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Highly Sensitive ZnO Nanorod- and Nanoprism-Based NO2 Gas Sensors: Size and Shape Control Using a Continuous Hydrothermal Pilot Plant Liang Shi,*,† Anupriya J. T. Naik,‡ Josephine B. M. Goodall,‡ Chris Tighe,‡ Rob Gruar,‡ Russell Binions,*,§ Ivan Parkin,*,‡ and Jawwad Darr*,‡ †

Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China Department of Chemistry, Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom § School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom ‡

ABSTRACT: Continuous hydrothermal flow synthesis of crystalline ZnO nanorods and prisms is reported via a new pilot-scale continuous hydrothermal reactor (at nominal production rates of up to 1.2 g/h). Different size and shape particles of ZnO (wurtsite structure) were obtained via altering reaction conditions such as the concentration of either additive H2O2 or metal salt. Selected ZnO samples (used as prepared) were evaluated as solid oxide gas sensors, showing excellent sensitivity toward NO2 gas. It was found that both the working temperature and gas concentration significantly affected the NO2 gas response at concentrations as low as 1 ppm.

1. INTRODUCTION ZnO nanostructures have received much attention for their unique optical, electronic, mechanical, and chemical properties and potential applications in optoelectronics, photonics, field emission, energy conversion, catalysis, and gas sensing.1,2 The discovery of room-temperature laser emission of ZnO nanorod arrays has triggered intense interest in the preparation and properties of ZnO 1D nanostructures.3 Until now, ZnO nanostructures have been fabricated via various gas-phase4−6 and solution-phase7−9 approaches. Gas-phase synthesis techniques usually involve rigorous experimental conditions such as high temperature, vacuum techniques, and the presence of a catalyst and sometimes poisonous gases. This involves either high-energy consumption or a relatively high cost for gas-phasebased approaches. In comparison to gas-phase approaches, solution-phase approaches have advantages of relatively lower operating temperatures, simplicity, and low cost. However, in solution-phase processes, organic surfactants, micelles, and polymers are often introduced to act as capping agents or templates to direct and promote oriented crystal growth. As a result, the residual organic or complexing agents can become impurities that complicate the experimental procedure, increasing the cost and ultimately reducing the industrial potential. Therefore, safe, more environmentally benign, and low-energy-consuming solution-phase routes to preparing metal oxides (such as ZnO nanostructures) with controlled size and morphology are of interest. Recently, continuous hydrothermal synthesis reactions have gained interest as a faster, safer, and more controllable method © XXXX American Chemical Society

for producing inorganic nanomaterials. In a typical CHFS reaction, a flow of supercritical (or superheated) water (critical temperature, Tc = 374 °C; critical pressure, Pc = 22.1 MPa) reacts with a room-temperature aqueous flow of metal salts in a continuous fashion, yielding the precipitation and crystallization of nanomaterials in the flow. Because supercritical water is a nonpolar, single-phase fluid possessing the properties of both a liquid and a gas, it offers a rapid medium for the precipitation of metal ions through a combination of hydrolysis and dehydration reactions to precipitate and oxidize metal ions rapidly to form oxide nanomaterials. Under these conditions, metal salt solubilities are significantly different than at ambient temperature; therefore, a supersaturated environment also favors nanomaterial syntheses through the rapid rate of metal ion precipitation.10,11 Compared to other solution-based nanoceramic synthesis approaches, CHFS can have many advantages including short reaction times (on the order of seconds), independent parameter control (pressure, temperature, etc.), and improved control over particle characteristics; some of these attributes are attractive for the commercialization of nanoceramic powders.12 Finally, because of the versatility of CHFS, a wide number of different materials including metal oxides have been made to date.13−15 Building upon the previous work of some of the authors,13,16,17 we report here the use of a new pilot-scale Received: June 20, 2013 Revised: July 10, 2013

