Reduced Graphene Oxide-Based Ordered Macroporous Films on a

Feb 1, 2016 - A new general method for the fabrication of a reduced graphene oxide (rGO)-based ordered monolayer macroporous film composed of a layer ...
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Reduced Graphene Oxide-Based Ordered Macroporous Films on a Curved Surface: General Fabrication and Application in Gas Sensors Shipu Xu,† Fengqiang Sun,*,†,‡,§ Zizhao Pan,† Chaowei Huang,† Shumin Yang,† Jinfeng Long,† and Ying Chen† †

School of Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R. China Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, South China Normal University, Guangzhou 510006, P. R. China § Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, P. R. China ‡

S Supporting Information *

ABSTRACT: A new general method for the fabrication of a reduced graphene oxide (rGO)-based ordered monolayer macroporous film composed of a layer of closely arranged pores is introduced. Assisted by the polystyrene microsphere monolayer colloid crystal by a simple solution-heated method, pure rGO, rGO-SnO2, rGO-Fe2O3, and rGO-NiO composite monolayer ordered porous films were examplarily constructed on the curved surface of a ceramic tube widely used in gas sensors. The rGO-oxide composite porous films could exhibit much better sensing performances than those of the corresponding pure oxide films and the composite films without the ordered porous structures in detecting ethanol gas. The enhancement mechanisms induced by distinctive rGO-oxide heterojunctions and porous structures as well as the effects of the rGO content and the pore-size on the sensitivity of the composite films were systematically analyzed and discussed. This study opens up a kind of construction method for an rGObased composite film gas sensor with uniform surface structures and high performance. KEYWORDS: reduced graphene oxide, monolayer colloid crystal template, ordered porous film, curved surface, gas sensor, rGO-semiconductor composite, assembly of graphene

1. INTRODUCTION Recently, graphene materials, including graphene oxide (GO) and reduced graphene oxide (rGO), have been widely used alone or as reinforcing additives in field-effect devices,1 sensors,2 catalysis,3 batteries,4 capacitors,5 and many others. For macroscopic applications, much attention has been focused on assemblies (e.g., papers, thin films, and porous films) of their sheets.6−8 Among these, a porous film is a kind of threedimensional (3D) hollow structure formed by bent sheet-like graphene materials, which can effectively avoid the overlap of the sheets in the assembly, expose as much surface as possible, and further enhance their applications. Many unordered porous films and several ordered porous films have been fabricated by self-assembly of chemically modified graphenes in solution,9,10 hydrothermal method,11 chemical vapor deposition method,12 and template-induced methods.13−17 However, subject to the method itself, the character of graphene materials or the relatively poor flexibility resulting from the big thickness, these porous films were difficult to directly grow or be fabricated on a curved surface with big curvatures. Moreover, few methods could simultaneously be employed for the fabrication of graphene material-incorporated composite porous films. These drawbacks obviously restricted some essential practical © XXXX American Chemical Society

applications of the graphene materials. As is well-known, many devices in reality require a substrate with a curved surface, for example, the commercially applied ceramic tube used in gas sensors. For another, graphene materials are also required to act as the intensifier to enhance the performance of a specific functional material in many cases. Therefore, a method for assembling porous graphene materials and their composites on a curved surface is necessary but remains a challenge. The ceramic tube with a curved surface (Figure 1A), as a traditional substrate used in gas sensors, has long been widely employed. For constructing such a gas sensor, a sensing film must be coated on the surface and connected to the two gold films on the ends of the tube. The monolayer colloid crystal composed of a layer of polymer microspheres has excellent flexibility, can cover the tube, and can be used as a template to fabricate ordered porous semiconductor sensing films.18−20 This construction method effectively promotes the device-todevice reproducibility. However, limited to the property of the material, the sensitivity of such a sensor is still relatively low. Received: November 30, 2015 Accepted: January 20, 2016

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DOI: 10.1021/acsami.5b11607 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 1. Fabrication Process of an rGO MOP Filma

Figure 1. (A) Schematic drawing of a ceramic tube used in gas sensors; (B) SEM image of polystyrene microspheres wrapped with GO.

