Photodecomposition of Metal Nitrate and Chloride Compounds Yields

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Photodecomposition of Metal Nitrate and Chloride Compounds Yields Amorphous Metal Oxide Films Jingfu He,† David M. Weekes,† Wei Cheng,† Kevan E. Dettelbach,† Aoxue Huang,† Tengfei Li,† and Curtis P. Berlinguette*,†,‡,§ †

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

S Supporting Information *

industries, such as electrochromic windows, are typically isolated by sputtering where a target material is deposited from the gas phase onto a substrate under high vacuum.18,19 Electrodeposition, which can be performed at room-temperature without a vacuum,20 can also be more expensive to execute than thermal decomposition. Consequently, sputtering and electrodeposition are not widely used to prepare electrocatalysts for commercial electrolyzers. Our research program has a long-standing interest in overcoming these shortcomings by exploring alternative solution-based methods to access amorphous metal-oxide films.6,12,21,22 We have previously documented that the UVlight driven decomposition of metal organic precursors (e.g., Fe(eh)3, Ni(eh)2, Ir(acac)3; eh = 2-ethylhexanoate; acac = acetylacetonate) offers access to metallic or metal-oxide amorphous thin films with rigid control of elemental ratios, uniform morphologies and metal distribution across the entire substrate. This process relies on UV light to trigger a ligand-tometal charge-transfer (LMCT) process to homolytically cleave the metal−ligand bonds to successively form amorphous phases of oxides. As part of our effort to elaborate on this photodecomposition technique, we have tested alternative metal precursors as a means of lowering fabrication costs and increasing the versatility of the technique (Scheme 1). We describe herein the unexpected discovery that UV radiation can effectively decompose widely available chloride and nitrate metal precursors, and that the organic ligands are not needed to facilitate the decomposition process. The versatility of this method is demonstrated by photolyzing thin films of first-row transition metals Fe, Co, Ni, Cu and Zn solution-deposited from aqueous solutions of the corresponding chloride and nitrate compounds. The resultant amorphous metal oxide thin films, denoted as MOnitrate and MOchloride, are shown to have nearly identical compositions and morphologies to films synthesized from the organic precursors MOorganic. The iron and nickel films, as well as the widely studied iron−nickel oxide,23 were also found to show OER activities that are commensurate with similar films prepared by more exotic techniques. We also demonstrate that this process offers a

ABSTRACT: UV light is found to trigger the decomposition of MClx or M(NO3)x (where M = Fe, Co, Ni, Cu, or Zn) to form uniform, amorphous films of metal oxides. This process does not elevate the temperature of the substrate and thus conformal films can be coated on a range of substrates, including rigid glass and flexible plastic. The formation of the oxide films were confirmed by a combination of powder X-ray diffraction, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy techniques. Amorphous oxide films of iron, nickel and a combination of iron and nickel demonstrated oxygen evolution reaction electrocatalytic activities commensurate with films of the same compositions prepared by widely used electrodeposition and sputtering methods. These results illuminate a potential route to amorphous oxides at scale using simple metal precursors without vacuum or heat. morphous phases of metal oxide films have attracted much attention owing to their unique properties in a range of applications that includes transistors, electrochromic windows and electrocatalysis.1−7 Indeed, the superior activity of thin amorphous oxide films containing late first-row transition metals (e.g., iron, nickel, cobalt) toward the oxygen evolution reaction (OER) relative to crystalline phases of similar compositions remains a promising strategy for energy storage and solar fuels applications.6−14 Notwithstanding, the economic viability of storing intermittent renewable energy as hydrogen fuels remains a challenge, and thus fundamental breakthroughs in catalytic activity and efficiency are still required.15,16 A key factor in identifying a commercially relevant electrocatalyst is whether it can be manufactured at scale. The ability to synthesize such films at moderate temperatures and ambient pressures is therefore desirable. The choice of precursor materials and management of byproducts also needs to be considered. It is for these reasons that the catalysts used in the chemical electrolyzer industries today are typically prepared by the thermal decomposition of metal halide compounds.17 The formation of amorphous metal coatings that are used in other

