Synergistic Effects of a Combination of Cr2O3-Functionalization and

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article 2

3

Synergistic effects of a combination of CrO-functionalization and UV-irradiation techniques on the ethanol gas sensing performance of ZnO nanorod gas sensors Sunghoon Park, Gun-Joo Sun, Changhyun Jin, Hyoun Woo Kim, Sangmin Lee, and Chongmu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11485 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Synergistic effects of a combination of Cr2O3-functionalization and UV-irradiation techniques on the ethanol gas sensing performance of ZnO nanorod gas sensors Sunghoon Parka, Gun-Joo Suna, Changhyun Jinb, Hyoun Woo Kimc, Sangmin Leed, Chongmu Leea,*1 a

Department of Materials Science and Engineering, Inha University, 253 Yonghyun-dong,

Nam-gu, Incheon 402-751, Republic of Korea b

School of Mechanical Engineering, Konkuk University, Hwayang-dong, Gwangjin-gu, Seoul

143-701, Republic of Korea c

Department of Materials Science and Engineering, Hanyang University, Haengdang 1-dong,

Seongdong-gu, Seoul 133-791, Republic of Korea d

Department of Electronic Engineering, Inha University, 253 Yonghyun-dong, Nam-gu,

Incheon 402-751, Republic of Korea (S) Supporting Information Abstract There have been very few studies on the effects of combining two or more techniques on the sensing performance of nanostructured sensors. Cr2O3-functionalized ZnO nanorods were synthesized using carbothermal synthesis involving the thermal evaporation of a mixture of ZnO and graphite powders followed by a solvothermal process for Cr2O3-functionalization. The ethanol gas sensing properties of multi-networked pristine and Cr2O3-functionalized ZnO nanorod sensors under UV illumination were examined to determine the effects of combining Cr2O3-ZnO heterostructure formation and UV irradiation on the gas sensing properties of *

Corresponding author. Tel.: +82 32 860 7536; Fax: +82 32 862 5546.

E-mail address: [email protected] (C. Lee). 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ZnO nanorods. The responses of the pristine and Cr2O3-functionalized ZnO nanorod sensors to 200 ppm of ethanol at room temperature by UV illumination at 2.2 mW/cm2 were increased by 3.8 and 7.7 times, respectively,. The Cr2O3-functionalized ZnO nanorod sensor also showed faster response/ recovery and better selectivity than the pristine ZnO nanorod sensor at the same ethanol concentration. This result suggests that a combination heterostructure formation and UV irradiation had a synergistic effect on the gas sensing properties of the sensor. The synergistic effect might be attributed to the catalytic activity of Cr2O3 for ethanol oxidation, as well as to the increased change in conduction channel width accompanying adsorption and desorption of ethanol under UV illumination due to the presence of Cr2O3 nanoparticles in the Cr2O3-functionalized ZnO nanorod sensor. Keywords: ZnO, nanorod, sensor, Cr2O3, UV

1. Introduction Zinc oxide (ZnO) is one of the most extensively studied materials because of its diverse applications such as a gas sensing material. ZnO gas sensors with various nanostructures have been assessed using a range of gases such as hydrogen, oxygen, ethanol (C2H5OH), acetone, H2S, NO2, and humidity.1-5 One-dimensional (1D) nanostructured ZnO sensor, in particular, have been studied intensively in recent years because of their high surface-to-volume ratios. Metal oxide semiconductor-based gas sensors such as ZnO have been assessed as sensing materials because of their many merits including high sensitivity, ease of fabrication and low cost.6,7 On the other hand, they also have demerits, such as high operating temperature and poor selectivity.8. Over the past decades several techniques such as metal catalyst doping,9-11 heterostructure formation,12-15 and ultraviolet (UV) light irradiation,16,17 have been developed 2


Page 2 of 28

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to overcome these shortcomings. Of the above techniques, heterostructure formation techniques to enhance the sensing performance of metal oxide semiconductor sensors have been intensively studied, particularly, in recent years. Heterostructure formation can be achieved by surface functionalization of metal oxide semiconductor nanostructures with oxide

semiconductor

nanoparticles12,13

or

metal

oxide

semiconductor

core-shell

nanostructures.14,15 These heterostructures are commonly synthesized by a combination of complicated dry and wet techniques. On the other hand, however, there have been very few studies on the effects of combining two or more of these techniques on the sensing properties of nanostructured sensors. In this study, multi-networked Cr2O3-functionalized ZnO nanorod sensors were fabricated and their ethanol gas sensing properties under UV illumination were examined to determine the effects of combining the Cr2O3-ZnO heterostructure formation and UV irradiation on the gas sensing properties of ZnO nanorods. Our results showed that a combination of the two techniques had a synergistic effect on the sensing performance. 2. Experimental 2.1 Preparation and Characterization of Cr2O3 Nanoparticle-Functionalized ZnO Nanorods Cr2O3 nanoparticle-functionalized ZnO nanorods were synthesized via a two-step process: carbothermal synthesis involving the thermal evaporation of a mixture of ZnO and graphite powders, followed by a solvothermal process for Cr2O3-functionalization. First, a 4nm thick gold thin film was deposited onto a Si (100) substrate by direct current (dc) magnetron sputtering. A quartz tube was mounted horizontally inside a tube furnace. An alumina boat containing a mixture of ZnO with a purity of 99.99 % and graphite powders 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

