Depth-Profiling Microanalysis of CoNCN Water-Oxidation Catalyst

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Depth-profiling Microanalysis of CoNCN Water-oxidation Catalyst using a #=46.9 nm Plasma-Laser for Nano-Ionization Mass Spectrometry Rafael Johannes Mueller, Ilya Kuznetsov, Yunieski Arbelo-Pena, Matthias Trottmann, Carmen S. Menoni, Jorge J. Rocca, Greta R. Patzke, and Davide Bleiner Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01740 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Analytical Chemistry

Depth-profiling Microanalysis of CoNCN Water-oxidation Catalyst using a λ= 46.9 nm Plasma-Laser for Nano-Ionization Mass Spectrometry Rafael Müller,† Ilya Kuznetsov,‡ Yunieski Arbelo,¶ Matthias Trottmann, Carmen S. Menoni,¶ Jorge J. Rocca,‡ ‡ Greta R. Patzke,† and Davide Bleiner∗,¶,† †University of Zurich, Department of Chemistry, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. ‡NSF Center for Extreme Ultraviolet Science and Technology & Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado 80523, USA. ¶Swiss Federal Laboratories for Materials Science and Technology (Empa), Überlandstrasse 129, CH-8600, Dübendorf, Switzerland. ABSTRACT: Nano-scale depth profiling analysis of a CoNCN-coated electrode for water oxidation catalysis was carried out using table-top extreme ultraviolet (XUV) laser ablation time-of-flight mass spectrometry. The self-developed laser operates at λ = 46.9 nm and represents factor of 4 reduction in wavelength with respect to the 193 nm excimer laser. The reduction of the wavelength is an alternative approach to the reduction of the pulse duration, to enhance the ablation characteristics and obtain smaller quasi-non-destructive ablation pits. Such a XUV-laser ablation method allowed distinguishing different composite components of the catalyst-Nafion blend, used to modify a screen-printed carbon electrode surface. Chemical information was extracted by fragment assignment and relative amplitude analysis of the mass spectrometry peaks. Pure Nafion and the exposed carbon substrate were compared as references. Material specific nonoverlapping fragments were clearly identified by the detected mass-to-charge peaks of Nafion and CoNCN. Three dimensional mapping of relevant mass peak amplitudes was used to determine the lateral distribution and to generate depth profiles from consecutive laser pulses. Evaluating the profiles of pristine electrodes gave insight into fragmentation behavior of the catalyst in a functional ionomer matrix and comparison of post catalytic electrodes revealed spots of thin localized Co residues.

lytic activity in various forms21 of oxides and hydroxides,22 perovskites,23,24 phosphates,25,26 polyoxometalates27 and complexes.28,29

The implementation of renewable hydrogen sources relies on the development of efficient catalysts for photoand electrocatalytic water splitting.1–3 The bottleneck of the overall efficiency is considered to be the thermodynamically demanding four-electron oxidation of water to molecular oxygen.4–7 Earth-abundant heterogeneous transition metal electro-catalysts improve the oxygen formation kinetics and are leading candidates for affordable large scale water electrolysis, both for the classical alkaline electrolyzers8,9 and direct photo-electrocatalytic10–12 approaches. In order to allow targeted catalyst design and rational development the catalytic mechanisms, reactive species and structure-activity relationships remain to be elucidated.13–15 For electrochemical water oxidation catalysts based on transition metals the formation of oxides or hydroxides on the electrode surface under turnover conditions is frequently debated and still a major concern.16–19 As a matter of fact, even a monolayer of surface species can significantly alter the reaction processes that take place at the electrode-electrolyte interface.20 Among the relatively abundant transition metals, cobalt shows cata-