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© XXXX American Chemical Society

Received: October 16, 2017

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DOI: 10.1021/jacs.7b11064 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society Scheme 1. Overview of Photodecomposition Reaction Process Used To Furnish Amorphous Metal Films Derived from Chloride, Nitrate and Organic Precursors

Figure 2. (a) FTIR spectra showing the disappearance of nitrate and O−H signals after the photodecomposition of a spin-cast film of Fe(NO3)3 on FTO. (b) XRF spectra showing the disappearance of the chloride signal after the photodecomposition of a spin-cast film of FeCl3 on FTO. (c) XPS surveys of FeOnitrate, FeOchloride and FeOorganic (inset: expansion of the Fe2p region). (d) Powder XRD diffractrograms of FeOnitrate, FeOchloride and FeOorganic on FTO before and after annealing at 600 °C. The diffractrogram of bare FTO is included for reference. 012, 104, 110 peaks of hematite (89-0599) are labeled in panel d.

Figure 3. (a) Cyclic voltammograms of FeOnitrate, FeOchloride and FeOorganic demonstrate nominal differences in reactivity despite using different precursors. (b) Cyclic voltammograms and (c) Tafel plots (Tafel slopes are indicated) for thin films of hematite, FeOnitrate, NiOnitrate and Fe0.2Ni0.8Onitrate on FTO glass.

Figure 1. SEM imaging showing top and tilted-side views of the asprepared two layer iron oxide films derived from the different precursors.

platform for depositing amorphous oxides on lightweight, flexible plastic materials.1,3,4,24−27 AMORPHOUS IRON OXIDE FILMS. The iron-based films FeOnitrate, FeOchloride and FeOorganic were prepared by first spincasting solutions of Fe(NO3)3 (1.0 M in H2O), FeCl3 (1.0 M in H2O) or Fe(eh)3 (0.3 M in hexanes), respectively, onto fluorinedoped tin oxide (FTO) substrates. Each of these precursor films were then exposed to noncoherent 185 and 254 nm UV light at ambient conditions to form the oxide films. The samples were placed 5.5 cm away from the light source delivering a radiation intensity of ∼10 mW/cm2 (see Supporting Information). These reaction conditions yield film thicknesses that are approximately 100−150 nm/layer based on cross-sectional SEM analyses.28 Thicker films were prepared for powder XRD analysis (vide inf ra) by making films with additional layers by following the same procedure in succession. The morphologies of each of the films, measured by top-view and side-view SEM imaging, show uniform surface structures (Figure 1), but the porosity of FeOchloride appears to be higher than those of the other two films. These differences in porosities are assumed to arise from the differences in reaction rates and gaseous byproducts released when decomposing the different precursors. The cracks observed in the SEM images are ascribed to film dehydration in the sample chamber that is under vacuum.29

The rate of Fe(eh)3 decomposition can be tracked by following the diminution of the ligand vibrational modes using Fourier transform infrared (FTIR) spectroscopy.6,30 This same protocol can be applied in the case of the nitrate precursor, where the vN−O modes centered at ca. 1340 and 1640 cm−1 report on the progression of Fe(NO3)3 decomposition (Figure 2). In the case of FeCl3, the loss of chloride was instead inferred by X-ray fluorescence spectroscopy (XRF) measurements (Figures 2 and S1).28 The conversion of each of these precursor films to the oxides is largely complete within minutes, but the films were retained under UV light for 3 h to ensure complete decomposition of the films. Nitrogen and chloride were not detected by X-ray photoelectron spectroscopy (XPS) in each of the oxide films (Figure 2c). The Fe2p regions of the XPS spectra for each of the three films showed a peak at 711 eV adjacent to a satellite peak at 719 eV,31 suggesting similar Fe3+ environments for the three films. The FeOnitrate, FeOchloride and FeOorganic films are characterized herein as amorphous on the basis that no reflections corresponding to any crystalline form of iron oxide was observed by powder XRD. These experiments were performed on thicker films (∼500 nm) prepared from five successive photodecomposed layers to increase the opportunity to detect any crystalline domains, yet none were observed. The same films B

