Materials and mechanisms of artificial photosynthesis
Yoshiyasu Matsumoto1 and Akihiko Kudo2
1
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
2
Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-1861,
Japan
The demand for developing clean energy systems has continued to rise. One of attractive means
to match the demand is water splitting with sunlight: solar energy is converted in the chemical
forms, hydrogen and oxygen, out of water. This uphill reaction is similar to photosynthesis by
green plants; thus this can be regarded as artificial photosynthesis.
Since the discovery of the Honda-Fujishima effect of water splitting[1] using a titanium dioxide
(TiO2) electrode in the early 1970s, numerous studies have been conducted and are still growing
on photocatalytic reactions of TiO2. Titanium dioxide serves as a model case for the mechanistic
studies of photocatalysis, but it has a serious drawback as a photocatalyst for water splitting: the
band gap is too large for absorbing visible photons in sunlight. Obviously it is necessary to
synthesize photocatalysts workable with sunlight. One of the authors (AK) has synthesized and
tested various heterogeneous photocatalyst materials for water splitting, including metal oxides,
metal (oxy)sulfides, and metal (oxy)nitrides.[2] Although these new sets of photocatalysts are
promising, their performances have to be further improved for practical use. Obstacles for
improving the efficiency of water splitting with those materials stem from lack of
understanding of the mechanisms for the photo-induced heterogeneous reactions, including
charge splitting dynamics, active sites for redox reactions, reaction intermediates, etc. We need
to explore the fundamentals of photocatalytic reactions with those materials to meet our goal. In
this talk, we describe the current status of those photocatalyst materials and preliminary results
of charge dynamics probed by transient absorption.
[1] A. Fujishima and K. Honda, Nature, 238, 37 (1972).
[2] A. Kudo and Y. Miseki, Chem. Soc. Rev., 38, 253-278 (2009).
EXCEEDING THE LIMIT IN SOLAR ENERGY CONVERSION
Xiaoyang Zhu
University of Texas at Austin
The Shockley-Queisser (SQ) limit, i.e., the maximum power conversion efficiency
of a conventional solar cell, results mainly from the loss of excess photon energy above
a semiconductor band gap to the heat bath. The first approach to exceed this
fundamental limit is to convert part of the excess photon energy to multiple electron-hole
pairs in a process called exciton fission or multiple exciton generation (MEG). I will
illustrate the harvesting of MEG via direct multi-electron transfer from the illusive “dark”
multiexciton state. The second approach is to harvest the hot electrons/holes before
they loss their excess energy. This must be a dynamic process occurring on ultrafast
time scales to compete with hot carrier cooling. I will discuss challenges in implementing
both approaches in solar cells with efficiency exceeding the SQ limit.
Water splitting under visible light using modified oxynitride particles
Kazuhiko Maeda
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
PRESTO/JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
E-mail: [email protected], Tel: +81-3-5841-1652, Fax: +81-3-5841-8838
Hydrogen production from water using heterogeneous photocatalysts is one of the attractive
candidates to realize a clean and sustainable energy system based on solar energy. Recently, several
types of photocatalytic systems, which work especially under visible light (λ > 400 nm), have been
developed.1 As schematically illustrated in Figure 1, they are mainly divided into two approaches;
one is developing new visible-light-responsive photocatalytic materials with an enough potential to
achieve overall water splitting, and the other one is applying a two-step excitation mechanism,
so-called Z-scheme system, based on two different photocatalysts.
Oxynitrides are attractive candidates as water-splitting photocatalysts, as they can harvest a wide
range of visible photons.1a So far, we have achieved overall water splitting using oxynitrides that
consist of d10 typical metals (e.g., (Ga1–xZnx)(N1–xOx)).2 The available wavelengths of light for these
d10-oxynitrides are up to about 500 nm at present.
On the other hand, d0 transition metal-based (oxy)nitrides such as CaNbO2N and Ta3N5 have
been proved to have enough potential for overall water splitting, and they are actually stable enough
to evolve H2 and O2 under visible light up to 600 nm with proper electron donors and acceptors.1a
Some of them have been successfully applied for a Z-scheme water splitting system, with an
apparent quantum yield of greater than 6% at 420 nm.3
References
1.