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Figure 1. Simplified schematic representation of the continuous hydrothermal flow synthesis pilot plant for making zinc oxide nanomaterials. configuration using a 514.5 nm Ar laser as the excitation source. Brunauer−Emmett−Teller (BET) surface area measurements were carried out using N2 in a Micrometrics ASAP 2420 instrument with six parallel analysis stations. The samples were degassed at 120 °C for 12 h in nitrogen before measurements. UV−vis absorption spectra of the nanopowders were measured using an Ocean Optics spectrophotometer (USB 4000) with a deuterium halogen light source (DH2000-BAL) and 6 μm fiber optic cables and spectrasuite software. Detailed morphology and microstructure analysis was carried out using transmission electron microscopy (Tecnai 20ST) and a highresolution transmission electron microscopy (JEOL 4000× transmission electron microscope at a 350 keV acceleration voltage). For the TEM and HRTEM observations, samples were prepared by dispersing the particles ultrasonically in 99.9% ethanol (Sigma-Aldrich, U.K.) and dropping them onto 400 mesh holey carbon film grids (Agar Scientific, U.K.). Gas sensor characterization of sample 4 was conducted. Thin films were deposited by drop coating slurry onto commercially produced 3 × 3 mm2 sensor substrates (City Technology, Portsmouth, U.K.). Drops of the ZnO nanoparticle slurry (6 μL × 0.225 g mL−1) were deposited onto the substrate using a calibrated Gilson pipet, with airdrying between separate depositions (the sensors were heat treated at 600 °C for 2 h in a furnace in air to improve substrate adhesion). The sensors consist of gold electrodes printed on top of an alumina tile and a platinum heater track printed on the reverse side of the tile. The gold electrodes were formed by laser trimming to produce an interdigitized section with an electrode gap of 150 μm and an electrode finger width of 50 μm. Contacts to the devices were formed by spot-welding a 50μm-diameter platinum wire to pads of the track material in the corner of the sensor chip. The sensor heater was kept at a constant resistance and hence a constant temperature by incorporating it into a Wheatstone bridge. Electrical experiments were formed on a custom-constructed test rig.22 Various concentrations (0−10 ppm) of NO2 were generated by dilution with a cylinder of compressed air (79% nitrogen, 21% oxygen) and a cylinder of 10 ppm nitrogen dioxide in synthetic air. The device’s gas response behavior was investigated at operating temperatures of between 300 and 500 °C in 50 °C increments. The lowest temperature at which the sensors provided a reasonable baseline resistance was found to be 300 °C.

CHFS reactor for the synthesis of crystalline ZnO nanoparticles. In addition, because of the relatively high processing flow rates used herein (unlike the rate for a comparable laboratory-scale process that has been reported before), the authors were able to study the effects of increasing Zn precursor and H2O2 concentrations on the growth of the asprepared ZnO nanomaterials. The results showed that a large variation in shape and size could be achieved as has been observed by other groups working in a similar area.18−21 These different crystal habits were analyzed via a number of techniques including electron microscopy, powder XRD, and BET surface area and were also investigated for NO2 gas sensing.

2. EXPERIMENTAL SECTION Zinc nitrate hexahydrate [Zn(NO3)2·6H2O, technical grade, (>98%)] was obtained from Fisher (U.K.), and potassium hydroxide pellets (analytical grade) were obtained from Merck (U.K.). H2O2 (30 wt % in H2O) was obtained from Sigma-Aldrich (U.K.). In all cases, 10 MΩ deionized water was used. Pure zinc oxide nanoparticles were made in the pilot-plant CHFS system (Figure 1), the basic design of which has been reported elsewhere.16 For the synthesis of zinc oxide, a roomtemperature aqueous solution of 0.2 M zinc nitrate hexahydrate with a certain amount of H2O2 aqueous solution was pumped (via pump P2) to meet an aqueous flow of 0.5 M KOH (pump P3) (also at room temperature) at a “Tee” piece (Figure 1). This mixed feed then met a feed of superheated water (at 400 °C and 24.1 MPa) in a confined jet mixer (“Reactor” in Figure 1), whereupon a continuous flow of particles was produced. Flow rates of 400, 200, and 200 mL min−1 were used for the superheated water feed, the mixed-ion feed, and the base feed, respectively. The aqueous nanoparticle slurry that is formed was then cooled as it passed through a cold-water-jacketed pipe and was continuously collected from the exit of the back pressure regulator (Swagelok KHB series). Volume ratios of H2O2 (30 wt % in H2O) to 0.2 M zinc nitrate hexahydrate were 0, 0.05, 0.1, 0.15, and 0.25 for asprepared samples labeled 1−5, respectively. Slurries were centrifuged (5000 rpm for 30 min), and then the supernatant was removed and replaced with 40 mL of clean deionized water with shaking to redisperse the solids. The centrifugation was repeated later to give a wet sludge that was then freeze-dried overnight. The solid products of each reaction were characterized by powder X-ray diffraction (Bruker D4 Diffractometer, Cu Kα1, λ = 1.540598 Å). Room-temperature Raman spectra were measured using a microlaser Raman spectrometer (Renishaw, in viva ) in a backscatter

