ratios of 10: 100,20: 100 and 30: 100, respectively. In a surfactant-aided solvothermal route, oleic acid (OA) was added to the titanium isopropoxide and toluene system, and produced monodisperse anatase TiO2 nanocrystals ranging from nanoparticles to nanodumbells, as a function of precursor ratio [87]. The Ti:Toluene ratio of 5 : 100 produced nanoparticles of 6 nm in diameter. The oleic acid is understood to direct the nanoparticle growth, and subsequently aided in producing nanodumbells in the [001] direction with a Ti: Toluene ratio of 10: 100, and a sliding Ti:OA ratio of 1-2-1: 5.

Sonochemical techniques include the use of ultrasonic waves to produce microscopic bubbles through acoustic cavitation. The ultrasonic technique has been applied to nanomater-ials of metals, alloys, carbides, oxides and colloids. After cavitational-bubble collapse, intense local heating is produced (^5000 K), high pressures (^1000 atm) and large heating and cooling rates (>109Ks TiO2 nanoparticles have been produced using the sonochemical method by various groups [90 98]. In one method, a combination of surfactants, industrially prepared nanoparticles and sonication was used to produce polyanaline-coated TiO2 nano-particles ranging in size from 30 200 nm [94]. Conductance measurements were made with varying degrees of polyanaline, and found that the hybrid polyanaline TiO2 nanoparticles increased in conductivity with increased analine wt%. The sonochemical route is rationalized to be a substitute for stirring in the case of colloidal synthesis, and can lead to better conductive behavior of these hybrid nanomaterials, in comparison to conventional stirring [94].

15.2.5 TiO2 Template-Driven Synthesis

Template-directed growth of TiO2 opals and photonic materials has been well studied, and includes a complimentary direction to the materials synthesis discussed previously in this section [99 104]. In general, these opal materials are produced by a templating mechanism, and will produce highly ordered crystalline structures. One synthetic protocol utilizes latex spheres laid on filter paper over a Buchner funnel, saturated in ethanol and adhered via an applied vacuum [104]. Titanium ethoxide was added dropwise to the latex beads under vacuum, allowed to dry in a vacuum desiccator for 3 24 hours, and then removed for annealing at 575 °C for 7 12hours in open-air conditions. The TiO2 opal produced via the templating technique shows a very well-ordered assembly with 320 360nm voids and is seen in Figure 15.6. Templated TiO2 also finds applications in advanced optoelectronics used as single-mode cavities for quantum electronics and quantum communications [104].

In a similar approach using a skeleton structure, a controlled CaF2 framework of TiO2 was produced from a template technique. The synthetic approach included the production of a polystyrene (PS) opal made by the slow evaporation of a PS solution made of 270 nm diameter particles. A 1 3 mm3 PS piece was then saturated with titanium isopropoxide, ethanol and an argon stream. The infiltrated opal was then exposed for several days to open-air conditions to hydrolyze the titanium precursor with residual water moisture. The composite constructed of PS and titania was than annealed at 300 °C and 700 °C to remove the PS template, and the resultant material was found to be highly opalescent. Figure 15.7 shows the TiO2 opal with its skeleton structure and polygon-like grid. The engineered material was subsequently modeled using a plane-wave expansion method and the bandgap was calculated to be 2.9 eV. This pseudogap was a product of the intrinsic atomic locations in the skeleton structure and was a compliment to the usual bandgap found for TiO2 of 3.2 eV. This dual bandgap material was reported for the first time in this article and the designed materials can be applied to photonic crystals of dielectrics [101].

Photonic Crystals Sem Opal

Figure 15.6 SEM image of the TiO2 inverse opal produced by polystyrene spheres and titanium isopropoxide. (Reprinted with permission from B.T. Holland, C.F. Blanford and A. Stein, Synthesis of macroporous minerals with highly ordered three dimensional arrays of spheroidal voids, Science, 281 (5376), 538 540, 1998. © 1998 American Association for the Advancement of Science.)

