Fabrication of Highly Sensitive Ethanol Chemical Sensor Based on

ARTICLE pubs.acs.org/JPCC

Fabrication of Highly Sensitive Ethanol Chemical Sensor Based on Sm-Doped Co3O4 Nanokernels by a Hydrothermal Method Mohammed M. Rahman,* Aslam Jamal, Sher Bahadar Khan, and Mohd Faisal Centre for Advanced Materials and Nano-Engineering (CAMNE) and Department of Chemistry, Faculty of Sciences and Arts, Najran University, P.O. Box 1988, Najran, 11001, Kingdom of Saudi Arabia ABSTRACT: Here, we report a large-scale synthesis of Sm-doped grain-shaped nanokernels of Co3O4 by a simple hydrothermal method at a low and active temperature of 150.0 °C. The synthesized Sm-doped Co3O4 nanokernels were characterized in detail in terms of their morphological, structural, and optical properties and efficiently applied as an ethanol chemical sensor. The morphological and structural examinations were executed by using field emission scanning electron microscopy (FESEM) attached with energy-dispersive spectroscopy, X-ray diffraction pattern, and Fourier transform infrared spectroscopy (FT-IR) measurements. It is noticed that the small nanostructures are assembled in such a simple approach that they acquired nanokernel-like morphologies. Detailed structural examinations also exposed that the calcined (at 400.0 °C) Sm-doped Co3O4 products are well-crystalline and comprising the face-centered cubic phase. The optical property, executed by UV
visible spectroscopy, demonstrated good optical properties for calcined Sm-doped Co3O4 nanokernels. Sm-doped Co3O4 is an attractive nanokernel to be employed in chemical sensing by a simple and reliable I
V method, where a toxic chemical (ethanol) is used as a target analyte. The ethanol sensor performances are explored, and the results displayed that the excellent sensitivity, constancy, stability, and reproducibility of the sensor improved comprehensively by Smdoped Co3O4 nanokernels with a fabricated surface coated with conducting binders onto silver electrodes (AgE). In the analytical investigation, the calibration plot is linear above the large concentration range (1.0 nM to 10.0 mM), where the sensitivity is around 2.1991 ( 0.10 μA cm
2 mM
1 with a very low detection limit (LOD) of 0.63 ( 0.02 nM based on the signal-to-noise ratio in a short response time. Consequently, on the basis of the sensitive features between structures, morphologies, and properties, it is confirmed that the morphologies and the optical behaviors can be improved to a large range in doped semiconductor nano-kernel and proficient chemical sensor applications in wide scale.

1. INTRODUCTION As one of the most interesting magnetic p-type semiconductors, the cobalt oxide nanostructure is recognized as an attractive material that has wide applications in various fields, such as doping, catalysts, solid-state sensors, and electrochemical devices.1
3 Doping is mostly operational to obtain high intelligibility, elevated electron communication, constancy, reliability, and high conductivity, which is typically suitable for applicable reasons. Sm-doped Co3O4 thin films have a prominent transmittance in the perceptible area and a low resistivity, where the large visual band gap can be controlled by using the Sm doping amount.4,5 Therefore, these have been achieved prospective applications in solar cells, antistatic coatings, solid-phase display devices, optical coatings, heaters, defrosters, biochemical sensors, etc.6
8 Various physical and chemical techniques have been utilized for samarium doping in cobalt oxides. These comprised magnetron sputtering,9 pulsed laser ablation,10 chemical beam deposition,11 sol
gel,12 electroless technique,13 spray pyrolysis,14 and others. The conventional physical techniques generally assembled goodquality perceptible and mesoporous films. Hence, they are extremely expensive and complicated to achieve in manufacturing. Chemical deposition techniques, on the other hand, are reasonably tiny charge techniques and can be basically scaled up r 2011 American Chemical Society

for manufacturing and developing purposes. Since the last two decades, chemical techniques have advanced to be a better substitute for material research in thin film nanostructure doped and undoped materials. Nanostructure materials of doped and undoped semiconductors have evoked much interest owing to their technological applications and attractive as well as exciting optical and structural properties.15 The distinctive physical and chemical properties of nanomaterials are owing to surface as well as size/shape effects; doped materials have been the subject of intense research, in terms of both scientific importance and industrial applications and present novel challenges for the physicist, technologist, scientist, chemist, etc.16
18 Multidimensional nanomaterials have drawn an enormous interest toward themselves owing to their extrinsic, remarkable and wonderful qualities in the electrical, optical, thermal, and mechanical properties as compared with their undoped materials. It is essential for the preparation of nanomaterials in order to achieve the exceptional quality of doped semiconductor structures. For these features, an extensive Received: March 9, 2011 Revised: April 3, 2011 Published: April 25, 2011 9503

