Improved photoelectrode performance of chemical solution-derived Bi2O3crystals: Via manipulation of crystal characterization

Article abstract of DOI:10.1039/d0ra08746g

Three-dimensional Bi2O3 crystals with various morphologies were successfully synthesized on F-doped tin oxide substrates with and without homoseed layers via chemical bath deposition (CBD) routes. The structural analysis reveals that control of the pH value of the reaction solution resulted in as-grown Bi2O3 crystals with nanosheet and plate morphologies. A lower pH value of the reaction solution engendered formation of a porous sheet-like morphology of Bi2O3; by contrast, a higher pH value of the reaction solution is favorable for formation of solid Bi2O3 plates on the substrates. Furthermore, a sputter coated Bi2O3 seed layer with dual α- and β-Bi2O3 phases plays an important role in the CBD-derived Bi2O3 crystallographic structures. The Bi2O3 crystals formed via CBD processes without a sputter coated Bi2O3 homoseed layer demonstrated a high purity in β-Bi2O3 phase; those grown with a homoseed layer exhibited a dual α/β phase. The photoactive performance results show that construction of an α/β-Bi2O3 homojunction in the CBD-derived Bi2O3 crystals substantially improved their photoactive performance. Comparatively, the porous Bi2O3 nanosheets with a dual α/β-Bi2O3 phase demonstrated the highest photoactive performance among various Bi2O3 crystals in this study. The superior photoactivity of the porous α/β-Bi2O3 nanosheets herein is attributed to their high light absorption capacity and photoinduced charge separation efficiency. The experimental results in this study provide a promising approach to design CBD-derived Bi2O3 crystals with desirable photoelectric conversion functions via facile morphology control and seed layer crystal engineering. This journal is

Full text of DOI:10.1039/d0ra08746g

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Improved photoelectrode performance of chemical
solution-derived Bi2O3 crystals via manipulation of
crystal characterization
Cite this: RSC Adv., 2020, 10, 45042
*
Yuan-Chang Liang
and Yu-Hsun Chou
Three-dimensional Bi2O3 crystals with various morphologies were successfully synthesized on F-doped tin
oxide substrates with and without homoseed layers via chemical bath deposition (CBD) routes. The
structural analysis reveals that control of the pH value of the reaction solution resulted in as-grown
Bi2O3 crystals with nanosheet and plate morphologies. A lower pH value of the reaction solution
engendered formation of a porous sheet-like morphology of Bi2O3; by contrast, a higher pH value of the
reaction solution is favorable for formation of solid Bi2O3 plates on the substrates. Furthermore, a sputter
coated Bi2O3 seed layer with dual a- and b-Bi2O3 phases plays an important role in the CBD-derived
Bi2O3 crystallographic structures. The Bi2O3 crystals formed via CBD processes without a sputter coated
Bi2O3 homoseed layer demonstrated a high purity in b-Bi2O3 phase; those grown with a homoseed layer
exhibited a dual a/b phase. The photoactive performance results show that construction of an a/b-Bi2O3
homojunction in the CBD-derived Bi2O3 crystals substantially improved their photoactive performance.
Comparatively, the porous Bi2O3 nanosheets with a dual a/b-Bi2O3 phase demonstrated the highest
photoactive performance among various Bi2O3 crystals in this study. The superior photoactivity of the
porous a/b-Bi2O3 nanosheets herein is attributed to their high light absorption capacity and
photoinduced charge separation efficiency. The experimental results in this study provide a promising
approach to design CBD-derived Bi2O3 crystals with desirable photoelectric conversion functions via
facile morphology control and seed layer crystal engineering.
Received 14th October 2020
Accepted 7th December 2020
DOI: 10.1039/d0ra08746g
rsc.li/rsc-advances
dened b and a polymorphs and their stabilization at room
temperature are complicated. Thus, the selective fabrication of b-
Introduction
Bi2O3 in a phase-pure form is still of great signicance.
Bi2O3 is an environmental friendly material, and it has received
much attention due to its unique electrical and optical prop-
erties, which leads to its extensive usage in various scientic
devices.1–3 Furthermore, it is a promising visible-light-driven
photocatalyst with a narrow band gap (2.0–2.9 eV), and the
valence band of bismuth oxide is made of a hybrid Bi 6s and O
2p orbit that decreases the symmetry of the band structure and
leads to the corresponding dipoles which could enhance its
photoactivity.4 Bi2O3 has different polymorphs;5 the monoclinic
a and tetragonal b phases have been reported to exhibit supe-
rior photoactivity among various polymorphs.2,6 Recently, several
works on synthesis of a-Bi2O3 crystals have been reported.7,8 By
contrast, a relatively few works on synthesis of b-Bi2O3 crystals are
proposed due to the difficulty of synthesizing the metastable b-
phase.9 The b-Bi2O3 has a higher photoactivity than a-Bi2O3
because of its lower band gap energy and higher optical
absorption in visible light region.10 Since the b-Bi2O3 phase
represents a metastable high temperature modication and is
known to transform readily to a-Bi2O3, controllable synthesis of
Bi2O3 crystals with various micro- and nanostructures,
including nanosheet, nanorod, nanober, and sphere have
been successfully synthesized via various chemical solution
routes.11–14 Among various morphologies, nanosheet semi-
conductor oxides have drawn substantial research interest due
to their unique geometry with outstanding electronic properties
and high specic surface area, which could provide large
amount of reaction sites and enable rapid charges generation
and transport.15,16 Till now, various synthetic methods have
been explored to prepare sheet- or plate-like b-Bi2O3 crystals
with various surface features.11,17 All the above sheet- or plate-
like Bi2O3 products exhibited outstanding visible-light-driven
light harvesting ability and photoactive performance. For
practical scientic applications, the cost of energy, time and raw
materials required for synthesizing Bi2O3 crystals processes
should be considered. Therefore, the chemical bath deposition
(CBD) because of its large area deposition ability, low cost
process, and easy process parameter control, is a promising
route to be used for developing Bi2O3 crystal growth.18 Notably,
several works have conrmed the importance of homoseed
layer for the crystal growth behavior of various chemical route
Department of Optoelectronics and Materials Technology, National Taiwan Ocean
University, Keelung 20224, Taiwan. E-mail: yuanvictory@gmail.com
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derived oxide crystals. For example, hydrothermal grown c-axis Bi2O3 and 13S-Bi2O3 represented Bi2O3 crystals formed at pH ¼
oriented zinc oxide nanorods were prepared on the c-axis 10 without a seed layer, pH ¼ 13 without a seed layer, pH ¼ 10
oriented ZnO homoseed layer.19 The WO3 homoseed layer with with a seed layer, and pH ¼ 13 with a seed layer, respectively in
desirable surface grain size shows important inuence on the this study.
vertical alignment and aggregation in chemical route-derived
The CBD route-derived Bi2O3 crystals herein are based on the
one-dimensional WO3 crystals.20 Furthermore, homoseed layer following reaction steps:
of CuO has an important inuence on the crystallinity and
In the nucleation process, bismuth nitrate reacts with the
morphology of CuO crystals grown via CBD routes.21 In addition complexing (TEA) reagent by dissociating the bismuth ions as:
to the crystal growth behavior affected by the seed layer, the
Bi(NO3)3 + N(CH2–CH2–OH)3 $ [BiN(CH2–CH2–OH)3]3+
+
seed layer also dominates the as-grown crystals' characteriza-
tion. However, systematical investigations of homoseed layer
effect on CBD deposition of sheet- or plate-like Bi2O3 crystals
and their resultant photoactive performance are not well
proposed. In this study, a Bi2O3 homoseed layer was formed via
sputtering deposition on substrates before the CBD Bi2O3
crystal growth. The present study introduces an important
scientic aspect: the usage of a homoseed layer to improve the
CBD process to change the growth and characteristics of two
types of 3D Bi2O3 crystals has not been proposed so far. The
effects of pre-formed Bi2O3 seed layer microstructures on the
crystal features of the CBD-derived Bi2O3 crystals are demon-
3NO3
(1)
[BiN(CH2–CH2–OH)3]3+ $ Bi3+ + N(CH2–CH2–OH)3 (2)
The pH value of the solution was adjusted to 10 or 13 with
NaOH solution herein to form Bi(OH)3:
NaOH $ Na+ + OHꢁ
(3)
(4)
Bi3+ + 3OHꢁ $ Bi(OH)3
Aer annealing in ambient air at the elevated temperature,
strated herein. Furthermore, the resultant crystal feature- the Bi(OH)3 converts into bismuth oxide (Bi2O3) via the
dependent photoactive performance of various 3D Bi2O3 crys- following reaction.
tals with and without a homoseed layer is also discussed. The
2Bi(OH)3 $ Bi2O3 + 3H2O
(5)
results herein are a good reference for design of chemical
solution derived 3D Bi2O3 crystals with desirable photoactive
performance with a homoseed layer engineering.