Figure 15.6 SEM image of the TiO2 inverse opal produced by polystyrene spheres and titanium isopropoxide. (Reprinted with permission from B.T. Holland, C.F. Blanford and A. Stein, Synthesis of macroporous minerals with highly ordered three dimensional arrays of spheroidal voids, Science, 281 (5376), 538 540, 1998. © 1998 American Association for the Advancement of Science.)

Batio3 Magnetron Sputtering

Figure 15.7 SEM image of skeleton structure of TiO2 produced by PS template with a titanium isopropoxide precursor. (Reprinted with permission from W.T. Dong, H. Bongard, B. Tesche and F. Marlow, Inverse opals with a skeleton structure: Photonic crystals with two complete bandgaps, Advanced Materials, 14(20), 1457 1460, 2002. © 2002 Wiley VCH Verlag GmbH & Co. KGaA.)

Figure 15.7 SEM image of skeleton structure of TiO2 produced by PS template with a titanium isopropoxide precursor. (Reprinted with permission from W.T. Dong, H. Bongard, B. Tesche and F. Marlow, Inverse opals with a skeleton structure: Photonic crystals with two complete bandgaps, Advanced Materials, 14(20), 1457 1460, 2002. © 2002 Wiley VCH Verlag GmbH & Co. KGaA.)

15.2.6 Sol-Gel WO3 Colloidal Synthesis

Synthesis of WO3 nanoparticle systems has been of considerable interest due to the ability to use such materials in electrochromics, photocatalysts and photoelectrochemical cells [105 110]. Colloidal WO3H2O can be produced by the passing of Na2WO4 aqueous solutions through a proton exchange column, and collected as a series of aliquots. After the acidification process, polymerization of tungsten oxoanions occurs to produce yellowish isomorphous crystalline WO3H2O tungstic-acid complexes [111]. The sol-gel techniques typically employed include the addition of directing agents such as the nonionic poly(ethylene glycol) (PEG). One study demonstrated that addition of PEG led to crystallographic orientation of the WO3 nanoparticles in the [200], [020] and [002] crystal directions, as investigated with X-ray diffraction (XRD) and Raman scattering spectroscopy [105]. Chapter 12 extensively covers WO3 for PEC hydrogen production. In a similar tungstic-acid preparation, quantum-confined WO3 colloids smaller than 50 A were synthesized with the use of oxalic acid as a stabilizing agent [112]. Besides investigating the electrochromic effects via UV-visible absorption spectroscopy, energy transfer and charge trapping were explored with picosecond laser flash photolysis using the dye thionine. It was discovered that the oxalic acid acts as an efficient hole scavenger after photoexcitation. In a related synthesis, the solution of 0.25 M Na2WO4 was acidified by passing through a proton-exchange column. To stabilize the growth of the WO3 H2O colloids, 0.520 g of carbowax (PEG) was added per 10 ml of solution and one drop of Triton X-100 was added per ml of tungstic acid [113]. The solution was then stirred for six hours and stored in a refrigerator. This sol-gel nanoparticle synthesis produced 30 50 nm WO3 nanoparticles of the monoclinic crystal phase. These particles have been extensively studied as active photoanodes in PEC cells for water splitting.

15.2.7 WO3 Hydrothermal Synthesis

Hydrothermal synthesis of WO3 systems has also been explored, including the use of tungsten precursors in concert with ligand additives [114 118]. In one WO3 nanoparticle synthesis, Na2WO4 and l( + )-tartaric acid were used, yielding WO3 nanoparticles of 100 nm in diameter, as shown in Figure 15.8 [118]. Another synthesis used the production of H2WO4 H2O by acidic precipitation of Na2WO4 and the formed gels were later washed and placed inside a Parr autoclave bomb. The autoclave bomb was heated to 125 °C, and the suspensions were laid onto indium tin oxide (ITO) conducting substrates. The morphology of the nanoparticles was 100 600 nm in diameter and produced agglomerates of hexagonal WO3 crystallites. The produced WO3 devices were used as NH3 sensors with concentrations ranging from 50 500 ppm [114].