Graphene materials have proven to be ideal reinforcing agents for many semiconductor gas sensors,21−23 and if they were introduced into such porous films, the performance of the film might be improved. However, this is still difficult because of the sheet-like morphology and big planar size of the graphene materials. Fortunately, we have recently found that flexible GO sheets show excellent affinity to polystyrene (PS) microspheres without any modification and can wrap them well in water, as shown in Figure 1B, which provides the possibility of introducing rGO sheets into the porous system by taking the PS microsphere colloidal crystal as a template. Inspired by these systems, we designed here a new templateinduced solution-heated method to fabricate rGO and rGOincorporated composite monolayer ordered porous (MOP) films directly on a ceramic tube used for gas sensors, aiming at providing a general method for the construction of a graphene material and its composite porous structures on a curved surface and exploring a practical application of such films in gas sensors.

a (A) Template floating on the surface of the GO solution; (B) template wrapped with GO sheets picked up with a ceramic tube; (C) GO-template composite on the tube; and (D) rGO MOP film on the tube. Process: ① immersing the tube into the solution slowly; ② putting it into an oven, drying at 120 °C, and crossing through a heating wire; and ③ heating the tube by the heating wire.

1 h. After the tube was taken out from the oven, a spring-like Ni/Cd alloy resistance wire crossed through its tunnel (Scheme 1C). The two ends of the wire were then connected to a power source. At 5.0 V of voltage, the wire generated heat to heat the tube wall (the surface temperature of the tube was measured to be approximately 340 °C). This new inside heating method differed from the traditional heat mode in a furnace and could help the film combine with the surface of ceramic tube better. Two hours later, the PS microspheres were burned away, the GO was reduced, and an rGO MOP film was finally formed on the ceramic tube (Scheme 1D). For the fabrication of an rGO-incorporated composite MOP film, a specific matter was directly dissolved into the 50 mg L−1 GO solution, and then the same procedure as that of the fabrication of the rGO MOP film was employed. All parameters, including the heating voltage and heating time, remained unchanged. 2.4. Characterizations of MOP Films. The morphologies of the as-prepared MOP films on the ceramic tubes were directly examined by scanning electron microscopy (SEM, Shimadzu SS-550). For preparing the TEM specimens, MOP films were scraped off and transferred onto carbon-coated TEM grids. A JEOL JEM 2010 transition electronic microscope was used for TEM analysis and HRTEM analysis. The compositions were characterized by an X-ray powder diffractometer (XRD, D/max2200 with Cu Kα radiation), and an X-ray photoelectron spectrometer (XPS, ESCALAB 250). Specimens for the XRD and XPS measurements were prepared on the glass substrates under the same conditions as those prepared on the ceramic tubes. 2.5. Gas-Sensing Test. The gas-sensing performance of the asprepared MOP films was tested on a commercial Gas Sensing Measurement System (WS-30A, Weisheng Instruments Co., Ltd., Zhengzhou, China), which is a static system using atmospheric air as the interference gas.27,28 Before the test, a gas sensor was heated at a certain temperature by tuning the heating voltage across a Ni-Cr alloy resistor inside the ceramic tube (Figure 1A), and a working voltage of 5 V was applied on the electrodes on the ceramic tube. Ethanol gas was chosen as the target gas to be detected. In the test process, first, a base value of the resistance of the sensor in ambient air was recorded by the computer connected with the system, and the test chamber was then closed. Second, a calculated volume of ethanol was introduced into the chamber by a microsyringe, and the signal on the variation of

2. EXPERIMENTAL SECTION 2.1. Fabrication of GO Sheets. The GO sheets were prepared based on the Hummers method.24 Briefly, 1.0 g of powdered flake graphite and 0.5 g of NaNO3 were added to 23 mL of concentrated H2SO4 in a flask cooled in an ice-bath under agitation. Then, 3.0 g of KMnO4 was added slowly to the suspension. The flask was removed and put into a water bath at a temperature of 35 °C. After agitation for 30 min, the suspension was heated to 98 °C accompanied by the slow addition of 46 mL of deionized water. Fifteen minutes later, the system was cooled to room temperature. Then, 140 mL of deionized water and 1 mL of 3% H2O2 were then successively added to reduce the residual KMnO4 and MnO2. After agitation for 10 min, the suspension was filtered. The filtrate was centrifuged at 5000 rpm for 1 h. Subsequently, the obtained precipitates were dispersed into deionized water and ultrasonically agitated for 1 h. Finally, the ultrasonic dispersion was centrifuged to obtain the GO sheets. 2.2. Preparation of Monolayer Colloidal Crystal Templates. Monodispersed PS-microsphere suspensions (2.5 wt % in water, surfactant free) were bought from Alfa Aesar Company. The ordinary glass substrate (1.5 × 1.5 cm2) was ultrasonically cleaned in acetone and then in ethanol for 1 h. Subsequently, the substrate was mounted on a custom-built spin coater. An amount of 10 μL of PS-microsphere suspension was dropped onto the substrate. A large-area monolayer (>1 cm2) colloidal crystal could be fabricated by a spin-coating method25 at a speed of 800 rotations per minute. 2.3. Fabrication Process of MOP Films. GO sheets were dispersed in deionized water to form a homogeneous transparent solution with concentration of 50 mg L−1. This solution would be directly used for the fabrication of an rGO MOP film, as shown in Scheme 1. A monolayer colloid crystal was slowly immersed into the GO solution,26 and then it was slowly stripped off the glass substrate and floated onto the surface of the solution (Scheme 1A). After 5 min, the colloidal crystal was picked up with the ceramic tube (Scheme 1B). Subsequently, the tube was put into an oven to be dried at 120 °C for B