The thermal annealing treatment of the as-deposited lm
will cause oxygen to remove hydroxide ions and form the crys-
talline Bi2O3 crystals22
Experiments
Sample preparation
Materials analysis
Various Bi2O3 crystals were synthesized by chemical bath Sample crystal structures were investigated by X-ray diffraction
deposition (CBD). Fluorine-tin-oxide (FTO) glass substrates (XRD; Bruker D2 PHASER) analysis using Cu Ka radiation
were ultrasonically cleaned sequentially in distilled water, (wavelength: 0.15406 nm) with a two theta scan range of 20–60ꢀ
acetone and isopropyl alcohol. For preparation of CBD reaction and scan rate of four degrees per min. The surface morphology
solution, the 0.1 M bismuth nitrate was dissolved in deionized of the thin lm sample was characterized by scanning electron
water to make 50 mL stock solution. A highly alkaline reaction microscope at an accelerating voltage of 15 kV (SEM; Hitachi S-
solution was prepared by further adding 3 mL triethanolamine 4800). A transmission electron microscope was used to study
(TEA) as a complexing agent and 0.2 M NaOH. The FTO the detailed microstructure of the Bi2O3 sample at 200 KV
substrates were immersed vertically in a 200 mL beaker and the (HRTEM; Philips Tecnai F20 G2). To prepare the TEM test piece,
pH value of the resulting solution was adjusted to 10 and 13 rst scrape off the nanostructures grown on the substrate into
with various addition contents of 0.1 M NaOH, and the subse- the absolute alcohol solution, and then drip a few drops from
ꢀ
quent CBD reaction of Bi2O3 crystals was carried out at 65 ꢀC for the solution into the copper mesh. Finally place it in a 90 C
2 hours. The as-deposited samples were further annealed at oven for more than 12 hours. The diffuse reectance spectra of
300 ꢀC for 1 hour in ambient air to obtain crystalline Bi2O3 the Bi2O3 samples were recorded by using UV-vis spectropho-
phase. For the preparation of seed layered CBD samples, the tometer (Jasco V750) with a scanning speed of 17 nm sꢁ1 at the
Bi2O3 homoseed layer was formed on the FTO substrates 300–700 nm range. The elemental binding states of the Bi2O3
through post-annealing the sputtering deposited metallic Bi crystals were explored by X-ray photoelectron spectroscopy (XPS
ꢀ
thin lms at 400 C in ambient air for 1 hour. The Bi metallic ULVAC-PHI, PHI 5000 VersaProbe) using Mg Ka radiation. The
thin lms were sputter deposited at room temperature with electrochemical and photoelectrochemical properties of the
a pure Ar atmosphere. The working pressure during Bi thin-lm samples were conducted by the potentiostat (SP150, BioLogic).
growth was maintained at 2 Pa, and the sputtering power of the The illumination excited from the 100 W Xe lamp combined
bismuth metallic target was xed at 30 W. The distance between with a 420 nm cut-off lter was used as the light source during
the FTO glass substrate and target is approximately 7 cm. Then the experiments. The Bi2O3 lm grown on the 1 cm ꢂ 1 cm FTO
the Bi2O3 seed layer coated FTO substrates were used to prepare glass was used as working electrode. Platinum and Ag/AgCl were
the Bi2O3 crystals via CBD with the pH values of 10 and 13 as used as counter and reference electrode respectively. The 0.5 M
mentioned before. The sample codes of 10-Bi2O3, 13-Bi2O3, 10S- solution of Na2SO4 was used as electrolyte. The PEC
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measurement is carried out in a voltage of 1 V, and every 20 Bi2O3 (201), b-Bi2O3 (222), b-Bi2O3 (203), and b-Bi2O3 (213) are
seconds is used as an interval for switching the lights. Electro- identied in the Bi2O3 seed layer (a-Bi2O3: JCPDS no. 041-1449).
chemical impedance spectroscopy (EIS) measurements were The Bi2O3 seed layer exhibited a dual a/b mixed phase feature.
performed using the aforementioned potentiostat and the same In addition to the Bragg reections of the originally identied b-
electrode conguration. Furthermore, the Nyquist plots of Bi2O3 phase as observed in Fig. 1(a) and (b), several visible Bragg
various samples were measured at the open circuit potential reections originated from the crystallographic planes of the
with the frequency from 0.1 Hz to 150 kHz.
monoclinic a-Bi2O3 phase were also identied in Fig. 1(c) and
(d). The XRD results demonstrate that the CBD-derived Bi2O3
crystals with a homoseed layer-assisted growth exhibited
a mixed a/b mixed phase feature at the same given CBD growth
conditions. The initially formed a-Bi2O3 phase in the seed layer
might promote the a-Bi2O3 crystal formation during the CBD
growth. The XRD results suggested that two crystal growth routes
involved a-Bi2O3 and b-Bi2O3 crystal nucleation and growth
respectively on the a-Bi2O3 and b-Bi2O3 phase regions in the seed
layer might occur and competed during the given CBD growth
conditions herein. Comparatively, the 13S-Bi2O3 lm demon-
strated a higher number of Bragg reections originated from the a-
Bi2O3 phase and the ratio of the integrated Bragg reection
intensity of the a-Bi2O3 over the all integrated Bragg reection
intensity (35.9%) is higher than that in the 10S-Bi2O3 lm (12.2%).
This reveals that a higher ratio of the a-Bi2O3 phase existed in the
13S-Bi2O3 lm. Notably, a higher pH value in the CBD solution has
been shown to be more favorable to the a-Bi2O3 growth environ-
ment as revealed in the earlier work by Chen et al.24 The higher a-
Bi2O3 phase content in the 13S-Bi2O3 lm than that of the 10S-
Bi2O3 lm might also accounted for the integrated effect of dual
phase seed layer together with the solution pH value during the
phase evolution of the CBD-derived Bi2O3 lms in this study.
Results and discussion
Fig. 1(a)–(d) show the XRD patterns of various Bi2O3 lms. In
addition to the Bragg reections originated from the FTO
substrate, Fig. 1(a) and (b) show Bragg reections centered at
approximately, 27.95ꢀ, 31.76ꢀ, 32.69ꢀ, 46.22ꢀ, 46.91ꢀ, 54.27ꢀ,
55.63ꢀ and 57.76ꢀ corresponding to the (210), (002), (220), (222),
(400), (203), (213) and (402) of the tetragonal structured b-Bi2O3
(b-Bi2O3: JCPDS no. 027-0050). No other crystalline phases were
detected, conrming the phase purity of the b-Bi2O3 product for
the 10-Bi2O3 and 13-Bi2O3 lms. Furthermore, a substantially
high intensity of the (201) Bragg reection appeared in Fig. 1(a)
and (b), revealing a preferred (201)-oriented b-Bi2O3 crystals
dominated the crystallographic feature of the CBD-derived 10-
Bi2O3 and 13-Bi2O3 thin lms herein. A similar (201)-oriented
crystallographic feature has been observed in the spray pyrolysis
technique and hydrothermal derived b-Bi2O3 crystals.17,23
Fig. 1(c) and (d) display the XRD patterns of the 10S-Bi2O3 and
13S-Bi2O3 lms, respectively. For a comparison, the XRD pattern
of the Bi2O3 seed layer was demonstrated in Fig. 1(e). The visible
Bragg reections originated from a-Bi2O3 (120), a-Bi2O3 (200), b-
Fig. 1 XRD patterns of various Bi2O3 films: (a) 10-Bi2O3. (b) 13-Bi2O3. (c) 10S-Bi2O3. (d) 13S-Bi2O3. (e) Sputtering deposited Bi2O3 seed layer.
