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Hydrogen-Doped Polymeric Carbon Monoxide at High Pressure Young-Jay Ryu, Choong-Shik Yoo, Minseob Kim, xue yong, John S Tse, Sung Keun Lee, and Eun Jeong Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 22, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Hydrogen-Doped Polymeric Carbon Monoxide at High Pressure Young-Jay Ryu,1 Choong-Shik Yoo,*1 Minseob Kim,1 Xue Yong,2 John Tse,2 Sung Keun Lee,3 Eun Jeong Kim3 1

Materials Science and Engineering, Department of Chemistry, and Institute of Shock Physics,

Washington State University, Pullman, Washington 99164 2

Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon,

Canada, S7N 5E2 3

School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea 151-742

* Correspondence: [email protected]; (509) 335 - 2712

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ABSTRACT The ability to control materials stability, bonding and transformation by thermo-mechanical and chemical means is significant for development of high-energy-density extended solids. We report that doping hydrogen (~10%) in carbon monoxide (CO) can greatly lower the polymerization pressure of CO and enhance the stability of recovered polymeric CO products at ambient conditions. Hydrogen-doped CO crystallizes into well-grown dendrites of β-CO-like phase at 3.2 GPa, which polymerizes to highly unsaturated black polymer (phase I) at ~4.7 (5.8) GPa. Upon further compression, this highly colored polymer transforms into a translucent 3D network structure (phase II) at 6-7 (10-17) GPa, then a transparent 2D layer structure (phase III) at 20-30 (30-60) GPa. A similar series of transformations are also found in pure CO but at the considerably higher transition pressures, as noted in parenthesis. All polymeric phases are recoverable at ambient conditions, exhibiting an array of phase stability and novel properties such as chemically unstable phase I, highly luminescent phase II, and highly transparent layered phase III. The density of recovered products ranges from ~2.3 g/cm3 to 3.6 g/cm3, depending on the pressure recovered. The recovered products are highly disordered but slowly decompose to crystalline solids of anhydrous polymeric oxalic acid, while exhibiting interesting crystal morphologies such as nm-cobs, nm-lamellar layers, and µm-bales. The present first-principles MD simulations suggest that the polymerization occurs at 6 (or 10) GPa in H2-doped (or pure) CO. While not directly participating in the reaction, the role of H2 molecules is to enhance the mobility of CO molecules leading to the polymerization.

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INTRODUCTION High energy density solids are metastable, so are the majority of materials in use today. The more metastable material contains the higher energy. Importantly, metastable materials, when made of low Z elements in dense 3D network structures (e.g. diamond and cubic-BN) exhibit fascinating thermal, mechanical and electro-optical properties such as high energy density, superhardness, superconductivity, magnetic correlation, and nonlinear optical properties.1-5 These low Z 3D network structures are both unique and ubiquitous at high pressures, but become highly metastable at ambient conditions.6 Hence, developing low Z extended solids for ambient uses poses great scientific and technological challenges. It requires that we understand and control materials structures, stabilities, and transitions along many transition pathways over relatively flat energy landscapes well beyond stable structures and equilibrium properties. This has yet to be accomplished despite five decades of high-pressure research and numerous materials discoveries at high pressures. In the past decade or two, a significant number of new materials and novel structures have been discovered by compressing simple molecular solids such as H2, N2, O2, and CO2.1-5,7-13 Yet, many of these materials are metastable upon releasing the pressure, limiting them within a regime of fundamental materials discoveries. Developing low Z extended solids amenable to ambient stabilization and scale up synthesis poses great scientific and technological challenges associated with the formidable transition pressures and high energy-states that make them metastable at low pressures. Nevertheless, it is important to note on large energy barriers (~ a few eV) required to break covalent bond network structures, which are in contrast to small thermal barriers (~ a few tenth of eV) associated with the melting (or sublimation) of low Z extended solids recently observed in CO2-V.14 Therefore, it is likely that the ambient

