ARTICLE pubs.acs.org/JPCC
Origin of Reaction-Induced Current in Pt/GaN Catalytic Nanodiodes J. Randall Creighton* and Michael E. Coltrin Sandia National Laboratories, P.O. Box 5800, MS-1086, Albuquerque, New Mexico 87185, United States
bS Supporting Information ABSTRACT: We have searched for chemicurrent generation in Pt/GaN nanodiodes during catalytic CO oxidation and observe a reaction-induced current that scales with reaction rate. But after considering (1) how the reaction-induced current depends on diode shunt resistance, (2) calculations of the Pt surface temperature rise during reaction, (3) direct experimental measurements of the Pt surface temperature rise during reaction, and (4) direct experimental measurements of the lateral temperature gradient during reaction, and thus the consequent thermoelectric current it produces, we conclude that the reactioninduced current is generated entirely from a thermoelectric voltage and is not true chemicurrent.
1. INTRODUCTION The direct conversion of chemical to electrical energy using Schottky diode structures has been reported by several research groups, beginning in about 1999.114 Some fraction of the energy released by an exothermic chemical reaction can be dissipated to form an excited (or hot) electron, which may be captured in an electronic circuit using the Schottky diode structure and thus produce a chemical current, or “chemicurrent”. Aside from being a very interesting phenomenon, chemicurrent generation has potential applications for chemical sensors and chemical energy conversion. Most of the early results reported transient chemicurrent generation by chemisorption or reaction of monolayer quantities of material and has been reviewed by Neinhaus4 and Wodtke et al.15 Quantum yields (electrons/atom or molecule) of the transient chemical reactions were generally quite low, ranging from 106 to 102, with the most exothermic reaction (O-atoms + Ag surface) having the highest yield.3 In 2005 there were several reports from Somorjai’s group of steady-state chemicurrent generation using Pt/TiO2, Pt/GaN, and Pd/TiO 2 Schottky diodes, and they coined the term “catalytic nanodiode” to describe their devices.57 Their early work utilized the catalytic oxidation of CO, CO + 1/2 O2 f CO2, which liberates ∼2.9 eV of energy per molecule of CO2 produced. If the catalytic metal is thin enough (typically a few nm), some of the hot electrons created by the exothermic chemical reaction may live long enough to migrate to the semiconductor side (GaN or TiO2) of the Schottky diode, as shown schematically in Figure 1a, thus yielding a chemicurrent. For the Pt/TiO2 nanodiode a remarkable quantum yield of ∼3/4 (3-electrons/ 4-CO2) was reported,5 although more recent results have generally been in the 104 to 5 103 range.812 The very high quantum yield originally reported5 prompted us to consider catalytic nanodiodes as a source of micropower, which could have the desirable attributes of small size, robustness, and r 2011 American Chemical Society
Figure 1. (a) Energy level diagram of the catalytic nanodiode, in this example with the CO oxidation reaction providing the energy source, (b) equivalent circuit diagram of nanodiode with chemicurrent ic; Rsh = shunt resistance, Rs = series resistance, RL = load resistance. We also included a possible thermoelectric current ite generated from a thermoelectric voltage (Vte).
reasonably high (projected) efficiency. We have fabricated a variety of Pt/GaN nanodiodes and report their performance during CO oxidation conditions in this paper. Like the previous work, we have found clear evidence of a current that is generated by a chemical reaction, which in this paper we will generically term a “reaction-induced current” (iri). However, unlike the previous reports, we do not believe the reaction-induced current from our devices is predominantly composed of chemicurrent (ic). To interpret out results, we have found it useful to consider the equivalent circuit of the nanodiode device, as shown in Figure 1b, which is analogous to a photodiode or solar cell circuit. But instead of light generating a photocurrent, we include a chemical reaction generating a chemicurrent; the main point being that the device should behave as a current source. It has Received: November 1, 2011 Revised: December 7, 2011 Published: December 09, 2011 1139
dx.doi.org/10.1021/jp210492k | J. Phys. Chem. C 2012, 116, 1139–1144
The Journal of Physical Chemistry C
Figure 2. Total pressure (red curve) and reaction-induced current (blue curve) during CO oxidation on the 5 nm Pt/GaN nanodiode at 270 °C. Approximately 4 Torr of CO was added during the time frame denoted with asterisks (*). The green curve (dPt/dt) is the reaction rate scaled and offset for comparison with the reaction-induced current.
been noted that during the operation of the nanodiode there is often an extraneous current generated from a thermoelectric voltage (Vte),811 which we have also included in Figure 1b. This thermoelectric current (ite) is generated from temperature gradients within the device and could arise from heat flowing from the substrate heater, and/or heat generated by the exothermic chemical reaction. In this paper we will describe how the reactioninduced current (iri) from our devices (1) has the characteristics of being derived from a voltage source and (2) can be quantitatively attributed to the temperature gradient (and the resulting Vte) produced by the reaction exothermicity. We conclude that the reaction-induced current we measure is a thermoelectric current; we have found no compelling evidence of true chemicurrent.
2. EXPERIMENTAL PROCEDURE We fabricated Pt/GaN nanodiodes using shadow mask techniques using GaN on sapphire as the substrate. The GaN films were grown by MOCVD on c-plane sapphire wafers to a typical thickness of ∼3 μm. The films were either moderately doped with silicon (using silane) to yield n-type carrier concentrations (Nd) of 2 1017 cm3 or heavily doped to Nd ∼5 1018 cm3. For some samples a thinner unintentionally doped (uid) film (∼200 nm) was grown as the top layer and exhibited a carrier concentration
