1D nanofiber composites of perylene diimides for visible-light-driven hydrogen evolution from water

Article abstract of DOI:10.1039/c4ra09258a

A series of novel nanocomposite structures have been fabricated by in situ deposition of TiO₂ layers and/or a co-catalyst (Pt) on one-dimensional (1D) self-assembled nanofibers of perylene diimide derivatives (PDIs). The PDI molecules were functionalized with dodecyl and/or phenylamino groups to compare the effect of nanofiber morphology and intramolecular charge transfer on the photocatalytic performance. Under visible-light irradiation (λ > 420 nm), hydrogen production for all composite systems has been detected through photocatalytic water splitting in aqueous solutions with sacrificial reagent methanol or triethanolamine, proving the applicability of organic nanofibers in the photocatalytic system. Compared to the well-defined nanofibril morphology obtained from dodecyl-substituted PDI molecules, donor-accepter type PDIs with electron-rich phenylamino moieties attached show much improved photocatalytic activity due to efficient inter- and intra-molecular charge transfer. This work provides insight into the role of molecular design and nanomorphology of organic semiconductor materials in the field of photocatalysis. This journal is

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1D nanofiber composites of perylene diimides for
visible-light-driven hydrogen evolution from
water†
Cite this: RSC Adv., 2014, 4, 48486
c
a
a
a
Shuai Chen,ab Daniel L. Jacobs, Jingkun Xu, Yingxuan Li, Chuanyi Wang
*
c
*
and Ling Zang
A series of novel nanocomposite structures have been fabricated by in situ deposition of TiO2 layers and/or
a co-catalyst (Pt) on one-dimensional (1D) self-assembled nanofibers of perylene diimide derivatives (PDIs).
The PDI molecules were functionalized with dodecyl and/or phenylamino groups to compare the effect of
nanofiber morphology and intramolecular charge transfer on the photocatalytic performance. Under
visible-light irradiation (l > 420 nm), hydrogen production for all composite systems has been detected
through photocatalytic water splitting in aqueous solutions with sacrificial reagent methanol or
triethanolamine, proving the applicability of organic nanofibers in the photocatalytic system. Compared
to the well-defined nanofibril morphology obtained from dodecyl-substituted PDI molecules, donor–
accepter type PDIs with electron-rich phenylamino moieties attached show much improved
photocatalytic activity due to efficient inter- and intra-molecular charge transfer. This work provides
insight into the role of molecular design and nanomorphology of organic semiconductor materials in the
field of photocatalysis.
Received 26th August 2014
Accepted 24th September 2014
DOI: 10.1039/c4ra09258a
www.rsc.org/advances
reduction of water to O2 and H2 respectively (Eg > 1.23 eV),
efficient charge separation to limit the back reaction of H2 and
Introduction
O2 to water and should be composed of inexpensive and
abundant materials.1–3 Typical photocatalytic systems oen use
large bandgap metal oxides that suffer from poor visible light
absorption or expensive sensitizing dyes that may not be pho-
tostable.10–12 Similarly, fast charge recombination in most suit-
able semiconductors occurs much faster than the redox
reactions of water, resulting in poor quantum yields.4,6 Addi-
tionally, traditional inorganic semiconductors (typically,
TiO2)2,13,14 and many new photocatalytic materials, such as
graphitic carbon nitride (g-C3N4),15 crystalline polyimides,16,17
carbon materials (carbon nanotubes, graphite oxide, graphene,
graphene oxide, reduced graphene oxide),18–22 as well as their
composites,23–26 are usually produced from high energy
consuming production processes which detract from the overall
goal of an alternative energy system. To meet these challenges,
increasing research efforts have now been put into the design
and development of new photocatalyst materials (or compos-
ites), as well as the novel photocatalytic mechanisms that may
lead to further improvement of the photoconversion efficiency
through nanoscale structural modication.27–29
Semiconductor-based photocatalytic splitting of water utilizing
solar irradiation has been recognized to be among the most
ideal approaches for a clean and renewable energy source.1–7
Unlike other solar energy technologies (photovoltaic and pho-
tothermal) where storing the converted energy can be compli-
cated, solar water splitting stores the energy in stable chemical
bonds (hydrogen and oxygen) which can then be used as a fuel
for on demand energy. Hydrogen is considered an ideal clean
and renewable fuel as the high energy capacity can be converted
and collected through either combustion or fuel cell systems
with water as the only by-product. However, despite four
decades of research since the initial discovery of photocatylatic
water splitting, low photoconversion efficiency limits its use in
practical applications.8,9 To realize cheap and efficient solar
water splitting, the photocatalytic materials should exhibit
strong visible light absorption to match the solar spectrum (Eg <
