Alkali Metal Doping Research Articles

Synergistic effect of alkali metal doping and thiocyanate passivation in CsPbBr3 for HTM-free all-inorganic perovskite solar cells.

All-inorganic CsPbBr3 perovskite solar cells (PSCs) without hole-transport materials (HTMs) have attracted widespread attention because of their significant environmental stability. However, the poor quality of perovskite film and the energetics mismatch between CsPbBr3 and charge-transport layers limit the further improvement of the CsPbBr3 PSC performance. To solve this issue, the synergistic effect of alkali metal doping and thiocyanate passivation in NaSCN and KSCN dopants is utilized to improve the properties of the CsPbBr3 film. The Na+ and K+ with smaller ionic radii are doped at the A-site of CsPbBr3 to cause a lattice contraction, which contributes to the formation of CsPbBr3 film with enhanced grain size and crystallinity. The SCN- exerts the function of passivating the uncoordinated Pb2+ defects of CsPbBr3 film, leading to a reduction of trap state density. The incorporation of NaSCN and KSCN dopants also adjusts the band structure of CsPbBr3 film to improve the interfacial energetics match of the device. As a result, the charge recombination is suppressed, and the charge transfer and extraction are effectively promoted, delivering a highly enhanced power conversion efficiency of 10.38% for the champion KSCN doped CsPbBr3 PSCs without HTMs compared to 6.72% efficiency for the original device. Moreover, the stability of the unencapsulated PSCs under ambient conditions with high humidity (85% RH, 25 °C) is distinctly improved, retaining 91.1% of the initial efficiency after 30 days of aging.

Nonstoichiometric Salt Intercalation as a Means to Stabilize Alkali Doping of 2D Materials.

Although doping with alkali atoms is a powerful technique for introducing charge carriers into physical systems, the resulting charge-transfer systems are generally not air stable. Here we describe computationally a strategy towards increasing the stability of alkali-doped materials that employs stoichiometrically unbalanced salt crystals with excess cations (which could be deposited during, e.g., insitu gating) to achieve doping levels similar to those attained by pure alkali metal doping. The crystalline interior of the salt crystal acts as a template to stabilize the excess dopant atoms against oxidation and deintercalation, which otherwise would be highly favorable. We characterize this doping method for graphene, NbSe_{2}, and Bi_{2}Se_{3} and its effect on direct-to-indirect band gap transitions, 2D superconductivity, and thermoelectric performance. Salt intercalation should be generally applicable to systems which can accommodate this "ionic crystal" doping (and particularly favorable when geometrical packing constraints favor nonstoichiometry).

Sintering behaviour and structure-thermal stability relationships of alkali-doped ternesite

The present study aims to address how alkali metal dopants affect (i) chemical reactions during sintering, (ii) phase assemblage and microstructure of resulting samples, (iii) structure and thermal stability of formed ternesite (C5S2S¯) crystals during the synthesis of ternesite. Our results show that the formation of C5S2S¯ was significantly facilitated by the addition of alkali metal dopants due to the fluxing effect of alkali bearing liquid phases. However, these alkali metal ions could be preferentially hosted by intermediate silicate phases, such as C2S, and caused the stabilization of these phases, thus indirectly affecting the final C5S2S¯ content. Li+ ions tended to occupy Ca sites and interstitial sites in ternesite lattices, stabilizing a rounded polyhedral habit of ternesite crystals. While the incorporation of Na+ and K+ ions was attributed to the monotonous Ca substitution and induced a nodule like morphology of ternesite crystals. Also a correlation between the structural modification of ternesite induced by the alkali metals doping and its thermal stability evolution was established.

