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Synthesis of zinc oxide nanorods via the formation of sea urchin structures and their photoluminescence after heat treatment Mattias E. Karlsson, Yann C. Mamie, Andrea Calamida, James M. Gardner, Valter Ström, Amir Masoud Pourrahimi, and Richard T. Olsson Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01101 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Synthesis of zinc oxide nanorods via the formation of sea urchin structures and their photoluminescence after heat treatment Mattias E. Karlsson,a Yann C. Mamie,a Andrea Calamida,a James M. Gardner,b Valter Ström,c Amir Masoud Pourrahimi*a and Richard T. Olsson*a A protocol for aqueous synthesis of ca. 1 µm long zinc oxide (ZnO) nanorods and their growth at intermediate reaction progression is presented, together with photoluminescence (PL) characteristics after heat treatment at temperatures up to 1000 °C. The existence of solitary rods after the complete reaction (60 min) was traced back to the development of sea urchin structures during the first 5 seconds of the precipitation. The rods primarily formed at later stages during the reaction due to fracture, which was supported by the frequently observed broken rod ends with sharp edges in the final material, in addition to tapered uniform rod ends consistent with their natural growth direction. The more dominant rod growth in the c-direction (extending the rods lengths’), together with the appearance of faceted surfaces on the sides of the rods, occurred at longer reaction times (>5 min), generated zinc-terminated particles more resistant towards alkaline dissolution. A heat treatment for 1 hour at 600 or 800 °C resulted in a smoothing of the rod surfaces and PL measurements displayed a decreased defect emission at ca. 600 nm, which was related to the disappearance of lattice imperfections formed during the synthesis. A heat treatment at 1000 °C resulted in significant crystal growth reflected as an increase in luminescence at shorter wavelengths (ca. 510 nm). Electron microscopy revealed that the faceted rod structure was lost for ZnO rods exposed to temperatures above 600 °C, whereas even higher temperatures resulted in particle sintering and/or mass redistribution along the initially long and slender ZnO rods. The synthesized ZnO rods was a more stable Wurtzite crystal structure than previously reported ball-shaped ZnO consisting of merging sheets, which was supported by the shifts in PL spectra occurring at ca. 200 °C higher annealing temperature, in combination with a smaller thermogravimetric mass loss occurring upon heating the rods to 800 °C.

Introduction The synthesis of zinc oxide (ZnO) nanorods for the emerging field of high voltage direct current (HVDC) polymer nanodielectrics is reported as a 1 L batch protocol. This ample volume allows for sufficient quantities to explore elongated ZnO as an inorganic phase for improved insulation properties 1 in polymer nanocomposites. The elongated and faceted ZnO nanorods are of interest in HVDC applications since they allow for exploring the effects of having particle surfaces with dominantly zinc crystal terminations, in combination with how their elongated shapes impact on the local electric fields and 2, 3 the trapping of electrical charges. ZnO particle applications 4, 5 6 also include sun-screen protections and solar technologies, 2, 7, 8 9, as well as active antibacterial surfaces and photocatalysts 10 where the morphologies and atomic plane terminations have been shown relevant for the function of the ZnO phase. It is thus not unlikely that the emerging nanodielectric polymer composites could also benefit from morphologies representing different specific atomic terminations, i.e. in addition to the extensive number of above applications currently using

industrial grade ZnO without specific ZnO crystal lattice 2, 11 An aqueous ZnO synthesis approach in the terminations. absence of organic chemicals can here be regarded as the most environment-friendly, but also foreseen to be of industrial relevance since it can be inexpensively scaled to larger amounts. The morphology of zinc oxide particles have been shown to vary extensively as dependent on the reaction conditions used 1, 2, 9, 12, 13 for small-scale aqueous precipitation. Time, temperature, reactant ratio and the use of a specific zinc ion salt (e.g. nitrates, chlorides, sulphates etc.) have been demonstrated to independently and significantly influence the 14 particle morphologies. Morphologies range from relatively spherical nanoprisms to nanometer thick sheets, which under certain conditions can grow into micrometer-sized particles with hierarchically organized nanostructures, e.g. porous 9, 14 flower- or star-shapes. Certain crystal growth directions can be favored by using surfactants as ‘capping agents’ that 0) crystal planes, resulting selectively adsorb on (0001) or (101 in growth of hexagonal sheet or rod-shaped particles, 15-18 respectively. Directional growth to specific morphologies has also been shown to occur in the absence of organic chemicals when specific reaction conditions govern growth of 2, 9, 19-22 selective crystal planes. However, while several growth 20, 22, 23 patterns have been suggested , the specific reaction paths’ from the early nucleation to ready-made particles are often held in darkness for different ZnO crystal shapes made in the absence of organic chemicals. A reason for this is the rather limited control over the early stages of the synthesis, which takes place in the time frame of minutes, but may well

