Co-reporter:F. Andrew Frame
The Journal of Physical Chemistry C June 17, 2010 Volume 114(Issue 23) pp:10628-10633
Publication Date(Web):Publication Date (Web): May 24, 2010
DOI:10.1021/jp101308e
Under visible light irradiation, CdSe-nanoribbons photocatalyze H2 evolution from aqueous sodium sulfite/sulfide solution with a quantum efficiency of 9.2% at 440 nm, whereas bulk CdSe is not active for the reaction. Photoelectrochemical measurements show that the activity of nano-CdSe is caused by a raised flatband potential (−0.55 V, NHE) which follows from the increased bandgap (2.7 eV) of this quantum confined material. In the presence of a sulfide ion, the flatband potential is fixed to −0.43 V (NHE), slightly below the sulfide redox potential (−0.48 V, NHE). When the nanoribbons are chemically linked to MoS2 nanoplates that were obtained by exfoliation and ultrasonication of bulk MoS2, the activity increases almost four times, depending on the mass percentage of MoS2. Cyclic voltammetry reveals that the enhancement from the MoS2 nanoplates is due to a reduction of the H2 evolution overpotential. In contrast, chemical linkage of Pt nanoparticles to the nanoribbons does not affect the photocatalytic activity.
Co-reporter:Frank E. Osterloh
ACS Energy Letters - New in 2016 February 10, 2017 Volume 2(Issue 2) pp:
Publication Date(Web):January 18, 2017
DOI:10.1021/acsenergylett.6b00665
The chemical literature often does not differentiate between photocatalytic (PC) and photosynthetic (PS) processes (including artificial photosynthesis) even though these reactions differ in their thermodynamics. Photocatalytic processes are thermodynamically downhill (ΔG < 0) and are merely accelerated by the catalyst, whereas photosynthetic processes are thermodynamically unfavorable (ΔG > 0) and require photochemical energy input to occur. Here we apply this differentiation to analyze the basic functions of PC and PS devices and to formulate design criteria for improved performance. As will be shown, the corresponding devices exhibit distinctly different sensitivities to their functional parameters. For example, under conditions of optimal light absorption, carrier lifetimes, and electrochemical rates, the performance of PCs is limited only by their surface area, while type 1 PS devices are limited by their carrier mobility and mass transport, and type 2 PS devices are limited by electrochemical charge-transfer selectivity. Strategies for the optimization of type 1 and 2 photosynthetic devices and photocatalysts are also discussed.
Co-reporter:Geetu Sharma, Zeqiong Zhao, Pranab Sarker, Benjamin A. Nail, Jiarui Wang, Muhammad N. Huda and Frank E. Osterloh
Journal of Materials Chemistry A 2016 vol. 4(Issue 8) pp:2936-2942
Publication Date(Web):29 Dec 2015
DOI:10.1039/C5TA07040F
As a visible light active p-type semiconductor, CuBi2O4 is of interest as a photocatalyst for the generation of hydrogen fuel from water. Here we present the first photovoltage and photocatalytic measurements on this material and DFT results on its band structure. Single crystalline CuBi2O4 nanoparticles (25.7 ± 4.7 nm) were synthesized from bismuth and cupric nitrate in water under hydrothermal conditions. Powder X-ray diffraction (XRD) confirms the CuBi2O4 structure type and UV-Vis spectroscopy shows a 1.75 eV optical band gap. Surface photovoltage (SPV) measurements on CuBi2O4 nanoparticle films on fluorine doped tin oxide yield 0.225 V positive photovoltage at >1.75 eV photon energy confirming holes as majority carriers. The photovoltage is reversible and limited by light absorption. When dispersed in 0.075 M aqueous potassium iodide solution, the CuBi2O4 particles support photochemical hydrogen evolution of up to 16 μmol h−1 under ultraviolet but not under visible light. Based on electrochemical scans, CuBi2O4 is unstable toward reduction at −0.2 V, but a pH-dependent photocurrent of 6.45 μA cm−2 with an onset potential of +0.75 V vs. NHE can be obtained with 0.01 M Na2S2O8 as a sacrificial electron acceptor. The photoelectrochemical properties of CuBi2O4 can be explained on the basis of the band structure of the material. DFT calculations show that the valence and conduction band edges arise primarily from the combination of O 2p and Cu 3d orbitals, respectively, with additional contributions from Cu 3d and Bi 6s orbitals just below the Fermi level. Trapping of photoelectrons in the Cu 3d band is the cause for reductive photocorrosion of the material, while the p-type conductivity arises from copper vacancy states near the VB edge. These findings provide an improved understanding of the photophysical properties of p-CuBi2O4 and its limitations as a proton reduction photocatalyst.
Co-reporter:Kathryn A. Newton
Topics in Catalysis 2016 Volume 59( Issue 8-9) pp:750-756
Publication Date(Web):2016 May
DOI:10.1007/s11244-016-0549-3
Tungsten(VI) oxide (WO3) is a robust n-type semiconductor with a 2.7 eV bandgap and proven activity for photoanodic water oxidation in the context of tandem water splitting photocatalysis. Here we present a systematic investigation of three types of WO3 particles to determine the influence of particle size and morphology on photocatalytic oxygen evolution, optical properties, energetics, and photocurrent generation. Nanodots (32 ± 16 nm), nanoplates (476 ± 98 nm by 58 ± 16 nm), and WO3 microcrystals (~2 μm) for the study were synthesized by calcination of WO3 powders or by hydrolysis of Na2WO4, followed by calcination. All samples crystallize in the monoclinic rhenium trioxide structure type and have band gaps between 2.75 and 2.87 eV. From 0.01 M aqueous NaIO4 under 610 mW cm−2 visible illumination (>400 nm), 30 mg of the WO3 dots, plates, and microcrystals evolve oxygen at 31.6, 16.5, and 2.9 μmol h−1, respectively. Photoelectrochemistry on WO3 particle films in aqueous K2SO4 at pH 3.5 confirms decreasing anodic photocurrents (25, 17.8, and 7.7 μA cm−2, respectively, at +1.0 V NHE) with decreasing particles size, and similar photoonset potentials of +0.25 V vs. NHE for all samples. This suggests that the photocatalytic activity differences among the WO3 series are controlled not by the energetics but by the kinetics of minority and majority carrier transport within the particles. The reactivity trends can be quantitatively described with the one-dimensional continuity model for charge generation, recombination, and transport.
Co-reporter:Frank E. Osterloh
ACS Energy Letters 2016 Volume 1(Issue 5) pp:1060
Publication Date(Web):October 31, 2016
DOI:10.1021/acsenergylett.6b00493
Co-reporter:Jing Zhao, Benjamin A. Nail, Michael A. Holmes, and Frank E. Osterloh
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 17) pp:3335-3340
Publication Date(Web):August 9, 2016
DOI:10.1021/acs.jpclett.6b01569
Surface photovoltage spectroscopy (SPS) was used to study the photochemistry of mercaptoethanol-ligated CdSe quantum dot (2.0–4.2 nm diameter) films on indium doped tin oxide (ITO) in the absence of an external bias or electrolyte. The n-type films generate negative voltages under super band gap illumination (0.1–0.5 mW cm–2) by majority carrier injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (−125 to −750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe–ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron–hole (polaron) pairs that are polarized by a space charge layer at the CdSe–ITO boundary. The space charge layer is 70–150 nm wide, based on thickness-dependent photovoltage measurements. The ability of SPS to directly measure built-in voltages, space charge layer thickness, sub-band gap states, and effective band gaps in drop-cast quantum dot films aids the understanding of photochemical charge transport in quantum dot solar cells.
