Analytica Chimica Acta 812 (2014) 206–214

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Heterogeneous nucleation for synthesis of sub-20 nm ZnO nanopods and their application to optical humidity sensing R. Majithia a , S. Ritter b , K.E. Meissner a,b,∗ a b

Materials Science and Engineering, Texas A&M University, TX, United States Department of Biomedical Engineering, Texas A&M University, TX, United States

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Colloidal

synthesis of onedimensional ZnO nanopods via a microwave-assisted technique. • Mechanism of growth of ZnO nanopods by heterogeneous nucleation on 0-D ZnO seeds is studied. • ZnO nanopods exhibit broad visible defect and near-band edge PL in the UV upon excitation at 350 nm. • The use of these ZnO nanopods as optical humidity sensors is demonstrated.

a r t i c l e

i n f o

Article history: Received 22 July 2013 Received in revised form 24 December 2013 Accepted 6 January 2014 Available online 13 January 2014 Keywords: Microwave-assisted Optical gas sensing ZnO nanostructures Heterogeneous nucleation

a b s t r a c t We present a novel method for colloidal synthesis of one-dimensional ZnO nanopods by heterogeneous nucleation on zero-dimensional ZnO nanoparticle ‘seeds’. Ultra-small ZnO nanopods, multi-legged structures with sub-20 nm individual leg diameters, can be synthesized by hydrolysis of a Zn2+ precursor growth solution in presence of ∼4 nm ZnO seeds under hydrothermal conditions via microwave-assisted heating in as little as 20 min of reaction time. One-dimensional ZnO nanorods are initially generated in the reaction mixture by heterogeneous nucleation and growth along the [0001] direction of the ZnO crystal. Growth of one-dimensional nanorods subsequently yields to an ‘attachment’ and size-focusing phase where individual nanorods fuse together to form multi-legged nanopods having diameters ∼15 nm. ZnO nanopods exhibit broad orange-red defect-related photoluminescence in addition to a near-band edge emission at 373 nm when excited above the band-gap at 350 nm. The defect-related photoluminescence of the ZnO nanopods has been applied towards reversible optical humidity sensing at room temperature. The sensors demonstrated a linear response between 22% and 70% relative humidity with a 0.4% increase in optical intensity per % change in relative humidity. Due to their ultra-small dimensions, ZnO nanopods exhibit a large dynamic range and enhanced sensitivity to changes in ambient humidity, thus showcasing their ability as a platform for optical environmental sensing. © 2014 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author at: 5045 ETB, 3120 TAMU, College Station, TX 77843, USA. Tel.: +1 979 458 0180. E-mail address: [email protected] (K.E. Meissner). 0003-2670/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2014.01.012

ZnO, an intrinsic n-type semiconductor, has numerous useful properties including a large direct band-gap (368 nm at room temperature) and large exciton binding energy (60 meV) that result in photoluminescence (PL) at room temperature, transparency in the visible range, piezoelectricity, and biocompatibility. Such

