Enhanced Methanol Oxidation on Strained Pt Films - The Journal of

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Enhanced Methanol Oxidation on Strained Pt Films Fernando M. F. Rhen, and Cian McKeown J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11290 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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The Journal of Physical Chemistry

Enhanced Methanol Oxidation on Strained Pt Films

Fernando M. F. Rhen* and Cian McKeown Bernal Institute, Department of Physics, University of Limerick, Ireland

Tel: +353-61-202290 Email: [email protected]

1 ACS Paragon Plus Environment


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Abstract:

Here we report a systematic investigation into the effect of strain on the electrocatalytic activity of films of Pt. We use an electroless deposition process to deposit the Pt onto sputtered Au substrates and characterise their structural and electrochemical properties. We found a direct correlation between the rate of methanol oxidation and the fractional surface coverage of methanol on the compressively strained catalyst surfaces. The fractional methanol surface coverage, ΘM, varied on the Pt surfaces with values ranging from 20.5 to 61.7%, depending on the level of compressive strain. A compressive strain of about ε = 0.110% resulted in a high fractional surface coverage of 61.7% with highest peak current density for methanol oxidation of 0.46 mA cm-2 and earliest peak potential of 0.67 V vs. Ag/AgCl. As the absolute value of the compressive strain increased, the fractional surface coverage and peak current density for methanol oxidation decreased. Therefore, the adsorption of molecular methanol was controlled by the strain generated in the Pt films, resulting in higher fractional surface coverage and ultimately enhancing the rate of oxidation of methanol.


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1. Introduction:

As the world’s reliance on fossil fuels continues, global energy demands will need to be met by alternative energy technologies.1 Direct methanol fuel cells (DMFCs) are attracting increasing interest as alternative power sources for portable electronics and transportation. This is due to their advantages over other power sources, including higher power density, low pollution and lower operating temperatures.2,3 Platinum shows the best catalytic properties towards the methanol oxidation reaction (MOR),4 which is a sluggish process that occurs at the anode of the DMFC. Despite its high catalytic activity, Pt is very susceptible to CO poisoning, which reduces the efficiency of the fuel cell.5

The mechanism of methanol oxidation on Pt has been studied previously6,7,8 and can be broadly described by the following steps; (1) methanol molecules are adsorbed onto the Pt surface; (2) the surface-bound methanol dissociates into several adsorbed species; (3) parallel reaction pathways exist to produce CO2. The oxidation of the surface-bound intermediaries to CO2 requires the presence of OH- species arising from the dissociation of water.9 It is at this step that the Pt surface can become poisoned by strongly adsorbed COads.

The rate of oxidation of methanol on Pt has been shown to be enhanced through alloying of Pt with one or more metal (Ru, Sn, Bi, Mo, Ni, Ir)10,11,12 or through the use of core-shell structures. These core-shell structures are comprised of a thin Pt shell with thickness of several monolayers deposited onto a metal core. Alloys of platinum and ruthenium are the best known catalysts for methanol oxidation in an acidic medium.13,14,15 Alloying Pt with Ru enhances methanol oxidation through multiple coexisting mechanisms. In the bifunctional mechanism16,17,18 the MOR is enhanced when water dissociation on Ru causes a reduction in



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COads on the Pt surface by oxidising it to CO2 before the COads is adsorbed too strongly. Via the ligand effect19,20,21 and/or the strain effect,22-29 the addition of the alloying metal causes a distortion in the Pt lattice, which modifies both the interatomic distances and the electronic properties of Pt. These effects are supported by the “d-band model” that relates the strain in catalyst surfaces to changes in the electronic structure.30 The “d-band model” states that tensile strain narrows the d-band width, shifting the d-band centre down and allowing for stronger bonding of adsorbates. Conversely, compressive strain widens the d-band, shifting the d-band centre up resulting in weaker adsorption.31, 32

