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Capillarity-Assisted Electrostatic Assembly of Hierarchically
Functional 3D Graphene: TiO2 Hybrid Photoanodes
Yen-Chang Chen, Hidetaka Ishihara, Wen-Jun Chen, Nicholas DeMarco, Andrew Siordia,
Yongsheng Sun, Oliver Lin, Chih-Wei Chu, and Vincent C. Tung*
Y.-C. Chen, Dr. H. Ishihara, W.-J. Chen, N. DeMarco,
A. Siordia, Prof. V. C. Tung
School of Engineering
University of California
Merced, CA 95343, USA
E-mail: ctung@ucmerced.edu
Y. Sun, O. Lin
Department of Materials Science
University of California
Davis, CA 95616, USA
C.-W. Chu
Research Center for Applied Sciences
Academia Sinica
Taipei 11529, Taiwan
DOI: 10.1002/admi.201500292
dimension can readily extend to tens of micrometers.[21,22] This
intriguing structural feature makes rGO an ideal candidate to
serve as a conductive pathway between the grainy TiO2 nano-
particles. While this discovery allows scientists to forge ahead
with a wealthy suit of experiments to rationally improve elec-
trical contacts, and energetics at interfaces simultaneously,
rGO sheets tend to aggregate during coassembly with TiO2
nanoparticles, thus leading to the formation of metal–semi-
conductor-type of Schottky junctions.[18,23]
In addition, multiple
layers of horizontally stacked rGO sheets are detrimental to the
carrier transport, which prefers the direction perpendicular to
the current collecting electrodes. As a result, the overall perfor-
mance is less forthcoming by virtue of the degrading junction
qualities and thus deteriorating fill factor (FF). In this light, it
is highly desirable to implement 3D rGO networks with high
aspect ratios to ensure uninterrupted carrier transport within
the nanostructured TiO2 active layers while preventing the for-
mation of problematic Schottky-type contacts.
Recent new insights into rGOs’ colloidal chemistry and
mechanical properties unlock new inroads to address this
formidable challenge.[24–26]
rGO remains in an isolated, single-
layer configuration in water without the need for foreign stabi-
lizers through controlling its surface chemistry.[21]
For instance,
the surface charge density of rGO can be systematically tuned
through chemically tailoring the nature of microenvironments,
e.g., pH.[26,27]
Under high pH, the negatively charged edges
render rGO sheets to repel from each other, thus preventing the
irreversible aggregation. On the other hand, previous studies
including our work on the dimensional transition of 2D rGO
sheets into 3D spherical nanostructures also suggest that the
atomic arrangement at edges of rGOs is distinctively hetero-
geneous. Thus, external stimuli arising near the edges will
directly trigger the transformation of rGOs along the preferred
facets, ultimately resulting in a wide variety of 3D, nonplanar
shapes.[28,29] For example, when drop casting from aqueous dis-
persions, freestanding rGO sheets can conform onto curvilinear
foreign objects,[25,30,31] and self-fold into various shapes[30,32] upon
anisotropic capillary compression, thus drastically reducing
the possibility of restacking into graphite-like stacks due to the
largely decreased van der Waals interaction between neighboring
rGO sheets.[29,33] Consequently, rGO is indeed a stimuli-respon-
sive, 2D nanomaterial with electrostatically tunable edges and
a mechanically deformable basal plane. We thus surmise that
enhancing of carrier transport while suppressing formation of
undesirable Schottky junctions can be achieved concurrently
if colloidal and capillary stimuli can be rationally engineered
during the synergistic assembly of rGO and TiO2 nanohybrids.
