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Untangling the Effect of Carbonaceous Materials on the
Photoelectrochemical Performance of BaTaO2N
Mirabbos Hojamberdiev,*Ronald Vargas, Lorean Madriz, Zukhra C. Kadirova, Kunio Yubuta,
Fuxiang Zhang, Katsuya Teshima, and Martin Lerch
Cite This: ACS Omega 2024, 9, 7022−7033
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ABSTRACT: The water oxidation reaction is a rate-determining
step in solar water splitting. The number of surviving photoexcited
holes is one of the most influencing factors affecting the
photoelectrochemical water oxidation efficiency of photocatalysts.
The solar-to-hydrogen energy conversion efficiency of BaTaO2N is
still far below the benchmark efficiency set for practical
applications, notwithstanding its potential as a 600 nm-class
photocatalyst in solar water splitting. To improve its efficiency in
photoelectrochemical water splitting, this study offers a straightfor-
ward route to develop photocatalytic materials based on the
combination of BaTaO2N and carbonaceous materials with
different dimensions. The impact of diverse carbonaceous
materials, such as fullerene, g-C3N4, graphene, carbon nanohorns, and carbon nanotubes, on the photoelectrochemical behavior
of BaTaO2N has been examined. Notably, the use of graphene and g-C3N4remarkably improves the photoelectrochemical
performance of the composite photocatalysts through a higher photocurrent and acting as electron reservoirs. Consequently, a
marked reduction in recombination rates, even at low overpotentials, leads to a higher accumulation of photoexcited holes, resulting
in 2.6- and 1.7-fold increased BaTaO2N photocurrent densities using graphene and g-C3N4, respectively. The observed trends in the
dark for the oxygen reduction reaction (ORR) potential align with the increase in the photocurrent density, revealing a good
correlation between opposite phenomena. Importantly, the enhancement observed implies an underlying accumulation
phenomenon. The verification of this concept lies in the evidence provided by oxygen reduction and is in line with photoredox
flux matching during photocatalysis. This research underscores the intricate interplay between carbonaceous materials and oxynitride
photocatalysts, offering a strategic approach to enhancing various photocatalytic capabilities.
1. INTRODUCTION
Photoelectrochemical (PEC) water splitting is one of the
potential processes to generate green hydrogen by involving
renewable energy.
1,2
However, the water oxidation reaction on
the photoanode requires the transfer of four electrons, in
comparison to the two-electron-transfer water reduction
reaction on the photocathode. Therefore, the sluggish water
oxidation reaction is the rate-determining step that governs the
rate of the water-splitting reaction.
3
BaTaO2N is a promising visible-light-active photocatalyst for
water oxidation due to its capability to absorb visible light up
to 660 nm, appropriate band-edge potential straddling the
water oxidation reaction potential, good stability in concen-
trated alkaline solution, and nontoxicity.
4,5
Under AM 1.5G
simulated sunlight based on an incident photon-to-current
conversion efficiency (IPCE) of 100% at <660 nm, the
photocurrent density and solar-to-hydrogen (STH) conversion
efficiency are assumed to reach approximately 18 mA cm−2and
24%, respectively.
6,7
To achieve higher efficiency in solar water
splitting over BaTaO2N, various strategies, such as band-gap
engineering via mono- and dual-substitution
8,9
and solid
solutions,
10,11
controlling the defect density,
12−14
fabricating
thin films,
15,16
tailoring the exposed surface, morphology, and
size,
17−19
etc., were applied. Photocurrent densities of ∼0.03,
20
>1.2,
21
2.05,
22
4.2,
6
∼4.5,
23
and 6.5 mA cm−2
24
at 1.2 V vs the
reversible hydrogen electrode were progressively achieved for
BaTaO2N, while incident photon-to-current efficiencies of 1%
at 500 nm,
20
>4% at 400 nm,
21
13% at 420 nm,
22
∼30% at 400
nm,
6
34−35% at 380−540 nm,
23
and ≈43% IPCE at 540 nm
24
at 1.2 V vs RHE steadily increased. Even though the half-cell
solar-to-hydrogen energy conversion efficiency of BaTaO2N
Received: November 8, 2023
Revised: December 6, 2023
Accepted: January 10, 2024
Published: January 29, 2024
Article
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reached 1.4% at 0.88 VRHE,

it is still far from the benchmark

efficiency set for practical application. Therefore, it is necessary

to further explore new ways to improve its efficiency in

photoelectrochemical water splitting.

As a straightforward approach, carbon-based nanomaterials

have been involved in improving the water-splitting perform-

ance of various photocatalysts due to their excellent

physicochemical, electrical, mechanical, and optical properties,

structural diversity, low cost, and easy synthesis. Carbon-based

nanomaterials have been applied as (i) supporting materials for

increasing the adsorption sites of active centers and the

homogeneous distribution of photocatalyst particles; (ii)

cocatalysts for improving the charge separation, reducing the

overpotential, providing the catalytic sites, and minimizing the

activation energy of hydrogen; (iii) photosensitizers for

enhancing the photoresponse of wide-band-gap photocatalysts

to visible light with a longer wavelength; and (iv) photo-

catalysts. The photoanode based on hydrogenated TiO2

nanorod arrays decorated with carbon quantum dots exhibited

an IPCE value of ∼66.8% and a photocurrent density of ∼3.0

mA cm−2at 1.23 V vs RHE under simulated sunlight, which

are 6-fold higher than that of pristine TiO2, because the

decorated carbon quantum dots acted as electron reservoirs to

trap photoexcited electrons and enhanced solar light harvesting

due to their upconversion effect.

The ZnFe2O4photoanode

showed an extremely weak transient photocurrent response,

whereas the carbon quantum dot-modified ZnFe2O4photo-

anode exhibited an eight times higher transient photocurrent

response because of the efficient separation of photoexcited

charge carriers.

The carbon quantum dot-modified Fe2O3

photoanode demonstrated a 27-fold enhancement in photo-

current density at 0.23 V in comparison to the Fe2O3-based

photoanode owing to the suppression of the recombination of

photoexcited charge carriers and enhanced light absorption

stemming from the upconversion of carbon quantum dots.

Wang et al.

enhanced the photoelectrochemical water

oxidation reaction of the BiVO4photoanode by involving

carbon spheres, and the photocurrent density and bulk carrier

separation efficiency of carbon sphere-modified BiVO4were

significantly higher than that of pristine BiVO4because carbon

spheres acted as electron reservoirs, promoting efficient

separation of photoexcited charge carriers. The photoelec-

trochemical performance of hexagonal and monoclinic WO3

toward water oxidation under light irradiation was boosted by

incorporating them with an amorphous nanoporous carbon

additive that facilitated the majority carrier transport through

the graphitic layers.

Carbonaceous materials with various dimensions have their

own advantages over others and can influence photo-

electrochemical performance differently.

In this study, we

aim to gain insights into the effect of carbonaceous materials,

such as fullerene, g-C3N4, graphene, carbon nanohorns, and

carbon nanotubes, on enhancing the photoelectrochemical

performance of BaTaO2N for water oxidation. Particularly, the

role of carbonaceous materials with various dimensions in the

improvement of the light absorption, separation, and transfer

of photoexcited charge carriers and photoelectrochemical

water oxidation kinetics is discussed here.

2. EXPERIMENTAL SECTION

2.1. Synthesis. BaTaO2N powders were synthesized by a

flux growth method using a localized NH3delivery system

KCl as a flux.

As a solute with a concentration of 10 mol %,

BaCO3(99.99%, chemPUR) and Ta2O5(99%, Alfa Aeser)

were manually mixed in a stoichiometric ratio with KCl

(>99.5%, Fluka) as the flux. The well-homogenized mixture

was placed in a platinum crucible and heated at 950 °C for 6 h

in a horizontal tube furnace with a heating rate of 400 °C h−1

and a natural cooling rate under an NH3flow rate of 12.5 L

h−1. Then, BaTaO2N powders were mixed with 20 wt %

fullerene (98%, chemPUR), g-C3N4,

graphene (99.5%,

chemPUR), carbon nanohorns (90%, Merck), and carbon

nanotubes (90%, Strem Chemicals), and the prepared samples

were denoted as BT, BT-FU, BT-CN, BT-GR, BT-NH, and

BT-NT.

2.2. Characterization. The X-ray diffraction (XRD)

patterns were acquired with a PANalytical X’Pert PRO

diffractometer with Cu Kαradiation. The microstructures of

the samples were examined by scanning electron microscopy

(SEM; JSM-7600F, JEOL). The bright-field and lattice images

and selected-area electron diffraction (SAED) patterns were

observed by transmission electron microscopy (TEM; EM-

002B, TOPCON). The ultraviolet−visible (UV−Vis) diffuse

reflectance spectra were recorded on an Evolution 220 UV−vis

spectrometer (Thermo Fisher Scientific).

2.3. Photoelectrochemical Measurements. To deter-

mine the photoelectrochemical behavior of the photoanodes,

the working electrodes were prepared. First, the BaTaO2N/

carbonaceous material composites were prepared by mixing

the as-synthesized BaTaO2N powders with 20 wt % fullerene

(98%, chemPUR), 20 wt % g-C3N4,

20 wt % graphene

(99.5%, chemPUR), 20 wt % carbon nanohorns (90%, Merck),

or 20 wt % carbon nanotubes (90%, Strem Chemicals), and

then, their corresponding suspensions (1.0 mg mL−1) were

prepared using an ethanol/water mixture with a 1:1 ratio under

ultrasonication for 30 min. The resulting suspensions were

evenly deposited onto the Metrohm-DropSens (110) screen-

printed electrodes by a dip-coating technique,

33,34

shielded by

a glass beaker, and dried at 80 °C using a heat gun for 10 min.

The main contact between BaTaO2N particles and carbona-

ceous materials is expected to be via electrostatics, and

connectivity was verified through electrochemical tests. In fact,

carbonaceous materials with <20 wt % did not give satisfactory

results. Before the photoelectrochemical measurements, cyclic

voltammetry (20 mV s−1) was conducted on the fabricated

electrodes (starting at 0 V vs RHE, with an upper limit of 2.0 V

vs RHE and a lower limit of −0.9 V vs RHE) in a

deoxygenated supporting electrolyte (0.1 M NaOH), typically

running 5 cycles or until a reproducible response was achieved.

The photoelectrochemical tests were conducted in a 0.1 M

NaOH aqueous solution, which was bubbled with N2for 10

min. The photoelectrochemical measurements were performed

by using a light-emitting diode (LED) lamp with a light

intensity of ∼100 mW cm−2(Solar Light, G2 V). The counter

electrode was a carbon ring concentric to the working

electrode, and the reference electrode used was Ag/AgCl.

