Desorption Transition Research Articles

Temperature-dependent epitaxial evolution of carbon-free corundum α-Ga2O3 on sapphire

Unintentionally doped carbon impurities from organometallic precursors are primary sources of carrier compensation and mobility degradation in wide bandgap semiconductors, leading to lowered performance of power electronic devices. To address this challenge, carbon-free α-Ga2O3 single-crystalline thin films were heteroepitaxially grown on sapphire substrates by using gallium inorganic precursors through a mist chemical vapor deposition technique. Determined through a temperature dependence of growth rates, three distinct growth regimes are identified: the surface reaction limited regime below 480 °C, the mid-temperature mass-transport limited regime (480 °C–530 °C) and the high temperature limited regime related to desorption or phase transition. With an optimized around 530 °C, the densities of screw and edge dislocations are reduced to 7.17 × 106 and 7.60 × 109 cm−2, respectively. Notably, carbon incorporation was eliminated in the α-Ga2O3 grown by inorganic GaCl3, as evidenced by the absence of carbon-related vibrational bands in Raman scattering analysis, while crystalline quality was comparable to that grown with organometallic precursors. The high solubility of GaCl3 in water is expected to enable the rapid growth of high purity α-Ga2O3 with improved electronic transport performances.

Sorption Hysteresis: A Statistical Thermodynamic Fluctuation Theory.

Hysteresis is observed commonly in sorption isotherms of porous materials. Still, there has so far been no unified approach that can both model hysteresis and assess its underlying energetics. Standard approaches, such as capillary condensation and isotherms based on interfacial equations of state, have not proved to be up to the task. Here, we show that a statistical thermodynamic approach can achieve the following needs simultaneously: (i) showing why adsorption and desorption transitions may be sharp yet continuous; (ii) providing a simple (analytic) isotherm equation for hysteresis branches; (iii) clarifying the energetics underlying sorption hysteresis; and (iv) providing macroscopic and nanoscopic perspectives to understanding hysteresis. This approach identifies the two pairs of parameters (determinable by fitting experimental data) that are required to describe the hysteresis: the free energy per molecule within the pore clusters and the cluster size in the pores. The present paper focuses on providing mechanistic insights to IUPAC hysteresis types H1, H2(a), and H2(b) and can also be applied to the isotherm types IV and V.

Adsorption properties of black phosphorene for actinides U, Th, and Pu: A first‐principles study

AbstractDensity functional theory (DFT) calculations were carried out in this work to systematically investigate the adsorption properties of monolayer black phosphorene (BP) for actinide uranium (U), thorium (Th), and plutonium (Pu) atoms. More specifically, the linear response method was used and the HubbardUvalues of 2.97 and 2.61 eV were fitted describing the strong lattice point Coulomb interactions of the U and Pu 5forbitals, respectively. From the DFT + Ucalculations, it was demonstrated that the U, Th, and Pu atoms can be favorably adsorbed on the hollow sites of the BP surface, with an adsorption energy of 3.48, 4.93, and 0.98 eV, respectively. By analyzing the electronic structure, charge transfer, and highest occupied molecular orbital, it was revealed that the U and Th adatoms can induce the generation of impurity states within the band gap of BP and stabilize their adsorption by strongp‐dcoupling with the phosphorus atoms. In addition, from the electron–phonon coupling calculations, it was revealed that the temperature for the stable adsorption of the U and Th adatoms can reach as high as 500 and 673 K. By using the adsorption rate equation, the critical temperature for the adsorption–desorption transition of U and Pu adatoms was estimated to be 632 and 185 K, respectively, while the Th adatom did not tend to desorb. Our results clearly indicate that BP is an ideal adsorption and separation material for actinides with promising potential applications, such as uranium extraction from seawater.

