DFT-D2 simulations of water adsorption and dissociation on the low-index surfaces of mackinawite (FeS)

The adsorption and dissociation of water on mackinawite (layered FeS) surfaces were studied using dispersion-corrected density functional theory (DFT-D2) calculations. The catalytically active sites for H2O and its dissociated products on the FeS {001}, {011}, {100}, and {111} surfaces were determined, and the reaction energetics and kinetics of water dissociation were calculated using the climbing image nudged elastic band technique. Water and its dissociation products are shown to adsorb more strongly onto the least stable FeS{111} surface, which presents low-coordinated cations in the surface, and weakest onto the most stable FeS{001} surface. The adsorption energies decrease in the order FeS{111} > FeS{100} > FeS{011} > FeS{001}. Consistent with the superior reactivity of the FeS{111} surface towards water and its dissociation products, our calculated thermochemical energies and activation barriers suggest that the water dissociation reaction will take place preferentially on the FeS nanoparticle surface with the {111} orientation. These findings improve our understanding of how the different FeS surface structures and the relative stabilities dictate their reactivity towards water adsorption and dissociation

Experimental and theoretical study into interface structure and band alignment of the Cu2Zn1–xCdxSnS4 heterointerface for photovoltaic applications

To improve the constraints of kesterite Cu2ZnSnS4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu2Zn1–xCdxSnS4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu2Zn1–xCdxSnS4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu2Zn1–xCdxSnS4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu2Zn1–xCdxSnS4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu2CdSnS4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances

Enhanced Photoresponse of FeS₂ Films: The Role of Marcasite-Pyrite Phase Junctions

The beneficial role of marcasite in iron-sulfide-based photo-electrochemical applications is reported for the first time. A spectacular improvement of the photoresponse observed experimentally for mixed pyrite/marcasite-FeS2 films can be ascribed to the presence of p/m phase junctions at the interface. Density functional theory calculations show that the band alignment at the phase boundary contributes to enhanced charge separation and transfer across the interface

Identification of Photoexcited Electron Relaxation in a Cobalt Phosphide Modified Carbon Nitride Photocatalyst

Transition metal phosphides have been recognized as efficient co-catalysts to boost the activity of semiconductor photocatalysts. However, a rigorous and quantitative understanding is still to be developed about how transition metal phosphides influence photoexcited electron dynamics. Here, we present a nanosecond time-resolved transient absorption spectroscopy (TAS) study of the photoexcited electron dynamics in carbon nitrides (g-C3N4) before and after Co and/or P modifications. Our spectroscopic study showed that Co or P lowered the initial electron density, whereas they promoted the photoexcited electron relaxation of g-C3N4, with their half-life times (t50%) of 2.5 and 1.8 ns, respectively. The formation of a CoP co-catalyst compound promoted the electron relaxation (t50%=2.8 ns) without significantly lowering the charge separation efficiency. Density functional theory (DFT) calculations were undertaken to explore the underlying fundamental reasons and they further predicted that CoP, compared to Co or P modification, better facilitates photoexcited electron transfer from g-C3N4 to reactants

The Transfer Hydrogenation of Cinnamaldehyde Using Homogeneous Cobalt(II) and Nickel(II) (E)-1-(Pyridin-2-yl)-N-(3-(triethoxysilyl)propyl)methanimine and the Complexes Anchored on Fe3O4 Support as Pre-Catalysts: An Experimental and In Silico Approach

The imino pyridine Schiff base cobalt(II) and nickel(II) complexes (C1 and C2) and their functionalised γ-Fe3O4 counterparts (Fe3O4@C1 and Fe3O4@C2) were synthesised and characterised using IR, elemental analysis, and ESI-MS for C1 and C2, and single crystal X-ray diffraction for C1, while the functionalised materials Fe3O4@C1 and Fe3O4@C2 were characterized using IR, XRD, SEM, TEM, EDS, ICP-OES, XPS and TGA. Complexes C1, C2 and the functionalised materials Fe3O4@C1 and Fe3O4@C2 were tested as catalysts for the selective transfer hydrogenation of cinnamaldehyde and all four pre-catalysts showed excellent catalytic activity. Complexes C1 and C2 acted as homogeneous catalysts with high selectivity towards the formation of hydrocinnamaldehyde (88.7% and 92.6%, respectively) while Fe3O4@C1 and Fe3O4@C2 acted as heterogeneous catalysts with high selectivity towards cinnamyl alcohol (89.7% and 87.7%, respectively). Through in silico studies of the adsorption energies, we were able to account for the different products formed using the homogeneous and the heterogeneous catalysts which we attribute to the preferred interaction of the C=C moiety in the substrate with the Ni centre in C2 (−0.79 eV) rather than the C=O (−0.58 eV)

Cu Electrodeposition on Nanostructured MoS2 and WS2 and Implications for HER Active Site Determination

