Single-crystalline Bi5O7NO3 nanofibers- Hydrothermal synthesis, ch

Single-crystalline Bi 5O 7NO 3nanofibers:Hydrothermal synthesis,characterization,growth mechanism,and photocatalytic properties
Shujie Yu a ,Gaoke Zhang a ,⇑,Yuanyuan Gao a ,Baibiao Huang b
a
Hubei Key Laboratory of Mineral Resources Processing and Environment,School of Resources and Environmental Engineering,Wuhan University of Technology,122Luoshi Road,Wuhan 430070,PR China b
State Key Laboratory of Crystal Materials,Shandong University,27Shanda Nanlu,Jinan 250100,PR China
a r t i c l e i n f o Article history:
Received 2August 2010Accepted 7October 2010
Available online 12October 2010Keywords:Bismuth Bi 5O 7NO 3
Hydrothermal synthesis Visible light Photocatalytic
a b s t r a c t
A new photocatalyst,namely single-crystalline Bi 5O 7NO 3nanofibers,was prepared by a facile hydrother-mal method in the presence of Triton X-100and ammonia.Bi 5O 7NO 3possessing a crystalline sheet mor-phology could be dissolved and transformed into nanofibers by controlling the reaction time.Samples were characterized by X-ray diffraction,UV–vis diffuse reflectance spectroscopy,X-ray photoelectron spectroscopy,scanning electron microscopy,transmission electron microscopy and high resolution trans-mission electron microscopy.The Bi 5O 7NO 3nanofiber growth mechanism is discussed in detail.The band gap energy of the as-prepared Bi 5O 7NO 3photocatalyst was about 2.70–2.90eV.Results of first-principle density functional theory calculations confirmed that Bi 5O 7NO 3had a narrow band gap.They revealed that the conduction band bottom was predominantly composed of Bi 6s,Bi 6p,N 2p and O 2p orbitals,while the valence band (VB)top primarily consisted of Bi 6p,Bi 6s and O 2p orbitals.The as-obtained Bi 5O 7NO 3nanofibers showed good photocatalytic activity and stability for the degradation of Rhodamine
B (RhB)under visible light irradiation,which may be ascribed to the highly mobile conduction band (CB)and VB charge carriers.
Ó2010Elsevier Inc.All rights reserved.
1.Introduction
In recent years,the design and preparation of novel nano-and microstructured one-dimensional (1D)materials (e.g.tubes,fibers,rods,and whiskers)has attracted much attention because of their unique electronic,optical,and catalytic properties [1–3].Among these materials,nanofibers have received special consideration due to their high surface-to-volume ratios [4–6].Nanofibers have been obtained by various methods such as hydrothermal [6],elec-trospinning [5,7]and sol–gel procedures [8].The hydrothermal method has been referred to as an environmentally friendly aqueous process [9],and has aroused interest due to the high degree of crys-tallinity and purity of the resulting product,the ease of which sample shape and size may be controlled,and finally the mild reaction con-ditions involved [6,9,10].Triton X-100is a surfactant which can facil-itate the formation of 1D materials.Nanofibers [11,12]and nanorods [13]have been successfully prepared with the aid of Triton X-100.Bismuth-based photocatalysts (e.g.BiOCl [14],Bi 3TaO 7[15],Bi 2-MoO 6[16,17],Bi 2WO 6[18],BiVO 4[19],BiOX (X =F,Cl,Br,I)[20],Na 0.5Bi 1.5O 2Cl [21]and Bi 5O 7I [22])have received recent interest due to their high chemical stabilities and visible-light photocata-lytic activities,both of which have been ascribed to their unique layered structures.Bismuth oxyhalides all have unique layered structures characterized by [Bi 2O 2]slabs interleaved by slabs of inorganic atoms [20,21].
Kodama [23]synthesized Bi 5O 7NO 3as an intermediate product by thermal decomposition.To date,there are no reports on the preparation of pure Bi 5O 7NO 3nanofibers and their photocatalytic activity.
In the present work,we have synthesized Bi 5O 7NO 3nanofibers by a facile hydrothermal method in the presence of Triton X-100and ammonia.The influence of reaction conditions on the forma-tion of the Bi 5O 7NO 3nanofibers was systematically investigated,and the obtained nanofibers showed excellent visible-light photo-catalytic activity.Furthermore,we have demonstrated a relation-ship between the electronic structure and photocatalytic activity of the Bi 5O 7NO 3photocatalyst using density functional theory calculations.