with a purity of 99.99 % was placed in the middle of the quartz tube furnace. The Au-coated Si(100) substrate was placed approximately 5 mm from the ZnO/graphite powder mixture. The furnace was heated to 900oC and maintained at that temperature for 1 h in a N2/O2 atmosphere with a constant flow rate of O2 of 5 cm3/min and that of N2 of 100 cm3/min. The total pressure was set to 1.0 Torr (133 Pa). Thermal evaporation was carried out for 1 h. The furnace was then cooled to

room temperature at 1 mTorr, and the products were removed. A 50-mM Cr2O3 precursor solution

was

then

prepared

by

dissolving

chromium

acetate

monohydrate

[Cr(CH3COO)2·H2O] in distilled water. 10 mL of a 28% NH4OH solution and 40 mL of H2O2 were then added to 50 mL of the Cr2O3 precursor solution. The mixed solution was stirred well by using a magnetic bar and then ultrasonicated for 30 min to make the solution uniform. The solution was then centrifuged at 5,000 rpm for 2 min to precipitate the Cr2O3 powders. The precipitated powders were collected by removing the liquid leaving the powders behind. The collected powders were rinsed in a 1:1-solution of isopropyl alcohol (IPA) and distilled water to remove the impurities. The rinsing process was repeated five times. Subsequently, the Cr2O3 precursor solution was dropped onto the ZnO nanorods on the substrate and the substrate was rotated at 1,000 rpm for 30 s to allow the decoration of Cr2O3 nanoparticles. After spin-coating, the Cr2O3-decorated ZnO nanorod sample was annealed in air at 500°C for 1 h. The morphology and crystal structure of the collected nanorod samples were examined by scanning electron microscopy (SEM, Hitachi S-4200,10 kV) and glancing angle X-ray diffraction (XRD, Philips X’pert MRD) using Cu-Kα radiation (λ = 0.15418 nm), respectively. 2.2 Sensor Device Fabrication and Sensing Tests. The pristine and Cr2O3-functionalized ZnO nanorods (50 mg) were dispersed ultrasonically in ethanol in two different beakers. Two sets of multi-networked nanorod 4


Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sensors were fabricated by spreading suspensions of the nanorods in ethanol over thermally oxidized Si substrates equipped with a pair of interdigited Pt electrodes with a gap of 20 µm. A flow-through technique was used to measure the gas sensing properties of the fabricated multi-networked sensors in a tube furnace with a resistance heater. The pristine or Cr2O3functionalized ZnO nanorod sensors were inserted in a dynamic gas flow chamber. Ethanol gas diluted with dry synthetic air was introduced into the quartz tube at a flow rate of 200 cm3/min at room temperature. The electrical and sensing measurements were then conducted at room temperature and 40 % relative humidity by a voltamperometric method. The electrical resistance of the gas sensors was determined in the dark and under UV light (λ = 365 nm) illumination at 0.9-3.7 mW/cm2 by measuring the electric current by using a Keithley source meter-2612 under a constant applied voltage of 1 V. Detailed procedures of the sensor device fabrication and sensing tests are described elsewhere.18

3. Results and Discussion Fig. 1(a) shows an SEM image of the ZnO nanorods functionalized with Cr2O3 nanoparticles. The nanorods with diameters of 50 - 300 nm and lengths up to a few tens of micrometers were oriented randomly. The diameters of the Cr2O3 particles decorating the nanorods ranged from 20 to 80 nm. An enlarged SEM image of a typical 1D nanostructure [Inset in Fig. 1(a)] showed that it had a rod-like morphology covered completely with large Cr2O3 particles, with little or no ZnO actually exposed to the air. The morphology was thus more like a core-shell structure than a nanoparticle-decorated one. Fig. 1(b) shows XRD patterns of the pristine and Cr2O3-functionalized ZnO nanorod samples. All the reflection peaks of the pristine ZnO nanorod sample were assigned to a wurtzite-structured ZnO phase (JCPDS card No. 84-1616). Several a few reflection peaks assigned to a rhombohedral5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

