Morphology Control of Zinc Oxide Nanocrystals via Hybrid Laser

Jul 12, 2010 - K. D. G. Imalka Jayawardena, James Fryar, S. Ravi P. Silva and Simon J. Henley*. Nano-Electronics Centre, Advanced Technology Institute...
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J. Phys. Chem. C 2010, 114, 12931–12937

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Morphology Control of Zinc Oxide Nanocrystals via Hybrid Laser/Hydrothermal Synthesis K. D. G. Imalka Jayawardena, James Fryar, S. Ravi P. Silva, and Simon J. Henley* Nano-Electronics Centre, AdVanced Technology Institute, UniVersity of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: June 22, 2010

We introduce a hybrid growth method for the rapid synthesis of ZnO nanocrystals with controllable morphologies through incorporating laser heating along with the traditional hydrothermal synthesis. The crystals are formed using a 248 nm KrF excimer laser irradiation of hydrothermal mixtures. Characterization of samples carried out using transmission electron microscopy, absorption spectroscopy, and photoluminescence spectroscopy indicate a critical fluence at which the reaction kinetics can be controlled to synthesize nanoparticles with a narrow size distribution while fluences above or below this value leads to nanocrystals with a range of morphologies and a broad size distribution.. A growth model is presented in order to explain the observed trend with varying laser fluence. At low fluence, Ostwald ripening is thought to control the growth process, while at the critical fluence a competing photothermal breakdown effect is thought to explain the size controlled formation of nanoparticles. At higher fluences, Ostwald ripening is thought to proceed at a rate higher than the photothermal breakdown leading to the formation of bi-, tri-, or tetrapod-like structures. The possibility of the photothermal breakdown process at the critical fluence is also supported through computational results. 1. Introduction Semiconducting nanomaterials have attracted strong interest among the scientific community due to their possible applications in a diverse range of fields. Among these nanomaterials, ZnO, a group II-VI semiconductor, has attracted special attention due to its wide direct band gap (∼3.37 eV) coupled with the large exciton binding energy (60 meV).1 Not only do such properties allow potential applicability in short wavelength optoelectronic devices2,3 but also as a room-temperature UV luminescence source.4 ZnO nanoparticles themselves have attracted considerable attention as quantum dots for spintronics5 as well as for other applications such as gas sensing.6 Traditionally, the growth of high-quality crystalline ZnO nanostructure is achieved through chemical vapor deposition (CVD), thermal decomposition, thermal evaporation,7–10 or pulsed laser ablation.11 Growth of ZnO nanocrystals based upon such methods usually involve maintaining a high temperature within the growth chamber. Despite the popularity of these techniques, the dependence of the final nanostructure on the stability of process parameters (such as minor temperature gradients during CVD growth)12 and the difficulty of incorporation of such growth techniques due to the high temperatures used, for device fabrication purposes has led to the growing popularity of the chemical synthesis of nanostructures among the research community.13,14 Despite its attractiveness as a lowtemperature growth technique, one of the disadvantages of the hydrothermal growth method has been the long growth times required to obtain the final nanostructures due to the low temperatures employed. In this paper, a novel technique is introduced where the rapid synthesis of morphologically controllable ZnO nanocrystals through the combination of an excimer laser as an energy source in a hydrothermal synthesis confuguration is achieved. In * To whom correspondence [email protected].