Co-reporter:J. Wang, J. Zhao and F. E. Osterloh
Energy & Environmental Science 2015 vol. 8(Issue 10) pp:2970-2976
Publication Date(Web):13 Jul 2015
DOI:10.1039/C5EE01701G
The application of inorganic nanostructures for solar water splitting is currently limited by our understanding of photochemical charge transfer on the nanoscale, where space charge layers are less effective for carrier separation. Here we employ surface photovoltage spectroscopy to measure the internal photovoltages in single crystalline platinum/ruthenium-modified Rh-doped SrTiO3 nanocrystals for the first time. Voltages of −0.88 V and −1.13 V are found between the absorber and the Ru and Pt cocatalysts, respectively, and a voltage of −1.48 V for a Rh:SrTiO3 film on an Au substrate. This shows that the Pt and Ru cocatalysts not only improve the redox kinetics but also aid charge separation in the absorber. Voltages of +0.4 V, +0.6 V, and +1.2 V are found for hole injection into KI, K4[Fe(CN)6], and methanol, respectively, and a voltage of −0.7 V for electron injection into K3[Fe(CN)6]. These voltages correlate well with the photocatalytic performance of the catalyst; they are influenced by the built-in potentials of the donor-acceptor configurations, the physical separation of donors and acceptors, and the reversibility of the redox reaction. The photovoltage data also allowed the identification of a photosynthetic system for hydrogen evolution (80 μmol g−1 h−1) under visible light illumination (>400 nm) from 0.05 M aqueous K4[Fe(CN)6].
Co-reporter:Matthew J. Greaney, Elsa Couderc, Jing Zhao, Benjamin A. Nail, Matthew Mecklenburg, William Thornbury, Frank E. Osterloh, Stephen E. Bradforth, and Richard L. Brutchey
Chemistry of Materials 2015 Volume 27(Issue 3) pp:744
Publication Date(Web):January 12, 2015
DOI:10.1021/cm503529j
We developed a simple and robust colloidal route for the installation of CdX2 (X = Cl, Br, I) ligands on the surface of CdSe nanocrystals, which effectively displace the native ligands and form stable suspensions. After colloidal ligand exchange, these nanocrystals can be easily solution cast into nanocrystal films. Photoelectrochemical measurements on solution-cast nanocrystal films reveal a striking influence of surface cadmium halide on photocurrent response, with mildly annealed, CdCl2-treated CdSe nanocrystals showing the greatest enhancement in photocurrent to above band gap illumination. The strong dependence of photoresponse on surface halide is thought to result from ligand-induced changes in the electronic structure of the nanocrystal samples. We arrive at this conclusion using a combination of ultrafast transient absorption, time-resolved photoluminescence, and surface photovoltage spectroscopies, which are being applied together for the first time to investigate nanocrystal trap states. From these measurements, we establish a trend for ligand-related sub-band gap states that accounts for electron and hole trapping at the nanocrystal surface. The nature of the electron and hole traps in the nanocrystal films are dependent on the thermal history of the sample as well as the specific halide surface treatment employed. After subjecting the nanocrystal films to mild thermal annealing, we find evidence that suggests a drastic reduction in electron trap states. Additionally, depending on the surface halide treatment employed, the energy of the hole trap states varies, with CdCl2 treatment resulting in energetically shallow hole trap states, and CdBr2 and CdI2 treatments leading to much deeper hole traps. Thus, judicious choice of cadmium halide surface treatment can be used to manipulate the trap state landscape of these ligand exchanged CdSe nanocrystals.
Co-reporter:Yuxin Yang, Jiarui Wang, Jing Zhao, Benjamin A. Nail, Xing Yuan, Yihang Guo, and Frank E. Osterloh
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 10) pp:5959
Publication Date(Web):February 20, 2015
DOI:10.1021/acsami.5b00257
The charge transfer properties of interfaces are central to the function of photovoltaic and photoelectrochemical cells and photocatalysts. Here we employ surface photovoltage spectroscopy (SPS) to study photochemical charge transfer at a p-silicon/n-BiVO4 particle interface. Particle films of BiVO4 on an aluminum-doped p-silicon wafer were obtained by drop-coating particle suspensions followed by thermal annealing at 353 K. Photochemical charge separation of the films was probed as a function of layer thickness and illumination intensity, and in the presence of methanol as a sacrificial electron donor. Electron injection from the BiVO4 into the p-silicon is clearly observed to occur and to result in a maximum photovoltage of 150 mV for a 1650 nm thick film under 0.3 mW cm–2 illumination at 3.5 eV. This establishes the BiVO4–p-Si interface as a tandem-like junction. Charge separation in the BiVO4 film is limited by light absorption and by slow electron transport to the Si interface, based on time-dependent SPS measurements. These problems need to be overcome in functional tandem devices for photoelectrochemical water oxidation.Keywords: photocatalyst; photoelectrochemistry; surface photovoltage spectroscopy; tandem; water splitting; z-scheme
Co-reporter:Benjamin A. Nail, Jorie M. Fields, Jing Zhao, Jiarui Wang, Matthew J. Greaney, Richard L. Brutchey, and Frank E. Osterloh
ACS Nano 2015 Volume 9(Issue 5) pp:5135
Publication Date(Web):April 14, 2015
DOI:10.1021/acsnano.5b00435
Nickel(II) oxide (NiO) is an important wide gap p-type semiconductor used as a hole transport material for dye sensitized solar cells and as a water oxidation electrocatalyst. Here we demonstrate that nanocrystals of the material have increased p-type character and improved photocatalytic activity for hydrogen evolution from water in the presence of methanol as sacrificial electron donor. NiO nanocrystals were synthesized by hydrolysis of Ni(II) nitrate under hydrothermal conditions followed by calcination in air. The crystals have the rock salt structure type and adopt a plate-like morphology (50–90 nm × 10–15 nm). Diffuse reflectance absorbance spectra indicate a band gap of 3.45 eV, similar to bulk NiO. Photoelectrochemical measurements were performed at neutral pH with methylviologen as electron acceptor, revealing photo-onset potentials (Fermi energies) of 0.2 and 0.05 eV (NHE) for nanoscale and bulk NiO, respectively. Nano-NiO and NiO-Pt composites obtained by photodepositon of H2PtCl6 catalyze hydrogen evolution from aqueous methanol at rates of 0.8 and 4.5 μmol H2 h–1, respectively, compared to 0.5 and 2.1 μmol H2 h–1 for bulk-NiO and NiO-Pt (20 mg of catalyst, 300 W Xe lamp). Surface photovoltage spectroscopy of NiO and NiO–Pt films on Au substrates indicate a metal Pt-NiO junction with 30 mV photovoltage that promotes carrier separation. The increased photocatalytic and photoelectrochemical performance of nano-NiO is due to improved minority carrier extraction and increased p-type character, as deduced from Mott–Schottky plots, optical absorbance, and X-ray photoelectron spectroscopy data.Keywords: nanoscale junction; p-type metal oxide; photocatalysis; photocorrosion; surface photovoltage spectroscopy;
Co-reporter:Rachel L. Chamousis and Frank E. Osterloh
Energy & Environmental Science 2014 vol. 7(Issue 2) pp:736-743
Publication Date(Web):27 Nov 2013
DOI:10.1039/C3EE42993H
Tetrabutylammonium (TBA) stabilized H[Ca2Nb3O10] nanosheets catalyze hydrogen evolution from aqueous methanol under illumination with UV light. Here we show that surface treatment with protons, potassium, and strontium potential-determining cations (PDIs) in aqueous solution modifies the electrostatic, energetic and photocatalytic properties of this nanomaterial. Attachment of cations to the nanocrystals was verified with elemental dispersive spectroscopy. Zeta potentials were measured as −40 mV (TBA+, pH = 4.8), −50 mV (K+, pH = 4.3), and −20 mV (Sr2+, pH = 4.4). Photoelectrochemical measurements in methanol containing 0.1 M tetraethylammonium chloride revealed anodic current photoonset potentials/Fermi energies ranging between −0.59 V (Sr2+) and −0.71 V (at pH = 7, vs. NHE). The photocatalytic proton reduction ability of the modified nanocrystals was assessed in aqueous methanol at pH = 1. Here, KxH1−x[Ca2Nb3O10] evolved hydrogen at 350 μmol H2 h−1, SrxH1−2x[Ca2Nb3O10] at 70 μmol H2 h−1, and H[Ca2Nb3O10] at 160 μmol H2 h−1. In addition, the photocatalytic activity was found to increase (20–160 μmol H2 h−1) with solution pH. These observed activity variations can be quantitatively understood using a linear free energy relationship between the proton reduction rate constant and the free energy of proton reduction. This shows that the photocatalytic activity of the nanocrystals depends on the electrochemical potentials/Fermi energies of the modified catalysts. The effect of the PDI charge on the nanomaterial energetics can be rationalized by considering the surface potential. The latter can be related to the particle surface charge and the concentration of counterions in solution using the Grahame equation. These results provide a quantitative basis for the understanding and manipulation of nanomaterial photocatalysts with PDIs.
