Bulk Hydrogen Research Articles

Unveiling subsurface hydrogen-bond structure of hexagonal water ice

The phase-resolved sum-frequency-generation (SFG) spectrum for the basal face of hexagonal ice is reported and is interpreted by molecular dynamics simulations combined with ab initio quantum calculations. Here, we demonstrate that the line shape of the SFG spectra of isotope-diluted OH chromophores is a sensitive indicator of structural rumpling uniquely emerging at the subsurface of hexagonal ice. In the outermost subsurface between the first (B1) and second (B2) bilayer, the hydrogen bond of ${\mathrm{O}}_{\mathrm{B}1}\ensuremath{-}\mathrm{H}\ensuremath{\cdots}{\mathrm{O}}_{\mathrm{B}2}$ is weaker than that of ${\mathrm{O}}_{\mathrm{B}1}\ensuremath{\cdots}\mathrm{H}\ensuremath{-}{\mathrm{O}}_{\mathrm{B}2}$ . This implies that subsurface O-O distance is laterally altered, depending on the direction of O-H bond along the surface normal: H-up or H-down, which is in stark contrast to bulk hydrogen bonds. This new finding uncovers how water molecules undercoordinated at the topmost surface influence on the subsurface structural rumpling associated with orientational frustration inherent in water ice.

Vibrational states of atomic hydrogen in bulk and nanocrystalline palladium studied by neutron spectroscopy

The vibrational states of hydrogen atoms in bulk and nanocrystalline palladium were examined in a wide energy region $0\ensuremath{\le}\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\le}300$ meV using neutron spectroscopy. In bulk ${\mathrm{PdH}}_{0.73}$, the vibrational excitations of H atoms were roughly reproduced by the quantum harmonic oscillator (QHO) model. In ${\mathrm{PdH}}_{0.42}$ nanocrystals with a diameter of 8 nm, however, additional vibrational excitations were found at energies above 80 meV. The energies and intensities of the additional states were not explained by the QHO but reasonably described as vibrations in a highly anharmonic trumpetlike potential. The additional excitations are attributed to the vibrations of H atoms at tetrahedral sites in the subsurface region stabilized by surface effects. Thus, this is an experimental work which clearly detects hydrogen vibration inside metal nanoparticles.

High-Pressure Phases of a S-Based Compound: Dimethyl Sulfide.

The high-pressure behavior of dimethyl sulfide was investigated at room temperature by Raman scattering measurements with pressures up to 30.1 GPa. Phase transitions at 1.3, 3.6-5.8, and 17.2 GPa were found and evidenced by the frequency shifts, pressure coefficients, and changes in fwhm of related modes. These phase transitions were suggested to result from the changes in the inter- and intramolecular bonding of the material. Interestingly, the CH3 groups was compelled to be frozen in positions at a relatively low pressure, suggested by the disappearance of the relative modes softening. In addition, the appearance of lattice mode can also be found at a modest pressure, which makes it possible to gain a superiority for this compound to further investigate the superconductivity with high transition temperatures of bulk hydrogen.

Bulk hydrogen stable isotope composition of seaweeds: Clear separation between Ulvophyceae and other classes.

Little is known about the bulk hydrogen stable isotope composition (δ2 H) of seaweeds. This study investigated the bulk δ2 H in several different seaweed species collected from three different beaches in Brazil, Australia, and Argentina. Here, we show that Ulvophyceae (a group of green algae) had lower δ2 H values (between -94‰ and -130‰) than red algae (Florideophyceae), brown algae (Phaeophyceae), and species from the class Bryopsidophyceae (another group of green algae). Overall the latter three groups of seaweeds had δ2 H values between -50‰ and -90‰. These findings were similar at the three different geographic locations. Observed differences in δ2 H values were probably related to differences in hydrogen (H) metabolism among algal groups, also observed in the δ2 H values of their lipids. The marked difference between the δ2 H values of Ulvophyecae and those of the other groups could be useful to trace the food source of food webs in coastal rocky shores, to assess the impacts of green tides on coastal ecosystems, and to help clarify aspects of their phylogeny. However, reference materials for seaweed δ2 H are required before the full potential of using the δ2 H of seaweeds for ecological studies can be exploited.

