A {110} facet predominated Bi6O6(OH)3(NO3)3·1.5H2O photocatalyst: Selective hydrothermal synthesis and its superior photocatalytic activity for degradation of phenol

Article abstract of DOI:10.1039/c5ra15629g

A basic bismuth(iii) nitrate photocatalyst with the composition of Bi₆O₆(OH)₃(NO₃)₃·1.5H₂O (BBN) was facilely synthesized using a hydrothermal strategy via incomplete hydrolysis of bismuth nitrate. Characterization of the composition, morphology, microstructure, optical absorption, BET surface area, and photocatalytic behavior was systematically explored. The results indicated that BBN architectures built up of multilayered meshing-teeth structures with predominant {110} side facets can be selectively obtained by fine-tuning the reaction parameters. The sample exhibits an obviously superior photocatalytic activity for the degradation of phenol compared with BBN sheets with dominant top {001} planes and commercial P25, with the rate constant k improved by 3.6 and 2.8 fold, respectively. The excellent photocatalytic behavior combined with the rather low BET surface area of 0.0453 m² g^-1 indicate that the highly reactive {110} facets in BBN are responsible for the photocatalysis. The active oxidation species and main intermediates in the phenol/BBN system are ascertained using scavenger experiments and high performance liquid chromatography (HPLC) techniques. Combining the band edge of BBN and the redox potentials of the active species, a possible migration mechanism of photogenerated e^-/h⁺ pairs on the surface of BBN is proposed. This work provides some new insights for the rational design and synthesis of active-facet exposed basic salt photocatalysts with excellent efficiency.

Full text of DOI:10.1039/c5ra15629g

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{110} facets predominated Bi6O6(OH)3(NO3)3·1.5H2O photocatalyst:
selectively hydrothermal synthesis and superior photocatalytic
activity for degradation of phenol
LiꢀMin Yang, GuoꢀYingZhang*, Yue Liu*, YanꢀYan Xu, ChunꢀMei Liu, JingꢀWang Liu
Tianjin Key Laboratory of Structure and Performance for Functional Molecules; Key
Laboratory of InorganicꢀOrganic Hybrid Functional Material Chemistry, Ministry of
Education; College of Chemistry, Tianjin Normal University, Tianjin 300387, China
* Corresponding author. Tel./fax: +86 22 23766532
Eꢀmail address:hxxyzgy@mail.tjnu.edu.cn (G.ꢀY. Zhang);
hxxyly@mail.tjnu.edu.cn(Y. Liu)
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Abstract: The basic bismuth (III) nitrate photocatalyst with composition of
Bi6O6(OH)3(NO3)3·1.5H2O (BBN) was facilely synthesized by hydrothermal strategy
via incomplete hydrolysis of bismuth nitrate. The characterizations of composition,
morphology, microstructure, optical absorption, BET, and photocatalytic behavior
were systematically explored. The results indicate that the BBN architectures built up
of multilayered meshingꢀtooth with predominated {110} side facets can be selectively
obtained by fineꢀtuning the reaction parameters. The sample exhibits obviously
superior photocatalytic activity for degradation of phenol compared with BBN sheets
of dominant top {001} planes and commercial P25 with rate constant k improved by
3.6 and 2.8 folds, respectively. The excellent photocatalytic behavior combined with
its rather low BET surface area of 0.0453 m2·gꢀ1 indicates the highly reactive {110}
facets in BBN for photocatalysis. The active oxidation species and main intermediates
in phenol/BBN system are ascertained by scavenger experiments and high
performance liquid chromatography (HPLC) technique. Combine the band edge of
BBN and the redox potentials of active species, the possible migration mechanism of
photogenerated eꢀ/h+ pairs on the surface of BBN is proposed. This work provides
some new insights for the rational design and synthesis of activeꢀfacet exposed basic
salt photocatalyst with excellent efficiency.
