Rapid Quantification of Film Thickness and Metal Loading for

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Rapid Quantification of Film Thickness and Metal Loading for Electrocatalytic Metal Oxide Films Kevan E. Dettelbach,† Michael Kolbeck,† Aoxue Huang,† Jingfu He,† and Curtis P. Berlinguette*,†,‡,§ †

Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC V6T1Z1, Canada Stewart Blusson Quantum Matter Institute, The University of British Columbia, 2355 East Mall, Vancouver, BC V6T1Z4, Canada § Department of Chemical & Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, BC V6T1Z3, Canada ‡

S Supporting Information *

ABSTRACT: The thicknesses and metal loadings of amorphous nickel, iron, and iridium oxide films widely used for solar fuel electrocatalysis were determined by cross-sectional scanning electron microscopy (SEM) and X-ray fluorescence (XRF) spectroscopy measurements. The thicknesses for a series of films, which were systematically varied from 10 to 400 nm using photodeposition techniques, were accurately measured by cross-sectional SEM using a protocol that successfully resolves the relevant catalyst layers. XRF measurements recorded on each of the films provided a strong linear correlation (R2 > 0.97) with the thicknesses determined by cross-sectional SEM. The electrochemical surface areas (ECSAs) determined by double-layer capacitance measurements, a technique widely used in the electrocatalysis community, showed a linear relationship for iridium oxide film thicknesses but not with those consisting of nickel and iron. These results highlight the limitations of using ECSA to determine catalyst film thicknesses and metal loadings. The noninvasive XRF technique is demonstrated to be a far superior method for reporting on the thickness and loadings of thin metal oxide films.

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INTRODUCTION There is a broad effort to rationally design electrocatalysts to mediate the electrolytic formation of hydrogen, oxygen, carbon fuels, and commodity chemicals.1−13 These processes each need to occur efficiently and selectively for extended time periods to be commercially viable. Realizing this goal requires a clear understanding of how catalyst coatings operate so that appropriate catalyst compositions and thicknesses can be deployed. The oxygen evolution reaction (OER) occurs at the anode for many electrolytic processes and is particularly important in the context of solar fuels where it represents the energetically demanding anodic reaction during electrolytic water splitting.1,2 There have been substantial efforts committed to elucidating the mechanism of the OER to develop more active and robust catalyst films.14−21 It remains a challenge to differentiate surface versus bulk sites of electrocatalyst films, yet this information would be extremely valuable for screening effective catalyst compositions. It is therefore important to develop meaningful ways to benchmark electrocatalyst performance to more effectively identify how catalyst composition affects catalyst activity and stability.22 The relative performance of an OER electrocatalyst is often determined by comparing the overpotentials needed to reach a certain rate of product formation (e.g., 10 mA cm−2). This extensive property will vary with the amount of material © 2017 American Chemical Society

present, which makes it a challenge to directly compare the intrinsic activities of different catalyst films. While differences in geometric surface areas can be handled by reporting current densities rather than current, this approach does not account for differences in material thickness, density, morphology, conductivity, or porosity. Indeed, all of these properties can impact the performance of a catalyst.23−27 A widely used method for resolving intrinsic activity is electrochemical surface area (ECSA). However, the specific capacitances of atomically smooth planar versions of materials are needed to make relevant comparisons of ECSA values. Unfortunately, it is not usually practical to fabricate such materials, and thus, estimates of the specific capacitance are used that can generate errors in the accuracy of the ECSA values by up to a factor of 7.22 We are of the opinion that benchmarking catalyst performance against the amount of electrocatalyst present is most relevant to the commercial evaluation of catalyst coatings. On this basis, catalyst activity should be normalized to account for mass loading, where activity is expressed per unit of mass of the deposited catalyst.4,7,15,23,25,28,29 For electrodeposited films, the amount of metal deposited on the film can be calculated by the total charge used to conduct the deposition.14,15,23,27,30 A Received: May 9, 2017 Revised: August 1, 2017 Published: August 24, 2017 7272

