ARTICLE pubs.acs.org/JPCC
Novel Synthesis and Gas Sensing Performances of CuOTiO2 Nanocomposites Functionalized with Au Nanoparticles Davide Barreca,*,† Giorgio Carraro,‡ Elisabetta Comini,§ Alberto Gasparotto,‡ Chiara Maccato,‡ Cinzia Sada,|| Giorgio Sberveglieri,§ and Eugenio Tondello‡ †
CNR-ISTM and INSTM, Department of Chemistry, Padova University, 35131 Padova, Italy Department of Chemistry, Padova University and INSTM, 35131 Padova, Italy § CNR-IDASC, SENSOR Lab, Department of Chemistry and Physics, Brescia University, 25133 Brescia, Italy Department of Physics and CNISM, Padova University, 35131 Padova, Italy
)
‡
ABSTRACT: CuOTiO2 nanocomposites were synthesized on Al2O3 substrates by a novel chemical vapor deposition (CVD) route, based on the sequential growth of CuO matrices (550 °C) and the overdispersion of TiO2 (400 °C), both performed under O2 þ H2O atmospheres. The obtained supported materials were subsequently functionalized with gold nanoparticles (NPs) by means of radio frequency (rf) sputtering. The system structure, nano-organization, and chemical composition were characterized by a multitechnique approach, using glancing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). For the first time, both CuOTiO2 and CuOTiO2Au nanosystems were tested as resistive gas sensors for toxic and flammable gases (CH3CH2OH, H2, and O3), revealing attractive performances even at moderate working temperatures. Interestingly, the functional response could be appreciably enhanced upon introduction of gold NPs, highlighting the present CuOTiO2Au nanosystems as appealing candidates in view of technological applications.
’ INTRODUCTION In the last two decades, the fabrication of cheap, low-sized, and reliable gas sensors with reduced power consumption and enhanced performances has attracted a remarkable interest due to the growing global concern on environmental monitoring, domestic/public safety, and automotive applications.14 In this context, several studies have focused on ethanol detection for biomedical and food industries, as well as breath sampling for fleet management companies and driver security purposes.58 On the other hand, a major challenge regards the efficient monitoring of flammable or toxic analytes, such as hydrogen, an attractive energy vector which is an odorless, colorless, and explosive gas.9,10 In addition, an increasing demand of ozone sensors has also been registered, due to their importance for the environment and human safety.5 In view of these applications, an advanced nano-organization enhances the sensitivity to surface-adsorbed species in comparison with conventional microstructured materials. Further synergistic contributions arising from the interrelations between electronic and nanoscale properties produce improved performances in terms of both detection limit and selectivity, two main practical issues which are strongly dependent on gassurface interactions.2,3,7,8,11,12 In this scenario, the engineering of supported sensing devices based on nanocomposite semiconductor (SC) metal oxides is undoubtedly a strategic issue.6,13 In particular, the development of chemical surface modifications, with an intimate contact between r 2011 American Chemical Society
the various components, is a valuable means to produce specific active sites.6,14,15 For instance, the sensing activity of SC metal oxides can be advantageously tailored by loading with noble metal nanoparticles (NPs; e.g., Pd, Pt, Au), resulting in an improved charge carrier separation with beneficial effects on sensitivity, response time, and operational temperatures.7,11,1619 The selection of appropriate modifiers is usually based on the target analytes and the possible interactions with the SC surface. Up to date, most of the pertaining works have been focused on n-type oxide SCs, which are almost ubiquitous for their intrinsic O defects. In a different way, investigation of p-type nanocomposite sensors is still lacking, although their development is highly demanded for various uses, such as sensor arrays for electronic noses.4,8,20 In this regard, CuOTiO2 nanocomposites represent one of the most appealing systems thanks to the synergistic combination between the single-oxide properties, such as the low band gap of p-type CuO4,2123 and the high reactivity of n-type TiO2.6,24,25 The pn junction resulting from their coupling is expected to produce an enhanced charge carrier lifetime, with beneficial impact both on catalytic and sensing performances, also arising from the TiO2 promotion of the involved chemical processes.6 A detailed discussion of photocatalytic activation/deactivation effects in CuOTiO2 Received: March 15, 2011 Revised: April 28, 2011 Published: May 12, 2011 10510
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The Journal of Physical Chemistry C systems has been recently reported by Li et al.26 Despite that various works have dealt with the introduction in TiO2 of metal and oxide species, including CuO,15,2632 chemical modifications of copper(II) oxide nanosystems have seldom been reported. In particular, only a work on the functionalization of nanostructured CuO sensors with Au and/or Pt NPs8 and a paper on powdered CuxOTiO2Au catalysts33 are available in the literature up to date, whereas no studies on CuO nanomaterials modified with TiO2 for gas sensing applications have ever been carried out. Yet, intriguing performances are expected upon introduction of metal NPs in such composites, thanks to their favorable influence on the system activity.8,34 In the present work, we report on the synthesis of CuO TiO2Au nanocomposite gas sensors based on a multistep vapor-phase process. The first stage is the preparation of porous CuO nanosystems on Al2O3 substrates by chemical vapor deposition (CVD), followed by the controlled growth of TiO2 NPs over copper oxides matrices. As a final step, gold nanoparticles were deposited over the above specimens by radio frequency (rf) sputtering at low temperatures and applied powers, in order to avoid detrimental alterations of the pristine CuOTiO2 nanocomposites. To the best of our knowledge, the above preparative strategy for gas sensors has never been reported in the literature up to date. A thorough insight into the system chemical and physical properties is presented, with particular attention on the spatial distribution of titanium dioxide and gold NPs in CuO matrices. For the first time, the performances of the prepared nanocomposites as gas sensors of ethanol, hydrogen, and ozone are reported and critically discussed, highlighting the role of TiO2 and Au nanoparticles on the system response, selectivity, and detection limits.
