Tarnishing Silver Metal into Mithrene - Journal of the American

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Tarnishing Silver Metal into Mithrene Brittany Trang, Matthew Yeung, Derek C. Popple, Elyse A. Schriber, Michael A. Brady, Tevye R. Kuykendall, and J. Nathan Hohman J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08878 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Journal of the American Chemical Society 1

Tarnishing Silver Metal into Mithrene Brittany Trang,1 Matthew Yeung,1 Derek C. Popple,1,2 Elyse A. Schriber,1 Michael A. Brady,1,3 Tevye R. Kuykendall,1 and J. Nathan Hohman*1 1

The Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California, 94720, United States

2

Department of Chemistry, University of California, Berkeley, Berkeley, California, 94720, United States

3

Advanced Light Source, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California, 94720, United States

ABSTRACT: Silver metal exposed to the atmosphere corrodes and becomes tarnished as a result of oxidation and precipitation of the metal as insoluble salt. Tarnish has so poor a reputation that the word itself connotes corruption and disrespectability; however, tarnishing is a facile synthetic approach for preparing thin metal-sulfide films on silver or copper metal that might be exploited to prepare more elaborate materials with desirable optoelectronic properties. In this work, we prepare luminescent semiconducting thin films of mithrene, a metal-organic chalcogenolate assembly, by replacing the tarnish-causing atmospheric sulfur source with diphenyl diselenide. Mithrene, or silver benzeneselenolate [AgSePh]∞, is a crystalline solid that contains both an organic supramolecular phase and a two-dimensional inorganic coordination polymer phase. This compound gradually accumulates as the sole product of silver metal corrosion. The chemical reaction is carried out on metallic silver thin films and yields crystalline films with thicknesses ranging from 5 – 100 nm. We use the large-area films ( > 6 cm2) afforded by this method to measure the optical properties of this compound. The mild-temperature, wafer-scale processing of hybrid chalcogenolate thin films may prove useful in the application of hybrid organic-inorganic materials in semiconductor devices and hierarchical architectures.

Introduction Hybrid organic-inorganic coordination polymers containing metal coordination centers and organic sidegroups have drawn considerable interest as highly configurable materials.1-8 Such compounds having metal chalcogenolate phases separated by organic ligands, which we refer to as metal-organic chalcogenolate assemblies (MOCHAs), are attractive for their ability to form three-dimensional (3D) ensembles of nanostructures having zero-, one-, or two-dimensional (0D, 1D, 2D) inorganic connectivities.9-16 Such low-dimensional MOCHA phases of group 11 metals (Cu, Ag, Au) are notable for their semiconducting properties, luminescence, thermochemistry, and mechanochemistry.17-27 A similar class of materials that has attracted a lot of attention are the 2D materials, like transition metal dichalcogenides (TMDs),28-32 which possess strong light-matter interactions and have been targeted for applications including photovoltaics, fast transistors, energy storage, and sensing.33-36 A notable distinction between TMDs and MOCHAs, however, is that TMDs are highly sensitive to coupling effects between the 2D layers and their substrate, or between individual 2D layers with each other when present as multilayers. The organic moieties incorporated in the threedimensional (3D) coordination polymer compounds isolate low-dimensional inorganic nanostructures from one another physically and electronically.18 Layered phases in this family of compounds are reminiscent of multi-quantum wells.37-39 Recently, we reported the optical properties of silver benzeneselenolate, [AgSePh]∞, a layered, self-assembling MOCHA we refer to informally as mithrene.13, 20 The compound exhibits robust blue photoluminescence in its as-

synthesized bulk crystal, requiring no exfoliation. The synthetic approaches for preparing [AgSePh]∞ and related MOCHAs have generally been solution-phase bulk crystal growth or crystal growth at biphasic liquid-liquid interfaces.18-20, 26 The thin-film forms of TMDs that are more compatible with device applications are usually grown with gas-phase techniques including chemical vapor deposition (CVD),40-45 atomic-layer deposition (ALD),46-48 and direct conversion of metal oxides.49 Molecular layer deposition (MLD) has been employed to produce similar periodic hybrid materials.50 It is reasonable to conclude that methods like these might be available to prepare thin-film MOCHAs; however, the high temperatures involved in creating TMD films are generally incompatible with the organic ligands present in hybrid materials. Our objective was to prepare thin-film forms of the [AgSePh]∞ MOCHA. For inspiration, we considered a mild, well-known chemical reaction that yields metal chalcogenide deposits on copper and silver - the formation of tarnish. The term ‘tarnish’ is generic for all of the chemical corrosion products of many metals, but is most commonly associated with acanthite (Ag2S), the black, insoluble sulfide of silver.51 Known since antiquity, this reaction and the product’s undesirable properties are generally considered a nuisance. Because tarnish and MOCHAs are both ultimately chalcogenide salts, we wondered whether the same chemical pathways might enable preparation of hybrid silver selenolate coordination polymers. Notably, there is some basis for this idea in the literature, where organosulfur vapors were employed to prepare MOCHAs. Allen and coworkers reported a direct conver-