structured Cr2O3 phase were observed in addition to the wurtzite-structured ZnO phase in the Cr2O3-functionalized ZnO nanorod sample. Figs. 1(c)-(e) show a low-magnification TEM image, a high-resolution TEM (HRTEM) image and corresponding selected area electron diffraction (SAED) pattern, respectively, of a typical Cr2O3-functionalized ZnO nanorod. The Cr2O3 nanoparticles show a sphere-like morphology. The fringe patterns with different crystallographic orientations in the HRTEM image indicate that the ZnO nanorods were polycrystalline, despite the clear spots in the SAED pattern. The concentric rings in the SAED pattern also suggests that the Cr2O3 nanoparticles were polycrystalline despite the fringes in the nanoparticles in the HRTEM image. Figs. 2(a) and 2(b) show the gas response transients of the pristine and the Cr2O3functionalized ZnO nanorods, respectively, towards ethanol gas under UV illumination at 3.7 mW/cm2 at room temperature. The gas response transients exhibited stable and reproducible response and recovery characteristics. Fig. 3(a) presents the responses of networked pristine and Cr2O3-functionalized ZnO nanorod sensors as a function of the ethanol concentration. The response of the pristine and Cr2O3-functionalized ZnO nanorod sensors to 200 ppm of ethanol were 424% and 1,095%, respectively. The response was defined as [Ra/Rg]×100 (%) for ethanol gas in this study, where Ra and Rg are the electrical resistances of the sensors in air and ethanol gas, respectively. Table 1 lists the responses of the various nanomaterials to ethanol gas.19-27 Only the sensing tests in this study (the first four rows in Table 1) were conducted at room temperature. A direct comparison of the responses of the different nanomaterials is therefore difficult because of the different test conditions, however, the Cr2O3-functionalized ZnO nanorod sensor exhibited a stronger response to ethanol than several of the other nanomaterials measured at high temperatures. The Cr2O3-functionalized ZnO nanorod sensor also exhibited a higher response [Fig. 3(b)] and faster response/ recovery 6


Page 6 of 28

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Fig. 3(c)) than the pristine ZnO nanorod sensor at the same ethanol concentration. A comparison of the response and recovery times of the pristine and functionalized nanorod sensors shows that the response time of the ZnO nanorod sensor to 200 ppm of ethanol decreased from 52 s to 26 s and from 192 s to 110 s, respectively, by Cr2O3-functionalization. Herein, the response and recovery times are defined as the times to reach 90 % of the resistance change upon exposure to ethanol and air, respectively. Figs. 4(a) and 4(b) presents the gas-response transient curves of the pristine and Cr2O3functionalized ZnO nanorod sensors, respectively, as a function of the UV illumination intensity. The figure shows the strong dependence of the responses to ethanol on the UV illumination intensity. Both types of sensors exhibited n-type semiconductor behavior. This suggests that the main conduction channel was the central region of the ZnO nanorod, even though the p-type Cr2O3 covers the n-type ZnO nanorod completely. The responses of the pristine ZnO nanorod sensor to ethanol were 111% and 424% in the dark (UV illumination intensity = 0) and under UV illumination at 3.7 mW/cm2, respectively (Table 1), i.e., the response was increased by 3.8 times by UV illumination. In contrast, the responses of the Cr2O3-functionalized ZnO nanorod sensor to ethanol were 143% and 1,095%, in the dark and under UV illumination at 3.7 mW/cm2, respectively (Table 1). In other words, the response increased 7.7 times by UV illumination. These results suggest that a combination of the two techniques: Cr2O3 functionalization and UV illumination had a synergistic effect on the response of the ZnO nanorod sensor to ethanol. Fig. 5 compares the selectivity of the two sensors to ethanol over other volatile organic compound (VOC) gases. Both the pristine and the Cr2O3-functionalized ZnO nanorod sensors exhibited a stronger response to ethanol gas than to the other gases. Table 2 lists the responses of the pristine and Cr2O3-functionalized ZnO nanorod sensor to several VOC gases 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

normalized to ethanol. The functionalized sensor showed lower normalized responses to the other VOC gases than the pristine ZnO nanorod sensor, suggesting that the functionalized sensor has superior selectivity for ethanol gas as compared to the pristine sensor. ZnO and Cr2O3 are n- and p-type semiconductors, respectively. When the pristine ZnO nanorods are exposed to air at room temperature, reactions occur between ZnO and the adsorbed oxygen molecules. Electrons are transferred from the conduction band to the adsorbed oxygen atoms, and ionic species, such as O−, O2−and O2− form as by-products of these reactions. Of these oxygen species, O2− is dominant at room temperature.28 A depletion layer forms on the surface of the ZnO nanorod because of the consumption of electrons in the ZnO nanorod. Upon exposure to ethanol gas, the ethanol gas adsorbs onto the ZnO nanorod surface, releasing electrons back to the conduction band of ZnO, as shown in the following equations:29 C2H5OH (gas) → C2H5OH (ads)