Keywords: Selective synthesis; Bi6O6(OH)3(NO3)3·1.5H2O; Photocatalysis; Active
facets; Carrier migration; Intermediates
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1. Introduction
Phenol and its derivatives are categorized as one of the most serious environmental
contaminants discharged by industrial plants due to their toxicity, carcinogenicity and
difficulty to degrade.1,2 In recent years, semiconductor photocatalysis has gradually
become one of the most effective technologies in the degradation of phenolic
wastewater since it can completely treat the pollutants to harmless end products under
mild conditions 3ꢀ6
The key issue for the photocatalytic degradation of phenol
.
contaminants in wastewater depends on the performance of photocatalytic materials
used. Developing a new efficient photocatalyst to meet future environmental
requirement is still one of the focuses of current studies.
Among the numerous semiconductors, bismuthꢀcontained materials have been
received great attention in environmental remediation over the past few years.
Bismuth (Bi) is a pꢀblock metal with a d10configuration, and the Bi 6s orbitals can
interact with the O 2p orbitals to form a preferable hybridized valence band (VB),
which favors the mobility of photogenerated holes in the VB and benefits the
enhancement of the photocatalytic performance of the Bi3+ꢀbased oxides.7 So far,
bismuth compounds with satisfactory photocatalytic properties for the degradation of
8,9
organic pollutants are gradually researched in the photocatalytic field such as Bi2O3
,
Bi2WO610,11, Bi2MoO612,13, BiVO414,15, BiOX (X=Cl, Br, I)16ꢀ18, and etc. Biꢀbased
basic salt is another family member of bismuth compounds and it has shown very rich
crystal chemistry with almost 15 different structures discovered.19 They were
preliminarily studied for medical application and as precursor for bismuth oxides,20
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and seldom attention has been given in the domain of photocatalysis.
Just recently, the basic bismuth nitrate nanosheets with complex composition of
Bi6O6(OH)3(NO3)3·1.5H2O was found to be a new type of photocatalyst for UVꢀlight
degradation of dyes such as malachite green and methyl orange.21,22 But for nanosized
photocatalyst, it is difficult to be thoroughly recollected from the degradation solution
and thus brings about secondary pollution. And organic dyes can often be seriously
degraded under UV illumination even in the absence of photocatalyst. Our recent
research has shown that BBN is also an efficient coꢀcatalyst of Bi2WO6 for
degradation of Rhodamine B due to bandꢀgap coupling effect.23 Although
doubleꢀlayered BBN architecture has been synthesized by Liu et al. for removal of
phenol but only exhibited comparable UV photocatalytic activity with classic P25.24
In the very limited reports, no systematical exploration has been carried out to clarify
the influence of preparation parameters on the structure, which is a prerequisite for
better understanding of the relationship between the specific structure and
photocatalytic performance. Usually, the photocatalytic reaction occurs at the
interface of the reactant and catalyst, and the photocatalytic activity is strongly
dependent on the surface morphology and exposed crystal facets.25ꢀ27 Pioneering
works on the synthesis of anatase28,29 and BiOCl30,31 with highly reactive facets have
paved a new way for the enhancement of photocatalytic performance. Undoubtedly,
facet engineering not only is an exciting direction to pursue for highly active
newꢀgeneration photocatalysts but also offers opportunities to investigate the
relationship between the surface properties and the photocatalytic properties.
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In this paper, we first report on the controllable synthesis of BBN architectures
built up of multilayered toothꢀmeshing with predominated {110} side surface and the
facetꢀdependent photoreactivity for degradation of phenol under UV light. The {110}
facets predominated BBN photocatalyst exhibits obviously superior photocatalytic
activity to less layered counterparts and individual BBN sheets with dominant {001}
crystal planes. And it also presents much higher photocatalytic behavior than P25 with
a rate constant improved by 2.8 folds. The migration direction of photoinduced
carriers at the reaction interface is further provided based on the results of active
species and intermediates.
