Low-temperature Selenization Research Articles

Efficient water splitting catalyst: Low-temperature selenization of Co and Ni hydroxide nanosheets on carbon cloth for enhanced electro-catalytic activity

The primary challenge in achieving full water splitting is the high cost of Pt/C-based materials and efficient electrocatalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). In this investigation, Co and Ni based selinides as a bifunctional electrocatalyst was produced by using electrodeposition technique on carbon cloth (CC) as supportive material following low-temperature selenization, Co and Ni hydroxide nanosheets were transformed into CoNiSe-based carbon cloth, which performed superb electro-catalytic activity for water splitting. Due to the unique morphology and collaborative effect of CoSe-CC/NiSe-CC, the Tafel slope values for HER and OER are 64.2 and 59.2 mVdec−1. which is comparable to the standard 48.6 and 58.4 mVdec−1 values of RuO2 and Pt/C. Remarkably, cobalt and nickel-based CC exhibit exceptional stability and catalytic activity towards overall water splitting in the basic medium as compared to higher-priced catalysts such as RuO2 and Pt/C. This composite also requires 1.6 V to drive a current density of 10 mA cm−2 for overall Water splitting in an alkaline medium.

NiSe2-CoSe2 with a Hybrid Nanorods and Nanoparticles Structure for Efficient Oxygen Evolution Reaction

Hetero-structure induced high performance catalyst for oxygen evolution reaction (OER) in the water splitting reaction has received increased attention. Herein, we demonstrated a novel catalyst system of NiSe2-CoSe2 consisting of nanorods and nanoparticles for the efficient OER in the alkaline electrolyte. This catalyst system can be easily fabricated via a low-temperature selenization of the solvothermal synthesized NiCo(OH)x precursor and the unique morphology of hybrid nanorods and nanoparticles was found by the electron microscopy analysis. The high valence state of the metal species was indicated by X-ray photoelectron spectroscopy study and a strong electronic effect was found in the NiSe2-CoSe2 catalyst system compared to their counterparts. As a result, NiSe2-CoSe2 exhibited high catalytic performance with a low overpotential of 250 mV to reach 10 mA·cm-2 for OER in the alkaline solution. Furthermore, high catalytic stability and catalytic kinetics were also observed. The superior performance can be attributed to the high valence states of Ni and Co and their strong synergetic coupling effect between the nanorods and nanoparticles, which could accelerate the charge transfer and offer abundant electrocatalytic active sites. The current work offers an efficient hetero-structure catalyst system for OER, and the results are helpful for the catalysis understanding.

PtSe2 outperforms Pt as a counter electrode in dye sensitized solar cells

In the present study, we show that PtSe2 films, prepared by soft selenization of pre-deposited Pt, is a very efficient counter electrode (CE) in dye sensitized solar cells (DSSCs). Devices based on PtSe2 achieve better photoconversion efficiency (9.5 ± 1.2%) than those employing bare Pt CE (9.18 ± 0.21%), even if the latter use three times more Pt loading than that used to prepare the PtSe2 CE. Films with of various Pt loadings have been prepared by electrodeposition and their catalytic properties have been investigated and compared with films of corresponding Pt loading which have been transformed to PtSe2 by low-temperature selenization. The improved performance of the PtSe2 CEs has been assigned to the higher availability of catalytic active sites and the more effective mechanism of interfacial electron transfer. This is a first attempt to explore the role of PtSe2 as the CE in DSSCs. The strong advantages of PtSe2 in relation to catalytic activity, stability, efficiency, combined with cost reduction, due to the lower mass loading required for a given performance, renders this noble-transition metal dichalcogenide a promising candidate to boost the performance of third generation photovoltaics, opening-up possibilities, at the same time, for other applications where Pt is used as the catalyst.