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properties have enabled the use of one-dimensional ZnO (1-D ZnO) nanostructures for multiple applications, most notably for gas sensing and energy harvesting [1–5]. 1-D ZnO nanostructures having ultra-small dimensions, defined as having two dimensions less than 20 nm, are widely regarded as promising structures for a multitude of nanotechnology applications. The large surface area to volume ratio in ultra-small 1-D ZnO nanostructures is expected to promote device design, making engineering applications of ZnO nanostructures more viable. Dimensions of ultra-small 1-D ZnO nanostructures have a significance impact on their PL properties. Upon UV excitation, ZnO structures, bulk or nanoscale, exhibit two distinct PL bands [6,7]. PL in the UV region (370–390 nm), commonly referred to as near band-edge (NBE) emission, occurs due to excitonic emission whereas PL in the visible and NIR region (450–750 nm), commonly referred to as the deep-level (DL) emission, occurs due to deeplevel defects in the ZnO crystal. Significant changes in PL properties are observed for 1-D ZnO having their smallest dimension ranging from 5 to 20 nm, i.e., ultra-small nanostructures. While still not quantum confined, ultra-small 1-D ZnO nanostructures in this size range possess a large percentage of surface states compared to larger nanostructures or bulk material, leading to alterations in their NBE and DL emissions [8–10]. While the precise role of surface states and their impact on PL of ultra-small 1-D ZnO nanostructures is still a topic of research [11,12], the PL properties exhibited by ultra-small nanostructures of ZnO provide an ideal platform for application of such nanostructures in optical gas sensing. Chemical gas sensing is an important area of application for ZnO nanostructures. Typically, gas sensors using 1-D ZnO nanostructures are configured as resistors whose conductance is altered by charge-transfer processes occurring at and near their surfaces in response to changes of the local environment [4,13–16]. It is well known that molecular oxygen adsorbed onto the surface of ZnO forms active surface complexes that act as electron acceptors and lead to generation of an electron depletion region [15,17]. The width of such a depletion region in ZnO nanostructures is sensitive to ambient gases and can cause changes in conductance of the nanostructures [4,18]. Proportionally, changes in electrical conductance are larger for ultra-small ZnO nanostructures where the width of the depletion region is comparable to the dimensions of the nanostructures, thus imparting higher sensitivity in response to ambient gases [18,19]. One of the biggest challenges with electrical gas sensors is the integration of the nanostructured sensor element into device design. Inconsistencies during device fabrication while making electrical contacts with the sensor element can lead to variations in device performance. Monitoring the PL response instead of electrical conductance provides an alternative motif for chemical gas sensing with ZnO nanostructures [20,21]. Optical motifs for gas sensing with ZnO nanostructures offer a distinct advantage over electrical readouts; optical sensing can be done remotely without the need for tethering or tedious device fabrication [22–24]. PL intensity of ZnO nanostructures, especially the surface-related DL emission band in the visible range of the electromagnetic spectrum, can be used for gas sensing. Visible PL in ZnO nanostructures is caused by deep-level donor–acceptor transitions that occur due to defect-related surface states [7]. Similar to electrical conductance, donor–acceptor transitions at and near the surface of ZnO nanostructures are affected by changes in the width of the electron depletion region in response to changes in the ambient environment of the ZnO nanostructures. Consequently, modification of the transitions alters the PL of ZnO nanostructures and provides optical signal transduction for chemical gas sensing. Ultra-small ZnO nanostructures possess a higher percentage of surface states as compared to larger nanostructures, thereby providing a platform

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for design of such optical gas sensors with potentially increased sensitivity. Nanostructures for optical gas sensing should ideally be synthesized colloidally. ZnO nanostructures grown on substrates via vapor transport processes [25,26] or wet-chemical approaches [27–30], as widely reported in the literature, provide non-radiative pathways for carrier recombination which are potentially detrimental to optical response. While recent reports have explored the nature of the PL response from larger 1-D ZnO nanostructures (i.e., smallest dimensions exceeding 50 nm) grown on substrates to gases like NO2 , ethanol and humidity [21,31–33], the application of colloidally grown ultra-small 1-D ZnO nanostructures to optical gas sensing has not been explored. In this work, we present a novel method for colloidal synthesis of ultra-small 1-D ZnO nanopods and their use for optical humidity sensing. Ultra-small 1-D ZnO nanopods, multi-legged structures with sub-20 nm individual leg diameters, are synthesized via heterogeneous nucleation using zero-dimensional (0-D) ZnO nanoparticle ‘seeds’ in solution-phase under hydrothermal conditions obtained via microwave irradiation. Multiple studies have demonstrated solution-phase growth of 1-D ZnO nanostructures adhered onto substrates via heterogeneous nucleation using sub-5 nm ZnO nanoparticles as seeds [27–30]. However, colloidal growth of 1-D ZnO nanostructures using 0-D seeds has been considered prohibitive due to the tendency of ZnO nanoparticles to form oligomeric chain-like aggregates [34]. Pacholski et al. demonstrated that sub-5 nm ZnO nanoparticles (seeds) can, upon refluxing for several hours, undergo an ‘oriented attachment’ wherein the crystal lattice planes of individual nanoparticles fuse together leading to formation of chain-like aggregates [35]. Oriented attachment leading to formation of larger aggregates is a major reaction pathway in ZnO [35], similar to the phenomenon observed in anatase and iron oxide nanoparticles [36,37]. This reaction pathway effectively competes with heterogeneous nucleation and growth in a reaction mixture consisting of a growth precursor solution and ZnO seeds. Formation of large aggregates in colloidal solution due to the oriented attachment of 0-D nanoparticles would deter the generation of 1-D nanostructures that are observed when seeds are physically adhered to a substrate. As an alternative to convective heating, microwave-assisted heating has the potential to drastically reduce the time required for synthesis of ZnO micro- and nano-structures [38,39]. Microwave irradiation heats by dipole polarization and ionic conduction, thereby directly interacting with reaction mixtures on a molecular level. This leads to faster, localized heating of the reaction mixture and an accelerated rate of reaction often making alternative reaction mechanisms feasible [40]. In this study, we demonstrate the advantages of the accelerated rate of reaction obtained via microwave irradiation for colloidal synthesis of ultra-small ZnO nanopods as well as the application of the material to optical humidity sensing.

