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New Insights Towards Electron Transport Mechanism of Highly Efficient p-type CuO (111) Nanocuboids Based HS Gas Sensor 2

Jayaseelan Dhakshinamoorthy, and Biji Pullithadathil J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11327 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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The Journal of Physical Chemistry

New Insights Towards Electron Transport Mechanism of Highly Efficient p-Type CuO (111) Nanocuboids Based H2S Gas Sensor Jayaseelan Dhakshinamoorthy, Biji Pullithadathil* Nanosensor Laboratory, PSG Institute of Advanced Studies, Coimbatore -641004, INDIA

ABSTRACT

Charge transport and adsorption kinetics of wet-chemically synthesized CuO nanocuboids have been explored. The growth direction of CuO nanocuboids was found to be (111) plane, which exhibited predominant surface catalytic activity towards the dissociation of H2S and O2. Temperature-dependent adsorption studies revealed the adsorption kinetics of (111) grown p-type CuO nanocuboids towards H2S gas. Adsorption of oxygen (O2) on the CuO (111) surface resulted in the formation of ionosorbed O2¯ species, which increased the hole density and enhanced the surface conductivity of CuO nanocuboids. H2S molecules were found to interact well with CuO (111) surface, donating electrons to the material and reduced the holeaccumulation layer width. Investigation of electrical characteristics of p-type CuO nanocuboids revealed absence of any structural phase transitions under H2S environment. The H2S sensing mechanism was found to be associated with local suppression/expansion of the hole-accumulation layer of p-CuO nanocuboids rather than the thermally activated carriers. Exposure to H2S gas molecules was found to decrease the band

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bending energy as a function of concentration. The distinctive (111) surface reactivity of p-CuO nanocuboids towards H2S and their unique electron transport properties makes them highly amenable for fabricating highperformance gas sensors.

1. INTRODUCTION In recent years, various p-type oxide semiconductors (such as NiO, CuO, Co3O4, Cr2O3 and Mn3O4 have been developed to detect various harmful gases (C2H5OH, HCHO, CO, NH3, (CH3)3N, H2, H2S, C6H4(CH3)2 and C6H5(CH3)).1 The response of a p-type metal oxide semiconductor is reported to be equal to the square root of that of an n-type oxide semiconductor gas sensor towards any particular gas, when both the materials were having similar morphology.2 This indicates that the p-type oxide semiconductors are promising candidates for trace level detection of various analyte gases. Intensive investigations are going on to improve the sensitivity and selectivity of p-type oxide semiconductors to deploy them as chemi-resistive gas sensors. Among other p-type materials, CuO exhibits conceptually different behavior under various gas environments.3 Especially, CuO nanostructures having large surface to volume ratio possess superior physical and chemical properties than their bulk counterparts.4 The adsorption of gaseous species on surface sites of CuO leads to formation of electrical conduction pathway, which alter its electronic behaviour.5 Hydrogen sulfide (H2S) is one of the highly toxic and flammable gases, which is widely used in chemical industries. Trace level detection of H2S is very crucial to avoid health hazards.6 High chemical affinity of CuO nanostructures with H2S during the interaction enhances the detection limit upto sub-ppm levels.7,8 It is highly essential, but still a challenging task, to determine H2S gas with high sensitivity and broad linear range with a detection limit of as low as few parts per million.7 For the detection of H2S gas, SnO2 loaded CuO was reported to have highest sensitivity.9 Consequently, a variety of hybrid materials, such as CuO–SnO2 bilayers, p-CuO/n-SnO2 hetero-structures and CuO-doped SnO2 films, have been developed for H2S gas sensing.10-12 Though CuO (5 wt.%): SnO2 films were found to be very successful for H2S gas


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sensing, they have poor detection limit (> 5 ppm). The CuO decorated ZnO nanotetrapods and SnO2 heterostructures also were showing poor response (< 30% at 10 ppm) towards H2S detection.13,14 Compared to hetero-structures, pure CuO thin film was showing superior response (> 200% at 5 ppm) towards H2S gas sensing as the underlying surface reaction kinetics makes CuO nanostructures as highly selective and ideal material for H2S gas sensing.15,16 Temperature plays a major role in gas sensing phenomenon, essentially affects the adsorption of gas on the sensing materials and the reaction between gas molecules and adsorbed oxygen species.17 So far, very few numerical models have been developed to understand the sensing mechanism of a gas sensor based on a p-type semiconductor as a function of operating temperature, oxygen pressure, gas concentration and semiconductor/gas interaction.18,19 To the best of our knowledge, the temperature dependent adsorption kinetics, electronic properties and their influence on the H2S sensing mechanism have not been explored so far. Herein, we report a comprehensive H2S gas sensing property analysis of CuO nanocuboids, to explore the sensing mechanism and gas/material interaction as a function of temperature. From temperature dependent current-voltage (I-V) analysis, the effective activation energy was calculated under H2S and air environment. Similarly, the electron transport mechanism of CuO nanocuboids was explored based on band bending calculations and I-V studies.