Self-generated FeSe2 and CoSe2 nanoparticles confined in N, S-doped porous carbon as efficient and stable electrocatalyst for oxygen evolution reaction

Transition metal selenides embedded into a nitrogen, sulfur co-doped carbon materials (TMS@NSC) are promising electrocatalysts to effectively address the sluggish kinetics of oxygen evolution reaction (OER). However, the OER catalytic activity of single metal TMS@NSC is still limited by insufficient active sites. Hence, FeSe2 and CoSe2 nanoparticles self-generated and confined in N, S-doped porous carbon (FeSe2/CoSe2@NSC) catalyst has been synthesized by in-situ polymerization and the subsequent low temperature selenization strategy, aiming to increase the active sites of TMS@NSC. The hybridized FeSe2/CoSe2@NSC catalyst has excellent OER catalytic activity and good durability in alkaline media, which only requiring an overpotential as low as 278 mV to drive a current density of 10 mA cm−2, outperforming most recently reported selenide catalysts and commercial RuO2 catalysts. The high catalytic activity of FeSe2/CoSe2@NSC catalyst is attributed to the rapid electron transfer between FeSe2 and CoSe2 in the unique N, S-doped graphene nanosheets, which effectively inhibits the aggregation of nanoparticles, fully exposes the active sites and provides pathway for oxygen release during the OER process. Moreover, the overall water splitting device assembled with FeSe2/CoSe2@NSC catalyst showed low overpotential of 1.57 V. This study provides a feasible strategy for the design of high active and stable TMS@NSC catalysts for OER and other energy conversion applications.

Improvement of CZTSSe film quality and superstrate solar cell performance through optimized post-deposition annealing

Improving the performance of kesterite solar cells requires high-quality, defect-free CZTS(Se) films with a reduced number of secondary phases and impurities. Post-annealing of the CZTS films at high temperatures in a sulfur or selenium atmosphere is commonly used to improve the quality of the absorbing material. However, annealing at high-temperatures can promote material decomposition, mainly due to the loss of volatile elements such as tin or sulfur. In this work, we investigate how the additional step of sulfurization at reduced temperatures affects the quality and performance of CZTSSe based solar cells. A comprehensive structural analysis using conventional and high resolution XRD as well as Raman spectroscopy revealed that the highest CZTSSe material quality with the lowest structural disorder and defect densities was obtained from the CZTS films pre-sulfurized at 420 °C. Furthermore, we demonstrate the possibility of using Sb2Se3 as a buffer layer in the superstrate configuration of CZTSSe solar cells, which is possible alternative to replace commonly employed toxic CdS as a buffer layer. We show that the additional low-temperature selenization process and the successful use of Sb2Se3 as a buffer layer could improve the performance of CZTSSe-based solar cells by up to 3.48%, with an average efficiency of 3.1%.

Phase evolution during annealing of low-temperature co-evaporated precursors for CZTSe solar cell absorbers

Systematic investigations into the phase evolution during reactive annealing of copper–zinc–tin–selenide (CZTSe) precursors for the fabrication of kesterite solar cell absorber layers have been paramount in understanding and suppressing the formation of secondary phases that deteriorate device performance. In this study, the phase evolution during annealing of low-temperature co-evaporated CZTSe precursors is investigated. A detailed analysis of films selenized at different temperatures is used to reveal the possible reaction pathway of CZTSe formation. Utilizing a combination of x-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy, it is shown that CZTSe formation starts by Cu out-diffusion to the surface and Cu–Se phase formation at a temperature of 350 °C. An intimate mixing of binaries and ternaries during low-temperature selenization is observed. On the contrary, only binaries are observed at high-temperature selenization. This suggests that the CZTSe formation pathway involves reaction schemes where (i) a competition between binary and ternary phases dominates at low-temperature and (ii) binary reactions dominate the process at high temperatures. However, the number of binary phases decreases with increasing selenization temperature until they become undetectable by XRD and Raman spectroscopy at a temperature of 540 °C (selenization time 10 min). Utilizing the presented selenization conditions, prototype solar cells with an efficiency of up to 7.5%, an open-circuit voltage of 407 mV, and a fill factor of 59%, could be demonstrated. The temperature-dependent current density–voltage characteristics indicate that the performance of the prototype devices is limited by bulk Schottky–Read–Hall recombination.