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addition to this, the UV absorption of ZnO also allows the possibility of restructering nanoctrystals through the use of a UV laser which as reported in this work has led to morphology controlled growth at a certain fluence window. 2. Experimental Methods Precursor solutions (25 mM) were prepared using zinc nitrate hexahydrate (Zn(NO3) · 6H2O, Fluka g99%) and hexamethylenetetramine (HMTA, Fluka g99%) in deionized water. The HMTA which was further diluted to ∼4 mM, and the 25 mM Zn(NO3) · 6H2O solution was then heated separately in a water bath until ∼90 °C was achieved, and 2 mL of the Zn(NO3) · 6H2O was then added to 12 mL of the diluted HMTA. Upon mixing, the solution was immediately irradiated using a Lambda-Physik LPX 210i KrF excimer laser operating at 248 nm with a pulse duration of 25 ns and a repetition rate of 40 Hz. The effect of the laser fluence was studied by irradiating separate mixtures prepared as above using fluences of 0, 170, 330, and 390 mJ cm-2. A rectangular shaped laser spot was formed at the bottom of the reaction vessel (area of ∼5 mm2) through the use of focusing optics with the penetration being clearly visible during the initial stages of the reaction. No deviation in the beam profile was evident over the growth period. The reactant vessel was translated unidirectionally throughout the process to achieve a more uniform reaction (Figure 1). For each laser fluence, the effect of irradiation time was also studied via irradiation of individual mixtures for periods of 0, 5, 7.5, 10, 12.5, and 15 min. Upon the completion of each process, the sample was quenched in cold water to prevent the reaction from proceeding any further. The products formed were then cleaned by centrifuging twice, with the liquid in the system being replaced by a new fill of deionized water after each centrifuging step to remove unreacted components in the system. The nanostructures formed were observed using an FEI Quanta 200F and a Philips XL30 scanning electron microscope

10.1021/jp103356v  2010 American Chemical Society Published on Web 07/12/2010

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Figure 1. Schematic of the growth system.

(SEM). A Philips CM200 transmission electron microscope for transmission electron microscopy (TEM) was also used to observe the nanostructures and to obtain a selected area electron diffraction pattern (SAED). For TEM observation, the samples were prepared by scooping up the samples while in deionized water onto a holey-carbon grid. The optical properties were analyzed using a Cary Eclipse fluorescence meter using a 330 nm filtered excitation and a Cary 5000 UV-vis-NIR spectrometer. A quartz cuvette was used for both the fluorescence and absorption measurements. With regards to the absorption measurement, the cuvette consisted of a 1 cm beam path with deionized water being used as the reference medium. Rapid measurements were carried out for both optical spectroscopic techniques prior to the sedimentation of nanostructures. 3. Results and Discussion 3.1. Structural Analysis. SEM micrographs of reference samples prepared by mixing Zn(NO3)2 · 6H2O and HMTA without laser irradiation are displayed in Figure 2. While Figure 2a displays the product after 5 min of growth, Figure 2b displays the product after 15 min growth. The structures display a clear change in morphology from “diamondlike” to a more cylindrical shape, there appears to be very little change in the dimensions of the sample over the growth time observed. In contrast, the laser-irradiated samples display a dramatic change in both morphology and dimensions as a function of fluence and growth time. TEM images of the nanocrystals formed by laser irradiation are shown in Figure 3, where each row represents a different applied fluence at a constant repetition rate of 40 Hz/pulse in each case. Parts a and b of Figure 3 (fluence of 170 mJ cm-2) demonstrate that at low fluence the nanostructures display a change in morphology from a more particlelike to a mixture of particles and nanorods with the former being evident at the lower growth duration 5 min and the latter at longer growth duration of 15 min. As can be seen from the transmission electron micrographs, a broad size distribution is present for both growth durations with nanocrystals with dimensions in the range of 20 to ∼100 nm present in both cases with the distribution broadening out more over increased growth time. It should be noted that although rods are also formed during the laser assisted growth process, the dimensions of these structures are much less than that formed in the reference samples indicating that the laser irradiation has an effect on the growth kinetics. Parts c and d of Figure 3 display samples synthesized at a higher fluence of 330 mJ cm-2 for 5 and 12.5 min growth durations, respectively. It is evident that with this fluence value

Figure 2. SEM images of reference ZnO particles grown for (a) 5 min and (b) 15 min. Except for the change in morphology, a significant difference between the dimensions of the nanocrystals cannot be seen for the different growth times.

