Reduced Graphene OxideâTiO2 Nanotube Composite

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Reduced Graphene Oxide–TiO2 Nanotube Composite: Comprehensive Study for Gas Sensing Applications Vardan Galstyan, Andrea Ponzoni, Iskandar Kholmanov, Marta Maria Natile, Elisabetta Comini, Sherzod Nematov, and Giorgio Sberveglieri ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01924 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Nano Materials

Reduced Graphene Oxide–TiO2 Nanotube Composite: Comprehensive Study for Gas Sensing Applications Vardan Galstyan†,*, Andrea Ponzoni‡, Iskandar Kholmanov§, Marta M. Natile∥, Elisabetta Comini†, Sherzod Nematov⊥, Giorgio Sberveglieri† †Sensor

Lab, Department of Information Engineering, University of Brescia, Via Valotti 9,

25133 Brescia, Italy ‡CNR

- National Institute of Optics (INO), Via Branze 45, 25123 Brescia, Italy

§Department

of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712,

USA ∥CNR-Institute

of Condensed Matter Chemistry and Technologies for Energy, Department of

Chemical Sciences, University of Padova, Via F. Marzolo 1, 35131 Padova, Italy ⊥The

Tashkent State Technical University, Universitetskaya 2, 100069 Tashkent, Uzbekistan

KEYWORDS: Graphene oxide, reduced graphene oxide, reduction effect, TiO2 nanotube, surface functionalization, gas sensing

ABSTRACT: Graphene oxide (GO) and reduced graphene oxide (RGO) have unique properties that can revolutionize performances of functional devices. Graphene-based materials can be coupled with metal oxide nanomaterials for gas sensing applications. In this work, we report the synthesis and the gas sensing properties of a composite material based on RGO loaded TiO2 nanotubes. To properly tune the reduction of GO to RGO we adopted a gas-phase process that can be applied in situ on each gas sensor device, allowing to track the process effects through the sensor conductance. We systematically investigated the gas-response dependence from RGO

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loading and its reduction, showing the occurrence of an optimal RGO concentration arising from the interplay of these two parameters. Hence, these two factors should be considered in parallel to functionalize the metal oxide materials with GO for the fabrication of chemical sensor devices. 1. INTRODUCTION The low-cost and energy-efficient chemical gas sensors based on one-dimensional (1D) nanostructures have received great attention for the fabrication of portable and mobile gas sensing systems for the environmental monitoring.1-2 1D oxide materials such as SnO2, TiO2, and ZnO exhibit promising sensing performance with several advantages, such as large surface area, good chemical stability and simple measuring electronics.1-3 However, the sensing performance of these materials has yet to reach to their full potential in capabilities and usage. The sensing mechanism of metal oxide chemiresistive gas sensors is based on the variation of their electrical conductivity due to the adsorption/desorption processes of reducing and oxidizing gases onto their surface. This mechanism was described in detail in our previous report.4 Polycrystalline TiO2 nanotubes (NTs) with their unique chemical, physical and structural properties which facilitate the surface reactions are one of the most studied wide band-gap oxide materials for gas sensors, light harvesting and energy storage devices, etc.1, 5-6 The investigations show that the tuning of the band-gap of TiO2 by doping with the different materials plays a significant role for improvement of its conductance and response.1,

7

The dopants also act as

charge carrier recombination centers affecting the material performance.8 In this regard, the preparation of composite materials based on TiO2 seems a very promising strategy for the fabrication of chemical gas sensors. Graphene is a two-dimensional (2D) honeycomb crystal nanostructure and has been widely explored for the fabrication of functional devices owing to its excellent electronic properties.9 Among the graphene based structures the graphene oxide (GO) is a very attractive due to its unique physical, chemical and structural properties. GO is an insulating material that contains oxygen groups on the plane of carbon atoms. It can be easily modified to reduced GO (RGO) by removing the oxygen-containing functional groups.10 RGO has been studied as a functional material to improve the performance of metal oxides.11 Several methods, such as chemical, electrochemical and thermal reductions have been reported for the reducing of GO.10, 12