In this work, we demonstrate that energetically favorable and
Since the world’s production of fossil fuels is diminishing,
the need to develop affordable renewable energy sources has
become more urgent than ever. Among the large extent research
directed at developing viable solutions over the last two decades,
photoelectrochemical (PEC) cells where massive energy can be
stored in the form of chemical bonds have been touted as the
key enabling technology to circumvent the hurdle.[1] In essence,
PEC technology provides an energy-conversion mechanism
through a chemical redox reaction in which light can be directly
harnessed into a chemical fuel, e.g., H2, from aqueous phase.[2]
To date, semiconducting inorganic metal oxides, such as ZnO,
ZnS, and TiO2, are the best-characterized material systems for
photoanodes of a PEC cell.[3–8]
Among all of these semicon-
ducting nanoparticles, TiO2 holds tantalizing prospect because
of solution processability, commercial availability, high catalytic
ability, and chemical stability against photo corrosion. While
nanostructured TiO2 have since been comprehensively investi-
gated and widely implemented as PEC photoanodes,[9–12]
it is
the spatial distribution of grain boundaries inherited from the
particulate nature of the TiO2 layers that impose unfavorable
energetic hurdles for charge carriers, leading to increased num-
bers of recombination centers and trap sites.
To improve both the charge carrier collection efficiency and
transport dynamics, recent research focuses have been directed
at the synergistic assembly of TiO2 nanoparticles with 2D gra-
phene sheets, especially reduced graphene oxide (rGO) due
to a combination of extended networks, favorable interfacial
energetics, scalable production, and aqueous solution pro-
cessability.[13–19]
rGO is a graphene derivative addressed with
oxygen-containing groups such as carboxylic acids at edges due
to the extensive oxidative exfoliation of graphite powders.[20]
The apparent thickness of an rGO sheet was measured to be
≈1 nm by atomic force microcopy (AFM) but its lateral
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mechanically deformable rGO:TiO2 (T-rGO) hybrid photoanodes
with tunable nanotextured morphology can be readily fabricated
by electrostatic assembly. The resulting T-rGO photoanodes
exhibit a combination of largely preserved FF, significantly
improved carrier transport, and exceptionally durable robustness.
The rational assembly of T-rGO begins with fine-tuning
the microenvironment of the colloidal dispersions, i.e., pH.
Figure 1a shows the zeta potentials of both TiO2 and rGO
measured from 0.1 mg mL−1
aqueous dispersions, respectively.
From a colloidal chemistry perspective, the final morphologies
of rGO deposition greatly hinges on the pH of colloidal disper-
sions emanating from the ionizable carboxylic moieties. At low
pH, rGO tends to be severely wrinkled and irreversibly aggre-
gated due to the strong π–π interactions between the neigh-
boring sheets. Upon increasing the pH, the negatively charged
functional groups propel rGO sheets from each other, thus
preserving the single-layer identity. Scanning electron micro-
scopy (SEM) along with atomic force microscopy (AFM) images
of spatially separated rGO sheets made by direct drop casting
(Figure S1a,b, Supporting Information) further confirms the
importance of electrostatically stabilized microenvironment.
In particular, a corresponding height profile measurement
delivers a peak-to-valley variation of only ≈1.1 nm. Meanwhile,
zeta potentials of the TiO2 dispersion remain positive before
reaching an isoelectric point at pH 6–7, where the net charge
of TiO2 changes its sign. This implies that T-rGO with opti-
mized spatial distribution can be readily formed if electrostatic
assembly takes place in the pH range of 4–6, denoted as section
II in Figure 1a. With this concept in mind, TiO2 nanoparticles
were directly diluted in an rGO dispersion (methanol, pH 5) as
demonstrated in Figure 1b. It is noted that anatase TiO2 is used
here due to the more efficient photocatalytic activity toward
water oxidation.[34]
The pH value was carefully monitored and
consistently maintained at ≈5 to first and foremost ensure the
sufficient mutual repulsion forces between rGO sheets. During
dilution, the molecular interactions immediately initiate the
electrostatic assembly between negatively charged rGOs and
positively charged TiO2 nanoparticles, leading to the formation
of a unique, alternating morphology where TiO2 nanoparticles
act as structural support between thin and corrugated rGO
sheets to effectively prevent the restacking of graphite-like mor-
phology (Figure S2a, Supporting Information). Further, unlike
most of the rGO scaffolds for which unwanted phase separation
is frequently observed upon assembling with foreign objects,
the electrostatically stabilized T-rGO hybrids remain stable dis-
persion for weeks without visibly discernible precipitation. The
stable dispersion can be readily and conveniently cast into thin
films that make it possible for morphological and subsequent
compositional characterization. In accordance with the SEM,
transmission electron microscopy (TEM; Figure S2b, Supporting
Information) image of the T-rGO hybrid further reveals spatial
distribution of densely packed TiO2 nanoparticles electrically
addressed on the basal plane of semitransparent rGO sheets. Fur-
ther, X-ray photoelectron spectroscopy (XPS) of the thin T-rGO
specimens shows all the characteristic signatures from both Ti
and C, as provided in Figure S3 (Supporting Information).
