Solar Light Harvesting Research Articles

Engineering the High Concentration of N3C Nitrogen Vacancies Toward Strong Solar Light-Driven Photocatalyst-Based g-C3N4

Graphitic carbon nitride (g-C3N4) has emerged as promising metal-free semiconductor-based photocatalysts for solar energy conversion. However, the photocatalytic performance of g-C3N4 is still moderate, due to the lack of the architecture design, which enables solar light harvesting, excited electron–hole separation as well as extending their availability for redox reactions, and far from practical application. Herein, we engineer the structure of g-C3N4 through a polyethylenimine (PEI)/alkaline coassisted technique. The resulting materials show the high concentration of both N3C nitrogen vacancies and hydroxyl groups on the g-C3N4 surface. These features significantly contribute to enhancing electron–hole generation and separation and harvest solar light absorption, which boosts the photocatalytic hydrogen production compared to bare g-C3N4 under solar irradiation.

Surface-disorder-engineered Zn2SnO4/SnO2 hollow microboxes with enhanced solar-driven photocatalytic activity

Engineering defects and/or amorphous shells is a promising strategy to improve the photocatalytic efficiency of the oxide materials. In this work, we developed a safe and easy magnesiothermic reduction method to prepare surface-disordered hybrid nanocomposites, taking Zn2SnO4/SnO2 hollow microboxes as an example. Our results demonstrate that the reduction treatment can successfully introduce oxygen vacancies and amorphous shells in the pristine Zn2SnO4/SnO2 sample, causing narrowing of the band gap and reinforced solar light harvesting. Moreover, the Mg-reduced Zn2SnO4/SnO2 heterojunction photocatalysts exhibit enhanced sunlight-driven hydrogen production activity and degradation performance of the organic pollutant. This strategy may disclose a new avenue for improving photocatalytic performance of defect-engineered heterojunction composites toward broader applications.

(Invited) Ultrafast Generation and Characterization of Hot Carriers in Plasmonic Nanostructures for Solar Light Harvesting

Photoexcitation of plasmonic materials can create energetic electrons and holes that are of strong interest for solar energy conversion, photocatalysis, and light harvesting applications. Since intraband photoexcitation can create energetic (other terminology includes “hot” or “nonthermal”) carriers that are up to the photon excitation energy above the Fermi energy, their potential for increasing the efficiency of optical energy conversion is clear. However, in practice there are several challenges to realizing the potential of hot carriers. First, the efficiency of hot carrier generation is low in many plasmonic materials. Second, the energetic carriers generally thermalize very quickly, on the order of 10-100 femtoseconds, thereby making their extraction before energy loss difficult. Third, the energetic carriers tend to produce small changes in the permittivity of the metal that require advanced spectroscopic experimentation or devices in order to characterize them. In this talk I begin with describing the principles of hot carrier generation and the large advantages of using nanostructured metals to improve their generation efficiency. This is followed by our efforts to spectroscopically characterize hot electrons on an ultrafast timescale, as well as to design new nanostructures for creating hot electrons in greater numbers. Physical insights into how to extract the hot carriers before loss of energy to thermalization is given, and our efforts to understand the anisotropic decay of hot carriers is described. Recent efforts to directly extract the energetic distribution of hot carriers as a function of time are also discussed. I further describe recent experiments on specific photocatalytic processes and new biomimetic nanostructures for light harvesting. These biomimetic structures enable unique opportunities for tuning structure and energy transport properties. This work focuses on the use of chromophore-doped peptide amphiphiles to create varied and potentially self-healing structures for transporting energy. A closing description of the research opportunities and capabilities at the Center for Nanoscale Materials user facility is also given. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

Efficient Photocatalytic Reduction of CO2 Using Carbon‐Doped Amorphous Titanium Oxide

