Insights into the impurities of Bi2WO6synthesized using the hydrothermal method

Article abstract of DOI:10.1039/d0ra07559k

Bismuth tungstate (Bi2WO6) nanomaterials are widely used as visible-light driven photocatalysts. However, limited attention has been paid to the purity of prepared Bi2WO6 nanoparticles, which may affect the photocatalytic performance and hinder in-depth study of Bi2WO6. In this work, the impurities of Bi2WO6 formed during the hydrothermal process under a wide range of acid-base conditions (from 1.5 M HNO3 to 0.5 M NaOH) were qualitatively analyzed and accurately quantified for the first time. After confirmation of Bi2WO6 stability, the impurities were dissolved using acid or base treatment, followed by measurements of the ion concentrations using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Furthermore, various characterization techniques including XRD, FE-SEM, TEM, UV-Vis DRS, XPS and FTIR were implemented to explore the change in morphology and optical properties of Bi2WO6 prepared in different acid-base environments, and to facilitate qualitative analysis of impurities. The hydrolytic properties of raw materials used for the synthesis of Bi2WO6 were also analyzed with UV-Vis transmittance observation. Following these analyses, the types and contents of impurities in Bi2WO6 prepared by the hydrothermal method under different acid-base conditions were determined. Results show that the primary impurity is WO3·0.33H2O (41.09%) for the precursor prepared in 1.5 M nitric acid solution. When the pH of the precursor was in the range of 0.97-7.01, the synthesized Bi2WO6 has relatively high purity, and the impure products were identified as BiONO3. Bi2O3 began to appear when pH reached 9.01 and it reached 18.88% when pH was 12.98. The final product was Bi2O3 exclusively for the precursor conditioned in 0.5 M NaOH solution. In addition, the accuracy of the proposed quantitative method using ICP-MS was validated for several scenarios by weight difference experiments. This journal is

Full text of DOI:10.1039/d0ra07559k

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Insights into the impurities of Bi WO synthesized
2
6
using the hydrothermal method
Cite this: RSC Adv., 2020, 10, 40597
ab
abc
c
d
d
Jiayou Liu,
Qianqian Nie,
Zhongchao Tan, Yulin Luo, Shuai Wang
ab
and Hesheng Yu*
Bismuth tungstate (Bi
limited attention has been paid to the purity of prepared Bi
photocatalytic performance and hinder in-depth study of Bi WO
formed during the hydrothermal process under a wide range of acid–base conditions (from 1.5 M HNO
to 0.5 M NaOH) were qualitatively analyzed and accurately quantified for the first time. After
confirmation of Bi WO stability, the impurities were dissolved using acid or base treatment, followed by
WO
2 6
) nanomaterials are widely used as visible-light driven photocatalysts. However,
WO nanoparticles, which may affect the
. In this work, the impurities of Bi WO
2
6
2
6
2
6
3
2
6
measurements of the ion concentrations using Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
Furthermore, various characterization techniques including XRD, FE-SEM, TEM, UV-Vis DRS, XPS and
FTIR were implemented to explore the change in morphology and optical properties of Bi
in different acid–base environments, and to facilitate qualitative analysis of impurities. The hydrolytic
properties of raw materials used for the synthesis of Bi WO were also analyzed with UV-Vis
transmittance observation. Following these analyses, the types and contents of impurities in Bi WO
prepared by the hydrothermal method under different acid–base conditions were determined. Results
show that the primary impurity is WO $0.33H O (41.09%) for the precursor prepared in 1.5 M nitric acid
solution. When the pH of the precursor was in the range of 0.97–7.01, the synthesized Bi WO has
relatively high purity, and the impure products were identified as BiONO . Bi began to appear when
pH reached 9.01 and it reached 18.88% when pH was 12.98. The final product was Bi exclusively for
2 6
WO prepared
2
6
2
6
3
2
2
6
Received 3rd September 2020
Accepted 31st October 2020
3
2 3
O
2
O
3
DOI: 10.1039/d0ra07559k
the precursor conditioned in 0.5 M NaOH solution. In addition, the accuracy of the proposed
rsc.li/rsc-advances
quantitative method using ICP-MS was validated for several scenarios by weight difference experiments.
5
efficient response to visible light. Bi WO and its modied
2
6
1
. Introduction
catalysts have been used for degradation of organic wastewater,
Many researchers consider photocatalysis to be a promising conversion of carbon dioxide (CO ), water splitting for hydrogen
technology for sustainable energy production and environ- production, and removal of nitric oxide (NO) from atmo-
2
1
2
6–11
12
mental remediation. Photocatalysts such as TiO , ZnO and sphere. Since Kudo rst synthesize Bi
SnO2 have been extensively studied; however, such catalysts solid state reactions, many methods have emerged for its
only respond to the ultraviolet light because of their wide band preparation including hydrothermal method,
gaps. Meanwhile, ultraviolet light accounts for approximately and ultrasonic-assisted hydrothermal method, solvothermal
2
WO
6
by conventional
2
3
13,14
15
microwave-
16
4
17
18
19
4
4
% of the solar energy on the surface of the planet earth and procedure, sol–gel process, grinding and calcination, and
20
4% of sunlight is visible. As a result, visible-light responsive co-precipitation. Among these methods, the hydrothermal
photocatalysts with a narrow band gap have attracted growing method is one of the most widely used because of its simplicity,
21,22
interest in research.
safety, and high yield.
Bismuth tungstate (Bi WO ) has recently gained consider-
During the hydrothermal synthesis process, the nal prod-
2
6
able attention because of its physicochemical stability and ucts may be affected by factors, such as hydrothermal reaction
23
temperature and pH value of precursors. Zhang and Zhu
explored the impact of hydrothermal reaction temperature on
the properties of Bi WO nanocatalysts. They observed that the
crystallinity and morphology of nal products were affected by
a
Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education,
2
6
Xuzhou 221116, China. E-mail: heshengyu@cumt.edu.cn
b
School of Chemical Engineering and Technology, China University of Mining and
the temperature and time during hydrothermal process.
