Photoaction Spectra Research Articles

Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting

Two different high potential Zn(II) porphyrin designs carrying either 4 or 5 meso pentafluorophenyl moieties as electron acceptor groups and a further electron withdrawing branch inserted in either the β (1) or meso (2) position were tested in photoelectrosynthetic cells for HBr splitting. Photoaction spectra in the presence of HBr showed that red photons up to 700 nm could be harvested and converted and that 2 performed better than 1, thanks to better electronic properties of the excited state, favored by the insertion of the benzothiadiazole electron withdrawing group. Photoanodic performances in the presence of HBr, however, remained low, due to inefficient regeneration of the oxidized sensitizer as a result of an insufficient driving force for Br− oxidation.

Surface modification of TiO2 with copper clusters for band gap narrowing

Surface modification of photocatalytic materials to give better activity and potentially extending the response into the visible spectrum, is an area of active research. In this work, DFT modelling suggests that surface modification of rutile and anatase TiO2 with partially oxidised copper clusters can induce a red shift in the photo-action spectrum. Copper clusters were synthesised and characterised separately before TiO2 nanoparticle surface modification. Characterisation of copper clusters and photocatalysts modified with copper clusters showed that ex-situ synthesis can control the size of surface clusters. Sub-nanometre clusters of copper maintained their size and morphology upon attachment to the photocatalyst surface. The copper clusters were determined to be a mixture of Cu(0) and Cu(I), and no significant change in the oxidation state was observed following surface modification or following photoelectrochemical measurements. Experimental measurements including UV–Vis spectroscopy and valence band XPS showed a small red shift the band gap correlating to the DFT predictions. Photoelectrochemical characterisation showed an enhancement in the UV photocurrent response and a small red shift in the effective band gap for the surface modified TiO2.

Integrating a Semitransparent, Fullerene-Free Organic Solar Cell in Tandem with a BiVO4 Photoanode for Unassisted Solar Water Splitting.

We report an unassisted solar water splitting system powered by a diketopyrrolopyrrole (DPP)-containing semitransparent organic solar cell. Two major merits of this fullerene-free solar cell enable its integration with a BiVO4 photoanode. First is the high open circuit voltage and high fill factor displayed by this single junction solar cell, which yields sufficient power to effect water splitting when serially connected to an appropriate electrode/catalyst. Second, the wavelength-resolved photoaction spectrum of the DPP-based solar cell has minimal overlap with that of the BiVO4 photoanode, thus ensuring that light collection across these two components can be optimized. The latter feature enables a new water splitting device configuration wherein the solar cell is placed first in the path of incident light, before the BiVO4 photoanode, although BiVO4 has a wider bandgap. This configuration is accessed by replacing the reflective top electrode of the standard DPP-based solar cell with a thin metal film and an antireflection layer, thus rendering the solar cell semitransparent. In this configuration, incident light does not travel through the aqueous electrolyte to reach the solar cell or photoanode, and therefore, photon losses due to the scattering of water are reduced. Moreover, this new configuration allows the BiVO4 photoanode to be back-illuminated, i.e., through the BiVO4/back contact interface, which leads to higher photocurrents compared to front illumination. The combination of a semitransparent single-junction solar cell and a BiVO4 photoanode coated with oxygen evolution catalysts in a new device configuration yielded an unassisted solar water splitting system with a solar-to-hydrogen conversion efficiency of 2.2% in water.

Solution Combustion Synthesis, Characterization, and Photocatalytic Activity of CuBi2O4 and Its Nanocomposites with CuO and α-Bi2O3

The ternary compound, CuBi2O4, a 1:1 stoichiometric derivative of the two component oxides CuO and Bi2O3, has attracted attention from the solar water splitting and photocatalysis communities as a p-type semiconductor responsive to visible light. This study demonstrates that solution combustion synthesis (SCS) can be used to prepare powders not only of this compound but also nanocomposites with either CuO or Bi2O3 in excess. This was simply done by tuning the SCS precursor mixture composition. The synthesized crystalline samples were characterized by powder X-ray diffraction (with Rietveld refinement for phase purity), diffuse reflectance UV–visible spectroscopy, electron microscopy, and photoelectrochemical (PEC) techniques. The band structure and photoactivity of these oxides were probed by linear sweep voltammetry and by measuring their photoaction spectra (internal photon-to-electron conversion efficiency vs wavelength). The photoactivity (attributed to hydrogen evolution and CO2 photoreduction) was c...

Photoelectrochemical versus Electrochemical Detection of H2O2 Using an All‐inorganic Sensor Based on Nanostructured Copper Oxide

AbstractAll‐inorganic nanostructured CuO sensors for both electrochemical and photolectrochemical detection of H2O2 were fabricated by electrodeposition and annealing, and compared in terms of their sensing properties. The calibration curves showed an excellent linearity in the H2O2 concentration range of 0.1–1.0 mM, with strong correlation in both electrochemical (R2>0.999) and photolectrochemical (R2>0.990) mode. The higher quality of the linear correlation in the electrochemical mode resulted in the lower limit of detection (LOD) of 0.06 mM, as compared to 0.18 mM in the photoelectrochemical mode. The sensitivity in the electrochemical and photoelectrochemical mode was −73.7 µA cm−2 mM−1 and −46.4 µA cm−2 mM−1, respectively. The signal‐to‐noise ratio, performance stability, and selectivity in the presence of various interference compounds (catechol, cysteine, dopamine, nitrate, and uric acid) were found to be better in the photoelectrochemical mode. The photoaction spectra of the photoelectrochemical CuO sensor revealed a maximum in visible light (λ∼450 nm) which is advantageous in terms of the possibility to use low‐cost and highly stable light sources (LEDs). The demonstrated differences of sensor operation in different modes show that a careful analysis is necessary for realistic evaluation of distinct advantages associated with operation of sensors based on nanostructured semiconductors in either electrochemical or photoelectrochemical mode.

(Invited) Biohybrid Photoelectrodes Exploiting Effective Photosystem I - Electrode Contacts

Hybrid systems using the unique features of natural photosynthesis are increasingly in the focus of current research. Particularly the two protein complexes photosystem I and II (PS) of the oxygenic photosynthesis have attracted the attention of researchers to build up new solar energy-converting systems. In such biohybrid systems an efficient coupling of PS with the electrode is essential. Besides the light-to-current conversion, the PS may also be used for light-driven enzymatic reactions resulting in photobioelectrocatalysis. Different basic principles can be used by establishing communication between the PS and electrode surfaces. Here we want to avoid shuttle molecules and generate a surface allowing direct electron transfer (DET) between the excited protein and the electrode. This has been studied with photosystem 1 (PSI) from Thermosynechococcus elongates.For efficient photocurrent generation a arther oriented immobilisation would be desirable which is complicated by the rather high molecular weight and the complex surface properties of PSI. In this context we have been focusing on the directional assembly of PSI on highly conductive graphene electrodes. In order to adjust the surface properties different aromatic molecules have been applied onto the graphene [1]. The different Π-systems serve as an artificial scaffold harbouring functional groups which interplay with the PSI. Particularly important is the hydrophilic-hydrophobic balance on the surface. In conclusion, an effective and rather directed assembly of PSI and a large photocurrent density has been obtained. It has to be mentioned here that photocurrent generation occurs already at open circuit potential with now additional driving force applied. Depending on the modifier used an unidirectional photocurrent generation can be achieved by this approach. Via the artificially modified graphene surface we have been able to create biohybrid systems with an improved photocurrent output. Here not only adsorption can be used for the functional assembly but also covalent linkage of PSI to the surface. A systematic investigation on the protein assembly and the resulting photoelectrochemical properties will be presented. For this quartz crystal microbalance experiments, chopped light voltammetry, constant potential measurements, photoaction spectra and experiments with methyl viologen have been performed. Moreover it can be demonstrated that the performance of such PSI electrodes can be further improved. Here nanoparticles are additionally coupled to the graphene-PSI system in order to enhance current generation under illumination. [1] S. C. Feifel, K. R. Stieger, H. Lokstein, H. Lux, F. Lisdat, Journal of Materials Chemistry A 3 (23) (2015) 12188-12196 [2] S. C. Feifel, H. Lokstein, M. Hejazi, A. Zouni, F. Lisdat, Langmuir 31 (38) (2015) 10590-10598