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The Journal of Physical Chemistry C range of chemical and physical methods have been used for the preparation of Co3O4 nanomaterials, such as polymer ignition, gel technology, coprecipitation, ball-milling, chemical vapor deposition, and others.18
22 Transition-metal-doped semiconductor metal oxides uncovered several applications because of the attractive properties owing to the changeable oxidation state of the transition-metal-doped semiconductor nanomaterials. Additionally, the main purpose of extraordinary nano-structural morphology leading transition-metal oxides is the modification of magnetic and electrical properties as well as chemical properties. It is well recognized that Co3O4 has an ordinary nanostructure and doped Co3O4 exhibits antiferromagnetism in the very lower temperature.23 In nanoscience and nanotechnology, the innovative nanosensors with doped-materials have been utilized to control keyfeatures in fabrication and improvement of very precise, perceptive, accurate, sensitive, and stable sensors. The exploration for even tiny devices capable of nanolevel imaging and controlling of nanomaterial; doping material; biological, chemical, and pathological samples; and chemical sensors has recently expanded the spotlight of awareness of the scientist, mainly for control monitoring, owing to the increasing necessity for environmental safety and health monitoring.24,25 Doped semiconductor metal oxides are the model materials for sensing due to high active surface areas and are extensively employed as sensors for the detection, recognition, and quantification of various hazardous pollutants as well as toxic chemicals or biomolecules.26,27 The organic volatile chemical ethanol causes damage of the brain and specific diseases of the stomach, liver, and erythrocytes. Consequently it is significant and a big challenge to sense ethanol proficiently and protect human health from insidious diseases and save the environment using analytical techniques.28 A large number of metal oxides have been considered as chemical sensors for the detection of different hazardous pollutants and toxic chemicals.29,30 Therefore, Sm-doped Co3O4 nanostructures have been offered as a mediator to detect and detoxify the organic ethanol chemical. The main attention of this present investigation is to fabricate and develop a highly sensitive chemical sensor (especially for ethanol) for detecting and quantifying hazardous pollutants using Sm-doped Co3O4 semiconductor nanokernels. Hence, well-crystalline Sm-doped Co3O4 nanokernel materials were prepared by a simple hydrothermal method and characterized by UV/visible, FT-IR spectroscopy, XRD, FESEM, and EDS analysis. For the purpose of environmental examining and organizing of chemical processes, many attempts have been made to introduce easy, simple, economical, consistent, and reliable sensors.31,32 Semiconductor sensors, owing to their many fastidious benefits over conventional chemical analysis techniques, such as high response, low charge, and portability, are extensively employed for the detection of contaminated or toxic pollutants, chemical process control, and monitoring of air/water contamination in the environment.33 Here, a simple hydrothermal method is undertaken to arrange Sm-doped Co3O4 nanokernels with a practically controlled nanokernel shape structure, which exposed a constant morphological improvement in nanostructure materials and potential applications. With most of the efficient involvement displayed on undoped material, there has been more and more concentration dedicated to exploring the doped counterparts. Semiconductor nanomaterials especially Sm-doped Co3O4 nano-kernel offer very sensitive transduction of the liquid/surface interactions to change the chemical as well as optical properties. The opportunity is to emergence a

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variety of structural morphologies proposes different prospects of modification the chemical-sensing possessions. A Sm-doped Co3O4 nanokernel is fabricated into a simple and efficient chemical sensor consisting of a side-polished silver electrode surface, and we measured the chemical-sensing performance considering ethanol at room conditions. To the best of our knowledge, this is the first report for detection of ethanol with calcined Sm-doped Co3O4 nanokernels using a easy, simple, convenient, and reliable I
V method in a short response time.

