This study analyses the sensing response of multi-walled carbon nanotubes for carbon dioxide sensing. The multi-walled carbon nanotubes were grown in-situ on tungsten interdigitated electrodes. Batches with varying nanotube diameters and network densities were tested in a controlled gas chamber and tested alongside commercially available carbon dioxide sensor. Results show the promising utilisation of as-grown multi-walled carbon nanotubes for the detection of carbon dioxide without the use of microheaters. This translates to the possibility of creating small environmental sensors that can be readily integrated into existing electronic devices and made into ubiquitous low power sensors.

The effects of the annealing temperatures and thicknesses on the shapes, sizes and arrangement of platinum (Pt) nanoparticles (NPs) on graphene and their sensing performance for hydrogen (H2) detection were investigated. It shows strong dependency of the annealing temperatures and thicknesses on the properties of NPs. It was found that the proposed technique is able to form the NPs with good size controllability and uniformity even for thick deposited layer, thus eliminating the requirement of very thin layer of below 5 nm for the direct NP synthesis by evaporation or sputtering. The transport properties of Pt NPs/graphene structure and its sensing performance on H2 at room temperature under various H2 concentration were evaluated. The results showed an acceptable sensing response, indicating an innovative approach to fabricate Pt NPs embedded graphene for gas sensing application.

In the present investigation, gold-silver@titania (Au-Ag@TiO2) plasmonic nanocomposite materials with different Au and Ag compositions were prepared using a simple one-step chemical reduction method and used as photoanodes in high-efficiency dye-sensitized solar cells (DSSCs). The Au-Ag incorporated TiO2 photoanode demonstrated an enhanced solar-to-electrical energy conversion efficiency of 7.33%, which is ∼230% higher than the unmodified TiO2 photoanode (2.22%) under full sunlight illumination (100 mW cm-2, AM 1.5G). This superior solar energy conversion efficiency was mainly due to the synergistic effect between the Au and Ag, and their surface plasmon resonance effect, which improved the optical absorption and interfacial charge transfer by minimizing the charge recombination process. The influence of the Au-Ag composition on the overall energy conversion efficiency was also explored, and the optimized composition with TiO2 was found to be Au75-Ag25. This was reflected in the femtosecond transient absorption dynamics in which the electron-phonon interaction in the Au nanoparticles was measured to be 6.14 ps in TiO2/Au75:Ag25, compared to 2.38 ps for free Au and 4.02 ps for TiO2/Au100:Ag0. The slower dynamics indicates a more efficient electron-hole separation in TiO2/Au75:Ag25 that is attributed to the formation of a Schottky barrier at the interface between TiO2 and the noble metal(s) that acts as an electron sink. The significant boost in the solar energy conversion efficiency with the Au-Ag@TiO2 plasmonic nanocomposite showed its potential as a photoanode for high-efficiency DSSCs.

This work was aimed to develop chemoresistive base sensor on silicon platform. Carbon nanotube (CNT) with low density was grown on passive interdigitated (IDE) based sensor which later can be functionalized with other material for the selectivity of the targeted goals. Low dense CNT will provide a bigger area of the functionalities element adhered to the wall hence increase the sensitivity of the sensor. In situ CNT growth was achieved using a PECVD machine with the time parameter was varied targeting for low dense CNT grow in-between the IDE fingers. SEM analysis reveals that low dense CNT was evidence for growth time as short as 30 s and as the time increase, the density was also increasing. Resistive probing of the grown CNT samples shows a drop in resistance compared to non-grown CNT which confirmed that the CNT successfully conducting the electron. CO2 gas testing shows that the low dense sensor shows better detection performance compared to high dense sensor. However, further study needs to be conducted to measure the level of amorphous carbon which determine the purity of the such CNT nanostructure.