AbstractCO2‐related solar to chemical conversions have gained extensive interest due to the great concerns on renewable energy utilization. Here, we have demonstrated a new synthetic route to C‐doped amorphous titanium oxide using a facile citric acid assisted sol‐gel method for efficient photocatalytic reduction of CO2. The synthesized amorphous material exhibits a mesoporous structure with high specific surface area and a significantly narrowed band gap of 2.1 eV, which are crucial for solar light harvesting and adsorption/chemical activation of CO2 for energy transformation. The amorphization, mesoporous structure, and the band structure of the C‐doped samples were also successfully tuned by controlling the annealing temperatures. The optimized catalyst annealed at 300 °C shows the highest specific surface area, favorable visible‐light response as well as the considerable performance for CO2 photoreduction. Moreover, the further treatment of Al reduction can induce numerous surface oxygen vacancies on the amorphous sample and thus efficiently restrain the recombination of photogenerated carriers. Of significant importance is that the Al‐reduced catalyst achieves excellent performance with the space‐time yield of CH4 and CO of 4.1 and 2.5 μmol g−1 h−1 for solar light, and 0.53 and 0.63 μmol g−1 h−1 for visible light, respectively. This sample is also stable for photocatalytic CO2 transformation.

A New Bi-Based Oxychloride Bi4Ti0.5W0.5O8Cl as a Photocatalyst

Synthesis of new mixed anion compounds under visible light has been considered as an elegant strategy of photocatalyst development for efficient solar light harvesting applications. Through combined experimental and theoretical studies, we show that a new single layer Sillen–Aurivillius perovskite Bi4Ti0.5W0.5O8Cl is a good photocatalyst under visible light. This compound prepared by a solid-state reaction method has an energy gap of 2.36 eV and the visible-light-driven photocatalytic activities for degrading Rhodamine B are much better than Bi4NbO8Cl. We conclude from density functional theory calculations that its promising water-splitting application is due to its direct band gap and more stronger hybridization between Bi-6s and O-2p orbitals than Bi4NbO8Cl.

Mild Deoxygenation of Sulfoxides over Plasmonic Molybdenum Oxide Hybrid with Dramatic Activity Enhancement under Visible Light.

Harvesting solar light to boost commercially important organic synthesis still remains a challenge. Coupling of conventional noble metal catalysts with plasmonic oxide materials which exhibit intense plasmon absorption in the visible light region is a promising option for efficient solar energy utilization in catalysis. Herein, we for the first time demonstrate that plasmonic hydrogen molybdenum bronze coupled with Pt nanoparticles (Pt/H xMoO3- y) shows a high catalytic performance in the deoxygenation of sulfoxides with 1 atm of H2 at room temperature, with dramatic activity enhancement under visible light irradiation relative to dark conditions. The plasmonic molybdenum oxide hybrids with strong plasmon resonance peaks pinning at around 556 nm are obtained via a facile H-spillover process. Pt/H xMoO3- y hybrid provides excellent selectivity for the deoxygenation of various sulfoxides as well as pyridine N-oxides, in which drastically improved catalytic efficiencies are obtained under the irradiation of visible light. Comprehensive analyses reveal that oxygen vacancies massively introduced via a H-spillover process are the main active sites, and the reversible redox property of Mo atoms and strong plasmonic absorption play key roles in this reaction. The catalytic system works under extremely mild conditions and can boost the reaction by the assistance of visible light, offering an ultimately greener protocol to produce sulfides from sulfoxides. Our findings may open up a new strategy for designing plasmon-based catalytic systems that can harness visible light efficiently.

Efficient Approaches on Photochemical CO2 Reduction to Alcohol by Solar Light with Functional Multi-layered Membrane Catalysts

ABSTRACTSimple and efficient approach for artificial photosynthesis of CO2 reduction into ethanol with flexible functional multi-layered membrane catalysts is suggested. The g-C3N4 and BiVO4 particle were synthesized by self-condensation and hydrothermal method. g-C3N4 membrane catalyst and g-C3N4/BiVO4 layered membrane catalyst were fabricated by casting and shaping of Nafion polymer mixture. XRD, FT-IR and XPS analyses proved that the intrinsic properties of g-C3N4 and BiVO4 were maintained after fabricating flexible functional multi-layered membrane catalyst. The interfacial contact between g-C3N4 and BiVO4 particles in flexible membrane catalyst for efficient transport of photogenerated electron was revealed by TEM and photoelectrochemical analysis. Finally, photochemical CO2 reduction reaction was performed with flexible functional multi-layered membrane catalysts. The g-C3N4 membrane catalysts produced 147 μM of ethanol during 12 hrs of CO2 reduction reaction while the g-C3N4/BiVO4 layered membrane catalysts produced 256 μM of ethanol during 12 hrs of CO2 reduction reaction. This is due to the higher solar light harvesting and efficient hole-charge separation from functional multi-layered BiVO4 membrane catalyst leading to the higher electron transport rate to g-C3N4 membrane catalysts, promoting the CO2 reduction reaction on the surface of g-C3N4 membrane catalyst.

(Invited) Functional Metal-Organic Frameworks: From Design to Implementation

Over the past two decades metal–organic frameworks (MOFs) have emerged as an intriguing class of solid-state materials. This has been fueled by the synthesis of thousands of discrete examples of these materials through the judicious choice of their components, inorganic nodes organic linkers. MOFs are held together by coordinate bonds which bestows MOFs with a highly attractive set of characteristics; including: (i) crystallinity, leading to precise structural determination; (ii) synthetic tunability—pore size, shape, and functionality; and (iii) porosity, MOFs possess some of the highest known surface areas and pore volumes reported to date, and can be ultramicroporous, microporous, and/or mesoporous. These characteristics give rise to new and unique physical and chemical properties that are driving researchers to investigate the suitability of MOFs in a handful of practical applications. Examples include gas storage, chemical separations, catalysis, sensing, conductivity, solar fuels conversion, light harvesting and energy conversion, and magnetic materials amongst others.

Identification of few-layer ReS2 as photo-electro integrated catalyst for hydrogen evolution

Efficient water splitting via photo-electrocatalytic (PEC) systems holds great promise for producing hydrogen fuel in future energy devices. The artificial integration of photosensitive semiconductors and co-catalysts into effective PEC electrodes is becoming significantly challenging. Here, few-layer ReS2 nanosheets (NSs) were found to have both favorable solar light harvesting and proton reducing kinetics based on the assessments for electronic band structure and Gibbs free energy (ΔGH*) of hydrogen adsorption. Furthermore, the naturally coupling between optical and catalytic functions was demonstrated using ReS2 NSs grown on conductive carbon fiber clothes (CFC) as a hydrogen evolution reaction (HER) cathode, where the simulated 1 sun irradiation (100 mW cm−2) could significantly improve the catalytic activity along with anodic shifts of ca. 37 mV in onset potential and ca. 39 mV in over potential, and near 3-fold increase in exchange current density. The comparative analyses on Tafel, turnover and active sites revealed that the light-improved HER performance was due to the photo-generated hot electron injection into edge active sites. This work identifies ReS2 as a naturally PEC function-coupling material, opening a prospect of utilizing light-activated catalysts or co-catalysts to integrate catalysis with solar energy in PEC systems.

A down-shifting Eu3+-doped Y2WO6/TiO2 photoelectrode for improved light harvesting in dye-sensitized solar cells.

Y1.86Eu0.14WO6 phosphors were prepared using a solid-state reaction method. Their optical properties were analysed, and they was mixed with TiO2, sintered, and used as a photoelectrode (PE) in dye-sensitized solar cells (DSSCs). The as-prepared photoelectrode was characterized by photoluminescence spectroscopy, diffuse reflectance, electrochemical impedance spectroscopy (EIS) and X-ray diffraction. The photoelectric conversion efficiency of the DSSC with TiO2:Y1.86Eu0.14WO6 (100:2.5) was 25.8% higher than that of a DSCC using pure TiO2 as PE. This high efficiency is due to the ability of the luminescent material to convert ultraviolet radiation from the sun to visible radiation, thus improving the solar light harvesting of the DSSC.

Microwave hydrothermal synthesis of BiP1 − xVxO4/attapulgite nanocomposite with efficient photocatalytic performance for deep desulfurization

Novel V doped BiPO4/attapulgite nanocomposites (BiP1−xVxO4/ATP) were prepared via microwave-hydrothermal method. XRD, TEM, FT-IR, UV–Vis and XPS were employed to characterize the powder products. It was found that the ATP nanofibers favored the controllable assembly of Bi-based nanoparticles on its surface. Adequate V doping generated BiP1−xVxO4 solid solution in conjunction with co-precipitated BiVO4, which formed dumbbell-shaped heterostructure of BiP1−xVxO4/BiVO4 as also proved by DFT calculations. The molar fraction of V doping played a significant role in the photocatalytic desulfurization. Results showed that the desulfurization rate achieved 90% under solar light irradiation when x was optimized to be 0.6, due to the reason that in addition to the contribution of ATP fibers with adsorption capacity, the staggered heterostructure BiP1−xVxO4/BiVO4 not only promoted the solar light harvesting, but also accelerated the migration of charges, thus improved the desulfurization efficiency of the powders.

The effect of cation modifier on improving the luminescent properties of borate glasses doped with Yb3 + and Er3 +

In this work we present the preparation and characterization of new fluoroborate glasses doped with Yb3+ and Er3+ and modified by Ca, Sr and Ba oxides. Glasses with compositions (100-x-y) (MO⋅2B2O3)+x NaF+y (YbF3:ErF3) (% in mol), where M is Sr, Ba, Ca, x=10–30% and y=0, 1 or 5% were prepared by the melt/quenching technique. Glass forming ability and stability were studied by thermal analysis and it is shown that calcium samples presented the highest stability. The nature of the modifier cation has also shown a direct influence on the absolute and relative intensities of the rare earth ions transitions in the visible and near-infrared regions. Though the calcium samples present lower absorption intensities, they display the best up- and down-conversion efficiencies. There are evidences that the incorporation of the fluoride compound leads to a fluorine rich environment on the first coordination sphere of the RE, decreasing the probability of OH– groups – well known quenchers of RE emissions. Though preliminary, the results in this study can become of relevance for the field of solar cell technology improvement via betterment of solar light harvesting in the infrared and ultraviolet regions.

Fabrication of mixed perovskite organic cation thin films via controllable cation exchange**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. 2015QNA09).

Here in this paper, we demonstrate a facile technique for creating the mixed formamidinium (, FA+) and methylammonium (, MA+) cations in the lead iodide perovskite. This technique entails a facile drop-casting of formamidinium iodide (FAI) solutions on as-prepared MAPbI3 perovskite thin films under the controlled conditions, which leads to controllable displacement of the MA+ cations by FA+ cations in the perovskite structure at room temperature. Uniform and controllable mixed organic cation perovskite thin films without a “bi-layered” or graded structure are achieved. By applying this approach to photovoltaic devices, we are able to improve the performances of devices through extending their optical-absorption onset further into the infrared region to enhance solar-light harvesting. Additionally, this work provides a simple and efficient technique to tune the structural, electrical, and optoelectronic properties of the light-harvesting materials for high-performance perovskite solar cells.

Solar Light Harvesting N-Graphene Quantum Dots Decorated TiO2 for Enhanced Photocatalytic Activity

N co-doped graphene quantum dots (N-GQDs) incorporated with nanocubic likes titanium dioxide (TiO2) are synthesized via facile hydrothermal method. The morphology features of the synthesized N-GQDs/TiO2 nanocomposites are seen to exhibited in cubic shape. The average lateral size of the synthesized nanocomposites is found to be ~5.1 nm. UV-Vis absorption spectra show vastly improvement in the visible light range when N-GQDs is introduced to TiO2. The photocatalytic activity clearly demonstrated its solar light harvesting capability of prepared nanocomposites by photodegrading 4-chlorophenol (4-CP) under sunlight irradiation.

