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Nanographite assembled films for sensitive NO2 detection
Sensors and Actuators B 161 (2012) 359–365
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nanographite assembled films for sensitive NO2 detection A. Serra ∗ , A. Buccolieri, E. Filippo, D. Manno Dipartimento di Scienza dei Materiali, Lab. Fisica delle Nanostrutture, Università del Salento, Via Monteroni, 73100 Lecce, Italy
a r t i c l e
i n f o
Article history: Received 17 June 2011 Received in revised form 17 October 2011 Accepted 18 October 2011 Available online 25 October 2011 Keywords: Nanographite Raman Gas sensor
a b s t r a c t Carbon based nanostructures are interesting building blocks to realize films for a lot of applications that include bio- and chemical-sensing. In this work we present a one-step electrochemical method to produce water-based stable nanographite colloid used to realize lined-up micro-patterned films. The high surfaceto-volume ratio makes these structures suitable for adsorbing gases. Morphological, compositional and structural characterization have been performed in order to determine the features of the films. Simple resistive nanographite devices were fabricated and tested for NO2 sensitivity. Moreover, the present approach represents a general and facile method of good potential for scale-up that could be extended to other materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the past few years the nano-structured carbon materials, have attracted much interest because of their potential applications in display [1], heat transfer media [2], fuel cell and secondary battery [3], shielding materials for electrostatics and electron wave [4], functional composite materials [5], and semiconductor and anti-wear materials [6]. Graphene and nanographite, as a low dimensional assembly of sp2 -bonded carbon atoms in a closely packed honeycomb, are of the most fascinating nanostructures with interesting physical, chemical, electrical and mechanical properties which qualify it as a promising nanomaterial in condensed-matter and high-energy physics, material science, and a wide range of technological applications, such as bioelectronics and biosensing [7–14]. However, the applications of carbon based materials are limited because they are easily aggregated each other due to strong van der Waals attraction forces that may frustrate the assembly of specific structures. To obtain a stable dispersion, an additive or chemical reagent is generally required [15]. Several authors [16,17] prepared graphite particle-based nanofluid with the assistance of electrochemical oxidation method, where anodic erosion occurred at the surface of graphite electrode immersed in ethanoic acid, sulfuric acid, and deionized water. The size range of oxidized graphite particles obtained was 40–870 nm in diameter. Unlike those previous studies, we simply immersed electrodes into deionized water without adding any surfactants in the combined electrochemical and sono-chemical processes, and
∗ Corresponding author. Tel.: +39 0832 297070; fax: +39 0832 297069. E-mail address: [email protected] (A. Serra). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.10.045
then we obtained stable carbon particle-based nanofluid with narrow primary size distribution; moreover, the production rate of our carbon nano colloid (CNC) fluid was soon reached. In this work we develop a single route electrochemical synthesis of nanographite particles in aqueous solution combined with subsequent chemical bath deposition (CBD). The obtained thin films reveals an interesting lined-up microstructure that provides a great potential for gas sensing applications. The simple resistive nanographite devices were fabricated and tested for NO2 sensitivity. Moreover, the present approach is a general and facile method with good potential for scale-up that could be extended to other materials. 2. Experimental Firstly nanographite colloidal solution was obtained in the following route. High purity graphite rod (SPI 99.99% purity, 5 mm diameter) was used as anode, and a stainless steel rod (SWAGELOK AISI 1016, 5 mm diameter) was used as cathode. The electrodes were immersed in a becker containing 200 mL deionized water (Millipore Milli-Q, 18.2 M cm) at a reciprocal distance of 10 mm. In the electrolysis process, the electric power applied to the electrodes was a constant voltage of 30 V. Simultaneously, the solution was forcedly dispersed by an ultrasonicator, which was the flat-type ultrasonic equipment (Flexonic-1200-35/72/100G, Mirae Ultrasonic Tech., Korea). To prevent the aggregation of nanoparticles, the ultrasonicator was continuously operated at the power output of 1000 W with the frequency of 100 ± 5 kHz during the production of nanographite. In this way we obtain a colloidal solution consisting of nanographite particles homogeneously dispersed in water. Then, nanographite particles were deposited on Corning glass commercial slide plates using chemical bath deposition (CBD)
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dynamic flow system implemented in our laboratory. As the carrier and the reference gases were used both air and argon at ambient pressure (RIVOIRA, 99.99% pure). Test measurements consisted of set of cycles in which increasing test gas concentrations at a flow of 100 sccm were separated by exposures to the reference gas. The relative humidity measured by means of a Keithley 6517A electrometer equipped with a 6517RH probe, at the top of test chamber, was of 30% at 25 ◦ C. Before test measurements, the samples were exposed to argon for 4 h at 200 ◦ C in the test chamber. Measurements were performed onto 5 different samples, in order to verify the repeatability of experiments. The sensor performance reported in the present work are the average values obtained by repeating 20 times the measurements. The reported error is the corresponding standard deviation. For evaluating the magnitude of the conductivity changes produced by the individual gas exposures, the resistance values measured at the end of successive dwell intervals in dry clean air and in reactive gas, respectively, were used. In this way, values of sensor response could be obtained, which were unaffected by time constants for the filling and emptying of the sensor chamber and also by temperature changes that might have occurred upon the switching of mass flow controllers. 3. Results Fig. 1. The experimental set up for chemical bath deposition (CBD) method, employed for the growth of the films.