(1)

C2H5OH (ads) + 6O- (ads) → 2CO2 (gas) + 3H2O (gas) + 6e-

(2)

The electrons released shrink the depletion layer, leading to a decrease in the resistance of the nanorod sensor. The enhanced sensing performance of the Cr2O3-functionalized ZnO nanorod sensor compared to the pristine one can be explained by a combination of electronic and chemical mechanisms. A critical difference between the pristine and Cr2O3-functionalized ZnO nanorod sensors is the presence of Cr2O3-ZnO p-n junctions in the functionalized sensors. Two electronic sensing mechanisms related to the presence of these p-n junctions: modulation of the surface depletion layer width at the heterojunction interface30,31 and modulation of the interfacial potential barrier height32,33 should be considered when explaining the enhanced sensing performance of the Cr2O3-functionalized ZnO nanorod sensor. Of these two 8


Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electronic mechanisms the modulation of the surface depletion layer width at the heterojunction interface might make a greater contribution to the sensor performance than the modulation of the interfacial potential barrier height, because the Cr2O3-ZnO interfaces are aligned parallel to the ZnO nanorod axis, i.e., the current flowing through the p-n junction is perpendicular to the main current flow direction along the ZnO nanorod axis. Therefore, the contribution to the performance of the sensor of the potential barrier modulation at the p-n junction is smaller than that of the surface depletion layer width modulation. In this paper, the modulation of the surface depletion layer width at the heterojunction interface is the primary explanation for the enhanced sensing performance of the Cr2O3-functionalized ZnO nanorod sensor. A possible model for the enhanced sensing performance of the ZnO nanorods is shown in Fig. 6. Fig. 6(a) presents a schematic diagram of the depletion layer, the conduction channel formed at the Cr2O3-ZnO interface and adjacent regions in a Cr2O3-functionalized ZnO nanorod in air and ethanol gas.34 The first schematic in Fig. 6(b) shows the energy band diagram of the Cr2O3-ZnO binary system in air. In air, a depletion layer and an accumulation layer form at the surfaces of the ZnO nanorod and Cr2O3 nanoparticle, respectively, due to electron transfer from the conduction band of ZnO to the adsorbed oxygen atoms on the ZnO side and the generation of holes on the Cr2O3 side, respectively. Comparing with the pristine ZnO nanorods, the Cr2O3-functionalized ZnO nanorods have a somewhat thicker depletion layer than the pristine ZnO nanorod due to the electron transfer from the ZnO nanorod to the Cr2O3 nanoparticle. However, in Fig. 6(b), in ethanol, the conduction path might actually be even wider because ethanol causes the Cr2O3 to become “less p-type”, causing the Cr2O3induced (p-n junction) contribution to the depletion region to be smaller, as compared to the pristine nanorod [the second schematic in Fig. 6(b)]. Consequently, the depletion layer width 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and thereby the conduction channel width change from WD1 to WD2, and from WC1 to WC2, respectively, accompanying the adsorption and desorption of ethanol gas, leading to a change in resistance. More importantly, a comparison of the pristine and Cr2O3-functionalized ZnO nanorods in Fig. 6(a) shows that in air, the conduction channel width in the functionalized nanorods, WC1(F) is smaller than that in the pristine nanorods, WC1(P) due to the electron transfer from ZnO to Cr2O3. In contrast, in ethanol gas, the conduction channel width in the functionalized nanorods, WC2(F), is larger than that in the pristine nanorods, WC2(P). Consequently, WC2(F)-WC1(F) is larger than WC2(P)-WC1(P), leading to a larger resistance change and an enhanced response of the Cr2O3-functionalized ZnO nanorods to ethanol gas as compared to the pristine nanorods. In other words, upon exposure to ethanol gas, a greater expansion in the conduction channel occurs in the Cr2O3-functionalized ZnO nanorods than in the pristine ZnO nanorods due to the influence of the p-type Cr2O3 nanoparticles. The chemical mechanism is related to the superior catalytic activity of Cr2O3 for the oxidation of ethanol.35,36

The Cr2O3 nanoparticles on the ZnO nanorod surface enhanced the

sensing performance of the functionalized sensor by promoting the oxidation of ethanol gas. The origin of the higher selectivity of the Cr2O3-functionalized ZnO nanorod sensor to ethanol might also be due to the superior catalytic activity of Cr2O3 for the oxidation of ethanol as compared to that for the oxidation of other gases at room temperature. The oxidation rate of a gas might depend on the following factors: the solid solubility of the gas in the material, the decomposition rate of the adsorbed molecules at the material surface, the charge carrier concentration in the material, the Debye length in the material, the catalytic activity of the material,37,38 and the orbital energy of the gas molecules.39-41 Of these, the superior catalytic activity of Cr2O3 for the oxidation of ethanol as compared to that for the oxidation of other gases at room temperature might be the main reason for the higher 10