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metastability of low Z extended solids is largely a manifestation of kinetics in origin, and a key to enhancing the stability at ambient conditions is to prevent the surface or interfacial melting of extended solids. A plausible way to overcome these challenges is to use a small amount of impurity molecules such as hydrogen, which can generate internal chemical pressure lowering the transition pressure and replace the danggling bonds of extended network structures by strong hetero-nuclear chemical bonds enhancing the stability at ambient conditions. Carbon monoxide is one of the first molecular systems found to transform into a highly disordered and colored (yellow to dark brown) nonmolecular “polymeric” solid above 5.5 GPa.15 This polymeric CO (or pCO) is a high-energy density solid,16 as predicted in its isoelectronic system of polymeric nitrogen (or cubic gauche form of nitrogen, cg-N).1 Yet, the crystal structure of pCO and the mechanism of polymerization are not well understood. This is largely due to an unstable nature of pCO; it is highly photosensitive, strongly hygroscopic, and chemically unstable at ambient conditions.17,18 Recently, we have studied the phase/chemical transformations of pure CO to 160 GPa and found the presence of three polymeric phases (depicted as pCO-I, II and III).19 The crystal structures of pCO phases, on the other hand, have not been determined, but inferred based on those predicted.20-22 This was due to highly disordered structures of pCO phases at high pressures and chemical instabilities of the recovered phases at ambient conditions. Adding a small amount of hydrogen molecules can alter the physical and chemical properties and stability of low Z extended solids such as polymeric CO in significant ways. These “impurity” atoms and molecules can form chemical bonds with surface atoms of extended solids, stabilizing the network structures by replacing the surface danggling bonds to strong covalent CH and OH bonds in the case of polymeric CO. They can also go into the interstitial

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sites of dense solid structures and create large chemical internal pressures, which can lower the transition pressure to extended solids. Theory predicts, for example, hydrogen-dominating group IV hydrides (e.g., CH4, SiH4, etc.) can metallize at substantially lower pressures (~100 GPa or less) than that of pure H2 (>400 GPa).23 In fact, the subsequent experiments found that SiH4 metallizes at 50 GPa, which becomes a superconductor at 100 GPa and Tc=17 K.24 It is also found that doping hydrogen in ice converts it to ice-X like phase at ~60 GPa,25 while pure ice transforms to polymeric ice-X with symmetrized hydrogen bonds at ~80-100 GPa.26 In this study, we have considered a small amount (10%) of hydrogen doped in carbon monoxide to investigate the effect of hydrogen doping on the stability of polymeric carbon monoxide. The results show a series of transformations in hydrogen-doped polymeric CO (or pCO:H) that are similar to those in pure pCO;19 those are, initially to phase I at ~4.7 (5.8) GPa then to phase II at ~7.2 (11) GPa and phase III at ~25 (50) GPa. The values in parenthesis signify the transition pressures of pure pCO. The recovered pCO:H phases exhibit enhanced chemical stabilities, high densities (2.3 – 3.6 g/cm3), and novel crystallization behaviors.

METHODS 1.

Experimental Methods The present study was based on a large number (more than two dozens) of high-pressure

experiments, all providing a consistent and reproducible set of results. For in-situ characterization and synthesis, we used both diamond anvil cells (DAC) and transparent large anvil press (TLAP).27 The TLAP cell was made of two large-flat diamond anvils supported by either tungsten carbide or sintered diamond seats in an opposed Bridgman-anvil configuration,

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used in combination of Paris-Edinburgh (PE) hydraulic press. In this configuration, using typical 0.3 carat diamond-anvils, the sample size was ~2-3 mm in diameter and 0.1-0.3 mm thick, thus producing a 1-5 mg of extended solids at a density of 3.0 g/cc. For comparison, a conventional DAC with similar size diamond anvils would produce a sample typically less than one µg. The mixture gas of 10% hydrogen (99.99% pure) and 90% carbon monoxide (>99.9% pure) is loaded in DAC or TLAP using a high-pressure gas loader at ~2000 atmospheres. The mixture sample was equilibrated for 24 hours in room temperature (26 ± 2°C) after the loading. The pressure was then increased in steps of 0.2 to 0.5 GPa using a helium-driven membrane DAC. The sample pressure was determined by the Ruby luminescence; the Raman spectra were occasionally acquired, both using a minimum laser-power (less than 10 mW). In addition, the micro-photographic sample image was recorded to examine any visual appearance change at every pressure point. The recovered samples were characterized by a scanning electron microscope (SEM, FEI Quanta 200F) and a transmission electron microscope (TEM, FEI Tecnai G20 T-20 Twin) at the Washington State University’s Franceschi Electron Microscopy and Image Center, as well as

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C Magic Angle Spinning (MAS) Nuclear Magnetic Resonance

(NMR) at Seoul National University. For the TEM investigation, we used a thin edge of the recovered sample using high-energy electron beams with accelerating voltages between 80 to 200 kV. During the operation, the search mode was used at an exposure time of 0.02 to 0.03 seconds to find region of interest and record the image with the required magnification typically ranging 5,000 to 100,000 times. The images were typically collected for five seconds. For the SEM, we used a relatively low accelerating voltage of 5 kV and a working distance of 6 to 10 mm, without coating the sample specimen.

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The

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C MAS NMR spectra were collected using a Varian 400 MHz solid-state NMR

spectrometer (9.4 T) at a Larmor frequency of 100.582 MHz for

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C using a 3.2 mm zirconia

rotor in a Varian double-resonance probe. The recycle delay for the NMR experiments was 5 s, with the rf pulse length being 1.3 µs (flip angle of ~ 30° for the central transition in solids). The magic-angle sample spinning speed of 17 kHz was employed. Nearly 49000 scans were averaged in the

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C MAS NMR spectra to achieve the current signal-to-noise ratio in the spectra.