the growth time does not affect the morphology of the nanocrystals produced which are particle shaped both growth durations. TEM of the samples prepared at this fluence for 5 and 12.5 min indicate an average particle size of 15.5 (5 min) and 18.7 nm (12.5 min) and size distributions lying in the range of ∼8-40 nm (5 min) ∼16-28 nm (12.5 min). Observation of the nanostructures grown at 390 mJ cm-2 for 15 min (Figure 3e) reveals the formation of bi-, tri-, and tetrapod structures as well as larger nanoparticles. Figure 3f reveals that these structures are nucleated from seed particles. The structures formed under a fluence of 390 mJ cm-2 are seen to vary in size from ∼40 nm particles to podlike structures that have rods with lengths of ∼400 nm indicating a broad size distribution. In brief, from the analysis of the TEM images, it can be concluded that there exists a critical laser fluence at which the formation of ZnO nanoparticles occurs and once the laser fluence is increased or decreased beyond this critical value, it will result in the formation of multipod or rodlike structures, respectively. To confirm the nature of material formed, a SAED analysis was carried out (Figure 4) taking the nanostructures formed at 330 mJ cm-2 as a mid-range example. Indexing of the polycrystalline-like diffraction rings indicates that the structure formed is wurtzite-structured ZnO. 3.2. Absorbance Spectra. Analysis of the absorbance spectra (Figure 5) of nanocrystals grown at different fluences for different periods of time reveals several different interesting features that provide support for the growth model to be presented later. One of the first and perhaps the most important features of interest is the high absorbance present for the 12.5 and 15 min growth duration samples prepared at 170 mJ cm-2 (Figure 5a,

Morphology Control of Zinc Oxide Nanocrystals

Figure 3. TEM images of ZnO seed particles grown with a laser fluence of (a) 170 mJ cm-2 for 5 min, (b) 170 mJ cm-2 for 15 min, (c) 330 mJ cm-2 for 5 min, (d) 330 mJ cm-2 for 12.5 min, (e) 390 mJ cm-2 for 15 min, and (f) seed-assisted growth of tripodlike structures at 390 mJ cm-2 for 15 min. While the lowest and the highest fluences of 170 and 390 mJ cm-2 have resulted in the formation of nanocrystals with a broad size distribution, the 330 mJ cm-2 sample has led to the growth of nanocrystals with a more uniform size distribution.

Figure 4. SAED pattern of nanocrystals grown with a laser fluence of 330 mJ cm-2 for 10 min.

parts iv and v) as well as for all growth durations under prepared at 390 mJ cm-2 (Figure 5c) in the wavelength region of 400-700 nm, which appears to be absent from the 330 mJ cm-2 (Figure 5b) absorbance spectra. The higher absorption away from the band edge features in samples synthesized at these

J. Phys. Chem. C, Vol. 114, No. 30, 2010 12933 higher laser fluences is indicative of scattering either due to the presence of a broad size distribution or irregularly shaped larger particles, both of which are supported by the evidence from TEM images with a broad size distribution seen at 170 mJ cm-2 (Figure 3b), while irregularly shaped particles are seen at 390 mJ cm-2 (Figure 3e). This is indicative of the fact that lower fluences result in the formation of more rodlike structures with uneven size distribution while the highest fluences seems to support the growth of bi-, tri-, and tetrapod-like structures. Another point of interest in the absorbance spectra is the presence of two absorbance peaks in the wavelength region of 400-250 nm with the first at 300 nm and the second lying usually between ∼355-363 nm. The peak at 300 nm is found at lower growth times and especially at lower laser fluences (Figure 5a, parts i-iii) and seen to become weaker as the fluence is increased up to 330 mJ cm-2. Upon increasing the fluence up to 390 mJ cm-2 (Figure 5c, parts i-ii), this peak is seen to reappear indicating a possible relationship with the growth process of the crystal structures. The 300-nm absorbance peak has been observed in the absorption spectrum of Zn(NO3)2 and has also been previously reported by Mack and Bolton15 as the signature for the presence of NO3- ions in the system. At lower fluences, there is a strong possibility that the reaction driving the formation of ZnO nanostructures may not occur completely leaving unreacted Zn(NO3)2 in the system leading to the 300-nm absorption peak at lower fluences. At higher fluences as in 390 mJ cm-2, photothermal destruction of the ZnO (discussed later) can result in the formation of Zn2+ leading to the reformation of Zn(NO3)2 and the reappearance of the 300-nm peak. The very weak intensity of the 300-nm peak at 330 mJ cm-2 may be indicative of the fact that this is the laser fluence at which nanocrystals are being formed most effectively. In the absorbance spectra, one particular “anomalous” feature seems to be the presence of a strong, broad absorbance peak closer to 400 nm particularly in the 390 mJ cm-2 annealed samples (Figure 5c). We attribute this absorbance to the rarely discussed “b band” absorption which is also seen in ZnO crystals that have been made yellowish through heat treatment in a Zn atmosphere.16 Vehse et al. showed that that displacement of Zn atoms into interstitial sites through interaction of a ZnO single crystal with electron or neutron radiation can result in the “b band”. Since during annealing with the excimer laser, the nanostructures will infact encounter high energy photons, it may be that this can result formation of Zni17 leading to the “b band” in the absorbance spectra. 3.3. Photoluminescence (PL) Spectra. The PL spectra (Figure 6) of the ZnO nanostructures grown at different fluences show two common features: the UV peak lying in the range of ∼2.9-3.5 eV and a broad peak approximately centered at ∼2.15 eV (the feature at ∼2.7 eV is due to fluorescence from the filters used). One of the important features of the PL spectra is the UV peak which lies at 3.3 eV, which is in agreement with the values provided in the literature.18 In all PL spectra, a yellow emission (∼576 nm/2.15 eV) is also seen. In the literature, the yellow emission has been attributed to the presence of OH- groups.19 When the growth is carried out under laser irradiation, OHgroup formation will occur as mentioned before because of the photodissociation of water.20 At lower fluences, the growth of structures each with a large surface area would mean that there would be a greater tendency for an increase in the surface area of crystalline surfaces with Zn2+ which can result in the formation of Zn(OH)2. At higher fluences, the thermal break-