Desorption of a Flexible Polymer with Activity from a Homogeneous Attractive Surface

The interfacial behavior of a flexible polymer with activity, which is named active Brownian polymer (ABPO), is studied by Langevin dynamics simulations. On the dependence of the adsorption strength and activity characterized by the Péclet number (Pe), the polymer displays two typical states on the surface: adsorption and desorption states. We find that the diffusion behavior of ABPO that is parallel to the surface yields the “active Rouse model” and the activity causes the adsorption–desorption transition at a certain adsorption strength. Particular attention is given to how the desorption time, τdes, changes with the activity. At intermediate activity, τdes displays an exponential decay with the inverse of the effective temperature, Teff ∝1 + Pe2/18, which is reminiscent of the mechanism of thermal activation. At higher activity, due to easily overcoming the attractive energy barrier, τdes ∝Pe–1 is found. At lower activity, a power-law dependence of τdes on the diffusion coefficient perpendicular to the surface (D⊥) is observed (τdes ∼D⊥–1.28). Further, we observed a non-monotonic dependence of desorption time on the rotation diffusion coefficient DR of the monomer and found that τdes exists on a scaling relation with the chain length N, τdes ∼ Nϕ, and the scaling exponent ϕ decreases with the increase of activity. Our results highlight that the activity can be used to regulate the polymer adsorption and desorption behavior.

Selective Adsorption of Confined Polymers: Self-Consistent Field Theory Studies

Selective adsorption of flexible polymers, where only a fraction of the “sticker” monomers can be adsorbed, confined to a cylindrical pore, is studied within the self-consistent field theory (SCFT). The selective adsorption, modeled with di-block, alternating, and random distributions of the sticker monomers along the chain, shows different adsorption–desorption transition behaviors: when the polymer chain possesses a high fraction of stickers, the di-block sequence adsorbs better than the alternating and the random sequence. On the other hand, when the polymer chain possesses a low fraction of stickers, the alternating sequence is the most adsorbed chain. Our theory indicates that the correlation between stickers and nonadsorbing monomers (neutrals) enhances the adsorption of polymers at a low fraction of sticker monomers, while it hinders the adsorption of polymers at a high fraction of sticker monomers, revealing the mechanism of adsorption efficiency determined by the sequence of sticker monomers in a polymer chain.

Adsorption Active Diblock Copolymers as Universal Agents for Unusual Barrier-Free Transitions in Stimuli-Responsive Brushes

We reconsider a recently proposed design for smart responsive brushes, which is based on a conformational transition in very dilutely embedded block copolymers with a surface active block (Qi et al., Macromolecules 53, 5326, 2020). Under certain conditions, the transition acquires an unusual character: it remains very sharp, but the barrier separating the adsorbed and desorbed states disappears completely. We show that these features are very robust with respect to changing almost all system parameters: the lengths of the inert and active blocks of the minority chain, the brush length, its density, and its polydispersity. The only relevant condition is that the inert block of the minority chain is long enough to extend outside the brush density profile. We develop an analytical theory that predicts the relevant characteristics of the transition and verify it with Monte Carlo simulations. We also show that the surprising universality of the transition properties is rooted in an underlying connection to the force-induced desorption transition, which is known to combine the features of the first- and second-order transitions with a pretransition fluctuation growth accompanied by phase coexistence.

Mechanistic insights and consequences of intrapore liquids in ethene, propene, and butene dimerization on isolated Ni2+ sites grafted within aluminosilicate mesopores