Approximately 1.5 mg of 20%

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C-enriched pCO synthesized at 7.2 GPa was used for the

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C

MAS NMR measurements. The background signals were collected under identical measurement conditions using an empty zirconia rotor and one and a half spacers. The background spectrum was subsequently subtracted from the

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C MAS NMR spectrum for each sample to yield the

NMR spectrum free from any background carbon signals. A similar method of background subtraction has been applied to yield a quantitative (artifact-free) NMR signal from a relatively small amount of samples (such as carbon species in fluid inclusion and Al-species in oxide thin films).28,29 Powder X-ray diffraction of the sample was performed at the 16IDB beamline at the Advanced Photon Source (APS) using a micro-focused (~10x10 µm) monochromatic synchrotron x-ray (λ = 0.3682 Å) and a 2D X-ray detectors (1M, PILATUS). The x-ray work was only performed above 30 GPa, where CO samples become stable under x-ray or laser illumination. Below this pressure, no x-ray or laser spectroscopic investigation is possible without photochemical decomposition. The measured x-ray diffraction data at high pressures, however, consists of a very broad ring – typical for a highly disordered solid; no further analysis has been performed. On the other hand, the recovered samples slowly recrystallize into small µm-size

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single crystallites, of which structure was determined using single crystal X-ray diffraction at the 12.2.2 beamline at the Advanced Light Source (ALS).

2.

Theoretical Methods Ab initio based molecular dynamics (AIMD) simulations in the NPT ensemble with

Langevin thermostats were performed.30 The Vienna ab initio simulation package, VASP, a plane wave basis electronic structure code employing projected augmented wave potentials for the atoms was used for all calculations.31,32 The Perdew, Burke, and Ernzerhof (PBE) exchangecorrelation functional was employed.33 A plane wave basis set with an energy cutoff 400 eV was used. In view of the large supercell and that the system is a large gap insulator, only one k-point (Γ) was used to sample the Brillouin Zone.

RESULTS 1.

Pressure-Induced Transformations in DAC Figure 1 shows the pressure-induced chemical and phase transformations in 10% H2 mixed

in CO to 60 GPa, accompanying dramatic changes in the visual appearance. These changes indicate that hydrogen-doped CO transforms to a black polymer above 4-5 GPa, analogous to the previously reported polymer in pure CO.15-18 We call it as phase I (or pCO:H-I). The black round area at the bottom represents the photochemically produced polymer (or pCO:H-I’) after laser illuminations (for Raman and pressure measurements). Note that this transition occurs at a substantially lower pressure than that in pure CO (typically above 5.5 GPa). The polymerization is completed at around 5.5-6.0 GPa, as the entire sample chamber becomes opaque. A small

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pressure drop was noticed upon the polymerization. This polymeric phase I, however, is only stable within a small pressure range (less than 2 GPa) and transforms into a transparent polymer (phase II or pCO:H-II) at ~6.5-7.2 GPa. The color of phase II slowly darkens as pressure increases and becomes nearly black or dark red at ~30 GPa. Remarkably, the phase II further transforms into a transparent solid (phase III or pCO:H-III) over the pressure range of 30-50 GPa. The phase III remains highly transparent to 85 GPa – the maximum pressure applied in the present study. These transitions in H2-doped CO are analogous to those recently observed in pure CO,19 but occur at considerably lower pressures, as compared in Fig. S1. The polymerization in pure CO starts at ~5.8 GPa (nearly completes at 7.0 GPa) to the previously known black polymeric phase I. Then, this black phase I transforms into translucent phase II at ~8-11 GPa, which deepens its color over ~11-30 GPa. Then, it again becomes translucent over a large pressure range 50-60 GPa, indicating an emergence of phase III. The phase III remains highly transparent to 160 GPa. The polymerization can also be induced photo-chemically at lower pressures, as apparent from large dark area at 5 GPa in Fig. 1. This photo-polymerized area expands as pressure increases and, interestingly, remains black to 20-30 GPa, where all other thermally polymerized area becomes translucent as they transform to phase II. Nevertheless, it eventually becomes transparent, as phase III emerges above 30-60 GPa and no longer photo-chemically active under laser or X-ray illumination. The pressure-induced changes in Raman spectra of hydrogen-doped CO (shown in Fig. 2) support the above-mentioned transformations. The polymerization to phase I at 5 GPa, for example, is evident from (i) the disappearance of hydrogen vibron νs(H2) at ~4226 cm-1, (ii) the