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be dictated already at the first few seconds during the precipitations. In fact, most of the studies regarding the timedependent growth of rod-shaped ZnO particles in the absence of organic chemicals have only been investigated for relatively long reaction times, i.e. from hours to days, or for methods 19, 22, 24-27 using hydrothermal growth techniques. In a previous work, we reported ball-shaped oxygenterminated micrometer sized ZnO particles that demonstrated significantly improved photocatalytic properties due to their 9 oxygen-terminated (67 %) hierarchical sheet morphologies. In this work we focus on rod-shaped particles with dominant zinc 0) and (011 0) planes along the terminations from the (101 2 sides of the ZnO rods (83 %). A 60 min aqueous synthesis 2 -1 protocol is presented that provides nanorods (9.9 m g ) with ca. 900 nm in length and 53 nm in width, i.e. an aspect ratio of ca. 17. The crystal growth pattern was, for the first time, studied by quenching aliquots as early as 5 seconds after the reaction initiation. From this, it was observed that none of the early quenched samples displayed solitary rods; the mechanism for the formation of the ZnO nanorods is discussed. Instead, all experiments indicated that the rods formed as a secondary product from initially nucleated sea urchin structures, which has never previously been reported in ZnO nanorod synthesis reactions. Heat treatment was further used to refine the surface of the rods; temperatures from 600 to 1000 °C were evaluated to determine the maximum temperature that could be used without extensive rod sintering. The photoluminescent properties of the ‘as synthesized’ and the ‘heat-treated’ ZnO nanorods are presented. Finally, an explanation to the formation of extended rods from an initially nucleated sea-urchin structure is suggested based on the difference in crystal lattice stability towards the alkaline synthesis medium.

Experimental Section Materials and synthesis of ZnO rod-shaped particles Zinc nitrate hexahydrate (Zn(NO3)2 · 6 H2O, ≥ 98 %, Sigma Aldrich), sodium hydroxide (NaOH, ≥ 98 %, Sigma Aldrich), Milli-Q water ("Type 1", following ISO 3696 and ASTM D1193– 91, 18.2 MΩ cm at 25 °C), sodium citrate dihydrate (Na3C6H5O7 · 2 H2O, ≥ 99 %, Alca) were used as received. The 900 nm ZnO rods were synthesized by adding a 4 M sodium hydroxide aqueous solution at 23 °C (250 mL) to a 0.067 M zinc nitrate hexahydrate aqueous solution (750 mL) kept at 80 °C under mechanical stirring. The synthesis was allowed to progress for 60 min at 80 °C. Synthesis of smaller 300 nm ZnO nanorods was carried out at 100 °C with otherwise identical conditions. Sampling and quenching at reaction times 5, 15, 60, 300 and 900 seconds were performed by transferring 1 mL solution from the reactor to a 2 mL Eppendorf tube containing 1 mL of 10 mM sodium citrate aqueous solution that was kept at 0 °C (using an ice bath). The precipitate from completed reactions (60 min) and quenched samples were centrifuged (Rotina 420 and Eppendorf MiniSpin, respectively) four times at 4500 rpm