2.Materials and methods 2.1.Preparation
Bi(NO 3)3Á5H 2O,ammonia (NH 3ÁH 2O,25%),NaOH,Triton X-100,terephthalic acid (TA)and 1,4-benzoquinone (C 6H 4O 2,BQ)were all of analytical grade and were used without further purification.In a typical process,1mmol of Bi(NO 3)3Á5H 2O was added to 70mL
0021-9797/$-see front matter Ó2010Elsevier Inc.All rights reserved.doi:10.1016/j.jcis.2010.10.012
Corresponding author.Fax:+862787887445.
E-mail address:gkzhang@ (G.Zhang).
of distilled water,with magnetic stirring used to form a homoge-neous solution.A white precipitate appeared immediately after adjusting the pH to12using NH3ÁH2O,and0.5mL of Triton X-100was then added.The reaction mixture was sealed in a 100mL Teflon-lined stainless steel autoclave,kept at180°C for 24h and then cooled to room temperature.The product was then collected,washed and dried at60°C.For comparison,the nano-sized Bi3TaO7catalyst was prepared by according to our previous study[15].
2.2.Characterization
The structure and phase composition of samples were analyzed using powder X-ray diffraction(XRD)on a D/MAX-RB powder X-ray diffractometer with Cu K a radiation.The accelerating voltage and applied current were40kV and80mA,respectively.Scanning electron microscopy(SEM,JSM-5610LV)andfield emission scan-ning electronic microscopy(FESEM,S-4800)were used to character-ize the product morphologies.Morphologies and microstructures were further examined with transmission electron microscopy (TEM)and high resolution transmission electron microscopy (HRTEM)imaging using a JEM2100F electron microscope with a 200kV accelerating voltage.Samples for TEM and selected area elec-tron diffraction(SEAD)analyses were dispersed in ethanol by ultra-sonication.UV–vis diffuse reflectance spectra(DRS)were measured using a Shimadzu UV-2550UV–vis spectrophotometer.X-ray photo-electron spectroscopy(XPS)was carried out using an ESCALAB II XPS system with a monochromatic Mg K a source and a charge neutral-izer.Binding energies were referenced to the C1s peak at284.8eV of the surface adventitious carbon.
2.3.Electronic structure calculation
First-principle calculations were carried out using CASTEP soft-ware within the generalized gradient approximation(GGA-PBE). The k-point meshes for Brillouin zone sampling were constructed using the Monkhorst–Pack scheme(k-point=2Â1Â4).A plane wave cutoff energy of340eV was used.
2.4.Photochemical experiments
The photocatalytic activity of the Bi5O7NO3nanofibers was eval-uated by the degradation of Rhodamine B(RhB)solution under vis-ible light irradiation(300W Dy lamp with a420nm cutofffilter), and the reaction temperature was held constant at25°C.Reaction solutions were prepared by adding0.075g of the nanofibers to a 50mL RhB solution(5mg/L).Solutions were magnetically stirred in the dark for5min to disperse the catalyst.After various irradi-ation times,5mL aliquots were collected and centrifuged to re-move the photocatalyst particles.The concentration of the RhB aqueous solution was determined by its absorbance at553nm using a UV–vis spectrophotometer(Unico UV-2102PC,Shimadzu). The total organic carbon(TOC)was measured via a multi N/C2100 TOC analyzer(Analytic Jena AG).The photocatalytic stability of the Bi5O7NO3nanofibers was evaluated by recycling and reusing the catalyst three times for the degradation of RhB under the same conditions.For comparison,the Bi3TaO7catalyst was used to de-grade RhB under the same conditions.
3.Results and discussion
3.1.Formation mechanism of Bi5O7NO3nanofibers
The XRD pattern of the precursor white precipitate prior to hydrothermal reaction is shown in Fig.1a.Diffraction peaks could be indexed to JCPDS76-2478(Bi2O3)and JCPDS01-0898(Bi(OH)3), and revealed that the precursor was a mixture of Bi2O3and Bi(OH)3.
