Tauc Analysis Research Articles

Synthesis and characterization of absorption-enhanced alumina solid particle materials for direct irradiation solar-thermal conversion

Based on solar-thermal application more than 1000 °C, a novel combination of sol-gel synthesis and stepwise calcination to prepare metal-ion doped alumina composite particles as heat transfer media were proposed, enhancing solar energy absorption and lowering preparation temperature. Investigations focused on the elemental distribution, phase composition, oxidation states, optical properties and thermal stability of the composites, which were doped with Cr, Co, Cu, Fe, and Mn metal ions in a 100:5 M ratio. The first-principles calculations elucidate the mechanism behind the impact of transition metal ions on alumina’s optical properties. Results show that introducing a small quantity of metal atoms results in new impurity energy levels and electron states around EF = 0. Specifically, the band gap energy of Cu- and Mn–Al2O3 decreases from 6.017 eV to 4.464 eV and 2.170 eV, respectively. Correspondingly, the absorptivity of these composite particles increases from around 20 %–76.6 %. And Tauc analysis indicates a decrease in the optical band gap of the particle materials from 5.37 eV to a range of 2.88–4.90 eV. Additionally, doping with Mn, Fe, and Cu accelerates the in-situ formation of α-Al2O3 crystals, with a phase transition temperature below 1000 °C and a crystallinity exceeding 93 %. Thermal stability tests show that the particles are highly stable, exhibiting a mere 0.49 % mass change between 30 °C and 1200 °C. The stepwise inert annealing strategy and metal-iron doping positively influence the optical performance of alumina, synthesizing long-term high-temperature stable alumina ceramic particles with absorptance enhancement and a low α-Al2O3 phase transition temperature. These attributes make a favorable choice for solar-thermal applications in direct irradiation.

Electronic structure that gives rise to the optical properties of zinc, cadmium and silver hexacyanocobaltates(III)

Transition metal hexacyanocobaltates(III) correspond to coordination polymers that present physical properties that can be used for technological applications. In this sense, properties like spin-crossover under different physical stimuli (light, temperature, pression, etc) or the colossal mechanical negative thermal expansion has been reported as astonishing properties of these materials (Goodwin et al., 2008; Goodwin et al., 2008; Avila et al., 2022) [1–3]. In this contribution, the electronic properties of hexacyanocobaltates(III) that contain Zn2+, Cd2+ and Ag1+ as metal ions, with a closed d-shell electronic configuration, are studied by means of combined UV–Vis spectroscopy and ab-initio calculations. The influence of the outer metals (Zn, Cd, Ag) when forming the coordination polymers as well as the effect of the inner octahedral moiety [Co(CN)6]3- have on the macroscopic electronic properties is discussed. Metal to ligand charge transfer transitions that produce the optical behavior are also clarified. Furthermore, the origin of the band gap transitions for Zn and Cd hexacyanocobaltates(III) is reported for the first time and supported by ab-initio calculations. New optical band gap energy values are proposed from a combined Urbach and Tauc analysis. Finally, the effects of metal substitution in Prussian blue analogues and the structural phase shift towards a zeolite-like phase on the band gap are analyzed.

MIL-100(Fe) /ZnO nanocomposite sensors: An enhanced ammonia selectivity and low operating temperature

This study aims to enhance the response and selectivity of zinc oxide (ZnO) gas sensors by its incorporating in MIL-100(Fe) as a support material. ZnO nanocomposites were synthesized by impregnating MIL-100 with varying weight percentages of zinc oxide (10% to 20%). The fabricated sensors underwent characterization through XRD, FTIR, DRS (UV-Vis), SEM/EDS, and NH3-TPD techniques to assess their material composition and sensor performance. DRS (UV-Vis) techniques revealed a significant decrease in the band gap, from Tauc analysis, accompanied by a 20% decrease in the Schottky barrier of the nanocomposite, compare to ZnO. Furthermore, the optimal operating temperature for the MIL-100/ZnO nanocomposites was identified as 240 °C, contrasting with pure ZnO sensors that exhibited an increasing response with temperature. Notably, the MIL-100/ZnO nanocomposites exhibited substantially improved selectivity for ammonia, displaying 3–6 times higher selectivity compared to pure ZnO sensors. Additionally, the response to ammonia increased threefold in the MIL-100/ZnO nanocomposites. This enhancement in selectivity and response can be attributed to the synergistic interaction between ammonia molecules and the Lewis acid sites present in the MIL-100 framework, facilitated by coordination bonds. The findings underscore the immense potential of MIL-100/ZnO nanocomposites as a promising solution for augmenting the selectivity of ZnO gas sensors. This work contributes valuable insights towards the advancement of gas sensing technology and paves the way for the development of highly selective gas sensors with extensive applications across various fields.

An eco-friendly bandgap engineering of semiconductor graphene oxide

Here, we demonstrate the tunability of the optical bandgap of graphene oxide (GO) and reduced graphene oxide (rGO) using an eco-friendly oxidation-reduction process. Based on UV–vis spectra and Tauc analysis, we observed a decrease in bandgap from 4.09 eV to 1.92 eV, effectively transforming GO from an insulating material to a semiconductor material. The transformation of GO into rGO is further characterized by FTIR, Raman, and TEM measurements. Additionally, electrical measurements showed that GO has a resistance of about 106, indicating its insulating nature. However, the reduction process significantly reduced the resistance of rGO to around 104, restoring its conductivity. Our results provide evidence for the potential use of GO and rGO in the semiconductor industry with tunable properties.

Temperature-dependent study of the fabricated ZnS/p-Si heterojunction

In the present work, ZnS thin film was deposited onto a p-type Si-substrate using the chemical bath deposition method, and subsequently Ag/n-ZnS/p-Si/Ag heterojunction device was fabricated to investigate its diode parameters in the temperature range of 0–60 °C. Cubic zinc blende phase of the prepared ZnS thin film was confirmed using grazing incidence X-ray diffraction (GIXRD); Scherrer analysis revealed a lower limit of crystallite size as 2.4 nm. Scanning electron microscopy (SEM) revealed the surface morphology of the prepared ZnS thin film, which exhibited good homogeneity and density. UV–Visible absorption spectroscopy revealed the good UV-region absorbance and visible-region transmittance of the deposited ZnS thin film. The corresponding energy band gap was obtained from Tauc analysis and found to be 3.88eV. Photoluminescent emission (PLE) spectroscopy showed the defect mediated radiative transitions in the deposited ZnS thin film. Current-voltage (I–V) measurements of the fabricated heterojunction device exhibited Schottky diode behaviour. Diode parameters of reverse saturation current, Schottky barrier height, ideality factor and series resistance were obtained from the I–V measurements in the temperature range of 0–60 °C; barrier height was obtained in the range of 0.87–0.74eV, and ideality factor was obtained close to unity for I–V measurements made at 30 °C and 60 °C.

Properties of GdSF and phase diagram of the GdF3 - Gd2S3 system

The objectives of this study were to refine the phase diagram of the GdF3-Gd2S3 system and to calculate their liquidus, and to synthesize GdSF and to study their properties. The GdSF compound (ST PbFCl, P4/nmm, a (Å) 3.83006(17), c (Å) 6.8529(3), has an optical band gap for a direct interband transition of 2.56 ​eV and is characterized by a pronounced increase in the Kubelka-Munk function in the region of this transition. Direct optical bandgap of GdSF is measured to be equal to 2.77 ​eV. Two indirect bandgaps are detected to be 1.54 and 2.4 ​eV. Meta-GGA simulations of band structure predicting 1.481 ​eV direct bandgap of GdSF are in good agreement with these features of the experimental absorption spectrum. To explain this complicated case, we argue that formally direct optical transitions to highly dispersive subbands contribute not to direct but to indirect bandgaps measured by Tauc analysis. The GdSF compound melts incongruently with the formation of a melt and γ-Gd2S3 compound at t ​= ​1280 ​± ​2°С, ΔН ​= ​40.6 ​± ​2.8 ​kJ/mol, ΔS ​= ​26.1 ​± ​1.8 ​J/mol∗K. The eutectic has a composition of 13 ​mol.% Gd2S3 (0.74 GdF3 ​+ ​0.26 GdSF), the melting characteristics of the eutectic are 1182 ​± ​2°С, ΔН ​= ​36.2 ​± ​2.5 ​kJ/mol, ΔS ​= ​24.9 ​± ​1.7 ​J/mol∗K. In the system GdF3 - Gd2S3 the balance equations for five phase transformations recorded by the DSC method were compiled. Convergence was achieved in the liquidus of the system constructed according to DSC data and calculated with the use of the Redlich-Kister equation.

Effective CO2 Capture and Selective Photocatalytic Conversion into CH3OH by Hierarchical Nanostructured GO-TiO2-Ag2O and GO-TiO2-Ag2O-Arg.

The attenuation of greenhouse gases, especially CO2, as one of the main causes of global warming and their conversion into valuable materials are among the challenges that must be met in the 21st century. For this purpose, hierarchical ternary and quaternary hybrid photocatalysts based on graphene oxide, TiO2, Ag2O, and arginine have been developed for combined CO2 capture and photocatalytic reductive conversion to methanol under visible and UV light irradiation. The material's band gap energy was estimated from the diffuse reflectance spectroscopy (DRS) Tauc analysis algorithm. Structural and morphological properties of the synthesized photocatalysts were studied using various analytical techniques such as Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The calculated band gaps for GO-TiO2-Ag2O and GO-TiO2-Ag2O-Arg were 3.18 and 2.62 eV, respectively. This reduction in the band gap showed that GO-TiO2-Ag2O-Arg has a significant visible light photocatalytic ability. The investigation of CO2 capture for the designed catalyst showed that GO-TiO2-Ag2O-Arg and GO-TiO2-Ag2O have high CO2 absorption capacities (1250 and 1185 mmol g-1, respectively, at 10 bar and 273 K under visible light irradiation). The amounts of methanol produced by GO-TiO2-Ag2O and GO-TiO2-Ag2O-Arg were 8.154 and 5.1 μmol·gcat1·h-1 respectively. The main advantages of this study are the high efficiencies and selectivity of catalysts toward methanol formation. The reaction mechanism to understand the role of hybrid photocatalysts for CO2 conversion is deliberated. In addition, these catalysts remain stable during the photocatalytic process and can be used repeatedly, proving to be enlightening for environmental research.

Extended dose response of 10B+CR-39 for thermal neutrons using spectrophotometric techniques

In this work, measurements were carried out in a standard thermal neutron facility using CR-39 detector combined with boron converter to study the dose response of CR-39 for thermal neutrons and the implementation of spectrophotometric techniques to extend the response towards high dose equivalents. The dose dependent variations in the response of 10B+CR-39 detectors versus etching and track parameters, structural and optical parameters were studied by image analysis, FTIR and UV–Vis spectral analysis. The track density increased linearly with the thermal neutron dose equivalent up to 0.46 mSv with a sensitivity of 7.393 × 105 tracks cm−2 mSv−1, above which saturation effect was observed. The bulk etch rate increased with the dose equivalent up to 4.761 mSv, beyond which it was found to be saturated. From FTIR spectral analysis, the absorbance peak intensities decreased with the neutron dose equivalent up to 4.761 mSv and saturation effects were observed above it. Similar result was also obtained from the UV–Vis spectral analysis, thereby demonstrating its application for thermal neutron dosimetry up to 4.761 mSv. Accordingly, the empirical relations were derived for dose estimation up to 4.761 mSv. From the Tauc analysis, the optical band gap for both direct and indirect transactions decreased with the thermal neutron dose. This study can be useful in determining the thermal neutron dose equivalent in a simple and effective manner using a CR-39 track detector combined with boron converters, facilitating its potential application in neutron dosimetry extended up to 4.761 mSv.