When the class of transition metal carbodiimides30–35 was investigated for water oxidation, CoNCN emerged as the most active binary material, with promising potential in mixed metal tuning.36 The cobalt-embedding carbodiimide environment represents a unique oxygen-free all-nitrogen coordination matrix that opens up interesting approaches to investigate the active surface sites with respect to their oxygen coordination. On the one hand it is inherently free of bulk oxygen that makes detection of surface oxygen species distinguishable and, on the other hand, it represents a solid state comparison to the highly investigated catalyst class of molecular Co complexes with ligand coordination via nitrogen groups [Co(NR)n].37,38 Some of these homogeneous catalysts were reinvestigated to find very small amounts of CoOx as true active catalyst species39–41 that may elude conventional analysis methods like Ultraviolet-visible spectroscopy

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(UV-Vis), Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX). Spatially resolved investigations42–44 of the electrode surface structure and chemical composition provide valuable information about surface species, but are not routinely accessible at the nano-scale. Indeed, leaching and nano-particle formation at the electrode surface must be considered and analyzed with advanced analytical technologies. Therefore, advanced spectroscopic tools are required that allow investigating the chemical surface stability of the water oxidation catalysts with nanoscale resolution and high sensitivity even of mixed materials. Typically, the reduction of the spot size turns into a degradation of the counting statistics, which implies poorer detection limits.45 Furthermore, the analytical challenge at nano-scale is to get access to signals free from contribution of the bulk.46–48 Instrumental innovation is thus necessary to enhance the microsampling efficiency and selectivity.49,50 Direct solid ionization using an XUV (also known as extreme ultraviolet or EUV) laser (λ = 46.9 nm ≙ 26 eV)51,52 coupled to time-of-flight mass spectrometry (TOFMS)53–58 offers the required capabilities of acquiring mass spectra with nanoscale resolution and minimal degree of fragmentation avoiding particle formation or inefficient inductively coupled plasma ionization.59 The ratio of elemental-to-molecular ions generated is favored by higher laser fluence achieved through increase of XUV energy.52 The reduction of the laser wavelength permits accomplishing two major advantages, i.e. (i) an enhancement of the spatial resolution, following the Abbe principle, and (ii) a thermal-free photolytic ablation. Fig. 1 shows the calculated optical penetration depth into Nafion and CoNCN at 90° as a function of wavelength in the XUV. One observes that the XUV spectral domain is characterized by a strong opacity, i.e. with a characteristic attenuation length-scale of approx. 10 nm. The ablation depth of a single XUV-laser pulse is thus drastically reduced with respect to what was reported previously with a state-of-the-art ArF excimer laser.60 With such an

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ultrathin ablation depth it was expected an insufficient mass-sampling to be able to detect a mass spectrum. A clear mass spectrum was easily obtained as shown here. As a matter of fact, the photolytic (direct ionization) ablation process improves the efficiency of the analytical process. In fact, the quantum nature of the photon absorption process with 26 eV photon energy (λ = 46.9 nm) leads to a single-photon ionization. Such single-photon ablation process at short wavelength is an alternative way to enhance the ablation efficiency, as compared to reduction of laser pulse duration to ps or fs, to induce a thermal-free multiphoton laser ablation. Photoionization is also advantageous versus electron impact, because once the photon has produced ionization it is annihilated. This prevents additional high-energy processes, like in the case of electron impact, causing unwanted fragmentation. Recently proof-of-principle XUV-laser TOFMS microanalyses have provided lateral resolution of 80 nm, depth resolution of 20 nm, mass spectral resolution of 1000 m/∆m and detection of rare isotopes in trace element abundance about 50 ppm depending on high mass cluster interference.51,52 The aim of this work was to move beyond proof-ofprinciple and to apply the analytical methods to a challenging scientific case in current chemistry. The research goal was to depth-profile the surface of CoNCN-Nafion coated electrodes utilizing XUV-laser TOFMS. The capabilities of the method were expanded to simultaneously probe a composite of ionomer binder and inorganic catalyst while maintaining the lateral micrometer resolution and depth sensitivity. Furthermore, the possibility of catalyst layer instability inspired us to pursue the highly surface sensitive investigations to achieve insight into the precise final state of the electrode and nature of degradation processes.

