W19O55 nano-/micron-rods (NMRs) were synthesized through calcining WO3 powders under a reducing
atmosphere with S vapor in a vacuum furnace. For comparison, the as-prepared W19O55 NMRs were
then annealed at 500 �C for 2 h to obtain WO3 NMRs. The decolourization of organic dyes methylene
blue and rhodamine B under visible light by the two kinds of NMRs reveals that the oxygen-deficient
W19O55 sample will present better comprehensive performance due to its stronger surface adsorption to
the dye molecules, which could be attributed to the oxygen vacancies.
1 Introduction
In the most recent decades, there has been great interest indeveloping semiconductor photocatalysts with high activitiesfor the degradation of air and/or water pollutants, watersplitting, and the production of synthetic fuel with solarenergy in an environmentally friendly way.1–7 To satisfy theincreasing demands of human beings, nanocrystalline semi-conducting oxides of appropriate band-gap are desirable, inwhich anatase TiO2 has been by far the most investigatedoxide. However, the universal use of anatase TiO2 is limiteddue to its wide band-gap (about 3.2 eV), which means that itcan be only utilized under ultraviolet light (l < 385 nm) toexcite the chemically active valence-electrons. It was reportedthat in photocatalysis via anatase TiO2 under sunlight, theconversion was limited to about 4% of the sunlight, thehighest content of ultraviolet light in it.1,8 To avoid the hugewaste of visible light in sunlight, the second generation ofhighly efficient visible active photocatalytic materials has beencurrently intensively sought, for which, the main effort hasbeen focussed on the doping of TiO2 by anionic or cationicimpurities, dye sensitization, and the coupling of TiO2 withsmaller band-gap semiconductors.2,9–14
Another try is to search for more single-component semi-conducting materials of high sensitivity to visible light,in which tungsten oxides occupy important positions. Forexample, WO3, with a band-gap of 2.8 eV, can absorb visible
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hemistry 2016
light to a large extent, and is of high stability but no toxicity,being the second most investigated semiconductor photo-catalyst.3,15 And, interestingly, nanoscaled tungsten oxidesphotocatalysts with controlled morphology possess enhancedphotocatalytic activities due to their high purity, novelmorphology and large specic surface area.16,17 Moreover,Schaub et al. conclusively identied the oxygen vacancies intitania as active sites for water dissociation under light.18
And many other papers on photocatalysis show that oxygen-vacancy can obviously enhance the visible light absorption ofsemiconducting TiO2.19,20 But until now, the role of the oxygenvacancy in tungsten oxides in their photocatalytic activities hasnot been clear, because WO3�x crystals with a variety of oxygen-decient stoichiometries, such as WO2.72 (W18O49), WO2.8
(W5O14), WO2.83 (W24O68) and WO2.9 (W20O58), can be easilyprepared, since they are stable, ordered phases with precisestoichiometries. In other word, it is even not clear if there isoxygen vacancy in WO3�x crystals. And a more recent studyrevealed that in gas sensing, the varied compositions (oxidizedor partially reduced) of WO3�x crystals would result in muchdifferent performances,16 and similar effect was also reportedin their photocatalysis: Szilagyi et al. indicated that the higheroxidized WO3 crystal sample would be a better photocatalystthan WO3�x samples.17 However, all these results are muchdifferent from our ndings presented in this paper.
In this work, oxygen-decient W19O55 nano-/micron-rods(NMRs) were synthesized through calcining WO3 powdersunder a reducing atmosphere with S vapor in a vacuum furnace.And the as-prepared W19O55 NMRs were then annealed at500 �C for 2 h to obtain WO3 NMRs. To study the role of oxygendeciency in tungsten oxides in their photocatalytic properties,the two kinds of structured tungsten oxides samples wereapplied as photocatalysts to decompose organic dyes methyleneblue (MB) and rhodamine B (RhB) in aqueous phase undervisible light.