3 eV), suitable energy levels and bandgap for oxidation and
aLaboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of
Physics & Chemistry, Key Laboratory of Functional Materials and Devices for Special
Environments, Chinese Academy of Sciences, Urumqi, 830011, China. E-mail:
cywang@ms.xjb.ac.cn; Fax: +86 991-3838957; Tel: +86 991-3835879
bThe Graduate School of Chinese Academy of Science, Beijing, 100049, China
cNanoInstitute of Utah and Department of Materials Science and Engineering,
University of Utah, Salt Lake City, UT, 84112, USA. E-mail: lzang@eng.utah.edu
† Electronic supplementary information (ESI) available: Experimental details,
Table S1 and Fig. S1–S9. See DOI: 10.1039/c4ra09258a
Among the new materials, small molecules based on PDI
show promise as efficient photocatalyts. PDI and its derivatives
are well-known n-type organic semiconductors that are
commonly used in organic electronics, sensitizers in dye-
sensitized solar cells and as the acceptors in bulk heterojunc-
tion organic photovoltaics due to their unique optoelectronic
48486 | RSC Adv., 2014, 4, 48486–48491
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properties.30–33 They have a bandgap around 2 eV with photo-induced exciton generation and dissociation and higher
outstanding photo-response in visible wavelengths that match electron mobility along the p–p stacking direction. The high
well with the solar spectrum. They exhibit large exciton and mobility along the stacking direction makes PDI nanobers a
charge diffusion lengths due to the extended p-electron delo- common material in OFET applications. The large surface area
calization through strong p–p overlap.34–37 Indeed, PDIs and reactivity of the PDI bers give them a unique advantage for
demonstrate the highest mobilities among common organic chemical sensors through interfacial charge transfer.33
semiconductors.38 Furthermore, chemical functionalization of Furthermore, 1D PDI-based heterostructures can be fabricated
PDIs can be used to tune the redox potentials and/or binding with organic or inorganic materials to form nanoscale p–n
properties leading to efficient semiconductor heterostructures junctions for large area and efficient photovoltaic cells.48,49
with energy levels suitable for photocatalytic-degradation or Combination of the large surface area, enhanced charge sepa-
water reduction to H2.38–40 Unlike most other organic molecules, ration and high surface reactivity to form a large number of
PDIs have unique stability in air and under heating making nano-composite structures makes PDI nanobers a very prom-
them a robust material for solar energy applications.31,32 Finally, ising system for photocatalytic activity. While PDI sensitized
in contrast to inorganic and carbonic counterparts, PDIs can be composites have been studied for photocatalytic applications,50
synthesized following simple synthetic routes from low-cost and there has been little investigation on the 1D self-assembly of
abundant materials in high yields and purity.30 For these PDIs for water splitting applications, to the best of our
reasons, PDIs are considered to be promising materials in knowledge.
photocatalytic systems and have already attracted several
In this study, we prepared a series PDI nanobers coated
investigations. For example, amorphous PDI was attached onto with co-catalyst Pt and/or TiO2 to study the effect of nano-
the surface of TiO2 nanoparticles, or another inorganic metal morphology on photocatalytic water splitting for hydrogen
oxide, mainly as a visible-light-responsive sensitizer for photo- generation under visible-light irradiation. Three PDI-based
degradation of pollutants.39,40 However, the efficiency is limited molecules (PDI-1 to PDI-3; Scheme 2) attached with either an
largely by the inefficient charge transport in the amorphous inert dodecyl chain or electron-donating N,N-dimethylaniline
PDI, resulting in losses before charge transfer reaction can group were used for comparative investigation of the effect of
occur at the TiO2 interface.39,40
side-chain modication on nanober growth and catalytic
In the last decade, orderly 1D nanostructures of PDIs fabri- activity.30,38,51–53 The molecules were either symmetric (PDI-1,
cated from simple solution-phase self-assembly have been PDI-2) or asymmetric (PDI-3) leading to different stacking
extremely attractive as building blocks for organic optoelec- orientations and ultimately different nanomorphologies. Co-
tronic devices in line with a large number of very appealing catalyst Pt and/or TiO2 were deposited on the PDI nanobers
results (Scheme 1).41–47 Strong p–p stacking of the large and at through a facile in situ solution deposition to form 1D hybrid
aromatic core of PDI drives the self assembly process. In nanocomposites (Scheme 3).2,27,54 A sacricial electron donor
comparison with the single-molecule and bulk-phase PDIs, molecule of CH3OH or triethanolamine (TEA) was used to
well-dened PDI-nanobers exhibit high purity, more efficient facilitate the H2 generation. Complete experimental details,
Scheme 2 Synthetic routes of PDI-based molecules.