(a) K. Maeda, K. Domen, J. Phys.
C.B.
e–
e–
Chem. C 2007, 111, 7851. (b) A.
h
Kudo, Y. Miseki, Chem. Soc. Rev.
(a) K. Maeda et al., J. Am. Chem.
Soc. 2005, 127, 8286. (b) K.
Maeda et al., Nature 2006, 440,
H2
H+
e–
Red
e–
H2O
+0.82
(O2/H2O)
e–
O2
V.B.
h+
Ox
V.B.
e–
O2
V.B.
h+
h+
H2 evolution
photocatalyst
O2 evolution
photocatalyst
K. Maeda et al., J. Am. Chem. Soc.
2010, 132, 5858.
H2
e–
C.B.
h
295.
3.
e–
(Ox/Red)
H2O
e–
C.B.
h
(H+/H2)
-0.41
H+
2009, 38, 253.
2.
Potential
(V vs. NHE)
pH 7
One-step photoexcitation
Two-step photoexcitation
Figure 1. Schematic energy diagrams of photocatalytic water splitting
for a one-step or two-step photoexcitation system.
"Photochemistry and Photodesorption from oxide supported metal
nanoparticles"
Hans‐Joachim Freund Fritz‐Haber‐Institut der Max‐Planck‐Gesellschaft Department of Chemical Physics Faradayweg 4‐6, D‐14195 Berlin, Germany freund@fhi‐berlin.mpg.de The group has been interested in photoinduced surface‐molecule bond breaking since the mid eighties of the last century. Fully energy (kinetic, rotational, vibrational) quantum state resolved desorption of molecules (NO, CO) from oxide surfaces as well as from oxide supported metal (Ag) nanoparticles have been studied using REMPI techniques and pulsed lasers (from nanoseconds to femtoseconds) as light sources for description. Photodesorption and Photochemistry were studied as functions of laser frequency covering a range that allowed to induce plasmon excitations as a function of particle sizes. Here we utilized knowledge drawn from investigations with the Photon‐STM, which was used to explore the dependence of plasmon energies as a function of particle size, alloying with Au and the support of the particles as well as its stoichiometry. An overview including very recent results will be presented. Quantum Dots – Artificial Atoms, Large Molecules or Small Pieces of Bulk?
Insights from Time-Domain Ab Initio Studies.
Oleg Prezhdo
University of Rochester
Quantum dots (QD) are quasi-zero dimensional structures with a unique
combination of solid-state and atom-like properties. Unlike either bulk or atomic
materials, QD properties can be modified continuously by changing QD shape and size.
Often, the bulk and atomic viewpoints contradict each other, leading to differing
predictions about the behavior of QDs. For example, the atomic view suggests strong
electron-hole and charge-phonon interactions, as well as slow energy relaxation due to
mismatch between electronic energy gaps and phonon frequencies. In contrast, the bulk
view advocates that the kinetic energy of quantum confinement is greater than
electron-hole interactions, that charge-phonon coupling is weak, and that the relaxation
through quasi-continuous bands is rapid.
QDs exhibit new physical phenomena. In particular, the so-called phonon
bottleneck to the electron and hole energy relaxation and generation of multiple excitons
upon absorption of a single photon can be used to improve efficiencies of photovoltaic
devices. Slowing down of the energy relaxation can result in extraction of hot electrons
and holes, thereby increasing the solar cell voltage. Generation of multiple electron-hole
pairs can increase the current.
Our state-of-the-art non-adiabatic molecular dynamics techniques, implemented
within time-dependent density functional theory, allow us to model the response of QDs
at the atomistic level and in real time. The studies provide a unifying description of
quantum dynamics in nanoscale materials, resolve the debated issues, and generate
theoretical guidelines for development of novel systems for solar energy harvesting and
other applications.
1.
