Carbon quantum dot sensitized integrated Fe2O3@g-C3N4 core-shell nanoarray photoanode towards highly efficient water oxidation

Article abstract of DOI:10.1039/c8ta01908h

The construction of integrated heterojunction system photoelectrodes for solar energy conversion is indubitably an efficient alternative due to their effectiveness in charge separation and optimizing the ability for reduction and oxidation reactions. Here, an integrated photoanode constructed with carbon quantum dot (CQD) sensitized Ti:Fe₂O₃@GCNN (where GCNNs are graphitic carbon nitride nanosheets) core-shell nanoarrays is demonstrated, showing an excellent photocurrent density as high as 3.38 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode (V_RHE), 2-fold higher than that of pristine Ti:Fe₂O₃, which is superior over that of recently reported promising photoanodes. In this ternary system (Ti:Fe₂O₃@GCNN-CQDs), each component plays a specific role in the process towards superior PEC water oxidation: (i) the vectorial hole transfer of Ti:Fe₂O₃ → g-C₃N₄ → CQDs; (ii) the introduction of CQDs leads to high catalytic activity for H₂O₂ decomposition contributing a high rate activity for water oxidation via a two-step-two-electron water-splitting process; (iii) the favorable electron transport behavior of CQDs. This controlled structure design represents one scalable alternative toward the development of photoanodes for high-efficiency water splitting.

Full text of DOI:10.1039/c8ta01908h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Carbon quantum dots sensitized integrated Fe2O3@g-C3N4 core-
shell nanoarrays photoanode towards highly efficient water
oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
Sha-Sha Yi, Jun-Min Yan,* and Qing Jiang
DOI: 10.1039/x0xx00000x
Constructing integrated heterojuction system photoelectrodes for solar energy conversion is indubitable to be an efficient
alternative method due to their effectiveness in charge separation and optimizing the ability for reduction and oxidation
reactions. Here, an integrated photoanode constructed by carbon quantum dots (CQDs) sensitized Ti:Fe2O3@GCNNs
(where GCNNs represents graphitic carbon nitride nanosheets) core-shell nanoarrays are demonstrated, showing a smart
photocurrent density as high as 3.38 mA cm-2 at 1.23 V versus reversible hydrogen electrode (VRHE), 2-fold higher than that
of the pristine Ti:Fe2O3, which is superior over that of recently reported promising photoanodes (Table S1). In this ternary
system (Ti:Fe2O3@GCNNs-CQDs), each of which plays a specific role in the process for superior PEC water oxidation that i)
the vectorial holes transfer of Ti:Fe2O3 → g-C3N4 → CQDs; ii) the introduction of CQDs has high catalytic activity for H2O2
decomposition contributing high rate activity for water oxidation via a two-step-two-electron water-splitting process; iii)
the favorable electron transport behaviors of CQDs. This controlled structure designing represents one scalable alternative
toward the development of photoanode for high-efficiency water splitting.
www.rsc.org/
as a model material for PEC water splitting due to the good
sunlight-harvesting capability with the desirable band gap of ~1.9-
2.2 eV,7 more positive potential of its valence band (VB) than
25 E°(O2/H2O)),11 excellent stability as well as natural abundance.12
Introduction
During natural photosynthesis, photosystem II (PS II) and
photosystem I (PS I) capture light energy to generate photoexcited
photons for participating water oxidation and reduction of
coenzyme called ferredoxinnicotinamide adenine dinucleotide
phosphate (NADP) on two photosystems individually, which a
process is achieved and connected in series with an electron
transfer chain.1-3 In such a careful arrangement of photosynthetic
constituent, the two photosynthetic reactions (reduction and
However, pure Fe2O3 limits low energy conversion efficiency with
poor charge transport and slow rapid carrier collection properties
that prevents efficient charge separation in bulk and surface.13 Thus,
it is highly desired to build of a heterojunction photoelectrode for
30 water oxidation to overcome this challenges, a structure that the
high separation of charge carriers as well as the fast water oxidation
kinetics can be achieved simultaneously.