2. EXPERIMENTAL SECTION 2.1 Synthesis of CuO nanocuboids All the chemicals used for the synthesis of CuO nanocuboids were of analytical grade and used without further purification. Briefly, 10 g of copper acetate (Cu(CH3COO)2·H2O, ≥ 99%, ACS grade, Merck) was dissolved in 300 mL of distilled water. 2 mL glacial acetic acid (CH3COOH, ≥ 96%, Merck) was added to this solution and heated at 80°C under stirring conditions. 30 mL of 8 M sodium hydroxide (NaOH, ≥ 99%, ACS grade, Merck) solution was added drop-by-drop into this stirring solution. While adding NaOH, the



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colour of the mixture was turned from greenish blue to dark brown to form a stable suspension. The dark brown suspension was cooled to room temperature and centrifuged several times with distilled water and ethanol to remove impurities and byproducts. Further, the purified material was kept for aging at 120°C for 48 h. The purified CuO nanopowder was grinded well and used for further characterization. 2.2 Characterization Techniques The optical properties of CuO nanocuboids were analysed using T90+ UV-Visible spectrophotometer (PG Instruments, UK) and Photoluminesence (PL) characterization was carried out using spectrofluorophotometer (RF-5301PC, Shimadzu, Japan). Raman spectra were acquired using an inVia Renishaw (UK) with He-Cd laser as the excitation light source at 514 nm and 1800 g/mm gratings in the backscattering configuration. X-ray diffraction (XRD) pattern was recorded using Seifert JSO Debyeflex (Japan), with CuKα radiation at a scanning rate of 0.02o/sec. The microstructure of synthesised CuO nanocuboids was characterized by High Resolution Transmission Electron Microscopy (JEOL, JEM-2010, Japan) at 200 kV attached with an energy dispersive spectrometer. The current-voltage (I-V) characteristics of CuO nanocuboids were analysed using digital multi-meter (Agilent, Model 34410A, USA) and source measuring unit (Keithely, modelSMU-2420, USA). X-ray fluorescence spectrums were acquired using X-ray fluorescence spectroscopy (EDX-720, Shimadzu, Japan). 2.3 Sensor fabrication and gas sensing measurements CuO nanocuboids based H2S gas sensor was fabricated by spin coating the material on Alumina substrate having pre-fabricated 16 finger gold inter-digitated arrays (IDA) electrode with a finger thickness of 200 µm, average spacing of 30 µm and sensing area is about 18 mm2. The CuO nanocuboids were dispersed in ethanol and coated over IDA transducer with a rotational speed of 1500 rpm. Further, the sensor was annealed at 200°C under air for 1 h.


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Figure 1. Schematic representation of in-house sensor test station used for the sensing property analysis of CuO nanocuboids based H2S gas sensor. The response of the sensor to H2S gas with various concentration ranges (2 to 10 ppm) was recorded using an in-house gas sensor test station fabricated in our laboratory. In brief, the sensor was mounted in a double walled stainless steel chamber (volume of 4500 cm3) with an in-built hot stage. The temperature was controlled by PID temperature controller (Eurotherm, 3216, UK). Required concentration of a H2S gas in the chamber was attained by introducing a derived quantity of H2S gas mixed with carrier gas (high pure Argon (Ar), 99.999%) using Owlstone gas generator unit (OVG-4, UK). The H2S gas pulse width was maintained as about 80s throughout experiments. The resistance of the sensor was continuously monitored as a function of time by an Agilent digital multimeter and Keithley source measuring unit with the help of LabVIEW based data acquisition system. The detailed schematics showing the experimental setup is shown in Figure 1. The recovery of the sensor was recorded by exposing it to normal atmospheric air. For reducing gas, relative sensor response (S) was calculated from the response curve using the relation,20



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S=

=

∆

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(1)

Where Rair is the initial electrical resistance of the sensor in normal air and Rgas is the measured realtime resistance upon exposure to H2S.