To understand the role of ammonia,the solution pH was ad-justed to12using NaOH instead of NH3ÁH2O,with the Bi5O7NO3 nanofibers then prepared according to the procedure described above.Fig.1b shows that the product obtained via use of NaOH was impure.The formation of pure Bi5O7NO3nanofibers was as-cribed to the presence of ammonia which acted as a nitrogen source for the synthesis of Bi5O7NO3.
Reaction temperature also had an important effect on Bi5O7NO3 formation.Fig.2shows XRD patterns for samples prepared at dif-ferent reaction temperatures.Pure Bi5O7NO3was obtained at tem-peratures above160°C.Diffraction peaks of the product broadened upon increasing the reaction temperature from160°C to180°C, and then sharpened upon further increasing to200°C.The
S.Yu et al./Journal of Colloid and Interface Science354(2011)322–330323
optimum temperature for producing a small grain size crystal according to the Scherrer equation was around180°C.
Triton X-100and the suspension pH value were also found to have an important influence on the Bi5O7NO3nanofiber morphol-ogy.Fig.3a,b and insert c shows SEM images of samples obtained under typical conditions without and with Triton X-100(0.2mL), respectively.The sample prepared without Triton X-100showed a rectangle sheet morphology of length around60–70l m,while the sample prepared with Triton X-100(Fig.3b)was composed of rectangle sheets and some nanorods with diameters of20–50nm on the surface of the sheets(circled in Fig.3b).This indi-cated that pure Bi5O7NO3nanofibers could not be obtained either without Triton X-100or with a lower Triton X-100concentration. The influence of Triton X-100on the nanofiber formation was as-cribed to steric stabilization from its large molecular size and long chain[24].Fig.3d,insert e and f shows FESEM images and a XRD pattern of the compound obtained under different pH values.A sample obtained at pH=10(Fig.3d and e)consisted of micro-rect-angular sheets and attached Bi5O7NO3nanorods.At pH=9(Fig.3f) the resultant product was impure.
To understand the Bi5O7NO3nanofiber formation mechanism, FESEM images of samples obtained after different reaction times were obtained,and are shown in Fig.4.At a reaction time of 12h(Fig.4a and b),the sample exhibited predominantly a rectan-gular sheet morphology with a few nanofibers.The rectangular sheets gradually disappeared as the reaction time was increased to16h(Fig.4c and d),and nanofibers formed with further increas-ing to20h(Fig.4e and f).The diameter of the nanofibers decreased
with increasing reaction time(Fig.4g and h).The relationship be-tween the reaction conditions and Bi5O7NO3nanofiber formation is described in Fig.S1.
From the above observations and the typical crystal structure of the material,nanofiber formation was ascribed to the presence of ammonia,Triton X-100and a sufficiently long reaction time.The Ostwald ripening[25,26]and hydrothermal recrystallization theo-ries[27]could explain the growth process,and the nanofiber for-mation could be divided into several steps.At the beginning of the hydrothermal reaction,crystalline nuclei were quickly gener-ated,and crystal growth followed due to a high reactant concentra-tion[25].Crystals of inhomogeneous size would redistribute themselves via Ostwald ripening processes in the hydrothermal conditions[26].During this process,the large rectangular sheet morphology crystals were formed,with smaller ones dissolving and recrystallizing along(010)planes to form nanofibers in the presence of Triton rger crystals dissolved due to Triton X-100and the critical reaction conditions(high temperature,high pressure and high pH)[26–28].The Bi5O7NO3crystal structure (Fig.S2)has many bismuth atoms on the(100)and(001)planes, with none on the(010)plane.Thus,dissolution of the large rect-angle sheet crystals was ascribed to the adsorption of Triton X-100 which formed a complex with Bi3+ions on the(100)and(001) planes[28].This mechanism is similar to that proposed by Chudi-nova et al.[11].Triton X-100molecules adsorbed on the(100)and (001)planes also resisted the growth of these planes and resulted in preferential growth along the(010)plane.Further increasing the reaction time leads to the larger rectangle sheet crystals 10203040506070
(f)
Bi
5
O
7
NO
3
Impurity
I
n
t
e
n
s
i
t
y
(
a
.
u
)
2 Theta (degree)
3
nanofibers obtained(a)without Triton X-100,(b)and insert(c)with0.2mL of Triton X-100;(d)and insert(e)FESEM at pH=10and(f)XRD pattern of the product obtained at180°C for24h at pH=9.