3. RESULTS The shape and size of the ZnO particles were investigated using transmission electron microscopy (TEM), which suggests that the samples are 1D ZnO nanorods or needlelike pyramidal nanostructures. Figures 2a shows a typical TEM image of ZnO B

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Figure 2. Representative (a) TEM and (b) HRTEM images of the asprepared ZnO nanorods in sample 3. The upper-right inset in panel b shows a 2D Fourier transform pattern of the lattice-resolved image, and the lower-right inset in panel b is a line profile from an area with the rectangular frame in panel b.

nanorods obtained from this pilot-plant CHFS process (sample 3). High-resolution TEM (HRTEM) images are shown in Figure 2b and indicate a lattice spacing of 0.26 nm, which corresponds to the d spacing of the (002) planes in wurtizite ZnO. This analysis reveals that [0001] is the growth direction of the ZnO nanorods (as expected), which is again confirmed in Figure 2b, showing the 2D Fourier transform pattern of the lattice, indexed to the [011̅0] zone of hexagonal ZnO (confirming the growth direction observed in Figure 2b). It has been reported that the ionic and polar wurtzite type ZnO is constructed as a thermodynamically stable hexagonal closepacked array of oxygen and zinc atoms in the space group P63mc with zinc atoms occupying the tetrahedral sites.23 As such, ZnO with a wurtsite structure has a basal “polar” oxygen (000I)̅ plane, a top polar zinc (0001) plane, and low-index planes (parallel to the c axis) consisting of nonpolar (01I0̅ ) planes. The “low-symmetry” nonpolar faces with 3-foldcoordinated atoms are the most stable; the polar planes are metastable. Anisotropic growth of the crystal along the [0001] direction is preferable because of the inherent asymmetry along the c axis.24 Thus the 1D nanostructure of ZnO such as a nanorod is favorable structurally under a range of synthesis conditions. Although the ZnO samples are all nanorods, their morphology showed some variances in shape as depicted in Figure 3. Samples 1−3 all showed a rodlike morphology with two flat ends, a typical crystal habit of ZnO. There is a different morphology for samples 4 and 5. With sample 4, one end of the ZnO rod is pointed whereas the other one is flat. As for sample 5, the dominant morphology is nanoprisms or pointed rods. Powder X-ray diffraction (XRD) was used to elucidate the crystal structures of the products. Figure 4 shows a typical powder XRD pattern of the as-prepared ZnO product. All diffraction peaks can be indexed to the hexagonal wurtzitestructured ZnO and match well to the reported values for ZnO (JCPDS card no. 36-1451). In Figure 4, it can be seen that the relative intensity of (0002) to (10I0̅ ) diffraction peaks varied from sample 1 to 5. It can be seen that the relative intensity of the (0002) peak increases gradually from sample 1 to 3 (which is the maximum) and then decreases from sample 3 to 5. This suggests that the growth orientation of the (0002) plane is promoted by the increasing H2O2 concentration. Thereafter, for higher concentrations of peroxide, the growth orientation of the (0002) plane is suppressed by excess H2O2. The average particles size of the as-prepared ZnO samples was estimated on the basis of the Scherrer equation,25 D =

Figure 3. Representative TEM images of the ZnO samples: (a) sample 1, (b) sample 2, (c) sample 4, and (d) sample 5.

Figure 4. Powder X-ray diffraction patterns of as-prepared ZnO nanopowder samples 1−5.