method. The experimental set up employed for the growth of the films can be seen from Fig. 1. The substrates were suspended vertically in the colloidal solution which was maintained at 50 ◦ C on a hot plate to facilitate evaporation of the solvent. The solution was stirred with the help of magnetic stirrer to avoid agglomeration of nanoparticles. As the solvent evaporates, carbon nanoparticles selfassemble into linear structures parallel to the solvent surface giving rise to homogeneous films. The average thickness of the films was measured by a TENCOR Alpha-Step profilometer. Atomic force microscopy (AFM) images were acquired by a Jeol 4210 STM-AFM microscope. The image was acquired by contact mode by using a cantilever with force constant of 0.15 N/m and a silicon carbide tip with radius of curvature less than 10 nm. All observations were performed in air at room temperature. Transmission electron microscopy (TEM) images and electron diffraction patterns were taken using an Hitachi 7100, at 100 kV representing the suitable acceleration voltage to obtain a good resolution and minimal radiation damage of the material. The specimens were prepared for transmission electron microscope observations by placing small droplets of nanographite solutions onto standard carbon supported 600-mesh copper grid and drying slowly in air naturally. Raman scattering measurements were obtained by backscattering geometry with a RENISHAW spectrometer coupled to a LEICA metallographic microscope. An argon-ion laser operated at a wavelength of 514.5 nm and a 10 mW incident power to avoid thermal effects provided excitation. Raman shifts were corrected by using silicon (1 1 1) reference spectra after each measurement. To analyze the data, we decomposed the measured spectra using a multiple-peak fitting procedure. Satisfactory fits have been achieved with the assumption of a Lorentzian central peak and other peaks described by the spectral response functions of a damped harmonic oscillators. In order to perform electrical characterization both four and two probe configuration were adopted, by using a BIO-RAD HL5500 system with gold probes of 100 m of diameter. In order to test the obtained thin films as gas sensing material, two ohmic gold contacts in the gap configuration were evaporated onto the films surface. The gas effect on electrical conductivity was measured in a
3.1. Morphological and structural analysis CBD based procedure was used to deposit 200 nm thick nanographite particles layers from aqueous solutions. The deposition of such films proceeds by the electrochemical formation of nanographite particles in water (homogeneous nucleation) and their subsequent adhesion on the glass substrate, which can be understood in terms of the “extended” DLVO theory [18,19]. In such a theory, the total interaction energy (Vtot ) between a flat substrate and a spherical nanoparticle is, in a first approximation: Vtot = Va + Ve were Va is the energy associated with the London-van der Waals interaction and Ve is the energy associated with the electrostatic interaction. Because the van der Waals and electrostatic forces do not decay at the same rate and have different dependencies, they act at different length scales. Often in practice, electrostatic forces dominate at larger separations while van der Waal forces dominate at shorter distances. The adhesion of the nanographite particles suspended in water onto a glass substrate can occurs because the total interaction energy results attractive, in addition the adhesion of larger nanoparticles (diameter > 60 nm) is favorite respect the adhesion of smaller nanoparticles (diameter < 50 nm). Typical transmission electron microscope image performed on nanographite particles is shown in Fig. 2a. To determine the nanoparticles size distribution digitalised TEM images were processed by Image Pro-Plus software and the obtained data were reported in the histogram of Fig. 2b. The histogram clearly reflects the convolution of the distribution of the nanoparticles constituting the film. In order to extract the individual size dispersion of nanoparticles, we performed a numerical deconvolution of the histogram. We assumes the histogram to be composed prevalently of one nanoparticle specie characterized by a Gaussian distribution of sizes spread around the average value d and half-width . The Gaussian curve with the best fit parameters was superimposed in the histogram of Fig. 2b. So, the observed nanographite particles are monodispersed with a size distribution ranging from 20 to 120 nm and an average size d = 79 nm with a standard deviation = 2 nm.
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Fig. 2. (a) Transmission electron microscope images performed onto nanographite particles and (b) size distribution histogram.
The morphological features of the obtained films were analyzed by atomic force microscopy. The AFM observations reveal that the film consists of regular and parallel lined-up structures (Fig. 3a) having a width of about 500 nm and a height of 50 nm. The shape of the contact line is determined by how the solvent and the substrate intersect. Ideally, a droplet on horizontal substrate forms a ring and a vertical substrate dipped into the solvent should form a line. In practical cases the formation of patterned deposits depend on wettability, capillarity, evaporation, and convective transport of the solution and diffusion of the solute [20]. There are a few unique features of dip coating for contact-line deposition. First, the contact line is straight, except at the edges, where the meniscus is bent. Second, the substrate is raised in a controllable manner; therefore, the dewetting speed and direction is uniform across the entire contact line. Third, the pinned meniscus is dragged by the rising substrate and becomes stretched. Each array is deposited during a “stick” event and the spacing results from the “slip” of the meniscus [21]. The lined-up structures, are aligned along the dewetting direction, which is the vertical pulling direction except at the edge. Our lined-up structures, as evident in the high magnification of Fig. 3b, are formed by nanocrystallites assembly, and the average size of nanoparticles turned up to be 75 nm with a standard deviation of 10 nm. 3.2. Raman spectroscopy In Fig. 4 is reported typical Raman spectrum acquired on obtained thin film. The most prominent features in the Raman spectra of graphitic materials are the so-called G band appearing
Fig. 3. (a) Typical AFM image of film surface and (b) high magnification of film surface. The films are formed by nano-crystallites assembly.
