Solar Conversion Applications Research Articles

Synergistic augmentation and rapid elimination of ciprofloxacin using a novel mesoporous nickel sulfide-adorned bismuth tungstate S-scheme heterojunction

Pharmaceutical contamination hazardously threatens the health of human beings and the ecological system. Heterogeneous photocatalysis provides a marvelous and effective strategy for directly breaking down these harmful contaminants during visible light exposure in the presence of a semiconducting photocatalyst. Even so, this approach is considerably challenged by inadequate photo-harvesting and swift recombination rates for photo-induced charge carriers of traditional photocatalysts. In this investigation, a novel NiS/Bi2WO6 step (S)-scheme heterostructure photocatalyst was constructed by surfactant-assisted sol-gel methodology and applied for ciprofloxacin (CIPF) oxidation beneath visible-light illumination. The Bi2WO6 catalyst was successfully decorated with diverse contents of NiS nanoparticles (NPs) to investigate their impact on the overall characteristics and catalytic performance of bismuth tungstate. The optimal photocatalyst, 9.0 wt% NiS/Bi2WO6, revealed astounding efficacy in CIPF elimination from water solution, with 100.0% degradation efficacy within 90-min visible light exposure. This novel photocatalyst achieved a degradation rate of ca. 0.966 μmol min−1, surpassing that attained employing bare Bi2WO6 by ∼8.0 times. Moreover, 9.0 wt% NiS/Bi2WO6 heterostructure has exhibited superior strength and stability with 98.0% sustainability. This outstanding improvement in photocatalytic proficiency and performance with high sustainability could be attributed to the enhanced transfer and separation rates of light-induced carriers occurring through the S-scheme route. This novel heterojunction design also enlarged the visible light-harvesting responsiveness. This study offers new insights into the design and building of promising S-scheme heterostructure photocatalytic systems for highly efficient solar conversion applications.

Dual modification of BiVO4 photoanode for synergistically boosting photoelectrochemical water splitting

Bismuth vanadate (BiVO4) is a promising nanomaterial for photoelectrochemical (PEC) water oxidation. However, the serious charge recombination and sluggish water oxidation kinetics limit its performance. Herein, an integrated photoanode was successfully constructed by modifying BiVO4 (BV) with In2O3 (In) layer and further decorating amorphous FeNi hydroxides (FeNi). The BV/In/FeNi photoanode exhibited a remarkable photocurrent density of 4.0 mA cm−2 at 1.23 VRHE, which is approximately 3.6 times larger than that of pure BV. And the water oxidation reaction kinetics has an over 200% increased. This improvement was mainly because the formation of BV/In heterojunction inhibited charge recombination, and the decoration of cocatalyst FeNi facilitated the water oxidation reaction kinetics and accelerated hole transfer to electrolyte. Our work provides another possible route to develop high-efficiency photoanodes for practical applications in solar conversion.

Synthesis of a new Cu2MgGeS4 quaternary compound thin film by a low-cost spray method: Structural, Raman and optical properties

• Cu 2 MgGeS 4 thin film was synthesized for the first time. • Tetragonal kesterite structure was confirmed. • Raman measurement was conducted for the first time for Cu 2 MgGeS 4. • An optical band gap of 1.80 eV was obtained. In this work, the new Cu 2 MgGeS 4 quaternary compound thin film was synthesized for the first time by spray-ultrasonic of Cu–Mg–Ge precursors onto glass-substrate followed by a sulfurization for 1 h at 500 °C. The EDX Mapping shows a uniform desitribution of Cu–Mg–Ge–S with a value near to the ideal stoichiometry of 2:1:1:4 of Cu 2 MgGeS 4 . X-ray diffraction displaying typical peaks relating to (1 1 2), (0 0 4), (1 0 5), (2 2 0) and (3 1 2)/(1 1 6) planes of the tetragonal kesterite Cu 2 MgGeS 4 phase ( I 4 ¯ ), the result was confirmed by the presence of all characteristic Raman peak of kesterite structure, especially the predominant peak at (361 cm −1 ). The optical-gap is around 1.80 eV with a high absorption in the visible solar range. These achievements make this new material a potential candidate for solar conversion applications.

Biosynthesis of cuprous oxide using banana pulp waste extract as reducing agent

In this study we synthesized cuprous oxide by biosynthesis process, replacing conventional reducing agents like sodium borohydride (NaBH4), hydrazine (N2H4) etc. with banana pulp waste extract. The particles obtained are highly oriented along the plane (111) and present octahedral morphology, which is excellent for electrical transport and photocatalytic degradation. TEM images show that particles are well crystallized and polycrystalline in nature with sizes in the range of 1–4 µm. According to FTIR spectra no remnants of banana pulp waste extract are present in the synthetized particles. The band gap calculated is 2.07 eV, which is suitable for solar conversion applications.

Spectral properties and microstructure of sputtered WOx films for solar conversion applications

Tungsten oxides (WOx) films have gained promising attention in terms of selective solar absorption due to its high intrinsic absorption properties. We fabricated a series of single‐layer WOx films on aluminum substrates by a magnetron sputtering system. The optical absorption properties of the film were investigated by spectrophotometer and ellipsometry. We found that the optical properties of the film were very sensitive to the change of the thickness. The result showed the highest α value can reach up 0.82 with the thickness of 26 nm at 0.6 Pa or 70 nm at 1.5 Pa, and both ε values was around 0.05, indicating the high spectral selectivity properties. The different reflectance evolutions presented a wide range of color appearances, such as yellow, reddish, cyan, and blue. Moreover, the surface morphologies and phase structures of single‐layer WOx films were investigated by SEM, XRD, and Raman. A WOx/SiO2 solar selective absorber coating indicated that the as‐obtained WOx film was a promising application in solar‐thermal conversion.

Structural, electronic, optical and elastic properties of XLa2S4 (X = Ba; Ca): Ab initio study

XLa2S4 (X = Ba; Ca) sulfides are widely used as materials for application in infrared region, but the theoretical studies on these materials are limited and incomplete. Using the augmented plane wave + local orbitals (APW + lo) method with different approximation functionals, we performed a systematic theoretical study on the structural, electronic, optical and elastic properties of XLa2S4 (X = Ba; Ca). The calculated band gaps Eg of BaLa2S4 and CaLa2S4 using the modified Becke-Johnson potential in the form proposed by Tran and Blaha (TB-mBJ approximation) are 2.63 eV and 2.80 eV, respectively, which agree well with the experimental data. The dependence of main optical properties such as the absorption coefficient, refractive index, extinction coefficient, electron energy-loss spectrum, and optical reflectivity of BaLa2S4 and CaLa2S4 on photon energy were calculated and discussed. The independent elastic constants and elastic moduli of BaLa2S4 and CaLa2S4 were determined by precise calculations. The degree of elastic anisotropy of XLa2S4 (X = Ba; Ca) has been evaluated from 3D curved surface representations of Young's modulus. The results of the band structure and optical properties calculations suggest that XLa2S4 (X = Ba; Ca) are potential materials for solar conversion applications.

Enhanced solar photoelectrochemical conversion efficiency of the hydrothermally-deposited TiO2 nanorod arrays: Effects of the light trapping and optimum charge transfer

The vertically aligned TiO2 nanorod arrays (NRA) with manipulated aspect ratio were hydrothermally synthesized by changing the amount of the titanium (Ti) precursor in the initial growth solution. FE-SEM images show the optimum morphology, density and aspect ratio of the well-aligned TB-1.2 NRs on the surface of the FTO substrate. The UV-vis-absorption measurements revealed that a sample prepared at TB-1.2 can provide an increased light trapping effect. PEC analyses demonstrated that the TiO2 nanorods deposited at TB-1.2 of Titanium butoxide show a relatively high PEC conversion efficiency (3.5 times) compared with the TB-0.8 prepared TiO2 at a 1.0 V versus RHE. The higher PEC performance is believed to be the result of an enhancement of the optimum aspect ratio, light trapping, an efficient charge separation, and the high carrier transport in the vertically aligned TiO2 NRs. Further, the PEC based organic dye degradation experiments showed 77% and 94% removal of Orange II and methylene blue respectively. Additionally, 109 μmol h−1 cm−2 hydrogen generations were attributed using optimized vertically aligned TiO2 NRA’s. Thus, the appropriate morphology manipulated the TiO2 NRAs are useful for solar conversion applications.

Direct evidence and enhancement of surface plasmon resonance effect on Ag-loaded TiO2 nanotube arrays for photocatalytic CO2 reduction

Surface plasmon resonance (SPR) effect has been utilized in many solar conversion applications because of its ability to convert visible photons into “hot electron” energy. However, the direct evidence and enhancement of this unique effect are still great challenges, limiting its practical applications. Here we present the direct evidence and enhancement of SPR effect using TiO2 nanotube arrays (TNTAs) loaded with Ag nanoparticles (NPs) as a proof-of-concept example. Particularly, electrochemical deposition method is applied to deposit Ag NPs into the inner space of TNTAs for enhancing SPR effect of Ag NPs, as demonstrated by Raman and light absorption spectroscopies. This enhanced SPR effect is because multi-scattered light within TNTAs can be effectively utilized by Ag NPs in the inner space of TNTAs. Moreover, combining synchronous-illumination X-ray photoelectron and electrochemical impedance spectroscopy characterization, we confirm that the SPR effect of Ag NPs can enhance photocatalytic performance of TNTAs mainly from two aspects: (i) injection of “hot electrons” from Ag NPs to TNTAs and (ii) acceleration of charge carrier migration on the TNTAs through a unique near field effect. The direct evidence and enhancement of SPR effect open new perspectives in design of functional plasmonic nanomaterials with high solar conversion efficiency.

(Invited) Controlling Solution Chemistry from Lab-Scale Spin Coating to Scalable Deposition for High-Performance Perovskite Solar Cells

Organic-Inorganic hybrid halide perovskites have recently emerged as a new class of light absorbers that have demonstrated a rapid progress and impressive efficiencies (>22%) for solar conversion applications. Despite the rapid progress demonstrated by these light absorbers, there is still a lack of understanding of some fundamental material/physical/chemical properties of these materials. There is also a significant processing gap between the lab-scale spin coating and scalable deposition methods toward future roll-to-roll manufacturing. The challenge results from the sensitivity of perovskite crystallization and film formation to the processing conditions, which depend strongly on the specific deposition method. In this presentation, I will present our recent studies toward a better understanding and control of perovskite nucleation, grain growth, and microstructure evolution using solution processing. The precursor chemistry and growth conditions are found to affect significantly the structural and electro-optical properties of perovskite thin films. We also find that the precursor ink chemistry (solvent, coordination chemistry) of perovskite is critical for scalable deposition; but this has largely been underexplored. We present a rational design of perovskite precursor film formation to achieve highly specular films using scalable deposition methods. Using these high quality perovskite films, we have achieved device efficiencies approaching the values obtained on small scale by spin coating. These findings make a significant advance towards commercialization of the perovskite photovoltaic technology. These results and others will be discussed.

Simultaneously efficient light absorption and charge transport of phosphate and oxygen-vacancy confined in bismuth tungstate atomic layers triggering robust solar CO2 reduction

The fundamental catalytic limitations for the photoreduction of CO2 still remain: low efficiency, poor charge transport and short lifetime of catalysts. To address the critical challenges, an efficient strategy based on spatial location engineering of phosphate (PO4) and oxygen-vacancy (Vo) confined in Bi2WO6 (BWO) atomic layers is employed to establish and explore an intimate functional link between the electronic structures and activities of Vo-PO4-BWO layers. Both theoretical and experimental results reveal, the Vo-PO4-BWO layers not only narrow the band gap from the UV to visible-light region but also reduce the resistance. The time-resolved photoluminescence decay spectra exhibit the increasing carrier lifetime for Vo-PO4-BWO layers, indicating the improved charge separation and transfer efficiency. As expected, the Vo-PO4-BWO layers with the simultaneously efficient light absorption and charge transport properties achieve much higher methanol formation rate of 157 μmol g-1 h-1, over 2 and 262 times larger than that of BWO atomic layers and bulk BWO. This work may reveal that the light absorption and spatial charge transport over atomic layers could benefit CO2 conversion and shed light on the design principles of efficient photocatalysts towards solar conversion applications.

Electrohydrodynamic-assisted Assembly of Hierarchically Structured, 3D Crumpled Nanostructures for Efficient Solar Conversions.

The tantalizing prospect of harnessing the unique properties of graphene crumpled nanostructures continues to fuel tremendous interest in energy storage and harvesting applications. However, the paper ball-like, hard texture, and closed-sphere morphology of current 3D graphitic nanostructure production not only constricts the conductive pathways but also limits the accessible surface area. Here, we report new insights into electrohydrodynamically-generated droplets as colloidal nanoreactors in that the stimuli-responsive nature of reduced graphene oxide can lead to the formation of crumpled nanostructures with a combination of open structures and doubly curved, saddle-shaped edges. In particular, the crumpled nanostructures dynamically adapt to non-spherical, polyhedral shapes under continuous deposition, ultimately assembling into foam-like microstructures with a highly accessible surface area and spatially interconnected transport pathways. The implementation of such crumpled nanostructures as three-dimensional rear contacts for solar conversion applications realize benefits of a high aspect ratio, electrically addressable and energetically favorable interfaces, and substantial enhancement of both short-circuit currents and fill-factors compared to those made of planar graphene counterparts. Further, the 3D crumpled nanostructures may shed lights onto the development of effective electrocatalytic electrodes due to their open structure that simultaneously allows for efficient water flow and hydrogen escape.

(Invited) Controlling Perovskite Structure and Grain Morphology for High-Performance Perovskite Solar Cells

Organometallic halide perovskites have recently emerged as a new class of light absorbers that have demonstrated a rapid progress and impressive efficiencies (>20%) for solar conversion applications. Despite the rapid progress demonstrated by these light absorbers, there is still a lack of understanding of some fundamental material/physical/chemical properties of these materials. In this presentation, I will present our recent studies toward a better understanding and control of perovskite nucleation, grain growth, and microstructure evolution using solution processing.1 The precursor chemistry and growth conditions are found to affect significantly the structural and electro-optical properties of perovskite thin films. Devices based on different grain sizes were investigated to correlate the grain sizes with device performance. The impact of grain size on charge carrier dynamics was also studied. Although we used the standard TiO2/perovskite/spiro-MeOTAD sandwich structure, we found evidence of a p-n junction rather than the commonly reported p-i-n structure for our studied devices.2 In addition, I will briefly discuss our recent investigation on stabilizing perovskite structures by tuning tolerance factor through solid state alloying.3 These results and others will be discussed. References 1 M. Yang, Y. Zhou, Y. Zeng, C.-S. Jiang, N. P. Padture, K. Zhu, Adv. Mater. 27, 6363 (2015). 2 C.-S. Jiang, M. Yang, Y. Zhou, B. To, S. U. Nanayakkara, J. M. Luther, W. Zhou, J. J. Berry, J. van de Lagemaat, N. P. Padture, K. Zhu, M. M. Al-Jassim, Nature Commun. 6, 8937 (2015). 3 Z. Li, M. J. Yang, J. S. Park, S. H. Wei, J. J. Berry, K. Zhu, Chem. Mater. 28, 284 (2016).

The interlace of Bi2S3 nanowires with TiO2 nanorods: An effective strategy for high photoelectrochemical performance

A netlike heterostructure is constructed by interlacing the Bi2S3 nanowires with well-aligned TiO2 nanorod arrays via a facile and effective solvothermal method. The winding Bi2S3 nanowires with several hundred nanometers long and 20–30nm wide are distributed in the interspace of TiO2 nanorods and cross-linked with these nanorods reducing the isolation of nanorods. The photoelectrochemical characterizations show that in addition to the high stability in air without any encapsulation, the netlike heterostructure exhibits an enhanced photoelectrochemical performance compared with TiO2 nanorods and controlled Bi2S3/TiO2 nanoparticle structure. The dual roles of Bi2S3 nanowires (1) as sensitizer for the enlargement of photoresponse range and (2) as multiple electron transport channels facilitating the fast separation of photogenerated electron-hole pairs are considered as key factors for the high energy conversion efficiency of 2.96%. This facile synthesis method offers an attractive strategy to further improve the photoelectrochemical performance of semiconductors and undoubtedly shows promising applications in solar conversion and storage devices.

Achieving tunable photocatalytic activity enhancement by elaborately engineering composition-adjustable polynary heterojunctions photocatalysts

We first achieve controllable synthesis of composition-adjustable polynary heterojunction photocatalysts via a facile in-situ treatment strategy at ambient conditions based on component transformation and chemical deposition. By controlling the chemical reaction degree, three-classes of heterojunction composites with diverse components were produced. The first group (G1) contains AgI/AgIO3 two phases. The second group (G2) consists of Bi7O9I3/AgI/AgIO3 three phases. While the last group (G3) is composed of Bi7O9I3/AgI. Along with different phase constitution, it is very intriguing to observe that photoabsorption edges of the composites can be orderly tuned from 400nm to 650nm, covering almost the whole visible light region, consistent with the color change of samples from white to light yellow, orange and brick red. The three series of composite photocatalysts all show greatly enhanced photocatalytic activity in photodegrading methyl orange (MO) and NO gas removal under simulated solar light or visible light irradiation compared with the AgIO3 precursor. Among them, the intense photoabsorption and desirable band alignment (three-rank heterojunction) are well balanced in Bi7O9I3/AgI/AgIO3, thus showing the optimal photoreactivity, approximately 128 times than that of AgIO3. Particularly, it also exhibits unsurpassed photocatalytic performance against diverse industrial pollutants and pharmaceutical, such as Rhodamine B, phenol, 2,4-dichlorophenol, bisphenol A, and tetracycline hydrochloride. Additionally, three types of photocatalytic mechanisms corresponding to the three series of photocatalysts are systematically investigated and illustrated. It is found that different phase composition has a huge impact on the separation and migration of charges as well as active species. The present work not only provides new perspective into manipulating component-adjustable heterojunctional photocatalysts, but also opens an alternative avenue for developing composite materials for solar-conversion applications.

Structural Investigation and Indium Substitution in the Thermoelectric Mn2.7Cr0.3Si4Al2−x In x Series

Following the recent discovery of the promising Mn2.7Cr0.3Si4Al2 thermoelectric compound (having, e.g., automotive, industrial, and solar conversion applications), structural characterization by x-ray single-crystal diffraction analysis has been performed. This layered material is composed of two distinct crystallographic sites where both (Mn, Cr) and (Al, Si) are randomly distributed. The deduced crystallographic parameters were then confirmed by powder x-ray diffraction analysis through a temperature dependence of the phase stability, showing at the same time chemical stability up to 873 K. Taking into account the two distinct crystallographic sites highlighted, samples possessing two guest elements, one on each site, were then synthesized to improve the thermoelectric properties. A solid solution is found in the system Mn2.7Cr0.3Si4Al2−xInx with x varying from 0 to 0.2. Thus, double-substituted samples were studied by x-ray diffraction, electrical, and thermal measurements. The present paper describes and discusses the experimental results obtained.

Photoelectrodes based upon Mo:BiVO4 inverse opals for photoelectrochemical water splitting.

BiVO4 has been regarded as a promising material for photoelectrochemical water splitting, but it suffers from a major challenge on charge collection and utilization. In order to meet this challenge, we design a nanoengineered three-dimensional (3D) ordered macro-mesoporous architecture (a kind of inverse opal) of Mo:BiVO4 through a controllable colloidal crystal template method with the help of a sandwich solution infiltration method and adjustable post-heating time. Within expectation, a superior photocurrent density is achieved in return for this design. This enhancement originates primarily from effective charge collection and utilization according to the analysis of electrochemical impedance spectroscopy and so on. All the results highlight the great significance of the 3D ordered macro-mesoporous architecture as a promising photoelectrode model for the application in solar conversion. The cooperating amplification effects of nanoengineering from composition regulation and morphology innovation are helpful for creating more purpose-designed photoelectrodes with highly efficient performance.

A facile and green synthesis route towards two-dimensional TiO2@Ag heterojunction structure with enhanced visible light photocatalytic activity

A facile and green route to the two-dimensional (2D) TiO2@Ag heterojunction structure has been developed for improving the visible light photocatalytic activity. The edible corn crispy was selected as the biomass template to fabricate 2D-TiO2@Ag heterojunction composites. The various analytical and spectroscopic methods including SEM, TEM, XRD, XPS, and FT-IR were employed to investigate the structure of the composites. The results show that Ag nanoparticles were evenly incorporated within the 2D-TiO2, and the increase of calcination temperature would make the interaction of Ag nanoparticles and 2D-TiO2 strong. Moreover, the introduction of Ag and a heterojunction structure could expand the visible light absorption region for composites. Based on the comparison of photocatalytic activity, it turns out that the 2D-TiO2@Ag composites can exhibit an improved significantly photocatalytic activity for methylene blue (MB) under visible light irradiation, which can be attributed to the retarding of the charge recombination and accelerating the electron transfer from the 2D-TiO2 to metal Ag. The 2D-TiO2@Ag heterojunction structure also has effective applications in solar conversion.

Dendritic Au/TiO2 nanorod arrays for visible-light driven photoelectrochemical water splitting

This paper describes the synthesis of TiO₂ branched nanorod arrays (TiO₂ BNRs) with plasmonic Au nanoparticles attached on the surface. Such Au/TiO₂ BNR composites exhibit high photocatalytic activity in photoelectrochemical (PEC) water splitting. The unique structure of Au/TiO₂ BNRs shows enhanced activity with a photocurrent of 0.125 mA cm(-2) under visible light (≥420 nm) and 2.32 ± 0.1 mA cm(-2) under AM 1.5 G illumination (100 mW cm(-2)). The obtained photocurrent is comparable to the highest value ever reported. Furthermore, the Au/TiO₂ BNRs achieve the highest efficiency of ∼1.27% at a low bias of 0.50 V vs. RHE, indicating elevated charge separation and transportation efficiencies. The high PEC performance is mainly due to the plasmonic effect of Au nanoparticles, which enhances the visible light absorption, together with the large surface area, efficient charge separation and high carrier mobility of the TiO₂ BNRs. The carrier density of Au/TiO₂ BNRs is nearly 6 times higher than the pristine TiO₂ BNRs as calculated by the Mott-Schottky plot. Based on the analysis by UV-Vis spectroscopy, electrochemical impedance spectroscopy, and photoluminescence, a mechanism was proposed to explain the high activity of Au/TiO₂ BNRs in PEC water splitting. The capability of synthesizing highly visible light active Au/TiO₂ BNR based photocatalysts is useful for solar conversion applications, such as PEC water splitting, dye-sensitized solar cells and photovoltaic devices.

Solar Photoconversion Using Graphene/TiO2 Composites: Nanographene Shell on TiO2 Core versus TiO2 Nanoparticles on Graphene Sheet

Size controlled nanographene oxides (NGOs; <50 nm) were prepared by a two-step oxidation process and NGOs were self-assembled with TiO2 nanoparticles to form the core/shell structure. Nanosized GO-coated TiO2 nanoparticles (NGOTs) were then reduced by a photocatalytic process under UV irradiation to obtain graphene-coated TiO2. This is clearly different from the typical graphene/TiO2 composite with the particles-on-a-sheet geometry and is the first study on the core/shell structure of its kind. The physicochemical properties of NGOs and the reduced NGOTs (r-NGOTs) were characterized by various analytical and spectroscopic methods (AFM, FT-IR, XPS, TEM, EELS, etc.). The photocatalytic and photoelectrochemical activities of r-NGOT were compared with a composite of r-GO/TiO2 that has TiO2 nanoparticles loaded on a larger graphene sheet (r-LGOT). The photocatalytic production of hydrogen was measured in the aqueous suspension of the composite photocatalyst under UV irradiation (λ > 320 nm), and the photoelectrochemical behaviors were characterized using the electrode coated with the composite photocatalyst. The rates of H2 production and photocurrent generation were higher with r-NGOT than r-LGOT, which indicates that the presence of r-GO shell on the surface of TiO2 facilitates the interfacial electron transfer. The direct contact between r-NGO and TiO2 is maximized in r-NGOT by retarding the charge recombination and accelerating the electron transfer. The geometry of the core/shell structure should be effective in the design of a graphene/TiO2 composite for solar conversion applications.

Mott insulators: An early selection criterion for materials for photoelectrochemical H2 production

For the last four decades, materials based on oxides have been the primary focus for studies of photo-catalysts for hydrogen production. So far, under visible-light illumination, this approach has not been much successful; only under ultraviolet radiation have oxides shown some limited success. Therefore, it is important to understand the fundamental problem of the photo-conduction properties in oxides at an electronic level. It can be shown that due to their highly localized electronic features, oxides that are Mott insulators are not suitable for photoelectrochemical (PEC) hydrogen production. Even with considerable doping/alloying, improving photo-current in these materials is unlikely. This represents a criterion by which future metal oxide candidates should be screened. Among metal oxides, the 3d elements have greater abundance and are receiving increased attention for application of solar conversion (photovoltaic and PEC), however the issues discussed here would appear to limit their usefulness for this purpose.