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emergence of a broad peak centered at ~1600 cm-1 from C=O/C=C stretching and a weak peak at ~652 cm-1 from O-C=O bending, and (iii) a weak broad feature at 3159 cm-1 from C-H stretching.34 Phase II is, on the other hand, evident from (i) the disappearance of all vibrational features and (ii) the appearance of strong laser-induced luminescence especially above 20 GPa; and phase III is from (i) a high degree of transparency of the sample including the photopolymerized area (phase I’) and (ii) a high level of photochemical stability. It is important to note that at ambient temperature, hydrogen doped CO polymerizes from β-CO-like solid, whereas pure CO polymerizes from δ-phase,35 as shown in Fig. 2b. Pure CO solidifies initially into hexagonal β-CO (P63/mmc) with a singlet νs(CO) at 2142 cm-1 at ~2.4 GPa, then to cubic δ-CO (Pm3n) with a characteristic doublet at 2149 and 2159 cm-1 at ~4.2 GPa. The both phases are optically isotropic, showing no apparent crystal boundaries. The polymerization occurs in this δ-CO at randomly distributed nucleation or reaction sites over the entire sample area and progresses gradually in pressure and time as shown in Fig. 2b. In contrast, hydrogen-doped CO crystallizes into well-developed dendrites at ~3.2 GPa with the β-like singlet vibron at 2147 cm-1. In fact, no δ-CO-like phase is evident in hydrogen-doped CO, signifying that highly mobile hydrogen molecules disturb CO molecules in β-CO from further ordering into δ-phase and prolong the stability of highly disordered β-CO.

2.

Synthesis and In-situ Characterization in TLAP The polymerization process can be further investigated with a milligram quantity of

samples using a TLAP.27 Figure 3 shows that hydrogen-doped CO crystallizes at ~2.5 GPa into well-developed dendrites (see the image in Fig. 3b at 3.2 GPa) with a β-like singlet vibron at

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2147 cm-1 (see Fig. S2). The polymerization occurs from these dendritic crystals, initially from the outside (see the images in 3c and 3d at 5.0 GPa) then to the inside (3e at 5.5 GPa). The Raman spectra (Fig. S2a) show no major difference in the CO vibron at 2145 cm-1 between the two regions, but show the H2 vibron (i.e., H2-rich) at 4209 cm-1 stronger in the outside together with a weak-broad side band at 3983 cm-1. This softening of hydrogen vibron by 226 cm-1 reflects the presence of weak bonding between H2 and β-CO lattice, presumably causing the initial darkening in the outside (at 3.2 and 5 GPa in Figs. 3c and 3e). The polymerization in the inside, however, progresses more rapidly to black phase I at ~5.5 GPa, whereas the outside becomes black at ~5.7 GPa as the inside becomes transparent. Eventually, both the inside and outside areas become transparent at ~6 and 7 GPa, respectively, signifying the transformation to phase II. Note at 7 GPa the entire sample, but the laser-illuminated area, becomes transparent. Again, no evidence was found for δ-like hydrogen-doped CO phase to exist before the polymerization (Fig. S2b). As expected, the polymerization depends on the concentration of H2; the dendritic growth seems unique at relatively low H2 concentration below 10%. We also found that the stability of 1D chain-like polymer (i.e., phase I) decreases as the concentration of H2 increases and above 30% H2 mixed in CO polymerizes directly to phase II (3D structure with 5-membered lactone rings) and phase III (2D layered structure), completely bypassing phase I (1D chain structure). These results are, in turn, consistent with the reduced pressure-stability region (5-7 GPa) of pCO:H-I than that of pCO-I (6 – 10 GPa). Hydrogen-doped polymeric products can be recovered at various stages of polymerization in a wide range of stability, color and morphology of recovered products. The polymer recovered at ~5-6 GPa (i.e., phase I) rapidly decompose or sublimes, leaving behind small spherical

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particles (Fig. 3i); whereas, one recovered above 6-7 GPa (i.e., phase II) shows highly glassy, translucent reddish-brown product, which remains stable and non-hygroscopic (2j-2k). This is unlike pure CO polymers recovered from the same pressure range. The density of recovered phase II is estimated in the range of 2.3 g/cm3 and 3.2 g/cm3. This estimation should be considered as a lower bound, considering an unaccounted amount of sample lost during the recovery. This density is substantially larger than the previous estimation 1.68 g/cm3,16 but is well in the range of the theoretically predicted value, 2.7 g/cm3.21 Therefore, it is likely that the previously recovered polymers of pure CO were mostly phase I, as those were recovered from the first formed black polymer well below 10 GPa. This in turn advocates for the theoretically suggested 3D network structure21,22 to be more closely related to that of transparent, high-density phase II. While pCO:H-I is chemically unstable, pCO:H-II and III exhibit substantially better stability and interesting optical properties. Recovered pCO:H-II, for example, luminesces strongly in red (600-750 nm) and absorbs the light in blue-green (400-550 nm) in the visible range (Fig. 4a), which is absent in pure pCO. Thus, it is likely that the photoluminescence of pCO:H-II is associated with the confined states of hydrogen impurities. MAS-NMR spectra of recovered solids (Fig. 4b) show the 13C peaks at 160 and 168 ppm (referenced to TMS), most likely from carboxylic carbons. The reference value of carboxylic carbons typically appears in the range of 160 ppm for -O-(C*=O)-H to 170 ppm for -O-(C*=O)C-.36 This result then supports the proposed 3D network structure with five-membered lactone rings, albeit the presence of hydrogen leading to some structural interference. Indeed, the intensity of the 160 ppm peak decreases as the number of NMR scans increases (Fig. S3), indicating gradual loss of weakly bonded hydrogen from the polymer. In contrast, the recovered