2. Materials and methods 2.1. Synthesis and characterization of ZnO nanopods For generation of ZnO nanopods, an aqueous reaction mixture of a precursor growth solution and 4 nm ZnO seeds was heated to hydrothermal conditions by microwave-irradiation in a single mode microwave cavity of a CEM Discover® system (North Carolina, USA) equipped with an IntelliventTM pressure device. A 2 mL volume of an aqueous equimolar growth mixture of Zn(NO3 )2 (99%, Sigma-Aldrich) and HMT (99%, Sigma-Aldrich), concentrations ranging from 10 to 25 mM, was mixed with 100 ␮L of 4 nm ZnO seeds solution at 2.9 ␮M concentration. The mixture was vortexed for 30 s

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and heated to 170 ◦ C in a 10 mL glass test tube under 100–150 psi of pressure. The reaction mixture required 50–60 s to reach the set point temperature of 170 ◦ C and was held for varying times from 2 to 20 min, as desired. The reaction mixture was cooled rapidly by flowing cold air into the microwave cavity. ZnO nanopods generated in the reaction mixture were centrifuged and washed in methanol before use in humidity sensing experiments. ZnO nanopods were characterized under a HR-TEM (Technai G2) equipped with a double tilt holder and JEOL 1200 for bright-field TEM. For XRD measurements, a Bruker-AXS D8 Advanced BraggBrentano geometry X-ray Powder Diffractometer (CuK␣ radiation; 40 kV, 40 mA) fitted with LynxEYE detector was used. Rietveld Analysis of the XRD patterns was performed on TOPAS software (v4) using a fundamental parameter approach. Crystal size values were measured from peak widths estimated using integral breadth measurements. Low temperature PL measurements on the ZnO nanopods were performed with a PTI QuantaMasterTM , which was equipped with a liquid nitrogen dewar. A 5 mm diameter NMR tube with ZnO nanopod samples dried inside was placed in the dewar and excited at 350 nm (slit width 5 nm) with a Xenon arc lamp. 2.2. Humidity sensing with ZnO nanopods A custom optical system was designed for humidity sensing experiments with ZnO nanopods. A Ti:Sapphire femtosecondpulsed laser (Coherent Mira 900, 140 fs pulse width, 78 MHz repetition rate) tuned to 700 nm was frequency doubled via a Type I BBO crystal to excite the sample at 350 nm. An Acton 2300i Spectrometer fitted with a Pixis 100 CCD camera was used as the detector and placed at a 90◦ angle to the excitation light. The sample chamber consisted of a 10 mm path length quartz cuvette with ZnO nanopods dried inside. The chamber was fitted with an inhouse designed humidifier capable of controlling humidity from 20 to 90% relative humidity (RH). For sensing experiments, ZnO nanopods were maintained in a dry nitrogen atmosphere (0% RH) and were subsequently exposed to air containing varying degrees of humidity. Humidity in the sample chamber was controlled by mixing dry nitrogen with 100% RH air. Flow rates for mixing were controlled by rotameters (Omega FL 4213-V and FL 4214-V for dry nitrogen and humid air, respectively). Total flow rate in the sample chamber during experiments was kept constant at ∼7 LPM. 3. Results and discussion 3.1. Synthesis of ZnO seeds ZnO nanoparticles (seeds), to be used in a subsequent reaction mixture for colloidal heterogeneous nucleation, were synthesized by alkaline hydrolysis using procedures reported elsewhere.[28,35] Briefly, zinc acetate salt (10 mM) was hydrolyzed with an alkali (NaOH, 30 mM) by refluxing in methanol at 60 ◦ C for a period of 2 h. While the ZnO nanoparticles used in this work were synthesized on a hot plate, microwave irradiation can also be used to generate similar nanoparticles (data not shown). Transmission Electron Microscopy (TEM) (Fig. 1A) indicates that the quasi-spherical ZnO nanoparticles generated by the hydrolysis reaction are 4.1 ± 0.8 nm in diameter (Fig. 1B). Also, High Resolution TEM (HR-TEM) images indicate that the ZnO nanoparticles are single wurzite crystals (Fig. 1B inset). PL measurements carried out on ZnO nanoparticles (PTI QuantaMasterTM , Xe Arc lamp excitation at 315 nm) show NBE emission centered at 360 nm accompanied by a broad green DL emission (Fig. 1C, red). The NBE emission from the nanoparticles is blue-shifted as compared to the bulk bandgap for ZnO (3.37 eV or 368 nm), indicating presence of quantum confinement effects.