DOI: 10.1021/acs.chemmater.7b01914 Chem. Mater. 2017, 29, 7272−7277

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Chemistry of Materials

Figure 1. (a) Each metal oxide film was fabricated by spin-casting metal precursor solutions of varying concentrations (w/w %) onto the substrate (FTO or iridium coated glass). These films were then exposed to ultraviolet radiation to decompose the precursor into the corresponding metal oxide film. (b) The thicknesses of nickel, iron, and iridium oxide films (determined by cross-sectional SEM) from precursor solutions with the indicated precursor concentrations (w/w %). Film Syntheses. All FTO electrodes and glass slides were cleaned prior to all experiments following 4 successive 10 min sonication steps in a neutral soap solution, then in distilled water, then in acetone, and finally in 2-propanol. After the last ultrasonic treatment, the substrates were rinsed with 2-propanol and subjected to ozone produced by UV irradiation for 10 min. The iridium coated glass substrates were prepared from clean microscope slides. A Leica EM MED020 Coating System was used to deposit 20 nm of iridium onto the glass slides. The thickness was monitored using an EM QSG 100 quartz crystal film thickness monitoring system. Solutions of metal precursor complexes were prepared by dissolving the appropriate ratio of metal complexes in organic solvent. For iron and nickel, the organic solvent is hexanes, and for iridium, the solvent is chloroform. Films were deposited on FTO (or iridium coated glass) by spin-coating at 3000 rpm for 60 s and then irradiated with UV light (5.8 W, 254 185 nm, Atlantic Ultraviolet) until the quantitative loss of the precursor’s absorbance bands indicates complete decomposition. Decomposition of the precursor was verified by Fourier transform infrared (FTIR) spectroscopy using a Bruker Alpha FTIR spectrometer. The samples on iridium coated glass had an additional 20 nm layer of iridium deposited on top of the metal oxide layer using the Leica system. Physical Methods. Electrochemical measurements were performed on a CH Instruments Workstation 660D potentiostat. The electrochemical measurements of iron and nickel samples were carried out in 1 M KOH, while iridium samples were measured in 1 M H2SO4. The electrolyte for the nickel samples was iron-free.25,40 The test cell was a single compartment, three-electrode cell made from a plastic beaker. Ag/AgCl was used as a reference electrode, and Pt wire was used as counter electrode. The Ag/AgCl electrode was calibrated against a reversible hydrogen electrode (RHE) using a Pt electrode for both the working and counter electrodes in the same 1 M KOH or H2SO4 electrolyte sparged with H2 (Praxair). All electrochemical potentials were corrected for uncompensated resistance. Samples that were to be studied by electrochemical means were deposited onto 3 × 3 cm FTO coated glass. The samples were each broken into three 1 × 3 cm pieces with the center piece being used for all subsequent studies. The sample was then tightly wrapped with Teflon tape to expose a 1 cm2 working area. Cyclic voltammograms were performed for all samples to determine the double-layer capacitance charging current density at scan rates of 10, 20, 30, 40, 50, and 100 mV s−1. An exception is the Ir10* film, which used scan rates of 1, 2, 3, 4, 5, and 10 mV s−1 because higher scan rates resulted in unreliable data. The

quartz microcrystal balance is also an effective means of determining the weight of the films.7,15,23,25,28,29 Other deposition methods such as photodeposition8,16,31−33 and sputtering2,34 do not have an easily measurable parameter to quantify, nor is it always practical to deposit a representative film on a quartz microcrystal balance. Ellipsometry can be an effective means of measuring film thickness, but it requires a knowledge of the indices of refraction that is not often known for amorphous layers.35 We demonstrate here the utility of X-ray fluorescence (XRF) analysis to document the metal loadings of widely studied OER electrocatalyst films. This method, which has proven effective in other fields,36−39 enables rapid acquisition of data without compromising the sample during analysis. We use XRF in this study to analyze amorphous films of iridium, nickel, iron, and mixed-metal Fe/Ni oxides with thicknesses ranging from ∼10− 400 nm. The thicknesses were unambiguously defined by crosssectional scanning electron microscopy (SEM) using a novel protocol that enables resolution of the active layers. XRF measurements recorded on each sample were found to linearly track the measured thicknesses. Notably, the ECSA did not provide a correlation to film thickness in all cases. These results provide a compelling case for researchers to consider using XRF as a means of analyzing catalyst loading and film thicknesses.