’ EXPERIMENTAL PROCEDURES Synthesis. In this work, polycrystalline Al2O3 slides (99.6%, thickness = 254 μm) were used as substrates and cleaned in dichloromethane and isopropyl alcohol before each growth experiment. A previously described cold-wall CVD apparatus equipped with an external precursor reservoir was used for CVD growth processes.34 CuO nanosystems were prepared starting from Cu(hfa)2 3 TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N0 ,N0 -tetramethylethylenediamine). The precursor was synthesized according to the literature,35 vaporized at 60 °C, and transported toward the growth surface by means of an O2 flow (purity = 6.0; flow rate = 100 sccm). Copper(II) oxide deposition was carried out for 120 min under optimized conditions (growth temperature = 550 °C, total pressure = 10 mbar). A separate auxiliary oxygen flow (100 sccm) was introduced in the reactor chamber after passing through a distilled water reservoir heated at 50 °C (H2O partial pressure ≈ 1.5 mbar).6,23,34 Gas lines and valves between H2O/precursor reservoirs and the reaction chamber were heated at 120 °C to prevent detrimental condensation phenomena. TiO2 dispersion over the as-prepared CuO nanosystems was performed by the same CVD apparatus, using as precursor Ti(O-iPr)2(dpm)2 (O-iPr = isopropoxy; dpm = 2,2,6,6-tetramethyl3,5-heptanedionate; Aldrich, 99.99%), vaporized at 80 °C. Depositions were carried under the following experimental conditions: growth temperature = 400 °C; total pressure = 10 mbar; duration = 5 min; carrier O2 flow rate = 40 sccm; auxiliary (O2 þ H2O) flow rate = 80 sccm. Water introduction and gas line heating were performed under the same conditions already described for CuO
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Table 1. Deposit Thickness for the Present Nanocomposite Specimens sample name
deposit thickness (nm)
CuO
170 ( 25
CuOAu CuOTiO2
170 ( 34 187 ( 27
CuOTiO2Au
188 ( 29
deposition. At the end of TiO2 growth, nanocomposite samples were cooled down to room temperature under flowing O2 before exposure to the outer atmosphere. As-prepared CuOTiO2 systems were subsequently inserted into a two-electrode (ν = 13.56 MHz) rf-sputtering instrumentation36 for gold deposition. The process was performed starting from a Au metal target (BAL-TEC AG, 99.99%) using Ar (purity = 6.0) plasmas, under the following conditions: substrate temperature = 60 °C; rf power = 5 W; total pressure = 0.38 mbar; Ar flow rate = 10 sccm; time = 10 min.7 For comparison purposes, CuOAu materials were also synthesized and characterized. In all cases, ex situ annealing treatments were intentionally avoided in order to prevent the possible formation of CuTiO ternary phases.37 Characterization. Glancing incidence X-ray diffraction (GIXRD) patterns were recorded by a Bruker D8 Advance diffractometer equipped with a G€obel mirror and a Cu KR source powered at 40 kV, 40 mA (incidence angle = 1.0°). The average crystallite dimensions were estimated by means of the Scherrer equation. Field emission scanning electron microscopy (FE-SEM) measurements were performed by means of a Zeiss SUPRA 40VP instrument, at a primary beam acceleration voltage of 3 kV. Atomic force microscopy (AFM) micrographs were obtained by an NT-MDT SPM Solver P47H-PRO instrument operating in tapping mode and in air. After a plane fitting procedure, root mean square (rms) roughness values were estimated on 5 5 μm2 micrographs. X-ray photoelectron spectroscopy (XPS) analyses were carried out by a Perkin-Elmer Φ5600ci spectrometer at pressures lower than 1 108 mbar, using a standard Al KR excitation source (hν = 1486.6 eV). The reported binding energies (BEs, standard deviation = (0.2 eV) were corrected for charging effects by assigning to the C1s line of adventitious carbon a position of 284.8 eV.38 Quantitation was performed using sensitivity factors provided by Φ V5.4A software. Copper Auger parameter was calculated as previously reported.23,38,39 Secondary ion mass spectrometry (SIMS) measurements were carried out by means of an IMS 4f mass spectrometer (Cameca) using a Csþ primary