Following the solution phase assembly, the T-rGO dispersion
was directly cast onto the carbon fiber electrodes (CFEs) that are
used as the electrically addressable back contacts. Upon drying,
the capillarity-induced mechanical force enables the soft rGO
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Figure 1. a) Zeta-potential measurements of surface charges of TiO2 and rGO sheets as a function of various pH. Hierarchically functional structures
took place when rGO is negatively charged, and TiO2 is positively charged, as shown in the section highlighted in yellow. b) Electrostatic assembly of
T-rGO at pH 5 where the difference of zeta potential reaches the peak. HRSEM images of T-rGO assembled at various pH ranges juxtapose distinctively
different assembling behaviors. Extended, 3D T-rGO networks begin to form when the binary hybrids are electrostatically assembled in c) section II
(pH 4.2–6). d) Upon annealing, rGO sheets begin conformally adhere to the rocky terrains of TiO2 as a result of isotropic-capillarity-induced mechanical
forces. Alternatively, rGO sheets are prone to aggregation when assembled in e) section I (pH 1–3.4) and f) section III (pH 7–12.1).

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sheets to intimately adapt to the rocky terrains of TiO2 nano-
particles as indicated in Figure 1c. The macroscopically well-
defined morphology characterized under high-resolution SEM
(HRSEM), however, becomes drastically textured when the CFEs
are preannealed at elevated temperatures. As shown in Figure 1d,
it was found that the coverage of rGO membranes not only
adheres more intimately to the contour of TiO2 matrix but also
develops a series of vertices, wrinkles, and ripples. Further, the
extent and length scales of this capillarity-induced self-organ-
izing feature are inherently dependent on the annealing temper-
atures and therefore allow for systematic manipulation of T-rGO
morphology. At a relative low annealing temperature, evapora-
tion-induced capillarity forces, especially in the lateral direction,
render the rGO sheets develop a series of wrinkles, ripples, and
even occasionally crumples before retraction process takes place.
The micro-morphology of resulting T-rGO composites comprise
largely jagged terrains, thus increasing the possibility of devel-
oping graphite-like stacks (Figure S2c, Supporting Information).
Annealing temperature at 200 °C for the duration of 30 min
was found to suppress the largely creased morphology of T-rGO
binary composites. As shown in Figure 1d, the evaporation-
driven retraction process enables the well-extended rGO net-
works to conformally propagate throughout the particulate TiO2
and help to divide large chunks of TiO2 nanoparticles into dis-
crete, island-like assemblies. The dimensions of the embedded
T-rGO nanodomains were determined around 300 nm
in average under HRSEM, well below the upper limit of diffu-
sion length of charge carriers. This range falls to ≈1–3 times the
previously reported minority carrier diffusion length (LD) in TiO2
(LD ≈ 70–100 nm). It is known that photogenerated carriers are
mostly dissipated to recombination prior to collecting electrodes
when domains of crystalline TiO2 are beyond 300 nm.[7,35]
On
the other hand, assembly of hybrid T-rGO at sections I (pH < 4)
and III (pH > 7) all leads to the formation of highly aggregated
morphology because of the acidic nature of rGO and incompat-
ible surface charges, as shown in Figure 1e,f.
Figure 2 demonstrates the fabrication process to complete
the construction of photoanodes. Sequential annealing in-
between the deposition of T-rGO layers was found to exhibit
the most buckled morphology necessary for achieving the
highest aspect rations. Various thicknesses can be readily
obtained through a combination of concentration and duration
of casting. To constitute a photoanode with high collection effi-
ciency and limited Schottky junctions, we leverage the design
from the morphological engineering of polymer photovoltaics
where individual constituents with opposite polarities form an
energetically effective network in a bicontinuous fashion.[36]
To
this end, 100 µL of T-rGO aqueous dispersion was first drop
cast to form the modification layer. Upon annealing, the res-
toration of π–π networks of rGOs forms a solid foundation for
successive deposition. Next, 550 µL of diluted TiO2 followed by
another 50 µL of T-rGO was deposited to complete the photo-
anodes. The alternating deposition procedure was found to
deliver a relatively thick film with thickness up to 3 µm as
experimentally confirmed by SEM in Figure 3a. In one of the
electrodes where part of the TiO2 was intentionally scratched
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Figure 2. Schematic illustration shows the fabrication process of multilayer CFE/T-rGO hybrid photoanodes.
Figure 3. a) Cross-sectional SEM image of T-rGO photoanodes. Inset shows an rGO sheet protruding out through a stack of TiO2 nanoparticles. Scale
bar is 100 nm. Corresponding EDX mappings of b) carbon, Kα; c) titanium, Lα; and the hybrid of both d) collectively reveal the spatial distribution of
individual elements. Electrons can be readily transported through the well-established bicontinuous rGO pathways.

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off, we noticed that rGO sheets not only effectively anchor the
TiO2 domains in a horizontal fashion but also establish vertical
transport pathways toward the current collecting CFEs (inset
of Figure 3a). To further elucidate the spatial distribution of
the rGO networks embedded within TiO2, we conducted an
energy dispersive X-ray spectroscopy (EDX) mapping elemental
analysis. We mapped out the relevant elemental (C in red, Ti
in blue, and the mixture of both) distribution with EDX in the
cross-sectional SEM as shown in Figure 3b–d. While the out-
line of titanium overlays very well with the corresponding SEM
image, the carbon map clearly shows the spatial distribution
of bicontinuous networks within the titanium map, thus con-
firming the establishment of transport pathways. Of particular
interest is the combination of SEM and the corresponding EDX
mapping that enables us to spatially resolve the possible car-
rier propagation routes, thus forming a powerful feedback loop
where the stimuli engineered assembly can be guided and then
optimized.
Figure 4a features the as-fabricated T-rGO photoanode sup-
ported on CFEs. The active area of 1 cm2
is defined by limiting
the deposition through hydrophobic polyimide tapes. Figure 4b
schematically illustrates the setup of PEC measurements com-
prised of a three-electrode configuration equipped with a
T-rGO-based working electrode, Pt counter electrode, and Ag/
AgCl reference electrode immersed in an aqueous 1.2 × 10−3
M
KOH electrolyte solution in tandem with a potentiostat. An AM
1.5G solar irradiation was utilized as a standard to compare
the effect of electrode structures on the photocurrent gener-
ated by the TiO2 absorber supported on various scaffolds.
Figure 4c shows the representative current–potential output
curves stemmed from pristine TiO2 (red line), T-rGO assem-
bled at pH 2 (blue line), T-rGO photoanodes synthesized at
pH 5 (black line), and T-rGO assembled at pH 11 (cyan line),
respectively. Upon illumination, all four photoanodes exhibit
increasing current densities as oxidation reactions take place at
the photoanode/electrolyte interfaces. The pristine TiO2 nano-
particle electrode shows a typical photoresponse, with short cir-
cuit current (Jsc) of 64.25 µA cm–2, fill factor (FF) of 52.40%,
and an open circuit voltage (Voc) of –0.87 V, whereas the T-rGO
assembled at pH 5 yields a nearly two times enhancement to
the overall Jsc of 111.13 µA cm–2, FF = 66.6%, and Voc = −0.96 V,
respectively. In particular, the T-rGO (pH 5) composites also
show a much steeper increase in the photocurrent with applied
voltage, suggesting a lower series resistance that allows electron
and hole pairs to split more readily compared to particulate
counterparts. In parallel, as shown in Figure S2a (Supporting
Information), the photon-to-electron transport dynamics has
been significantly enhanced since the structural support (TiO2)
is now a functional building block of the overall assembly. This
unique morphology effectively reduces the distribution of con-
ductive metal–semiconducting junctions that are known to
create shunt pathways, as is evident in the T-rGO photoanodes
assembled in sections I III as denoted in Figure 1a.
As can be seen in the SEM images (Figure 1e,f), the
Schottky-type contact and undesirable aggregation are primarily
responsible for the transport pathways in the case of T-rGO
photoanodes assembled at sections I III. Evidently, the incor-
poration of aggregated rGO sheets among the particulate TiO2
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Figure 4. (a) The as-fabricated hybrid T-rGO photoanode. b) Schematic depicts the experimental setup of PEC measurements under an AM 1.5G irra-
diation. c) Output current–potential characteristics and d) time-dependent light pulse response collectively demonstrates the much improved carrier
transport at interfaces when incorporating T-rGO hybrid configuration.

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grain boundaries further aggravates the mediocre performance
with FF = 49.30% and Voc = –0.68 V for T-rGO assembled at pH
2, and with FF = 48.30% and Voc = –0.73 V for T-rGO assembled
at pH 11, in accordance with the previous reports.[18] Table 1
summarizes the effect of the morphological tailoring of rGO
distribution on the output characteristics. The magnitude of the
photocurrent generation is further examined through a pulse
photocurrent response as a function of time (Figure 4d). T-rGO
photoanodes (pH = 5; black open square) display a similar trend
with the current–potential characterization, delivering signifi-
cantly enhanced, rapid, and consistent photoresponses. Noted
that the slow transient characteristics observed in Figure 4d are
attributed to inherited limitation of the shutter switch and can
be readily improved through the acquisition or upgrading of the
new setup. Nevertheless, the much-enhanced photoresponse is
in a good agreement with our morphological observation, sug-
gesting that in the proximity of dissociated electrons there are
vertically percolated and horizontally extended graphitic path-
ways where transport takes place. Electrons can immediately
propagate to the CFEs without circumventing energetic barriers
between grain boundaries, substantially improving charge col-
lection efficiency at the electrode/liquid interfaces. To further
underscore the importance of 3D bicontinuous networks, con-
trol devices made of only one layer of T-rGO followed by the
deposition of pristine TiO2 were fabricated. As can be seen
in Figure S4a–d (Supporting Information), the carbon map
shows that sporadically spatial distributions of graphitic net-
works are mostly limited at the vicinity of underlying substrates
with slightly meandering line signals propagating in a vertical
fashion. Dissociated carriers will encounter numerous grain
boundaries/traps before propagating toward the CFEs. As a
result, output characteristics along with pulse photoresponses
only exhibits a moderate increase when compared with the
multilayered T-rGOs (Figure S4e,f, Supporting Information).
In addition to the improved carrier transport and preserved
junction quality, the rational assembly of T-rGO photoanodes
also shows surprisingly good performance durability under
strain conditions such as iterative bending. T-rGO photoanodes
built up on CFEs are highly flexible and can sustain numerous
cycles of bending tests without affecting the structural integ-
rity of the devices. To systematically examine the mechanically
durable nature of the T-rGO hybrid electrodes, we tested their
PEC performance after experiencing bending cycles as shown
in Figure 5a. CFE/T-rGO photoanodes were bent at 60° for 5 s
and then relaxed to their original position through the comput-
erized stage. Figure 5b shows the Jsc of T-rGO and pristine TiO2
under bending cycles up to 500 times, respectively. As expected,
the T-rGO electrodes exhibit exceptional PEC stability regard-
less of the iterative bending, indicating excellent mechanical
stability and durability. In parallel, the flexibility endurance of
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Table 1. Summary of I–V characteristics encompasses pristine TiO2,
T-rGO assembled at various pH, and GO/TiO2 hybrids from the litera-
ture (last column).[18]
Pristine
TiO2
T-rGO,
pH 2
T-rGO,
pH 5
T-rGO,
pH 11
GO/TiO2
Voc [V] –0.87 –0.68 –0.96 –0.73 –0.86
JSC [µA cm−2] 64.25 22.90 111.13 31.60 38.00
FF [%] 52.40 49.30 66.60 48.30 NA
Efficiency [%] 0.029 0.008 0.070 0.011 NA
Figure 5. a) The flexible CFE/T-rGO photoanode being bent with an automated stage. b) Bending the photoanode of T-rGO-based photoanode has
limited impacts on its performance as can be seen from the durability tests when under bending cycles up to 500 times. The T-rGO device retains
≈95% of its initial Jsc after the bent state while the performance of pristine TiO2 drops drastically after only tenth of cycles. HRSEM images show that
the landscape of initially packed TiO2 nanoparticles c) begins to fall apart or even developing cracks d) after 100 bending cycles. On the other hand, the
seemingly transparent rGOs networks e) not only preserve the structural integrity but act like electrical conduits to facility the efficient carrier transport
even between discrete TiO2 assemblies f).

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the electrodes was tested while keeping the electrodes under
the bent state. Remarkably, the Jsc was reversibly maintained
with nearly 95% retention of the initial Jsc in average while the
pristine TiO2 counterparts deteriorate abruptly with only 65%
retention of its original Jsc. It is known that structural integ-
rity of nanoparticles is mechanically held only by the weak
intermolecular force which scales with the contacting area
between two interacting bodies. Thus, any mechanical interrup-
tion, such as bending or twisting, will significantly impact the
overall assembly. Indeed, SEM images taken before and during
bending reveal striking morphological features. As shown in
Figure 5c,d, assembly of pristine TiO2 nanoparticles begins to
fall apart and even develops a myriad of slits and cracks upon
bending. In contrast, although microscopic defects begin to
form, the electrical pathway remains intact as the seemingly
invisible rGO membranes help to facilitate electron transport
across slits and cracks of TiO2 layers (Figure 5e,f; rGO mem-
brane is false-colored in light blue to enhance the contrast).
This superior performance makes T-rGO promising for many
emerging flexible electronics where the great challenges lies
in the design of eclectically conductive, catalytically functional,
and mechanically durable interfaces.
In summary, we demonstrate that rational engineering of
external stimuli, such as electrostatic and capillary cues, can lead
to the formation of nanoscopically alternating and microscopi-
cally corrugated T-rGO hybrid photoanodes with energetically
favorable and structurally durable interfaces as confirmed by the
HRSEM and corresponding EDX mapping. The advantageous
morphological features were found to significantly improve
Jsc and to largely preserve FF. The facile, general, and scalable
assembling route makes it possible to alleviate the constraints
of integrating 2D nanomaterials as reinforcements into nano-
particle matrices for macroscale applications. Further, we found
that the rGO embedded within the TiO2 nanoparticles serves
as both good electrical interconnect and mechanical binder,
thus synergistically enhancing the mechanical durability, PEC
reactivity, and electrical conductivity under iterative bending
cycles, delivering 95% retention in Jsc. Moreover, the facile, low-
temperature and high-throughput solution phase assembly of
3D, nanostructured T-rGO hybrid scaffolds presented here also
embodies a very visible nexus to many flexible electronics for
energy harvesting and storage applications, such as the electron
modification layer for low temperature, all-solution processed
flexible perovskite photovoltaics, where the formidable challenge
is the requirement of both high-temperature sintering process
and ultrahigh vacuum conditions for crystalline TiO2 layers.[37]
Experimental Section
Materials: TiO2 nanoparticles (anatase) with the diameter smaller
than 25 nm and ammonium hydroxide (NH4OH; 28%–30%) were used
as received without further purification (Sigma-Aldrich).
Synthesis of rGO: The rGOs in this study were synthesized by
longitudinal unzipping multiwalled carbon nanotubes (MWCNTs,
Sigma-Aldrich) with the diameters of 110–170 nm. In a typical synthesis,
300 mg of MWCNTs were first dispersed in concentrated 100 mL H2SO4
solution for pre-oxidation and followed by slowly adding 1.5 g of KMnO4.
Noted that both solvents are of highly explosive nature and should
proceed with caution. The brown color mixture was stirred at room
temperature for 1 h and then raised to 65 °C for another hour. Next,
the reaction was stopped by adding 3 mL of 10% H2O2 in the ice bath.