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Figure 5. Absorbance spectra of ZnO nanocrystals (a) grown with a laser fluence of 170 mJ cm-2, (b) grown with a laser fluence of 330 mJ cm-2, and (c) grown with a laser fluence of 390 mJ cm-2.

Figure 6. PL spectra of ZnO seed particles (a) grown with a laser fluence of 170 mJ cm-2, (b) grown with a laser fluence of 330 mJ cm-2, and (c) grown with a laser fluence of 390 mJ cm-2.

down will result in the increase of the Zn2+ crystalline surface area leading to the presence of this band at higher fluences. The highest relative intensity for the yellow band is seen for the 330 mJ cm-2 laser fluence synthesized samples and can be explained by the higher surface area of the smaller nanocrystals achieved at this fluence compared to 170 and 390 mJ cm-2. 3.4. Growth Mechanism for ZnO Growth under Laser Irradiation. In the hydrothermal growth of II-VI as well as III-V semiconducting nanomaterials, the traditionally accepted view has been that the growth of these nanostructures occurs as a result of the diffusion-controlled “Ostwald Ripening” process.21 During the growth of nanocrystals, there exists a critical equilibrium size. Particles that are smaller than this critical size tend to dissolve while particles larger than the critical parameter grow in size. When the particles are larger than the critical size, growth of the crystals that are closer in size to the critical size occur at a rate greater than that of the crystals that are comparably larger than the critical size (focusing) as can be seen from the growth rate curve in the schematic Figure 7. However, depletion of the monomer concentration leads to an increase in the critical size above the average particle size, which results in the shrinking of the crystals that are smaller than the critical size while leading to the growth of crystals larger than the critical size. This defocusing effect is known as the Ostwald Ripening effect. Prevention of such a defocusing effect, for

Figure 7. Schematic of the effect of photothermal breakdown (green line) and OH- generation (red line) on the morphology of the nanocrystals formed at different fluences.

example, through rapid cooling during the initial stages of the growth, can lead to a narrow size distribution.22 Viswanathe et al. have on the other hand argued that the growth of ZnO nanocrystals depends not only on the Ostwald Ripening process but also on the reaction kinetics which determines the arrival rate of the species to the nanostructures for further growth.23 In terms of the morphology of the material, a fast continuous growth rate is known to result in faceted rodlike growth along

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the c axis of the crystal as the growth direction is higher along this direction, while a slower growth rate has been reported to lead to spherical shaped particles.22 Other than the above factors, photothermal size reduction of nanoparticles is also thought to be a possible mechanism for size control during the laser synthesis nanoparticles as discussed by Inasawa et al.24 During the hydrothermal synthesis of ZnO nanocrystals the reaction sequence for the growth of the nanostructures using Zn(NO)3 and HMTA is well reported25 and is as follows

