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Proceeding Paper

The Influence of Temperature and Visible Light Activation on the NO2 Response of WO3 Nanofibers Prepared by Electrospinning †

1
Department of Industrial Engineering, University of L’Aquila, Via Gronchi 18, 67100 L’Aquila, Italy
2
Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia, Italy
*
Author to whom correspondence should be addressed.
Presented at the 8th GOSPEL Workshop. Gas Sensors Based on Semiconducting Metal Oxides: Basic Understanding & Application Fields, Ferrara, Italy, 20–21 June 2019.
Proceedings 2019, 14(1), 1; https://doi.org/10.3390/proceedings2019014001
Published: 18 June 2019

Abstract

:
Aim of this work is to compare the electrical responses to 100–400 ppb NO2 gas concentrations of WO3 electrospun nanofibers both activated by thermal (in the temperature range 25–100 °C) and/or visible light at different wavelengths (Red λ = 670 nm, Green λ = 550 nm, and Purple-Blue λ = 430 nm). WO3 nanofibers were prepared by mixing a W-O sol-gel transparent solution with a polymeric solution made of PVP and DMF, electospun and subsequently annealed at 450 °C. Regarding gas sensing measurements, Purple Blue light resulted the most effective light source as respect to the others. Light illumination at room temperature revealed to improve both base line recovery and response time, whereas temperature enhances relative response, with a maximum at 75 °C. Light-radiating room temperature gas detection yields a satisfactory response notwithstanding a slight reduction of sensor gas sensitivity. Light induced electrical response mechanisms is presented and discussed.

1. Introduction

Thermal activation mode at different operating temperatures (OT) represents so far one of the most common strategies to increase the catalytic activity of metal oxides sensors (MOX) toward gas response [1]. However, drawbacks of the thermal activation mode are yet represented by power consumption and shortened life time of the components. MOX gas response by light activation mode at room temperature has been more recently reported for NiO [2], TiO2 [3], In2O3 [4], and WO3 [5] respectively. Considering that literature reports have already shown that visible light activation can be easily achieved at room temperature by utilizing WO3 thick films [5], in this paper we report room temperature NO2 gas responses of 1D electrospun WO3 nanofibers thermally and light activated at different wavelengths.

2. Results and Discussion

Figure 1 compares the SEM images for the as deposited (a,b) and annealed (c,d) WO3 NFs at low (left side) and high magnification (right side), deposited on Si3N4 substrates. The formation of a continuous 3D-network of interconnected homogenous nanofibers of around 50 nm diameter is highlighted. After annealing at 450 °C for 1 h, fine nanograins of around 20 nm are visible with a well-developed crystalline structure.
Electrical responses to NO2 gas were measured in dark conditions and at different visible light sources (Red λ = 670 nm, Green λ = 550 nm, and Purple-Blue λ = 430 nm) in the temperature range 25–100 °C. Figure 2 shows the electrical responses of WO3 to 400 ppb NO2 at 25 °C in dark and illuminated conditions. It turns out that the base line resistance (BLR) decreases by switching from dark, red, green and blue light respectively. This behavior can be explained considering that all the investigated light sources yield enough energy to cause the oxygen desorption from the WO3 surface with associated release of previously-trapped electrons into the conduction band. Furthermore, another evidence coming out from Figure 2 is that by desorbing in dry air, the recovery of the base line is strongly enhanced by light-radiating the sensor surface. To give a figure of the sensor base line recovery ability we introduce the recovery percentage (RP) given by the percentage ratio (ΔD/ΔA,) × 100, where D and A stands for desorption and adsorption respectively (see Figure 2). It turns out that the RPs increase from 9% (dark), to 38% (Red), 55% (green) and 92% (blue). Figure 3 shows a comparison between the electrical responses of WO3 nanofibers under dark and purple blue light at different operating temperatures (OT) in the range 25 °C–100 °C and different NO2 gas concentrations (100 ppb–400 ppb). At 25 °C the base line recovery is very poor when desorbing in dark, but it significantly improves under blue light, as previously demonstrated in Figure 2, thus assigning the best performance to purple blue light. Regarding temperature, under both dark and light conditions, heating resulted to enhance the relative response (RR), with a maximum at 75 °C, and the recovery percentage.
However, an inhibiting influence played by light on the relative response is revealed, in particular at 75 °C. We may conclude that light activation mode increases the recovery percentage, whereas thermal activation enhances the relative response. To explain the higher RRs in dark as respect to light illumination conditions, we have to consider again that light is expected to activate the desorption of adsorbed oxygen from WO3 surface. Considering now that NO2 sensing is due to the reaction between NO2 and the oxygen adsorbed on WO3 surface, by illuminating the sensor surface, less oxygen species are available to react with NO2, eventually decreasing the relative response of the sensor.

