ROOM TEMPERATURE HYDROGEN GAS SENSOR

Abstract

A room temperature hydrogen gas sensor comprising tin(IV) oxide and platinum layered on an electrode substrate is described. The tin(IV) oxide may be polycrystalline with an average layer thickness of 10-700 nm, and topped with platinum having an average layer thickness of 1-15 nm. The room temperature hydrogen gas sensor may be used to detect and measure levels of H.sub.2 gas at room temperature and at concentrations of 50-1800 ppm, with fast response and high stability. A method of making the room temperature hydrogen gas sensor is also described, and involves sputtering to deposit tin(IV) oxide and platinum on a substrate, which is then subjected to low-temperature annealing step.

Claims

1: A room temperature hydrogen gas sensor, comprising: at least two electrodes on a substrate, the electrodes separated by 10-500 m; a SnO.sub.2 layer in contact with the at least two electrodes on the substrate, the SnO.sub.2 layer having an average thickness of 10-700 nm; and a platinum layer in contact with the SnO.sub.2 layer, the platinum layer having an average thickness of 1-15 nm.

2: The hydrogen gas sensor of claim 1, wherein the SnO.sub.2 layer consists essentially of SnO.sub.2, and the Pt layer consists essentially of Pt.

3: The hydrogen gas sensor of claim 1, wherein the at least two electrodes are substantially planar.

4: The room temperature hydrogen gas sensor of claim 1, wherein the platinum layer has an RMS surface roughness of 0.1-4 nm.

5: The room temperature hydrogen gas sensor of claim 1, wherein the substrate comprises silica.

6: The room temperature hydrogen gas sensor of claim 5, having a transmittance of 30-50% for a wavelength in a range of 420-500 nm.

7: The hydrogen gas sensor of claim 1, wherein the SnO.sub.2 layer comprises polycrystalline SnO.sub.2 having an average grain size of 5-20 nm.

8: A method of making the hydrogen gas sensor of claim 1, comprising: sputtering SnO.sub.2 onto the at least two electrodes on the substrate to produce an amorphous SnO.sub.2 layer, sputtering platinum onto the amorphous SnO.sub.2 layer to produce a deposited platinum layer; and annealing the amorphous SnO.sub.2 layer and the deposited platinum layer at 130-250 C.

9: The method of claim 8, wherein the annealing is at 140-170 C. for 1-5 hours.

10: The method of claim 8, wherein the SnO.sub.2 is sputtered by a RF sputtering mode, and the Pt is sputtered by a DC sputtering mode.

11: A method of using the room temperature hydrogen gas sensor of claim 1, comprising: contacting the platinum layer with a first gas sample comprising hydrogen gas, and measuring a first resistivity across the at least two electrodes, wherein the first resistivity is decreased by 70-99.9% relative to a second resistivity arising from a second gas sample, the second gas sample substantially free of hydrogen gas.

12: The method of claim 11, wherein the first gas sample comprises 50-1800 ppm hydrogen gas.

13: The method of claim 11, wherein the first gas sample has a temperature of 0-50 C. and a pressure of 0.9-1.1 atm.

14: The method of claim 13, wherein the first gas sample has a temperature of 20-35 C.

15: The method of claim 11, wherein the second gas sample comprises 300-5,000 ppm of at least one gas selected from the group consisting of NH.sub.3, n-butane, O.sub.2, CO.sub.2, and N.sub.2.

16: The method of claim 11, wherein the decrease in the first resistivity has a response time of 0.5-100 s.

17: The method of claim 11, wherein the second gas sample comprises 0.1-99 vol % of at least one gas selected from the group consisting of O.sub.2, CO.sub.2, H.sub.2O, Ar and N.sub.2, relative to a total volume of the second gas sample, or consists essentially of O.sub.2, CO.sub.2, H.sub.2O, Ar and/or N.sub.2.

18: The method of claim 11, wherein the decrease in the first resistivity has a recovery time of 200-400 s.

19: The method of claim 11, wherein the room temperature hydrogen gas sensor is in contact with 500-5,000 ppm H.sub.2 gas for 1-6 months before the contacting with the first gas sample.

20: The method of claim 11, which has a repeatability of at least 99%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

[0031] FIG. 1 shows an experimental setup used for testing a room temperature hydrogen gas sensor.

[0032] FIG. 2 shows XRD (X-Ray Diffraction) patterns of samples S1-S6.

[0033] FIG. 3A shows 3D, 2D, and linear AFM surface profiles and calculated RMS surface roughness of the S1 sample.

[0034] FIG. 3B shows 3D, 2D, and lincar AFM surface profiles and calculated RMS surface roughness of the S2 sample.

[0035] FIG. 3C shows 3D, 2D, and linear AFM surface profiles and calculated RMS surface roughness of the S6 sample.

[0036] FIG. 4 shows UV-Vis transmittance spectra of samples S1-S6.

[0037] FIG. 5A is an XPS (X-Ray Photoelectron Spectroscopy) survey spectrum of the S3 sample.

[0038] FIG. 5B is a representative resolved Sn 3d XPS spectrum of the S3 sample.

[0039] FIG. 5C is a representative deconvoluted Pt 4fXPS spectrum of the S3 sample.

[0040] FIG. 5D is a representative resolved O1s XPS spectrum of the S3 sample.

[0041] FIG. 6A shows the resistance of sample S1 over time while being exposed to increasing concentrations of H.sub.2 gas (250 to 1750 ppm) at room temperature.

[0042] FIG. 6B shows the resistance of sample S3 over time while being exposed to increasing concentrations of H.sub.2 gas (250 to 1750 ppm) at room temperature

[0043] FIG. 7 shows the response obtained from samples S1-S6 at different concentrations of H.sub.2 gas at room temperature.

[0044] FIG. 8 is a repeatability test of sample S3 with 1000 ppm H.sub.2 gas at room temperature.

[0045] FIG. 9 is a plot of both response time and recovery time of sample S3 at different H.sub.2 concentrations at room temperature.

[0046] FIG. 10 displays the response of sample S3 exposed at RT to different concentrations (750, 1250, 1750 ppm) of H.sub.2 gas.

[0047] FIG. 11 shows the response of sample S3 in the presence of 1250 or 1750 ppm H.sub.2 gas over a temperature range of RT to 500 C.

[0048] FIG. 12 shows the response of sample S3 in the presence of different gases at RT.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the disclosure are shown.

[0050] The present disclosure will be better understood with reference to the following definitions. As used herein, the words a and an and the like carry the meaning ofone or more. Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. It will be further understood that the terms comprises and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0051] As used herein, the words about, approximately, or substantially similar may be used when describing magnitude and/or position to indicate that the value and/or position described is within a reasonable expected range of values and/or positions. For example, a numeric value may have a value that is +/0.1% of the stated value (or range of values), +/1% of the stated value (or range of values), +/2% of the stated value (or range of values), +/5% of the stated value (or range of values), +/10% of the stated value (or range of values), +/15% of the stated value (or range of values), or +/20% of the stated value (or range of values). Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

[0052] As used herein, compound is intended to refer to a chemical entity, whether as a solid, liquid, or gas, and whether in a crude mixture or isolated and purified.

[0053] As used herein, composite refers to a combination of two or more distinct constituent materials into one. The individual components, on an atomic level, remain separate and distinct within the finished structure. The materials may have different physical or chemical properties, that when combined, produce a material with characteristics different from the original components. In some embodiments, a composite may have at least two constituent materials that comprise the same empirical formula but are distinguished by different densities, crystal phases, or a lack of a crystal phase (i.e. an amorphous phase).

[0054] The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material. For example, Ni(NO.sub.3).sub.2 includes anhydrous Ni(NO.sub.3).sub.2, Ni(NO.sub.3).sub.2.6H.sub.2O, and any other hydrated forms or mixtures. CuCl.sub.2 includes both anhydrous CuCl.sub.2 and CuCl.sub.2.2H.sub.2O.

[0055] In addition, the present disclosure is intended to include all isotopes of atoms occurring in the present compounds and complexes. Isotopes include those atoms having the same atomic number but different mass numbers. By way of general example, and without limitation, isotopes of hydrogen include deuterium and tritium. Isotopes of carbon include .sup.13C and .sup.14C. Isotopes of oxygen include .sup.16O, .sup.17O, and .sup.18O. Isotopes of tin include .sup.112Sn, .sup.114-120Sn, .sup.122Sn, and .sup.124Sn. Isotopes of platinum include .sup.190Pt, .sup.192Pt, .sup.194pt, .sup.195Pt, .sup.196pt, and .sup.198Pt. Isotopically-labeled compounds of the disclosure may generally be prepared by conventional techniques known to those skilled in the art or by processes analogous to those described herein, using an appropriate isotopically-labeled reagent in place of the non-labeled reagent otherwise employed.

[0056] According to a first aspect, the present disclosure relates to a room temperature hydrogen gas sensor. The room temperature hydrogen gas sensor comprises at least two electrodes on a substrate, a SnO.sub.2 layer in contact with the at least two planar electrodes on the substrate, and a platinum layer in contact with the SnO.sub.2 layer.