(CH2)6N4 + 6H2O f 6HCHO + 4NH3

(1)

NH3 + H2O S NH+ 4 + OH

(2)

2OH- + Zn2+ S Zn(OH)2 S ZnO + H2O

(3)

Under conventional hydrothermal synthesis of ZnO nanocrystals, growth and the morphology is thought to be determined by the Zn2+/OH- ratio as well as the presence of HMTA, HCHO, and NH3. While a decrease in the Zn2+/OH- ratio leads to the formation of narrow nanorods, the role of HMTA is less clear but together with HCHO and NH3 is assumed to adsorb on specific crystalline planes of ZnO influencing the morphology.25 If the TEM images in Figure 3 are observed for the dimensions of the nanocrystals grown with the assistance of laser irradiation in comparison to the reference samples, it can be seen that the former are considerably smaller in comparison to the latter. This can be taken as an indication on the effect of the laser irradiation on the growth process. There is a strong possibility that, in addition to changing the rate at which reactions 13 proceed, laser irradiation can also affect the morphology of the product through photothermal breakdown. The final morphology can therefore be thought of as an outcome of two competing processes due to the laser: increased reaction rate against the photothermal breakdown. From the TEM images for the 170 mJ cm-2 fluence based growth (parts a and b of Figure 3), it has already been discussed that not only is there a broadening of size distribution in the laser assisted growth over time but also a reduction in the nanocrystal size compared to the reference sample. As the broadening of size distribution can only occur through an Ostwald ripening process driven by depletion in reactant concentration, it can be inferred that the excimer laser irradiation is involved in changing the reaction rate. Among the species present in the reactant solution, Zn(NO3)2 · 6H2O displays the highest absorption near 248 nm. Therefore, it is highly likely that the energy supplied by the excimer laser to the system is being absorbed by Zn(NO3)2 · 6H2O, which then dissipates this energy to the surrounding region resulting in an increased breakdown of HMTA according to reaction 1. This in turn would drive eqs 2 and 3 in the forward direction leading to the spontaneous nucleation of large amount of ZnO, which is followed by their growth. The considerable reduction in dimensions of samples prepared at this fluence compared to the reference samples can be attributed to the increased formation of ZnO nuclei. The depletion of the reactant concentration over time would lead to the Ostwald ripening process resulting in a broad size distributed end product. Contrary to the growth at 170 mJ cm-2, the TEM images for the growth carried out at 330 mJ cm-2 revealed a narrow size distributed sample with very little change over time in the mean

particle size (parts c and d of Figure 3). Although the increased fluence would lead to an increase in the reaction rate leading to the formation of rodlike structures, photothermal breakdown of the nanostructures become a possible mechanism for size control once the critical fluence for the breakdown process is exceeded. The narrow size distribution with an extremely slow increase in the nanoparticle size over time can be thought of as being due to the rate of HMTA breakdown competing with the photothermal breakdown process without a clear domination of either process on the growth rate over the times studied. At the highest fluence studied, it is suggested that the reemergence of nanorodlike structures is due to the reaction acceleration dominating over the rate of photothermal breakdown. The nanoparticle formed by the breakdown could act as the seed for ZnO bi-/tri-/tetrapod structures observed. As the formation of bi-, tri-, and tetrapod-like structures in ZnO26 and CdS27 have been reported to occur through a seed with zinc blende structure, it is plausible that some of the ZnO nanocrystals formed at such high fluences may also possess this structure. A schematic diagram outlining the above processes is given in Figure 7. 3.5. Photothermal Breakdown of ZnO Nanocrystals. In explaining the growth of ZnO nanocrystals, we have discussed the possibility of photothermal destruction of ZnO nanocrystals due to laser irradiation. Although it has been mentioned that a KrF excimer laser is capable of delivering sufficient energy to break the Zn-O bond, the high (2248 K) melting temperature of bulk ZnO warrants the study of this material under laser irradiation. In observation of the TEM images, it can be seen that the growth of the nanoparticles starts off as very small elongated nanoparticles which then proceeds to form nanorods which are then found out to be broken down into nanoparticles of nearly uniform size distribution at 330 mJ cm-2, followed by the growth of larger nanostructures again at 390 mJ cm-2. Here we have attempted to predict the growth parameters for nanoparticles based along the same arguments presented by Inasawa et al.24 We state the assumptions used that we have made for the calculation with respect to ZnO: 1. A ZnO particle is a spherical single crystal. 2. The nanoparticles have the same thermodynamic properties of the bulk except for the melting temperature, which we have varied based on literature values. 3. The laser has a uniform intensity across the cross section of the particles. 4. There is a uniform temperature inside the particle. 5. Each nanocrystal is surrounded by a temperature boundary layer with a thickness equal to that of the nanocrystal radius. 6. The temperature is uniform outside the boundary layer. On the basis of the calculation carried out by Inasawa et al.,24 the time required for melting is given by

