Effect of Doped Alkali Metal Ions on the SO_(2) Capture Performance of MnO_(2) Desulfurization Materials at Low Temperature

Sulfur dioxide(SO_(2))emissions from diesel exhaust pose a serious threat to the environment and human health.Thus,desulfurization technology and the performance of desulfurization materials must be improved.In this study,MnO_(2) was modified with various alkali metal ions using the impregnation method to enhance its SO_(2) capture performance.The composites were characterized intensively by scanning electron microscopy,energydispersive X-ray spectroscopy,X-ray diffraction spectroscopy,and Brunauer-Emmett-Teller theory.The SO_(2) capture performance of these composites were measured via thermogravimetry,and the effect of doping with alkali metal ions on the SO_(2) capture performance of MnO_(2) was investigated.Results showed that the SO_(2) capture performance of MnO_(2) could be enhanced by doping with alkali metal ions,and the MnO_(2) composite doped with LiOH(2.0 mol/L)had the best SO_(2) capture capacity(124 mgSO_(2)/gMaterial),which was 18%higher than that of pure MnO_(2).Moreover,the type and concentration of alkali metal ions had varying effects on the SO_(2) capture performance of MnO_(2).In our experiment,the SO_(2) capture performance of the MnO_(2) doped with NaOH,LiCl,Na2CO3,K2CO3,and Li2CO3 composites were worse than that of pure MnO_(2).Therefore,the influences of the type and concentration of alkali metal ions to be doped into desulfurization materials must be considered comprehensively.

A comparison study of alkali metal-doped g-C_3N_4 for visible-light photocatalytic hydrogen evolution

Photocatalytic hydrogen production based on semiconductor photocatalysts has been considered as one of the most promising strategies to resolve the global energy shortage.Graphitic carbon nitride(g‐C3N4)has been a star visible‐light photocatalyst in this field due to its various advantages.However,pristine g‐C3N4usually exhibits limited activity.Herein,to enhance the performance of g‐C3N4,alkali metal ion(Li+,Na+,or K+)‐doped g‐C3N4are prepared via facile high‐temperature treatment.The prepared samples are characterized and analyzed using the technique of XRD,ICP‐AES,SEM,UV‐vis DRS,BET,XPS,PL,TRPL,photoelectrochemical measurements,photocatalytic tests,etc.The resultant doped photocatalysts show enhanced visible‐light photocatalytic activities for hydrogen production,benefiting from the increased specific surface areas(which provide more active sites),decreased band gaps for extended visible‐light absorption,and improved electronic structures for efficient charge transfer.In particular,because of the optimal tuning of both microstructure and electronic structure,the Na‐doped g‐C3N4shows the most effective utilization of photogenerated electrons during the water reduction process.As a result,the highest photocatalytic performance is achieved over the Na‐doped g‐C3N4photocatalyst(18.7?mol/h),3.7times that of pristine g‐C3N4(5.0?mol/h).This work gives a systematic study for the understanding of doping effect of alkali metals in semiconductor photocatalysis.

Defect Engineering in Earth-Abundant Cu_(2)ZnSnSe_(4) Absorber Using Efficient Alkali Doping for Flexible and Tandem Solar Cell Applications

To demonstrate flexible and tandem device applications,a low-temperature Cu_(2)ZnSnSe_(4)(CZTSe)deposition process,combined with efficient alkali doping,was developed.First,high-quality CZTSe films were grown at 480℃by a single co-evaporation,which is applicable to polyimide(PI)substrate.Because of the alkali-free substrate,Na and K alkali doping were systematically studied and optimized to precisely control the alkali distribution in CZTSe.The bulk defect density was significantly reduced by suppression of deep acceptor states after the(NaF+KF)PDTs.Through the low-temperature deposition with(NaF+KF)PDTs,the CZTSe device on glass yields the best efficiency of 8.1%with an improved Voc deficit of 646 mV.The developed deposition technologies have been applied to PI.For the first time,we report the highest efficiency of 6.92%for flexible CZTSe solar cells on PI.Additionally,CZTSe devices were utilized as bottom cells to fabricate four-terminal CZTSe/perovskite tandem cells because of a low bandgap of CZTSe(~1.0 eV)so that the tandem cell yielded an efficiency of 20%.The obtained results show that CZTSe solar cells prepared by a low-temperature process with in-situ alkali doping can be utilized for flexible thin-film solar cells as well as tandem device applications.

Alkali metal cation doping of metal-organic framework for enhancing carbon dioxide adsorption capacity

Metal-organic frameworks （MOFs） have attracted much attention as adsorbents for the separation of CO2 from flue gas or natural gas. Here, a typical metal-organic framework HKUST-I（also named Cu-BTC or MOF-199） was chemically reduced by doping it with alkali metals （Li, Na and K） and they were further used to investigate their CO2 adsorption capacities. The structural information, surface chemistry and thermal behavior of the prepared adsorbent samples were characterized by X-ray powder diffraction （XRD）, thermo-gravimetric analysis （TGA） and nitrogen adsorption-desorption isotherm analysis. The results showed that the CO2 storage capacity of HKUST-1 doped with moderate quantities of Li＋, Na＋ and K＋, individually, was greater than that of unmodified HKUST-1. The highest CO2 adsorption uptake of 8.64 mmol/g was obtained with 1K-HKUST-1, and it was ca. 11% increase in adsorption capacity at 298 K and 18 bar as compared with HKUST- 1. Moreover, adsorption tests showed that HKUST-1 and 1K-HKUST-1 displayed much higher adsorption capacities of CO2 than those of N2. Finally, the adsorption/desorption cycle experiment revealed that the adsorption performance of 1K-HKUST-1 was fairly stable, without obvious deterioration in the adsorption capacity of CO2 after 10 cycles.

Influence of alkali metal doping on surface properties and catalytic activity/selectivity of CaO catalysts in oxidative coupling of methane

Surface properties （viz. surface area, basicity/base strength distribution, and crystal phases） of alkali metal doped CaO （alkali metal/Ca= 0.1 and 0.4） catalysts and their catalytic activity/selectivity in oxidative coupling of methane （OCM） to higher hydrocarbons at different reaction conditions （viz. temperature, 700 and 750 ℃; CH4/O2 ratio, 4.0 and 8.0 and space velocity, 5140-20550 cm^3 ·g^-1·h^-1） have been investigated. The influence of catalyst calcination temperature on the activity/selectivity has also been investigated. The surface properties （viz. surface area, basicity/base strength distribution） and catalytic activity/selectivity of the alkali metal doped CaO catalysts are strongly influenced by the alkali metal promoter and its concentration in the alkali metal doped CaO catalysts. An addition of alkali metal promoter to CaO results in a large decrease in the surface area but a large increase in the surface basicity （strong basic sites） and the C2＋ selectivity and yield of the catalysts in the OCM process. The activity and selectivity are strongly influenced by the catalyst calcination temperature. No direct relationship between surface basicity and catalytic activity/selectivity has been observed. Among the alkali metal doped CaO catalysts, Na-CaO （Na/Ca = 0.1, before calcination） catalyst （calcined at 750 ℃）, showed best performance （C2＋ selectivity of 68.8% with 24.7% methane conversion）, whereas the poorest performance was shown by the Rb-CaO catalyst in the OCM process.

Lithium-assisted synergistic engineering of charge transport both in GBs and GI for Ag-substituted Cu2ZnSn(S,Se)4 solar cells

Although silver(Ag) substitution offers several benefits in eliminating bulk defects and facilitating interface type inversion for Cu2ZnSn(S,Se)4(CZTSSe) photovoltaic(PV) technology, its further development is still hindered by the fairly low electrical conductivity due to the significant decrease of acceptors amount.In this work, a versatile Li–Ag co-doping strategy is demonstrated to mitigate the poor electrical conductivity arising from Ag through direct incorporating Li via postdeposition treatment(PDT) on top of the Ag-substituted CZTSSe absorber. Depth characterizations demonstrate that Li incorporation increases ptype carrier concentration, improves the carrier collection within the bulk, reduces the defects energy level as well as inverts the electric field polarity at grain boundaries(GBs) for Ag-substituted CZTSSe system. Benefiting from this lithium-assisted complex engineering of electrical performance both in grain interior(GI) and GBs, the power conversion efficiency(PCE) is finally increased from 9.21% to 10.29%. This systematic study represents an effective way to overcome the challenges encountered in Ag substitution,and these findings support a new aspect that the synergistic effects of double cation dopant will further pave the way for the development of high efficiency kesterite PV technology.

Sodium-doped carbon nitride nanotubes for efficient visible light-driven hydrogen production

Sodium-doped carbon nitride nanotubes （Nax-CNNTs） were prepared by a green and simple two-step method and applied in photocatalytic water splitting for the first time. Transmission electron microscopy （TEM） element mapping and X-ray photoelectron spectroscopy （XPS） measurements confirm that sodium was successfully introduced in the carbon nitride nanotubes （CNNTs）, and the intrinsic structure of graphitic carbon nitride （g-C3N4） was also maintained in the products. Moreover, the porous structure of the CNNTs leads to relatively large specific surface areas. Photocatalytic tests indicate that the porous tubular structure and Na＋ doping can synergistically enhance the hydrogen evolution rate under visible light （λ 〉 420 nm） irradiation in the presence of sacrificial agents, leading to a hydrogen evolution rate as high as 143 μmol·h-1 （20 mg catalyst）. Moreover, other alkali metal-doped CNNTs, such as Lix-CNNTs and Kx-CNNTs, were tested; both materials were found to enhance the hydrogen evolution rate, but to a lower extent compared with the Nax-CNNTs. This highlights the general applicability of the present method to prepare alkali metal-doped CNNTs; a preliminary mechanism for the photocatalytic hydrogen evolution reaction in the Nax-CNNTs is also proposed.

Alkalis-doping of mixed tin-lead perovskites for efficient near-infrared light-emitting diodes

Substitution of lead(Pb)with tin(Sn)is a very important way to reduce the bandgap of metal halide perovskite for applications in solar cells,and near infrared(NIR)light-emitting diodes(LEDs),etc.However,mixed Pb/Sn perovskite becomes very disordered with high trap density when the Sn molar ratio is less than 20%.This limits the applications of mixed Pb/Sn perovskites in optoelectronic devices such as wavelength tunable NIR perovskite LEDs(Pe LEDs).In this work,we demonstrate that alkali cations doping can release the microstrain and passivate the traps in mixed Pb/Sn perovskites with Sn molar ratios of less than 20%,leading to higher carrier lifetime and photoluminescence quantum yield(PLQY).The external quantum efficiency(EQE)of Sn_(0.2)Pb_(0.8)-based NIR Pe LEDs is dramatically enhanced from 0.1%to a record value of 9.6%(emission wavelength:868 nm).This work provides a way of making high quality mixed Pb/Sn optoelectronic devices with small Sn molar ratios.

Defect-rich ultrathin poly-heptazine-imide-framework nanosheets with alkali-ion doping for photocatalytic solar hydrogen and selective benzylamine oxidation

Polymeric carbon nitride(CN)as a metal-free photocatalyst holds great promise to produce high-value chemicals and H_(2) fuel utilizing clean solar energy.However,the wider deployment of pristine CN is critically hampered by the poor charge carrier transport and high recombination.Herein,we develop a facile salt template-assisted interfacial polymerization strategy that insitu introduces alkali ions(Na+,K+)and nitrogen defects in CN(denoted as v-CN-KNa)to simultaneously promote charge separation and transportation and steer photoexcited holes and electrons to their oxidation and reduction sites.The photocatalyst exhibits an impressive photocatalytic H_(2) evolution rate of 8641.5μmol·g^(−1)·h^(−1)(33-fold higher than pristine CN)and also works readily in real seawater(10752.0μmol·g^(−1)·h^(−1))with a high apparent quantum efficiency up to 18.5%at 420 nm.In addition,we further demonstrate that the v-CN-KNa can simultaneously produce H_(2) and N-benzylidenebenzylamine without using any other sacrificial reagent.In situ characterizations and DFT calculations reveal that the alkali ions notably promote charge transport,while the nitrogen defects generate abundant edge active sites,which further contribute to efficient electron excitation to trigger photoredox reactions.

Enhanced near-infrared light-induced photoresponse via transition of monocrystalline phase and surface reconstruction

Rare-earth-doped upconversion(UC)materials are ideal candidates for solar photovoltaic conversion and NIR response devices due to their unique spectral conversion properties.However,their low efficiency remains a tremendous challenge for practical applications.Here,we constructed an efficient NIR light-responsive device by coating a Si-photoresistor with a transparent gel consisting of UC powders and an organic polymer matrix.We show that reasonable introduction of alkali metal ions(Na~+,K~+,and Cs~+)into the lattice of UC crystals results in the improvement of photoelectricity conversion efficiency,due to the high crystallinity and surface reconstruction caused by alkali metal ion doping.