DOI: 10.1021/acsami.5b11607 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Related characterizations on the rGO MOP film. (A) PS monolayer colloidal crystal template; (B) SEM image of the GO-template composite; (C) SEM image of the rGO MOP film on the ceramic tube; (D) XRD patterns of GO sheets and rGO MOP film; (E) C 1s XPS spectrum of the pristine GO sheets; and (F) C 1s XPS spectrum of the final rGO MOP film.

assembled into the MOP film and experienced the heating process, this peak disappeared, but another broad peak centered at ∼23° emerged, indicating the reduction of GO to rGO.32 Regarding the reduction of GO in ambient air, Zangmeister undertook an intensive study and found that most oxygencontaining groups could be efficiently removed at 220 °C.33 Here, the heating temperature was higher than 220 °C, and the GO could be more easily reduced. For confirmation of the reduction, XPS measurements of pristine GO sheets and the asfabricated MOP film were carried out further. Figure 2E shows the high-resolution and curve fit C 1s spectrum of the pristine GO sheets. Four peaks centered at 284.8, 286.8, 287.3, and 288.7 eV are observed corresponding to C−C, C−O, CO, and OH-CO groups, respectively. In comparison, the C 1s XPS spectrum of the MOP film (Figure 2F) shows that only C−C and C−O groups are preserved. Obviously, CO and OH-CO groups had been removed by the heat-treatments; the GO was partially reduced and converted to rGO. Because of the removal of partial superficial oxygen functional groups, the interlayer spacing of rGO was decreased compared with that of GO (e.g., d002 = 0.882 nm for GO and d002 = 0.392 nm for rGO). 3.2. rGO-Incorporated Composite MOP Films. The fabrication process of the composite MOP films was the same as that (Scheme 1) of the pure rGO MOP film, except that a specific water-soluble salt was previously dissolved into the asprepared GO solution. For exploration of the MOP films in the application of gas sensors, the fabrication of composites of rGO and the conventional sensing semiconductors n-type SnO2 and Fe2O3 and p-type NiO were taken as examples. SnCl4, Fe(NO3)3, and Ni(CH3COO)2 were chosen as precursors and solved into the same concentration (50 mg L−1) of GO solutions. During the drying process (similar to Scheme 1C), the metallic ions (Mn+) were hydrolyzed into hydroxides (eq 1) and deposited in the voids of the colloidal crystal accompanied by the deposition of GO sheets (Scheme 2A). Because the surfaces of GO sheets were bonded with many carboxyl ions (−COO−), and these ions were also hydrolyzed by combining the H+ ions, there would be more hydroxyl ions (OH−) than other places far from the sheets in the solution (eq 2). Hydroxide particles would preferentially generate and grow on surfaces of GO sheets. Subsequently, in the heating process (Scheme 2B), the hydroxides were decomposed into the

resistance of the sensor was simultaneously recorded. Third, after the signal stabilized, the chamber was opened to remove the gas, and the signal on the resistance was also simultaneously recorded until it reached steady state. The same procedure was followed for the recycling test.

