Syntheses, characterizationsna and water-electrolysis properties of 2D α- and β-PdSeO3 bulk and nanosheet semiconductors

Article abstract of DOI:10.1016/j.jssc.2021.122018

In contrast to alkaline water electrolysis, water electrolysis in an acidic environment has higher energy efficiency, more compact cell design, higher gas purity, and less sensitivity to environmental impurity. Herein, the syntheses of the known 2D α- and β-PdSeO₃ were improved. The bulk materials were exfoliated into nanosheets using an ultrasonic-assisted liquid-phase exfoliation similar to that of preparing graphene nanosheets. Their band-gaps, valence band maximums (VBMs) and conduction band minimums (CBMs) were determined using Mott-Schottky plots and the linear potential scan method in acidic aqueous solution, 0.2 ?M Na₂SO₄ aqueous solution and non-aqueous solution. Their transient photocurrent properties were also examined, along with their Electrochemical Impedance Spectra (EIS). The bulk materials are stable in pH ?= ?0–14 value water solution for at least 48 ?h. α- and β-PdSeO₃ nanosheets exhibit moderate catalytic activities for hydrogen evolution reactions in 0.5 ?M ?H₂SO₄ (η₁₀ (overpotential at 10 ?mA ?cm^-2: 185 and 209 ?mV, respectively; catalyst loading: 0.28 ?mg ?cm^-2), much better than the performances of the bulk materials. In addition, β-PdSeO₃ bulk and nanosheets materials show good and similar OER catalytic activities in 0.5 ?M ?H₂SO₄ (η₁₀: 381 and 405 ?mV, respectively; catalyst loading: 0.28 ?mg ?cm^-2). Thus, β-PdSeO₃ nanosheets were found to be a good overall water splitting catalyst in acidic solution. The above catalysts all show long-term stability. In contrast, α-PdSeO₃ does not have OER catalytic property. To our knowledge, Pd-based acidic overall water splitting catalysts are very rare.

Full text of DOI:10.1016/j.jssc.2021.122018

Journal of Solid State Chemistry 297 (2021) 122018
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Syntheses, characterizationsna and water-electrolysis properties of 2D α-
and β-PdSeO3 bulk and nanosheet semiconductors☆
,c,*
Yusheng Wu a, Lin Wang a, Hongyan Zhang a, Jie Ding a, Min Han b, Min Fang a
,
,
,***
Jianchun Bao b **, Yong Wu a
a Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science,
Nanjing Normal University, Nanjing, 210023, PR China
b Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, China
c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In contrast to alkaline water electrolysis, water electrolysis in an acidic environment has higher energy efficiency,
Acidic overall watersplitting
Hydrogen evolution reaction
Oxygen evolution reaction
Palladium
PdSeO3
Ultra-thin 2D layers
more compact cell design, higher gas purity, and less sensitivity to environmental impurity. Herein, the syntheses
of the known 2D α- and β-PdSeO3 were improved. The bulk materials were exfoliated into nanosheets using an
ultrasonic-assisted liquid-phase exfoliation similar to that of preparing graphene nanosheets. Their band-gaps,
valence band maximums (VBMs) and conduction band minimums (CBMs) were determined using Mott-
Schottky plots and the linear potential scan method in acidic aqueous solution, 0.2 M Na2SO4 aqueous solution
and non-aqueous solution. Their transient photocurrent properties were also examined, along with their Elec-
trochemical Impedance Spectra (EIS). The bulk materials are stable in pH ¼ 0–14 value water solution for at least
48 h.
H2SO4
better than the performances of the bulk materials. In addition, β-PdSeO3 bulk and nanosheets materials show
good and similar OER catalytic activities in 0.5 M H2SO4 10: 381 and 405 mV, respectively; catalyst loading:
0.28 mg cm-2). Thus, β-PdSeO3 nanosheets were found to be a good overall water splitting catalyst in acidic
solution. The above catalysts all show long-term stability. In contrast, -PdSeO3 does not have OER catalytic
property. To our knowledge, Pd-based acidic overall water splitting catalysts are very rare.
α- and β-PdSeO3 nanosheets exhibit moderate catalytic activities for hydrogen evolution reactions in 0.5 M
(
η10 (overpotential at 10 mA cm-2: 185 and 209 mV, respectively; catalyst loading: 0.28 mg cm-2), much
(η
α
1. Introduction
electrolysis with electricity generated by the above mentioned renewable
energy sources is widely considered as a sustainable and cost-effective
technology for producing high purity hydrogen on a large scale in the
future and huge efforts have been investigated into electrocatalytic water
splitting [5–13]. However, at present, only a small quantity of hydrogen
(<5% of global hydrogen production) is produced by industrial water
electrolysis, due to the poor efficiency or stability of both the oxygen
evolution reaction (OER) and hydrogen evolution reaction (HER) elec-
trocatalysts [14,15].
Exploring clean sustainable energy is an extremely urgent need in the
modern society, owing to the fast depletion of remaining fossil fuel res-
ervoirs and numerous environmental issues [1–3]. These issues could be
mitigated by converting renewable energy sources such as waste heat,
wind, and solar power into electricity, which is then converted to H2 by
water electrolysis. Hydrogen is clean and has high energy density. It is
also a staple of chemical industry [4]. Compared to the present major
hydrogen production method of steam reformation of fossil fuels, water
☆
Electronic supplementary information (ESI) available: Some experimental details, additional tables and figures.
* Corresponding author. Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of
Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, PR China.
** Corresponding author.
*** Corresponding author.
E-mail addresses: fangmin@njnu.edu.cn (M. Fang), baojianchun@njnu.edu.cn (J. Bao), wuyong@njnu.edu.cn (Y. Wu).
https://doi.org/10.1016/j.jssc.2021.122018
Received 16 December 2020; Received in revised form 11 January 2021; Accepted 24 January 2021
Available online 6 February 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
The water-splitting process involves two half reactions: the HER and the
OER. The key challenge of this technology is to reduce the large over-
potentials for both cathodic HER and anodic OER. Up to now, it is still
difficult to find two such electrocatalysts that can couple in an integrated
electrolyzer for overall water splitting, because their best working con-
ditions often mismatch [16]. On the other hand, coupling the two cata-
lysts in two differing conditions, in which they work best, complicates
their integration into a single water splitting device, because they require
different accessories, and preparation and optimization procedures [16].
So, developing efficient bifunctional electrocatalysts that can work well
for both HER and OER under the same condition may address these is-
sues, and may make it feasible to easily integrate them into a single
working electrolyzer [16].
In contrast to alkaline water electrolysis, proton exchange membrane
(PEM) water electrolysis in an acidic environment has higher energy
efficiency, more compact cell design, higher gas purity, and less sensi-
tivity to environmental impurity [17–19]. However, only handful
bifunctional electrocatalysts were discovered, which can be applied in
acidic electrolyte because of instability and corrosion during OER elec-
trocatalysis under such harsh conditions [20–23].
Recently, two-dimensional (2D) layered materials have drawn enor-
mous attention due to their unique layered structures and intriguing
electrical characteristics, which brings the unprecedented broad appli-
cations in the fields ranging from electronic [24–27], spintronic [24,28,
29], optical [29–33], optoelectronic [34–40], thermal [41–43], magnetic
[44], quantum devices [45–51] to energy storage [52–54] and catalysis
[55–59], including water electrolysis [60–64]. Zhang et al. obtained
amorphous lithium-incorporated 2D palladium phosphosulfide nanodots
[65], which exhibit excellent electrocatalytic activity towards the
hydrogen evolution reaction with an overpotential of 91 mV at 10 mA
cm-2, a Tafel slope of 29 mV dec-1 in 0.5 M H2SO4, and outstanding
long-term stability. To our knowledge, no other 2D Pd-containing com-
2. Experimental
2.1. Materials and general methods
All chemicals and reagents were obtained from commercial sources
and used without further purification. Pd(NO3)2⋅2H2O (99.8%, Alfa-
Aesar), selenic acid H2SeO4 aqueous solution (40 wt% in H2O, 1.407
g/mL, Aldrich), selenious acid H2SeO3 (97%, Alfa-Aesar) and potassium
nitrate (99.994%, Alfa-Aesar). Nafion D-520 dispersion (5% w/w in
water and 1-propanol) were purchased from Alfa Aesar (Lot# Z31E033,
CAS: 31175-20-9). Ultrapure water obtained from a Millipore water
purification system (ꢀ18.23 MΩ cm, Millipore) was used in electro-
chemical experiments. Commercial Pt/C (20 wt%) was provided by Alfa
Aesar Chemical Co., Ltd. All other chemicals and reagents were of
analytical grade from commercial sources and used without further pu-
rification. The room-temperature powder X-ray diffraction (PXRD)
spectra were recorded on a Rigaku D/Max 2500/PC diffractometer at 40
kV and 100 mA with a Cu-target tube (λ ¼ 1.54178 Å) and a graphite
monochromator. The XRD diffraction patterns were recorded in the 2θ
range between 4 and 48ꢁ with a step size (2θ) of 0.02ꢁ and at a scanning
rate of 10ꢁ minꢂ1. Thermogravimetry analysis (TGA) experiments were
carried out using a SDT 2980 thermal analyzer under Ar flow of 15 mL
minꢂ1 oveꢂr1the temperature range from 25 to 800 ꢁC at a heating rate of
10 ꢁC min UV–Vis-IR diffuse reflectance spectral measurements were
.
taken using a Varian Cary 5000 UV–Vis–NIR spectrophotometer using
barium sulphate (BaSO4) as a reference. Fourier transform infrared (FT-
IR) spectra were recorded on a Bruker tensor 27 Fourier transform
infrared spectrometer. The spectra were measured using KBr plates in the
wavenumber range of 4000–400 cm-1. Raman spectra were recorded on a
JY LabRam HR Evolution instrument at a laser excitation wavelength of
633 nm at room temperature.
The morphology of the samples was observed using a HITACHI
H7650 transmission electron microscope (TEM) and a LEO1530VP field
emission scanning electron microscope (SEM). Thickness test measure-
ments were performed by a Pico Scan 5500 atomic force microscopy
(AFM) with a Molecular Imaging Pico Scan 5500 AFM in an AC mode.
AFM samples were prepared by dropping a diluted NNs/isopropanol
dispersion on a freshly cleaved mica substrate and sucking the redundant
dispersion with a piece of filter paper and then drying at ambient tem-
perature. The surface elemental analysis of samples was performed by X-
ray photoelectron spectroscopy (XPS) using a Thermo ESCALAB Xiþ with
pounds have been applied in water electrolysis.
layered materials that were synthesized in 2007 [66,67]. In 2018, Li et al.
predicted that the -PdSeO3 monolayer is a promising photocatalyst
α- and β-PdSeO3 are 2D
α
which can split water without using sacrificial agent based on density
functional theory (DFT) computations [68]. The calculations also predict
that the exfoliation of the bulk 2D material should be easy, and require
similar energy as that required for the exfoliation of graphene. In 2020,
Liu and Zhang et al. did obtained 2D α-PdSeO3 nanosheets (NNs) by using
the quaternary ammonium intercalation-assisted electrochemical exfo-
liation method and obtained good activity and stability towards photo-
catalytic H2 evolution from water [69]. Since photocatalysts can often be
used also as electrocatalysts, and Pd-based materials have been used as
water-splitting electrocatalysts [70–73], whether PdSeO3 can also be a
water-splitting electrocatalyst; and if it is, how the structural differences
Al Kα radiation. All binding energies were corrected based on the C 1s
peak (284.8 eV).
2.2. Synthesis of α-PdSeO3 bulk material
and the layer thickness of
water-splitting properties are worth investigations.
Herein, we improved the syntheses of - and β-PdSeO3 bulk materials,
and obtained their NNs based on the ultrasonic-assisted liquid-phase
exfoliation similar to that of making graphene NNs. The bulk materials
are stable in pH ¼ 0–14 value aqueous solution for at least 48 h. The
water electrolytic properties of the above four materials were investi-
α- and β-PdSeO3 affect the electrocatalytic
The experiment was carried out in a 50 mL open single-necked
spherical flask. 5.0 mL selenic acid aqueous solution (19 mmol) was
heated at 200 ꢁC for 35–40 min using an oil bath and cooled down natu-
rally to 25–60 ꢁC. The final liquid was colourless, transparent and slightly
viscous and the condensed liquid is about 1.0 mL of its original volume.
In a typical synthesis, Pd(NO3)2⋅2H2O (90 mg, 0.34 mmol), KNO3
(88.6 mg, 0.876 mmol), concentrated H2SeO4 (0.200 mL, about 3.9
mmol assuming it does not change to H2SeO3) and deionized water (0.50
mL, 28 mmol) were added in a glass vial (height 4 cm and diameter 1.1
cm) and stirred for 30 min. Since Pd(NO3)2⋅2H2O, KNO3 would absorb
water, their weighing process was carried out in a glove box under argon
atmosphere (H2O < 10 ppm). The overall molar ratio of Pd(NO3)2:
H2SeO4: KNO3: H2O is 1 : 11: 2.6 : 84. The vial containing the reaction
mixture was transferred to a 20 mL PTFE-lined steel autoclave, put into
an air circulation oven at room temperature, and heated at 200 ꢁC for 2 d.
The oven was then allowed to cool down naturally. Collected products
were washed several times with methanol and water, then dried over-
α
gated.
hydrogen evolution reactions in 0.5 M H2SO4, much better than the ac-
tivities of the bulk materials; and -PdSeO3 NNs have better performance
α- and β-PdSeO3 NNs exhibit moderate catalytic activities for
α
than that of β-PdSeO3 NNs. In addition, β-PdSeO3 NNs and bulk material
show good and similar OER catalytic properties in 0.5 M H2SO4. Thus,
β-PdSeO3 NNs were found to be a good overall water splitting catalyst in
0.5 M H2SO4 solution at a catalyst loading of 0.28 mg cm-2
(η10: 209 mV
(HER) and 405 mV (OER)). To our knowledge, only a handful overall
water-splitting electrocatalysts can operate in acidic solutions [21–23,
74–80]; most of them are Ir- or Ru-based; and there is only one Pd-based
compound (Sr2PdO3) [80], which has worse catalytic activity than that of
β-PdSeO3 NNs. The above catalysts all show long-term stability. In
night in vacuum oven at 60 ꢁC for 8 h, obtaining crystalline
α-PdSeO3
bulk material (50.3 mg, 0.216 mmol) of 64% yield. Plastic spatulas
should be used instead of metal ones during the synthetic process.
contrast, α-PdSeO3 does not show OER catalytic properties.
2

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
2.3. Synthesis of β-PdSeO3 bulk material
otherwise. Measurements were performed on an electrochemical work-
station (CHI 760E, CH Instruments Inc., Shanghai) using a typical three-
electrode setup with an electrolyte solution of 0.5 M H2SO4, a platinum
wire (OER) or a graphite rod (HER) as the counter electrode, an Ag/AgCl
electrode (saturated KCl) as the reference electrode, and the modified
glassy carbon electrode with the diameter of 3.0 mm (GCE, geometric
area: 0.071 cm2) as the working electrode in a N2- (HER) or O2- (OER)
saturated solution. To prepare the working electrode, 2.5 mg of catalyst
Solution 1 was made in glove box by mixing H2SeO3 (2.00 g, 15.4
mmol), H2SeO4 aqueous solution (1.00 mL, 3.86 mmol), and 3.0 mL H2O
(molar ratio: H2SeO3: H2SeO4 ¼ 4.0 : 1); and stirred for 15 min. In a
typical synthesis, Pd(NO3)2⋅2H2O (100.0 mg, 0.375 mmol), KNO3 (88.3
mg, 0.873 mmol), 0.25 mL solution 1 and deionized water (0.25 mL,
13.9 mmol) were added in a glass vial and stirred for 30 min.
Pd(NO3)2⋅2H2O, KNO3, and solution 1 were added in the glove box.