Fig. 1. (A) TEM image of ZnO nanoparticles used in this work as seeds for colloidal heterogeneous synthesis. (B, inset) HR-TEM image showing a single ZnO nanoparticle. (B) Size distribution of ZnO nanoparticles with an average diameter of 4.1 ± 0.8 nm. (C) Absorption and PL spectra of the ZnO nanoparticles.

UV–Vis absorption measurements (Hitachi U-4100) show a first exciton peak at 341 nm supporting the existence of quantum confinement effects in the 4 nm ZnO nanoparticles (Fig. 1C, blue). In quantum confined ZnO nanoparticles, the absorption onset gives a measure of bandgap and can be used to predict nanoparticle radius [41]. Based on effective mass model calculations (figure S1), the absorption onset of ∼365 nm observed in UV–Vis absorption measurements corresponds to particle diameter of ∼4.2 nm, supporting the TEM measurements.

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Fig. 2. SEM images of ZnO nanopods generated by heterogeneous nucleation on ZnO seeds. (A) A cluster of ZnO nanopods. (B) An individual five-legged structure.

3.2. Synthesis of ZnO nanopods Ultra-small ZnO nanopods were generated from an aqueous reaction mixture consisting of a precursor growth solution and 4 nm ZnO nanoparticle seeds under hydrothermal conditions in a single-mode CEM Discover® microwave reactor. The growth solution consisted of an equimolar mixture of Zn(NO3 )2 and hexamethylenetetramine (HMT) at concentrations ranging from 10 to 25 mM. A mixture of Zn salts and HMT has been widely used as a precursor growth solution for aqueous homogeneous synthesis of ZnO micro and nanostructures [28–30,42,43]. The following sets of reaction are known to occur during ZnO formation with Zn(NO3 )2 and HMT: HMT decomposes and supplies OH− ions for reaction as so: C6 H12 N4 + 6H2 O → 6H2 CO + 4NH3 NH3 + H2 O → NH4 − +OH− Zn2+ ions react with the OH− by two reversible competing mechanisms:Zn2+ + OH−  ZnO ↓ + H+ Zn2+ + 2OH−  Zn(OH)2 ↓ In the reaction system discussed here, HMT served as a source of OH− ions in solution by means of decomposition causing hydrolysis of Zn2+ salts to form ZnO crystals [44,45]. This crystallization occurs in presence of ∼4 nm ZnO ‘seeds’ causing heterogeneous nucleation and leading to varied nanostructure morphology than typically observed in homogenous systems. The reaction mixture was heated to hydrothermal conditions at 170 ◦ C for a period of up to 20 min in high pressure glass test tubes by single-mode microwave irradiation. After 20 min of reaction time, ZnO nanopods, multi-legged structures with individual leg diameters of 15.44 ± 1.89 nm and lengths of 250–300 nm, were generated (Fig. 2A). Electron microscopy indicated that these nanopods consisted of a varying number of legs with up to 5 individual legs isolated on a single nanopod (Fig. 2B). No significant changes in diameters or aspect ratios (values close to 25) of individual nanopod legs were observed in the range of concentrations studied as a part of this work. Unless otherwise mentioned, ZnO nanopods generated after 20 min of reaction time were used for further studies in this work. Powder X-ray diffraction (XRD) studies were performed on ZnO nanopods to analyze the crystallinity of the structures. An XRD pattern (Fig. 3) of the ZnO nanopods can be indexed to the wurzite ZnO phase (JCPDS # 01-079-2205), as expected. A Rietveld analysis