Physico‐chemical properties of international simple glass (ISG) nuclear waste simulants: Luminescence baseline study

AbstractBorosilicate glass has been adopted internationally for the treatment of nuclear waste with the object of long‐term stabilization through vitrification. Aiming to facilitate experimental comparisons across laboratories, the six‐component international simple glass (ISG) was developed as benchmark. The original ISG produced by Mo‐SCI Corporation (Rolla, MO) was distributed and characterized for different physical properties thus serving as reference for further studies. Still, photoluminescence (PL) properties, which may be useful for scrutinizing radiation‐induced damage, have not been reported. Further, the original ISG contained significant iron impurities, which may present interference in various studies (e.g., optical). Consequently, two new ISG analogs lacking iron impurities (labeled ISG‐1 [closest composition to original ISG] and ISG‐2 [some MgO added at expense of CaO]) were synthesized by Corning Inc. (Corning, NY) (also doped with lanthanum for subsequent corrosion tests involving atom probe tomography), which remain to be fully characterized. Accordingly, this work was undertaken to perform a comprehensive study comparing the three ISG specimens. Various characterizations were then performed on the pristine glasses: X‐ray diffraction and vibrational spectroscopy (structural properties); static‐leach product consistency test (PCT, dissolution behavior); dilatometry and calorimetry measurements (thermal properties); and optical absorption with Tauc and Urbach plots analysis, followed by PL spectroscopy with decay kinetics assessment (optical properties). While the general structural features appeared similar among the glasses, the deconvolution of Raman spectra suggested a lower degree of connectivity in the silica network for ISG‐2. In addition, some differences were indicated from the PCT results and thermal properties assessed, which were discussed based on compositional variations. Further on, the optical properties were shown to be the most distinct. The optical absorption of the original ISG was characterized by Fe2+/Fe3+ impurities not detected for ISG‐1 and ISG‐2. The glasses all exhibited two main PL features (∼1.8 and 2.5 eV) with multiexponential decay behavior apparently of intrinsic origin. In addition, ISG‐1 and ISG‐2 showed a third high‐energy (∼3.4 eV) fast‐decaying contribution tentatively credited to tin impurities, which appeared most significantly for ISG‐1. The results are expected to serve as baseline for future studies simulating the effects of radioactive element decay.

Mixed Tin-Titanium Oxides by Atomic Layer Deposition on Planar Substrates: Physical and Electronic Structure

Ternary atomic layer deposition of Sn-doped TiOx (Sn:TiOx) and Ti-doped SnOx (Ti:SnOx) was performed using tetrakisdimethylamidotitanium (IV) and tin(IV) with water at 150 °C on (100) Si and quartz substrates. The physical structure of the films was investigated using scanning electron, atomic force, and Kelvin probe force microscopies as well as spectroscopic ellipsometry, Raman spectroscopy, and X-ray diffraction. All as-deposited films are amorphous and possessed flat and conformal topographies with low surface roughness (<1 nm). Annealed Sn:TiOx and Ti:SnOx crystallize into the tetragonal anatase TiO2 and cassiterite SnO2 structures, respectively, without large variations to the film topography. The chemical composition of the as-deposited and annealed films was investigated through X-ray photoelectron spectroscopy. While dopants are homogenously mixed, the ternary ALD to form Sn:TiOx and Ti:SnOx surprisingly does not follow the rule of mixtures, despite similarities in both the growth rates of binary SnOx and TiOx and the precursors used. This finding is also supported by spectroscopic ellipsometry. We postulate the deviation from the rule of mixtures is due to non-ideal surface chemistry of the binary ALD of SnOx. The optical bandgap of all films was quantified by the Tauc analysis and the valence band position was investigated by valence band X-ray photoelectron spectroscopy. Intra-bandgap states above the valence band maximum were observed for as-deposited Ti:SnOx and Sn:TiOx and ascribed to 5 s states from Sn2+ ions. The presence of these intra-bandgap states induces enhanced visible absorption and bathochromic shifts to the apparent bandgap for all as-deposited films. Annealing the films removes the intra-bandgap states and increases the optical bandgap. Simplified band diagrams created using the data available demonstrated that the conduction and valence band positions did not vary between the positions of pure TiO2 or SnO2 with monotonic increases to the dopant incorporation.

ABIDE: An Accurate Predictive Model of Liver Decompensation in Patients With Nonalcoholic Fatty Liver-Related Cirrhosis.