[0057] As described here, room temperature may refer to a temperature in a range of 18-24 C., preferably 20-22 C., or about 20 C., or about 25 C. However, in certain cases, and depending on weather, air conditioning, heating, ventilation, and personal preferences, room temperature may refer to a temperature lower than 18 C., for example, 15 or 16 C., or to a temperature greater than 24 C., for instance, 27 C. In one embodiment, room temperature may refer to more than one temperature in one of the ranges as described previously. For instance, a room temperature gas sensor may have a temperature of 20 C., while coming in contact with a room temperature gas having a temperature of 22 C. A small difference in temperatures may arise from the gas sensor being attached to a housing, casing, wall, or some other object that has a heat sink effect or a higher heat capacity. In another aspect, the term room temperature refers to the ambient temperature of a sample or environment that is in contact with the gas sensor.

[0058] In one embodiment, the substrate may be planar, and may have a rectangular shape, a circular shape, or some other shape. In one embodiment, the substrate may have a planar side with a surface area of 0.1-100 cm.sup.2, preferably 0.25-50 cm.sup.2, more preferably 0.5-10 cm.sup.2, even more preferably 0.7-8 cm.sup.2. However, in some embodiments, the substrate may have a planar side with a surface area smaller than 0.1 cm.sup.2 or larger than 100 cm.sup.2. The substrate may have a thickness of 0.10-20 mm, preferably 0.15-15 mm, more preferably 0.17-10 mm, however, in some embodiments, the substrate may have a thickness of less than 0.10 mm, or greater than 20 mm. In an alternative embodiment, the substrate may be curved, grooved, knurled, or shaped into some other non-planar arrangement.

[0059] The substrate may be a sapphire substrate, a quartz substrate, a magnesium oxide single crystal substrate, a ceramic substrate, an alumina substrate, a silicon substrate (e.g. silicon wafer or silicon oxide), a silicon nitride substrate, or some other substrate. In one embodiment, the substrate comprises silica (SiO.sub.2), and preferably in one embodiment, the substrate consists essentially of silica, meaning that at least 98 wt %, preferably 99 wt %, more preferably at least 99.9 wt % of the substrate is silica, relative to a total weight of the substrate. The silica may be amorphous silica, fumed silica, quartz, or some other type of silica. In alternative embodiments, the substrate may be a type of glass such as flint glass, soda lime glass, or borosilicate glass. In one embodiment, the substrate may be a glass coverslip or a glass slide for a microscope. In an alternative embodiment, the substrate may not necessarily be silica, but may be some other substance having a low electrical conductivity and/or considered an electrical insulator. Defined here, an insulator refers to a solid material with a high electrical resistivity that may prevent an electric current from flowing between two points. The electrical resistivity of the insulator may be at least 10.sup.2 .Math.m, preferably at least 10.sup.3 .Math.m, more preferably at least 10.sup.4 .Math.-m at 20 C.

[0060] In one embodiment, the at least two electrodes may be separated by 10-500 m, preferably 20-450 m, more preferably 50-300 m, even more preferably 70-250 m. In one embodiment, the electrodes may be separated by a minimum distance of the abovementioned ranges. The electrodes may comprise an electrically-conductive material such as indium tin oxide alloy, platinum-iridium alloy, iridium, titanium, titanium alloy, stainless steel, gold, cobalt alloy, copper, aluminum, tin, iron, and/or some other metal or metal alloy. In a preferred embodiment, the electrodes comprise gold. In another preferred embodiment, the electrodes comprise platinum. In other embodiments, the electrodes may comprise a non-metallic electrically-conductive material, such as graphene or a polyelectrolyte. As defined here, an electrically-conductive material refers to substance with an electrical resistivity of at most 10.sup.6 .Math.m, preferably at most 10.sup.7 .Math.-m, more preferably at most 10.sup.8 .Math.m at a temperature of 20-25 C. In one embodiment, a part of the electrically conductive material of the electrode may extend away from the substrate in order to connect with a power source to form part of a circuit. In one embodiment, the electrodes may be arranged in an interwoven, interdigitated, or comb-like pattern on the substrate. In one embodiment, two or more interdigitated patterns of electrodes may be electrically connected to each other, forming a continuous, single electrode. In another embodiment, two or more interdigitated electrodes may be electrically isolated from one another, and may function as parallel detector circuits within the room temperature hydrogen gas sensor. In one embodiment, the at least two electrodes are substantially planar. However, in other embodiments, the electrodes may be located on a surface of a curved or angled substrate, and may be non-planar. In another related embodiment, one or more of the at least two electrodes may be deposited on a location of a substrate having a high surface roughness, for instance an RMS greater than 20 nm, preferably greater than 50 nm, in which the at least two electrodes would not be considered planar. In this case, the electrodes may be formed on nanoparticles or a nano-pattered substrate. In one embodiment, the electrodes may have an average thickness of 100-500 nm, preferably 150-450 nm, more preferably 200-400 nm. The electrodes may be in the form of ribbons, wires, dots, or some other shape.

[0061] In one embodiment, the electrodes may be patterned on the substrate using known methods such as, for example, photolithography or electron beam lithography, and then wet or dry etching. Alternatively, a lift-off technique may be used, in which the electrode patterning is achieved by the dissolution of photoresist followed by deposition of a metallic layer of a photolithographically or e-beam lithographically defined photoresist layer.

[0062] In one embodiment, the SnO.sub.2 layer has an average thickness of 10-700 nm, preferably 25-550 nm, more preferably 40-450 nm, even more preferably 50-400 nm. However, in some embodiments, the SnO.sub.2 layer may have an average thickness of less than 10 nm or greater than 700 nm. In one embodiment, the SnO.sub.2 layer may have a thickness that varies by less than 50 nm, preferably less than 35 nm, more preferably less than 25 nm of the average thickness. However, in some embodiments, the SnO.sub.2 layer may have a thickness in some parts that is more than 50 nm or less than 50 nm of the average thickness.

[0063] In one embodiment, the SnO.sub.2 layer consists essentially of SnO.sub.2, meaning that the SnO.sub.2 layer comprises at least 97 wt %, preferably at least 99 wt %, more preferably at least 99.9 wt % SnO.sub.2 relative to a total weight of the SnO.sub.2 layer. In one embodiment, the SnO.sub.2 layer may comprise one or more compounds that are not SnO.sub.2, for instance, the SnO.sub.2 layer may comprise 1-4 wt %, or 2-3 wt % SnO (tin(II) oxide), relative to a total weight of the SnO.sub.2 layer. In other embodiments, other semiconducting metal compounds or metal oxides may be used in place of or with the SnO.sub.2. These include, but are not limited to, In.sub.2O.sub.3, ZnO, WO.sub.3, Co.sub.2O.sub.3, TiO.sub.2, NiO, ZrO.sub.2, Fe.sub.2O.sub.3, Al.sub.2O.sub.3, Ga.sub.2O.sub.3, Nb.sub.2O.sub.5, and Sb.sub.2O.sub.3, n or any other semiconducting metal oxide, or a combination of one or more metals including In.sub.2O.sub.3 with ZnO, SnO.sub.2 with ZnO, or any other combination of metals.

[0064] In one embodiment, the SnO.sub.2 layer comprises polycrystalline SnO.sub.2. Polycrystalline, as used herein, refers to material composed of multiple crystal grains that are typically separated by high-angle grain boundaries, i.e., boundaries between adjacent grains crystallographically misoriented by greater than 10, preferably greater than 12, more preferably greater than 15. In one embodiment, the polycrystalline SnO.sub.2 of the SnO.sub.2 layer may be substantially, or even completely, free of any biaxial texture (e.g., a preferred grain-to-grain orientation).

[0065] In one embodiment, the SnO.sub.2 layer comprises polycrystalline SnO.sub.2 having an average grain size of 5-20 nm, preferably 5.5-15 nm, more preferably 6-10 nm, though in some embodiments, the SnO.sub.2 layer may comprise polycrystalline SnO.sub.2 having an average grain size of less than 5 nm or greater than 20 nm. In one embodiment, the SnO.sub.2 layer may comprise monocrystalline SnO.sub.2, or a mixture of amorphous SnO.sub.2 and polycrystalline SnO.sub.2.

[0066] In one embodiment, the grain size may be thought of as the longest distance through a central region of a crystal grain that connects opposite facing surfaces of the crystal grain. In one embodiment, the SnO.sub.2 layer may have a lattice parameter or lattice constant (a) of 4.60-4.75 , preferably 4.62-4.72 , more preferably 4.68-4.70 . In one embodiment, the SnO.sub.2 layer may have a lattice parameter or lattice constant (c) of 3.16-3.20 , preferably 3.16-3.19 , more preferably 3.17-3.18 . In one embodiment, the SnO.sub.2 layer may show X-ray diffraction peaks corresponding to (110), (101), (200), (211), (220), (310), (112), and/or (321) SnO.sub.2 crystal faces.

[0067] In one embodiment, the platinum layer has an average thickness of 1-15 nm, preferably 2-12 nm, more preferably 3-10 nm, even more preferably 4-8 nm. However, in some embodiments, the platinum layer may have an average thickness of less than 1 nm or greater than 15 nm. In an alternative embodiment, Pt nanoparticles having an average diameter of 1-15 nm, preferably 3-10 nm may be deposited on the SnO.sub.2 layer. In another related alternative embodiment, other nanostructures of Pt or some other noble metal may be deposited on the SnO.sub.2 layer, such as nanorods, nanowires, nanocubes, or nanoplatelets.

[0068] In one embodiment, the platinum layer has an RMS surface roughness of 0.1-4 nm, preferably 0.5-3.5 nm, more preferably 1-3 nm, even more preferably 1.2-2.5 nm. However, in other embodiments, the platinum layer may have an RMS surface roughness of less than 0.1 nm or greater than 4 nm.