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sion of silver nanoparticles to multilayered silver nalkanethiolate crystals using an organic thiol, establishing a strong basis for this idea.23-24 Earlier, Sinclair reported that exposing silver to methyl and ethyl disulfides results in a solid, tarnish-like deposit.52 Here, we report convenient, solvent-free reactions between silver metal and diphenyl diselenide (DPSe) at mild temperatures that form multilayered, luminescent, crystalline mithrene, henceforth [AgSePh]∞. We note that the reaction between gas-phase DPSe and silver metal that results in the subsequent deposition of the MOCHA appears to be corrosion-like. To test this corrosion hypothesis, we examine water’s role and show it to be critically important to the MOCHA formation. The structure of [AgSePh]∞ and the progression of the reaction are characterized by synchrotron grazingincidence wide-angle X-ray scattering (GIWAXS). The role of the reagents’ oxidation states reveals several chemical mechanisms that form the corresponding MOCHA. We believe that the mild-temperature, large-area synthesis of hybrid chalcogenide thin films shown here is an important step towards incorporation of hybrid materials in devices or hierarchical architectures.

Results and Discussion Silver has an affinity for organosulfur53-55 and organoselenium56-61 reagents, both of which rapidly adsorb and form self-assembled monolayers (SAMs) on the reactive metal surface. To begin, we compare and contrast the known structures of silver benzeneselenolate SAMs62 with silver benzeneselenolate MOCHA.13 Figure 1 schematically depicts the idealized structure and organization of both of these related compounds, shown side by side to give a sense of scale. Like SAMs of the better-known thiols on gold, the PhSe-Ag SAMs on silver form spontaneously from either PhSeH or DPSe reagents.62 Zharnikov and coworkers suggest oxidative addition to the diselenide bond as the critical mechanism.62 In both structures, the coordination bonds between the chalcogen and silver enforce the intermolecular spacing of the organic assembly. In the SAM, this assembly is restricted to a single layer (Fig. 1a), while the hybrid salt manifests as a multilayered repeat of trilayer organic-inorganic-organic 2D subunits (Fig. 1b).

Figure 1: a) Idealized representations of a silver benzenethiolate selfassembled monolayer on Ag(100) and b) mithrene, [AgSePh]∞. c) The two structures compared in cross section. Blue/grey = Ag, black = C, white = H, orange = Se, and clear grey spheres represent the van der Waals volume of the organic moiety.

Initially, we attempted to form [AgSePh]∞ by heating a silver substrate for three days at 80 °C in or above ethanolic solutions of DPSe in a glass vial, but no reaction was noted. Leaving the silver film in a solution of DPSe at room temperature over a period of several days led a yellowing of the silver metal, attributed to the gradual buildup of a mixture of insoluble silver benzeneselenolate species. A low yield of crystalline [AgSePh]∞ was observed—its presence confirmed by its characteristic blue luminescence.20 This reaction also yielded an undesirable amorphous, non-luminescent metal-organic coordination polymer. These results are shown in supplementary Figures S1-S2. Eliminating the solvent and baking the silver film along with a quantity of the solid precursor DPSe resulted in a dramatic color change from the reflective silver metal to a deep golden yellow. Figure 2 presents a schematic overview of the formation of hybrid coordination polymer thin films by tarnishing. To avoid splattering of the solid reagent, we devised a simple “developing chamber” using an oven-safe lidded glass jar (v.i. Figs. 4a-c). The [AgSePh]∞ thin film that forms dramatically alters the appearance of the silver. As the film thickens, it takes on a green to yellow-gold color. Thicker films have a matte finish because of the polycrystalline film’s low reflectivity in the optical range.


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Figure 2: Schematic overview of hybrid tarnish reaction wherein silver is converted to [AgSePh]∞ in a single procedural step. a) Silver metal attracts a thin layer of water from the atmosphere. b) A vapor of diphenyl diselenide (DPSe) tarnishes the metal into a film of crystalline [AgSePh]∞. c) A proposed redox pathway involving the oxidation of silver metal, reduction of DPSe, and subsequent precipitation of [AgSePh]∞.

Scanning electron microscopy (SEM) and confocal photoluminescence (PL) microscopy (Fig. 3) show [AgSePh]∞ grown by a solution-phase method at an immiscible biphasic liquid-liquid interface20 and by the tarnish method. The same square-like morphology of the individual crystallites observed in biphasic-solution-grown [AgSePh]∞ (Fig. 3a) is observed in the [AgSePh]∞ film grown by the tarnish method (Fig. 3c). PL emission maps comparing the isolated crystallites and the tarnished film are compared in Figure 3b and 3d. The optical properties of [AgSePh]∞ have been previously shown to be consistent across many morphologies.20 The individual crystals shown in Figure 3b exhibit fairly uniform emission profiles, as do the tarnished films shown in Figure 3c, which we found to exhibit remarkable optical uniformity over large areas. The PL emission spectra (Fig. 3e) for solution-grown [AgSePh]∞ crystals and the tarnished films both show a single peak centered at 467 nm and are nearly identical. Confirmation of the crystal structure was achieved using synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS). The high luminosity of the synchrotron X-ray source enabled us to obtain considerable detail regarding the relative orientation of the crystallites in the films despite a large deviation from a bulk reference sample. Figure 3f shows GIWAXS data obtained on a representative [AgSePh]∞ thin film prepared by the tarnish method, a bulk powder obtained using a solution phase method, and a calculated pattern from the reference single crystal structure.13 The monoclinic [AgSePh]∞ exhibits a large number of peaks in its XRD spectrum. Most prominent are reflections related to the basal plane, corresponding to the (002), (004), and (006) reflections. The reflections at higher values of q (>15 nm-1) appear at lower intensity and with broadening in the thin film sample relative to the bulk powder, a consequence of the high degree of preferred orientation strongly favoring reflections associated with the basal plane.20

Figure 3: a, b) SEM and confocal emission maps of [AgSePh]∞ crystallites reveal uniform blue photoluminescence. c,d) SEM and confocal emission maps of an [AgSePh]∞ surface. e) The continuous polycrystalline film of [AgSePh]∞ exhibits uniform emission intensity over a large area. f) Synchrotron grazing-incidence wide-angle X-ray scattering results reveal a match between crystal structure for [AgSePh]∞ prepared via bulk and tarnished film approaches. Smoothing at high q in the tarnished film is attributed to preferred orientation of the crystals to the substrate. The predicted theoretical pattern is simulated from the single-crystal structure.

Water’s Role in Silver Tarnishing by Diphenyl Diselenide Silver, as with other metals, attracts a thin layer of water from the atmosphere that serves as an electrochemical electrolyte and enables corrosion under atmospheric conditions.63-65 In the case of silver, the adsorbed water plays a role in tarnish formation by absorbing atmospheric sulfur carbonyl (0.5 ppb)



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that first equilibrates to hydrogen sulfide and then precipitates with silver ion as Ag2S.66 The liquid water also serves a secondary role as it provides a mobile phase by which chemical species can be transported to locations remote from where they were formed. To test the role of water on the formation of [AgSePh]∞, we examined films prepared in both dry and humid conditions. Samples consisted of silicon wafers with 100 nm of silver metal deposited by thermal evaporation. Figures 4a-c show our experimental setup. A dry atmosphere was achieved by adding a calcium sulfate desiccant (Drierite) to the reaction vessel (Fig. 4a). Conversely, the humidity was increased by adding a small quantity (0.5 mL) of water to a small vial placed in the vessel (Fig. 4c). The ambient-air reaction was performed with the humidity available in room air (Fig. 4b). The vessels were maintained at 80 °C for 72 hours. A control sample was heated in a reaction vessel with ambient air but without DPSe. Figure 4d shows the as-prepared series of samples and Figure 4e shows the reflectivity of samples measured using a UV-vis absorption spectrometer in diffuse reflectance mode. The tarnished samples are antireflective in the visible range, and exhibit a peak absorption at 434 nm, consistent with literature reports for [AgSePh]∞.13, 20 The desiccated and control samples were nearly indistinguishable by optical reflectivity, demonstrating that scavenging water from the reaction vessel inactivates the reaction. The higher optical density of the saturated-water sample compared to the room-air experiments suggest that the rate of formation depends on the amount of humidity in the environment.

Figure 4: Developing chambers for a tarnishing experiment testing the role of humidity on the tarnishing reaction, using a) a dry vessel, accomplished by addition of Drierite, b) a vessel with room air, and c) a vessel saturated with water. d) Photograph showing as-prepared samples. e) UV-vis diffuse reflectance spectra.

Efforts to produce exceptionally thin films on silicon wafers (1 – 10 nm Ag on Si(100)) showed no reaction. We attempted to enrich the humidity of the environment and did observe the formation of product crystals; however, we also noted considerable etching of the silicon wafer. This unexpected result persisted even in the absence of the DPSe. Enhancement of silicon etching by silver nanoparticles has been previously examined in HF/H2O2 etching solutions,67-68 although we are not aware of any reports that silver can initiate etching of silicon by pure water at elevated temperatures (80 °C). We attribute the inactivation of the tarnishing to consumption of of water by this Ag/Si reaction. We saw no such chemical interference on silicon passivated by a thick thermal oxide, or on other substrates including glass, quartz, or mica. These results are found in the supporting information, Figs. S5,S6.