at about 1582 cm−1 (see Fig. 5a), the D band [22] at about 1350 cm−1 , the D -band at about 1617 cm−1 and the G -band at about 2700 cm−1 . The G band is a doubly degenerate (iTO and LO, one transverse and one longitudinal optic phonon modes, respectively) phonon mode (E2g symmetry) at the BZ center that is Raman active for sp2 carbon networks. The D and D bands are defect induced Raman features, and thus these bands cannot be seen for a highly crystalline graphite. The integrated intensity ratio ID /IG for the D band and G band is widely used for characterizing the defect quantity in graphitic materials. Since the work of Tuinstra and Koenig [23], the ratio of the D and G band intensities (ID /IG ) was used for many years to estimate La in disordered carbon materials. Knight and White [24] later summarized the Raman spectra of various graphitic systems measured using the = 514.5 nm (Elaser = 2.41 eV) laser line and derived an empirical expression which allows the determination of La from the (ID /IG ) ratio. According to this work, assuming that the ratio ID /IG is inversely proportional to the fourth power of Elaser , we used
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Fig. 4. Typical Raman spectrum acquired on a nanographite thin film.
Fig. 6. Typical resistance behavior of the films as a function of temperature. The resistance value was collected varying the temperature of the film between 20 and 200 ◦ C at a ramp rate of 10 ◦ C per minute under a gas flow of dry air at a flow rate of 100 sccm, under different cycle.
the following equation for the determination of the crystallite size La using any laser line in the visible range: La (nm) =
560 4 Elaser
I −1 D
(1)
IG
where Elaser is the excitation laser energy in eV used in the Raman experiment. Considering the laser line wavelength (laser ) in nm units, Eq. (1) can be rewritten as: La (nm) = (2.4 × 10−10 )4laser
560 4 Elaser
I −1 D
IG
(2)
We obtain a crystallite size of about 80 nm, according to atomic force microscopy and TEM analysis. The second feature appearing at approximately 2700 cm−1 (see Fig. 5b) for Elaser 2.41 eV is usually called the G -band in the graphite literature because this feature is also found in all sp2 carbon materials. This feature occurs in the second order Raman spectrum and is due to a double-resonance process involving two phonons of opposite wave vector. Notice that this G band in our spectrum has been fitted using a single Lorentzian curve having center at 2707 cm−1 and FWHM of about 70 cm−1 , in agreement with the typical spectra of turbostratic graphite samples. Then, we can conclude that the films are made by turbostratic nanographite particles, that is, graphene layers piled up without stacking order along the c axis. 3.3. Effect of temperature on device resistance
Fig. 5. (a) Raman spectrum details in the G-band region and (b) Raman spectrum details in the G -band region.
The temperature of the film device was swept between 20 and 200 ◦ C at a ramp rate of 10 ◦ C per minute under a gas flow of dry air at a flow rate of 100 sccm. In order to perform electrical characterization both two and four probe configuration were used. The experimental data indicate that the charge transfer mechanism is operative at the films surface with a limited role of the electrical contacts. Obtained results in the different configurations are fully stackable. The resistance of the films decreased with increasing temperature as shown in Fig. 6. The device exhibited good stability on sweeping the temperature with only a small shift observed in resistance after the first heating sweep. This initial shift may be due to desorption of adsorbed gases on the surface or annealing of the contacts. As reported in literature [25,26], the resistance of
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Fig. 7. Dynamic response curves for a nanographite thin film at different NO2 concentrations, the working temperature was 100 ◦ C.
graphite is generally thought to decrease with increasing temperature and increase again at temperatures above 1000 ◦ C. In some reports the initial resistance decrease is sub linear and in some cases super linear, reversals of the trend are also observed at differing temperatures with the difference probably due to varying defect concentrations arising from the differing production methods [25,26]. A decrease in resistance with temperature is characteristic of intrinsic semiconductors and is generally agreed to be due to the increase in thermally generated electron–hole pairs. No different conduction regimes were observed at various operating temperatures and atmospheres, the current–voltage characteristics of the nanographite thin films exhibit a linear regime in the ± 20 V voltage range. Therefore, in the present study, all the sensor experiments are performed using 1 V polarization voltage to minimize the metal/material contact effect and to study gas/material interactions primarily. No appreciable variation in the physical and structural properties was observed after gas testing on analyzed samples. 3.4. Response of the films towards NO2 Upon exposure of the film device to NO2 a decrease in resistance [27], indicating the film has a majority hole carriers (p-type material), is observed at all attempted temperatures and concentrations. In Fig. 7 the dynamic resistance variation curve is reported for a nanographite thin film at different NO2 concentrations, the working temperature was 100 ◦ C. Similar curves were obtained for the other samples, and this one is reported as an example. As evident, the resistance of films decreases remarkably after exposure to NO2 gas. As one can see the injection of NO2 leads to a decrease in the resistance. The resistance reaches its starting value, of about 1 k after NO2 is shut off and dry air gas is let in. At low NO2 concentrations the device demonstrates an extremely large response, as evident, at only 1 ppm of NO2 the resistance variation decreases until 500 . Fig. 8 shows the resistance variations at different temperatures and NO2 concentrations. For a better comparison the resistance variation is normalized at 1. As evident the resistance variation is dependent both on working temperature and gas concentrations. The resistance variation increases with increasing of temperature and gas concentration. The resistance value returns to its initial value when the gas is switched-off. Even in these conditions the resistance of film decreases in interaction with NO2 . So we can conclude that the adsorption process is always reversible.