324S.Yu et al./Journal of Colloid and Interface Science354(2011)322–330
gradually dissolving and reforming into the homogeneous nanofibers.
3.2.Characterization of the Bi 5O 7NO 3nanofibers
An XRD pattern of a typical sample is shown in Fig.5.The struc-ture of Bi 5O 7NO 3has been previously reported [23,29],with Meyer et al.[29]concluding that Bi 5O 7NO 3belonged to the monoclinic space group P121/c.Our XRD pattern of Bi 5O 7NO 3was refined from this monoclinic structure using the Reflex of Materials Studio 4.3,and the refinement is shown in Fig.S3.The structure was refined to Rwp =5.74%and Rp =10.47%.The lattice parameters are a =8.55Å,b =23.38Å,c =5.54Å,and were consistent with those reported by Meyer et al.[29].As shown in Fig.5,all diffraction peaks could be indexed to this result.Bi 5O 7NO 3was found to grow
preferentially orientated with the (141),(080)and (023)crystal planes.
The morphology of a typical nanofiber sample was investigated using FESEM and TEM techniques.Fig.6a and b shows FESEM images of Bi 5O 7NO 3at different magnifications.The as-prepared Bi 5O 7NO 3exhibited a nanofiber morphology,which had a smooth uniform surface and fiber diameters of 45–200nm.The TEM image in Fig.6c shows a nanofiber with a diameter of about 60nm,con-sistent with the FESEM result from Fig.6b.Fig.6d shows a typical HRTEM image for a selected part of a single nanofiber,and it exhib-its many lattice planes with extremely high crystallinity.The mea-sured interplanar spacing for all lattice fringes was 1.168nm,which indexed to the (020)lattice plane of Bi 5O 7NO 3.This plane was parallel to the preferential growth plane (080)which was the strongest peak in the XRD pattern in Fig.5.Fig.6e shows a se-lected area electron diffraction (SAED)pattern also taken from a single nanofiber (shown in Fig.6f),which revealed that the nanof-ibers had a single crystal structure.
To evaluate the electronic environment on the Bi 5O 7NO 3nano-fiber surface,XPS studies were carried out with a typical high res-olution scanning XPS spectrum of Bi 5O 7NO 3shown in Fig.7.From the Bi 4f region in Fig.7a,the spectrum contains peaks at 161.9eV and 167.2eV were attributed to the binding energies of the Bi 4f7/2and Bi 4f5/2levels,respectively.These binding energies are sim-ilar to those observed for BiOCl nanofibers [30],thus bismuth ions in Bi 5O 7NO 3existed in the +3valence state.The N 1s region in Fig.7b contains peaks at 394.1eV and 399.2eV,and the low peak intensity arose due to the low content (2.65at.%)of nitrogen in Bi 5O 7NO 3.The peak at 394.1eV was ascribed to the presence of Bi-N bonds [31–36].The peak at 399.2eV arose due to the special layered structure [31–33,37]and was ascribed to N–O bonds on the nanofiber surface.The O 1s region contains three peaks as shown in Fig.7c.The higher binding energy peak at around 531.3eV was attributed to bridging oxygen atoms from Bi–O–Bi bonds [38,39].The peaks at 527.6eV and 528.4eV were attributed to O–N bonds and non-bridging oxygen atoms in Bi–O bonds,respectively [38].
3.3.Photocatalytic properties of Bi 5O 7NO 3nanofibers
Photocatalytic activity of the Bi 5O 7NO 3nanofibers was evalu-ated by degrading RhB under visible light irradiation (k >420nm).Fig.8a shows UV–vis absorption spectra of a RhB solution during the photocatalytic degradation by the typical Bi 5O 7NO 3nanofiber catalyst.The characteristic absorption peak of RhB at 553nm underwent a rapid intensity decrease [22,40]and the absorption band blueshifted,which was caused
by
Fig. 4.Low and high-magnification FESEM images of samples obtained after different reaction times:(a),(b)12h;(c),(d)16h;(e),(f)20h;(g)and (h)24h.
