Reduced Graphene OxideâTiO2 Nanotube Composite

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Cite This: ACS Appl. Nano Mater. 2018, 1, 7098−7105

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,⊥ and Giorgio Sberveglieri† †

Sensor Laboratory, Department of Information Engineering, University of Brescia, Via Valotti 9, 25133 Brescia, Italy CNR, National Institute of Optics, Via Branze 45, 25123 Brescia, Italy § Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ∥ 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

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‡

S Supporting Information *

ABSTRACT: Graphene oxide (GO) and reduced graphene oxide (RGO) have unique properties that can revolutionize the 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 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 one to track the process effects through sensor conductance. We systematically investigated the gas-response dependence from the RGO 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. KEYWORDS: graphene oxide, reduced graphene oxide, reduction effect, TiO2 nanotube, surface functionalization, gas sensing

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 environmental monitoring.1,2 1D oxide materials such as SnO2, TiO2, and ZnO exhibit promising sensing performances 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 their full potential in capability 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 surfaces. 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 surface reactions, are one of the most studied wide-band-gap © 2018 American Chemical Society

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 different materials plays a significant role for improvement of its conductance and response.1,7 The dopants also act as chargecarrier recombination centers affecting the material performance.8 In this regard, the preparation of composite materials based on TiO2 seems to be 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 due to its excellent electronic properties.9 Among the graphene-based structures, graphene oxide (GO) is very attractive because of its unique physical, chemical, and structural properties. GO is an insulating material that contains Received: October 25, 2018 Accepted: December 4, 2018 Published: December 4, 2018 7098

DOI: 10.1021/acsanm.8b01924 ACS Appl. Nano Mater. 2018, 1, 7098−7105

Article

ACS Applied Nano Materials

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 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 the 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

oxygen groups on the plane of carbon atoms. It can be easily modified to reduced GO (RGO) by removing the oxygencontaining 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 reduction of GO.10,12 RGO structures studied for chemical gas-sensing applications showed a 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 a 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 by 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 authors17,22,23 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 toward NO2. So far, the effective development of a strategy for the fabrication of a composite structure requires the investigation of two main issues, namely, variation of the GO concentration and reduction of GO. The study of these two effects in parallel on the metal oxides’ gas-sensing response is crucial for improvement of the oxide material sensing performance and 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 hydrogen (H2), which is an extremely clean fuel. It is considered to be 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 a colorless, odorless, flammable, and explosive gas. Consequently, the use of H2 involves dangers associated with 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 concentration effects of GO on the response of TiO2 NTs. We have chosen H2 as the 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 GO with the metal oxide materials to fabricate highperformance gas sensors and other functional devices.

Figure 1. Schematic illustration of the fabrication of a composite structure for the electrical and gas-sensing studies.

RGO (RGO−TiO2), we used an aqueous dispersion of GO with a concentration of 0.03 mg/mL. Afterward, we drop-casted 0.3 μL of an aqueous dispersion of GO onto 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 platinum parallel electrodes with a thickness of 500 nm were deposited onto the surface of prepared structures by means of radiofrequency magnetron sputtering. Before the deposition of platinum electrodes, a titanium/tungsten (10/90 wt %) adhesion layer of about 90 nm thickess was applied. To control the sample temperature, a platinum heater was deposited onto the backside of the alumina substrates. To enhance control over the reduction of the prepared GO and composite materials, we applied the reduction process directly to 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 rises to 400 °C and is kept constant for 4 h, and finally the RT conditions are restored. The same treatment has been applied to pure TiO2 for comparison. As better detailed in the Results and Discussion section, measuring the sensor signal during the thermal treatment allows real-time feedback on the applied reduction process. Because 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 the loading effects through a comparison of different samples, even if they are nominally identical, may give the nontrivial 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 a LEO 1525 scanning electron microscope (SEM) equipped with a field-emission gun. Raman spectroscopy (WITec Micro-Raman Spectrometer Alpha 300; λ = 488 nm; 100× objective) was used to characterize the samples. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PerkinElmer PHI 5600ci spectrometer with a standard Al Kα source working at 250 W. Both extended (survey, 187.85 eV pass energy, 0.5 eV/step, and 0.025 s/step) and detailed (for O 1s and C 1s, 23.5 eV pass energy, 0.1 eV/step, and 0.1 s/step) spectra were recorded. The working pressure was less than 7 × 10−7 Pa. The standard deviation in the binding energy (BE) values is ±0.1 eV.