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Fig. 8. Normalized resistance variation of a film under different working temperature and NO2 gas concentrations.
To assess the operating temperature and selectivity characteristics of the sensor, we tested the resistance of the films, at different temperatures, also in the presence of ethanol and CO. These gases, according to data of literature, lead to a depletion of holes from the conduction band and hence raising the resistance of material. Fig. 9 represents the typical response characteristics of the sensor as a function of operating temperatures to 1 ppm NO2 and 100 ppm of CO and C2 H5 OH. The sensor response to oxidizing gases (such as NO2 ) is defined for a p-type material as the ratio of the sensor resistance in air (R0 ) to that in an air–NO2 mixture (R) R0 /R, while response to reducing gases (such as CO and C2 H5 OH) is defined as the ratio of R/R0 . The response increased with the increasing of temperature. When temperature increases to 100 ◦ C, the response reaches a maximum value of 1.17, and the temperature at which the response exhibits a maximum value is termed as optimal operating temperature. As the temperature further increases, the response of sensor decreased. This trend is also found with other gases, therefore the response induced from carbon monoxide and ethanol is very negligible in the same temperature range. The sensor response for ethanol and carbon monoxide ranges from 1.001 to 1.007. For
Fig. 9. Sensor response induced by 1 ppm of NO2 and 100 ppm of CO and C2 H5 OH, as a function of working temperature.
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these reasons, the nanographite based sensor, limited to gases analyzed, show a reasonable selectivity. A decrease in resistance upon exposure to NO2 , indicative of a ptype material, is reported for both carbon nanotube sensors (CNT) [28] and few layers graphene devices at 1–10 ppm NO2 [29]. Hall measurements have shown that NO2 adsorption on graphite sheets dopes the graphite with holes [25]. It is postulated here that sufficient electrons are bound on exposure to low concentrations (less than 10 ppm) of electron withdrawing NO2 to make holes the majority carriers. Indeed it has been theorized that upon NO2 adsorption on few layers of graphene, the acceptor states are almost entirely localized at the adsorbed NO2 molecule and the associated electrons have a considerably lower mobility than those in the graphene bands [30]. The acceptor level is reported to be fully occupied with one electron effectively transferred from the single layer graphite sheet for each adsorbed molecule of NO2 [26]. We cannot excluded that humidity can play a role in the mechanism of charge transfer especially at room temperature. This effect has been studied in the literature especially for the graphene. In fact, Leenaerts et al. [31] predicted that for H2 O, there is a fractional charge transfer of 0.025 electrons/molecule from graphene to the adsorption. However, this role should decrease with increasing temperature. Our experimental data show an increasing performance of the sensor as temperature increases, incompatible with a mechanism of hole doping due to moisture, therefore it is reasonable to assume that our data are rather compatible with a process of hole-doping due to nitrogen dioxide. The exact mechanism behind hole-doping in graphite based compounds remains unclear and further studies are needs. Our sensor performance is comparable with the best results obtained on few layers of graphene and on single carbon nanotubes [32,33]. In addition, the sensor response of our nanographite films are far superior to those obtained on graphite based devices [34,35]. The measurements were repeated for three months, with identical results, showing a promising stability of the films.
4. Conclusions In summary, simple resistive nanographite devices were fabricated and tested for NO2 detection. The obtained films consist of turbostratic nanographite particles, that is, graphene layers piled up without stacking order along the c axis. The chemical bath deposition (CBD) permitted us to obtain micro lined-up nanographite device extremely sensitive to low concentrations of NO2 . Upon exposure of the film device to NO2 a decrease in resistance, indicating the film has a majority hole carriers (ptype material), is observed at all attempted temperatures and gas concentrations. The adsorption process following the sensor activity is always reversible. The films response, relatively high also at room temperature, reaches the optimum at operating temperature of 100 ◦ C. Electrical measurements support the view that charge transfer is the primary mechanism in chemical response. It is thought that this low carrier concentration in the nanographite film device allows for extremely sensitive detection of low concentration of NO2 . Our sensor performance is comparable with the best results obtained on few layers of graphene and on single and assembled carbon nanotubes, and are far superior to those obtained on graphite based devices. Experiments are underway to further understand the sensor characteristics of this material also in different working conditions (as humidity and pressure) and to enhance its performance as an effective chemical sensor.
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the Laboratory of Physics of Nanostructures of the University of Salento, deals with analytical techniques for characterization of materials.
Biographies
Daniela Manno is associate professor in experimental physics at University of Salento. Her current research interests include morphological and structural characterization of materials of biomedical and technological interest by means of transmission electron microscopy and scanning probe microscopy; synthesis of nano metal particles for use in the field of sensors, biosensors and drug-delivery and development of methods for physical characterization of materials at the nanoscale.
