Solar Light Harvesting Research Articles

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.

Two-Dimensional GeSe as an Isostructural and Isoelectronic Analogue of Phosphorene: Sonication-Assisted Synthesis, Chemical Stability, and Optical Properties

Monochalcogenides of germanium (or tin) are considered as isoelectronic and isostructural analogues of black phosphorus. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted liquid phase exfoliation (LPE) of bulk microcrystalline powders in organic solvents. The thickness of the GeSe sheets is dependent on the exfoliation conditions, and highly crystalline few-layer GeSe sheets of 4–10 layer stacks with lateral sizes over 200 nm were obtained. In ambient atmosphere, the LPE sheets deposited on the substrate demonstrate strong resistance against degradation, while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions. Density functional theory calculation together with optical characterizations confirm the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect. In addition, we show that the few-layer GeSe sheets with favorable optical bandgap allow for efficient solar light harvesting fo...

Local vs Nonlocal States in FeTiO3 Probed with 1s2pRIXS: Implications for Photochemistry.

Metal–metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron–titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core–hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

(Invited) Nanophotonic Structures for Solar Light Harvesting and the Generation of Hot Plasmonic Electrons

Future nanophotonic architectures for optical energy conversion and photocatalysis will require nanostructures with roles that work together cooperatively, such as structures with a large absorption cross section combined with features that enable guiding, propagating, and converting energy. Ultrafast optical responses in nanostructures are potentially important for modulating energy flow and also for characterizing and minimizing energy dissipation pathways in nanophotonic structures. Also, ultrafast extraction of hot plasmonic electrons from metal nanostructures is an important opportunity for increasing efficiency of solar energy conversion and photocatalysis. Efforts to greatly improve the efficiency of hot electron generation through plasmonic metamaterials, and characterization of hot electron dissipation pathways are described. I further describe efforts to induced long-range energy propagation in nanostructures. Nanostructured materials inspired by natural photosynthetic membranes represent an opportunity for efficient and directional energy transport. In natural photosystems, light harvesting complexes can transport excitons to the reaction center core with near unity conversion of absorbed photons to separated charge. A key to this process is the high rate of exciton hopping and the directionality of exciton flow due to an energy level waterfall effect of excitons in complexes as the reaction center core is approached. Efforts to induce similar behavior in biomimetically inspired nanostructures are described. Finally efforts to image exciton flow in light harvesting nanoscale materials are introduced. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

II3 V2 (II: Zn, Cd; V: P, As) Semiconductors: From Bulk Solids to Colloidal Nanocrystals.

II3 V2 semiconductors have become increasingly popular for a variety of applications including solar light harvesting, near-IR imaging, and low energy light detection. The bulk physical and electronic structure of these materials is highlighted, followed by an in-depth survey on progress in synthesizing these semiconductors as colloidal nanocrystals. Interestingly, no universal synthetic approach has yet been developed to access all compounds within this family. A discussion on how the complex crystal structure of these materials translates to small domain sizes will highlight current challenges in the characterization of II3 V2 nanocrystals. Finally, potential avenues for further research will be proposed as a way to advance this field towards greater utilization in light harvesting applications.

Matildite Contact with Media: First-Principles Study of AgBiS2 Surfaces and Nanoparticle Morphology.

Motivated by the interest in AgBiS2 material for solar light harvesting applications, a detailed bulk first-principles quantum mechanical study of its surface properties is presented. Density functional theory based calculations with the Perdew-Burke-Ernzerhof functional have been carried out for different surface orientations and terminations of the matildite polymorph. From the results, two particularly stable facets are predicted to dominate Wulff shaped AgBiS2 nanoparticles. These are the (001) type nonpolar surface and the (111) polar terminations where facets are exposed containing solely Ag or S atoms. The Wulff equilibrium shape is predicted to be a cube with only two edges capped. This particular shape explains a previously reported surface enrichment of Ag with respect to Bi of ∼1.5. The (001) surfaces display an ionic character similar to bulk AgBiS2, with a low work function of 4.31 eV, although the inspection of the density of states (DOS) reveals a bandgap increased by 0.3 eV compared to bulk. This surface effect could explain the bulk wavelength overestimation of the absorption coefficient decay, as previously determined. Last but not least, the DOS of the (111) polar termination reveals a metallic character, where Fermi level is located below that on the (001) surfaces. Possible implications of the different electronic structure of these surfaces in the reported photoactivity are discussed.