Non-Stechiometric Titanates Embellished with a Catalyst and Employed for Visible-Light Water Splitting

Titanates are known from their photocatalytic properties, but they need additional catalysts to work efficiently also they have very high electronic conductivity when they are prepared under reducing conditions in the hydrogen flame or under thermal oxidation in high temperature. We prepared these non-stechiometric titanates as materials for visible-light water and/or small organic molecules oxidation. Specific conditions of synthesis of non-stechiometric titanates, created by oxidation of the metal at high temperature in oxygen flow, allowed to get unique properties of such films like: large surface area, high conductivity, red-shift in the absorption spectrum and stability over large range of pHs. Adding to these properties, the affinity of polyoxometalate (POMs) species to the formed structures, and their ability to catalyse the water oxydation reaction, the system of TiO2-x – POM seems to be suitable for water splitting purposes under visible light irradiation. Morover, these sub-stoechiometric titanium dioxide / titanes films exhibited red-shifted IPCE photo-action spectrum. These results demonstrated the possibility to form defective TiO2 through thermal oxidation of the titanium metal as alternative to the reduction by H2.

Solution Combustion Synthesis, Characterization, and Photoelectrochemistry of CuNb2O6 and ZnNb2O6 Nanoparticles

This study reports on the solution combustion synthesis of two different ternary niobium oxides, namely, p-CuNb2O6 and n-ZnNb2O6. Such ternary oxides are attractive candidates in the “Holy Grail” search for efficient and stable semiconductors for solar energy conversion and environmental remediation. We demonstrate how this time- and energy-efficient method is capable of synthesizing high surface area and crystalline nanoparticles of the above compounds with enhanced optoelectronic properties. The synthesized crystalline samples were characterized by powder X-ray diffraction (with Rietveld refinement for phase purity), diffuse reflectance UV–visible and Raman spectroscopy, electron microscopy, and photoelectrochemical (PEC) techniques. The band structure of these oxides was probed by linear sweep voltammetry and by measuring their photoaction spectra (internal photon to electron conversion efficiency vs wavelength). The obtained bandgap energy values (1.9 and 3.2 eV for the Cu- and Zn-containing compounds...

Sunlight inactivation of viruses in open-water unit process treatment wetlands: modeling endogenous and exogenous inactivation rates.

Sunlight inactivation is an important mode of disinfection for viruses in surface waters. In constructed wetlands, for example, open-water cells can be used to promote sunlight disinfection and remove pathogenic viruses from wastewater. To aid in the design of these systems, we developed predictive models of virus attenuation that account for endogenous and exogenous sunlight-mediated inactivation mechanisms. Inactivation rate models were developed for two viruses, MS2 and poliovirus type 3; laboratory- and field-scale experiments were conducted to evaluate the models' ability to estimate inactivation rates in a pilot-scale, open-water, unit-process wetland cell. Endogenous inactivation rates were modeled using either photoaction spectra or total, incident UVB irradiance. Exogenous inactivation rates were modeled on the basis of virus susceptibilities to singlet oxygen. Results from both laboratory- and field-scale experiments showed good agreement between measured and modeled inactivation rates. The modeling approach presented here can be applied to any sunlit surface water and utilizes easily measured inputs such as depth, solar irradiance, water matrix absorbance, singlet oxygen concentration, and the virus-specific apparent second-order rate constant with singlet oxygen (k2). Interestingly, the MS2 k2 in the open-water wetland was found to be significantly larger than k2 observed in other waters in previous studies. Examples of how the model can be used to design and optimize natural treatment systems for virus inactivation are provided.

Anodized TiO2 Nanotubes: Effect of anodizing time on film length, morphology and photoelectrochemical properties

The influence of anodizing time on the length, morphology and photoelectrochemical properties of TiO2 Nanotubes (NTs) has been investigated. An optimum anodizing time of 20min at 30V leads to 1.1µm long NTs films, generating a photocurrent density (Jphoto) of 460µA/cm2. Anodized films grown for a time shorter than 10min show only a thin (100nm) porous layer, instead of well-defined NTs, and exhibit lower photocurrent densities. Similarly, Jphoto is lower in NTs grown for 1–2h, partly because the NTs are damaged after extended anodization and partly because their length (l) already exceeds the electron diffusion length (Ln), found to be 0.8±0.1µm. The Incident Photon to Current Efficiency (IPCE) study reveals how the microstructural changes (morphology and length) affect the behavior of the anodic films over the photoaction spectrum. IPCE analysis confirms the superiority of 1.1µm long and well-defined NTs, with a maximum value of 18% recorded at 340nm. In contrast, the ability to collect electrons decreases for longer and partially damaged NTs, especially in the region 360–300nm. A small response above 400nm is observed in all the photoelectrodes, due to oxygen vacancies induced during annealing.

Quantum-Confined CdTe Films Deposited by SILAR and Their Photoelectrochemical Stability in the Presence of Se2– as a Hole Scavenger

Quantum-confined CdTe films were deposited by successive ionic layer adsorption and reaction (SILAR) on nc-TiO2 and on a conducting oxide electrode (FTO) from aqueous solutions of Cd2+ and Te2– prepared in situ under inert atmosphere. The films were characterized with UV–visible absorption, SEM, EDX, and XRD. CdTen films exhibited a zinc-blende structure and a red-shift in absorbance with increasing SILAR cycles (n) consistent with quantum size effects and featured either a mesoporous morphology on FTO or followed the contours of the titania nanoparticles on nc-TiO2 films. The films’ photoelectrochemical behavior was studied in the presence of Se2– compared to S2– as hole scavengers. The incident-photon-to-current conversion efficiency reached ca. 16% at 460 nm and 9% at 500 nm at CdTe10/nc-TiO2 in alkaline Se2– electrolyte compared to 1% at 460 nm or 0.5% at 500 nm in S2–. CdTe10 films examined after acquiring a photoaction spectrum in Se2– still exhibited zinc-blende structure, EDX analysis showed Cd an...

Sunlight Inactivation of MS2 Coliphage in the Absence of Photosensitizers: Modeling the Endogenous Inactivation Rate Using a Photoaction Spectrum

The endogenous sunlight inactivation rates of MS2 coliphage in photosensitizer-free water were measured (kobs) under different light conditions and compared to modeled inactivation rates (kmod) computed using a previously published action spectrum. Experiments were conducted under simulated and natural sunlight. There was generally good agreement between modeled and observed MS2 sunlight inactivation rates in the summer and winter, suggesting that the action spectrum can be used to predict changes in the inactivation rate caused by diurnal and seasonal changes in natural sunlight irradiance. However, we show that a major source of uncertainty in the predictions is the ability to accurately measure or model the comparatively weak and highly variable solar irradiance between 280 and 300 nm, a range to which the inactivation rate is very sensitive. The action spectrum was also used to predict the endogenous inactivation rates of MS2 at different depths in a column of strongly humic-colored [i.e., solar ultraviolet (UV)-attenuating] wetland water under simulated sunlight; we observed fairly good agreement between kobs and kmod, suggesting that the action spectrum can be used to estimate the decrease in the endogenous inactivation rate caused by spectrally selective sunlight attenuation in the water column.

Comparative Photoactivity and Stability of Isolated Cyanobacterial Monomeric and Trimeric Photosystem I

Photoactivity of native trimeric, and non-native monomeric Photosystem I (PSI) extracted from Thermosynechococcus elongatus is compared in a photoelectrochemical system. The PSI monomer is isolated by disassembling a purified PSI trimer using a freeze-thaw technique in presence of the short-chain surfactant, octylthioglucoside. Photoactive electrodes are constructed with PSI, functioning as both light absorber and charge-separator, embedded within a conductive polymer film. Despite structural differences between PSI trimers and monomers, electrodes cast with equal chlorophyll-a concentration generate similar photoactivities. Photoaction spectra show that all photocurrent derived from electrodes of PSI and conductive polymer originates solely from PSI with no photocurrent caused by redox mediators in the conductive polymer film. Longevity studies show that the two forms of PSI photodegrade at the same rate while exposed to equal intensities of 676 nm light. Direct photo-oxidation measurements indicate that PSI's monomeric form has fewer light harvesting antennae than trimer, and also shows energy sharing between monomeric subunits in the trimer. The structure of PSI is also found to impact cell performance when applying light at incident powers above 1.5 mW/cm(2) due to the reduced optical cross-section in the monomer, causing saturation at lower light intensities than the trimer.

Photovoltaic Performance of Spray-Coated Zinc Oxide Nanoparticles Sensitized With Metal-Free Indoline Dyes

Photovoltaic properties of nano-sized zinc oxide (ZnO) films sensitized with a conventional ruthenium complex (N719) and two metal-free organic indoline dyes (D-149 and D-205) were compared. The ZnO nanoparticles were deposited on transparent conductive aluminum-doped ZnO coated glass substrates (AZO) by spray-coating deposition method and then annealed in air at 500 °C. Using the ZnO-coated AZO as transparent conductive substrates, dye-sensitized solar cells (DSCs) were prepared with the N719, D149 and D205 dyes as the sensitizers. The photoaction spectra of the incident photon-to-current conversion efficiency (IPCE) of the DSCs revealed that the indoline-sensitized solar cells were higher and broader than the ruthenium-sensitized solar cell in the photo-absorption behavior. Under AM 1.5 simulated sunlight (1000 W m-2), the indoline-sensitized ZnO solar cells yielded solar-to-electric energy conversion efficiency of 3.02 and 2.26% for the D-205 and D-149 respectively, while the N719 sensitized ZnO recorded only 0.97%. The superior performance of the indoline-sensitized solar cells was attributed to mainly higher sunlight harvesting efficiency of these metal-free organic dyes.

Single-step fabrication of all-solid dye-sensitized solar cells using solution-processable precursor

A one-pot, solution-processable fabrication method of dye-sensitized bulk heterojunction solar cell is employed to provide an alternate and analogous fabrication perspective for all-solid dye-sensitized solar cells (DSSCs) in a single step. All-solid DSSCs of ZnO/dye/polythiophene derivative (P3HT) are fabricated from a soluble precursor solution of diethylzinc (DEZ), dye and P3HT. Owing to its air and moisture sensitivity, DEZ is decomposed on P3HT layer to form bicontinuous structures of ZnO and P3HT between which dyes are sandwiched. The photoaction spectrum of all-solid DSSC shows that it covered wavelengths up to 750 nm, leading to the conclusion that panchromatic photon harvesting and electron collection are possible due to the synergistic photon harvesting by both the P3HT and the dye sensitizer. ZnO/dye/P3HT interfaces of all-solid DSSC solar cells prepared by a single-step fabrication process.

Ruthenium Sensitizer Functionalized by Acetylacetone Anchoring Groups for Dye-Sensitized Solar Cells

A new ruthenium sensitizer Ru-acac (cis-bis(thiocyanato)(2,2′-bipyridyl-4,4′-di(3-pentane-2,4-dione)(4,4′-dinonyl-2′-bipyridyl)ruthenium(II)) functionalized by acetylacetone (acac) anchoring groups was synthesized and studied in ZnO-based dye-sensitized solar cells (DSCs). In comparison with the parent ruthenium complex bearing classical carboxylic acid groups (coded Z907), Ru-acac exhibits blue-shifted metal-to-ligand charge transfer (MLCT) absorption and emission bands as well as a cathodic shifted oxidation potential (E(RuIII/II)). Quantum chemistry time-dependent density functional theory (TD-DFT) calculations revealed that the acac groups are less conjugated with the pyridine units than carboxylic acid groups, entailing a lower electronic coupling with the semiconductor (SC) conduction band. Finally, the photovoltaic performances of both ruthenium complexes were investigated in DSCs by recording the J/V characteristics under solar spectrum simulation (AM1.5), the photoaction spectra, and by electrochemical impedance measurements. We showed that Z907 display higher photoconversion efficiency than Ru-acac owing to a lower electronic coupling with the SC and a different bending of the conduction band. Interestingly, the acac group provides a covalent binding to the ZnO surface but does not etch the structure as carboxylic acid usually does.