[0069] In one embodiment, the Pt layer may completely cover the exposed SnO.sub.2 layer. However, in other embodiments, the Pt layer may cover only 50-95 area %, preferably 60-90 area %, more preferably 70-85 area % of the total area of the exposed surface of the SnO.sub.2 layer. In one embodiment, the Pt layer may be non-porous, though in some embodiments, the Pt layer may have pores, some of which may extend to the SnO.sub.2 layer. In the embodiments where the SnO.sub.2 layer does not cover all of the exposed substrate and electrodes, the Pt layer may be in direct contact with a part of the electrode and/or substrate.

[0070] In one embodiment, the room temperature hydrogen gas sensor, subjected to X-ray crystallography analysis, may show no measurable signal from the Pt layer due to its thinness and/or amorphous (non-crystalline) form. Thus, in some embodiments, the Pt layer may comprise crystalline Pt, and in other embodiments, the Pt may be in an amorphous state.

[0071] In one embodiment, the Pt layer consists essentially of Pt, meaning that the Pt layer comprises at least 97 wt %, preferably at least 99 wt %, more preferably at least 99.9 wt % Pt, even more preferably at least 99.99 wt % Pt relative to a total weight of the Pt layer.

[0072] In an alternative embodiment, a different metal may be used in place of or along with platinum. This metal may be palladium, gold, silver, aluminum, copper, iron, nickel, ruthenium, or some other metal or metal alloy.

[0073] In one embodiment, the Pt layer, or some other noble metal layer, may further comprise a polymer such as a polyamide, a polyacrylamide, a polyacrylate, a polyalkylacrylate, a polystyrene, a polynitrile, a polyvinyl, a polyvinylchloride, a polyvinyl alcohol, a polydiene, a polyester, a polycarbonate, a polysiloxane, a polyurethane, a polyolefin, a polyimide, or heteropolymeric combinations thereof. The polymer may be present at a weight percentage of 1-25 wt %, preferably 2-20 wt %, more preferably 3-8 wt % relative to the total weight of the Pt layer. In one embodiment, the Pt layer (or layer of some other material) on the SnO.sub.2 layer may function as a molecular diffusion barrier, which is selectively permeable to diffusion of hydrogen gas to the exclusion of oxygen.

[0074] In one embodiment, both the SnO.sub.2 layer consists essentially of SnO.sub.2, and the Pt layer consists essentially of Pt.

[0075] In one embodiment, the SnO.sub.2 layer may completely coat the electrodes and substrate, however, in other embodiments, the SnO.sub.2 layer may cover only 50-90 area %, preferably 60-80 area % of the area of the total area of the exposed face of the electrodes and substrate. In one embodiment, the SnO.sub.2 layer may be non-porous, though in some embodiments, the SnO.sub.2 layer may have pores, some of which may extend to the substrate and/or electrode surface.

[0076] In one embodiment, the SnO.sub.2 layer and/or the Pt layer may be partially or completely coated with materials such as nanostructured barium cerate, strontium cerate, or other proton conducting membranes or hydrogen permeable membranes to provide an effective barrier against non-hydrogen gases in the environment, yet enable only hydrogen to diffuse to interior of the room temperature hydrogen gas sensor, thereby acting as a selective membrane for hydrogen in the sensor element.

[0077] Preferably, the room temperature hydrogen gas sensor produces a change in electrical conductivity or resistivity upon exposure to hydrogen gas. Given Ohm's law, at a fixed electric potential (voltage), the conductivity is inversely proportional to the resistivity. Thus, the room temperature hydrogen gas sensor may be thought of as detecting a change in conductivity (i.e. current) or a change in resistivity. These changes may result from the adsorption of hydrogen gas molecules onto the surface of the room temperature hydrogen gas sensor. In view of that, the room temperature hydrogen gas sensor may also be referred to as a chemiresistive hydrogen gas sensor. However, in other embodiments, the room temperature hydrogen gas sensor may exhibit other measurable changes in physical properties such as optical transmittance, electrical capacitance, magneto-resistance, photoconductivity, and/or any other detectable property change accompanying the exposure of the room temperature hydrogen gas sensor to hydrogen. The room temperature hydrogen gas sensor may further include a detector constructed and arranged to convert the detectable change of physical property to a perceivable output, e.g., a visual output, auditory output, tactile output, and/or auditory output.

[0078] In a further embodiment, where the substrate comprises transparent silica, preferably quartz such as a quartz slide, the room temperature hydrogen gas sensor has a transmittance of 30-50%, for a wavelength in a range of 420-500 nm. Preferably the room temperature hydrogen gas sensor has a transmittance of 37-48% for a wavelength in a range of 420-480 nm. More preferably the room temperature hydrogen gas sensor has a transmittance of 42-45% for a wavelength in a range of 440-460 nm. However, in some embodiments, the transmittance may be lower than 30% or greater than 50% for a wavelength in a range of 420-500 nm. In some embodiments, the transmittance may be generally lower due to the substrate absorbing, reflecting, and/or scattering light, as compared to a substrate of transparent silica or quartz. In some embodiments, the transmittance of the room temperature hydrogen gas sensor may be generally higher at greater wavelengths and lower at shorter wavelengths, however, in alternative embodiments, the reverse may be true.

[0079] According to a second aspect, the present disclosure relates to a method of making the room temperature hydrogen gas sensor of the first aspect. This method involves the steps of sputtering SnO.sub.2 onto the at least two electrodes on the substrate to produce an amorphous SnO.sub.2 layer, sputtering platinum onto the amorphous SnO.sub.2 layer to produce a deposited platinum layer; and annealing the amorphous SnO.sub.2 layer and the deposited platinum layer.

[0080] In one embodiment, the SnO.sub.2 layer and/or the Pt layer may be deposited by a sol-gel process. The sol-gel process is a versatile solution process for making ceramic and glass materials. In general, the sol-gel process involves the transition of a system from a liquid sol (mostly colloidal) into a solid gel phase. Applying the sol-gel process, it is possible to fabricate ceramic or glass materials in a wide variety of forms: ultra-fine or spherical shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. The starting materials used in the preparation of the sol are usually inorganic metal salts or metal organic compounds such as metal alkoxides. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or a sol. Further processing of the sol enables one to make ceramic materials in different forms. Thin films can be produced on a piece of substrate by spin-coating or dip-coating. When the sol is cast into a mold, a wet gel will form. With further drying and heat-treatment, the gel is converted into dense ceramic or glass articles. If the liquid in a wet gel is removed under a supercritical condition, a highly porous and extremely low density material called aerogel is obtained. As the viscosity of a sol is adjusted into a proper viscosity range, ceramic fibers can be drawn from the sol. Ultra-fine and uniform ceramic powders are formed by precipitation, spray pyrolysis, or emulsion techniques. In one embodiment, the SnO.sub.2 and/or the Pt may be deposited by electron beam deposition, chemical vapor deposition, wet deposition, or some other technique. In one embodiment, the SnO.sub.2 and/or the Pt may be sputtered, for instance, by a RF sputtering mode, a magnetron sputtering mode, or a DC sputtering mode. In one embodiment, the SnO.sub.2 is sputtered by a RF sputtering mode, and the Pt is sputtered by a DC sputtering mode.

[0081] Where the SnO.sub.2 and/or Pt are sputtered, a sputtering chamber may be used that is evacuated to a base pressure of less than 3.5110 Torr, preferably less than 3.010.sup.6 Torr. Then, the sputtering chamber is filled with argon gas, or a gas mixture comprising 5-20 vol %, preferably 10-18 vol %, more preferably 12-16 vol % oxygen in argon gas, relative to a total volume of the gas mixture. The pressure of the argon or the gas mixture (i.e. working pressure) may be maintained in the range of 0.5-10 mTorr, preferably 1-6 mTorr in the sputtering chamber during sputtering. A sputtering power may be set to a value in the range of 10 to 500 W, preferably 20 to 180 W. An SnO.sub.2 source may be used for sputtering the SnO.sub.2 onto the substrate, and a Pt source may be used for sputtering the Pt onto the SnO.sub.2. The distance between the target and the substrate may be 5-20 cm, preferably 7-15 cm, more preferably 8-12 cm. The substrate may be maintained at room temperature, or at 20-35 C., preferably 22-32 C., more preferably 26-30 C. In one embodiment, the SnO.sub.2 may be sputtered for 0.5-4 h, preferably 1-3 h, more preferably 1.5-2.5 h, and the Pt may be sputtered for 15 s-2 min, preferably 30 s-1.5 min, or about 1 min.

[0082] Following the deposition of the SnO.sub.2 and Pt, the amorphous SnO.sub.2 layer and the deposited Pt layer may be annealed in an oven at a temperature of 130-250 C., preferably 135-220 C., more preferably 138-210 C., even more preferably 140-200 C., to produce the room temperature hydrogen gas sensor. In another embodiment, the temperature may be 130-200 C., preferably 135-180 C., more preferably 140-170 C., even more preferably 145-155 C., or about 150 C. However, in some embodiments, the annealing may be carried out at temperatures lower than 130 C., such as 128 C., or greater than 250 C., such as 275-325 C., 425-475 C., 550-650 C., or even greater temperatures.