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RGO structures studied for chemical gas sensing applications showed p-type response.13-14 In recent investigations, RGO has been integrated with metal oxides in order to obtain composites with improved gas sensing properties.15-20 In these studies the composites with relatively large amount of RGO were investigated.18-21 Thus, the sensing response of the composite materials was almost dominated by the response of RGO and increased with increasing the concentration of RGO in the composites. In addition, the optimal operating temperature of the obtained composites was significantly lower, 100 °C or below, down to room temperature (RT),16-19 compared to that of pure metal oxide chemical gas sensors (typically 200–500 °C). Several authors,17,22,23 demonstrated that the increase in the concentration of RGO and the decrease of the composite operation temperature to 100 °C and below improved the material response towards NO2. So far, the effective development of a strategy for the fabrication of a composite structure requires the investigation of two main issues, namely the variation of GO concentration and the reduction of GO. The study of these two effects in parallel on the metal oxides` gas sensing response is crucial for the improvement of oxide material sensing performance and the other functional features as well. Literature investigations showed that the TiO2 NTs are attractive materials for the detection of reducing gases.22 One of them is the hydrogen (H2) that is an extremely clean fuel. It is considered as a green and renewable future energy source and has numerous applications, such as fuel cells, power generators, chemical production, automobiles and aerospace.23-26 H2 is colorless, odorless, flammable and explosive gas. Consequently, the use of H2 involves dangers associated with the H2 production, storage and transport.27 Herein, we report the preparation and investigations of the sensing properties of RGO functionalized TiO2 NTs. We have studied the reduction and the concentration effects of GO on the response of TiO2 NTs. We have chosen the H2 as case-study gas for our investigations. The selectivity of the obtained structures was studied as well. The obtained results indicate that we have developed a novel efficient approach for coupling of GO with the metal oxide materials to fabricate high performance gas sensors and the other functional devices. 2. EXPERIMENTAL SECTION Fabrication of samples. The pure TiO2 NTs were prepared using the following procedures. The metallic titanium films with the thickness of 500 nm were deposited on 2 mm × 2 mm ×



0.75 mm alumina substrates by means of radio frequency (RF) (13.56 MHz) magnetron sputtering. To improve the adhesion of the metallic films to the substrates the temperature of substrates during the sputtering process was kept at 300 °C. The sputtering power was 75 W. Then, TiO2 NTs were obtained by electrochemical anodization of metallic titanium films in a two electrode system Teflon cell. The anodization process was carried out in NH4F and H2O containing glycerol at room temperature. A platinum foil was used as the counter electrode. Asprepared tubular structures were crystallized by thermal annealing under a 50 vol% O2 and 50 vol% Ar gas atmosphere at 400 °C for 6 h. Aqueous G–O dispersions were made by sonication of graphite oxide produced using a modified Hummers’ method.28 Briefly, graphite powder (SP1, Bay Carbon) was oxidized into graphite oxide using sulfuric acid and potassium permanganate. Aqueous dispersions of individual graphene oxide (GO) platelets were prepared by stirring graphite oxide solids in pure water (17.5 MΩ, Barnstead) for 3 h, and then sonicating the mixture (VWR B2500A-MT, a bath sonicator) for 45 min. The fabrication procedure of composite structure for the electrical and gas sensing studies is illustrated schematically in Figure 1. To fabricate the composite structures based on TiO2 NTs and RGO (RGO–TiO2) we used an aqueous dispersion of GO with the concentration of 0.03 mg/ml. Afterwards, we drop casted 0.3 µl aqueous dispersion of GO on the surface of TiO2 tubular arrays obtained on alumina substrates using a dispenser with a precision of 0.003 µl (Gilson Company, Inc, USA). The concentration of GO at each drop casting was 2.25 ng/mm2 with respect to the concentration of GO aqueous dispersion. To carry out the electrical measurements two Pt parallel electrodes with the thickness of 500 nm were deposited on the surface of prepared structures by means of RF magnetron sputtering. Before the deposition of Pt electrodes a Ti/W (10/90 wt%) adhesions layer about 90 nm thick was applied. To control the samples temperature a Pt heater was deposited on the backside of the alumina substrates. To enhance the control over the reduction of the prepared GO and composite materials, we applied the reduction process directly on gas sensor devices through a thermal treatment carried out in a pure Ar atmosphere according to the following procedure: samples are first kept at RT, then their temperature was risen to 400 °C and kept constant for 4 h, finally the room temperature conditions are restored. The same treatment has been applied to pure TiO2 for comparison. As