The effect of cobalt (Co) and nickel (Ni) nanoparticle catalysts on the growth of carbon nanotubes (CNTs) were studied, where the CNTs were vertically grown by plasma enhanced chemical vapour deposition (PECVD) method. The growth conditions were fixed at a temperature of 700 °C with a pressure of 1000 mTorr for 40 minutes with various thicknesses of sputtered metal catalysts. Only multi-walled carbon nanotubes are present from the growth as large average diameter of outer tube (∼10–30 nm) were measured for both of the catalysts used. Experimental results show that high density of CNTs was observed especially towards thicker catalysts layers where larger and thicker nanotubes were formed. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. The average diameter of particle size increases from 4 to 10 nm for Co and Ni catalysts. A linear relationship is shown between the nanoparticle size and the diameter of tubes with catalyst thicknesses for both catalysts. The average growth rate of Co catalyst is about 1.5 times higher than Ni catalyst, which indicates that Co catalyst has a better role in growing CNTs with thinner catalyst layer. It is found that Co yields higher growth rate, bigger diameter of nanotube and thicker wall as compared to Ni catalyst. However, variation in Co and Ni catalysts thicknesses did not influence the quality of CNTs grown, as only minor variation in IG/ID ratio from Raman spectra analysis. The study reveals that the catalysts thickness strongly affects not only nanotube diameter and growth rate but also morphology of the nanoparticles formed during the process without influencing the quality of CNTs.

Relative Humidity (RH) is considered as one of the important variable to be controlled during testing and characterizing of metal oxide gas sensor. This is because the metal oxide can easily react with the hydrogen and oxygen molecule of the water vapor which associated with RH level. In this paper, the effect of using nitrogen as carrier to feed the analyte to the sensor inside a 40 liter capacity container has been studied. When Nitrogen gas was supplied to the chamber, the RH value dropped significantly from initial ~67 % RH to ~7 % RH within one hour duration. As an alternative, a simple bubbler system was introduced to minimize the drop of RH. The bubbler which was filled with 100% distilled water or mixed with a certain percentage of glycerol are able to maintain the RH to a certain level for a certain period of time.

In this work, the piezoresistive effects of defective graphene used on a flexible pressure sensor are demonstrated. The graphene used was deposited at substrate temperatures of 750, 850 and 1000 °C using the hot-filament thermal chemical vapor deposition method in which the resultant graphene had different defect densities. Incorporation of the graphene as the sensing materials in sensor device showed that a linear variation in the resistance change with the applied gas pressure was obtained in the range of 0 to 50 kPa. The deposition temperature of the graphene deposited on copper foil using this technique was shown to be capable of tuning the sensitivity of the flexible graphene-based pressure sensor. We found that the sensor performance is strongly dominated by the defect density in the graphene, where graphene with the highest defect density deposited at 750 °C exhibited an almost four-fold sensitivity as compared to that deposited at 1000 °C. This effect is believed to have been contributed by the scattering of charge carriers in the graphene networks through various forms such as from the defects in the graphene lattice itself, tunneling between graphene islands, and tunneling between defect-like structures.

In this paper, the effect of nickel (Ni) catalyst on the growth of carbon nanotubes (CNTs) was studied where the CNTs were vertically grown by plasma enhanced chemical vapor deposition (PECVD) method. The growth conditions were fixed at a temperature of 700°C with a pressure of 1000mTorr for 40 minutes with various thicknesses of sputtered Ni catalyst. Experimental results show that high density of CNTs was observed especially towards thicker catalyst layers where larger and taller nanotubes were formed. The growth rate increases by ~0.7 times with increasing catalyst thickness from 4nm to 10nm. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. From the Raman results, we found that only slight variation in the intensity ratio of G-band over D-band as increasing catalyst thicknesses. The minor difference in G/D ratio indicates that the catalyst thickness does not significantly influence the quality of CNTs grown.

The article describes a facile method for the preparation of a conjugate composed of silver nanoparticles and graphene oxide (Ag@GO) via chemical reduction of silver precursors in the presence of graphene oxide (GO) while sonicating the solution. The Ag@GO was characterized by X-ray photoelectron spectroscopy, X-ray powder diffraction, and energy-dispersive X-ray spectroscopy. The nanocomposite undergoes a color change from yellow to colorless in presence of Hg(II), and this effect is based on the disappearance of the localized surface plasmon resonance absorption of the AgNPs due to the formation of silver-mercury amalgam. The presence of GO, on the other hand, prevents the agglomeration of the AgNPs and enhances the stability of the nanocomposite material in solution. Hence, the probe represents a viable optical probe for the determination of mercury(II) ions in that it can be used to visually detect Hg(II) concentrations as low as 100 μM. The instrumental LOD is 338 nM.