Previous studies have attempted to separate the ligand effect and the strain effect in order to fully understand the role of strain in the enhancement of methanol oxidation. Strain has been induced in thin films catalysts without the addition of an alloying metal in a number of ways. Gsell et al.24 produced a local strain in close-packed Ru (0001) single crystal surfaces by implanting inert gas bubbles via ion sputtering and annealing at high temperatures. They found that localised tensile strain resulted in enhanced adsorption of oxygen molecules on the single crystal Ru surface.24, 33

Strain has also been induced in thin metal films by growing them on single crystal substrates. The pseudomorphic growth onto lattice-mismatched substrates generates strain in the films due to changes in the film’s lattice parameter. Enhanced reactivity of strained Cu monolayers was shown by Kampshaff et al.34 and Otero et al.35 on single crystal Pd (110) and Ru (0001) substrates, respectively. Zhang et al.36 studied the effect of strain on the kinetics of oxygen reduction on Pt monolayer on Pd (111) single crystal substrates. Strained monolayers of Pt on various single crystal substrates were studied by Li et al.37 as electrocatalysts for the oxidation of methanol and ethanol. They found enhanced reactivity in the dehydrogenative


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adsorption of alcohol molecules and the dissociation of water due to the strain generated in the Pt monolayers. The role of strain alone in monolayer catalysts is difficult to determine due to possible electronic interaction of the single crystal substrate. Investigations using coreshell catalysts have produced conflicting explanations on the role of the alloying metal in the enhancement of the MOR.29,37

Recently, Pt thin films with thicknesses greater than one monolayer have been investigated in an attempt to understand the role of strain without electronic interference from the substrate material. El Jawad et al.38 used molecular beam epitaxy (MBE) to deposit multilayer Pt films on W (111) substrates. They observed a strain effect on the adsorption of OH- species and the oxidation of COads. Similarly, model Pt thin film electrodes were prepared by Temmel et al. 39 using a pulsed laser deposition (PLD) method in order to investigate CO oxidation on strained Pt surfaces. Single crystal (100)- and (111)- oriented strontium titanate (STO) substrates were used for epitaxial deposition.

Both MBE and PLD are complex deposition techniques that require high vacuum pressures or high operating temperatures to prepare thin film catalysts and are not suitable for the preparation of thick metal films. In this study we use a low temperature, one-step deposition method known as electroless deposition to grow films of Pt and study the effect of strain on their catalytic properties. Among many methods for the preparation of films, electroless deposition emerges as a highly effective method for the synthesis of metals.40 It is an autocatalytic plating process that requires no external power supply to initiate growth. Due to its highly conformal growth processes, electroless deposition has been used to grow highsurface-area 3D nanostructures.41 Previously, 3D nanostructures have exhibited advantageous catalytic properties due to their enhanced surface area and tuneable physical properties.42,43,44



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As well as high surface area nanostructures, electroless deposition can be used to grow uniform films of metal at relatively low temperatures (T < 100 oC) in a controlled manner.40

Growing films of Pt of varying thickness onto Au substrates allowed for a systematic investigation into the effect of strain on the electrocatalytic activity, as well as ensuring there was no electronic contribution from the substrate. Using (111) oriented Au substrates, we induce a strain in Pt films and investigate the effect of surface strain on the adsorption and oxidation of methanol.

2. Experimental:

The deposition parameters for the electroless deposition of Pt films were the same as those described by Koslov et al.40,45 The deposition bath contains DNP platinum metal salts (Diamminedinitritoplatinum (II) 3.4 wt. % in dilute ammonium hydroxide, Sigma-Aldrich). Hydrazine hydrate was used as a reducing agent (Sigma-Aldrich). Addition of acetic acid (ACS reagent, ≥99.7%, Sigma-Aldrich) was employed to control the pH and a water bath was used to keep the temperature of the deposition constant. A Ti adhesion layer (15 nm) and Au seed layer (35nm) were grown by RF magnetron sputtering onto a Si (100) wafer. The Au substrates were submerged in the electroless deposition bath individually and left for appropriate amounts of time, ranging from thirty minutes to three hours, in order grow a series of Pt films with a range of film thicknesses.