for 8 min with intermediate change of the liquid phase by Milli-Q water and dispersing the particles by thorough shaking. Synthesis of ZnO ball-shaped particles and dissolution experiment The ball-shaped ZnO particles were synthesized by adding a 1 M aqueous NaOH solution (250 mL) to a 0.067 M zinc nitrate hexahydrate aqueous solution (750 mL) kept at 60 °C under mechanical stirring followed by 1 h reaction time prior to identical cleaning methods as for the nanorods described above. The resistance to dissolution of synthesized ZnO nanorods and ball-shaped ZnO particles where investigated by adding 20 mg of each particle in a 2.5 M aqueous NaOH solution (5 mL). The experiment was carried out over 15 min at identical stirring speed for both vials. Post-treatment of synthesized ZnO nanorods The obtained ZnO particles were dried either at atmospheric pressure at 80 °C and further ground to a fine powder with a pestle and mortar or freeze-dried, using a CoolSafe freeze dryer (Scanvac) operated at ca. 0.1 mbar and −96 °C over a time period of 48 hours. The samples were frozen as aqueous suspensions of nanorods prior to freezedrying by immersion of the sample containers in liquid nitrogen. A heat treatment of the nanorods was carried out at 600, 800 and 1000 °C in air for 1 h by using a H14-GAXP furnace from Micropyretics Heaters International Inc. Particle characterization The morphology of synthesized particles was studied by using a Hitachi S–4800 field emission scanning electron microscope (SEM). The acceleration voltage and emission current were 5 kV and 10 µA, respectively. Sputtering (Cressington 208 HR) for 20 s with Pt/Pd was performed on the particles prior to SEM observation by using a current of 80 mA. The specific surface area was measured by Brunauer-EmmettTeller nitrogen adsorption/desorption (Micromeritics ASAP 2000 at 77 K). A TGA/DSC 1 from Mettler Toledo was used to measure the mass loss during heating (40-800 °C). The powder particles (10 ± 1 mg) were placed in an aluminum oxide -1 crucible (70 μL) and the heating rate was 10 °C min in oxygen -1 (50 mL min ). X-ray diffraction (XRD) was performed on ‘as synthesized’ and heat treated nanorods by using a Siemens Crystalloflex D5000 with Cu Kα radiation, 0.02 ° step size, 35 kV and 40 mA. The crystal size (D) was calculated from the 28 Scherrer equation ,

D=

kλ β cos(θ )

(1)

where k is a shape factor (k=0.89), λ is the X-ray wavelength (0.15406 nm), β is the full width at half maximum of the intensity of the diffraction peak and θ is the Bragg diffraction angle. The lattice parameters a and c were 29 calculated by using the equation

4sin 2 θ

λ

2

=

4 ( h 2 + hk + k 2 ) 3a

2

+

l2 c2

(2)

2



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where h, k and l are the Miller indices. Photoluminescence characterization was carried out at room temperature on ZnO nanorods heat treated at different temperatures, using a Varian Cary Eclipse spectrophotometer (Xe lamp, 250 nm excitation wavelength). The nanorods were dispersed by ultrasonication (2510-DTH, Branson) in deionized water -1 2 (1 g L ) and then added to a quartz cuvette (10 × 10 mm ).

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Fig. 1. The micrographs show the ZnO rods for different post synthesis treatments: freeze-drying (a-e) and conventional drying at 80 °C followed by grinding (f). The characteristic rod ends: tapered and irregularly broken are shown in (d) and (e), respectively. The X-ray diffractograms for the nanorods heat-treated at 600, 800 and 1000 °C under air are shown in (g). The distribution in length of freeze-dried rods and the volume for the same mass of grinded rods compared to freeze-dried rods are presented as insets in (b) and (f), respectively.