Multiple pathways for permanent deactivation of boron-oxygen defects in p-type silicon

In this work, it is shown that there are at least two separate pathways for the permanent deactivation of boron-oxygen defects – a purely thermal pathway involving the dissociation of defect precursors, and another mechanism based on passivation of the fully-formed defect during an illuminated annealing process (regeneration). Based on investigations on fired and non-fired p-type Czochralski silicon wafers, a thermal reduction in the net concentration of BO defects is confirmed and found to be caused by rapid cooling following a belt-furnace firing process. This thermal deactivation occurs independently of any subsequent permanent deactivation induced by illuminated annealing and is likely related to the dissociation of the defect and subsequent loss of defect precursors to other species. Further, by varying the surface dielectric present on the wafers during a rapid thermal process, it is demonstrated that in the absence of hydrogen in the wafer bulk, applying an illuminated annealing process does not result in any significant permanent deactivation of defects. In contrast, thermal deactivation occurs independently of the presence of hydrogen in the wafer bulk. This demonstrates that permanent deactivation via the thermal pathway and that via the illuminated annealing (regeneration) pathway occur independently of each other and have different underlying mechanisms. The implications of multiple pathways for permanent deactivation are discussed and the well-known three-state model is revisited in light of this information. A fourth state to represent the end point of thermal deactivation (State D) is proposed for more accurate modelling of BO defect kinetics.

Carbon and hydrogen isotope fractionation during aerobic biodegradation of quinoline and 3-methylquinoline.

Compound-specific isotope analysis has been used extensively to investigate the biodegradation of various organic pollutants. To date, little isotope fractionation information is available for the biodegradation of quinolinic compounds. In this study, we report on the carbon and hydrogen isotope fractionation during quinoline and 3-methylquinoline aerobic microbial degradation by a Comamonas sp. strain Q10. Degradation of quinoline and 3-methylquinoline was accompanied by isotope fractionation. Large hydrogen and small carbon isotope fractionation was observed for quinoline while minor carbon and hydrogen isotope fractionation effects occurred for 3-methylquinoline. Bulk carbon and hydrogen enrichment factors (ε bulk) for quinoline biodegradation were -1.2±0.1 and -38±1‰, respectively, while -0.7±0.1 and -5±1‰ for 3-methylquinoline, respectively. This reveals a potential advantage for employing quinoline as the model compound and hydrogen isotope analysis for assessing aerobic biodegradation of quinolinic compounds. The apparent kinetic isotope effects (AKIEC) values of carbon were 1.008±0.0005 for quinoline and 1.0048±0.0005 for 3-methylquinoline while AKIEH values of hydrogen of 1.264±0.011 for quinoline and 1.0356±0.0103 for 3-methylquinoline were obtained. The combined evaluation of carbon and hydrogen isotope fractionation yields Λ values (Λ=Δδ2H/Δδ13C≈εHbulk/εCbulk) of 29±2 for quinoline and 8±2 for 3-methylquinoline. The results indicate that the substrate specificity may have a significant influence on the isotope fractionation for the biodegradation of quinolinic compounds. The substrate-specific isotope enrichment factors would be important for assessing the behavior and fate of quinolinic compounds in the environment.

Copper in ultrapure water, a scientific issue under debate

The corrosion properties of copper in ultrapure water have been studied experimentally by submerging copper samples (99.9999%) in pure water for up to 29 months. The surface was first electropolished at ambient temperature, then exposed to hydrogen gas treatment at 300–400°C, thereby reducing the bulk hydrogen content to 0.03ppm. These copper samples, the water and the glassware were all then subjected to precise chemical analysis. Great care was taken to avoid contamination. After exposure, only ∼6μg/L copper had accumulated in the water phase. Electron spectroscopy could not detect Cu2O or any other oxidation products containing copper.

Properties of immobile hydrogen confined in microporous carbon

We report results of vibrational neutron spectroscopy investigation aimed to identify the state of hydrogen adsorbed in ultramicroporous carbon. The mobility of hydrogen confined in carbon pores was probed as a function of temperature and pressure using inelastic neutron scattering, and the molecular translational and rotational motions were studied. At low loading rotation of H2 molecules adsorbed in the smallest carbon pores (∼4–5 Å) is severely hindered, suggesting that the interaction between H2 and the host matrix is anisotropic. At higher loading, H2 molecules behave as a nearly free rotor, implying lower anisotropic interactions with adsorption sites. At 77 K where bulk H2 is a gas, deconvolution of elastic/quasielastic signal provide evidence of pressure-dependent fractions of immobile (solid-like) and partially mobile (liquid-like) hydrogen, which correlate with the excess adsorption isotherm at 77 K. Effective H2 density in pores changes from solid-like to liquid-like with increasing pressure at 77 K. Surprisingly, immobile and partially mobile H2 is present even at temperatures as high as ∼110 K where bulk hydrogen exists only in gas form.