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product of pure pCO shows the peak at 223 ppm,16,17 likely from carbons in cumulated ketones (see Table S1). For comparison, carbon peaks appear at 198 ppm in 1,2 ketone, 210 ppm in 1,4 ketone, and 220 ppm in seven-membered cyclic ketone. This interpretation is then better fit to the theoretically proposed linear chain P21/m structure –(*C=O)p- for phase I. Furthermore, the 223 ppm peak in pure pCO disappears rapidly, while a very sharp new feature emerges at 151 ppm. The latter peak is likely from >*C=CH2 in a small hydrocarbon such as HO-*C(OH)=CH2, while the rapid disappearance of the 223 ppm peak signifies a chemically unstable and hygroscopic nature of pure pCO-I as expected from its highly cumulated ketone structure. In fact, the liquid 13C NMR spectrum of phase I (dissolved in DMSO) shows the peak at 202 ppm, most likely representing carbons in aldehyde (H-(C*=O)-) originated from chemically decomposed phase I.

3.

Crystallization of Recovered Polymers Recovered hydrogen-doped polymers (nominally phase II) show interesting crystallization

(Fig. 5): (a-d, d’) TEM images and (a’-c’) Selected Area Diffraction (SAD) patterns. The recovered product consists largely of twisted thin layers of amorphous solids as shown in (a). Yet, there are also some small (nm-sized) crystallites with interesting crystal morphologies. Those include nm-cobs made of small single crystallites (less than 20 nm) embedded in amorphous solid (b), nm-lamellar layers separated by ~3-4 Å (c), and µm-bales made of rolled nm-layers and containing H2 bubbles (d). These SAD patterns obtained from recovered single and polycrystalline samples can be indexed to a wide range of crystal structures with the density ranging from 2.3 g/cm3 to 3.6 g/cm3 (see Figs. S4 and S5), which can be compared well with

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those estimated based on the recovered weight and volume (2.3-3.2 g/cm3), as well as the theoretically predicted density of 2.7 g/cm3.21 Interestingly,

the

recovered

hydrogen-doped

phase

II

undergoes

spontaneous

decomposition, which occurs “slowly” over an hour to days in air. This process occurs exothermically as apparent on the surface of and nearby the sample in DAC as well as from the remnant of burn stains surrounding the recovered product from TLAP (Fig. S6). Furthermore, the crystals are formed over a large range of distance, reflecting a large release of mass momentum. This behavior is somewhat analogous to the violent crystal trembling often observed during the β−δ HMX transition just prior to decomposition.37 It is also important to note that these single crystals are only intermediate phases formed during the decomposition pathway, eventually, to carbon and carbon dioxide. Thus, polymeric CO should release a substantially larger amount of energy than that observed in this recrystallization process. However, a quantitative determination of the exothermic energy release from recovered polymers has been challenging, because it decomposes over a large temperature range of 100-400 oC, which makes thermal analysis inconclusive. The crystal morphology of decomposed crystallites varies greatly, ranging from small (~10-20 µm in size) single crystallites to a few µm size particles of initially rod (or needle)- and later plate-like crystals (Fig. S7). The Raman spectra of rod- and plate-like crystals (Fig. S8) indicate that they are, respectively, anhydrous oxalic acids and oxalic acid dihydrate. For example, the Raman spectrum of plate-like crystals is nearly identical to that of oxalate acid dihydrate, including the νs(OH) peak of H2O at 3450 cm-1. The Raman spectra of rod-like crystals, on the other hand, shows no νs(OH) peak, but the νs(CH) at 3120 cm-1 and νb(HCH) at 700 cm-1 – both absent in the plate-like crystals. In additions, the νs(CC) at 840 cm-1 is strong in

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the plate-like crystals but appears only as a weak side band of the main νb(O-C=O) at 860 cm-1 in the rod crystals. Similarly, there are apparent differences in all observed peak positions between the plate and rod crystals. Importantly, hydrogen molecules are still present in the rod-like crystals as evident from the νs(H2) at ~4200 cm-1, but absent in the plate-like crystals later formed. The plate-like crystal can be grown, over a month, sufficiently large (a few-30 µm) for single-crystal x-ray diffraction and is indeed found as oxalic acid dehydrates in an orthorhombic cell (Pnma, SG 62, see Fig. S9 and Table S2), as found in the Raman spectrum.