5
On this concept, we attempt to construct the well and integrated
heterojunction photoanode for PEC water oxidation, which consists
35 of titanium-doped hematite (Ti:Fe2O3) nanoarrays, graphitic carbon
nitride nanosheets (GCNNs), and ultra-small carbon quantum dots
(CQDs). In this system, Ti is doped as a foreigner in Fe2O3 to
effectively boost the bulk charge separation by adjusting the band-
bending.14 Recently, graphitic carbon nitride (g-C3N4) has received
40 great attention for direct photocatalytic water splitting on account
of its appropriate bandgap (~ 2.7 eV) and high stability.15 Moreover,
GCNNs shows much higher negative conduction band (CB) and
lower VB potentials as compared to bulk g-C3N4 because of its
quantum confinement effect.16 Thus imagine that GCNNs can be
45 introduced as a second semiconductor with Ti:Fe2O3 to form
heterojuction photoanode for superior water oxidation, which is
not reported so far. Furthermore, it is reported that CQDs can boost
charge carriers separation as well as shift the onset potential of the
photoelectrode, resulting in a higher photocurrent density.17 On the
50 other hand, Kang et al18 have confirmed that the CQDs possess high
catalytic activity for H2O2 decomposition on photocatalytic process
10 oxidation reactions) take place simultaneously in time but
separately in space, thus addressing the high quantum efficiency
derived from the efficient charge separation in spatial.2 Inspired by
this natural photosynthesis, photoelectrochemical (PEC) water
splitting based heterojunction artificial photosynthesis for chemical
15 fuel is so fascinating, that it is a promising solution to critical
problems facing the modern industrial development including of
unrenewable energy sources, environmental pollution as well as the
issues of effective storage and transport.4-6 Much research based on
7-10
various photoanodes materials, such as TiO2, Fe2O3, BiVO4, WO3
20 et al, have been devoted to achieving high PEC performance.
Especially Fe2O3, it has emerged and gained considerable attention
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of g-C3N4, then we imagine that their combination can also be wavelength increased, which probably originates from the
applied to the PEC process. characteristic of CQDs itself, deriving from its surface defect
In this paper we implement the concept of natural emission.21 In addition, graphitic carbon nitride (g-C3N4)
photosynthesis using Ti:Fe2O3 nanoarrays as the core and GCNNs as demonstrates its morphology of ultra-thin nanosheets (Fig. S2),
the shell to form core-shell structure photoanode for PEC water 40 denoted as GCNNs. Furthermore, Fig. S3 compares the UV-vis DRS
splitting through a facile step-by-step route. To extract the best and PL emission spectra of GCNNs and bulk g-C3N4, and the results
possible performance of Ti:Fe2O3, CQDs is selected as the sensitizer indicate that the blue-shifted Eg and PL spectra for GCNNs can be
owing to its high catalytic activity for H2O2 decomposition as well as ascribed to the quantum confinement effect with the conduction
the favorable electron transport behaviors. As results, the ultimate band (CB) and valence band (VB) shifting in opposite directions.16,22
5
10 Ti:Fe2O3@GCNNs-CQDs photoanode exhibits surprising high 45 The preparation process of Ti:Fe2O3@GCNNs-CQDs photoanode
photocurrent density (3.38 mA cm-2 at 1.23 VRHE) and efficiency with is demonstrated in Fig. 1A. Clearly, Ti-doped Fe2O3 (Ti:Fe2O3)
respect to Ti:Fe2O3. The underlying mechanism is systematic studied nanoarrays are primarily fabricated on fluorine-doped tin oxide
and the results show that each of which plays a specific role in the (FTO) via a simple calcination-assisted hydrothermal approach.
process for superior PEC water oxidation.
Then, dicyandiamide is chosen as the precursor of GCNNs grown on
50 Ti:Fe2O3 by method of vapour phase deposition method in furnace
in air at 550℃ for 3 h to obtain the integrated Ti:Fe2O3@GCNNs
core-shell structure. Finally, the sensitization of CQDs is conducted
using a simple spin-coating strategy followed by drying at 100℃
overnight. In this way, the well-controlled CQDs sensitized
55 integrated Ti:Fe2O3@GCNNs core-shell nanoarrays are successfully
synthesized. Similarly, Ti:Fe2O3@GCNNs and Ti:Fe2O3/CQDs can also
be obtained by the method above except for the processes of CQDs
sensitization and GCNNs spin-coating process, respectively. In Figs.
1B and 1C, the typical field-emission scanning electron microscopy
60 (FESEM) of the top-view and cross-sectional-view images of Ti:Fe2O3
demonstrate its morphology of nanoarrays made up of numerous
nanorods, which diameter and length are ~100 and 500 nm,
respectively. The HRTEM image reveals the growth direction of the
Fe2O3 nanorod along (104) with the lattice fringes of 0.37 nm (Fig.
65 1D), furthermore, Fe2O3 shows its single crystalline nature (Fig. S4).