Progression of the Tarnishing Reaction The interactions between silver and sulfur have been recognized since antiquity and are technologically relevant for preservation, chemistry, medicine, and engineering applications.69-78 Acanthite tarnish gradually accumulates on evaporated silver metal at an approximate rate of about half a nanometer per day.79 Bennet found that the reaction is fastest on


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rough surfaces devoid of Ag2S, and observed a tendency for silver sulfide to deposit in clumps, rather than in a continuous passivating film.79 Sharma reported that the relatively fast initial rate diminished to a steady rate of ~30-35 Å/month over a one year timescale.80 In our studies of [AgSePh]∞ formation on thermally evaporated silver, we found a similar tendency for crystals to nucleate and to grow without fully passivating the metal until the film became quite thick. Figure 5 shows photographs and SEM images of [AgSePh]∞ films formed at 80°C with added moisture on 100-nm-thick films of thermally evaporated silver metal. The macroscopic coloration of the thin films (Fig. 5a) varies as a function of thickness. The color change is apparent after 24 hours and the sample takes on greenish notes as the silver layer thins and the hybrid coordination polymer thickens. SEM micrographs were collected after 1, 3, 4 and 8 days to show the progression of the reaction. The crystal habit of [AgSePh]∞ is also apparent even after 24 hours, where the deposits resting on the rough silver substrate already reveal the faceting of the crystal. The

segregation of products and the uneven coverage is consistent with the formation of silver sulfide films on similarly thermally evaporated silver metal.79 The crystalline layer continues to thicken as the experiment progresses, and after eight days, a polycrystalline film having well-defined step edges is observed. Step edges are observed by SEM in the mature samples at high magnification. Cross-sectional analysis reveals a polycrystalline layer that is approximately 255-nm-thick (Figs. 5b, 5c). Focusing the electron beam on the edge of the sample induced film recession from the edge and exposed the underlying silver (Fig. 5b). A preferred orientation of the crystalline product is evident in the SEM images, where the product is largely coplanar to the substrate. Plastic deformation and deviation from this substrate coplanarity, as well as some vertical orientation of crystallites, is observed as crystals enlarge and the film thickens over time. This is especially evident in the cross-sectional image in Fig. 5c.

Figure 5: a) Progression of the tarnishing reaction of silver into [AgSePh]∞ over 8 days (top). SEM micrographs collected after 1, 3, 4 and 8 days (bottom). [AgSePh]∞ is accented yellow in the SEM images using false color to distinguish the coordination polymer from the metal. b) SEM image of the cross section after 8 days reveals the orientation of individual crystallites. c) SEM cross section showing film thickness and void spaces present in films grown under these conditions.

The evolution of the films and their crystallinity can be tracked by GIWAXS. The plot of intensity versus wavevector in Figs. 6a and 6b show the effect of film thickness as the reaction progresses. Thicker films of the MOCHA correspond with more intense peaks, especially for the well-resolved peaks associated with the basal plane. Intensity of the basal plane reflections (002), (004), and (006) increases with thickness. The feature associated with metallic silver around 27 nm1 diminishes over time. The intensity-versus-scatteringwavevector plots also show that [AgSePh]∞ becomes more crystalline as the growth time increases. A change in ordering is evident in the relative intensity of the (002) reflection, centered at a scattering wavevector of 8.78 nm-1. The (002) reflection grows in intensity, and the peak maximum tends to shift

slightly to larger q vectors over the course of the reaction. Since these curves were normalized for exposure time in the GIWAXS experiment, the backgrounds are nearly constant, and thus the ratio of peak maximum intensities directly correlates with the degree of crystallinity for the coordination polymer thin films. The small shift can originate from an increase in crystalline order over long growth times, broadening due to secondary X-ray reflection off the substrate, or the existence of a closely related second crystallographic phase. Representative 2D plots of the GIWAXS results are shown in days 1, 3, and 7 in Figures 6c-e. In the thinnest samples, prominent Debye-Scherrer rings associated with the silver metal are observed, corresponding to the thermally evaporated precursor film. This feature is most prominent at 1 day (Fig. 6c). As the [AgSePh]∞ film thickens and silver metal is consumed, this feature diminishes (Fig. 6d) until it is no longer observed (Fig.



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6e). Out-of-plane reflections slightly increase in intensity with time, consistent with the observation of crystal deformation in textured, mature [AgSePh]∞ films.

Figure 6. a, b) GIWAXS linecuts of the progression of the [AgSePh]∞ tarnishing reaction. b) Magnified region associated with the basal plane reflections. c-e) 2D GIWAXS images for 1 day, 3 day, and 7 day tarnishing experiments, respectively.

glass were estimated by assuming stoichiometric conversion and product coplanarity and accounting for the volumetric expansion of the material, from the 10.5 g/cm3 for silver metal to 2.7 g/cm3 for [AgSePh]∞. The GIWAXS spectra shown in Figure 7d, e confirm the presence of the [AgSePh]∞ phase with no evidence for residual silver particles in the converted films. Because the films are exceptionally thin, only the reflections associated with the basal plane are resolved. Despite the exceptionally low signal originating from the thinnest sample, the (002) reflection is still observed, although the intensity shifts to a lower value of q suggesting stacking disorder in these exceptionally thin films. Although unresolved, the background at higher q (q > 15 nm-1) is elevated, correlating with the position of reflections in the bulk powder. Together, these results are consistent with a thin or sparse crystalline layer that is predominantly oriented face-on to the substrate, with some contribution of out-of-plane reflections attributed to non-planar crystallites. The weak scattering in this region is attributed to the same high-q reflections observed in thicker films and in isotropic powders samples (see Figs. 3e and 6a).20

The orientation of tarnished films deviate considerably from the well-mixed mithrene powders we have previously studied.20 The films’ preference for a face-on orientation is evident both from the images in Fig. 5 and the relative intensities of reflections associated with the basal plane and higher angle features in the GIWAXS results in Fig. 6. The relative intensity of the high-q peaks can be used as a proxy for coverage and crystal orientation.20 In tarnished films, higher angle reflections are diminished relative to the powder, but are still observed. The considerable volumetric expansion of the MOCHA film as it forms from the higher density metal is considered a likely contributing factor to driving the polymer crystals out of alignment as they grow against one another.

Thin Films on Glass Substrates for Optical Characterization Thin films of silver (1 – 10 nm) deposited on glass cover slips were employed as the limiting reagent for the gas phase transformation into [AgSePh]∞. Photographs of the converted [AgSePh]∞ coverslips are shown in Figure 7a. We observe that the optical density of the thin films increases with thickness of deposited silver. The absorption spectra of the films before and after the tarnish reaction are shown in Figures 7b and 7c. Silver metal has a broad absorption in the optical range, which is consistent with a large distribution of individual particle sizes. The absorption redshift with increasing silver thickness is attributed to the larger average size of silver particles (Fig 7b). After 3 days of tarnishing, the broad optical absorption observed for the metal is absent, replaced by the characteristic absorption spectrum of [AgSePh]∞, shown in Figure 7c. The intensity of the absorption feature at 434 nm was plotted against number of [AgSePh]∞ layers (Fig. 7c, inset) and shown to have a linear relationship (with r2 = 0.998), supporting the presumption that the silver is fully converted in all five examples. As with thicker films, these samples were all uniformly emissive at 467 nm (see Fig. S3), and no such emission was observed on untarnished samples. Thicknesses of the films on

Figure 7: a) Glass cover slips with silver films converted to [AgSePh]∞. Thicknesses in nm correspond to the deposited thickness of the unreacted silver films. Number of layers are estimated assuming stoichiometric conversion of silver metal. b) Absorption spectra of the unreacted silver metal with varying thicknesses. c) Absorption spectra of the converted [AgSePh]∞ films. Inset: absorption intensity at 434 nm as a function of number of layers. d) GIWAXS spectra of the converted [AgSePh]∞ films. e) GIWAXS spectra from d) scaled to highlight basal plane reflections (002), (004), and (006). The differences in relative background are attributed to sample alignment at the synchrotron beamline.


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Role of Reagent Oxidation State on the Mechanism of Coordination Polymer Formation Silver has a complex surface chemistry in the presence of water, with silver metal, silver (I) oxide, and Ag+ all coexisting.78 We now consider the role of reagent oxidation state on the formation of [AgSePh]∞ and present experiments designed to disentangle the competing or complementary reactions occurring as the MOCHA forms. Two distinct chemical pathways for the formation of the same product are shown: a redox reaction between silver metal and DPSe and an acid-base reaction between silver oxide and PhSeH. Relevant reactions for both pathways are presented, and the role of the formation of selenolate SAMs on silver is considered. The Tarnishing Redox Reaction The role of oxygen on the progression of the chemical reaction was tested by carrying out the experiment in a tube furnace with and without water, using an argon carrier gas. As long as water was present, [AgSePh]∞ formed readily. Oxygen was not found to play a direct role in the chemical reaction. We therefore assume an oxidation-reduction reaction between silver metal and DPSe, described by equations 1-3: (1) Ag(s) Ag+(aq) + e(2) PhSeSePh(g) + 2e- 2PhSe-(aq) (3) PhSe-(aq) + Ag+(aq) AgSePh(aq) These reactions are assumed to be essential for the nucleation of the crystalline polymeric product. For subsequent crystal growth, there are several possibilities, including alternating deposition of the ionic and molecular species at the reactive interfaces of the crystal, or deposition of small clusters of AgSePh that subsequently integrate into the crystal.81 In either case, the reaction is described by the balanced equation 4: (4) 2Ag(s) + PhSeSePh(g) 2[AgSePh](s) We note that there is no buildup of corrosion byproducts (e.g. change in pH) that one observes in the formation of tarnishes from atmospheric sulfur sources. Although molecular oxygen was not found to play an essential role in the formation of the MOCHA, it may play a secondary role involving the oxidation of the silver metal.78 The Role of Silver (I) Oxide Silver (I) oxide at surfaces is unstable in aqueous solutions, and its dissolution would contribute to an increase in Ag(OH)2- concentration in an adherent aqueous layer.82 The dissolution of silver metal in solution is well-known.78, 83 The charge and mass-transport across defects in silver oxide layers can lead to the formation of a nanoscale corrosion cell, a localized center of corrosion activity in partially passivated surfaces.84 Silver surfaces treated by a UVozone treatment are readily oxidized, affording additional passivation relative to a native oxide on freshly deposited metal. We found that the presence of thick silver oxide, with its +1 silver oxidation state, had a considerable passivating effect on the progression of the tarnish reaction. Thick layers largely inhibited the reaction with DPSe, while thinner layers