2. Experimental section
Preparation
All the reagents were of analytical grade and used without further purification.
BBN with predominant {110} facets was prepared as follows: 1mmol Bi(NO3)3·5H2O
was dissolved in 17.0 mL distilled water. After stirred for 30 min, the resulting
suspension was transferred into a 25 mL Teflonꢀlined stainless steel autoclave and
heated at 120 °C for 8 h. After the autoclave cooled to room temperature naturally, the
solid products were collected by centrifugation, washed repeatedly with deionized
water and absolute ethanol for several times, and then dried at 60 °C in air. For
comparison, a series of parallel experiments was designed. All temperature series
samples were treated from 80 to 200 °C for 8 h and time series samples were prepared
at 120 °C for different hours, keeping other conditions unchanged (Tab. S1).
Characterization
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The crystal phase and composition of the samples were characterized by Xꢀray
diffraction (XRD) on a Bruker D8ꢀAdvance diffractometer with Cu Kα (λ = 0.15418
nm) radiation. Morphologies and microstructures of the prepared products were
examined with a fieldꢀemission scanning electron microscopy (FEI, NOVA Nano
SEM 230) and a highꢀresolution transmission electron microscopy (FEI, Tecnai G2
F20). BET surface area measurement was obtained through N2 adsorptionꢀdesorption
isotherms collected at liquid nitrogen temperature using a Micromeritics ASAP2020
surface area and porosity analyzer. The UVꢀvis diffuse reflectance spectra were
recorded on a JASCO Vꢀ550/Vꢀ570 UVꢀvis spectrophotometer fitted with an
integrating sphere accessory.
Photocatalytic tests
Photocatalytic activities of the samples were evaluated for the degradation of
phenol aqueous solution under UV irradiation (300W Hg lamp) in a photoreactor
equipped with water circulation facility (Fig. S1). In each test, 10.0 mg of
photocatalyst was added into phenol solution with the initial concentration of 10ꢀ4 M
in a quartz tube. Before illumination, the suspension was magnetically stirred in dark
for 30 min to establish an adsorptionꢀdesorption equilibrium. Then at given time
intervals, one quartz tube was taken out and the photocatalyst was immediately
separated by centrifugation to analyze the supernate by a Shimadzu 2550 UVꢀvis
spectrophotometer.
The degradation intermediates of phenol were monitored with HPLC of Agilent
1100 series (Palo Alto, CA, USA) equipped with an Agilent Zorbax Eclipse
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XDBꢀC18 column (150 mm × 4.6 mm, 5ꢁm). During the sample analysis, the column
was maintained at 30 °C. The intermediates were detected in an isocratic elution
program. Methanol and water with volume ratio of 30:70 were used as mobile phase
at a flowꢀrate of 1 mL/min and the injection volume was 20 ꢁL.
3. Results and discussions
Phase, morphology and microstructure analysis
Fig.1 shows the typical XRD pattern of the hydrothermal sample selectively
prepared at 120 °C for 8 h. All diffraction peaks match well with those of tetragonal
basic bismuth (III) nitrate BBN (a = b = 3.818 Å, c = 17.149 Å, JCPDS: 53ꢀ1038). No
sights of impurity phases such as Bi, BiO(NO3), Bi2O3,and Bi(NO3)3 are detected,
showing the high purity of the product. And the sharp and narrow diffraction peaks
indicate the high crystallinity of BBN. But it is interestingly noted that the intensity
ratios of the diffraction peaks show obviously difference with that of the standard
values. The (002) peak is greatly inhibited but the (110) and (102) peaks are
dramatically strengthened. In particular, the diffraction intensity ratio of (110)/(002)
planes in the asꢀobtained BBN sample is 2.5, promoted by 25 folds compared with the
standard value of 0.1. And the intensity ratio of (102)/(002) has also been enhanced by
12.2 times. The XRD result demonstrates the preferential growth of specific crystal
plane in BBN.
