Pd-based optical H2 sensors (i.e., sensors employing Pd either as the main sensing material or for only catalyzing the dissociation of H2 before this is absorbed by the main sensing material) rely on probing either (1) intensity changes of the transmittance/reflectance or (2) frequency shifts of the localized surface plasmon resonance (LSPR) upon exposure of the sensing element to H2. Transmission/reflection sensors (discussed here) rely on changes in the optical properties of the sensing material upon exposure to H2, which can be easily probed by a simple light detector or a conventional optical spectrophotometer.
) is a clean energy carrier that provides a promising alternative to fossil fuels as its only by-product when consumed is water 
. Despite the unique eco-friendly nature and high energy density, the high flammability over a wide concentration range (4–75%) and low ignition energy (0.0017 mJ) of H2
are important concerns for its safe handling and use 
. This, in turn, raises the need to develop reliable sensors capable of measuring H2
over a wide range of concentrations (from a few tens of ppb to a few percent) under different environmental conditions.
The most common types of H2
sensors can be classified as (1) electrical or (2) optical 
. Electrical H2
sensors available in the market have detection limits down to a few hundreds of ppm and response times in the order of tens of seconds. Recent efforts have yielded electrical H2
sensors that exhibit superior performance compared to their commercially available counterparts, with reported limit of detection (LOD) down to a couple of tens of ppb, and response times of less than one second 
Electrical sensors  rely on changes in the resistance (or conductance) of their sensing elements that are induced by concentration variabilities of H2 in the gas they are exposed to. The most common electrical sensors employ metal oxide semiconductors (e.g., ZnO, NiO, and TiO2) as sensing elements ,. The resistance of these materials is sensitive to the depletion of electrons on their surface that is caused by the adsorption of target gas molecules , providing a measurable variable related to the concentration of specific gases. The thickness of the electron depletion region increases as a function of the number of gaseous H2 molecules adsorbing on the surface of the metal oxide (which is proportional to the concentration of H2 in the vicinity of the sensing material), and returns to that corresponding to the initial/nominal resistance of the material when H2 is removed.
Apart from MOS-based hydrogen sensors, other categories of sensors for measuring the concentration of H2
in gases can be classified into three main categories: conductometric, amperometric, and electrochemical 
. In addition, sensors that utilize the high thermal conductivity of hydrogen, which is about 7.5 times greater than that of air, have also been developed and tested. Such sensors can exhibit a limit of detection down to 500 ppm, while they are also able to operate at temperatures below 0 °C 
. Last, but not least, sensors that utilize the catalytic reaction of H2
to increase the temperature and thus the resistance of the sensing material (known as combustion-based sensors), can exhibit good sensitivity and a linearity for concentrations up to 4% 
Electrical sensors that rely on H2
absorption (instead of adsorption as in the case of metal oxide semiconductors discussed above) by transition metals have also been proposed and tested 
. Such sensors are, in principle, more selective compared to those relying on adsorption as not many species can penetrate the lattice of the transition metals. In this respect, Pd, which exhibits high H2
solubility, can be employed to attribute high selectivity towards H2
, thus providing a very promising material for sensing purposes 
Although materials based on Pd thin films have been successfully synthesized and tested as electrical H2
sensors where the conductivity of the sensing material varies proportionally with concentration 
, they show certain constraints that can limit their use in real-life applications. At H2
concentrations above 2%, for instance, the expansion of the Pd lattice, which is associated to the phase transition of the material, can degrade the properties of the sensing element, attributing poor irreversibility and stability to the sensor. At lower concentrations (less than 1%), the sensors exhibit long response times due to the slow diffusivity of hydrogen atoms in the crystal lattice, thereby limiting their use for H2
Sensors that rely on changes of the optical properties of their sensing material upon exposure to H2
provide a highly promising alternative to their electrical counterparts. An important advantage of optical sensors is the lack of electrical contacts that could induce sparks under harsh operating conditions, and consequently ignite the sampled gas with catastrophic consequences. This is particularly important when sensing H2
in an environment containing O2
, which is the case for H2
sensors designed for safety purposes. Although optical H2
sensors reported in the literature have been tested down to a few ppm or less , in principle they can reach concentrations down to the ppt regime 
Pd-based thin films may exhibit cracking caused by the volume expansion/shrinkage upon hydrogenation/dehydrogenation cycling. Such cracks can affect the optical properties of the films in a non-reversible manner, thereby limiting their application as optical H2
sensors. This may be prevented by alloying the Pd and by applying intermediate layers such as Ti on the substrate. Nanoparticle-based materials are much less prone to cracking upon hydrogenation/dehydrogenation, providing a more favorable alternative to thin films. In the case of ANP(aggregated nanoparticle)-based materials, cracks are typically formed during their synthesis (i.e., before exposure to H2
, providing room for the materials to expand or shrink upon hydrogenation and dehydrogenation, respectively, in a reversible way. For INP(isolated nanoparticle)-based materials, cracking issues are completely avoided as a result of their non-continuous nature (having freedom levels that expand in all directions), while attributing other highly favorable properties to the resulting sensors in terms of sensitivity and response/recovery times 
Another limitation of Pd-based sensors is CO poisoning, which impairs the stability and accuracy of the sensor. At low concentrations, CO adsorbs on specific sites of the Pd lattice that progressively covers the entire material 
. Eventually, a C layer is formed on the surface of the Pd material, forming a barrier for H2
to reach the Pd surface.