Results and Discussion Morphology of rod-shaped ZnO particles Fig. 1 shows the rod-shaped particles after one hour of synthesis at a temperature of 80 °C, followed by freeze-drying the particles into a powder. Similar rod morphologies have previously been prepared by synthesis conditions making use 0) planes, allowing the of ‘capping agents’ that adsorb on (101 hexagonal structure to expand more dominantly in the cdirection of the rods, i.e. resulting in large aspect ratio 15, 17, 18 values. The zinc ions are dominantly present as 22Zn(OH)2/Zn(OH)4 at pH>7, where Zn(OH)2 and Zn(OH)4 are the nucleation and growth units, respectively, and a ‘capping agent’ was not required under the applied conditions in this 24, 27, 30, 31 work. From Fig. 1a it can be seen that the rods were organized as clusters with an average diameter of ca. 5-10 µm. Nevertheless, all clusters consisted of randomly organized solitary nanorods with an average length of ca. 900 nm and width of 53 nm, although an increase in synthesis temperature to 100 °C was an efficient way to favor growth of smaller rods with dimensions of ca. 300 x 50 nm (Fig. S1, in the supporting information). From Fig. 1b it can be seen that the organization of the rods after the freeze-drying frequently displayed parallel rod-to-rod alignments, and multiples of these rod associations worked to interconnect and bridge the spherical clusters. Fig. 1c shows one of the bridges and demonstrates that the crystal growth occurred with almost perfect formation of equally wide external planes facing outwards along the ZnO crystals and faceted edges. The organization of the ZnO rods was; however, lost for particles dried in a conventional oven under atmospheric pressure before grinding into a powder (Fig. 1f)

and the parallel alignment of the rods between the clusters (Fig. 1b) was associated with the ice crystal growth and boundary formations for the given freezing conditions. Fig. 1d shows tapered rod ends that were formed due to the different growth rate of the crystal planes, i.e. 1}>{101 0}, where the (0001) represents the fastest (0001)>{101 growing plane perpendicular to the spiky rod tippet in 0} planes along comparison to the much slower growing {101 32, 33 the sides of the rods. However, a dominant feature of the powder sample was rod ends with uneven and sharp morphologies, see Fig. 1e. Although completely flat rod ends may form under certain conditions, the broken nature of the rod ends was taken as evidence of their formation as the result 34 of fracture (Fig. S2). It could thus be concluded that the aspect ratio of ca. 17 did not solely represent the natural growth of the rods by condensation reactions since the rods displayed the different characteristics at the rod ends; tapered vs. irregularly broken ends. Interestingly, although the freezedried (FD) samples showed an interconnected structure of spherical clusters, a lower powder density could not be observed when comparing the powder volume for the same mass of material as for a grinded (GR) sample (inset in Fig. 1f). Fig. 1g shows the X-Ray diffractograms of the pure ZnO phase corresponding to the hexagonal Wurtzite structure. Overall, the prepared ZnO rods were uniform in their morphology and the need for a ‘capping agent’ can accordingly be eliminated if the reaction conditions are tuned to naturally favor the rod formation.

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Fig. 2. The micrographs show the evolution of the ZnO rod shapes with time during the first 15 min of synthesis. The first 1 mL aliquot was taken 15 s after addition of the alkaline medium (a), 1 min (b), 5 min (c) and 15 min (d) after the addition. Histograms are presented in nm length scale and only represent the formations of rods.

Growth of rod-shaped ZnO particles To investigate the initial stages of the rod formation, 1 mL liquid aliquots were taken at different time intervals from the reaction initiation (by the addition of the alkaline medium, time = 0 s). The aliquots at 5, 15, 60, 300 and 900 seconds were rapidly mixed with an aqueous solution of sodium citrate held at 0 °C. The times were selected due to the dominant reaction progression during the first 15 minutes (900 s), where after changes mostly occurred in the lengths’ of the rods, increasing from an average of ca. 500 to 900 nm (see the size distributions in Fig. 2d and Fig. 1b, respectively). The motivation for sodium citrate solutions was based on the citrate molecules ability to adsorb selectively on the (0001) crystal plane, hindering the growth in the normal direction of this plane, i.e. preventing further extensional growth of the 15, 16 rods. The quenching of the reaction at 0 °C was made to further minimize possible growth of the rods after the sampling and the samples were then freeze-dried prior to microscopy. As can be observed from Fig. 2a, already after 15 s signs of rod-shaped morphologies had formed. At this point the average rod length was ca. 210 nm (see inset) and all rods showed uneven surfaces with a grainy structure. The grainy structure was identified as nucleation of smaller inorganic complexes, which did not show exclusive preference towards