Antonio Serra is associate professor of applied physics at the University of Salento since 2005. His current research interests include PVD deposition of nanostructured metal-oxide films, electrical and optical spectroscopy for advanced bio-technological applications. He is currently president of the University Degree in Technology for Cultural Heritage. Alessandro Buccolieri graduated in chemistry at the University of Bari in 2001, Ph.D. in chemistry and physics of Territory at University of Salento, is technical officer at
Emanuela Filippo received her physics degree and Ph.D. degree both from University of Lecce, Italy in 2000 and 2005, respectively. Since 2000, she works at the Department of Material Science as fellow. Her research interest includes metal nanostructures synthesis and characterization and the study of thin films for applications in technology.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Nanographite assembled films for sensitive NO2 detection A. Serra ∗ , A. Buccolieri, E. Filippo, D. Manno Dipartimento di Scienza dei Materiali, Lab. Fisica delle Nanostrutture, Università del Salento, Via Monteroni, 73100 Lecce, Italy
a r t i c l e
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Article history: Received 17 June 2011 Received in revised form 17 October 2011 Accepted 18 October 2011 Available online 25 October 2011 Keywords: Nanographite Raman Gas sensor
a b s t r a c t Carbon based nanostructures are interesting building blocks to realize films for a lot of applications that include bio- and chemical-sensing. In this work we present a one-step electrochemical method to produce water-based stable nanographite colloid used to realize lined-up micro-patterned films. The high surfaceto-volume ratio makes these structures suitable for adsorbing gases. Morphological, compositional and structural characterization have been performed in order to determine the features of the films. Simple resistive nanographite devices were fabricated and tested for NO2 sensitivity. Moreover, the present approach represents a general and facile method of good potential for scale-up that could be extended to other materials. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the past few years the nano-structured carbon materials, have attracted much interest because of their potential applications in display [1], heat transfer media [2], fuel cell and secondary battery [3], shielding materials for electrostatics and electron wave [4], functional composite materials [5], and semiconductor and anti-wear materials [6]. Graphene and nanographite, as a low dimensional assembly of sp2 -bonded carbon atoms in a closely packed honeycomb, are of the most fascinating nanostructures with interesting physical, chemical, electrical and mechanical properties which qualify it as a promising nanomaterial in condensed-matter and high-energy physics, material science, and a wide range of technological applications, such as bioelectronics and biosensing [7–14]. However, the applications of carbon based materials are limited because they are easily aggregated each other due to strong van der Waals attraction forces that may frustrate the assembly of specific structures. To obtain a stable dispersion, an additive or chemical reagent is generally required [15]. Several authors [16,17] prepared graphite particle-based nanofluid with the assistance of electrochemical oxidation method, where anodic erosion occurred at the surface of graphite electrode immersed in ethanoic acid, sulfuric acid, and deionized water. The size range of oxidized graphite particles obtained was 40–870 nm in diameter. Unlike those previous studies, we simply immersed electrodes into deionized water without adding any surfactants in the combined electrochemical and sono-chemical processes, and
∗ Corresponding author. Tel.: +39 0832 297070; fax: +39 0832 297069. E-mail address: [email protected] (A. Serra). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.10.045
then we obtained stable carbon particle-based nanofluid with narrow primary size distribution; moreover, the production rate of our carbon nano colloid (CNC) fluid was soon reached. In this work we develop a single route electrochemical synthesis of nanographite particles in aqueous solution combined with subsequent chemical bath deposition (CBD). The obtained thin films reveals an interesting lined-up microstructure that provides a great potential for gas sensing applications. The simple resistive nanographite devices were fabricated and tested for NO2 sensitivity. Moreover, the present approach is a general and facile method with good potential for scale-up that could be extended to other materials. 2. Experimental Firstly nanographite colloidal solution was obtained in the following route. High purity graphite rod (SPI 99.99% purity, 5 mm diameter) was used as anode, and a stainless steel rod (SWAGELOK AISI 1016, 5 mm diameter) was used as cathode. The electrodes were immersed in a becker containing 200 mL deionized water (Millipore Milli-Q, 18.2 M cm) at a reciprocal distance of 10 mm. In the electrolysis process, the electric power applied to the electrodes was a constant voltage of 30 V. Simultaneously, the solution was forcedly dispersed by an ultrasonicator, which was the flat-type ultrasonic equipment (Flexonic-1200-35/72/100G, Mirae Ultrasonic Tech., Korea). To prevent the aggregation of nanoparticles, the ultrasonicator was continuously operated at the power output of 1000 W with the frequency of 100 ± 5 kHz during the production of nanographite. In this way we obtain a colloidal solution consisting of nanographite particles homogeneously dispersed in water. Then, nanographite particles were deposited on Corning glass commercial slide plates using chemical bath deposition (CBD)
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dynamic flow system implemented in our laboratory. As the carrier and the reference gases were used both air and argon at ambient pressure (RIVOIRA, 99.99% pure). Test measurements consisted of set of cycles in which increasing test gas concentrations at a flow of 100 sccm were separated by exposures to the reference gas. The relative humidity measured by means of a Keithley 6517A electrometer equipped with a 6517RH probe, at the top of test chamber, was of 30% at 25 ◦ C. Before test measurements, the samples were exposed to argon for 4 h at 200 ◦ C in the test chamber. Measurements were performed onto 5 different samples, in order to verify the repeatability of experiments. The sensor performance reported in the present work are the average values obtained by repeating 20 times the measurements. The reported error is the corresponding standard deviation. For evaluating the magnitude of the conductivity changes produced by the individual gas exposures, the resistance values measured at the end of successive dwell intervals in dry clean air and in reactive gas, respectively, were used. In this way, values of sensor response could be obtained, which were unaffected by time constants for the filling and emptying of the sensor chamber and also by temperature changes that might have occurred upon the switching of mass flow controllers. 3. Results Fig. 1. The experimental set up for chemical bath deposition (CBD) method, employed for the growth of the films.
method. The experimental set up employed for the growth of the films can be seen from Fig. 1. The substrates were suspended vertically in the colloidal solution which was maintained at 50 ◦ C on a hot plate to facilitate evaporation of the solvent. The solution was stirred with the help of magnetic stirrer to avoid agglomeration of nanoparticles. As the solvent evaporates, carbon nanoparticles selfassemble into linear structures parallel to the solvent surface giving rise to homogeneous films. The average thickness of the films was measured by a TENCOR Alpha-Step profilometer. Atomic force microscopy (AFM) images were acquired by a Jeol 4210 STM-AFM microscope. The image was acquired by contact mode by using a cantilever with force constant of 0.15 N/m and a silicon carbide tip with radius of curvature less than 10 nm. All observations were performed in air at room temperature. Transmission electron microscopy (TEM) images and electron diffraction patterns were taken using an Hitachi 7100, at 100 kV representing the suitable acceleration voltage to obtain a good resolution and minimal radiation damage of the material. The specimens were prepared for transmission electron microscope observations by placing small droplets of nanographite solutions onto standard carbon supported 600-mesh copper grid and drying slowly in air naturally. Raman scattering measurements were obtained by backscattering geometry with a RENISHAW spectrometer coupled to a LEICA metallographic microscope. An argon-ion laser operated at a wavelength of 514.5 nm and a 10 mW incident power to avoid thermal effects provided excitation. Raman shifts were corrected by using silicon (1 1 1) reference spectra after each measurement. To analyze the data, we decomposed the measured spectra using a multiple-peak fitting procedure. Satisfactory fits have been achieved with the assumption of a Lorentzian central peak and other peaks described by the spectral response functions of a damped harmonic oscillators. In order to perform electrical characterization both four and two probe configuration were adopted, by using a BIO-RAD HL5500 system with gold probes of 100 m of diameter. In order to test the obtained thin films as gas sensing material, two ohmic gold contacts in the gap configuration were evaporated onto the films surface. The gas effect on electrical conductivity was measured in a
3.1. Morphological and structural analysis CBD based procedure was used to deposit 200 nm thick nanographite particles layers from aqueous solutions. The deposition of such films proceeds by the electrochemical formation of nanographite particles in water (homogeneous nucleation) and their subsequent adhesion on the glass substrate, which can be understood in terms of the “extended” DLVO theory [18,19]. In such a theory, the total interaction energy (Vtot ) between a flat substrate and a spherical nanoparticle is, in a first approximation: Vtot = Va + Ve were Va is the energy associated with the London-van der Waals interaction and Ve is the energy associated with the electrostatic interaction. Because the van der Waals and electrostatic forces do not decay at the same rate and have different dependencies, they act at different length scales. Often in practice, electrostatic forces dominate at larger separations while van der Waal forces dominate at shorter distances. The adhesion of the nanographite particles suspended in water onto a glass substrate can occurs because the total interaction energy results attractive, in addition the adhesion of larger nanoparticles (diameter > 60 nm) is favorite respect the adhesion of smaller nanoparticles (diameter < 50 nm). Typical transmission electron microscope image performed on nanographite particles is shown in Fig. 2a. To determine the nanoparticles size distribution digitalised TEM images were processed by Image Pro-Plus software and the obtained data were reported in the histogram of Fig. 2b. The histogram clearly reflects the convolution of the distribution of the nanoparticles constituting the film. In order to extract the individual size dispersion of nanoparticles, we performed a numerical deconvolution of the histogram. We assumes the histogram to be composed prevalently of one nanoparticle specie characterized by a Gaussian distribution of sizes spread around the average value d and half-width . The Gaussian curve with the best fit parameters was superimposed in the histogram of Fig. 2b. So, the observed nanographite particles are monodispersed with a size distribution ranging from 20 to 120 nm and an average size d = 79 nm with a standard deviation = 2 nm.
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Fig. 2. (a) Transmission electron microscope images performed onto nanographite particles and (b) size distribution histogram.
The morphological features of the obtained films were analyzed by atomic force microscopy. The AFM observations reveal that the film consists of regular and parallel lined-up structures (Fig. 3a) having a width of about 500 nm and a height of 50 nm. The shape of the contact line is determined by how the solvent and the substrate intersect. Ideally, a droplet on horizontal substrate forms a ring and a vertical substrate dipped into the solvent should form a line. In practical cases the formation of patterned deposits depend on wettability, capillarity, evaporation, and convective transport of the solution and diffusion of the solute [20]. There are a few unique features of dip coating for contact-line deposition. First, the contact line is straight, except at the edges, where the meniscus is bent. Second, the substrate is raised in a controllable manner; therefore, the dewetting speed and direction is uniform across the entire contact line. Third, the pinned meniscus is dragged by the rising substrate and becomes stretched. Each array is deposited during a “stick” event and the spacing results from the “slip” of the meniscus [21]. The lined-up structures, are aligned along the dewetting direction, which is the vertical pulling direction except at the edge. Our lined-up structures, as evident in the high magnification of Fig. 3b, are formed by nanocrystallites assembly, and the average size of nanoparticles turned up to be 75 nm with a standard deviation of 10 nm. 3.2. Raman spectroscopy In Fig. 4 is reported typical Raman spectrum acquired on obtained thin film. The most prominent features in the Raman spectra of graphitic materials are the so-called G band appearing
Fig. 3. (a) Typical AFM image of film surface and (b) high magnification of film surface. The films are formed by nano-crystallites assembly.