[0083] In one embodiment, the annealing temperature may affect the crystal grain size of the SnO.sub.2 in the SnO.sub.2 layer, and thus change the sensitivity or functioning of the room temperature hydrogen gas sensor in H.sub.2 detection. The amorphous SnO.sub.2 layer and the deposited Pt layer may be annealed in an atmosphere of air, or in an atmosphere consisting essentially of an inert gas, such as N.sub.2 or argon.

[0084] In one embodiment, the annealing may be carried out for 1-5 hours, preferably 2-4 hours, more preferably 2.5-3.5 hours, or about 3 hours, however, in some embodiments, the annealing may be carried out for less than 1 hour or greater than 5 hours. In a preferred embodiment, the annealing is carried out for 1-5 hours at a temperature of 140-170 C., preferably 2-4 hours at a temperature of 145-155 C., even more preferably about 3 hours at a temperature of about 150 C. In one embodiment, for the annealing step, the amorphous SnO.sub.2 layer and the deposited Pt layer may be placed in an oven heated at 130-250 C. or any of the above annealing temperature ranges. In another embodiment, the amorphous SnO.sub.2 layer and the deposited Pt layer may be placed in an oven at room temperature, and then heated to one of the above annealing temperatures at a rate of 0.5-10 C./min, preferably 1.0-5 C./min, more preferably 1.5-4 C./min. However, in some embodiments, the oven may be heated at a rate slower than 0.5 C./min or faster than 10 C./min. In one embodiment, following the annealing time, the oven may be turned off with the room temperature hydrogen gas sensor inside and allowed to cool to room temperature. In another embodiment, the room temperature hydrogen gas sensor may be taken out and placed in a room temperature environment in order to cool. In another embodiment, the room temperature hydrogen gas sensor may be cooled with a stream of inert gas, such as nitrogen or argon.

[0085] In one embodiment, in the entire method of making the room temperature hydrogen gas sensor as described above, starting with the sputtering, the amorphous SnO.sub.2 layer and the deposited platinum layer are exposed to temperatures no greater than 200 C., preferably no greater than 180 C., more preferably no greater than 160 C., even more preferably no greater than 155 C. In other words, the method of making the room temperature hydrogen gas sensor does not involve heating, annealing, or exposing any materials to temperatures greater than 200 C., preferably no greater than 180 C., more preferably no greater than 160 C., even more preferably no greater than 155 C.

[0086] In an alternative embodiment, the substrate, electrodes, and amorphous SnO.sub.2 layer may be annealed before depositing the Pt layer. In another alternative embodiment, crystalline particles of SnO.sub.2, produced by annealing or some other process, may be deposited on the substrate and electrodes, without any further requirement for annealing.

[0087] The room temperature hydrogen gas sensor, or its precursor material (substrate, amorphous SnO.sub.2 layer, etc.) at any step of its synthesis, may be characterized by a variety of techniques. Exemplary techniques include, but are not limited to, electron microscopy (TEM, SEM), atomic force microscopy (AFM), ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), Rutherford backscattering spectrometry (RBS), dual polarization interferometry, time-of-flight secondary ion mass spectrometry (ToF-SIMS), electron energy loss spectroscopy (EELS), high-angle annular dark field (HAADF), near infrared (NIR) spectroscopy, nuclear magnetic resonance (NMR), or combinations thereof.

[0088] According to a third aspect, the present disclosure relates to a method of using the room temperature hydrogen gas sensor of the first aspect. This method involves contacting the platinum layer with a first gas sample comprising hydrogen gas, and measuring a first resistivity across the at least two electrodes. Here, the first resistivity is decreased by 70-99.9% relative to a second resistivity arising from a second gas sample, where the second gas sample is substantially free of hydrogen gas. The second gas sample may be measured before and/or after the first gas sample. In some embodiments, the second gas sample may be considered a gas blank sample, as it is intended to not produce a detection signal as would as gas sample comprising H.sub.2 gas. In some embodiments, the first resistivity is decreased by 75-95%, preferably 78-92%, more preferably 80-90% relative to a second resistivity. In some embodiments, the first resistivity is decreased by 10-75%, preferably 20-60%, more preferably 30-50% relative to a second resistivity. However, in some embodiments, the first resistivity may be decreased by smaller than 10% or larger than 99.9% relative to a second resistivity. In some embodiments, this percentage decrease may be considered a response of the room temperature hydrogen gas sensor. Preferably, the magnitude of the percentage decrease is dependent on the concentration of H.sub.2 gas in contact with the room temperature hydrogen gas sensor. For instance, in one embodiment, a H.sub.2 gas concentration of 200-500 ppm may produce a response of 45-85%, preferably 50-82%, more preferably 70-80%. A H.sub.2 gas concentration of 600-900 ppm may produce a response of 65-95%, preferably 80-90%, more preferably 85-92%. A H.sub.2 gas concentration of 1,100-1,400 ppm may produce a response of 70-97%, preferably 85-96%, more preferably 90-95%. A H.sub.2 gas concentration of 1,500-2,000 ppm may produce a response of 75-99.9%, preferably 80-99%, more preferably 92-98%.

[0089] In one embodiment, a gas sample may originate from an ambient indoor environment, for example, of a residence, a factory, a store, a hospital, a car, or some other indoor environment. In another embodiment, a gas sample may come from an outdoor environment, or from a cave, a mine, or a geothermal vent. In another embodiment, a gas sample may come from a vessel or tubing of a laboratory or a chemical processing plant, where H.sub.2 may be a main product, a byproduct, or a contaminant. In one embodiment, the H.sub.2 may be the product of water splitting or may be used in a hydrogen fuel cell to extract power.

[0090] In one embodiment, the gas sample may be diluted, concentrated, pressurized, depressurized, dried, heated, or cooled before contacting the room temperature hydrogen gas sensor.

[0091] In one embodiment, the room temperature hydrogen gas sensor may be housed in a casing designed for portability. In another embodiment, the room temperature hydrogen gas sensor may be housed in a casing for fixing or securing to a wall or to connect with a vessel or tubing. In one embodiment, the room temperature hydrogen gas sensor may be operated continually, similar to other emergency detectors (such as a smoke detector), and may have a set threshold of H.sub.2 gas concentration beyond which an audible and/or visible alarm is triggered.

[0092] In one embodiment, the method of using the room temperature hydrogen gas sensor further comprises a calibration process. For instance, gas samples comprising known concentrations of H.sub.2 may be brought into contact with the room temperature hydrogen gas sensor, and the corresponding response may be measured. A person having ordinary skill in the art would be able to construct a calibration curve or plot based on the measured response of the room temperature hydrogen gas sensor when in contact with the different known gas samples.

[0093] In one embodiment, the first gas sample comprises 50-1800 ppm hydrogen gas, preferably 200-1700 ppm, more preferably 500-1250 ppm hydrogen gas. However, in some embodiments, the first gas sample may comprise less than 50 ppm hydrogen gas or greater than 1800 ppm hydrogen gas.

[0094] In one embodiment, the first gas sample has a temperature of 0-50 C., preferably 15-40 C., more preferably 20-35 C., even more preferably 22-28 C. However, in some embodiments, the first gas sample may have a temperature of less than 0 C. or greater than 50 C.

[0095] In one embodiment, the first gas sample may have a total pressure of 0.9-1.1 atm, preferably 0.92-1.08 atm, more preferably 0.95-1.05 atm. However, in some embodiments, the first gas sample may have a total pressure of less than 0.9 atm or greater than 1.1 atm.

[0096] In one embodiment, the second gas sample comprises 300-5,000 ppm, preferably 350-3,000 ppm, more preferably 400-2,000 ppm, even more preferably 500-1,500 ppm of at least one gas selected from the group consisting of NH.sub.3, n-butane, O.sub.2, CO.sub.2, and N.sub.2. However, in some embodiments, the second gas sample may comprise less than 300 ppm or greater than 5,000 ppm of at least one gas selected from the group consisting of NH.sub.3, n-butane, O.sub.2, CO.sub.2, and N.sub.2, or may comprise some other gas, such as methane. In one embodiment, the second gas sample may comprise N.sub.2 at a concentration of 100-5,000 ppm, preferably 500-2,000 ppm, more preferably 800-1,200 ppm. In one embodiment, the second gas sample may comprise n-butane at a concentration of 100-5,000 ppm, preferably 500-2,000 ppm, more preferably 800-1,200 ppm. In one embodiment, the second gas sample may comprise NH.sub.3 at a concentration of 50-1,000 ppm, preferably 100-800 ppm, more preferably 200-600 ppm. In one embodiment, the second gas sample may comprise CO.sub.2 at a concentration of 100-5,000 ppm, preferably 500-2,000 ppm, more preferably 800-1,200 ppm.

[0097] In one embodiment, the second gas sample may have a total pressure of 0.9-1.1 atm, preferably 0.92-1.08 atm, more preferably 0.95-1.05 atm. However, in some embodiments, the second gas sample may have a total pressure of less than 0.9 atm or greater than 1.1 atm.

[0098] In one embodiment, a gas sample in contact with the room temperature hydrogen gas sensor includes hydrogen gas and at least one compound selected from the group consisting of NH.sub.3, n-butane, O.sub.2, CO.sub.2, N.sub.2, pentane, butene, and pentene, wherein a hydrogen selectivity of the hydrogen gas sensor is at least 70% by mole, preferably at least 80% by mole, more preferably at least 85% by mole. As used herein, the term hydrogen selectivity refers to a ratio of a number of moles of the hydrogen gas that are adsorbed onto the room temperature hydrogen gas sensor relative to the total number of moles of gas molecules that are adsorbed onto the zinc oxide nanostructured thin film. For example, a hydrogen selectivity of the 80% by mole refers to an embodiment wherein 80% of all species adsorbed onto the room temperature hydrogen gas sensor are hydrogen. The hydrogen selectivity may be related to the specific surface area and the concentration of oxygen vacancies of the room temperature hydrogen gas sensor.