4.

Chemical Mechanisms for Polymerization and Decomposition A model composed of 32 CO molecules, constructed from the P213 ambient structure of

solid CO, with 3 H2 molecules filled randomly in the interstitial sites was constructed to describe the polymerization of CO using ab initio MD simulations. We compressed the system to 1, 3, 6, 10, 15, 20, and 24 GPa, all at 300 K (Fig. 6). The simulation results indicate no apparent chemical reaction at 1.0 and 3.0 GPa, within the time scale of the simulation (~10 ps). At 6.0 GPa CO molecules start to polymerize, while H2 molecules are not involved in the reaction. Interestingly, the product mainly composed of carbonyl chains with a fused five-member lactone rings with C=C at 1.32 Å and C-O at 1.36 Å. In comparison, substituting H2 molecules for CO molecules, we found that the polymerization occurs at substantially higher pressures at 10 GPa, catalyzed at the H2 defect sites as there are more free volume for the surrounding CO molecules. Further compression leads to the formation of more linked cyclic 4-member lactones (Fig. 6). At 15 GPa a cyclic epoxide appears; yet, there is still no reaction with H2 molecules. Only at 20 GPa a H2 molecule is found to react with the C-C bond of the epoxide at 20 GPa, opening the

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ring and forming two C-H bonds. No further consumption of H2 is found even if the model is compressed to 40 GPa. In this dense state, the bonding architecture becomes more like a 3D network and the most noticeable new species formed is a fused bicyclic 5-member lactone ring. To understand the role of H2, the atomic positions accumlated at 6.0 GPa have been ananylized. Selected snaphots of the interstitial filled CO:H2 model in the early stage of the polymerzation are shown in Fig. S10. Interestingly, although H2 molecules did not participate directly in the reaction, the role it plays is similar to that in the substitutial model. The smaller H2 were found rapidly rotating and diffused through the structure upon compression (Fig. S10ac). These motions perturb the CO molecules in the vicinity (see the region highlighted with the blue circle in Fig. S10a-c) and increase their mobility, which in turn initiate the polymerization (see Fig. S10d). The reaction product is unlike pure CO, in which chain-like polymers (pCO-I) are formed. Instead, the presence of the H2 led to the formation of a lactone ring similar to that of phase II. Therefore, the theory predicts that the experimentally observed products of hydrogendoped CO may also differ from that of pure CO. In fact, the reduced pressure-stability of pCO:HI than that of pCO-I and at higher H2 concentrations seem to reflect the theoretical prediction. The structure at 20 GPa was then fully optimized and the model was relaxed to 0 GPa at 300 K from a series of NPT calculations. The high-pressure structure is recoverable and, therefore, metastable within the time scale of the simulation. To study the stability of the recovered product, a vacuum region was created by doubling the length of the c-axis and a microcanonical ensemble (NVE) MD calculation was performed at 300K. The product decomposed into a variety of small molecules and molecular fragments such as H2, CO, CO2, and short chain polymers with the presence of 4-member lactone rings (Fig. S11). From the

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energy difference of the recovered and decomposed samples, the heat evolved with the decomposition is estimated to be 917 J/g at 300 K.

DISCUSSION The pressure-induced color (or transparency) changes of hydrogen-doped pCO suggest the presence of three polymeric phases, analogous to those of pure pCO: (i) highly colored relatively low-density phase I in the predicted linear chain structure21,38 or other 2D ladder structures,16,17 (ii) translucent high-density phase II in the 3D network structure,20-22 and (iii) transparent phase III in the 2D layer structure. However, there is a significant difference between hydrogen-doped pCO:H and pure pCO phases. That is, while pure pCO-I can be recovered as yellow-to-brownish hygroscopic solids, hydrogen-doped pCO:H-I is highly unstable and immediately sublimes at ambient conditions. As a result, the recovered products are mostly phase II and III when recovered from the pressure above 6 GPa. The structural and chemical characteristics of recovered phase II and III are found as the following: •

Hydrogen-doped pCO:H phase II and III exhibit strong light absorption in blue and emission in red, absent in pure pCO phases.

•

While the recovered products are primarily amorphous solids, they contain small (less than 20-40 nm) particles with interesting crystal morphologies such as nm-cobs (~2.36 g/cm3) nm-lamellar layers (3.62 g/cm3), and µm-bale (likely a rolled form of nm-lamellar layers).