(Kλ)/β(cos θ), where K (assumed to be 0.89) is the shape factor of the average crystallite, λ is the wavelength for Kα1 (1.540 Å), β is the full width at half-maximum of the diffraction line, and θ is the Bragg angle. The variation in the crystallite size (from powder XRD) and BET surface area of the five samples is shown in Figure 5 and suggests a decrease in crystallite size from sample 1 to 3 and then increases gradually again. As expected, the BET surface area of the ZnO samples shown in Figure 5 has the opposite behavior with respect to size, with the maximum BET surface area for sample 3 (smallest crystallite size). Room-temperature UV−vis absorption spectra (not shown) showed a sharp absorbance peak at around 363 nm as expected. The band gap of the ZnO nanoparticle is calculated to be 3.42 eV, consistent with the reported value for bulk ZnO,26,27 suggesting excellent crystal quality of the ZnO nanoparticles. No blue shift was observed in the UV−vis absorbance spectrum, revealing that the nanoscaled ZnO particles were C

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Figure 5. Variation in crystallite size (by powder XRD) (■) and BET surface area (△) for samples 1−5.

not small enough to show observable quantum-confinementrelated effects. Raman spectroscopy was used to study the vibrational properties of the ZnO samples. All of the samples showed bond vibrations at 332, 383, 437, and 585 cm−1.28−30 The gas response of thin films of sample 4 to nitrogen dioxide (NO2) was investigated. The gas response is defined as the ratio between R (the resistance when exposed to NO2) and R0 (the point immediately prior to exposure to NO2). The resistance was measured between the two electrodes of an interdigitized gold electrode structure on an alumina tile. Figure 6 shows the gas response curve (R/R0 vs time) of the ZnO sensor to differing concentrations of NO2 in flowing air at

Figure 7. Gas response (R/R0) of the ZnO sensor to different concentrations of NO2 measured at different working temperatures.

different working temperatures. It is obvious for all curves that the gas response values grow with increasing gas concentration. However, the gas response values begin to plateau as the NO2 gas concentration increases. This is likely because the sensor surface becomes saturated at higher test gas concentrations. A possible explanation can be given as follows. When the NO2 gas concentration is low, the available adsorption sites on the surface of ZnO nanoparticles are enough to let each NO2 molecule interact through a surface reaction with ZnO. In this case, the gas response is mainly dependent on the NO2 concentration as long as adsorption sites are abundant. However, if the NO2 concentration increases, there are insufficient adsorption sites and NO2 molecules have to compete for adsorption sites. As a result, the gas response is surface-reaction-rate-determining.31 The temperature-dependent response of the ZnO sensor to NO2 under different gas concentrations is shown in Figure 6. The gas response increased with increasing temperature up to 350 °C and then decreased. The optimum working temperature that gave the largest gas response herein was 350 °C, which corresponds to the optimum conditions where the largest number of active electrons are involved in surface reactions. The gas response (sensor conductance) is believed to be affected by many factors, including absorbed oxygen species, rates of adsorption and desorption, and the charge-carrier concentration, which are all functions of temperature.32 All of the factors together determined the quantity of active electrons in the surface reaction. Therefore, the gas response becomes temperature-dependent, and an optimum temperature is usually induced.

Figure 6. Gas response (R/R0) of ZnO sensors upon exposures to differing concentrations of NO2 in flowing air at a working temperature of 350 °C.

a working temperature of 350 °C. Dry air was first used to purge the testing chamber to make the baseline constant (not shown), and then NO2 was introduced into the testing chamber. The ZnO sensor shows a considerable response to NO2 even at the low concentration of 1 ppm. Once the NO2 was introduced, the gas response increased quickly with time and a near plateau was reached within minutes, suggesting the rapid response to NO2 and a near steady state being achieved. After the NO2 flow was shut off, the gas response recovered to the original baseline level. The steady-state response for the ZnO sensor increased with an increase in NO2 concentration. It was found that the working temperature and gas concentration have significant effects on the gas response. Figure 7 shows the gas response (R/R0) of the ZnO sensor versus NO2 concentration plots in the range of 1−10 ppm at

4. DISCUSSION 4.1. Materials Characterization. In the hydrothermal process presented once the zinc precursor solution, KOH solution, and superheated water meet in the reactor, the following reactions occur: Zn 2 + + 4OH− → Zn(OH)4 2 − → ZnO2 2 − + 2H 2O