[0099] In one embodiment, the decrease in the first resistivity has a response time of 0.5-100 s, preferably 20-90 s, more preferably 40-80 s. However, in some embodiments, the response time may be shorter than 0.5 s or longer than 100 s. As defined here, the response time is the time needed by the room temperature hydrogen gas sensor to attain 90% of its saturation state value (i.e., the saturation state value may be thought of as the maximum response for a specific gas sample). The recovery time is defined as the time required for the maximum response to return to this 90% saturation state value once the particular gas sample is removed or exchanged with a gas producing essentially no response signal. In one embodiment, the decrease in the first resistivity may have a recovery time of 200-400 s, preferably 250-380 s, more preferably 280-360 s. However, in some embodiments, the recovery time may be shorter than 200 s or longer than 400 s. Generally, in some embodiments, as the concentration of H.sub.2 in a gas sample increases, the recovery time increases and/or the response time decreases. However in some embodiments and/or certain concentration ranges, the concentration of H.sub.2 may increase while the response time and/or recovery time may be essentially unchanged. In alternative embodiments, the response time and/or the recovery time may be defined by the time it takes the response signal to reach a percentage lower than or greater than 90% of the saturation state value.

[0100] In one embodiment, the second gas sample comprises 0.1-99 vol %, preferably 1-90 vol %, more preferably 10-80 vol %, even more preferably 15-70 vol %, or 20-60 vol %, or 0-10 vol %, 10-20 vol %, 20-30 vol %, 30-40 vol %, 40-50 vol %, 50-60 vol %, 60-70 vol %, 70-80 vol %, 80-90 vol %, 90-99 vol % of at least one gas selected from the group consisting of O.sub.2, CO.sub.2, H.sub.2O, Ar, and N.sub.2, relative to a total volume of the second gas sample. In another embodiment, the second gas sample consists essentially of O.sub.2, CO.sub.2, H.sub.2O, Ar, and/or N.sub.2. Where the second gas sample consists essentially of O.sub.2, CO.sub.2, H.sub.2O, Ar, and/or N.sub.2, the second gas sample may comprise 99.999 vol %, preferably 99.9999 vol %, more preferably 99.99999 vol % O.sub.2, CO.sub.2, H.sub.2O, Ar, and/or N.sub.2 relative to a total volume of the second gas sample. In other words, where the second gas sample consists essentially of O.sub.2, CO.sub.2, H.sub.2O, Ar, and/or N.sub.2, the second gas sample comprises O.sub.2, CO.sub.2, H.sub.2O, Ar, and/or N.sub.2 and less than 100 ppm of other gases, preferably less than 10 ppm of other gases, more preferably less than 1 ppm of other gases. In one embodiment, the second gas sample may be air, for example, from an indoor or outdoor environment. The air may comprise 75-80 vol % N.sub.2, 18-22 vol % O.sub.2, 0-1.2 vol % Ar, 0-0.05 vol % CO.sub.2, and 0-2 vol % H.sub.2O.

[0101] In one embodiment, the room temperature hydrogen gas sensor is in contact with 500-5,000 ppm H.sub.2 gas, preferably 500-5,000 ppm H.sub.2 gas, more preferably 500-5,000 ppm H.sub.2 gas for a time period before the contacting with the first gas sample. The time period may be 1-6 months, preferably 1.5-5 months, more preferably 2-4 months. However, in some embodiments, the time period may be shorter than 1 month or longer than 6 months, and the concentration of H.sub.2 gas may be less than 500 ppm or greater than 5,000 ppm.

[0102] In one embodiment, the method has a repeatability of at least 99%, preferably, at least 99.5%, over a time period of at least one hour, preferably at least one month, more preferably, at least one year. In one embodiment, the method has a repeatability of at least 99%, preferably at least 99.5% for at least 4 separate instances of contacting with H.sub.2 gas, preferably at least 10 separate instances, more preferably at least 100 separate instances, even more preferably at least 1,000 separate instances. In other embodiments, the method may have a repeatability of at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% or at least 95% for any of the previously mentioned number of instances or time intervals. As defined here, repeatability refers to the percentage of a response, relative to the response of an initial measurement, for an identical gas sample or control being measured more than once. For instance, a room temperature hydrogen gas sensor may have an initial response of 75 k for a gas sample of 1500 ppm H.sub.2 in N.sub.2. After numerous measurements, the same gas sample may then produce a response of 72 k, which indicates a repeatability of 72/75100%=96%.

[0103] In one embodiment, the method may further comprise a step of cleaning or recharging the sensor. Here, the sensor may be heated above room temperature, for instance, to 80-120 C., 120-200 C., or 200-500 C. and/or may be contacted with one or more compounds such a solvent or an acid, in order to remove impurities, though in other embodiments, the cleaning may involve exposure to light irradiation, such as with UV light. In one embodiment, the cleaning or recharging may increase the repeatability of a room temperature hydrogen gas sensor.

[0104] The room temperature hydrogen gas sensor may further be utilized to detect and/or determine a concentration of other gaseous compounds that affect its electrical resistance upon adsorption. Exemplary gaseous compounds without limitations may include carbon monoxide, nitrogen monoxide, nitrogen dioxide, methane, ethane, methanol, ethanol, hydrogen sulfide, etc. In view of that, the room temperature hydrogen gas sensor may also be used to detect exhaust gases or toxic gases, for example, in automobile industries and/or in air pollution control systems.

[0105] In an alternative embodiment, a room temperature hydrogen gas sensor may be used in the field of batteries, fuel cells, photo-chemical cells, heated hydrogen sensors, semiconductors (such as field effect transistors), magnetic semiconductors, capacitors, data storage devices, biosensors (such as redox protein sensors), photovoltaic, liquid crystal screens, plasma screens, touch screens, OLEDs, antistatic deposits, optical coatings, reflective coverings, anti-reflection coatings, and/or reaction catalysis. Similarly, in one embodiment, the room temperature hydrogen gas sensor may be coated with another material. For example, the room temperature hydrogen gas sensor may be coated with a layer of gold. A gold-coated nanostructured room temperature hydrogen gas sensor may then be used for detection of H.sub.2 or some other analyte using surface enhanced Raman spectroscopy (SERS).

[0106] In one embodiment, two or more separate substrates having electrodes with SnO.sub.2 and Pt layers may be connected in series and/or parallel in order to create an array of room temperature hydrogen gas sensors.

[0107] The examples below are intended to further illustrate protocols for preparing, characterizing the room temperature hydrogen gas sensor, and uses thereof, and are not intended to limit the scope of the claims. In the examples, sample and sensor may be used interchangeably to describe the room temperature hydrogen gas sensor.

Example 1

Fabrication of RT (Room Temperature) Hydrogen Sensor

[0108] Nano-structured SnO.sub.2 and Pt/SnO.sub.2 thin films were synthesized by a DC/RF sputtering technique (model NSC-4000, Nanomaster, USA) on quartz substrates as well as on SiO.sub.2 substrates with pre-interdigitated Au electrodes (200 nm thick with 250 m interspace distances). The quartz substrates were used to carry out the structural, compositional, morphological, and optical analysis while the pre-interdigitated Au electrodes were used to study the gas sensing properties. Before loading the substrates into the deposition chamber, the substrates were sonicated in acetone for 35 min. followed by blow-drying with N.sub.2 gas and drying in an oven at 120 C. for 30 min.

[0109] The SnO.sub.2 and Pt targets were cleaned by pre-sputtering for 3 min and 1 min, respectively. The sputtering conditions for the fabrication of pristine SnO.sub.2 and Pt/SnO.sub.2 are listed in Table 1. For the synthesis of heated Pt/SnO.sub.2 films, a layer of SnO.sub.2 thin film was fabricated by RF sputtering followed by sputter deposition of a very thin layer of Pt. Finally, heat treatments at 150 C., 300 C., 450 C., and 600 C. were carried out for a duration of 3 h in argon atmosphere to ensure stability, as well as enhance the conductivity and the crystallinity of the grown samples. The fabricated sensors have been designated: as-deposited SnO.sub.2 (S1), as-deposited Pt/SnO.sub.2 (S2), Pt/SnO.sub.2 post annealed at 150 C. (S3), Pt/SnO.sub.2 post annealed at 300 C. (S4), Pt/SnO.sub.2 post annealed at 450 C. (S5), and Pt/SnO.sub.2 post annealed at 600 C. (S6). Various characterization techniques were performed to investigate the structural, morphological, compositional, and optical properties. The topography of the films was investigated with Atomic Force Microscopy (AFM, Dimension Icon, Bruker) operating in the ScanAsyst mode. Structural characterization of the prepared films was carried out using XRD (Rigaku Miniflex 600 X-Ray Diffraction (XRD), with Cu K irradiation at =1.5406 ). The 2 range was set to 20-80 with a scan speed of 1/min. Optical transmittance was carried out using a double beam UV/Vis spectrophotometer (Jasco V-570) in the wavelength range of 200-800 nm. The films' chemistry was studied by X-ray photoelectron spectroscopy (XPS, Model: ESCALAB250Xi).