Nonalcoholic fatty liver disease (NAFLD) is an increasingly important cause of liver cirrhosis and subsequent complications. We retrospectively developed and validated a model to predict hepatic decompensation in patients with NAFLD and cirrhosis and compared this with currently available models. Baseline variables from an international cohort of 299 patients with biopsy-proven NAFLD with compensated cirrhosis were examined to construct a model using competing risk multivariate regression and Akaike/Bayesian information criteria. Validation was performed in 244 patients with biopsy-proven NAFLD cirrhosis from the United States. Prognostic accuracy was compared with the NAFLD fibrosis score (NFS), fibrosis-4 (FIB-4), Model for End-Stage Liver Disease (MELD), Child-Turcotte-Pugh (CTP), and albumin-bilirubin (ALBI)-FIB-4 score using time-dependent area under the curve (tAUC) analysis. During a median follow-up of 5.6years (range 2.4-14.1) and 5.4years (range 1.5-13.8), hepatic decompensation occurred in 81 and 132 patients in the derivation and validation cohorts, respectively. In the derivation cohort, independent predictors of hepatic decompensation (Aspartate aminotransferase/alanine aminotransferase ratio, Bilirubin, International normalized ratio, type 2 Diabetes, andEsophageal varices) were combined into the ABIDE model. Patients with a score ≥4.1 compared with those with a score <4.1 had a higher risk of decompensation (subhazard ratio, 6.7; 95% confidence interval [CI], 4.0-11.2; P<0.001), a greater 5-year cumulative incidence (37% vs. 6%, P<0.001), and shorter mean duration to decompensation (3.8 vs 6.7years, P<0.001). The accuracy of the ABIDE model at 5years was good in the derivation (tAUC, 0.80; 95% CI, 0.73-0.84) and validation cohorts (0.78; 95% CI, 0.74-0.81) and was significantly more accurate than the NFS (0.72), FIB-4 (0.74), MELD (0.69), CTP (0.72), and ALBI-FIB-4 (0.73) (all P<0.001). In patients with NAFLD and compensated cirrhosis, ABIDE, a predictive model of routine clinical measures, predicts future hepatic decompensation.

Electrodeposition in Ionic Liquids for Controlled Synthesis of New Class of Ionic Semiconductors Based on Binary and Ternary Chalcogenide Perovskites for Thin Film Photovoltaic Solar Cells