For the fabrication of W19O55 NMRs, WO3 and S powders wereloaded separately into quartz boats in an Ar gas lled vacuumtube furnace. In typical processing, 0.5 g commercially-boughtreagent-grade light green WO3 powder (Tianjin Fuchen Chem-icals, China) was placed at the centre of the furnace, whileanother boat with 0.5 g Aladdin-reagent S powder was located at10 cm apart from the WO3 powder on the upstream of thefurnace. Before heating, the quartz tube was evacuated andushed repeatedly with Ar gas for several times. Then thefurnace was heated up to 1050 �C at a rate of 15 �C min�1, andheld there for 1 h. Aer that, the furnace was cooled naturally toroom temperature. Finally, a dark blue powder product can becollected in the boat. For annealing, a part of the as-synthesizeddark blue powder was heated at 500 �C for 2 h in a mufflefurnace, and the powder was turned back to light green again.
The phase composition of the samples was identied by X-ray diffraction (XRD, D/max-RB, Cu Ka radiation, and l ¼1.5418 A) in a continuous scanning mode with a rate of 6�
min�1, and their corresponding chemical bonds wereconrmed by Raman spectroscopy (Horiba HR800 equippedwith a 532 nm wavelength Nd-YAG laser). The morphology andcrystalline structure were examined by eld emission scanningelectron microscopy (FE-SEM, S4800), and transmission elec-tron microscopy (TEM, Tecnai G2 F20 U-TWIN). The chemicalcomposition was measured by an energy-dispersive X-ray (EDX)spectroscope attached to the TEM, and X-ray photoelectronspectroscopy (XPS, non-monochromated Mg Ka radiation,photon energy 1253.6 eV) in which the spectrometer was cali-brated by the binding energy of C 1s line (285 eV). The UV-visabsorption spectra were recorded on a Cary 5000 UV-vis spec-trometer (Varian) equipped with a DRA-CA-30I integratingsphere for solid-phase characterization. The electron spinningresonance (ESR) spectra were collected at room temperatureusing a JEOL JES-FA200 spectrometer operating with a micro-wave frequency of 9.44 GHz. Diphenylpicrylhydrazyl was usedfor the g value calibration. Specic surface area was obtained bythe Brunauer–Emmett–Teller (BET) N2 adsorption methodusing a Micromeritics ASAP2020 surface analyzer. The photo-luminescence (PL) measurements were conducted at roomtemperature by using a Fluorolog-Tau-3 spectrometer witha He–Cd laser (325 nm) as the excitation source. The electro-chemical impedance spectroscopy (EIS) of the samples wasperformed with an electrochemical workstation (CHI 660D, CHInstrument Company, China) under visible light irradiation (a300 W Xe arc lamp system).
The photocatalytic activity was evaluated by the degradationof MB and RhB in aqueousmedia under visible light irradiation.For the irradiation, the reaction test tubes were located axially ina ring whirligig test tube rack (XPA-7, China) with a visible lamp(8 W, halogen lamp) in the centre. The wavelength of the lightfrom the lamp is approximately 400 to 790 nm. The lamp wasused at a distance of 100 mm from the aqueous solution ina dark box. The initial concentration of the MB solution was setat 5 mg L�1 and that of RhB was 4 mg L�1 in all the experiments.
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The amount of the photocatalysts samples was 0.05–0.2 g per 50ml solution. The reactor was placed in the dark box for 30 minso that the photocatalyst particles could absorb the dyes mole-cules thoroughly. Aer that, the visible light irradiation waslaunched. During the degradation of organic dyes, a 50 ml testtube was used as the reactor and placed on the ring whirligigdevice stirring by magnetic force. The suspension was thenirradiated with visible light for 20, 40, 60, 80, 100 and 120 min,respectively. The supernatant samples were collected regularlyfrom the reactor and the remnant powders dispersed in themwere removed by centrifuging. The prepared clear, transparentsolutions were analyzed by UV/vis spectroscopy (SP-752) fordetermining the dyes concentration in the solutions. Aer theanalysis, the clear solution with the sediment during centri-fuging was immediately poured back into the test tube to keepthe irradiation reaction in almost the same state.