at about 1582 cm−1 (see Fig. 5a), the D band [22] at about 1350 cm−1 , the D -band at about 1617 cm−1 and the G -band at about 2700 cm−1 . The G band is a doubly degenerate (iTO and LO, one transverse and one longitudinal optic phonon modes, respectively) phonon mode (E2g symmetry) at the BZ center that is Raman active for sp2 carbon networks. The D and D bands are defect induced Raman features, and thus these bands cannot be seen for a highly crystalline graphite. The integrated intensity ratio ID /IG for the D band and G band is widely used for characterizing the defect quantity in graphitic materials. Since the work of Tuinstra and Koenig [23], the ratio of the D and G band intensities (ID /IG ) was used for many years to estimate La in disordered carbon materials. Knight and White [24] later summarized the Raman spectra of various graphitic systems measured using the = 514.5 nm (Elaser = 2.41 eV) laser line and derived an empirical expression which allows the determination of La from the (ID /IG ) ratio. According to this work, assuming that the ratio ID /IG is inversely proportional to the fourth power of Elaser , we used
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Fig. 4. Typical Raman spectrum acquired on a nanographite thin film.
Fig. 6. Typical resistance behavior of the films as a function of temperature. The resistance value was collected varying the temperature of the film between 20 and 200 ◦ C at a ramp rate of 10 ◦ C per minute under a gas flow of dry air at a flow rate of 100 sccm, under different cycle.
the following equation for the determination of the crystallite size La using any laser line in the visible range: La (nm) =
560 4 Elaser
I −1 D
(1)
IG
where Elaser is the excitation laser energy in eV used in the Raman experiment. Considering the laser line wavelength (laser ) in nm units, Eq. (1) can be rewritten as: La (nm) = (2.4 × 10−10 )4laser
560 4 Elaser
I −1 D
IG
(2)
We obtain a crystallite size of about 80 nm, according to atomic force microscopy and TEM analysis. The second feature appearing at approximately 2700 cm−1 (see Fig. 5b) for Elaser 2.41 eV is usually called the G -band in the graphite literature because this feature is also found in all sp2 carbon materials. This feature occurs in the second order Raman spectrum and is due to a double-resonance process involving two phonons of opposite wave vector. Notice that this G band in our spectrum has been fitted using a single Lorentzian curve having center at 2707 cm−1 and FWHM of about 70 cm−1 , in agreement with the typical spectra of turbostratic graphite samples. Then, we can conclude that the films are made by turbostratic nanographite particles, that is, graphene layers piled up without stacking order along the c axis. 3.3. Effect of temperature on device resistance
Fig. 5. (a) Raman spectrum details in the G-band region and (b) Raman spectrum details in the G -band region.
The temperature of the film device was swept between 20 and 200 ◦ C at a ramp rate of 10 ◦ C per minute under a gas flow of dry air at a flow rate of 100 sccm. In order to perform electrical characterization both two and four probe configuration were used. The experimental data indicate that the charge transfer mechanism is operative at the films surface with a limited role of the electrical contacts. Obtained results in the different configurations are fully stackable. The resistance of the films decreased with increasing temperature as shown in Fig. 6. The device exhibited good stability on sweeping the temperature with only a small shift observed in resistance after the first heating sweep. This initial shift may be due to desorption of adsorbed gases on the surface or annealing of the contacts. As reported in literature [25,26], the resistance of
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Fig. 7. Dynamic response curves for a nanographite thin film at different NO2 concentrations, the working temperature was 100 ◦ C.
graphite is generally thought to decrease with increasing temperature and increase again at temperatures above 1000 ◦ C. In some reports the initial resistance decrease is sub linear and in some cases super linear, reversals of the trend are also observed at differing temperatures with the difference probably due to varying defect concentrations arising from the differing production methods [25,26]. A decrease in resistance with temperature is characteristic of intrinsic semiconductors and is generally agreed to be due to the increase in thermally generated electron–hole pairs. No different conduction regimes were observed at various operating temperatures and atmospheres, the current–voltage characteristics of the nanographite thin films exhibit a linear regime in the ± 20 V voltage range. Therefore, in the present study, all the sensor experiments are performed using 1 V polarization voltage to minimize the metal/material contact effect and to study gas/material interactions primarily. No appreciable variation in the physical and structural properties was observed after gas testing on analyzed samples. 3.4. Response of the films towards NO2 Upon exposure of the film device to NO2 a decrease in resistance [27], indicating the film has a majority hole carriers (p-type material), is observed at all attempted temperatures and concentrations. In Fig. 7 the dynamic resistance variation curve is reported for a nanographite thin film at different NO2 concentrations, the working temperature was 100 ◦ C. Similar curves were obtained for the other samples, and this one is reported as an example. As evident, the resistance of films decreases remarkably after exposure to NO2 gas. As one can see the injection of NO2 leads to a decrease in the resistance. The resistance reaches its starting value, of about 1 k after NO2 is shut off and dry air gas is let in. At low NO2 concentrations the device demonstrates an extremely large response, as evident, at only 1 ppm of NO2 the resistance variation decreases until 500 . Fig. 8 shows the resistance variations at different temperatures and NO2 concentrations. For a better comparison the resistance variation is normalized at 1. As evident the resistance variation is dependent both on working temperature and gas concentrations. The resistance variation increases with increasing of temperature and gas concentration. The resistance value returns to its initial value when the gas is switched-off. Even in these conditions the resistance of film decreases in interaction with NO2 . So we can conclude that the adsorption process is always reversible.