beam (14.5 keV, 12 nA) and by negative secondary ion detection. Measurements were performed in high mass resolution (HMR) configuration to avoid mass interference artifacts. Charge compensation was achieved by means of an electron gun, avoiding to cover the sample surface by a metal film. The erosion rate dependence on the local matrix composition was evaluated by measuring the depth of the sputtered crater at the end of each analysis through a Tencor Alpha step profiler. The nanodeposit thickness values (see Table 1) were obtained by the substrate Al signal, after correcting the measured profile by the substrate roughness contribution. Gas sensing tests were carried out by the flow-through method in a thermostatic sealed chamber with controlled temperature 10511
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The Journal of Physical Chemistry C (20 °C) and relative humidity (40%). A detailed description of the experimental setup and the contact geometry has already been reported.40 Measurements were performed at atmospheric pressure using dry air as a carrier gas (flow rate = 0.3 slm). After a prestabilization for 8 h for each working temperature, the sensor resistance was measured as a function of the analyte concentration using the voltamperometric technique at a constant bias voltage (1 V). The sensor response (uncertainty = 5%) was calculated as the relative resistance/conductance variation upon exposure to the target gases for reducing (CH3CH2OH, H2)/ oxidizing (O3) analytes, respectively.7,19,41 Both response and recovery times were determined as previously reported.2,6 Under repeated cycling, the maximum response deviation did not exceed 10%, indicating a good reproducibility.
Figure 1. Representative GIXRD patterns for CuO and CuO TiO2 samples. The Al2O3 substrate diffraction peaks are marked by asterisks (/).
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’ RESULTS AND DISCUSSION In this work, the attention was initially devoted to the optimization of the CuO nanosystem preparative conditions, with particular attention on the obtainment of single-phase matrices with suitable porosity for the subsequent TiO2 and Au dispersion. Figure 1 reports GIXRD patterns of CuO and CuOTiO2 nanosystems synthesized adopting the previously described parameters. As can be noticed, both spectra presented only the typical diffraction peaks of monoclinic CuO (tenorite phase). In particular, the patterns showed the reflections at 2θ = 32.5°, 35.5°, 38.7°, 46.3°, and 48.8°, due to the (110), (111)/(002), (111)/(200), (112), and (202) planes, respectively.42 The relative intensities were comparable to those of the powder spectrum, indicating thus the absence of any texturing effect. Upon titania dispersion, no signals related to crystalline TiO2 or to mixed CuTiO phases could be observed, due to the high TiO2 dispersion and its relatively low amount.6,43 For the same reasons, no appreciable differences were observed in GIXRD patterns recorded after gold deposition (not reported). These observations suggested that the adopted synthesis conditions were mild enough to ensure the dispersion of low-sized TiO2 and Au NPs in copper(II) oxide matrices, obtaining an intimate contact between the nanocomposite single components. Irrespective of the processing conditions, the mean CuO crystallite size was evaluated to be 30 nm and did not undergo any significant variation upon TiO2 and Au deposition. The system nano-organization and morphology were investigated by the complementary use of FE-SEM and AFM. Planeview SEM micrographs showed that pure CuO morphology (Figure 2a), in line with previous findings,44 was characterized by the presence of evenly distributed “nano-flowers” (mean lateral
Figure 2. Selected plane-view FE-SEM and AFM micrographs for CuO (a and c) and CuOTiO2Au (b and d) nanocomposites. Higher magnification FE-SEM images are reported in the insets. 10512
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Figure 3. Representative SIMS depth profiles for CuO (a), CuOAu (b), CuOTiO2 (c), and CuOTiO2Au (d) nanosystems deposited on Al2O3.