Co-reporter:Po Wu, Jiarui Wang, Jing Zhao, Liejin Guo and Frank E. Osterloh
Journal of Materials Chemistry A 2014 vol. 2(Issue 47) pp:20338-20344
Publication Date(Web):16 Oct 2014
DOI:10.1039/C4TA04100C
Graphitic carbon nitride (g-C3N4) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C3N4 is limited by structure defects generated during the calcination process. Specifically we find that the photocatalytic hydrogen production rate from aqueous methanol is inversely related to the calcination temperature (520–640 °C). The highest activity of 0.301 mmol h−1 g−1 is observed for the sample prepared at the lowest processing temperature. Surface photovoltage (SPV) spectroscopy shows that the maximum photovoltage is reduced (from 1.29 V to 0.62 V) as the processing temperature is increased, in accordance with higher defect concentrations and faster electron–hole recombination. The defects also produce additional optical absorption in the visible spectra and cause a red shifted, weakened photoluminescence (PL). Based on the sub-gap signal in the SPV and PL spectra, defect energy levels are +0.97 V and −0.38 V (vs. NHE) in the band gap of the material. According to Fourier transform infrared (FTIR) spectra, the defects are due to amino/imino groups in the g-C3N4 lattice.
Co-reporter:Rachel L. Chamousis, Lilian Chang, William J. Watterson, Rick D. Montgomery, Richard P. Taylor, Adam J. Moule, Sean E. Shaheen, Boaz Ilan, Jao van de Lagemaat and Frank E. Osterloh
Journal of Materials Chemistry A 2014 vol. 2(Issue 39) pp:16608-16616
Publication Date(Web):2014/08/21
DOI:10.1039/C4TA03204G
Living organisms use fractal structures to optimize material and energy transport across regions of differing size scales. Here we test the effect of fractal silver electrodes on light distribution and charge collection in organic semiconducting polymer films made of P3HT and PCBM. The semiconducting polymers were deposited onto electrochemically grown fractal silver structures (5000 nm × 500 nm; fractal dimension of 1.71) with PEDOT:PSS as hole-selective interlayer. The fractal silver electrodes appear black due to increased horizontal light scattering, which is shown to improve light absorption in the polymer. According to surface photovoltage spectroscopy, fractal silver electrodes outperform the flat electrodes when the BHJ film thickness is large (>400 nm, 0.4 V photovoltage). Photocurrents of up to 200 microamperes cm−2 are generated from the bulk heterojunction (BHJ) photoelectrodes under 435 nm LED (10–20 mW cm−2) illumination in acetonitrile solution containing 0.005 M ferrocenium hexafluorophosphate as the electron acceptor. The low IPCE values (0.3–0.7%) are due to slow electron transfer to ferrocenium ion and due to shunting along the large metal–polymer interface. Overall, this work provides an initial assessment of the potential of fractal electrodes for organic photovoltaic cells.
Co-reporter:Jiarui Wang and Frank E. Osterloh
Journal of Materials Chemistry A 2014 vol. 2(Issue 24) pp:9405-9411
Publication Date(Web):29 Apr 2014
DOI:10.1039/C4TA01654H
Chemical modification of BiVO4 nanoparticles (Scheelite, EG = 2.62 eV) with chemically deposited Co3O4 nanoparticles improves the photocatalytic water oxidation activity by a factor of 17 to 11 mmol g−1 h−1 under visible light (AQE 10% at 435 nm) and to 1.24 mmol g−1 h−1 under sunlight from aqueous 0.02 M NaIO4. This activity ranks among the highest among known visible light driven water oxidation photocatalysts. Based on systematic electrochemical, photoelectrochemical, and surface photovoltage measurements, the high photocatalytic activity can be attributed to the electrocatalytic properties of the Co3O4 cocatalyst and to the formation of a heterojunction at the BiVO4–Co3O4 interface.
Co-reporter:Po Wu, Jiarui Wang, Jing Zhao, Liejin Guo and Frank E. Osterloh
Chemical Communications 2014 vol. 50(Issue 98) pp:15521-15524
Publication Date(Web):20 Oct 2014
DOI:10.1039/C4CC08063G
A high rate of 2.23 mmol h−1 g−1 (quantum efficiency of 6.67% at 400 nm) for visible light driven photocatalytic H2 evolution can be achieved with g-C3N4 by alkalization of the solution to a pH of 13.3, due to accelerated transfer of photoholes to the sacrificial donor.
Co-reporter:Frank E. Osterloh ; Michael A. Holmes ; Jing Zhao ; Lilian Chang ; Steven Kawula ; John D. Roehling ;Adam J. Moulé
The Journal of Physical Chemistry C 2014 Volume 118(Issue 27) pp:14723-14731
Publication Date(Web):June 6, 2014
DOI:10.1021/jp500226u
Surface photovoltage (SPV) spectra are reported for separate films of (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and for regioregular and regiorandom poly(3-hexylthiophene) (P3HT):PCBM bulk heterojunctions, as a function of wavelength, film thickness, thermal annealing, and substrate. In PCBM films, two photovoltage features are observed at 1.1–1.4 eV (F1) and 1.4–2.3 eV (F2), which are assigned to excitation of charge transfer states at the interface (F1) and in the bulk (F2) of the film. In BHJ films, five different photovoltage features are observed at 0.75–0.9 eV (F1), 0.9–1.3 eV (F2), 1.3–1.8 eV (F3), 1.8–2.0 eV (F4), and 2.0–2.4 eV (F5). This data can be analyzed on the basis of optical absorbance and fluorescence spectra of the films, and using SPV spectra for PCBM and P3HT only films, and for a BHJ film containing P3HT nanofibers for comparison. SPV features are assigned to states at the polymer–substrate interface (F1 and F2), the P3HT:PCBM charge transfer state (F3), the self-ionized (CT) state of P3HT (F4), and the band gap transition of P3HT (F5). This interpretation is also consistent with molecular orbital energy diagrams and electron microscopy-derived topological maps of the films. Photovoltage sign and substrate dependence can be understood with the depleted semiconductor model. Features F1–4 are caused by polarization of electrostatically bound charge pairs by the built-in electric field at the substrate–BHJ interface, whereas F5 is due to transport of free charge carriers through the film and through the substrate film interface. This work will promote the understanding of photochemical charge generation and transport in organic photovoltaic films.
Co-reporter:Jing Zhao and Frank E. Osterloh
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 5) pp:782-786
Publication Date(Web):February 9, 2014
DOI:10.1021/jz500136h
Photochemical charge generation, separation, and transport at nanocrystal interfaces are central to photoelectrochemical water splitting, a pathway to hydrogen from solar energy. Here, we use surface photovoltage spectroscopy to probe these processes in nanocrystal films of HCa2Nb3O10, a proven photocatalyst. Charge injection from the nanoparticles into the gold support can be observed, as well as oxidation and reduction of methanol and oxygen adsorbates on the nanosheet films. The measured photovoltage depends on the illumination intensity and substrate material, and it varies with illumination time and with film thickness. The proposed model predicts that the photovoltage is limited by the built-in potential of the nanosheet–metal junction, that is, the difference of Fermi energies in the two materials. The ability to measure and understand these light-induced charge separation processes in easy-to-fabricate films will promote the development of nanocrystal applications in photoelectrochemical cells, photovoltaics, and photocatalysts.Keywords: Fermi level; Kelvin probe; metal−semiconductor junction; nanocrystal films; photocatalysis; surface photovoltage spectroscopy; water splitting;
Co-reporter:Frank E. Osterloh
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 19) pp:3354-3359
Publication Date(Web):September 8, 2014
DOI:10.1021/jz501740n
The Shockley–Queisser analysis provides a theoretical limit for the maximum energy conversion efficiency of single junction photovoltaic cells. But besides the semiconductor bandgap no other semiconductor properties are considered in the analysis. Here, we show that the maximum conversion efficiency is limited further by the excited state entropy of the semiconductors. The entropy loss can be estimated with the modified Sackur–Tetrode equation as a function of the curvature of the bands, the degeneracy of states near the band edges, the illumination intensity, the temperature, and the band gap. The application of the second law of thermodynamics to semiconductors provides a simple explanation for the observed high performance of group IV, III–V, and II–VI materials with strong covalent bonding and for the lower efficiency of transition metal oxides containing weakly interacting metal d orbitals. The model also predicts efficient energy conversion with quantum confined and molecular structures in the presence of a light harvesting mechanism.Keywords: Disorder; Effective Density of States; Effective Mass; Perovskite Solar Cell; Photosynthesis; Quantum Dot; Quasi Fermi level;
Co-reporter:Frank E. Osterloh
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 15) pp:2510-2511
Publication Date(Web):August 7, 2014
DOI:10.1021/jz501342j
Co-reporter:Frank E. Osterloh
Chemical Society Reviews 2013 vol. 42(Issue 6) pp:2294-2320
Publication Date(Web):16 Oct 2012
DOI:10.1039/C2CS35266D
The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.
Co-reporter:Frank E. Osterloh, Michael A. Holmes, Lilian Chang, Adam J. Moulé, and Jing Zhao
The Journal of Physical Chemistry C 2013 Volume 117(Issue 51) pp:26905-26913
Publication Date(Web):December 2, 2013
DOI:10.1021/jp409262v
Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap (<2.0 eV) and super-band-gap (>2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model.
Co-reporter:Jing Zhao, Michael A. Holmes, and Frank E. Osterloh
ACS Nano 2013 Volume 7(Issue 5) pp:4316
Publication Date(Web):April 16, 2013
DOI:10.1021/nn400826h
The ability to adjust the mechanical, optical, magnetic, electric, and chemical properties of materials via the quantum confinement effect is well-understood. Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor–solution interface. Specifically, it is found that the hydrogen evolution rate from illuminated suspended CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler–Volmer equation for reaction kinetics. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction with semiconductor nanocrystals. Electrochemical data further indicate that proton reduction occurs at cadmium sites on the dots, and that charge separation in these nanocrystals is controlled by surface effects, not by space charge layers.Keywords: Butler−Volmer; photolysis; quantum confinement; quantum dot; solar fuel; space charge layer; water splitting
Co-reporter:Troy K. Townsend, Nigel D. Browning and Frank E. Osterloh
Energy & Environmental Science 2012 vol. 5(Issue 11) pp:9543-9550
Publication Date(Web):20 Aug 2012
DOI:10.1039/C2EE22665K
NiOx (0 < x < 1) modified SrTiO3 (STO) is one of the best studied photocatalysts for overall water splitting under UV light. The established mechanism for this and many other NiOx containing catalysts assumes water oxidation to occur at the early transition metal oxide and water reduction at NiOx. Here we show that NiOx–STO is more likely a three component Ni–STO–NiO catalyst, in which STO absorbs the light, Ni reduces protons, and NiO oxidizes water. This interpretation is based on systematic H2/O2 evolution tests of appropriately varied catalyst compositions using oxidized, chemically and photochemically added nickel and NiO nanoparticle cocatalysts. Surface photovoltage (SPV) measurements reveal that Ni(0) serves as an electron trap (site for water reduction) and that NiO serves as a hole trap (site for water oxidation). Electrochemical measurements show that the overpotential for water oxidation correlates with the NiO content, whereas the water reduction overpotential depends on the Ni content. Photodeposition experiments with NiCl2 and H2PtCl6 on NiO–STO show that electrons are available on the STO surface, not on the NiO particles. Based on photoelectrochemistry, both NiO and Ni particles suppress the Fermi level in STO, but the effect of this shift on catalytic activity is not clear. Overall, the results suggest a revised role of NiO in NiOx–STO and in many other nickel-containing water splitting systems, including NiOx–La:KTaO3, and many layered perovskites.
Co-reporter:Mollie R. Waller, Troy K. Townsend, Jing Zhao, Erwin M. Sabio, Rachel L. Chamousis, Nigel D. Browning, and Frank E. Osterloh
Chemistry of Materials 2012 Volume 24(Issue 4) pp:698
Publication Date(Web):February 16, 2012
DOI:10.1021/cm203293j
Here we investigate the structure, photophysics, and photocatalytic water splitting properties of single-crystalline WO3 nanosheets (0.75 nm × 90 ± 38 nm), obtained by exfoliation from Bi2W2O9. Upon delamination, the nanosheets undergo a structural change from tetragonal symmetry in the parent material to monoclinic, as confirmed by powder X-ray diffraction and electron microscopy. Diffuse reflectance optical spectra show band gap energies consistent with quantum confinement in nano-WO3 (EG = 2.88 eV) and Bi2W2O9(EG = 2.81 eV), relative to bulk WO3 (EG = 2.68 eV). Surface photovoltage measurements on nano-WO3 films on a F:SnO2 substrate demonstrate photochemical carrier formation under band gap excitation and irreversible trapping of holes. Photochemical oxygen formation is observed with 50 mg of the material in aqueous AgNO3 and (NH4)2Ce(NO3)6 solutions under full spectrum (>250 nm) or visible only (>400 nm) irradiation. The highest initial O2 evolution rates (69.7 μmol h–1 for bulk and 35.5 μmol h–1 for nano-WO3) are observed under >250 nm illumination in the presence of 8.3 mM AgNO3(aq). Quantum efficiencies (at 375 nm) reach 1.43% and 1.55% for bulk and nano-WO3, respectively. Electrochemical measurements reveal large water oxidation overpotentials (0.96 V) for both nano- and bulk-WO3. On the basis of photo-onset measurements, the conduction band edges in nano-/bulk-WO3 are at +0.11/+0.23 V, respectively. Overall, the data show that the photoelectrochemical water oxidation ability of WO3 is maintained in 0.75 nm nanocrystal WO3 sheets, although more energetic photons are required because of the extended band gap.Keywords: catalysis; nanocrystal; nanosheet; photocatalysis; quantum confinement; solar energy conversion; tungsten oxide; water oxidation; water splitting; WO3;
Co-reporter:Michael A. Holmes, Troy K. Townsend and Frank E. Osterloh
Chemical Communications 2012 vol. 48(Issue 3) pp:371-373
Publication Date(Web):14 Nov 2011
DOI:10.1039/C1CC16082F
The photocatalytic hydrogen production of CdSe nanocrystals (1.75–4.81 nm) in the presence of aqueous sodium sulphite depends exponentially on the bandgap of the particles, confirming that the material’s activity is controlled by the degree of quantum confinement.