impacted coverage, thickness, and crystal size of the coordination polymer product. Substitution of DPSe with PhSeH The PhSeH reagent provides an opportunity to test the role of selenium oxidation state on the reaction. Treating silver metal with PhSeH in oxygenfree conditions did not lead to the formation of the MOCHA, regardless of the presence of water. In the presence of oxygen, this reaction produced a complex mixture of products. A dark deposit formed on the silver wafer, with color variation ranging from purple to brown. Additionally, the silver was heavily corroded, delaminating from the silicon substrate. It is important to note that PhSeH is a weakly acidic reagent with a pkA of 5.9. This acidity likely contributes to the aggressive corrosion of the silver metal noted in this pairing.85-86 Furthermore, PhSeH is also unstable at the conditions of the reaction, as it is susceptible to air oxidation to diphenyl diselenide. Reactions performed in this manner occur in the presence of all oxidation states and can be expected to be rather poorly controlled.87 Substitution of Silver (0) with Silver (I) Oxide We next considered reactions between silver (I) oxide powder and both the DPSe and PhSeH reagents. Unsurprisingly, exposing the silver (I) oxide to DPSe resulted in no reaction; both reagents are in the most-oxidized of their common forms. On the other hand, silver oxide is well-known to react with inorganic strong acids (e.g. HCl) to produce the insoluble silver halide and water.88 Benzeneselenol contains active protons that would be expected to react with silver oxide. Indeed, a simple combination of silver oxide powder and an excess of PhSeH reacted vigorously at 80 °C, producing a yellow solid after less than five minutes of reaction time, consistent with an acid-base reaction. The resulting product generally had a yellow-black appearance, as the product contained both the MOCHA product and residual silver oxide. Adding isopropyl alcohol to this product and mixing overnight enabled the reaction to go to completion, yielding a bulk suspension of the yellow MOCHA polymer. The resulting nanocrystalline product matches [AgSePh]∞ by its XRD spectrum and exhibits the same optical transitions as the material prepared under tarnishing conditions. This second reaction pathway is consistent with an acidbase reaction, which is shown in eq. 5. (5) Ag2O(s) + 2PhSeH(l) 2[AgSePh](s) + H2O(l) Both reactions described by equations 4 and 5 yielded the same coordination polymer product, [AgSePh]∞.13, 20 Although the end product is the same, the oxidation state of the reagent plays an important role in the mechanistic pathway converting silver and its oxide into hybrid chalcogenolates. Because silver metal preferentially reacts with DPSe over PhSeH, and vice versa for silver oxide, more elaborate control over the reactivity may be afforded by systematic variation of the silver oxide thickness and defect density. It is reasonable to presume that alternative pathways including, for instance, cleavage of the diselenide akin to that observed in dithiolate reactions, might be at play in complex mixtures of all of the reagents.89 The results of the study of reagent oxidation states are collected in Table 1.



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Table 1. Summary of reagent oxidation states and outcomes. Organochalcogen

Silver Source

Outcome

DPSe(g) DPSe(g) PhSeH(g) PhSeH(l)

Ag Ag2O Ag Ag2O

[AgSePh]∞ No Reaction Delamination [AgSePh]∞

Relationship Between MOCHAs and SAMs Self-assembled monolayers on metals prepared from chalcogenols are generally indistinguishable from those prepared from dichalcogenides, despite the difference in the oxidation state.90 However, the experiments described above demonstrate the importance of ligand selection for the preparation of the MOCHA. Because SAMs rapidly form on metals even in low concentrations of precursor, it is likely that the majority of sites on the silver surface are functionalized by silver benzeneselenolate ligands. SAMs have previously been shown to be responsible for inducing surface reconstructions as they form, the notable example being the well-understood formation of adatom complexes in gold organothiolates SAMs.91-94 We postulate such a complex may occur in the silver selenolate system, with an oxidative addition of silver to DPSe taking the form of eq. 6: (6) PhSeSePh(g) + Ag0(s) 2PhSe-Ag+·Ag0(s) The formation of such a surface-bound complex would explain charge transfer from the metallic silver to reduce the diselenide bond in a single adsorption step, which is consistent with our observations described above. If such a complex were suitably weakly bound, it could detach from the substrate and become solvated by the adherent electrolyte. This idea is consistent with the poor passivation afforded by acanthite tarnishes. A complementary MOCHA-forming mechanism for the PhSeH, which would require reductive elimination of dihydrogen, was not observed. This suggests that a concerted oxidation of the metal by the DPSe is critical for the preparation of the MOCHA. This mechanism would suggest a potential route involving an adsorbed organoselenolate species as an intermediate for the preparation of the MOCHA. Diphenyl disulfide did not react reliably under the tarnishing conditions to produce the silver benzenethiolate. This observation is consistent with observations in the field of SAM research that n-alkylselenolates have higher affinities and stronger interactions with metal surfaces than thiolates.56-61 Considering the passivating effect of organochalcogenolate SAMs, the fact that the [AgSePh] ∞ MOCHA formed at all came as a welcome surprise. Self-assembled monolayer formation is generally considered self-limiting; once all available surface binding sites have been occupied by the reactive head group (the chalcogen atom), the reaction stops, passivating the surface against further reaction. So uniform is this chemical barrier presented by a SAM that they are used to protect surfaces and reduce the rate of etching and corrosion.55, 95-106 The preference for MOCHA formation from DPSe over DPS may be related to the increased strength of metal-Se interactions, to their more dynamic binding configurations, and to their higher stability relative to sulfur.56, 59, 107-109

