Highly Sensitive NH₃ Sensors Using CVD and Epitaxial Graphene Functionalised with Vanadium(V) Oxide: A Comparative Study ^†

Abstract

Exceptionally sensitive and selective graphene-based chemiresistive gas sensors were produced as a result of graphene functionalisation with a sub-nanometer V₂O₅ layer by using the method of pulsed laser deposition. Two different types of graphene were used—epitaxial graphene on SiC and CVD graphene on Si/SiO₂—and both showed remarkable enhancement of sensing properties in terms of response and recovery speed, response magnitude and selectiveness towards NH₃ gas. The epitaxial graphene-based sensor was demonstrating the highest relative response towards ammonia amounting to 80% for 0.1 ppm NH₃.

1. Introduction

Since its discovery, graphene as a monolayer-thick (semi)conducting material was investigated as a gas sensing system, with great potential for miniaturization, low-power operation and cheap production [1]. However, realization of practical sensing systems functioning in real atmosphere with the high concentration of oxygen and water vapor is still challenging. Enhancement of sensing properties of graphene is possible by promoting gas adsorption through introducing defects, atomic dopants, or by functionalisation with nanoparticles [2]. A pulsed laser deposition of a sub-nanometer thick layer of catalytically active V₂O₅ gives CVD graphene remarkable sensitivity and selectivity towards ammonia gas [3]. Even greater improvement would be expected in the case of epitaxial graphene, because of its higher sensitivity to chemical doping as a result of its lower intrinsic doping level [4].

In the current work, the chemical vapour deposited graphene on Si/SiO₂ (Si/SiO₂/CVDG), and the epitaxial graphene grown on SiC (SiC/EG) were functionalised by laser deposited ultra-thin (0.6 nm) V₂O₅ layer. The effect of PLD process on graphene defect creation was investigated by Raman spectroscopy. The NH₃ gas sensing properties are compared for pristine and V₂O₅ deposited graphenes. Responses of both types of graphene sensors to other polluting gases NO₂, CO, SO₂ and O₃ were researched to test the selectivity.

2. Materials and Methods

Chemical vapour deposited graphene was grown on a commercial 25 µm thick polycrystalline copper foil in a home-built CVD reactor. The grown graphene film was transferred onto Si/SiO₂ substrate carrying Au (60 nm) electrodes deposited through a shadow mask by magnetron sputtering. The graphene growth and transfer processes were executed as described earlier [3]. Epitaxial graphene (EG) was grown by sublimation method on Si-terminated 4H–SiC (0001) substrates at 2000 °C in argon gas at 1 bar [5]. Au (200 nm) contact pads were made by evaporation on top of SiC/EG substrate. In the PLD process, a ceramic V₂O₅ target was ablated with a KrF excimer laser at wavelength 248 nm and laser pulse energy density of 5.0 J/cm², in the presence of 5 × 10⁻² mbar of O₂. The total of 120 laser pulses with a repetition rate of 5 Hz was used for V₂O₅ layer deposition. Sensor substrates were kept at the room temperature during deposition.

Gas sensitivity was measured the room temperature in a 7 cm³ stainless steel sample chamber, equipped with a gas mixing system [6]. The voltage applied to the electrodes was 100 mV. In the experiments with UV light excitation, the 365 nm light intensity on the sample was ~15 mW/cm².

3. Results and Discussion

According to the analysis of vanadium valence state by X-ray photoelectron spectroscopy (XPS) and a mass thickness measurement by X-ray fluorescence (XRF), the material deposited on graphene is predominately in the highest stable oxidation state of vanadium, V₂O₅, and has an average thickness of ~0.6 nm [3].

As indicated by the Raman spectrum (Figure 1), the defectiveness of CVD graphene increased drastically as a result of PLD, which is an expected result of high energy atomic bombardment with PLD plasma species. The rise of defectiveness by PLD is evident from the emergence of prominent defect-related D and D’ bands, and a height decrease of G and 2D bands at the same time. By contrary, for EG on SiC substrate the typical Raman peaks indicate that graphene remained intact after identical PLD treatment. This implies a much larger resilience against the defect creation, probably due to much tighter graphene-to-support contact in case of EG on SiC. This interpretation is also supported by the fact that the electrical conductivity of SiC/EG sample did not change after the PLD of V₂O₅, whereas the conductivity of CVD graphene sample dropped substantially.