the (0001) plane. The microscopy also revealed that a significant amount of disorganized material was distributed in between associated rods (Fig. 2a). This material was also present in the 60 seconds sample (Fig. 2b), but was absent in the aliquots taken at longer synthesis times. In contrast to the 15 seconds sample, the 60 seconds sample revealed tapered morphologies but without the faceted edges. Fig. 2c shows the 5 min sample wherein the smaller disorganized zinc oxide complexes were absent, i.e. only rod morphologies could be found. It is notable that although rod formation had occurred at this point, no signs of the faceted morphologies could be observed for the 5 min sample (compare Fig. 2c and Fig. 1c). Fig. 2d shows that the rods doubled in lengths’ during the following 10 min to ca. 500 nm, from the ca. 210-260 nm sampled the first 5 minutes (Fig. 2a-c). The faceted rod-shapes could thus be concluded to primarily form in the time period from ca. 15 to 60 min, wherein the average length of the rods increased to ca. 900 nm. In comparison, the relatively complete rod formation in 15 min by immediate wet precipitation at 80 °C was accordingly faster than previously reported methods based on a drop wise addition of the alkaline source to the zinc salt solution, followed by increasing the temperature to 90 °C and maintained refluxing for 30-60 35, 36 min. Table 1 summarizes the characteristics of all precipitated sample morphologies.

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Table 1. Summary of nanorod growth for quenched (5 s - 15 min) and complete reaction progress (60 min). Reaction time

Disorganized

Faceted

Average

Average

(s)

material

surfaces

length (nm)

width (nm)

5

yes

no

133

43

15

yes

no

213

33

60

yes

no

223

48

300 (5 min)

no

no

257

51

900 (15 min)

no

yes

478

53

3600 (60 min)

no

yes

893

53

All samples were identically transferred as 1 mL aliquots over to an Eppendorf tube in the timeframe of a second and quenched (0 °C) in a citrate solution.

To isolate the earliest possible formation of the rods, sampling the precipitated product immediately after mixing the solutions was made, i.e. within the first 5 seconds. Fig. 3 shows that rods did not initiate their formation as unique/individual rod structures but formed as a result of rod growth from a nucleation center. The nucleation center is visible in Fig. 3a, where the rods only existed as grown radially splaying outwards from the nucleation center. At this point the average rod length was ca. 130 nm and constituted ca. 15 % of the final rod length after the 60 min synthesis. The rods were entirely covered with smaller ca. 10 nm grains evenly formed on their surfaces.

Fig. 3. The micrographs show aliquots quenched 5 s after addition of the alkaline medium in large (a) and low (b) magnification.

Fig. 3a also shows that the grainy surface occasionally functioned as support for growth of thinner rods splaying