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better detailed in the Results and Discussion section, measuring the sensor signal during the thermal treatment allows a real time feedback on the applied reduction process. Since the goal of the present work is to investigate the effects of RGO loading, different RGO amounts have been added sequentially to the same sample for each sample, applying every time the Ar treatment and measuring the gas sensing properties. In this way the loading effects can be tracked from null loading to the maximum one for the same sample. This procedure is important to avoid ambiguities that may arise from reproducibility issues. Indeed, assessing loading effects through the comparison of different samples, even if they are nominally identical, may give the non-trivial problem of decoupling differences related to reproducibility issues from differences induced by the RGO functionalization. Materials characterization. The morphologies of the samples were examined by means of LEO 1525 scanning electron microscope (SEM) equipped with field emission gun. Raman spectroscopy (WITec Micro-Raman Spectrometer Alpha 300, λ=488 nm, 100× objective) was used to characterize the samples. XPS measurements were carried out with a Perkin-Elmer PHI 5600ci spectrometer with a standard Al Kα source working at 250 W. Both extended spectra (survey, 187.85 eV pass energy, 0.5 eV/step, 0.025 s/step) and detailed spectra (for O 1s and C 1s, 23.5 eV pass energy, 0.1 eV/step, 0.1 s/step) were recorded. The working pressure was less than 7 × 10−7 Pa. The standard deviation in the BE values is ± 0.1 eV. Electrical and gas sensing characterization. The gas sensing properties of samples were studied in a computer controlled thermostatic test chamber by flow-through technique. The detailed description of the experimental setup has already been reported.29 The carrier gas was synthetic air with a flow rate of 0.2 l/min. The concentration of analyte gas during the measurements was controlled by the aforementioned computer controlled gas flow system. The studies were performed focusing on H2 in three different concentrations of (120, 240 and 480 ppm) at the operating temperatures of 100, 200 and 300 °C. The samples were stabilized for 10 h before each gas sensing measurement at the each selected operating temperature. The test chamber was purged with the synthetic air after every exposure to test gases for 85 min to allow a full recovery of samples. The conductance of structures was monitored by means of the voltamperometric technique at the applied constant voltage of 1 V. The conductance values were recorded every 30 s. The sensing response (S) of samples was calculated according to the following equation: S =

|(𝐺𝑓 ― 𝐺0)| G0

=

⌈∆G⌉ G0

, where G0 is the sample conductance in air, and Gf is the



sample conductance in presence of analyte gas. The response and recovery times were defined as the time to reach 90% of (𝐺𝑓 ― 𝐺0) when the gas was introduced and to recover to 70% of the original conductance in air.

Figure 1. Schematic illustration of the fabrication of composite structure for the electrical and gas sensing studies. 3. RESULTS AND DISCUSSION Materials characterization. Figure 2 presents the morphological analyses of pure TiO2 and RGO–TiO2 samples. As can be seen in Figure 2(a) the TiO2 NTs with the diameter of 30 nm were obtained on alumina substrates. We made a few scratches on the surface of samples to carry out the surface and cross-sectional analysis. The obtained GO sheets are shown in the Figure 2(b). Figure 2(c, d and f) show the SEM images of RGO–TiO2 composite material, where the surface regions of TiO2 structure covered with the RGO (Figure 2(c, d and f)) and the regions of pure TiO2 (Figure 2(d and f)) are indicated. In Figure 2(d and e) is shown a cross-sectional view of TiO2 tubular array with an average length of tubes of about 1 µm.