15.2.8 WO3 Solvothermal and Sonochemical Synthesis

Solvothermal routes to WO3 nanocrystallites are limited in scope, but follow similar protocols seen in hydrothermal synthetic routes [119 121]. In general, solvothermal routes for WO3 in the literature appear to form nanowires and nanorods preferentially over nanoparticle formation. A solvothermal approach to the production of CaWO4 did produce nanoparticles in conjunction with a side product of WOx [121].

Wo3 Nanowire

Figure 15.8 SEM image of WO3 nanoparticles made via a hydrothermal method with Na2WO4 and l( +) tartaric acid. (Reprinted with permission from Q.J. Sun, H.M. Luo, Z.F. Xie et al., Synthesis of monodisperse WO3.2H2O nanospheres by microwave hydrothermal process with l( +) tartaric acid as a protective agent, Materials Letters, 62(17 18), 2992 2994, 2008. © 2008 Elsevier.)

Figure 15.8 SEM image of WO3 nanoparticles made via a hydrothermal method with Na2WO4 and l( +) tartaric acid. (Reprinted with permission from Q.J. Sun, H.M. Luo, Z.F. Xie et al., Synthesis of monodisperse WO3.2H2O nanospheres by microwave hydrothermal process with l( +) tartaric acid as a protective agent, Materials Letters, 62(17 18), 2992 2994, 2008. © 2008 Elsevier.)

Sonochemical techniques using ultrasonic irradiation in the synthesis of WO3 nanoparticles is also a rare occurrence and is seen based on a reaction of tungsten hexacarbonyl in diphenylmethane [122]. Overall, the solubility of tungsten hexacarbonyl can be aided by sonication in diphenylmethane (DPM). The residual black solid that resulted was then centrifuged and washed three times with pentane. After heating in open-air conditions at 1000 °C, the black solid became yellowish green, the color usually associated with WO3. TEM images revealed the particles to be 50 70 nm in diameter after annealing, and of the triclinic crystal phase.

15.2.9 WO3 Template Driven Synthesis

Template-driven synthesis of mesoporous and nanostructured WO3 for electrochromic devices, sensing and energy production has been explored with a number of techniques [123 129]. As a gas sensor, WO3 has been widely explored and block copolymers have been exploited to assist silica template formation for producing hexagonal and cubic mesoporous structures. The silica template and the nanoporous WO3 produced are seen in Figure 15.9. The WO3 nanostructures were found to be of a mixed phase of both triclinic and monoclinic, and the nanopores were in the size range of 10 nm in width, in close agreement to the silica-template pore diameter.

Utilizing natural biopolymer templates such as chitosan, which are extracted from the shells of shrimp and other sea crustaceans, WO3 nanoparticles can also be synthesized [125]. Chitosan is a linear polysaccharide composed of randomly distributed b-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). In the synthesis, 1 g of ammonium metatungstate was dissolved in 60 ml of acetic acid diluted with water (1: 1), stirred until dissolved, and then 3g of chitosan was added and left at room temperature for three hours. This chitosan/WO3 gel was than annealed at 400 600 °C, and

Figure 15.9 HRTEM images of the silica mold (a) and its resulting WO3 nanoporous structures (b d). (Reprinted with permission from E. Rossinyol, J. Arbiol, F. Peiro et al., Nanostructured metal oxides synthesized by hard template method for gas sensing applications, Sensors and Actuators B, Chemical, 109(1), 57 63, 2005. © 2005 Elsevier.)

Figure 15.9 HRTEM images of the silica mold (a) and its resulting WO3 nanoporous structures (b d). (Reprinted with permission from E. Rossinyol, J. Arbiol, F. Peiro et al., Nanostructured metal oxides synthesized by hard template method for gas sensing applications, Sensors and Actuators B, Chemical, 109(1), 57 63, 2005. © 2005 Elsevier.)

dispensed in water for TEM characterization. Depending on annealing temperature the WO3 nanoparticles ranged in size from 30 50 nm and consisted of the triclinic phase at 400 500 °C; the nanoparticles formed a large agglomerated structure of the hexagonal phase on annealing at 600 °C. The WO3 nanoparticles were fashioned into electrochemical cells and cyclic voltammetry was performed to cathodically produce hydrogen at the electrode surface.

Control over pore size and geometric positioning is a key attribute in these structures and can be accomplished by the utilization of anodized aluminum oxide (AAO) [126]. With the motivation to make a high surface-to-volume ratio gas-sensing device, an AAO was produced by the sputtering of aluminum on Si substrates, then anodizing them in malonic acid electrolytes with a an applied voltage of 95 V and a current of 6 mA cm 2. In a secondary step to coat the exposed aluminum with compact alumina, a follow up anodization at 110 V followed. To increase surface-to-volume ratios with the AAO, a third preparatory step was taken, wherein a chemical bath of phosphoric and chromic acid at 50 °C was used for upwards of 9 minutes. Deposition of the WO3 was performed using rf magnetron sputtering onto the

Wo3 Sputter

Figure 15.10 SEM images of the template WO3 gas sensing devices produced with anodized aluminum templates. (Reprinted with permission from V. Khatko, G. Gorokh, A. Mozalev et al, Tungsten trioxide sensing layers on highly ordered nanoporous alumina template, Sensors and Actuators B, Chemical, 118 (1 2), 255 262, 2006. © 2006 Elsevier.)

Figure 15.10 SEM images of the template WO3 gas sensing devices produced with anodized aluminum templates. (Reprinted with permission from V. Khatko, G. Gorokh, A. Mozalev et al, Tungsten trioxide sensing layers on highly ordered nanoporous alumina template, Sensors and Actuators B, Chemical, 118 (1 2), 255 262, 2006. © 2006 Elsevier.)

AAO at a power of 100 Wand a chamber pressure of 5 x 10 3mbarofanAr:O2mixture(1: 1). The resulting AAO/WO3 nanoporous structures after chemical-bath dipping in seen in Figure 15.10. Pore diameter was found to change from 50 170 nm depending on chemical etching time via SEM imaging. The nanoporous structures were originally amorphous and after annealing at 400 °C, the monoclinic WO3 crystal phase was detected by XRD. AAO templates have been used in the synthesis of 1D nanostructures, as discussed further later.

15.2.10 ZnO Sol-Gel Nanoparticle Synthesis

Sol-gel techniques have been widely used in the course of producing ZnO nanoparticle systems for fundamental spectroscopic investigations [130 135]. In one procedure, 37.7 mg of zinc perchlorate (Zn(ClO4)2H2O) in 40.5 ml of methanol, was added to a NaOH solution in 40.5 ml of methanol, allowed to stir overnight at 25 °C and stored in a dark environment [131]. The nanoparticles had a mean particle diameter of 3.4nm, and were tracked for size changes by UV-visible absorption. Subsequent photoluminescence (PL) studies were conducted to probe the states of fluorescence as a function of excess Zn2 + ions in solution. With extremely small particles of 7 30 A, the quantum yield of ZnO particles was found to depend on diameter, most likely due to the changes in surface trap density [135]. In a similar sol-gel synthesis, zinc acetate was used as a precursor for producing ZnO nanoparticles. SEM images of the ZnO colloids in a hexagonal packing arrangement can be seen in Figure 15.11. The mean nanoparticle diameter was found to be 60 nm and the nanoparticles were in a close-packed hexagonal structure. The growth mechanism is believed to be a two-stage process of nucleation of 3 4nm ZnO nanopar-ticles, and then subsequent agglomeration that is very rapid, and this in turn allows for spherical geometries to form into larger 60 nm nanoparticles.

Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook


Post a comment