The metal substrate is another important factor to consider when thinking about the interplay between adsorbed layers and corrosion products. Our study is mostly concerned with rough, thermally evaporated silver substrates, whereas SAMs are stereotypically thought of as well-ordered molecular films on Ag(111). Bennet noted that the Ag(111) facet tarnished so slowly that it might be considered inactive in this context.79 Further study into the atomistic detail of this reaction at various interfaces presents an interesting route for future investigation.

CONCLUSIONS Preparing a thin-film silver sulfide coating on silver metal is among the simplest synthetic procedures, requiring little more than placing a piece of silver on a table and embarking on an extended vacation. Our route to hybrid chalcogenolates retains this intrinsic simplicity, requiring minimal sample preparation or synthetic expertise. The multilayered luminescent material mithrene, or silver benzeneselenolate [AgSePh]∞, was prepared directly from silver metal and DPSe using a convenient, solvent-free approach. These conditions normally produce SAMs, but here we showed that the reaction can continue to the layered hybrid chalcogenolate in the absence of organic solvents. This facile reaction proceeds at mild temperatures without the need for high or low pressures or special atmospheres. The mild reaction conditions and reagents employed are amenable to a variety of self-assembly applications. Because of the interesting optoelectronic properties of the crystal, using [AgSePh]∞ as a substrate for molecular attachment at step edges presents opportunities to interconnect hybrid material systems. For example, the films described here might be usable as substrates for subsequent preparation of MOCHAmetal, MOCHA-MOCHA, or MOCHA-molecule heterostructures. The interesting luminescent properties of [AgSePh]∞ make it an interesting candidate for studying (or utilizing) energy transfer across such well-defined interfaces.

Materials and Methods Benzeneselenol (PhSeH), triphenyl phosphine, tetrahydrofuran, and silver (I) oxide were used as received from Sigma Aldrich. Diphenyl diselenide DPSe was used as received from both TCI (Portland, OR) and Sigma Aldrich. Silicon wafers were used as received from University Wafers (Boston, MA). Glass coverslips (1” x 1”, 0.17-0.25 mm thick), glass jars, dram vials, Drierite, isopropyl alcohol, and acetone were used as received from VWR. Quartz substrates 1” x 1”) were used as received from Ted Pella (Redding, CA). Metal films of the desired thicknesses were deposited onto substrates using an MBraun thermal evaporator within a nitrogen glovebox equipped with a Sigma Instruments SQC-310 deposition controller and quartz crystal microbalance. Titanium adhesion layers were deposited to a thickness of 5-7 nm prior to coating with silver, and without breaking vacuum. Such adhesion layers were employed to prevent uneven coverage of products, attributed to delamination of silver metal from the silicon wafer and acceleration of the reaction by undercut-


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ting. After metal deposition, silver samples were reserved in a covered container until use.

was performed three times to purify the product from all traces of residual benzeneselenol.

Because all of the benzene chalcogenols (PhSH, PhSeH, PhTeH) are liquids at room temperature, toxic, susceptible to oxidation, and are strong-smelling, they can be difficult to work with. A small release can render an under-ventilated space quite unpleasant, a consequence of human detection thresholds at the part per trillion. It is advantageous to use instead the solid dichalcogenides DPSe and DPTe, which are neither foul smelling nor air sensitive. Once melted in a sealed reaction vessel, they produce a vapor. Safety precaution: although not strongly smelling while solid, heated samples should be only opened in a ventilated environment, or be first allowed to cool, to prevent exposure to DPSe precursors still in the gas phase.

Safety precautions: benzeneselenol is toxic and strongsmelling, and suitable engineering controls should be employed to prevent unplanned release and exposure. Jars are not appropriate reactors for the benzeneselenol reagent because they are inadequate to contain the reagent at elevated temperature and present a considerable toxic exposure and stench hazard.