Structure engineering of solution-processed precursor films for low temperature fabrication of CuIn(S,Se)2 solar cells

Fabrication of flexible Cu(In,Ga)(S,Se)2 (CIGS) thin-film solar cell via precursor solution is a promising approach due to its perfect compatibility with roll-to-roll production. So far, polyimide (PI) is the most suitable substrate for flexible CIGS solar cell because of many advantages such as low cost, light weight, good insulativity and free of metallic impurities. However, PI has not been used in solution-processed approach because PI cannot tolerate the annealing process (selenization) at high temperature (≥550 °C) which is normally required for most of highly efficient CIGS solar cells fabricated via precursor solution. Here, we report low temperature (450 °C) fabrication of CuIn(S,Se)2 (CISSe) absorber by carefully engineering the structure of solution-processed precursor film. By adding extra Cu2S into solution-processed CuInS2 (CIS) precursor films through structure engineering of Mo/In2S3/Cu2S (bilayer), Mo/CIS/Cu2S/(CIS/Cu2S)3/CIS (multilayer), enough Cu2−xSe is formed in low temperature selenization which facilitates the formation process of CISSe absorber. Characterizations using XRD, Raman, SEM and EDX show that absorbers fabricated from precursor films with new structures favor the sufficient selenization, resulting in much better morphology and higher photovoltaic performance than the absorber fabricated from precursor film with normal structure of Mo/CIS (single layer). After preliminary optimization, efficiencies of 5.01% and 6.13% have been achieved in CISSe solar cells fabricated from precursor films with new structures which is only 3.83% from normal structure. Our results demonstrate the feasibility of fabricating efficient chalcopyrite solar cells via precursor solution in low temperature selenization.

Phase Variations and Layer Epitaxy of 2D PdSe2 Grown on 2D Monolayers by Direct Selenization of Molecular Pd Precursors.

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe2 and Pd2Se3 are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensors. This research presents a feasible approach to synthesize PdSe2 and Pd2Se3 with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS2. We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by low-temperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd2Se3 phase, to continuous few-layer PdSe2 films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe2 with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe2 and monolayer Pd2Se3 are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe2 and underlying 2D substrate was also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.

High-Performance Rechargeable Aluminum–Selenium Battery with a New Deep Eutectic Solvent Electrolyte: Thiourea-AlCl3

Aluminum-sulfur batteries (ASBs) have attracted substantial interest due to their high theoretical specific energy density, low cost and environmentally friendly while the traditional sulfur cathode and ionic liquid have very fast capacity decay, limiting cycling performance because of the sluggishly electrochemical reaction and side reactions with the electrolyte. Herein, we demonstrate, for the first time, excellent rechargeable aluminum-selenium batteries (ASeBs) using a new deep eutectic solvent, thiourea-AlCl3 as an electrolyte and Se nanowires grown directly on a flexible carbon cloth substrate (Se NWs@CC) by a low-temperature selenization process as a cathode. Selenium (Se) is a chemical analogue of sulfur with higher electronic conductivity and lower ionization potential that can improve the battery kinetics on the sluggishly electrochemical reaction and the reduction of the polarization where the thiourea- AlCl3 electrolyte can stabilize the side reaction during the reversible conversion reaction of Al-Se alloying processes during the charge-discharge process, yielding a high specific capacity of 260 mAh g-1 at 50 mA g-1 and a long cycling life of 100 times with high columbic efficiency nearly 93 % at 100 mA g-1. The working mechanism base on the reversible conversion reaction of the Al-Se alloying processes, confirmed by the ex-situ Raman, XRD and XPS measurements was proposed. This work provides new insights into the development of rechargeable aluminum-chalcogenide (S, Se, and Te) batteries.

Cable-like carbon nanotubes decorated metal-organic framework derived ultrathin CoSe2/CNTs nanosheets for electrocatalytic overall water splitting

The high cost and low reserves of noble metals greatly hinder their practical applications in new energy production and conversion. The exploration of cost-effective alternative electrocatalysts with the ability to drive hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is extremely significant to promote overall water splitting. Herein, ultrathin CoSe2/CNTs nanocomposites have been synthesized by a facile two-step method, where the ultrathin Co-MOF (metal organic-framework) decorated with cable-like carbon nanotubes (CNTs) (Co-MOF/CNTs) was initially fabricated, and followed a low-temperature selenization process. The ultrathin CoSe2 nanosheets as well as the superior conductivity of CNTs synergistically resulted in abundant active sites and enhanced conductivity to boost the electrocatalytic activity. The as-prepared CoSe2/CNTs electrocatalysts exhibited an overpotential of 190 mV and 300 mV vs. reversible hydrogen electrode (RHE) at a current density of 10 mA/cm2 for the HER and OER in alkaline solution, respectively, and demonstrated superior durability. Furthermore, the as-prepared bifunctional CoSe2/CNTs electrocatalysts can act as cathode and anode in an electrolyzer, showing a cell voltage of 1.75 V at 10 mA/cm2 for overall water splitting.

Hierarchical nickel-cobalt selenide nanoparticles/nanosheets as advanced electroactive battery materials for hybrid supercapacitors

Transition metal selenides are very ideal as electroactive battery material for hybrid supercapacitors owing to their high electrochemical activity and metallic electrical conductivity, but their environmental friendly fabrication and complex structure construction is still a big challenge. Here, a simple low-temperature selenization method has been developed to green synthesize a series of Ni–Co selenide samples with a hierarchical nanoparticle/nanosheet structure. The hierarchical structure has been constructed by the recrystallization of sample during the selenization process, which can be greatly influenced by the Co composition. The specific hierarchical structure provides more electroactive sites for charge storage, and the optimized synergy between the Ni and Co compositions further greatly tunes the charge storage performance, as a result, the Ni0.67Co0.33Se2 shows the best performance with a specific capacity of 447 C g−1 at 1 A g−1, which is much higher than the Ni–Co selenides with the other Ni to Co ratios and the Ni–Co oxides. The Ni0.67Co0.33Se2 retains 97% of the specific capacity after 2000 cycles, demonstrating its excellent cycling stability. A hybrid supercapacitor has been assembled using Ni0.67Co0.33Se2 as the positive electrode, which shows both high specific energy and specific power performances.

Nanoscale electrical property enhancement through antimony incorporation to pave the way for the development of low-temperature processed Cu2ZnSn(S,Se)4 solar cells

The Sb-assisted grain growth and nanoscale band-bending enhancement directly decrease the selenization temperature down to 470 °C.

Accelerated oxygen evolution kinetics on nickel–iron diselenide nanotubes by modulating electronic structure

Development of cost-effective, highly active and durable electrocatalysts is highly demanded for oxygen evolution reaction (OER), which is an indispensable process involved in water splitting cells and metal-air batteries. Here, we accelerate the sluggish OER kinetics by controlling the morphology and engineering electronic structure of FexNi1-xSe2 catalysts. FexNi1-xSe2 nanotubes with tunable stoichiometry were fabricated through the removal of sacrificial templates (Cu2O nanowires) and subsequent low-temperature selenization process. By precise tuning of Fe doping amount in FexNi1-xSe2 nanotubes, the optimized Fe0.125Ni0.875Se2 nanotubes show a low overpotential of 253 mV at a current density of 10 mA cm−2, a small Tafel slope of 49 mV dec−1, and superior durability. By combining analysis of the experimental results with density-functional-theory (DFT) simulations, we reveal that the in-situ formed amorphous hydroxyl group layers on the surface of Fe0.125Ni0.875Se2 (Fe0.125Ni0.875Se2–OH) enable highly efficient oxygen evolution catalysis. The Fe dopant can achieve a near-optimal adsorption energy for OER intermediates (OH*, O*, and OOH*) on the oxidized surface of Fe0.125Ni0.875Se2–OH, which is responsible to the enhanced OER performance of Fe0.125Ni0.875Se2. This work paves a new way for exploring low cost and highly active OER electrocatalysts based on ternary metal selenides with hollow structure.