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents and Apparatus. Ethanol, samarium chloride (SmCl3 3 6H2O), cobalt chloride (CoCl2), ethyl acetate, disodium phosphate, butyl carbitol acetate, ammonia hydroxide (25%), monosodium phosphate, and all other chemicals used were of analytical grade and purchased from Sigma-Aldrich Company. A stock solution of 1.0 M ethanol was made in double distilled water. The calcined Sm-doped cobalt oxide nanokernel was investigated with UV/visible spectroscopy (Lamda-950, PerkinElmer, Germany). FT-IR spectra were measured for calcined doped nanomaterials with a spectrophotometer (Spectrum-100 FT-IR) in the mid-IR range, which was obtained from PerkinElmer, Germany. The powder X-ray diffraction (XRD) prototypes were evaluated with an X-ray diffractometer (XRD, X’Pert Explorer, PANalytical diffractometer) prepared with Cu
KR1 radiation (λ = 1.5406 nm) using a generator voltage of 40 kV and current of 35 mA applied for the measurement. The morphology of Smdoped Co3O4 nanokernels was examined on an FE-SEM instrument (FESEM, JSM-7600F, Japan). Elemental analysis was investigated using EDS from JEOL, Japan. The I
V technique was employed by an Electrometer (Kethley, 6517A, Electrometer, USA) in room conditions. 2.2. Synthesis of Sm-Doped Co3O4 Nanokernels. The largescale production of Sm-doped Co3O4 nanokernels was accomplished by a simple hydrothermal process at low temperature by using samarium chloride (SmCl3 3 6H2O), cobalt chloride (CoCl2), and ammonium hydroxide (NH4OH). In a typical reaction process, 0.1 M SmCl3 3 6H2O was dissolved in 50.0 mL of deionized (DI) water and then mixed with 50.0 mL of a DI solution of 0.1 M CoCl2 under continuous stirring. The pH of the solution was adjusted to 10.1 by the addition of NH4OH, and the resultant mixture was stirred continuously for 10 min at room temperature. After stirring, the resultant solution was then heated for 16 h in a Teflon autoclave in the oven. The heating temperature of the solution was controlled manually throughout the reaction process, where the oven temperature reached 150.0 °C. After 16 h, the reaction was completed, and the solution was kept for cooling at room conditions. The final products were obtained as a precipitate, which was washed with DI water, ethanol, and acetone several times sequentially and dried at roomtemperature, for further structural and optical characterizations. The growth mechanism of the nanokernels can be determined on the basis of chemical reactions and nucleation, as well as the growth of Co3O4 crystals. The perceptible reaction mechanisms are proposed for achieving the doped nanomaterial oxides, which are added below.
þ
SmCl3 3 6H2 OðsÞ f Sm3þ ðaqÞ þ 3ClðaqÞ þ 6HðaqÞ þ 6OHðaqÞ ðiÞ

9504


NH4 OHðaqÞ f NH4 þ ðaqÞ þ OHðaqÞ

ðiiÞ


CoCl2ðsÞ f Co2þ ðaqÞ þ 2ClðaqÞ

ðiiiÞ

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Figure 1. (a) UV/visible spectroscopy, (b) plot for band-gap energy (Ebg), (c) FT-IR, and (d) Raman spectroscopy of calcined Sm-doped Co3O4 nanokernels.
3Co2þ ðaqÞ þ 8OHðaqÞ f Co3 O4ðaqÞ þ 4H2 O 2þ
Sm3þ ðaqÞ þ CoðaqÞ þ 8OHðaqÞ f Sm 3 Co3 O4ðsÞ þ 4H2 O

ðivÞ ðvÞ

SmCl3ðaqÞ þ 4NH4 OHðaqÞ þ 3Co2þ ðaqÞ f Sm 3 Co3 O4ðsÞ þ 4NH4 þ þ 2Cl
ðviÞ ðaqÞ þ H2 O The precursors of SmCl3 3 6H2O and CoCl2 are soluble in alkaline medium (NH4OH reagent) according to eqs i
iii. After adding NH4OH into the mixture of metal oxides solution, it was stirred vigorously for 10 min at room temperature. The solution pH was then adjusted (10.1) using NH4OH and put into a Teflon autoclave cell and placed in an oven at 150.0 °C for 16 h. In this stage, the reaction is improved slowly, according to eqs iv
vi. After cooling, the desired precipitate was acquired, and then the solution was washed thoroughly with acetone and kept for drying at room conditions. During the total synthesis process, NH4OH acts a pH buffer to control the pH value of the solution and the slow donation of OH
. When the concentrations of the Co3þ and OH
ions go above a critical value, the precipitation of Co3O4 nuclei starts. As there is a high concentration of Sm3þ ions in the solution, the nucleation of Co3O4 crystals becomes easier due to the lower activation energy barrier of heterogeneous nucleation. However, as the concentration of Sm3þ is sustained, a number of larger Co3O4 crystals with a particle-like morphology form among the nanostructures. The shape of calcined Co3O4 particles is approximately reliable with the growth pattern of Co3O4 crystals.34 Finally, the as-grown Sm-doped Co3O4 nanostructure products were calcined at 400.0 °C for 5 h in the furnace (Barnstead Thermolyne, 6000 Furnace, USA). The calcined doped nanomaterials were characterized in detail in

terms of their morphological, structural, and optical properties, and were applied for ethanol sensing. 2.3. Fabrication and Detection of Ethanol Using SmDoped Co3O4 Nanokernels. Phosphate buffer solution (PBS, 0.1 M, pH 7.0) is made by mixing 0.2 M Na2HPO4 and 0.2 M NaH2PO4 solution in 100.0 mL of deionized water. The silver electrode (AgE, surface area = 0.0216 cm2) is fabricated by calcined Sm-doped Co3O4 nanokernels with butyl carbitol acetate (BCA) and ethyl acetate (EA) as a conducting binding agent. It is then transferred into the oven at 65.0 °C for 12 h until the film is completely uniform and dried. An electrochemical cell is constructed with Sm-doped Co3O4 nanokernel coated AgE as a working electrode, and Pd wire is used as a counter electrode. Ethanol (1.0 M) is diluted at different concentrations in DI water and used as a target chemical. The amount of 0.1 M PBS is kept constant in the beaker at 10.0 mL throughout the chemical analysis. The analyte solution is produced with various concentrations of ethanol from 1.0 nM to 1.0 M. The sensitivity is calculated from the slope of voltage versus current from the calibration plot. An electrometer is used as a voltage source for the I
V method in the two-electrode system.