Co-reporter:Rachel L. Chamousis ; Frank E. Osterloh
ChemSusChem 2012 Volume 5( Issue 8) pp:1482-1487
Publication Date(Web):
DOI:10.1002/cssc.201200016
Abstract
A solar-energy-driven biomass fuel cell for the production of electricity from wastewater using only air and light as additional resources is described. The device consists of a photoelectrochemical cell that contains a nanostructured titanium dioxide or tungsten trioxide film as photoanode and a platinum air electrode as cathode, in separate compartments. The TiO2 or WO3 films are fabricated from TiO2 nanocrystals or from sodium tungstate solutions on top of fluorine-doped tin dioxide. Devices were tested with electrolyte only, synthetic wastewater, or with aqueous glucose solution, under irradiation with sunlight, broad spectral illumination, and monochromatic light. Measured light conversion efficiencies were between 0.007 % and 1.7 %, depending on conditions. The highest efficiency (1.7 %) and power output (0.73 mW cm−2) are determined for TiO2 electrodes under 395 nm illumination. In contrast to TiO2, the WO3 electrodes are active under visible light (>440 nm), but the IPCE value is low (2 %). Apart from limited visible-light absorption, the overall performance of the device is limited by the substrate concentration in the water and by transport resistance through the cell.
Co-reporter:Troy K. Townsend, Nigel D. Browning, and Frank E. Osterloh
ACS Nano 2012 Volume 6(Issue 8) pp:7420
Publication Date(Web):July 22, 2012
DOI:10.1021/nn302647u
SrTiO3 (STO) is a large band gap (3.2 eV) semiconductor that catalyzes the overall water splitting reaction under UV light irradiation in the presence of a NiO cocatalyst. As we show here, the reactivity persists in nanoscale particles of the material, although the process is less effective at the nanoscale. To reach these conclusions, Bulk STO, 30 ± 5 nm STO, and 6.5 ± 1 nm STO were synthesized by three different methods, their crystal structures verified with XRD and their morphology observed with HRTEM before and after NiO deposition. In connection with NiO, all samples split water into stoichiometric mixtures of H2 and O2, but the activity is decreasing from 28 μmol H2 g–1 h–1 (bulk STO), to 19.4 μmol H2 g–1 h–1 (30 nm STO), and 3.0 μmol H2 g–1 h–1 (6.5 nm STO). The reasons for this decrease are an increase of the water oxidation overpotential for the smaller particles and reduced light absorption due to a quantum size effect. Overall, these findings establish the first nanoscale titanate photocatalyst for overall water splitting.Keywords: complete water splitting; nanoscale; photocatalyst; photolysis; solar fuel
Co-reporter:Erwin M. Sabio ; Rachel L. Chamousis ; Nigel D. Browning
The Journal of Physical Chemistry C 2012 Volume 116(Issue 4) pp:3161-3170
Publication Date(Web):January 25, 2012
DOI:10.1021/jp209006n
The layered Dion–Jacobson phase KCa2Nb3O10 is known to catalyze photochemical water reduction and oxidation under UV light in the presence of sacrificial agents. The same reactions are catalyzed by tetrabutylammonium hydroxide-supported HCa2Nb3O10 nanosheets obtained by chemical exfoliation of the parent phase. Here we describe a factorial study into the effects of nanoscaling, sacrificial charge donors, cocatalysts, and cocatalyst deposition conditions on the activity of these catalysts. In water, nanoscaling leads to a 16-fold increase in H2 evolution and an 8-fold increase in O2 evolution over the bulk phase under the same conditions. The sacrificial electron donor methanol improves H2 production by 2–3 orders of magnitude to 20–30 mmol of H2/h/g, while the electron acceptor AgNO3 increases O2 production to 400 μmol of O2/h/g. Rates for H2 and O2 evolution further depend on the presence of cocatalysts (Pt or IrOx) and, in the case of H2, inversely on their particle size. To rationalize these findings and the increased activity of the nanoscale particles, we propose a kinetic model for photocatalysis with semiconductor particles. The model calculates the electronic rate of the catalysts as a product of terms for charge generation, charge and mass transport, chemical conversion, and charge recombination. The analysis shows that the activity of the catalysts is limited mainly by the kinetics of the redox reactions and by the rate of charge transport to the water–catalyst interface. Mass transport in the solution phase does not play a major role, and neither does surface charge recombination.
Co-reporter:Troy K. Townsend, Erwin M. Sabio, Nigel D. Browning and Frank E. Osterloh
Energy & Environmental Science 2011 vol. 4(Issue 10) pp:4270-4275
Publication Date(Web):17 Aug 2011
DOI:10.1039/C1EE02110A
Alpha-Fe2O3 is cheap and abundant, and has a visible light indirect (phonon assisted) band gap of 2.06 eV (600 nm) due to a d–d transition, and a direct band gap at 3.3 eV (375 nm), associated with the ligand to metal charge transfer process. Here we describe results on using freely dispersed Fe2O3 nanocrystals for photocatalytic water oxidation. Three morphologies of hematite were compared, including bulk-type-α-Fe2O3 (Bulk-Fe2O3, 120 nm), ultrasonicated Bulk-Fe2O3 (Sonic-Fe2O3, 44 nm), and synthetic Fe2O3 (Nano-Fe2O3, 5.4 nm) obtained by hydrolysis of FeCl3·6H2O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe2O3 also containing traces of β-FeOOH. UV/Vis diffuse reflectance revealed an absorption edge near 600 nm (EG = 2.06 eV) for all materials. Cyclic voltammetry gave the water oxidation overpotentials (versusNHE at pH = 7, at 1.0 mA cm−2) as η = +0.43 V for Nano-Fe2O3, η = +0.63 V for Sonic-Fe2O3, and η = +0.72 V for Bulk-Fe2O3. Under UV and visible irradiation from a 300 W Xe-arc lamp, all three materials (5.6 mg) evolved O2 from water with 20.0 mM aqueous AgNO3 as sacrificial electron acceptor. The highest rates were obtained under UV/Vis (>250 nm) irradiation with 250 μmol h−1 g−1 for Bulk-Fe2O3, 381 μmol h−1 g−1 for Sonic-Fe2O3 and 1072 μmol h−1 g−1 for Nano-Fe2O3. Turnover numbers (TON = moles O2/moles Fe2O3) were above unity for Nano-Fe2O3 (1.13) and Sonic-Fe2O3 (1.10) but not for Bulk-Fe2O3 (0.49), showing that the nanoscale morphology was beneficial for catalytic activity.
Co-reporter:F. Andrew Frame ; Troy K. Townsend ; Rachel L. Chamousis ; Erwin M. Sabio ; Th. Dittrich ; Nigel D. Browning
Journal of the American Chemical Society 2011 Volume 133(Issue 19) pp:7264-7267
Publication Date(Web):April 27, 2011
DOI:10.1021/ja200144w
Rutile IrO2 is known as being among the best electrocatalysts for water oxidation. Here we report on the unexpected photocatalytic water oxidation activity of 1.98 nm ± 0.11 nm succinic acid-stabilized IrO2 nanocrystals. From aqueous persulfate and silver nitrate solution the nonsensitized particles evolve oxygen with initial rates up to 0.96 μmol min–1, and with a quantum efficiency of at least 0.19% (measured at 530 nm). The catalytic process is driven by visible excitations from the Ir-d(t2g) to the Ir-d(eg) band (1.5–2.75 eV) and by ultraviolet excitations from the O-p band to the Ir-d(eg) (>3.0 eV) band. The formation of the photogenerated charge carriers can be directly observed with surface photovoltage spectroscopy. The results shed new light on the role of IrO2 in dye- and semiconductor-sensitized water splitting systems.
Co-reporter:Troy K. Townsend;Erwin M. Sabio; Nigel D. Browning; Frank E. Osterloh
ChemSusChem 2011 Volume 4( Issue 2) pp:185-190
Publication Date(Web):
DOI:10.1002/cssc.201000377
Abstract
Layered K4Nb6O17 is a known UV-light-driven photocatalyst for overall water splitting, with a band gap of 3.5 eV. Following ion exchange and exfoliation with tetrabutylammonium hydroxide, the layered material separates into nanosheets that coil into 1.0±0.5 μm long and 10±5 nm wide nanoscrolls to reduce their surface energy. Pt and IrOx (x=1.5–2) nanoparticles were photochemically deposited onto the surface of the nanoscrolls to produce two- and three-component photocatalysts. Under UV irradiation, the nanostructures produced H2 from pure water and aqueous methanol, with turnover numbers ranging from 2.3 and 18.5 over a 5 h period. The activity of the catalysts for H2 evolution can be directly correlated with the varying overpotentials for water reduction (210–325 mV). From water, no oxygen is evolved. Instead, the formation of surface-bound peroxides in a 1:1 stoichiometry with H2 is observed. Slow photochemical oxygen evolution can be achieved with the sacrificial electron acceptor AgNO3, and under an electrochemical bias. The electrochemical water oxidation overpotentials are ca. 600 mV across the series of scrolls. From the photo onset potential the conduction band edge for the unmodified scrolls is estimated as −0.75 V at pH 7. Deposition of a co-catalyst is found to depress this value by 58 mV (IrOx), 148 mV (Pt/IrOx), and 242 mV (Pt). However, because water oxidation remains rate-limiting, this does not affect the overall performance of the catalysts.
Co-reporter:Mark R. Allen, Arthur Thibert, Erwin M. Sabio, Nigel D. Browning, Delmar S. Larsen and Frank E. Osterloh
Chemistry of Materials 2010 Volume 22(Issue 3) pp:1220
Publication Date(Web):November 5, 2009
DOI:10.1021/cm902695r
K2Ti4O9 has been known as a photocatalyst for the oxidation of methanol under UV irradiation. Here we study the evolution of morphology, optical, and photocatalytic properties of this titanate as it is converted into H2Ti4O9 and subsequently exfoliated into individual tetrabutylammonium (TBA)-supported [Ti4O9]2- nanosheets. We find that proton exchange and exfoliation are accompanied by a red shift of the optical absorption edge and fluorescence maximum, suggesting a reduction of the bandgap in the series K2Ti4O9 (3.54 eV), H2Ti4O9 (3.25 eV), TBA2Ti4O9 (3.00 eV). Neither compound is active for photochemical water splitting, even after photochemical deposition of platinum nanoparticles. However, in aqueous methanol, all platinated compounds are moderately active for H2 evolution upon bandgap irradiation, and in 0.01 M AgNO3, they all produce moderate quantities of O2. From the onset potentials for photoelectrochemical methanol oxidation, the values for the valence band edges at pH = 7 are deduced to lie between −0.23 and −0.53 V (NHE) for the nonplatinated compounds, and at +0.08 V and −0.30 V for the platinated compounds. This Pt-induced decrease of negative charge on the titanates is likely due to Fermi level equilibration of metal and semiconductor. Its effect can also be seen in a shift of the onset potentials for electrochemical water oxidation, as measured by cyclic voltammetry. Transient absorption data reveal that photogenerated electrons become trapped in mid band gap states, from which they decay exponentially with a time-constant of 43.67 ± 0.28 ms, much slower than observed for 68 ± 1 ns for TiO2 nanocrystals (Degussa, P25).
Co-reporter:Han Zhou, Erwin M. Sabio, Troy K. Townsend, Tongxiang Fan, Di Zhang and Frank E. Osterloh
Chemistry of Materials 2010 Volume 22(Issue 11) pp:3362
Publication Date(Web):May 11, 2010
DOI:10.1021/cm903839t
We present a layer-by-layer assembly approach for the construction of core−shell structures with photocatalytic activity for hydrogen evolution from aqueous methanol. Submicrometer silica spheres and ultrasonicated (TBA, H)Ca2Nb3O10 and PA2K2Nb6O17 nanosheets are used as the building blocks to assemble core−shell structures with single and double (homostacked and heterostacked) nanosheet layers via sequential electrostatic coupling with poly(diallyldimethylammonium) chloride (PDDA). The lateral nanosheet distribution on the SiO2 spheres is observed with SEM while the stacking is directly observed with TEM. Diffuse reflectance UV/vis spectra reveal the nanosheet absorbance edge at ∼350 nm. All core−shell structures are active for photocatalytic H2 evolution from aqueous methanol solution with gas evolution rates comparable or smaller than observed for individually dispersed nanosheets. Heterostacks were more active than homostacks, with the latter being comparable to single layers (at equal mass). Loading with Pt nanoparticles increases H2 evolution rates, but reduces the activity differences between homostacked and heterostacked samples.
Co-reporter:Erwin M. Sabio, Miaofang Chi, Nigel D. Browning and Frank E. Osterloh
Langmuir 2010 Volume 26(Issue 10) pp:7254-7261
Publication Date(Web):January 4, 2010
DOI:10.1021/la904377f
Photolabeling was employed to probe charge separation and the distribution of redox-active sites on the surface of nanosheets derived from the layered photocatalysts KCa2Nb3O10. Electron microscopy reveals 1−50 nm particles of silver, gold, iridium oxide, and manganese dioxide particles and small atomically sized clusters of platinum and IrOx on the nanosheet surfaces and along the edges. The sizes, shapes, and particle densities vary with the deposition conditions, i.e., the precursor concentration and the presence of sacrificial agents. Overall, the study shows that photogenerated electrons and holes are accessible throughout the nanosheets, without evidence for spatial charge separation across the sheet.
Co-reporter:Frank E. Osterloh
Chemistry of Materials 2008 Volume 20(Issue 1) pp:35
Publication Date(Web):December 11, 2007
DOI:10.1021/cm7024203
Photochemical splitting of water into H2 and O2 using solar energy is a process of great economic and environmental interest. Since the discovery of the first water splitting system based on TiO2 and Pt in 1972 by Fujishima and Honda, over 130 inorganic materials have been discovered as catalysts for this reaction. This review discusses the known inorganic catalysts with a focus on structure–activity relationships.