The morphology and microstructure of BBN are identified by SEM and TEM. The
panoramic SEM image in Fig.2a indicates that the sample consists of monodispersed
globularꢀlike structure mostly ranging from 20 to 37 ꢁm in diameter with coarse
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surface. The closeꢀup view of a few BBN architectures in Fig.2b shows that the
spheres are built up of multilayered toothꢀmeshing of 10ꢀ15 layers. The coarse surface
comes from the ragged arrangement of triangleꢀshaped tooth with side length of 1ꢀ3
ꢁm. Further investigation was carried out by TEM to give detailed insights into the
microstructures of BBN. The cogꢀwheel outline of an isolated BBN particle shown in
the TEM image (Fig.2c) agrees well with the toothꢀmeshing structure as indicated in
SEM. Fig.2d shows the HRTEM image of the edge part projected from the top surface
of a BBN meshingꢀtooth. Clear lattice fringe with spacing of 0.270 nm corresponds to
{110} crystal planes and implies the high crystallinity of the meshingꢀtooth. The
selectedꢀarea electron diffraction (SAED) in Fig.2e with wellꢀordered dot pattern
further confirms the single crystal nature of the building unit and can be well indexed
to {110} equivalent planes. The ED pattern was projected from the [001] zone axis of
a BBN meshingꢀtooth. Therefore, the exposed side surface of the tooth structure is
dominant {110} facets, while the top surface of the tooth unit belongs to {001} facets.
The TEM data coupled with XRD result strongly confirm that the BBN sample with
multiꢀlayered meshingꢀtooth exhibits predominated {110} crystal planes.
Influence of preparation parameters
To gain a better understanding of the formation process of the BBN superstructures,
products obtained at different growth stages were collected for SEM and XRD
measurement. As shown in Fig.3a, the precursor presents orderly rods with length of
10ꢀ25 ꢁm and width of 2ꢀ5 ꢁm, respectively. As the reaction proceeds for 30 min (Fig.
3b), the rods become disordered and tend to construct into a few clusters. After 50
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min of reaction, some multilayered toothꢀmeshing architectures have appeared with
the coexistence of precursor rods and transitional clusters. The fast transformation
from clusters to toothꢀmeshing superstructure may disclose the metastable property of
the intermediate morphology. The XRD patterns show that the products before 50 min
are complex and difficult to be indexed due to the complicated hydrolysis of bismuth
nitrate.32 After 2 h of treatment, the rods and clusters disappear completely and all the
particles present toothꢀmeshing structure but with much less layers and nonꢀuniform
diameters. At this stage, the XRD pattern can be ascribed to tetragonal BBN and the
(110) and (102) peaks have showed preferential growth compared with the standard
file. Finally, BBN architectures of wellꢀdefined multilayered toothꢀmeshing are
formed via the Ostwald ripening process33 at 8h. And the prevailed (110) diffraction
has been in sharp contrast to the great inhibition of (002) peak. It seems that the more
exposed side faces in the multilayered meshingꢀtooth structures account for the
obvious change in facet orientation. And it has been ascertained by HRTEM that the
side surface belongs to {110} planes.
The influences of hydrothermal temperature on the morphology and structure of
BBN were also investigated. As shown in Fig.4a, the sample obtained at 80 °C is still
randomly constructed rods with complicated XRD pattern. At elevated temperature of
100ꢀ120 °C, the samples are distinguished from the former with morphologies of
multilayered toothꢀmeshing (Fig.4bꢀc). But the architectures formed at 120 °C are
more uniform and have obviously more layers of meshingꢀtooth. Although the BBN
products both exhibit preferential (110) planes, vivid comparison gives a 1.3ꢀfold
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higher intensity ratio of (110)/(002) planes at 120 °C. Further increase of
hydrothermal temperature leads to great changes in both morphology and structure.