The stability, selectivity, and reactivity conferred by intrapore liquids onto Ni2+ species grafted within mesoporous aluminosilicates for the dimerization of C2-C4 alkenes are reported and interpreted by the preferential stabilization of dimer desorption transition states, which inhibit further growth during the surface sojourn that forms these dimers. The marked decrease in deactivation rate constants leads to catalyst half-lives > 600 h for all alkenes and to turnover rates (per Ni) at sub-ambient temperatures (240–260 K) that exceed those at the higher temperatures previously used for Ni-based solid catalysts (350–500 K). These effects appear at the relative pressures required for alkene capillary condensation within Ni-Al-MCM-41 catalysts with different mesopore diameters and are also evident when the intrapore liquids consist of unreactive alkanes, indicative of solvation effects conferred by non-covalent interactions mediated by dispersion forces. For all alkenes, turnover rates (per Ni) are unaffected by Ni content until each acidic proton in the Al-MCM-41 is replaced by one Ni atom, consistent with the involvement of grafted (Ni-OH)+ monomers as active centers and excess Ni leading to inactive NiO species. The stable nature of these catalysts allows accurate mechanistic assessments and a demonstration of the kinetic relevance of the CC coupling elementary steps on essentially bare (Ni-OH)+ centers, leading to rates described by second-order dimerization constants (αn′) and adsorption constants (βn′) for each alkene (Cn); these parameters are influenced only slightly by intrapore liquids, because they reflect free energies of formation of surface-bound species and transition states from gaseous precursors. These αn′ and βn′ parameters increase with alkene size, with the enthalpic component of αn′ increasing from 21 to 46 kJ mol−1 and the activation entropy becoming more negative (−103 to −187 J (mol K)−1), consistent with CC coupling transition states that occur late along the reaction coordinate and resemble its bound dimer product. The low C3n/C2n ratios observed in the presence of intrapore liquids (<0.04, for n = 2–4) reflect the endothermic nature of dimer desorption events, for which product-like transition states benefit from the solvation of desorbing products by a non-polar liquid phase, thus allowing detachment from Ni centers before subsequent growth. The inhibition of such growth events accounts not only for the unique dimer selectivity observed, but also for the unprecedented stability conferred by intrapore liquids by inhibiting the formation of oligomers that bind more strongly with increasing chain size, as evident from the βn′ values reported here for alkene reactants.

Critical adsorption of multiple polyelectrolytes onto a nanosphere: splitting the adsorption-desorption transition boundary.

Employing extensive Monte Carlo computer simulations, we investigate in detail the properties of multichain adsorption of charged flexible polyelectrolytes (PEs) onto oppositely charged spherical nanoparticles (SNPs). We quantify the conditions of critical adsorption-the phase-separation curve between the adsorbed and desorbed states of the PEs-as a function of the SNP surface-charge density and the concentration of added salt. We study the degree of fluctuations of the PE-SNP electrostatic binding energy, which we use to quantify the emergence of the phase subtransitions, including a series of partially adsorbed PE configurations. We demonstrate how the phase-separation adsorption-desorption boundary shifts and splits into multiple subtransitions at low-salt conditions, thereby generalizing and extending the results for critical adsorption of a single PE onto the SNP. The current findings are relevant for finite concentrations of PEs around the attracting SNP, such as the conditions for PE adsorption onto globular proteins carrying opposite electric charges.

Angle-Dependent Atomic Force Microscopy Single-Chain Pulling of Adsorbed Macromolecules from Planar Surfaces Unveils the Signature of an Adsorption-Desorption Transition.

The adsorption–desorption behavior of polymer chains is at the heart of macromolecular surface science and technology. With the current developments in atomic force microscopy (AFM), it has now become possible to address the desorption problem from the perspective of a single macromolecule. Here, we report on desorption of single polymer chains on planar surfaces by AFM-based single molecule force spectroscopy (SMFS) as a function of the pulling angle with respect to the surface-normal direction. SMFS experiments were performed in water with various substrates using different polymers covalently attached to the AFM probe tip. End-grafting at the AFM tip was achieved by surface-initiated polymerization using initiator functionalized tips. We found that the desorption force increases with a decreasing pulling angle, i.e., an enhanced adhesion of the polymer chain was observed. The magnitude of the desorption force shows a weak angular dependence at pulling angles close to the surface normal. A significant increase of the force is observed at shallower pulling from a certain pulling angle. This behavior carries the signature of an adsorption–desorption transition. The angular dependence of the normalized desorption force exhibits a universal behavior. We compared and interpreted our results using theoretical predictions for single-chain adsorption–desorption transitions.