3. RESULTS AND DISCUSSION 3.1. UV/visible, FT-IR, and Raman Spectroscopies. The optical property of the calcined Sm-doped Co3O4 nanokernels is one of the important characteristics for the evaluation of their photocatalytic activity. The optical absorption spectra of Smdoped Co3O4 nanostructure materials are accomplished by using a UV
vis spectrophotometer in the visible range (200.0
800.0 nm). UV/visible absorption is a method in which the outer electrons of atoms or molecules absorb radiant energy and 9505

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undergo transitions to high energy levels. In this procedure, the spectrum obtained owing to optical absorption can be analyzed to acquire the energy band gap of the semiconductor nanomaterials. The optical absorption measurement was carried out at ambient conditions. From the absorption spectrum, it has been analysized that the lower cutoff wavelength for the calcined Smdoped Co3O4 is about 241 nm, which is presented in Figure 1a. Also, the spectrum does not show any absorption peak in the wavelength range of 400
700 nm. Therefore, the doped material may be helpful for the development of nonlinear optical sensors in this wavelength region, as the lack of absorption peaks is the major prerequisite for the nanomaterials to confirm nonlinear properties. By using the UV/visible spectrum, the optical band gap of nanomaterials was measured by Tauc’s formula, which shows the relationship among the absorption coefficient and the incident photon energy of Sm-doped Co3O4 semiconductors. The Tauc’s equation (eq vii) is presented below as Rhν ¼ Aðhν
Eg Þn

ðviiÞ

where R is the absorption coefficient, A is a constant, and n is equal to 1/2 for a direct transition semiconductor and 2 for indirect transition semiconductors. Accordingly, to calculate the optical band gap (Ebg) for the calcined Sm-doped Co3O4, a plot of (Rhν)2 against the incident photon energy (hν) has been displayed in Figure 1b. The direct band-gap energy was estimated by extrapolating the straight-line segment of the plot “(Rhν)2” versus “(hν)” to a zero absorption coefficient value. From Figure 1b, the value of the band gap (Ebg) was found to be 3.57 eV. As can been observed, the optical band gap of Sm-doped Co3O4 nanokernels is 3.57 eV, which is in good conformity with the doped cobalt oxide band structure.35,36 This proposes that the calcined Sm-doped Co3O4 nanokernel is semiconducting with direct transitions at the energy. The Ebg is accredited to the interband doped transition and considered as the accurate energy gap. The calcined Sm-doped Co3O4 nanokernels are also characterized from the atomic and molecular vibrations. To predict the motivated recognitions distinctly, FT-IR spectra simply in the region of 400
4000 cm
1 are investigated. Figure 1c exhibits the FT-IR spectrum of the calcined Sm-doped Co3O4 nanomaterials. It presents bands at 665, 748, 1630, and 3486 cm
1. These observed vibration bands (at 665 and 748 cm
1) could be assigned as metal
oxygen (Sm
O and Co
O modes) stretching vibrations,37 which confirm the formation of doped nanostructure materials. The additional observed vibration bands may be assigned to O
H stretching (3486 cm
1) and O
H bending (1630 cm
1) vibrations. The absorption bands at 1630 and 3486 cm
1 normally exhibit from water, which is usual for semiconductor nanomaterials. Finally, the observed vibration bands at low-frequency regions suggested the formation of Smdoped Co3O4 nanokernels. Raman spectroscopy is a spectroscopic method utilized to reveal vibrational, rotational, and other low-frequency phases in a Raman-active compounds. It depends on inelastic scattering, or Raman scattering, of monochromatic light, generally from a laser in the visible, near-infrared, or near-ultraviolet ranges. The laser light communicates with molecular vibrations, phonons, or other excitations in the modes, showing in the energy of the laser photons being shifted up or down. The shift in energy represents information regarding the phonon modes in the system, where

Figure 2. (a) Powder X-ray diffraction pattern, (b, c) low- to highmagnified FE-SEM images, and (d, e) energy-dispersive spectroscopy of calcined Sm-doped Co3O4 nanokernels.

