A novel room-temperature formaldehyde gas sensor based on walnut-like WO3 modification on Ni-graphene composites

Abstract

Ternary composite with great modulation of electron transfers has attracted a lot of attention from the field of high-performance room-temperature (RT) gas sensing. Herein, walnut-like WO₃-Ni-graphene ternary composites were successfully synthesized by the hydrothermal method for formaldehyde (HCHO) sensing at RT. The structural and morphological analyses were carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). SEM and TEM studies confirmed that walnut-like WO₃ nanostructures with an average size of 53 ± 23 nm were functionalized. The Raman and XPS results revealed that, due to the deformation of the O-W-O lattice, surface oxygen vacancies O_v and surface-adsorbed oxygen species O_c were present. The gas-sensing measurement shows that the response of the WO₃-Ni-Gr composite (86.8%) was higher than that of the Ni-Gr composite (22.7%) for 500 ppm HCHO at RT. Gas-sensing enhancement can be attributed to a p-n heterojunction formation between WO₃ and Ni-Gr, O_c, spill-over effect of Ni decoration, and a special walnut-like structure. Moreover, long term stability (%R = 61.41 ± 1.66) for 30 days and high selectivity in the presence of other gases against HCHO suggested that the proposed sensor could be an ideal candidate for future commercial HCHO-sensing in a real environment.

Keywords: WO3-Ni-Gr composite; long-term stability; pn heterojunction-based gas sensors; room-temperature formaldehyde sensor; spill-over effect.

FIGURE 1

Schematic of the syntheses of Ni-Gr and WO₃-Ni-Gr composites, sensor fabrication, and testing.

FIGURE 2

(A,B) SEM images of Ni-Gr composites. Wrinkled, aggregated graphene sheets are present (C,D) SEM images of WO₃-Ni-Gr composites. A bumpy surface approximating the walnut-like surface confirms the attachment of WO₃ nanostructures. (E–J) EDS-mapping represents the presence of C, O, Ni, and W.

FIGURE 3

(A) XRD patterns of Ni-Gr (bottom) and WO₃-Ni-Gr (top) composites. Characteristic diffraction peak C (002) at 26.4° represent the graphitic nature of Gr. Minute diffraction peaks of Ni (010) and Ni (002) show that the Ni-Gr composites are formed. 23.0°, 23.5°, and 24.3° which correspond to (002), (020), and (200) reflection of WO₃ highlighted by the inset confirmed that WO₃ nanostructures were successfully modified on the Ni-Gr composite. (B) The Raman spectra of Ni-Gr (bottom) and WO₃-Ni-Gr (top) composites. G and D bands were present for Ni-Gr composites. WO₃ functionalization in the WO₃-Ni-Gr composite was confirmed by the emergence of new Raman modes of WO₃ at 240 cm⁻¹, 335 cm⁻¹, 431 cm⁻¹, 805 cm⁻¹, and 950 cm⁻¹. (C) XPS survey scan of the WO₃-Ni-Gr composite discloses that only W (4f, 4d state), O (1s), C (1s), and Ni (2p, 3s, and 3p) were present. (D) TEM images, (E,F) HRTEM image of Ni-Gr composites. Layered structures of Gr with dark spots of Ni show the Ni-Gr composite formation. (G) TEM images of the WO₃-Ni-Gr composite. The irregular dark portion highlighted by yellow circles is due to WO₃ nanostructures with an average size of 53 ± 23 nm. (H) HRTEM images of WO₃-Ni-Gr composites at 20 nm resolution. The inset shows the HRTEM image at 10 nm resolution. (I) HRTEM images of WO₃-Ni-Gr composites at 5 nm resolution. HRTEM images show that crystallographic planes of (002) Gr, (220) and (002) of Ni, and (002), (020), and (200) of WO₃ are present for the WO₃-Ni-Gr composite.

FIGURE 4

(A) UV-Vis diffuse reflectance spectra of WO₃, Ni-Gr, and WO₃-Ni-Gr composites. (B) Corresponding Kubelka–Munk plot of WO₃ (C) the WO₃-Ni-Gr composite for band gap estimation. (D–G) High-resolution XPS spectra of (D) W (4f), (E) Ni (3p3/2), (F) C (1s), and (G) O (1s).

FIGURE 5

(A) Transient responses toward varying concentrations of formaldehyde (HCHO), 10–500 ppm for Ni-Gr (bottom), and 1–500 ppm for the WO₃-Ni-Gr composite (top) at room temperature. (B) The relationship between the response and the concentration (up to 500 ppm) for both sensors against HCHO. Inset of Figure (B) shows the linear trend for both composites for 1 ≤ C ≤ 100), where C is the concentration in ppm. (C) Logarithm of % response $\left(S_g-1\right)$ as a function of the logarithm of HCHO concentration in ppm. The linear relationship with a slope of 0.59 shows that the absorbed oxygen species on the surface of WO₃-Ni-Gr are mainly $O_2^{-}$ species. (D) t_res and t_rec for both WO₃-Ni-Gr and Ni-Gr sensors are plotted as a function HCHO concentration (ppm). The fast sensing speed of the WO₃-Ni-Gr based sensor toward HCHO is observed in comparison to the Ni-Gr sensor.

FIGURE 6

(A) Transient responses of the WO₃-Ni-Gr composite toward 50 ppm of formaldehyde (HCHO) for different temperatures such as 25°C, 75°C, and 150°C. The response of the sensor to HCHO gas increases as the temperature increases. (B) Histogram of the sensing response of WO₃-Ni-Gr composites against various VOCs such formaldehyde, NH₃, IPA, ethanol, acetone, and CO. Inset of Figure (B) shows the selectivity of theWO₃-Ni-Gr composite toward 100 ppm HCHO in the presence 100 ppm of other interfering gases such as C₂H₅OH, IPA, and NH₃. Negligible change in the response value in the presence of other interfering gases was observed. (C) The histogram of the sensing response for 100 ppm HCHO at 1st, 15th, 20^th, and 30th days. It is obvious that the sensor was stable for a long time.

FIGURE 7

(A) Transient responses (% R) of the WO₃-Ni-Gr sensor to 500 ppm of HCHO at 30 % RH, 70 %, and 90 % RH. (B) Plot of responses (% R) and base-line resistance as a function of RH.

FIGURE 8

Schematic diagram for the passible sensing mechanism of (A) Ni-Gr based sensor. Ni-Gr composites exhibit the p-type behavior. In air-ionized chemisorbed oxygen species, due to the capture of electrons will increase the hole concentration (small resistance), while on HCHO exposure, the captured electrons are released back to the Ni-Gr composite and will decrease the hole concentrations (high resistance). (B) A possible mechanism of the WO₃-Ni-Gr heterojunction formation between WO₃ and Ni-Gr. Electrons from WO₃ migrate to Ni-Gr to form the depletion region and heterojunction. (C) Schematic of a possible sensing mechanism of the WO₃-Ni-Gr heterojunction on air and HCHO exposure.