outwards from the larger rods, which had formed first from the nucleation center. It was also apparent from the aliquot that a heterogeneous nucleation and uneven mass condensation within the first 5 s had formed the origin of the relatively wide size distribution of the final nanorods. Many different mechanisms have been proposed to explain 20, 23, 24, 27, the formation of the rod-shaped ZnO morphologies. 30, 37-39 27 Hsieh et al. reported that hydrothermally grown ZnO rods in absence of ‘capping’ agents formed as individual rods 2in the [0001] direction by attraction of negative Zn(OH)4 ion species onto the (0001) plane, followed by condensation to ] direction was suggested to be ZnO. The growth in the [0001 prevented by repulsion between the established negatively ) plane (consisting of O2-) and the negatively charged (0001 charged unit Zn(OH)42- in the suspension.27 The early stage of the hydrothermal reaction was further concluded to consist of nucleation and formation of hexagonal ZnO sheet nanoparticles, which formed the basis for the extensional growth into hexagonally faceted rods in the [0001] direction.27 In this work, the hexagonal sheets referred to as the starting point of the formation of the solitary rods were never observed for aliquots isolated within the first 5 seconds, possibly due to a much faster progression of the reaction. Usui et al.37 reported the formation of solitary rods without evidence of formation of the hexagonal sheets. The presence of hexagonal sheets in the early stages for rapidly progressing reactions is thus an open question, although it may be presumed that some ground for the anisotropic growth must have existed. It was also notable that the aliquots isolated within the first 5 seconds showed no particles with faceted edges (see Fig. 3), which is in contrast to the faceted hexagonal sheets isolated by Hsieh et al.27, where only the hexagonal sheet formation required ca. 40 seconds to occur. The growth into extended rods has further been related to a large supply of Zn(OH)42- ions.37, 40 However, a large supply of Zn(OH)42- ions has also been shown to result in sea urchin structures, whereas smaller concentrations favored solitary rods.41, 42 Interestingly, Wahab et al. reported a transformation of solitary rods into sea urchin structures with increasing heat treatment times during the sol-gel reaction, using hydrazine as reductant.23, 43 Much of the background to the early formation of solitary rods and/or rods organized into sea-urchin structures is thus still unknown although several theories have been proposed, including twinning phenomena already suggested in the 1940s.22, 41, 44 From a theoretical perspective the crystallographic planes of the ZnO rods have been associated with different surface energies, as reported by Ge et al., ranging from ca. -2.8 to -1.7 kJ/mol.20 However, the planes are not always perfectly smooth, as demonstrated in Fig. 3 (as well as in previous reports) during the aqueous sol-gel formation of ZnO particles.9, 14 The presence of uneven planes with different reactivity, as well as the rate of the ZnO formation, may both affect the coordination chemistry occurring during the condensation of zinc species onto the crystal planes. It is also clear that the counter ions in the aqueous phase affect the specific deposition, since only using a different zinc salt, i.e.

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varying the zinc salt counter ions, resulted in completely 9, 14 different morphologies under the same reaction conditions. In our work it is concluded that the heterogeneous nucleation initially favored a formation of sea urchin structures with rods splaying from their nucleation centers (see Fig. 3). However, the dominant product after 60 min synthesis was solitary rods mostly formed by fracture (see Fig. 1 and Fig. S2). The absence of remains from the nucleation centers in the final material was suggested to stem from their ability to dissolve into smaller species that could favor the formation of more extended rods at the later stages of the reactions, i.e. when the rod length doubled from ca. 500 to 900 nm (after 15 min reaction). This hypothesis was supported by results confirming a greater resistivity against dissolution of dominantly zinc-terminated crystal planes compared to dominantly oxygen-terminated ZnO crystal surfaces. The experiment was carried out by separately evaluating the dissolution of 20 mg zinc- and oxygen-terminated particles in 5 mL of a 2.5 M aqueous NaOH solution (see Fig. S3 and the enclosed video in the supporting information). Selective dissolution of ZnO nucleation centers has previously been described for spherical ZnO particles.45 Accordingly, as the zinc-terminated faceted sides along the rods (present after 15 min of growth) merged over to the more spherically shaped cores of the sea urchin centers, a presence of more oxygen rich surfaces existed, which in turn was more prone to attack by the alkaline medium. The more sensitive nature of the ) plane has previously been described oxygen-terminated (0001 by Nicholas et al. using a 2 M sodium hydroxide solution to etch single crystals of ZnO (as observed from AFM analysis).46 Effect of heat-treatment of freeze-dried ZnO nanorods Fig. 4 shows the freeze-dried samples heat-treated at: a) 600 °C, b) 800 °C, c) 1000 °C for 1 hour. After one hour at 600 °C, the rods had merged at the junction points of crossing rods, or as parallel rods (Fig. 4a and d). The rod ends were smooth and their irregular fractured appearance, together with the