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Figure 2. SEM images of the obtained samples. (a) The surface morphology of pure TiO2 NTs with different resolutions, (b) SEM image of GO sheets on silicon substrate, (c) the morphology of the obtained RGO–TiO2 composite structures (after the drop casting of GO sheets on TiO2 surface and its thermal treatment) with different resolutions, (d) a scratched sector of RGO–TiO2 composite surface for the cross sectional analysis. An RGO sheet over the scratched region of composite is shown with the yellow dashed line. (e) The cross-section image of the TiO2 tubular structures, (f) a sector of RGO–TiO2 composite surface taken from image (c), where pure TiO2 area and the TiO2 area functionalized with the RGO are clearly visible.

Figure 3. Raman spectra of (a) pure TiO2 NTs, and (b) RGO on an alumina substrate. Raman spectroscopy studies of TiO2 NTs used in our experiments show the predominantly anatase structure with clearly observable Raman active modes at 144 cm-1, 399 cm-1, 519 cm-1



and 639 cm-1, as exhibited in Figure 3(a). Raman spectroscopy was also used to characterize the RGO platelets on nanotubular TiO2 samples. The RGO exhibits intense G (~1580 cm-1) and D (~1365 cm-1) peaks in its Raman spectrum (Figure 3(b)). The G peak is a primary in-plane vibrational mode corresponding to the in-plane bond-stretching optical vibration of sp2hybridized carbon atoms. The Raman D band of graphene is activated by the defects that cause an intervalley double resonance involving transitions near two inequivalent K points at neighboring corners of the first Brillouin zone of graphene.30 Defects in RGO are formed due to the decrease in size of the in-plane sp2 domains after extensive oxidation and ultrasonic exfoliation processes.31-32 The effect of thermal treatment as a function of GO loading was also evaluated by XPS analysis. The C 1s XPS peak of GO sample and RGO samples obtained with the GO concentration of 6.75 and 15.75 ng/mm2, respectively, are showed in Figure 4. The C1s peak of a GO sample deposited on the metal oxide surface is characterized by four contributions at 284.3 eV (graphitic C–C, C=C and C–H), 285.5 eV (C in C–OH), 286.4 (C in epoxide C–O–C) and 288.4 eV (C in carbonyl C=O and carboxylate O=C–OH) respectively. The presence of different oxygen-containing functional groups, in particular epoxide groups, on GO is in agreement with literature.33-35 A successive thermal treatment at 400 °C for 4 h in pure Ar atmosphere of GO sample produces several changes depending on the GO concentration. In the sample obtained with 6.75 ng/mm2 of GO, the contributions due to epoxide (286.4 eV) and carbonyl and carboxyl groups (288.4 eV) disappear suggesting a successful reduction. Unlike sample with 6.75 ng/mm2 of GO, in the sample with higher concentration of GO (15.75 ng/mm2) it is evident an increase of the contributions at higher BEs characteristic of oxygen-containing groups respect to graphitic C. The poor stability of RGO tick layers could be the reason of this different behavior. At lower GO concentration, in fact, the substrate surface could play a key role in stabilizing the deposited RGO layers, but when a ticker layer of RGO is deposited, the interface stabilizing effect of substrate is loss and the highly active RGO surface layers easily reacts with the atmospheric moisture and CO2 forming these oxygen-containing groups.


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Figure 4. C 1s XPS spectra for GO, RGO (6.75 ng/mm2) and RGO (15.75 ng/mm2). Spectra are normalized with respect to their maximum and minimum. Fitting is also reported (dotted line). Electrical and gas sensing characterization. The reduction process carried out in Ar atmosphere was characterized in situ by measuring the electrical conductance of the material under treatment. Figure 5(a) presents the conductance variation of pure TiO2, RGO and RGO– TiO2 samples during their thermal treatment in pure Ar atmosphere. For all samples, the conductance increases upon the temperature increase from room temperature to 400 °C and reaches a nearly steady state value. When the room temperature condition is restored (while keeping the pure Ar atmosphere) the conductance of pure TiO2 recovers its initial (before the 400 °C treatment) value, suggesting that this process do not appreciably alter the TiO2 matrix. Differently, the conductance of pure RGO and RGO–TiO2 samples stabilizes to a higher value,