Scheme 3 Schematic diagram of 1D hybrid nanocomposites: (a) Pt/
PDI-X nanofibers fabricated by in situ deposition of Pt on PDI-X
Scheme 1 Schematic diagram for recent development (fabrication, nanofibers; (b) Pt/TiO2/PDI-X nanofibers fabricated by in situ deposi-
morphology and application) of 1D self-assembly of PDIs.
tion of TiO2 and then Pt on PDI-X nanofibers.
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including material synthesis, characterization, and photo-
catalytic tests, are available in the ESI.†
Results and discussion
Molecular characterization
The PDI molecules 1–3 were chosen to optimize both self-
assembly and photocatalytic activity through side chain modi-
cation. The dodecyl chain (R2) was used to improve solubility
in CHCl3 for enhanced self assembly process. Conversely, the
aniline functional group (R1) acts as an electron donor to
effectively increase intramolecular charge separation under
illumination, aiming to promote the photocatalytic activity.
Substitution at the imide position of PDI more or less inuences
the thermodynamic stability (Fig. S1†) but should not result in
modication of the molecular orbitals due to a node in the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) at the imide.30,33,55
Indeed, functionalization for all PDI molecules showed no
signicant change in the molecular energy levels as determined
from optical spectroscopy (Fig. S2†) and cyclic voltammetery
(Table S1†). Thus, major differences in H2 generation across the
three PDIs can be attributed to differences in molecular
packing, nanoscale morphology, and intramolecular charge
separation efficiency.
Fig. 1 SEM images of nano-assemblies fabricated from PDIs at low
(up) and high (down) magnification.
extended 1D charge carrier pathways, large surface area for high
reactivity and long term stability, making them all suitable for
comparative photocatalytic experiments.
Preparation of nanocomposites
To realize a functional photocatalytic system, nanocomposites
(Scheme 3) composed of PDI nanobers coated with co-catalysts
Pt and TiO2 were fabricated. TiO2 was employed as electron
transfer relay that accepts electron from PDI and transfers it to
Pt, which provides active sites for reduction of water to H2. Such
TiO2 mediated electron transfer helps suppress the electron–
hole recombination. The expected photocatalytic mechanism is
shown in Scheme 4. In this system, PDIs are used as central
photocatalyst that responds to visible light (Fig. S3†). Photo-
excitation of PDI promotes electron transition from the HOMO
to the LUMO, leaving a hole (positive charge) located at HOMO.
For PDI-1 and PDI-3, which are both attached with aniline group
(a strong electron donor), rapid intramolecular electron transfer
from aniline can ll in the hole, i.e., neutralize the positive
charge, producing a long-range (thus long lived) charge sepa-
ration between the PDI backbone and aniline substituent.
Further, 1D organized p–p stacking between PDI molecules
enables extensive delocalization of the electron along the long
Nanoscale self-assembly
Nanobers of PDIs were fabricated through solution phase self-
assembly, which was performed via bisolvent interfacial trans-
fer of PDIs.33,53 Briey, a large volume of a poor solvent, CH3OH,
was injected slowly into a small volume of high concentrations
of PDI in a good solvent, CHCl3. Self-assembly occurs as the
solubility of PDI decreases in the mixture solvent, mainly driven
by the strong intrinsic co-facial p–p stacking of the aromatic
core in the long axis of the nanostructure, in conjunction with
the hydrophobic association between the side chains along in
the short axis.6,32,40 Thus, there are several degrees of freedom in
the fabrication method to control the nanostructure
morphology, such as solubility in different solvents and aging
conditions. In this study, all PDI samples formed well-dened
nanober structures through a slow injection technique where
400 mL of CH3OH was slowly added dropwise to 100 mL of
CHCl3 (100 mmol LÀ1) solution followed by aging for 24 h at
room temperature. This slow injection technique produces a
more thermodynamically stable structure. Fig. 1 shows the
different nanomorphologies obtained with the three different
PDI molecules using the slow injection technique. PDI-1 and
PDI-2 formed short nanorods and long nanobelts respectively,
whereas PDI-3 produced longer but thicker nanowires (several
to tens of microns long). Alternatively, a quick injection and
dispersion method was also employed which forms shorter and Scheme 4 Proposed photocatalytic H2 evolution mechanism for Pt/
less ordered nanostructures of PDI-3 (Fig. S5†).56
All of the PDI nanostructures were very stable in solution and
could be transferred to polar or nonpolar solid surfaces for
TiO2/PDI-1(3) nanofiber composites, for which the hole left in the
HOMO of PDI can be filled in by rapid intramolecular electron transfer
from the aniline group. For PDI-2, which is not attached with an
electron donor, the hole can be scavenged by the sacrificial reducing
further characterization and functionalization. Despite the
reagent. Energy values (vs. vacuum level) are taken from references
different nanomorphologies, all PDI nanostructures form given in the article.
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axis of nanober (a band like charge transport process).33,38 This
intermolecular charge delocalization not only suppresses the
charge recombination, but more importantly enhances the
interfacial electron capture by TiO2. In the presence of sacri-
cial donor reagent, such as CH3OH or TEA, the positive charge
at the aniline moiety can be effectively scavenged (neutralized),
thus allowing for continuous ow of electrons along the nano-
bers. In comparison, for PDI-2, which is not connected with an
electron donor, the hole le in the HOMO of PDI can only be
scavenged by the sacricial reducing reagent, an intermolecular
process that is much less efficient than the intramolecular
electron transfer from aniline to PDI as in the case of PDI-1 or
PDI-3. This means that the electron–hole recombination within
PDI-2 would strongly competes with the interfacial electron
transfer from PDI to TiO2, leading to decreased photocatalysis
efficiency as observed in Fig. 2 and 3.
To examine the role of 1D nanomorphology of PDI, photo-
catalytic reduction of water was investigated with both PDI
sensitized Pt/TiO2 nano-particle composites and Pt/TiO2 coated
PDI nanober composites. Commercially available P25 TiO2
nanoparticles were used for fabricating the PDI sensitized
nanocomposites (see ESI† for experimental detail), here to be
referred as PDI-X/Pt/P25. For the Pt/TiO2 coated PDI nanober
composites, here to be referred as Pt/TiO2/PDI-X. TiO2 was rst
deposited on the PDI nanober surface by reduction of titanium
isoproproxide in a PDI nanober solution of CHCl3/CH3OH. Pt
nanoparticles were deposited onto either P25 TiO2 nano-
particles, TiO2 coated PDI nanobers or bare PDI nanobers by
in situ photo-reduction from H2PtCl6$6H2O in an aqueous
solution. The in situ deposition of TiO2 and Pt onto PDI bers
was proven to not damage the PDI nanomorphoology as seen in
Fig. S6 and S7.†
Fig. 2 (a) The time courses of H2 evolution over PDI-X(0.5 wt%)/Pt/
P25 nanoparticles and (b) Pt(0.5 wt%)/PDI-X nanofibers with TEA
(10 vol%) as the sacrificial reagent.
highest performance followed by PDI-3. PDI-1 and PDI-3
molecules both contain the aniline functional group, which acts
as an intramolecular electron donor.38 In bicomponent system
of PDI nanobers coated with Pt co-catalyst, similar results are
obtained, as shown in Fig. 2b, where the H2 generation rate of
Pt(0.5 wt%)/PDI-1 nanobers is more than double that of the
other PDI systems. Thus, it is hypothesized that the intra-
molecular electron transfer from aniline to PDI enhances the
photocatalytic activity as illustrated in Scheme 4. In addition,
the better photocatalytic performance of Pt/PDI-X nanobers
than PDI-X/Pt/P25 nanoparticles is largely due to the nanober
morphology that possesses extended p–p stacking structure
enabling efficient charge transfer and separation in the former
system as discussed above, whereas in the later, the weak
physical adsorption between PDI-X and Pt/P25 particle results
in inferior interfacial charge transfer and separation.
TiO2 was coated on the surface of the PDI nanobers to
enhance photocatalytic activity through interfacial charge
separation. The weight ratio of TiO2 to PDI nanober was
optimized to balance surface coverage and dispensability. An
optimized photocatalyst material was found to be Pt(0.5 wt%)/
TiO2(5 wt%)/PDI-3 nanober (Fig. S9†). This material was
capable of producing 100 mmol gÀ1 H2 within 24 h. Unlike the
experiments shown in Fig. 2 (where the photocatalysis perfor-
mance of PDI-1 is better than that of PDI-3), here PDI-3-based
nanocomposites showed a better result than that of PDI-1
(Fig. 3). Based on the SEM imaging (Fig. 1), it can be seen that
For efficient photocatalytic systems, the catalyst should be
well dispersed and stable in the aqueous solvent. The presence
of the PDI molecules allow for improved dispersion of the
sensitized Pt/P25 nanocomposites in aqueous solution
compared to pristine TiO2 nanoparticles. Conversely, TiO2
coating on PDI nanober surface acts as a protective layer to
improve the mechanical and chemical stability of the long and
exible PDI nanobers in solution during the photocatalytic
tests. To maintain good dispensability and avoid clumping of
PDI nanobers during the in situ hydrolysis process, an opti-
mized weight ratio 5 wt% of TiO2 to PDI nanobers was deter-
mined (Fig. S9†).