The solution was then vacuum filtered through a polytetrafluoroethylene
(PTFE) membrane with the pore size of 5 µm, and the as-filtered rGO
cake was further washed by 10% HCl solution and aliquots of acetone.
Finally, the rGO film was then dissolved in deionized water (DI-H2O) to
produce a concentration of 60 µg mL−1.
TiO2 Solutions Preparation and T-rGO Assembly: Stable TiO2
dispersion was prepared by directly suspending TiO2 nanoparticles
in methanol (5 mg mL−1
). Next, the solution was continuously and
vigorously sonicated using a VWR tabletop sonicator for 30 min until no
visibly distinguishable precipitations. To prepare the binary composites,
199.94 mg of TiO2 nanoparticles were first diluted with 38.8 mL, pH
5, 1.55 µg mL−1
methanol-based rGO solution. It is noted that the pH
value was carefully monitored and maintained at pH 5. At this point, the
surface of rGOs remains negatively charged while the surface charge of
TiO2 nanoparticles turns positive, thus synergistically created a T-rGO
assembly. Upon assembly, no visible precipitation was observed in the
resulting binary dispersion. Next, the addition of 1.2 mL ammonium
hydroxide solution tunes the pH value to 11. The increasingly high pH
value was found to further propel the rGOs from aggregation. The final
weight ratio between TiO2 and rGO is 3333:1. Control experiment was
prepared accordingly except tuning the pH value of rGO solution.
Measurement of PEC Cells: Hybrid photoanodes were prepared by
sequential casting of a total volume of 100 µL T-rGO suspension on
CFEs preheated at 200 °C, followed by another 550 µL of TiO2 colloidal
suspensions. Finally, another 50 µL of T-rGO was deposited to complete
the photoanodes. Control photoanodes were prepared in an analogous
manner, except for adding 600 µL of pristine TiO2 onto the pre-formed
T-rGO layers. Pt wire and Ag/AgCl were used as counter and reference
electrodes, respectively. To ensure electrical contact, the CFE/T-rGO
working electrode was connected through a toothless alligator clip,
which was then connected to a tandem working station comprised
of a CH Instruments and a photovoltaic characterization setup (QE-5
IPCE, ENLI Tech, Taiwan). 1.2 × 10−3
M KOH solution was used as the
electrolyte, which was made from dissolving 61.5 mg KOH (reagent
grade, Sigma-Aldrich) into 900 mL DI water and 100 mL ethylene glycol
(anhydrous, Sigma-Aldrich). Ethylene glycol was added to adjust the pH
value to 8 as well as increase the electrolyte conductivity. The working
electrode was illuminated by a 150 W simulated Xenon light source with
an AM 1.5 global illumination filter to get an intensity of 100 mW cm−2
.
Linear sweep voltammetry sequences were performed to identify the
photocurrent density as well as the open circuit potential of the devices.
In addition, photocurrent densities in response with light switch tests
were measured through bulk electrolysis with coulometry technique. To
minimize the overestimation of device performances, ten devices were
used to calculate the average values of PEC characteristics. The current-
density–potential plot of a device that shows closest performance to the
average was chosen to represent the group in the figures.
Characterizations: Microscopic and morphologic observations were
performed using SEM (Zeiss Ultra 55, FEG), TEM (JEOL, JEM-2100),
AFM (AFM, XE-70, Park Systems), and XPS (PHI 5400).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Y.-C.C. and H.I. contributed equally to this work. The authors gratefully
acknowledge the research award from the Doctoral New Investigator
Award from ACS Petroleum Fund (ACS PRF 54717-DNI10, V.C.T.).
Characterization in this work was performed as a user project (No. 3192)
at the Molecular Foundry, Lawrence Berkeley National Lab, supported
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by the Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231. V.C.T. is indebted
to Mohammed Ibrahem at the Academia Sinica for the help on TEM,
Yang Liu and Prof. Jennifer Lu for the help on AFM, Ronald Magpantay
for synthesis of rGOs, Jaskiranjeet Sodhi for the initial preparation of
manuscript, Dr. Yi Liu and Teresa Chen at the Molecular Foundry for
assistance on device measurements.
Received: June 4, 2015
Revised: July 19, 2015
Published online: September 5, 2015
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