Thin film photovoltaic (PV) solar cells based on Cu(In,Ga)(S,Se)2 (CIGS), CdTe and Cu2ZnSnS4 (CZTS) have been in the forefront of the pursuit for sustainable and pervasive solar electricity utilization. Recently, researchers have focused on alternative semiconductor photo-absorbers that can be synthesized as thin films by simple low cost solution based methods. Efforts in this direction led to solar cells based on the hybrid organic-inorganic perovskite photo-absorbers having the general formula RNH3MX3 where R= CnH2n+1, M= Pb, Sn and X= Cl, I or Br showing significantly high (24.2%) conversion efficiency. However, the light induced instability, mechanical durability and thermal degradation problems though largely elucidated, still require intensive efforts towards alleviation. The key factors for high efficiency of hybrid perovskite cell are high absorption coefficient, defect-tolerance and large carrier diffusion lengths. This is attributed to their ionic character derived from higher cation-anion coordination resulting in enhanced Coulomb interaction. The entirely inorganic chalcogenide perovskites are the ionic semiconductor counterparts which are purported to have similar optoelectronic properties sans the stability issues but have not been investigated in detail. The perovskite ionic semiconductors with formula unit ABX3 (A=Ca, Ba or Sr ; B=Ti or Zr; X= S, Se) have direct 0.8-2.4 eV bandgap, high photo-absorption and large carrier mobility. These appear attractive for photovoltaic solar cell application but are needed to be investigated as thin films by emphasizing the synthesis-property correlations. In this work we have investigated electrodeposition synthesis in ionic liquid medium of the transition metal layered chalcogenide TiS2 and p-type CaTiS3 perovskite chalcogenide semiconductor thin films which have application in solar cells, the former as a hole conductor and the later as an efficient photo-absorber.In this work we describe an ab initio study of scalable electrodeposition of TiS2 and CaTiS3 polycrystalline semiconductor thin films from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C and establish their structure and optoelectronic properties dependent upon the electrodeposition variables. The ionic liquid electrodeposition, not studied in detailed for semiconductor layers, is unique due to large stable potential window, high solubility for metal salts and devoid of hydrogen interference usual in aqueous medium. Further, it enables electrodeposition of semiconductor with elements having high reduction potential like Ca and Ti. We synthesized these films by electrodeposition in a 3-cell assembly with Pt reference and counter electrode and transparent conducting glass (FTO) as substrate. Precursors TiCl4, CaCl2 and Na2S2O3 served as source of Ti, Ca and S and initial cyclic voltammetry (CV) studies with variable precursor composition have established deposition potentials of Ca and Ti as -0.68V and -0.79V vs Pt, respectively.Detailed CV study with variable Ca:Ti precursor ratio coupled with film composition analysis establishes deposition potential and precursor composition for stoichiometric CaTiS3 film synthesis. X-ray diffraction showed as-electrodeposited films are polycrystalline. The Raman spectra includes numerous peaks in the 50-1200 cm-1 range which are interpreted as characteristic vibration modes of CaS at 170 and 220 cm-1. The variation in the intensity of the A1g mode at 359 cm-1 originating from TiS2 was used to calibrate the deposition potential relative to film composition. Tauc analysis of the uv-vis transmission spectra showed direct bandgap of 1.5 eV which is ideal for terrestrial solar radiation absorption. A correlation of the electrochemical deposition potential with the inclusion of secondary phases was noticed. This also resulted in the increase in the bandgap energy indicating possibility for adjusting bandgap for specific solar cell structures. The TiS2 layered chalcogenide with potential use as hole transport layer in chalcogenide perovskites was likewise electrodeposited. Detailed analysis of the reduction peaks in CV plots shows peak at -0.76V vs Pt corresponding to TiS2 formation and a rapidly rising current wave corresponds to under potential deposition. The direct electrodeposition of TiS2 is by reduction of Ti at substrate and instant sulfurization in presence of S2- ions. The granular growth is directed by FTO which provides electrochemical reduction and nucleation sites for TiS2 polycrystalline film growth as established by X-ray diffraction. All Raman peaks are interpreted belonging to TiS2 and partial phase mixing with the TiS3 depending on the electrodeposition potential was seen. Thus, the observed decrease in the bandgap with increase in potential can be used for integrating TiS2 in solar cells as hole conductor as well as an absorber depending on the configuration, This paper will report on the detailed effect of the variables used in ionic liquid electrodeposition and consequent structural and optoelectronic properties of the two important ionic semiconductors for assessment of their use in solar cells.

Development of an algorithm for precise and automated determination of optical band gap from Tauc analysis: case studies using alpha-irradiated CR-39 detectors

Tauc analysis is popularly used for the determination of optical band gap by extrapolating the linear portion of the Tauc’s plot and finding its intersection point at energy axis. In most of the studies, the data points in the linear portion of the Tauc’s plot are selected manually, which is time-consuming and the reproducibility in the selection process gets affected. To overcome this, an automated determination of band gap has been proposed for ensuring the reproducibility as well as for faster selection process. In this work, we report the development of an efficient algorithm which can provide the information of the optical band gap by Tauc analysis. The algorithm is based on finding the linear portion around maximum slope of the Tauc’s plot. A computer program (autoTauc) based on the algorithm was written and was validated through CR-39 irradiated to four different alpha fluences. The extrapolated linear line obtained through linear regression (using the program) was observed to pass through the linear portion of the Tauc’s plot. The optical band gap was found to be decreasing linearly with the alpha fluence for both direct and indirect transitions. Also the direct band gap was found to be higher than the indirect band gap.

The optical properties of the nonlinear crystal BaGa4Se7

Large crystals BaGa4Se7 (BGSe), promising as a nonlinear optical converters in the mid-IR, were grown and investigated using methods of optical spectroscopy. The crystals are transparent in the range 0.465 ÷ 22 μm. The Tauc analysis showed that BGSe is a direct bandgap dielectric with Eg = 2.73 and 2.91 eV at 300 and 80 K, respectively, with dEg/dT = −8.07 × 10−4 eV/K. Vibrational modes are observed in the range 150–350 cm−1 in the Raman spectra: Thus, two-phonon absorption determines the transparency edge in the mid-IR. Main extended defects (the (001) microtwins and point defects (impurity centers such as Ga–O and different native centers) are revealed. BGSe crystals are photosensitive in the range of 1.5–4.0 eV. Luminescence in BGSe is quenched by two orders of intensity as temperature increases from 77 to 300 K. The possibility of controlling the luminescence parameters by post-growth annealing in a suitable atmosphere demonstrated. Pyroelectric luminescence in the 100–440 K range confirms the noncentrosymmetric structure of this crystal.

Photoelectrochemical Solar Water Splitting: The Role of the Carbon Nanomaterials in Bismuth Vanadate Composite Photoanodes toward Efficient Charge Separation and Transport.

Photoelectrochemical performance of bismuth vanadate (BiVO4) photoanode is limited by poor charge separation and transport properties. The roles of carbon nanotube, reduced graphene oxide, or graphitic carbon nitride in BiVO4 composite photoanode were investigated toward enhancing light absorption and reducing overall impedance during photoelectrochemical water oxidation process. X-ray diffraction and Tauc analysis showed that BiVO4 retains its monoclinic phase, n-type semiconductor nature, and band gap in all carbon nanomaterials-incorporated composite photoanodes. It was observed that the carbon nanomaterials incorporation in BiVO4 film increases its surface porosity, ultimately leading to enhanced light absorption. The BiVO4 photoanode with reduced graphene oxide and graphitic carbon nitride showed same bulk charge separation efficiency, whereas the latter showed better charge transfer. It was found that the graphitic carbon nitride formed composite with BiVO4 to provide enhanced light absorption efficiency, i.e., 89% in 350-505 nm range. The BiVO4 with graphitic carbon nitride photoanode showed the best performance with a photocurrent of 2.2 mA cm-2, charge separation efficiency of 67%, and photocurrent of 4.0 mA cm-2 with cobalt-phosphate surface catalyst at 1.23 VRHE for water oxidation under 1 sun illumination. The Mott-Schottky and impedance measurements confirmed the shift of conduction band position toward hydrogen reduction potential and reduction in film resistance, respectively, with carbon nanomaterials addition, and the shift was most significant for graphitic carbon nitride. It is concluded that by concomitant formation of junction during photoanode fabrication between carbon nanomaterials, BiVO4, and fluorine-doped tin oxide glass substrate, better charge separation, transport, and light absorption can be achieved.

Electrochromic effect in Q-carbon

Here, we report the electrochromic effect in undoped Q-carbon. This unique phase of carbon, Q-carbon, is formed by pulsed laser melting of amorphous carbon and subsequent ultrafast quenching process. The excess amount of unpaired electrons near the Fermi energy level in the Q-carbon causes a 48% increase in the optical absorption at 265 nm with an applied electric field of 10 V. The Kelvin probe force microscopy (KPFM) also indicates that the Fermi level of Q-carbon is situated ∼40 meV higher than that in the neighboring diamond-like carbon region. It is also observed that, with an increase in the tip voltage from 0 to 10 V in KPFM, there occurs an increase in the image phase contrast thereby indicating an increase in the concentration of electrons. This causes an increase in optical absorbance with the application of an electric field in Q-carbon. The direct and indirect optical band gaps in Q-carbon are calculated to be 3.82 and 2.93 eV, respectively, using the Tauc analysis. High-resolution scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction pattern depict the formation and amorphous nature of Q-carbon. Raman and electron energy-loss spectroscopy of Q-carbon reveal over 75% sp3-bonded carbon (rest sp2) and excess electrons near the Fermi level. This discovery of electrochromic effect in Q-carbon shows that highly non-equilibrium synthesis can be elegantly used to fabricate smart windows with immense energy-saving applications.