Interface Science 354(2011)322–330325
N-demethylation of RhB[22,40].The sharp intensity decrease and shift of the major adsorption band within40min indicated the Bi5O7NO3nanofibers possessed a high visible-light photocatalytic activity.
Degradation rates of RhB solution under visible light illumina-tion with and without a photocatalyst present(i.e.the photolysis of RhB),and in the absence of light with the photocatalyst are shown in Fig.8b.The photolysis of RhB was extremely slow un-der visible light illumination without a photocatalyst,and the RhB concentration decreased by only9%after120min with a photocatalyst in the absence of light.However,when the Bi5O7-NO3nanofibers were used as the photocatalyst,99.6%of RhB was degraded after120min under visible light irradiation,which is higher than that of the Bi3TaO7catalyst(80.0%).The results suggest that the decrease of the RhB absorption intensity was predominantly caused by the photocatalytic degradation rather than the adsorption over the Bi5O7NO3nanofibers and the Bi5O7-NO3nanofibers showed good photocatalytic activity for the deg-radation of RhB.
The extent of mineralization of the RhB can be reflected by the reduction of TOC.The TOC variations of RhB solution during the photocatalytic degradation by the Bi5O7NO3nanofibers under vis-ible light irradiation are shown in Fig.8c.After degradation for 120min,the TOC concentration of RhB solution decreased from 11.96mg/L to7.07mg/L,which indicates that the RhB was miner-alized to CO2
partly.
and(c)TEM images of a typical Bi5O7NO3nanofiber sample,(d)HRTEM image and(e)SEAD pattern taken from a single
The Bi5O7NO3nanofibers can be separated easily from the RhB solution by a simple precipitation procedure and be reused.The photocatalitic stability of the Bi5O7NO3nanofibers for the degrada-tion of RhB is shown in Fig.8d.The photocatalytic degradation rate of RhB still reached99.0%after three recycles,and the TOC concen-tration of RhB solution decreased to7.75mg/L,which shows that the Bi5O7NO3nanofibers has good stability for the degradation of organic pollutants.
Photocatalytic degradation rates of RhB solution by Bi5O7NO3 samples obtained under different reaction conditions are shown in Fig.S4.The degradation rate of RhB exhibited a clear increase with both increasing Triton X-100concentration and pH value for the hydrothermal reaction.The photocatalytic degrada-tion of RhB and FESEM analysis suggested that the increasing degradation rate was due to the formation of Bi5O7NO3
nanofibers.
3.4.Visible light-driven degradation mechanism
Previous studies have shown that both hydroxyl radicals(ÅOH) and the oxidizing species such as superoxide radical anions(OÅÀ
2
) are the most important factors affecting photo-degradation[41–47].The high photocatalytic efficiency of the as-prepared Bi5O7NO3 was ascribed to the special configuration of the samples structure,
as shown in Fig.S2.There are two2
1
½BiO þlayers per unit cell which are oriented parallel to the(010)plane.These layers are
connected by1
1
½Bi4O8 4Àunits parallel to the[001]axis.The 2
1
½BiO þlayers and1
1
½Bi4O8 4Àunits form a framework structure. Channels arefilled by double columns of nitrate groups with col-umn axes parallel to[100][29].The Bi5O7and NO3slabs are well-ordered and piled up one-by-one along the b-axis to form the unique structure[22,29,48].Accelerated separation of elec-tron–hole pairs upon photoexcitation was ascribed to the perma-nent static electricfields between the Bi5O7and N–O3layers. When the aqueous RhB/Bi5O7NO3suspension was irradiated with visible light,the electron–hole pairs react with O2and OH–directly
or indirectly to form OÅÀ
2
andÅOH oxidation species[22,49,50].Tere-phthalic acid photoluminescence(TA-PL)probing technique has been widely used in the detection ofÅOH radicals[47,51].2-Hydro-xyl-terephthalic acid(TAOH),which is generated when TA captures theÅOH radicals,performs a strongfluorescence at around426nm on the excitation of its own312nm absorption band.Thus,ÅOH radicals can be detected indirectly by monitoring thefluorescence intensity changes of the TA solution with Bi5O7NO3nanofibers un-der visible light irradiation.As shown in Fig.9a,thefluorescence intensity at426nm is low,which shows thatÅOH radicals are not
the major active radicals for the degradation of RhB.The OÅÀ
2
radi-cals can be detected by1,4-benzoquinone(C6H4O2,BQ)which is
the quencher of OÅÀ
2
radicals[46,52].The degradation of RhB over Bi5O7NO3nanofibers under visible light irradiation with or without BQ is shown in Fig.9b.After degradation for120min,the degrada-tion rate of RhB reached99.6%without BQ,while only30.2%of RhB
was degraded with BQ,which indicates that the OÅÀ
2
radicals were the main oxidative species responsible for the degradation of RhB.