DOI: 10.1021/jacs.7b11064 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Table 1. OER Electrocatalytic Activities for Amorphous Metal Oxide Films Prepared by Different Synthetic Methods Overpotential (V) at j = 10 mA/cm2 Material

Photodecomposition of MClx

Photodecomposition of M(NO3)x

Electrodeposition11,20

Sputtering34,35

a-FeOx a-NiOx a-Fe0.2Ni0.8Ox

0.43 0.41 0.26

0.42 0.41 0.25

0.43 0.43 0.28

0.43 0.45 0.32

formed the other corresponding amorphous phases of MOnitrate and MOchloride, respectively. The rates of formation of NiOnitrate and NiOchloride were tracked by FTIR and XRF spectroscopies, respectively, to confirm that formation of the oxide films were complete within 16 h. Indeed, the precursors of all the metals showed complete decomposition within 16 h (Figures S4 and S5). This method is also effective for making mixed-metal systems. To demonstrate this point, the binary films Fe0.2Ni0.8Onitrate and Fe0.2Ni0.8Ochloride were each isolated by photodecomposing spin-cast aqueous precursor solutions of 0.2 M Fe(NO3)3 and 0.8 M Ni(NO3)2, and 0.2 M FeCl3 and 0.8 M NiCl2, respectively, for 6 h. Energy-dispersive X-ray (EDX) spectroscopy shows a uniform distribution of each metal across the substrate (Figure S4), with relative Fe to Ni ratios of 1:4 being aligned with that of the precursor solutions. ELECTROCHEMICAL CHARACTERIZATION. Amorphous metal oxide films are known to be active OER electrocatalysts.6,7,11,13,14 We therefore tested the redox behavior and OER electrocatalytic properties of the amorphous iron, nickel and iron−nickel films derived from the nitrate and chloride precursors. The cyclic voltammograms (CVs) for each the iron films display the similar featureless oxidative profiles until the onset of catalytic water oxidation at ∼0.3 V. Tafel slopes of ∼35 mV/dec were also measured to be the same across the series, thereby supporting the notion that the same amorphous metal oxide is prepared regardless of the choice of precursor salts. We then analyzed the electrochemical behaviors of other nitrate-derived thin films, NiOnitrate and Fe0.2Ni0.8Onitrate (Figure 3). The CV for NiOnitrate exhibits a large redox wave at 0.09 V and Tafel slope of 65 mV/dec, consistent with amorphous films of nickel oxide prepared by sputtering and electrodeposition.20,34,35 The mixed-metal film Fe0.2Ni0.8Onitrate, one of the most active OER catalyst compositions in alkaline media,12 was also found to produce excellent OER behavior, with a small Tafel slope of ∼30 mV/dec that yields an overpotential of 0.25 V at 10 mA/cm2. These results collectively show electrochemical behavior that is consistent with amorphous films prepared by sputtering and electrodeposition despite using cheap, inorganic starting materials (Table 1). The stabilities of the films are also reasonably high at a current density of 10 mA/cm2 (e.g., a mere 25 mV increase in electrode potential was required to maintain a constant current density for Fe0.2Ni0.8Onitrate over 12 h, as shown in Figure S6). AMORPHOUS METAL OXIDE FILMS ON PLASTIC ELECTRODES. To demonstrate how operating at ambient temperature enables a broader substrate scope, photodeposited films of FeOnitrate and Fe0.2Ni0.8Onitrate were prepared on indium tin oxide-coated polyethylene (PET-ITO). FTIR measurements confirmed the loss of the nitrate ligands within 4 h of UV irradiation (Figure 4). The OER onset potential for FeOnitrate and Fe0.2Ni0.8Onitrate films coated on the plastic substrates are anodically shifted by 90 mV relative to the measurements on glass (Figures S6) due to the much higher sheet resistance of the PET-ITO substrate (60 Ω/sq) compared with FTO glass. Given