2. EXPERIMENTAL SECTION Fabrication of the Samples. Pure TiO2 NTs were prepared using the following procedures. Metallic titanium films with a thickness of 500 nm were deposited on 2 mm × 2 mm × 0.75 mm alumina substrates by means of radio-frequency (13.56 MHz) magnetron sputtering. To improve adhesion of the metallic films to the substrates, the temperature of the 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 RT. A platinum foil was used as the counter electrode. As-prepared tubular structures were crystallized by thermal annealing under a 50 vol % O2/50 vol % Ar gas atmosphere at 400 °C for 6 h. 7099


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ACS Applied Nano Materials Electrical and Gas-Sensing Characterization. The gas-sensing properties of samples were studied in a computer-controlled thermostatic test chamber by a 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 H 2 in three different concentrations of (120, 240, and 480 ppm) at 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 synthetic air after every exposure to test gases for 85 min to allow full recovery of the samples. The conductance of the structures was monitored by means of the voltamperometric technique at an applied constant voltage of 1 V. The conductance values were recorded every 30 s. The sensing response (S) of the samples was calculated according to the following equation: S =

|(Gf − G0)| G0

=

⌈ΔG ⌉ , G0

clearly observable Raman-active modes at 144, 399, 519, and 639 cm−1, as exhibited in Figure 3a. Raman spectroscopy was

Figure 3. Raman spectra of (a) pure TiO2 NTs and (b) RGO on an alumina substrate.

where G0 is the sample conductance

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 3b). The G peak is a primary in-plane vibrational mode corresponding to the in-plane bond-stretching optical vibration of sp2-hybridized 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 because of 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 peaks of the GO and RGO samples obtained with GO concentrations of 6.75 and 15.75 ng/mm2, respectively, are shown in Figure 4. The C 1s peak of the 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 eV (C in epoxide C−O−C), and 288.4 eV (C in carbonyl CO and carboxylate OC−OH). The presence of different oxygen-containing functional groups, in particular epoxide groups, on GO is in agreement with the literature.33−35 A successive thermal treatment at 400 °C for 4 h in pure Ar atmosphere of the GO sample produces several changes depending on the GO concentration. In the sample obtained with 6.75 ng/mm2 GO, the contributions due to epoxide (286.4 eV) and carbonyl and carboxyl groups (288.4 eV) disappear, suggesting a successful reduction. Unlike the sample with 6.75 ng/mm2 GO, in the sample with a higher concentration of GO (15.75 ng/mm2), an increase of the contributions at higher BEs characteristic of oxygen-containing groups with respect to graphitic C is evident. The poor stability of RGO thick layers could be the reason for 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 thicker layer of RGO is deposited, the interface stabilizing effect of substrate is lost and the highly active RGO surface layers easily react with the atmospheric moisture and CO2, forming these oxygen-containing groups. Electrical and Gas-Sensing Characterization. The reduction process carried out in an Ar atmosphere was characterized in situ by measuring the electrical conductance of the material under treatment. Figure 5a presents the conductance variation of the pure TiO2, RGO, and RGO− TiO2 samples during their thermal treatment in a pure Ar atmosphere. For all samples, the conductance increases with

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

3. RESULTS AND DISCUSSION Materials Characterization. Figure 2 presents the morphological analyses of pure TiO2 and RGO−TiO2 samples.