size = 600 nm) resulting in a high active area, a key feature for the subsequent dispersion of TiO2 and Au NPs. As a matter of fact, after TiO2 and Au deposition (Figure 2b), no appreciable variations involving aggregate coalescence/collapse could be detected, validating the capability of the adopted synthetic protocol in functionalizing CuO nanodeposits without morphological alterations. Similar results have already been reported concerning Au/Co3O4 and ZnO/TiO2 nanosystems.6,7 The surface corrugation could be appreciated also from AFM images (Figure 2c and 2d), evidencing a globular and porous texture, with negligible differences on going from pure CuO to CuOTiO2Au specimens. In line with this observation, the evaluated rms roughness was ≈60 nm for all the analyzed samples. In order to investigate the surface and in-depth chemical composition, XPS and SIMS analyses were performed, focusing in particular on copper, titanium, and gold chemical states in CuOTiO2Au nanocomposites. In line with the above observations, the surface presence of copper (≈15 at. %) proved the high dispersion of TiO2 and Au NPs in the CuO matrix, without any complete CuO coverage. The formation of copper(II) oxide was confirmed by the Cu2p3/2 position (BE = 933.9 eV), the Auger parameter (R = 1851.4 eV), and the presence of shake-up satellites typical of the Cu(II) oxidation state.15,23,26,39,45 After TiO2 deposition, a shoulder at BE = 932.2 eV could be observed, pointing out the occurrence of a partial surface reduction from Cu(II) to Cu(I).23,39,45 This apparently unexpected phenomenon could be attributed to oxygen deficiencies in Ti(IV) centers of the deposited TiO2 NPs, acting as O getters and inducing, in turn, a partial Cu(II) f Cu(I) reduction.4649 It is worth noting that the surface presence of the Cu(I)/Cu(II) redox couple could be beneficial in view of gas sensing applications, since it might enable the detection of both oxidizing and reducing analytes with an improved efficiency. Two components contributed to the surface O1s line, one major band at 530.1 eV, attributed to CuOCu bonds, and a
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second one at higher BE, centered at 531.4 eV, mainly due to CuOH species.38,39,46,47 The latter, related both to air exposure and to the use of water vapor in the reaction atmosphere, likely played a key role in titania dispersion, since OH species act as nucleation sites for the CVD precursor and enhance the in-depth TiO2 dispersion into CuO matrices.6 In line with the above observations, Ti2p3/2 BE values (458.6 eV) proved the presence of TiO2 as such (≈8 at. % at the system surface) and enabled us to discard the formation of CuTiO ternary phases.6,26,38,39 In addition, the presence of metallic gold NPs was confirmed by the BE of the Au4f7/2 photoelectron peak, centered at 84.2 eV.7,36,38,39 The mean Au surface content was evaluated to be ≈13 at. %. Subsequently, the attention was devoted to analyzing the indepth sample composition by SIMS depth profiling (compare Figure 3), that provided important information on the mutual dispersion of the various components. Despite that the morphology of the present samples enabled only a limited accuracy in thickness determination, it was possible to identify the depositto-substrate interface by operating a deconvolution of the SIMS profiles, taking into account both the film roughness and the instrumental response.41 The determined thickness values, along with the corresponding uncertainties, are reported in Table 1. As a general trend, C contamination was negligible (lower than 40 ppm), pointing out to a high purity of the synthesized specimens. For bare CuO (Figure 3a), a homogeneous distribution of Cu and O throughout the nanodeposit thickness could be observed, confirming the uniform formation of copper(II) oxide. Upon gold functionalization (Figure 3b and Table 1), no significant variation of the overall thickness took place. Notably, the Au ionic yield trend was parallel to that of Cu, indicating an efficient and homogeneous dispersion of Au NPs throughout the CuO nanodeposit. Since Au deposition was performed at 60 °C, significant thermal effects could be discarded and the observed behavior could be attributed to the synergy between the matrix porosity and the inherent infiltration power of vapor-phase techniques,6 further aided by the plasma bombardment in the present case. TiO2 dispersion into pure CuO (specimen CuOTiO2, Figure 3c) resulted in a ≈10% increase of the overall deposit thickness (Table 1). As can be noted, Cu and Ti profiles displayed a similar trend throughout the sample, suggesting a notable intermixing between copper and titanium oxides. Since TiO2 deposition was carried out at 400 °C, the above phenomenon could be explained by interdiffusion processes. The observed tailing of Cu and Ti signals within the Al2O3 substrates resulted from both thermal diffusion events and the high alumina roughness.6,41 As far as Au deposition over CuOTiO2 is concerned (Figure 3d), Au and Cu signals presented once again the same shape and in-depth profile. Since SIMS artifacts, such as mass interference, could be excluded due to the adopted HMR configuration, the above results point out to the obtainment of CuOTiO2Au nanocomposites with a very uniform in-depth composition, resulting from a homogeneous dispersion of titania and gold NPs into the pristine copper(II) oxide matrix. Taken together, these features, demonstrating an intimate contact between CuO, TiO2, and Au, are extremely promising in view of gas sensing applications thanks to the possibility of exploiting the mutual electronic interactions between the various components. Gas sensing performances of the present nanomaterials were initially screened in the detection of various analytes, both 10513
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Figure 5. Responses as a function of working temperature for CuOTiO2 (a) and CuOTiO2Au (b) sensors toward selected CH3CH2OH, H2, and O3 concentrations (100 and 1000 ppm and 300 ppb, respectively). Figure 4. Selected dynamic responses exhibited by CuO-based nanocomposites to different CH3CH2OH (a) and H2 (b) square concentration pulses.