infrared spectroscopy yields similar, but complementary, information. Raman spectroscopy is generally established and utilized in material chemistry, because the information is specific to the chemical bonds and symmetry of metal
oxygen stretching or vibrational modes. Figure 2d confirms the Raman spectrum where key aspects of the wavenumber are employed at about 485 cm
1 (Eg) and 792 cm
1 (A1g) for metal
oxygen (Sm
O and Co
O) stretching vibrations. These large bands can be assigned to a cubic phase of Sm-doped Co3O4 nanokernels. At 792 cm
1, higher wavenumber shift is revealed owing to the different dimensional effects of the nanokernels.38 3.2. XRD, FESEM, and EDS Analyses. The crystallinity and crystal phases of the calcined Sm-doped Co3O4 nanokernels were investigated. X-ray diffraction patterns of doped nanokernels are represented in Figure 2. The Sm-doped Co3O4 nanokernel samples were examined and exhibited as face-centered cubic shapes. The doped sample was calcined at 400.0 °C in the furnace to start the formation of nanocrystalline phases. Figure 2a reveals the characteristic crystallinity of the calcined Sm-doped Co3O4 nanokernels and their aggregative arrangement. All the reflection peaks in this prototype were initiated to correspond with the Sm-doped Co3O4 phase having a face-centered cubic geometry (JC-PDF no. 076-1802). The phases demonstrated the key feature peaks with indices for calcined crystalline Sm-doped Co3O4 at 2θ values of 31.96(220), 34.86(311), 36.89(222), 45.89(400), 53.23(422), and 65.31(440). The face-centered cubic lattice parameters are 8.072, with point group Fd3m, using Cu KR1 radiation (λ = 1.5406). These confirmed that there is a major number and amount of crystalline Sm-doped Co3O4 present in nanokernels.39 High-resolution FESEM images of calcined Sm-doped Co3O4 nanokernels are displayed in Figure 2b,c. The images are 9506

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The Journal of Physical Chemistry C composed of nanostructure materials with an aggregated nanostructure in kernel (grain) shapes. The average length of the Smdoped Co3O4 nanokernels is 213.68 nm in the range of 135.48
310.48 nm, which is presented in Figure 2b. The average diameter is also calculated in the range of 22.0
39.0 nm, which is close to 27.0 ( 5 nm. In Figure 2c, it is exhibited noticeably from the FESEM images that the result of the simple solution methodology of synthesized doped products are nanostructures of Smdoped Co3O4, which flourished in an extraordinary shape, in highdensity, and obtained a nanostructure in round kernel shapes. It is also suggested that almost all of the nanostructures have spherical grain-like shapes of the aggregated Sm-doped Co3O4 nanokernels. The energy-dispersive spectroscopy (EDS) investigation of Sm-doped Co3O4 nanokernels indicates the presence of Sm, Co, and O in the pure calcined doped material. It is clearly displayed that calcined synthesized materials controlled only samarium, cobalt, and oxygen elements, which presented in Figure 2d,e. The composition of samarium, cobalt, and oxygen is 52.56, 25.99, and 21.45%, respectively. No other peak related with any impurity has been detected in the EDS, which confirms that the nanokernel doped products are composed of only samarium, cobalt, and oxygen. 3.3. Detection of Ethanol Using Sm-Doped Co3O4 Nanokernels. The potential application of Sm-doped Co3O4 nanokernels as chemical sensors (especially for ethanol) has been explored for measuring and detecting hazardous and risky chemicals, which are not environmental friendly. Improvement of doping of these nanokernel materials as chemisensors is in the primary stage, and only insufficient quantities of reports are available.40 The nanokernels of Sm-doped Co3O4 sensors have advantages, such as consistency in air, nontoxicity, chemical stability, electrochemical activity, simplicity to assemble or construct, and biosafe characteristics. As in the case of ethanol sensors, the main reason is that the current response in the I
V method of Sm-doped Co3O4 nanokernels significantly changes when aqueous ethanol is adsorbed. The calcined Smdoped Co3O4 nanokernels were employed for modification of chemical sensors, where ethanol was measured as the target analyte. The fabricated surface of the Sm-doped Co3O4 nanokernel sensor was prepared with conducting binders on the silver electrode surface, which is presented in Figure 3a. The fabricated electrode was moved into the oven at low temperature (65.0 °C) for 12 h to make it smooth, dry, stable, and with a totally uniform surface. Theoretical I
V signals of the chemical sensor are expected with doped thin film as a function of current versus potential for ethanol, which is presented in Figure 3b. The electrical responses of target ethanol are investigated by a simple and reliable I
V technique using Sm-doped Co3O4 nanokernel fabricated AgE film, which is presented in Figure 3c.The time holding of the electrometer was set for 1.0 s. A considerable amplification in the current response with applied potential is perceptibly confirmed. The simple, reliable, possible reaction mechanism is generalized in Figure 3d in the presence of ethanol on Sm-doped Co3O4 sensor surfaces by the I
V method. The ethanol is converted to water and carbon dioxide in the presence of doped semiconductor nanomaterials by releasing electrons (6e
) to the reaction system (conduction band, C.B.), which improved and enhanced the current responses against potential during the I
V measurement at room conditions. Figure 4a displays the current responses without (gray-dotted) and with (dark-dotted) coating of Sm-doped Co3O4 nanokernels on AgE working electrode surfaces. With the nanokernel fabricated surface, the current signal is reduced compared with that