TABLE-US-00001 TABLE 1 RF and DC experimental sputtering conditions for as-deposited SnO.sub.2 and Pt/SnO.sub.2 thin films. SnO.sub.2 thin Parameter Films Pt thin Films Sputtering mode RF DC Source material SnO.sub.2 (99.99) Pt (99.99) Base pressure (Torr) 1.6 * 10.sup.6 3.1 * 10.sup.6 Working pressure (Torr) 5.6 * 10.sup.3 5.6 * 10.sup.3 Flow of Ar (SCCM) 60 60 Flow of O.sub.2 (SCCM) 10 0 Power (W) 150 30 Target-substrate distance (cm) 10 10 Substrate temperature (K) 301 301 Sputtering time (min.) 120 1

Example 2

Gas Sensing Measurements

[0110] The chemical sensing system 9 used in this work is schematically shown in FIG. 1. The H.sub.2 sensing behavior of the fabricated sensors were investigated using a test-stage, namely LINKAM stage 1 (Model HFS-600E-PB4, UK), procured from Linkam Scientific Instruments. The LINKAM stage temperature could be heated between RT and 600 C. by a temperature controller 4 and could be cooled rapidly with temperature stability <0.1 C. using a water cooling 3. The test-stage was connected to two mass flow controllers 8 (MFCs) controlled via an external X PH-100 power hub supply 5: one MFC was for 1% H.sub.2/balance N.sub.2 6, and one MFC was for dry air 7. The H.sub.2 concentration (C.sub.H.sub.2: in ppm) in the chamber was measured via the following formula:

[00001]$C_{H_2}=C\frac{F_{H_2}}{F_T}$

where C is the hydrogen concentration in the cylinder (10,000 ppm), F.sub.H.sub.2 is the flow of 1% H.sub.2/balance N.sub.2, and FT is the total flow (hydrogen balance with nitrogen and the flow from the dry-air tank). Prior to each test, the chamber was purged with 50 sccm (standard cubic centimeter per minute) for at least 1 h to stabilize the system before being supplied with the H.sub.2. The response of the sensor is defined as:

[00002]$\mathrm{Response}.Math..Math.\left(\%\right)=\frac{R_0-R_g}{R_0}100$

Here, the R.sub.0 and R.sub.g are the sensor resistance in air and in H.sub.2 test gas, respectively. The resistance was measured by an Agilent B1500A Semiconductor Device Analyzer (SDA) 2. The sensor response was investigated in the 250-1750 ppm concentration range of H.sub.2 gas in dry air at an operating temperature range of RT500 C.

Example 3

Characterization of RT Hydrogen Sensor

[0111] The XRD patterns of the obtained thin films are shown in FIG. 2. The identification of the diffracted SnO.sub.2 peaks was based on the International Center for Diffraction Data, card no. 041-1445. As shown in sample S2 in FIG. 2, the as-deposited Pt/SnO.sub.2 film has an amorphous structure with very weak (110), (101), and (211) diffraction peaks. Upon heat treatment at 150 C., the amorphous structure of the film was transformed into the polycrystalline tetragonal rutile structure of SnO.sub.2, as indicated by the occurrence of broad diffraction peaks along the (110), (101), (200), and (211) growth directions in sample S3. Further annealing at 300 C. or 450 C. (S4 and S5 films) led to improved crystallinity of the films, as can be seen from the sharpness of the above-mentioned diffraction peaks. Furthermore, four more diffraction peaks corresponding to the (220), (310), (112), and (321) direction were observed. This re-crystallization could be attributed to post annealing minimizing the energy of growth along these directions. Upon increasing the annealing temperature to 600 C., the intensities of the diffraction peaks were not only enhanced, but a SnO phase was formed, as indicated by the appearance of the new (101) peak. The absence of any diffraction peaks due to Pt in the as-deposited Pt/SnO.sub.2 and heated Pt/SnO.sub.2 films indicates Pt's amorphous nature in these films and/or its very small doping percentage.

[0112] The average grain size (D) of the nanostructured SnO.sub.2 in the annealed Pt/SnO.sub.2 films (S3-S6 films) was calculated using Scherrer's equation:

[00003]$D=\frac{K.Math..Math.}{.Math..Math.\mathrm{Cos}.Math..Math.}$

where is the wavelength of the incident X-ray, K is a constant, is the full width at the half of the peak maximum, and is the Bragg angle. See P. Bindu, S. Thomas. Estimation of lattice strain in ZnO nanoparticles: X-ray peak profile analysis. J. Theor. Appl. Phys. 8 (2014)123-134, incorporated herein by reference in its entirety. The values of the average grain size are listed in Table 2. The agglomeration of particles increases with annealing, which explains the increase of the grain size as annealing temperature increases. See J. Ungula, B. F. Dejene, and H. C. Swart. Effect of annealing on the structural, morphological and optical properties of Ga-doped ZnO nanoparticles by reflux precipitation method. Results in Physics 7 (2017) 2022-2027, incorporated herein by reference in its entirety. In addition, the lattice constants (a) and (c) of the SnO.sub.2 were calculated using formula:

[00004]$\frac{1}{d^2}=\frac{h^2+k^2}{a^2}+\frac{1}{c^2}$

where hkl are the Miller indices. See A. S. Manikandan, K. B. Renukadevi. Influence of fluorine incorporation on the photocatalytic activity of tin oxide thin films. Materials Research Bulletin 94 (2017) 85-91, incorporated hereby by reference in its entirety. The results, listed in Table 2, are in a good agreement with the standard values for SnO.sub.2 (a=4.73 , c=3.18 ). See Joint Committee on Powder Diffraction Standards (JCPDS), International Centre for Diffraction Data: Newtown Square, Pa., USA, 1997, incorporated herein by reference in its entirety.

TABLE-US-00002 TABLE 2 Grain size, lattice parameters, and RMS of the fabricated thin films. Lattice Sample Grain size (nm) parameter (a) Lattice parameter (c) RMS S1 5 S2 4 S3 7 4.690 3.172 S4 39 4.728 3.178 S5 42 4.736 3.185 S6 63 4.741 3.192 2

[0113] FIGS. 3A, 3B, and 3C show AFM images (3D, 2D, and linear surface profiles) and the calculated root mean square (RMS) surface roughness values of the S1, S2, and S6 samples, respectively. It can be seen that all surfaces are of porous structure consisting of grains with small crystal sizes and voids between them. The 2D AFM images of the S1 and S2 films show voids between the grains, which become closely packed in the S6 film. It can also be seen from the 3D AFM images that all three sample films have a columnar structure. The RMS surface roughness for S1, S2, and S6 are 5, 4, and 2 nm, respectively. The very low values of the RMS surface roughness indicate the uniform and homogeneous surface of the fabricated films.

[0114] The transmittance spectra of the fabricated films are shown in FIG. 4. The interference fringes appearing in the visible range originate from light interference at the film-air and the substrate-film interfaces. See S. Cho. Effect of growth temperature on structural. Electrical, and optical properties of Gd-doped zinc oxide films. Phys. Status Solidi A 3 (2014) 709-13, incorporated herein by reference in its entirety. As can be seen from FIG. 4, the S1 (pristine SnO.sub.2) thin film is highly transparent in the visible and the infra-red regions of the spectrum. However, decoration with Pt (S2 film) led to a significant decrease in the transparency of SnO.sub.2. This could be attributable to the optical scattering caused by the Pt decoration of the SnO.sub.2 surface. See J. Lee, and B. Park. Transparent conducting ZnO:Al, In and Sn thin films deposited by the sol-gel method. Thin Solid Films 426 (2003) 94-99, incorporated herein by reference in its entirety. Upon heat treatment, the transparency of the S2 film was enhanced (S3-S6 films).

[0115] The chemical composition of the fabricated films was performed by XPS (X-Ray Photoelectron Spectroscopy). A typical XPS survey spectrum, and spectra of Sn 3d, Pt 4f, and O is core-level spectra for the S3 sample are shown in FIGS. 5A-5D. Only the related core-levels of the constituent elements (Sn, Pt, and O) are observed in the survey spectrum (FIG. 5A). The Sn 3d core level spectrum comprises doublet peaks assigned to Sn 3d.sub.5/2 and Sn 3d.sub.3/2 due to spin-orbit splitting. The oxidation state in each film was determined by deconvoluting the Sn 3d XPS spectra. For samples S1, S2, and S3, each deconvoluted spectrum consisted of two peaks located in the binding energy range of 486.9-487.1 eV and 494.5-497.0 eV, corresponding to Sn 3d.sub.5/12 and Sn 3d.sub.3/2 of the Sn (IV) oxidation state. See Q. Ni, D. W. Kirk, and S. J. Thorpe. Characterization of the mixed oxide layer structure of the Ti/SnO.sub.2Sb.sub.2O.sub.5 anode by photoelectron spectroscopy and impedance spectroscopy. Journal of the Electrochemical Society 162 (2015) H40-H46; and Y. Wang, M. Aponte, N. Leon, I. Ramos, R. Furlan, and N. Pinto. Synthesis and characterization of ultra-fine tin oxide fibers using electrospinning. J. Am. Ceram. Soc., 88 (2005) 2059-2063, each incorporated herein by reference in their entirety. No additional peaks belonging to other oxidation states were observed in the resolved Sn 3d spectra of these samples, confirming the stoichiometry of the obtained films. FIG. 5B shows representative resolved Sn 3d XPS spectrum for S3. For samples S4, S5 and S6, each Sn 3d XPS spectrum was resolved into two pairs of Sn 3d.sub.5/2 and Sn 3d.sub.3/2 peaks. Two peaks positioned at high binding energies were assigned to Sn(IV) oxidation state, and two small peaks positioned at low binding energies were ascribed to Sn(II) oxidation state. See Q. Ni et al.; Y. Wang et al.; and C. D. Wagner, A. V. Naumkin, A. Kraut-Vass, J. W. Allison, C. J. Powell, J. R. Jr. Rumble, NIST, Standard Reference Database 20, Version 3.4 (web version) (http://srdata.nist.gov/xps/) 2003, each incorporated herein by reference in their entirety. The binding energy and the weight of each component in Sn 3d.sub.52 region for each film are listed in Table 3. It can be seen clearly from Table 3 that the weight of the Sn(II) component increased with annealing to reach almost 16% of the total Sn 3d peak in sample S6. Such a result is evidenced by the appearance of the XRD diffraction peak of the said compound in the XRD spectrum of this sample.

