3.1. Si-Field Effect Gas Sensor
Recent investigations have demonstrated the large potential of monolayer-protected nanoparticles for gas sensing applications [20
]. Several studies have been focused on the use of gold-nanoparticle (Au-NP) based gas sensor for the detection of NOx
. The central thread at the basis is the proven good affinity of NOx
with gold surface, as evidenced by Lu and co-workers [21
Thermally evaporated gold thin film has been used for the monitoring of NOx
, with little or negligible sensor response towards other interfering gases, such as H2
and CO [22
]. A straightforward relationship between the grain size of the gold catalytically layer and the sensing response has been envisaged in recent studies, focused on the size-dependent sensor response of Au-NP based sensors [23
]. Au-doped micro-porous silicon layers have been employed for NOx
monitoring, showing comparable responses toward both NO2
and NO species [24
]. In 2001, Steffes et al.
demonstrated the improvement in the sensing behavior of In2
, after doping with Au-NPs [25
Subsequently, Langmuir-Schaeffer deposited thiol encapsulated Au-NP films were proposed as sensing material for the detection of NO2
down to concentration levels of 0.5 ppm; sensing response was proven to be affected by the chemical composition of the capping molecule and the particle size [26
]. Furthermore, Parthangal and co-workers assembled an hybrid system comprising nanostructured gold and zinc oxides nanowires, that allowed the detection of both reducing (methanol) and oxidizing (nitrogen dioxide) gasses at high temperatures [27
In our laboratories, we have recently employed electrochemically synthesized Au-NPs as catalytically active materials in field-effect transistor (FET) sensors for NOx
monitoring. The synthesis of nanostructured gold was carried out according to the so-called Sacrificial Anode Electrolysis (SAE), first reported in 1994 in a seminal study by M.T. Reetz [28
]. Au-NPs with a core-shell structure were electrosynthesized in presence of quaternary ammonium chloride dissolved in THF/acetonitrile mixed solution (mixing ratio 1:3). In this process, the ammonium salt acts as both the supporting electrolyte and the NP stabilizer, forming the particle shell and thus giving rise to a stable Au-NPs colloidal solution. In similar systems, it has been proven that the thickness of the NP outer shell approximately corresponds to the length of the surfactant alkyl chains [29
]. Before the use as a gate material in FET sensors, Au-NPs were subjected to a thermal treatment at 200 °C for 1 hour, to increase the nanomaterial conductivity and enhance its stability. The material, after heating, was still nanostructured, with a spherical morphology, although a moderate increase in the NP mean core diameter could be detected (up to 50 nm, see Figure 4
for a Scanning Electron Microscopy micrograph, SEM, of annealed Au-NPs) with respect to the pristine materials (5 nm) [30
X-ray Photoelectron Spectroscopy (XPS) was used for the surface chemical characterization of both pristine and annealed materials. Carbon (92.3 ± 0.3%atomic
), nitrogen (3.9 ± 0.3%atomic
), and chlorine (2.4 ± 0.3%atomic
), were the most abundant elements detected on the surface of the pristine materials, due to the high amount of surfactant present in the electrolytic environment. Gold (0.1 ± 0.1%atomic
), and oxygen (1.3 ± 0.3%atomic
), were present at lower concentration and the carbon to gold elemental ratio was close to 103
. After annealing, a higher surface concentration of gold was detected. Indeed, Au, C, N, atomic concentrations were respectively equal to 1.3 ± 0.1%, 34.6 ± 0.3%, 3.2 ± 0.3%, and the C/Au surface atomic ratio was 102
. Moreover, the surface of annealed materials showed a significant abundance of signals due to the silicon substrate (Si and O atomic concentrations were equal to 24.7 ± 0.3% and 36.2 ± 0.3%, respectively), which is in agreement with the morphology shown in Figure 4
, outlining the presence of holes in the NP active layer, exposing the surface of the underlying SiO2
High-resolution XP spectra of pristine and annealed Au-NPs films are shown in Figure 5
. The Au4f region of the pristine material (top-left panel) is composed by two doublets, relevant to two chemical states.