Cu electrodeposition in both underpotential and overpotential regimes on nanostructured MoS2 and WS2 prepared by plasmaenhanced atomic layer deposition has been studied in detail. A combination of electrochemical methods, advanced characterization by X-ray absorption spectroscopy (XAS) as well as theoretical modelling were employed to reveal Cu adsorption modes on transition metal dichalcogenides (TMDs) from initial stages until bulk deposition. Since Cu UPD on TMDs has been used recently to evaluate the number of electrochemically active sites (NAS) for H2 evolution reaction, we evaluate and discuss here the implications of the Cu electrodeposition phenomena on nanostructured MoS2 and WS2 gauging the general applicability of the Cu UPD method for number of HER active sites determination in TMDs. Although an apparently better correlation of HER current density with Cu UPD charge than with double layer capacitance is found, the Cu UPD method cannot be used quantitatively because of the absence of a clear H UPD phenomenon on the studied nanostructured TMDs. This is in contrast to platinum group metal catalysts where H UPD and Cu UPD sites are strongly correlated

Revealing the electronic structure, heterojunction band offset and alignment of Cu2ZnGeSe4: a combined experimental and computational study towards photovoltaic applications

Cu2ZnGeSe4 (CZGSe) is a promising earth-abundant and non-toxic semiconductor material for large-scale thin-film solar cell applications. Herein, we have employed a joint computational and experimental approach to characterize and assess the structural, optoelectronic, and heterojunction band offset and alignment properties of CZGSe solar absorber. The CZGSe films were successfully prepared using DC-sputtering and e-beam evaporation systems and confirmed by XRD and Raman spectroscopy analyses. The CZGSe films exhibit a bandgap of 1.35 eV, as estimated from electrochemical cyclic voltammetry (CV) measurements and validated by first-principles density functional theory (DFT) calculations, which predicts a bandgap of 1.38 eV. A fabricated device based on the CZGSe as light absorber and CdS as a buffer layer yields power conversion efficiency (PCE) of 4.4% with VOC of 0.69 V, FF of 37.15, and JSC of 17.12 mA cm−2. Therefore, we suggest that interface and band offset engineering represent promising approaches to improve the performance of CZGSe devices by predicting a type-II staggered band alignment with a small conduction band offset of 0.18 eV at the CZGSe/CdS interface

Solution-processed Cd-substituted CZTS nanocrystals for sensitized liquid junction solar cells

The Earth-abundant kesterite Cu2ZnSnS4 (CZTS) exhibits outstanding structural, optical, and electronic properties for a wide range of optoelectronic applications. However, the efficiency of CZTS thin-film solar cells is limited due to range of factors, including electronic disorder, secondary phases, and the presence of anti-site defects, which is key factor limiting the Voc. The complete substitution of Zn lattice sites in CZTS nanocrystals (NCs) with Cd atoms offers a promising approach to overcome several of these intrinsic limitations. Herein, we investigate the effects of substitution of Cd2+ into Zn2+ lattice sites in CZTS NCs through a facile solution-based method. The structural, morphological, optoelectronic, and power conversion efficiencies (PCEs) of the NCs synthesized have been systematically characterized using various experimental techniques, and the results are corroborated by first-principles density functional theory (DFT) calculations. The successful substitution of Zn by Cd is demonstrated to induce a structural transformation from the kesterite phase to the stannite phase, which results in the bandgap reducing from 1.51 eV (kesterite) to 1.1 eV (stannite), which is closer to the optimum bandgap value for outdoor photovoltaic applications. Furthermore, the PCE of the novel Cd-substituted liquid junction solar cell underwent a four-fold increase, reaching 1.1%. These results highlight the importance of substitutional doping strategies in optimizing existing CZTS-based materials to achieve improved device characteristics

The surface chemistry of NOx on mackinawite (FeS) surfaces: a DFT-D2 study

We present density functional theory calculations with a correction for the long-range interactions (DFT-D2) of the bulk and surfaces of mackinawite (FeS), and subsequent adsorption and dissociation of NOx gases (nitrogen monoxide (NO) and nitrogen dioxide (NO2)). Our results show that these environmentally important molecules interact very weakly with the energetically most stable (001) surface, but adsorb relatively strongly onto the FeS(011), (100) and (111) surfaces, preferentially at Fe sites via charge donation from these surface species. The NOx species exhibit a variety of adsorption geometries, with the most favourable for NO being the monodentate Fe–NO configuration, whereas NO2 is calculated to form a bidentate Fe–NOO–Fe configuration. From our calculated thermochemical energy and activation energy barriers for the direct dissociation of NO and NO2 on the FeS surfaces, we show that NO prefers molecular adsorption, while dissociative adsorption, i.e. NO2 (ads) → [NO(ads) + O(ads)] is preferred over molecular adsorption for NO2 onto the mackinawite surfaces. However, the calculated high activation barriers for the further dissociation of the second N–O bond to produce either [N(ads) and 2O(ads)] or [N(ads) and O2(ads)] suggest that complete dissociation of NO2 is unlikely to occur on the mackinawite surfaces