reducing (ethanol, hydrogen, carbon monoxide, methane) and oxidizing (ozone, nitrogen dioxide). As a matter of fact, the systems were almost insensitive to carbon monoxide (10 100 ppm), methane (1001000 ppm), and nitrogen dioxide (110 ppm), as already reported by some of us in the case of pure CuO nanomaterials.41 In a different way, significant responses (g0.5) were detected toward ethanol, hydrogen, and ozone. These data indicated a good selectivity of the present nanosystems, a main practical advantage in view of technological utilization.6,9,19 The analysis of sensing performances toward CH3CH2OH, H2, and O3 revealed an appreciable reproducibility even upon repeated cycling. Figure 4 presents the isothermal dynamic responses for various nanosystems to square concentration pulses of CH3CH2OH and H2 at 300 and 200 °C, respectively. For all specimens, the measured current dropped off in the presence of both reducing gases, a typical behavior for p-type SCs, due to a decrease of p-type carrier content upon reaction between the target gas and the adsorbed oxygen species.4,7,15,41,50 This observation is in line with the results of compositional and structural characterization, indicating the presence of p-type CuO as the main phase in all the analyzed nanosystems. Interestingly, the measured conductance variations were proportional to the target gas concentration and the recovery of the air value was almost complete upon switching off CH3CH2OH or H2 pulses, indicating a reversible interaction between the sensing elements and the analytes. On gas injection, the experimental curves decreased sharply and subsequently more slowly up to the end of the pulses, a phenomenon that could be better appreciated in ethanol detection, especially for CuOTiO2Au systems at the highest gas concentrations.
This behavior indicated that the analyte chemisorption on the system surface was the rate-determining step in the corresponding conductance variations, as recently reported for 1D ZnO nanoassemblies.2 Typical response and recovery times were evaluated to be of the order of 1 min. In order to attain a deeper insight into the system performances and into the role of Au NPs, the responses of CuOTiO2 and CuOTiO2Au nanosystems to selected concentrations of different analytes (CH3CH2OH, H2, and O3) are compared in Figure 5. Interestingly, in the case of reducing species (CH3CH2OH and H2), the presence of gold resulted in a response improvement (compare Figure 5a and 5b). In particular, regarding H2 detection, a shift of the minimum working temperature down to 100 °C took place upon functionalization with Au NPs. For both CuOTiO2 and CuOTiO2Au, the optimal working temperature was 200 °C and an increase up to 400 °C corresponded to a progressively lowered response, likely due to a minor H2 chemisorption under these conditions. In the case of ethanol, a qualitatively similar behavior was observed, with the best operating temperature of 300 °C and a decreased extent of surface reaction at 400 °C. These data were in line with recent results on p-type copper and cobalt oxide nanosensors.7,41 The interaction of O3, an oxidizing analyte, with CuOTiO2, produced higher responses than the other gases (Figure 5a). Whereas in this case a maximum-like behavior, with the best response at 200 °C, was observed, for CuOTiO2Au nanosystems the maximum response was registered at 100 °C and progressively decreased upon increasing the working temperature (Figure 5b), indicating a change in the selectivity pattern with respect to the pristine system. A similar effect of Au NPs in ozone detection has been recently reported by Korotcenkov et al.18 This behavior confirmed the valuable catalytic activation of gold nanoparticles, enabling an effective analyte detection at temperatures as low as 100 °C. Notably, the response under 10514
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Figure 6. Dependence of CuOTiO2 and CuOTiO2Au sensor responses on CH3CH2OH and O3 concentrations.