3 Results and discussion3.1 Composition, structure and growth mechanism
The recorded XRD pattern of the as-synthesized product iscompared in Fig. 1a with that of its corresponding annealedsample. The diffraction peaks of the as-synthesized product canbe well indexed to the known phase of monoclinic W19O55
(JCPDS card no. 45-0167), with lattice constants of a ¼ 12.1, b ¼3.793 and c ¼ 22.45 A, and b ¼ 94.5�. According to the JCPDSdata, the strongest peak at 2q ¼ 23.472� can be labelled as thereection of (010) plane, indicating that (010) is the preferentialgrowth direction of the present W19O55 sample. On the otherhand, the diffraction peaks, peak intensities and cell parame-ters of the annealed sample are in good agreement with those ofthe crystalline monoclinic phase of WO3 (JCPDS no. 43-1035).And the strongest three diffraction peaks at 2q ¼ 23.119�,23.586� and 24.38� can be indexed to the reections from (002),(020) and (200) planes of monoclinic WO3.
The XRD results could be conrmed by Raman spectra (seeFig. 1b). Generally, compared with XRD, Raman signal is moresensitive to the presence of WO3.16,17 In the recorded Ramanspectrum of the annealed WO3 sample, the main bands at 812and 709 cm�1 were characteristic of the O–W–O stretchingvibrations, the bands at 326 and 275 cm�1 were identied asO–W–O deformation vibrations and the peak at 134 cm�1 wasa lattice vibration of the monoclinic WO3 structure.21,22 On theother hand, the Raman peaks of the as-synthesized monoclinicW19O55 sample were of much low intensity and some peakswere not even identied, which reected the distorted structureof monoclinic W19O55. Moreover, the shi of the Raman bandsalso evidenced the presence of reduced tungsten atoms (W4+
and W5+) in the W19O55 sample.23 Because the chemical bondsof W6+ were stronger than those of the reduced tungsten atoms(i.e., W5+ and W4+), so the Raman peaks of W6+ bonds wouldappear at higher energies, that is, higher wave numbers in thespectrum, which can be conrmed by the fact that the defor-mation vibration peak shied from 319 cm�1 for W19O55
sample to 326 cm�1 for the annealed WO3 one.Fig. 2 shows the FE-SEM images of both samples with
different magnications. The low-magnication one as shown
Fig. 1 Typical XRD pattern (a) and Raman spectrum (b) of the as-synthesized product in comparison with that of its corresponding annealedsample.
Fig. 2 Typical SEM images of the as-synthesized product withdifferent magnifications (a and b). For comparison, those of its cor-responding annealed sample (c and d) are also presented.
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in Fig. 2a indicates that the as-synthesized W19O55 productmainly consists of numerous straight NMRs, randomlydistributed in the sample, typically with a length of dozens ofmicrometer and a diameter from 100 to 500 nm. With a closerview (Fig. 2b), it can be found that some of the NMRs arebundled together, which might comprise two or more indi-vidual parallel-aligned nanorods. Aer annealing at 500 �C for 2
Fig. 3 Typical TEM image (a), EDX spectrum (b) and HRTEM image with
h in air, the sample presents similar morphology with the as-synthesized product (see Fig. 2c and d), implying that theNMRs would not be damaged, destroyed or deformed duringannealing. It should be noted that the small particles on theannealed sample were examined as WO3 by EDX, which mightbe the fractural fragments produced by grinding duringpreparing the sample for annealing.
TEM morphology of the as-synthesized NMRs (see Fig. 3a) isconsistent with the observation from SEM images presentedabove. The NMRs have smooth surface and homogenousdiameter along the longitudinal direction. The as-presentedNMR in Fig. 3a is of 188 nm in diameter. And no particles canbe found at the tips or roots of the prepared NMRs by bothSEM and TEM observations, implying that a vapor–solid (VS)mechanism might be responsible for the growth of the NMRs.24
Typical EDX spectrum recorded on the as-synthesized NMRs(see Fig. 3b) reveals that they consist of W and O with an O/Wratio of 2.16, although Cu and C peaks were also detected,which are originated from the TEM grid to support the samples.This result suggests that the as-synthesized NMRs are of oxygen-decient tungsten oxide. In addition, no peak of S could beidentied from the EDX spectrum although S powder was usedas the starting material. This is because the applied S powderacted as a reducing agent in the system, which reacted withWO3, producing SO2 gas and then going away with the carrier Argas. A typical HRTEM image of the as-synthesized NMRs isshown in Fig. 3c, where the inset represents the selected areaelectron diffraction (SAED) pattern. From the well-resolved
the inset of SAED pattern (c) from the as-synthesized W19O55 NMRs.