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

As can be seen in Figure 2a, the TiO2 NTs with a diameter of 30 nm were obtained on alumina substrates. We made a few scratches on the surface of the samples to carry out the surface and cross-sectional analyses. The obtained GO sheets are shown in Figure 2b. Parts c, d, and f of Figure 2 show the scanning electron microscopy (SEM) images of a RGO−TiO2 composite material, where the surface regions of the TiO2 structure are covered with RGO (Figure 2c,d,f) and the regions of pure TiO2 (Figure 2d,f) are indicated. In Figure 2d,e is shown a cross-sectional view of a TiO2 tubular array with an average length of the tubes of about 1 μm. Raman spectroscopy studies of TiO2 NTs used in our experiments show the predominant anatase structure with 7100


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RGO sheets connecting the TiO2 NTs with each other facilitate charge transport through the tubular arrays.38 Then, we investigated the conductance variation of the RGO and RGO−TiO2 samples depending on the concentration of GO (Figure 5b). The conductance of the RGO−TiO2 samples drastically increased by about 3 orders of magnitude with increasing RGO loading, until 15.75−18 ng/mm2. We did not observe big changes of the RGO−TiO2 conductance at higher concentration of RGO (from 18 to 33.75 ng/mm2). The conductance of the RGO sample increased drastically until 9 ng/mm2 GO and remained nearly constant at 15.75 ng/mm2. Then, there was a drastic decrease and increase of the conductance at 18 and 24.75 ng/mm2, respectively. In Figure 5b, it can be seen that the conductance change feature of the 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 the RGO conductance followed by the small variations. Given the larger electrical conductivity of RGO with respect to GO,14 the observed behavior is consistent with the improved reduction of GO observed with XPS for small RGO concentrations compared to larger concentrations. These analyses indicate that, with an increase of the RGO concentration in the RGO−TiO2 composite, RGO deeply affects the RGO−TiO2 conductance. We studied the sensing performance of the RGO−TiO2 samples in the temperature range of 100−300 °C, starting from the pure TiO2 samples and increasing the concentration of GO until 40.5 ng/mm2 (Figure S1). The samples demonstrated the best gas-sensing performance at the sensor temperature of 200 °C. Therefore, we will have the further discussions based on the sensing measurements carried out in these conditions. Figure 6a 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 of the RGO loading studied (from 0 to 40.5 ng/mm2), with an optimal response recorded for 4.5 ng/mm2 GO, which is about 1 order of magnitude higher than the response exhibited by pure TiO2 (zero droplets in Figure 4). Increasing the RGO loading beyond 4.5 ng/mm2 caused the response to decrease by more than 2 orders of magnitude, making the material less responsive than the original TiO2 layer but keeping the ntype sensing character (Figure 6a). No significant changes in the response were observed at higher concentrations of RGO (from 15.75 to 40.5 ng/mm2). As for the RGO sample, this is not sensitive to the H2 until 6.75 ng/mm2 GO. The RGO showed a p-type weak response toward H2 after the dropcasting of 6.75 ng/mm2 GO (Figure 6a). The p-type response is typical for the RGO chemical sensors.39 Furthermore, the RGO sample obtained with 6.75 ng/mm2 GO showed the highest response compared to the RGO samples prepared with other concentrations of GO. The dynamic response of the RGO samples obtained with 4.5 and 6.75 ng/mm2 GO is shown in Figure 6c. The improved conductance of the RGO sample could be reasonably related with the percolation threshold reached, which predicts 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 GO, and after 15.75 ng/mm2, the RGO sample has a response similar to that of the RGO−TiO2 structure functionalized with 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

Figure 4. C 1s XPS spectra for GO, RGO of 6.75 ng/mm2, and RGO of 15.75 ng/mm2. Spectra are normalized with respect to their maximum and minimum. Fitting is also reported (dotted line).