The surface morphology of the Au seed layer and the electrolessly deposited Pt films were investigated using scanning electron microscopy (SEM, Hitachi SU-70) at an accelerating voltage of 20 kV. The films were fractured and mounted in a 90o holder in order to measure


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the thickness using the SEM (Hitachi SU-70). The crystallographic structures of the films were investigated using X-ray diffraction (XRD, Analytical X’Pert MPD Pro) with Cu kalpha radiation (λ = 1.5418 Å). Electrochemical experiments were carried out using a multichannel potentiostat (CHI Instruments, Model 1000C) in a standard gas-tight three electrode cell. A large area Pt wire and Ag/AgCl (in 3 M KCl) were used as counter and reference electrode, respectively. A testing area was created on each of the working electrode surfaces with inert varnish. The three electrode configuration was submerged in the electrolyte which was purged with pure N2 gas for long periods prior to each experiment to ensure it was completely de-aerated.

Pt working electrodes were electrochemically treated by holding at a constant potential of 2 V vs. Ag/AgCl for 120 seconds prior to the cyclic voltammetry measurements. The potential was then cycled between -0.2 and 1.2 V vs. Ag/AgCl at a scan rate of 100 mV s-1 until reproducible cyclic voltammograms were obtained. Cyclic voltammetry (CV) was then carried out in N2-saturated 0.5 M H2SO4 electrolyte at a scan rate of 50 mV s-1 to determine the electrochemical surface area (ECSA) of the electrodes. The evaluation of the methanol oxidation reaction (MOR) on the catalyst surfaces was carried out using CV in N2-saturated 0.5 M H2SO4 + 1 M CH3OH electrolyte with a scan rate of 50 mV s-1.



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3. Results and Discussion:

Electroless deposition of Pt on the sputtered Au films began immediately as the substrates were submerged in the deposition baths. Hydrogen evolution was initially observed after substrate submersion.46 Figure 1 shows representative scanning electron micrographs of the films prepared by electroless deposition. The silicon substrate with a thin film of Au is shown in Figure 1 (a). Figure 1 (b) shows the surface of a Pt film, deposited on the Au substrate. Here, we can see that a continuous film of Pt is formed across the surface of the Au substrate. By comparing the two SEM images at the same magnification, the mean crystallite size of the Pt film was visibly smaller than that of the Au substrate.

The thickness of the Pt films was determined from cross-sectional SEM imaging, as shown in Figure 1 (c), and has a thickness of 40.5 ± 2.1 nm. The Au seed layer can be seen in the same image. The tightly packed nanocrystallite morphology of the Pt film can be seen similar to the top-down SEM images shown in Figure 1 (b). Five films were grown with thicknesses of 40.5 ± 2.1 nm, 53.5 ± 6.0 nm, 63.5 ± 6.8 nm, 88.9 ± 6.1 nm, 100.3 ± 8.6 nm. For simplicity, the average values of the Pt film thickness will be used throughout the text. (i.e. 41 nm, 54 nm, 64 nm, 89 nm and 100 nm).


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Figure 1. Scanning electron microscope images showing top-down surface of (a) blank Au substrate and (b) a 41 nm thick Pt film. The cross-sectional SEM image is shown in (c) of a 41 nm thick Pt film deposited on Au substrate.

The crystallographic structure of the electrolessly deposited platinum films was investigated using X-ray diffraction (XRD), as shown in Figure 2. Diffraction peaks at 38.187o and 81.754o were observed on films and correspond to Au (111) and Au (222) diffraction indexes (JCPDS # 04-0784). The peaks at 37.022o, 41.445o and 53.051o correspond to α-Ti (002), αTi (101) and α-Ti (102) (JCPDS # 05-682) and are denoted by a diamond. The peak at 69.066o corresponded to the Si (400) peak of the Si (100) wafer (JCPDS # 27-1402), denoted by a black circle.



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Figure 2. X-ray diffractammograms of (a) Au substrate before deposition and the Pt films with thickness of (b) 41 nm, (c) 54 nm, (d) 64 nm, (e) 89 nm and (f) 100 nm. The crystallographic structure of pure Pt is shown in (g). Peaks from the Ti adhesion layer and the underlying Si (100) substrate are denoted by ♦ and ●, respectively.