As shown in Fig.4d, the compact BBN architectures become cracked with 3D
meshingꢀtooth gradually replaced by flatted sheets at 160 °C. And the corresponding
XRD pattern gives an obviously strengthened (002) diffraction peak. At a high
temperature of 200 °C, the BBN architectures have been seriously destroyed,
accompanied by the growth of isolated and irregular sheets with lateral size of 20ꢀ40
ꢁm. And the XRD pattern displays predominant (002) peak with a ratio of (002)/(110)
approaching the standard value. The results further indicate that the exposed side
surface of the meshingꢀtooth should be mainly attributed to {110} facets. So
appropriate hydrothermal temperature and time are both key factors to achieve BBN
structures with preferentially oriented {110} facets.
Optical properties
To confirm the optical absorption property of the BBN architectures, UVꢀvis DRS
spectrum was measured with the result shown in Fig.5. According to the spectrum, the
BBN presents the photoabsorption to UV light with a threshold of 375 nm. The steep
shape of the spectrum indicates that the light absorption is not due to the transition
from the impurity level but comes from the intrinsic bandꢀgap transition.34 Thus the
BBN with composition of Bi6O6(OH)3(NO3)3·1.5H2O behaves like a bulk
semiconductor. For a crystalline semiconductor, the optical absorption near the band
edge follows the equation αhν =A(hν − Eg)n/2, where α, hν, A, and Eg are the
absorption coefficient, photonic energy, proportionality constant, and band gap,
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respectively.35 And the value of n decides the characteristics of the transition in a
semiconductor.36 According to previous literature24 and the clear region of linearity in
(αhν)1/2 vs. hν, the value of n for BBN is ascertained to be 4 as indirect transition. As
can be expected based on the absorption spectra, the sample is white in color. The
intercept of the tangent to the plot gives a good approximation of the band gap energy
of 3.3 eV. The DRS spectrum indicates that the BBN can be used as a photocatalyst
under UV irradiation.
Photocatalytic properties
The photocatalytic degradation of phenol over temperature and time series samples
was evaluated under the irradiation of a 300 W Hg lamp with results shown in Fig.6.
The contrast test shows that only ca. 6.5 % of phenol was degraded in the dark in the
presence of BBN, indicating that the adsorption of phenol can be negligible. The
blank test in the absence of BBN presents reversely raised absorption at 270 nm in the
initial stage (Fig. S2). The phenomenon has been reported in the study of UV
photolysis of phenol and it is ascribed to the excessively accumulated intermediates
with similar structure to phenol.37 The slightly decreased absorbance after 60 min
irradiation in the absence of BBN shows the ineffectiveness of direct photolysis. The
BBN sample prepared at 120 °C for 8h gives the best photocatalytic behavior with
98.1% removal of phenol in 45 min, which is much better than commercial P25 under
the same conditions. As shown in Fig. 6b, the pseudo firstꢀorder kinetics constants k
of the phenol degradation over BBN is calculated to be 0.08945 minꢀ1, improved by a
factor of 2.8 in comparison with that of P25 (0.03192 min−1). It should be noted that
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the BET surface area of BBN is detected merely 0.045 m2·gꢀ1, which is considerably
smaller than 43.26 m2·gꢀ1 of the most widely used P25. The superior photocatalytic
activity and rather low surface area may suggest the preferential orientation of
reactive facets in BBN. The asꢀobtained sample also exhibits higher degradation
efficiency than the reported doubleꢀlayered BBN architecture.24 For example, 38% of
phenol has been degraded after 15 min irradiation over the multilayered BBN in
compared with the value of only 6.0% over doubleꢀlayered BBN. And there is still 18%
of phenol left after 45 min photocatalysis for doubleꢀlayered BBN in contrast to the
little residue of 1.9% in our work. The results may imply the important role of the side
face of the BBN meshingꢀtooth in photocatalysis, which is further confirmed by the
following photocatalytic results over temperature and time series BBN samples.