Fig. 3. An X-ray diffractogram obtained for ZnO nanopods. Variation in peak widths corresponding to (100), (101) & (002) planes indicate an anisotropy in crystal size.

performed on the diffractogram, after deconvolution of the instrument response to account for the variation in full-width at half maximum of peaks at the 31.7, 34.5 and 36.25 2- angles, gave a measure of anisotropic peak broadening observed in the XRD pattern of the ZnO nanopods. Fitting individual peaks in the XRD pattern with the fundamental parameter approach showed that crystallite size (∼40 nm) is largest for the peak at 34.5◦ 2- which corresponds to the (002) plane for wurzite ZnO. (Table 1) Crystallite sizes along the (100) and (101) planes, 31.7◦ and 36.25◦ 2-, respectively, were estimated to be ∼11–12 nm (Table 1) and match closely with individual leg diameters of the ZnO nanopods. ZnO is known to have three types of fast growth directions including the ±[0001] direction.[46] A sharper (002) peak in the XRD pattern of the ZnO nanopods, indicating a larger crystal size along the [002] direction, thus corresponds to growth along the length of the individual legs of each nanopod structure. This is confirmed by HR-TEM images of individual nanopod legs (Fig. 4A). Interplanar spacing along the length of individual legs of ZnO nanopods corresponds to the (002) d-spacing for wurzite ZnO

Table 1 Crystal size estimations obtained via Rietveld analysis of X-ray diffractogram for different crystal planes of ZnO nanopods. Crystal plane

Crystal size (nm)

Standard deviation (nm)

(100) (002) (101)

11.81 40.19 12.7

0.08 0.3 0.05

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Fig. 4. (A) HR-TEM image showing an individual leg of a ZnO nanopod growing along the [0001] direction. (B, inset) A single ZnO nanopod with 3 legs which has a ‘hole’ in the center. (B) All legs are joined and each leg grows along the [002] direction.

(0.52 nm). It is important to note that the crystal size perpendicular to the (002) plane, estimated from the XRD pattern, is smaller than the length of individual nanopod legs observed on TEM roughly by a factor of 5. Estimation of crystal sizes from XRD patterns can result in values smaller than the actual nanoparticle sizes, as measured by TEM, due to presence of crystal defects such as line dislocations and grain boundaries. As predicted by lower crystal size values along the [002] direction, such defects are present in individual legs of ZnO nanopods and were observed with HR-TEM (figure S2). HR-TEM images of ZnO nanopods reveal that individual nanopod legs consist of single crystals (Figs. 4B & S2). However, the nanopods as a whole are polycrystalline structures with visible grain boundaries between individual legs. At times, nanopods show gaps or holes between the individual legs (Fig. 4B inset). In conjunction with the fact that ZnO nanopods possess a varying number of legs, such observations suggest that individual ZnO nanorods are fused together with random orientation after they individually grow from the ZnO seeds to form nanopods. To further explore this hypothesis, ZnO nanostructures were synthesized at shorter reaction times of 2, 10 and 15 min and imaged via TEM. At 2 min, ZnO nanorods with diameters of 36.7 ± 7.9 nm are observed alongside sub-5 nm ZnO seeds (Fig. 5A). TEM images suggest that nucleation from ZnO seeds and growth of ZnO nanopods is incomplete and ongoing at this stage in the reaction. Also, no multi-legged nanopods are observed. At 10 min, multi-legged nanopods with individual leg diameters of 36.3 ± 20.9 nm are generated alongside nanorods indicating an ongoing crystal growth phase (Fig. 5B). Appearance of multi-legged structures in TEM images at this stage of the reaction indicates initiation of an ‘oriented crystal attachment’ phase. Such oriented crystal attachment, in which crystal planes of individual single crystal nanorods fuse with each other to form polycrystalline nanopods, becomes dominant as nanopods with varying number of individual legs are observed after 15 and 20 min into the reaction. The temporal evolution observed during the course of the reaction suggests that 1-D ZnO nanorods nucleate and grow from 0-D ZnO seeds and then subsequently undergo ‘oriented attachment’ as has been previously observed for sub-5 nm nanoparticles by Pacholski et al. [35] Another important feature to note is that the leg diameters of nanostructures (nanorods or nanopods) focus down to 12.4 ± 2.2 nm