3. RESULTS AND DISCUSSION 3.1. Pure rGO MOP Film. Figure 2A shows a monolayer colloid crystal template composed of closely packed 750 nm PS microspheres on a glass substrate. After it was transferred into the GO solution (Scheme 1A), it was naturally divided into two parts by the solution surface (Figure S1 in the Supporting Information). One protruded out of the solution, and the lower part was immersed in the solution. Generally, the aromatic structures interact strongly with the basal plane of the graphite surface through π−π stacking.29,30 A π−π stacking should also occur between the aromatic system of π-electrons of PS and the π-electrons system of the GO. Part of the GO sheets would thereby be adsorbed onto the surface of PS microspheres in the solution. When the colloidal crystal was picked up with the ceramic tube, it could wrap the tube well because of its excellent flexibility (Scheme 1B). The periodically distributed voids, naturally formed by the adjacent microspheres, of the colloidal crystal could be filled with the solution under the capillary force. In the subsequent drying process, water was vaporized, and GO sheets suspended in the solution gradually deposited onto the surfaces of PS and the tube (Scheme 1C). The corresponding SEM image of this moment is shown in Figure 2B. Obviously, the lower part of the microspheres had been wrapped with the GO sheets, whereas the upper part still showed the smooth spherical surface. During the heat treatment at around 340 °C, PS template microspheres were gradually burned away at the same time GO was reduced, resulting in the formation of the rGO MOP film on the ceramic tube (Scheme 1D). Figure 2C shows the SEM image of the MOP film. Each pore has open diameter of ∼720 nm that is near the diameter of the template microsphere. Adjacent pores are arranged hexagonally on such a curved surface. The overlying rGO sheets constitute the pore walls. Figure 2D shows the XRD spectra of pristine GO and the asfabricated rGO MOP film. A single diffraction peak was found in the GO powder sample at ∼10° (corresponding to the reflection of the (002) plane).31 After the GO sheets were C

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file No. 70-4177), Fe2O3 (JCPDS file No. 87-1165), and NiO (JCPDS file No. 73-1519) in the corresponding composite MOP films are clearly observed, explaining that the metal oxides had been formed by heat treatments. No other substances can be detected in every MOP film, which means that the metallic ions had been completely transformed into the corresponding metal oxides. According to the compositions of precursors (i.e., 50 mg L−1 GO and a specific concentration of metallic ions), the contents of rGO can be calculated as ∼0.06, ∼0.62, and ∼0.17 wt % in rGO-SnO2, rGO-Fe2O3, and rGONiO MOP films, respectively. Because of the relatively low content in all MOP films, rGO could not be detected by the XRD measurements. Figure 4A shows the morphology of the as-fabricated rGOSnO2 composite MOP film. In large area, the film is composed of closely packed and hexagonally arranged pores. The uniform film is well-covered on the whole surface of the tube (inset of Figure 4A). An enlarged image shows that the pore size and the pore arrangement are same as those of the pure rGO MOP film (Figure 4B). Fine SnO2 particles and rGO sheets were blended together to constitute the pore walls. Because of the close contacts between some GO sheets and template PS microspheres during the formation of the film, rGO sheets on the surface of the as-formed pore walls expose a smooth surface, and SnO2 particle aggregates locate in interstices among rGO sheets. Figure 4C shows the TEM image of a fragment patch of the rGO-SnO2 MOP film scraped from the ceramic tube. Providentially, it shows the back of the spherical pores. A broken edge displays a layered structure of a pore wall (Zone 1 in Figure 4C); in the selected area electron diffraction (SAED) pattern (inset of Figure 4C), diffraction rings matching to (110), (101), and (211) planes of SnO2 and diffraction dots resulting from the diffraction of rGO sheets can be identified.34,35 As is obvious, the rGO sheets had been incorporated into the pore walls of the MOP film. The diffraction rings reveal the polycrystalline structure of the SnO2 particles. An enlarged TEM image shows that SnO2 nanoparticles have a size of 3−8 nm and are packed into a nanoporous structure (Figure S2). Upon further observation of the high resolution TEM image of the film (Figure 4D), SnO2 nanoparticles (displaying lattice fringes) are found to be wellattached to the surface of the rGO sheet. Lattice fringes with distances of 0.34, 0.28, and 0.18 nm are from the (110), (101), and (200) crystallographic planes of SnO2, respectively. The other two MOP films, i.e., rGO-Fe2O3 (Figure 4E) and rGONiO (Figure 4F), have similar surface morphologies and inner structures (shown in Figures S2) to that of the rGO-SnO2 MOP film. The chemical compositions and oxidation states of existing elements in the as-prepared composite MOP films were determined by XPS. Figure 5A shows the survey scan XPS spectrum of the rGO-SnO2 MOP film and confirms that only three elements (i.e., C, O, and Sn) exist. The C 1s XPS spectrum (Figure 5B) is similar to that of the pure rGO MOP film, indicating that GO incorporated in the composite still experienced a similar thermal deoxygenation process and was reduced to rGO. The high-resolution XPS spectrum in the vicinity of the Sn 3d peak (inset of Figure 5A) shows two distinct peaks at binding energies of 486.5 and 494.9 eV that correspond to the 3d3/2 and 3d5/2 states of Sn4+, respectively.36 The asymmetric O 1s spectrum (Figure 5C) can be resolved into three peaks that correspond to O2−, O−, and O2− with binding energies of 530.0, 530.6, and 531.5 eV, respectively.