Page 10 of 28

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


selectivity of the functionalized ZnO nanorod sensor to ethanol gas. Fig. 6 also shows differences in the depletion layer width between in the dark and under UV illumination. Upon exposure to UV light with a photon energy larger than the band gaps of ZnO and Cr2O3, electron-hole pairs are generated in the Cr2O3 nanoparticles and ZnO nanorods. The light causes the generation of e-h pairs throughout the material. If it is near the surface of the ZnO, two forces determine the diffusion direction. Based on the band diagram, holes would diffuse toward the interface, whereas electrons would diffuse away from the interface (in the ZnO), However, there is also a higher concentration of electrons in the bulk than in the depletion region, such that the concentration gradient/extra negative charge would push them toward the depletion region, creating some equilibrium. If holes do diffuse toward the interface from the ZnO, then the ZnO near the interface would be “less n-type” and the conduction band minimum (CBM) would be further from the Fermi level, In contrast, in the case of the adsorption of ethanol or in ethanol, if photogenerated electrons in Cr2O3 diffuse toward the interface, as predicted by the bending in the CBM, they would make that section “less p-type”, in which case the CBM would be closer to the Fermi level. In other words, the change in resistance in the sensor accompanying the adsorption and desorption of ethanol gas will increase under UV illumination due to the generation of additional carriers. In summary, UV-excitation results in an increase in the accumulation layer width and a decrease in the depletion layer width, i.e., an increase in the conduction channel width with increasing carrier concentration by lowering the interface state density. Therefore, UV-light irradiation enhances the gas-sensing properties of the sensors. Considering all the above results in this study, we might conclude that a combination of the two techniques: heterostructure formation and UV light irradiation had a synergistic effect on the response of the ZnO nanorod sensor to ethanol. 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4. Conclusions The response of the ZnO nanorod to ethanol gas at room temperature was enhanced significantly by combining Cr2O3-functionalizion and UV illumination. The responses of the pristine and Cr2O3-functionalized ZnO nanorod sensors to 200 ppm of ethanol at room temperature increased 3.8 and 7.7 times, respectively, by UV illumination at 2.2 mW/cm2. The Cr2O3-functionalized ZnO nanorod sensor also exhibited faster response/ recovery and better selectivity than the pristine ZnO nanorod sensor at the same ethanol concentration. These results suggest that a combination of heterostructure formation and UV irradiation had a synergistic effect on the gas sensing properties of the sensor. The synergistic effect might have been caused by the catalytic activity of Cr2O3 for ethanol oxidation as well as by the increase in conduction channel width accompanying adsorption and desorption of ethanol under UV illumination due to the presence of Cr2O3 nanoparticles in the Cr2O3-functionalized ZnO nanorod sensor. The substantial increase in the response of the Cr2O3-functionalized ZnO nanorod sensor under UV irradiation have been caused by the larger change in resistance caused by the increased number of carriers participating in the reactions with ethanol molecules due to photo-generated electron-hole pairs. We believe that this study shows the possibility of realizing high-performance gas sensors using a combination of heterostructure formation and UV irradiation. It also elucidates the underlying mechanism for the enhanced sensing performance of the p-n heterostructured gas sensors under UV illumination.

■ ASSOCIATED CONTENT (S) Supporting Information 12


Page 12 of 28

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table S1 shows the raw data of the responses of the pristine and Cr2O3 nanoparticledecorated ZnO nanorod gas sensors at 25oC as a function of the ethanol concentration under UV illumination (365 nm, 3.7 mW/cm2). Tables S2 and S3 show the raw data of the response and recovery times, respectively, of the pristine and Cr2O3 nanoparticle-decorated ZnO nanorod gas sensors at 25oC under UV illumination (365 nm, 3.7 mW/cm2).Table S4 shows the raw data of the responses of pristine and Cr2O3 nanoparticle-decorated ZnO nanorod gas sensors to 200 ppm of ethanol gas at 25oC for different UV illumination intensities. Table S5 shows the raw data of the responses of the pristine and Cr2O3 nanoparticle-decorated ZnO nanorod gas sensors at 25oC to 200 ppm of different gases under UV illumination (365 nm, 3.7 mW/cm2). These materials are available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *Phone: +82 32 860 7536. Fax: +82 32 862 5546. E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgment This study was supported by Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