these conditions was even higher than that recently reported for 1D ZnO nanomaterials,5 paving the way to the efficient and selective detection of oxidizing species at low temperatures. The experimental data presented so far can be interpreted in relation to the possible gas sensing mechanisms. As already noted, the present electrical behavior is typical for p-type SC sensors. For such systems, the initial O2 chemisorption in air can be described as follows:50,51 O2 ðgÞ þ 2e a 2O ðadsÞ
ð1Þ
The subsequent processes depend on the reducing (eqs 2 and 3) or oxidizing (eq 4) nature of the target gas analytes:50,51 CH3 CH2 OH þ 6O ðadsÞ a 2CO2 ðgÞ þ 3H2 OðgÞ þ 6e ð2Þ H2 þ O ðadsÞ a H2 OðgÞ þ e
ð3Þ
O3 þ e a O3 ðadsÞ a O ðadsÞ þ O2 ðgÞ
ð4Þ
In cases 2 and 3, the electron release in the conduction band leads to a decreased amount of the majority hole carriers in the surface charge layers, determining the observed conductance drop-off (Figure 4).50 Conversely, as concerns ozone detection (case 4), the electron transfer to physisorbed O3, that acts as an electron trap, might yield O3 and, ultimately, O species. As a result, the potential barrier height at grain boundaries is increased and, for p-type sensors, a conductance enhancement occurs.52,53 The above-proposed schemes enable a clearer interpretation of the sensing performance improvement induced by gold NPs that could be related to the CuO/TiO2/Au intimate contact already evidenced by SIMS analyses (see above). As a result, a noticeable catalytic promotion of Au nanoparticles on the involved surface reactions took place,8,17 irrespective of the target analyte. Concerning the detection of H2 and CH3CH2OH (cases 2 and 3), the performance enhancement for CuOTiO2Au versus CuOTiO2
nanocomposites could be related to an improved charge carrier separation due to the high dispersion of Au NPs. In particular, the Schottky barrier character of the interface between Au (electron sink) and the composite oxide enabled an increased lifetime of electrons released in processes 2 and 3.6,7,54 Conversely, in the detection of O3, Au NPs can facilitate and activate reactions 4 even at lower temperatures, with a promotional effect on the detection efficiency even at 100 °C.8,17 The response versus concentration trends for the detection of ethanol and ozone are shown in Figure 6 for CuOTiO2 and CuOTiO2Au nanocomposites. As regards ethanol, the responses of the present composite nanosystems were appreciably better than those previously reported for pure CuO.4 In all cases, the experimental trends displayed a linear dependence on concentration throughout the investigated range, without any appreciable saturation, in agreement with the above-reported observations. The experimental data could be well-fitted by the typical relation for metal oxide SC sensors: response = A[gas concentration]B, where A is a constant typical of the sensing element and B values are usually 1 or 1/2, depending on the charge of the surface species and the stoichiometry of the involved reactions.4,6,41,55 As already observed, the response increase induced by the introduction of gold nanoparticles was significantly higher in the detection of ozone. Assuming the validity of the previous equation even at low gases concentration, and considering 0.5 as the minimum significant response value, the detection limits for the target gases could be estimated by analyzing the data reported in Figure 6. In particular, as concerns the CuOTiO2Au sensor, the extrapolated detection limits were 6 ppm and 5 ppb for CH3CH2OH (300 °C) and O3 (100 °C). Interestingly, the value for ethanol is appreciably lower than the typical one for commercial breath analyzers (200 ppm).1,40 As concerns ozone, Au-free CuOTiO2 composites presented a detection limit of 30 ppb, evidencing that gold NPs have a favorable impact not only on the sensing response but also on the pertaining detection limits. Regarding H2, the present value for the CuOTiO2Au sensor (10 ppm at 200 °C) was 1 order of magnitude inferior to that reported for 1D ZnO nanomaterials.2