2. Pd-Based Thin Films for Optical H2 Sensors
The most investigated family of materials for optical H2
sensing is that of Pd thin films (i.e., films having thicknesses from few tens to few hundreds of nanometers) deposited on flat transparent substrates. In such films, Pd has a double role: (1) it acts as a catalyst to dissociate molecular hydrogen and (2) it changes its reflection/transparency upon hydrogen absorption, which is the property probed to determine the concentration of H2
in the sample. Pd thin films can be produced by conventional methods including Physical Vapour Deposition (PVD) or sol–gel techniques 
Pure Pd thin films, deposited on SiO2
substrates, have been proposed as optical material for sensing H2
since the late 1980s 
. The transition from the α to the β phase that can induce a lattice volume increase can potentially cause material deformation and cracking. In principle, this is seen as a disadvantage because it can lead to a non-repeatable behavior of the sensor. Nevertheless, deformations and cracks of Pd thin films can be used to optimize the performance of the sensing elements when exposed to a H2
-containing gas as they can increase the surface to volume ratio of the material. For example, reduced Pd thin films have been shown to exhibit pronounced changes in their optical properties compared to their oxidized counterparts, because cracks increase the available sites for absorption upon reduction 
In an attempt to improve the performance of Pd thin film sensors in terms of sensitivity and limit of detection, a number of studies have tried to cap an elastomer with a Pd thin film. A Pd-capped elastomer (PCE) sensor exploits the deformation of the sensing element to radically change the absolute reflectance, showing up to ~60% increase in the reflectance compared to samples without an elastomer, over the entire visible spectrum when exposed to air containing 4% H2
. This material deformation changes the specular (mirror-like reflection) surface of the Pd thin film to a diffusing (where incident rays are scattered in many angles) surface, thereby enhancing the light scattering efficiency of the material upon absorption of H2
. Sensors employing PCE materials have been reported to have a response time of 14 s, when exposed to 4% H2
in air, and a recovery time of 10 s 
An important ability of Pd thin films is that they can dissociate molecular into monoatomic hydrogen. Using this capability, Pd thin films have been employed as a top layer of bi-layer structures for optical H2
. In such bi-layer structures, monoatomic hydrogen produced by the dissociation of H2
at the surface of the Pd thin film is subsequently absorbed by a second layer typically consisting of oxides or transition metals that play the role of the sensing elements. Such material architectures can support improved sensing performance in terms of intense optical changes and detection limits compared to pure Pd systems 
provides an excellent bi-layer thin-film material architecture for transmission/reflection H2
sensing. In a H2
-containing environment, the H2
adsorbed on the surface of the Pd thin film dissociates and diffuses to the WO3
layer which is converted to tungsten bronze (HWO3
), exhibiting a noticeable change in its optical properties as it becomes opaque (dark blue) 
. In those systems (also referred to as gasochromic sensors), the thickness of the Pd layer should be small enough (~3–4 nm) to ensure high optical transparency. The transmittance of a 760 nm thick Pd-capped WO3
film before and after 10 min exposure to 1% H2
, can exhibit a large change in the visible and near-infrared regime.
Other Pd-capped thin films that can provide promising optical H2
sensors employ Yttrium (Y) and Magnesium (Mg) as capped materials 
. These elements undergo a metal to semiconductor transition upon hydrogenation, inducing color changes by interference effects. We should note here that Y reacts with oxygen if not capped, becoming transparent upon oxidation. As a result, by capping it with Pd in principle offers good selectivity towards H2 when this is present in an oxygen-containing environment. Color changes in the Y
states correspond to H2
threshold concentrations ranging from 5 to 1000 ppm, whereas the time required for these transitions can vary from 10 to 100 s depending on the concentration of H2
in the system, which can be considered too slow for certain applications. In addition, those systems are strongly hysteretic, posing another limitation for their use.