periodic lattice fringe in HRTEM image, it was measured thatthe spacing of the observed lattice planes is approximately 0.37nm, which is consistent with the interplanar distance of the(010) plane of monoclinic W19O55. This result is also in accor-dance with the calculated result from SAED pattern. Also itshows that the growth direction of these NMRs is along the [010]axis, the close-packed plane of crystalline monoclinic W19O55,which further conrms the XRD results.
From the information presented above we can logicallysuggest the growth mechanism of the as-synthesized W19O55
NMRs. The presence of S vapor took away the oxygen from theWO3 vapor, forming volatile W19O55 and allowing locally theformation and nucleation of W19O55 crystallites. The crystalsgrew along the b-axis of the W19O55 monoclinic unit cellthrough a VS mechanism, with [010] as the preferential growthplane, along which the fastest growth rate could be probablyachieved. When more W19O55 precipitated on the initial crys-tals, W19O55 NMRs were nally formed.
The stoichiometry of the tungsten oxide samples is furtherevaluated by XPS through analyzing the chemical binding statesof W and O. The W4f spectrum of the as-synthesized productcan be deconvoluted into four peaks (see Fig. 4a). The strongpeaks located at 35.67 and 37.85 eV are corresponding to W4f7/2and W4f5/2 of W
6+ oxidation state, respectively, while the weakpeaks at 34.16 and 37.03 eV can be assigned to those of W5+
oxidation state. The percentages of W6+ andW5+ were calculated
Fig. 4 XPS spectra of the as-synthesized product: (a) deconvolution of thstates, and (b) O 1s with peaks showing three chemical binding states.displayed: (c) W4f, indicating only one oxidation state (W6+), and (d) O 1
8064 | RSC Adv., 2016, 6, 8061–8069
as 76.7% and 23.3%, respectively. Thus the calculated meanvalence of W in the as-synthesized product was 5.76, implyingthat the composition of this product is about WO2.88, which isbasically consistent with the XRD result on them (W19O55 orWO2.89). Meanwhile, the spectrum of O 1s of the as-synthesizedproduct (see Fig. 4b) can be tted into three components. Therst component (major component), has a binding energy of530.4 eV (62.5%), which is assigned to the oxygen atoms inWO3.25 The second one with a binding energy of 531.6 eV(19.2%) has been attributed to the oxygen atoms in oxygen-decient tungsten oxides.26 The binding energy of the thirdcomponent is at 533 eV (18.3%), which can be assigned to theoxygen atoms in water molecules adsorbed on the samplesurface.27 When it comes to the annealed sample, as can be seenfrom Fig. 4c and d, its W4f spectrum consists of only twopronounced peaks, W(4f7/2) at 35.73 eV and W(4f5/2) at 37.87 eV,revealing that only W6+ oxidation state existed on thesample surface. Its O 1s spectrum can only be tted into twocomponents: the peak assigned to oxygen atoms in WO3
with a binding energy of 530.5 eV, and the one at 532.8 eVattributed to the oxygen atoms in water molecules adsorbed onthe sample surface. And when compared with W19O55 NMRs,the intensity of oxygen atoms in WO3 peak in the maincomponent increased from 62.5% to 76.3%, and the peak at531.6 eV, which appeared in the spectra of W19O55 owing to theoxygen atoms in the oxygen-decient tungsten oxides, almost
e W 4f core-level with peaks corresponding to W6+ and W5+ oxidationFor comparison, those of its corresponding annealed sample are alsos, revealing two chemical binding states.
Fig. 6 UV-vis absorption spectrum of the as-synthesized product(W19O55 NMRs) in comparison with that of its corresponding annealedsample (WO3 NMRs).
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completely disappears. This result indicates that the as-synthesized W19O55 NMRs were fully oxidized into WO3 aerannealing under the designed conditions, which is in accor-dance with the result from XRD and Raman analyses presentedabove.
To investigate the unpaired electron and defects in thesamples, ESR spectra were recorded at room temperature,which are shown in Fig. 5. The ESR spectrum of the as-synthesized product exhibited a sharp signal at g ¼ 2.28,while that of the annealed sample shows no obvious signals.The sharp g signal for the as-synthesized oxygen-decientW19O55 NMRs should be attributed to the oxygen vacancies inthem, because it presents a similar value with those of thealready reported metal oxides, in which the peak assigned tooxygen vacancies was always with a g factor of about 2.01.28–31
And in metal oxides, most of the excessive electrons are local-ized at the oxygen vacancies sites, and usually, one oxygenvacancy bounds one extra electron. However, in oxygen-decient tungsten oxides, the situation is more complicated.Di Valentin et al.32 analyzed experimentally and computation-ally the spectroscopic data of oxygen-decient tungsten oxides,indicating that sometimes the oxidation state of the under-coordinated W ions is still formally +6 while two extra elec-trons are trapped in one vacancy void, which can be expressedby W6+/VO(2e
�)/W6+ (VO is the oxygen vacancy). Because of thetwo charged centres (two unpaired electrons) in one oxygenvacancy (VO
2+), there is a shi of g value in the present oxygen-decient W19O55 NMRs from the reported values for metaloxides in literature.32 Moreover, there is no signal in the ESRspectrum of the annealed WO3 sample, implying that theannealed sample was completely oxidized, containing very littledoubly charged oxygen vacancy (VO
2+) or even no oxygenvacancy. This result is also in good agreement with the XPSspectra.
3.2 Photocatalytic properties
The UV-vis absorption spectra as shown in Fig. 6 have been usedto characterize the optical absorbance in the prepared tungstenoxides, helping to understand their photocatalytic properties.The recorded UV-vis absorption spectrum reveals that the
Fig. 5 ESR pattern of the as-synthesized product (W19O55 NMRs) incomparison with that of its corresponding annealed sample (WO3
annealed sample (WO3 NMRs) has a strong absorption band tolight in the wavelength range from 200 to 410 nm, which is inaccordance with the already reported UV-vis absorption spectraof WO3 materials in literature.17,33 For the as-synthesizedW19O55 NMRs, the UV-vis absorption spectrum presentsa more complicated prole than that of its correspondingannealed counterpart. Besides the strong absorption band from200 to 404 nm, which is similar with that of WO3, anotherstrong wide absorption band (centred at around 600 nm) can beobserved in the visible region from 470 to 740 nm. Theabsorption above 500 nm of the as-synthesized W19O55 NMRscould be attributed to the defect states in them.34,35 In litera-tures, it was reported that the oxygen vacancies in tungstenoxides would give rise to three types of defect states: a donor-like state within the fundamental band-gap, a hyper-deepresonant state in the valence band, and a high-lying resonantstate in the conduction band.35,36 Here in this study, thestronger absorption in the visible region observed in the UV-visabsorption spectrum is attributed to the state of oxygenvacancies in the conduction band of the as-synthesized W19O55
NMRs.17,34–37 Correspondingly, when applied for photocatalysis,because of their stronger absorption to the visible light, theycould be a better photocatalyst than their annealed WO3
counterparts. However, the new, quite deep energy levels intheir band-gap might also act as the recombination centres,38,39
which would lower their photocatalytic efficiency. So the pho-tocatalytic properties of the as-synthesized W19O55 NMRs maybe very complex.
For the photocatalysis test in the present study, two steps areinvolved in the decolourization of the dyes (MB and RhB): theabsorption and photodegradation of the dye molecules. Fig. 7illustrates the decolourization performances of 200 mg W19O55
NMRs photocatalyst on 50 ml 5 mg L�1 MB and 4mg L�1 RhB inthe dark (for the absorption) and under visible light irradiation(for the photodegradation) in comparison with those of it cor-responding annealed WO3 counterparts, as done in literaturefor photocatalysis test.17 Before the visible light irradiation, inthe rst, absorption step, the as-synthesized W19O55 NMRs andtheir corresponding annealed WO3 counterparts presenteddifferent absorption effects. The decolourization efficiency of
Fig. 7 The decolourization performances of 200 mg W19O55 NMRsphotocatalyst on 50 ml 5 mg L�1 MB and 4 mg L�1 RhB in the dark andunder visible light irradiation in comparison with those of it corre-sponding annealed WO3 counterparts, clearly revealing the absorptioneffect differences on the dye molecules between W19O55 and WO3
NMRs.