In addition the dual loading of GCNNs and CQDs on Ti:Fe2O3, the
frame structure of the compound photoelectrode is highly
maintained dues to the thin layers of GCNNs together with the
ultra-small of CQDs (Figs. 1E and 1F). Furthermore, the TEM image
70 of Ti:Fe2O3@GCNNs-CQDs is performed for much clearly to see its
structure, showing that GCNNs nanolayers with thin thickness are
coated on the surface of Ti:Fe2O3 to form the integrated core-shell
hybrid; CQDs are firmly and uniformly deposit on the surface of
GCNNs layers (Figs. 1G and S5). By comparing the selected area
75 electron diffraction (SAED) images of Ti:Fe2O3 and
Ti:Fe2O3@GCNNs-CQDs (Fig. S4), we confirm the co-existence of
Ti:Fe2O3 and GCNNs in the later. Further observation by the
elemental mapping images obtained by scanning TEM-energy-
dispersive X-ray (STEM-EDX) are demonstrated in Fig. 1H, showing
80 the presence of Fe, O, C, N, and Ti elements, and imply that GCNNs
layers are uniformly coated on the Ti:Fe2O3 nanorods surface.
15 Results and discussion
Fig. 1 (A) Scheme illustration for the preparation of Ti:Fe2O3@GCNNs-CQDs
nanoarrays. (B) Top-view and (C) cross-sectional-view FESEM images of
Ti:Fe2O3. (D) HRTEM images of Ti:Fe2O3. (E) Top-view, (F) cross-sectional-
20 view FESEM images, and (G) TEM image of Ti:Fe2O3@GCNNs-CQDs. (H)
STEM-EDX element mapping for Ti:Fe2O3@GCNNs-CQDs.
As illustrated in Fig. S1A, the transmission electron microscopy
(TEM) image of the carbon quantum dots (CQDs) product shows its
morphology of ultra-small particles with a diameter of 1.70 nm. In
25 Fig. S1B, the combination of clear peaks at 1379 cm-1 (D-bond) and
1593 cm-1 (G-bond) for Raman spectrum of pristine CQDs are easily
observed.19 To explore the electronic structure and optical
properties of CQDs, the ultraviolet-visible diffuse reflectance
spectra (UV-vis DRS) and photoluminescence (PL) emission spectra
30 are performed. As shown in Fig. S1C, CQDs demonstrates its
intrinsic light absorption edge of 550 nm with the energy band gap
(Eg) of 2.22 eV, based on the Kubelka-Munk method.20 Fig. S1D
records the PL spectra of CQDs with the excitation wavelengths
range from 300 to 500 nm, showing that the centered wavelength
35 of the emission peak shifts to higher values as the excitation
To investigate the structural, composition and interaction in
modified Ti:Fe2O3 specimens, X-ray diffraction (XRD), Raman, and X-
ray photoelectron spectroscopy (XPS) techniques are systematically
85 investigated. The XRD patterns of the obtained four photoanodes
23
are presented in Fig. 2A, except for the peaks of Fe2O3 and SnO2
(be from FTO substrate), there is no characteristic peaks
corresponding to GCNNs and CQDs. In Fig. 3B, the intrinsic Raman
bands of Fe2O3 peaking at 222, 242, 286, 440, 496, 608, and 1316
90 cm-1 are found for the Ti:Fe2O3 based four photoanodes.24 For
specimens of Ti:Fe2O3/CQDs and Ti:Fe2O3@GCNNs-CQDs, there
appears characteristic peak corresponding to CQDs, locating at
2 | J. Name., 2012, 00, 1-3
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1595 cm-1 (Fig. S1B). While there is no Raman peak for GCNNs, that the Fe 2p and O 1s spectra for Ti:Fe2O3@GCNNs-CQDs show
during that the laser wavelength of 532 nm cannot make it excited. slightly the increasing BEs than that of Ti:Fe2O3 (Fig. S8), indubitably
It is interest to observe that the slightly changing of Raman shift at manifesting the interaction among CQDs, GCNNs and Ti:Fe2O3. Fig.
222 cm-1 is found for modified Ti:Fe2O3-based three photoanodes in 35 2E shows the high-resolution XPS C 1s spectra, it can be clearly seen
5
comparison with the Ti:Fe2O3 (Fig. S6), powerfully suggesting that
that three peaks with BEs at 284.6, 286.1, and 288.0 eV are
observed, corresponding to theC-C, C-O in CQDs21 (or C-N in
GCNNs),15 and C=O in CQDs16 (or N-C=N in GCNNs),15 respectively.
In the region of N 1s spectrum (Fig. 2F), three strong peaks with
40 their BEs at 398.5, 399.7, and 400.6 eV, respectively, attributed to
the pyridinic nitrogen (C-N=C), pyrrolic nitrogen (C-N-C), and
quaternary nitrogen (N-(C)3) of GCNNs16,33 can be observed, which
means that the modified Ti:Fe2O3 contains the component of
GCNNs. The Ti 2p XPS spectrum recorded in Fig. S9 suggests that Ti
A)
B)
D)
F)
G (1595)
♦
Fe2O3
♣
♥
Fe2O3 PDF#33-0664
SnO2 PDF#46-1088
♥
♥
♣
♥
♥
♥
♣
♥
(d)
(d)
(c)
♣
♦
♦
♦
♦
♦
♦
(c)
(b)
(a)
(b)
(a)
20
30
40
50
60
70
400
800
1200 1600 2000
Raman shift /cm-1
45 exists in the form of Ti .