Improvement in low-temperature and instantaneous high-rate output performance of Al-free AB5-type hydrogen storage alloy for negative electrode in Ni/MH battery: Effect of thermodynamic and kinetic regulation via partial Mn substituting

Abstract A series of Al-free Mn-modified AB 5 -type hydrogen storage alloys have been designed and the effects of thermodynamic stability and electrochemical kinetics on electrochemical performance via Mn substituting have been investigated. Compared with high-Al alloys, the Al-free alloys in this study have better low-temperature performance and instantaneous high-rate output because of the higher surface catalytic ability. After partial substitution of Ni by Mn, both the hydrogen desorption capacity and plateau pressure decrease, and correspondingly results in an improved thermodynamic stability which is adverse to low-temperature delivery. Additionally, with the improvement of charge acceptance ability and anti-corrosion property via Mn substitution, the room-temperature discharge capacity and cycling stability increase slightly. However, Mn adversely affects the electrochemical kinetics and deteriorates both the surface catalytic ability and the bulk hydrogen diffusion ability, leading to the drop of low-temperature dischargeability, high-rate dischargeability and peak power ( P peak ). Based on the thermodynamic and kinetic regulation and overall electrochemical properties, the optimal composition is obtained when x = 0.2, the discharge capacity is 243.6 mAh g −1 at −40 °C with 60 mA g −1 , and the P peak attains to 969.6 W kg −1 at −40 °C.

Block copolymers containing polyacrylamide and polyacrylic acid: Bulk structure and hydrogen bonds system

ABSTRACTAsymmetric block copolymers PAAc-b-PAAm (DBCs) and PAAm-b-PAAc-b-PAAm (TBCs) comprised poly(acrylic acid) blocks of constant chain length and polyacrylamide blocks of variable chain length were synthesized using RAFT/MADIX technique. Their properties in a bulk state were studied by differential scanning calorimetry and the hydrogen bond system was investigated by Fourier transform infrared spectroscopy. It was found that DBCs and TBCs possess a homogeneous amorphous structure, whose temperatures of glass transition and destruction beginning increased with growth of PAAm block length. The H-bond system between PAAc and PAAm blocks was most developed in TBC sample and formed manly by the mixed cyclic dimmers of carboxyl and amide groups.

Copper dry etching by sub-atmospheric-pressure pure hydrogen glow plasma

Copper (Cu) dry etching is demonstrated using a narrow-gap hydrogen plasma generated at 13.3 kPa (100 Torr) for applications in the Cu wiring technology of integrated circuits. A localized hydrogen plasma is generated around the apex of a fine pipe electrode. The Cu etching can be observed only when the process gas contains hydrogen, and the etching rates decrease with decreased hydrogen concentration. The plasma heating effect owing to plasma localization is negligible for the Cu etching because no etching occurs in the presence of pure N2 plasma whose volume is almost equal to that of the pure H2 plasma. Furthermore, the influences of physical sputtering and vacuum ultraviolet irradiation on the Cu etching are confirmed to be insignificant by exposing the samples to rare-gas plasma. The maximum Cu etching rate of 500 nm/min can be achieved at a stage temperature of 0 °C. However, the Cu etching rate has no obvious dependence on the stage temperature in a range from −20 to 330 °C. In contrast, the etching rates for Si and SiO2 at a stage temperature of 0 °C are 100 μm/min and 50 nm/min, respectively. The Cu etching rate is 10 times higher than that of SiO2, which implies that this etching technique has potential applications for Cu wiring on an SiO2 layer. The Cu surface etched by the hydrogen plasma is roughened and exhibits many round pits and bumps, which seems to be owing to excessive incorporation of the diffused hydrogen in the Cu bulk.