Fig. 8 The decolourization performance of 50 mg W19O55 NMRsphotocatalyst on 50 ml 5 mg L�1 MB in the dark, and under visible lightirradiation without absorption equilibrium, in comparison with that of itcorresponding annealed WO3 counterparts, clearly revealing thephotodegradation effect difference on MB between W19O55 and WO3
NMRs.
Fig. 9 N2 adsorption–desorption isotherm of the as-synthesizedproduct (W19O55 NMRs) in comparison with that of its correspondingannealed sample (WO3 NMRs).
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the as-synthesized W19O55 NMRs on MB is 90.5%, much higherthan that resulted by the annealed sample (63.2%). And on RhB,the situation was the same: the decolourization efficiency of theas-synthesized W19O55 NMRs (71.5%) was also much higherthan that of the annealed WO3 NMRs (2.4%). Because themorphologies of these two NMR samples are identical, theenhanced decolourization efficiency of the as-synthesizedW19O55 NMRs on the organic dyes should be attributed to itshigher content of oxygen vacancies than that in the annealedsample. In the second, photodegradation step, the decolourizedMB and RhB by the annealed WO3 NMRs sample aer 120 minincreased from 63.2% to 89.3% and from 2.4% to 40.5%,respectively, presenting a signicant degradation effect on bothorganic dyes in aqueous solution under visible light. However,the decolourized RhB by the as-synthesized W19O55 NMRsincreased from 71.5% to 88.6% aer 120min, much slower thanthat by the annealed WO3 NMRs, indicating that the annealedWO3 NMRs performs better than the as-synthesized W19O55
NMRs during the photodegradation process of RbB. Moreover,because of the high absorption performance (90.5% MBdecolourized in the absorption step), the decolourized MB bythe as-synthesized W19O55 NMRs during the degradation stepunder visible light was very little. Therefore, only based on suchresult, the photocatalytic degradation effect of the as-synthesized W19O55 NMRs on MB under visible light couldnot be claried yet. And the comparison on the photo-degradation activity and/or process of both catalysts on MBneeds further study.
Fig. 8 compares the decolourization rates of MB by the as-synthesized W19O55 NMRs and their corresponding annealedWO3 counterparts in the dark, and under visible light irradia-tion without absorption equilibrium. In this case, the appliedamount of the photocatalysts was reduced from 200 to 50 mg inorder to prevent from the appearance of a 100% removal of MB.From this gure, an acceleration for the dye removal could beobserved in the photocatalysis process under visible light whenthe annealed sample was used. Approximately 28.3% of the dyewas decolourized with an exposure time of 140 min under
8066 | RSC Adv., 2016, 6, 8061–8069
visible light, while the efficiency of the dye removal was only15.3% for the same exposure time in the dark, further con-rming that the annealed WO3 NMRs have signicant photo-degradation effect on MB under visible light. However, thephotodegradation effect of the as-synthesized W19O55 NMRs onMB was much weaker than that of the annealed WO3 sample,presenting a decolourization effect on the dye only slightlyincreased from 33.7 to 36%, when the condition turns from thedark to visible light. In a word, the annealed WO3 NMRs alsoperforms better than the as-synthesized W19O55 NMRs duringthe photodegradation process of MB.
In summary, the better comprehensive performance ondecolourizing the organic dyes methylene blue and rhodamineB under visible light by the oxygen-decient W19O55 NMRs overWO3 NMRs should be attributed to its stronger surfaceadsorption to the dye molecules, due to the existence of largeamount of oxygen vacancies.