Sensitive determination of brassinosteroids by solid phase boronate affinity labeling coupled with liquid chromatography-tandem mass spectrometry

Brassinosteroids (BRs) are regarded as the sixth plant hormone that is widely distributed in the plant kingdom. Sensitive quantification of BRs will be greatly benefit to illuminate the detail mechanisms about how BRs play crucial role in plant developmental processes such as cell division, cell expansion, cytodifferentiation, seed germination, vegetative growth and resisting biological or abiotic stress. In the current study, we developed a method for rapid and sensitive determination of endogenous BRs in plant tissues by combining LC–MS and a novel sample preparation strategy, in which the plant tissue extract was supplied to solid phase boronate affinity labeling and extraction, followed by desorption and salt-induced phase transition extraction for further purification. Under the optimized conditions, good linearity was obtained for 6 BR with correlation coefficients (r) ranging from 0.9988 to 0.9999. The limits of detection (LODs, S/N=3) ranged from 1.4 to 2.8pgmL−1. The recoveries were between 93.4% and 116.2% with the relative standard deviations (RSDs) ranging from 2.8% to 15.8%. Finally, the developed method was successfully applied to the analysis of 6 endogenous BR in various plant tissues including 20mg FW Oryza sativa shoot, 10mg FW Oryza sativa root, 20mgFW Arabidopsis thaliana shoot, 4 Arabidopsis thaliana flowers (2.8mg) and one Brassica napus stamen (3.0mg) with concentration ranging from 0.26 to 157.28ngg−1FW.

Critical adsorption of polyelectrolytes onto planar and convex highly charged surfaces: the nonlinear Poisson–Boltzmann approach

We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems.

Abnormal polymer transport in crowded attractive micropost arrays.

We investigate polymer diffusion in a quasi-two-dimensional environment decorated with attractive cylindrical posts using Langevin dynamics simulation. We find that the polymer diffusivity has non-monotonic dependence on the post array density. This diffusive behavior strongly depends on the adsorption-desorption transition and the critical adsorption strength εc. For ε < εc, the polymer undergoes normal diffusion and the diffusivity decreases as the post density increases due to the reduction of the void volume. For ε > εc, polymer dynamics is strongly mediated by post adsorption, and we observe a regime where the polymer diffusivity increases as the post density increases. The polymer diffusivity reaches a maximum, which can be attributed to cross-post translation enabled by large polymer conformation fluctuations. We find both cross-post transport and polymer conformation fluctuations strongly depend on the post absorption strength and the chain length.

Adsorption of a homopolymer chain onto a heterogeneous stripe-patterned surface studied using a directed walk model of the polymer

Adsorption of a homopolymer chain onto a heterogeneous stripe-patterned surface is studied in the framework of the three-dimensional partially directed walk model of the polymer by using the generating function approach. Both annealed random and quenched regular (periodic) stripe-patterned surfaces are considered. For random surfaces the adsorption–desorption transition temperature is analyzed. Comparison of the transition points for random and the ‘equivalent’ regular surfaces (having the same average stripe width) shows that the transition temperature is higher for the random surface. For regular stripe-patterned surfaces, the phase diagram is derived and the temperature dependences of the main conformational and thermodynamic characteristics of the adsorbed chain are obtained. It is demonstrated that adsorption is accompanied by the orientation of the polymer along the stripes. At low temperature (in the strong adsorption regime), the polymer chain is localized onto a single adsorbing stripe. When the surface consists of narrow adsorbing stripes separated by wide nonadsorbing stripes, an additional second peak on the heat capacity temperature dependence emerges, which indicates that adsorption occurs as a two-stage process where chain localization on a single adsorbing stripe is completed prior to its full adsorption on this stripe. This effect is enhanced with increasing distance between adsorbing stripes.

Transitions of tethered chain molecules under tension.