Photocatalytic H2 production
The presence of sacricial reagents and noble-metal Pt was
found necessary for efficient H2 generation under visible-light
(l > 420 nm) irradiation.2,54 In this work, 0.5 wt% of co-catalyst
Pt to TiO2 and 10 vol% of TEA in aqueous solution were deter-
mined to be optimal (Fig. S8†) and chosen for the remainder of
the experiments.
As shown in Fig. 2a, PDI-sensitization of all three molecules
on Pt(0.5 wt%)/P25 nanoparticles enhanced H2 generation from
water. This enhancement is attributed to increased visible light
Fig. 3 The time courses of H2 evolution over Pt(0.5 wt%)/TiO2(5 wt%)/
PDI-X nanofiber composites using TEA (10 vol%) as the sacrificial
absorption. Among these PDIs, however, PDI-1 exhibits the reagent.
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the PDI-3 nanobers formed a longer and more ordered nano- numerous options to optimize the property and function at
structure compared to PDI-1. Therefore, it is hypothesized that nanoscale. In the future, inspired by the present results, more
the improved nanomorphology enhanced the photocatalytic suitable PDI molecules with selective donor moieties will be
activity. To further test this, PDI-3 nanorods fabricated through studied to satisfy the needs of controllable 1D self-assembly and
the fast injection technique (Fig. S5†) was investigated for the adjustable composite fabrication. Similarly, the energy level
same photocatalysis as comparison. As expected, the short difference between the HOMO of PDIs and conduction band of
nanorods demonstrated much lower efficiency of H2 production TiO2 or Fermi level of Pt should be critically considered to
under the same photoreaction conditions (Fig. 3).
achieve much improved charge separation. Moreover, more
promising results are anticipated by introducing well-tailored
supramolecular nanostructures of bay-substituted or core-
extended PDIs (donor–accepter type having hydrophobic/
Mechanism speculation
To explain the superior performance of the Pt/TiO2/PDI-3 hydrophilic characteristics), as well as constructing organic
nanober system, both the nanomorphology and molecular nanober based hybrid nanocomposites like p/n hetero-
design are taken into consideration. By looking at only the stuctures or co-assembled crystalline donor–acceptor
nanomorphology, as seen in the SEM images in Fig. 1, PDI-2 heterojunctions.
nanoribbons show the highest aspect ratio, and thus largest
surface area. Based on nanomorphology alone, PDI-2 should
show the best performance. However, the shorter PDI-1 nano-
Acknowledgements
rods and thicker PDI-3 nanobers outperform PDI-2 by nearly
Financial support by the National Natural Science Foundation
25%. It is speculated that intramolecular charge transfer from
of China (Grant no. 21173261), the “One Hundred Talents”
the electron rich amine group is responsible for the perfor-
program of Chinese Academy of Sciences, International Science
mance enhancement by suppressing the electron–hole recom-
& Technology Cooperation Program of Xinjiang Uygur Autono-
bination, despite the inferior nanomorphology. Furthermore,
mous Region (20126017), and the “Cross Cooperation Program
by optimizing both nanomorphology and molecular design with
for Creative Research Teams” of Chinese Academy of Sciences is
the asymmetric PDI-3 system, another nearly 25% performance
gratefully acknowledged.
enhancement was obtained. This optimized design takes
advantage of both intra- and inter-molecular charge transfer to
promote H2 generation by suppressing the back charge
recombination.
Notes and references
Indeed, the mechanism illustrated in Scheme 4 also explains
why PDI-1 outperforms PDI-3 in the bicomponent system with
no TiO2 deposition (Fig. 2b). When no TiO2 is present, the
energy level alignment between the LUMO of the PDI and work
function of the Pt is poor suggesting slow electron transfer. The
extended charge carrier lifetime stemming from the larger p–p
system in PDI-3 may not be enough to suppress the charge
carrier recombination. Therefore, the overall photocatalysis
becomes mostly determined by the intramolecular electron
transfer from the aniline group to PDI, for which the positive
charge formed at the aniline can be neutralized by reacting with
the sacricial reagent. Since PDI-1 has two aniline groups, its
photocatalytic performance is increased.
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