The hydrothermal temperature has a significant influence on the photocatalytic
activity of BBN. As shown in Fig.6aꢀb, the degradation of phenol increases at first
and then decreases with the improvement of temperature, and the best activity is
achieved at 120 °C. Its rate constant k is as fast as 2.4 and 3.6 times of BBN samples
obtained at 100 and 200 °C, respectively. From the above SEM and XRD analysis, it
is known that the BBNꢀ120 °C presents the most layers of meshingꢀtooth with
predominated {110} side surfaces. However, the BBNꢀ200 °C exhibits prevailed
microsheets with extremely diminished side faces but dominant {100} top surfaces.
And the BET surface area of the sheetꢀshaped BBN is 0.245 m2·gꢀ1, which is 5.4ꢀfolds
higher than that of BBNꢀ120 °C. The much improved photocatalytic activity
combined with the lowered surface area confirms the high reactivity of the
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predominated {110} facets in BBNꢀ120 °C. Fig.6cꢀd shows that reaction time is
another important factor for the photocatalytic activity of BBN. The 50ꢀmin sample
gives a rather low photocatalytic activity due to the partial formation of toothꢀmeshing
architectures as shown in Fig.4. The degradation of phenol is accelerated over BBN
for prolonged hydrothermal time with more layers of meshingꢀtooth. But a conversely
lowered activity was detected for BBNꢀ12h with a rate constant k only 34% of that of
BBNꢀ8h. The structure and morphology analysis (Fig.S3) imply that BBNꢀ12h shows
decreased layers of meshingꢀtooth and much inhibited (110) planes. The results
further confirm the highly reactive {110} tooth surface of BBN.
Effects of process parameters
The photocatalytic performance of a catalyst is affected by various process
parameters. The influences of BBN dosage and initial concentration of phenol
solution were investigated. As shown in Fig.7a, the degradation efficiency of phenol
is firstly increased and then decreased with the enhancement of BBN amount and the
best BBN dosage is fixed at 10 mg. The corresponding rate constant k (inset) over 10
mg of BBN is improved by 3.5 and 3.0 times compared with that of 5 and 20 mg of
BBN, respectively. Obviously, lower catalytic activity is observed when a smaller
amount of catalyst is used because less catalytic active sites are available. However,
excessive BBN powder would cause an increase in the opacity of the degradation
solution and light scattering and even a decrease in the number of catalytic surface
active sites caused by aggregation of photocatalyst particles.38 So there is often an
optimum amount of photocatalyst to achieve the maximum photocatalytic efficiency.
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And this phenomena have also been reported in previous literatures39,40
.
Fig.7b presents the influence of the initial phenol concentration (C0) on the
photocatalytic activity. It can be seen that lower phenol concentration gives a more
efficient degradation. A higher phenol concentration of 1.5 × 10ꢀ4 M gives a decreased
degradation ratio by ca. 20%. It is believed that as the C0 of phenol increases, the
surface active sites of BBN would be over loaded by additional phenol molecules.
Resultantly, the photoꢀgeneration of reactive oxygen species would be reduced. In
addition, more incident photons would be intercepted by increased phenol molecules
before they reach and react with the photocatalyst.41 So 10 mg of BBN per 10 mL
phenol solution (10ꢀ4 M) was fixed to be the optimum photocatalytic parameters.