Low temperature crystallization of Cu2ZnSnSe4 thin films using binary selenide precursors

In the present paper, a novel process for synthesis of Cu2ZnSnSe4 thin films via low temperature selenization (350 A degrees C) of multiple stacks of binary selenides has been reported. Further, the influence of selenization temperature (250-450 A degrees C) on the physical properties of Cu2ZnSnSe4 thin films was studied and discussed herein. The Rietveld refinement from X-ray diffraction data of Cu2ZnSnSe4 films grown at a selenization temperature of 350 A degrees C was found to be single phase with kesterite type crystal structure and having lattice parameters a = 5.695 , c = 11.334 . Raman spectra recorded using multi excitation wavelength sources under non-resonant and near resonant conditions confirms the formation of single phase Cu2ZnSnSe4 films. Secondary ion mass spectroscopic (SIMS) analysis demonstrated that composition of elements across the thickness is fairly uniform. Energy dispersive X-ray analysis measurement reveals that the obtained films are Cu-poor and Zn-rich. The scanning electron micrographs of binary selenide stacks selenized at a temperature of 350 A degrees C shows randomly oriented cylindrical grains. The optical absorption studies indicated a direct band gap of 1.01 eV. The films showed p-type conductivity with electrical resistivity of 4.66 a''broken vertical bar cm, Hall mobility of 15.17 cm(2) (Vs)(-1) and carrier concentration of 8.82 x 10(16) cm(-3).

30×40 cm2 flexible Cu(In,Ga)Se2 solar panel by low temperature plasma enhanced selenization process

A progressing non-toxic plasma-enhanced solid Se vapor selenization process (PESVS) technique, compared with hydrogen-assisted Se vapor selenization (HASVS) to achieve a large-area (30×40cm2) Cu(In,Ga)Se2 (CIGS) solar panel with enhanced efficiencies from 10.8% to 13.2% (14.7% for active area), was demonstrated. The bonding of Se was partially broken by ICP plasma treatment and these Se radicals are helpful to enhance reaction activity for following selenization process at an extremely low temperature of 330°C. The effects of plasma steps, plasma power, selenization temperature and optimized conditions were thoroughly studied in detail. The remarkable enhancement of the efficiency is ascribed to the better crystallinity, enlarged grain size, less Se vacancy and uniform depth distribution of Ga. From reaction kinetics point of view, PESVS provides extra energy to crack Se, resulting in the decrease in reaction activation energy. The PESVS methodology was also applied to low temperature (450°C) selenized CIGS thin film solar panel with uniform conversion efficiency more than ~10%. Furthermore, a large-area flexible stainless steel substrate with remarkable conversion efficiency of ~6.8% without Na addition was demonstrated. We believed that this work can provide a facile approach of low temperature selenization on flexible substrate applications or fast selenization for throughput consideration, thus stimulating the mass-production in large scale CIGS PV industry.

Dynamics of the phase formation process upon the low temperature selenization of Cu/In-multilayer stacks

