Reduced Graphene Oxide-Coated Si Nanowires for Highly Sensitive and Selective Detection of Indoor Formaldehyde
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Although significant developments have been made in the low-concentration formaldehyde monitoring in indoor air by using gas sensors, they still suffer from insufficient performance for achieving ppb-level detection. In this work, <100> oriented Si nanowires (SiNWs) with high specific surface area were prepared via metal-assisted chemical etching method (MACE), and then were uniformly coated with graphene oxide (GO) followed by the subsequent reductive process in H2/Ar atmosphere at 800 °C to obtain reduced graphene oxide (RGO). The RGO coating (RGO@n-SiNWs) obviously enhances SiNWs sensitivity to low-concentration formaldehyde, benefiting from the increased specific surface area, the sensitization effect of RGO, and the formation of p-n junction between SiNWs and RGO. Specifically, RGO@n-SiNWs exhibits a high response of 6.4 to 10 ppm formaldehyde at 300 °C, which is about 2.6 times higher than that of pristine SiNWs (~ 2.5). Furthermore, the RGO@n-SiNWs show a high response of 2.4 to 0.1 ppm formaldehyde which is the largest permissive concentration in indoor air, a low detection limit of 35 ppb obtained by non-linear fitting, and fast response/recovery times of 30 and 10 s. In the meanwhile, the sensor also shows high selectivity over other typical interfering gases such as ethanol, acetone, ammonia, methanol, xylene, and toluene, and shows a high stability over a measurement period of 6 days. These results enable the highly sensitive, selective, and stable detection of low-concentration formaldehyde to guarantee safety of indoor environment.
KeywordsSi nanowires Reduced graphene oxide Sensitivity Selectivity Formaldehyde
High-resolution transmission electron microscopy
International Agency for Research on Cancer
Metal-assisted chemical etching
National Institute for Occupational Safety and Health
Reduced graphene oxide
Reduced graphene oxide-coated n-type silicon nanowires
Reduced graphene oxide-coated silicon nanowires
Sick building syndrome
Scanning electron microscopy
Transmission electron microscopy
Volatile organic compounds
World Health Organization
X-ray photoelectron spectroscopy
Nowadays, as one of the toxic volatile organic compounds (VOCs) in newly built house environment, formaldehyde (HCHO) is seriously threatening human health [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], which is considered to be one of the major sources of sick building syndrome (SBS) [13, 14], and a carcinogen by International Agency for Research on Cancer (IAIC) . Therefore, several standards have been set up to avoid the risk induced by the pollution of indoor air. In the literatures, the upper limit of formaldehyde concentration established by the National Institute for Occupational Safety and Health (NIOSH) is 0.1 ppm in the living room and 1 ppm in industrial production workshop . Meanwhile, the World Health Organization (WHO) also established a safe standard of 0.08 ppm averaged over 30 min for long-term exposure in formaldehyde vapor . Therefore, the successful detection of low-concentration HCHO makes a great stride to ensure the safety of living environment.
Although many schemes have been developed for detecting low-concentration HCHO, including liquid chromatograph (LC) [16, 17], spectroscopy , etc., these techniques have limitations for portable use and real-time monitoring due to their bulky sizes and complicated analysis processes . Currently, gas sensors based on the semiconductor nanostructures (e.g., In2O3 [19, 20], Cr2O3 , SnO2 [21, 22, 23]) are extensively employed in the detection of low-concentration HCHO, owing to their high sensitivity, fast response, and excellent chemical stability [2, 10, 19, 24, 25, 26, 27, 28, 29, 30, 31, 32]. These sensors based on semiconductor nanostructures offer significant advantages compared with LC and spectroscopy, such as easy miniaturization for portable use, low cost, and in-situ detection. However, their responses to HCHO need to be further improved at the ppb level though they are good at ppm level. For example, Chen et al. reported Ga-doped In2O3 nanofiber sensors which showed a high response (defined as Ra/Rg, where the Ra and Rg are the resistances of the sensor in air and in HCHO) of 52.4 to 100 ppm HCHO, while < 1.5 to 0.1 ppm, which needs to be enhanced to meet the response requirement of practical utilization limitation of Ra/Rg = 2 . Therefore, it is an urgent affair to find an efficient route to enhance the sensitivity for reaching the safe detection limitation. Silicon nanowires (Si NWs) have been selected as one of semiconductor materials to be used in chemical sensors. For example, biosensors based on chemically modified Si NW field effect transistors have been reported and demonstrated a superior sensitivity and selectivity to proteins . However, this sensor fabrication needs a high cost and complicated process as the sensitivity has to be improved by the filed effect.