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Figure 3. Schematic views of (a) fabricated with Sm-doped Co3O4 nanokernels with conducting binders (EC and BCA), (b) I
V detection methodology (theoretical), (c) outcome of I
V experimental result, and (d) reaction mechanism of ethanol in the presence of semiconductor Sm-doped Co3O4 nanokernels.

without the fabricated surface, which indicates that the surface is slightly inhibited with doped nanomaterials. The current changes for the nanomaterial-modified film before (gray-dotted) and after (dark-dotted) injecting of 50.0 μL of ethanol (1.0 nM) in 10.0 mL of PBS solution, which is presented in Figure 4b. This considerable change of surface current is investigated in every injection of the target ethanol into the electrochemical solution by the electrometer. A 10.0 mL portion of the 0.1 M PBS solution is originally transferred into the cell, and the low to high concentration of ethanol was added dropwise consecutively from the store solution. I
V responses with the calcined Sm-doped Co3O4 nanokernel-modified electrode surface are evaluated from the different concentrations (1.0 nM to 1.0 M) of ethanol, which is revealed in Figure 4c. It shows the current changes of fabricated films as a function of ethanol concentration in room conditions. It is also observed that, at low to high concentration of the target ethanol, the current response is improved gradually. The apparent current changes at a higher potential range (potential, þ1.0 V to þ1.5 V) based on analyte concentration are magnified and presented clearly in Figure 4d. A large range of analyte concentration is selected to examine the probable analytical limit, which is calculated in 1.0 nM to 1.0 M. The calibration curve was plotted from the variation of ethanol concentrations, which is presented in Figure 4e. The sensitivity is calculated from the calibration curve, which is close to 2.1991 ( 0.10 μA cm
2 mM
1. The linear dynamic range of this sensor displays from 1.0 nM to 10.0 mM (linearity, R = 0.9065), and the detection limit was considered as 0.63 ( 0.02 nM [3  noise (N)/slope(S)]. Generally, the value of resistance for the nanomaterial dopedsensors are decreased with increasing the electron communication features, which is the fundamental characteristics of the semiconductor nanomaterials at room conditions.41 Actually, oxygen adsorption demonstrates an important responsibility in the electrical properties of the Sm-doped Co3O4 with nanokernel structures. Oxygen ion adsorption removes the conduction electrons and increases the resistance of Sm-doped Co3O4 nanokernels. Reactive oxygen species such as O2
and O
are adsorbed on the doped material surface, where the quantity of such chemisorbed oxygen species strongly depends on structural as well as physical properties of doped semiconductor nanomaterials. At room conditions, O2
is chemisorbed, while in the nanokernel morphology, O2
and O
are chemisorbed, and the O2
disappears rapidly.42 Here, the ethanol-sensing mechanism of the Smdoped Co3O4 sensor is originated based on the semiconductor oxides, due to the oxidation or reduction of the semiconductor oxide 9507

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Figure 5. Mechanism of Sm-doped Co3O4 nanokernel ethanol sensors at ambient conditions.

Figure 4. I
V responses of (a) with and without the coating, (b) in the absence and presence of 1.0 nM ethanol in 10.0 mL of PBS solution, (C) concentration variations (1.0 nM to 1.0 M) of the analyte, and (d) magnified plot of concentration variation at þ1.2 to þ1.5 V. (e) Calibration plot of Sm-doped Co3O4 nanokernels fabricated on AgE surfaces. Potential was taken between 0.0 and þ1.5 V.

nanokernels, according to the dissolved O2 in the bulk solution or surface air of the neighboring atmosphere, according to eqs viii and ix. e
ðSm-doped Co3 O4 Þ þ O2 f O
2

ðviiiÞ


e
ðSm-doped Co3 O4 Þ þ O
2 f 2O

ðixÞ

These reactions are attained in the bulk system, air/liquid interface, or adjacent atmosphere owing to the small carrier concentration, which improved the resistance. The ethanol sensitivity toward Sm-doped Co3O4 (e.g., MOx) could be attributed to the high oxygen deficiency, which increases the oxygen adsorption. The larger the quantity of oxygen adsorbed on the sensor surface, the larger would be the oxidizing potentiality and the faster would be the oxidation of ethanol. The reactivity of ethanol would have been very large as compared with other chemicals with the surface under indistinguishable conditions.43
45 When ethanol reacts with the adsorbed oxygen on the exterior/interior of the layer, it oxidized to carbon dioxide and water, releasing free electrons (6e
) in the conduction band, which could be expressed through the following reaction x.
CH3 CH2 OHðadsÞ þ 6O
ðadsÞ f 2CO2 þ 3H2 O þ 6e ðC:B:Þ