[0116] Similarly, the Pt 4f core-level spectrum is composed of Pt 4f.sub.7/2 and Pt 4f.sub.5/2 regions. The reported binding energies of the metallic Pt(0), Pt(II), and Pt(IV) oxidation states in the Pt 4f.sub.7/2 region are in the ranges of 71.2-72.0 eV, 72.2-73.0 eV, and 73.2-75.3 eV, respectively. See C. Li, Z. Wang, X. Sui, L. Zhang and D. Gu. Graphitic-C.sub.3N.sub.4 quantum dots modified carbon nanotubes as a novel support material for a low Pt loading fuel cell catalyst. RSC Adv. 6 (2016) 32290-97; N. An, X. Yuan, B. Pan, Q. Li, S. Li and W. Zhang. Design of a highly active Pt/Al.sub.2O.sub.3 catalyst for low temperature CO oxidation. RSC Adv. 4 (2014) 38250-57; C. Dablemont, P. Lang, C. Mangeney, J. Piquemal, V. Petkov, F. Herbst, G. Viau. FTIR and XPS study of Pt nanoparticle functionalization and interaction with alumina. Langmuir 24 (2008) 5832-41; and R. Brandiele, C. Durante, E. Gradzka, G. A. Rizzi, J. Zheng, D. Badocco, P. Centomo, P. Pastore, G. Granozzi and A. Gennaro. One step forward to a scalable synthesis of platinum-yttrium alloyed nanoparticles on mesoporous carbon for oxygen reduction reaction. J. Mater. Chem. A 4 (2016) 12232-12240, each incorporated herein by reference in their entirety. The Pt 4f XPS spectrum of each sample (S2-S6) was deconvoluted into three components, corresponding to the above-mentioned oxidation states of Pt (each component was a doublet composed of Pt 4f.sub.7/2 and Pt 4f.sub.5/2 peaks). A representative deconvoluted Pt 4f XPS spectrum of the sample S3 is shown in FIG. 5C. Table 3 summarizes the observed binding energy and the weight of each component in the Pt 4f.sub.7/2 region for each film. The binding energy position of the Pt (II) overlaps with that of Pt(OH).sub.2, which could originate from humidity. See V. JohAnek, M. Vaclavu, I. Matolinovi, I. Khalakhan, S. Haviar, V. Matolin. High low-temperature CO oxidation activity of platinum oxide prepared by magnetron sputtering. Applied Surface Science 345 (2015) 319-328, incorporated herein by reference in its entirety. Hence, the remarkable increase of the Pt(0) weight in the annealed samples (S3-S6) could be attributed to the decomposition of Pt(OH).sub.2 into Pt and PtO (Pt(II)) contained in the as-deposited Pt/SnO.sub.2 film (S2) due to the following reaction: Pt(OH).sub.2.fwdarw.Pt+PtO+H.sub.2O. Upon further annealing at 300 C. and 450 C., the weights of Pt(0), Pt(II), and Pt(IV) remained unchanged. However, increasing the annealing temperature to 600 C. led to the complete decomposition of Pt(IV) into Pt and PtO. See L. K. Ono, B. Yuan, H. Heinrich, and B. R. Cuenya. Formation and thermal stability of platinum oxides on size-selected platinum nanoparticles: support effects. J. Phys. Chem. C 114 (2010) 22119-22133, incorporated herein by reference in its entirety.

[0117] Finally, the O1s XPS spectrum of each sample was resolved into three components centered at lower (O.sub.I), intermediate (O.sub.II), and higher (O.sub.III) binding energies, except that S2 and S6 were resolved into only two components (O.sub.I and O.sub.II). FIG. 5D shows a representative resolved O1s XPS spectrum for S3. The binding energy value and the weight content of each peak in each film are summarized in Table 3. The reported binding energies of O.sub.I, O.sub.II, and O.sub.III components were in the range 530.5-530.7 eV, 531.5-531.7 eV, and 532.5-532.8 eV, which are assigned to stoichiometric metal-oxide bonds, defects and/or oxygen deficient regions within the lattice, and adsorbed water on the film surface, respectively. See D. Hu, B. Han, R. Han, S. Deng, Y. Wang, Q. Li and Y. Wang. SnO.sub.2 nanorods based sensing material as an isopropanol vapor sensor. New J. Chem., 38(2014) 2443-2450; and L. Cheng, S. Y, T. T. Wang, and J. Luo. Synthesis and enhanced acetone sensing properties of 3D porous flower-like SnO.sub.2 nanostructures. Mater. Lett. 143 (2015) 84-87, each incorporated herein by reference in their entirety.

TABLE-US-00003 TABLE 3 XPS characteristics in the Sn 3d.sub.5/2, Pt 4f.sub.7/2, and O1s of the pristine SnO.sub.2, the as-deposited Pt/SnO.sub.2, and the annealed Pt/SnO.sub.2 thin films. Peak Binding Energy (eV) Weight (%) Sample Region Sn (IV) Sn (II) Sn (IV) Sn (II) S1 Sn3d.sub.5/2 487.1 100 S2 486.9 100 S3 486.9 100 S4 487.2 485.5 94.5 5.5 S5 487.0 485.3 90.7 9.3 S6 486.9 485.8 84.4 15.6 Peak Binding Energy (eV) Weight (%) Sample Region Pt (0) Pt (II) Pt (IV) Pt (0) Pt (II) Pt (IV) S2 Pt4f.sub.7/2 71.5 72.6 73.3 49.4 39.3* 11.3 S3 71.5 72.3 73.2 67.1 22.8 10.1 S4 71.5 72.6 73.5 67.3 21.7 11.0 S5 71.6 72.7 73.4 69.4 20.8 9.8 S6 71.4 72.3 73.3 74.4 25.6 Peak Binding Energy (eV) Weight (%) Sample Region O.sub.I O.sub.II O.sub.III O.sub.I O.sub.II O.sub.III S1 O1s 530.6 531.6 532.3 79 13 8 S2 530.5 531.7 532.4 80 13 7 S3 530.3 531.5 84 16 S4 530.4 531.7 532.5 77 21 3 S5 530.6 531.5 76 24 S6 530.5 531.6 532.3 68 31 1 *A mixture of Pt(II) and Pt(OH).sub.2.

Example 4

Gas Sensing Properties

[0118] The RT response to various concentrations of H.sub.2 (250, 750, 1250, and 1750 ppm) of sensors S1 and S3, which represent pure SnO.sub.2 and Pt/SnO.sub.2 subjected to post annealing treatment at 150 C., respectively, is shown in respective FIGS. 6A and 6B. As can be observed, the S1 sensor failed to detect H.sub.2 at all H.sub.2 concentrations. This is in concurrence with the literature which claims that pure sputtered SnO.sub.2 films usually work for H.sub.2 detection at temperatures higher than 200 C. See I. H. Kadhim, H. A. Hassan, and Q. N. Abdullah. Hydrogen gas sensor based on nanocrystalline SnO.sub.2 thin film grown on bare Si substrates. Nano-Micro Lett. 8 (2016) 20-28, incorporated herein by reference in its entirety. In contrast, the S3 sensor showed excellent response at RT, and its response amplitudes increased with increasing H.sub.2 concentration. Moreover, it was observed that the electrical resistance of the S3 sensor decreases upon exposure to the H.sub.2 gas and completely returns to its original value upon the removal of H.sub.2 gas, displaying the n-type semiconducting sensing behavior.

[0119] The RT response of the fabricated sensors at different concentrations was studied. FIG. 7 plots the derived sensor responses at RT as a function of H.sub.2 concentrations (250 ppm, 750 ppm, 1250 ppm, and 1750 ppm). As can be noticed, zero-response, low response, and high response were observed for S1 (pure SnO.sub.2), S2 (as deposited Pt/SnO.sub.2), and other sensors (annealed Pt/SnO.sub.2 sensors), respectively. The switch from the zero-response to the low-response is attributed to the chemical and electronic sensitization of the Pt component in the S2 sensor. Moreover, the sensing response of the S2 sensors increases with increasing H.sub.2 concentration from 250 to 1750 ppm. A similar behavior was also observed in other sensors (annealed Pt/SnO.sub.2 sensors). Interestingly, among all annealed sensors, the response of the S3 sensor at all H.sub.2 levels was higher than other sensors followed by S4, S5, and then S6. Compared to the S2 sensor, the S3 sensor exhibited higher response due to the increase in the amount of metallic Pt, resulting from decomposition of Pt(OH).sub.2 to Pt and PtO as confirmed by XPS analysis. On the other hand, the S3 sensor a showed higher response than the other annealed sensors (S4-S6). This is due to the formation of fully stoichiometric SnO.sub.2 in this sample, as evidenced by XPS results. In addition, the smaller grain size in the S3 sensor could lead to an augmentation of the surface area, which promotes more active sites on the sensor surface, resulting in the enhancement of the gas-sensing performance. It was observed that the increase in the post annealing temperature led to the decrease of the gas response due to the increase of the partial formation of a p-type SnO compound in the S4, S5, and S6 sensors. The formation of SnO was confirmed by XPS analysis of sensors S4, S5, and S6 and by the XRD spectrum of sensor S6. The reason behind the degradation of the sensing signal in these sensors is due to the presence of p-type SnO compound and will be discussed later.