Recently, the incorporation of graphene with nanostructured semiconductor gas sensors becomes a promising approach to improve the sensitivity, due to its high specific surface area and exceptional sensitivity to gases . Compared with the sensitization effect of conventional noble metals (e.g., Pt, Pd, and Au nanoparticles) [35, 36, 37], this strategy can not only possess the merits of low cost and high efficiency but also enlarge the surface area and improve the electron transport. For example, reduced graphene oxide (RGO)-SnO2 , RGO-Cu2O , graphene-SnO2  have demonstrated excellent enhancement of gas sensitivity. However, many reports put the semiconductor nanostructures on the surface of RGO or graphene to form simple contact, of which the efficient contact area is too restricted to achieve the maximization of sensitivity. Therefore, it is significant to search an efficient and feasible strategy to realize core-shell structures based on RGO and semiconductor.
In this work, highly sensitive and selective detection of low-concentration HCHO was achieved by a core-shell structure of RGO-coated silicon nanowires (SiNWs), with increased specific surface area twice as large as SiNWs. Specifically, the response of reduced graphene oxide-coated n-type silicon nanowires (RGO@n-SiNWs) increases about 2.6× toward 10 ppm HCHO (~ 6.4) than that of pristine SiNWs (~ 2.5) at the best operation temperature of 300 °C, which is attributed to the excellent sensitization effect of RGO. The as-fabricated sensors can reach a superior application detection limitation of as low as 35 ppb, and the response/recovery times are as fast as 30/10 s. Besides the improved sensitivity, the selectivity is high over typical interfering gases (e.g., ethanol, acetone, ammonia, methanol, xylene, and toluene) and the stability is good in a period of 6 days. All of the results made a significant stride toward using reduced graphene oxide-coated silicon nanowires (RGO@SiNWs) for the low concentration HCHO detection in indoor environment.
Materials and Methods
Fabrications of SiNWs Arrays
n (100) and p (100) Silicon wafers (0.005–0.02 Ωcm and 0.001–0.005 Ωcm) were employed as starting wafers (3.0 cm × 3.0 cm). Before the etching process, the Si wafers were cleaned in acetone for 10 min, ethanol for 10 min, and deionized (DI) water for 10 min in turn. The cleaned starting wafers were immersed in oxidant solution containing H2SO4 (97%, Sigma-Aldrich) and H2O2 (35%, GR 30 wt.% in H2O, Aldrich) in a volume ratio of 3:1 for 30 min to remove the organic contaminants on the surface. After the cleaning step, the samples were then immersed into 5% HF solution for 8 min at room temperature to dissolve the thin oxide layer formed on the surface and thus the fresh Si surfaces were H-terminated. Next, the cleaned Si wafers were immediately transferred into an Ag coating solution containing 0.005 M AgNO3 (99.99%, Aladdin) and 4.8 M HF (Aladdin, GR 40%), which was slowly stirred for 1 min at room temperature (~25 oC). After a uniform layer of Ag nanoparticles (AgNPs) was deposited on the surfaces, the AgNPs-coated wafers were washed with deionized water to remove the extra Ag+ ions. Then, the wafers were etched in the etching solution (H2O2 = 0 .4 M and HF = 4 .8 M) for 30 min at room temperature in the dark. Finally, the samples were dipped in the aqueous solution of HNO3 (70%, Sigma-Aldrich) to dissolve the Ag catalyst, and then rinsed with deionized water for several times to remove residual layer. The fabricated SiNWs were slowly scraped by a sharp blade.
SiNWs Functionalized with RGO
The graphene oxide (GO) dispersion was synthesized by the modified Hummer’s method , and then was ultrasonically dispersed in 60 mL DI water for 3 h to prepare the GO solution (30 mg). In a typical synthesis, the obtained SiNWs (0.2 g) were firstly dispersed in the mixture of DI water (10 mL) and ethanol (30 mL), then ethylenediamine (400 μL) was dropwise added. After the ultrasonic treatment for 20 min, 20 mL GO solution was added to the above solution and kept vigorous stirring. Subsequently, the product was collected by centrifugation and washed with ethanol for several times, then dried at 60 °C to obtain GO@SiNWs. Finally, the GO@SiNWs was reduced in H2/Ar atmosphere at 800 °C (2 °C min−1) to obtain RGO@SiNWs.