Department of Mechanical & Mechatronics Engineering, University of Waterloo, 200 Specically, low temperature will lead to poor crystallinity,
Technology, Xuzhou 221116, China
c
University Avenue West, Waterloo N2L 3G1, Canada
while high temperature will result in low photocatalytic activity.
Additionally, it is well acknowledged that pH value also affects
d
Advanced Analysis and Computation Center, China University of Mining and
Technology, Xuzhou 221116, China
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2 6 3 3 2
several physical properties of Bi WO in a hydrothermal prep- related studies and investigations using Bi(NO ) $5H O and/or
24
aration. Huang et al. produced Bi WO nanomaterials with Na WO $2H O as raw materials.
2
6
2
4
2
different morphologies by adjusting the pH level of the
precursor solution. At pH 8, the uffy microspheres-like Bi WO
2
6
25
2. Experimental
exhibited the highest photocatalytic activity. Hu et al. reported
that the pH value of precursor solution affected the specic 2.1 Materials
surface area and the porosity of their Bi WO nanocatalysts. Lin
et al. reported that pH value impacted on the particle sizes of
the synthesized Bi WO photocatalysts. Moreover, the pH value
2
6
Reagents used in this research include Bi(NO
3
)
3
2
$5H O
14
(
(
$99.0%), Na WO $2H O ($99.5%), ethanol ($99.7%), NaOH
$96.0%) and concentrated HNO (65–68.0%). They are all of
2
4
2
2
6
3
of precursor could affect the composition of nal products.
analytical grade and were purchased from Sinopharm Chemical
Reagent Co. These chemicals were used as received without
further purication. Ultrapure water with a resistivity of 18.2
Some researchers prepared WO /Bi WO composite in a strong
3
2
6
2
6,27
acid environment.
Others discovered Bi3.84W0.16O6.24 in
28
Bi WO nanomaterials prepared under alkali conditions.
2
6
ꢀ1
MU cm produced by Direct-Q 5 UV System (Merck Millipore
Therefore, it is likely that the acidity and/or alkalinity of prep-
aration conditions may have a strong impact on the nal
products. However, most researchers focused on properties like
morphology and specic surface area; few attempted to deter-
mine the purity of Bi WO nanomaterials prepared at different
Germany) was used in this study.
2
.2 Synthesis of Bi
conditions
The Bi WO nanomaterials were prepared using hydrothermal
2 6
WO in different acid and alkali
2
6
synthesis conditions.
2
6
It is therefore necessary to qualitatively and quantitatively method. The molar ratio between bismuth and tungstate was
analyze impurities to better understand the effects of synthesis maintained at 2 : 1. The preparation procedures of precursor
conditions on the purity of Bi
Bi WO can be qualitatively analyzed by some characterization slightly from those under dilute acid and base conditions. To
technologies such as XRD, SEM and FTIR.
have indicated that the impurities in Bi
hydrothermal method mainly resulted from the hydrolysis of sodium tungstate dihydrate (Na WO $2H O) were rst weighed
2 6
WO . The impure components in suspension under strong acid or mild alkali condition differs
2
6
2
9,30
Earlier studies prepare Bi
2
WO
6
under strong acid condition, 1.0914 g of
$5H O) and 0.3711 g of
2
WO
6
prepared via bismuth nitrate pentahydrate (Bi(NO
3
)
3
2
2
4
2
26,31,32
raw materials.
Therefore, accurate and comprehensive using an analytical balance (METTLER TOLEDO-ME104E/02).
understanding of the hydrolysis properties of Bi(NO ) $5H O They were then transferred to two beakers containing 30 mL
3
3
2
and Na
qualitative analysis of impurities.
To the best of our knowledge, however, there is not relevant 30 minutes separately, before they were mixed up for another 20
research on the quantication of impurities in Bi WO nano- minutes. Nanomaterials synthesized under such strong acid
2
WO
4
$2H
2
O at different pH levels is also essential to of nitric acid solution, the concentration of which ranged from
0.5 M to 1.5 M. These two solutions were magnetically stirred for
2
6
materials. One feasible strategy is to take advantage of the conditions are denoted as 0.5MHNO, 1.00MHNO, 1.25MHNO,
difference in physio-chemical properties between impurities and 1.5MHNO; the numerical numbers stand for HNO3
and Bi WO . As far as we know, the hydrolysate of the raw concentrate. As for mild base condition, the same amount of
2
6
material for Bi WO preparation can be dissolved in an acid or Bi(NO ) $5H O and Na WO $2H O were dissolved in 30 mL of
2
6
3 3
2
2
4
2
33,34
35,36
hot alkali solution,
while Bi
2
WO
6
is relatively stable.
0.5 M NaOH solution, respectively. The subsequent procedures
Hence it is reasonable to carry out quantitative analysis by are the same as those under strong acid conditions and the
dissolving impurities from the Bi
The objective of the current work is to study the impurities of
prepared Bi WO
synthesized Bi WO using hydrothermal method from 1.5 M $2H O were dissolved in 25 mL of 0.3 M HNO solution and in
2
WO
6
samples.
corresponding nanomaterials are denoted as 0.5MNAOH.