Research status and development trend of layered K<italic><sub>x</sub></italic>MnO<sub>2</sub>-based cathode materials for potassium-ion batteries

<p indent="0mm">Potassium-ion batteries have the advantages of abundant resources, low price, and similar principles to lithium-ion batteries, and are currently promising candidates to replace lithium-ion batteries as large-scale energy storage systems. Cathode materials play a key role in the performance of potassium-ion batteries, among which layered K<italic>_x</italic>MnO₂-based cathode materials have attracted much attention due to their high specific capacity, environmental friendliness, low cost, and unique two-dimensional structure. However, the repeated intercalation/deintercalation of large-sized K⁺ during the charging and discharging process will lead to the structural damage, and the Jahn-Teller effect generated by Mn³⁺ can easily cause structural distortion, resulting in a rapid capacity decrease of potassium-ion batteries, which much hinders their commercial application. Beginning with the control of the layered structure of K<italic>_x</italic>MnO₂ materials, this review systematically summarizes the preparation methods of K<italic>_x</italic>MnO₂-based cathode materials, and focuses on the doping modification of K<italic>_x</italic>MnO₂-based cathode materials, including single metal doping, dual metal doping and alkali metal doping. And this paper summarizes the main challenges existing in the current research and further looks forward to its future development direction, which is expected to provide a theoretical reference for the development of high-efficiency and stable potassium-ion batteries.

Dirac Nodal Line in Hourglass Semimetal Nb3SiTe6.

Glide-mirror symmetry in nonsymmorphic crystals can foster the emergence of novel hourglass nodal loop states. Here, we present spectroscopic signatures from angle-resolved photoemission of a predicted topological hourglass semimetal phase in Nb3SiTe6. Linear band crossings are observed at the zone boundary of Nb3SiTe6, which could be the origin of the nontrivial Berry phase and are consistent with a predicted glide quantum spin Hall effect; such linear band crossings connect to form a nodal loop. Furthermore, the saddle-like Fermi surface of Nb3SiTe6 observed in our results helps unveil linear band crossings that could be missed. In situ alkali-metal doping of Nb3SiTe6 also facilitated the observation of other band crossings and parabolic bands at the zone center correlated with accidental nodal loop states. Overall, our results complete the system's band structure, help explain prior Hall measurements, and suggest the existence of a nodal loop at the zone center of Nb3SiTe6.

Engineering doping and defect in graphitic carbon nitride by one-pot method for enhanced photocatalytic hydrogen evolution

Hydrogen energy, as one clean energy, is one of the important ways to decarbonize in the future. The advancement of doping and constructing defect engineering in graphitic carbon nitride (g-C3N4) show promise for artificial photosynthesis for H2 evolution using solar energy. Unfortunately, it is still difficult to generate g-C3N4 that has been co-modified by doping and defect and investigate their synergistic effects. In this study, potassium thioacetate was used first-time as a potassium source and dehydrogenation agent to prepare g-C3N4 decorated by K+ ions and cyano groups with enhanced H2 evolution by a one-step method in an air atmosphere. The presence of K+ ions can increase the density of electron clouds in the delocalized π bond of the heptazine ring. The cyano group may successfully delocalize the solitary valence electrons in conjugated heterocycles since it is a potent electron-withdrawing group. These outcomes allow the modified catalysts to achieve improved H2 evolution activity, reduced interfacial transfer resistance, and increased light absorption and photocurrent. Finally, K(0.05)-CN's photocatalytic hydrogen evolution rate (HER) rose to 1319 μmol h−1 g−1, which is five-fold better than that of CN, and the normalized HER reached 118 μmol h−1 m−2, which was much higher than other reports. This research not only offers a fresh approach to creating photocatalysts with good H2 evolution efficiency but also advances a deep understanding of how structural defects and alkali metal doping affect photocatalytic activity.

Enhanced Performance of Ternary CuGaSe 2 Thin‐Film Photovoltaic Solar Cells and Photoelectrochemical Water Splitting Hydrogen Evolution with Modified p–n Heterointerfaces (Adv. Mater. Interfaces 25/2022)

Efficient Photovoltaic Solar Cell and Hydrogen Evolution Using CuGaSe2-based Photoabsorber Layers In article number 2201266, Shogo Ishizuka, Riku Okamoto, Shigeru Ikeda, and co-workers develop an interface modification technique using Cu-deficient phases, alkali-metal doping, and an n-type buffer material for enhancing performance of CuGaSe2 thin-film photovoltaic solar cells and photoelectrochemical water splitting hydrogen evolution.