An applied tension force changes the equilibrium conformations of a polymer chain tethered to a planar substrate and thus affects the adsorption transition as well as the coil-globule and crystallization transitions. Conversely, solvent quality and surface attraction are reflected in equilibrium force-extension curves that can be measured in experiments. To investigate these effects theoretically, we study tethered chains under tension with Wang-Landau simulations of a bond-fluctuation lattice model. Applying our model to pulling experiments on biological molecules we obtain a good description of experimental data in the intermediate force range, where universal features dominate and finite size effects are small. For tethered chains in poor solvent, we observe the predicted two-phase coexistence at transitions from the globule to stretched conformations and also discover direct transitions from crystalline to stretched conformations. A phase portrait for finite chains constructed by evaluating the density of states for a broad range of solvent conditions and tensions shows how increasing tension leads to a disappearance of the globular phase. For chains in good solvents tethered to hard and attractive surfaces we find the predicted scaling with the chain length in the low-force regime and show that our results are well described by an analytical, independent-bond approximation for the bond-fluctuation model for the highest tensions. Finally, for a hard or slightly attractive surface the stretching of a tethered chain is a conformational change that does not correspond to a phase transition. However, when the surface attraction is sufficient to adsorb a chain it will undergo a desorption transition at a critical value of the applied force. Our results for force-induced desorption show the transition to be discontinuous with partially desorbed conformations in the coexistence region.

On the existence of a hysteresis loop in open and closed end pores

We have studied the adsorption of argon at 87 K in slit pores of finite length with a smooth graphitic potential, open at both ends or closed at one end. Simulations were carried out using conventional GCMC (grand canonical Monte Carlo) or kMC (kinetic Monte Carlo) in the canonical ensemble with extremely long Markov chain, of at least 2 × 108 configurations; selected simulations with much longer Markov chains do not show any change in the results. When the pore width is in the micropore range (0.65 nm), type I isotherms are obtained for both pore models and for both simulation methods. However, wider pores (1, 2 and 3 nm in width) all exhibit hysteresis loops in the GCMC simulations, while in the canonical ensemble simulations, the isotherms pass through a sigmoid van der Waals type loop in the transition region. This loop locates the true equilibrium transition. For the pores with one closed end, this transition is close to, or coincides with, the adsorption branch of the GCMC hysteresis loop, but for the open-ended pores, it is more closely associated with the desorption branch. In a separate study of adsorption hysteresis in an infinitely long slit pore, using both simulation techniques, the van der Waals loop follows the adsorption branch of the GCMC isotherm to the transition, then reverts to a long vertical section that falls midway between the two hysteresis branches and finally moves to the desorption transition close to the evaporation pressure. An examination of molecular distributions inside the pores reveals two coexisting phases in the canonical simulations, whereas in the grand canonical simulations, the molecules are uniformly distributed along the length of the pores.

Effect of ligand chain length on hydrophobic charge induction chromatography revealed by molecular dynamics simulations

Hydrophobic charge induction chromatography (HCIC) is a mixed-mode chromatography which is advantageous for high adsorption capacity and facile elution. The effect of the ligand chain length on protein behavior in HCIC was studied. A coarse-grain adsorbent pore model established in an earlier work was modified to construct adsorbents with different chain lengths, including one with shorter ligands (CL2) and one with longer ligands (CL4). The adsorption, desorption, and conformational transition of the proteins with CL2 and CL4 were examined using molecular dynamics simulations. The ligand chain length has a significant effect on both the probability and the irreversibility of the adsorption/desorption. Longer ligands reduced the energy barrier of adsorption, leading to stronger and more irreversible adsorption, as well as a little more unfolding of the protein. The simulation results elucidated the effect of the ligand chain length, which is beneficial for the rational design of adsorbents and parameter optimization for highperformance HCIC.

Photoluminescence measurements and molecular dynamics simulations of water adsorption on the hydrophobic surface of a carbon nanotube in water vapor.

Hydrophilicity or hydrophobicity is a macroscopic property of the surface, and its atomic scale understanding has not been established. We have studied adsorption of water molecules on the "hydrophobic" carbon nanotube surface at room temperature in water vapor. Based on optical measurements of individual single-walled carbon nanotubes suspended between micropillars in water vapor together with molecular dynamics simulations, we found that water molecules form a stable adsorption layer of 1-2ML thickness on the nanotube surface and they show rapid adsorption and desorption transition at a critical pressure. This adsorption layer is created by lateral hydrogen bonding of water molecules confined in the weak van der Waals potential of the surface. In spite of hydrophobic hydration, carbon nanotubes exhibit hydrophobicity macroscopically.