Experimental Synthesis of CoNCN The synthesis of CoNCN followed the procedure by Ressnig et al.36 via the precursor Co(NCNH)2 which was mixed with a eutectic salt mixture of LiCl-KCl, heated to 400◦C under N2 for 2.5 h and cooled to room temperature. The product was isolated from the salt cake by dissolution in Millipore water, filtration and washing with water. A dark brown powder was obtained consisting of particles with size distribution up to 10 µm, as determined from SEM images (Fig. S-1a and S-1c). Electrode modification and characterization Screen-printed electrodes (SPE) by DropSens with a circular carbon working electrode area of 4 mm diameter were used for catalyst deposition. The heterogeneous catalyst was applied by dropcasting catalyst-Nafion ink on the working electrode. The ink was prepared by sonicating a suspension of 5.0 ± 0.2 mg CoNCN and 42.2 µL

Figure 1: Calculated XUV penetration depth in the materials 2 composing the WOC catalyst. The Henke code can go as high as to 40nm, but one can extrapolate to 46.9nm of Plus Environment ACS values Paragon approx. 10nm and 5nm for Nafion and CoNCN, respectively.

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Nafion (5%wt sol., neutralized with KOH) in 320 µL ethanol for 10 min.

were accelerated to 1.5 keV energy due to the potential difference between the sample and the zone plates, which

Figure 3: Schematics of the extreme ultraviolet time-offlight mass spectrometer (XUV TOFMS) setup.

Figure 2: Comparison of EDX and Raman mapping of the post catalytic CoNCN-Nafion SPE (scale bars = 70 µm. (a) SEM image displaying an electrode section with inhomogeneous catalyst coverage. (b) EDX map showing the presence of C (green), Co (red) and N (blue). (c) Confocal optical microscope image of the same electrode area. d) Raman map (scanning step width = 10 µm = one pixel), black = lower speak signal intensity due to increased fluorescence on the area where EDX shows the most Nafion layer material, red to yellow = increasing signal to noise ratio of the post-catalytic CoNCN Raman peaks (see SI Fig. S-3).

The final ink contained 5.4 mg/mL Nafion and 13.8 mg/mL catalyst. The dropcasted ink volume for a catalytic layer was 2 µL. SEM and focused ion beam (FIB) crosssection analysis with EDX of an analogous catalyst layer microstructure is shown in the SI section 1 and 2. For the post-catalytic sample EDX (Jeol JSM-6010, 15kV) and Raman mapping (inVia Qontor, 785 nm) were performed on a representative electrode area to provide complementary information about the electrode surface state after electroanalysis (Fig. 2). The post catalytic SPE surface appears to be non-uniformly covered with CoNCN microparticles also present in the incompletely covered regions, as evident from the Raman map Fig. 2d.

XUV TOFMS setup Fig. 3 shows the experimental setup utilized for the characterization of pre- and post-catalytic CoNCN samples. The capillary discharge XUV-laser (λ = 46.9 nm ≙ 26 eV) generated pulses of about 10 µJ with duration (FWHM) of about 2 ns (see SI Fig. S-4) and a pulse to pulse energy fluctuation of 10%.61–63 Grazing incidence optics were used to collimate the XUV-laser and focus it on the sample using a zone plate lens. The ablated ions