Plastic spatulas should be used instead of metal ones during the synthetic
process. The overall molar ratio of Pd(NO3)2: H2SeO3: H2SeO4: KNO3:
H2O is 1 : 2.6: 0.64 : 2.3: 72. The vial was transferred to a 20-mL PTFE-
lined steel autoclave, put into an air circulation oven at room tempera-
ture, and heated at 220 ꢁC for 4 d. The oven was then allowed to cool
down naturally. Other procedures are the same as that of the synthesis of
powders and 10
dispersed in 1.0 mL of a mixed water-alcohol (1: 1 v/v) solution to form a
homogeneous ink. 8.0 L aliquot of the catalyst ink was drop-cast onto
the glassy carbon electrode in three portions (3.0, 3.0 and 2.0 L) for the
μL of Nafion D-520 dispersion were ultrasonically
μ
μ
electrochemical measurements. After each dropping, the electrode was
dried in a 60 ꢁC oven for 10 min. Before each experiment, we scanned
each electrocatalyst by CVs in the range of -0.5–0 V (HER) or 1.0–1.5 V
(OER) for 40 cycles at a rate of 0.1 V s-1 for activity activation. The HER
and OER activities of the catalysts were evaluated using LSV at a scan rate
of 5 mV s-1. HER and OER potentials were converted to the reversible
hydrogen electrode (RHE) based on the following formula: ERHE ¼ EAg/
α-PdSeO3, obtaining crystalline β-PdSeO3 bulk material (64.7 mg, 0.277
mmol) of 73.6% yield, which was used for preparation for β-PdSeO3 NNs.
β-PdSeO3 could also be prepared using only H2SeO3. Solution 2 was
made in glove box by mixing H2SeO3 (1.00 g, 7.70 mmol) and 2.00 mL
H2O, and stirred for 15 min. In a typical synthesis, Pd(NO3)2⋅2H2O
(113.1 mg, 0.424 mmol), KNO3 (88.6 mg, 0.877 mmol), 0.300 mL so-
lution 2 and deionized water (0.400 mL, 22.2 mmol) were added in a
glass vial and stirred for 30 min. Pd(NO3)2⋅2H2O, KNO3, and solution 2
were added in the glove box. The overall molar ratio of Pd(NO3)2:
H2SeO3: KNO3: H2O is 1 : 2.7: 2.1 : 95. The vial was transferred to a 20-
mL PTFE-lined steel autoclave, put into an air circulation oven at room
temperature, and heated at 200 ꢁC for 4d. The oven was then allowed to
cool down naturally. Other procedures are the same as that of the syn-
þ 0.197 þ 0.059 pH.
AgCl
2.7. Photoelectrocatalytic (PEC) HER experiments
Experiments were carried out in an outer irradiation type photo-
reactor (50 mL quartz glass) in a N2 environment. Working electrode in a
typical HER measurement was replaced with sample-modified ITO glass.
The ITO glass (1.0 x 5.0 cm2, only one side is conductive) are pretreated
by ultrasonically washing with acetone, ethanol and water for 20 min,
respectively. 2.83 mg catalyst was dispersed in a 1.0 mL aqueous solution
thesis of α-PdSeO3, obtaining crystalline β-PdSeO3 bulk material (72.9
mg, 0.310 mmol) of 73.3% yield.
containing 500
520 dispersion under ultra-sonication for 2.0 h under 400 W. Afterwards,
50 L x 2 of the slurry was dropped onto the pretreated ITO electrode (the
μL deionized water, 500 μL ethanol and 10 μL Nafion D-
2.4. Synthesis of PdSeO3 nanosheets (NNs)
μ
bottom 1 .0 x 1.0 cm2 area out of the 1.0 x 5.0 cm2, and the rest area was
pasted with insulating paper), and dried at 60 ꢁC for 20 min after each
dropping (twice in total). The irradiation light source used was a 300 W
Xe lamp without light filter. The intensity of light reached the ITO
electrode was kept as 100 mW⋅cm-2 to simulate the full spectra of solar
light. Before irradiation, the system was thoroughly degassed to remove
air by bubbling N2 for 30 min.
The exfoliation NNs were prepared by a ultrasonic homogenizer
(SCIENTZ-IID). These NNs were prepared according to a published
method for the preparation of graphene [81]. Typically, a PdSeO3 pow-
der (20 mg) was dispersed in 40 mL isopropanol solvent. The mixture was
placed in a centrifugation tube (50 mL). It was ultra-sonicated for 4s at
300 W, stopped for 2s, and continued for 1 h, the crusher was then shut
off for 20–40 min; the above process was repeated 4 times at room
temperature. Liquid supernatant was obtained after centrifuged at 3000
rpm for 5.0 min, followed by centrifuged at 12000 rpm for 20 min and
removal of the supernatant. The NNs were collected after washing with
water and ethanol several times, and then freezing dried, obtaining about
1.8 mg product.
3. Results and discussion
3.1. Syntheses
Syntheses of
-PdSeO3 and obtained its structure based on single-cyrstal X-ray
diffraction [66]. We made -PdSeO3 as shown in Fig. 1 based on their
α-PdSeO3 Ling and Albrecht-Schmitt first synthesized
α
2.5. Electrochemical characterization
α
method with some modifications. We concentrated the selenic acid
aqueous solution (40%) using an oil bath instead of a heating plate and
the reaction time was reduced from 4 d to 2 d. Without concentrating the
All measurements in aqueous solution were carried out using three-
electrode systems with a saturated Ag/AgCl electrode as the reference
electrode. Experiments for linear sweep voltammetry (LSV) in aqueous
solution and photocurrent measurements were carried out using a Pt wire
as the counter electrode and the working electrode is the PdSeO3
modified ITO glass (sheet resistance 20–25 Ω/square). Electrochemical
impedance spectroscopy (EIS) analyses were carried out using a graphite
rod as the counter electrode, and sample modified glassy carbon as the
working electrode. A 0.382 V amplitude AC signal over a frequency range
from 100 kHz to 1 Hz were performed on an electrochemical workstation
(CHI 760E). Photocurrent measurements were performed with a Zahner
PEC workstation (Zahner, Germany). Detail procedures, Mott-Schottky
plot measurements, LSV in nonaqueous solutions and methods of mak-
ing modified electrodes are given in Supporting Information.
selenic acid aqueous solution, α-PdSeO3 could not be obtained. If the
heating period is too long (>40 min), some white precipitate would form
when the concentrated solution was cooled down to room temperature.
Addition of such slurry would result in PdSe2O5 [66] and other uniden-
tified product (Fig. S1). While too short concentration time (<35 min)
will lead to the appearance of β-PdSeO3 [67] alongside α-PdSeO3, sug-
gesting that a higher water content favour the formation of β-PdSeO3.
Although it is reported that selenic acid decomposes above 200 ꢁC ac-
cording to eq. (1) [82], we made α
-PdSeO3 at 200 ꢁC for 2 d. Thus, we
propose that under prolonged heating, eq. (1) does occur, producing
H2SeO3, which then react with Pd(NO3)2 to produce PdSeO3 or PdSe2O5
(when H2SeO3 is in large excess) according to eq. (2) or (3).
2.6. HER or OER measurements
2H2SeO4 → 2H2SeO3 þ O2
(1)
(2)
All polarization curves were iR-corrected in this work unless noted
H2SeO3 þ Pd(NO3)2 → PdSeO3 þ 2HNO3
3

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
3.2. Syntheses of β-PdSeO3
Arndt and Wickleder synthesized β-PdSeO3 by reacting palladium
powder and 1 mL of H2SeO4 in a glass ampoule at 350 ꢁC for 3 d followed
by cooling down slowly at a rate of 3 ꢁC/h. Here, we reported a much
simpler method to prepare β-PdSeO3. It was obtained using a mixture
with the overall molar ratio of Pd(NO3)2: H2SeO4: H2SeO3: KNO3: H2O is
1 : 2.6: 0.64 : 2.3: 72 at 220 ꢁC for 4 d. Its PXRD pattern are consistent
with the simulated PXRD pattern based on the single-crystal structure,
indicating pure bulk β-PdSeO3 crystals have been obtained (Fig. 1). It can
be synthesized using H2SeO3 alone (Table S1 No. 2 and Fig. S2(i)), can be
synthesized at 180 ꢁC with lower yield (Table S1 No. 13 and Fig. S2(o)),
and can be obtained within 2 d (Table S1 No. 6 and Fig. S2(f)).
The effect of various factors on the synthesis of β-PdSeO3 were
investigated (Table S1 and Fig. S2). Without adding H2SeO3, no PdSeO3
was synthesized. Adding KNO3 is beneficial to increase the yield of
PdSeO3 (Table S1 No 3). Excess water would reduce crystallinity
(Table S1 No. 4 versus No. 9, Fig. S2(c) versus Fig. S2(k)). Without adding
KNO3, pure β-PdSeO3 can also be prepared (Table S1 and Fig. S2(b)).
Fig. 1. PXRD patterns of
α and β-phase PdSeO3 bulk materials and the exfoli-
ated
α and β-phase PdSeO3 NNs compared to the simulated patterns based on the
cif files of single-crystal structures.
3.3. Syntheses of PdSeO3 NNs
2H2SeO3 þ Pd(NO3)2 → PdSe2O5 þ 2HNO3 þ H2O
(3)
Since Li et al. predict that the cleavage of
which is close to that of graphene (0.37 J/m2) [68]. The van der Waals
interactions between layers in β-PdSeO3 are similar to those in -PdSeO3
since the nearest atom-atom distances are similar (see the structure
section). We therefore made - and β-PdSeO3 NNs following the method
α
-PdSeO3 is only 4.2 J/m2,
α
α
of making graphene [81]. SEM images (Fig. 2(a)-(b)) indicate that the
bulk materials have layered structures. TEM images (Fig. 2(c)-2(f))
indicate that PdSeO3 NNs were obtained. Several NNs overlapping with
each other (darker places in Fig. 2(c)–(d)) and wrinkled sheets
(Fig. 2(e)–(f) and Fig. S5) were also observed. The thickness of the bulk
materials is greater than 1 μm (Fig. 2(a)–(b)). The thickness of α-PdSeO3
is about 15–20 nm; the thickness of β-PdSeO3 is about 13 nm and some
place can be as thin as 4 nm as shown in Fig. 3.
The energy-dispersive X-ray spectroscopy (EDX) elemental analyses
of the bulk materials are given in Fig. S14, showing an expected Pd to Se
ratio of 1 : 1.
3.4. Structures of
α-PdSeO3 and β-PdSeO3
The single crystal X-ray structure of
α-PdSeO3 and β-PdSeO3 have
been reported [66,67]. They have layered structures as shown in Fig. 4.
Although the atomic connectivities of these two phases are the same
(Fig. 4(a) and (c)), the steric arrangements of atoms are different
(Fig. 4(b) and (d)). Both structures contain planar [PdO4] units. Pd–O
distances are 2.008(2) and 2.021(2) Å in
β-PdSeO3. The O1–Pd–O2 are 88.96(7) and 85.14ꢁ in
respectively. Se–O1 and Se–O2 distances are 1.689(3) and 1.785(3) Å for
-PdSeO3, 1.6911 and 1.7632 Å for β-PdSeO3. The neutral layers are held
α
-PdSeO3, 2.014 and 2.012 Å in
α- and β-PdSeO3,
α
together only by weak Van der Waals interactions with respect to the
observed distances between the atoms of adjacent sheets (O–O distances
of 2.929(3) Å (O–O), 3.008(3) Å (Se–O), and 3.9017(5) Å (Se–Se) for
α
-PdSeO3; 3.1390-3.1586 Å (O–O), 3.1590 Å (Se–O), and 3.7021
Å(Se–Se) for β-PdSeO3.. The thicknesses of single layers of -and
α
β-PdSeO3 are 5.1 and 5.5 Å, respectively; and the distances between the
layers are less than 0.71 and 0.74 Å, respectively.
3.5. FT-Raman and FT-IR spectra of
α-PdSeO3 and β-PdSeO3 materials
The FT-IR and FT-Raman spectra of bulk and NNs of
α- and β-PdSeO3
were obtained and are shown in Fig. 5. The IR spectra of the NNs are
consistent with those of the bulk materials. The IR spectra shows that the
samples contain adsorbed water and methanol. The adsorbed methanol
has higher vibrational frequencies than those of gas phase methanol
(Fig. 5(I)). Methanol does not give IR bands in the <800 cm-1 region. The
Fig. 2. Morphology of
-PdSeO3 (a) and bulk β-PdSeO3 crystals (b), (c) and (e)TEM images of
NNs (NNs), (d) and (f) TEM images of β-PdSeO3 NNs.
α
-PdSeO3 and β-PdSeO3. (a)–(b) SEM images of bulk
α
α-PdSeO3
4

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 3. AFM images and the corresponding thickness (b) (d) of
α
-PdSeO3 ((a)–(b)) and β-PdSeO3 ((c)–(d) NNs.
- and β-PdSeO3 bulk materials, X-ray photoelectron spectros-
IR and Raman bands of
α
- and β-PdSeO3 are summarized in Table 1 and state of α
their assignments are straightforward by comparison with literature data.
copy (XPS) spectra were obtained as shown in Fig. 6. C1s (248.1 eV) is
α
- and β-PdSeO3 consists of planar PdO4, Pd–O–Se, and SeO3 units. The
bands due to PdO4 were assigned by comparison with compounds con-
used as a benchmark to calibrate the position of all element peaks. The
high-resolution Pd 3d spectrum of α-PdSeO3 shows that Pd 3d3/2 and Pd
taining also PdO4 units [83–87]. In the PdO structure, the Pd atoms are
3d5/2 bands are at 343.0 and 337.6 eV (Fig. 6(b)), respectively, which can
be assigned to the Pd2þ state and nearly the same as those reported by
Zhang et al. (343.1 and 337.7) [69] and those of β-PdSeO3 (Fig. 6(b)).
The bands associated with the Se 3d3/2 and Se 3d5/2 bands are at 59.9 and
58.4 eV (Fig. 6(c)), respectively, and are assigned to the Se4þ state, which
disagree with the assignments of Zhang et al. [69] They assigned the
broad peak at about 59 eV to only Se 3d5/2. Since Se0 3d3/2 and 3d5/2
bands appears at 56.5 and 55.6 [93], we think the Se 3d3/2 in PdSeO3
should appear at a position close to that of Se 3d5/2. And we successfully
simulated the broad peak into two bands with an area ratio of 2:3 to be Se
3d3/2 and 3d5/2, respectively. The Se 3d3/2 and 3d5/2 band positions in
planar coordinated with four
tetrahedral-coordinated with four Pd [86]. Pd–O bond length in PdO is
2.01 Å [87], which is slightly shorter than the P–O bond lengths in - and
β-PdSeO3 (2.008-2.021 Å). McBride et al. assigned the Raman bands of
PdO [86]. G. Kliche obtained and assigned the IR spectrum of PdO [83].
The sharp band observed at 652 cm-1 in the Raman spectrum of PdO is
assigned to the B1g stretching mode. Baran et al. obtained the IR spectrum
of a PdO4 unit containing compound Nd2BaPdO5 [84]. They observed the
antisymmetric stretchings of the PdO4 unit at 645 and 622 (vs).
The symmetry of the selenite ion is similar to that of sulfite, which is
tetrahedral with one vacant orbital, thus giving C3V symmetry. The
selenite ion SeO23ꢂ thus has four fundamentals: the ν1 symmetric
stretching vibration (approximately 790–806 or 760–855 cmꢂ1); the ν2
symmetric bending vibration (approximately 430–461 cmꢂ1); the doubly
degenerate ν3 antisymmetric stretching vibration (approximately
714–769 or 680–775 cmꢂ1); the ν4 doubly degenerate antisymmetric
bending vibration (approximately 387–418 cmꢂ1). All vibrations are
Raman and infrared active [88]. The symmetry of the SeO23ꢂ ions is
remarkably lower in PdSeO3 than in the free anions. Thus, the bands
would not be degenerate and would split. We assigned the bands due to
SeO23ꢂ (Table 1) by comparing the spectra and assignments of similar
selenite compounds, e.g. Pb2Cu5(UO2)2 (SeO3)6(OH)6⋅2H2O [88] (Se–O:
1.68-1.73 A) [89], Er2(SeO3)3 and Dy3(SeO3)4F (Se–O: 1.65-1.68 and
O
atoms, while
O
atoms are
α
β-PdSeO3 are similar to those of α-PdSeO3 (60.0 and 59.2 eV, respec-
tively). The crystal structures show that both PdSeO3 have two kinds of
O, one is three-coordinated (Oa) and one is two-coordinated (Ob) and
their molar ratio is 1:2. We successfully simulated the O1s spectra to
three peaks, fixing the peak area ratio of Oa and Ob to be 1:2, the other
peak is due to adsorbed water molecules. XPS spectra shows that
α
-PdSeO3 adsorbed more water than that by β-PdSeO3, consistent with
the IR spectra shown in Fig. 5. The bands of Oa1s, Ob1s of - and
β-PdSeO3 and O1s in H2O and methanol bands appear at 534.6, 534.7
-PdSeO3); 530.5 and 530.8 (β-PdSeO3); and 532.5 and 532.7 (H2O and
methanol) eV. In summary, the XPS spectra indicate that the composition
of the surfaces of - and β-PdSeO3 are consistent with the bulk materials.
α
(
α
α
1.70-1.72 Å) [90] and others [91,92]. The Se–O distances in
α- and
3.7. Stability of α- and β-PdSeO3 bulk materials
β-PdSeO3 are 1.69, 1.76 (β) and 1.79 ( ) Å.
α
Since Se–O–Se symmetric stretching vibration were found at 540-577
cm-1 in Se2O25ꢂ salts [92] and Pd has higher mass than Se, we assigned the
peak at 510-521 cm-1 to the symmetric stretching of Pd–O–Se unit.
α
- and β-PdSeO3 bulk materials are stable in pH ¼ 0–14 aqueous so-
lutions for at least 48 h (Fig. 7). The IR spectra of selected samples after
immersing in acidic or basic aqueous solutions for 48 h were also taken
(Fig. S13), and shows the same patterns as those of as-synthesized sam-
ples, again indicating the high stability of PdSeO3 materials. The ther-
mogravimetric (TG) and the differential thermal analyses (DTA) were
3.6. XPS of
α-PdSeO3 and β-PdSeO3 bulk materials
To verify the surface elemental composition and elemental oxidation
carried out with these materials (Fig. S3). Bulk α-PdSeO3 bulk material
5

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 5. FT-IR and Raman spectra of α- and β-phase PdSeO3 bulk and NNs ma-
terials in the range of 3600-50 cm-1 (I) and 1000-100 cm-1 (II). IR spectra of gas
phase methanol and net water were obtained from https://webbook.nist.
gov/chemistry/.
Table 1
IR and Raman bands of α
- and β-PdSeO3 and tentative assignments.a
α
- PdSeO3
β-PdSeO3
IR
Raman
780 s
IR
Raman
794 w
Assign.
874 vw
828 m
745 s, br
ν1(SeO23ꢂ) symmetric stretching
ν1(SeO23ꢂ) symmetric stretching
ν3(SeO23ꢂ) antisymmetric stretching
Fig. 4. Structures of α-PdSeO3 ((a)–(b)) and β-PdSeO3 ((c)–(d). Viewing along a
and b axes and the structures of one layer ((b) and (d)). Unit cell edges
are shown.
806 m
728 s,br
672 vw
620 s
589 m
568 w
637 m
607 m
578 m
521 s
638 s, br
531 m
636 m
620 m
PdO4 stretching mode
PdO4 stretching mode
PdO4 stretching mode
Pd–O–Se symmetric stretching
ν2(SeO23ꢂ) symmetric bending
ν1(SeO23ꢂ) antisymmetric bending
decomposes at 416 ꢁC, and bulk β-PdSeO3 bulk material decomposes
drastically at 464 ꢁC. Thus, β-PdSeO3 bulk material was found to be more
510 s
436 w
417 s
405 w,br
312 w
281 m
stable than α-PdSeO3 bulk materials. β-PdSeO3 bulk material was also
found to be stable at 310 ꢁC for at least 12h as indicated by PXRD results
shown in Fgi. S4. Both materials decompose to PdO as indicated by PXRD
pattern (Fig. S4). The weight loss after decomposition also fit very well as
expected according to eq. (4). The theoretical weights of PdO left are
283 s
211 w
176 m
141 s
202 s
128 w
95 m
A1 vibrational mode of of PdO4 [87]
52.1% for α- and β-PdSeO3, respectively, consistent with the observed
a
weights in TG/DTA curves (52.9 and 52.0%, respectively) as shown in
Fig. S3. When using 13.033 mg sample, the obtained TG curve of
β-PdSeO3 (Fig. S3b) are flat before the decomposition temperature, and
gave excellent fit with the theoretical weight percentage of PdO. When
measuring its TG curve with <5 mg sample, the TG curve gradually went
s ¼ strong, m ¼ medium, br ¼ broad, w ¼ weak, vw ¼ very weak.
down even before decomposition temperature as also observed with
-PdSeO3 (Fig. S3(a), 4.183 mg). Thus, if a TG curve gradually goes
down, the sample might not be decomposing. Such phenomena are
α
6

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 6. (a) Full spectrum survey and (b–d) high resolution XPS spectra of PdSeO3 bulk materials.
Fig. 8. UV–Vis absorption spectra of the
α and β-phase PdSeO3 bulk materials.
greater than 500 nm in the range of 500–800 nm. If they are direct band-
gap semiconductor, the bandgaps of - and β-PdSeO3 bulk materials
would be 2.05 and 2.11 eV, respectively, determined by plots of (
)2 vs
where is the absorbance of the light, and h is the light energy
α
Fig. 7. PXRD patterns of as-synthesized α- and β-PdSeO3 bulk materials, and the
patterns after immersed in pH ¼ 0 and pH ¼ 14 aqueous solutions for 48 h.
αhυ
hυ
α
υ
derived based on the wavelength of the light (Fig. S8); if they are indirect
simply caused by instrumental factors, which can be detected by using
large amounts of sample (13 mg in our case).
band-gap semiconductors, the bandgaps would be 1.40 and 1.35 eV,
1/2
respectively, determined by plots of (
αhυ
)
vs hυ based on UV–vis
reflectance spectra (Fig. S8) [94,95]. They are found to be direct
band-gap semiconductors based on electrochemical studies (see below).
PdSeO3 → PdO þ SeO2
(4)
3.9. Determination of band-edge positions
3.8. UV–Vis diffuse reflectance spectra of bulk α- and β-PdSeO3 materials
The conduction band minimum (CBM) (LUMO) and valence band
maximum (VBM) (HOMO) positions of materials were also determined
by an EC method, the linear potential scan method [94,96–99]. In this
method, the potential corresponding to the first onset oxidation current
and the potential corresponding to the first onset potential of reduction
The ultraviolet–visible (UV–vis) diffuse reflectance spectra of
α
- and
β-PdSeO3 bulk materials in the form of absorbance are given in Fig. 8.
Their adsorption of light is strong and nearly constant in the range of
250–500 nm and gradually decreases when the wavelength increases to
7

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
current can be observed by separate cathodic and anodic scans, respec-
tively; and are regarded as the energy levels of the HOMO (CBM) and
LUMO (VBM) of the material, respectively. The band gap (Eg) is the
difference between the two on-set potentials. In our study, the linear
potential scans were also carried out using ITO working electrodes
without modifying with samples under the same condition as back-
ground curves to compare with those scans using the sample modified
electrodes.
3.10. Why the values of CBM and VBM determined by LSV are different
when using different electrolyte?
The VBM and CBM of α-PdSeO3 NNs were determined as -0.71 and
1.98 eV, thus the bandgap is 2.69 eV, which is very close to the value
determined using the ultraviolet–visible (UV–Vis) absorption spectrum
of the exfoliated PdSeO3 NNs obtained by Liu and Zhang et al. [69]. This
is also close to the value predicted for the PdSeO3 monolayer at 2.84 eV
[68]. Liu and Zhang et al. [69] determined the CBM and VBM and Fermi
Level at -3.99, -6.65 and (-6.65 þ 1.93) eV with respect to vacuum. Since
the NHE is -4.44 eV with respect to vacuum [68], these values are -0.45,
2.21 and 0.28 eV, respectively with respect to NHE. When we carried out
the LSV experiments to determine the band-edge positions,
sample-modified ITO electrodes were applied as the working electrode
using three electrode system. It is helpful to draw energy level digrams to
understand LSV results. The EF(ITO) equals þ0.40 V vs NHE [106,107].
The work function of ITO is -4.59 eV vs vacuum [108], thus its CBM is
þ0.19 vs NHE [109]. The band gap of ITO is in a range of 3.5–4.3, thus its
VBM is in a range of 3.7–4.5 V vs NHE. Based on the above information,
the important energy levels for a pH ¼ 0 aqueous solution/PdSeO3/ITO
working electrode system were presented in Fig. 11.
The CBM and VBM of α- and β-PdSeO3 bulk materials and NNs were
measured in pH ¼ 0, 2, 4H2SO4 aqueous solutions, and the results are
given in Table 2 (also in Fig. 9 and Fig. S6). The band-edge positions
remain constant in the pH range of 0–4. The CBM and VBM of α- and
β-PdSeO3 bulk materials were also measured in 0.2 M Na2SO4 aqueous
solution as described by Wang et al. [97] and in non-aqueous solutions
(0.10 M Bu4NPF6 in acetonitrile) according to the literature [100,101];
and the results are given in Table 2 and Fig. S7. However, in neutral 0.2
M Na2SO4 solution and non-aqueous solution the conduction band
maximum was found to be considerably lower (-0.6- -0.7 eV) than the
values obtained in the acidic aqueous solution (-0.08- 0.05 eV). The band
gaps of
α- and β-PdSeO3 NNs were much greater than those of bulk
materials, consistent with observations on other layered materials [102].
The CBMs of NNs have higher energies than those of bulk materials,
while VBMs of NNs have lower energies than those of bulk materials. Li
et al. reported that when the number of layers increase, the CBM values
Both ITO and PdSeO3 are n-type semiconductors and they form a step-
scheme heterojunctions [110]. Since the Fermi level of
higher than that of ITO, when they are in contact, the electrons in
-PdSeO3 NNs transfers to ITO across their interface until their Fermi
α-PdSeO3 NNs is
of
α
-PdSeO3 increases [68], which is consistent with the above experi-
α
mental results. The band-gap values determined by LSV method of
α- and
levels are the same (Fig. 11(b)), resulting in band bending and internal
electric field [110]. When a LSV experiment of negative scan is carried
out, a negative bias potential (-Δφ) is applied to ITO, the EF of ITO would
raise, the band-bending would decrease as shown in Fig. 11(c), the
β-PdSeO3 bulk materials are consistent with the values determined based
on UV–Vis reflectance spectra assuming they are direct band-gap semi-
conductors, but not with the values obtained (1.35-1.40 eV) assuming
they are indirect band-gap semiconductors (Fig. S8). Thus, the experi-
electrons can flow from ITO to α-PdSeO3 NNs to form a current [111].
mental values suggest that both
band-gap materials.
α- and β-PdSeO3 bulk materials are direct
However, to form a continuous current also requires that the crossover
electrons can reduce a species (e.g. Hþ) in the electrolyte. At the begin-
ning, the energy of electrons on ITO is still too low to do the reduction as
shown in Fig. 11(c) and no current flow can be observed [112]. To form a
current, a species in the electrolyte should be oxidized on the surface of
the counter electrode. This is usually not a problem since the counter
electrode in a three electrode system will often swing to extreme po-
tentials at the edges of the solvent window, where it oxidizes or reduces
the solvent or supporting electrolyte [113]. When the bias potential
continuously reaches to more negative potential, the bands of ITO and
MottꢂSchottky plots of
α- and β-PdSeO3 bulk materials are obtained
in 0.2 Na2SO4 aqueous solution as shown in Fig. 10. The negative slope of
the C-2 versus potential E indicate that they are n-type semiconductor
[103]. By extrapolating the linear part of the Mott-Schottky plots to the X
axis, the flat band potentials of α- and β-PdSeO3 bulk materials were
found to be -0.77 and -0.80 V versus.
Ag/AgCl. It is known that the conduction band potentials (ECB) for n-
type semiconductor is more negative about 0.1–0.3 eV than its flat band
potential [104,105]. Thus, their ECB were estimated as -0.9 V (vs NHE)
using this method. Based on EVB ¼ ECB þ Eg and the Eg values obtained
from the UV–Vis reflectance spectra (2.05 and 2.11 eV) (see above), the
α
-PdSeO3 NNs would become flat, and then bend to the reversed di-
rections [103]. When the EF of ITO becomes higher than the CBM of
-PdSeO3 NNs, electrons would transfer from ITO to the conduction
α
EVB values of
α- and β-PdSeO3 bulk materials were calculated to be 1.2 V
bands (CBs) of PdSeO3, and then to reduce Hþ in the solution to form
current. Thus, the on-set potential of this current would be the CBM of the
PdSeO3. For other cases, if there is lacking of a readily reducible species
in the electrolyte, the current would not form immediately, the value
would be measured higher than its real values. Thus, our reasoning
suggest that CBM values measured in neutral or non-aqueous solution for
(vs NHE). These values are consistent with values obtained from LSV
carried out in 0.2 M Na2SO4 solution and non-aqueous solution. How-
ever, this likely is not the true values for their band-edge positions (see
discussions in Why the values of CBM and VBM determined by LSV are
different when using different electrolyte? section).
Table 2
Theoretical and experimental determined band gaps (Eg in eV) and band positions (eV).
pH
0
2
4
Na2SO4 aq.a
Non-aqueousb
UV–Visc
BG (eV)
2.05
NHEd (eV)
BG (eV)
NHE (eV)
BG (eV)
NHE (eV)
BG (eV)
NHE (eV)
BG (eV)
NHE (eV)
BG (eV)
α
bulk
CBM
VBM
CBM
VBM
CBM
VBM
CBM
VBM
-0.09
1.74
-0.10
–
-0.72
2.02
-0.73
2.01
1.83
-0.03
1.79
-0.09
1.83
-0.71
1.96
-0.73
1.87
1.82
-0.03
1.80
-0.10
1.86
-0.71
1.95
-0.73
1.91
1.83
-0.68
1.79
-0.60
1.78
2.47
-0.77
1.59
-0.58
1.54
2.36
β bulk
А NNs
В NNs
–
1.90
2.67
2.60
1.94
2.61
2.64
2.38
2.12
2.11
2.74
2.74
a
0.2 M Na2SO4 was used as electrolyte.
b
using an Ag/Agþ (0.01 M AgNO3, 0.09 M Bu4NPF6 in acetonitrile) electrode as the reference electrode, 0.10 M acetonitrile solution of Bu4NPF6 as electrolyte. c.
Derived based on UV–Vis diffuse reflectance spectra (Fig. S8) assuming to be direct-band semiconductors. d Potentials obtained versus Ag/AgCl and Ag/Ag þ electrode
(nonaqeous) were added 0.197 and 0.54 V [101], respectively. e.Single layer as reported in Ref. [68].
8

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 9. Cathodic and anodic linear potential scans for searching the positions of the CB and VB edges of
0 ((a)–(b)); β-PdSeO3 bulk material in 0.2 M Na2SO4 aqueous solution ((c)–(d)).
α
and β-PdSeO3 bulk materials as well as NNs at pH ¼
PdSeO3 bulk materials are quite negative, which might likely be due to
the very low concentration of Hþ. Thus, using a high concentration acidic
solution would be beneficial to avoid such situations.
Similarly, if a LSV scan of positive bias potential is carried out, the EF
of ITO would decrease, however, this would not result in current flow
from α-PdSeO3 NNs to ITO since the EF of α-PdSeO3 NNs is still higher
than the oxidation potential of water (Fig. 11(c)). When the EF of ITO
would continuously decrease to an energy level below the VBM of
α
-PdSeO3 NNs, the electrons would flow from the valence bands (VBs) of
to the ITO, and H2O would transfer electron to the VBs of -PdSeO3 NNs.
In some other cases, if there is lacking of a readily oxidizable species in
the electrolytes, current would not form until the VBM of -PdSeO3 NNs
α
α
were also brought to a much lower energy level during the scan, so that
the species in the electrolyte can be oxidized. This would result in higher
VBM values than its true value. The EF of the sample would always try to
reach equilibrium with the ITO electrode. Thus, it would decrease as the
EF of ITO decreases. To reduce the phenomenon (since this will change
the band-edge values), a fast scan rate would help. Moreover, if the
interested area on the ITO surface is not completely covered with the
sample or some of ITO surface was exposed to the solution, when the
CBM of ITO is below 1.23 V, not 2.21. H2O would transfer electron to the
CBM of ITO to form current, and one would thought it is due to VBs of the
sample, and obtained a CBM value much smaller than its true value. Thus
electrode preparation need to be meticulous and no immersion of ITO
parts should be in the electrolyte.
The above analysis suggests that values obtained in neutral Na2SO4
solution and non-aqueous solution may not be reliable, and to do such
experiments, a readily reducible (for CBM measurements) or oxidizable
species (for VBM measurements) might need to be added into the elec-
trolyte. How to accurately measure the CBM and VBM of a semiconductor
material using LSV is still need further investigations.
3.11. Photoelectric responses
The photoelectrical properties of the exfoliated α- and β-PdSeO3 NNs
and bulk materials were studied using a three-electrode setup under 365
nm and 470 nm light illumination at -0.5, -0.1, -0.25, 0.1, 0.25 and 0.5 V.
Photocurrents were observed only when applying 0.5 V bias potential
versus Ag/AgCl (saturated KCl) as shown in Fig. 12. Under illumination,
the photocurrent increases with anodic potential as commonly observed
Fig. 10. Mott-Schottcky plots of (a) α-PdSeO3 and (b) β-PdSeO3 bulk materials
in 0.2 M Na2SO4 aqueous solution.
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Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 12. Photocurrent density–time curves for the exfoliated
α- and β-PdSeO3
NNs (a) and and β-PdSeO3 bulk materials (b) in a solution of 0.1 M Tris-HCl
α
(pH ¼ 7.4) with λ ¼ 365 and 470 nm light at a bias voltage of 0.5 V versus
Ag/AgCl (saturated KCl).
Fig. 11. (a) The energy levels of ITO and α-PdSeO3 NNs before contact. (b) The
band-edge bending at the interface after contact. The CBM and VBM values
determined by Liu and Zhang et al. was used [69].
stoichiometric [119]. The current could not last for the entire light illu-
mination time when a longer wavelength or bulk materials were applied,
suggesting that the charge recombination rates were even greater than
the rate of charge generation through light excitation in these cases.
The photocurrent (105 and 410 nA cm-2) generated using
α- and
in a n-type semiconductor [114–117]. Thus, α- and β-PdSeO3 were found
β-PdSeO3 NNs are significantly greater than those (36 and 202 nA/cm2)
to be n-type semiconductors. This is consistent with the negative slopes of
the Mott-Schottky plots of these materials (Fig. 10), which also suggests
that they are n-type semiconductors.
using of bulk materials under 365 nm light illumination. However, the
photocurrent (26 nA cm-2) generated using
α
-PdSeO3 NNs are smaller to
that (36 nA/cm2) of
α-PdSeO3 bulk material under 470 nm light illumi-
The photocurrent density increases upon irradiation at shorter
nation. When the layer is thin, the light absorption would be poorer than
the thicker material, such less electron and holes were generated in the
wavelength (365 nm) for all materials except for α-PdSeO3 bulk material
(36–57 nA/cm2), indicating that the photogenerated electrons and holes
usually combine faster when applying longer wavelength. This obser-
vation was also reported by others [114,118,119]. The photocurrent
response was stable over the course of the experiment, meaning the
sensitization process was regenerative/photocatalytic and not
α
-PdSeO3 NNs than those generated in the
Photocurrents of β-PdSeO3 materials are greater than those of
α
-PdSeO3 bulk material.
α-PdSeO3
at 365 nm light irradiation except for the current generated under 470
nm light irradiation (same or smaller). This is consistent with the fact that
10

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
β-PdSeO3 bulk material absorbs more light than α-PdSeO3 bulk material
2M ꢂ H*⇌H2 þ 2M
(7)
in the range of 250–550 nm as shown in Fig. 8 β-PdSeO3 bulk materials
did not show current at 470 nm illumination is due to its rather poor
conductivity as shown in Fig. 13, which results in the localization of the
light generated electrons and holes, leading to fast charge recombination.
However, the photogenerated electrons and holes in its NNs could go
to surroundings to produce current. In contrast, the conductivity of
Where H* is the adsorbed hydrogen atom and M is the the electrode
material surface.
The performances of
they are not the best HER electrocatalysts and their catalytic activities are
α- and β-PdSeO3 NNs in acid media indicate that
moderate [122,125,127–130]. The performance of α-PdSeO3 NNs (η10 ¼
α-PdSeO3 bulk material is good enough to produce current.
185 mV, Tafel slope ¼ 88 mV dec-1) are similar to that of a MoS2/ul-
trathin graphene heterostructure (η10 ¼ 180 mV, Tafel slope ¼ 79 mV
dec-1) made by Zhao, Lyu and Zhang et al. [131].
3.12. Electrocatalytic hydrogen evolution
Most Pd-based HER electrocatalysts are Pd alloys or composites of Pd
metal and other materials [71–73,126,132]. Few reports are on other Pd
containing materials. In acidic media (0.5 M H2SO4), PdP2@CB (carbon
To measure the electrocatalytic activity of the NNs and bulk of α- and
β-PdSeO3 toward water splitting, the synthesized samples were drop-cast
black) gave an η
10 of 27.5 mV and a Tafel slope of 29.5 mV⋅dec-1, which is
onto glassy carbon electrodes. The electrocatalytic HER performance of
the NNs and bulk of
using a standard three-electrode system. The NNs samples are found to be
more active than the bulk materials, and -PdSeO3 is more active than
β-PdSeO3 (Fig. 13(a)). -PdSeO3 NNS shows the highest catalytic activity
among all catalysts (η10 ¼ 185 mV), followed by the 209 mV of β-PdSeO3.
The - and β-PdSeO3 bulk materials have overpotentials of 283 and 477
mV at 10 mAcm-2, respectively. The commercial Pt/C (20 wt%) gave a
10 of 24 mV and a Tafel slope of 29 mV⋅dec-1 obtained under the same
α- and β-PdSeO3 in acidic solutions were then tested
an outstanding performance [133]; Pd3P2S8 gave an η10 of 175 mV and a
Tafel slope of 123 mV⋅dec-1 with long-term stability [65]; Li–Pd3P2S8
gave an η10 of 91 mV and a Tafel slope of 29 mv⋅dec-1 [65], while
α
η
10 of 92 mV and a Tafel slope of 107 mV⋅dec-1
α
palladium black gave an
[65].
α
The HER performance of α-PdSeO3 NNs in 1.0 M KOH and 1.0 M PBS
(pH ¼ 7.0) (Fig. S11) were also tested, showing overpotentials of 0.591
and 0.682 V, indicating it is poor HER electrocatalyst in alkaline [134,
135] and neutral solutions [136,137].
η
experimental conditions, which are consistent with the reported values
(30 mV⋅dec-1) [120,121]. We did this experiment to show that our
experimental method is valid as we produced the literature result.
The above results are consistent with the Electrochemical impedance
spectroscopy (EIS) results of these materials (Fig. 13(e)). The semicircle
of NNs display a much smaller arc radius compared to the bulk materials,
substantiating that the 2D structure of PdSeO3 NNs can markedly pro-
mote the charge transfer through the solid/liquid interfaces. The
3.13. Photoelectrocatalytic hydrogen evolution reaction
Photoelectocatalytic HER experiments were carried out by replacing
the glass carbon working electrode with a sample-modified ITO glass.
The current density became much lower, and overpotentials were much
higher than those obtained when using glassy carbon electrode. Upon
light irradiation (Xe lamp, full spectra, 100 mW cm-2), the overpotential
α
-PdSeO3 NNs generate the smallest semicircle radius, thus achieving the
lowest charge transfer resistance among all the samples. Thus, it may
have the most efficient charge transport during the electrochemical HER
process. Without iR drop compensation, η10 values are only slightly
higher (less than 8 mV as shown in Fig. S9).
To understand the electrocatalytic HER kinetics, the Tafel slope was
determined by fitting the linear regions of Tafel plots to the Tafel equa-
of α
-PdSeO3 NNs at 1.0 mA cm-2 reduced significantly (from 212 mV to
13.6 mV) in contrast to a much smaller reduction using β-PdSeO3 NNs
(61–26 mV) (Fig. 14). These results consistent with the better conduc-
tivity of
α-PdSeO3 NNs which would facilitate the charge separation
(Fig. 13). Without iR drop compensation, the overpotentials at 2 mA cm-2
are only slightly higher (less than 6 mV) as shown in Fig. S10.
tion:
η
¼ b log j þ a, where η is the overpotential, j is the current density,
and b is the Tafel slope. The Tafel slope (b) designates the potential
difference necessary to increase or decrease the current density by 10-
fold, which indicates the mechanism of the HER process [122]. A
smaller Tafel slope usually indicates that current density can increase
faster with smaller overpotential change (i.e., faster reaction rate con-
stant), which implies good electrocatalytic kinetics [123,124]. As shown
3.14. Electrocatalytic oxygen evolution reactions
The four PdSeO3 materials were tested for OER electrocatalytic
properties in acidic, neutral and basic solutions and the results are given
in Fig. 15 and Fig. S12. α-PdSeO3 bulk and NNs materials gave over-
potentials over 1.0 V in all pH condition, which are poor performance.
However, β-PdSeO3 bulk and NNs materials gave overpotentials of 381
mV and 405 mV at 10 mA cm-2, respectively; Tafel slopes of 342 and 369
mV⋅dec-1, respectively in 0.5 M H2SO4 solution; stability at least for 6 h,
which are good performances considering the high pH value and low
catalyst-loading (0.28 mg cm-2). Surprisingly, the β-PdSeO3 bulk mate-
rials outperformed the β-PdSeO3 NNs. Under light irradiation using Xe
lamp and light intensity of 100 mW⋅cm-2 did not improve the perfor-
mances (Fig. 15(a)–(b)), which is probably again due to their poor con-
ductivities (see above).
The Tafel behavior, especially the Tafel-slope, is an important kinetic
parameter to reveal changes in the apparent OER mechanism [138]. The
value of the Tafel slope usually ranges from 30 to 120 mV dec-1 and can
help to determine whether the slowest step of the reaction is from the
first or second electron transfer or the recombination step. In our case,
much higher Tafel slope values are observed, indicating additional con-
tributions related to the oxide-surface processes, which might be a po-
tential drop in the anode charge layer, or a blockage of the electrode
surface by the bubble accumulations [139–143]. The high Tafel slope
values, 200–500 mV dec-1, were also reported in literature [80,143].
Water oxidation catalysts (WOCs) are essential to electrochemical
hydrogen production, but (electro)chemical instability remains a critical
in (Fig. 13(b)), the Tafel slope of
mV dec-1, this value is significantly smaller than those of β NNs (103 mV
dec-1), bulk (108 mV dec-1) and β bulk (172 mV dec-1), highlighting the
faster reaction kinetics of the NNs. After 1000 cycle testing, the over-
potential of -PdSeO3 NNs only increased 9 mV (Fig. 13(c)). As shown in
Fig. 13(d),
-PdSeO3 NNs can yield a stable overpotential at 10 mA cm-2
α-PdSeO3 NNs shows a Tafel slope of 88
α
α
α
α
for at least 6 h in 0.5 M H2SO4 solution, indicating the good long-term
operating stability.
The Tafel plot slope of PdSeO3 materials are much greater than the 29
and 39 mV⋅dec-1, which are for the chemical-desorption (Tafel reaction
as shown eq. (3)) rate limiting and the electrochemical-desorption
(Heyrovsky reaction as shown in eq. (2)) rate limiting HER processes,
respectively. If the rate limiting step is the first step-the hydrogen
adsorption step (the Volmer reaction (eq. (1)), the Tafel slope would be
about 120 mV dec-1 [122,125]. The distinct HER Tafel slopes of these
PdSeO3 materials suggest their different HER kinetics and intrinsic cat-
alytic activities [15,126].
H3Oþ þ M þ eꢂ⇌M ꢂ H* þ H2O
Hþ þ eꢂ þ M ꢂ H*⇌H2 þ M
(5)
(6)
11

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 13. (a) iR compensated polarization curves obtained in 0.5 M H2SO4 solution at a scan rate of 5 mV s-1 (b) Corresponding tafel slopes of
materials and NNs. (c) Polarization curves of -PdSeO3 NNs before and after 1000 cycles. (d) Chronoamperometric test of
-PdSeO3 NNs at 10 mA cm-2 for 6h. (e)The
EIS Nyquist plots for - and β-PdSeO3 bulk materials and NNs.
α- and β-PdSeO3 bulk
α
α
α
Sr2PdO3 gave a η10 ¼ 450–550 mV, Tafel slope ¼ 454 mV⋅dec-1 in 0.1 M
HClO4 solution [80], which is less active than β-PdSeO3.
4. Conclusions
The syntheses of 2D α- and β-PdSeO3 bulk materials were greatly
improved. They are found to be n-type direct band-gap semiconductors.
Their transient photocurrent experiments suggest that they only weakly
respond to light irradiation. Their photocurrent intensities depend on the
conductivities and light absorbance properties of the materials. Stronger
conductivity and light absorbance would increase the photocurrent. In
addition, short wave-number light would produce stronger photocurrent,
which might be due to higher mobility of the photo-generated charges
produced by such light so that the charge recombination would be
supressed. The band-gaps, valence band maximums (VBMs) and con-
duction band minimums (CBMs) of α- and β-PdSeO3 bulk and NNs were
determined using Mott-Schottky plots and the linear potential scan
method in acidic solution, 0.2 M Na2SO4 and non-aqueous solution. We
found the electrolyte would affect the results. Although using LSV
method to determine CBM and VBM has been widely applied, our finding
indicate that how to reliably determine the CBM and VBM positons using
LSV method still need to be investigated.
Fig. 14. iR-compensated JꢂV curves of
α and β-phase PdSeO3 NNs measured
with or without light illumination of 100 mW/cm2 in a solution of 0.5 M H2SO4.
α
-
and β-PdSeO3 NNs exhibit moderate catalytic activities for
hydrogen evolution reactions in 0.5 M H2SO4, much better than the bulk
materials; and -PdSeO3 NNs have better performance than that of
issue for WOCs especially under acidic conditions [123,144,145]. In
principle, water electrolysis in acidic media has several advantages over
that in alkaline media, including favorable reaction kinetics, easy prod-
uct separation, and low operating pressure [19]. However, acidic water
electrolysis poses higher requirements for the catalysts, especially the
ones for the oxygen evolution reaction. It is a grand challenge to develop
highly active, durable, and cost-effective catalysts for acidic water
oxidation [19]. The state-of-the-art catalysts for OER in acidic media are
usually iridium-based metal or metal oxides [17–19,77,146]. To our
knowledge, the best OER electrocatalyts in acidic solution in terms of
overpotential and Tafel slope are a Mn-doped RuO2 prepared by Wang
and Chen et al. (η10 ¼ 158 mV, Tafel slope ¼ 43 mV⋅dec-1 in 0.5 M H2SO4
solution) [147]. The best Pd-based acidic water oxidation electro-
catalysts are Ir44Pd10 prepared by Xia et al. [148] (η10 ¼ 226 mV, Tafel
slope ¼ 54–69 mV⋅dec-1 in 0.1 M HClO4 solution) and Pd–Ru nano-
particles prepared by Tilley et al. [149] (η10 ¼ 225 mV, Tafel slope ¼ 61
mV⋅dec-1 in 0.1 M HClO4 solution). Besides Pd-metal-based acidic water
oxidation electrocatalyst, few Pd compounds were reported as acidic
α
β-PdSeO3 NNs. In addition, β-PdSeO3 NNs and bulk material show good
and similar OER catalytic properties in 0.5 M H2SO4. Thus, β-PdSeO3 NNs
were found to be an overall water splitting catalyst in 0.5 M H2SO4 so-
lution. The above catalysts all show long-term stability. In contrast,
α
-PdSeO3 does not show OER catalytic properties. Pd-based but not Pd-
metal-based acidic overall water splitting catalysts are very rare. Only
a strontium Palladium Perovskite Sr2PdO3 was applied in acidic OER
process [80], showing worse activity than that of β-PdSeO3. We also
found that PEC HER can be improved if the catalyst is more conductive.
Usually, thin layer materials were found to have better performance
than the thicker materials due to better charge transfer properties.
However, the β-PdSeO3 bulk material were found to have slightly better
performance than β-PdSeO3 NNs in the acidic OER reactions, which is a
surprising result deserving further investigations. This result might be
related with the slow kinetics of OER reactions.
Again, we found that a photocatalyst can be used as an electcatalyst
water oxidation electrocatalyst.
A strontium Palladium Perovskite
(α-PdSeO3). We think this is because that the energy provided by light
12

Y. Wu et al.
Journal of Solid State Chemistry 297 (2021) 122018
Fig. 15. (a)–(b) iR compensated OER polarization curves of β-PdSeO3 bulk (a) and NNs (b) obtained in 0.5 M H2SO4 solution at a scan rate of 5 mV s-1 (c) Corre-
sponding tafel slopes. (d)–(e) Chronoamperometric test of β-PdSeO3 bulk material (d)and β-PdSeO3 nns (e)at 9.2 mA cm-2 for 6h.
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CRediT authorship contribution statement
Yusheng Wu: Investigation, Data curation, Writing - original draft.
Lin Wang: Investigation. Hongyan Zhang: Investigation. Jie Ding:
Investigation. Min Han: Methodology. Min Fang: Conceptualization,
Methodology, Supervision, Writing - review & editing. Jianchun Bao:
Conceptualization, Methodology, Supervision. Yong Wu: Conceptuali-
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgements
We thank National Natural Science Foundation of China (NSFC) (Nos.
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