(1)

ZnO2 2 − + H 2O ↔ ZnO + OH−

(2)

During the hydrothermal flow synthesis process, increasing [OH−] induced by an aqueous flow of 0.5 M KOH causes Zn2+ ions to convert into soluble Zn(OH)42− species that become D

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preferred morphology for ZnO is hexagonal with crystals elongated along the c axis (±[0001] direction) (Figure 9). Herein, H2O2 promotes growth in fast growth directions as it increases the solubility of Zn species in solution. Therefore, with the increasing concentration of H2O2 from samples 1−3, the increase in the ratio of peroxide to zinc precursor contributes to the growth of rods along the [0001] direction (due to free Zn ions remaining in solution owing to the increased solubility of Zn in the hydrothermal solution). It is suggested that under a yet higher [H2O2] to [Zn] ratio the sharpening of one end of the ZnO nanorod in sample 4 could be induced by the suppression of the (0002) plane and is likely a result of the fast growth rate, driven by the promotion of soluble Zn species due to the higher concentration of H2O2. The {01I1̅ } planes appear between the sharp point and the {01I0̅ } planes (as shown in Figure 4), forming a pyramidal point. Here, the growth rate of {01I0̅ } planes is larger than that of {01I1̅ } planes, and the growth of {01I0̅ } begins to dominate. As for sample 5, the growth of {01I0̅ } planes is very quick and leads to their disappearance. Here, {01I1̅ } planes are still retained as a result of their slow growth rate. Finally, a tapered pyramid-like nanoneedle shape with a (000I)̅ basal plane and a {01I1̅ } lateral plane is formed. So from samples 4 and 5, the size increased and the growth of the (0002) plane is suppressed by the increased growth rate of {01I0̅ } planes. 4.2. Sensor Characterization. The most widely accepted theory of metal oxide semiconductor gas sensing states that “when a metal oxide semiconductor gas sensor is exposed to air, oxygen species are adsorbed on the surface of the sensor material and go on to be ionized by electrons from the materials conduction band to form species such as O2−”.33 As such, in air, the measured resistance of ZnO and other n-type semiconductors will increase because of the lower concentration of free electrons in the materials conduction band.32 The ability of the sensor material to absorb and ionize oxygen species is key to sensor performance. The good performance observed here by the ZnO sensor is likely to be the result of two factors. First, a reduced crystallite size, between 20 and 40 nm in diameter, was observed (Figures 2 and 3). This causes an important effect: principally the surface of the crystallites becomes more reactive and more likely to absorb oxygen and form ionized oxygen species.34−36 The surface area to volume ratio of the film is also likely to be increased (Figure 5). This means that the amount of absorbable and ionizable oxygen is increased relative to that of a sensor prepared by conventional screen-printed methods. The ZnO sensors prepared in this work were tested against NO2 gas at a variety of concentrations and operating temperatures (Figures 6−8). The highest gas response of 128 was observed at an operating temperature of 350 °C toward 10 ppm NO2 gas. This response compares favorably to other reports of nanoscaled ZnO gas sensors reported in the literature. Our sensors give a gas response of over 80 when exposed to 4 ppm NO2, which compares with a gas response of 3 for ZnO nanotubes exposed to 5 ppm NO2 gas,37 a gas response of 2 for 1 ppm NO2 gas for hydrothermally produced nanorods,38 a gas response of 50 or 60 for 5 ppm NO2 gas for ZnO nanowires,39,40 a gas response of 4 for 8.5 ppm NO2 gas for a ZnO-based nanobelt sensor,41 or a gas response of 11 for 1 ppm NO2 gas for a doped ZnO thin film.42 Indeed our sensors give a favorable response when compared to quantumdot-based devices.43 Interestingly, the optimal operating

the growth unit of ZnO. As such, ZnO nanostructures are precipitated from the alkaline hydrothermal environment by the above chemical reactions, the dehydration reaction occurring due to the reaction temperature of ca. 335 °C. In this case, the nucleation and crystal growth phases differ slightly from those reported in previous publications of some of the authors as a result of the addition of very large concentrations of H2O2 to some of the syntheses, which appears to affect both stages of particle formation (Figure 9). In an early stage of the

Figure 8. Gas response (R/R0) of the ZnO sensor measured at different temperatures for different concentrations of NO2.