Co-reporter:F. Andrew Frame, Elizabeth C. Carroll, Delmar S. Larsen, Michael Sarahan, Nigel D. Browning and Frank E. Osterloh
Chemical Communications 2008 (Issue 19) pp:2206-2208
Publication Date(Web):27 Mar 2008
DOI:10.1039/B718796C
CdSe
nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na2S/Na2SO3 solution under irradiation with ultraviolet and visible light.
Co-reporter:Michael C. Sarahan, Elizabeth C. Carroll, Mark Allen, Delmar S. Larsen, Nigel D. Browning, Frank E. Osterloh
Journal of Solid State Chemistry 2008 Volume 181(Issue 7) pp:1678-1683
Publication Date(Web):July 2008
DOI:10.1016/j.jssc.2008.06.021
The layered hexaniobate K4Nb6O17 is known as a photocatalyst for methanol dehydrogenation and hydrogen evolution from water under ultraviolet (UV) light. Here we show that the activity is retained in propylammonium- (PA) or tetrabutylammonium- (TBA) stabilized H2K2Nb6O17 nanosheets and TBA-stabilized H4Nb6O17 nanoscrolls that can be obtained by exfoliation of K4Nb6O17 followed by cation exchange. The catalytic activity of the exfoliated systems is comparable to K4Nb6O17, with scrolls being most active in water, and PA sheets giving enhanced H2 rates due to sacrificial electron donor action of PA. Femtosecond absorption spectra for TBA scrolls and PA sheets exhibit broad features between 450 and 700 nm due to trapped holes and electrons. Electron–hole recombination follows approximately second-order kinetics, with rates of decay similar for sheets and scrolls. In addition, catalysts were characterized with UV/vis and fluorescence spectroscopy and transmission electron microscopy.Nanosheets and nanoscrolls composed of individual niobate layers from K4Nb6O17 are active for hydrogen generation from water and aqueous methanol under UV irradiation.
Co-reporter:Mark Allen, Erwin M. Sabio, Xiubin Qi, Bokuba Nwengela, M. Saif Islam and Frank E. Osterloh
Langmuir 2008 Volume 24(Issue 13) pp:7031-7037
Publication Date(Web):May 29, 2008
DOI:10.1021/la8004085
LiMo3Se3 nanowire film sensors were fabricated by drop-coating a 0.05% (mass) aqueous nanowire solution onto microfabricated indium tin oxide electrode pairs. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM), the films are made of a dense network of 3−7 nm thick nanowire bundles. Immersion of the films in 1.0 M aqueous solutions of group 1 or 2 element halides or of Zn(II), Mn(II), Fe(II), or Co(II) chlorides results in an increase of the electrical resistance of the films. The resistance change is always positive and reaches up to 9% of the base resistance of the films. It occurs over the course of 30−240 s, and it is reversible for monovalent ions and partially reversible for divalent ions. The signal depends on the concentration of the electrolyte and on the size and charge of the metal cation. Anions do not play a significant role, presumably, because they are repelled by the negatively charged nanowire strands. The magnitude of the electrical response and its sign suggest that it is due to analyte-induced scattering of conduction electrons in the nanowires. An ion-induced field effect can be excluded based on gated conductance measurements of the nanowire films.
Co-reporter:Nidhal N. Akl;Olga Trofymluk;Xiubin Qi;Jin Y. Kim ;Alexra Navrotsky
Angewandte Chemie International Edition 2006 Volume 45(Issue 22) pp:
Publication Date(Web):26 APR 2006
DOI:10.1002/anie.200503950
Giant holes: Macroporous solids with high electrical conductivity and adjustable pore sizes can be synthesized by the covalent cross-linking of LiMo3Se3 nanowires with citrate-stabilized Ag nanoparticles (see picture, cit=citrate). The ease of synthesis and the possibilities for functionalization with redox-active groups make these xerogels potentially useful in applications such as batteries, capacitors, and electrochemical catalysts.
Co-reporter:Nidhal N. Akl;Olga Trofymluk;Xiubin Qi;Jin Y. Kim ;Alexra Navrotsky
Angewandte Chemie 2006 Volume 118(Issue 22) pp:
Publication Date(Web):26 APR 2006
DOI:10.1002/ange.200503950
Riesenlöcher: Makroporöse Feststoffe mit hoher elektrischer Leitfähigkeit und variablen Porengrößen sind durch kovalentes Verknüpfen von LiMo3Se3-Nanodrähten mit Citrat-stabilisierten Ag-Nanopartikeln zugänglich (siehe Bild, cit=citrat). Die einfache Synthese und die Möglichkeit, redoxaktive Gruppen einzuführen, machen diese Xerogele für Anwendungen wie Batterien, Kondensatoren und elektrochemische Katalysatoren interessant.
Co-reporter:Frank E. Osterloh and Daniel P. Hewitt
Chemical Communications 2003 (Issue 14) pp:1700-1701
Publication Date(Web):12 Jun 2003
DOI:10.1039/B302266H
The cluster salt [N(C2H5)4]4[Cd10S4Br4(Sp-Tol)12] reacts with six equivalents of sulfur in dimethylformamide at 140 °C to produce polycrystalline (wurtzite type) CdS nanoparticles of 5.8 nm mean diameter, which on their surface are ligated with p-tolylthiolate ligands, dimethylformamide and water; UV/vis, PL, 1H- and 113Cd-NMR spectra recorded during various stages of the synthesis indicate that the CdS nanoparticles are formed by a cluster condensation process.
Co-reporter:Timothy L. Shelton, Nicholas Harvey, Jiarui Wang, Frank E. Osterloh
Applied Catalysis A: General (5 July 2016) Volume 521() pp:
Publication Date(Web):5 July 2016
DOI:10.1016/j.apcata.2015.11.041
•Surface photovoltage spectroscopy (SPS) observes photochemical oxidation reactions on Fe2O3 nanorod films without applied bias.•Photovoltages of up to −130 mV are generated when photoholes are trapped in surface states and react with water.•Reactions with sacrificial electron donors increase the voltage up to −400 mV.•Oxygen increases the photovoltage, suggesting it can promote water oxidation by increasing the concentration of surface holes.Surface photovoltage spectroscopy (SPS) was used to observe photochemical charge separation and oxidation reactions on Fe2O3 nanorod arrays under zero applied bias. Nanorod films were grown from FeCl3 under hydrothermal conditions followed by calcination at 550 °C. A negative photovoltage of up −130 mV is observed under 2.0–4.5 eV (0.1 mW cm−2) illumination, confirming 2.0 eV as the effective bandgap of the material, and electrons as majority carriers. SPS in the presence of air, nitrogen, water, oxygen, and under vacuum suggest that the photovoltage is associated with the oxidation of surface water and with reversible surface hole trapping on the 1 min time scale and de-trapping on the 1 h time scale. O2 promotes water oxidation by increasing the concentration of surface holes. Sacrificial donors KI, H2O2 or potassium hydroxide increase the voltage to −240 and −400 mV, due to improved hole transfer. Cobalt oxide and Co-Pi cocatalysts quench the voltage, which is tentatively attributed to the removal of surface states and enhanced e/h recombination. An energy diagram is used to relate the experimental photovoltage to the built-in potentials at the respective interfaces.