’ CONCLUSIONS This work has been devoted to an unprecedented preparation of CuOTiO2Au nanocomposites by means of a hybrid CVD/ rf-sputtering approach. The proposed strategy involved the CVD of CuO nanomaterials on polycrystalline Al2O3, which resulted in porous systems thanks to the rough substrate surface and the unique morphology of CuO matrices. Subsequently, the CVD dispersion of TiO2 NPs, followed by Au rf-sputtering under mild conditions, was performed. The adopted synthetic protocol enabled us to avoid solid state reactions between the two oxides and to produce pure and large-area nanocomposites, with a high dispersion of TiO2 and Au into the pristine CuO matrices and an intimate contact between the single components. These features had a strategic relevance in view of gas sensing applications. Indeed, functional tests at 100400 °C for the detection of toxic/flammable gases, both reducing (H2, CH3CH2OH) and oxidizing (O3), provided appealing results in terms of responses and detection limits. It was observed that the simultaneous presence of TiO2 and Au NPs appreciably enhanced the overall sensing performances. Such an effect was related to the formation of a high interfacial area pn heterojunction between p-type CuO and n-type TiO2, increasing charge carrier lifetime. The Schottky-type barrier character of the goldoxide interface, as 10515
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by funding from the European Community’s Seventh Framework Program (FP7/2007-2013; Grant Agreement No. ENHANCE-238409). PRIN-COFIN 2008 and Padova University PRAT 2008/2010 projects are also acknowledged for financial assistance. Thanks are due to Mr. A. Ravazzolo and Dr. A. Parfenova (CNR-ISTM and Padova University, Padova, Italy) for technical and synthetic assistance. ’ REFERENCES (1) Comini, E. Anal. Chim. Acta 2006, 568, 28. (2) Barreca, D.; Bekermann, D.; Comini, E.; Devi, A.; Fischer, R. A.; Gasparotto, A.; Maccato, C.; Sberveglieri, G.; Tondello, E. Sens. Actuators, B 2010, 149, 1. (3) Chen, P.-C.; Shen, G.; Zhou, C. IEEE Trans. Nanotechnol. 2008, 7, 668. (4) Wang, C.; Fu, X. Q.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Nanotechnology 2007, 18, 145506. (5) Barreca, D.; Bekermann, D.; Comini, E.; Devi, A.; Fischer, R. A.; Gasparotto, A.; Maccato, C.; Sada, C.; Sberveglieri, G.; Tondello, E. CrystEngComm 2010, 12, 3419. (6) Barreca, D.; Comini, E.; Ferrucci, A. P.; Gasparotto, A.; Maccato, C.; Maragno, C.; Sberveglieri, G.; Tondello, E. Chem. Mater. 2007, 19, 5642. (7) Barreca, D.; Comini, E.; Gasparotto, A.; Maccato, C.; Pozza, A.; Sada, C.; Sberveglieri, G.; Tondello, E. J. Nanosci. Nanotechnol. 2010, 10, 8054. (8) Gou, X.; Wang, G.; Yang, J.; Park, J.; Wexler, D. J. Mater. Chem. 2008, 18, 965. (9) Gasparotto, A.; Barreca, D.; Fornasiero, P.; Gombac, V.; Lebedev, O. I.; Maccato, C.; Montini, T.; Tondello, E.; van Tendeloo, G.; Comini, E.; Sberveglieri, G. ECS Trans. 2009, 25, 1169. (10) Wang, Y. Q.; Zhang, Z. J.; Zhu, Y.; Li, Z. C.; Vajtai, R.; Ci, L. J.; Ajayan, P. M. ACS Nano 2008, 2, 1492. (11) Janata, J. Principles of Chemical Sensors, 2nd ed.; Springer: London, 2010. (12) Li, W. Y.; Xu, L. N.; Chen, J. Adv. Funct. Mater. 2005, 15, 851. (13) Savage, N. O.; Akbar, S. A.; Dutta, P. K. Sens. Actuators, B 2001, 72, 239. (14) Krivetskiy, V. V.; Ponzoni, A.; Comini, E.; Badalyan, S. M.; Rumyantseva, M. N.; Gaskov, A. M. Inorg. Mater. 2010, 46, 1100. (15) Ruiz, A. M.; Cornet, A.; Shimanoe, K.; Morante, J. R.; Yamazoe, N. Sens. Actuators, B 2005, 109, 7. (16) Chiarello, G. L.; Aguirre, M. H.; Selli, E. J. Catal. 2010, 273, 182. (17) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C.-Y.; Meethong, N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Science 2006, 312, 885.
ARTICLE
