HF Adsorption Research Articles

Novel dialkylphosphinic acid modified Merrifield resins: Synthesis, characterization, and their adsorption and separation behaviors for zirconium and hafnium

Deep separation of Zr and Hf has always been a great challenging due to their very similar physicochemical properties. In this study, we established a method to synthesize two novel extraction chromatographic resins which graft dialkylphosphinic acid groups through amide group. The dialkylphosphinic acid modified resins were characterized by FT-IR, EA, SEM-EDS, XPS, and BET. Their adsorption and separation behaviors for Zr and Hf were investigated, and the adsorption mechanism was studied. The Zr and Hf adsorption onto Mer-CON-POOH-272 and Mer-CON-POOH-HYY2 both fit pseudo-second order model and Langmuir isotherm model. Mer-CON-POOH-HYY2 has stronger affinity to Zr and Hf than Mer-CON-POOH-272. Mer-CON-POOH-272 has a theoretical maximum loading capacity of 5.663 mg·g−1 Zr and 11.111 mg·g−1 Hf, while Mer-CON-POOH-HYY2 has a higher theoretical maximum loading capacity of 8.376 mg·g−1 Zr and 13.157 mg·g−1 Hf. Dialkylphosphinic acids are the functional groups of Mer-CON-POOH-272 and Mer-CON-POOH-HYY2 that interact with Zr and Hf during adsorption. They adsorb Zr and Hf through cation exchange mechanism, which can be expressed as M(SO4)32– + 2HL ⇌ M(SO4)L2 +.2H+ + 2SO42-. With increasing H2SO4 concentration, the Zr and Hf extraction percentages and their separation efficiency reduce. The DFT calculation reveals that the adsorption process is accompanied by charge transfer. Adsorption occurs mainly at P = O and P-O positions. The affinity of the two resins for Hf is higher than that of Zr, which is the reason why the resin preferentially adsorbs Hf.

Adsorption of HF and H2S on α-Al2O3 (0001) Surfaces: A DFT Study

Ex-service SF6 adsorbents in SF6 gas-insulated electric equipment contain many toxic substances. Inside, HF and H2S are two typical toxic gases. Based on the first principle, the interaction process between HF/H2S and α-Al2O3 (0001) surfaces was calculated using the density functional theory (DFT). The results showed that the adsorption of HF on α-Al2O3 (0001) is stronger than that of H2S. Under the five adsorption sites, the adsorption effect of HF-H and HF-F was similar. At O-2 site, the adsorption energy of H2S-H adsorption configuration is significantly higher than that of the other four sites. The density of states (DOS) indicated that new peaks appeared after adsorption. The DOS and partial density of states (PDOS) indicated that the adsorption of HF and H2S occurs via chemical adsorption. The DOS and PDOS shifted to the right when the S atom was approaching, proving that the system shifts to instability. Compared with the energy gap of α-Al2O3 (0001), HF and H2S adsorption systems decreased significantly. The energy gap of the HF adsorption system was 1.173 eV larger than that of the H2S system and the geometry was relatively stable, which is consistent with the DOS and PDOS adsorption calculation results. Thus, the adsorption of HF and H2S on α-Al2O3 (0001) surfaces was clearly different. The findings of this study may provide theoretical guidance for the adsorption of other gases or developing a new adsorbent.

Sensing performance of CdPc monolayer toward the SF6 decomposition gases: A DFT study

The rapid and effective detection of SF6 decomposition species is beneficial to diagnose the faults type and severity of gas insulated switchgear (GIS). In this work, the adsorption behaviors of SF6 decomposition products (HF, H2S SO2, SOF2 and SO2F2) on the Cd decorated phthalocyanine (CdPc) monolayer were systematically studied by using the first-principles calculations, the electronic properties of different adsorption systems and their sensing characteristics were also discussed in detail. The results show that the CdPc monolayer is both thermodynamically and dynamically stable, the H2S and SO2 are all chemically adsorbed on the CdPc monolayer, but the large adsorption strength restrict its application in gas sensor. By contrast, the CdPc monolayer has the appropriate adsorption strength for HF, SOF2 and SO2F2 gases with the Wads of −0.60 to −0.71 eV, as proved by the analysis of charge transfer, charge density difference and density of state. Besides, the work function of CdPc monolayer undergoes remarkable change after HF, SOF2 and SO2F2 adsorption, and the corresponding recovery time is predicted to be 0.01 s, 1.01 s and 0.46 s at room temperature. Therefore, CdPc monolayer has a great potential to be a chemi-resistive material with high sensitivity for HF, SOF2 and SO2F2 detection in GIS.

A Mechanistic Study of the HF Pulse in the Thermal Atomic Layer Etch of Crystalline and Amorphous HfO2

HfO2 is a high-k material that is crucial in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of this material at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow the fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique that enables a high level of control over film thickness. Thermal ALE, which is growing in importance, uses self-limiting halogenation (e.g. HF exposure) producing a non-volatile modified layer. Subsequent ligand exchange reactions remove up to a monolayer of the metal oxide. This modern approach for controlled etching is the reverse of ALD. In gate oxides, the use of amorphous materials is better than crystalline films as the lack of grain boundaries allow for a high breakdown voltage. Amorphous materials have a lower density than crystalline materials.Experimental studies have shown that HfO2 amorphous films have higher etch rates per cycle than crystalline HfO2 films regardless of the fluorination reactant. Given that it is difficult to investigate ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the etch of different materials. This contribution presents first-principles density functional theory modelling to examine the etch chemistry of crystalline and amorphous films of HfO2. HF molecules adsorb on the surface of HfO2 by forming hydrogen bonds and may remain intact or dissociate to form Hf-F and O-H bonds. The adsorption of one HF molecule at the bare surface of both the crystalline and amorphous models results in dissociative adsorption at all binding sites. For multiple HF adsorption, we find mixed molecular and dissociative adsorption of HF molecules at the bare surfaces of the crystalline and amorphous models of HfO2. We also present a thermodynamic analysis approach to compare reaction models representing the self-limiting (SL) and continuous spontaneous etch (SE) processes taking place during an ALE pulse.

Hydroxylation and fluorination on γ-Al2O3(110) surfaces: First-principles and transition state calculations

In this paper, the adsorption mechanism of HF and H2O on the γ-Al2O3 and hydroxylated surfaces were built and calculated. The calculated results show that the main active adsorption sites are concentrated near the octahedral hole sites. There are two types of chemical adsorption of H2O: ▲Al-O-H2 and ▲Al-O-H*▲Oa-H. The adsorption energy range is -0.30~-1.75eV. With the increase of adsorption energy, H2O gradually inclines to the surface to form OH. The chemisorption form of HF is ▲Al-F and ▲O-H, with the adsorption energy in the range of -0.61 to -2.41eV. At low coverage, there were more active adsorption sites on the surface, and the interaction between OH and HF adsorption was less. At high coverage, the active site occupied by OH tended to be saturated, and HF could not break bonds to form Al-F, so the adsorption effect decreased. When the HF coverage is about 0.4, the binding is the most stable, and the adsorption energy is -2.97eV.

First-Principles Study of Au-Doped InN Monolayer as Adsorbent and Gas Sensing Material for SF6 Decomposed Species.

As an insulating medium, sulfur hexafluoride (SF6) is extensively applied to electrical insulation equipment to ensure its normal operation. However, both partial discharge and overheating may cause SF6 to decompose, and then the insulation strength of electrical equipment will be reduced. The adsorption properties and sensing mechanisms of four SF6 decomposed components (HF, SO2, SOF2 and SO2F2) upon an Au-modified InN (Au-InN) monolayer were studied in this work based on first-principles theory. Meanwhile, the adsorption energy (Ead), charge transfer (QT), deformation charge density (DCD), density of states (DOS), frontier molecular orbital and recovery property were calculated. It can be observed that the structures of the SO2, SOF2 and SO2F2 molecules changed significantly after being adsorbed. Meanwhile, the Ead and QT of these three adsorption systems are relatively large, while that of the HF adsorption system is the opposite. These phenomena indicate that Au-InN monolayer has strong adsorption capacity for SO2, SOF2 and SO2F2, and the adsorption can be identified as chemisorption. In addition, through the analysis of frontier molecular orbital, it is found that the conductivity of Au-InN changed significantly after adsorbing SO2, SOF2 and SO2F2. Combined with the analysis of the recovery properties, since the recovery time of SO2 and SO2F2 removal from Au-InN monolayer is still very long at 418 K, Au-InN is more suitable as a scavenger for these two gases rather than as a gas sensor. Since the recovery time of the SOF2 adsorption system is short at 418 K, and the conductivity of the system before and after adsorption changes significantly, Au-InN is an ideal SOF2 gas-sensing material. These results show that Au-InN has broad application prospects as an SO2, SOF2 and SO2F2 scavenger and as a resistive SOF2 sensor, which is of extraordinary meaning to ensure the safe operation of power systems. Our calculations can offer a theoretical basis for further exploration of gas adsorbent and resistive sensors prepared by Au-InN.

First-principles studies of HF and HCl adsorption over graphene.

In this paper, the adsorption characteristics of HF and HCl over graphene were studied by the first-principles method. The results showed that the adsorption of HCl over graphene was a weak chemical adsorption, while HF was a weak physical adsorption. The density of states showed that HCl and graphene at - 4.3eV are relative to the Fermi level. At the same time, there is no obvious change and hybridization between HF-graphene system near the Fermi level. Furthermore, when HCl and HF molecules adsorbed over the graphene simultaneously, two optimal adsorption structures would be chosen to investigate how HCl and HF molecules jointly affected adsorption properties. The result showed that two gas molecules adsorbed over graphene could enhance the adsorption effect and influenced electronic distribution. Graphical abstract HF and HCl over graphene belong to weak physical and chemical adsorption separately. Two gases on graphene surface can be enhanced.

DFT Study of HF and H2O Adsorption on Zn and Ga-Doped Single-Walled Carbon Nanotube

Quantum chemistry calculations were performed using density functional theory (DFT) to evaluate electronic and sensing properties in the presence and absence of gas molecules HF and H2O of pristine and dope carbon nanotubes (CNTs) zigzag (6, 0), and CNTs doped with gallium and zinc, which have a significant effect on improving the sensing properties. The results appear that the gas molecules (HF and H2O) show weak physisorption on ZnGa-doping CNT with adsorption energy (Ead) ranging from –0.95 to ‒0.21 eV, while a powerful chemisorption molecule on pristine CNT ranging from 0.05 to 0.4 eV. Where we note that the total energy of the cases above increased dramatically at add dopants and with adsorption of gas molecules with total energy (Etotal) ranging from –103 461 to –0.49651 eV. Through our results, we can recommend the use of ZnGa-doped CNT as a gas chemical sensor.

First principle study of HF molecule adsorption on TiO2 (110) surface

Titanium and its alloy components are one of the most important technological materials, which has found extensive application in various industries. However, surface defects play a key role in the mechanical properties of these components. Currently, wet chemical etching is one of the most important procedure for surface processing due to the presence of HF since it can etch metal oxide. Therefore, there still a need to investigate the etching mechanism. In this work, adsorption of HF on TiO2(110) surface has been studied using density functional theory to investigate the fundamental process of etching. HF molecule is adsorbed on the TiO2 surface by dissociation to form Ti-F and O-H species. The interaction between HF and TiO2surface become more favorable at high HF coverage. The pre-adsorbed of water molecule is favorable for HF adsorption process, which is in good agreement with experimental results. Fluorination processes show the formation and desorption of water intermediate at 0.50 ML coverage. We also investigated the relation between the work function and Mulliken charge for HF adsorption. The results indicate that the interaction of F on the surface attracts electrons due to its higher electronegativity than oxygen. Our results suggest that adsorption of HF is considered chemisorption process.

Detection of HF and H2S with pristine and Ti-embedded twin graphene: A density functional theory study

Adsorption of HF and H2S on pristine and Ti-embedded twin graphene sheets is studied utilizing density functional theory (DFT) calculations to understand the capability of these materials for detecting these toxic gases. The optimal configurations, adsorption energies, and charge transfer are calculated. The small adsorption energies and large adsorption distance indicated that HF and H2S are weakly physisorbed on pristine twin graphene sheets. The adsorption strength increases after decorating twin graphene with a Ti atom. Electronic properties including band structures and density of states reveal that both pristine and Ti-embedded twin graphene sheets possess semiconducting properties. Adsorption of HF and H2S decreased the energy band gap of both pristine and Ti-embedded twin graphene sheets. It was found that embedding Ti to twin graphene increases the sensitivity of the material to HF and H2S adsorption, which is mainly due to the stronger integration between adsorbed molecules and Ti-embedded sheet. Hence, current research introduces Ti-embedded twin graphene as a high sensitive, and low cost gas sensor candidate for detecting the toxic gases.