the

(2015R1D1A1A01057029). 13


Ministry

of

Education


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

References [1] Hsueh, T.J.; Chang, S.J.; Hsu, C.L.; Lin, Y.R.; Chen, I.C. Highly Sensitive ZnO Nanowire Ethanol Sensor with Pd Adsorption, Appl. Phys. Lett. 2007, 91, 053111– 053111-3. [2] Wagh, M.S.; Jain, G.H.; Patil, D.R.; Patil, S.A.; Patil, L.A. Modified Zinc Oxide Thick Flm Resistors as NH3 Gas Sensor, Sens. Actuators B 2006, 115, 128–133. [3] Wollenstein, J.; Plaza, J.A.; Cane, C.; Min, Y.; Bottner, H.; Tuller, H.L. A Novel Single Chip Thin Film Metal Oxide Array, Sens. Actuators B 2003, 93, 350–355. [4] Mridha, S.; Basak, D. Investigation of a p-CuO/n-ZnO Thin Film Heterojunction for H2 Gas-Sensor Applications, Semicond. Sci. Technol. 2006, 21, 928–932. [5] Sun, Y.; Ndifor-Angwafor, N.G.; Riley, D.J.; Ashfold, M.N.R. Synthesis and Photoluminescence of Ultra-Thin ZnO Nanowire/Nanotube Arrays Formed by Hydrothermal Growth, Chem. Phys. Lett. 2006, 431, 352–357. [6] Kolmakov, A.; Zhang, Y.; Cheng, G.; Moskovits, M. Detection of CO and O2 Using Tin Oxide Nanowire Sensors, Adv. Mater. 2003, 15, 997–1000. [7] Liu, Y.; Koep, E.; Liu, M. A Highly Sensitive and Fast-Responding SnO2 Sensor Fabricated by Combustion Chemical Vapor Deposition, Chem. Mater. 2005, 17. 3997– 4000. [8] Chaabouni, F.; Abaab, M.; Rezig, B. Metrological Characteristics of ZnO Oxygen Sensor at Room Temperature, Sens. Actuators B 2004, 100, 200–204. [9] Wan, Q.; Wang, T. Single-Crystalline Sb-Doped SnO2 Nanowires: Synthesis and Gas Sensor Application, Chem. Commun. 2005, 1, 3841–3843. [10] Kim, H.; Jin, C.; Park, S.; Kim, S.; Lee, C. H2S Gas Sensing Properties of Bare and Pd14


Page 14 of 28

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Functionalized CuO Nnorods, Sens. Actuators B 2012, 161, 594–599. [11] Kolmakov, A.; Klenov, D.; Lilach, Y.; Stemmer, S.; Moskovits, M. Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles, Nano Lett. 2005, 5, 667–673. [12] Rakshit, T.; Mondal, S.P.; Manna, I.; Ray, S.K. CdS-Decorated ZnO Nanorod Heterostructures for Improved Hybrid Photovoltaic Devices, ACS Appl. Mater. Interfaces 2012, 4, 6085–6095. [13] Na, C.W.; Woo, H.-S.; Kim, I.-D.; Lee, J.-H. Selective Detection of NO2 and C2H5OH Using a Co3O4-dDecorated ZnO Nanowire Network Sensor, Chem. Commun. 2011, 47, 5148–5150. [14] Park, S.; An, S.; Mun, Y.; Lee, C. UV-Enhanced NO2 Gas Sensing Properties of SnO2Core/ZnO-Shell Nanowires at Room Temperature, ACS Appl. Mater. Interfaces 2013, 5, 4285–4292. [15] Singh, N.; Ponzoni, A.; Gupta, R.K.; Lee, P.S.; Comini, E. Synthesis of In2O3–ZnO Core–Shell Nanowires and their Application in Gas Sensing, Sens. Actuators B 2011, 160, 1346–1351. [16] Cao, C.; Hu, C.; Wang, X.; Wang, S.; Tian, Y.; Zhang, H. UV Sensor Based on TiO2 Nanorod Arrays on FTO Thin Film, Sens. Actuators B 2011, 156, 114–119. [17] Fan, S.; Srivastava, A.; Dravid, V. UV-Activated Room-Temperature Gas Sensing Mechanism of Polycrystalline ZnO, Appl. Phys. Lett. 2009, 95, 142106. [18] Park, S.; An, S.; Ko, H.; Lee, S.; Lee, C. Synthesis, Structure, and UV-Enhanced Gas Sensing Properties of Au-Functionalized ZnS Nanowires, Sens. Actuators B 2013, 188, 1270–1276. [19] Pourfayaz, F.; Mortazavi, Y.; Khodadadi, A.; Ajami, S. Ceria-Doped SnO2 Sensor Highly 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Selective to Ethanol in Humid Air, Sens. Actuators B 2008, 130, 625–629. [20] Neri, G.; Bonavita, A.; Micali, G.; Donato, N.; Deorsola, F.A.; Mossino, P.; Amato, I.; De Benedetti, B. Ethanol Sensors Based on Pt-doped Tin Oxide Nanopowders Synthesised by Gel-Combustion, Sens. Actuators B 2006, 117, 196–204. [21] Hemmati, S.; Firoozb, A.A.; Khodadadi, A.A.; Mortazavi, Y. Nanostructured SnO2-ZnO sensors: Highly Sensitive and Selective to Ethanol, Sens. Actuators B 2011, 160, 1298– 1303. [22] Ahn, H.; Park, J.-H.; Kim, S.B.; Jee, S.H.; Yoon, Y.S.; Kim, D.J. Vertically Aligned ZnO Nanorod Sensor on Flexible Substrate for Ethanol Gas Monitoring, Electrochem. Solid State Lett. 2011, 13, J125–J128. [23] Hu, P.; Du, G.; Zhou, W.; Cui, J.; Lin, J.; Liu, H.; Liu, D.; Wang, J.; Chen, S. Enhancement of Ethanol Vapor Sensing of TiO2 Nanobelts by Surface Engineering, Appl. Mater. Interface 2010, 2, 3263–3269. [24] Zhang, W.H.; Zhang, W.D.; Chen, L.Y. Highly Sensitive Detection of Explosive Triacetone Triperoxide by an In2O3 Sensor, Nanotechnology 2010, 21, 315502. [25] Li, Y.J.; Li, K.M.; Wang, C.Y.; Kuo, C.I.; Chen, L.J. Low-Temperature Electrodeposited Co-Doped ZnO Nanorods with Enhanced Ethanol and CO Sensing Properties, Sens. Actuators B 2012, 161, 734–739. [26] Zhan, S.; Li, D.; Liang, S.; Chen, X.; Li, X. A Novel Flexible Room Temperature Ethanol Gas Sensor Based on SnO2 Doped Poly-Diallyldimethylammonium Chloride, Sensors 2013, 13, 4378–4389. [27] Park, S.; Ko, H.; An, S.; Lee, W.I.; Lee, S.; Lee, C. Synthesis and Ethanol Sensing Properties of CuO Nanorods Coated with In2O3, Ceram. Int. 2013, 39, 5255–5262. [28] Wu, R.-J.; Lin, D.-J.; Yu, M.-R.; Chen, M.H.; Lai, H.-F. Ag@SnO2 Core–Shell Material 16