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It is a widespread concern to address the antibiotics in water with low-cost and eco-friendly photocatalysts that could efficiently harvest solar light. Herein, we designed an efficient photocatalyst by integrating the lamellar g-C3N4 into Znln2S4 microflowers through a one-step hydrothermal method. The as-synthesized g-C3N4/Znln2S4 heterojunction photocatalysts exhibited evidently enhancement on the photocatalytic activities for the degradation of tetracycline (100 mL, 20 mg/L) compared with pristine g-C3N4 and Znln2S4. Significantly, g-C3N4/Znln2S4 composite with loading 50 wt.% g-C3N4 showed the highest photocatalytic performance (almost 100% degradation within 120 min), which was around 40 and 22.8 times higher than that of g-C3N4 and Znln2S4, respectively. This enhanced photocatalytic activity of g-C3N4/Znln2S4 is mainly attributed to the formation of heterostructure that can efficiently promote the transfer of photoinduced electrons and holes between g-C3N4 and ZnIn2S4, restricting the recombination of electron-hole pairs. In addition, a possible mechanism was also proposed.

Plasmonic nano-printing: large-area nanoscale energy deposition for efficient surface texturing

The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plasmonic nano-printing. Irreversible nanoscale modifications are delivered by surface plasmon polariton (SPP) using: (i) fast scan and (ii) cylindrical focusing of femtosecond laser pulses for a high patterning throughput. The mechanism of ripple formation on ZnS dielectric is experimentally proven to occur via surface wave at the substrate–plasma interface. The line focusing increase the ordering quality of ripples and facilitates fabrication over wafer-sized areas within a practical time span. Nanoprinting using SPP is expected to open new applications in photo-catalysis, tribology, and solar light harvesting via localized energy deposition rather scattering used in photonic and sensing applications based on re-scattering of SPP modes into far-field modes.

Graphite carbon nitride/ZnIn2S4 heterojunction photocatalyst with enhanced photocatalytic performance for degradation of tetracycline under visible light irradiation

It is a widespread concern to address the antibiotics in water with low-cost and eco-friendly photocatalysts that could efficiently harvest solar light. Herein, we designed an efficient photocatalyst by integrating the lamellar g-C3N4 into Znln2S4 microflowers through a one-step hydrothermal method. The as-synthesized g-C3N4/Znln2S4 heterojunction photocatalysts exhibited evidently enhancement on the photocatalytic activities for the degradation of tetracycline (100 mL, 20 mg/L) compared with pristine g-C3N4 and Znln2S4. Significantly, g-C3N4/Znln2S4 composite with loading 50 wt.% g-C3N4 showed the highest photocatalytic performance (almost 100% degradation within 120 min), which was around 40 and 22.8 times higher than that of g-C3N4 and Znln2S4, respectively. This enhanced photocatalytic activity of g-C3N4/Znln2S4 is mainly attributed to the formation of heterostructure that can efficiently promote the transfer of photoinduced electrons and holes between g-C3N4 and ZnIn2S4, restricting the recombination of electron-hole pairs. In addition, a possible mechanism was also proposed.

Nitrogen doped carbon quantum dots as a green luminescent sensitizer to functionalize ZnO nanoparticles for enhanced photovoltaic conversion devices

A simple and facile hydrothermal approach for the synthesis of highly green luminescent nitrogen doped carbon quantum dots (NCQDs) is reported. The prepared NCQDs show narrow size distribution of 2–3nm, abundant surface functional groups and good water solubility. To showcase the potential of these NCQDs as a solar light harvesting material, we designed NCQDs sensitized ZnO nanoparticles based photoanodes for photovoltaic solar cells. As a result, NCQDs sensitized ZnO photoanodes exhibited 75% higher photoconversion efficiency when compared to that of bare ZnO photoanodes. The characterization and optical properties of NCQDs sensitized ZnO photoanodes is also discussed.

Near-Unity Emitting Copper-Doped Colloidal Semiconductor Quantum Wells for Luminescent Solar Concentrators.

Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.