UV–vis diffuse reflectance spectra(DRS)of Bi5O7NO3obtained at180°C under different conditions are shown in Fig.10.The band gap energy was estimated to be about 2.70–2.90eV[53,54]. Fig.10a shows UV–vis DRS of the compound prepared with0–0.5mL of Triton X-100.The absorption edge was blueshifted with an increasing amount of Triton X-100.UV–vis DRS of the com-pounds prepared under different pH values are shown in Fig.10b.The absorption edge of the sample prepared at pH=10 was blueshifted compared with that of the sample obtained at pH=12.From the FESEM results presented in Fig.3,the blueshift of the absorption edge was attributed to quantum size effects resulting from a decreasing catalyst grain size.
Fig.11shows the band structure and density of states of Bi5O7-NO3calculated by CASTEP.Fig.11a shows the band gap estimation of Bi5O7NO3to be about 2.34eV,which was smaller than the experimentally obtained value of2.70eV.Fig.11a also shows that Bi5O7NO3is an indirect bandgap semiconductor.The lowest unoc-cupied state lied at the G point,while the highest occupied state was at the F point[22].Fig.11b and c shows the total and partial density of states of Bi5O7NO3.The conduction band(CB)bottom predominantly consisted of O2p,N2p,Bi6s and Bi6p orbitals, while the broad valence band(VB)top predominantly consisted of O2p,Bi6p and Bi6s orbitals.This result was clearly different from that of the Bi6p orbital contributing mainly at the bottom of the CB in Bi5+/Bi3+-containing oxides[22,55].A strong dispersion observed in the hybridized sp orbital in the CB and VB of Bi5O7NO3
S.Yu et al./Journal of Colloid and Interface Science354(2011)322–330327
suggested that the photoexcited electrons had a high mobility on the sp bands.This may also have led to suppression of the elec-tron–hole pair recombination and a high photooxidation activity of the material [55,56].This explanation accounts for the high pho-tocatalytic activity of Bi 5O 7NO 3.4.Summary
Single-crystalline Bi 5O 7NO 3nanofibers were prepared by a fac-ile hydrothermal method.The Bi 5O 7NO 3nanofiber formation was ascribed to the special layered structure of Bi 5O 7NO 3,the presence of Triton X-100and ammonia,and a sufficiently long reaction time.
Bi 5O 7NO 3sheets could be dissolved and transformed into nanofi-bers.The Bi 5O 7NO 3nanofiber growth process was explained using the Ostwald ripening and hydrothermal recrystallization theories.The as-obtained Bi 5O 7NO 3nanofiber material exhibited a high pho-tocatalytic activity for the degradation of RhB dye under visible light irradiation.The band gap energy was estimated to be about 2.70–2.90eV.The accelerated separation of electron–hole pairs upon photoexcitation was ascribed to permanent static electric fields between Bi 5O 7and N–O 3layers.DFT calculations showed that Bi 5O 7NO 3was an indirect semiconductor,and DOS indicated that the top of the VB and bottom of the CB contained a large por-tion of O 2p and Bi 6p orbitals.The band structure of the material
328S.Yu et al./Journal of Colloid and Interface Science 354(2011)322–330
showed that the charge carriers in the CB and VB were highly mo-bile.This may have explained the high photocatalytic activity of Bi 5O 7NO 3.
Acknowledgments
This work was supported by the National Natural Science Foun-dation of China (50872103),National Basic Research Program of China (973Program)2007CB613302,the Key Project of Chinese Ministry of Education (No.108164)and the Project-sponsored by SRF for ROCS,SEM.
Appendix A.Supplementary material
Supplementary data associated with this article can be found,in the online version,at doi:10.1016/j.jcis.2010.10.012.
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