4+ 34 In summary, these results prove that the
2θ /degree
C)
Ti:Fe2O3@GCNNs-CQDs photoanode is indeed successfully
synthesized.
Raw
Fe 2p
O 1s
531.2 eV
Fitting
O=C-O*
O-H
Fe-O
533.1 eV
709.7 eV
710.7 eV
713.5 eV
717.6 eV
724.5 eV
A)
5
4
3
2
1
0
B)
(a)
(c)
(b)
(d)
(a)
(c)
(b)
(d)
5
4
3
2
1
0
3.38 mA cm-2
Raw
Fitting
530.1 eV
2.75 mA cm-2
Fe-O
Fe3+
720
Fe 2p3/2,sat
714
2.49 mA cm-2
1.66 mA cm-2
732
726
708
536
534
532
530
528
526
Binding energy /eV
Binding energy /eV
E)
Raw
Fitting
C-C
C-O
C=O
Raw
C 1s
N 1s
399.7 eV
284.6 eV
Fitting
C-N=C
C-N-C
N-(C)3
0.6
0.8
1.0
1.2
1.4
1.6
0
0
0
100
200
Time /s
300
400
Potential /V vs. RHE
0.4
0.3
0.2
0.1
0.0
D)1.2
1.1
C)
286.1 eV
398.5 eV
(a)
(c)
(b)
(d)
(a)
(b)
(c)
(d)
288.0 eV
400.6 eV
1.0
292 290 288 286 284 282 280
Binding energy /eV
404
402
400
398
396
394
Binding energy /eV
0.9
0.6 0.7 0.8 0.9 1.0 1.1 1.2
Potential /V vs. RHE
100
200
Time /s
300
400
Fig.
2 (A) XRD patterns and (B) Raman spectra of (a) Ti:Fe2O3, (b)
Ti:Fe2O3@GCNNs, (c) Ti:Fe2O3/CQDs, and (d) Ti:Fe2O3@GCNNs-CQDs
specimens. XPS spectra of (A) Fe 2p, (B) O 1s, (C) C 1s, and (D) N 1s for
E)80
F)
80
60
40
20
0
4
1.23 V vs. RHE
(a)
(b)
(c)
(d)
60
40
20
0
10 Ti:Fe2O3@GCNNs-CQDs photoelectrode.
3
2
1
0
the strong interactions among GCNNs, CQDs andTi:Fe2O3 leads to
an increased probability of charge exchange and transfer among
them25 for high separation of photogenerated charge carriers in PEC
water oxidation. Moreover, the Raman signal at 660 cm-1 can only
15 be found for Ti:Fe2O3 (Fig. S7), which manifests the incorporation of
Ti atoms into the Fe2O3, ascribing to the disorder-sensitive mode
generated by the Ti4+ dopant incorporation.24,26 Fig. 2C-F shows the
XPS spectra of Fe 2p, O 1s, C 1s, and N 1s, respectively, for
Ti:Fe2O3@GCNNs-CQDs specimen. The Fe 2p detailed high-
20 resolution XPS spectra is shown in Fig. 2C, and the Fe 2p1/2 and Fe
2p3/2 binding energies (BEs) of 724.5 and 710.7 eV are fully
consistent with typical values reported for Fe2O3.27 In addition, the
BEs of 713.5 and 709.7 eV together with the satellite peak of Fe
2p3/2,sat centered at 717.6eV are all characteristics of Fe3+.28,29 Fig.
25 2D illustrates the O 1s spectrum region, which can be decomposed
into three well-fitted peaks locating at 530.1, 531.2 and 533.1 eV,
can be assigned to Fe-O, O-H, and O=C-O* groups, respectively.30-32
Notably, the pronounced shoulder for O-H group in
Ti:Fe2O3@GCNNs-CQDs (Fig. 2D) compared with Ti:Fe2O3 (Fig. S8)
30 together with the O=C-O* group indicates the existence of CQDs in
this composite photoelectrode.31 Besides, it is worth mentioning
400
450
500
550
600
650
Wavelength /nm
400
450
500
550
600
650
1
2
3
4
5
6
Wavelength /nm
Time /h
Fig.