Reversible band structure engineering in semimetal SrIrO3 films using atomic hydrogen

We report on the atomic hydrogen annealing to in situ manipulate the electronic structures in semimetal SrIrO3 films. Carriers are found to be effectively doped into epitaxial SrIrO3 films by atomic hydrogen annealing without degrading the lattice structure, as revealed by transport and structural measurements. Using angle-resolved photoemission spectroscopy, the low-lying hole bands exhibit a nearly rigid band shift toward the higher binding energy upon atomic hydrogen annealing, which can be further restored by the ozone annealing. Unlike the typical in situ alkali-metal surface doping method, our atomic-hydrogen-treated SrIrO3 films are stable in air, which warrants ex situ characterizations. Our work shows a reversible and effective in situ carrier doping method to explore exotic phases in strongly correlated systems, such as the potential superconductivity in iridates.

Enhancing carrier transport in flexible CZTSSe solar cells via doping Li strategy

The passivation of non-radiative states and inhibition of band tailings are desirable for improving the open-circuit voltage (Voc) of CZTSSe thin-film solar cells. Recently, alkali metal doping has been investigated to passivate defects in CZTSSe films. Herein, we investigate Li doping effects by applying LiOH into CZTSSe precursor solutions, and verify that carrier transport is enhanced in the CZTSSe solar cells. Systematic characterizations demonstrate that Li doping can effectively passivate non-radiative recombination centers and reduce band tailings of the CZTSSe films, leading to the decrease in total defect density and the increase in separation distance between donor and acceptor. Fewer free carriers are trapped in the band tail states, which speeds up carrier transport and reduces the probability of deep-level defects capturing carriers. The charge recombination lifetime is about twice as long as that of the undoped CZTSSe device, implying the heterojunction interface recombination is also inhibited. Besides, Li doping can increase carrier concentration and enhance build-in voltage, leading to a better carrier collection. By adjusting the Li/(Li + Cu) ratio to 18%, the solar cell efficiency is increased significantly to 9.68% with the fill factor (FF) of 65.94%, which is the highest FF reported so far for the flexible CZTSSe solar cells. The increased efficiency is mainly attributed to the reduction of Voc deficit and the improved CZTSSe/CdS junction quality. These results open up a simple route to passivate non-radiative states and reduce the band tailings of the CZTSSe films and improve the efficiency of the flexible CZTSSe solar cells.

Alkali metal doping in B–C3N4 extends carrier lifetime and increases the CO2 adsorption: DFT study and time-domain Ab initio analysis

Carbon nitride (C3N4) has been widely used in photocatalytic applications. Its performance is unsatisfactory due to high band gap. In order to address these challenges, we have investigated the effect of alkali metals (Na, Li) on B-doped C3N4 having nitrogen vacancy (Nv). DFT calculations show that doping of alkali metals into B doped C3N4 (B–C3N4) can decrease its band gap. Na atom doped into the surface of B–C3N4 transfers all of its charge to atoms on the surface. There is transfer of charge from surface to CO2 when it is adsorbed on Na/B–C3N4. In Li doped B– C3N4 system (Li/B– C3N4), Li atom form bond with two N atoms. There is transfer of charge from Li–N bonds to CO2, which would facilitate the stronger adsorption of CO2. Using nonadiabatic molecular dynamics (NAMD) simulations, we show that recombination rate of charge carriers decreases in alkali doped samples. Our results provide new insight into tuning of band gap of C3N4 in visible/infrared region of the spectrum. Na/B-co-doped C3N4 is considered to be an efficient photocatalyst as it has lowest band gap, shows reasonable adsorption for CO2 and lifetime of charge carriers is extended by a factor of three as compared to B–C3N4.

Deciphering the Role of Alkali Metals (Li, Na, K) Doping for Triggering Nonlinear Optical (NLO) Properties of T-Graphene Quantum Dots: Toward the Development of Giant NLO Response Materials.

Nanoscale nonlinear optical (NLO) materials have received huge attention of the scientists in current decades because of their enormous applications in optics, electronics, and telecommunication. Different studies have been conducted to tune the nonlinear optical response of the nanomaterials. However, the role of alkali metal (Li, Na, K) doping on triggering the nonlinear optical response of nanomaterials by converting their centrosymmetric configuration into noncentrosymmetric configuration is rarely studied. Therefore, to find a novel of way of making NLO materials, we have employed density functional theory (DFT) calculations, which helped us to explore the effect of alkali metal (Li, Na, K) doping on the nonlinear optical response of tetragonal graphene quantum dots (TGQDs). Ten new complexes of alkali metal doped TGQDs are designed theoretically. The binding energy calculations revealed the stability of alkali metal doped TGQDs. The NLO responses of newly designed complexes are evaluated by their polarizability, first hyperpolarizability (βo), and frequency dependent hyperpolarizabilities. The Li@r8a exhibited the highest first hyperpolarizability (βo) value of 5.19 × 105 au. All these complexes exhibited complete transparency in the UV region. The exceptionally high values of βo of M@TGQDs are accredited to the generation of diffuse excess electrons, as indicated by NBO analysis and PDOS. NCI analysis is accomplished to examine the nature of bonding interactions among alkali metal atoms and TGQDs. Our results suggest alkali metal doped TGQD complexes as potential candidates for nanoscale NLO materials with sufficient stability and enhanced NLO response. This study will open new doors for making giant NLO response materials for modern hi-tech applications.

Influence of Alkali Metal Doping and BN Substitution on the Second-Order Nonlinear Optical Properties of Graphyne: A Theoretical Perspective.

The electronic and nonlinear optical (NLO) properties of BN-substituted graphynes and the corresponding alkali-doped hybrid systems have been determined using density functional theory. When the carbon atoms in the graphyne are replaced by BN pairs, the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (Egap) increases to some extent, and the static first hyperpolarizabilities (β0) of the novel systems hardly increase. However, when an alkali atom is introduced on the surface of BN-substituted graphyne, the doping effect can effectively modulate the electronic and NLO properties. Doping the alkali atom can significantly narrow the wide Egap of BN-substituted graphynes in the range of 1.03-2.03 eV. Furthermore, the doping effect brings considerable β0 values to these alkali-doped systems, which are 52-3609 au for Li-doped systems and 3258-211 053 au for Na/K-doped ones. The result reveals that the β0 values of alkali-doped complexes are influenced by the atomic number of alkali metals and the proportion of BN pairs. The nature of the excellent NLO responses of alkali-doped complexes can be understood by the low excitation energy of the crucial excited state and the analysis of the first hyperpolarizability density. Besides, these alkali-doped complexes have a deep-ultraviolet working region. Therefore, the combined effect of alkali metal doping and BN substitution can be an excellent strategy to design novel high-performance NLO materials based on graphyne.

Hydrogen Storages Based on Graphene Nano-Flakes: Density Functional Theory Approach

Carbon materials such as graphene, carbon nanotubes, fullerene, and graphene nanoflakes (GNFs) are used for hydrogen storage. The doping of alkali metals to these materials generally increases the accumulation density of molecular hydrogen (H2). However, the reason why the doping enhances the ability of the H2 storage of GNF is not clearly known, although there are some explanations. In addition, the information on the storage capacity of GNF is ambiguous. In the present review article, we introduce our recent theoretical studies on the interaction of GNF with H2 molecules carried out to elucidate the mechanism of hydrogen storage in alkali-doped GNFs. As alkali metals, lithium (Li), sodium (Na), and potassium (K) were examined, and the abilities of hydrogen storage were discussed. Next, the mechanism of Li-diffusion on GNF, which plays a crucial role in Li-battery, was presented. There are several unanswered questions. In particular, does lithium diffuse randomly on GNF? Or is there a specific diffusion path? We present our study, which elucidates the factors governing lithium diffusion on GNF. If the dominant factor is known, it is possible to arbitrarily control the diffusion path of lithium. This will lead to the development of highly functional battery materials. Finally, the molecular design of H adsorption–desorption reversible storage devices based on GNF will be introduced. Elucidating the mechanism of hydrogen storage, Li-diffusion on GNF, and molecular design of storage device is important in understanding the current molecular devices and provide a deeper insight into materials chemistry.

Enhanced non-linear optical response of alkali metal-doped nitrogenated holey graphene (C2N)

In the quest of better non-linear optical (NLO) materials, a DFT method is employed on alkali metals doped C2N to investigate the optoelectronic characteristics. To understand the optoelectronic properties the charge distribution is a key factor for which frontier molecular orbitals (FMOs), transition density matrix (TDM), density of states (DOS), electron density difference map (EDDM) and molecular electrostatic potential(MEP) analyses were carried out. Interaction energy (Eint) is computed to delve into thermodynamic stability. The projected results of all the complexes is lying in domain of effective NLO materials, like constricted EH-L, reduced Eopt, bathochromic shift in ʎmax and above all upgraded α0 and β0. When compared to C2N, doped complexes display extraordinary NLO response with 1st hyperpolarizability ranging from 2.4 × 104 (K@C2N) to 8.02 × 104 a.u. (Li@C2N). Furthermore, alkali metal doping also lessens the energy gap (EH-L)from 2.3 eV to 2.06 eV in Li@C2N. The nature of vibrations and interactions are probed via IR and NCI analysis. Doped molecules (Li@C2N-K@C2N) do possess greater dipole moment (2.48 D to 5.56 D) than C2N (2.04 D). All characteristics support the use of all doped complexes in integrated NLO devices, paving the way for new approach to computational designing of super-efficient NLO materials.

Specific alkali metal sites as CO2 traps in activated carbon with different pore size for CO2 selective adsorption: GCMC and DFT simulations

Activated carbon is a kind of promising adsorbent for post-combustion CO2 adsorption and separation. However, due to the physical adsorption properties, the use of the reported activated carbon for CO2 adsorption capacity and CO2/N2 selectivity presents a huge challenge without deep cooling of flue gas to room temperature or even lower temperature and low CO2 partial pressure. To solve the above problem, the CO2 uptake in practical flue gas on M−doped (M = O, N, S and alkali metal) graphite surfaces (GS) with various pore widths (0.35–3 nm) were studied by density functional theory (DFT) and grand canonical Monte Carlo simulation (GCMC). The results show that the doping of oxygen, nitrogen, sulfur groups into graphite surface have no significant increase in CO2 uptake and CO2/N2 selectivity, while the doping of alkali metal into graphite surface can significantly improve the CO2 uptake and CO2/N2 selectivity in pores of less than 0.6 nm. Especially GS-Na shows the highest CO2 uptake and CO2/N2 selectivity, up to 20.7 mmol mL−1 (0.35 nm) and 1040 (0.4 nm), respectively. The above findings emphasize the importance of the combination of pore widths less than 0.6 nm and alkali metal groups in determining the CO2 uptake. By studying the influence of pore size and functional groups on CO2 adsorption, the internal enhancement mechanisms, including CO2 pore-filling adsorption and electrostatic interaction, are clarified. Meantime, the mechanism of functional groups and pore widths synergistically increasing the CO2 uptake was revealed. These results would provide theoretical guidance for the design and preparation of carbon-based adsorbents for CO2 uptake.

Thermal Stability of K-Doped Organometal Halide Perovskite for Photovoltaic Materials

Organometal halide perovskites (OHPs) have garnered considerable attention as materials for next-generation solar cells. Although the power conversion efficiencies of solar cells with OHPs are comparable to those of the existing solar cells, their durability must be improved for practical use based on a deep understanding of their basic properties. During the last decade, various methods, such as composition control, alkali metal doping, and surface passivation, were used to increase the durability of OHPs. In this study, the effects of K-doping on the thermal stability of mixed OHPs are investigated. The activation energies of the mixed OHPs with and without K-doping are found to be 128.4 ± 5.2 and 92.9 ± 9.8 kJ/mol, respectively. Furthermore, the Johnson–Mehl–Avrami model is adopted to the in situ X-ray diffraction measurement data, the results of which indicate that K-doped OHPs can be stabilized to such an extent that they do not decompose after 10,000 h of heating at 85 °C.

Photocatalytic performance of alkali metal doped graphitic carbon nitrides and Pd-alkali metal doped graphitic carbon nitride composites

Although there are several reports about the beneficial performance of metal doped graphitic carbon nitrides (g-C3N4) in various photocatalytic reactions, the effect of different alkali metal dopants has not been studied systematically. Series of undoped, Li-, Na- and K-doped samples was synthetized and used for the preparation of novel type of photocatalysts, composed of palladium nanoparticles supported on alkali metals-doped graphitic carbon nitride. Palladium loading was achieved via citrate reduction of palladium precursor in the presence of the carbon nitride material. Several physicochemical characterization methods such as attenuated total reflection infrared-(ATR-IR), diffuse reflectance UV–visible spectroscopy, high-resolution transmission microscopy (HRTEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) were used to get information about the morphology and composition of the novel composites. According to the optical characterization results, the band gap of g-C3N4 decreased upon Na- and K doping but increased after Li-doping. The palladium nanoparticles were distributed over the support and appeared in dispersed (undoped g-C3N4, Li-g-C3N4) or agglomerated (Na-g-C3N4, K-g-C3N4) form with individual particle size below 5 nm. The catalytic performance of these composite materials was tested in two processes: (i) photodegradation of methyl orange and (ii) hydrogen production from methanol. The Na- doping of graphitic carbon nitride was found to be a key issue to enhance hydrogen production. The carbon nitride matrix was found to be stable during the photocatalytic experiments, while the Pd component underwent certain changes during the photocatalytic reforming reaction of methanol. Our results indicated that the really acting metallic Pd cocatalyst was formed mainly in situ.

Impurity Controlled near Infrared Surface Plasmonic in AlN.

In this work, we used multi-scale computational simulation methods combined with density functional theory (DFT) and finite element analysis (FEA) in order to study the optical properties of substitutional doped aluminium nitride (AlN). There was strong surface plasmon resonance (SPR) in the near-infrared region of AlN substituted with different alkali metal doping configurations. The strongest electric field strength reached 109 V/m. There were local exciton and charge transfer exciton behaviours in some special doping configurations. These research results not only improve the application of multi-scale computational simulations in quantum surface plasmons, but also promote the application of AlN in the field of surface-enhanced linear and non-linear optical spectroscopy.

Unusual Site-Selective Doping in Layered Cathode Strengthens Electrostatic Cohesion of Alkali-Metal Layer for Practicable Sodium-Ion Full Cell.

P2-type Na0.67 Ni0.33 Mn0.67 O2 is a dominant cathode material for sodium-ion batteries due to its high theoretical capacity and energy density. However, charging P2-type Na0.67 Ni0.33 Mn0.67 O2 to voltages higher than 4.2 V (vs. Na+ /Na) can induce detrimental structural transformation and severe capacity fading. Herein, stable cycling and moisture resistancy of Na0.67 Ni0.33 Mn0.67 O2 at 4.35 V (vs. Na+ /Na) are achieved through dual-site doping with Cu ion at transition metal site (2a) and unusual Zn ion at Na site (2d) for the first time. The Cu ion doping in 2a site stabilizes the metal layer, while more importantly, the unusual alkali-metal site doping by Zn ion serves as O2- Zn2+ O2- "pillar" for enhancing electrostatic cohesion between two adjacent transition metal layers, preventing the crack of active material along the a-b-plane and restraining the generation of O2 phase upon deep desodiation. This unique dual-site-doped [Na0.67 Zn0.05 ]Ni0.18 Cu0.1 Mn0.67 O2 cathode exhibits a prominent cyclability with 80.6% capacity retention over 2000 cycles at an ultrahigh rate of 10C, demonstrating its great potential for practical applications. Impressively, the full cell devices with [Na0.67 Zn0.05 ]Ni0.18 Cu0.1 Mn0.67 O2 and commercial hard carbon as cathode and anode, respectively, can deliver a high energy density of 217.9Wh kg-1 and excellent cycle life over 1000 cycles.

Revisiting the Structural Evolution of Hydrotalcite‐Derived Mixed Metal Oxides upon Alkali Metal Doping and Its Impact on Base Catalysis

AbstractThe structural evolution of classic coprecipitation‐derived Mg−Al mixed oxide (MMO) upon doping of different alkali metals in a wide level (1–20 wt.%) was revisited. The pristine lateral MMO of aggregates into particulates, accompanied with the dramatic loss of the surface areas and the formation of aluminates. These phenomena become severer with the decrease of the atomic radius. The formation of NaAlO2 likely occurs via the gradual dealumination of the MMO, which is highly dependent on both the dopant content and the activation temperature. This leads to ultimately a new ensemble with MgO as the core decorated by NaAlO2 in the outer surfaces at high Na doping level and above 973 K. When evaluated in the base‐catalyzed transesterification and acylation model reactions, the Na‐doped MMO show enhanced performance and a plateau at increasing doping that might be explained by an interplay between the number and strength of strong basicity.