Scheme 2. Formation Process of an rGO-Oxide Composite MOP Filma

a

(A) State of the template and the precursor solution on the ceramic tube during the drying process. (B) rGO-oxide pore-walls after removal of the template.

corresponding oxides (eq 3). GO was reduced, and the rGOoxide MOP films were obtained after removal of the PS microspheres. Some rGO sheets were stabilized on the surface of pore walls, and others were buried in the oxide matrix composed of nanoparticles. Δ

Mn + + nH 2O ↔ M(OH)n ↓ +nH+

(1)

Δ

−COO− + H 2O ↔ −COOH + OH−

(2)

n H 2O↑ (3) 2 Figure 3 shows the XRD patterns of rGO-SnO2, rGO-Fe2O3, and rGO-NiO MOP films from the precursor solutions containing 0.6 mol L−1 (M) Sn4+, 0.1 M Fe3+, and 0.4 M Ni2+, respectively. The main diffraction peaks of SnO2 (JCPDS Δ

M(OH)n → MOn/2 +

Figure 3. XRD spectra of rGO-oxide MOP films. D

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Figure 4. Morphologies of rGO-oxide composite films. (A) SEM image of a large area of rGO-SnO2 MOP film and the ceramic tube covered with this film (inset); (B) an enlarged SEM image of the rGO-SnO2 MOP film; (C) TEM image of the rGO-SnO2 MOP film and the SEAD pattern (inset); (D) HRTEM image of Zone 1 in (C); and SEM images of (E) rGO-Fe2O3 and (F) rGO-NiO MOP films.

Figure 5. Survey scan (A), C 1s (B), and O 1s (C) XPS spectra of the rGO-SnO2 MOP film and survey scan XPS spectra of rGO-Fe2O3 (D) and rGO-NiO (E) MOP films. The insets in (A), (D), and (E) show the high resolution XPS spectra of Sn 3d, Fe 2p, and Ni 2p, respectively.

rGO in the film, the peak corresponding to the C−O groups cannot be shown in this spectrum. The XPS survey scan spectra

These binding energies are characteristic of ionized oxygen species at the SnO2 surface.37 Because of the lower content of E

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Figure 6. A photo of an rGO-oxide MOP film sensor (A) and the response of each sensor to 50, 100, and 200 ppm ethanol gas for (B) rGO-SnO2 MOP film and MC film, (C) rGO-Fe2O3 film, and (D) rGO-NiO MOP film. Insets of (B−D) show the sensitivity comparisons among the corresponding composite MOP film, composite MC film, and the pure oxide MOP film.

(Figure 5D and E) of the other two MOP films can also confirm the existence of rGO and the corresponding oxides. In the Fe 2p spectrum (inset of Figure 5D), peaks at binding energies of 711.6 and 724.9 eV are attributed to the 2p3/2 and 2p1/2 states of Fe3+, respectively;38 in the Ni 2p spectrum (inset of Figure 5E), peaks at 854.0 and 861.5 eV (corresponding to 2p3/2 and its statellite) and peaks at 872.5 and 879.4 eV (corresponding to 2p1/2 and its statellite) are all attributed to Ni2+.39 C 1s and O 1s spectra of these two films are similar to those of the rGO-SnO2 MOP film (Figure S3). 3.3. Application of MOP Films in Gas Sensors. 3.3.1. Response of MOP Film Sensors to Ethanol Gas. When the electrodes of the ceramic tube covered with a MOP sensing film were welded with a specific support, an available gas sensor was constructed (Figure 6A). The detection of ethanol gas was taken as an example to explain the sensing performances of the MOP films. For rGO-SnO2, rGO-Fe2O3, and rGO-NiO MOP film sensors, according to their responses at different temperatures (Figure S4), the working temperature was optimized at 170, 110, and 150 °C, respectively, by controlling the heating voltage on the Ni/Cd alloy wire in the ceramic tube. The pure rGO MOP and every pure oxide MOP film (also fabricated by a similar process) showed no response to the ethanol gas. However, every rGO-based composite MOP film exhibited excellent response to the introduced ethanol gas. For rGO-SnO2 (Figure 6B) and rGO-Fe2O3 (Figure 6C) sensors, the resistance decreased, whereas for the rGO-NiO sensor (Figure 6D), it increased, meaning that rGO-SnO2 and rGO-Fe2O3 still had the character of an n-type semiconductor, and rGO-NiO had the character of a p-type semiconductor. The response and recovery times for every rGO-oxide MOP sensor in detecting 50−200 ppm ethanol gas were all in the range of 5−10 s. The sensitivity is defined as S = Rair/Rgas for n-