3
Solar-light-driven PEC performances of (a) Ti:Fe2O3, (b)
50 Ti:Fe2O3@GCNNs, (c) Ti:Fe2O3/CQDs, and (d) Ti:Fe2O3@GCNNs-CQDs
photoanodes, respectively, for water oxidation in 1 M KOH at pH 13.6: (A)
LSV plots recorded under AM 1.5G illumination, 100 mW cm-2 (dark currents
are shown in the dash lines); (B) I-t curves at 1.23 VRHE; (C) ABPE spectra; (D)
potential (VRHE) versus time for photoelectrodes, at 1 mA cm-2; (E) IPCE
55 spectra obtained at 1.23 VRHE (inset: APCE spectra). (F) Long-time stability
test recorded on 1.23 VRHE under AM 1.5G illumination for Ti:Fe2O3@GCNNs-
CQDs photoanode.
To evaluate how the modification of CQDs and GCNNs affect the
PEC water-oxidation of our Fe2O3 photoelectrodes, we perform
60 systematically their performances in a typical three-electrode cell
using a simulated solar light (AM 1.5G). In our study, Ti:Fe2O3 is
employed as a model photoanode, for that the Ti-dopant can
effectively boosts the charge separation by reason of anodic-shifted
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flat band potential (Efb) (Fig. S10) that can boost band-bending,14 boosted water-oxidation kinetics as well as the fast electrons
resulting in significant performance improvement of photocurrent transport owning to its high conductivity. The long-term current
density. As a result, the 0.5 at.% Ti dopant Fe2O3 (labelled as 60 density versus time (I-t) performance for Ti:Fe2O3@GCNNs-CQDs is
Ti:Fe2O3) bring out the maximum current density of 1.66 mA cm-2 at conducted (Fig. 3F); the current density is well matched with the
1.23 VRHE (versus reversible hydrogen electrode) (Fig. S11). To gain LSV result, with highly stable behavior for more than 5 h.
5
the higher PEC behaviour, we optimize the Ti:Fe2O3 through
B) 40
100
A)
controlled experiments by comparing the loading amounts of
GCNNs (Fig. S12) and CQDs (Fig. S13), respectively. Notably, the
optimized Ti:Fe2O3@GCNNs and Ti:Fe2O3/CQDs show much
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)
80
60
40
20
0
30
10 enhanced photocurrent densities of 2.75 and 2.49 mA cm-2 at 1.23
VRHE, respectively, as compared with Ti:Fe2O3. Significantly, the
linear sweep voltammetry (LSV) plot in Fig. 3A reveals that
Ti:Fe2O3@GCNNs-CQDs exhibits an prominent photocurrent density
of 3.38 mA cm-2 at the same potential, which is two and about eight
15 times higher than that of Ti:Fe2O3 and Fe2O3 (Fig. S11), respectively.
These results confirm that our fabricated hybrid structure in favor
of charge separation and transportation for PEC water-oxidation
(vide-infra), with their results are comparable to the high
performance of hematite reported recently (Table S1). Notably, an
20 apparent cathodic shift on onset potentials observed for both
specimens of Ti:Fe2O3/CQDs and Ti:Fe2O3@GCNNs-CQDs, shifting
ca. 70 mVRHE which can be much more apparently seen in spectrum
of first-order derivative of the photocurrent density as a function of
voltage35 (Fig. S14). The above phenomena shed light on that the
25 CQDs can perfect the kinetic of Fe2O3 for water oxidation.
Moreover, the steady state photocurrent based on amperometric I-
20
10
0
0.8
1.0
1.2
1.4
1.6
0.6
0.8
1.0
1.2
1.4
1.6
Potential /V vs. RHE
Potential /V vs. RHE
5
4
3
2
1
0
D)10
C)
(a)
(a)
(b)
(c)
(d)
(b)
(c)
(d)
8
6
4
2
0
0.6
0.8
1.0
1.2
1.4
0
1
2
3
4
5
Potential /V vs. RHE
Z' /k
ꢀ
65 Fig. 4 (A) Surface charge injection efficiency, (B) bulk charge separation
efficiency, (C) M-S plots collected at a frequency of 1 kHz in dark, and (D) EIS
Nyquist plots under AM 1.5G illumination for based Ti:Fe2O3 photoanodes of
(a) Ti:Fe2O3, (b) Ti:Fe2O3@GCNNs, (c) Ti:Fe2O3/CQDs, and (d)
Ti:Fe2O3@GCNNs-CQDs, respectively.
t
curves at 1.23 VRHE (Fig. 3B) together with the transient
photocurrent responses under chopped light (Fig. S15) are further
conducted to analyse the photocurrent density improvement by
30 incorporation of CQDs and GCNNs. Gratifyingly, these results are in
high accordance with the LSV results.