indicating the occurrence of non-reversible effects. This is expected from GO/RGO literature: GO is an insulating material and its conductivity is improved upon reduction.36 The conductance improvement of RGO–TiO2 samples is related with the presence of RGO sheets on the surface of TiO2 NTs.37 The RGO sheets connecting the TiO2 NTs with each other facilitating the charge transport through the tubular arrays.38 Then, we investigated conductance variation of RGO and RGO–TiO2 samples depending on the concentration of GO (Figure 5(b)). The conductance of RGO–TiO2 samples drastically increased by about 3 orders of magnitude with increasing the RGO loading, till the 15.75–18 ng/mm2. We did not observe big changes of RGO–TiO2 conductance at higher concentration of RGO (from 18 to 33.75 ng/mm2). The conductance of RGO sample increased drastically till the 9 ng/mm2 of GO and remained nearly constant at 15.75 ng/mm2. Then, there was a drastic decrease and increase of conductance at 18 and 24.75 ng/mm2, respectively. In Figure 5(b) it can be seen that the conductance change feature of RGO–TiO2 structure at higher concentration of RGO (since 18 ng/mm2) is coincident with the points when we observed the drastic decrease and increase of RGO conductance followed with the small variations. Given the larger electrical conductivity of RGO with respect to GO,14 the observed behaviour is consistent with the improved reduction of GO observed with XPS for small RGO concentrations compared to larger concentrations. These analyses indicate that with the increase of RGO concentration in the RGO–TiO2 composite, the RGO deeply affects the RGO–TiO2 conductance.


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Figure 5. (a) The conductance dependence vs time of the pure TiO2, RGO, and RGO–TiO2 structures during their thermal treatment, where the treatment intervals at room temperature (RT) and at 400 °C are indicated. (b) The RT conductance change of the RGO, and RGO–TiO2 structures depending on the concentration of GO. The conductance values were measured after each drop casting and thermal treatment procedure. We studied the sensing performance of the RGO–TiO2 samples in the temperature range 100– 300 °C, starting from the pure TiO2 samples and increasing the concentration of GO till 40.5 ng/mm2 (Figure S1). The samples demonstrated the best gas sensing performance at the sensor temperature of 200 °C. Therefore, we will make the further discussions based on the sensing measurements carried out in these condition. Figure 6(a) presents the response of the RGO and RGO–TiO2 samples depending on the concentration of GO. The RGO–TiO2 structure showed an n-type response for all the RGO loading studied (from 0 till 40.5 ng/mm2), with an optimal response recorded for 4.5 ng/mm2 of GO, which is about one order of magnitude higher than the response exhibited by the pure TiO2 (zero droplets in Figure 4). Increasing the RGO loading beyond 4.5 ng/mm2 cause the response to decrease by more than 2 orders of magnitude, making the material less responsive than the original TiO2 layer, but keeping the n-type sensing character (Figure 6(a)). No significant changes in response changes were observed at the higher concentrations of RGO (from 15.75 to 40.5 ng/mm2). As for the RGO sample, this is not sensitive to the H2 till 6.75 ng/mm2 of GO. The RGO showed p-type weak response towards H2 after the drop casting of 6.75 ng/mm2 GO (Figure 6(a)). The p-type response is typical for the RGO chemical sensors.39 Furthermore, the RGO sample obtained with 6.75 ng/mm2 of GO showed the highest response comparing to the RGO samples prepared with the other