The diffraction patterns show the effect of deposition time on the deposited films. Pt was grown preferentially in the (111) direction due to (111) textured Au film used as a seed layer for the deposition. The (222) diffraction peak also supports this idea. While the five films show similar diffraction peaks for a FCC Pt (111) and (222) crystal orientations, the exact angular position of these peaks are different for each of the deposited films, which can be seen clearly in Figure 3. We observed a shift in the angular position of the Pt (111) peak for all of the films. We found the Pt (111) peak at 39.672o, 39.780o, 39.942o, 40.022o and 39.995o for films of thickness 41 nm, 54 nm, 64 nm, 89 nm and 100 nm, respectively. Instrumental


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uncertainty of the XRD was ruled out due to the common Au (111) to all the samples, which we use as an internal standard, at 38.187o. Therefore, Pt (111) peaks shifted to more positive values as the film thickness increased.

Figure 3. X-ray diffractammograms of the Pt films of with thickness 41 nm, 54 nm, 64 nm, 89 nm and 100 nm. All XRD scans share a common peak at 38.187o, corresponding to the underlying Au (111) layer.

The angular position of diffraction peaks corresponding to Pt (111) and Pt (222) indexes were used to calculate the lattice parameter of the films, which are shown in Figure 4 as a function of film thickness. It can be seen that the lattice constant decreases as the thickness of the films increased. The mean crystallite size was also estimated from the diffraction patterns using Scherer’s equation:



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=

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(1)

Where τ = mean crystallite size, K = Shape factor (0.94), λ = X-ray wavelength (1.5418 Å), β = line broadening at FWHM and θ = diffraction angle. The values of mean crystallite size are shown as a function of film thickness in the inset of Figure 4. While we observed a slight growth of the crystallites as the film thickness increased, all crystallite sizes were smaller than 8 nm. The shortening of the lattice parameter with increasing film thickness resulted in a compression of the atomic lattice, which is known to cause compressive strain in thin film catalysts and alter their chemical properties.27,28 The strain, ε in the Pt lattice was calculated using the ratio of the change in lattice constant and the bulk Pt lattice constant, according to the following:

ε=

∆a af - a0 = a0 a0

(2)

Where ε = lattice strain, af = lattice constant of film and a0 = bulk Pt lattice constant (3.924 Å). Table 1 shows the calculated values of lattice constant and lattice strain in the Pt films.


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Figure 4. The calculated values of lattice constant as a function of film thickness. The inset shows the value of mean crystallite size, τ, as a function of film thickness. The inset shows the mean crystallite size value, τ, which was calculated by using equation (1).

Table 1: Experimental values of af and ε for the Pt films. D (nm)

af (Å)

ε (%)

41

3.927

0.074

54

3.920

-0.110

64

3.909

-0.392

89

3.906

-0.445

100

3.905

-0.481

The film with thickness of 41 nm was found to be under tensile strain. This can be rationalised by the fact that the thin Pt films were grown on the Au substrate, whose lattice 13 ACS Paragon Plus Environment


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constant was larger (4.08 Å). This lattice mismatch caused a tensile stress in the film. As the thickness of the films increased the effect of the Au substrate was reduced and the strain in the Pt films became compressive due to the forced growth in the (111) direction. This is evidenced by the negative values of lattice strain calculated for all the other samples. The growth of oriented Pt on the Au substrate was observed for all films even though the initial effect of lattice mismatch became reduced as the film thickness increased.

Cyclic voltammetry measurements were carried out to investigate the electrocatalytic activity of the films in a standard three electrode cell configuration and the results are shown in Figure 5. The current density was obtained by normalising the measured current to the ECSA, which was determined by calculating the charge due to monolayer hydrogen adsorption in the hydrogen region (ranging from -0.16 to 0.15 V vs. Ag/AgCl) and relating that with the charge density of an ideal Pt surface (210 µC cm-2).


