Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals

<table>Band diagram of the Schottky Junction explaining the sensing mechanism. The barrier height reduces upon exposure to the reducing gas. <svg height="11.075" id="M40" style="vertical-align:-3.25792pt" version="1.1" viewbox="0 0 66.599998 11.075" width="66.599998" xmlns="http://www.w3.org/2000/svg">
<g transform="matrix(1.25,0,0,-1.25,0,11.075)">
<g transform="translate(72,-63.14)">
<text transform="matrix(1,0,0,-1,-71.95,66.44)">
<tspan style="font-size: 12.50px; " x="0" y="0">𝜓</tspan>
</text>
<text transform="matrix(1,0,0,-1,-62.56,63.31)">
<tspan style="font-size: 8.75px; " x="0" y="0">M</tspan>
<tspan style="font-size: 8.75px; " x="7.7805281" y="0">A</tspan>
</text>
<text transform="matrix(1,0,0,-1,-47.96,66.44)">
<tspan style="font-size: 12.50px; " x="0" y="0">,</tspan>
<tspan style="font-size: 12.50px; " x="5.2012482" y="0">𝜓</tspan>
</text>
<text transform="matrix(1,0,0,-1,-33.36,63.31)">
<tspan style="font-size: 8.75px; " x="0" y="0">M</tspan>
<tspan style="font-size: 8.75px; " x="7.7805281" y="0">G</tspan>
</text>
</g>
</g>
</svg> are the work function of M in air and in gas respectively. <svg height="14.5375" id="M41" style="vertical-align:-3.25792pt" version="1.1" viewbox="0 0 61.762501 14.5375" width="61.762501" xmlns="http://www.w3.org/2000/svg">
<g transform="matrix(1.25,0,0,-1.25,0,14.5375)">
<g transform="translate(72,-60.37)">
<text transform="matrix(1,0,0,-1,-71.95,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">Φ</tspan>
</text>
<text transform="matrix(1,0,0,-1,-62.41,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">B</tspan>
<tspan style="font-size: 8.75px; " x="5.548768" y="0">A</tspan>
</text>
<text transform="matrix(1,0,0,-1,-50.04,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">,</tspan>
<tspan style="font-size: 12.50px; " x="5.2012482" y="0">Φ</tspan>
</text>
<text transform="matrix(1,0,0,-1,-35.29,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">B</tspan>
<tspan style="font-size: 8.75px; " x="5.837584" y="0">G</tspan>
</text>
</g>
</g>
</svg> are the barrier height of the junction in air and in gas, respectively. <svg height="14.5375" id="M42" style="vertical-align:-3.25792pt" version="1.1" viewbox="0 0 49.474998 14.5375" width="49.474998" xmlns="http://www.w3.org/2000/svg">
<g transform="matrix(1.25,0,0,-1.25,0,14.5375)">
<g transform="translate(72,-60.37)">
<text transform="matrix(1,0,0,-1,-71.95,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">E</tspan>
</text>
<text transform="matrix(1,0,0,-1,-64.31,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">C</tspan>
</text>
<text transform="matrix(1,0,0,-1,-57.97,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">,</tspan>
<tspan style="font-size: 12.50px; " x="5.2012482" y="0">E</tspan>
</text>
<text transform="matrix(1,0,0,-1,-45.13,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">C</tspan>
<tspan style="font-size: 8.75px; " x="5.837584" y="0">G</tspan>
</text>
</g>
</g>
</svg>  are the conduction band in air and in gas, respectively. <svg height="14.5375" id="M43" style="vertical-align:-3.25792pt" version="1.1" viewbox="0 0 50.262501 14.5375" width="50.262501" xmlns="http://www.w3.org/2000/svg">
<g transform="matrix(1.25,0,0,-1.25,0,14.5375)">
<g transform="translate(72,-60.37)">
<text transform="matrix(1,0,0,-1,-71.95,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">E</tspan>
</text>
<text transform="matrix(1,0,0,-1,-64.31,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">V</tspan>
</text>
<text transform="matrix(1,0,0,-1,-57.49,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">,</tspan>
<tspan style="font-size: 12.50px; " x="5.2012482" y="0">E</tspan>
</text>
<text transform="matrix(1,0,0,-1,-44.64,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">V</tspan>
<tspan style="font-size: 8.75px; " x="5.9863682" y="0">G</tspan>
</text>
</g>
</g>
</svg> are the valence band in air and in gas, respectively. <svg height="14.5375" id="M44" style="vertical-align:-3.25792pt" version="1.1" viewbox="0 0 51.400002 14.5375" width="51.400002" xmlns="http://www.w3.org/2000/svg">
<g transform="matrix(1.25,0,0,-1.25,0,14.5375)">
<g transform="translate(72,-60.37)">
<text transform="matrix(1,0,0,-1,-71.95,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">E</tspan>
</text>
<text transform="matrix(1,0,0,-1,-64.31,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">F</tspan>
</text>
<text transform="matrix(1,0,0,-1,-58.95,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">,</tspan>
<tspan style="font-size: 12.50px; " x="5.2012482" y="0">F</tspan>
</text>
<text transform="matrix(1,0,0,-1,-46.79,60.55)">
<tspan style="font-size: 8.75px; " x="0" y="0">F</tspan>
<tspan style="font-size: 8.75px; " x="4.8661122" y="0">G</tspan>
</text>
</g>
</g>
</svg>are the fermi level in air and in gas, respectively. <svg height="10.475" id="M45" style="vertical-align:-0.0pt" version="1.1" viewbox="0 0 14.0125 10.475" width="14.0125" xmlns="http://www.w3.org/2000/svg">
<g transform="matrix(1.25,0,0,-1.25,0,10.475)">
<g transform="translate(72,-63.62)">
<text transform="matrix(1,0,0,-1,-71.95,63.67)">
<tspan style="font-size: 12.50px; " x="0" y="0">M</tspan>
</text>
</g>
</g>
</svg> is Pt, Pd, or Rh.</table>

Journal of Sensors

fig2

Figure 2

Figure 2: Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals 