Effects of element doping and H2O presence on CO2 adsorption using hexagonal boron nitride

Effects of Phosphorus Doping on Amorphous Boron Nitride's Chemical, Sorptive, Optoelectronic, and Photocatalytic Properties

Amorphous porous boron nitride (BN) represents a versatile material platform with potential applications in adsorptive molecular separations and gas storage, as well as heterogeneous and photo-catalysis. Chemical doping can help tailor BN's sorptive, optoelectronic, and catalytic properties, eventually boosting its application performance. Phosphorus (P) represents an attractive dopant for amorphous BN as its electronic structure would allow the element to be incorporated into BN's structure, thereby impacting its adsorptive, optoelectronic, and catalytic activity properties, as a few studies suggest. Yet, a fundamental understanding is missing around the chemical environment(s) of P in P-doped BN, the effect of P-doping on the material features, and how doping varies with the synthesis route. Such a knowledge gap impedes the rational design of P-doped porous BN. Herein, we detail a strategy for the successful doping of P in BN (P-BN) using two different sources: phosphoric acid and an ionic liquid. We characterized the samples using analytical and spectroscopic tools and tested them for CO2 adsorption and photoreduction. Overall, we show that P forms P-N bonds in BN akin to those in phosphazene. P-doping introduces further chemical/structural defects in BN's structure, and hence more/more populated midgap states. The selection of P source affects the chemical, adsorptive, and optoelectronic properties, with phosphoric acid being the best option as it reacts more easily with the other precursors and does not contain C, hence leading to fewer reactions and C impurities. P-doping increases the ultramicropore volume and therefore CO2 uptake. It significantly shifts the optical absorption of BN into the visible and increases the charge carrier lifetimes. However, to ensure that these charges remain reactive toward CO2 photoreduction, additional materials modification strategies should be explored in future work. These strategies could include the use of surface cocatalysts that can decrease the kinetic barriers to driving this chemistry.

Co-adsorption enhancement of formaldehyde/carbon dioxide over modified hexagonal boron nitride for whole-surface capture purification

Simultaneous capture of formaldehyde (HCHO) and carbon dioxide (CO2) in indoor air is promising of achieving indoor-air purification. Of all potential adsorbents, hexagonal boron nitride (h-BN) is one of the most suitable species owing to facile formation of attraction points. Therefore, in this study, performances of HCHO and CO2 being adsorbed over pure/modified h-BN are systematically investigated via density functional theory (DFT) calculations. Minutely speaking, direct interaction between HCHO and CO2, single-point adsorption enhancement of HCHO over modified h-BN, co-adsorption reinforcement of HCHO/CO2 as well as relevant thermodynamic characteristics are major research contents. According to calculation results, there is relatively strong attraction between HCHO and CO2 owing to hydrogen bonds, which is in favor of co-adsorption of HCHO/CO2. As to single-adsorption of HCHO, C-doped h-BN shows better adsorption features than P-doped h-BN and C/P-doped h-BN is slightly weakened in adsorption ability due to surficial deformation caused by P atoms. For co-adsorption of HCHO/CO2, CO2 is the protagonist via formation of quasi-carbonate with the help of delocalized π-orbital electrons. Regarding effects of temperatures on adsorption strengths, they depend on interelectronic interactions among dopant atoms and finally derives from dispersion of π bonds across adsorbents. Overall, this study provides detailed mechanisms for co-capture of HCHO/CO2 to accomplish indoor-air purification.

Construction adsorption and photocatalytic interfaces between C, O co-doped BN and Pd-Cu alloy nanocrystals for effective conversion of CO2 to CO

Photocatalytic reduction of carbon dioxide (CO₂) into fuels is an auspicious route to alleviate the energy and environmental crisis brought by the continuous depletion of fossil fuels. The CO₂ adsorption state on the surface of photocatalytic materials plays a significant role in its efficient conversion. The limited CO₂ adsorption capacity of conventional semiconductor materials inhibit their photocatalytic performances. In this work, a bifunctional material for CO₂ capture and photocatalytic reduction was fabricated by introducing palladium (Pd)-copper (Cu) alloy nanocrystals onto the surface of carbon, oxygen co-doped boron nitride (BN). The elemental doped BN with abundant ultra-micropores had high CO₂ capture ability, and CO₂ was adsorbed in the form of bicarbonate on its surface with the presence of water vapor. The Pd/Cu molar ratio had great impact on the grain size of Pd-Cu alloy and their distribution on BN. The CO₂ molecules tended to be converted to carbon monoxide (CO) at interfaces of BN and Pd-Cu alloys due to their bidirectional interactions to the adsorbed intermediate species while methane (CH₄) evolution might occur on the surface of Pd-Cu alloys. Owing to the uniform distribution of smaller Pd-Cu nanocrystals on BN, more effective interfaces were created in the Pd₅Cu₁/BN sample and it gave a CO production rate of 7.74 μmolg^-1h^-1 under simulated solar light irradiation, higher than the other PdCu/BN composites. This work can pave a new way for constructing effective bifunctional photo-catalysts with high selectivity to convert CO₂ to CO.

Novel Janus MoSiGeN4 nanosheet: adsorption behaviour and sensing performance for NO and NO2 gas molecules

A novel Janus MoSiGeN4 nanosheet is proposed for detecting poisonous gas molecules. Herein, the adsorption behaviour and sensing performance of both sides of the MoSiGeN4 monolayer to NO and NO2 gas molecules were investigated by first-principles calculations. Firstly, it is found that the MoSiGeN4 monolayer exhibits structural stability and indirect gap semiconductor characteristics. The largest adsorption energy of NO2 molecules on the MoSiGeN4 monolayer is -0.24 eV, which is higher than the -0.13 eV for NO molecules. Of course, the physisorption between gas molecules and the MoSiGeN4 monolayer appears with slight charge transfer. It is confirmed that NO molecules and NO2 molecules act as electron donors and electron acceptors, respectively. Meanwhile, the generation of small band gaps and impurity levels in the electronic structures after gas adsorption is in favour of the enhancement of electronic conductivity. Furthermore, the longest recovery times of NO and NO2 molecules are predicted to be 0.15 and 10.67 ns at room temperature, and the lateral diffusion at the surface requires crossing a large energy barrier. These findings provide indisputable evidence for further design and fabrication of highly sensitive gas sensors based on the MoSiGeN4 monolayer.

CO2 adsorption enhancement over Al/C-doped h-BN: A DFT study

Reducing energy barriers of CO₂ being chemisorbed on hexagonal boron nitride (h-BN) is a kernel step to efficiently and massively capture CO₂. In this study, aluminum/carbon (Al/C) atoms are used as dopants to alter surface potential fields of h-BN, which aims at lowering energy barriers of adsorption processes. Through theoretical calculations, direct-adsorption structures/properties of CO₂, joint-adsorption structures/properties of CO₂/H₂O, transition state (TS) energy barriers, effects of temperatures on adsorption energies/TS energy barriers and changes of reaction rate constants over different adsorbents are investigated in detail in order to reveal how doping of Al/C atoms promotes CO₂ adsorption strength over doped h-BN. According to DFT calculation results, the average adsorption energy of CO₂ being directly adsorbed on Al/C-doped h-BN arrives at -59.43 kJ/mol, which is about 5 times as big as that over pure h-BN. As to the average adsorption energy of CO₂/H₂O and relevant TS energy barrier, they are modified to -118.89 kJ/mol and 40.23 kJ/mol over Al/C-doped h-BN in contrast with -33.91 kJ/mol and 1695.11 kJ/mol over pure h-BN, respectively. What is more, based on thermodynamic analyses and reaction dynamics, the average desorption temperatures of CO₂(/H₂O) are promoted over doped h-BN and the temperature power exponent is negatively correlated with the activation energy in the Arrhenius equation form. The complete understanding of this study would supply crucial information for applying Al/C-doped h-BN to effectively capturing CO₂ in real industries.

Fe-Catalyzed CO2 Adsorption over Hexagonal Boron Nitride with the Presence of H2O

The energy barrier of CO₂ chemically adsorbed on hexagonal boron nitride (h-BN) is relatively big. In order to cut down the energy barriers and facilitate fast adsorption of CO₂, it is necessary to apply catalysts as a promoter. In this study, single-atom iron is introduced as the catalyst to reduce the energy barriers of CO₂ adsorbed on pure/doped h-BN. Through density functional theory calculations, catalytic reaction mechanisms, stability of single-atom iron fixed on adsorbents, CO₂ adsorption characteristics, and features of thermodynamics/reaction dynamics during adsorption processes are fully investigated to explain the catalytic effects of single-atom iron on CO₂ chemisorption. According to calculations, when CO₂ and OH^- get into activated states (i.e., CO₂^•- and ^•OH) with the help of single-atom iron, their chemical activities will be promoted to a large degree, which makes the transition state (TS) energy barrier of HCO₃^- to decrease by 92.54%. In the meantime, it is proved that single-atom iron could be stably fixed on doped h-BN with the binding energy larger than 2 eV to achieve sustainable catalysis. With the presence of single-atom iron, TS energy barriers of CO₂ adsorbed on h-BN with the presence of H₂O decreased by 94.39, 78.87, and 30.63% over pure h-BN, 3C-doped h-BN, and 3N-doped h-BN, respectively. In the meantime, thermodynamic analyses indicate that TS energy barriers are mainly determined by element doping and temperatures are a little beneficial to the reduction of TS energy barriers. With the above aspects combined, the results of this study could supply crucial information for massively and quickly capturing CO₂ in real industries.