C. F. Craig, W. R. Duncan, O. V. Prezhdo “Trajectory surface hopping in the time-dependent Kohn-Sham
theory for electron-nuclear dynamics”, Phys. Rev. Lett., 95 163001 (2005).
2.
O. V. Prezhdo “Photoinduced dynamics in semiconductor quantum-dots: insights from time-domain ab
initio studies”, Acc. Chem. Res., 42, 2005 (2009).
3.
S. A. Fischer, C. M. Isborn, O. V. Prezhdo, “Excited states and optical absorption of small semiconducting
clusters: dopants, defects and charging”, Chem. Science, 2, 400 (2011).
Crystal plane-dependent surface physical chemistry and catalysis of oxide
nanocatalysts
Weixin Huang
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical
Physics, University of Science and Technology of China, Hefei, 230026, China
* E-mail address: [email protected]
Oxides have wide applications in heterogeneous catalysis, either as catalysts or as supports. Crystal
planes exposed on the surface of an oxide nanocrystal decide the surface composition and geometric
structure and thus greatly influences its surface reactivity and catalytic performances. Crystal planes
exposed on the surface of an oxide nanocrystal are determined by its shape. Recently the controlled
synthesis of oxide nanocrystals with a uniform shape has achieved great progresses, and accordingly, a
novel concept is now being developed to tune the catalytic performance of oxide nanocrystals by
controlling their shapes [1]. Successful examples include shape-dependent catalytic performances of
CeO2 nanocrystals [2], Au/CeO2 nanocatalysts [3], and Co3O4 nanocrystals [4]. In this presentation,
employing cuprous oxide nanocrystals with different uniform shapes, we will comprehensively
demonstrate the crystal plane-dependent surface physical chemistry and catalysis of oxide
nanocatalysts, including reducibility, oxidation ability, and catalytic performance in CO oxidation and
partial oxidation of propene [5,6].
References
1. J. A. van Bokhoven, ChemCatChem 1, 363 (2009).
2. K. Zhou, X. Wang, X. Sun, Q. Peng, Y. Li, J. Catal. 229, 206 (2005).
3. R. Si, M. Flytzani-Stephanopoulos, Angew. Chem. Int. Ed. 47, 2884 (2008).
4. X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta, W. J. Shen, Nature 458, 746 (2009).
5. H. Bao, W. Zhang, D. Shang, Q. Hua, Y. Ma, Z. Jiang, J. Yang, W. Huang, J. Phys. Chem. C 114, 6676
(2010).
6. Q. Hua, D. Shang, W. Zhang, K. Chen, S. Chang, Y. Ma, Z. Jiang, J. Yang, W. Huang, Langmuir 27,
665 (2011).
Adsorption of molecules on TiO2: insights from density functional theory
with van der Waals corrections
Wissam Alsaidi
TiO2 based devices have many important applications in catalysis and photocatalysis.
For solar energy applications, sensitization of TiO2 nanoparticles with appropriately
chosen dyes can lead to significant red shifts of the absorption spectrum thus improving
their efficiency for solar conversion. Continuous improvements of these devices
necessitates a fundamental understanding of the interactions between the molecular
adsorbates and TiO2 surfaces. Local and semi-local density functional theory (DFT)
approaches, which are the standard methods for investigating large scale systems, fail
to describe van der Waals interactions (vdW). Although, vdW interactions are weak
compared to covalent or ionic bonding, these ubiquitous forces play a prominent role in
several molecular and solid structures, in particular the phys-adsorption of large
organic molecular systems (such as dyes) on TiO2 surfaces. Recently, there has been a
surge of interest in correcting standard DFT methods for the missing vdW interactions,
and have led to several promising approaches in terms of accuracy and practicality. I
will review state-of-the-art methods for including vdW interactions in standard DFT
methods. I will then describe our study with vdW-corrected DFT methods of the
interaction of CO2 with wet and dry rutile (110) and anatase (101). For the adsorption
of CO2 on oxidized and defective dry rutile (110), vdW-corrected DFT results are in
excellent agreement with experiment, and show that CO2 adopts both tilted and flat
configurations on the five-fold coordinated Ti sites, with no appreciable charge transfer
with the TiO2 surface. Preliminary results of the interaction between large acenes and
rutile (110) will also be presented.