after 15 min of reaction time (Fig. 5C). Such size focusing of leg diameters seems to occur concurrently with the oriented attachment phase and yields nanopods with leg diameters of ∼15 nm after 20 min of reaction time (Fig. 5D). As an alternate competing reaction mechanism to heterogeneous nucleation and growth, oriented attachment of ZnO nanoparticles for formation of 1-D nanostructures has been previously reported by Pacholski et al. [35]. However, such attachment is usually a very slow process taking several hours for generation of 1-D nanostructures [35]. In this work the accelerated rate of reaction for nucleation and growth of ZnO nanostructures, obtained via microwave-assisted hydrothermal synthesis, presumably changes the dynamics between the two competing mechanisms and promotes colloidal hetergeneous nucleation and growth of 1-D ZnO nanorods over oriented attachment and fusion of nanorods. In negative control experiments containing only ZnO seeds and DI water instead of a growth solution, initiation of oriented ZnO nanoparticle attachment is only seen after 20 min of reaction time. Alternatively, the absence of ZnO seeds leads to generation of sub-micron (∼380 nm diameter) ZnO structures [39], highlighting the role of heterogeneous nucleation from 0-D seeds in the generation of nanopods having legs with ∼15 nm diameters.

3.3. Optical properties of ZnO nanopods and humidity sensing ZnO nanostructures have been shown to luminescence in the UV (NBE emission) and visible (DL emission) regions upon UV photoexcitation [7]. Upon excitation at 350 nm (PTI QuantaMaster, TM Xe Arc lamp), the PL spectra of ZnO nanopods (generated after 20 min of reaction time) show a large defect-related DL emission in the orange-red region of the visible spectrum (Fig. 6). Spectra taken at liquid nitrogen temperature (77 K) show a very broad defect-level band for the ZnO nanopods (full-width at half maximum greater than 140 nm) peaked at ∼615 nm. Similar orange-red emission has previously been observed for nanosized ZnO needle-like structures [7,47] and attributed to donor–acceptor transitions involving Zn vacancy complexes [48]. In addition, weak emission in the UV region (peak at 374 nm) corresponding to the NBE is also observed for the ZnO nanopods.

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Fig. 5. TEM images of ZnO nanostructures synthesized at (A) 2, (B) 10 and (C) 15 min of reaction time. A gradual temporal evolution from single nanorods (and unreacted seeds) to nanopods with small leg diameters is observed. The temporal evolution observed in individual leg diameters of ZnO nanopods (D) indicates that after an initial nucleation and growth phase to form nanorods, a concurrent size focusing and oriented attachment is observed leading to generation of multi-legged nanopods.

The strong defect-level PL observed in the colloidally synthesized ultra-small ZnO nanopods, having ∼15 nm diameter, makes them ideal candidates as optical chemical gas sensors. The electron depletion layer caused by adsorption of ambient oxygen, ionized as O− or O2− , on ZnO nanostructure surfaces is comparable to the diameters of the nanopods synthesized in this work. The depletion layer model postulated for ZnO nanostructures with such ultrasmall diameters predicts the electrical sensitivity, as measured by percent change in electrical conductance, in response to changes in the ambient chemical environment to be higher than that for structures with larger (>50 nm) diameters [18]. In analogy to higher electrical sensitivity stemming from the increased ratio of surface states to total available states in these types of nanostructures, the PL of the nanostructures will also be impacted by this phenomenon and produce highly sensitive optical gas sensors. As a proof of concept, we demonstrate the use of ZnO nanopods generated in this work as optical humidity sensors at room temperature. Previous reports on optical sensing with larger ZnO structures (diameters ∼300 nm) grown on substrates have shown very low responses to humidity [21]. For humidity sensing