A comprehensive dual beam approach for broadband control of ultrafast optical nonlinearity in reduced graphene oxide

A unique delay-dependent dynamic switching of optical nonlinearity in terms of saturation absorption (SA) and excited state absorption (ESA) in graphene oxide (GO) and reduced graphene oxide (RGO2 and RGO100) is achieved with an optical pump and white light super-continuum probe only above a threshold pump intensity. Infrared and chemical reduction are used to obtain RGO2 and RGO100 respectively. The switching regime of probe wavelength can be modulated by varying either pump wavelength or degree of reduction. When pumped at 415 nm, the threshold pump intensity to obtain switching property decreases to 9 GW/cm2 for RGO2 from 18 GW/cm2 in GO and the tunability range shifts from 471 to 526 nm for as grown GO to 519–623 nm in maximally reduced RGO2. Though the saturation intensity of intrinsic non-degenerate two photon absorption (nd-TPA) is found to be lower in GO (4.3 GW/cm2) than RGO2 (18.2 GW/cm2), nd-TPA coefficient increases from 0.0015 cm/GW (GO) to 0.0026 cm/GW (RGO2) with increasing reduction. The detailed kinetics of the scattering processes show variation as a function of pump power, probe wavelength and degree of reduction. A model band diagram based on amorphous-carbon model and a Tauc analysis are used to explain the unusual nonlinear optical properties.

Multifunctional Fischer Aminocarbene Complexes as Hole or Electron Transporting Layers in Organic Solar Cells.

A new series of Fischer carbenes have been synthetized and examined as hole-transporting or electron-transporting layers (HTLs or ETLs) in the fabrication of organic solar cells (OSCs). The synthesis of three Fischer aminocarbene complexes with the general formula [Cr(CO)5{C(NHCH2)Ar}] (Ar = 2-pyridyl (3a), 3-pyridyl (3b) and 4-pyridyl (3c)) is reported. The molecular structure of complex 3b has been confirmed by X-ray analysis. In order to study the possible applications of the three Fischer aminocarbenes in OSCs, thin films of these complexes were prepared using a vacuum deposition process. These organometallic films were chemically and morphologically characterized by IR spectroscopy, SEM, AFM and XRD. According to the IR and Tauc analysis, the vacuum deposition process generates thin films free of impurities with an activation energy of 4.0, 2.7 and 2.1 eV for 3a, 3b y 3c, respectively. The UV-vis spectra of the amorphous aminocarbene films show that they are practically transparent to the visible radiation of the electromagnetic spectrum. This is due to the fact that their absorption is located mainly in the ultraviolet range. Two OSCs with bulk-heterojunction configuration were manufactured in order to prove the use of the aminocarbenes as ETL o HTL. The aminocarbene [Cr(CO)5{C(NHCH2) 4-pyridyl}] (3c) proved to be suitable as ETL with a fill factor (FF) of 0.23 and a short circuit current density (JSC) of 1.037 mA/cm2.

High Throughput Light Absorber Discovery, Part 2: Establishing Structure-Band Gap Energy Relationships.

Combinatorial materials science strategies have accelerated materials development in a variety of fields, and we extend these strategies to enable structure-property mapping for light absorber materials, particularly in high order composition spaces. High throughput optical spectroscopy and synchrotron X-ray diffraction are combined to identify the optical properties of Bi-V-Fe oxides, leading to the identification of Bi4V1.5Fe0.5O10.5 as a light absorber with direct band gap near 2.7 eV. The strategic combination of experimental and data analysis techniques includes automated Tauc analysis to estimate band gap energies from the high throughput spectroscopy data, providing an automated platform for identifying new optical materials.

High Throughput Light Absorber Discovery, Part 1: An Algorithm for Automated Tauc Analysis.

High-throughput experimentation provides efficient mapping of composition-property relationships, and its implementation for the discovery of optical materials enables advancements in solar energy and other technologies. In a high throughput pipeline, automated data processing algorithms are often required to match experimental throughput, and we present an automated Tauc analysis algorithm for estimating band gap energies from optical spectroscopy data. The algorithm mimics the judgment of an expert scientist, which is demonstrated through its application to a variety of high throughput spectroscopy data, including the identification of indirect or direct band gaps in Fe2O3, Cu2V2O7, and BiVO4. The applicability of the algorithm to estimate a range of band gap energies for various materials is demonstrated by a comparison of direct-allowed band gaps estimated by expert scientists and by automated algorithm for 60 optical spectra.