We introduce a new high-temperature adsorption calorimeter that approaches the ideal limit of a heat detector whereby the signal at any time is proportional to the heat power being delivered to the sample and prove its sensitivity for measuring pulse-to-pulse heats of half-reactions during atomic layer deposition (ALD) at 400 K. The heat dynamics of amorphous Al2O3 growth via sequential self-limiting surface reaction of trimethylaluminum (TMA) and H2O is clearly resolved. Calibration enables quantitation of the exothermic TMA and H2O half-reactions with high precision, −343 kJ/mol TMA and −251 kJ/mol H2O, respectively. A time resolution better than 1 ms is demonstrated, allowing for the deconvolution of at least two distinct surface reactions during TMA microdosing. It is further demonstrated that this method can provide the heat of reaction versus extent of reaction during each precursor’s half-reaction, thus providing even richer mechanistic information on the surface processes involved. The broad applicability of this novel calorimeter is demonstrated through excellent signal-to-noise ratios of less exothermic ALD half-reactions to produce TiO2 and MnO.

We report measurements of adsorption isotherms and the determination of the isosteric heats of adsorption of several small gases (H, D, Ne, N, CO, CH, CH, Ar, Kr, and Xe) on the metal-organic framework (MOF) NU-1000, which is one of the most thermally stable MOFs. It has transition-metal nodes of formula Zr(μ-OH)(μ-O)(OH)(OH) that resemble hydrated ZrO clusters and can serve as catalysts or catalyst supports. The linkers in this MOF are pyrenes linked to the nodes via the carboxylate groups of benzoates. The broad range of adsorbates studied here allows us to compare trends both with adsorption on other surfaces and with density functional calculations also presented here. The experimental isotherms indicate similar filling of the MOF surface by the different gases, starting with strong adsorption sites near the Zr atoms, a result corroborated by the density functional calculations. This adsorption is followed by the filling of other adsorption sites on the nodes and organic framework. Capillary condensation occurs in wide pores after completion of a monolayer. The total amount adsorbed for all the gases is the equivalent of two complete monolayers. The experimental isosteric heats of adsorption are nearly proportional to the atom-atom (or molecule-molecule) Lennard-Jones well-depth parameters of the adsorbates but ∼13-fold larger. The density functional calculations show a similar trend but with much more scatter and heats that are usually greater (by 30%, on average).

The nature and energy of the reactions between calcium vapor and the internal surfaces of the metal–organic framework (MOF) NU-1000 have been studied by adsorption microcalorimetry, low energy He+ ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and Kohn–Sham density functional theory (DFT). NU-1000 is one of the most stable MOFs with transition-metal-oxide nodes, and thus it is of interest as a potential catalyst or catalytic support when modified with other metals. The reaction heats of Ca with NU-1000 are high below 2 monolayers (ML) Ca coverage (570–366 kJ/mol), attributed (based on DFT) to Ca reacting first with free benzoic acid functionalities or water impurities, then with H2O and OH groups on the Zr6 nodes to produce Ca­(OH)2 clusters. With higher Ca doses, the heat of Ca reaction decreases asymptotically to the sublimation enthalpy of bulk Ca (178 kJ/mol), attributed to the formation of Ca­(solid) nanoparticles on the external surface, which only occurs after all of the H2O and OH groups are titrated deeply enough (∼20 nm) such that slow Ca diffusion prevents further reaction.

The adsorption of Ca vapor on phenyl-C 61 -butyric acid methyl ester (PCBM) at 300 K has been studied by X-ray photoelectron spectroscopy (XPS), lowenergy He + ion scattering spectroscopy (LEIS), adsorption microcalorimetry, and atomic beam/surface scattering. This interface commonly occurs in some of the highest efficiency organic solar cells. It is found that over 10 nm of the PCBM undergoes aggressive reaction with the Ca vapor to make the Ca carboxylate of PCBM. This thick reacted layer, which was not previously known to be present, lies at the interface between the metallic Ca film and PCBM and is expected to influence charge transfer across that interface during photovoltaic operation. The heat of Ca adsorption is very high below 0.03 ML (800−850 kJ/mol) due to reaction of Ca with impurities. Between 0.05 and 0.4 ML, the heat of adsorption is 624 kJ/mol and nearly constant. This heat is assigned to the reaction of Ca with subsurface methyl ester groups to form the Ca carboxylate of PCBM. This assignment is supported by the shift of the O 1s XPS peak of PCBM toward lower binding energy (BE) due to this reaction with Ca, and the absence of Ca LEIS signal, below 0.4 ML coverage. Conversely, the C 1s XPS peak shifts toward higher BE due to downward band bending. Beyond 0.4 ML, the heat of adsorption decreases nearly exponentially to the sublimation enthalpy of Ca (178 kJ/mol) by 3 ML, attributed to the formation of Ca(solid) nanoparticles on the surface and eventually a continuous Ca film. This model is supported by LEIS. Impinging Ca atoms face a kinetic competition between diffusing subsurface to react with methyl ester groups of PCBM and the formation and growth of three-dimensional Ca clusters on the surface. The total extent of reaction of Ca with subsurface ester groups to make the Ca carboxylate of PCBM is equivalent to ∼14 layers of reacted PCBM molecules or ∼13 nm of reacted depth.