A 3D Chemically Modified Graphene Hydrogel for Fast, Highly Sensitive, and Selective Gas Sensor

Reduced graphene oxide (RGO) has proved to be a promising candidate in high‐performance gas sensing in ambient conditions. However, trace detection of different kinds of gases with simultaneously high sensitivity and selectivity is challenging. Here, a chemiresistor‐type sensor based on 3D sulfonated RGO hydrogel (S‐RGOH) is reported, which can detect a variety of important gases with high sensitivity, boosted selectivity, fast response, and good reversibility. The NaHSO3 functionalized RGOH displays remarkable 118.6 and 58.9 times higher responses to NO2 and NH3, respectively, compared with its unmodified RGOH counterpart. In addition, the S‐RGOH sensor is highly responsive to volatile organic compounds. More importantly, the characteristic patterns on the linearly fitted response–temperature curves are employed to distinguish various gases for the first time. The temperature of the sensor is elevated rapidly by an imbedded microheater with little power consumption. The 3D S‐RGOH is characterized and the sensing mechanisms are proposed. This work gains new insights into boosting the sensitivity of detecting various gases by combining chemical modification and 3D structural engineering of RGO, and improving the selectivity of gas sensing by employing temperature dependent response characteristics of RGO for different gases.


S-2
. Analysis of the response of the S-RGOH sensor to 4 ppm NO 2 divides it into two stages: rapid response stage (with large slope) and slow response stage (with small slope). The red and blue linear fitted lines correspond to rapid and slow response stages respectively.

Calculation of sensitivity, noise level (RMS Noise ) and LOD of the 3D S-RGOH based NO 2 sensor. [1, 2]
Step 1: Plot the response versus NO 2 concentration curve, followed by performing the linear fitting of the curve. The slope (sensitivity) and standard error can be obtained from the fitted line ( Figure S4a).
Step 2: Implement 5 th order polynomial fit for the response ΔR/R% versus time curves at the baseline before NO 2 exposure ( Figure S4b). Step 3: Take 11 data points at the baseline before NO 2 exposure (Table S2).
Step 4: Calculate regular residual (A i -A) and statistical parameters of 5 th order polynomial fit of response ΔR/R% versus time (sec) curves, where A i and A are the measured data point and corresponding value calculated from the fitted curve respectively (Table S2).

Calculation of the concentration of various saturated organic vapors:
The concentration of 10 mL methanol, ethanol, acetone, toluene and chloroform can be calculated according to the following formula: where C is the concentration of the gas to be formulated (ppm), Q is the liquid volume (10 mL), D is the liquid density, P is the liquid purity, V is the chamber volume (1 L), M is the material molecular weight, T B is the chamber temperature (RT) and T R is the environment temperature (RT). As such, the detailed concentration is calculated as below:   The characterization of microheater: Because of the low thermal conductivity of the SiO 2 layer (∼1 W/(m· K)) and the relative high thermal conductivity of Si layer (∼149 W/(m· K)) on the Si/SiO 2 substrate, [9] the imbedded microheater can introduce a local heating effect to elevate the temperature of S-RGOH on the opposite side of substrate quickly with little power consumption and small device size. Importantly, the heat dissipation was fast after the direct current (DC) voltage is removed due to the local heating effect with small total heat generated. Thus, the temperature of the substrate can quickly return to the original state (< 51 s for 45 °C).