Impact of the Confinement on the Intra-Cage Dynamics of Molecular Hydrogen in Clathrate Hydrates

We have studied the diffusive mobility of hydrogen molecules confined in different size cages in clathrate hydrates. In clathrate hydrate H2 molecules are effectively stored by confinement in two different size cages of the nanoporous host structure with accessible volumes of about 0.50 and 0.67 nm diameters, respectively. For the processes of sorption and desorption of the stored hydrogen the diffusive mobility of the molecules plays a fundamental role. In the present study we have focused on the dynamics of the H2 molecules inside the cages as one aspect of global guest molecule mobility across the crystalline host structure. We have found that for the two cage sizes different in diameter by only 34 % and in volume by about a factor of 2.4, the dimension can modify the diffusive mobility of confined hydrogen in both directions, i.e. reducing and surprisingly enhancing mobility compared to the bulk at the same temperature. In the smaller cages of clathrate hydrates hydrogen molecules are localized in the center of the cages even at temperatures >100 K. Confinement in the large cages leads to the onset already at T=10 K of jump diffusion between sorption sites separated from each other by about 2.9 Å at the 4 corners of a tetrahedron. At this temperature bulk hydrogen is frozen at ambient pressure and shows no molecular mobility on the same time scale. A particular feature of this diffusive mobility is the pronounced dynamic heterogeneity: only a temperature dependent fraction of the H2 molecules was found mobile on the time scale covered by the neutron spectrometer used. The differences in microscopic dynamics inside the cages of two different sizes can help to explain the differences in the parameters of macroscopic mobility: trapping of hydrogen molecules in smaller pores matching the molecule size can to play a role in the higher desorption temperature for the small cages.

A new chemisorption methodology involving H2 and CO for estimation of surface Pd atoms in supported catalysts

A new room temperature static hydrogen chemisorption methodology has been developed for the estimation of surface Pd atoms in supported catalysts. The methodology involves adsorption of H2 on clean and CO covered Pd surface. Following the methodology, the problem related to hydrogen absorption in the bulk of palladium (bulk hydrogen) is nullified truly and the real amount of the Pd atoms exposed on the surface is determined. It is established that chemisorbed CO over-layer does not block/restrict passage of hydrogen into the bulk of Pd. It is also revealed that room temperature evacuation does not remove bulk hydrogen completely.

Hydrogen diffusion in bulk and nanocrystalline palladium: A quasielastic neutron scattering study

The diffusion dynamics of hydrogen in bulk and nanocrystalline palladium has been examined using quasielastic neutron scattering (QENS). With respect to bulk ${\mathrm{PdH}}_{0.73}$, two relaxation processes were found. For both processes, the variation of the relaxation times with momentum transfer was well reproduced by a model of jump diffusion between adjacent octahedral sites. Upon cooling the fast relaxation fraction decreases. The result suggests that the slow relaxation corresponds to jumps between the ground states and the fast one between excited states. In nanocrystalline ${\mathrm{PdH}}_{0.47}$ with a size of 8 nm, we found a fast diffusion process with a smaller activation energy in addition to the one observed in the bulk sample. This process could be due to the motion of hydrogen atoms in the subsurface region where the potential energy surface is substantially modified by surface strain/distortion effects.

The Pliensbachian-Toarcian paleoclimate transition: New insights from organic geochemistry and C, H, N isotopes in a continental section from Central Asia

The Taskomirsay section (South Kazakhstan) is a unique Pliensbachian-Toarcian sequence of lignites, clayey layers and silty-sandstones deposited in a fluvial/lacustrine environment with nearby swampy areas. This period, characterized by a drastic climate change, has been particularly studied in Western Tethyan marine environments, whereas very few studies focused on continental settings. Paleoflora analyses, associated with a multi-isotope approach, based on well-preserved Type-III bulk organic nitrogen isotopes (δ15Norg) and hydrogen isotopic composition (δ2H) of n-alkanes, were developed to document paleoclimatic changes in the area. Sporomorph associations and fossil woods revealed a globally warm- to cool-temperate climate – characterized by Xenoxylon, a conifer morphogenus biogeographically related to cool/humid settings – apart from slightly less humid and warmer conditions in the early Toarcian. Warmer conditions are supported by reconstructed Mean Annual Air Temperatures (MAATs), based on the first branched glycerol dialkyl glycerol tetraethers (brGDGTs) ever recorded in the Early Jurassic. Nevertheless, no drastic changes were recorded in the δ15Norg values; its signal being attributed to tenuous equilibrium between water- and nutrient-availability via intense N-recycling. Based on n-alkane distributions, sources of organic matter were separated in two pools: (i) a purely terrestrial (n-C27) and (ii) an “aquatic” pool (n-C23) constituted of vegetation that thrived under almost permanent water supply. The n-alkane δ2H values (−248 to −151‰) as well as their amounts and average chain lengths (ACL) are in agreement with cool-temperate conditions in the Pliensbachian and less humid/warmer conditions in the early Toarcian. The isotopic difference between δ2H values of n-C27 and n-C23 (Δ2Hter-aq) suggests enhanced seasonality during the Pliensbachian-Toarcian transition and low seasonality in the early Toarcian, in agreement with temperate climate-regime. Finally, contrasted response to paleoclimate changes between markers suggests different spatial integration of those proxies. The role of sea-level variations for δ2H values might also resolve this contrasted response.