3. RESULTS AND DISCUSSION CuO nanocuboids were prepared by precipitation method. The possible CuO precipitate formation route is explained by the following chemical reactions,21,22 Cu + 2NaOH → CuOH + 2Na ∆

CuOH → CuO + H O

(2) (3)

The purified CuO nanopowder was further characterized by high resolution-transmission electron microscopy (HR-TEM). Figure 2a and 2b shows the TEM images of cuboid-like CuO nanostructures with an average length and width of 26 nm and 13 nm respectively. More detailed crystallographic analysis on the CuO nanocuboids were performed by selected area electron diffraction (SAED) and HR-TEM analysis. The d-spacing of CuO lattice was found to be 0.23 nm corresponding to the (111) crystal planes of CuO (Figure 2c). The SAED pattern of CuO nanocuboids demonstrated regular circular dot array (Figure 2d), indicating their polycrystalline nature. The 111, (111), (110) and (200) crystal planes observed in SAED patternconfirm the monoclinic phase of CuOnanocuboids The characteristics bands of CuO were observed in Raman spectra at 288, 337 and 622 cm−1, corresponding to the standard Ag (297 cm−1) and two Bg (345 cm−1 and 632 cm−1) modes respectively, which confirmed the phase purity of the synthesized CuO nanocuboids [Figure S1 in supporting information]. Similarly, X-ray diffraction pattern peaks [Figure S2 in supporting information] observed at 35.7° and 38.9° were assigned to the reflections of the 111and


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Figure 2. (a-b) Representative TEM images (c) high resolution TEM image and (d) SAED pattern of the CuO nanocuboids. Insets in (a) and (b) show the corresponding width and length distribution of CuO nanocuboids respectively.



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Figure 3. (a) High resolution TEM images of the CuO nanocuboid (b) Filtered image (c) corresponding FFT image and (d) simulated crystal structure of monoclinic CuO. (111) planes of monoclinic CuO with crystallite size of 15 nm, which matched with size distribution obtained from HR-TEM image analysis. The HR-TEM images (Figure 3a) of the nanocuboid show the wellresolved inter-planar distances of CuO as d(111) = 2.3 Å and d111=2.5 Å. The long axis of the nanocuboid was found to be parallel to the d(111) direction, suggesting the growth of nanorods along (111)


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direction (Figure 3b). For CuO, the surface with (111) plane has lowest specific surface energy, which is the energetically favourable surface for nucleation and growth.23 Figure 3c shows the corresponding Fast Fourier Transform (FFT) analysis of HR-TEM image of CuO nanocuboids. The angle between the 111direction and the long axis (111) was found to be ∼62°. As shown in Figure 3d, the (111) plane is likely to be terminated with a Cu or O atomic layer with a large d-spacing of 2.3 Å. The angle between (111) and 111planes of CuO nanocrystal was closely matching with simulated crystal structure. These results clearly depict the (111) surface as the predominant growth surface in the CuO nanocuboids. Especially, the (111) surface of CuO exhibits a strong catalytic activity towards the dissociation of H2S and O2.24,25 Therefore, it is reasonable to propose that the (111) surface with dangling bonds adsorb more H2S gas molecules, which make them as a good candidate for H2S gas sensing. Figure 4(a) inset shows the sensor response of the CuO nanocuboids towards 2 ppm H2S gas at different working temperatures. The resistance of the nanocuboids increased, when they were exposed to H2S gas environment, owing to their p-type characteristics. The sensor response towards 2 ppm of H2S was calculated to be 0.92 at 200°C. The optimum operating temperature of the CuO nanocuboids to H2S gas was found to be 200°C. In order to study the sensing properties of the CuO nanocuboids, a time dependent gas sensing studies against different concentrations of H2S at 200°C was carried out as shown in Figure 4(a). The relative sensor response as a function of concentration ranging from 1 to 10 ppm were calculated and depicted in Figure 4(b). As the reaction kinetics progressed with time, response of the sensor was perceived to be linear depending on the H2S concentrations as shown in Figure 4a and 4b. Enhanced sensor response was observed even at lower concentrations (2 ppm) of H2S gas suggesting the possibility of sub-ppm detection. The selectivity properties of the CuO nanocuboids were analyzed against cross-interfering gases which may exist in the atmosphere and are summarized in Figure S3 [See supporting information]. The gas responses towards 2 ppm of NH3, C5H8, SO2 and 4 ppm of C2H6O were found to be less (

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