′ ) tm + tm

4πr3F ∆H 3M πr2

F - 4πrκ(Tm - Tw) τ

(4)

Here, tm′ is the duration of melting of the particle, tm is the time taken to reach the melting temperature, r is the radius of the particle, which is varied, F is the density of ZnO taken as 5675 kg m-3, ∆H, the bond enthalpy (248 kJ mol-1), M is the molar weight (81.39 g mol-1), F is the laser fluence (330 mJ

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cm-2), τ is the laser pulse duration (25 ns), Tm is the melting temperature of ZnO, Tw is the temperature of water (363 K), κ is the thermal conductivity of water (0.673 W m-1 K-1), and Cp is the specific heat of ZnO (5.122 J mol-1 K-1). If the temperature is thought to increase until the boiling point of the material, the time taken is then given by

′ tb ) tm

A - B(Tb - Tw) 1 ln B A - B(Tm - Tw)

(5)

where

A)

3FM 4τFCpr

B)

3Mκ FCpr2

(6)

while the temperature variation of the particle over time is governed by

T ) Tw +

[

(

)]

Fr 3Mκ 1 - exp t 4τκ FCpr2

(7)

The term introduced by Inasawa et al.24 to model the transparency of the nanoparticles is ignored here as for all nanoparticle sizes discussed; this term is approximately equal to 1 for ZnO. The decrease in the laser fluence due to absorption by water or Zn(NO3)2 has also been ignored. To understand the possibility as well as the conditions required for photothermal breakdown of ZnO nanocrystals, the temperature vs time behavior for different particle sizes ranging from 15-50 nm has been simulated using eq 7 for a laser fluence of 330 mJ cm-2 (Figure 8). It can be noticed that when the particles that are surrounded by a water layer of a temperature of 363 K (Figure 8a), the particles are unable to reach the melting temperature of the bulk ZnO (2248 K). Nanocrystals can easily reach this melting point if it is assumed to be surrounded by a (steam) layer of water vapor (κ ) 0.0799 W m-1 K-1 as used by Inasawa et al.24) as shown in Figure 8b). Calculations carried out using eq 4 also reveal that the time spent at the melting point for different particle sizes are ∼238 ps (15 nm), 263 ps (20 nm), 338 ps (30 nm), 420 ps (40 nm), and 505 ps (50 nm). From the above calculations, it can be seen that while the nanocrystals are barely able to reach the bulk melting temperature if the surrounding water layer is assumed to remain at 363 K; it can also be seen that the particles would definitely melt if surrounded by a water vapor (steam) layer. However, both of these situations may be considered as extreme cases with the actual water layer surrounding the nanoparticles being at an in between temperature and in a transition between liquid and vapor. Theoretical investigations on the effect of size and shape on melting temperature of ZnO nanostructures also reveal a dramatic reduction in the melting point in comparison to the bulk with decreasing size (20 nm diameter particles are shown to have a melting point of ∼1900 K while 50 nm diameter nanoparticles have a melting point closer to that of the bulk).28 This photothermal breakdown process would explain the lack (at 12.5 min) or the presence of only very small quantities (at 5 min for 330 mJ cm-2) of particles greater than 30 nm for 330 mJ cm-2.