Figure 9. Suggested growth directions for ZnO rods and pyramids with increasing concentration of hydrogen peroxide.

reaction, with the flow of OH− ions, large quantities of nuclei were generated as a result of supersaturation and homogeneous nucleation in the solution. The newly formed nuclei aggregate to minimize the surface energy. Meanwhile, crystal growth is initiated and follows the intrinsic anisotropic character of hexagonal ZnO. From our observations and detailed investigations of the crystal habit of ZnO reported in the literature, the authors observe the following: under hydrothermal alkaline conditions, the rates of ZnO crystal growth differ as a function of the crystallographic plane. The rates of addition to the terminating atoms on each of these planes resulting in growth perpendicular to the plane are reported to occur in the following series as (0001) > (01II̅ )̅ > (01I0̅ ) > (01I1̅ ) > (000I)̅ ; as such, the E

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temperature for our sensors at 350 °C is somewhat higher than others reported, where it is typically found to be 225 °C. Overall, continuous throughput hydrothermal synthesis and drop-coating deposition techniques represent facile ways to prepare nanoscaled metal oxide gas sensors. Control of key materials properties such as doping and microstructure can be readily achieved. These techniques have the key advantage of being easily integrated into the microelectronics industry; as such, simple integration into MEMS- and silicon-based devices ought to be readily achievable.

(6) Zhang, Y.; Wang, Z.; Lu, F.; Zhang, Y.; Xiao, Y.; Zhang, L. Property Modulation of Zinc Oxide Hierarchical Architectures in Photoluminescence and Raman Scattering. Appl. Phys. Lett. 2006, 89, 113110−113113. (7) Sounart, T. L.; Liu, J.; Voigt, J. A.; Hsu, J. W. P.; Spoerke, E. D.; Tian, Z.; Jiang, Y. B. Sequential Nucleation and Growth of Complex Nanostructured Films. Adv. Funct. Mater. 2006, 16, 335−344. (8) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Complex and Oriented ZnO Nanostructures. Nat. Mater. 2003, 2, 821−826. (9) Zhang, T. R.; Dong, W. J.; Keeter-Brewer, M.; Konar, S.; Njabon, R. N.; Tian, Z. R. Site-Specific Nucleation and Growth Kinetics in Hierarchical Nanosyntheses of Branched ZnO Crystallites. J. Am. Chem. Soc. 2006, 128, 10960−10968. (10) Adschiri, T.; Hakuta, Y.; Sue, K.; Arai, K. Hydrothermal Synthesis of Metal Oxide Nanoparticles at Supercritical Conditions. J. Nanopart. Res. 2001, 3, 227−235. (11) Sue, K.; Hakuta, Y.; Smith, R. L.; Adschiri, T.; Arai, K. Solubility of Lead(II) Oxide and Copper(II) Oxide in Subcritical and Supercritical Water. J. Chem. Eng. Data 1999, 44, 1422−1426. (12) Chen, M.; Ma, C. Y.; Mahmud, T.; Darr, J. A.; Wang, X. Z. Modelling and Simulation of Continuous Hydrothermal Flow Synthesis Process for Nano-Materials Manufacture. J. Supercrit. Fluids 2011, 59, 131−139. (13) Chaudhry, A. A.; Goodall, J.; Vickers, M.; Cockcroft, J. K.; Rehman, I.; Knowles, J. C.; Darr, J. A. Synthesis and Characterisation of Magnesium Substituted Calcium Phosphate Bioceramic Nanoparticles Made via Continuous Hydrothermal Flow Synthesis. J. Mater. Chem. 2008, 18, 5900−5908. (14) Galkin, A. A.; Kostyuk, B. G.; Lunin, V. V.; Poliakoff, M. Continuous Reactions in Supercritical Water: A New Route to La2CuO4 with a High Surface Area and Enhanced Oxygen Mobility. Angew. Chem., Int. Ed. 2000, 39, 2738−2740. (15) Weng, X.; Brett, D.; Yufit, V.; Shearing, P.; Brandon, N.; Reece, M.; Yan, H.; Tighe, C.; Darr, J. A. Highly Conductive Low Nickel Content Nano-Composite Dense Cermets from Nano-Powders Made via a Continuous Hydrothermal Synthesis Route. Solid State Ionics 2010, 181, 827−834. (16) Boldrin, P.; Hebb, A. K.; Chaudhry, A. A.; Otley, L.; Thiebaut, B.; Bishop, P.; Darr, J. A. Direct Synthesis of Nanosized NiCo2O4 Spinel and Related Compounds via Continuous Hydrothermal Synthesis Methods. Ind. Eng. Chem. Res. 2007, 46, 4830−4838. (17) Elouali, S.; Bloor, L. G.; Binions, R.; Parkin, I. P.; Carmalt, C. J.; Darr, J. A. Gas Sensing with Nano-Indium Oxides (In2O3) Prepared via Continuous Hydrothermal Flow Synthesis. Langmuir 2012, 28, 1879−1885. (18) Ohara, S.; Mousavand, T.; Sasaki, T.; Umetsu, M.; Naka, T.; Adschiri, T. Continuous Production of Fine Zinc Oxide Nanorods by Hydrothermal Synthesis in Supercritical Water. J. Mater. Sci. 2008, 43, 2393−2396. (19) Svendergaard, M.; Bvejesen, E. D.; Christensen, M.; Iversen, B. B. Size and Morphology Dependence of ZnO Nanoparticles Synthesized by a Fast Continuous Flow Hydrothermal Method. Cryst. Growth Des. 2011, 11, 4027−4033. (20) Chen, L.-Y.; Yin, Y.-T. Facile Continuous Flow Injection Process for High Quality Long ZnO Nanowire Arrays Synthesis. Cryst. Growth Des. 2012, 12, 1055−1059. (21) Li, S.; Gross, G. A.; Guenther, P. M.; Koehler, J. M. Hydrothermal Micro Continuous-Flow Synthesis of Spherical, Cylinder-, Star- and Flower-like ZnO Microparticles. Chem. Eng. J. 2011, 167, 681−687. (22) Naisbitt, S. C.; Pratt, K. F. E.; Williams, D. E.; Parkin, I. P. A Microstructural Model of Semiconducting Gas Sensor Response: The Effects of Sintering Temperature on the Response of Chromium Titanate (CTO) to Carbon Monoxide. Sens. Actuators, B 2006, 114, 969−977. (23) Warren, B. E. X-ray Diffraction; Addison-Wesley: Reading, MA, 1969.