Photocatalytic mechanism
To evaluate the role of the primary reactive species, scavenger experiments were
performed by adding individual scavengers to the phenol/BBN system and the results
were shown in Fig. 8. The addition of tertꢀbutanol (TBA), a wellꢀknown scavenger of
•
OH radicals,42 into the photoreaction system causes a decreased degradation
efficiency of 16.6% after 45 min irradiation. The weak inhibition indicates that OH
•
may be not the major species in the oxidation process. However, the introduction of
holes scavenger ethylene diamine tetraꢀacetic acid (EDTA)42 gives a greatly depressed
degradation efficiency of 50.1% at 45 min. And the presence of benzoquinone (BQ), a
ꢀ
superoxide radical (O2 ·) scavenger,43 leads to the most strong inhibition for phenol
degradation. In addition, the photocatalysis was carried out with continuous
N2ꢀsparging due to the nonꢀvolatility of phenol. Under the anoxic condition, the
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degradation of phenol is also significantly suppressed. The presence of oxygen is
important and its effect is to primarily act as an efficient eꢀ trap, leading to the
generation of O2 · and preventing the recombination of eꢀ and h+.44,45 The similar
ꢀ
results with added BQ and in N2ꢀsaturated suspension together confirm the most
ꢀ
important effect of O2 ·. Accordingly, the reactive species are believed to play their
ꢀ
oxidation roles in a sequence of O2 ·, h+, and
•OH in phenol/BBN system.
The photocatalytic oxidation of phenol often involves complicated processes.
HPLC was utilized to monitor the evolution of intermediates during phenol
degradation over the BBN with dominant {110} facets. Fig.9aꢀc shows typical HPLC
chromatograms of the phenol degradation solutions before and after irradiation for
different time. The original solution exhibits only one strong peak at the retention
time of 9.5 min corresponding to phenol (Fig. 9a). The intensity of phenol peak
decreases gradually with prolonged irradiation and disappears after 45min (Fig. 9bꢀc),
indicating the full degradation of phenol and in good consistence with the results
shown in Fig.6a. In addition to the underꢀdegraded phenol, three major aromatic
products were detected in the initial stage with retention time of 2.2, 3.1 and 4.6 min,
respectively. And the intermediates are identified to be hydroquinone, resorcinol and
catechol by contrasting to the chromatogram of standard materials.
The corresponding concentrationꢀtime curves calculated from the internal standard
calibration with good linear correlation (Tab. S2) were shown in Fig.9d. Accompanied
with continuously decreasing concentration of phenol (Fig. S4), the concentration of
intermediates all enhances at earlier stage and then reduces quickly. In specific,
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catechol and hydroquinone achieve the maximum concentration of 0.00113 and
0.00045 mg/ml at 8 min and resorcinol reaches a peak concentration of only 0.00029
mg/ml at 3 min. According to the substitution rules, ·OH radicals attack the phenol
molecules with higher probability in 2 and 4 positions versus OH group of the
aromatic ring. So it is rational that the mꢀpositioned resorcinol presents a
concentration of the minimum. In previous reports on traditional TiO2, benzoquinone
is also a common organic byproduct generated during the degradation of phenol.46 It
is different that no obvious benzoquinone peak is detected in the phenol/BBN system.
ꢀ
The O2 · and h+ have been ascertained to be the dominant active species, which are
nonselective for organic degradation with strong oxidation ability. It may be inferred
that the life span of benzoquinone formed at different stages of the reaction is short
ꢀ
due to the fast oxidation by O2 · and h+. The transient presence of benzoquinone with
low concentration would make it difficult to be detected.24 The aromatic intermediates
would undergo further photocatalytic oxidation quickly to form nontoxic aliphatic
acids by ring cleavage47and subsequently be completely transformed to CO2 and H2O.