In order to explore the inuence of oxygen vacancy onenhancing the adsorptivity, the BET surface areas of the as-synthesized product (W19O55 NMRs) and its correspondingannealed sample (WO3 NMRs) have been investigated. Fig. 9
Fig. 11 Photoluminescence spectrum of the as-synthesized product(W19O55 NMRs) in comparison with that of its corresponding annealedsample (WO3 NMRs).
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displays the N2 adsorption–desorption isotherms of the W19O55
and WO3 NMRs, and both isotherms are characteristic of a typeIV isotherm with a type H3 hysteresis loop,40 indicating theyhave mesoporous structures. And the BET surface areas of theW19O55 and WO3 NMRs were determined to be ca. 4.72 and2.79 m2 g�1, respectively. It is clear that both samples havesmall specic surface area, and the annealing at 500 �C pre-sented no signicant inuence on the BET surface area of thesamples. However, under the same pressure, the W19O55 NMRsadsorbed much more N2 than the WO3 NMRs. For example, thevolume of the adsorbed N2 by the as-synthesized W19O55 NMRswas 10.57 cm3 g�1, much higher than that by the annealed WO3
NMRs (1.40 cm3 g�1), when the relative pressure was 1.0. TheW19O55 NMRs showmuch stronger adsorbability on N2 than theWO3 NMRs, but their BET surface areas are similar, suggestingthat the vacant surface sites in the W19O55 NMRs enhancedtheir adsorbability.
The EIS analysis is a powerful tool to study the chargetransfer process, and the recorded results are displayed inFig. 10. Our EIS experiments were carried out in the frequencyfrom 0.1 to 100 kHz at dc bias of 0.3 V. The Nyquist plots of theas-synthesized product (W19O55 NMRs) and its correspondingannealed sample (WO3 NMRs) both present semicircles. Butwith more oxygen vacancies in W19O55 NMRs, the arc radius ofits EIS Nyquist plot becomes smaller, indicating a decrease inthe solid state interface layer resistance and the charge transferresistance on the surface. Therefore, the charge carriers transferefficiency over W19O55 NMRs is much higher than that over theannealed WO3 NMRs.
The PL spectra of the as-synthesized W19O55 NMRs and theircorresponding annealed WO3 counterparts have also beencollected for further understanding the photoexcited energy/electron transfer and recombination processes. In tungstenoxides, the oxygen vacancies can be neutral (V0
O), singly charged(VO
1+) or doubly charged (VO2+). Thus the PL emission can be
associated with the decay of an electron from the conductionband (CB) directly back to the valence band (VB), from the CBinto a charged vacancy state (VO
1+ or VO2+), or from the vacancy
state (V0O or VO
1+) to the VB.41 Fig. 11 shows the PL spectra of theas-synthesized W19O55 and annealed WO3 NMRs. Both samples
Fig. 10 Electrochemical impedance spectrum of the as-synthesizedproduct (W19O55 NMRs) in comparison with that of its correspondingannealed sample (WO3 NMRs).
had six emission peaks. The peak at 402 nm indicates an opticalband gap Eg ¼ 3.08 eV for them, and the one at 427 nm wasassigned to a band–band recombination. And an electronexcited into the conduction band can decay non-radiatively intoa (VO
2+ and VO1+) state, hence forming a (VO
1+ and V0O)* centre,
and the emission peaks at 460 and 495 nm are associated withthe decay of an electron from the unrelaxed (denoted by *)vacancy state (VO
1+ and V0O)* to the VB, respectively.41 The lower-
energy green emission peaks at 537 and 583 nm were attributedto the decay of an electron from the vacancy state (V0
O and VO1+)
to the VB, respectively.42 The luminescence intensity of the peakat 537 nm of the WO3 NMRs proved that there were still someoxygen vacancies sites (V0
O) in the sample. And compared withthe WO3 NMRs, the as-synthesized W19O55 NMRs had signi-cantly lower luminescence intensity of the peaks at higher than460 nm, because the large amount oxygen vacancies in theW19O55 NMRs were mainly the doubly charged vacancies VO
2+
(proved by ESR), which would turn into (VO1+)* by the adsorp-
tion of light. This result implies that a smaller portion of theabsorbed light could be used to generate holes and electrons,which would take part in the photocatalytic processes. Inaddition, because the unrelaxed vacancies (VO
1+)* are the siteswhere the electron recombines with a hole,42 so although theW19O55 NMRs have stronger light absorption than WO3 NMRs,their photocatalytic activity in photo-degradation step was stillinferior.