faceted sides along the rods, were no longer possible to 47 distinguish. Zhang et al. reported similar changes in the ZnO morphology occurring with a heat treatment at 600 °C, whereas such morphology changes could not be observed with a heat treatment at 400 °C. A heat treatment for 1 hour at 800 °C resulted in completely merged rods, sometimes forming sheet-shaped structures of in-plane parallel rods as in Fig. 4e. The presence of circa 30 nm wide, evenly distributed grains observed on the surface of the sheets, was taken as an indication that a heat induced mass transfer along the rods was initiated between 800 and 1000 °C, compare Fig. 4f. Extensive inter-particle sintering was apparent after 1 hour at 1000 °C. At this point, the borders between the individual rods had completely disappeared; see Fig. 4c and f, and the entire sample consisted of an ensemble of unified rods in a network. The porous nature of the sample, i.e. the volume of the rod superstructure, remained however almost the same as a result from the freeze-drying of the samples. In agreement with the previous work by Zhang et al.47 it was also observed that the rod sintering generated shorter and wider rods with a heat treatment at 1000 °C, compare Fig. 4a and 4c. The calculated unit cell lattice parameters for all the samples corresponded to a=3.25 Å, c=5.21 Å, i.e. the values for ZnO (from inorganic crystal structure database; collection # 067849), and only peaks corresponding to the hexagonal Wurtzite crystal structure were present after all the heat treatments (Fig. 1g). For the sample heat treated at 600 °C the crystal size remained the same, i.e. 30 nm (see Table 2). The heat treatment at 800 and 1000 °C resulted in an increase to ca. 32 and 38 nm, respectively. Table 2 also shows the ratio of the crystal size based on the peaks corresponding to the 0) plane, i.e. crystal size ratio of the c- and (0002), and the (101 a-directions. The ratio approached unity with increasing temperatures as the crystals became equiaxed, in accordance with previously reported results for heat treatment of ZnO nanostructures.9, 48

Fig. 4. The micrographs show the heat treatment of ZnO nanorods for 600 °C (a and d), 800 °C (b and e) and 1000 °C (c and f).

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Table 2. Crystal size for heat-treated ZnO nanorods from X-ray diffraction measurements.

Heat treatment

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[nm] a

/

Reference

29.9

1.20

600 °C

30.1

1.20

800 °C

32.1

1.12

1000 °C

37.6

1.00

b

a The crystal size was calculated by the Scherrer equation (eq 1) based on the (101 1) diffraction peak (i.e. both a and c direction). b The ratio of the crystal size 0) peaks. from the Scherrer equation was calculated from the (0002) and (101

Photoluminescence characteristics Fig. 5 shows the photoluminescence spectra for ZnO nanorods heat treated at different temperatures, together with the freeze-dried reference sample. The UV-emission peak at 390 nm, corresponding to recombination of free excitons, was present for all samples and was thus consistent with previous 9, 49 reports. The broad emission peak of lower intensity, showing a maximum at ca. 600 nm in the visible region (400700 nm), was associated with defects in the ZnO structure for 50 the reference nanorod sample. This 600 nm emission decreased in intensity with the heat treatments at 600 and 800 °C, whereas a broad green emission peak at ca. 510 nm emerged for the sample heat-treated at 1000 °C. Similar observations have previously been reported for heat-treated 51-53 ZnO nanostructures , but the origin to the shift towards 50 lower wavelength’s has not been entirely explained. A decrease in emission around 600 nm was suggested due to fusion of hydroxyl groups in the Zn(OH)2, i.e. by condensation 49 of water in the nanorods. A weight loss was herein confirmed by thermogravimetric measurements (see Fig. S4) and showed a 0.9 wt% mass loss at 800 °C.

Fig. 5. Photoluminescence spectra for heat-treated ZnO nanorods.