Page 16 of 28

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for Use in Fast-Response Ehanol Sensor at Room Operating Temperature, Sens. Actuators B 2013, 178, 185–191. [29] Li, J.; Fan, H.; Jia, X.; Yang, W.; Fang, P. Enhanced Blue-Green Emission and Ethanol Sensing of Co-doped ZnO Nanocrystals Prepared by a Solvothermal Route, Appl. Phys. A 2010, 98, 537–542. [30] Liu, Y.; Zhu, G.; Chen, J.; Xu, H.; Shen, X.; Yuan, A. Co3O4/ZnO Nanocomposites for Gas-Sensing Applications, Appl. Surf. Sci. 2012, 265, 379–384. [31] Wang, W.; Li, Z.; Zheng, W.; Huang, H.; Wang, C.; Sun, J. Cr2O3-Sensitized ZnO Electrospun Nanofibers Based Ethanol Detectors, Sens. Actuators B 2010, 143, 754–758. [32] Patil, D.; Patil, L.; Patil, P. Cr2O3-Activated ZnO Thick Film Resistors for Ammonia Gas Sensing Operable at Room Temperature, Sens. Actuators B 2007, 126, 368–374. [33] Park, S.; Ko, H.; Kim, S.; Lee, C. Role of the Interfaces in Multiple Networked OneDimensional Core–Shell Nanostructured Gas Sensors, ACS Appl. Mater. Interfaces 2014, 6, 9595–9600. [34] McCafferty, E. A Surface Charge Model of Corrosion Pit Initiation and of Protection by Surface Alloying, J. Electrochem. Soc. 1999, 146, 2863–2869. [35] Carmian, R.; Piri, F. Synthesis and Investigation the Catalytic Behavior of Cr2O3 Nanoparticles, J. Nanostructures 2013, 3, 87–92. [36] Sato, T.; Breedon, M.; Miura, N. Improvement of Toluene Selectivity via the Application of an Ethanol Oxidizing Catalytic Cell Upstream of a YSZ-based Sensor for Air Monitoring Applications, Sensors 2012, 12, 4706–4714. [37] Parrondo, J.; Santhanam, R.; Mijangos, F.; Rambabu, B. Electrocatalytic performance of In2O3-Supported Pt/C Nanoparticles for Ethanol Electro-Oxidation in Direct Ethanol Fuel Cells, Int. J. Electrochem. Sci. 2010, 5, 1342–1354. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[38] Choi, K.I.; Kim, H.R.; Kim, K.M.; Liu, D.; Cao, G.; Lee, J.H. C2H5OH Sensing Characteristics of Various Co3O4 Nanostructures Prepared by Solvothermal Reaction, Sens. Actuators B 2010, 146, 183–189 [39] Li, Y.; Xu, J.; Chao, J.; Chen, D.; Ouyang, S.; Ye, J.; Shen, G. High-Aspect-Ratio SingleCrystalline Porous In2O3 Nanobelts with Enhanced Gas Sensing Properties, J. Mater. Chem. 2011, 21, 12852–12857. [40] Wen, Z.; Tianmo, L. Gas-Sensing Properties of SnO2-TiO2-Based Sensor for Volatile Organic Compound Gas and its Sensing Mechanism, Physica B 2010, 405, 1345–1348. [41] Feng, C.; Li, W.; Li, C.; Zhu, L.; Zhang, H.; Zhang, Y.; Ruan, S.; Chen, W.; Yu, L. Highly Efficient Rapid Ethanol Sensing Based on In2-xNixO3 Nanofibers, Sens. Actuators B 2012,) 83–88.