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Fig. 8. Normalized resistance variation of a film under different working temperature and NO2 gas concentrations.
To assess the operating temperature and selectivity characteristics of the sensor, we tested the resistance of the films, at different temperatures, also in the presence of ethanol and CO. These gases, according to data of literature, lead to a depletion of holes from the conduction band and hence raising the resistance of material. Fig. 9 represents the typical response characteristics of the sensor as a function of operating temperatures to 1 ppm NO2 and 100 ppm of CO and C2 H5 OH. The sensor response to oxidizing gases (such as NO2 ) is defined for a p-type material as the ratio of the sensor resistance in air (R0 ) to that in an air–NO2 mixture (R) R0 /R, while response to reducing gases (such as CO and C2 H5 OH) is defined as the ratio of R/R0 . The response increased with the increasing of temperature. When temperature increases to 100 ◦ C, the response reaches a maximum value of 1.17, and the temperature at which the response exhibits a maximum value is termed as optimal operating temperature. As the temperature further increases, the response of sensor decreased. This trend is also found with other gases, therefore the response induced from carbon monoxide and ethanol is very negligible in the same temperature range. The sensor response for ethanol and carbon monoxide ranges from 1.001 to 1.007. For
Fig. 9. Sensor response induced by 1 ppm of NO2 and 100 ppm of CO and C2 H5 OH, as a function of working temperature.
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these reasons, the nanographite based sensor, limited to gases analyzed, show a reasonable selectivity. A decrease in resistance upon exposure to NO2 , indicative of a ptype material, is reported for both carbon nanotube sensors (CNT) [28] and few layers graphene devices at 1–10 ppm NO2 [29]. Hall measurements have shown that NO2 adsorption on graphite sheets dopes the graphite with holes [25]. It is postulated here that sufficient electrons are bound on exposure to low concentrations (less than 10 ppm) of electron withdrawing NO2 to make holes the majority carriers. Indeed it has been theorized that upon NO2 adsorption on few layers of graphene, the acceptor states are almost entirely localized at the adsorbed NO2 molecule and the associated electrons have a considerably lower mobility than those in the graphene bands [30]. The acceptor level is reported to be fully occupied with one electron effectively transferred from the single layer graphite sheet for each adsorbed molecule of NO2 [26]. We cannot excluded that humidity can play a role in the mechanism of charge transfer especially at room temperature. This effect has been studied in the literature especially for the graphene. In fact, Leenaerts et al. [31] predicted that for H2 O, there is a fractional charge transfer of 0.025 electrons/molecule from graphene to the adsorption. However, this role should decrease with increasing temperature. Our experimental data show an increasing performance of the sensor as temperature increases, incompatible with a mechanism of hole doping due to moisture, therefore it is reasonable to assume that our data are rather compatible with a process of hole-doping due to nitrogen dioxide. The exact mechanism behind hole-doping in graphite based compounds remains unclear and further studies are needs. Our sensor performance is comparable with the best results obtained on few layers of graphene and on single carbon nanotubes [32,33]. In addition, the sensor response of our nanographite films are far superior to those obtained on graphite based devices [34,35]. The measurements were repeated for three months, with identical results, showing a promising stability of the films.
4. Conclusions In summary, simple resistive nanographite devices were fabricated and tested for NO2 detection. The obtained films consist of turbostratic nanographite particles, that is, graphene layers piled up without stacking order along the c axis. The chemical bath deposition (CBD) permitted us to obtain micro lined-up nanographite device extremely sensitive to low concentrations of NO2 . Upon exposure of the film device to NO2 a decrease in resistance, indicating the film has a majority hole carriers (ptype material), is observed at all attempted temperatures and gas concentrations. The adsorption process following the sensor activity is always reversible. The films response, relatively high also at room temperature, reaches the optimum at operating temperature of 100 ◦ C. Electrical measurements support the view that charge transfer is the primary mechanism in chemical response. It is thought that this low carrier concentration in the nanographite film device allows for extremely sensitive detection of low concentration of NO2 . Our sensor performance is comparable with the best results obtained on few layers of graphene and on single and assembled carbon nanotubes, and are far superior to those obtained on graphite based devices. Experiments are underway to further understand the sensor characteristics of this material also in different working conditions (as humidity and pressure) and to enhance its performance as an effective chemical sensor.
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the Laboratory of Physics of Nanostructures of the University of Salento, deals with analytical techniques for characterization of materials.
Biographies
Daniela Manno is associate professor in experimental physics at University of Salento. Her current research interests include morphological and structural characterization of materials of biomedical and technological interest by means of transmission electron microscopy and scanning probe microscopy; synthesis of nano metal particles for use in the field of sensors, biosensors and drug-delivery and development of methods for physical characterization of materials at the nanoscale.
Antonio Serra is associate professor of applied physics at the University of Salento since 2005. His current research interests include PVD deposition of nanostructured metal-oxide films, electrical and optical spectroscopy for advanced bio-technological applications. He is currently president of the University Degree in Technology for Cultural Heritage. Alessandro Buccolieri graduated in chemistry at the University of Bari in 2001, Ph.D. in chemistry and physics of Territory at University of Salento, is technical officer at
Emanuela Filippo received her physics degree and Ph.D. degree both from University of Lecce, Italy in 2000 and 2005, respectively. Since 2000, she works at the Department of Material Science as fellow. Her research interest includes metal nanostructures synthesis and characterization and the study of thin films for applications in technology.