Using Single-Layer HfS2 as Prospective Sensing Device Toward Typical Partial Discharge Gas in SF6-Based Gas-Insulated Switchgear

We performed density functional theory study on the adsorption properties of five typical SF6 decomposition products in gas-insulated switchgear on single-layer HfS2. The adsorption structure, adsorption energy, and electron transfer were discussed. We also considered the physical and chemical interactions between the adsorbed gas molecule and the HfS2 monolayer with the discussion of the charge density difference configurations, the electron localization functional configurations, and the density of states (DOSs). To assess the sensing property of single-layer HfS2-based gas sensor, we built a simulated device with three parts, including left lead, scattering region, and right lead and the response to different gas molecules was evaluated. The results indicated that the SO2F2 molecule exhibited the largest adsorption energy but the HF molecule had the largest electron transfer. Only H2S adsorption leads to the obvious decrease of the bandgap. All the gas molecules could have chemical interactions with the HfS2 monolayer with different degrees based on the DOS analysis. For the simulated sensing device, according to the nonequilibrium Green’s function method, it showed better response and selectivity in the voltage range between 1.2 and 1.6 V. The device exhibited the maximum current after HF adsorption and the minimum current after SO2F2 adsorption.

A Promising Gas Sensor Based on Monolayer $\alpha $-SbN to Detect SO2 Among SF6 Decompositions

The adsorption of five decomposed components (SO2, SOF2, SO2F2 , H2S, and HF) of SF6 on monolayer -SbN was investigated using the density functional theory. Based on the adsorption configurations, we built a simulated gas sensing device using monolayer -SbN, and the response to five gases was discussed and compared. The results showed that SO2 and HF adsorption exhibited relatively larger adsorption energy and electron transfer with states overlaps in the density of states. Only SO2 adsorption leads to the obvious decrease of band gap from about 1.47 to 1.15eV. The currentvoltage property demonstrates that the simulated gas sensor only had high response to SO2 among these five gases. Our work suggests that there is a promising prospect for detecting SO2 among SF 6 decompositions using a monolayer -SbN based gas sensor.

New challenge of microporous metal-organic frameworks for adsorption of hydrogen fluoride gas

To avoid the health threat and environmental pollution as well as to preserve the life of industrial equipment, it is very important and necessary to find an efficient method for detecting and eliminating HF gas. In the present work, four MOFs including Cu-BTC, ZIF-8, MIL-53(Al) and MIL-101(Cr) with various surface area, functionalities, and pore sizes were selected for their HF adsorption capacities studied under unique conditions. As comparison, commercial Al2O3 powder and 13X zeolite were also investigated under the same conditions. It is resulted that the MOFs have higher HF uptakes than commercial Al2O3 and 13X zeolite. MIL-53(Al) have a highest HF capacity, and the HF adsorption of MIL-101(Cr) can be regenerated up to 90%. To the best of our knowledge, this is the first work of studying the HF adsorption behaviors of MOFs, which provides a new perspective of MOFs in a new field of practical purify application.

A Mathematical Model Forecasting HF Adsorption onto Cu-Coated Wafers as a Function of the Airborne Concentration and Moisture

In order to better understand and model the whole sorption behavior of the HF when in contact with a Cu surface inside a FOUP and thus control the related yield losses, Cu-coated wafers have been exposed to varying HF (ppbv) and H2O (% RH) airborne concentrations. These experiments have yielded a HF-Cu sorption empirical-mathematical model that may be used as an industrial tool for queue-time or fluoride surface concentration predictions. Besides, the formation of CuF2 only in case of corrosion is evidenced by the XPS measurements whereas the key role of the H2O is further confirmed by the desorption experiments. The H2O retained by the surface is actually responsible for promoting the HF adsorption/desorption onto/from Cu and enables a HF threshold concentration that separates the two well differenced uptake regimes; adsorption and corrosion.

Adsorptive capacity and evolution of the pore structure of alumina on reaction with gaseous hydrogen fluoride.

Brunauer-Emmet-Teller (BET) specific surface areas are generally used to gauge the propensity of uptake on adsorbents, with less attention paid to kinetic considerations. We explore the importance of such parameters by modeling the pore size distributions of smelter grade aluminas following HF adsorption, an industrially important process in gas cleaning at aluminum smelters. The pore size distributions of industrially fluorinated aluminas, and those contacted with HF in controlled laboratory trials, are reconstructed from the pore structure of the untreated materials when filtered through different models of adsorption. These studies demonstrate the presence of three distinct families of pores: those with uninhibited HF uptake, kinetically limited porosity, and pores that are surface blocked after negligible scrubbing. The surface areas of the inaccessible and blocked pores will overinflate estimates of the adsorption capacity of the adsorbate. We also demonstrate, contrary to conventional understanding, that porosity changes are attributed not to monolayer uptake but more reasonably to pore length attenuation. The model assumes nothing specific regarding the Al2O3-HF system and is therefore likely general to adsorbate/adsorbent phenomena.