Synthesis of arrays containing porphyrin, chlorin, and perylene-imide constituents for panchromatic light-harvesting and charge separation.

Achieving solar light harvesting followed by efficient charge separation and transport is an essential objective of molecular-based artificial photosynthesis. Architectures that afford strong absorption across the near-UV to near-infrared region, namely panchromatic absorptivity, are critically important given the broad spectral distribution of sunlight. A tetrapyrrole–perylene pentad array was synthesized and investigated as a means to integrate panchromatic light harvesting and intramolecular charge separation. The pentad consists of three moieties: (1) a panchromatically absorbing triad, in which a porphyrin is strongly coupled to two perylene-monoimides via ethyne linkages; (2) a perylene-diimide electron acceptor; and (3) a chlorin hole-trapping unit. Integrating the three components with diphenylethyne linkers generates moderate electronic coupling for intramolecular energy and hole/electron transfer. The construction of the array relies on a stepwise strategy for incorporating modular pigment building blocks. The key building blocks include a trans-A2BC porphyrin, a chlorin, a perylene-monoimide, and a perylene-diimide, each bearing appropriate (halo, ethynyl) synthetic handles for Pd-catalyzed Sonogashira coupling reactions. One target pentad, three tetrads, four triads, and four monomeric benchmark compounds were synthesized from six building blocks (three new, three reported) and 10 new synthetic intermediates. Four of the tetrapyrrole-containing arrays are zinc chelated, and four others are in the free base form. Absorption and fluorescence spectra and fluorescence quantum yields were also measured. Collectively, investigations of the arrays reveal insights into principles for the design of novel reaction centers integrated with a panchromatic antenna for artificial photosynthetic studies.

Redox Conditions Affect Ultrafast Exciton Transport in Photosynthetic Pigment-Protein Complexes.

Pigment-protein complexes in photosynthetic antennae can suffer oxidative damage from reactive oxygen species generated during solar light harvesting. How the redox environment of a pigment-protein complex affects energy transport on the ultrafast light-harvesting time scale remains poorly understood. Using two-dimensional electronic spectroscopy, we observe differences in femtosecond energy-transfer processes in the Fenna-Matthews-Olson (FMO) antenna complex under different redox conditions. We attribute these differences in the ultrafast dynamics to changes to the system-bath coupling around specific chromophores, and we identify a highly conserved tyrosine/tryptophan chain near the chromophores showing the largest changes. We discuss how the mechanism of tyrosine/tryptophan chain oxidation may contribute to these differences in ultrafast dynamics that can moderate energy transfer to downstream complexes where reactive oxygen species are formed. These results highlight the importance of redox conditions on the ultrafast transport of energy in photosynthesis. Tailoring the redox environment may enable energy transport engineering in synthetic light-harvesting systems.

Efficient photocatalytic hydrogen production over Rh and Nb codoped TiO2 nanorods

It is important to tune the electronic structure of TiO2 to harvest solar light for efficient photocatalytic hydrogen production. As a widely used approach, doping enables TiO2 to respond to visible light, but generally does little to improve the photocatalytic performance because of the serious charge recombination. In this work, (Rh, Nb) codoped TiO2 nanorods (with Ti0.996Nb0.002Rh0.002O2 as an example) are found to be capable for both obvious absorption of visible light and efficient separation of photogenerated carriers. Consequently, this photocatalyst displays ultra-high activity for hydrogen production under the irradiation of either UV or visible light. The hydrogen evolution rate under simulated sunlight is as high as 7.85 mmol g−1 h−1 in methanol solution and 0.99 mmol g−1 h−1 in pure water, far higher than that for pristine TiO2 nanorods and Pt-loaded P25, respectively. This study demonstrates that Rh/Nb codoping is a promising strategy for enhancing the photocatalytic efficiency of TiO2.

Designing CdS Mesoporous Networks on Co-C@Co9 S8 Double-Shelled Nanocages as Redox-Mediator-Free Z-Scheme Photocatalyst.

Designing porous nanostructures with unprecedented functionalities and an effective ability to harvest the maximum energy region of the solar spectrum and suppress the charge-carrier recombination rate offers promising potential for sustainable energy production. Although several functional porous nanostructures have been developed, high-efficiency materials are still needed. Herein, we report a new, highly active, noble-metal-free, and redox-mediator-free Z-scheme photocatalyst, CdS/Co-C@Co9 S8 , for H2 production through water splitting under solar irradiation. The designed photocatalytic system contains open 3 D CdS mesopores as a light absorber for wider solar-light harvesting. Metal-organicframework-derived cobalt nanocrystal-embedded few-layered carbon@Co9 S8 double-shelled nanocages were used as a co-semiconductor to hamper the photo charge-carrier recombination by accelerating the photogenerated electrons and holes from the other semiconductor. The optimized catalyst shows a H2 evolution rate of 26.69 mmol g-1 h-1 under simulated solar irradiation, which is 46 times higher than that of the as-synthesized CdS mesoporous nanostructures. The apparent quantum yield reached 7.82 % at λ=425 nm in 5 h. The outstanding photocatalytic activity of CdS/Co-C@Co9S8 reflects the favorable suppression of the charge-carrier recombination rate, as determined by photoluminescence, photocurrent, and impedance analyses. We believe that the findings reported here may inspire the design of new noble-metal-free porous nanohybrids for sustainable H2 production.

Enhancement in the solar light harvesting ability of tungsten oxide thin films by annealing in vacuum and hydrogen

Thin films of tungsten oxide were fabricated by RF-sputtering. The films were annealed in vacuum and hydrogen at different temperatures to enhance the solar light absorption in the visible and near infrared regions by creating oxygen vacancies and localized surface plasmon resonance. The effect of vacuum and hydrogen annealing on structural, morphology, chemical, electrical and optical properties were investigated. Vacuum-annealed and hydrogen-annealed films behaved like degenerate semiconductor and showed strong absorption in the near-infrared region due to localized surface plasmon resonance. The solar light harvesting properties of as-deposited, vacuum-annealed and hydrogen-annealed tungsten oxide films were examined by measuring their photocurrent response.

Preparation and characterization of metals supported on nanostructured TiO2 hollow spheres for production of hydrogen via photocatalytic reforming of glycerol

Nanostructured TiO2 hollow spheres (THS) were prepared via a simple hydrothermal method with titanium butoxide, ethanol, urea, and ammonium sulphate. The effects of Ti/ethanol, and reflux temperature on the morphological properties of the nanostructured THS were investigated. An impregnation method was subsequently employed to load metals such as Cu, Co, Cr, Ag, and Ni on the optimized THS, followed by calcination in H2/N2 at 450°C for 4h. The morphological properties of the prepared samples were characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ultraviolet-visible spectroscopy (UV/vis). The SEM and TEM pictures showed that the Ti/ethanol ratio of 1:10 resulted in the formation of uniform hollow spheres. The XRD spectre revealed that phase transformation took place as the reflux temperature was increased, with pure anatase TiO2 hollow spheres being formed at 200°C. The BET surface areas of the calcined photocatalysts were in the range of 80.6–116m2/g−1. The UV/vis spectra of the photocatalysts showed that loading of transition metals reduced the band gap of the THS. The activities of the prepared catalysts were tested for hydrogen production via photocatalytic reforming of glycerol under solar irradiation. The improved hydrogen evolution from photocatalytic reforming of glycerol was attributed to: the high surface area which enhanced the adsorption of glycerol onto the surface of photocatalysts; high crystallinity and the reduced band gap which improved the solar light harvesting; the hollow chamber within the TiO2 spheres which produced multiple reflection of the light harvested, thus producing efficient electron/hole pair formation; and the detailed composition of the solids retarded the electron/hole recombination by trapping the electrons generated during the photo excitation of the photocatalysts, and thereby promoted their activity.