Synergetic Enhancement of Light Harvesting and Charge Separation over Surface-Disorder-Engineered TiO2 Photonic Crystals

Summary Solar light harvesting and charge separation are both critical to the solar-to-energy conversion in photocatalysis, but it is difficult to simultaneously achieve both in a single-component material. This paper describes the design and synthesis of dual-functional surface-disorder-engineered titanium dioxide photonic crystals (TiO 2 PCs) for efficient solar light harvesting and charge separation in photocatalytic hydrogen evolution. The slow photon effect of TiO 2 PCs enhances the photon-matter interaction to increase light harvesting, and the disorder-engineered surface layer can form band tail states and trap centers to promote the separation and inhibit the recombination of charge carriers. The synergetic enhancement of light harvesting and charge separation in this single-component material leads to a high performance toward photocatalytic hydrogen evolution. This approach provides a promising strategy for designing dual-functional materials with a high solar-to-energy conversion efficiency for various photocatalytic processes.

Bidirectional Perfect Absorber Using Free Substrate Plasmonic Metasurfaces

Much effort has been made to achieve metasurface absorbers with polarization‐insensitive, wide‐angle, multiband, or broadband performance from gigahertz to terahertz to optical ranges. However, there are well‐known challenges in optimizing the performance of metasurface absorbers, due to the existence of a theoretical limit. It is urgent to extend and improve the optical properties of metasurface absorbers to satisfy the requirements for practical applications. Here, a novel free substrate metasurface absorber is demonstrated that has high spatial frequencies on the surface and can prevent light transmission and reflection by transforming free‐space incident waves into evanescent waves. Experimental and simulation results show that the proposed metasurface absorber can realize bidirectional perfect absorption and that the absorption of the light incident on one side of the metasurface is independent of the light incident on the other. Hence, the bidirectional perfect absorption does not depend on coherence of the beams. The proposed metasurface provides new degrees of freedom to realize a perfect absorber with twice the absorption capacity. It could have a wide range of applications and profound implications in photodetection, solar light harvesting, and sensing.

A facile synthesis of broad plasmon wavelength tunable silver nanoparticles in citrate aqueous solutions by laser ablation and light irradiation

A complete light based synthesis of silver nanoparticles of different shapes exhibiting strong and broad band plasmon resonances is demonstrated in aqueous solutions of single reagent tri-sodium citrate (TSC). This method involves synthesis of almost spherical nanoparticles using liquid phase pulsed laser ablation (LPPLA) followed by their photo-mediated shape transformation into triangular nanoplates by visible light irradiation. UV–vis spectroscopy and Mie theory based analytical calculations show that this method can produce high concentrations of nanoparticles at optimum TSC concentration. LPPLA method is shown to be potential for producing relatively smaller size, isolated spherical silver nanoparticles at optimum TSC concentration without aid of strong reducing agents like borohydrides. This method has also resulted interconnected nanoparticle networks in pure water or at very low TSC concentrations. These isolated spherical nanoparticles were further shown to act as seed particles for high yield synthesis of nanoplates. The concentration of TSC played a vital role in producing both highly stable spherical nanoparticles and their photo-mediated shape transformation into triangular nanoplates. At the optimum TSC concentration of about 10mM, pH about 7.5 and visible light irradiation of about 24h, the produced triangular nanoplates have shown strong plasmon absorption in visible to near NIR region, useful for solar light harvesting applications. Further during the light irradiation, presence of additional silver ions in the nanoparticle solution initially increased the growth of spherical nanoparticles which finally resulted in high yield formation of triangular nanoplates.

In Situ Gold‐Loaded Fluorinated Titania Inverse Opal Photocatalysts for Enhanced Solar‐Light‐Driven Hydrogen Production

AbstractThis work reports the preparation of in situ gold‐loaded fluorinated titania inverse opal (Au/F TiO2 IO) photocatalysts for enhancing solar‐light‐driven hydrogen evolution. The use of trifluoroacetic acid facilitates the surface fluorination as well as the in situ deposition of gold nanoparticles (AuNPs) at the defect sites on the TiO2 surface. The thermal treatment partially fuses the gold and titania nanoparticles leading to enhanced hot electron injection when illuminated under solar light. Au/F TiO2 IO‐215 prepared using polystyrene (PS) spheres of 215 nm size are demonstrated to be able to enhance the hydrogen evolution five‐fold relative to nanocrystalline titania photocatalyst (Au/F nc‐TiO2) due to the electronic band gap (EBG) absorption of TiO2 within the photonic band gap (PBG). Au/F TiO2 IO‐460; with the blue edge of the PBG closer to the localized surface plasmon resonance (LSPR) region of AuNPs displayed four‐fold enhancement in the hydrogen evolution with respect to Au/F nc‐TiO2. The solar light driven hydrogen evolution rates obtained for Au/F TiO2 IO‐215 and Au/F TiO2 IO‐460 photocatalysts (58.86 and 49.43 mmol h−1 g−1 respectively) are remarkably higher than the rates of inverse‐opal‐assisted hydrogen evolution experiments reported in literature. This work demonstrated a strategy for synergistically enhancing the solar light harvesting by the restructuring of Au/F TiO2 photocatalysts into inverse opals. The extended hydrogen yield by Au/F TiO2 IOs is on the order of millimoles, making them potential photocatalysts for solar hydrogen production.