Characterization of SiNWs and RGO@SiNWs
The morphology of SiNWs and RGO@SiNWs was observed by scanning electron microscopy (SEM, JSM-7001F+INCA X-MAX) and transmission electron microscopy (TEM, JEM-2100F). Besides, the crystal structure was studied by X-ray diffraction (XRD, X’Pert PRO MPD). Additionally, in order to analyze the surface area and pore size distribution, nitrogen absorption-desorption isotherm was performed on a specific area and a pore-size analyzer (SSA-7300, BUILDER) by the Brunauer–Emmett–Teller (BET) method and Barett–Joyner–Halenda (BJH) model, respectively. For the confirmation of the existence of RGO, Raman spectrum was performed by a Raman spectrometer (Thermo Scientific DXR2). Besides, the elemental analyses were performed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Kα radiation).
Devices Fabrication and Measurement
As-prepared RGO@SiNWs (~ 5 mg) was mixed with ethanol (~ 100 μL), and dispersed uniformly by ultrasonic. The dispersed solution was coated onto a ceramic plate with Pt wires (i.e., heater and measurer), and aged under a voltage of 5 V for 3 days in air. Finally, the prepared devices were measured in a gas sensor analyzer (Winsen WS-30A, China). Formaldehyde was produced by the evaporation of formaldehyde solution (40 wt%) at heating holder in chamber. Ethanol, acetone, ammonia, methanol, xylene, and toluene were produced by pure liquid ethanol, acetone, ammonia, methanol, xylene, and toluene respectively. Response is defined as Ra/Rg, where Ra and Rg are the resistances of the sensor in pure air and in formaldehyde gases. Response/Recovery times are defined as the time needed to change to 90% of the total response.
Results and Discussions
Throughout this process, Ag nanoparticles directly seize electrons from Si because of the higher electronegativity of Ag compared with Si, creating a hole-rich region around the Ag nanoparticles. Then, H2O2 is reduced by Ag nanoparticles and Si is oxidized to be SiO2, which is dissolved quickly by HF solution .
Next, the as-etched SiNWs were functionalized by RGO. Figure 1g is the SEM image of RGO@n-SiNWs and Fig. 1h is the zoomed SEM images of RGO@n-SiNWs, which proved that RGO was compactly and uniformly wrapped on the surface of NWs. There would be a formation of p-n junction between RGO and SiNWs, which is important for the enhancement of sensors sensitivity discussed in the following sections.
Finally, the mechanism of sensitivity enhancement induced by the combination of n-SiNWs and RGO was discussed. The combination of RGO and n-SiNWs can form a p-n junction, as a result of the p-type characterization of RGO with a narrow band gap (0.2 eV~2 eV) . This p-n junction formed between SiNWs and RGO has been reported in many previous reports . To understand how this p-n junction improve the sensitivity, the schematic diagram of band structure is described in Fig. 8b. As illustrated in band structure diagram in Fig. 8b, the electrons are transferred from SiNWs and stored in RGO, forming a depletion layer and a built-in electric field. The electron depletion and built-in voltage would enhance the chemical reaction in Eq. (4) and facilitate the electron transfer, thus enhances the gas sensing performance.
In summary, SiNWs with high specific surface area are prepared via metal-assisted chemical etching method (MACE), and then are wrapped by reduced graphene oxide (RGO) to form a p-n junction. After wrapping RGO, the specific surface area increases by 1× demonstrated by N2 absorption-desorption isotherm. More importantly, due to the formed p-n junction, the RGO@n-SiNWs reveals an outstanding sensitivity and high selectivity toward low concentration HCHO at 300 °C. The response of RGO@n-SiNWs increases about 2× toward 10 ppm HCHO (~ 6.4) at 300 °C than that of pristine n-SiNWs (~ 2.5). The application detection limitation can reach 35 ppb (Ra/Rg = 2) obtained by non-linear fitting absolutely meeting the safe standard of indoor air. These results provide a promising possibility to precisely detect the low-concentration HCHO, enabling the monitoring the indoor environment.
The work was financially supported by the National Key R&D Program of China (2016YFC0207100), the Natural Science Foundation of Shandong Province, China (ZR2018JL021 and ZR2014EMQ011), and the National Natural Science Foundation of China (51402160 and 51602314). The work was also supported by the Taishan Scholar Program of Shandong Province, China, and the Opening Project of Key Laboratory of Microelectronic Devices and Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences.
Availability of Data and Materials
The datasets supporting the conclusions of this manuscript are included within the manuscript.
LFS, LQL,YX, LPY, and AQW conducted the extensive experiments and analyzed the data. FYW and NH supervised the project and wrote the manuscript. JJS, YW, and YFC helped to review and discuss the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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