ðxÞ

In the reaction medium, these reactions referred to oxidation of the reducing carriers. These methods improved the carrier concentration and, consequently, decreased the resistance on contact to reducing liquids/analytes. At the room conditions, the exposure of the metal oxide surface to reducing liquid/analytes results in a surface-mediated incineration procedure. The abolition of ionosorbed oxygen amplifies the electron concentration and hence the surface conductance of the film.46 The reducing analyte (ethanol) provides electrons to the Sm-doped Co3O4 nanokernel surface. Consequently, the resistance is reduced; and hence, the conductance is increased. This is the reason why the analyte response (current) amplifies with escalating potential. Thus produced electrons contribute to the rapid increase in conductance of the thick film. The Sm-doped Co3O4 unusual regions dispersed on the surface would improve the capability of the material to absorb more oxygen species, giving high resistance in air ambient, which is presented in Figure 5. In another approach, the utmost ethanol response of nanokernels was accredited to the larger chemical communication with the sensing surface owing to the greater surface area and mesoporous capacity. The high ethanol response of the Smdoped Co3O4 nanokernels can be investigated in more detail relative to the probable chemical-sensing mechanism. It is a p-type semiconductor nanomaterial. The oxide surface of a p-type semiconductor is readily covered with chemisorbed oxygen.47 Therefore, at the sensing condition, the adsorption of negatively charged oxygen can generate the holes for conduction. The subsequent ethanol-sensing reactions might be considered according to the charges of the adsorbed oxygen species under the statement of full oxidation of C2H5OH according to the following eqs xi and xii. 1 o O2ðg=lÞ T O
ðadsÞ þ h 2 o C2 H5 OHðgÞ þ 6O
ðadsÞ þ 6h f 2CO2ðgÞ þ 3H2 OðgÞ

ðxiÞ ðxiiÞ

That is, the oxidation reaction with reducing ethanol amplifies the resistivity of the surface regions of the p-type Sm-doped Co3O4 nanokernels, which, in turn, enhances the sensor resistance. The resistive contacts among the nanomaterials control the chemisensor resistance. Accordingly, the ethanol response is significantly dependent upon the dimensions of the nanokernels, the large active surface area, and the nanoporosity. According to the charge accumulation reproduction of p-type semiconductors, the conduction occurs along the conductive as well as active sensor surface.48
50 The response time was approximately 10 s for the Sm-doped Co3O4 nanokernel coated sensor to achieve a saturated steadystate current. The prominent sensitivity of the sensor can be attributed to the good absorption (porous surfaces fabricated 9508

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The Journal of Physical Chemistry C with binders), adsorption ability, high catalytic activity, and good biocompatibility of the Sm-doped Co3O4 nanokernels. The expected sensitivity of the fabricated sensor is relatively better than previously reported ethanol sensors based on other composite or material modified electrodes.51,52 Owing to the profound surface area, the nanomaterials proposed a beneficial nanoenvironment for the chemical detection and recognition with excellent quantity. The prominent sensitivity of Sm-doped Co3O4 nanokernels affords high electron communication features that improved the direct electron movement between the active sites of nanokernels and electrodes. The modified thin Sm-doped Co3O4 nanokernel sensor film had a better reliability as well as stability. Furthermore, due to the high dynamic surface area, the nanokernels of Sm-doped Co3O4 imposed favorable atmospheres/surroundings for the ethanol detection (by adsorption) with large quantity. The high sensitivity of nanokernels presented high electron communication features that improved the direct electron communication between the active sites of Sm-doped Co3O4 and the AgE electrode. Smdoped Co3O4 nanokernels exhibit several approaches in providing chemical-based sensors, and encouraging improvement has been accomplished in the research section. Despite this development, there are still a number of important apprehensions that are in need of additional investigation before the production of this sensor can be expanded commercially for the established applications. As for the nanokernels, Sm-doped Co3O4 materials propose a method to an original fabrication of chemical sensors and a premeditative endeavor has to be promoted for doped nanostructure materials to be used extensively for large-scale purposes.