70 To gain insights into the effect of surface coupling of CQDs and
GCNNs on enhancement of photocurrent density of Ti:Fe2O3, the
Furthermore, an analysis of the applied bias photon-to-current
efficiency (ABPE) characteristics derived from the LSV plots show
that the composite electrode configuration (Ti:Fe2O3@GCNNs-
35 CQDs) shows its ABPE value of 0.33%, which is much higher than
the values obtained for all the other three photoanodes (Fig. 3C). To
evaluate how the extent of coupling of GCNNs and CQDs affects the
over potential of the Ti:Fe2O3 photoanode, we compare the
potentials by providing the same current density (1 mA cm-2) for
surface charge injection efficiency (ηinjection
) and bulk charge
separation efficiency (ηseparation are also carried out by
)
a
comparison study between JH2O and JSO32- (the photocurrent density
75 achieved by oxidation of H2O and Na2SO3) as well as the theory
current density (Jabs), a product of the AM 1.5G spectrum and the
light harvesting efficiency (LHE) of the photoelectrode. The
2-
comparison of the JH2O and JSO3 for each photoanode, the LHE
together with the Jabs are shown in Figs. S16 and S17. As results,
80 the ηinjection and ηseparation are plotted in Figs. 4A and 4B, respectively,
and the well-constructed Ti:Fe2O3@GCNNs-CQDs hybrid
photoanode can able to reach ηinjection of 87.3% at 1.23 VRHE, which
is much higher than those of Ti:Fe2O3 (62.4%), Ti:Fe2O3@GCNNs
(80.0%), and Ti:Fe2O3/CQDs (71.9%), respectively. The observed
85 enhancement of ηinjection for Ti:Fe2O3@GCNNs and Ti:Fe2O3/CQDs
suggests that the GCNNs and CQDs can augment the separation of
photogenerated electron-hole pairs, illuminating that more holes
can arrive the electrode/electrolyte interface for participating the
PEC water oxidation reaction, compared to Ti:Fe2O3 electrode.
90 Furthermore, the above tendency is also observed for ηseparation at
1.23 VRHE for Ti:Fe2O3@GCNNs-CQDs with respect to Ti:Fe2O3 (Fig.
4B), Ti:Fe2O3@GCNNs, and Ti:Fe2O3/CQDs, with their ηseparation
values to be 28.5, 21.0, 25.5, and 26.4%, respectively. These results
indicate that the formed heterojunction in Ti:Fe2O3@GCNNs-CQDs
95 hybrid can control the charges behavior, which can facilitate the
electron-hole pairs separation in bulk of Ti:Fe2O3.36
40 each photoelectrode (Fig. 3D), which is
a measurement for
characterizing their charge separation and transportation
behaviours. As results, the over potential decreases in the order of
Ti:Fe2O3 < Ti:Fe2O3@GCNNs < Ti:Fe2O3/CQDs < Ti:Fe2O3@GCNNs-
CQDs, with their values of 1.07, 1.03, 0.99, and 0.96 VRHE
,
45 respectively. To further identify our well-designed hybrid structure,
the incident photon-to-current efficiency (IPCE) and absorbed
photon-to-current efficiency (APCE) spectra are also conducted
under PEC water-oxidation conditions. These constructed
Ti:Fe2O3@GCNNs-CQDs hybrid photoanode is optimized to exhibit
50 both the high IPCE and APCE values in the wavelength range from
400 to 650 nm compared to that of Ti:Fe2O3 (Fig. 3E), which is
consistent with the LSV plots. The maximum IPCEs are achieved at
400 nm, which are 34.8, 63.2, 58.8, and 77.4% for Ti:Fe2O3,
Ti:Fe2O3@GCNNs, Ti:Fe2O3/CQDs, and Ti:Fe2O3@GCNNs-CQDs,
55 respectively. In
a word, the enhanced PEC water-oxidation
performances of Ti:Fe2O3@GCNNs-CQDs are expected to be
assigned to the reasons of the enhanced charge separation, the
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To further discuss the effect of GCNNs and CQDs on the electrical
properties of Ti:Fe2O3, Mott-Schottky (M-S) plots and
electrochemical impedance spectroscopy (EIS) measurements are
conducted on the photoelectrodes. Fig. 4C compares the M-S plots
of Ti:Fe2O3-based four photoelectrodes in dark condition with 1 kHz
frequency in 1 M KOH, revealing that all the photoelectrodes show
the positive slope, indicating their n-type semiconductor nature.
The charge carrier density, ND, is derived from the slope of the M-S
5
plots, producing values of 1.3×1020
, , , and
5.8×1020 3.0×1020
10 68.7×1020 cm-3 for Ti:Fe2O3, Ti:Fe2O3@GCNNs, Ti:Fe2O3/CQDs, and
Ti:Fe2O3@GCNNs-CQDs, respectively. This suggests that loading of
CQDs and GCNNs can increase donor density of Ti:Fe2O3
independently, and a further higher donor density is achieved by
coupling of them simultaneously due to better charge separation
15 and transport which inhibits bulk/surface recombination.37 By
extrapolating the M-S plots to the x-axis (i.e., 1/C2 = 0) to achieve
the intercept, that the value represents the Efb of a photoelectrode.
Clearly, in comparison with the Ti:Fe2O3 specimen, the other three
Ti:Fe2O3-based hybrid photoanodes exhibit an positive shift of Efb,
20 which suggests that the composite photoanodes have decreased
band bending, which is beneficial to the facilitated
electrode/electrolyte interface charge transfer as well as the
increased semiconductor/electrolyte interface.8,9
Fig. 5 The generation of H2 and O2 with Ti:Fe2O3 (left) and Ti:Fe2O3@GCNNs-
60 CQDs (right) photoanodes and the concurrent i-t curves at 1.23 VRHE under
AM 1.5G illumination.
To achieve highly efficient PEC conversion, the pivotal factor
focuses on maximize extracting the photogenerated charge carriers
to the surface while impressing their recombination. Therefore, we
65 analyse the energy band positions of each component in
Ti:Fe2O3@GCNNs-CQDs photoanode, for fully understanding the
working mechanism of the transportation of these photogenerated
charge carriers. Ti:Fe2O3, GCNNs and CQDs demonstrate their
optical Eg values of 2.05, 3.05 and 2.22 eV, respectively, as shown
70 respectively in Figs. S19, S3A and S1C. As for Ti:Fe2O3 and GCNNs
specimens, the Efb values are first performed to estimate their ECB
positions base on method of M-S plots, which are -0.15 and -1.39 V
vs. Ag/AgCl (Figs. S10 and S20), respectively. Because of the small
difference between Efb and ECB for n-type semiconductor nature,41
75 the ECB potential levels for Ti:Fe2O3 and GCNNs are calculated to be
0.05 and -1.19 V vs. NHE,10 respectively. Additionally, according to
the cyclic voltammetry (CV) curve in Fig. S21, the LUMO level of
CQDs can be estimated to be -0.83 V vs. Ag/AgCl, that is -0.63 V vs.
NHE. Thus, the EVB (or HOMO) level for Ti:Fe2O3, GCNNs and CQDs
80 are reckon up to be 2.1, 1.86 and 1.59 V vs. NHE, respectively, by
equation of EVB = ECB + Eg.41 To observe clearly the Eg, ECB (or LUMO)
and EVB (or HOMO) of Ti:Fe2O3, GCNNs and CQDs, their values are
summarized in Table S3.
In Fig. 4D, EIS technique is conducted under an open circuit
25 potential (OCP) condition and AM 1.5G irradiation, which can used
to expound the charge separation and transportation process in this
photoanodes. The equivalent circuit shown in Fig. S18 is used for
fitting the EIS data, in which Rs, Rtrap, and Rct express as the series
resistance at the interface between the FTO substrate and bulk
30 Ti:Fe2O3, the bulk charge trapping resistance, as well as the surface
charge transfer resistance at the photoelectrode/electrolyte
interface,37,38 respectively, with their fitting results listed in Table
S2. It can be clearly seen that the Rs is found dramatically decreased
with loading GCNNs and CQDs on Ti:Fe2O3 in sequence with respect
35 to pristine Ti:Fe2O3, which suggests that the electron leakage case
on exposed FTO is well protected.39 Moreover, the Rtrap for
Ti:Fe2O3@GCNNs-CQDs is 30 Ω which is much smaller than that of
Ti:Fe2O3 (589 Ω), because of the effective surface coupling by
GCNNs and CQDs, indicating the high efficiency of formed
40 heterojunction for water oxidation.37 Meanwhile, it is also notable
that a smallest Rct value for Ti:Fe2O3@GCNNs-CQDs is observed
relatives to the other photoanodes, suggesting that the charge
transfer resistance is dramatically decreased after loading of GCNNs
and CQDs dues to their favorable electron transport behaviors, a
45 significant process for higher PEC performance owing the fast
charge transfer characteristic.
In our LSV results (Fig. 3A), the much enhanced photocurrent
85 density responses of Ti:Fe2O3@GCNNs and Ti:Fe2O3/CQDs are
achieved with respect to Ti:Fe2O3, indicates the higher separation
efficiency of photogenerated electron-hole pairs in integrated
heterojunction system photoanodes (Fig. S22), which can also be
proved by the photoluminescence (PL) spectra (Figs. S23 and S24).