Tarnishing reactions were performed by sealing solid DPSe, with a silver substrate in a jar. The silver-coated substrates were placed on the floor of the glass jar. ~50 mg of the appropriate precursor compound or a mixture thereof was placed into a small (2 mL) glass dram vial. The precursor and vial were placed inside the jar, the jar was sealed, and the jar was then placed inside the oven. As the oven is unlit, all reactions were performed in the dark, and no effort was made to shield the samples from room lighting before or after the tarnishing process. As the precursor melted, a vapor of the organic compound filled the jar. The glass dram vial reserved the liquefied precursor and prevented splattering during melting. After the specified amount of time (24 – 192 h), the jars were taken from the oven and the samples were removed. After tarnishing reactions, samples were stored in a drawer, shielded from light. Films deposited on silicon were found to darken after several days exposed to room light, and so were stored in the dark. The jar and precursor can be reused. Care should be taken that the surface of the sample is not blocked by multiple samples within the jar to prevent “shadowing” effects. For experiments increasing the humidity, an additional vial containing ~0.5 mL 18.2 MΩ deionized water was added. Steady-state condensation and evaporation within the vessel was not noted to inhibit the reactions. Drying the atmosphere was achieved by adding 4 g Drierite to the floor of the jar. The silver-coated silicon substrate was placed on top of the Drierite. Surface oxidation was performed by placing an evaporated silver film into a UV-Ozone cleaner for a 30 minute cycle. The transformation of silver oxide to [AgSePh]∞ was achieved by combining 50 mg Ag2O with 100 uL of neat benzeneselenol in a dram vial with a chemically inert lid. This vial was then sealed within a secondary container and then placed in an oven at 80 °C for 20 minutes. The sample was removed from the oven and 1.5 mL ethanol added and stirred with a disposable plastic spatula. The reaction was left to rest for 1-3 days to allow settling. The solution was decanted to remove the majority of residual benzeneselenol. The product was suspended in fresh isopropyl alcohol by mixing. Resuspension in isopropyl alcohol, centrifugation, and decanting

The bulk reference sample of [AgSePh]∞ was prepared by a gram-scale approach. An oven dried round bottom flask equipped with a stir bar was charged with silver nitrate (1.4 g, 0.008 mol) and triphenylphosphine (4.4 g, 0.017 mol) in 250 mL of dry tetrahydrofuran. The solution was stirred for 16 h under nitrogen at ambient temperature giving a cloudy, white suspension. DPSe (1.3 g, 0.004 mol) in 80 mL of dry tetrahydrofuran was then added slowly to the flask at -50 °C. The reaction was stirred while warming slowly to room temperature in which a deep yellow solution resulted. The solution is layered with 75 mL of diethyl ether and stirred rapidly until solution was clear and colorless and bright yellow crystals have precipitated. The solvent was decanted and the solid was purified by the addition of fresh isopropyl alcohol followed by sonication and centrifugation to separate the crystalline pellet and supernatant. The crystals were then dried under vacuum giving a canary yellow fine powder (2.3 g isolated). UV-visible measurements were obtained in transmission mode for samples on glass and quartz, and diffuse reflectance spectra were obtained for samples on silicon. These experiments were carried out on an Agilent Technologies Cary-5000 UV-Vis-NIR spectrophotometer. Photoluminescence spectra were obtained on an Edinburgh Instruments FL980 Spectrometer using an excitation wavelength of 350-380 nm. A density of 2.7 g/mol was calculated from the crystal structure of silver benzeneselenolate13 using the commercially available software package CrystalMakerX. Grazing-incidence wide-angle X-ray scattering (GIWAXS) was performed at Beamline 7.3.3 of the Advanced Light Source at Lawrence Berkeley National Laboratory. Patterns were recorded using an incident photon energy of 10 keV (wavelength of 0.12398 nm) at an angle of 0.15°, a sample-todetector distance of ~300 mm, and a Pilatus 2-M detector. Spectra are normalized to exposure time. 2D patterns were reduced by circular integration and plotted using the Nika2 plug-in for Igor Pro. Ewald sphere corrections were performed using Xi-cam.110

ASSOCIATED CONTENT Description of additional experiments involving solutionphase DPSe as a reagent. SEM and EDS analyses of the silverinitiated etch of silicon by water at 80 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION



Page 10 of 15 10

Corresponding Author *[email protected]

ORCID Brittany Trang

0000-0002-2499-8990

J. Nathan Hohman

0000-0002-9777-6432

ACKNOWLEDGMENT This work was performed at the Molecular Foundry, and used resources of the Advanced Light Source and beamline 7.3.3, which are supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (DOE) under contract No. DE-AC02-05CH11231. This work was also supported by the DOE Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Berkeley Lab Undergraduate Research (BLUR) program managed by Workforce Development and Education at Berkeley Lab. The authors warmly thank Drs. Yi Liu, David Prendergast, and Adam Schwartzberg for helpful discussions, Drs. Mary Collins and Lorenzo Maserati for synthetic support, and Dr. Behzad Rad for confocal microscopy support.

ABBREVIATIONS MOCHA, metal-organic chalcogenolate assembly; SAM, selfassembled monolayer; DPS, diphenyl disulfide; DPSe, diphenyl diselenide; GIWAXS, grazing incidence wide-angle X-ray scattering; TMD, transition metal dichalcogenide; SEM, scanning electron microscopy; PL, photoluminescence; 0D, zero-dimensional; 1D, one-dimensional; 2D, two-dimensional; 3D, threedimensional


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