Figure 4. (a) FTIR spectra showing the disappearance of nitrate signals during the photodecomposition of Fe(NO3)3 and Fe/Ni(NO3)2 spincoated on PET-ITO (inset: image of FeOnitrate on PET-ITO). (b) Cyclic voltammograms and (c) Tafel pots for thin films of FeOnitrate (red) and Fe0.2Ni0.8Onitrate (blue) on PET-ITO.

were then annealed at 600 °C for 60 min, which produced reflections corresponding to hematite (α-Fe2O3). The formation of FeOorganic is known to occur through a lightdriven ligand-to-metal charge-transfer (LMCT) process that homolytically cleaves the metal−ligand bond to form an organic radical species that proceeds through a series of reactions to ultimately generate volatile carbon species and CO2. To determine the mechanism for the decomposition of the nitrate and chloride precursors, we first ruled out thermally driven decomposition during photolysis being responsible for oxide formation: Heating films of Fe(NO3)3 and FeCl3 in an oven at 60 °C for 24 h did not decompose FeOnitrate or FeOchloride. We also successfully decomposed Fe(NO3)3 and FeCl3 using a UV light source that precludes ozone formation to confirm that ozone is not required for decomposition; however, the rates of precursor decomposition were accelerated 3-fold in the presence of ozone to confirm that ozone may not be innocent during the reaction process. Photodecomposition of Fe(NO3)3 in a nitrogen-filled purge box devoid of oxygen showed slower but still complete decomposition of the precursor (Figure S2), but FeCl3 did not convert to the oxide after 14 days of photolysis. We therefore conjecture that Fe(NO3)3 follows reaction chemistry known to the environmental science community where metal nitrates in aqueous solution under UV light form OH• radicals and NO2 gas.32,33 This chemistry appears to offer a novel way of accessing solid-state metal oxide films from aqueous solutions that had not been previously contemplated. We currently assume that decomposition of FeCl3 follows a pathway akin to the metal−organic complexes, where excitation into the absorption band at 380 nm (Figure S3) drives homolytic bond cleavage to mediate metal oxide film formation. EXTENDING THE SYNTHESIS TO ADDITIONAL METALS. The same general photodecomposition procedure applied to M(NO3)x or MClx, where M = Co, Ni, Cu, or Zn, successfully C