type semiconductors, and as S = Rgas/Rair for p-type semiconductors, where Rair and Rgas are the resistances of the sensor in the air and the air mixed with ethanol gas, respectively. Obviously, the sensitivities increased with the increase of the gas concentration for all sensors (insets of Figure 6B−D); for example, in detecting 50, 100, and 200 ppm ethanol gas, the rGO-SnO2 MOP film sensor showed sensitivities of around 28.7, 43.1, and 53.7, respectively. To further evaluate the MOP films used in gas sensors, in control experiments, rGO-based composite powders from the same precursors as those of the corresponding MOP films were prepared first by directly heating the precursor solutions and then manually coating onto the surface of the ceramic tubes to obtain the sensing films without the ordered porous structures. The heat treatments of the powders were the same as those of the MOP films. This kind of film was cited as an MC film. Despite the fussy fabrication process, these sensors showed obviously lower sensitivities than those of the corresponding MOP film sensors (Figure 6B and its inset), though they had been optimized. The sensitivity of the rGO-SnO2 MC film from powders was lower than 1/10 that of the MOP film, whereas the same type of rGO-Fe2O3 and rGO-NiO MC films showed no response to the ethanol gas. In addition to the excellent sensitivity, the composite MOP film sensors also exhibited excellent stability (Figure S5); rGO-SnO2 and rGO-Fe2O3 MOP films had good selectivity for detecting ethanol gas (Figure S6). 3.3.2. Enhanced Sensing Mechanism. Generally, the sensing mechanism of oxide semiconductor sensors is based on the fact that adsorbed negative oxygen ions (O2−, O−, or O2−) on the surface of the oxides will interact with target gas molecules and result in the conductivity change of the sensing films.40−42 The rGO sheets synthesized by chemical modF

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Scheme 3. Schematic Illustration of the Relevant Information on the Sensing Mechanism of the rGO-Oxide MOP Filma

a (A) Existential state of rGO sheets and oxide matrix; (B) enlarged Zone 1 in (A); (C) a manually coated rGO-oxide film composed of sheet-like composite particles; (B1 and B2) heterojunction regions in the side of the oxide matrix and the surface state for rGO-oxide MOP film in air; (B1r and B2r) response of the corresponding MOP film to ethanol gas; and (B1f and B2f) the final state of the MOP film in ethanol gas.

(Scheme 3B1r), decreasing the width of the first CDL (Scheme 3B1f); at the same time, ethanol molecules might be adsorbed onto the naked defects of rGO exposed to the gas to inject electrons,48 decreasing the width of the second CDL. Because of the injection of electrons, the third CDL would also be decreased, just like the reported enhancement mechanism of the p-n junction in the composite of SnO2 and a carbon nanotube.49 The resistance of the composite film was thereby significantly decreased. Because of the existence of three CDLs and the interplay between them, the rGO-oxide MOP film had a more significant change in resistance and much better sensitivity than those of the corresponding pure oxide MOP film. For the rGO-NiO MOP film, it has p-type character and holes play as the charge carriers. Although the p-type NiO also adsorbed oxygen negative ions, different from the n-type oxides, a hole accumulation layer could form at the surface (Scheme 3B2). The hole accumulation layer was beneficial to the conductivity of the film and could be called a charge accumulation layer (CAL). When rGO was introduced into the NiO matrix, p-p junctions could form. Electrons were transferred from rGO (work function: 4.5 eV)50 to NiO (work function: 5.0−5.6 eV),51 resulting in the formation of two kinds of charge (hole) depletion layer (CDL) in the side of NiO matrix.52 The first located near the surface and the second located inside. The introduced ethanol gas would react with the oxygen negative ions (Scheme 3B2r), injecting electrons to decrease the hole accounts and the width of CAL (Scheme 3B2f). At the same time, the adsorbed ethanol gas on the naked rGO sheets consumed the holes in the rGO, releasing electrons. These electrons would then be transferred into the NiO through the junction, consuming the holes and increasing the width of the first CDL. Accompanying the decrease of hole concentration near the surface of the composite film, holes in

ification possess p-type semiconductor characteristics.43 When the rGO combined with the oxides, p-n or p-p junctions could form, putting essential impact on the conductivity change and the sensitivity of the composite film.35,44−46 The composite MOP film reported here had a special heterojunction structure caused by the specific distribution of rGO sheets in the oxide matrix (just as shown in Figure 4). For more clearly illustrating the effect of the structure, the oxide matrix was simplified as a continuous whole to represent the particle aggregate, as shown in Scheme 3A. A typical region (Zone 1) was enlarged and simplified as a flat structure (Scheme 3B). Obviously, the rGO sheet on the surface of the pore walls could provide one surface to take part in the formation of a heterojunction and one surface exposed to the air, whereas all of the surface of the sheet embedded into the oxide matrix could take part in the formation of the heterojunction. This structure was different from that of most films (e.g., the MC films) composed of sheetlike rGO-based composite particles (Scheme 3C). Extra rGO sheets on the surface of pore walls would lead to a specific sensing mechanism when the composite sensing film worked. For rGO-SnO2 and rGO-Fe2O3 MOP films possessing n-type character, electrons played as charge carriers. When they were exposed to air, there should be three types of charge depletion layers (CDL) in the side of the oxide matrix, as shown in Scheme 3B1. The first depletion layer was formed due to the adsorption of ionized oxygen and located at the surface of the oxide;47 the second was caused by the transfer of electrons from oxide to the rGO sheet on the surface of the pore wall during the formation of a p-n heterojunction, and the third was similar to the second but caused by the p-n heterojunction located in the oxide matrix. In these CDLs, current was not allowed to pass through, resulting in the high resistance of the film. When reductive ethanol gas was introduced, electrons trapped by the oxygen ions were released and flowed back into the composites G

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ACS Applied Materials & Interfaces the matrix (including holes near the p-p junction) would diffuse to the surface driven by the broken equilibrium of hole concentration, resulting in the width increase of the second CDL. The decreased width of the conductive CAL and the increased width of the nonconductive CDLs significantly increased the resistance of the composite film. Compared with the pure NiO MOP film with only a CAL, the rGO-NiO MOP film obviously had much higher sensitivity to ethanol gas. Besides the effect of the special heterojunctions, the morphology of the MOP film also embodied the advantage in improving sensing performances when compared with manually coated films (MC films). As shown in Scheme 1C, the MC films were composed of sheetlike particles; adjacent particles had to randomly connect with each other by the physical contacts. Many overlaps among the particles must be generated, reducing the exposed active surfaces. Different from these, in the MOP composite films, all rGO sheets were stabilized into the films; rGO sheets and the oxide particles formed a continuous whole, and in particular, the ordered pores increased the surface areas (Figure S7), which effectively enhanced the stabilities and sensitivities of the corresponding devices. 3.3.3. Effect of rGO Content on Sensitivity. Taking the rGO-SnO2 sensor as an example, the effect of rGO content on the response of the final sensor to 200 ppm ethanol gas was shown in Figure 7A and B. In a specific range, the sensitivity of

maximum. Thus, 0.06% rGO content should be optimized for the rGO-SnO2 sensor. Lower than this content, although the surface area of SnO2 exposed to air still increased, the number of heterojunctions was reduced because of the decreased number of rGO sheets. The film thereby suffered fewer impacts from the heterojunctions and showed decreased sensitivity. Other rGO-oxide MOP film sensors obey similar paths, except for the difference in the optimal content of rGO. 3.3.4. Effect of Pore Size on Sensitivity. In addition, the pore-size could also put an impact on the sensitivity of the rGO-oxide MOP films. Insets (1) and (2) in Figure 8 show the

Figure 8. Response of rGO-SnO2 MOP film sensors with different pore sizes to 200 ppm of ethanol gas. Insets (1) and (2) exhibit the SEM images of the MOP films from 200 and 1000 nm template microspheres, respectively. The diameter of the template microspheres was employed to represent the pore size.

morphologies of rGO-SnO2 MOP films fabricated with 200 and 1000 nm PS microsphere colloid monolayers, respectively. They have similar pore structures to that of the MOP film from 750 nm PS microspheres but different pore sizes. The diameters of pore openings in every MOP film are close to those of the corresponding template microspheres. According to the results from geometric calculations, all MOP films on the same ceramic tube have approximately equal naked surface area (i.e., ∼2.5 times of the flat surface), but the film and pore wall thickness simultaneously increase with the increase of the size of the template microsphere. This results in sensitivity variation of different MOP films. Generally, in addition to being adsorbed on the surface of an oxide film, oxygen ions can also diffuse through spaces formed by adjacent particles or grain boundaries to go to the inside of the film and be adsorbed. The thicker the film and pore wall, the deeper the diffusion. When the film was employed to detect ethanol gas, the oxygen ions in deep were difficult to remove. The removal ratio of oxygen ions and the resulting sensitivity of the film would be decreased with an increase in the film thickness in a specific range. As shown in Figure 8, the MOP film sensor from 200 nm PS template microspheres has a sensitivity of ∼66.1 in detecting 200 ppm ethanol gas, and as the diameter of the microsphere increased to 750 and 1000 nm, the sensitivity of the resulting films decreased to ∼53.7 and ∼34.6, respectively.

Figure 7. (A) Responses of rGO-SnO2 MOP film sensors containing different contents of rGO; (B) summary of the sensitivity varying with rGO content.

the sensor regularly varied with the change of the rGO content. When the content was higher than 0.16%, the resulting sensor had no response to the ethanol gas. As it was lowered to 0.08 and 0.06%, the sensitivity of the corresponding sensor was increased to ∼20.0 and ∼53.7, respectively. However, after that, the continuous decrease of the content would lead to decreased sensitivity. For example, the sensitivity of the sensor loaded with 0.04% rGO was decreased to ∼28.9. The reason for this was thought to be as follows. According to the sensing mechanism of the rGO-oxide MOP film, both the distribution of rGO and the heterojunction were closely related to the sensitivity of a sensor. When the rGO content was too high (>0.16%) in the composite, more rGO sheets would locate at the surface of the pore walls, decreasing the exposed SnO2 surface area and the number of adsorbed oxygen ions. At the same time, the amount of SnO2 particles was relatively low, and a smaller number of heterojunctions could form in the film, weakening the enhancement effect of the heterojunction. As a result, the film had low sensitivity and even no response to the ethanol gas. Upon decreasing the rGO content (from 0.16 to 0.06%), the number of rGO sheets located at the surface decreased, and the exposed SnO2 surface area increased; the number of SnO2 particles and heterojunctions also increased, which resulted in gradually increased sensitivity until the

4. CONCLUSIONS In summary, taking the monolayer colloid crystal as template by the solution-heated method, rGO, rGO-SnO2, rGO-Fe2O3, and rGO-NiO MOP films had been fabricated on the curved surface based on the affinity of GO with the PS microspheres in water, the flexibility of the template, and the reduction of GO through thermal deoxygenation. The as-fabricated composite films had been directly used as sensors to detect ethanol gas and could exhibit highly increased sensitivity because of the effects of the corresponding heterojunctions and the ordered porous H

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structures. Variations of rGO content and pore size in certain ranges could put regular impacts on the sensitivity of composite films. The method is easily manipulated, reproduced well, lowcost, and general. Theoretically, any rGO-based composite MOP film and the corresponding device could be fabricated on various substrates once suitable water-soluble precursors were employed. In addition to the application in gas sensors, the rGO-MOP films might be further used in energy storage/ conversion, biological scaffolds, or supporting frameworks of catalysts.

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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/acsami.5b11607. Details on the formation process of the pure rGO MOP film, TEM images of rGO-oxide composite MOP films, C 1s and O 1s XPS spectra of rGO-Fe2O3 and rGO-NiO MOP films, optimization of working temperature of sensors, stability of sensors, selectivity of sensors, and specific surface area of the rGO-SnO2 MOP film (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-20-39310187. Fax: 86-2039310187. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was cosupported by the Research Project of Chinese Ministry of Education (No. 213029A), the National Natural Science Foundation of China (No. 21571068), and the Special funds for Discipline Construction in Guangdong Province (No. 2013KJCX0057).

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REFERENCES

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