Intercalation of solid hydrogen into graphite under pressures

We present the Raman spectral evidence of pressure-induced intercalation of solid hydrogen into graphite to 60 GPa. The intercalation is evident by the emergence of two characteristic Raman bands of hydrogen (νo1 and νo2), which appear upon the solidification of hydrogen and disappear as all sp2-hybridized graphitic carbons convert to sp3-hybridized hexagonal diamond at 57 GPa. The νo1 and νo2 frequencies of intercalated hydrogen, 4250 and 4270 cm−1 at 10 GPa, are substantially higher than the νo of bulk hydrogen, 4228 cm−1 at the same pressure, indicating the presence of strong repulsive interactions between intercalated hydrogen molecules and graphite layers and, thereby, strong internal chemical pressures. Based on the spectral blue shift of intercalated hydrogen vibrons, we estimate the internal pressure to be ∼1 GPa at 10 GPa and ∼10 GPa at 50 GPa.

Site-specific hydrogen diffusion rates during clinopyroxene dehydration

The rate of hydrogen diffusion in clinopyroxene is relevant to interpreting hydrogen (“water”) concentrations in xenoliths, phenocrysts, and clinopyroxene-hosted melt inclusions to provide insight into the deep-earth water cycle and volcanic explosivity. Here, we determine bulk and site-specific hydrogen diffusivities in two diopsides and an augite by heating initially homogeneous water-bearing samples in a 1-atm CO/CO2 gas-mixing furnace at 800–1000 °C and oxygen fugacity at the quartz–fayalite–magnetite buffer and observing H-loss profiles. The O–H stretching range between wavenumbers 3000 and 4000 cm−1 in FTIR spectra is resolved into 4–6 peaks, each of which is assumed to represent a distinct defect site for the hydrogen, to determine peak-specific diffusivities using our previously published whole-block method. For the diopside from the Kunlun Mts. in China, Arrhenius relations are reported for peaks at 3645, 3617, 3540, 3443, and 3355 cm−1 based on measurements at 816, 904, and 1000 °C. Bulk and site-specific diffusivities are determined for the same set of peaks at 904 °C for the second diopside (Jaipur). The augite (PMR-53) was a triangular thin slab, and hydrogen diffusivities were determined for bulk hydrogen and peaks at 3620, 3550, 3460, and 3355 cm−1 in the thickness direction at 800 °C. Bulk hydrogen diffusivity in the Jaipur diopside is consistent with previous work, and hydrogen diffusivity in augite PMR-53 is slightly lower than the fast direction diffusivities measured || [100] and [001]* in Jaipur diopside. Both diopsides show 1–2 orders of magnitude differences in the peaks-specific diffusivities, with the fastest diffusivities at 3450 cm−1 and the slowest at 3645 cm−1. However, the hydrogen diffusivities in Jaipur diopside are 2–4 orders of magnitude higher than those in Kunlun diopside for bulk hydrogen and all peaks. Thus, peak-specific differences cannot by themselves adequately explain the 5 orders of magnitude range in hydrogen diffusivities observed experimentally in different diopsides. The results are broadly consistent with a previously proposed increase in hydrogen diffusivity in diopside with Fe up to 0.6–0.8 a.p.f.u., although there may be an opposing relationship with Al(IV). The results of this study and others predict high water diffusivities in Fe-bearing mantle xenolith clinopyroxene, on the order of ~10−9 to 10−11 m2/s at 1000 °C. The common observation of hydrogen zonation in mantle xenolith olivine, but not in clinopyroxene implies that hydrogen diffusion is much faster in olivine than in pyroxene, which then requires the operation of the fastest diffusion mechanism quantified in olivine and diffusivities in clinopyroxene at the lower end of this 2 orders of magnitude range. Such high diffusivities strongly suggest that water in mantle xenoliths has at least partially equilibrated with the host magma, and that the diffusion profiles observed in mantle xenolith olivine reflect only the final stage of ascent after water begins to exsolve.