Theoretical study on the morphology of MgF2 nanocrystals at finite temperature and pressure

Periodic density functional theory and surface thermodynamics were combined in a model for the relative stability of rutile-structured MgF2 surfaces in a mixed H2O/HF gas phase. We performed DFT calculations for surfaces (001), (100), (101) and (110), considering H2O and HF adsorption, as well as hydroxylation of the surfaces. The influence of temperature and composition of the gas phase on the surface energies was explored using ab-initio thermodynamics. We found that the relative stability of the surfaces, which depends on their composition, is governed by the thermodynamic equilibrium between gas phase and solid. As a consequence, the morphology of the crystallites strongly varies depending on these parameters.

Effect of substitutional carbon-doping in BNNTs on HF adsorption: DFT study

We employed density functional calculations to investigate the adsorption behavior of HF gas on the side walls of pure and carbon-doped boron nitride nanotubes (BNNTs). The HF adsorption over the pure BNNT opens a door for its functionalization without causing significant changes in its electronic properties. The substitutional doping of carbon atom on the BNNT considerably enhances its affinity towards HF where the effect of the dopant concentration plays a vital role. The change in electronic properties of the doped BNNT on HF adsorption is significant enough to consider it a potential sensor for HF detection.

Study of Interactions of Hf and SiO2 Film for High-k Materials

Hf adsorption on thin SiO2 films was carried out using an ultrahigh-vacuum system with e-beam evaporation. The interaction of adsorbed Hf and SiO2 films was studied by Auger electron spectroscopy (AES), atomic force microscopy (AFM), and current–voltage (I–V) measurements. The leakage current of the SiO2 films increased with Hf adsorption temperature. The dielectric properties of the SiO2 layer were markedly degraded by Hf adsorption even at 300 °C. Additionally, Hf adsorbed on a SiO2 (6.3 nm)/Si substrate at 900 °C penetrated through the SiO2 layer and reacted with the Si substrate forming large concavities. Otherwise, Si diffused from the SiO2/Si interface to the surface of adsorbed Hf.

Comparison of S, Pt, and Hf adsorption on NiAl(1 1 0)

First-principles periodic slab density-functional theory (DFT) calculations with a plane-wave basis are used to predict the properties of S, Pt, and Hf adsorption on NiAl(1 1 0). Stable adsorption sites are identified, and adsorbate binding energies and structures are predicted. We find that while S adsorbs in a threefold site, the metals prefer to adsorb in the Ni–Ni twofold bridge site. The latter finding is consistent with scanning tunneling microscopy experiments for adsorption of various transition metals on NiAl(1 1 0) by Ho and coworkers. S is predicted to easily diffuse between threefold sites. We find that Pt and Hf both induce significant changes in the local surface structure, changing twofold bridge sites into fourfold coordination sites by drawing next-nearest-neighbor atoms nearly equidistant with the nearest-neighbor atoms. We find Pt favors interaction with Al slightly more than Ni, while Hf shows a particularly strong affinity for Ni compared to Al. We also predict that Hf may diffuse one-dimensionally along Ni rows with a barrier of ≈0.6 eV.

Field electron emission study of Ti and Hf adsorption layers on W

Adsorption, nucleation, and crystal growth of hafnium and titanium deposited from vapor in ultra-high vacuum onto thermally cleaned tungsten emitters have been studied by field electron emission microscopy methods. The Ti adsorption on W surface was typical of metals of low intersolubility. The local work function decreases monotonically under Ti adsorption and saturates. Local work function values at saturation were equal to 3.0 eV on the (0 1 6)W face, 3.5 eV on the (0 1 1)W and (1 1 1)W faces, 3.75 eV on the (0 0 1)W face, and 3.95 eV on the (1 1 2)W face. The Hf interaction with W was much stronger and much more anisotropic than the Ti–W interaction. The formation of an intermetallic compound between Hf and W caused strong decrease of the local work function on the {0 0 1}W faces, resulting in an unusually high electron emission from these areas.