Figure 8. Temperature variation of nanoparticles of different sizes under 330 mJ cm-2 laser irradiation (a) with a surrounding water layer temperature of 363 K and (b) with a surrounding water layer temperature of 853 K. While melting of the nanocrystals can be seen to be impossible under the former condition, all particles are seen to melt under the latter condition.

4. Conclusions In conclusion, we have shown that ZnO nanocrystals with controllable morphologies can be fabricated using a hybrid hydrothermal/laser heating growth process. Lower fluences lead to a critical particle size in excess of the average particle size resulting in a broader size distribution. Higher fluences lead to the photothermal degradation as well as the formation of a large number of reactant units leading to bi/tri/tetra-pod nanostructures. Shape-controlled nanoparticles are observed within a narrow fluence window based around 330 mJ cm-2. These observations are supported by calculations that also reveal possibility of photothermal breakdown of all nanoparticles at higher fluences. Acknowledgment. The authors thank the EPSRC (UK) for funding this work and the studentship awarded as well as the Laser Chemistry, Spectroscopy and Dynamics Group, School of Chemistry, University of Bristol, for their contributions to this work. References and Notes (1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16 (25), R829–R858. (2) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4 (3), 423–426. (3) Lau, S. P. Appl. Phys. Lett. 2005, 87 (1), 013104. (4) Tang, Z. K. Appl. Phys. Lett. 1998, 72 (25), 3270–3272. (5) Janssen, N. Nano Lett. 2008, 8 (7), 1991–1994. (6) Carotta, M. C. Sensors Actuators B 2009, 137 (1), 164–169. (7) Yang, P. D. AdV. Funct. Mater. 2002, 12 (5), 323–331. (8) Park, W. I. Appl. Phys. Lett. 2002, 80 (22), 4232–4234. (9) Xu, C. K. Solid State Commun. 2002, 122 (3-4), 175–179. (10) Umar, A.; Suh, E. K.; Hahn, Y. B. J. Phys. D: Appl. Phys. 2007, 40 (11), 3478–3484.

Morphology Control of Zinc Oxide Nanocrystals (11) Fuge, G. M.; Holmes, T. M. S.; Ashfold, M. N. R. Chem. Phys. Lett. 2009, 479 (1-3), 125–127. (12) Wen, B.; Huang, Y.; Boland, J. J. J. Phys. Chem. C 2008, 112 (1), 106–111. (13) Radovanovic, P. V. J. Am. Chem. Soc. 2002, 124 (51), 15192– 15193. (14) Kim, S. Abstracts of Papers of the American Chemical Society 2002, 224, U443–U443. (15) Mack, J.; Bolton, J. R. J. Photochem. Photobiol., A 1999, 128 (13), 1–13. (16) Vehse, W. E. Phys. ReV. 1968, 167 (3), 828–836. (17) Schmidt-Mende, L.; MacManus-Driscoll, J. L. Mater. Today 2007, 10 (5), 40–48. (18) Srikant, V.; Clarke, D. R. J. Appl. Phys. 1998, 83 (10), 5447–5451. (19) Kwok, W. M. Appl. Phys. Lett. 2006, 89 (18), 183112.

J. Phys. Chem. C, Vol. 114, No. 30, 2010 12937 (20) Carrington, T. J. Chem. Phys. 1964, 41 (7), 2012–2018. (21) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120 (21), 5343–5344. (22) Peng, X. G. Nature 2000, 404 (6773), 59–61. (23) Viswanatha, R. Phys. ReV. Lett. 2007, 98 (25), 255201. (24) Inasawa, S.; Sugiyama, M.; Koda, S. Jpn. J. Appl. Phys. Part 1 2003, 42 (10), 6705–6712. (25) Sun, Y.; Riley, D. J.; Ashfold, M. N. R. J. Phys. Chem. B 2006, 110 (31), 15186–15192. (26) Lazzarini, L. ACS Nano 2009, 3 (10), 3158–3164. (27) Jun, Y. W. J. Am. Chem. Soc. 2001, 123 (21), 5150–5151. (28) Guisbiers, G.; Pereira, S. Nanotechnology 2007, 18 (43), 435710.

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