18


Page 18 of 28

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Responses of the different nanomaterials to ethanol gas

Materials

T (oC)

Ethanol concentration (ppm)

Response (%)

Comment

Pristine ZnO nanorods

25

200

111

dark

Cr2O3-functionalized ZnO nanorods

25

200

143

dark


25

200

424

UV

Cr2O3-functionalized ZnO nanorods

25

200

1,095

UV

200

5,000

16

300

100

311

250-450

200

18,500

270

69

270

200-400

300

390,000

200-400

300

120,000

25

200

80

300

50

382

25

200

224

350

50

987

TiO2 nanotubes ZnO nanorods Ce-SnO2 nanopowders In2O3 nanorcrystals SnO2-ZnO(0.05) composite nanopowders ZnO-SnO2(0.05) composite nanopowders SnO2-PDDAC* CuO/In2O3 nanorods Ag-SnO2 nanobelt Co-ZnO nanorods

* PDDAC: Poly-Diallyldimethylammonium Chlori

19


Reference Present work Present work Present work Present work [21] [22] [23] [24] [25] [25] [26] [27] [28] [29]


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Table 2. Response of the pristine and Cr2O3-functionalized ZnO nanorods to various VOC gases normalized to ethanol gas

Materials

Ethanol

Acetone

Methanol

Toluene

Benzene


1.0

0.83

0.70

0.62

0.43

Cr2O3functionalized ZnO nanorods

1.0

0.69

0.47

0.39

0.42

20


Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Fig. 1. (a) SEM image of Cr2O3-functionalized ZnO nanorods. Inset, enlarged SEM image of a typical Cr2O3-functionalized ZnO nanorod. (b) XRD pattern of the pristine ZnO nanorods and Cr2O3-functionalized ZnO nanorods. (c) Low-magnification TEM image of a typical Cr2O3-functionalized ZnO nanorod. (d) HRTEM image and (e) the corresponding SAED pattern of a Cr2O3-functionalized ZnO nanorod. Fig. 2. Gas response transients of (a) the pristine ZnO nanorods and (b) Cr2O3-functionalized ZnO nanorods towards ethanol gas at room temperature under UV illumination at 3.7 mW/cm2. Fig. 3. (a) Response, (b) response time, and (c) recovery time of the pristine ZnO nanorods and Cr2O3-functionalized ZnO nanorods towards ethanol gas at room temperature under UV illumination at 3.7 mW/cm2. Fig. 4. Gas response transients of (a) the pristine ZnO nanorods and (b) the Cr2O3functionalized ZnO nanorods to 200 ppm of ethanol at room temperature under UV illumination at different intensities Fig. 5. Comparison of the response of the pristine and the Cr2O3-functionalized ZnO nanorods to ethanol with those to other VOC gases. Fig. 6. (a) Schematic diagrams of a pristine ZnO nanorod. (b) Schematic diagram of a Cr2O3functionalized ZnO nanorod in the dark and under UV illuminationin in air and ethanol showing the depletion layer and conduction path. (c) Energy band diagram of a Cr2O3-ZnO junction in the dark and under UV illumination showing the depletion layer and potential barrier forming at or near the junction. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figures

Fig. 1.

Park et al.

22


Page 22 of 28

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. Park et al.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3. Park et al. 24


Page 24 of 28

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4. Park et al.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5. Park et al.

26


Page 26 of 28

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. Park et al.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents graphic

28


Page 28 of 28

Synergistic Effects of a Combination of Cr2O3-Functionalization and

Recommend Documents