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Figure 5. Cyclic voltammograms of Pt films measured at a scan rate of 50 mV s-1 in 0.5 M H2SO4 (a,b,c,d, and e) and in H2SO4 + 1 M CH3OH (f, g, h, i and j) with corresponding film thickness of 41 nm, 54 nm and 64 nm (d) 89 nm and (e) 100 nm, respectively.

The mechanism of methanol oxidation, which has been described previsouly7,8 can be divided into three distinct potential regions in the forward scan of the cyclic voltammograms, see Figure 5 (f), (g), (h), (i) and (j). In the first region, extending from -0.16 to 0.15 V vs. Ag/AgCl, hydrogen adsorption on the electrocatalyst surfaces was suppressed when compared to CVs measured in 0.5 M H2SO4 alone. This was due to the adsorption of methanol on the Pt surface.15 At ~0.2 V vs. Ag/AgCl, methanol oxidation began with an observed increase in current density. This current density reached a maximum at about 0.7 V vs. Ag/AgCl. At potentials larger than 0.7 V vs. Ag/AgCl, a reduction in the methanol



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oxidation is observed and is attributed to the formation of hydroxyl species on the catalyst surface.

Figure 6 shows linear anodic sweeps of the CVs measured in Figure 5 (f), (g), (h), (i) and (j) for the films. The 54 nm thick film, under a tensile strain of -0.110 %, exhibited the highest current density and earliest onset potential for methanol oxidation of 0.46 mA cm-2 and 0.67 V vs. Ag/AgCl, respectively. The inset in Figure 6 shows the evolution of strain with film, which reveals that the strain has a great effect on the oxidation current density of methanol. In contrast, the film with a strain of 0.074% exhibited the lowest activity for the oxidation of methanol with a peak current density of 0.08 mA cm-2 at 0.74 V vs. Ag/AgCl, which is only slightly above the initialisation current density of 0.05 mA cm-2, defined by Alia et al. and used as a baseline of catalytic activity measurement for MOR catalysts.47 The low catalytic activity of the 41 nm thick film was associated with the tensile strain in the catalyst film.

In order to understand the role of compressive lattice strain in the enhancement of methanol oxidation, we investigated the ability of the electrode to adsorb molecular methanol by comparing the activity of the catalysts in the ‘hydrogen region’ of the cyclic voltammograms (from -0.16 to 0.15 V vs. Ag/AgCl) in electrolytes with and without methanol. Figure 7 shows the adsorption and desorption of hydrogen on electrodes in the 0.5 M H2SO4 electrolyte (represented by a solid line). The charge associated with hydrogen adsorption in this region was used to calculate the ECSA of the working electrodes. We determined ECSA values of 0.023 cm2, 0.027 cm2, 0.031 cm2, 0.023 cm2 and 0.022 cm2 for films with thickness 41 nm, 54 nm, 64 nm, 89 nm and 100 nm respectively. These values were larger than the working area of the electrodes, which we measured to be 0.010 cm2, 0.013 cm2, 0.014 cm2,


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0.012 cm2 and 0.010 cm2 for films with thickness 41 nm, 54 nm, 64 nm, 89 nm and 100 nm respectively. We calculated roughness factors between 1.9 and 2.3 for the catalyst films.

Figure 6. Linear sweep voltammograms of Pt with different strain measured at scan rate of 50 mV s-1 in N2-saturated 0.5 M H2SO4 + 1 M CH3OH. LSVs are shown for Pt films with thicknesses of 41 nm (solid line), 54 nm (short dashed line), 64 nm (dash dot dotted line), 89 nm (dashed line) and 100 nm (dotted line). The inset shows the dependence of strain on the film thickness.

The dashed lines in Figure 7 show reduction of the area of catalyst associated with hydrogen adsorption due to the presence of methanol in the electrolyte. The reduction in hydrogen adsorption is attributed to methanol molecules being adsorbed onto the Pt surface, thereby blocking possible Pt-H bonding sites.15, 48, 49 We found that the fractional surface coverage of methanol varied with the thickness of Pt films.