It is well known that the effective separation and transfer of photoinduced electrons
and holes are important factors for a desired photocatalyst. And the key to cause the
migration of carriers is the difference between the redox potential of active species
and the positions of valence band (VB) and conduction band (CB) of the
semiconductor. Combined the band edges of BBN with the ascertained oxidative
species, a proposed migration mechanism of photogenerated carriers on the surface of
BBN is illustrated in Fig.10. The BBN is excited by UV irradiation to generate
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electrons in CB and holes in VB, which then migrate to the surface of photocatalyst. It
is reported that BBN exhibits nꢀtype characteristics with CB potential at ꢀ0.90 V,22
which is more negative than the redox potential of O2/O2 · (ꢀ0.28V vs. NHE).48 Then
ꢀ
forced by the potential difference, the CB electrons can be quickly captured by the
ꢀ
adsorbed molecular oxygen in water to form O2 ·, which has been confirmed to be the
most important active species. While the VB holes are believed to play vital roles
through two different routes at the phenol/BBN interface. The major one is the direct
degradation to phenol due to the strong oxidative ability of BBN holes with calculated
VB position of 2.40 V based on the estimated Eg of 3.3 eV in DRS spectra. And the
direct oxidation has been affirmed by the greatly inhibited degradation efficiency with
added EDTA. In addition, the hole can also oxidize the OHꢀ or H2O in phenol solution
to form •OH owing to the slightly anodic VB position compared with the redox
potential of •OH/OH− (2.38 V vs. NHE).49 And •OH has also been confirmed to be
one of the active species in phenol/BBN although not the most important. Then the
•OH invades into phenol molecules in the initial stage of photocatalysis, leading to the
formation of benzenediol intermediates as monitored by HPLC. Then with the
ꢀ
synergism of active species of O2 ·, h+, and •OH, the phenol and intermediates would
subsequently decomposed into nontoxic alphatic acids47,50 and finally transformed to
H2O and CO2.
4. Conclusions
In summary, the BBN photocatalyst Bi6O6(OH)3(NO3)3·1.5H2O with predominately
exposed {110} facets was selectively prepared. The sample exhibit much higher
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photocatalytic activities to phenol than that of BBN with dominant {001} planes and
commercial P25 with rate constants k improved by 3.6 and 2.8 folds, respectively. It
suggests that {110} facets should be the reactive highꢀenergy crystal planes in BBN.
And the work provides a better understanding of the correlation among preparation
parameter, specific structure and photocatalytic performance of the novel
photocatalyst. The scavenger experiments and HPLC technique disclose the dominant
active species and main intermediates in the initial stage of photocatalysis. And the
possible migration mechanism of photogenerated carriers on the surface of BBN is
proposed.
Acknowledgements
The project was supported by National Natural Science Foundation of China (No.
21303122), the Program for Innovative Research Team in University of Tianjin
(TD12ꢀ5038), and the Open Project Program of Tianjin Key Laboratory of Structure
and Performance for Functional Molecules in 2014.
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Figure captions:
Fig. 1 XRD pattern of the asꢀsynthesized BBN sample and the bottom is the JCPDS
standard.
Fig. 2 (a) Panoramic FEꢀSEM image of the product, (b) magnified FEꢀSEM image, (c)
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TEM image of an isolated structure, (d) HRTEM image of the edge part in c,
and (e) the corresponding SAED pattern.
Fig. 3 SEM images and XRD patterns of time series samples: (a) precursor, (b) 30
min, (c) 50 min, (d) 2 h, and (e) 8 h.
Fig. 4 SEM images and XRD patterns of temperature series samples: (a) 80°C, (b)
100°C, (c) 120°C, (d) 160°C, (e) 200°C,and (f) corresponding XRD patterns.
Fig. 5 UVꢀvis diffuse reflectance spectrum of BBN sample with Eg analysis in inset.
Fig. 6 Photocatalytic degradation of phenol and corresponding pseudo firstꢀorder
plots over: (a, b) temperature series samples, and (c, d) time series samples.
Fig. 7 The degradation of phenol over BBN under different operating conditions: (a)
BBN dosage, (b) the initial concentration of phenol.
Fig. 8 Photocatalytic degradation of phenol over BBN with the addition of scavengers
of different reactive species and in N2ꢀsaturated solutions.
Fig. 9 (aꢀc) HPLC chromatograms of phenol photodegradation by BBN for different
time, (d) concentrations of intermediates formed from phenol during the
photocatalysis.
Fig.10 Schematic illustration of the charge transfer on the BBN surface and the
degradation process of phenol
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Fig.1
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Fig.2
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Fig.3
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Fig.4
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Fig.5
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Fig.6
29