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EXPERIMENTAL SECTION

Materials. Ni(II) 2-ethylhexanoate, Fe(III) 2-ethylhexanoate, and Ir(III) acetylacetonate were purchased from Strem Chemicals. Hexanes (98.5%+), H2SO4 (95−98%), and chloroform (99.8%) were purchased from Sigma-Aldrich. KOH electrolyte solutions were obtained by dissolving the appropriate amount of KOH (85%, SigmaAldrich) in deionized water. Because iron impurities can greatly influence the electrochemical behavior of nickel-based catalysts,25,26 we removed those impurities from the electrolyte by the addition of Ni(OH)2, which was precipitated from a Ni(NO3)2 (99.9985%, Alfa Aesar) aqueous solution using 1 M KOH; the solution was agitated for 3 days prior to use. The iron-free electrolyte was used exclusively with the nickel samples. 7273

DOI: 10.1021/acs.chemmater.7b01914 Chem. Mater. 2017, 29, 7272−7277

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Chemistry of Materials double-layer capacitance charging current density was calculated to be half of the difference between current densities of the forward and reverse sweeps measured at the center of the sweep. A plot of these current densities versus the scan rate they were taken at gives the double-layer capacitance as the slope. XRF measurements were taken with a Thermo Fisher Scientific Niton XL3t analyzer utilizing a shielded test stand. This benchtop stand served to provide a fixed distance from the sample to the X-ray source and detector, giving a consistent spot size for the X-ray beam. The X-ray source was run with an accelerating voltage of 50 kV and a current of 40 μA. Scan time was 30 s for each sample. Scanning electron microscopy was carried out on all samples using an FEI Helios NanoLab 650 dual beam scanning electron microscope. A platinum block (length = 10 μm, width = 2 μm, height = 1 μm) was deposited onto the sample in situ using an ion beam current of 20 pA. A pit just slightly intersecting with the platinum block was bored out with an ion beam current of 0.8 nA. The edge of the resulting pit was cleaned using an ion beam current of 80 pA. Cross-sectional images were taken at an angle of 52 degrees with an accelerating voltage from 1 to 3 kV and a current of 50 pA. Images of the nickel and iron samples were taken using backscattered electrons, and the iridium samples were imaged using secondary electrons. All images were corrected for tilt. The mass of iron deposited onto glass slides was measured using inductively coupled plasma mass spectrometry (ICP-MS) by AGAT Laboratories. The samples were digested by aqua regia before being injected into the instrument.

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Table 1. Metal Oxide Film Thicknesses, Metal Loadings, and ECSA Values metal loadingb (cps μA−1) sample Ni2.5 Ni5 Ni10 Ni15 Ni30* Fe2.5 Fe5 Fe10 Fe15 Fe30* Ir1.25 Ir2.5 Ir3.75 Ir5 Ir10*

film thickness (nm)a 25 55 105 225 382 18 32 46 115 291 13 16 23 43 71

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2 3 4 5 4 3 3 6 5 4 3 3 4 4 3

glass 1.3 3.2 6.7 11.3 21.6 0.8 1.8 2.9 4.6 12.0 1.3 3.7 5.2 8.0 13.9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 0.1 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.3

FTO

ECSA (mF cm−2)c

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.007 0.014 0.013 0.013 0.009 0.010 0.009 0.009 0.010 0.012 3.86 10.06 14.04 16.00 32.57

1.7 3.0 7.0 11.0 24.2 1.0 1.5 3.2 4.5 12.3 1.8 3.3 5.4 7.4 15.9

0.1 0.2 0.2 0.2 0.3 0.1 0.3 0.2 0.2 0.3 0.1 0.1 0.1 0.2 0.2

a

Determined by cross-sectional SEM on the iridium-coated glass samples. bDetermined by XRF. cDetermined by double-layer capacitance measurements on the FTO samples.