3. Conclusions

We have prepared WO3 electrospun nanofibers and tested to sub-ppm NO2 concentrations by light and thermal activation modes. Room temperature gas sensitivity was comparable in dark and light conditions. A strong enhancement of both base line recovery and response times was displaced under light conditions, suggesting 2D WO3 fibers to be suitable for ppb NO2 detection at room temperature.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Cantalini, C.; Lozzi, L.; Passacantando, M.; Santucci, S. The comparative effect of two different annealing temperatures and times on the sensitivity and long-term stability of WO3 thin films for detecting NO2. IEEE Sens. J. 2003, 3, 171–179. [Google Scholar] [CrossRef]
  2. Geng, X.; Lahem, D.; Zhang, C.; Li, C.-J.; Olivier, M.-G.; Debliquy, M. Visible light enhanced black NiO sensors for ppb-level NO2 detection at room temperature. Sens. Actuators B Chem. 2019, 45, 4253–4261. [Google Scholar] [CrossRef]
  3. Zampetti, E.; Macagnano, A.; Bearzotti, A. Gas sensor based on photoconductive electrospun titania nanofibers operating at room temperature. J. Nanopart. Res. 2013, 15, 1566. [Google Scholar] [CrossRef]
  4. Wagner, T.; Kohl, C.-D.; Malagù, C.; Donato, N.; Latino, M.; Neri, G.; Tiemann, M. UV light-enhanced NO2 sensing by mesoporous In2O3: Interpretation of results by a new sensing model. Sens. Actuators B Chem. 2013, 187, 488–494. [Google Scholar] [CrossRef]
  5. Giberti, A.; Malagù, C.; Guidi, V. WO3 sensing properties enhanced by UV illumination: An evidence of surface effect. Sens. Actuators B Chem. 2012, 165, 59–61. [Google Scholar] [CrossRef]
Figure 1. SEM images of electrospun WO3 nanofibers. Panels (a) and (b) are respectively low and high magnification of as deposited NFs. Panels (c) and (d) represent the NFs after 1h annealing at 450 °C.
Figure 1. SEM images of electrospun WO3 nanofibers. Panels (a) and (b) are respectively low and high magnification of as deposited NFs. Panels (c) and (d) represent the NFs after 1h annealing at 450 °C.
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Figure 2. WO3 nanofibers responses at 25 °C to 400 ppb NO2 under different illuminating conditions.
Figure 2. WO3 nanofibers responses at 25 °C to 400 ppb NO2 under different illuminating conditions.
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Figure 3. Comparison of the electrical responses in dark conditions and under purple-blue light (λ = 430 nm) at different temperatures and NO2 gas concentrations.
Figure 3. Comparison of the electrical responses in dark conditions and under purple-blue light (λ = 430 nm) at different temperatures and NO2 gas concentrations.
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MDPI and ACS Style

Paolucci, V.; Emamjomeh, S.M.; Anselmi-Tamburini, U.; Cantalini, C. The Influence of Temperature and Visible Light Activation on the NO2 Response of WO3 Nanofibers Prepared by Electrospinning. Proceedings 2019, 14, 1. https://doi.org/10.3390/proceedings2019014001

AMA Style

Paolucci V, Emamjomeh SM, Anselmi-Tamburini U, Cantalini C. The Influence of Temperature and Visible Light Activation on the NO2 Response of WO3 Nanofibers Prepared by Electrospinning. Proceedings. 2019; 14(1):1. https://doi.org/10.3390/proceedings2019014001

Chicago/Turabian Style

Paolucci, Valentina, Seyed Mahmoud Emamjomeh, Umberto Anselmi-Tamburini, and Carlo Cantalini. 2019. "The Influence of Temperature and Visible Light Activation on the NO2 Response of WO3 Nanofibers Prepared by Electrospinning" Proceedings 14, no. 1: 1. https://doi.org/10.3390/proceedings2019014001

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