Fig. 6 shows the powder samples under UV-light (365 nm) after immersion in liquid nitrogen. The sample without thermal treatment emitted a strong orange color, the 600 and 800 °C samples emitted a weaker slightly violet and white color respectively, whereas the sample heat-treated at 1000 °C showed a strong yellow color. The observed colors are qualitatively in agreement with the photoluminescence spectra acquired at room temperature. It was concluded that the more intense emissions associated with the colors for the reference and the 1000 °C samples (Fig. 6) appeared to be correlated to the presence of defects visible as the broad peaks in the PL spectra centered at 600 and 510 nm, respectively. Due to the spectrally distinct emissions from the reference and 1000 °C samples, it is unlikely that these emissions centered at 600 and 510 nm, respectively, originate from the same set of structural/electronic defects in ZnO. The changes in visible emission by heat treatment during PL measurements were qualitatively in agreement with a previous study of ZnO ball-shaped particles, with hierarchical nanostructures consisting of many interconnected sheets.9 However, for ball-shaped particles the disappearance of the broad peak at 580 nm occurred already with a 400 °C heat treatment, and a broad peak at 520 nm became present already after a heat treatment at 800 °C. The shifts in PL characteristics for the nanorods accordingly occurred at approx. 200 °C higher temperature, suggesting that the nanorods are structurally more stable already before and during the heat treatments, and that the electronic defects are also more stable. This is consistent with the crystal growth occurring between 800 to 1000 °C (see Table 2) compared to from 600 °C for the previously reported ball-shaped particles.9 The mass loss related to condensation of hydroxyl groups was also larger for the ball-shaped particles, 1.3 wt% compared to 0.9 wt% for the nanorods at 800 °C. It was therefore concluded that the nanorods grown dominantly in the c-direction were less prone to crystal reorganization by condensation, as compared to the sheet-like morphologies in ball-shaped ZnO particles.

Fig. 6. Emitted light from the ‘as synthesized’, as well as the same nanorod batch produced samples heat-treated at: 600, 800 and 1000 °C for 1 hour in air.

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Conclusions

Polytechnique Fédérale de Lausanne (EPFL, Switzerland) for providing the opportunity to experience research work within the undergraduate program. The Swedish Centre for Smart Grids and Energy Storage (SweGRIDS), the Swedish Energy Agency and ABB AB are gratefully acknowledged for their financial support.

It is demonstrated by in-situ sampling during aqueous growth of ZnO nanorods that the formation of the ca. 1 µm rods was preceded by a heterogeneous nucleation yielding sea urchin shaped particles. Repetitive sampling from 5 s to 60 min during the synthesis together with microscopy characterization revealed that the ZnO rods stemmed from the fracture of the radially growing sea urchin spines. These conclusions were supported by microscopy observations of very frequent uneven broken rod ends, although evenly tapered slanting rod ends were also present in the final material. Interestingly, the formation of faceted edges along the rods after 60 minutes of synthesis occurred after ca. 15 min of reaction, as the rods doubled in length. It was therefore proposed that this later stage of the growth was associated with dissolution of a ZnO phase with origin from the centers of the small sea urchins. The dissolution of nucleated sea urchin centers (cores) was in agreement with the separately evaluated and much faster dissolution rate of dominantly oxygen terminated crystal planes (using 1 µm large ball-shaped particles for comparison) as compared to the presented dominantly zinc-terminated crystal planes in the nanorods. The ZnO nanorod powders were further heat treated at 600, 800 and 1000 °C in order to determine the maximum temperature for surface refining of the rods while avoiding significant fusion of individual rods. A heat treatment of 600 °C was here found as an upper limit for preserving the rod-like structure. Thermogravimetric analysis on the ZnO rods during a heat treatment up to 800 °C showed a small weight loss of 0.9 wt% associated with Zn(OH)2 condensation of water and crystal growth. A strong correlation was found between the morphology of the rods and the photoluminescence spectra, explained by the disappearance and appearance of crystal defects, i.e. with thermal annealing. Finally, the protocol presented shows that ZnO nanorods can be approached with economically viable and scalable aqueous synthesis conditions in the absence of capping agents, e.g. for applications as inorganic filler in insulating polymer nanodielectrics or optoelectronic applications.

Associated Content Supporting Information Available: Electron microscopy images (Fig. S1, S2 and S3), movie from dissolution experiment and mass loss from thermogravimetry (Fig. S4). This material is available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding Author * E-mail: [email protected]; Fax: +46 8 208856; Tel: +46 87907637, alt. [email protected].

Acknowledgements The authors acknowledge the exchange program between the Royal Institute of Technology (KTH, Sweden) and École

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For Table of Contents Only Keywords: zinc oxide, aqueous synthesis, nanorods, annealing, photoluminescence

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Synthesis of zinc oxide nanorods via the formation ... - ACS Publications

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