Figure 5. (a) Conductance dependence versus time of the pure TiO2, RGO, and RGO−TiO2 structures during their thermal treatment, where treatment intervals at RT and 400 °C are indicated. (b) RT conductance changes 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.

increasing temperature from RT to 400 °C and reaches a nearly steady state value. When the RT condition is restored (while maintaining a pure Ar atmosphere), the conductance of pure TiO2 recovers its initial (before the 400 °C treatment) value, suggesting that this process does not appreciably alter the TiO2 matrix. Differently, the conductance of the pure RGO and RGO−TiO2 samples stabilizes to a higher value, indicating the occurrence of nonreversible effects. This is expected from the GO/RGO literature: GO is an insulating material, and its conductivity is improved upon reduction.36 The conductance improvement of the RGO−TiO2 samples is related with the presence of RGO sheets on the surface of TiO2 NTs.37 The 7101


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significant role in gas adsorption at the very low concentrations of GO droplets. Moreover, when the concentration of GO increases, the RGO sheets cover each other and form relatively thick layers of RGO on the TiO2 NTs. According to XPS analysis, these formed RGO thick layers feature a weaker reduction than that featured by the RGO thinner (lower concentration) 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 a significant effect on the RGO−TiO2 sensing performance. Hence, to improve the sensing properties of the RGO− TiO2 composite material and to find the optimal operation conditions, aforementioned issues should be considered in parallel with the structure working temperature. In addition, the conductance of the RGO−TiO2 sample after the gas test was recovered to its initial baseline value as the conductance of pure TiO2 (Figure 6b). This fact proves the stability of the RGO−TiO2 structure after its thermal treatment and reduction of GO in an Ar atmosphere. This is another important property for the reversible interaction between the analyte gas and composite structure at 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 an n-type response at all concentrations of GO. Consequently, when the structure is in the test chamber under ambient conditions, the oxygen is ionosorbed onto 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− and O2−) species.44 When H2 is introduced to the test chamber, it interacts with the ionosorbed oxygen species and creates free electrons on the surface of the material, increasing its conductance (Figure 6b).43 The response of the composite RGO−TiO2 sample (obtained with the optimal concentration of GO, 4.5 ng/mm2) was increased compared to 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 a crucial role for improvement of the material sensing response.41

Figure 6. (a) Response versus concentration of GO dependence of RGO and RGO−TiO2 toward 480 ppm of H2 at 200 °C. (b) Dynamical response of TiO2 and RGO−TiO2 (obtained with the optimal concentration of GO, 4.5 ng/mm2) toward 120, 240, and 480 ppm of H2 at 200 °C. (c) Dynamical response of RGO samples (obtained with 4.5 and 6.75 ng/mm2 GO) toward 120, 240, and 480 ppm of H2 at 200 °C.

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 the XPS results, showing the key role played by the RGO− TiO2 interface. The response of the 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 the composite material was mainly determined by modulation of the barrier height and barrier width formed between TiO2 and RGO, which was further modulated by gas adsorption.39,41 The RGO−TiO2 conductance increased drastically until 15.75 ng/mm2 GO (Figure 5b). Nevertheless, the structure showed an optimal response at 4.5 ng/mm2 (Figure 6a). The RGO−TiO2 conductance enhancement and depletion of layer formation (between 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 TiO2 and RGO plays a

Table 1. Sensing Parameters of the TiO2 and RGO−TiO2 (Obtained with 4.5 ng/mm2 GO) Samples toward 120, 240, and 480 ppm of H2 at 200 °C sample TiO2

RGO−Ti O2

H2 concentration (ppm)

response

response time (s)

recovery time (s)a

120 240 480 120

1.3 2.2 3.5 5.4

1050 900 990 1320

≤300 ≤300 ≤300 ≤300

240 480

13.2 37.6

1470 1110

≤300 ≤300

a

The recoded recovery times of both structures were less than the purge time (300 s) of the H2 into the test chamber. For this reason, the recorded exact values even by the computer-controlled gas-test program cannot indicate the precise response time; we can only state that the recovery time is lower or comparable with the chamber purging time.

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the fabrication of high-performance chemical sensors for specific gases based on the developed approach in the present study. We will introduce different dopant materials in the composite structure and investigate the sensing properties (response, selectivity, long-term stability, etc.) of the obtained materials for a wide range of concentrations of different gases, including the effect of humidity.