experiments, a customized optical system consisting of a doubled Ti:Sapphire laser at 700 nm for excitation and an Acton 2300i spectrometer with Pixis 100 Peltier cooled CCD camera as a detector was designed. The sample chamber, consisting of ZnO nanopods dried inside a quartz cuvette, was initially maintained under a dry nitrogen atmosphere (0% RH) before exposure to humidity. PL intensity collected at room temperature was integrated across the entire defect band (450–690 nm) and monitored in response to changes in humidity. A small red shift (∼15 nm) was observed in the center wavelength of the defect band collected at room temperature when compared to spectra obtained at liquid nitrogen temperature (figure S3). ZnO nanopods undergo quasi-reversible photobleaching when excited with light above bandgap energies. Reports of similar effects observed with ZnO nanoparticles have been attributed to reversible charging of ZnO during illumination.[49] In this work, the integrated PL intensity in the defect band of ZnO nanopods decreased by ∼36% of its original value upon continuous excitation with 350 nm light at a fluence of ∼475 mW cm−2 (figure S4). The decay in the PL intensity was found to stabilize after ∼4 min

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Fig. 6. PL spectrum of ZnO nanopods used in this work obtained at 77 K. ZnO nanopods exhibit broad orange-red defect-related PL in addition to a NBE PL at 373 nm when excited above band-gap energies at 350 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

of continuous excitation. Upon periodic photoexcitation of the ZnO nanopod sample, the decay in PL intensity was found to reduce dramatically resulting in a loss of only ∼15% after 5 min. Periodic excitation involved illumination of the sample for an ‘on’ time of 1 second followed by a recovery ‘off’ time of 20 s. Additionally, it was found that the fluence of the excitation laser light has a significant impact on signal decay. Upon reducing the laser fluence from ∼475 mW cm−2 to ∼160 mW cm−2 , the total loss in PL signal intensity reduced ∼2 fold after 5 min of continuous excitation (data not shown). For periodic excitation at a lower excitation fluence of ∼160 mW cm−2 , the signal decay averaged only ∼0.2% per minute (Fig. 7A, first 5 min) indicating stable PL for lower excitation fluence. To reduce artifacts in measurement of changes in optical signal due to such quasi-reversible photobleaching effects, a periodic excitation scheme with 1 second exposure every 20 s at a laser light fluence of ∼160 mW cm−2 was used for humidity sensing experiments.

ZnO nanopods, dried inside a quartz cuvette, were stabilized under dry nitrogen for a period of 30 min. For each experiment, the stability of the PL signal, with an average signal decay ∼0.2% per minute, was confirmed for an initial period of 5 min. The sample was subsequently exposed to varying degrees of humidity (in a random fashion) for a period of 25 min per humidity level. A desired humidity level was obtained in the sample chamber by mixing dry nitrogen and humid air (100% RH) with total flow rate kept constant at ∼7 LPM. Each humidity level was followed by exposure to dry nitrogen (0% RH) for 25 min to allow for stabilization of the local sample environment for subsequent experiments. The defect-level PL intensity of the ZnO nanopods was found to increase upon exposure to humid air. The maximum change in integrated PL intensity, observed after a 25 min exposure to humid air, varied from a 30% increase for 85% RH to 8% increase for 22% RH (Fig. 7A). It is important to note that the humidity response dynamics of the ZnO nanopod sensor showed two distinct phases. An initial rapid increase in total signal intensity upon exposure to humidity lasted for ∼2 min and accounted for ∼60% of the total response observed at the end of 30 min. This was followed by a phase exhibiting a slower increase in PL signal. Results also indicated the increase in the PL intensity was completely reversible upon exposure to dry nitrogen. It is noteworthy that such sensor dynamics observed for changes in PL signal are similar to the dynamics observed in ZnO nanostructure gas sensors interrogated via changes to electrical conductance in response to reductive gases such as ethanol vapor [4]. The calibration curve for observed optical responses, a plot of maximum observed response in PL intensity versus %RH, follows a sigmoid curve with the sensor saturating at >75% RH (Fig. 7B). Sensitivity of the ZnO nanopods sensor defined as (maximum response in PL intensity)/(RH) shows a linear behavior in the range of 22–70% RH with a 0.4% increase in optical intensity per % change in RH. ZnO nanopods synthesized in this work show a larger dynamic range and enhanced optical sensitivity (by a factor of ∼5) in response to ambient humidity changes compared to similar studies on larger nanostructures [21]. Such enhanced sensitivity most likely arises due to a large ratio of active surface states contributing to DL emission in the ZnO nanopods with ultra-small diameters. Future studies will focus on the investigation of this hypothesis as well as a more in-depth analysis of ZnO nanopods and other ZnO materials with ultra-small dimensions as environmental sensors.