All these results indicate that the oxides vacancies in tung-sten oxides NMRs can act not only as adsorbents, which canpromote the adsorption of MB and RhB, but also as the centresfor the recombination of photo-electrons and photo-vacancies,which will weaken the photocatalytic decolourization effectson MB and RhB under visible light irradiation. For theabsorption, the decolourization rate on MB and RhB by theannealed WO3 NMRs is prompt at the initial stage, but there-aer becomes slower near the equilibrium, because at theinitial stage, a number of vacant surface sites (mainly neutralV0O, which is why in the ESR spectrum, the annealed WO3 NMRs
did not exhibited a sharp g signal) might be also available on theannealed WO3 NMRs for adsorption, and aer a lapse of timefor the absorption of MB and RhB, the remaining vacant surface
Fig. 12 Schematic diagram of the proposed reaction mechanism forphotocatalytic degradation of organic dyes over the as-synthesizedproduct (W19O55 NMRs) under visible light irradiation.
RSC Advances Paper
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sites are difficult to be occupied due to the repulsive forcesbetween the solute molecules on solid bulk phases.43 On theother hand, there are much more surface and even interiorvacancies in the as-synthesized W19O55 NMRs than those in theannealed WO3 NMRs, and the oxygen vacancies acting as activesites on the exterior or interior surface favor the adsorption oforganic dyes into the W19O55 NMRs, presenting a more rapiduptake.44
During the photo-degradation step, based on the aboveresults, a reaction mechanism for the photocatalytic selectiveoxidation of organic dyes over the W19O55 NMRs can beproposed, which is schematically displayed in Fig. 12. The oxygenvacancy states narrow the band gap from 3.08 to 2.12 eV, so uponthe irradiation of visible light, the oxygen vacancy can largelypromote the visible light absorption and the generation of thephotoexcited electron–hole pairs over the surface of W19O55
NMRs. The electrons excited into oxygen vacancies would decayinto a (VO
2+) state, hence forming an unrelaxed (VO1+)* centre,
where the electron recombines with a hole easily. The adsorbedorganic dye molecules in solution then interact with the photo-generated holes to form corresponding inorganic ions and/orcompounds, such CO2 and H2O. And the photo-generated elec-trons can also be captured by oxygen, producing superoxideradicals (cO2
�), which are able to selectively oxidize the organicdyes.6,19 However, as a number of oxygen vacancies act as therecombination centres of the photoelectrons with photo-generated holes, it would consume a part of the photoelectronsand photo-generated holes, thus the photodegradation efficiencyof such photocatalysts might be weakened.38,39
4 Conclusions
W19O55 NMRs were synthesized by simple thermal evaporation ofWO3 under reductive S atmosphere, and WO3 NMRs were ob-tained by annealing the as-synthesized W19O55 product in air forcomparison. The as-synthesized W19O55 NMRs display muchhigher adsorbability to MB and RhB than their correspondingannealed WO3 counterparts in the dark, which can be ascribed tothe larger amount of oxygen vacancies in the W19O55 NMRs. But
8068 | RSC Adv., 2016, 6, 8061–8069
the photo-degradation process of the as-synthesizedW19O55 NMRson MB and RhB is slower than that of the annealed WO3 samples,due to the more recombination centers of photoelectrons andholes in the W19O55 NMRs. Therefore, the comprehensivelyexcellent decolourization of MB and RhB by the as-preparedW19O55 NMRs might be attributed to the strong adsorption toMB and RhB molecules on the surface of W19O55 NMRs.
Acknowledgements
The authors would like to thank the nancial support for thiswork from the National Natural Science Foundation of China(grant no. 61274015, 11274052 and 51172030), Excellent AdviserFoundation in China University of Geosciences from theFundamental Research Funds for the Central Universities, andFund of State Key Laboratory of Information Photonics andOptical Communications (Beijing University of Posts andTelecommunications).
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