(18) Korotcenkov, G.; Cho, B. K.; Gulina, L.; Tolstoy, V. Sens. Actuators, B 2009, 138, 512. (19) Labidi, A.; Gillet, E.; Delamare, R.; Maaref, M.; Aguir, K. Sens. Actuators, B 2006, 120, 338. (20) Wisitsoraat, A.; Tuantranont, A.; Comini, E.; Sberveglieri, G.; Wlodarski, W. Thin Solid Films 2009, 517, 2775. (21) Barreca, D.; Fornasiero, P.; Gasparotto, A.; Gombac, V.; Maccato, C.; Montini, T.; Tondello, E. ChemSusChem 2009, 2, 230. (22) Kim, T. W.; Ha, H.-W.; Paek, M.-J.; Hyun, S.-H.; Choy, J.-H.; Hwang, S.-J. J. Mater. Chem. 2010, 20, 3238. (23) Barreca, D.; Gasparotto, A.; Maccato, C.; Tondello, E.; Lebedev, O. I.; van Tendeloo, G. Cryst. Growth Des. 2009, 9, 2470. (24) Park, S.-H.; Ryu, J.-Y.; Choi, H.-H.; Kwon, T.-H. Sens. Actuators, B 1998, 46, 75. (25) Zhu, B. L.; Xie, C. S.; Wang, W. Y.; Huang, K. J.; Hu, J. H. Mater. Lett. 2004, 58, 624. (26) Li, G.; Dimitrijevic, N. M.; Chen, L.; Rajh, T.; Gray, K. A. J. Phys. Chem. C 2008, 112, 19040. (27) Ruiz, A. M.; Cornet, A.; Shimanoe, K.; Morante, J. R.; Yamazoe, N. Sens. Actuators, B 2005, 108, 34. (28) Ruiz, A. M.; Dezanneau, G.; Arbiol, J.; Cornet, A.; Morante, J. R. Chem. Mater. 2004, 16, 862. (29) Yoong, L. S.; Chong, F. K.; Dutta, B. K. Energy 2009, 34, 1652. (30) Gombac, V.; Sordelli, L.; Montini, T.; Delgado, J. J.; Adamski, A.; Adami, G.; Cargnello, M.; Bernal, S.; Fornasiero, P. J. Phys. Chem. A 2010, 114, 3916. (31) Teleki, A.; Bjelobrk, N.; Pratsinis, S. E. Sens. Actuators, B 2008, 130, 449. (32) Dutta, P. K.; Ginwalla, A.; Hogg, B.; Patton, B. R.; Chwieroth, B.; Liang, Z.; Gouma, P.; Mills, M.; Akbar, S. J. Phys. Chem. B 1999, 103, 4412. (33) Sangeetha, P.; Zhao, B.; Chen, Y.-W. Ind. Eng. Chem. Res. 2010, 49, 2096. (34) Barreca, D.; Gasparotto, A.; Maragno, C.; Tondello, E.; Bontempi, E.; Depero, L. E.; Sada, C. Chem. Vap. Deposition 2005, 11, 426. (35) Bandoli, G.; Barreca, D.; Gasparotto, A.; Seraglia, R.; Tondello, E.; Devi, A.; Fischer, R. A.; Winter, M.; Fois, E.; Gamba, A.; Tabacchi, G. Phys. Chem. Chem. Phys. 2009, 11, 5998. (36) Barreca, D.; Gasparotto, A.; Maccato, C.; Tondello, E. Nanotechnology 2008, 19, 255602. (37) Lu, F.-H.; Fang, F.-X.; Chen, Y.-S. J. Eur. Ceram. Soc. 2001, 21, 1093. (38) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley and Sons: New York, 1990. (39) Moulder, J. F.; Stickle, W. F.; Sobol, P. W.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992. (40) Barreca, D.; Gasparotto, A.; Maccato, C.; Maragno, C.; Tondello, E.; Comini, E.; Sberveglieri, G. Nanotechnology 2007, 18, 125502. (41) Barreca, D.; Comini, E.; Gasparotto, A.; Maccato, C.; Sada, C.; Sberveglieri, G.; Tondello, E. Sens. Actuators, B 2009, 141, 270. (42) JCPDS, pattern no. 45-937, 2000. (43) Wu, J. C. S.; Lin, H. M.; Lai, C.-L. Appl. Catal., A 2005, 296, 194. (44) Gu, A. X.; Wang, G. F.; Zhang, X. J.; Fang, B. Bull. Mater. Sci. 2010, 33, 17. (45) Barreca, D.; Gasparotto, A.; Tondello, E. Surf. Sci. Spectra 2007, 14, 41. (46) Xin, B.; Wang, P.; Ding, D.; Liu, J.; Ren, Z.; Fu, H. Appl. Surf. Sci. 2008, 254, 2569. (47) Mungkalasiri, J.; Bedel, L.; Emieux, F.; Dore, J.; Renaud, F. N. R.; Maury, F. Surf. Coat. Technol. 2009, 204, 887. (48) Lopez, R.; Gomez, R.; Llanos, M. E. Catal. Today 2009, 148, 103. (49) Yates, H. M.; Brook, L. A.; Ditta, I. B.; Evans, P.; Foster, H. A.; Sheel, D. W.; Steele, A. J. Photochem. Photobiol., A 2008, 197, 197. (50) Zhang, Y.; He, X.; Li, J.; Zhang, H.; Gao, X. Sens. Actuators, B 2007, 128, 293. 10516
dx.doi.org/10.1021/jp202449k |J. Phys. Chem. C 2011, 115, 10510–10517
The Journal of Physical Chemistry C
ARTICLE
(51) Shishiyanu, S. T.; Shishiyanu, T. S.; Lupan, O. I. Sens. Actuators, B 2006, 113, 468. (52) Miyata, T.; Hikosaka, T.; Minami, T. Surf. Coat. Tecnol. 2000, 126, 219. (53) Korotcenkov, G.; Blinov, I.; Ivanov, M.; Stetter, J. R. Sens. Actuators, B 2007, 120, 679. (54) Shafiei, M.; Yu, J.; Arsat, R.; Kalantar-zadeh, K.; Comini, E.; Ferroni, M.; Sberveglieri, G.; Wlodarski, W. Sens. Actuators, B 2010, 146, 507. (55) Madou, M. J.; Morrison, S. R. Chemical Physics of Surfaces; Academic Press Inc.: New York, 1988.
10517
dx.doi.org/10.1021/jp202449k |J. Phys. Chem. C 2011, 115, 10510–10517