For dilute acid and base conditions ranging from pH 0.97 to
2
6 3 3 2 2 4
nanocatalysts. For this purpose, we rst pH 12.98, 1.4552 g of Bi(NO ) $5H O and 0.4948 g of Na WO -
2
6
2
3
nitric acid (HNO ) to 0.5 M sodium hydroxide (NaOH) solutions. 55 mL of ultrapure water, respectively. These two solutions were
3
Then we identied the compounds of the impurities and other then magnetically stirred for 30 minutes separately, before they
properties of the samples according to XRD, UV-Vis DRS, XPS were mixed up for another 20 minutes. The pH of the precursor,
FTIR, FE-SEM, TEM results and hydrolysis properties of raw which was monitored by a pH meter (S470-K, METTLER
materials for preparation of Bi
2
WO
6
. Aer that, the stability of TOLEDO), was then adjusted to 0.97, 2.99, 4.99, 7.01, 9.01,
. The
Bi WO was determined experimentally to ensure the feasibility 10.00, 11.05, 12.01 and 12.98 with NaOH and/or HNO
2
6
3
of quantitative experiments. Later, impurities in the catalysts corresponding nanomaterials synthesized under these condi-
were dissolved into strong acid or hot alkaline solutions. tions are dened as 0.97BWO, 2.99BWO, 4.99BWO, 7.01BWO,
Following that, the concentrations of key elements in the 9.01BWO, 10.00BWO, 11.05BWO, 12.01BWO, and 12.98BWO,
solutions were measured using ICP-MS, followed by conversion respectively.
into mass weight of corresponding impurities. Finally, the
accuracy of the quantitative method based on the ICP-MS PTFE autoclave, and heated at 180 C for 24 h. Aer naturally
technique was then veried by weight difference experiments. cooling down to room temperature, the solid was collected by
The precursor suspension was then transferred to a 100 mL
ꢁ
Overall, the present work is expected to shed light onto Bi
2
WO
6
-
centrifuge, followed by washing 3 times with ultrapure water
and ethanol, respectively. The nanomaterials collected were
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ꢁ
then dried at 60 C for 12 h; the resultant nanomaterials were to ensure Bi
stored for subsequent use.
2
WO
6
stability in the corresponding acid and base
environments. For this purpose, 0.97BWO nanomaterials were
treated with acid and alkaline solutions, respectively, following
the same procedure as described in ICP-MS analysis, which can
be found in the next section. Crystal structure stability and
morphology stability were analyzed by examining the XRD
patterns and FE-SEM images of 0.97BWO before and aer
treatment with acid or base solvents. In addition, chemical
stability was observed based on the change in the mass of dis-
solved Bi and W elements during a period of 7 days.
2
.3 Sample characterization
Crystal phase of the prepared photocatalysts was analyzed by
the X-ray diffraction technique (XRD, Bruker AXS D8 Advance,
˚
Cu Ka, l ¼ 1.5406 A, 40 kV, 40 mA) at 2q angles from 5 to
ꢁ ꢁ
0 ,and a scan speed of 10 per min. Their optical properties
8
were determined using UV-Vis diffused reectance spectra (UV-
Vis DRS, Hitachi U-3900H) over the wavelengths in the range of
75–800 nm. X-ray photoelectron spectroscopy (XPS) was per-
formed on ESCALAB 250Xi using Al Ka radiation to determine
the chemical valence and relative percentage of surface
elements. The data of binding energy for all samples were
calibrated with the C 1s signal at 284.8 eV. Field emission
scanning electron microscope (FE-SEM, Hitachi-SU8220), which
was operated at 10 kV acceleration voltage and 9800 nA emis-
Nanomaterials prepared at strong acid (1.5MHNO) and base
12.98BWO) precursors were also analyzed to further conrm
2
(
the stability of Bi WO . The XRD patterns and FE-SEM images
2
6
of 1.5MHNO before and aer alkali treatment was compared.
The stability of 12.98BWO was conrmed by XRD results before
and aer soaking with acid.
sion current, was used to observe the morphology of the 2.6 Quantitative analysis of impurities
samples. Additionally, transmission electron microscopy (TEM)
2 6
Upon conrmation of Bi WO stability, Inductively Coupled
and high-resolution transmission electron microscopy (HR-
TEM) observations were carried out on a Tecnai G2 F20
instrument in the bright eld. Furthermore, chemical bonds of
samples were determined using Fourier Transform Infrared
Spectroscopy (FTIR, Bruker Vertex 80v) over the range of 400–
Plasma Mass Spectrometry (ICP-MS, Agilent 7900) was used to
determine the soluble bismuth and tungstate ions dissolved in
solutions. Data is used for the quantication of impurities in
the Bi WO nanomaterials synthesized under different acid and
2
6
alkali conditions. The samples for ICP-MS analysis were
prepared following these steps: for those prepared at pH value
equal to or above 0.97, and the 0.5MNAOH nanomaterials,
ꢀ
1
4
000 cm .
2
.4 Observation of critical point of hydrolysis
$5H O and Na WO
were subject to severe hydrolysis in 1.25MHNO, 1.00MHNO, 0.5MHNO and 0.97BWO samples were
samples were soaked in pH 0.5 HNO
3
solution for one week to
The raw materials [Bi(NO
the preparation of Bi WO
solution, producing insoluble precipitates.
reactions contribute largely to the impurities in the Bi
produced during hydrothermal process. Therefore, under- rated by a centrifuge operating at 4500 rpm for 3 min; then the
standing the hydrolysis properties of Bi and WO4 ions at supernatant was then sampled and diluted for ICP-MS analysis.
different pH levels is important to impurity analysis.
The ICP-MS results were further veried by weight difference
The critical pH value of hydrolysis is dened as the pH value tests. A specic amount of 1.5MHNO, 0.97BWO and 12.98BWO
3
)
3
2
2
4 2
$2H O] used for dissolve potential acid-soluble impurities. However, 1.5MHNO,
2
6
9
,27,31
The hydrolysis treated by boiling 0.5 M NaOH solution for 1 min to dissolve
WO
alkali-soluble impurities. The resultant suspension was sepa-
2
6
3
+
2ꢀ
3
+
at which ions begin to hydrolyze. The critical pH values for Bi
and WO
samples were treated using base or acid solution. The treated
ions were determined using the transmittance catalysts were then cleaned in a sintered disc lter. The
2
ꢀ
4
pattern of the UV-Vis spectrophotometer (Hitachi U-3900H). 1.5MHNO nanomaterials aer alkaline treatment were washed
The precipitate may affect the transmittance intensity of using ultrapure water during ltration. However, the 0.97BWO,
a solution. Therefore, transmittance observations can be used 12.98BWO samples soaked in acid were washed 5 times using
to identify the critical pH values. At the critical pH value, pH 0.5 HNO3 solution, followed by a thorough rinse with
3
+
transmittance drops abruptly due to the presence of ultrapure water. This way, the hydrolysis of Bi ions le on
precipitates.
solids could be prevented, ensuring the accuracy of nal anal-
ꢁ
The procedure of UV-Vis transmittance observation was ysis. Aer drying at 60 C for 12 hours, the weight difference
described as follows. Firstly, Bi(NO
completely dissolved in a strong acidic solution and ultrapure
water, respectively. The obtained transparent Bi(NO and
Na WO solutions were then gradually regulated using NaOH
3 3 2 4
) and Na WO were before and aer acid or base treatment was recorded.
3 3
)
3
. Results and discussion
2
4
and HNO solutions to reach a series of pH values with a small 3.1 XRD patterns of Bi WO nanomaterials
3
2
6
interval. The transmittance of these Bi(NO3)3 and Na WO
solutions was recorded at each pH value.
2
4
Fig. 1 and 2 show the XRD patterns of the catalysts prepared
under different acid and base conditions. All samples, except
for 12.98BWO and 0.5MNAOH, exhibited diffraction peaks that
match up with Bi WO standard card (JCPDS No.: 39-0256). The
2.5 Catalysts stability
2
6
A new approach based on acid and base treatment is proposed peaks appear to be sharp, indicating that the primary
herein to quantify the impurities in the Bi WO nanomaterials compound of the samples is highly crystalline Bi WO . Fig. 1
produced by hydrothermal method. In this method, it is critical also shows that the phase of WO $0.33H O (JCPDS No.: 35-0270)
2
6
2
6
3
2
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2 6
7.01 to 11.05, the XRD spectrums showed the phase of Bi WO
only. When the pH value increased to 12.01, however, a new
crystal phase of Bi3.84W0.16O6.24 (JCPDS No.: 43-0447), besides
Bi WO , appeared in the prepared nanomaterials. Moreover,
2
6
the intensity of the diffraction peak of Bi3.84
stronger than that of Bi WO . This may be attributed to the
in the precursor suspension, which is
0.16
W O6.24 was
2
6
2
ꢀ
solubility of [WO
much greater than that of [Bi
4
]
2
+
2
O
2
]
in a strong alkaline envi-
2
+
ronment. Therefore, [Bi
the new phase of Bi3.84W0.16O6.24
2.98, however, the phase of Bi WO disappeared and only
2 2
O ]
can precipitate rapidly, forming
28,37
.
As the pH value reached
1
2
6
Bi3.84W0.16O6.24 existed. In this paper, Bi3.84W0.16O6.24 is regar-
ded as another crystalline phase of Bi WO rather than impu-
rities because of its strong stability and catalytic ability.
2
6
Synthesized Bi3.84
W
0.16
O6.24 could be converted to a-Bi
2
O
3
(
JCPDS No.: 76-1730) when the precursor was further condi-
Fig. 1 XRD patterns of the catalyst synthesized in acidic environment.
tioned by 0.5 M NaOH solution. The formation of a-Bi
resulted from the high concentration of OH in the precursor,
leading to the production of Bi(OH) . Bi(OH) further trans-
formed into a-Bi O under hydrothermal reaction conditions.
Consequently, Bi ions were unable to participate in the
2
O
3
ꢀ
3
3
38
2
3
+
3
2
ꢀ
synthesis of Bi3.84
According to the XRD patterns, WO
6.24, and a-Bi
increasing precursor alkalinity in addition to Bi
W
0.16
O
6.24 and Bi
2
WO
6
with WO
4
.
3
2
$0.33H O,
Bi3.84
W
0.16
O
2
O
3
were observed sequentially with
WO . The
2
6
results not only demonstrated the successful synthesis of
Bi WO , but also strongly proved that different substances
2
6
could be produced under different acid–base conditions.
3.2 Optical properties of catalysts
The UV-Vis DRS was used to determine the photo-response
range of nanomaterials synthesized under different acid and
alkali conditions. It can be seen from Fig. 3 that all samples
showed high photo-absorption at around 380 nm, and their
absorption bands were approximately 450 nm. This indicates
that all the synthesized samples are responsive to visible light
Fig.
2 XRD patterns of the catalyst synthesized in alkaline
environments.
is present in the nanomaterials synthesized in strong acid
environments including 1.5 M, 1.25 M, and 1.0 M HNO solu-
3
tions. The presence of WO $0.33H O is consistent with the
3
2
results of SEM analysis in the following section. In such cases,
+
the H ion concentrations in the precursor were so high that
ꢀ
+
WO
tion of H
thermal process to generate WO
difficult to observe the peaks of WO $0.33H O in the XRD
4
ions combined with H to produce insoluble precipita-
WO . H WO then dehydrated during the hydro-
$0.33H O. However, it was
2
4
2
4
3
2
3
2
spectrum of the 0.5MHNO samples. The samples of 0.5MHNO,
.97BWO, 2.99BWO, and 4.99BWO showed only characteristic
0
peaks of Bi
2
WO
6
, and the intensity of the strongest (131) peak at
ꢁ
28.299 changed signicantly from about 1572 to about 10 631
as the acidity decreased. This nding might be attributed to the
increase in crystallite size and enhanced crystallinity
28
perfection.
Fig. 2 shows the XRD patterns of catalysts synthesized in
neutral and alkaline environments. For pH values ranging from
Fig. 3 UV-Vis DRS spectra of photocatalysts synthesized in different
acid and base conditions.
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regardless of the pH value of the precursor. In addition, the
optical band gap of the as-prepared photocatalysts were calcu-
39
lated using Kubelka–Munk (KM) method, and the results are
included in the legend of Fig. 3.
Fig. 3 shows that the band gaps of different catalysts,
including 0.97BWO, 2.99BWO, 4.99BWO, 7.01BWO, 9.01BWO,
10.00BWO and 11.05BWO, varied slightly between 2.76–2.80 eV.
This range is consistent with those in other studies on
40,41
Bi
.5MHNO are 446 nm, 451 nm and 454 nm corresponding to
band gaps of 2.78 eV, 2.75 eV and 2.73 eV, respectively. The
2
WO
6
.
The absorption edges of 1.5MHNO, 1MHNO and
0
band gap of 12.98BWO (which is Bi3.84
W
O6.24) is 2.82 eV,
0.16
42
corresponding to previous report as well. The results in this
section indicate that all samples have visible light response
capabilities and that the band gap lies between 2.73 eV and
2.82 eV.
3.3 Morphology and structure of prepared nanomaterials
Morphologies of the samples were characterized by FE-SEM and
TEM techniques. It can be seen from Fig. 4a that the 1.5MHNO
sample appears like a ower with dewdrops. Specically, the

ower-like base is made up with large nanosheets, which are
decorated by small “nanocrumbs”. The large nanosheets are
composed of Bi WO and the nanocrumbs are WO $0.33H O.
2
6
3
2
The presence of these two substances is further conrmed with
the XRD pattern of 1.5MHNO. As the acidity decreases, regular
large nanosheets disappear, and are replaced by irregular ocs
consisting of small nanosheets for precursor prepared in 0.5 M
HNO3 solution (Fig. 4b). Fig. 4c shows the morphology of
Bi WO prepared at pH 0.97. Numerous small nanosheets
2
6
accumulate to form large wheat spike-like shapes (see the inset
on the top right). Further accumulation of these spike-like
structures resulted in 3D spheres, the diameters of which are
about 4–7 mm.
The morphologies of the synthesized catalysts are similar to
each other when the pH values of the precursors fall into the Fig. 4 SEM images of samples synthesized at different acid and alkali
environments: (a) 1.5MHNO; (b) 0.5MHNO; (c) 0.97BWO; (d)
range of 4.99–7.01 (Fig. 4d and e). Both appear to be building
blocks, formed by interspersion between small rectangular
pieces. The side lengths of these blocks are approximately 0.6–
4.99BWO; (e) 7.01BWO; (f) 9.01BWO; (g) 11.05BWO; (h) 12.01BWO; (i)
1
2.98BWO; (j) 0.5MNAOH.
1
.0 mm. The lower the acidity is, the thicker the blocks are.
Fig. 4f and g illustrate that the shapes of samples 9.01BWO
spheres and rods with various sizes when the precursor is
conditioned using 0.5 M NaOH solution (Fig. 4j). The trans-
and 11.05BWO are like square bricks. These square bricks of
Bi WO are randomly plied up without obvious aggregation. As
the pH value reaches 12.01, new crystal phase of
Bi3.84W0.16O6.24 emerges. Two different morphologies can be
clearly observed in Fig. 4h. The sheet shape is for the Bi WO
ꢀ
formation arises from the fact that a large amount of OH ions
2
6
3
+
in the precursor directly combines with Bi ions to form
Bi(OH) , which is then converted into Bi aer dehydration
during hydrothermal process.
a
3
2 3
O
43
2
6
Fig. 5 presents the TEM images of 1.5MHNO and 12.01BWO
samples. The overall TEM image of 1.5MHNO in Fig. 5a shows
large nanosheets decorated by small “nanocrumbs”. The
section inside the circle was further analyzed using HR-TEM
and the regular octahedron shape is for Bi3.84
0.16
W O6.24. It also
shows that the Bi3.84 6.24 crystal is the dominate phase.
W O
0.16
This nding agrees with the XRD patterns of 12.01BWO
samples presented in Fig. 2, which show two crystal phases.
Fig. 4i shows the morphology of 12.98BWO nanomaterials. It
(
Fig. 5b), which reveals the lattice interplanar spacing (0.31
nm) of the large nanosheets corresponding to the (131) plane of
Bi WO . The nanocrumbs sprinkled onto the large nanosheet
2 6
does not show the structure of the Bi WO sheet, but regular
octahedral crystals of Bi3.84W0.16O6.24 and some irregular small
crystals. According to the FTIR results in the following section
2
6
have interplanar distances of 0.32 nm and 0.37 nm, corre-
sponding to the crystal lattice spacing of the (220) and (200)
31,38
and those in earlier studies,
these irregular crystals are likely
planes of WO $0.33H O, respectively. These ndings are
Bi . Finally, the synthesized crystals become irregular Bi
2
O
3
2
O
3
3
2
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Fig. 5 TEM and HRTEM of samples: (a) TEM and (b) HRTEM of 1.5MHNO; (c) TEM of 12.01BWO.
consistent with the SEM image shown in Fig. 4a. Fig. 5c shows 532.34 eV. They are assigned to lattice oxygen, hydroxyl groups
11
the TEM image of 12.01BWO. Both granular and aky crystals and H O molecules adsorbed on the samples surface. Gener-
2
can be clearly seen. These SEM and TEM images clearly show ally, a high ratio of the peak area of hydroxyl groups to that of
the change of morphology and transformation of dominate lattice oxygen indicates a high concentration of oxygen vacan-
44
compounds with the variation from strong acid to mild base cies. According to this rule, it can be speculated that the
circumstances.
concentration of oxygen vacancy depends upon the acidity of
precursor environment. The 7.01BWO samples have the highest
concentration of oxygen vacancy, while the 0.5MNAOH samples
have the smallest one.
3.4 XPS analyses
Fig. 6 shows the percentage and chemical valence states of
As shown in Fig. 6b, the spectrum of Bi 4f had two peaks
surface elements determined by XPS. The high-resolution XPS located at around 158.74 eV and 164.05 eV, which correspond to
spectra of O 1s are shown in Fig. 6a. Through Gaussian distri- Bi 4f7/2 and 4f5/2, respectively. Moreover, the difference between
bution, the O 1s peaks of samples could be deconvoluted into the two binding energies is about 5.31 eV, indicating the exis-
3
+
10
three peaks at 529.19–529.87 eV, 529.91–530.46 eV and 531.10– tence of Bi in all samples. In Fig. 6c, the peaks around 34.96
Fig. 6 High-resolution XPS spectrums of (a) O 1s, (b) Bi 4f and (c) W 4f of samples; (d) atomic percentage of surface elements on samples.
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31,49
and 37.10 eV are tted into W 4f7/2 and 4f5/2, respectively. The Bi
2
O
2
(OH)NO
3
.
This conclusion can also be veried by
peak distance of 2.14 eV indicates that W in these samples are comparing the FTIR spectra of 0.97BWO before and aer the
6+
45
mainly presented as W . Moreover, the absence of W peaks in acid treatment. Aer acid soaking, most of the impurities
the 0.5MNAOH sample conrms that 0.5MNAOH samples are (BiONO and Bi O (OH)NO ) in 0.97BWO was dissolved, which
3
2
2
3
ꢀ
1
Bi
2
O
3
.
corresponds to a sudden decrease in the peak at 1384 cm
Fig. 6d shows the XPS atomic percentage of surface elements while others remained almost unchanged. This peak becomes
on different samples. The content of O element changes insig- weaker with the increasing alkalinity, and it disappears at pH
ꢀ
nicantly with the variation in precursor acidity. When the 9.01. With the escalation of OH concentration in the
acidity gradually increases, the W element gradually increases, precursor, BiONO
3 2 2 3
and Bi O (OH)NO gradually transform into
whereas the Bi element gradually decreases. This nding is Bi(OH) , which could be further converted to Bi O via hydro-
3
2 3
ꢀ1
associated with gradual transformation of the main substances thermal reaction. Furthermore, the tiny peak at 1384 cm of
from Bi O into Bi3.84W0.16O6.24, into Bi WO , and into Bi WO / 1.5MHNO is probably due to the hydrolysis of minor residual of
2
3
2
6
2
6
3
+
WO
3
$0.33H
2
O with increasing acidity.
3
Bi ions, which forms BiONO precipitates on nanocatalyst
surface during washing with ultrapure water.
The nanomaterials synthesized at pH value greater than 7.01
3
.5 FTIR analyses
ꢀ1
ꢀ1
has no obvious peaks between 3400 cm and 3800 cm , which
Fig. 7 shows the FTIR spectra of the as-prepared samples. The
absorption peaks mainly range from 400 to 1000 cm . Specif-
ically, the adsorption band at 441 cm and 578 cm are owing
50–52
are the characteristic peaks of O–H.
It is believed that most
ꢀ
ꢀ1
1
Bi(OH) was converted to Bi O , which is the main impurity at
3
2 3
ꢀ
1
pH $ 9.01.
to the bending vibrations of Bi–O bond; the strong peaks at
ꢀ
1
ꢀ1
7
27 cm and 820 cm are attributed to the W–O and W–O–W
16,46
3.6 Stability analysis
bond of Bi
2
WO
6
, respectively.
These peaks prove the
successful synthesis of Bi
2
WO nanomaterials.
6
ꢀ
The stability of the prepared Bi WO is evaluated in terms of
crystal structure, morphology, and chemical composition.
Fig. 8a shows the XRD patterns of original 0.97BWO and
2
6
1
ꢀ1
The minor peaks at 1001 cm and 1608 cm of 1.5MHNO's
FTIR spectrum are assigned to the W]O valence vibrations and
d(O–H) stretching vibrations of coordinated water, respectively.
They are likely attributed to the crystal phase of WO $0.33H O.
Additionally, the weak peak at 1634 cm corresponds to the
0.97BWO before and aer treatment with acid or base solutions,
47
3
2
which shows that the three XRD patterns are barely affected by
the treatment. They are almost the same, and can match the
ꢀ
1
48
O–H bond of the adsorbed water molecules. The peaks at
Bi
2 6
WO standard card (JCPDS No.: 39-0256) well. It indicates
ꢀ1
ꢀ1
5
00 cm and 787 cm are likely originated from the vibrations
of Bi–O and W–O bonds of Bi3.84 6.24. Unlike the charac-
teristic peaks in Bi WO , the intensity of the Bi–O peak is
greater than that of W–O. This may result from the increase of
that the crystal structure of as-prepared Bi WO is not damaged
2
6
W
0.16
O
during the acid or base treatment.
2
6
On the contrary, Fig. 8b clearly demonstrates that XRD
pattern of 1.5MHNO changes aer alkali treatment. The
disappearance of the characteristic peaks of WO $0.33H O
Bi–O bond in Bi3.84W0.16O6.24
.
3
2
ꢀ1
It is worth noting that the small peak at 1384 cm corre-
sponds to the vibration of N–O bond.
hydrolysis products of Bi(NO $5H
impurity indicates that WO
solved in a boiling alkaline solution, but Bi
3
$0.33H
2
O in 1.5MHNO can be dis-
WO is stable. The
4
6,48
It was ascribed to the
O, such as BiONO and
2
6
3
)
3
2
3
disappearance of the “nanocrumbs” scattered on the ower-like
Bi WO in the inset SEM images supports this conclusion. For
2
6
Bi3.84W0.16O6.24 (Fig. 8c), XRD spectrum remains the same
before and aer acid leaching, indicating that Bi3.84
0.16
W O6.24 is
relatively stable.
The chemical stability of Bi
Fig. 8d shows the variation of Bi and W elements leached from
g of 0.97BWO sample over 7 days. As soaking time increases,
the mass of dissolved Bi element in the HNO solution at pH 0.5
rst increased and then leveled off. Similar trend is observed for
2 6
WO is veried by ICP-MS data.
1
3

the change of W element dissolved in 0.5 M NaOH solution. The
constant quantity of the leaching elements aer 3 days strongly
proves the chemical stability of Bi WO . Meanwhile, undam-
2 6
aged microspheres of 0.97BWO in the inset SEM images of
Fig. 8d indicates morphological stability.
2 6 0.16
In summary, Bi WO and Bi3.84W O6.24 are stable enough
to resist dissolution during acid and base treatments. There-
fore, the Bi and W elements measured by ICP-MS in the
following leaching experiment are believed to be derived from
soluble impurities rather than from Bi
2
WO
6
and
Fig. 7 The FTIR spectra of photocatalysts synthesized in different acid
and base conditions.
0.16 6.24
Bi3.84W O .
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Fig. 8 (a) XRD of 0.97BWO and 0.97BWO after acid or alkali treatment; (b) XRD patterns and morphologies of 1.5MHNO before and after boiling
alkali solution treatment; (c) XRD patterns of 12.98BWO before and after acid treatment; (d) mass of the Bi and W elements dissolved in acid or
base solution, respectively, and the morphologies of 0.97BWO after acid or base treatment.
3.7 Determination of impurities
primary impurities include WO
3 2 3 2 2
$0.33H O, BiONO , Bi O (OH)
NO , and Bi O . Eqn (1)–(6) describe the formation of such
impurities. When the pH of the precursor is lower than 1.13,
3
2 3
The impurities in Bi WO prepared under a wide range of acid–
2
6
base conditions were qualitatively analyzed according to the
characterization results and hydrolysis properties of the raw
materials. The mass percentage of impurities was calculated
using ICP-MS results, and the accuracy was veried by several
weight difference experiments.
2
ꢀ
+
WO
transformed to WO
reaction (eqn (1) and (2)). Bi ions are subject to hydrolysis to
form BiONO and Bi (OH)NO precipitates when the pH is
4
ions will combine with H ions to form H
2 4
WO , which is
3
$0.33H O in the subsequent hydrothermal
2
3
+
3
2
O
2
3
greater than 0.67 (eqn (3) and (4)). Since the degree of hydrolysis
3
.7.1 Hydrolysis properties. The knowledge on the hydro-
3
+
2ꢀ
in the second step (eqn (4)) is much lower than that in the rst
lysis reactions of Bi and WO4 ions is critical to the analysis
of possible impurities in the prepared nanomaterials. Fig. 9
3
+
step (eqn (3)), the hydrolysis products of Bi ions are mainly
BiONO instead of Bi O (OH)NO . In a relatively strong base
3
2
2
3
shows the transmittances of Bi(NO
3 3 2 4
) solution and Na WO
solution, impurities could progressively turn to Bi
2
O
3
, which is
solution at different pH values. According to Fig. 9a, it is
3
+
transformed from Bi(OH)
3
(eqn (5) and (6)).
observed that the critical pH is 0.67 for the hydrolysis of Bi
3
+
ion. When the pH value is less than 0.67, Bi ion is present in
a free state; otherwise, Bi ion is subject to hydrolysis to form
such precipitates as BiONO and Bi O (OH)NO . Fig. 9b shows
42ꢀ
+
WO
+ 2H / H
2
WO
4
(1)
(2)
(3)
(4)
(5)
(6)
3
+
3
1
3
2
2
3
H WO / WO $0.33H O + 0.67H O
2
4
3
2
2
2
ꢀ
that the critical pH value of hydrolysis of WO
is 1.13. When
ion is at a free state in solution;
begins to hydrolyze and forms H WO
4
2
ꢀ
pH is greater than 1.13, WO
Bi(NO ) + H O 4 BiONO + 2HNO
4
3 3
2
3
3
2
ꢀ
otherwise, WO
4
2
4
53
precipitate. In summary, there is not a pH value at which the
precursor of the hydrothermal reaction is a clear solution at
room temperature. And both ions are hydrolyzed at pH value
between 0.67–1.13.
2BiONO
3
+ H
2
O 4 Bi
2
O
2
(OH)NO
3
+ HNO
3
ꢀ
3ꢀ
Bi(NO
3
)
3
+ 3OH / Bi(OH)
3
+ 3NO
2
Bi(OH)
3
/ Bi
2
O
3
+ 3H
2
O
It is believed that the impurities in the Bi WO nano-
2
6
materials prepared using hydrothermal reaction are primarily
3
+
2ꢀ
hydrolysis products and derivatives of Bi and WO4 ions. The
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3 3 2 4
Fig. 9 Transmittance of (a) Bi(NO ) solution and (b) Na WO solution at different pH values.
3.7.2 Qualitative analysis. The XRD patterns in Fig. 1 show
Fig. 10 shows a concise diagram based on the preceding
that WO $0.33H O is the primary impurity in 1.5MHNO, analyses presented to describe the transformation of samples
3
2
1
.25MHNO, 1.00MHNO samples. Although the XRD pattern of and their impurities with the change of acid and alkali envi-
0
.5MHNO does not show the phase of WO $0.33H O, the ronments. Both the main compounds and the impurities in the
3
2
2
ꢀ
WO
4
ions were observed using ICP-MS aer the 0.5MHNO samples undergo transformation. The main compounds begin
WO 6.24 at precursor pH of
O in the 0.5MHNO sample. The pres- around 12. Further increase in alkalinity leads to the production
sample was treated in boiling NaOH solution. It indicates the to convert from Bi
existence of WO $0.33H
ence of BiONO
is unlikely, because Bi ions do not hydrolyze of Bi
in the pH value lower than 0.67 according to Fig. 9.
WO $0.33H
to Bi3.84W O
6 0.16
2
3
2
3
+
O .
2 3
The impurities then gradually shi from
O to BiONO around pH 1, and to Bi at near pH
3
3
2
3
2 3
O
3
+
2ꢀ
Fig. 9 indicates both Bi and WO4 ions can be hydrolyzed 7.
at pH 0.97. Their major hydrolysis products, WO $0.33H O and
3.7.3 Quantitative analysis. Summarizes the mass ratio of
3
2
BiONO , could appear in the catalysts. The presence of corresponding impurities quantied using ICP-MS technique.
3
WO
384 cm
BiONO
For the pH values between 2.99 and 7.01, the hydrolysis of the hydrolysis of WO
3
$0.33H
2
O is proven by ICP-MS measurements. The peak at The impurity of WO
3
$0.33H
in the FTIR spectra conrms the presence of decreased from 41.09% to 1.99% as the acidity weakens from
1.5 M HNO to pH 0.97. It is well known that high acidity favors
2
O in the synthesized catalysts
ꢀ
1
1
3
.
3
2
ꢀ
in the precursor. The higher the degree
hydrolysis, the greater the content of WO $0.33H O.
4
2
ꢀ
2ꢀ
4
WO
4
ions becomes negligible. BiONO
3
,
which is the of WO
3
2
3
+
predominant product of Bi hydrolysis, constitutes to the When the pH value of the precursor solution is between 0.97
impurity in the samples. The FTIR spectra of samples prepared and 10.00, however, the content of impurity, which contains
within this pH range have a clear peak at 1384 cm , indicating Bi ions, in the synthesized catalyst does not change signi-
the existence of BiONO
. As indicated by the FTIR results the cantly. When the pH value increases to 11.05, the impurity
hydrolysate BiONO
disappears when synthesizing catalyst content starts to increase exponentially, and all the hydro-
under alkaline (pH $ 9.01) conditions. The presence of Bi(OH)
thermal reaction product was converted to Bi when the
ꢀ
1
3+
3
3
O
2 3
3
seems unlikely because there is not obvious vibration charac- precursor prepared using 0.5 M NaOH solution. Sample
teristic peak of O–H. Earlier studies indicate that impurities are 0.5MNAOH can be completely dissolved into the soaking solu-
3
8,54–56
very likely Bi O ,
which could be converted from Bi(OH)3 tion of pH 0.5 HNO
through hydrothermal reactions.
conclude that no Bi
The Bi ions were identied by ICP-MS aer the 12.98BWO worth noting that the 0.97BWO sample contains both
sample was soaked in acid solution, indicating the presence of WO $0.33H O and BiONO , which conrms our previous
acid soluble impurities. The aforementioned FTIR analysis assumption.
proves that the acid-soluble impurity in 12.98BWO is likely
Weight difference experiments were conducted before and
Bi . It is believed that the impurity is d-Bi
(ref. 57) because aer purication to validate our identication analysis and the
the positions of strong diffraction peaks of d-Bi
in XRD newly developed quantication method using ICP-MS. The
3
; therefore, it is deemed reasonable to
2
3
2
WO was produced in the sample. It is
6
3
+
3
2
3
2
O
3
2 3
O
2 3
O
pattern (JCPDS No.: 27-0052 or JCPDS No.: 52-1007) are almost results are tabulated in the last column of Table 1. ICP-MS
overlapping with those of the primary crystal phase of provides similar results to the weight difference experiments.
Bi3.84W0.16O6.24
.
Most discrepancies are within 3.5%, except for the 12.98BWO
samples, which is 11%. This indicates the analysis of impure
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Fig. 10 Diagram of the transformation of the samples and their impurities with the change of acid and alkali environment.
Table 1 The mass fraction of impurities in various samples
3 3 2 2 4 2
and investigations using Bi(NO ) $5H O and/or Na WO $2H O
as raw materials.
Mass weight (%)
weight
Mass weight
%)
ICP-MS
(
difference
experiment
Conflicts of interest
Samples
Impurities
There are no conicts to declare.
1
1
1
0
0
.5MHNO
.25MHNO
MHNO
.5MHNO
.97BWO
WO
WO
WO
WO
WO
3
3
3
3
3
$0.33H
$0.33H
$0.33H
$0.33H
$0.33H
2
2
2
2
2
O
O
O
O
41.09
14.27
9.90
39.75
—
—
—
—/3.63
Acknowledgements
5.99
O/ 1.99/3.58
This work is nancially supported by “the Fundamental
Research Funds for the Central Universities” (No.:
2019XKQYMS59). The authors would like to acknowledge the
technical support from Advanced Analysis and Computation
Center within China University of Mining and Technology.
BiONO
BiONO
BiONO
BiONO
3
3
3
3
2.99BWO
4.99BWO
7.01BWO
9.01BWO
10.00BWO
11.05BWO
12.98BWO
0.5MNAOH
3.52
2.75
2.80
1.50
2.03
4.90
18.88
102.90
—
—
—
—
—
—
21.26
100.00
Bi
Bi
Bi
Bi
Bi
2
2
2
2
2
O
3
O
3
O
3
O
3
O
3
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