The doublet falling at lower Binding Energy (BE; BEAu4f7/2
= 83.0 ± 0.1 eV) values is attributed to nano-sized Au(0) [31
]. The second doublet (BEAu4f7/2
=84.5 ± 0.1 eV) is attributed to (NR4
]. The relative abundance of these chemical environments showed a certain sample-to-sample variation. After the thermal annealing, the Au4f region changed significantly and only the nano-Au(0) doublet (BEAu4f7/2
= 83.7 ± 0.1 eV) was detected. Noteworthy, the slight BE increase observed for this feature is in agreement with the microscopy results, showing a NP size increase upon annealing.
The C1s region of both pristine and annealed nano-films showed the presence of two chemical states, the most abundant one (BE = 284.8 ± 0.1 eV) is due to aliphatic carbon, while the second one (BE = 286.2 ± 0.2 eV) to carbon bound to nitrogen. Both the signals were due to the tetra-alkyl-ammonium salt, used for the Au-NPs preparation. The N1s region was composed by two chemical environments: the first one (BE = 401.8 ± 0.1 eV) is attributed to quaternary nitrogen, while the second one (BE = 399.0 ± 0.2 eV) is attributed to amine species formed by the Hofmann's degradation of the quaternary ammonium during the electrolysis. Sample heating favored further degradation of NR4+ into lower amines (NR3, NHR2, etc.), too, and the latter became the main chemical environment in annealed films.
The capacitive FET sensor devices employed consisted of p-doped Si as the semiconductor with a thermally grown SiO2 as the insulator. The ohmic backside contact consisted of evaporated, annealed Al. Bonding pads of evaporated Cr/Au were then deposited on the insulator. The sensor chip, a ceramic heater, and a Pt-100 element for temperature control, were mounted on a 16-pin holder and electric contacts made from the sensor to the pins with gold bonding.
A fixed volume (0,5 μL) of the colloidal gold solution was drop-cast on the SiO2
surface of the capacitor, partially overlapping the bonding pad and subjected to the thermal heating. Figure 6
shows the schematic diagram of the device.
Then, the 16-pin holders were mounted in aluminum blocks connected to a gas line, with a computer-controlled gas mixing system used to regulate the concentration of gases flowed over the sensor surface. Au-NPs-based gas sensors were exposed to NO or NO2 gases, in a nitrogen/oxygen carrier gas flow.
A typical calibration curve, recorded in the case of NO2
at 175 °C is shown in the panel a) of Figure 7
. The sensor showed similar responses in presence of NO. Au-NP sensors were able to detect NOx
in a concentration range comprised between 50 and 200 ppm. After the NOx
pulse, the sensor was not able to fully recover back to the initial baseline, since some irreversible interaction takes place at the nanoparticle surface between NOx
and the nanostructured gold. Similar evidences were reported in several other studies and were explained in light of strong interactions between NOx
and gold-based nanoparticles. For instance, these evidences can be found in a study by Filippini et al.
, who used un-stabilized Au islands [37
], and in a paper by Hanwell et al.
, who used thiol-capped AuNPs [20
]. Furthermore, a high response and recovery time was observed by Steffes et al.
with a working temperature below 350 °C in a gold-modified In2
]; similar results were also reported by Penza et al.
thin films activated by gold [39
Moreover, Vitale et al.
found spectroscopic evidences of NOx
coordination to organometallic Pd(II)
/thiol-gold nanoparticles hybrids [40
], while Koel studied the absorption or bonding geometries of Nx
species, i.e., nitrogen dioxide (NO2
), dinitrogen trioxide (N2
), and dinitrogen tetroxide (N2
), on Au(111) by means of infrared reflection-absorption spectroscopy [41
Finally, in the above-cited work of Lu and al. a theoretical investigation by means of ab-initio and density functional calculations supported the stability of several gold-NO2
complexes is also reported. [21
In the panel b) of Figure 7
, a typical experiment aiming to quantify the NO2
/interferent response ratio is reported. The sensor was exposed to different gases: CO, H2
It is noteworthy that the system used in Figure 7
, although less sensitive to NOx
than other ones (see for details Table 2
, summarizing the state of art Au-based sensor performances for NOx
detection, in terms of detection limit), proved to be extremely selective, as no response at all was detected in case of CO, and hydrocarbons. Small signals were recorded in case of NH3