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Fig. 2 shows top- and cross-sectional views of SEM micro- ion; the thicker Bi(OH)3 will be formed during the CBD. The
graphs of various Bi2O3 lms prepared by various process initially formed thicker Bi(OH)3 crystals will result in formation
procedures. In Fig. 2(a), the as-synthesized 10-Bi2O3 lm is of the thicker Bi2O3 crystals aer the postannealing procedure.
composed of many nanosheets perpendicular to the substrate Fig. 2(c) and (d) show the SEM images of the 10S-Bi2O3 and 13S-
surface and crossing each other. These sheet-like structures Bi2O3 lms grown with the assistance of the homo-Bi2O3 seed
have the thickness in the range of 40–70 nm and width in the layers, respectively. In Fig. 2(c), the Bi2O3 nanosheets grown on
range of 2.5–3.5 mm. The Bi2O3 nanosheets contained numerous the homo-Bi2O3 seed layer showed similar crystal morphology
tiny pores. Furthermore, the Bi2O3 nanosheets exhibited as revealed in Fig. 2(a). Moreover, the size of the nanosheets
a rugged peripheral morphology, and the composition is loose herein is almost the same with those grown without a homo-
from the cross-sectional view observation. By contrast, the 13- Bi2O3 seed layer, but 10S-Bi2O3 nanosheets are more evenly and
Bi2O3 lm consisted of numerous plates with a high growth densely dispersed on the substrate. The Bi2O3 seed layer herein
density and shape uniformity as observed in Fig. 2(b). The Bi2O3 promoted the CBD-derived Bi(OH)3 crystal growth to be more
plates were in a rectangular morphology and the surface of uniformly along the seed layer. By contrast, the 13S-Bi2O3 lm
peripherals was smooth. Comparatively, the distribution exhibited a feathery crystal feature which differed from the 13-
density of the Bi2O3 plates over the area of the substrate is Bi2O3 lm (Fig. 2(d)). The feathery crystals had a thickness
substantially higher than that of the 10-Bi2O3 lm. From ranged from 1.0–1.5 mm and their length was approximately
Fig. 2(b), the thickness and width of the plates can be evaluated 3.0–4.0 mm. These feathery crystals were densely contacted to
to be approximately in the ranges of 500–700 nm and 2.5–3.5 each other; moreover, several feathery crystals are bundled and
mm, respectively. The SEM observations herein demonstrated arranged in a specic crystal orientation. This might be asso-
that the 13-Bi2O3 plates are thicker than that of the 10-Bi2O3 ciated with the crystallographic orientation of the homo-seed
nanosheets, revealing the higher pH reaction solution caused layer that affect the resultant CBD-derived crystal growth
an increased Bi2O3 crystal thickness. This can be understood orientation herein. It has also been shown that the homo-WO3
from earlier eqn (4) that the higher the pH value of the reaction seed layer promotes growth density and orientation of CBD-
solution, the more the bismuth ion reacts with the hydroxide derived H2WO4$nH2O precursor crystals and therefore, highly
Fig. 2 SEM top and cross-sectional views of various Bi2O3 films: (a) 10-Bi2O3. (b) 13-Bi2O3. (c) 10S-Bi2O3. (d) 13S-Bi2O3.
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Fig. 3 TEM analyses of the 10-Bi2O3 film: (a) low-magnification image of 10-Bi2O3 nanosheet. (b and c) High-resolution TEM (HRTEM) images
taken from the local regions of the sample in (a). (d) Selected area electron diffraction (SAED) pattern of the sample in (a). (e) Energy dispersive X-
ray spectroscopy (EDS) spectrum of the sample in (a).
oriented and dense WO3 nanostructures are formed aer orientation from a homoseed layer has been shown in the CBD-
postannealing procedures. Furthermore, a similar promotion of derived Fe2O3 and TiO2 oxide crystal growth. The improved
crystal distribution density and consistency of growth growth density and consistency of growth orientation in CBD-
Fig. 4 TEM analyses of the 10S-Bi2O3 film: (a) low-magnification image of 10S-Bi2O3 nanosheet. (b and c) HRTEM images taken from the local
regions of the sample in (a). (d) EDS spectrum of the sample in (a).
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derived FeOOH and Ti(OH)4 precursor crystals respectively on be easily distinguished from the grayscale contrast image of
the Fe2O3 and TiO2 thin homoseed layers result in formation of particles around the sample. Fig. 3(b) and (c) are high-resolution
dense and highly-oriented Fe2O3 and TiO2 crystals aer proper TEM (HRTEM) images taken from the outer regions of the
post annealing procedures.25,26 Notably, from earlier XRD nanosheet. The visible lattice fringes with multiple orientations
analyses, the formation of 13S-Bi2O3 lm is related to the crystal are observed in the Bi2O3 nanosheet, which reveals its poly-
structure of monoclinic a-Bi2O3 and tetragonal b-Bi2O3. A crystalline feature. The distinguishable interplanar spacings of
markedly higher phase content of a-Bi2O3 in the 13S-Bi2O3 lm approximately 0.31 nm and 0.27 nm are corresponded to the
might explain the observed substantial morphology change of (201) and (220) planes of tetragonal b-Bi2O3, respectively. Fig. 3(d)
the Bi2O3 lm derived from the pH ¼ 13 reaction solution on displays the selected area electron diffraction (SAED) pattern in
the homoseed layer. However, this morphology change is not which the sharp and bright spots were arranged in concentric
visibly observed for the Bi2O3 lm formed from the pH ¼ 10 circles with various radii. The (201), (002), (220), (222), and (421)
reaction solution with the assistance of homoseed layer crystal crystallographic planes were determined to correspond to the
growth. The relative low phase content of a-Bi2O3 in the 10S- tetragonal b-Bi2O3 phase herein. The composition of the Bi2O3
Bi2O3 lm might not substantially affect its crystal growth mode nanosheet was obtained by energy dispersive spectroscopy (EDS)
and the resultant morphology in this study.
analysis (Fig. 3(e)); the EDS spectrum indicated that the ratio of
Fig. 3(a) shows a low-magnication TEM image of 10-Bi2O3 Bi/O is approximately 0.71, and the C and Cu signals in the
nanosheet. The nanosheet exhibited a rugged peripheral spectrum were originated from the TEM grid.
morphology. Tiny pores existed in the nanosheet. The nano-
Fig. 4(a) presents a low-magnication TEM image of 10S-
sheet consisted of many nanoscaled Bi2O3 particles which can Bi2O3 nanosheet. Similar to Fig. 3(a), the nanostructure exhibits
Fig. 5 TEM analyses of the 13-Bi2O3 film: (a and b) low-magnification images of the 13-Bi2O3 plates. (c and d) HRTEM images taken from the
local regions of the sample in (a). (e) SAED pattern of the sample in (a). (f) EDS spectrum of the sample in (a).
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Fig. 6 TEM analyses of the 13S-Bi2O3 film: (a and b) low-magnification images of the 13S-Bi2O3 plates. (c and d) HRTEM images taken from the
local regions of the sample in (a). (e) SAED pattern of the sample in (a). (f) EDS spectrum of the sample in (a). (g) A summarized TEM morphology
chart of various Bi2O3 crystals.
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sheet morphology with an irregular periphery; moreover, crystallographic planes. A well crystalline quality of the Bi2O3 plate
abundant pores existed in the structure. Fig. 4(b) and (c) display was formed in this study. The EDS spectrum taken from the Bi2O3
HRTEM images taken from the outer regions of the nanosheet. plate (Fig. 5(f)), revealing that the atomic ratio of Bi/O is 0.69.
The distinct, ordered lattice fringes with multiple orientations
Fig. 6(a) and (b) show the low-magnication images of the
are observed in the nanosheet. The interplanar spacings of 13S-Bi2O3 feathery crystals. The feathery crystals are in a cone
approximately 0.32 nm and 0.25 nm are corresponded to the b- shape with two different aspect ratios can be observed herein.