Similar to Y, Mg films also exhibit limitations for optical H2
sensing that can be overcome by capping them with Pd. Mg exhibits optical transition within a narrow pressure range around the pressure plateau that limits the sensitivity of the resulting sensors, but this can be overcome by doping it with other elements, such as Ni, Ni-Zr, and Ti 
. Moreover, Mg also shows strong hysteresis upon hydrogenation/dehydrogenation cycling, mainly associated to the complex phase transformation, which is also responsible for the slow response and relatively high inaccuracy of the resulting sensors. Despite that, Pd-capped Mg thin films can be a choice for a single-use eye-readable H2
Pd-capped transition metals, such as Hf and Ta, have also been proposed as H2
sensing materials. Hf exhibits steeper optical changes over a wider H2
pressure range, whereas Ta shows less abrupt changes at low H2
Pa), compared to those of pure Pd or PdAu thin films. In the case of Hf, the material does not exhibit hysteresis upon hydrogenation/dehydrogenation cycling, due to the occurrence of a coherent transition from HfH1.4
(referred to as the δ
. This transition appears at the range where the optical transmission spans six orders of magnitude in pressure, attributing a very high spanning range to the resulting sensor. In general, Pd-capped Hf thin films exhibit a response time that ranges from ~5 s to 10 min as the H2
pressure decreases from 5 to 10−2
Pa at 120 °C. Note here that the lowest H2
pressure tested was due to limitations of the testing setup and not of the sensing material.
Ta shows a higher H2 solubility compared to Hf, associated with the absence of any structural changes upon hydrogenation. Pd-capped Ta thin films have demonstrated a repeatable hysteresis-free optical response in the range of 10−2 to 104 Pa, with the intensity of the optical change being wavelength-dependent. Such films have also been reported to have response and recovery times of 7 and 20 s, respectively, when exposed to H2 pressures ranging from 10 to 300 Pa. In general, Ta-based thin films exhibit short/sub-second response times at close to room temperatures compared to their Hf-based counterparts, large H2 sensing range, making them highly attractive for many applications.
3. Toward Manufacturing of Pd-Based Thin Film Optical H2 Sensors for Real-Life Applications
The important limitations of Pd-based H2
sensing materials can be addressed to a certain extent by different synthesis approaches including the use of coatings, mixed materials and by nanostructuring. The hysteresis effect typically exhibited by flat and ANP thin films can be addressed by alloying and/or controlling their thickness. In addition, small (i.e., a few nanometers) INPs can also suppress hysteresis. Polymeric coatings applied on thin films have also been shown to limit CO poisoning and enhance the chemical stability. This approach can in principle be used on other types of materials, including INP- and CN (complex nanostructures)-based materials 
. Alloy PdAuCu materials structures have also been shown to address this issue and improve the chemical stability of the sensor, suppressing the CO poisoning.
Cracking, which is typically observed in flat and ANP thin films, is oftentimes seen as a limitation because it spoils the uniformity of the materials. Nevertheless, this can be considered as advantageous as cracks increase the surface to volume ratios of the materials, thereby enhancing the overall sensor sensitivity. Cracks are by definition avoided when using INP- and ANP-based sensing materials, as the building blocks of the very materials are isolated and thus do not form large “crackable” crystal structures 
An important consideration defining whether a specific sensor material can be industrially produced is the easiness, the repeatability, and the cost of manufacturing. Thin film sensors have rather established, straightforward, and easy ways of manufacturing. Common PVD techniques, such as sputtering and e-beam evaporation have been successfully used to synthesize Pd-based thin films for optical H2 sensors. Furthermore, sol–gel has been used as a promising fabrication technique for depositing Pd thin films.
All in all, the criteria for selecting the most appropriate method for manufacturing sensing materials strongly depend on the specific demands of the final sensors. PVD methods can be used to produce conventional thin film layers, the properties of which can be tailored by alloying. To increase the surface to volume ration of thin films and thus further enhance their performance of thin films requires methods for nanostructuring, including aerosol-based or chemical solution methods. Both are rather inexpensive methods for synthesizing sensing elements, which is crucial for industrial production. Lithography-based or combination of PVD and CVD fabrication can be used to produce Pd-based architectures that can yield highly sensitive (with low-enough LOD values) and fast response sensors, but at a rather high cost.