Phase reactions occurring during a low temperature selenization of thin In/Cu-multilayer stacks were investigated by ex-situ x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS). Therefore, dc-sputtered In/Cu-multilayers onto molybdenum coated soda lime glass were selenized in a high vacuum system at temperatures between 260 and 340 °C with different dwell times and selenium supply. The combination of the results of the phase analysis by XRD and the measurements of the in-depth elemental distribution by EDS allowed a conclusion on the occurring reactions within the layer depth. We found two CuInSe2 formation processes depending on the applied temperature. Already, at a heater temperature of 260 °C, the CuInSe2 formation can occur by the reaction of Cu2−xSe with In4Se3 and Se. At 340 °C, CuInSe2 is formed by the reaction of Cu2−xSe with InSe and Se. Because both reactions need additional selenium, the selenium supply during the selenization can shift the reaction equilibria either to the metal binaries side or to the CuInSe2 side. Interestingly, a lower selenium supply shifts the equilibrium to the CuInSe2 side, because the amount of selenium incorporated into the metallic layer is higher for a lower selenium supply. Most likely, a larger number of grain boundaries are the reason for the stronger selenium incorporation. The results of the phase formation studies were used to design a two stage selenization process to get a defined structure of an indium selenide- and a copper selenide-layer at low temperatures as the origin for a controlled interdiffusion to form the CuInSe2-absorber-layer at higher temperatures. The approach delivers a CuInSe2 absorber which reach total area efficiencies of 11.8% (13.0% active area) in a CuInSe2-thin-film solar cell. A finished formation of CuInSe2 at low temperature was not observed in our experiments but is probably possible for longer dwell times.

Characteristics of Cu(In,Ga)Se2 Films Prepared by Atmospheric Pressure Selenization of Cu-In-Ga Precursors Using Ditert-Butylselenide as Se Source

In this study, copper indium gallium diselenide [Cu(In,Ga)Se2; CIGS] films were prepared by selenization of Cu-In-Ga metallic precursors using ditert-butylselenide (DTBSe) under atmospheric pressure. Based on the results of θ-to-2θ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), it was found that the films selenized at 300 or 400°C for 60 min showed the presence of Kirkendall voids along with the XRD signitures of pure copper (Cu) and certain intermediate binary selenides, copper indium diselenide (CIS) and CIGS depending on temperature while only a single CIGS structure was detected in those films selenized at 500 or 600°C for 60 min. A 5-temperature selenization process was found to enable the formation of CIGS structure with better crystalline quality and thickness uniformity. It is believed that intermediate binary and ternary selenides are formed sequentially with increasing completeness during low-temperature selenization stages of the 5-temperature selenization process. This enhances the subsequent formation of CIGS structure at high-temperature selenization stages of the 5-temperature selenization process with improved structural and morphological properties.

Low-Temperature Direct Conversion of Cu–In Films to CuInSe2 via Selenization Reaction in Supercritical Fluid

In this study, we achieve the direct conversion of metallic Cu-In films to compound semiconductor CuInSe(2) films, at quite low temperature around 300 °C using less hazardous metalorganic selenium source in supercritical fluid (SCF). We investigated the effects of temperature and fluid (ethanol) density, and found that supercritical ethanol plays a crucial role in this low-temperature selenization reaction. Such SCF-assisted direct conversion reactions can facilitate large-scale, low-temperature, and rapid synthesis of CuInSe(2) films, which can potentially lead to the low-cost production of solar cells.

Morphological and Structural Changes in Cu(In,Ga)Se2 Thin Films by Selenization Using Diethylselenide

The morphological and structural changes in Cu(In,Ga)Se2 (CIGS) thin films during selenization using diethylselenide (DESe) are investigated. The surface morphology and extra phase existence of CIGS thin films strongly depend on the heating profile temperature and time of each step during selenization. Conventional high-temperature (515 °C) selenized CIGS thin films formed grains of approximately 2–3 µm, although the Cu–In–Ga metallic precursor was very smooth. On the other hand, the precursor selenized at a low temperature (about 400 °C) exhibited a homogeneous surface morphology because the precursor was formed from Cu–Se and (In,Ga)–Se alloys to diffuse selenide into a Cu–In–Ga metallic precursor. The high-temperature selenized CIGS thin films after low-temperature selenization had a homogeneous surface morphology with appropriate-sized grains. These results indicate that the heating profile “during” selenization was very important in forming Cu–Se and (In,Ga)–Se alloys from a Cu–In–Ga metallic precursor.