•

The recovered products have a conjugated lactonic moiety -C-(C=O)-O- as evident from the present Raman (Fig. 2) and the previous IR spectra of pure CO in Refs. [16-18], as well as the –O-(C=O)-H and –C-(C=O)-C-, as evident from the SS 13C NMR data (Fig. 4b).

17



•

The recovered products slowly undergo spontaneous decomposition (or recrystallization) in air, initially to rod-like crystals of oxalic acid anhydrides, then to plate-like crystals of oxalic acid dihydrate (1.65 g/cm3) as absorbing H2O in air.

Based on these findings, we first conclude that the local and crystal structures of hydrogen-doped CO polymers are indeed analogous to those of pCO-II and pCO-III, and these amorphous polymers crystallize into oxalic acid anhydrides in various chain lengths, which slowly decompose (or recrystallize) into oxalic acids and eventually oxalic acid dihydrates (see Fig. 7). Second, the crystal structures of pCO-II and III consist of fundamental chemical moiety of sp2 C=O and sp3 C-O bonds in 3D and 2D network structures, respectively. Incorporating hydrogen molecules, however, has two counter-effecting consequences: (i) to form C-H and O-H bonds with the surface atoms of these extended polymers, replacing the danggling bonds and (ii) to truncate the 3D (phase II) and 2D (phase III) network polymers, primarily made of conjugated five- and six- membered rings, to a less degree of polymeric units including various oxalic anhydrides in different 1D chain lengths. The former stabilizes the polymer products against the reactions with water, whereas the latter makes it slowly decomposed to oxalic acids and eventually oxalic acid dihydrates in air. The pressure-induced phase/chemical transformations and associated color changes in hydrogen-doped CO underscore an interesting structure-bonding relationship of pCO:H phases and a stepwise polymerization from C≡O in molecular phase, to highly conjugated, unsaturated C=O in the 1D chain or 2D layer (phase I), and to mostly saturated C-O in the 3D network (phase II) and fully saturated 2D layer structures (phase III). The opaqueness of phase I, for example, is likely due to a small band gap (or strong light absorption in the visible spectral range) of conjugated, unsaturated C=O in the 1D chain structure. The band gap of phase I is expected to

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open up wide, as it transforms to phase II in a 3D network structure made of primarily sp3 hybridized C-O and C-C single bonds and occasionally sp2 C=O bonds. The pressure-induced darkening of phase II can then be understood in terms of the pressure-induced bandgap close of sp2 C=O bonds (Fig. S12). Upon further compression to phase III, the sp2 C=O bonds become more polarized and convert into sp3 C-O bonds in a 2D layer structure, once again resulting in a wide band gap, highly transparent solid. While the stepwise transitions observed in both pure and hydrogen-doped carbon monoxide are consistent with the sequence of theoretically predicted transitions,21 it is important to recognize that the pressure-induced 3D-to-2D structural transition is in contrast to more commonly observed 2D-to-3D structural transition such as in the graphite-to-diamond transition. Nevertheless, there is an increasing number of recent theoretical calculations, which suggest the stabilization of 2D layer structures at very high presssures. Those include the structures proposed for polymeric N in Pba2, H2O in P21,39,40 and recently discovered H2-IV.7,41 In this regard, the observed 3D-to-2D transition of polymeric CO is not surprising, but may advocate the pressureinduced ionization as the electrostatic stabilization (or structural packing) energy overcomes the electron delocalization (or chemical bonding) energy at very high density. Finally, the present study shows the significant role of hydrogen in development of polymeric carbon monoxide amenable to scale up synthesis and ambient stabilization. First, a small addition of hydrogen creates a relatively large internal chemical pressure in carbon monoxide and thereby lowers the transition pressure, as suggested theoretically23,42 and found experimentally in other hydrogen mixtures.24,43 Second, the presence of highly mobile hydrogen stimulates the reaction in β-CO, which otherwise is chemically inert.15 Third, hydrogen enhances the stability of recovered network structures via chemical passivation of the danggling bonds.

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This is in many ways analogous to the role of hydrogen used in the CVD diamond growth process. Fourth, the presence of hydrogen alters the optical and electronic properties of polymeric carbon monoxide, which makes it strongly photo-luminescent for example. Importantly, the ability to modify the stability, bonding, and transitions of solids by thermomechanical (pressure and temperature) and chemical (doping) means opens new opportunities to development of highly metastable, high-energy-density, low Z extended solids amenable to stabilization and scale up synthesis at ambient conditions.