The response and recovery times of the TiO2 and RGO− TiO2 (obtained with the optimal concentration of GO, 4.5 ng/ mm2) samples toward different concentrations of H2 at 200 °C are reported in 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 latter is about 300 s for our experimental setup, which should than be considered as the lower measurable limit. On the basis of the sensor signal versus time data (Figure 6b), 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 the obtained samples was studied toward 120 ppm of acetone (C3H6O), H2, carbon monoxide (CO), and 30 ppm of ammonia (NH3) at an operating temperature of 200 °C (Figure 7). A small decrease in the response of the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01924.

■

Additional information regarding the investigation of the samples’ sensing properties to find their optimal operating temperature and response versus concentration of the GO dependence of a RGO−TiO 2 composite toward 120 ppm of H2 at 100, 200, and 300 °C (Figure S1a), toward 240 ppm of H2 at 100, 200, and 300 °C (Figure S1b), and toward 480 ppm of H2 at 100, 200, and 300 °C (Figure S1c) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vardan Galstyan: 0000-0002-0615-3097 Andrea Ponzoni: 0000-0001-9955-5118 Marta M. Natile: 0000-0001-5591-2670 Notes

Figure 7. Response of the TiO2 and RGO−TiO2 (obtained with 4.5 ng/mm2 GO) structures toward 120 ppm of C3H6O, H2, and CO and 30 ppm of NH3 at an operating temperature of 200 °C.

The authors declare no competing financial interest.

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REFERENCES

(1) Galstyan, V.; Comini, E.; Faglia, G.; Sberveglieri, G. TiO2 Nanotubes: Recent Advances in Synthesis and Gas Sensing Properties. Sensors 2013, 13, 14813−14838. (2) Spencer, M. J. S. Gas Sensing Applications of 1D-nanostructured Zinc Oxide: Insights from Density Functional Theory Calculations. Prog. Mater. Sci. 2012, 57, 437−486. (3) Yang, D. J.; Kamienchick, I.; Youn, D. Y.; Rothschild, A.; Kim, I. D. Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading. Adv. Funct. Mater. 2010, 20, 4258−4264. (4) Galstyan, V.; Comini, E.; Baratto, C.; Faglia, G.; Sberveglieri, G. Nanostructured ZnO Chemical Gas Sensors. Ceram. Int. 2015, 41, 14239−14244. (5) Galstyan, V.; Vomiero, A.; Concina, I.; Braga, A.; Brisotto, M.; Bontempi, E.; Faglia, G.; Sberveglieri, G. Vertically Aligned TiO2 Nanotubes on Plastic Substrates for Flexible Solar Cells. Small 2011, 7, 2437−2442. (6) Ellis, B. L.; Knauth, P.; Djenizian, T. Three-Dimensional SelfSupported Metal Oxides for Advanced Energy Storage. Adv. Mater. 2014, 26, 3368−3397. (7) Galstyan, V.; Comini, E.; Baratto, C.; Ponzoni, A.; Ferroni, M.; Poli, N.; Bontempi, E.; Brisotto, M.; Faglia, G.; Sberveglieri, G. Large Surface Area Biphase Titania for Chemical Sensing. Sens. Actuators, B 2015, 209, 1091−1096. (8) Choi, W. Y.; Termin, A.; Hoffmann, M. R. The Role of MetalIon Dopants in Quantum-Sized TiO2: Correlation Between Photoreactivity and Charge-Carrier Recombination Dynamics. J. Phys. Chem. 1994, 98, 13669−13679. (9) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652−655.

composite structure toward C3H6O was obtained compared to the response of the pristine TiO2 NTs. The response of the composite structure toward H2 increased 4.1 times compared to the response of the pristine TiO2. No significant change was observed in the response of the samples toward NH3. Instead, the response of the composite toward 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.

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 the gas-sensing properties of RGO−TiO2 composites. The RGO platelets improve 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 improvement of the material-sensing response. The RGO concentration variation should be considered as well to find out its optimal value for TiO2 functionalization. The obtained results indicate that 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 other functional properties as well. The goal of our next work is 7103


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