Sum-frequency spectroscopy studies on surface photo-reactions
Weitao Liu
Sum-frequency spectroscopy studies on surface photo-reactions
Sum-frequency spectroscopy is a highly sensitive surface probing technique that has
many unique features. In the talk I'll describe our study on the photo-reaction of
surfaces of poly(vinyl cinnamate) (PVCi). PVCi and its derivatives form a well-known
class of photopolymers. It is known that uv irradiation of PVCi and derivatives induces
surface structural anisotropy that can align liquid crystal (LC) fi lms, but there is a long
standing controversy on whether dimerization or isomerization of the cinnamoyl side
chains is the dominant photo-reaction process. Using sum-frequency spectroscopy
technique, we obtained vibrational spectra directly related to the molecular structure of
the polymer surface, and showed that polarized uv irradiation dimerizes rather than
isomerizes the protruding cinnamoyl side chains at the surfaces, creating signi cant
surface structural anisotropy needed in many applications.
Lesson from the Action of Individual Molecules
- Single Molecular Spectroscopy at Surfaces Maki Kawai
Executive Director, RIKEN
and
Department of Advanced Materials Science, University of Tokyo
Ultimate spatial resolution of scanning tunneling microscope (STM) enables us
to observe the inner electronic, vibrational [1-5] and spin [6] structures of a molecule
adsorbed on solid surfaces. Vibrational spectrum of a single molecule provides useful
information not only for the chemical identification of the molecule [1] but also for
investigating how molecular vibrations can couple with the relevant dynamical
processes [2, 3]. The response of vibrationally mediated molecular motion to applied
bias voltage, namely an “action spectrum”, can reveal vibrational modes that are excited
through STM inelastic tunneling processes, because the molecular motion is induced
only via the inelastic tunneling processes [4]. Thus, the action spectrum would be a
candidate for detecting which vibrational mode is actually excited and associated with
molecular motions. The mechanism to excite vibrational modes of molecules is revealed
to be a resonant mechanism [5]. A theoretical analysis of the action spectrum even
enabled us to learn about the assorted excitation relevant for the reaction to occur [7].
Life-time of the vibrational excited state was found to lengthen by inserting insulator
thin film of MgO, decoupling the adsorbate and metal, enabled to split water molecule
by exciting the OH stretching mode [8].
References :
[1] Y. Kim, T. Komeda, and M. Kawai, Phys. Rev. Lett. 89 (2002) 126104. S. Katano, M. Trenary, Y.
Kim and M. Kawai, Science 316 (2007) 1883.
[2] T. Komeda, Y. Kim, M. Kawai, et al., Science 295 (2002) 2055.
[3] T. Okada, Y. Kim and M. Kawai, submitted for publication (2011).
[4] Y. Sainoo, Y. Kim, T. Okawa, et al., Phys. Rev. Lett. 95 (2005) 246102.
[5] M. Ohara, Y. Kim and M. Kawai, Phys. Rev. Lett. 100 (2008) 136104.
[6] N. Tsukahara, K. Noto, M. Ohara, S. Shiraki, N. Takagi, Y. Takata, J. Miyawaki, M. Taguchi, A.
Chainani, S. Shin and M. Kawai, Phys. Rev. Lett. 102 (2009) 167203.
N. Tsukahara, S. Shiraki, S. Itou, N. Ohta, N. Takagi, and M. Kawai, Phys. Rev. Lett. 106 (2011)
187201.
[7] K. Motobayashi, Y. Kim, H. Ueba and M. Kawai, Phys. Rev. Lett. 105 (2010) 076101.
[8] H-J. Shin, J. Jung, K. Motobayashi, S. Yanagisawa, Y. Morikawa, Y. Kim and M. Kawai, Nature
Materials 9 (2010) 442-447.