Hydrogen detected by the naked eye!
Researchers at King’s College London have developed an inexpensive, robust and ultrasensitive hydrogen detector based on a new type of optical metamaterial with sensitivity so large, 2% Hydrogen can even be detected by the naked eye!
Creating a safe environment
Hydrogen is increasingly used in a host of industrial applications, particularly in the petrochemical and chemical industries and as engineering challenges are overcome and costs reduced, it is increasingly being used as a fuel in conventional vehicular transportation. Hydrogen poses a number of hazards to safety from potential detonations and fires when mixed with air (at less than 4% concentration) to being an asphyxiant in its pure, oxygen-free form. Furthermore, fires sustained by hydrogen are not only extremely hot, but are almost invisible and can thus lead to severe burns. Hydrogen is also detrimental to many metals (hydrogen embrittlement) due to its solubility which can cause leaks and lead to explosions.
Metamaterial Hydrogen Sensor
Academics at Kings have used their expertise in the fabrication and characterisation of state-of-the-art optical metamaterials to develop an inexpensive hydrogen sensitive metamaterial that represents a leap forward in terms of sensitivity. Research has been focused on developing a highly sensitive optical hydrogen sensor for a number of years, however these efforts have suffered either due to a lack of sensitivity or expensive fabrication strategies. The team at King’s have neatly eradicated these issues by taking advantage of the ultra-high sensing capability of optical metamaterials based on gold nanorod arrays which are fabricated using scalable, self-assembled principles. The metamaterial, based on the optical interaction between arrays of plasmonic (gold/palladium) nanorods, takes advantage of the engineered metamaterial response; when exposed to Hydrogen, not only does the optical properties of each component nanorod change, but also the interaction between them - a combination which leads to superior sensitivity.
Current Market Sensors
Current hydrogen sensing technologies are based on the change of conductivity caused by hydrogen exposure, and utilise a variety of mechanisms listed below.
Thermal conductivity sensors – rely on a temperature-induced change of resistance in a heated element (requiring electronic control) on exposure to hydrogen, as it measures the conduction of heat to a surrounding gas. Major disadvantage: sensitive to a number of other vapours.
Metal-Oxide Sensors – Utilise a wide-band gap material and are typically fabricated using Tin oxide. Rely on the adsorption of a gas which changes the electron density and hence the conductivity changes (usually at elevated temperature). Major disadvantage: huge variability, cost of fabrication.
Catalytic sensors – relies on the combustion of hydrogen on a platinum catalyst which increases the temperature causing a change in resistance. Major disadvantage: dangerous at high hydrogen concentrations.
Conversely, the hydrogen sensor described here exhibits none of the aforementioned disadvantages, and requires no micromachining to fabricate a robust, highly sensitive platform for hydrogen detection.
Advantages of Metamaterial based Hydrogen Sensing
- Inexpensive Metamaterial Fabrication – Based on Anodisation and Electrodeposition.
- Scalable fabrication process – Multiple sensors from a single wafer.
- Simple optical setup - usable in transmission or reflection for enhanced sensitivity.
- Inherently Safe – Remote source and detector using optical fibre; no electrical connections in sensing area.
- Fast Cycle Time – can even be reset in <30 seconds using the detection laser at increased intensity.
- Room Temperature Operation – no special heating required.
- Low Power Consumption – can be operated simply using an inexpensive laser source – even a battery powered laser pen provides sufficient light intensity.
Patent status
PCT Application (WO2014184530) PLASMONIC HYDROGEN DETECTION
Technical development
Ultra-high sensitivity to hydrogen at low concentrations (40% change in 2% Hydrogen) has been demonstrated. The aim is to license the technology to develop and launch a commercial product.
Engineering the Sensor
The metamaterial hydrogen sensor is fabricated using self-assembled processes and is therefore scalable, requires no micromachining and the maximum required size of the sensor can easily be the same as a laser spot size (approx. 1 mm) without any special optics other than a delivery fibre. A typical laboratory sample is approximately 1cm2. The fabrication process begins with the anodisation of thin film aluminium, followed by the electro-deposition of gold, to form a plasmonic metamaterial. Finally, a short chemical etch is performed to create a shell, which is then filled via the electro-deposition of palladium finalising the material. The overall thickness of the layer is less than 300 nm, resulting in negligible material costs per sensor. Without optimising the parameters of the metamaterial, the sensor can readily detect concentrations as low as 0.1% (well within the noise limits of the sensor). The response time to 2% Hydrogen gas is under 30 seconds, again, without optimisation via sensor geometry.
Metamaterial Sensing Results
For illustration purposes, a schematic of the hydrogen sensitive metamaterial is shown in Figure 1(a) below. Figure 1(b) shows two images of the sensor taken using reflected light, both when the sensor is exposed to 2% hydrogen in nitrogen, and when exposed to 100% nitrogen –the large change in the reflected intensity on hydrogen exposure can easily be detected by eye. The sensor may be employed in any geometry, showing a 38% change in transmitted light intensity on exposure to the 2% H2 in N2 mix (Figure 1c) and a 30 % change in reflected intensity (Figure 1d).
Figure 1: Schematic (a) and (b)-(d) optical characterisation. (b) CCD images of the reflection from the Hydrogen sensor when exposed to Hydrogen (ON) and Nitrogen (OFF) – the image area is designated by the dashed circle. (c) and (d) Spectra showing the change in transmission reflection respectively, on exposure to Hydrogen.
It can be easily seen from examining the time response of the sensor (legend in Figure 1c and 1d), that even at 2% H2 concentration, that the optical response is rapid, particularly (about 5% after 30 seconds) considering the volume in the experimental gas cell and low flow rate (about 100 ml) at atmospheric pressure. Further optimisation is expected to increase this temporal response significantly (optical signal modulation, metamaterial geometry). In addition, mild heating of the metamaterial, using the same wavelength of light used as the probe, is shown to reset the sensor to initial conditions after less than 45 seconds for re-use.