RESULTS AND DISCUSSION

to displace core electrons. The resulting vacancies are filled by electrons in higher orbitals, which emit characteristic X-ray radiation that is detected by the instrument. The penetration depth and escape path associated with this technique are approximated to be ∼10 μm, which is sufficient to analyze the vast majority of thin films used in the electrocatalyst field. The XRF instrument can be set to detect specific elements (nickel, iron, and iridium in our case) and reports quantity in counts per sec per μA. This value refers to the filament current of the X-ray source, which is 40 μA for our measurements. Each of our samples was analyzed for 30 s where the collected data was averaged over the entire detection period. The error for such measurements is typically in the range of 1−3% with thicker films tending toward a lower error. Before deposition of the metal oxide films, each blank substrate was analyzed by XRF, and the resultant values were subtracted from all subsequent measurements for that sample. These background measurements contained errors on the order of 3%. Substrates containing trace amounts of iridium contained errors closer to 7%. Figure S2 shows the XRF data plotted against the precursor concentration for the nickel, iron, and iridium oxide films. Each film shows a linear relationship between the XRF measurements and the precursor concentration. We then calibrated the XRF readings against films of known thicknesses. Cross-sectional scanning electron microscopy (SEM) was used to unambiguously determine film thicknesses. We are not aware of this method being used to correlate film activity to film thickness. Using the same precursor concentrations as we described earlier, we deposited films of nickel, iron, and iridium oxide onto glass microscope slides that had first been coated with ∼20 nm of iridium metal. This substrate was chosen because it provides a smooth surface and provides excellent contrast against the metal oxide films of interest. An additional 20 nm of iridium was coated on top of the photodeposited metal oxide film to enhance conductivity and minimize charging effects by the electron and ion beams. A protective layer of platinum was then deposited in situ over the site of measurement to protect the sample from the focused ion

The films were prepared by our photodeposition method that conveniently offers access to uniform and conformal oxide films with a wide range of transition metals.8,16,32 The photodeposited films under investigation were produced from metal carboxylate precursor solutions spin-cast onto fluorine doped tin oxide coated glass (FTO) or iridium coated glass substrates. The amorphous FeOx, NiOx and IrOx films were derived from the commercially available precursors Fe(III) 2-ethylhexanoate, Ni(II) 2-ethylhexanoates, and Ir(III) acetylacetonate. Each precursor coating was exposed to UV light under ambient conditions for 12 h (48 h for the nickel-based films) to fully drive off the ligands and furnish the desired amorphous metal oxide film (Figure 1a). The NiOx and FeOx films were then annealed for 1 h at 100 °C, while the iridium films were annealed at 300 °C for 1 h. X-ray diffractograms for each of the samples showed only peaks corresponding to the FTO substrate (Figure S1); it is on this basis that we characterize the films as amorphous. The film thicknesses were controlled by the concentrations of the metal precursors, where the concentration is expressed as the weight percentage (w/w %) of the precursor and the solvent (hexanes) used to dissolve the precursor for spincoating. The films are denoted herein according to this weight percentage of the metal precursor; e.g., Ni2.5 corresponds to a 2.5 w/w % of Ni(2-ethylhexanoate)2 in hexanes, or 0.034 g of Ni(2-ethylhexanoate)2 in 1.31 g of hexanes. Certain films were created with two layers deposited from solutions of half of the listed concentration and are indicated herein with an asterisk (*); e.g., Ni30* consists of one photolyzed layer of 15 w/w % Ni(2-ethylhexanoate)2 in hexanes under a second layer of photolyzed 15 w/w % Ni(2-ethylhexanoate)2. The precursor solutions used to prepare the series of NiOx, FeOx, and IrOx films used for this study are shown in Figure 1, while a full listing of the films investigated in this study is provided in Table 1. XRF was used to determine the metal loading of each film. This technique works by exposing the sample to X-ray radiation 7274