Co-reporter:Po Wu, Jiarui Wang, Jing Zhao, Liejin Guo and Frank E. Osterloh
Journal of Materials Chemistry A 2014 - vol. 2(Issue 47) pp:NaN20344-20344
Publication Date(Web):2014/10/16
DOI:10.1039/C4TA04100C
Graphitic carbon nitride (g-C3N4) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C3N4 is limited by structure defects generated during the calcination process. Specifically we find that the photocatalytic hydrogen production rate from aqueous methanol is inversely related to the calcination temperature (520–640 °C). The highest activity of 0.301 mmol h−1 g−1 is observed for the sample prepared at the lowest processing temperature. Surface photovoltage (SPV) spectroscopy shows that the maximum photovoltage is reduced (from 1.29 V to 0.62 V) as the processing temperature is increased, in accordance with higher defect concentrations and faster electron–hole recombination. The defects also produce additional optical absorption in the visible spectra and cause a red shifted, weakened photoluminescence (PL). Based on the sub-gap signal in the SPV and PL spectra, defect energy levels are +0.97 V and −0.38 V (vs. NHE) in the band gap of the material. According to Fourier transform infrared (FTIR) spectra, the defects are due to amino/imino groups in the g-C3N4 lattice.
Co-reporter:Frank E. Osterloh
Chemical Society Reviews 2013 - vol. 42(Issue 6) pp:NaN2320-2320
Publication Date(Web):2012/10/16
DOI:10.1039/C2CS35266D
The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.
Co-reporter:F. Andrew Frame, Elizabeth C. Carroll, Delmar S. Larsen, Michael Sarahan, Nigel D. Browning and Frank E. Osterloh
Chemical Communications 2008(Issue 19) pp:NaN2208-2208
Publication Date(Web):2008/03/27
DOI:10.1039/B718796C
CdSe
nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na2S/Na2SO3 solution under irradiation with ultraviolet and visible light.
Co-reporter:Michael A. Holmes, Troy K. Townsend and Frank E. Osterloh
Chemical Communications 2012 - vol. 48(Issue 3) pp:NaN373-373
Publication Date(Web):2011/11/14
DOI:10.1039/C1CC16082F
The photocatalytic hydrogen production of CdSe nanocrystals (1.75–4.81 nm) in the presence of aqueous sodium sulphite depends exponentially on the bandgap of the particles, confirming that the material’s activity is controlled by the degree of quantum confinement.
Co-reporter:Po Wu, Jiarui Wang, Jing Zhao, Liejin Guo and Frank E. Osterloh
Chemical Communications 2014 - vol. 50(Issue 98) pp:NaN15524-15524
Publication Date(Web):2014/10/20
DOI:10.1039/C4CC08063G
A high rate of 2.23 mmol h−1 g−1 (quantum efficiency of 6.67% at 400 nm) for visible light driven photocatalytic H2 evolution can be achieved with g-C3N4 by alkalization of the solution to a pH of 13.3, due to accelerated transfer of photoholes to the sacrificial donor.
Co-reporter:Rachel L. Chamousis, Lilian Chang, William J. Watterson, Rick D. Montgomery, Richard P. Taylor, Adam J. Moule, Sean E. Shaheen, Boaz Ilan, Jao van de Lagemaat and Frank E. Osterloh
Journal of Materials Chemistry A 2014 - vol. 2(Issue 39) pp:NaN16616-16616
Publication Date(Web):2014/08/21
DOI:10.1039/C4TA03204G
Living organisms use fractal structures to optimize material and energy transport across regions of differing size scales. Here we test the effect of fractal silver electrodes on light distribution and charge collection in organic semiconducting polymer films made of P3HT and PCBM. The semiconducting polymers were deposited onto electrochemically grown fractal silver structures (5000 nm × 500 nm; fractal dimension of 1.71) with PEDOT:PSS as hole-selective interlayer. The fractal silver electrodes appear black due to increased horizontal light scattering, which is shown to improve light absorption in the polymer. According to surface photovoltage spectroscopy, fractal silver electrodes outperform the flat electrodes when the BHJ film thickness is large (>400 nm, 0.4 V photovoltage). Photocurrents of up to 200 microamperes cm−2 are generated from the bulk heterojunction (BHJ) photoelectrodes under 435 nm LED (10–20 mW cm−2) illumination in acetonitrile solution containing 0.005 M ferrocenium hexafluorophosphate as the electron acceptor. The low IPCE values (0.3–0.7%) are due to slow electron transfer to ferrocenium ion and due to shunting along the large metal–polymer interface. Overall, this work provides an initial assessment of the potential of fractal electrodes for organic photovoltaic cells.
Co-reporter:Geetu Sharma, Zeqiong Zhao, Pranab Sarker, Benjamin A. Nail, Jiarui Wang, Muhammad N. Huda and Frank E. Osterloh
Journal of Materials Chemistry A 2016 - vol. 4(Issue 8) pp:NaN2942-2942
Publication Date(Web):2015/12/29
DOI:10.1039/C5TA07040F
As a visible light active p-type semiconductor, CuBi2O4 is of interest as a photocatalyst for the generation of hydrogen fuel from water. Here we present the first photovoltage and photocatalytic measurements on this material and DFT results on its band structure. Single crystalline CuBi2O4 nanoparticles (25.7 ± 4.7 nm) were synthesized from bismuth and cupric nitrate in water under hydrothermal conditions. Powder X-ray diffraction (XRD) confirms the CuBi2O4 structure type and UV-Vis spectroscopy shows a 1.75 eV optical band gap. Surface photovoltage (SPV) measurements on CuBi2O4 nanoparticle films on fluorine doped tin oxide yield 0.225 V positive photovoltage at >1.75 eV photon energy confirming holes as majority carriers. The photovoltage is reversible and limited by light absorption. When dispersed in 0.075 M aqueous potassium iodide solution, the CuBi2O4 particles support photochemical hydrogen evolution of up to 16 μmol h−1 under ultraviolet but not under visible light. Based on electrochemical scans, CuBi2O4 is unstable toward reduction at −0.2 V, but a pH-dependent photocurrent of 6.45 μA cm−2 with an onset potential of +0.75 V vs. NHE can be obtained with 0.01 M Na2S2O8 as a sacrificial electron acceptor. The photoelectrochemical properties of CuBi2O4 can be explained on the basis of the band structure of the material. DFT calculations show that the valence and conduction band edges arise primarily from the combination of O 2p and Cu 3d orbitals, respectively, with additional contributions from Cu 3d and Bi 6s orbitals just below the Fermi level. Trapping of photoelectrons in the Cu 3d band is the cause for reductive photocorrosion of the material, while the p-type conductivity arises from copper vacancy states near the VB edge. These findings provide an improved understanding of the photophysical properties of p-CuBi2O4 and its limitations as a proton reduction photocatalyst.
Co-reporter:Jiarui Wang and Frank E. Osterloh
Journal of Materials Chemistry A 2014 - vol. 2(Issue 24) pp:NaN9411-9411
Publication Date(Web):2014/04/29
DOI:10.1039/C4TA01654H
Chemical modification of BiVO4 nanoparticles (Scheelite, EG = 2.62 eV) with chemically deposited Co3O4 nanoparticles improves the photocatalytic water oxidation activity by a factor of 17 to 11 mmol g−1 h−1 under visible light (AQE 10% at 435 nm) and to 1.24 mmol g−1 h−1 under sunlight from aqueous 0.02 M NaIO4. This activity ranks among the highest among known visible light driven water oxidation photocatalysts. Based on systematic electrochemical, photoelectrochemical, and surface photovoltage measurements, the high photocatalytic activity can be attributed to the electrocatalytic properties of the Co3O4 cocatalyst and to the formation of a heterojunction at the BiVO4–Co3O4 interface.
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