Fig. 7. (A) Response of defect-related PL intensity integrated from 450 to 690 nm of the PL spectra to variations in ambient levels of humidity. (B) A calibration curve for maximum humidity response at various levels of humidity shows a sigmoid behavior.

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4. Conclusions In this work we demonstrate a novel method for colloidal synthesis of ZnO nanopods with ultra-small dimensions via microwave-assisted heterogeneous nucleation under hydrothermal conditions. Size-focused ZnO nanopods with ∼15 nm individual leg diameters can be synthesized using ∼4 nm ZnO nanoparticle seeds in as little as 20 min of reaction time. Individual legs of each ZnO nanopod are shown to be single crystals with growth taking place along the c-axis or [0001] direction. In addition to a NBE emission at 373 nm, ZnO nanopods exhibit broad orange-red defect-related PL when excited above band-gap energies. ZnO nanopods generated in this work can be used as optical humidity sensors by monitoring changes in intensity of the defectrelated PL in response to variations in ambient humidity levels. As optical humidity sensors, ZnO nanopods with ultra-small dimensions exhibit a large dynamic range and high sensitivity to changes in ambient humidity levels. The sensitivity of the optical sensors to humidity was approximately 5× greater than previous reports using larger nanostructures. Our results indicate that 1-D ZnO nanostructures with ultra-small dimensions have great potential as sensitive room temperature optical sensors for environmentallyrelevant chemical gases like NO2 and CO. Acknowledgments The authors would like to thank Dr. Amanda Young (MIC-TAMU) and Dr. Nattamai Bhuvanesh for their advice and important discussions for PL and XRD data collection, respectively. RM gratefully acknowledges support from the National Science Foundation CBET0933719. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.01.012. References [1] S. Shukla, E. Agorku, H. Mittal, A. Mishra, Synthesis, characterization and photoluminescence properties of Ce3+-doped ZnO-nanophosphors, Chem. Pap. 68 (2014) 217–222. [2] X. Wang, J. Song, J. Liu, Z.L. Wang, Direct-current nanogenerator driven by ultrasonic waves, Science 316 (2007) 102–105. [3] M. McCune, W. Zhang, Y. Deng, High efficiency dye-sensitized solar cells based on three-dimensional multilayered ZnO nanowire arrays with caterpillar-like structure, Nano Lett. 12 (2012) 3656–3662. [4] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (2004) 3654–3656. [5] N.D. Hoa, One-dimensional Semiconducting Metal Oxides: Synthesis, Characterization and Gas Sensors Application, Intelligent Nanomaterials, John Wiley & Sons, Inc., 2012, pp. 39–88. [6] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M. Reshchikov, S. Do an, V. Avrutin, S.J. Cho, H. Morkoc, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301. ´ Y.H. Leung, Optical properties of ZnO nanostructures, Small 2 [7] A.B. Djuriˇsic, (2006) 944–961. [8] L. Wischmeier, T. Voss, S. Börner, W. Schade, Comparison of the optical properties of as-grown ensembles and single ZnO nanowires, Appl. Phys. A: Mater. Sci. Process. 84 (2006) 111–116. [9] L. Wischmeier, T. Voss, I. Rückmann, J. Gutowski, A. Mofor, A. Bakin, A. Waag, Dynamics of surface-excitonic emission in ZnO nanowires, Phys. Rev. B 74 (2006) 195333. [10] L. Liao, H.B. Lu, J.C. Li, H. He, D.F. Wang, D.J. Fu, C. Liu, W.F. Zhang, Size dependence of gas sensitivity of ZnO nanorods, J. Phys. Chem. C 111 (2007) 1900–1903. ´ X. Chen, Y.H. Leung, A.M.C. Ng, ZnO nanostructures: growth, prop[11] A.B. Djuriˇsic, erties and applications, J. Mater. Chem. 22 (2012) 6526–6535. [12] D. Stichtenoth, C. Ronning, T. Niermann, L. Wischmeier, T. Voss, C.J. Chien, P.C. Chang, J.G. Lu, Optical size effects in ultrathin ZnO nanowires, Nanotechnology 18 (2007) 435701. [13] K.J. Choi, H.W. Jang, One-dimensional oxide nanostructures as gas-sensing materials: review and issues, Sensors 10 (2010) 4083–4099.

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