were set 2.13 mm apart. The zone plate had a central opening of 50 µm in order to allow the ions to be detected and analyzed by their time of flight using a micro-channel plate (MCP) detector system (chevron configuration). The detector is: a dual a MCP detector (Galileo MCP40B), with 40 mm in diameter active area and 2.31 × 106 electron/ion gain. The sample-to-reflectron distance is roughly 1 m. The entire system occupies an optical table space of 0.6 × 3 m. The mass spectra are processed and stored by a personal computer digitizer (GaGe® EON CS121G1, 12 bits, 1 GS/s). With 50Ω input impedance the amplitude is limited to ±5 V. A typical full mass range TOF spectrum is 100 µs (ca. 0 to 650 m/z) long and for high mass range spectra the first 20 µs are skipped (ca. 35 to 650 m/z). Raw TOF-MS spectra were calibrated and analyzed using the MATLAB software package including the bioinformatics toolbox and direct peak integration was done by signal amplitude summation and baseline subtraction. Typically an XUV-irradiation pattern of 16 points arranged as a 4×4 grid on each electrode was selected as trade-off between sampling duration and representability. The laser spot size was 1 µm while the center-to-center distance between the adjacent points was 3 µm. On all the spots 10 consecutive laser pulses were delivered to acquire depth profiles. Averaging of all 160 mass spectra per sample provided a representative response with improved signal-to-noise ratio. Peak areas of single pulse spectra were obtained by integration over the baseline corrected corresponding full peak width and extracted to 3D matrices. 2D map stacks were generated by interpolation of the resulting 4×4 grids of peak amplitude for each pulse. Calculation of the optical penetration depth was done with the online code available at Center for X-ray Optics of the Lawrence Berkeley National Laboratory, implementing the algorithm in Henke et al.64

Results and Discussion Laser-ionization Mass Spectrometry of CoNCN



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Figure 4: Averaged mass spectra of pre- (top) and post-catalytic (bottom) CoNCN-Nafion SPEs, a) and c) low mass range measurements (0–40 m/z), b) and d) high mass range measurements (40–160 m/z). The mass spectrum of the pre-catalytic sample shows CoNCN as well as Nafion peaks while the post-catalytic spectrum is clearly dominated by substrate carbon fragment peaks.

Fig. 4 shows the averaged XUV TOFMS of pre- and post-catalytic CoNCN. Most of the mass-to-charge (m/z) peaks in the low mass range (Fig. 4a and 4c) belong to singly charged mono atomic ions and are easily identified. In the high mass range (Fig. 4b and 4d), ionic clusters consisting of several atoms are found and peak assignment is based on reference comparison and probable composition of CoNCN and Nafion fragments. In the spectrum of the pre-catalytic samples (Fig. 4b), peaks which are specific for CoNCN fragmentation can be found: CoCN+ (84.93 m/z), Co2+ (117.87 m/z) and Co2CN+ (143.87 m/z). Additionally, several specific Nafion fragment peaks CxFy(Oz)+ are detected. The strongest Nafion peaks are CF+ (31.00 m/z), CF2+ (50.00 m/z), CF3+ (69.00 m/z), C3F3+ (93.00 m/z), C2F4+ (99.99 m/z), C2F5+ (118.99 m/z) and C3F5+ (130.99 m/z) and can be used as representative Nafion indicators (compare Fig. 5). The relative Nafion peak amplitude distribution of non-overlapping signals in the spectrum of the CoNCN@Nafion layer corresponds to the fragmentation pattern of the Nafion reference, even for the separate spectra (not shown).

averaged spectra with FWHM ranging from 0.002 to 0.06 m/z (= 6 – 12 ns). Some peaks show asymmetric shapes but strictly the resolution is at the limit of resolving the contributions. The comparison of relative peak amplitudes herein is of qualitative nature. Nevertheless, by assuming similar fragmentation as measured in reference mass spectra of pure Nafion or blank carbon SPE, relative peak correlation shows that the maximum amplitude contribution of the possibly interfering fragments is expected to be negligible, i.e. below 1% for C2FO+ to Co+ and C12+ to Co2CN+. The observed TOFMS spectra therefore allow a clear distinguishing of the catalyst layer components, i.e. from each other and from the substrate. Occurrence of the mixed species CoF+ and CoNF+ suggests that ion clusters assemble during the ablation process by condensation or collisions, as cobalt and fluorine are not directly chemically linked in the as-prepared coating and fluorine is bound covalently in the Nafion polymer. The mass peaks obtained from the post-catalytic SPE differ substantially from the pre-catalytic ones. Most of the peaks can be assigned to carbon substrate fragment multiplets CnHm+ with highest amplitudes for hydrogen atom addition m = 0–3. Both the CoNCN and Nafion specific peaks are not visible in the averaged mass spectra, which suggests that on this particular spot the deposited catalyst particles and Nafion binder detached from the electrode to a large extent during electrocatalytic water oxidation. While SEM-EDX and Raman imaging are well suited for the micrometer range investigation of samples, as shown in Fig. 2, they are not sufficient to determine whether a very thin nanometers-range layer is still pre-