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concentrations of GO. The dynamic response of the RGO samples obtained with 4.5 and 6.75 ng/mm2 of GO is shown in Figure 6 (c). The improved conductance of RGO sample could be reasonably related with the reached percolation threshold, which predict a sharp conductance/connection dependence on the density of the most conducting (RGO sheets in this case).40 The RGO response is decreased from 9 to 15.75 ng/mm2 of GO and after the 15.75 ng/mm2 the RGO sample has similar response to that of RGO–TiO2 structure functionalized with the higher concentrations of GO (from 15.75 to 40.5 ng/mm2). These results clearly show that the composite material (RGO–TiO2) features can be considered dominated neither by RGO nor by the TiO2 layer, differently, its sensing properties are reasonably ascribed to the RGO–TiO2 interface. These data are in good agreement with XPS results showing the key role played by the RGO–TiO2 interface. The response of RGO–TiO2 composite was much higher than the response of pure TiO2. This fact is related with the depletion layer formed between the n-type TiO2 and RGO sheets, which creates more active centers for H2.15 Consequently, the response of composite material was mainly determined by the modulation of the barrier height and barrier width formed between the TiO2 and RGO, which was further modulated by the gas adsorption.39,

41

The RGO–TiO2

conductance increased drastically till 15.75 ng/mm2 of GO (Figure 5(b)). Nevertheless, the structure showed an optimal response at 4.5 ng/mm2 (Figure 6 (a)). The RGO–TiO2 conductance enhancement and the depletion layer formation (between the TiO2 and RGO) due to the presence of RGO sheets on the TiO2 NTs define the response of the composite material. The created depletion layer between the TiO2 and RGO plays significant role for the gas adsorption at the very low concentration of GO droplets. Moreover, when the concentration of GO increases the RGO sheets cover each other and forming relatively tick layers of RGO on TiO2 NTs. According to XPS analysis, these formed RGO thick layers feature a weaker reduction than the one featured by the RGO thinner (a lower concentrations) layers, which may be responsible for the reduced response amplitude.42 The obtained results show that the effective reduction of GO and its concentration change have significant effect on RGO–TiO2 sensing performance. Hence, to improve the sensing properties of RGO–TiO2 composite material and to find out the optimal operation conditions aforementioned issues should be considered in parallel with the structure working temperature.


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In addition, the conductance of RGO–TiO2 sample after the gas test was recovered to its initial baseline value as the conductance of pure TiO2 (Figure 6(b)). This fact proving the stability of RGO–TiO2 structure after its thermal treatment and reduction of GO in Ar atmosphere. This is another important property for the reversible interaction between the analyte gas and the composite structure at the relatively high operating temperatures. The sensing mechanism of the composite is based on the adsorption/desorption processes of gaseous compounds occurring at the surface of the material followed by its conductance change.41,

43

The composite structure

showed n-type response at all concentrations of GO. Consequently, when the structure is in the test chamber under the ambient conditions the oxygen is ionosorbed on its surface extracting electrons and thus decreasing the conductance of the material. The detection process is related with the reactions between the H2 and the ionosorbed oxygen (O ― , O2 ― ) species.44 When the H2 is introduced to the test chamber it interacts with the ionosorbed oxygen species, creates free electrons on the surface of the material increasing its conductance (Figure 6(b)).43 The response of the composite RGO–TiO2 sample (obtained with the optimal concentration of GO, 4.5 ng/mm2) was increased compared with the pristine TiO2 NTs from 4.1 to 10.7 times depending on the concentration of H2 (Table 1). In particular, the heterojunction formed between the TiO2 NTs and RGO plays crucial role for the improvement of the material sensing response.41 The response and recovery times of the TiO2 and RGO−TiO2 (obtained with the optimal concentration of GO, 4.5 ng/mm2) samples towards different concentrations of H2 at 200 °C are reported in the Table 1. It is worth noting that these parameters are related to both the sensor response/recovery times and the filling/purging-times of the test chamber hosting sensors. The latters are about 300 seconds for our experimental setup, which should than be considered as the lower measurable limits. Based on sensor signal vs time data (Figure 6(b)), the observed sensor recovery time is lower than 300 s, meaning that it is mainly dominated by the setup kinetics, with sensors showing recovery times comparable or lower than those of the test chamber. The selectivity of obtained samples was studied towards 120 ppm of acetone (C3H6O), H2, carbon monoxide (CO) and 30 ppm of ammonia (NH3) at operating temperature of 200 °C (Figure 7). A small decrease in the response of the composite structure towards C3H6O was obtained compared to the response of the pristine TiO2 NTs. The response of the composite structure towards H2 increased 4.1 times compared to the response of the pristine TiO2. No significant change was observed in the response of the samples towards NH3. Instead, the



response of the composite towards CO decreased 2 times compared to TiO2. Thus, the presence of RGO, its reduction and proportion may affect the selectivity of the RGO−TiO2 composite structure as well.