Resist patterning method has been used to enable selective ZnO nanorods grown via facile hydrothermal process. The growth region of the ZnO nanorods was controlled by pre-coating the seed layer on the Silicon base substrate. Using the plasma process, the seed layer which is not coated with a resist layer will be etched out. Therefore, when the samples completely undergone the hydrothermal process, there will be no nanorods grow in that specific area. The grown ZnO nanorods was in well array with flat hexagonal tip and wurtzite crystal structure. This technique is can be applied for application which require integration of nanostructure in specific critical areas such as an interdigitated electrodes (IDE) for various gas sensor applications.

There are few known parameters which govern tungsten trioxide (WO3) hydrothermal synthesis process which includes material source concentration, synthesis temperature, duration, pH value and additive level. Using design of experiments (DOE) approach, a systematic experimental procedure was conducted to investigate the effect of each parameter to the final morphology of the synthesized nanostructure. Despite the response obtained from this study is in qulitative form, the analysis still can be done to identify the combination of variables that most likely can produce either 1-D, 2-D or 3-D nanostructure. This insight is essential before further optimization of the process can be done in order to predict the behavior of the WO3 hydrothermal synthesis process.

We report on the development of a carbon dioxide gas sensor from the room-temperature reduction of graphene oxide via hydrogen plasma. The hydrogen plasma contains radicals and atoms which give dissociation energies for oxygen functional groups, which is capable of reducing the graphene oxide. The sample morphology, degree of reduction, chemical bonding and gas sensing capability were systematically studied. The effective removal of oxygen functional groups at the edges and both basal planes while restoring C=C bonds has been observed by AFM, XPS and Raman analysis. The C/O ratio increased from 0.81 to 7.9 and the resistance decreased significantly from 33kΩ to 1.6kΩ after the reduction process. The fabricated rGO-F20 sensor shows the highest CO2 gas sensing response of 71% and 15% at 1500ppm in N2 (37% RH) and air environment (68% RH), respectively. In addition, the sensor shows a good repeatability performance with the sensing and recovery time of about 4min when exposed to 750 and 769ppm CO2 concentration. The repeatability performance was measured in air environment at 68% RH without external assisted recovery. This simple, room-temperature reduction process and sensing capability, low cost fabrication process of a graphene sensor could lead to the implementation of a practical indoor air quality monitor.

This paper presents a functionalized, horizontally oriented carbon nanotube network as a sensing element to enhance the sensitivity of a pressure sensor. The synthesis of horizontally oriented nanotubes from the AuFe catalyst and their deposition onto a mechanically flexible substrate via transfer printing are studied. Nanotube formation on thermally oxidized Si (100) substrates via plasma-enhanced chemical vapor deposition controls the nanotube coverage and orientation on the flexible substrate. These nanotubes can be simply transferred to the flexible substrate without changing their physical structure. When tested under a pressure range of 0 to 50 kPa, the performance of the fabricated pressure sensor reaches as high as approximately 1.68%/kPa, which indicates high sensitivity to a small change of pressure. Such sensitivity may be induced by the slight contact in isolated nanotubes. This nanotube formation, in turn, enhances the modification of the contact and tunneling distance of the nanotubes upon the deformation of the network. Therefore, the horizontally oriented carbon nanotube network has great potential as a sensing element for future transparent sensors.

Horizontally networked carbon nanotubes grown on Au–Fe catalyst nanoparticles were achieved through the decomposition of acetylene by plasma-enhanced chemical vapor deposition (PECVD) method. It has been ascertained that conditions governing the formation of the bilayer Au–Fe catalyst nanoparticles dictate the resultant growth of the nanotubes. From the FESEM and AFM studies, the average nanoparticle size was found to increase in the range of 13.3–87.0 nm as the catalyst film thickness increases. A net-like structure of multi-walled carbon nanotubes network with the remains of Au clusters on the substrate was observed for the thicker catalyst, suggesting the absence of nanotube growth on these Au clusters due to catalyst deactivation. Possible mechanisms of nanotube growth dependent catalyst formation are discussed in the paper, which explain the role of Au film in the growth stage. The influence of ammonia to acetylene gas ratios on the sheet resistance, sheet concentration, average sheet Hall coefficient, and magnetoresistance of the grown nanotubes was discussed.