4. CONCLUSIONS The simple hydrothermal method is an extensive proposition to prepare Sm-doped cobalt oxide nanokernels with the most exceptional morphologies. Such a controlled doping has a broad postulation and makes it difficult to understand the crystalline and electronic properties of Sm-doped Co3O4. We also efficiently assembled an enormously sensitive ethanol sensor based on a calcined Sm-doped Co3O4 ingrained AgE with conducting coating binders for the first time. Sm-doped Co3O4 nanokernels are fundamentally produced using reducing mediators in the alkaline phase, which displays an easy, simple, efficient, reliable, and inexpensive approach. The performance of the developed highly sensitive ethanol sensor is excellent in terms of sensitivity, detection limit, linear dynamic ranges, and in a short response time. The Sm-doped Co3O4 nanokernel structures have been prepared by a very easy and simple solution method with a low cost and displayed high sensitivity for ethanol sensing. Therefore, it is concluded that the chemical-sensing properties of doped nanomaterials are of enormous significance for the application of Sm-doped Co3O4 efficient chemical sensors. The ethanol chemical sensor is carried out by a reliable I
V technique in room conditions, and the analytical parameters were inspected thoroughly in terms of response time, sensitivity, limit of detection, and storage ability. These involvements accessed significant research activity regarding the synthesis, structural and optical characterization, and chemical-sensing application of Sm-doped cobalt oxide. The crystalline structure, shapes, optical properties, and band gap were executed by XRD, FESEM, and UV
visible methods. This original attempt resulted in a well-organized reliable technique of proficient chemical sensor development for environmental hazardous chemicals and biomedical health care fields in large scale.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] and [email protected]. Phone: þ00966-59-6421830.

’ ACKNOWLEDGMENT The authors are thankful to the Deanship of Scientific Research, Najran University, Najran, Kingdom of Saudi Arabia, for all financial support. The Centre for Advanced Materials and Nano-Engineering (CAMNE), Najran University, Najran, is highly acknowledged. ’ REFERENCES (1) Li, Y. G.; Tan, B.; Wu, Y. Y. J. Am. Chem. Soc. 2006, 128, 14258. (2) Yu, Y.; Chen, C. H.; Shui, J. L.; Xie, S. Angew. Chem., Int. Ed. 2005, 44, 7085. (3) Xu, R.; Wang, J.; Li, Q.; Sun, G.; Wang, E.; Li, S.; Gu, J.; Ju, M. J. Solid State Chem. 2009, 182, 3177. (4) Jeong, S. H.; Park, B. N.; Yoo, D. G.; Boo., J. H. J. Korean Phys. Soc. 2007, 50, 622. (5) Takamura, H.; Koshino, Y.; Kamegawa, A.; Okada, M. Solid State Ionics 2006, 177, 2185. (6) Shi, X.; Han, S.; Sanedrin, R. J.; Galvez, C.; Ho, D. G.; Hernandez, B.; Zhou, F.; Selke, M. Nano Lett. 2002, 2, 289. (7) Jiao, F.; Frei., H. Angew. Chem., Int. Ed. 2009, 48, 1841. (8) Nishino, J.; Ohshio, S.; Kamata, K. J. Am. Ceram. Soc. 1992, 75, 3469. (9) Lee, Y. E.; Kim, Y. J.; Kim., H. J. J. Mater. Res. 1998, 13, 1260. (10) Srikant, V.; Sergo, V.; Clarke, D. R. J. Am. Ceram. Soc. 1995, 78, 1935. (11) Sato, H.; Minami, T.; Takata, S.; Miyata, T.; Ishii, M. Thin Solid Films 1993, 236, 14. (12) Ohyama, M. J. Am. Ceram. Soc. 1998, 81, 1622. (13) Raviendra, D.; Sharma, J. K. J. Appl. Phys. 1985, 58, 838. (14) Aktaruzzaman, A. F.; Sharma, G. L.; Malhotra, L. K. Thin Solid Films 1991, 198, 67. (15) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. (16) Morgado, E. B.; Marinkovic, A.; Jardim, P. M.; de-Abreu, M. A. S.; Rizzo, F. C. J. Solid State Chem. 2009, 182, 172. (17) Si, S. F.; Li, C. H.; Wang, X.; Peng, Q.; Li, Y. D. Sens. Actuators, B 2006, 119, 52. (18) Awschlom, D. D.; Smyth, J. F.; Grinstein, G.; DiVincenzo, D. P.; Loss, D. Phys. Rev. Lett. 1992, 68, 3092. (19) Callegari, A.; Tonti, D.; Chergui, M. Nano Lett. 2003, 3, 1565. (20) Lai, T.; Lai, Y.; Lee, C.; Shu, Y.; Wang, C. Catal. Today 2008, 131, 105. (21) Ahmed, J.; Ahmad, T.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Colloid Interface Sci. 2008, 321, 434. (22) Jiu, J.; Ge, Y.; Li, X.; Nie, L. Mater. Lett. 2002, 54, 260. (23) Niederberger, M.; Garnweitner, G.; Buha, J.; Polleux, J.; Ba, J.; Pinna, N. J. Sol
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