Bi2O3 (201) and a-Bi2O3 (200), respectively. HRTEM images The granular particle assembled surface feature of the 13-Bi2O3
reveal well crystalline feature of the nanostructure. The plate was not easily observed in Fig. 6(a) and (b). Comparatively,
composition of the Bi2O3 nanosheet was further evaluated by the 13S-Bi2O3 feathery crystals exhibited dense and at surface
EDS spectrum (Fig. 4(d)); the Bi/O ratio of the nanostructure is feature. Fig. 6(c) and (d) display HRTEM images taken from the
approximately 0.71 in this study.
outer region of the feathery crystal in Fig. 6(a). Notably, the
The low-magnication TEM images in Fig. 5(a) and (b) show lattice fringes are quite regularly arranged and are visibly
that the morphology of the Bi2O3 plates formed at pH ¼ 13 (13- distinguishable at the corner regions. The lattice spacings with
Bi2O3) differed from that of the 10-Bi2O3 nanosheet. The 0.32 nm and 0.25 nm corresponded to the (201) plane of b-Bi2O3
periphery of 13-Bi2O3 plates was relatively at and the plates and the (200) plane of a-Bi2O3, respectively. The result further
were more rectangular shaped. The rectangular 13-Bi2O3 plate conrmed that a phase junction between a-Bi2O3 and b-Bi2O3
was made of Bi2O3 particles with a size of approximately 50 nm was obtained in the 13S-Bi2O3 feather crystal.27 Fig. 6(e) shows the
and can be observed in these TEM images. These Bi2O3 particles SAED pattern of the 13S-Bi2O3 feathery crystal in Fig. 6(a).
were rmly assembled to form the Bi2O3 plates, and the granular According to the SAED pattern, the (201), (002), (220), and (222)
feature is still visibly distinguished on the surface. Fig. 5(c) and (d) crystallographic planes were determined to correspond to the
are HRTEM images taken from the outer local regions in Fig. 5(a). tetragonal b-Bi2O3 phase. Moreover, the (002), (111), (120), (200),
The clearly lattice fringes with multiple orientations are also (022) and (140) crystal planes corresponded to the monoclinic a-
observed in the outer regions of the HR images where is the Bi2O3 phase were also identied in the SAED pattern. The SAED
thinner regions of the Bi2O3 plate. The granular particle images analysis revealed that the a- and b-Bi2O3 phases coexisted in the
can also be distinguished in these HR images. The interplanar 13S-Bi2O3 lm. The EDS spectrum in Fig. 6(f) supported the high
spacings with 0.32 nm and 0.27 nm corresponded to the (201) and composition purity of the as-synthesized, and the Bi/O atomic
(220) planes of tetragonal b-Bi2O3, respectively herein. Fig. 5(e) ratio is approximately 0.68. Finally, a TEM morphology compar-
reveals the SAED pattern taken from the selected Bi2O3 plate in ison chart of various Bi2O3 crystals was summarized in Fig. 6(g),
Fig. 5(a). The pattern exhibited several distinct diffraction rings the effect of homoseed layer on the morphology evolution is
consisting of sharp and bright spots that originated from the especially profound in the CBD-derived Bi2O3 crystals synthe-
tetragonal b-Bi2O3 (201), (220), (222), (400), and (203) sized at pH ¼ 13 reaction solution.
Fig. 7 XPS survey spectra of various Bi2O3 films: (a) 10-Bi2O3. (b) 13-Bi2O3. (c) 10S-Bi2O3. (d) 13S-Bi2O3.
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Fig. 7 shows typical XPS survey spectra of various Bi2O3 lms. lms; more loose and porous structure for the Bi2O3 lms
In these XPS survey spectra, the main binding feature peaks can synthesized at a lower pH value of the CBD reaction solution
be assigned to originate from Bi, and O, proving the purity of might accounted for the observed results.
the prepared Bi2O3 sample. Notably, the existence of carbon
Fig. 10(a) shows the optical absorption properties of various
signal in XPS survey spectra is associated with the exposure of Bi2O3 lms. For the 10-Bi2O3 and 13-Bi2O3 lms, a sharp drop of
samples to ambient air, engendering surface carbon contami- the absorption edge was observed, and the onset was at
nation. Fig. 8 shows the high-resolution XPS spectra of Bi 4f approximately 537 nm, which is consistent with the intrinsic
taken from various Bi2O3 lms. The Bi 4f7/2 peak at 157.1 eV and band-gap absorption of tetragonal b-Bi2O3.6 The result shows
Bi 4f5/2 at 162.4 eV could be observed with a separation of 5.3 eV, that b-Bi2O3 lms have good absorption in visible light region.
which was a characteristic of Bi3+ in Bi2O3 crystals according to The 10-Bi2O3 and 13-Bi2O3 lms have the similar absorption
the previous report.4 No distinct shoulder peaks appeared in Bi range of light wavelength, but the difference in light absorption
4f spectra, revealing bismuth was only in +3 oxidation state, and intensity is very obvious. The 10-Bi2O3 lm seems to have
no +5 and metallic states are present in the as-synthesized Bi2O3 signicant light utilization efficiency, which might be more
lms.28 Fig. 9 presents the high-resolution O 1s spectra. The effective in photoactive applications. By contrast, 10S-Bi2O3 and
asymmetric XPS spectra of the O 1s could be deconvoluted into 13S-Bi2O3 lms have an absorption edge (about 526 nm and
two subpeaks, which were oxygen in the lattice (Bi–O) at 512 nm, respectively) slightly lower than that of the 10-Bi2O3
approximately 528.1 eV (peak I) and surface-absorbed oxygen at and 13-Bi2O3 lms, respectively. The absorption edges of the
529.5 eV (peak II).4 Comparatively, Fig. 9(a) and (c) demonstrate 10S-Bi2O3 and 13S-Bi2O3 thin lms were between that of pure a-
the size of the subpeak at the higher binding energy is higher Bi2O3 and b-Bi2O3 phases, indicating that the existence of dual
than that of Fig. 9(b) and (d). This revealed that the surface- a/b-Bi2O3 phase in the samples.29 Notably, no much difference
absorbed oxygen content of the 10-Bi2O3 and 10S-Bi2O3 lms in the light absorption ability was found for the 10S-Bi2O3 and
is larger than that of the 13-Bi2O3 and 13S-Bi2O3 lms. An 10-Bi2O3 thin lms; this might be attributed to the fact that the
increased pH value of the CBD reaction solution to grow Bi2O3 content of a-Bi2O3 crystallite in the 10S-Bi2O3 lm is low, and b-
lms decreased the surface-absorbed oxygen size of the Bi2O3 Bi2O3 phase still dominated the light absorption property of the
thin lms accordingly. The relative surface-absorbed oxygen 10S-Bi2O3 lm. By contrast, the a-Bi2O3 phase content in the
content in the crystalline Bi2O3 lms was further evaluated 13S-Bi2O3 lm was markedly increased to affect the overall light
according to the intensity area ratio of Gaussian deconvolution absorption characterization of the 13S-Bi2O3 lm. This induced
subpeaks: (peak II)/(peak I + peak II). The relative surface- the marked blue-shi of absorption edge of the 13S-Bi2O3 lm
absorbed oxygen contents of the 10-Bi2O3, 13-Bi2O3, 10S-Bi2O3, and resulted in its absorption edge closer to that of pure a-Bi2O3
and 13S-Bi2O3 lms are approximately 31.6, 19.8, 33.4, and phase in comparison with that of the 10S-Bi2O3. The optical
23.1%, respectively. We inferred that this diversity may be absorption result of various Bi2O3 lms is consistent with the
caused by the different crystal morphology of various Bi2O3 XRD results. The bandgap energy (Eg) was evaluated using the
Fig. 8 High-resolution XPS spectra in the Bi 4f region of various Bi2O3 films: (a) 10-Bi2O3. (b) 13-Bi2O3. (c) 10S-Bi2O3. (d) 13S-Bi2O3.
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Fig. 9 High-resolution XPS spectra in the O 1s region of various Bi2O3 films: (a) 10-Bi2O3. (b) 13-Bi2O3. (c) 10S-Bi2O3. (d) 13S-Bi2O3.
Fig. 10 (a) UV-vis absorption spectra of various Bi2O3 films. (b) Evaluation of band gap of various Bi2O3 films. (c) Urbach energy plots of various
Bi2O3 films.
Kubelka–Munk function equation30 and the results are further where a0 is a constant and DE is the Urbach energy. A large
shown in Fig. 10(b). The bandgap energy of the 10-Bi2O3 and 13- Urbach energy would have greater tendency to convert weak
Bi2O3 thin lms was 2.5 and 2.43 eV, respectively. These bonds into defects. This is evident from the experimental
bandgap energies are consistent with the reported values of observations of the correlation between the defect density and
pure b-Bi2O3 phase.31 Furthermore, the evaluated bandgap Urbach energy in the ZnO and TiO2.16,34 The evaluated Urbach
energies of the 10S-Bi2O3 (2.53 eV) and 13S-Bi2O3 (2.64 eV) lms energies of the 10-Bi2O3, 13-Bi2O3, 10S-Bi2O3 and 13S-Bi2O3
are between that of reported values of pure a-Bi2O3 and b- lms are approximately 0.2078, 0.1254, 0.2263 and 0.1723 eV,
Bi2O3,32 which is in agreement with the aforementioned respectively. It is found that the Urbach energy of the Bi2O3
discussions. Notably, Urbach energy corresponds to the width lms formed at pH ¼ 10 is generally greater than the Bi2O3 lms
of localized states, and it is used to characterize the disorder formed at pH ¼ 13. This is attributable to the fact that the Bi2O3
degree in crystalline material systems. Fig. 10(c) shows that the lms formed at the reaction solution with a lower pH value
absorption coefficient near the absorption edge has an expo- resulted in the intermediate stage formed Bi(OH)3 precursor
nential behavior and obeys the empirical Urbach rule33 given by crystals with a porous and loose structure. This may increase
ꢀ
ꢁ
the possibility of initial crystal defects in the prepared Bi2O3
thin lms aer the postannealing procedure in this study.
The morphology effect on electrochemically active surface
area (ECSA) size of porous (10-Bi2O3 and 10S-Bi2O3) and solid
hv
aðvÞ ¼ a0 exp
(6)
DE
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Fig. 11 Cyclic voltammetry curves of various Bi2O3 thin-film electrodes measured at the non-faradaic region of ꢁ0.20–0.20 V vs. NHE with
different scan rates, varying from 0.1–0.5 V sꢁ1: (a) 10-Bi2O3. (b) 10S-Bi2O3. (c) 13-Bi2O3. (d) 13S-Bi2O3. (e) Scan rate dependent current density
curves of various Bi2O3 thin-film electrodes.
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(13-Bi2O3 and 13S-Bi2O3) Bi2O3 lms are further investigated;
Fig. 12(a) shows transient photoresponse curves of various
the relevant results provide overall electro-catalytic activity of Bi2O3 lms. The Bi2O3 lms exhibited perfect switching
the Bi2O3 electrode materials.35 The ECSA of the 10-Bi2O3, 13- behavior under the chopping visible light irradiation. Notably,
Bi2O3,10S-Bi2O3, and 13S-Bi2O3 lms was compared through the all the Bi2O3 photoelectrodes showed clear spike-like transient
values of electrochemical double-layer capacitance (Cdl) because response. The possible cause of this spike transient feature is
the Cdl is positively proportional to ECSA of the sample.36 associated with the recombination of the photoinduced elec-
Fig. 11(a)–(d) show cyclic voltammetry (CV) curves at non-faradaic trons and holes at the surface states of the oxides.39,40 The
potential regions (ꢁ0.2–0.2 V vs. NHE) with different scan rates maximum steady-state photocurrent density value achieved by
for various Bi2O3 lms. All the Bi2O3 lms showed the rectangular the Bi2O3 photoelectrodes under irradiation follows the order:
CV curves and peak separation took place with scanning speed 10S-Bi2O3 > 10-Bi2O3 > 13S-Bi2O3 > 13-Bi2O3. The photocurrent
from 0.1 to 0.5 V sꢁ1. Notably, the 10-Bi2O3 and 10S-Bi2O3 lms densities of the 10-Bi2O3 and 13-Bi2O3 photoelectrodes with
exhibited the higher current density at the constant potential a pure b-Bi2O3 phase are 0.0039 mA cmꢁ2 and 0.0012 mA cmꢁ2
,
than did the 13-Bi2O3 and 13S-Bi2O3 lms. The double-layer respectively. Comparatively, 10S-Bi2O3 and 13S-Bi2O3 photo-
charging current (Dj) is equal to ja ꢁ jc, in which ja is anodic electrodes with a dual a/b-Bi2O3 phase showed markedly
current and jc is cathodic current at the middle potential (0 V) enhanced photocurrent density under irradiation with respect
against the CV scan rate. The correlation between the double- to that of the 10-Bi2O3 and 13-Bi2O3 photoelectrodes, respec-
layer charging current and electrochemical double-layer capaci- tively. The maximum steady-state photocurrent densities of the
tance follows Dj ¼ vCdl, in which v is the scan rate.37 A plot of Dj as 10S-Bi2O3 and 13S-Bi2O3 photoelectrodes are approximately
a function of v yields a straight line with a slope equal to Cdl value 0.0078 mA cmꢁ2 and 0.0034 mA cmꢁ2, respectively; these
of the Bi2O3 lm (Fig. 11(e)). According to Fig. 11(e), the Cdl values photocurrent densities are around 2 and 3 times higher than
of 10-Bi2O3 and 10S-Bi2O3 thin lms are closed to be 0.024 mF that of the 10-Bi2O3 and 13-Bi2O3 photoelectrodes, respectively.
cmꢁ2 and 0.027 mF cmꢁ2, respectively. This is associated with the Notably, the 10S-Bi2O3 photoelectrode achieved the highest
similar morphology of the 10-Bi2O3 and 10S-Bi2O3 lms through photocurrent density in this study. The signicant increase in
earlier SEM observations. By contrast, the 13-Bi2O3 and 13S-Bi2O3 photocurrent density of the Bi2O3 photoelectrode under irradi-
lms exhibited smaller Cdl values of approximately 0.008 mF ation indicates the improved separation ability of photo-
cmꢁ2 and 0.013 mF cmꢁ2, respectively. The Cdl value of the Bi2O3 generated electron (eꢁ)–hole (h+) pairs in semiconductor
lms formed at pH ¼ 10 is almost 2–3 times higher than that of oxides.2,3 The transient photocurrent density vs. time curves
the Bi2O3 lms formed at pH ¼ 13 herein. Thus, the ECSA of the results herein demonstrated that the 10S-Bi2O3 photoelectrode
four Bi2O3 lms follows the trend 10S-Bi2O3 > 10-Bi2O3 > 13S- exhibited the superior photoinduced electron–hole separation
Bi2O3 > 13-Bi2O3. The higher ECSA of the 10S-Bi2O3 and 10-Bi2O3 efficiency among various Bi2O3 photoelectrodes. Notably, the
lms herein revealed more surface active sites exposed during photocurrent density of the Bi2O3 photoelectrodes with a dual
electrochemical measurements and maintained a sufficient a/b-Bi2O3 phase is signicantly higher than that of the Bi2O3
electrochemical reaction between thin-lm electrode and elec- photoelectrodes with a pure b-Bi2O3 phase under the similar
trolyte ions. This thus might deliver improved photo-/ synthesis solution pH value but with and without seed layer
electrocatalytic performance of the 10-Bi2O3 and 10S-Bi2O3 assisted growth, indicating that the a/b homojunction is effec-
lms in comparison with that of the 13-Bi2O3 and 13S-Bi2O3 tive to separate the photoinduced electron–hole pairs.2,41 This is
lms.38 The morphology effect on electrochemical active of related to the proper type II energy band arrangement between
various as-synthesized Bi2O3 lms with and without a porous a-Bi2O3 and b-Bi2O3, which markedly enhances photoinduced
structure is visibly demonstrated herein.
charge separation efficiency in the polymorphic Bi2O3 lm.2 It is
Fig. 12 (a) Transient photocurrent density versus time curves of various Bi2O3 photoelectrodes under chopped illumination at 1 V. (b) Nyquist
plots of various Bi2O3 photoelectrodes under visible light irradiation. (c) The possible equivalent circuit used for Rct evaluation of various Bi2O3
photoelectrodes.
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worth noting that 10-Bi2O3 photoelectrode has better photo- The Mott–Schottky plot according to the slope in the quasi-
response performance than did 13S-Bi2O3 thin-lm photo- linear region of Fig. 13(a) indicated that the at band poten-
electrode. One possible reason can be explained by the porous tial of 10-Bi2O3 is around 0.48 eV (vs. normal hydrogen elec-
structure of the 10-Bi2O3 lm observed earlier. The Bi2O3 lms trode, NHE). The at-band potential of 13-Bi2O3 is estimated at
with a porous structure could lead to a larger specic surface 0.49 eV (Fig. 13(b)). In general, the conduction band (CB)
area which contributed to more favorable adsorption of OHꢁ potentials are more negative by about ꢁ0.1 eV than the at
over the surfaces and lead to an improved light harvesting potentials for n-type semiconductors.48,49 The CB potentials of
ability of the porous lm compared with the solid one.42 The 10-Bi2O3 and 13-Bi2O3 are evaluated to be approximately 0.38 eV
aforementioned both reasons can explain the higher photo- and 0.39 eV, respectively. Moreover, according to the earlier
active performance of the 10-Bi2O3 photoelectrode than that of bandgap energy results of the Bi2O3 lms, the VB potentials of
the 13S-Bi2O3 photoelectrode. Fig. 12(b) shows the Nyquist plots 10-Bi2O3 and 13-Bi2O3 could be calculated as 2.88 eV and 2.82 eV
of various Bi2O3 photoelectrodes under irradiation. Small semi- (vs. NHE) according to the equation ECB ¼ EVB ꢁ Eg, respectively,
circular radii are typically attributed to photoelectrodes that which also match well with the values in the literature.50,51
exhibit small charge-transfer resistance; moreover, due to the Notably, 10S-Bi2O3 and 13S-Bi2O3 are polymorphic structures
small charge transfer resistance, there will be a higher separa- (dual a/b-Bi2O3 phases), the position of the conduction band
tion efficiency of photogenerated electrons and holes in mate- cannot be known from the current Mott–Schottky plots, so they
rial systems.2,16 From Fig. 12(b), the semi-circular radii of the are not further discussed. The at band potentials of 10S-Bi2O3
Bi2O3 photoelectrodes with a seed layer assisted crystal growth and 13S-Bi2O3 in Fig. 13(c) and (d) are between the reported
were smaller than those of the Bi2O3 photoelectrodes prepared reference values of the a-Bi2O3 and b-Bi2O3 phases,41 supporting
under the same process condition but without a Bi2O3 seed layer a a-Bi2O3/b-Bi2O3 junction existed in the 10S-Bi2O3 and 13S-
assisted growth, indicating that the charge-transfer resistance Bi2O3 thin lms. Furthermore, the carrier density of the oxide
of the Bi2O3 lms was substantially improved via a seed layer semiconductor is inversely proportional to the slope of the
assisted crystal growth during CBD. The earlier mentioned straight-line portion in the Mott–Schottky plot according to the
coexistence of a/b-Bi2O3 phases in the 10S-Bi2O3 and 13S-Bi2O3 proposed relationship of 1/C2 versus the applied potential.52 The
photoelectrodes might explain their lower internal charge tangent slope size in Fig. 13(a)–(d) shows the order: 10S-Bi2O3 <
transfer resistance than that of the 10-Bi2O3 and 13-Bi2O3 10-Bi2O3 < 13S-Bi2O3 < 13-Bi2O3. It is evidently that an increased
photoelectrodes, respectively. The relatively lower internal charge carrier density was found in the 10S-Bi2O3 and 13S-Bi2O3
charge transfer resistance can accelerate electron transfer and lms than that of their counterparts without a seed layer assisted
inhibit photoinduced electron/hole recombination.43 Compar- growth. This result might indicate that the formation of a-
atively, the Nyquist plot radius of the 10S-Bi2O3 photoelectrode b homojunction in the Bi2O3 lm could efficiently create a suffi-
was the smallest among various Bi2O3 photoelectrodes. The cient space charge layer to enhance the charge carrier trans-
multiple factors associated with a thin layer morphology, high portation in comparison with that of a pure b-Bi2O3 thin lm.
porosity, good light-capturing ability, and coexistence of a/b- Fig. 13(e) shows possible energy band diagram of the 10-Bi2O3
Bi2O3 phases might accounted for the observed results herein. and 13-Bi2O3 lms with a pure b-Bi2O3 phase according to the
Fig. 12(c) exhibits the possible equivalent circuits for a quanti- earlier analysis results. By contrast, for the 10S-Bi2O3 and 13S-
tative analysis of interfacial charge transfer ability of various Bi2O3 thin lms in which dual a/b-Bi2O3 phases existed in the
Bi2O3 photoelectrodes. As the illustrations shown, the solution lms, their homojunction energy band diagrams were con-
resistance Rs depends on the concentration and conductivity of structed with the assistance of a-Bi2O3 energy band diagram from
the electrolyte.44 The C is an ordinary double layer capacitance literatures.29 A possible schematic for visible-light-driven elec-
for the electrode surface.45 Rct represents the electron transfer tron–hole separation and transport at the homojunction inter-
resistance, and it can be estimated through the tting of arc face of both 10S-Bi2O3 and 13S-Bi2O3 is shown in Fig. 13(e). Since
radii of the Nyquist curves. Rct (a) is a charge transfer resistance the CB edge potential of a-Bi2O3 is more negative than that of b-
across the formed a-Bi2O3 electrolyte interface, and C (a) is an Bi2O3, the photoinduced electrons on the CB of the a-Bi2O3 will
ordinary double layer capacitance for the formed a-Bi2O3 transfer easily to the CB of the b-Bi2O3 via the well-developed
surface.46 In the current work, the separately evaluated Rct interface. Similarly, the photoinduced holes on the VB of the b-
values of the 10-Bi2O3, 13-Bi2O3, 10S-Bi2O3, and 13S-Bi2O3 are Bi2O3 move to the VB of the a-Bi2O3 due to the large difference in
approximately 1000, 6369, 618 and 1196 ohm. Overall, the PEC VB edge potentials retarding the recombination of the photoin-
and EIS results herein behaved that the Bi2O3 lms with a dual duced electron–hole pairs in the a/b-Bi2O3 heterojunction of the
a/b-Bi2O3 phase (10S-Bi2O3 and 13S-Bi2O3) exhibited superior 10S-Bi2O3 and 13S-Bi2O3 lms. Thus, the formation of the a-Bi2O3
charge separation and transportation efficiencies, in turn phase in the b-Bi2O3 matrixed lm via a seed layer assisted
reduced the interfacial resistance in comparison with that of growth could function as traps to capture the photoinduced holes
their counterparts (10-Bi2O3 and 13-Bi2O3).
and the homojunction of a/b-Bi2O3 could act as an active center
To understand the origin of the enhanced PEC activity in the for hindering the rapid recombination of photoinduced electron–
a/b-Bi2O3 composite photoelectrodes, the Mott–Schottky plots hole pairs. For the reasons mentioned above, it is known that
of various Bi2O3 lms are measured (Fig. 13(a)–(d)). The Mott– 10S-Bi2O3 and 13S-Bi2O3 will enhance the charge separation and
Schottky plots of various Bi2O3 lms with a positive slope thus the enhanced PEC and EIS performance in comparison with
indicated the n-type characteristic with electron conduction.47 their counterparts of the 10-Bi2O3 and 13-Bi2O3, respectively.
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Fig. 13 Mott–Schottky plots of various Bi2O3 photoelectrodes: (a) 10-Bi2O3 (b) 13-Bi2O3. (c) 10S-Bi2O3. (d) 13S-Bi2O3. (e) Possible photo-
generated electron–hole pathways between two polymorphic Bi2O3 semiconductors (a-Bi2O3 and b-Bi2O3) in this study.
Bi2O3 under the similar processes conditions. The sputter
coated a/b-Bi2O3 homoseed layer induced formation of a-Bi2O3
Conclusions
phase in the b-Bi2O3 matrix during the CBD crystal growth. The
PEC and EIS results show that the photoactive performance of
the CBD-derived Bi2O3 crystals is enhanced via a homoseed
layer crystal growth engineering. The proper type II band
alignment between a-Bi2O3 and b-Bi2O3 phase improved the
efficiency of photoinduced charge separation in the seed layer
assisted grown Bi2O3 crystals. Moreover, the porous structure of
In summary, the porous b-Bi2O3 nanosheets and solid b-Bi2O3
plates were successfully synthesized by CBD routes with reac-
tion solution of pH ¼ 10 and pH ¼ 13, respectively. These b-
Bi2O3 crystals were vertically aligned on the substrates with
a high crystallinity. The use of sputter coated a/b-Bi2O3 homo-
seed layer during CBD crystal growth shows an important effect
on the resultant crystallographic structures of the CBD-derived
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the Bi2O3 nanosheets formed at a lower pH value of reaction 11 H. Liu, M. Luo, J. Hu, T. Zhou, R. Chen and J. Li, b-Bi2O3 and
solution improved their specic surface area size and light-
trapping performance; therefore, the porous a/b-Bi2O3 nano-
sheets demonstrated the highest photoactive performance in
Er3+ doped b-Bi2O3 single crystalline nanosheets with
exposed reactive {0 0 1} facets and enhanced photocatalytic
performance, Appl. Catal., B, 2013, 140–141, 141–150.
this study. The porous and dual phase Bi2O3 nanosheets 12 S. Anandan, G.-J. Lee, P.-K. Chen, C. Fan and J. J. Wu,
synthesized herein may have the potential for applications in
photoactive devices.
Removal of Orange II Dye in Water by Visible Light
Assisted Photocatalytic Ozonation Using Bi2O3 and Au/
Bi2O3 Nanorods, Ind. Eng. Chem. Res., 2010, 49(20), 9729–
9737.
Conflicts of interest
13 C. Wang, C. Shao, L. Wang, L. Zhang, X. Li and Y. Liu,
Electrospinning
photocatalytic properties of Bi2O3 nanobers, J. Colloid
Interface Sci., 2009, 333(1), 242–248.
preparation,
characterization
and
There are no conicts to declare.
Acknowledgements
14 S. Kong, Z. An, W. Zhang, Z. An, M. Yuan and D. Chen,
Preparation of Hollow Flower-Like Microspherical b-Bi2O3/
BiOCl Heterojunction and High Photocatalytic Property for
Tetracycline Hydrochloride Degradation, Nanomaterials,
2020, 10(1), 57.
This research was funded by Ministry of Science and Tech-
nology of Taiwan. Grant No. MOST 108-2221-E-019-034-MY3.
15 X. Huang, C. Tan, Z. Yin and H. Zhang, 25th Anniversary
Article: Hybrid Nanostructures Based on Two-Dimensional
Nanomaterials, Adv. Mater., 2014, 26(14), 2185–2204.
References
1 Q. Wang, S. Yu, Z. Tan, R. Zhang, Z. Li, X. Gao, B. Shen and
H. Su, Synthesis of monodisperse Bi2O3-modied CeO2 16 Y.-C. Liang, C.-S. Hung and W.-C. Zhao, Thermal Annealing
nanospheres with excellent photocatalytic activity under
Induced Controllable Porosity and Photoactive Performance
visible light, CrystEngComm, 2015, 17, 671–677.
of 2D ZnO Sheets, Nanomaterials, 2020, 10(7), 1352–1366.
2 Y.-C. Liang and K.-J. Chiang, Growth temperature-dependent 17 Z. Zhang, D. Jiang, C. Xing, L. Chen, M. Chen and M. He,
phase evolution and photoactivities of sputtering-deposited
crystalline Bi2O3 thin lms, CrystEngComm, 2020, 22, 4215–
4227.
Novel AgI-decorated b-Bi2O3 nanosheet heterostructured Z-
scheme photocatalysts for efficient degradation of organic
pollutants with enhanced performance, Dalton Trans.,
2015, 44(25), 11582–11591.
3 Y.-C. Liang and K.-J. Chiang, Coverage Layer Phase
Composition-Dependent Photoactivity of One-Dimensional 18 Y. Wang, L. Jiang, D. Tang, F. Liu and Y. Lai,
TiO2–Bi2O3 Composites, Nanomaterials, 2020, 10, 1005–1018.
4 H. Cheng, B. Huang, J. Lu, Z. Wang, B. Xu, X. Qin, X. Zhang
and Y. Dai, Synergistic effect of crystal and electronic
Characterization of porous bismuth oxide (Bi2O3)
nanoplates prepared by chemical bath deposition and post
annealing, RSC Adv., 2015, 5(80), 65591–65594.
structures on the visible-light-driven photocatalytic 19 Y. Yin, Y. Sun, M. Yu, X. Liu, B. Yang, D. Liu, S. Liu, W. Cao
performances of Bi2O3 polymorphs, Phys. Chem. Chem.
Phys., 2010, 12, 15468–15475.
5 Y.-C. Wu, Y.-T. Huang and H.-Y. Yang, Crystallization
and M. N. R. Ashfold, Controlling the hydrothermal growth
and the properties of ZnO nanorod arrays by pre-treating the
seed layer, RSC Adv., 2014, 4(48), 44452–44456.
mechanism and photocatalytic performance of vanadium- 20 Y.-C. Liang and C.-S. Hung, Effects of sputtering deposited
modied bismuth oxide through precipitation processes at
room temperature, CrystEngComm, 2016, 18, 6881–6888.
6 Y. Qiu, M. Yang, H. Fan, Y. Zuo, Y. Shao, Y. Xu, X. Yang and
homoseed layer microstructures on crystal growth behavior
and photoactivity of chemical route-derived WO3
nanorods, CrystEngComm, 2019, 21, 5779–5788.
S. Yang, Nanowires of a- and b-Bi2O3: phase-selective 21 C. Zhu and M. J. Panzer, Seed Layer-Assisted Chemical Bath
synthesis and application in photocatalysis, CrystEngComm,
2011, 13, 1843–1850.
7 Y. Xiong, M. Wu, J. Ye and Q. Chen, Synthesis and
Deposition of CuO Films on ITO Coated Glass Substrates
with Tunable Crystallinity and Morphology, Chem. Mater.,
2014, 26(9), 2960–2966.
luminescence
microcrystals, Mater. Lett., 2008, 62(8–9), 1165–1168.
properties
of
hand-like
a-Bi2O3 22 T. P. Gujar, V. R. Shinde, C. D. Lokhande, R. S. Mane and
S.-H. Han, Bismuth oxide thin lms prepared by chemical
8 A. M. Abu-Dief and W. S. Mohamed, a-Bi2O3 nanorods:
synthesis, characterization and UV-photocatalytic activity,
Mater. Res. Express, 2017, 4(3), 035039.
bath deposition (CBD) method: annealing effect, Appl. Surf.
Sci., 2005, 250(1–4), 161–167.
23 K. Barrera-Mota, M. Bizarro, M. Castellino, A. Tagliaferro,
´
9 L. Liu, J. Jiang, S. Jin, Z. Xia and M. Tang, Hydrothermal
synthesis of b-bismuth oxide nanowires from particles,
CrystEngComm, 2011, 13, 2529–2532.
A. Hernandez and S. E. Rodil, Spray deposited b-Bi2O3
nanostructured lms with visible photocatalytic activity for
solar water, Photochem. Photobiol. Sci., 2015, 14(6), 1110–
1119.
10 M. Jalalah, M. Faisal, H. Bouzid, J.-G. Park, S. A. Al-Sayari and
A. A. Ismail, Comparative study on photocatalytic 24 X. Y. Chen, H. S. Huh and S. W. Lee, Controlled synthesis of
performances of crystalline a- and b-Bi2O3 nanoparticles
under visible light, J. Ind. Eng. Chem., 2015, 30, 183–189.
bismuth oxo nanoscale crystals (BiOCl, Bi12O17Cl2, a-Bi2O3,
45056 | RSC Adv., 2020, 10, 45042–45058
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
and (BiO)2CO3) by solution-phase methods, J. Solid State
Chem., 2007, 180(9), 2510–2516.
Oxygen Evolution Reaction, J. Am. Chem. Soc., 2013,
135(45), 16977–16987.
25 D.-D. Qin, C.-L. Tao, Z.-Y. Yang, T. E. Mallouk, N. Bao and 38 Y. Tang, X. Fang, X. Zhang, G. Fernandes, Y. Yan, D. Yan,
C. A. Grimes, Facile Solvothermal Method for Fabricating
Arrays of Vertically Oriented a-Fe2O3 Nanowires and Their
Application in Photoelectrochemical Water Oxidation,
Energy Fuels, 2011, 25(11), 5257–5263.
X. Xiang and J. He, Space-conned earth-abundant
bifunctional electrocatalyst for high-efficiency water
splitting, ACS Appl. Mater. Interfaces, 2017, 9(42), 36762–
36771.
26 A. M. Selman and Z. Hassan, Inuence of deposition 39 Y.-C. Liang and C.-S. Hung, Design of Hydrothermally
temperature on the growth of rutile TiO2 nanostructures
by CBD method on seed layer prepared by RF magnetron
sputtering, Superlattices Microstruct., 2013, 64, 27–36.
Derived Fe2O3 Rods with Enhanced Dual Functionality Via
Sputtering Decoration of a Thin ZnO Coverage Layer, ACS
Omega, 2020, 5, 16272–16283.
27 Y. Shi, L. Luo, Y. Zhang, Y. Chen, S. Wang, L. Li, Y. Long and 40 A. Watanabe and H. Kozuka, Photoanodic Properties of Sol–
F. Jiang, Synthesis and characterization of a/b-Bi2O3 with
enhanced photocatalytic activity for 17a-ethynylestradiol,
Ceram. Int., 2017, 43(10), 7627–7635.
Gel-Derived Fe2O3 Thin Films Containing Dispersed Gold
and Silver Particles, J. Phys. Chem. B, 2003, 107(46), 12713–
12720.
28 D. Barreca, F. Morazzoni, G. Andrea Rizzi, R. Scotti and 41 J. Hou, C. Yang, Z. Wang, W. Zhou, S. Jiao and H. Zhu, In situ
E. Tondello, Molecular oxygen interaction with
a spectroscopic and Bi2O3 : spectromagnetic investigation,
Phys. Chem. Chem. Phys., 2001, 3(9), 1743–1749.
synthesis of a–ß phase heterojunction on Bi2O3 nanowires
with exceptional visible-light photocatalytic performance,
Appl. Catal., B, 2013, 142–143, 504–11.
29 Y. Ma, Q. Han, T.-W. Chiu, X. Wang and J. Zhu, Simple 42 X. Yang, X. Lian, S. Liu, C. Jiang, J. Tian, G. Wang, J. Chen
thermal decomposition of bismuth citrate to Bi/C/a-Bi2O3
with enhanced photocatalytic performance and adsorptive
ability, Catal. Today, 2018, 340, 40–48.
and R. Wang, Visible light photoelectrochemical properties
of ß-Bi2O3 nanoporous lms: a study of the dependence on
thermal treatment and lm thickness, Appl. Surf. Sci.,
2013, 282, 538–543.
30 Y.-C. Liang and T.-W. Lung, Growth of Hydrothermally
Derived CdS-Based Nanostructures with Various Crystal 43 H. Huang, K. Xiao, K. Liu, S. Yu and Y. Zhang, In Situ
Features and Photoactivated Properties, Nanoscale Res.
Lett., 2016, 11(1), 264.
Composition-Transforming Fabrication of BiOI/BiOIO3
Heterostructure: Semiconductor p–n Junction and
Dominantly Exposed Reactive Facets, Cryst. Growth Des.,
2015, 16(1), 221–228.
´
31 T. A. Gadhi, S. Hernandez, M. Castellino, P. Jagdale,
´
T. Husak, A. Hernandez-Gordillo, A. Tagliaferro and
N. Russo, Insights on the role of b-Bi2O3/Bi5O7NO3 44 M. Ahila, E. Subramanian and D. Pathinettam Padiyan,
heterostructures synthesized by
method for the sunlight-driven photocatalytic degradation
of dyes, Catal. Today, 2018, 321, 135–145.
a
scalable solid-state
Inuence of annealing on phase transformation and
specic capacitance enhancement in Bi2O3, J. Electroanal.
Chem., 2017, 805, 146–158.
32 D. Chen, S. Wu, J. Fang, S. Lu, G. Zhou, W. Feng, Y. Fan, 45 N. F. Atta, A. H. Ibrahima and A. Galal, Nickel oxide
Y. Chen and Z. Fang, A nanosheet-like a-Bi2O3/g-C3N4
heterostructure modied by plasmonic metallic Bi and
oxygen vacancies with high photodegradation activity of
organic pollutants, Sep. Purif. Technol., 2018, 193, 232–241.
nanoparticles/ionic liquid crystal modied carbon
composite electrode for determination of
neurotransmitters and paracetamol, New J. Chem., 2016,
40, 662–673.
33 F. Urbach, The Long-Wavelength Edge of Photographic 46 L. Shan, G. Wang, D. Li, X. San, L. Liu, L. Dong and Z. Wu,
Sensitivity and of the Electronic Absorption of Solids, Phys.
Rev., 1953, 92(5), 1324.
34 B. Choudhury and A. Choudhury, Oxygen defect dependent
Band alignment and enhanced photocatalytic activation of
a/b-Bi2O3 heterojunctions via in situ phase transformation,
Dalton Trans., 2015, 44(17), 7835–7843.
variation of band gap, Urbach energy and luminescence 47 H.-Y. Wang, J. Chen, F.-X. Xiao, J. Zheng and B. Liu, Doping-
property of anatase, anatase–rutile mixed phase and of
rutile phases of TiO2 nanoparticles, Phys. E, 2014, 56, 364–
371.
induced structural evolution from rutile to anatase:
formation of Nb-doped anatase TiO2 nanosheets with high
photocatalytic activity, J. Mater. Chem. A, 2016, 4(18), 6926–
6932.
35 B. Qin, H. Yu, X. Gao, D. Yao, X. Sun, W. Song, B. Yi and
Z. Shao, Ultrathin IrRu nanowire networks with high 48 X. Li, J. Yu, J. Low, Y. Fang, J. Xiao and X. Chen, Engineering
performance and durability for the hydrogen oxidation
reaction in alkaline anion exchange membrane fuel cells, J.
Mater. Chem. A, 2018, 6, 20374–20382.
heterogeneous semiconductors for solar water splitting, J.
Mater. Chem. A, 2015, 3(6), 2485–2534.
49 X. Yue, S. Yi, R. Wang, Z. Zhang and S. Qiu, A novel
architecture of dandelion-like Mo2C/TiO2 heterojunction
photocatalysts towards highperformance photocatalytic
hydrogen production from water splitting, J. Mater. Chem.
A, 2017, 5(21), 10591–10598.
36 H. Hu, B. Guan, B. Xia and X. W. Lou, Designed Formation of
Co3O4/NiCo2O4 Double-Shelled Nanocages with Enhanced
Pseudocapacitive and Electrocatalytic Properties, J. Am.
Chem. Soc., 2015, 137(16), 5590–5595.
37 C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, 50 K. K. Bera, M. Chakraborty, M. Mondal, S. Banik and
Benchmarking Heterogeneous Electrocatalysts for the
S. K. Bhattacharya, Synthesis of a-b Bi2O3 heterojunction
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photocatalyst and evaluation of reaction mechanism for
degradation of RhB dye under natural sunlight, Ceram.
Int., 2019, 46(6), 7667–7680.
nanorods with exceptional cationic and anionic dye
degradation properties, Sol. Energy, 2019, 190, 531–542.
52 H. Farsi, S. Moghiminia, A. Roohi and S.-A. Hosseini,
51 A. Lebedev, F. Anariba, X. Li, D. Seng Hwee Leng and P. Wu,
Rational design of visible-light-driven Pd-loaded a/b-Bi2O3
Preparation,
characterization
and
electrochemical
behaviors of Bi2O3 nanoparticles dispersed in silica matrix,
Electrochim. Acta, 2014, 148, 93–103.
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