DOI: 10.1021/acs.chemmater.7b01914 Chem. Mater. 2017, 29, 7272−7277

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Chemistry of Materials beam (FIB) used to excavate a pit on the sample. Images of the resulting cross-section were collected and corrected for the tilt angle. The two iridium layers provide the visual contrast needed to resolve the oxide layers, thereby enabling the metal oxide film thicknesses to be easily measured between the two iridium layers. Figure 2 shows a composite image of the five

Figure 2. Cross-sectional SEM images of nickel samples (A) Ni30*, (B) Ni15, (C) Ni10, (D) Ni5, and (E) Ni2.5. Images were taken in backscattering mode to enhance contrast and resized to a common scale (the brightness/contrast of B and D was slightly adjusted to better match the other images). Inset: Schematic of layer architecture prior to FIB milling.

different thicknesses of nickel oxide. The thicknesses of the iron and iridium oxide films are shown in Figures S3 and S4, and all measured thicknesses are catalogued in Table 1. The technique for measuring the thickness of the metal oxide layer is detailed in Figure S5. The uncertainty in the thickness measurements depends on the sharpness of focus achieved which, in turn, is dependent on the conductivity of the sample and the operator. The relative error of these thickness measurements is greater than that determined by XRF analysis. The metal loadings for each of the three films deposited on iridium-coated glass were measured by XRF (cps μA−1) and plotted against the SEM-determined film thickness (Figure 3). All three films exhibited a linear relationship between metalloading and film thickness with a high degree of accuracy (as demonstrated by R2 values >0.97) within the range of thicknesses tested for each type of material (approximately 20−400 nm for NiOx; 20−300 nm for FeOx; 10−70 nm for IrOx). The linear equations correlating XRF count to film thickness were derived and are displayed on each plot. The gradient and intercept values provide a calibration constant (in cps μA−1 nm−1) and background value (in cps μA−1), respectively, that enable the thickness of a material (of known composition) to be accurately determined in a single XRF measurement that lasts merely 30 s. The equations and R2 values when the trendlines are forced through zero do not deviate significantly: y = 0.056x (R2 = 0.991), y = 0.042x (R2 = 0.986), and y = 0.198x (R2 = 0.974) for the NiOx, FeOx, and IrOx films, respectively. On the basis of a limit of detection for the XRF instrument being three times the height of the background noise, the minimal detectable thickness is expected to be 1.9, 1.4, and 0.4 nm for the FeOx, NiOx, and IrOx films, respectively. The sample is not damaged during this measurement, and no additional sample preparation steps are necessary so that samples analyzed by XRF can be subsequently analyzed by other spectroscopic or electrochemical methods. Similar XRF measurements were also performed on thin film oxides

Figure 3. Metal loading as determined by XRF with increasing film thickness for films on iridium coated glass. Thickness was measured using cross-sectional SEM. There is a linear relationship between the XRF measurements and the thickness of the films for all three materials.