New tunable ruthenium complex dyes for TiO2 solar cells

The light-to-electricity conversion efficiencies of the new (Bu4N)3[Ru(dcbpy)2(tmtH2)], (Bu4N)4[Ru(dcbpy)2(tmtH)] and (Bu4N)5[Ru(dcbpy)2(tmt)] complexes, where dcbpy=2,2′-bipyridyl-4,4′-dicarboxylate and tmt=2,4,6-trimercapto-1,3,5-triazine, were comparable with those obtained with the N719 dye. Furthermore, the contribution of two MLCT and a LMCT charge-transfer band to the energy conversion process was revealed by deconvolution of the photoaction spectra, indicating that they are the first ruthenium dyes presenting long distance photo-injection from a ligand-to-metal charge-transfer excited state.

Improving the layer morphology of solution-processed perylene diimide organic solar cells with the use of a polymeric interlayer

Herein we demonstrate a method to improve the power conversion eciency (PCE) parameter of organic photovoltaic (OPV) devices based on the electron acceptor N,N’-bis(1ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) blended with the electron donor poly(indenofluorene)-aryloctyl (PIF-Aryl). The device parameters of the short-circuit current, open-circuit voltage and fill factor are found increased after the insertion of a thin poly [9, 9-dioctylfluorene-co-N[4-(3-methylpropyl)]-diphenylamine] (TFB) photoactive interlayer between the hole-collecting electrode and the photoactive layer of the device. Unlike to most of the cases where interlayers serve as charge extractors, in our system the polymeric interlayer serves as a morphology modifying agent that drives the PDI component to segregate better at the interface with the device cathode; that is at the carrier-collecting electrode interface, which is not in physical contact with the interlayer. The processes of energy/charge transfer of the TFB excitons to/with the PIF-Aryl:PDI top-layer are also addressed. Charge transfer reactions dominate at the TFB/PIF-Aryl:PDI interface but no significant contribution in the photocurrent generation is seen in the photoaction spectra of the bilayer device.

Synthesis of donor-substituted meso-phenyl and meso-ethynylphenyl BODIPYs with broad absorption

We report the synthesis of meso-ethynylphenyl BODIPYs and compare their properties with the corresponding meso-phenyl derivatives. Both types of BODIPYs carry a 2-cyano-3-acrylic acid anchoring moiety and either methyl groups or 4,4′-dimethoxytriphenylamine (MeOTPA) donor groups at positions 3 and 5. All compounds were characterized by NMR, UV/vis and cyclic voltammetry. The MeOTPA-substituted BODIPYs show an excellent panchromatic absorption with high molar extinction coefficients over the whole UV/vis range up to the near-IR region. The most impressive absorption was exhibited by the MeOTPA-substituted meso-ethynylphenyl BODIPY which strongly absorbs up to 1030 nm. By cyclic voltammetry measurements, all compounds were identified to be electrochemically stable in solution. Further, it was observed that the value of the LUMO level can be tuned by the meso-substituent. The HOMO level is determined by the donor substituents (−5.41 ± 0.03 eV and −4.84 ± 0.01 eV for BODIPYs with methyl groups and MeOTPA donor groups, respectively). These findings were further supported by DFT calculations. To evaluate the potential of the BODIPYs as sensitizers, the incident photon-to-current conversion efficiencies of solid-state dye-sensitized solar cells were measured. The photoaction spectra clearly show that the BODIPYs contribute to the photocurrent generation over their entire absorption region.

Solution processable thin film organic photovoltaic cells based on far red sensitive soluble squaraine dyes

Symmetrical squaraine (SQ) dyes with varying alkyl chain lengths have been synthesized towards their utilization as sensitizer for enhancing the photon harvesting in far-red region of solar spectrum. It has been observed that energy level of the SQ-dyes can be systematically tuned only by alkyl chain substitution. It has also been shown that SQ-dyes behave like p-type donors forming a p–n heterojunction with soluble fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) as n-type organic electron acceptor. Thin film organic photovoltaic p–n heterojunction devices have been fabricated using soluble SQ-dyes with PCBM without using any thermal evaporation technique for the active layers. A perusal of the photo-action spectra clearly corroborates that at least 0.25eV energy barrier is necessary for the facile electron transfer from donor moiety to the acceptor after the photo-excitation.