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The fractional surface coverage of methanol compared to the original ECSA, ΘM, is a measure of the adsorption of methanol on the Pt surface. By integrating the area under the curves shown in Figure 7 as a solid line and dashed line, we calculated the percentage of methanol surface coverage, ΘM, for all of the films. The double layer capacitance in this region was calculated and subtracted from the overall area to ensure that only activity was due to hydrogen adsorption/desorption. The values of ΘM, shown in Figure 7 ranged from 20.5 to 61.7%.

Figure 7. The fractional surface coverage of methanol on compressively strained Pt catalyst surfaces is shown. Methanol coverage is evidenced by the reduction in hydrogen adsorption in the presence of methanol in the electrolyte. Cyclic voltammograms were measured on the films at a scan rate of 50 mV/s with and without methanol (1 M CH3OH).


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Figure 8 shows the relationship between the fractional surface coverage of methanol and the peak methanol oxidation current density on compressively strained Pt films. The initial methanol adsorption occurred at low potentials (-0.16 to 0.15 V vs. Ag/AgCl) while the peak current density for methanol oxidation was taken at 0.67 V vs. Ag/AgCl. Our data shows that there is a linear relationship between the initial methanol adsorption (i.e the fractional methanol surface coverage) and the activity for methanol oxidation for the films under compressive strain. This is clear evidence that the enhancement of methanol oxidation is directly associated with the increase of the surface coverage by methanol molecules.

Figure 8. The peak methanol oxidation current density as a function of methanol surface coverage on compressively strained Pt films measured at 0.67 V vs. Ag/AgCl. A linear relationship is observed for the films under compressive strain.



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In Figure 9, the fractional surface coverage, ΘM, and the peak methanol oxidation current density, jMOR, are shown as a function of the film strain (both tensile and compressive) for all the films. The left axis shows the ability of the Pt catalysts to adsorb methanol and the right axis shows the current density at 0.67 V vs. Ag/AgCl, which were taken from the LSV data in Figure 6. We found a direct correlation between the activity for both initial methanol adsorption and peak current density for methanol oxidation on Pt films.

Figure 9. Methanol surface coverage and peak methanol oxidation are shown as a function of lattice strain. The left-hand axis shows the fractional surface coverage of methanol on the Pt catalysts as a function of the strain in the films, denoted by black squares. The right-hand axis displays the peak current density for methanol oxidation at 0.67 V vs. Ag/AgCl. The two axes show the same trend across all Pt films.


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We observed that the enhanced activity for the oxidation of methanol is due to the higher surface coverage of methanol in films under compressive strain. The fractional methanol surface coverage, ΘM, varied as a function of the strain induced in the films during growth, which results in the highest coverage for a strain of -0.11 %. Similar behaviour was observed for the methanol oxidation current density with a value of 0.46 mA cm-2 and earliest onset potential for methanol oxidation of 0.67 V vs. Ag/AgCl at the same compressive strain. Furthermore, the strain is expected to change surface energy and hence the availability active surface sites for adsorption of methanol. Therefore, as we observe a high coverage for slightly negative strain, our data suggest that the adsorption does not vary monotonically with induced strain.

4. Conclusion:

We present an investigation into the effect of strain on the electrocatalytic activity of (111) oriented Pt films, which were grown by electroless deposition. Changes in lattice parameters show that films are strained with values varying from initially tensile to increasingly compressive as thickness is increased. We found that both the peak current density for methanol oxidation and fractional surface coverage of methanol depend on the strain. Furthermore, the peak current density for methanol oxidation is directly proportional to the surface coverage with values ranging from 20.5 % to 61.7%. Slight compressive strain leads to high activity of MOR with an experimental value of strain of -0.11% resulting in peak current density of 0.46 mA cm-2 at 61.7 % methanol surface coverage. Therefore, we show that electroless deposition is an effective method for growing Pt films with tuneable strain and that the role of the strain is to control the fractional surface coverage of methanol on Pt electrodes.



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5. Acknowledgements:

This research is supported by Science Foundation Ireland grant number 12/IP/1692 and the HEA PRTLI4 programme (INSPIRE). The authors are thankful to Professor Edmond Magner and his team for access to characterization facilities.


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