Controllable synthesis of Au@SnO2 core–shell nanohybrids with enhanced photocatalytic activities

Combination of semiconductors with plasmonic nanostructures is an effective route to promote the solar light harvesting as well as the efficiency of photocatalysis. In the present work, the Au@SnO2 hybrid nanostructures with Au nanorods as the cores and highly crystallized SnO2 nanoparticles as the shells were fabricated by a facile hydrothermal method. A critical factor, which influences the coating state of the SnO2 shells over Au NRs, was found to be the concentration of CTAB agent in the system and the corresponding mechanism was also proposed. The photocatalytic activities of the Au@SnO2 nanohybrids were examined by degradation of rhodamine B (RhB) dyes at room temperature. The Au@SnO2 nanohybrids exhibited much higher catalytic activities than that of the commercial SnO2 NPs, which could be attributed to the localized electric field enhancement effect of Au nanorods plasmon and charges transfer between the Au nanorods and SnO2.

Electronic and optical properties of Cr-, B-doped, and (Cr, B)-codoped SrTiO3

Energy band engineering of semiconductors plays a crucial role in exploring high-efficiency visible-light response photocatalysts. Herein, we systematically study the electronic properties and optical response of Cr-, B-doped SrTiO3, and (Cr, B)-codoped SrTiO3 by using first-principles calculations to explore the mechanism for its superior photocatalytic activities in the visible light region. Special emphasis is placed on uncovering the synergy effects of nonmetal B dopant with metal Cr dopant at different cation sites. It is found that the electronic properties and optical absorption of SrTiO3 can be dramatically engineered by mono- or co-doping. In particular, the intermediate levels lying in the bandgap of the codoped SrTiO3 relay on the Cr impurity doped at Sr or Ti cation sites. Moreover, the (Cr@Sr, B@O)-SrTiO3 retains the charge balancing without the generation of unexpected oxygen vacancies, and is more desirable for solar light harvesting due to its higher absorption than others in the entire visible light. The findings can rationalize the available experimental results and are helpful in designing SrTiO3-based photocatalysts with high-efficiency performance.

Controlled synthesis, characterization, and optical properties of ZnIn2S4 and CdIn2S4 nanostructures with enhanced performance for solar cell applications

In this paper, we present the design, synthesis and characterization of ZnIn2S4 (ZIS), CdIn2S4 (CIS) nanostructures and their nanocomposites to enhance the solar cell high light harvesting efficiency via the simple chemical Polyol method. Results show that pure ZIS and CIS nanostructures could significantly enhance the separation of photo exited electron-hole pairs, thereby increasing the photo current density. The CIS/ZIS and ZIS/CIS composites exhibit superior photovoltaic performance under light illumination compared to the unalloyed ZIS and CIS. The improvement of performance is ascribed to the uniform distribution of ZIS nanoparticles around CIS and vice versa.

Design of CdTeSe–Porphyrin–Graphene Composite for Photoinduced Electron Transfer and Photocurrent Generation

Considerable attention has been paid to designing graphene based hybrid materials for developing new light-harvesting systems. Here, we have synthesized CdTeSe alloy nanorods and modified the surface of CdTeSe nanorods by porphyrin molecules to attach to graphene surface through π–π stacking. In this CdTeSe nanorod–porphyrin–graphene hybrid system, porphyrin acts as an antenna material which harvests light and consequently transfers electrons to the adjacent conduction band of nanorods and finally to reduced graphene oxide (RGO). An ultrafast spectroscopic study suggests that the rate of electron transfer from porphyrin to RGO via conduction band of CdTeSe nanorods is 17.4 × 10–2 ps–1. It is to be noted that this ternary hybrid system exhibits 240-fold enhancement of photocurrent under visible light irradiation which reveals that this new type of graphene based inorganic and organic hybrid system is very promising for solar light harvesting.