90 Furthermore, it is reported that CQDs shows high catalytic activity
for H2O2 decomposition, an intermediate of PEC water oxidation,
which higher reaction rate may be achieved in this process.18,27 As
state above, the proposed schematic diagram illustrating the
mechanism of Ti:Fe2O3@GCNNs-CQDs for PEC water splitting is
95 given in Scheme 1. Under AM 1.5G illumination, photogenerated
electron-hole pairs are firstly obtained respectively in specimens of
Ti:Fe2O3, GCNNs and CQDs. Clearly, VB-holes of Ti:Fe2O3 can rapidly
transfer to VB-holes of GCNNs, and subsequently to VB of CQDs;
To gain in-depth insight into the superior PEC performance of
water oxidation, the comparison rate of H2 or O2 generation on
Ti:Fe2O3 and Ti:Fe2O3@GCNNs-CQDs photoanode at 1.23 VRHE under
50 AM 1.5G illumination is demonstrated in Fig. 5. The O2 generation
rate on Ti:Fe2O3@GCNNs-CQDs photoanode is found to be 2-fold
larger than that of Ti:Fe2O3, corresponding to 27.0 and 11.9
1, respectively. The faradaic efficiencies (
) of O2 generation for
Ti:Fe2O3@GCNNs-CQDs and Ti:Fe2O3 photoanodes are calculated to
55 be 91.2 and 90.1%, respectively. Note that these values are
ꢁ
mol h-
η
η
slightly lower than expected (100%), which probably due to the gas
leak during collection.40
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ARTICLE
Journal Name
then the VB-holes of CQDs could contribute to a high rate activity that the delicate structure presented herein provide new insight
for water oxidation by the stepwise two-electron/two-electron 30 into the design of high PEC systems.
process: (i) 2H2O → H2O2 + H2; (ii) 2H2O2 → 2H2O + O2.18 To confirm
Conclusions
In summary, we presented an integrated artificial photosynthesis
system of Ti:Fe2O3@GCNNs-CQDs, which processes smart high
photocurrent density, high stability, and efficient surface charge
35 injection and bulk charge separation efficiency. The resulting
ternary system shows a water oxidation photocurrent density of
3.38 mA cm-2 at 1.23 VRHE, which is 2-fold higher than that of
Ti:Fe2O3 and
a comparable value to that of many reported
promising photoanodes. There are mainly two factors leading to the
40 highly efficient PEC performance, which is that the greatly
enhanced separation of charge carriers at interface of
Ti:Fe2O3@GCNNs deriving from the delicate heterojunction
constructed; the high catalytic behaviour of CQDs for H2O2
decomposition leading to rapidly oxidized to O2 through two-step-
45 two-electron reaction partway. Our sunlight-driven PEC water-
splitting device paves the way toward to design solar-to-fuel
conversion efficiency photoelectrodes with low cost, highly efficient
as well as stable.
Conflicts of interest
50 There are no conflicts to declare.
Acknowledgements
This work is supported in part by the National Natural Science
Foundation of China (51522101, 51631004, 51471075, and
51401084); Specialized Research Fund for the Doctoral Program of
55 Higher Education of China (20110061120040); and Program for
Innovative Research Team (in Science and Technology) in University
of Jilin Province.
5
Scheme 1. (A) Schematics of the CQDs modified heterojunction system of
Ti:Fe2O3@GCNNs core-shell heterostructure used for PEC water splitting. (B)
The energy band schematic of our heterojunction structure.
this suppose, LSV plots based on CQDs integrated photoanodes are
performed in 0.05 M (pH = 7.0) phosphate buffered 25 mM H2O2
10 solution in dark condition (Fig. S25), with its results suggest the fact
that CQDs shows indeed the catalytic ability for H2O2
decomposition, in agreement with the previous report.18 At the
same time, CB-electrons of GCNNs can be quickly injected to CB of
Ti:Fe2O3, then flow to the FTO substrate, and ultimately to the Pt
15 counter electrode for water reduction. In addition, we find that the
CB-electrons of CQDs are leaved, which also can be used for water
reduction (Fig. S26), due to the suitable CB level position and high
conductivity of CQDs as well as its higher transfer rate of electrons
than holes. As such, charge recombination in Ti:Fe2O3 can be
20 effectively suppressed after introducing of GCNNs and CQDs, and
therefore more holes can be released for water oxidation. We
therefore attribute the superior PEC performances of our
Ti:Fe2O3@GCNNs-CQDs photoanode to: i) the delicate
heterojunction designing for efficient charge separation and
25 transportation; ii) the introduction of CQDs to contribute high rate
activity for water oxidation by the stepwise two-electron/two-
electron process; iii) the favorable electron transport behaviors of
CQDs that boosts charge separation. We are therefore confident
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Journal of Materials Chemistry A
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Table of Contents
The carbon quantum dots (CQDs) sensitized integrated Ti:Fe2O3@g-C3N4 core-shell
nanoarrays demonstrate superior PEC water oxidation performance can be ascribed to the
efficient charge separation efficiency derived from the well-confined heterojunction structure
together with the high catalytic activity of CQDs for H2O2 via a two-step-two-electron
water-splitting process.
1