DOI: 10.1021/jacs.7b11064 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

(9) Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das, C.; Tallarida, M.; Schmeisser, D.; Strasser, P.; Driess, M. J. Am. Chem. Soc. 2014, 136, 17530−17536. (10) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977−16987. (11) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2015, 137, 4347−4357. (12) Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Trudel, S.; Berlinguette, C. P. J. Am. Chem. Soc. 2013, 135, 11580−11586. (13) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. J. Am. Chem. Soc. 2015, 137, 1305−1313. (14) Risch, M.; Ringleb, F.; Kohlhoff, M.; Bogdanoff, P.; Chernev, P.; Zaharieva, I.; Dau, H. Energy Environ. Sci. 2015, 8, 661−674. (15) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (16) Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; Moore, T. A.; Moser, C. C.; Nocera, D. G.; Nozik, A. J.; Ort, D. R.; Parson, W. W.; Prince, R. C.; Sayre, R. T. Science 2011, 332, 805− 809. (17) Fierro, S.; Kapałka, A.; Comninellis, C. Electrochem. Commun. 2010, 12, 172−174. (18) Wen, R.-T.; Granqvist, C. G.; Niklasson, G. A. Nat. Mater. 2015, 14, 996−1001. (19) Gillaspie, D. T.; Tenent, R. C.; Dillon, A. C. J. Mater. Chem. 2010, 20, 9585−9592. (20) Louie, M. W.; Bell, A. T. J. Am. Chem. Soc. 2013, 135, 12329− 12337. (21) Salvatore, D. A.; Dettelbach, K. E.; Hudkins, J. R.; Berlinguette, C. P. Sci. Adv. 2015, 1, e1400215. (22) Larrazábal, G. O.; Martín, A. J.; Mitchell, S.; Hauert, R.; PérezRamírez, J. ACS Catal. 2016, 6, 6265−6274. (23) Speck, F. D.; Dettelbach, K. E.; Sherbo, R. S.; Salvatore, D. A.; Huang, A.; Berlinguette, C. P. Chem. 2017, 2, 590−597. (24) Kim, M.-G.; Kanatzidis, M. G.; Facchetti, A.; Marks, T. J. Nat. Mater. 2011, 10, 382−388. (25) Yu, X.; Smith, J.; Zhou, N.; Zeng, L.; Guo, P.; Xia, Y.; Alvarez, A.; Aghion, S.; Lin, H.; Yu, J.; Chang, R. P. H.; Bedzyk, M. J.; Ferragut, R.; Marks, T. J.; Facchetti, A. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 3217− 3222. (26) Hagendorfer, H.; Lienau, K.; Nishiwaki, S.; Fella, C. M.; Kranz, L.; Uhl, A. R.; Jaeger, D.; Luo, L.; Gretener, C.; Buecheler, S.; Romanyuk, Y. E.; Tiwari, A. N. Adv. Mater. 2014, 26, 632−636. (27) Nadarajah, A.; Wu, M. Z. B.; Archila, K.; Kast, M. G.; Smith, A. M.; Chiang, T. H.; Keszler, D. A.; Wager, J. F.; Boettcher, S. W. Chem. Mater. 2015, 27, 5587−5596. (28) Dettelbach, K. E.; Kolbeck, M.; Huang, A.; He, J.; Berlinguette, C. P. Chem. Mater. 2017, 29, 7272−7277. (29) Esswein, A. J.; Surendranath, Y.; Reece, S. Y.; Nocera, D. G. Energy Environ. Sci. 2011, 4, 499−504. (30) Miller, F. A.; Wilkins, C. H. Anal. Chem. 1952, 24, 1253−1294. (31) Fujii, T.; De Groot, F.; Sawatzky, G. A.; Voogt, F. C.; Hibma, T.; Okada, K. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 3195− 3202. (32) Jacobi, H.-W.; Annor, T.; Quansah, E. J. Photochem. Photobiol., A 2006, 179, 330−338. (33) Mack, J.; Bolton, J. R. J. Photochem. Photobiol., A 1999, 128, 1−13. (34) Klaus, S.; Louie, M. W.; Trotochaud, L.; Bell, A. T. J. Phys. Chem. C 2015, 119, 18303−18316. (35) Zhang, D.; Meng, L.; Shi, J.; Wang, N.; Liu, S.; Li, C. Electrochim. Acta 2015, 169, 402−408.

the fewer options available for depositing amorphous oxides on plastics,3,24 the successful decomposition of these metal compounds by UV light is a promising option for preparing materials for applications that extend beyond OER applications. We demonstrate that exposure of films of halide and nitrate compounds of Fe, Co, Ni, Cu and Zn to UV light at ambient temperatures and atmospheric pressures yields uniform films of amorphous metal oxides. Mixed-metal-oxide films with a uniform distribution of each metal can also be produced by this method. The complete decomposition of each of the metal nitrate and chloride compounds was confirmed by FTIR or XRF in under 16 h. The OER activities of select films were measured to validate that the metal oxide films derived from nitrate and chloride compounds display performances commensurate with those produced by the more widely used electrodeposition and sputtering methods. The low cost of our precursors and the scalable nature of photodecomposition offers an alternative method for manufacturing amorphous metal oxides on rigid and plastic substrates at scale.

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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/jacs.7b11064. Description of materials and methods, XRF data of metal chloride decomposition process, UV−vis spectra of iron oxide, FTIR of Co, Ni, Cu, Zn and binary Fe−Ni samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Tengfei Li: 0000-0002-8378-7130 Curtis P. Berlinguette: 0000-0001-6875-849X 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 (RGPIN 337345-13), Canadian Foundation for Innovation (229288), Canadian Institute for Advanced Research (BSE-BERL-162173) and Canada Research Chairs for financial support. K.E.D. was supported by an NERC PGS D scholarship. A.H. and T.L. were supported by the University of British Columbia with a Four Year Doctoral Fellowship (4YF).

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REFERENCES

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DOI: 10.1021/jacs.7b11064 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX