Enhanced NH3-Sensitivity of Reduced Graphene Oxide Modified by Tetra-α-Iso-Pentyloxymetallophthalocyanine Derivatives
© Li et al. 2015
Received: 18 July 2015
Accepted: 8 September 2015
Published: 24 September 2015
Three kinds of novel hybrid materials were prepared by noncovalent functionalized reduced graphene oxide (rGO) with tetra-α-iso-pentyloxyphthalocyanine copper (CuPc), tetra-α-iso-pentyloxyphthalocyanine nickel (NiPc) and tetra-α-iso-pentyloxyphthalocyanine lead (PbPc) and characterized by Fourier transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–vis), Raman spectra, X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), and atomic force microscope (AFM). The as-synthesized MPc/rGO hybrids show excellent NH3 gas-sensing performance with high response value and fast recovery time compared with bare rGO. The enhancement of the sensing response is mainly attributed to the synergism of gas adsorption of MPc to NH3 gas and conducting network of rGO with greater electron transfer efficiency. Strategies for combining the good properties of rGO and MPc derivatives will open new opportunities for preparing and designing highly efficient rGO chemiresistive gas-sensing hybrid materials for potential applications in gas sensor field.
It is well known that the carbon nanotubes (CNTs) are considered as an excellent candidate for gas-sensing applications, due to unique electronic and structural characteristics of CNTs. Lots of research results have been reported on the CNT materials used as gas-sensing devices and show excellent gas-sensing properties toward NH3, NO, CO, CH4, and NO2 [1, 2]. As a material similar to CNTs, graphene and reduced graphene oxide (rGO) are the novel nanoscaled materials, atomic-thick layer of sp2-bonded carbon atoms, which has two-dimensional honeycomb nanostructure. Because of its unique nanostructure, graphene has exceptional mechanical, thermal, and electrical properties, high electron mobility, and outstanding conductivity at room temperature [3, 4]. These properties make it promising as one of the most appealing carbon materials for electrochemical devices, optoelectronic devices, and sensing devices.
RGO has been demonstrated as a promising chemical-sensing material because of the following merits [5, 6]. Firstly, rGO has a large specific surface area. All atoms of rGO sheet can be considered as surface atoms, and they are capable of absorbing gas molecules, which are a benefit for the charge transfer and provide an enough sensing area for gas adsorption and desorption . Secondly, its inherently low electric noise which makes the charge transfer is more stable than 1D structure such as CNTs. Moreover, rGO possesses the most advantages of CNTs and low price, which makes rGO more suitable for the large-scale research and application. RGO has shown excellent performance for detecting NH3, NO2, NO, and warfare and explosive agents [6, 8–10]. For instance, Fowler et al.  reported the development of useful rGO sensors from chemically converted graphene for NO2, NH3, and 2, 4-dinitrotoluene. Hu et al. presented a useful gas sensor based on chemically rGO, which can be used as an excellent sensing material and shows excellent responsive repeatability to DMMP . Lu et al. demonstrated high-performance gas sensors based on partially reduced graphene oxide (rGO) sheets via low-temperature thermal treatments . They observed that the rGO showed p-type semiconducting behavior in ambient conditions and was responsive to low-concentration NO2 and NH3 gases diluted in air at room temperature.
However, there are many problems about the development of rGO-based sensors, such as the slow recovery time, poor selectivity, less-than-ideal solubility, and half-baked film method. Functionalized modifications play more and more important roles in improving the sensing performance of rGO-based sensors, due to their synergetic combination of rGO and functionalized molecules , such as polymers , metals , and metal oxides . A flourishing research area is focused on the functionalization of rGO with metalophthalocyanine (MPc) complexes for enhancing the electronic, optical, and sensing properties [15, 16]. However, few reports have been concentrated on the application of the MPc/rGO hybrids toward the development of gas sensors.
MPc has been widely studied as an excellent sensing material due to the following virtues [17, 18]. First of all, it has high sensitivities as well as great thermal and chemical stability. In addition, MPc includes a planar π-conjugated skeleton and adjustable structure. Its center metal, peripheral, and axial substituent group can be changed, which provides possibilities for us to design the target hybrids according to our ideas. More importantly, the peripheral active ground of MPc and the in-plane system with a large π-conjugated structure provide a profitable tendency to combine the large basal plane of carbon materials through noncovalent or covalent modification and improve the target hybrids’ gas-sensing properties.
Functionalization of CNTs with MPc has been extensively studied in many groups and exhibits admirable sensing properties, which revealed that MPc plays important roles in enhancing the sensing performance of CNT-based gas sensors [1, 2]. However, MPc can not easily stably grow on 1D CNTs, due to their high curvature. Herein, we developed a method to evenly load the MPc on rGO and MPc/rGO hybrid gas sensors. The key to this method is to choose tetra-α-iso-pentyloxyphthalocyanine copper, nickel, and lead with excellent solubility, large π-conjugate system, and superior NH3-sensing performance in the previous studies . MPc molecules have been successfully anchored on the surface of rGO sheets by noncovalent π–π stackable interaction. The resultant MPc/rGO hybrids exhibited prominent gas-sensing response and fast recovery performance. And the sensing mechanism was also deduced in this paper.
Fourier transform infrared (FT-IR) spectra were recorded on a PE instruments Spectrum One FT-IR spectrometer using the KBr pellet method in the range of 500–4000 cm−1. Raman spectroscopy was recorded on a JobinYvon HR800 Raman Spectrometer with excitation from the 450-nm laser source. Ultraviolet–visible (UV–vis) spectra were conducted on a Perkin–Elmer Lambda 900 UV/VIS/NIR spectrophotometer. X-ray photoelectron spectra (XPS) measurements were carried out on a Kratos AXIS Ultra DLD system using monochromated Al Kα X-ray source (1486.6 eV). The morphology and microstructure of the products were characterized using a transmission electron microscope (TEM) with a JEM 2100 instrument at 200 kV utilizing a JEOL FasTEM system and scanning electron microscopy (SEM) with a Hitachi S4800. The surface morphology and thickness of the film deposited on silicon wafer was investigated by a tapping-mode atomic force microscope (AFM, Digital instrument Nanoscope IIIa).
The sensing device was fabricated by dip-dropping the DMF suspension of MPc/rGO hybrids onto 5 × 5 mm interdigitated electrode using a microsyringe. These electrodes were fabricated using standard lithography technology and made in the basal of Al2O3 through sputtering the conductor layer (Au), which have been illustrated by us before . The DMF suspensions of MPc/rGO hybrids (0.5 mg/ml) were prepared by dispersing the MPc/rGO hybrids in DMF. The MPc/rGO hybrids suspensions were sonicated at room temperature for 2 h to make sure that the hybrids were dispersed evenly.
The sensor testing was carried out using a homemade gas-sensing measurement system as illustrated in our previous report [2, 20]. NH3 (99.9 %) was purchased from Guangming Research and Design Institute of Chemical Industry, PR China. The electrical resistance of the sensors was measured with a CUST · G2 gas-sensing test system (Advanced Sensor Technology Laboratory of Jilin University, China) by applying a constant DC voltage (3 V) and recording the change in resistance passing through the sensor at a 1-s interval by a computer. In a typical sensing measurement procedure at room temperature 23 °C, (i) sensors were placed in a 10-L volume test chamber provided with a two-way stopcock joined with a pump and a trap. The test chamber was first evacuated, (ii) followed by injection of the NH3 gas of the required concentration by a micro syringe with the sweeping clean-air through the first inlet for determination of sensor response for 15 min. (iii) Then the second valve was opened and clean-air was passed in the chamber through an air drying cylinder for recovery determination. After the measurement, both inlet valves were closed so that all the test gas was driven away by the running pump. An electric fan was installed and kept the gas uniform and gas output. The sensing performance of as-fabricated MPc/rGO devices was carried out under external environmental condition (i.e., room temperature 23 °C, the relative humidity 50 % RH). The response of sensors upon exposure to NH3 was defined as Response (%) = 100 × (△R/R a) = 100 × (R g − R a)/R a, where R a is the resistance of the sensors before exposure to NH3 and R g is the resistance in the NH3/air mixed gas. The response and recovery times of the films were defined as the times needed to reach 90 % of the final resistance.
Results and Discussion
UV–vis Absorption Spectra
The response of the sensors as a function of gas concentration is shown in Fig. 8c. For MPc/rGO hybrid sensors, the response to NH3 gas is much higher than that of a pure rGO sensor at any corresponding NH3 concentration. For CuPc/rGO hybrids, a response of 11.5 % could be observed with the concentration of NH3 at 400 ppm. Even in response to as low as 0.8 ppm NH3, a response of 2.46 % could be obviously observed. Analogously, for PbPc/rGO and NiPc/rGO hybrids, the responses of 11.4 and 10.2 % could also be observed with the concentration of NH3 at 400 ppm, the responses of 1.88 and 1.28 % were still observed for 0.8 ppm NH3, respectively. In the meantime, the lowest detectable concentration of the MPc/rGO hybrids sensor is down to 400 ppb NH3. But for the pure rGO, only a response of 4.83 % can be observed with the concentration of NH3 at 400 ppm. The response of MPc/rGO hybrid sensors to 400 ppm NH3 is 2.1–2.4 times than that of pure rGO sensor. The response of MPc/rGO hybrid sensors is more than six times than that of the pure rGO sensor when the NH3 gas concentration is decreased to 0.8 ppm. Moreover, the response we got is much better than in the recently reported works on ammonia sensing by carbon nanomaterials [6, 24]. For example, Niu et al. in 2014 got a response of 5.2 % with phosphorus-doped graphene naonosheets in presence of 100 ppm of NH3, and Ghosh et al. in 2013 got a response of 5.5 % using chemically reduced graphene oxide in presence of 200 ppm of NH3. From these studies, analyses and comparisons between pure rGO and MPc/rGO hybrid sensors, we can draw a conclusion that the MPc/rGO hybrid sensors exhibit prominent response and recovery characteristic to wide ranges of concentration of NH3 gases, in particular to lower concentration of NH3 gases.
Selectivity is also an important factor for the gas sensors. Therefore, the responses of pure rGO and MPc/rGO hybrid sensors to some interferential gases are shown in Fig. 8d. The sensors show a very weak response to these gases, including CO2, CH4, H2, and CO. It is clearly seen that the MPc/rGO sensors show excellent sensitivity and selectivity to NH3. The MPc/rGO hybrids can be considered as an outstanding candidate for gas-sensing applications.
The gas mechanism for MPc/rGO hybrid sensors is briefly described as absorbed NH3 gas molecules inducing charge transfer interaction. As we known, NH3 is a strong electron donor , since MPc derivatives are well-known p-type semiconductor, NH3 can be chemisorbed on the MPc, which leads to electrons transfer from NH3 to MPc. The red shift in UV–vis, binding energy shift in XPS and G band shift in Raman were observed in MPc/rGO hybrids, which powerfully demonstrate the large conjugated π system of the hybrids and electron transfer interaction from MPc to rGO sheets. When the MPc derivatives modified on the rGO encounter NH3, the transferred electrons from NH3 to MPc are easily extracted by rGO. Since rGO is a well-known p-type semiconductor, the electron charge transfer results in a decrease of the hole carrier density, hence causing a marked increase of the resistance. Moreover, the response order of MPc/rGO hybrids to NH3 coincides with our previous studies of the gas-sensing properties of the individual MPc molecules , which indicates that the attachment of MPc plays an important role in gas-sensing performance of MPc/rGO hybrids. The possible reasons for the improved gas-sensing property of MPc/rGO hybrids are discussed. Firstly, the principle of gas sensing for the resistance-type sensors is based on the conductance variations of the sensing element. Thus, the superior electrical property of rGO contributes to the improved conductivity of hybrids, leading to a better sensing behavior. Secondly, benefited from the existence of rGO, the large surface area facilitates the gas adsorption and diffusion on the active surface . Thirdly, the attachment of MPc derivatives onto the surface of the rGO results in the specific capture and migration of electrons from MPc to rGO. The role of rGO as an electron mediator further facilitates the electron transfer from NH3 to MPc molecules. Meanwhile, the four iso-pentyloxy grounds of MPc may donate electrons to the phthalocyanine π-system as electron donor grounds, weakening MPc interactions with electron donating NH3 and reducing the adsorption between phthalocyanine macrocycles and NH3. Therefore, MPc/rGO hybrids exhibit better recoverability than pure rGO by noncovalent modification between rGO and MPc. Therefore, the significantly enhanced electron transfer, electrical conductivity, and gas adsorption due to the combination of rGO and MPcs results in the excellent sensing performance of MPc/rGO hybrids.
In summary, three novel hybrids based on reduced graphene oxide (rGO) and tetra-α-iso-pentyloxyphthalocyanine copper, nickel, and lead (CuPc, NiPc and PbPc) have been successfully fabricated and the ammonia gas-sensitive properties are studied for the first time. FT-IR, UV–vis, XPS, Raman spectra, TEM, and AFM results demonstrate that the MPc molecules were successfully anchored on the surface of rGO sheets through π–π stacking and form electron transfer interaction from MPc to rGO. MPc/rGO hybrids were explored as gas sensors and exhibit improved sensing performances to NH3 gases at room temperature in comparison to that of pure rGO. The MPc/rGO hybrids exhibited high response value, fast recovery behavior, good reproducibility, selectivity, and stability to NH3 gases. The enhanced sensing properties are attributed to the synergistic effect of MPc and rGO in the hybrids with strong electron transfer interaction, superior electrical conductivity, and gas adsorption activity. Strategies for combining various MPcs and nanoscale rGO will open new opportunities for designing and developing low power, low cost, and portable gas sensors.
This work was supported by National Natural Science Foundation of China (51202061, 51002046), Overseas Science Foundation of Heilongjiang Province (LC08C06), Natural Science Foundation of Heilongjiang Province (B201308), Foundation of Education Department of Heilongjiang Province (12521399, 12511384), Opening Project Foundation of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education.
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