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Figure 6. (a) The response vs concentration of GO dependence of RGO and RGO–TiO2 towards 480 ppm of H2 at 200 °C. (b) The dynamical response of TiO2 and RGO–TiO2 (obtained with the optimal concentration of GO, 4.5 ng/mm2) towards 120, 240 and 480 ppm of H2 at 200 °C. (c)



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The dynamical response of RGO samples (obtained with 4.5 and 6.75 ng/mm2 of GO) towards 120, 240 and 480 ppm of H2 at 200 °C. Table 1. The sensing parameters of the TiO2 and RGO−TiO2 (obtained with 4.5 ng/mm2 of GO) samples towards 120, 240 and 480 ppm of H2 at 200 °C. The recoded recovery times of both structures were less than the purge-time (300 s) of the H2 into the test chamber. Due to this reason, the recorded exact values even by the computer controlled gas-test program can not indicate the precise response time, we can only state the recovery time is lower or comparable than the chamber purging time. Sample TiO2 RGO–TiO2

H2 concentration (ppm) 120 240 480 120 240 480

Response

Response time (s)

Recovery time (s)

1.3 2.2 3.5 5.4 13.2 37.6

1050 900 990 1320 1470 1110

≤300 ≤300 ≤300 ≤300 ≤300 ≤300

Figure 7. The response of TiO2 and RGO−TiO2 (obtained with 4.5 ng/mm2 of GO) structures towards 120 ppm of C3H6O, H2, CO, and 30 ppm of NH3 at the operating temperature of 200 °C. 4. CONCLUSIONS In summary, we have prepared a composite structure composed of RGO and TiO2 NTs and studied its sensing properties considering the compositional and structural effects of each material. We have demonstrated that the concentration of RGO has significant effects on gas


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sensing properties of RGO–TiO2 composites. The RGO platelets improve the charge transport through the TiO2 tubular arrays enhancing the composite conductance. Meanwhile, a depletion layer is formed between the n-type TiO2 and RGO platelets which plays an essential role for the improvement of the material sensing response. The RGO concentration variation should be considered as well to find out its optimal value for the TiO2 functionalization. The obtained results indicate that the GO concentration variation, its reduction and the reduction conditions should be carefully considered for its applications to improve the gas sensing performance of metal oxides and the other functional properties as well. The goal of our next work is the fabrication of high performance chemical sensors for specific gases based on the developed approach in present study. We will introduce different dopant materials in the composite structure and we will investigate the sensing properties (response, selectivity, long-term stability) of the obtained materials for the wide range of concentrations of different gases including the effect of humidity. ASSOCIATED CONTENT Supporting Information Additional information regarding the investigation of the samples' sensing properties to find their optimal operating temperature. The response vs concentration of GO dependence of RGO–TiO2 composite towards 120 ppm of H2 at 100, 200 and 300°C (Figure S1(a)), the response vs concentration of GO dependence of RGO–TiO2 composite towards 240 ppm of H2 at 100, 200 and 300°C (Figure S1(b)), the response vs concentration of GO dependence of RGO–TiO2 composite towards 480 ppm of H2 at 100, 200 and 300°C (Figure S1(c)). AUTHOR INFORMATION * Corresponding

Author

E-mail address: [email protected] REFERENCES (1)

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Table of content graphic for manuscript: The schematic illustration of the RGO–TiO2 composite structure and the dynamical response of the TiO2 and RGO–TiO2.


Reduced Graphene OxideâTiO2 Nanotube Composite

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