Molecular simulation studies of hydrogen enriched methane (HEM) storage in Covalent Organic Frameworks

Instead of separating hydrogen and methane mixture, such as synthetic gas, to obtain pure hydrogen and methane as clean fuel, hydrogen enriched methane (abbreviated as HEM) storage in 33 different Covalent Organic Frameworks (COFs) were studied for the first time near ambient temperatures using Grand Canonical Monte Carlo (GCMC) simulation. The use of HEM for on-board combustion engine is also known to be able to improve combustion performance as well as decrease noxious emissions. HEM adsorption performance in the COFs was mainly evaluated from three different aspects: volumetric energy density of combustion of stored HEM, gravimetric energy density of combustion of stored HEM and hydrogen selectivity. Several properties of the COFs, such as surface area, porosity, pore size were calculated for establishing the correlation with the HEM adsorption performance. The effect of temperature, initial hydrogen/methane bulk composition and hydrogen and methane/hydrogen’s heat of adsorption (HOA) in COFs on the performance of HEM adsorption were also investigated. Our work suggested there exists a complex interplay of the properties of the COFs, temperature and bulk composition which influence the energy density of the adsorbed HEM as well as methane and hydrogen ratio in the adsorbed phase.

Hydrogen and chlorine abundances in the Kimberley formation of Gale crater measured by the DAN instrument on board the Mars Science Laboratory Curiosity rover

AbstractThe Dynamic Albedo of Neutron (DAN) instrument on board the Mars Science Laboratory Curiosity rover acquired a series of measurements as part of an observational campaign of the Kimberley area in Gale crater. These observations were planned to assess the variability of bulk hydrogen and neutron‐absorbing elements, characterized as chlorine‐equivalent concentration, in the geologic members of the Kimberley formation and in surface materials exposed throughout the area. During the traverse of the Kimberley area, Curiosity drove primarily over the “Smooth Hummocky” unit, a unit composed primarily of sand and loose rocks, with occasional stops at bedrock of the Kimberley formation. During the Kimberley campaign, DAN detected ranges of water equivalent hydrogen (WEH) and chlorine‐equivalent concentrations of 1.5–2.5 wt % and 0.6–2 wt %, respectively. Results show that as the traverse progressed, DAN observed an overall decrease in both WEH and chlorine‐equivalent concentration measured over the sand and loose rocks of the Smooth Hummocky unit. DAN measurements of WEH and chlorine‐equivalent concentrations in the well‐exposed sedimentary bedrock of the Kimberley formation show fluctuations with stratigraphic position. The Kimberley campaign also provided an opportunity to compare measurements from DAN with those from the Sample Analysis at Mars (SAM) and the Alpha‐Particle X‐ray Spectrometer (APXS) instruments. DAN measurements obtained near the Windjana drill location show a WEH concentration of ~1.5 wt %, consistent with the concentration of low‐temperature absorbed water measured by SAM for the Windjana drill sample. A comparison between DAN chlorine‐equivalent concentrations measured throughout the Kimberley area and APXS observations of corresponding local surface targets and drill fines shows general agreement between the two instruments.

The effect of the crystallographic orientation of ZnO on the surface adsorption and bulk diffusion of hydrogen

The thermal reactions of surface and subsurface hydrogen atoms on ZnO have been investigated. When the (0001) and surfaces of ZnO are exposed to atomic hydrogen at 370 K, surface-bound hydrogen atoms are formed on both surfaces. As the hydrogen exposure increases, H atoms diffuse into the bulk of ZnO. Upon heating, recombinative desorption of surface H atoms occurs at ∼450 K on both surfaces. Bulk H atoms also evolve as H2 molecules. In the case of ZnO(0001), bulk hydrogen is completely removed below 750 K. Evolution of bulk hydrogen on the face of ZnO takes place at much higher temperatures. Our observations confirm that the migration of H atoms in the ZnO crystal lattice takes place mostly along the c-axis.