The potential with respect to the Ag/AgCl electrode was

converted to that relative to the reversible hydrogen electrode

(RHE) using the Nernst relationship

E E(vs RHE) (vs Ag/AgCl) 0.059 pH 0.197= + × +

(1)

The photoelectrochemical measurements were performed

using a Metrohm-DropSens potentiostat (μSTAT200). The

volume of the solution was 100 μL, and the geometric area

exposed to light was 0.13 cm2. The photoelectrochemical

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measurements were performed in two modes: (i) potentiody-
namic by linear scanning voltammetry (LSV) at 4 mV s−1and
(ii) potentiostatic by chronoamperometry (CA) at 1.2 and 1.5
V (vs RHE). The power density (P) was calculated according
to eq 2
P J E E( )
0
= ×
(2)
where Jis the photocurrent density, E0= 1.23 V vs RHE, and E
is the potential.
36
In addition, for each fabricated electrode,
cyclic voltammetry (CV) was performed under dark conditions
at 20 mV s−1in a 0.1 M NaOH aqueous solution with
dissolved O2(∼1 mM). The O2concentration was fixed after
air bubbling for 5 min at room temperature (25 °C) and
measured with the HACH sensor.
3. RESULTS AND DISCUSSION
3.1. Material Characterization. The synthesized Ba-
TaO2N powders were analyzed by X-ray diffraction. Figure 1
shows the XRD pattern of synthesized BaTaO2N powders
along with an entry from the ICDD-PDF-2 powder pattern
database. As shown, all of the intense reflections in the XRD
pattern can be readily indexed to the cubic perovskite
BaTaO2N phase with a space group of Pm3m(No. 221) and
unit cell parameters of a=b=c= 4.1128 Å and α=β=γ=
90°(ICDD PDF#84−1748).
37
No reflections assignable to
the impurity crystalline phase are noted, indicating the high
phase purity of the synthesized BaTaO2N powders.
Afterward, the synthesized BaTaO2N powders were
mechanically mixed with carbonaceous materials having
different dimensions and examined by scanning and trans-
mission electron microscopies. The SEM images in Figure 2
show that the synthesized BaTaO2N powders have an
idiomorphic crystal morphology and an average crystal size
of 286 nm. Apparently, some BaTaO2N crystals are aggregated,
creating a high number of grain boundaries that may hinder
majority carrier charge transport.
38
On the contrary, Yabuta et
al.
39
pointed out that grain boundaries in aggregated particles
do not act as recombination centers for photoexcited charge
carriers but contribute to the prolongation of carrier lifetime.
The BaTaO2N crystal is surrounded by facets. Particularly, the
{110} planes exhibit edges or planes as facets as can be seen in
Figure 3a. This morphological behavior suggests that
BaTaO2N crystals have a high crystallinity. Despite their
mechanical mixing, BaTaO2N crystals have close contact with
the involved carbonaceous materials, such as fullerene, g-C3N4,
graphene, carbon nanohorns, and carbon nanotubes (Figure
3). Such contacts are anticipated to facilitate the separation
and transfer of photoexcited charge carriers in BaTaO2N
crystals, promoting photoelectrochemical water oxidation
performance.
Figure 1. XRD pattern of BaTaO2N powders (sample BT).
Figure 2. SEM images of BT (a), BT-FU (b), BT-CN (c), BT-GR (d), BT-NH (e), and BT-NT (f).
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Figure 4 shows the UV−vis diffuse reflectance spectra of BT,
BT-FU, BT-CN, BT-GR, BT-NH, and BT-NT samples.
BaTaO2N powders (sample BT) indicate an onset of light
absorption at a wavelength of approximately 665 nm, which
corresponds to an optical band-gap energy of 1.87 eV. No
background absorption beyond the absorption-edge wave-
length is observed, suggesting the low defect density. Unlike
the spectrum of the sample BT, the BT-FU and BT-CN
samples exhibit two and three absorption edges in their
corresponding UV−vis diffuse reflectance spectra, respectively.
For BT-FU, three absorption edges located at the wavelengths
of 700, 665, and 600 nm are associated with the light
absorption edges of BaTaO2N powders and fullerene.
40
For
the BT-CN, two absorption edges positioned at the wave-
lengths of 665 and 460 nm are related to the light absorption
edges of BaTaO2N powders and g-C3N4, respectively.
41
The
BT-GR, BT-NH, and BT-NT samples show light absorption
with different intensities beyond the absorption-edge wave-
length of BaTaO2N due to the response of graphene,
42
carbon
nanohorns,
43
and carbon nanotubes
44
to visible light beyond
665 nm. This implies that the BT-GR, BT-NH, and BT-NT
samples have the capability to absorb more visible light beyond
665 nm in comparison to other samples.
3.2. Photoelectrochemical Performance. Combining
the BaTaO2N powders with carbonaceous materials with
different dimensions in the fabrication of photoanodes is a
straightforward strategy to understand the photoelectrochem-
ical behavior of photocatalysts that hinder the collection and
transfer of photoexcited charge carriers.
28,29
Figure 5a shows
the LSV results of the BT, BT-FU, BT-CN, BT-GR, BT-NH,
and BT-NT photoanodes. With the addition of each
carbonaceous material, a relevant difference in the photo-
current densities was observed. Particularly, the difference in
the photocurrent densities of BT-GR, BT-CN, and BT is
obvious. Interestingly, no significant difference was observed
for BT-FU, BT-NH, and BT-NT in the range of 0.6 and 1.2 V
vs RHE. However, a trend in the photocurrent density in the
range of 1.2 and 2.0 V vs RHE was elucidated. At high
overpotentials, an increasing trend in the photocurrent
densities was noted in the following order: 0.723 μA cm−2
(BT) < 0.802 μA cm−2(BT-FU) < 0.816 μA cm−2(BT-NT) <
0.874 μA cm−2(BT-NH) < 1.279 μA cm−2(BT-CN) < 1.879
μA cm−2(BT-GR). Clearly, the BT-GR and BT-CN samples
exhibited 2.6-fold and 1.7-fold higher photocurrent densities at
1.2 vs RHE compared to the BT sample. These results are
consistent with previous reports on other types of photoanodes
based on BiVO4/carbon spheres
28
and WO3/nanoporous
carbon.
29
Gomis-Berenguer et al.
29,45
observed a varying
trend in the photoelectrochemical response of WO3/nano-
porous carbon-based photoanodes for photocatalytic water
oxidation with different weight ratios of nanoporous carbon,
Figure 3. TEM images of BT (a), BT-FU (b), BT-CN (c), BT-GR (d), BT-NH (e), and BT-NT (f).
Figure 4. UV−vis diffuse reflectance spectra of BT, BT-FU, BT-CN,
BT-GR, BT-NH, and BT-NT.
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and 20 wt % was found to be the most favorable content of
nanoporous carbon. It is noteworthy to mention that the
enhanced PEC performance of BT-CN, beyond the intrinsic
properties of carbonaceous material, is anticipated not solely
due to its typical characteristics but also because of the
synergistic photoelectrochemical response arising from the
inherent photocatalytic processes exhibited by g-C3N4.
33,46,47
Also, the photoactive nature of g-C3N4contributes to the
observed improvements, creating a distinctive charge accumu-
lation region within the BT-CN composite.
46
Consequently,
the photocatalytic behavior of g-C3N4significantly influences
and augments the overall photoelectrochemical response in the
BT-CN composite, further enhancing its performance.
Furthermore, it has been reported that in both darkness and
light,
47,48
g-C3N4can promote the oxygen reduction reaction
(ORR), and the arguments to be presented later on ORR
remain consistent for the BT-CN.
It should be noted that at low overpotentials, the
photocurrent density of BaTaO2N (sample BT) is limited by
the recombination and dynamics of photoexcited charge
carriers.
12
With the incorporation of graphene and g-C3N4,
the photocurrent density of BaTaO2N at lower overpotentials
was improved, and the onset potentials for BT-GR and BT-CN
were 0.8 and 0.9 V vs RHE, respectively. Therefore, the
improved photocurrent density provides evidence that
carbonaceous materials can lower the recombination rate of
photoexcited charge carriers. Gomis-Berenguer et al.
29
enhanced the photoelectrochemical performance of hexagonal
and monoclinic WO3by incorporating them with nanoporous
carbon, resulting in an increased photocurrent even at low
overpotentials due to the lowered recombination rate
stemming from the delocalization of electrons in nanoporous
carbon. Also, the IPCEs of hexagonal and monoclinic WO3
were enhanced two and three times at different potentials and
with different amounts of nanoporous carbon, respectively. In
another study, Wang et al.
28
employed carbon spheres as an
electron reservoir, which was fed by electrons photoexcited in
the BiVO4photocatalyst, improving the photoelectrochemical
performance substantially and leading to the photocurrent
density of 2.20 mA cm−2at 1.0 V vs RHE, which is about 6.5
times larger than the photocurrent density obtained for the
BiVO4photoanode. Considering that the BT-GR and BT-CN
samples exhibit a notably higher photocurrent even at low
overpotentials, a comparative discussion with BT is made
based on chronoamperometric results and power density
curves versus photocurrent density. Other photocatalysts do
not show a large difference in the resulting photocurrent at 1.2
V vs RHE.
Figure 5b presents the power density vs photocurrent
density curves for BT, BT-GR, and BT-CN photoanodes. In all
cases, a parabolic-shaped contour is resolved, indicating the
existence of a photocurrent density value where power density
is maximized. The obtained results lead to finding the best
possible operating conditions in the photoelectrochemical
water-splitting cells. Since the photocurrent density is greater
at high overpotentials, the application of electrochemical
assistance can be an effective strategy to improve the
photoelectrochemical performance of BaTaO2N. The external
assistance of a photoelectrochemical cell at the photocurrent
density that maximizes the power density suggests operating
conditions where the transformation of solar energy into
hydrogen is greater.
36
Another relevant aspect of Figure 5b is
that the difference between the behavior of the fabricated
photoanodes is increased, giving clear evidence that graphene
and g-C3N4can outperform in enhancing the photo-
electrochemical performance of BaTaO2N in comparison to
Figure 5. Photocurrent density vs potential (a), powder density vs photocurrent density (b), photocurrent density vs time under illumination−dark
cycle at 1.2 V vs RHE (c) and 1.5 V vs RHE (d), photocurrent density vs time at 1.5 V vs RHE for 60 min (e), and normalized current vs potential
(dark condition) (f).
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other carbonaceous materials used in this study. In fact, the
photocurrent density (power density) that maximized the
solar-to-chemical energy conversion efficiency is 0.30 μA cm−2
(0.13 μW cm−2) for BT, which was increased to 0.91 μA cm−2
(0.22 μW cm−2) for BT-GR and 0.61 μA cm−2(0.15 μW
cm−2) for BT-CN. These conditions are the ones that must be
imposed externally to implement these photocatalysts in
reactors where the photocatalysis is electrochemically assisted
so that the hydrogen evolution reaction (HER) can
conveniently close the charge balance at an appropriate
cathode.
Figure 5c,d shows the CA results of the BT, BT-GR, and
BT-CN samples. In all cases, the photocurrent density as a
function of time is higher under light irradiation than in the
dark, defining a transient where the photocurrent spike is
observed at the onset of light irradiation. It should be noted
that when the light is turned off, the photocurrent density
drops abruptly, registering a negative overshoot. The observed
behavior in the CA results is characteristic of semiconductor-
electrolyte interfaces, where the recombination of photoexcited
charge carriers is important.
49
A similar behavior was
previously observed for BaTaO2N
6,12,24
and TiN-modified
LaTiO2N
50
photoanodes. Therefore, the described qualitative
evidence from the CA analysis confirms that the incorporation
of graphene and g-C3N4into BaTaO2N changes the delicate
balance between the electron transfer and recombination of
photoexcited charge carriers. This favors an efficient electron
transfer and a photocurrent collection. Particularly, the
photocurrent density at low overpotentials increases because
the photoexcited electrons pass into the carbonaceous
materials, separating the charges and influencing the
recombination rate. At high overpotentials, a greater tendency
to transfer electrons to carbonaceous materials is evident. The
obtained results are consistent with the behavior of graphene
51
and g-C3N4
33
in photocatalysis. At this point, it is important to
clarify that the recombination processes can manifest in various
ways. One manifestation is evident in the overall changes in the
net photocurrent: a higher photocurrent corresponds to a
reduced recombination rate. Another aspect of recombination
is reflected in the transient current profiles during the light−
dark cycles. As described earlier, the appearance of spikes upon
light illumination and a negative overshoot upon light
extinction indicate recombination via surface and deep-level
states. Notably, both BT-GR and BT-CN exhibit an increased
photocurrent compared to that of BT, suggesting a reduction
in recombination. However, the presence of peaks and negative
overshoots in the transient response or CA results of BT-GR,
BT-CN, and BT indicates that the recombination processes
involving surface states persist.
To verify the effect of the potential on the complex
dynamics of charge carriers and the stability of the electrodes
giving a higher photocurrent (BT-GR and BT-CN),
chronoamperometric tests were performed first at different
potentials and then for a longer time. Figure 5c,5d shows the
respective chronoamperometric responses obtained at 1.2 and
1.5 V (vs RHE) during a light−dark cycle. As mentioned, the
shape of the current transient is associated with the balance
between the electrons collected and those lost by recombina-
tion, affecting the kinetics of charge carriers. Higher bias
increases the driving force to extract photoexcited electrons,
thereby minimizing the current spikes and overshoots typically
observed in the chronoamperometric response under light−
dark cycles. In addition, Figure 5e shows the transient current
at high polarization for over 60 min, enabling the observation
of that current value. Although the photocurrent value is
initially higher, it reaches a steady state, demonstrating the
stability of the fabricated photoanodes over time scales longer
than the conducted PEC measurements.
It is confirmed that the photoexcited electrons move from
BaTaO2N to the carbonaceous material upon light irradiation,
boosting the charge separation and favoring the accumulation
of photoexcited holes in the valence band of BaTaO2N.
Subsequently, the photoelectrochemical performance for water
oxidation is enhanced. Apparently, the improvement in the
photoelectrochemical performance of BaTaO2N depends on
the contact between carbonaceous materials and BaTaO2N.
Furthermore, the efficiency of carbonaceous materials may be
related to their ability to accumulate, transfer, and donate
photoexcited electrons due to their surface chemistry,
dimensions, and electrocatalytic properties.
The discussion of the results turns to an interpretative
analysis of the observed trends. In particular, the alignment
between the oxygen reduction reaction (ORR) potential in the
dark and the increased photocurrent density uncovers a
rational connection between an apparently contrasting
phenomenon. This observation becomes crucial when
considering the observed improvement, which suggests an
underlying accumulation phenomenon. If the performance of
the composite photocatalyst is enhanced by charge accumu-
lation in the carbonaceous phase after light excitation, then the
same composite should be able to differentiate in the dark by
performing reduction processes. To validate this concept, we
turn to the compelling evidence provided by the ORR.
Therefore, cyclic voltammetry (CV) was performed in the dark
in the presence of the same electron acceptor (dissolved O2)
for carbonaceous material-modified BaTaO2N photoanodes.
The CV results are listed in Figure 5f. The electrochemical
response arises from the juxtaposition of two signals: (i) the
capacitive signal from the fabricated photoanodes
52
and (ii)
the signal due to the oxygen reduction reaction (ORR).
53
The
ORR peak potential values were determined to be −0.46,
−0.58, −0.60, −0.61, and −0.63 V vs RHE for BT-GR, BT-
CN, BT-NH, BT-NT, and BT-FU, respectively. The obtained
potential values are closer to the value of the reduction
potential (one electron) of O2(−0.33 V vs RHE
53
) and can be
assigned to higher electrocatalytic activity for the above-
mentioned reaction.
54
Then, the BaTaO2N/carbonaceous
materials are expected to be a better electron donor compared
to BaTaO2N. Thus, when the photoanodes fabricated based on
the combination of BaTaO2N and carbonaceous material are
illuminated and subjected to an electrochemical gradient, it is
anticipated that the extraction of electrons can be favored. For
all photoanodes, the intensifying tendency in the photocurrent
density is consistent with the trend measured for the ORR
potential in the dark. That is, the more positive the ORR
potential, the more likely the BaTaO2N/carbonaceous
materials are to be better electron donors. Therefore, the
reduction processes during photocatalysis must be improved.
The findings from this study can be used as a descriptor to
optimize the design of novel photocatalytic materials based on
photocatalysts and carbonaceous materials.
Finally, it is worth noting that the increase in the
photocurrent and the decrease in the onset potential observed
for BT-GR and BT-CN are related to the respective good and
large contact between BaTaO2N and graphene or g-C3N4. In
addition to the fact that both graphene and g-C3N4have high
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electron accumulation and charge retention properties, these
characteristics are particularly associated with their two-
dimensional structures.
55,56
Therefore, the depolarization of
these carbonaceous materials, which are enriched with
electrons during the photocatalytic process, depends on their
electrocatalytic activity. It can be stated that the electrocatalytic
performance of graphene and g-C3N4is relatively high in both
ORR and water reduction reactions.
57,58
The fact that BT-GR
defines a higher photocurrent density than BT-CN, at both
high and low overpotentials, may also be related to the
electrochemical properties of graphene
59,60
and g-C3N4.
61,62
It
is inferred that the higher conductivity and electron transfer
properties of graphene
59,60
in comparison to that of g-C3N4
61
increase the efficiency of electron collection during the
photoelectrochemical reaction in the presence of BaTaO2N
(Figure 5a,5c,d). Thus, graphene assists in lowering the energy
loss in BT-GR. In all cases, the observed transduction of the
properties of graphene and g-C3N4to BT-GR and BT-CN
photoanodes could enhance the photoelectrochemical per-
formance of BaTaO2N.
The surface chemistry and interfacial effects between
heterogeneous photocatalysts and surface water molecules
are important for efficient solar water splitting. The electronic
and structural properties of photocatalyst/water and photo-
catalyst/carbonaceous material/water interfaces depend on the
electronic structure of a photocatalyst and band potentials
matching with water redox potentials.
63
The specific aqueous
interface structure, chemical surface complexation, and the
structure and morphology of nanocrystals have significant
impacts on the band-edge positions of photoelectrode
materials, Ohmic contacts, and Schottky barriers. In addition,
the hydrophilic surfaces with surface oxygen vacancies at the
terminated surfaces of metal oxides can form a hydrated layer
on the surfaces and interfaces of photoelectrodes. However, it
should be mentioned that it is still difficult to investigate the
adsorption of water molecules using heterogeneous, multia-
Figure 6. Visualization of the adsorption of water molecules onto BT-FU (a), BT-CN (b), BT-GR (c), BT-NH (d), BT-NT-horizontal (e), and
BT-NT-vertical (f). The green, light blue, red, dark blue, and gray colors represent barium, tantalum, oxygen, nitrogen, and carbon, respectively.
Isosurface field density: water in turquoise blue.
Figure 7. (a) Relationship between differential energy of adsorption of water molecules, redox potential, and photocurrent density and (b)
relationship between the energy of adsorption of water molecules, redox potential, and photocurrent density of BT-FU, BT-CN, BT-GR, BT-NH,
and BT-NT composites.
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tomic structure interface models by high-cost, first-principles
molecular dynamics (FPMD) and density functional theory
(DFT) simulations. Using implicit-solvent models with the
Monte Carlo method provides a faster and inexpensive
approach to exploring the interaction of the photocatalyst
surface/interface with water molecules with regard to the
surface photocatalytic reaction, which is strongly dependent on
the stability of the photocatalyst, the adsorption of water
molecules, and the formation of clusters on the photocatalyst
surface.
64
Therefore, the structure and adsorption energetics of
the BaTaO2N/carbonaceous material interface and the
interaction with water molecules were studied using the
Monte Carlo method (BIOVIA, Adsorption Locator mod-
ule)
65,66
and experimental structural data,
37,67
and the results
are presented in Figures 6,7, and 8.
The modeled composites consisted of BaTaO2N (BT) with
a crystallographic plane of (110) and different carbonaceous
materials: g-C3N4(CN) with a crystallographic plane of (002),
multiwalled carbon nanotubes (NT), C60-fullerene (FU),
carbon nanohorns (NHs), or graphene (GR) in the same
periodic box. The predominant crystallographic planes of
BaTaO2N and C3N4were confirmed by the XRD results. The
layer of carbonaceous materials consisted of 250 carbon atoms,
except for 240 carbon atoms for fullerene. The direct contact
of carbonaceous materials and BaTaO2N is strongly related to
the morphology of carbonaceous materials. The decrease in the
values of the carbon surface affinity (EBT/C) in the composite
indicates the stability of the BT/carbon interface (Table 1).
The BT-GR composite has the most stable interface
interaction (−221.29 kcal mol−1) compared with BT-FU
(−97.75 kcal mol−1), BT-CN (−156.71 kcal mol−1), BT-NH
(−58.75 kcal mol−1), and BT-NT (−40.25 kcal mol−1). The
affinities of water molecules were evaluated by distribution
field density (Figure 6), differential energy of adsorption of
water molecules (Figure 7), and energy distribution of water
molecules (Figure 8) adsorbed between the predominant
surface of BaTaO2N (110) and carbonaceous materials with
different morphologies: spherical (buckyballs) C60-fullerene,
single-sheet graphene, two-dimensional (2D) layered graphite-
like structure (g-C3N4(002)), tubular multiwalled carbon
nanotube, and conical single-walled carbon nanohorn.
Figure 6 shows the interfaces between BaTaO2N and
carbonaceous materials and the formation of a stable layer of
adsorbed molecules on the composite surface. The distribution
field density of adsorbed water molecules depends on the
localization of the carbon nanoparticles. Carbon nanoparticles
have a random spreading in the periodic box except for carbon
nanotubes modeled in horizontal (Figure 6e) and vertical
(Figure 6f) positions. Generally, the addition of carbonaceous
materials increased the Eads value of BaTaO2N (−98.42 kcal
mol−1) (Table 1 and Figure 7b), and the highest Eads values
were observed for the BT-NT composite with carbon
nanotubes placed in horizontal and vertical positions to the
surface of BaTaO2N. The calculated Eads values have the
following order: −245.58 kcal mol−1for BT-NT (vertical) <
−202.52 kcal mol−1for BT-NT (horizontal) < −179.57 kcal
mol−1for BT-FU < −160.21 kcal mol−1for BT-GR < −151.76
kcal mol−1for BT-CN < −127.56 kcal mol−1for BT-NH <
−98.43 kcal mol−1for BT, while the differential adsorption has
a different order: −1.61 kcal mol−1for BT-GR < −0.60 kcal
mol−1for BT-CN < −0.18 kcal mol−1for BT-FU < −0.06 kcal
mol−1for BT-NT (vertical) < −0.02 kcal mol−1for BT-NT
(horizontal) < −0.01 kcal mol−1for BT = −0.01 kcal mol−1for
BT-NH. The highest value of differential adsorption (dEads/
dNi) is in good agreement with the experimental data (Figure
7a). Computational simulation and experimental data revealed
that the water molecules can form stable hydrated multilayers
Figure 8. (a) Energy distribution of adsorbed water molecules and (b) formation of water molecule layers on BT-FU, BT-CN, BT-GR, BT-NH,
and BT-NT composites. The green, light blue, red, dark blue, and gray colors represent barium, tantalum, oxygen, nitrogen, and carbon,
respectively.
Table 1. Energy Parameters of the Adsorption of Water
Molecules
structure
water adsorption
energy, Eads
(kcal mol−1) dEads/dNi
carbon surface affinity,
EBT/C (kcal mol−1)
BaTaO2N
(110)
−98.42 −0.01
BT-FU −179.56 −0.18 −97.75
BT-CN −151.76 −0.59 −156.71
BT-GR (002) −160.20 −1.61 −221.28
BT-NH −127.55 −0.01 −58.75
BT-NT
(horizontal)
−202.52 −0.01 −40.25
BT-NT
(vertical)
−245.58 −0.06
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on the flat units of graphene.
68
The highest peak observed at
an adsorption energy of −2.15 kcal mol−1(Figure 8) indicates
a large number of water molecules adsorbed on the surface of
the BT-GR composite. In the cases of BT-FU, BT-NH, and
BT-NT composites, the presence of multiple peaks noted at
different adsorption energies confirms a number of water
molecules adsorbed on different adsorption sites in comparison
to the BT-GR and BT-CN composites.
However, the highest values of Eads for BT-NT, BT-NH, and
BT-FU (Figure 7b) are the subject of controversy regarding
the mode of adsorption of water molecules. The increased
value of Eads can be explained by the impact of possible
adsorption of a part of water molecules in the internal channel
of nanotubes, nanohorns, and fullerene.
68−71
It is known that
the carbon nanotube channel is strongly hydrophobic but can
filled up with water molecules forming a hydrogen-bonded
chain.
70
The structure of carbon nanohorns has a closed horn
tip at one side and an open end on the other side, allowing the
water molecules to enter.
70
Although fullerene is hydrophobic,
the C60 nanoclusters have a hydrophilic nature that can
increase the Eads value of BT-FU.
71
The water molecules prefer
to be adsorbed around the intrinsic vacancies of a single sheet
of g-C3N4as clusters. The adsorption of water molecules on
both sides of g-C3N4does not affect the flat structure.
However, the adsorption of water molecules on one side of a
single g-C3N4sheet leads to the transformation of the flat
structure with indirect semiconductor properties to buckle the
structure with direct semiconductor properties. Therefore, the
band structure of g-C3N4is finally changed due to the
adsorption of water molecules.
8
A previous study
63
found that
the exothermic adsorption of water molecules in the presence
of chemisorbed oxygen causes the dissociation of water
molecules on the hydroxylated surfaces of photocatalysts via
a partial proton transfer mechanism at the interface. In
addition, the surface anions can accept protons from water
molecules, while equivalent OH ions form bonds with surface
cations. The N-sites can also be protonated even more rapidly
than the surface oxygen sites.
63
Finally, the water molecules
can be adsorbed dissociatively on the surface oxygen vacancies,
leading to the formation of surface hydroxyls and oxidative
intermediates (H*for H2evolution and HO*, O*, and HOO*
for O2evolution).
8
The dissociation of water molecules is dependent on the
termination of the photocatalyst surface. BaTaO2N has
negative charges on the oxygen-terminated surfaces and
positive charges on the barium-terminated surfaces. The
water molecules prefer to be adsorbed by interaction with
surface oxygen and on the top side of the Ba atom on the
(110) surface, forming the Ta−O···H2O and H2O···Ba bonds
because the coordination of water molecules to the BaTaO2N
surface is energetically more favorable than to carbonaceous
materials due to electrostatic forces. The adsorption of water
oxygen atoms located on the exposed cation and anion sites
varies less proportionally to the number of oxygen atoms
missing from the normal Ta(O,N6) octahedral coordination
and the Ba cation, which can be 12-fold surrounded by anions.
The most stable surfaces are shown to be along the (110)
crystallographic planes that have more metal cations exposed.
The analysis of energy distribution of adsorbed water
molecules (Figure 8) confirmed the shift to higher adsorption
energy in the case of the BT-GR composite due to the
interaction of the conjugated π-system of graphene and the
BaTaO2N surface. Although the BT-NT (horizontal), BT-NH,
and BT-FU composites also have the possibility to form similar
interactions, the BT-GR composite has a more pronounced
effect on the ability of water adsorption.
The adsorption of water molecules on the BaTaO2N/
carbonaceous material surfaces was found to be a favorable
exothermic process. The surface reactivity of the BT-GR
composite is significantly higher in comparison to that of BT-
FU, BT-NT, and BT-NH, and the possible dissociation of
water molecules is highly favorable on the BT-GR and BT-CN
surfaces. The experimental results confirmed that the ORR
potential is correlated with the photocurrent density and
adsorption of water molecules at the interface between
BaTaO2N and carbonaceous materials (Figure 7). Considering
the localization of the distribution field density of adsorbed
water molecules, it is clear that the adsorption of water
molecules is favored onto surfaces of BT-GR and BT-CN
composites. Thus, the incorporation of graphene can lead to a
more positive ORR potential and better electron donors, which
should be in line with the photoredox flux-matching conditions
during the photocatalysis process, suggesting the improvement
of the reduction processes during photocatalysis.
72
Therefore,
the BT-GR composite exhibited a higher water oxidation
reaction (WOR) performance.
4. CONCLUSIONS
In this study, the photoelectrochemical performance of
BaTaO2N was enhanced by involving carbonaceous materials,
such as fullerene, g-C3N4, graphene, carbon nanohorns, and
carbon nanotubes. The difference in the photoelectrochemical
performance of BaTaO2N/carbonaceous materials and Ba-
TaO2N could be attributed to the capability of each
carbonaceous material for the collection and transfer of
photoexcited electrons, thus decreasing the recombination of
photoexcited charge carriers. Particularly, the photocurrent
density of BaTaO2N was 2.6- and 1.7-fold increased using
graphene and g-C3N4due to the efficient electron transfer,
electron reservoir capacity, and accumulation of a greater
number of photoexcited holes on the valence band. The
comparison of trends between the ORR potential in the dark
and the increase in the photocurrent density validates the
charge accumulation process in the carbonaceous phase,
highlighting that the subsequent photocurrent collection was
not compromised. This implies that the more positive the
ORR potential is, the better electron donors the BaTaO2N/
carbonaceous materials are expected to be, suggesting the
improvement of the reduction processes during photocatalysis.
Computational simulation of the adsorption of water
molecules onto the surfaces of BaTaO2N/carbonaceous
materials revealed that the incorporation of graphene can
enhance the water oxidation reaction performance of
BaTaO2N. The photoelectrochemical performance of other
photocatalysts can also be improved using carbonaceous
materials having different dimensions, morphologies, and
surface and optoelectronic properties.
■AUTHOR INFORMATION
Corresponding Author
Mirabbos Hojamberdiev −Institut fur Chemie, Technische
Universität Berlin, 10623 Berlin, Germany; orcid.org/
0000-0002-5233-2563; Email: khujamberdiev@tu-
berlin.de,[email protected]
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https://doi.org/10.1021/acsomega.3c08894
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7030

Authors
Ronald Vargas −Instituto Tecnológico de Chascomus
(INTECH), Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET) and Escuela de Bio y Nanotecnologías,
Universidad Nacional de San Martín (UNSAM),
B7130IWA Chascomus, Provincia de Buenos Aires,
Argentina; orcid.org/0000-0002-4890-8187
Lorean Madriz −Instituto Tecnológico de Chascomus
(INTECH), Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET) and Escuela de Bio y Nanotecnologías,
Universidad Nacional de San Martín (UNSAM),
B7130IWA Chascomus, Provincia de Buenos Aires,
Argentina; orcid.org/0000-0001-7476-7114
Zukhra C. Kadirova −Uzbekistan−Japan Innovation Center
of Youth, 100095 Tashkent, Uzbekistan; orcid.org/0000-
0002-2112-1886
Kunio Yubuta −Department of Applied Quantum Physics and
Nuclear Engineering, Kyushu University, Fukuoka 819-0395,
Japan
Fuxiang Zhang −State Key Laboratory of Catalysis, Dalian
National Laboratory for Clean Energy, iChEM, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China; orcid.org/0000-0002-7859-0616
Katsuya Teshima −Department of Materials Chemistry and
Research Initiative for Supra-Materials, Shinshu University,
Nagano 380-8553, Japan; orcid.org/0000-0002-5784-
5157
Martin Lerch −Institut fur Chemie, Technische Universität
Berlin, 10623 Berlin, Germany; orcid.org/0000-0002-
4065-4928
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.3c08894
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This project received funding from the European Union’s
Horizon 2020 research and innovation program under the
Marie Sklodowska-Curie grant agreement no. 793882. This
study was also supported by the CAS President’s International
Fellowship Initiative Grant No. 2022VSB0003. The authors
thank Reiko Shiozawa (Shinshu University, Japan) for the
SEM analysis.
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