Some mass-to-charge peaks may be interfering in principle, yet not likely considering clustering probability and ion stability: Co+ (58.93 m/z) with C2FO+ (58.99 m/z) or CN2F+ (59.00 m/z); CoCN+ (84.93 m/z) with CF3O+ (84.99 m/z); Co2CN+ (143.87 m/z) with C12+ (144.00 m/z). The resolution needed to distinguish these peaks is determined by the ratio of mass to mass-difference ( / 1000) and compared to the resolution of ∆

. /

the measurements, which is defined by using the peaks FWHM as ∆m. Our system experimentally showed a resolution of m/∆m = 500 – 1000 for m/z = 1 – 60 for non-


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sent on the post-catalytic SPE. The XUV TOFMS results perfectly complement those methods, which through their higher penetration-depths give rise to a larger bulk or background signal contribution.

high amplitude. To clarify the correlation, the ratio and difference of the C+ and N+ peak have been calculated and a relative excess of C+ is found for the first laser pulse. The spatial correlation with Co+ is less defined.

Carbon chain fragment multiplets C1-20H0-3+, including m/z), are indicative of the carbon substrate and are not detected in mass-spectra of the pre-catalytic SPEs (compare Fig. 4 and Fig. 5). The low amplitude of carbon chain fragments in the pre-catalytic layer and the spatially correlated occurrence of fragments derived from Nafion, CoNCN and impurities is confirmed by mapping the corresponding signals of a larger map, see Fig. S-5 in the SI.

Distribution of C+ and C2+ shows significant difference, which is in line with C+ as major ablation product from CoNCN and Nafion while C2+ is a fragment observed as a product from ablation of the carbon substrate but not of the Nafion carbon backbone. The C2+ signal of the carbon electrode only appears in the post-catalytic sample indicating that the layer on the pre-catalytic SPE is not fully ablated by the laser pulses. Clearly the 10 consecutive XUV-laser pulses do not form a pit that penetrates the full

C2+ (24.00

Figure 5: Averaged mass spectra of pre-catalytic CoNCN-Nafion SPEs vs scaled and offset pure Nafion and blank carbon reference spectra from a) low and b) high mass range measurements. The stars indicate nominal masses the of Nafion fragment peaks with high amplitude.

10 µm thick carbon substrate, as no aluminium is detected in any sample. With that an average ablation rate well below 1 µm/pulse is indicated.

Depth profiling micro-analysis Plotting significant mass peak areas from spectra of consecutive laser pulses as 2D map stack as shown in Fig. 6, allows observation of spatial correlations on the x/y surface as well as depth profiles in z direction with increasing XUV pulse number from top to bottom. From the depth profile of the pre-catalytic SPE (Fig. 6a), it can be seen that the Nafion layer covering the CoNCN particles is ablated in a single pulse revealing the CoNCN underneath in the following pulses. The peak amplitudes of C+, F+ and O+ decrease in factors of 2 to 5 after the first pulse, while the peak amplitudes of N+ and Co+ sharply increase. The O+ detected on the surface most likely originates from the sulfonate groups of Nafion, although contribution of residual trapped moisture or other impurities cannot be excluded. The spatial distributions of the C+ and N+ peaks on the amplitude maps of the pre-catalytic sample resemble each other quite well suggesting their occurrence in the same compound at the same spots. The option of coincidental correlation has to be noted as well as the potential overlap of CH2+ and N+ at m/z = 14. This overlap is unlikely as no significant contribution of substrate carbon or CH+ is found in the spectra where the peak at m/z = 14 shows

On the post-catalytic electrode sample (Fig. 6b), there is barely any fluorine-containing fragment detectable and Co+, O+ and N+ are only observed at the very surface. These could be residuals of the catalyst layer that detached during electrocatalytic water oxidation, a low amount of material electrodeposited after catalyst layer degradation or (re)adsorbed CoNCN particles. The latter is most likely as the post-catalytic sample surface is devoid of Nafion residues, but Co and N are found spatially correlated. A plausible explanation would be protonation and swelling of the Nafion membrane and dissolution or detachment by oxygen formation.

Conclusions Time-of-flight mass spectra of XUV-laser ablated preand post-catalytic CoNCN coated screen-printed electrodes for water oxidation have successfully been recorded and reveal significant chemical depth profiling information. It was shown that the mass-to-charge peaks corresponding to N+ as well as the ones correlated to the



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Figure 6: Depth profile representation of mass fragment peak areas by stacked 2D maps for a) pre-catalytic and b) post-catalytic CoNCN@Nafion coated SPEs. Below the peak assignment the rounded nominal mass and the average peak area is noted. The peak area magnitude is represented by color and is normalized to the corresponding signal maximum and set to 1 mV×ns for lower + + + signals. Note the significant decrease of peak area and the adjusted scale for N , F and Co in the post catalytic sample.

fragmentation of CoNCN (CoCN+ and Co2CN+) and Nafion (CF+, CF3+ and C3F5+) are practically absent from the averaged mass spectra of the selected post-catalytic sample spots. Together with the increase in C2+ amplitude the results may indicate that these spots are devoid of catalyst layer. On the other hand Co+ and N+ are clearly detected on the first pulse, which shows that even small amounts of superficial catalyst remains are detectable above the substrate background. This complements EDX and Raman mapping, which describe the overall electrode surface more representative, but with less depth resolution. The confirmed localized absence of Nafion after electrocatalysis provides valuable information to address possible layer-degradation and redeposition processes in a targeted way. In fact, operational catalytic stability was observed with CoNCN@Nafion SPEs for the duration of electroanalytical experiments (1600 s cyclic voltammetry and 2400 s chronoamperometry) and is presented in another study focusing on operando analysis).65 Meanwhile catalyst-Nafion layer detachment during electrocatalytic water oxidation has been observed when polished rotating disc electrodes were used as substrate for the Nafion ink as part of our parallel mechanistic investigations. A similar effect of apparent sudden electrode deactivation has recently been reported for glassy carbon electrodes.66 Understanding the cause and process of electrode transformations is essential for the progress of catalyst development, especially for technological applications. To this

end, also investigation of reference materials of different types and the impact of morphology and mixed materials requires additional attention. With the growing need for nanoscale surface analyses, further development of lab scale XUV systems is necessary. Overall, the achieved sensitivity and spatial resolution fulfilled the required high demands of micro- to nanoscale electrode surface characterization. This paves the way for a wider use of benchtop XUV TOFMS on applications beyond the field of electrocatalysis.

Conflict of interest There are no conflicts of interest to declare.

Acknowledgement The authors thank the support of the University Research Priority Program Solar Light to Chemical Energy Conversion (LightChEC) of the University of Zurich, the Swiss National Science Foundation (Sinergia Grant No. CRSII2 160801/1), and the US Department of Energy, Office of Science, Basic Energy Sciences under Award (DEFG02-04ER15592). We thank Karla Lienau (University of Zurich) for providing catalyst material.

SUPPORTING INFORMATION CoNCN@Nafion SEM analysis and FIB cross section, Raman mapping spectra, electroanalytical details, XUV-laser pulse profile, additional CoNCN@Nafion surface mapping.


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Depth-Profiling Microanalysis of CoNCN Water-Oxidation Catalyst

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