CONCLUSIONS We have investigated the effect of hydrogen doping on CO polymerization. The results indicate that doping hydrogen (10%) in carbon monoxide alters its stability and properties in significant ways, while the general mechanism of CO polymerization remains unchanged from pCO-I to pCO-II to pCO-III. The significant role of hydrogen doping can be summarized as the following: First, H2 guest molecules lead to increase repulsive interactions, promote the formation of polymeric CO phases at lower pressures as suggested theoretically23,42 and found experimentally in other hydrogen-containing mixtures.24,25,43 Second, the presence of highly diffusive hydrogen enhances the mobility of CO molecules in the β-phase, leading to the polymerization without being directly participated in the reaction. Third, doping hydrogen enhances the stability of polymeric CO through chemical passivation of danggling bonds of the 2/3D network structures. Fourth, the presence of hydrogen alters the optical and electronic properties of polymeric carbon monoxide, which makes it strongly photo-luminescent in a visual spectral range, probably

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arising from the transitions associated with hydrogen confined states. Finally, the present study on hydrogen-doped CO has implications to development of high energy density low Z extended solids amenable to scale up synthesis and ambient stabilization.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publications website, including (A) twelve supplementary figures and captions and (B) two supplementary tables.

AUTHOR INFORMATION Corresponding author: C. S. Yoo, e-mail: [email protected] Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been done in support of the DARPA projects (Grant No. W31P4Q-12-1-0009 and Contract No. HR0011-14-C-0035), NSF-DMR (Grant No. 1203834), DTRA (HDTRA1-12-010020), and NNSA (DE-NA 0003342). We sincerely appreciate Dr. Judah Goldwasser and Dr. John Paschkewitz for their support and encouragement. We also thank Drs. G. M. Borstad and H. Y. Chang at WSU; Dr. C. Beavers and Dr. A. MacDowell at ALS for their assistance for single crystal x-ray diffraction experiments; and Dr. J. Smith at HPCAT/APS for powder x-ray

21



diffraction experiment. The C-13 NMR measurement on H-doped pCO at SNU was performed in support of the NRF, Korea (2014-053-046).

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Figure Captions

Fig. 1 Microphotograph images of 10% hydrogen-doped carbon monoxide at various pressures, showing the pressure-induced polymerization of β-phase CO to black solid phase I at ~4 GPa, translucent phase II at 6-7 GPa, and transparent phase III at 20-30 GPa. The onset pressures of transformations can be estimated based on the transparency change as illustrated in Fig. S1.

Fig. 2 (a) Raman spectra of hydrogen-doped CO under pressures, showing the stepwise pressure induced polymerizations. Note that the vibrational band at ~1600 cm-1 is most likely from photoinduced polymerization to phase I’, whereas the phase II is most evident from the weak feature at ~3100 cm-1. Note on the strongly enhanced background, especially above 20 GPa, which is due to laser-induced luminescence of phase II. (b) Raman spectra of hydrogen-doped CO (red) in comparison with pure CO (black), showing the polymerization occurs from β-CO in hydrogendoped CO, whereas from δ-CO in pure CO.

Fig. 3 Microscopic images of 10% hydrogen-doped carbon monoxide in a transparent largevolume cell, showing (a-b) a formation of well-grown dendrite crystals at ~3 GPa, (c-h) multistep polymerization processes, and (i-l) the recovered polymers at various pressures noted in the frame. The number on each image indicates the pressure in GPa.

Fig. 4 (a) The absorption and luminescence spectra of hydrogen-doped polymer recovered from ~7 GPa. The sharp feature at 514.5 nm in the absorption spectrum is from the excitation laser.

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(b) MAS 13C NMR spectra of hydrogen-doped polymeric CO recovered from ~7 GPa, showing the characteristic peaks for carboxylic carbons at 160 ppm for -O-(C*=O)-H and 168 ppm for O-(C*=O)-C-.

Fig. 5 TEM (a-d and d’) and SAD (a’-c’) images of hydrogen-doped CO polymer recovered from various pressures, showing a wide range of crystal morphologies: (a) typical thin twisted layers of amorphous solids, (b) novel nm-cobs made of aggregates of small single crystallites embedded in amorphous solid, (c) single crystal nm-lamella layers, and (d) a µm-size bale of nm-layer containing H2 gas in bubbles. Also, see Figs. S5 and S6 for further analysis of the structure and morphology.

Fig. 6 The snapshots of ab initio MD simulations performed on the P213 structure of CO molecules with H2 molecules randomly distributed in the interstitial sites, showing the initial structure at 0 GPa and the final structures at 1.0, 3.0, 6.0, 15, 20, and 40 GPa.

Fig. 7 Reaction mechanism for spontaneous decomposition of recovered hydrogen-doped phase II and III in 5, 6-membered rings in 3D and 2D network solids to various forms of oxalic acids, respectively.

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Hydrogen-Doped Polymeric Carbon Monoxide at ... - ACS Publications

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