deposited on FTO. The results showed that films deposited on FTO and iridium coated glass have similar metal loadings for the same precursor concentrations (Figure S2), demonstrating the applicability of this technique to different substrate materials. Given the importance of mixed-metal oxide catalysts,16 we measured the thickness and XRF signal of a series of ∼50 atom % iron−nickel oxide films (Fe0.5Ni0.5Ox). These films also displayed a linear relationship between thickness and the metal loading for each of iron and nickel (Figure S6). Overall, our results provide a compelling case for XRF to be applied as a routine technique in determining catalyst loading. ECSA is a standard benchmarking technique used for thin films to help predict the electrochemical performance of the sample.24 For ECSA to be an accurate measure of the bulk material, however, electron and ion conductivity cannot be inhibited. If the material is a poor electronic conductor in a thick film, only the material near the electrode may be measured as electrochemically active. Conversely, only the portion of the film exposed to the electrolyte will be deemed active if ion transport through thick films is limited. We therefore set out to determine the ECSA of our films with variable thicknesses by measuring the double layer capacitance of the films (Figure S7). These values were then plotted against the actual thicknesses determined by cross-sectional SEM (Figure 4). The iridium films follow a linear relationship between ECSA and thickness; however, the ECSA values recorded on nickel and iron films did not yield a meaningful 7275

DOI: 10.1021/acs.chemmater.7b01914 Chem. Mater. 2017, 29, 7272−7277

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Chemistry of Materials

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double layer capacitance measurements for nickel, iron, and iridium films, XRF and ECSA readings as a function of film thickness for iron-nickel films, and XRF readings for iron as function of metal loading as determined ICPMS (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Curtis P. Berlinguette: 0000-0001-6875-849X

Figure 4. ECSA of nickel (black squares), iron (black circles), and iridium (blue triangles) oxide films of variable thicknesses showing a linear correlation only for the iridium data.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Canadian Natural Science and Engineering Research Council (Grant RGPIN 337345-13), Canadian Foundation for Innovation (Grant 229288), Canadian Institute for Advanced Research (Grant BSE-BERL162173), and Canada Research Chairs for financial support. K.E.D. was supported by an NERC PGS D scholarship. A.H. was supported by the University of British Columbia with a Four Year Doctoral Fellowship (4YF). The authors thank the Centre for High-Throughput Phenogenomics for access to SEM facilities.

correlation with film thickness. We also did not find a correlation of the Fe0.5Ni0.5Ox ECSA values with thickness (Figure S8). These results highlight the potential pitfalls of using ECSA as a determinant of film thickness, as is widely done in the community. Building from the analyses by the Boettcher15 and Hu6 groups, we postulate that the ECSA values do not increase with the thickness of the iron and nickel oxide films due to limited electronic conductivities. These porous films presumably allow electrolyte to permeate throughout the film, but poor conductivity would confine the active region to near the electrode interface.6 Higher thicknesses would therefore not increase the ECSA values, which is corroborated by our measurements (Figure 4) and previous studies.6 Indeed, Boettcher and co-workers showed that only a very thin layer close to the electrode surface was electrochemically active (at low overpotentials) for iron oxide films on gold substrates.15 It is worth highlighting that in both studies, catalyst loading was quantified by mass and not by measuring the actual thickness of the films. Our results build on their results to also make the connection with measured thickness. We also found a strong correlation with the XRF metal count and the mass loadings determined by ICP-MS for five iron oxide films (Figure S9).

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CONCLUSIONS XRF is demonstrated here to be a competent method for measuring metal loading and film thickness when calibrated against independently determined thicknesses for oxide layers of nickel, iron, and iridium as well as an iron−nickel mixed metal oxide. This method presumably is effective for quantifying other metals, particularly those of relevance to the solar fuels community. We also show that ECSA measurements provide a good correlation to film thickness for iridium-based films but not for those based on iron and nickel. These results show that conductivity and mass transport limitations can lead to starkly different responses; thus, caution must be used for such analyses. We contend that XRF analysis is a more effective tool for quantifying metal loadings and thicknesses of thin metal oxide films.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01914. X-ray diffractograms, XRF readings as a function of precursor concentrations, cross-sectional SEM images, 7276

DOI: 10.1021/acs.chemmater.7b01914 Chem. Mater. 2017, 29, 7272−7277

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DOI: 10.1021/acs.chemmater.7b01914 Chem. Mater. 2017, 29, 7272−7277