Spontaneous desorption and phase transitions of self-assembled alkanethiol and alicyclic thiol monolayers chemisorbed on Au(1 1 1) in ultrahigh vacuum at room temperature

Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface structure and adsorption conditions of hexanethiol (HT) and cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) as a function of storage period in ultrahigh vacuum (UHV) conditions of 3×10−7Pa at room temperature (RT). STM imaging revealed that after storage for 7days, HT SAMs underwent phase transitions from c(4×2) phase to low coverage 4×√3 phase. This transition is due to a structural rearrangement of hexanethiolates that results from the spontaneous desorption of chemisorbed HT molecules on Au(111) surface. XPS measurements showed approximately 28% reduction in sulfur coverage, which indicates desorption of hexanethiolates from the surfaces. Contrary to HT SAMs, the structural order of CHT SAMs with (5×2√3)R35° phase completely disappeared after storage for 3 or 7days. XPS results show desorption of more than 80% of the cyclohexanethiolates, even after storage for 3days. We found that spontaneous desorption of CHT molecules on Au(111) in UHV at RT occurred quickly, whereas spontaneous desorption of HT molecules was much slower. Thermal desorption spectroscopy (TDS) results suggest CHT SAMs in UHV at RT can desorb more efficiently than HT SAMs due to formation of thiol desorption fragments that result from chemical reactions between surface hydrogen atoms and thiolates on Au(111) surfaces. This study clearly demonstrated that organic thiols chemisorbed on gold surfaces are desorbed spontaneously in UHV at RT and van der Waals interactions play an important role in determining the structural stability of thiolate SAMs in UHV.

Polyelectrolyte Adsorption onto Oppositely Charged Interfaces: Image-Charge Repulsion and Surface Curvature

We analyze theoretically the influence of low-dielectric boundaries on the adsorption of flexible polyelectrolytes onto planar and spherical oppositely charged surfaces in electrolyte solutions. We rationalize to what extent polymer chains are depleted from adsorbing interfaces by repulsive image forces. We employ the WKB (Wentzel-Kramers-Brillouin) quantum mechanical method for the Green function of the Edwards equation to determine the adsorption equilibrium. Scaling relations are determined for the critical adsorption strength required to initiate polymer adsorption onto these low-dielectric supports. Image-force repulsion shifts the equilibrium toward the desorbed state, demanding larger surface charge densities and polyelectrolyte linear charge densities for the adsorption to take place. The effect is particularly pronounced for a planar interface in a low-salt regime, where a dramatic change in the scaling behavior for the adsorption-desorption transition is predicted. For the adsorbed state, polymers with higher charge densities are displaced further from the interface by image-charge repulsions. We discuss relevant experimental evidence and argue about possible biological applications of the results.

Mechanical desorption of a single chain: Unusual aspects of phase coexistence at a first-order transition

The phase transition occurring when a single polymer chain adsorbed at a planar solid surface is mechanically desorbed is analyzed in two statistical ensembles. In the force ensemble, a constant force applied to the nongrafted end of the chain (that is grafted at its other end) is used as a given external control variable. In the z-ensemble, the displacement z of this nongrafted end from the surface is taken as the externally controlled variable. Basic thermodynamic parameters, such as the adsorption energy, exhibit a very different behavior as a function of these control parameters. In the thermodynamic limit of infinite chain length the desorption transition with the force as a control parameter clearly is discontinuous, while in the z-ensemble continuous variations are found. However, one should not be misled by a too-naive application of the Ehrenfest criterion to consider the transition as a continuous transition: rather, one traverses a two-phase coexistence region, where part of the chain is still adsorbed and the other part desorbed and stretched. Similarities with and differences from two-phase coexistence at vapor-liquid transitions are pointed out. The rounding of the singularities due to finite chain length is illustrated by exact calculations for the nonreversal random walk model on the simple cubic lattice. A new concept of local order parameter profiles for the description of the mechanical desorption of adsorbed polymers is suggested. This concept give evidence for both the existence of two-phase coexistence within single polymer chains for this transition and the anomalous character of this two-phase coexistence. Consequences for the proper interpretation of experiments performed in different ensembles are briefly mentioned.