5. CONCLUSIONS A new pilot-plant continuous hydrothermal flow reactor was used as a rapid and controllable single-step synthesis method for the synthesis of crystalline ZnO nanorods and nanoprisms. The size and shape of the as-prepared ZnO nanostructures could be tuned by varying the amounts of peroxide and zinc precursor used in this continuous hydrothermal synthesis. High concentrations of peroxide are reasoned to have affected the crystallite growth, resulting in the synthesis of both ZnO rods and prismatic crystallites as a function of peroxide concentration. Our observations suggest that the depletion of Zn ions in solution occurs at high peroxide concentrations (in the later stages of growth), resulting in a sharpening of the rods. A study of the NO2 gas-sensing properties of the as-prepared ZnO nanostructures was carried out. The as-prepared ZnO sample gave a gas response of 128 to a gas exposure of 10 ppm NO2 and a good response to the lowest concentrations tested. The magnitude of the response was significantly larger than others we have found reported in the literature. The use of CHFS is demonstrated to be an excellent method for the synthesis of nanoscaled materials used in gas sensing.

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AUTHOR INFORMATION

Corresponding Author

*(L.S.) Phone: 86-551-3607234. E-mail: [email protected]. Phone: 20-7679-4345. (R.B.) E-mail: [email protected]. (I.P.) E-mail: [email protected]. (J.D.) E-mail: j.a.darr@ucl. ac.uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Leeds EPSRC Nanoscience and Nanotechnology Research Equipment Facility (LENNF) accessed through EPSRC grant EP/F056311/1 for assistance with TEM imaging.

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REFERENCES

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dx.doi.org/10.1021/la402339m | Langmuir XXXX, XXX, XXX−XXX