[0120] The repeatability of the sensor devices is another important aspect in the evaluation of their suitability. FIG. 8 illustrates the five-cyclic response and recovery curves of the S3 sensor towards 1000 ppm H.sub.2 at RT. As can be observed, the sensor operating at RT was capable of complete recovery upon removal of H.sub.2. The deviation in the response was less than 1%, thus ensuring good repeatability.

[0121] The response time is defined as the time needed by the sensor to attain 90% of its saturation state value, while the recovery time is defined as the time required for the signal to decay by 90%. See S. Basu, Y. Wang, C Ghanshyam and P. Kapur. Fast response time alcohol gas sensor using nanocrystalline F-doped SnO.sub.2 films derived via sol-gel method. Bull. Mater. Sci. 36 (2013)521-33, incorporated herein by reference in its entirety. The gas concentration-dependent recovery and response times of the S3 sensor at RT are shown in FIG. 9. From this figure, it is observed that the response time of the S3 sensor decreases sharply from about 140 s at 250 ppm H.sub.2 to 45 s at 1750 ppm H.sub.2. The decrease of the response time in higher concentrations of H.sub.2 could be attributed to the presence of the large number of H.sub.2 molecules on the sensor surface, which causes more reactions to take place. Therefore, the sensor needs a shorter time to reach equilibrium. See Y. Wang, B. Liu, S. Xiao, H, Li, L. Wang, D. Cai, D. Wang, Y. Liu, Q. Li, T. Wang. High performance and negative temperature coefficient of low temperature hydrogen gas sensors using palladium decorated tungsten oxide. J. Mater. Chem. A 3 (2015) 1317-1324, incorporated herein by reference in its entirety. In contrast, the recovery time was increased from 110 s at 250 ppm to about 140 s at 750 ppm of H.sub.2 and saturated thereafter. This phenomenon could be due to more reactions taking place between residual H.sub.2 captured on the surface of the sensor and sample surface components, which large number of reactions results in long recovery periods. See S. Nasirian, H. Moghaddam. Hydrogen gas sensing based on polyaniline/anatase titania nanocompositc. International Journal of Hydrogen Energy 39 (2014) 630-642, incorporated herein by reference in its entirety.

[0122] To examine the long-term stability of the S3 sensor fabricated by post annealing treatment (150 C.) of sputtered SnO.sub.2 films catalyzed by an ultra-thin layer of Pt, the gas sensing properties were recorded once again after three months. FIG. 10 displays the response of the S3 sensor exposed at RT to the different concentrations (750, 1250, 1750 ppm) of the H.sub.2 gas. As can be observed, the response of the S3 sensor was stable within a range of 0.7% and no degradation of the sensing signal was recorded over a period of three months verifying a good stability of the fabricated sensor.

[0123] In order to investigate the capability of the sensor to work in high temperature environments, the H.sub.2 sensing performance of the S3 sensor at different operating temperatures ranging from RT to 500 C. was investigated. The responses of the sensor toward 1250 ppm and 1750 ppm of H.sub.2 at different operating temperatures were calculated, and the results are plotted in FIG. 11, which depicts that the sensor exhibits the strongest response at RT (room temperature). At elevated temperatures (100 C.-300 C.), the response value of the sensor declined when sensing the two H.sub.2 concentrations. This is might be due to the formation of relatively fewer active oxygen species on the surface of the sputtered SnO.sub.2 film at that range of temperatures. See W. Zhang, M. Hu, X. Liu, Y. Wei, N. Li, and Y. Qin. Synthesis of the cactus-like silicon nanowires/tungsten oxide nanowires composite for room-temperature NO.sub.2 gas sensor. Journal of Alloys and Compounds 679 (2016) 391-99; Q. A. Drmosh and Z. H. Yamani. Hydrogen sensing properties of sputtered ZnO films decorated with Pt nanoparticles. Ceramics International 42 (2016)12378-84; and I. Kocemba, and J. Rynkowski. The influence of catalytic activity on the response of Pt/SnO.sub.2 gas sensors to carbon monoxide and hydrogen. Sensors and Actuators B 155 (2011) 659-66, each incorporated herein by reference in their entirety.

[0124] In the real application, there are different possible interfering gases. Hence, the selectivity, which is defined as the ability of the sensor to identify specific gas among several other gases, is an important factor that needs to be considered in the evaluation of sensor performance. To examine the selectivity of the S3 sensor, the responses to 250 ppm H.sub.2, 400 ppm NH.sub.3, 1000 ppm n-butane, and 1000 N.sub.2 at RT were compared. It is noted that the S3 sensor has a higher response to H.sub.2 than that to other gases (NH.sub.3, n-Butane, and N.sub.2) under the same concentration, indicating that the S3 is much more sensitive and selective to H.sub.2.

[0125] The gas sensing properties of the sensors fabricated by this method are compared with the previously demonstrated metal/metal oxide H.sub.2 gas sensors prepared by DC or RF sputtering techniques, as displayed in Table 4. It is clear that the sensors of the present invention have significant advantages over most of the previous work presented in the table. When compared with other sputtered metal/metal oxide H.sub.2 gas sensors, the sensors of the present invention have significant advantages, especially considering their room temperature operating conditions.

TABLE-US-00004 TABLE 4 Gas sensing properties of the S3 sensor in comparison with various other metal/metal oxide sensors fabricated by DC and RF sputtering techniques. Response and recovery Operating Maximum Concentration time Sensor Fabrication method Temp. ( C.) response (S) (ppm) (sec.) Ref. Pt/WO.sub.3 WO.sub.3 by RF sputtering and Pd 80 and no 400.sup.(2) 5,000 NA/NA * by DC sputtering response at 200 C., 250 C., and 300 C. Micro-sized Pd@SnO.sub.2 + SnO.sub.2 by DC reactive sputtering 180 3.sup.(3) 100 50/50 for ** gamma rays ranging and Pd by DC sputtering 50 ppm from a few mGy up to 100 kGy Pt nanoparticles @ZnO ZnO by DC reactive sputtering 300 99.sup.(1) 1,200 187/834 *** and Pd by DC followed by heat treatment Au nanopartiules @ZnO ZnO by DC reactive sputtering 400 60.sup.(1) 600 252/639 and Au by DC followed by heat treatment Pd decorated MnO.sub.2 porous alumina was prepared 300 20.sup.(2) 100 4/35 nanowalls on porous by electrochemical anodization alumina method, MnO.sub.2 by DC reactive, and Pd by DC sputtering Pt-ITO composite RF sputtering of 0.5 wt. % 300 0.008.sup.(2) 1000 5/NA Pd-catalyzed ITO targets Heated Pt/SnO.sub.2 SnO.sub.2 by RF sputtering and Pd RT 97.sup.(1) 1,250 67/350 This by DC sputtering followed by work heat treatment @ 150 C.
(1): S=[(R.sub.0R.sub.g)/R.sub.0*100]; (2): S=R.sub.0/R.sub.g; (3): S=V.sub.g/V.sub.0; and (4): S=(I.sub.0I.sub.g)/I.sub.0, where R, V, and I are the electrical resistance, potential, and current, respectively. The subscripts 0 and g refer to the electrical parameter of the sensor when exposed to air, or to H.sub.2, respectively. References: * S. J. Ippolito, S. K. Kandasamy, K. Kalantar, W. Wlodarski. Hydrogen sensing characteristics of WO.sub.3 thin film conductometric sensors activated by Pt and Au catalysts. Sensors and Actuators B 108 (2005) 154-158; **N. V. Duy et al.; *** Q. A. Drmosh and Z. H. Yamani. Hydrogen sensing properties of sputtered ZnO films decorated with Pt nanoparticles. Ceramics International 42 (2016)12378-84.; Q. A. Drmosh, and Z. H. Yamani. Synthesis characterization, and hydrogen gas sensing properties of AuNs-catalyzed ZnO sputtered thin films. Applied Surface Science 375 (2016)57-64.sup.; .sup. A. Sanger, A. Kumar, A. Kumar, R. Chandra. Highly sensitive and selective hydrogen gas sensor using sputtered grown Pd decorated MnO.sub.2 nanowalls. Sensors and Actuators B 234 (2016) 8-14; .sup. and C. Ling, Q. Xue, Z. Han, H. Lu, F. Xia, Z. Yan, L. Deng. Room temperature hydrogen sensor with ultrahigh-responsive characteristics based on Pd/SnO.sub.2/SiO.sub.2/Si heterojunctions. Sensors and Actuators B 227 (2016) 438-447, each incorporated herein by reference in their entirety.