Compared to unmodified graphene, large improvement of sensor response to the reducing NH₃ gas was achieved for both types of graphene sensors (Figure 2). The SiC/EG showed by an order of magnitude larger response, as compared to CVDG sensor. For instance, the relative response to 100 ppm NH₃ was 295% and 31% in case of EG and CVDG sensors, respectively. The conductivity of different types of graphene sensors changes in opposite direction after introducing NH₃, due to different types of majority charge carriers—electrons in Si/SiO₂/CVDG and holes in SiC/EG.

In addition, considerably faster characteristic response times were measured on both types of sensors after deposition (by 4 to 6 times). Again, the improvement of the response and recovery speed was more drastic for the SiC/EG sensor. Enhancement of gas response can be caused by increased density of energetically favourable adsorption sites or/and increased charge transfer from NH₃ adsorbate molecules [4]. Strong adsorption ability of NH₃ on V₂O₅ and possible redox reactions on the surface of V₂O₅ are well known [7]. Although perfect graphene is relatively inert with respect to chemisorption, owing to low concentration of dangling bonds and lack of charged atoms on the surface [2], the as-made, or pristine graphene almost always shows considerable gas sensitivity to NO₂ and NH₃, in conjunction with extremely slow sensor recovery at RT [1,2,8]. High adsorption energy binding sites may exist in freshly produced graphene, such as uncontrollably introduced defects or dopant atoms in the graphene lattice. In the course of functionalisation a high number of point defects is formed by bombardment with high-energy atoms of PLD plasma. The induced defects are further modified through the adding of functionalising oxide material [3,9].

The recovery kinetics and the dependence of sensor response magnitudes on NH₃ concentration are in accordance with Langmuir type two-site adsorption model [10]:

$$S_x=\frac{G_x-G_0}{G_0}={\alpha }_1\frac{x{b}_1}{1+x{b}_1}+{\alpha }_2\frac{x{b}_2}{1+x{b}_2},$$

(1)

where b₁ and b₂ are affinity constants, α₁ and α₂ are transduction coefficients.

Fitting the data with the model resulted in two important consequences regarding functionalised graphene sensors:

• Adsorption sites with comparable affinity constants b₁ and b₂ on both types of graphene indicate the similar effect of deposited V₂O₅ nanophase on the surface.
• Both transduction coefficients α₁ and α₂ are much smaller in case of CVD graphene sensor, which means less influence of gas adsorption on relative conductivity, and hence, lower relative gas response of the CVD graphene sensor. Taking into account that the gas adsorption mainly modifies the charge carrier concentration rather than mobility [1], the substantially larger response of EG sensor is probably arising from lower initial carrier doping of EG as compared to free electron concentration in CVD graphene.

The selectivity with respect to NH₃ of both CVDG and EG sensors increases after the PLD of V₂O₅, when compared to the response to several other polluting gases. For instance, in case of EG, the response to NH₃ increases significantly by functionalisation, and at the same time, the response to NO₂ gas is reduced, thus improving the selectivity. The functionalisation with V₂O₅ slightly increases the response to CO and SO₂ gases for both types of sensors, both gases being reducing. As expected from the strongly oxidizing properties, reactions to O₃ and NO₂ gases are much alike.

4. Conclusions

CVD-grown graphene (Si/SiO₂/CVDG) and the epitaxial graphene grown on SiC (SiC/EG) were functionalised by laser deposited ultra-thin V₂O₅ layer (0.6 nm). Compared to pristine graphene sensors, large and selective enhancement of sensor response to the reducing NH₃ gas was achieved for both types of graphenes. The effect of metal oxide modifier was more remarkable for EG which showed, as compared to similarly functionalised CVDG sensor, by an order of magnitude larger response (295% vs. 31% for 100 ppm NH₃). The recovery kinetics and dependence on NH₃ concentration are in accordance with Langmuir type two-site adsorption model. The results imply the presence of similar adsorption sites on both types of materials, indicating a similar role of deposited V₂O₅ nanophase on the surface. The substantially larger response of EG sensor is probably the result of the lower initial carrier doping in EG.