PAPER www.rsc.org/dalton | Dalton Transactions

Room temperature, template-free synthesis of BiOI hierarchical structures:Visible-light photocatalytic and electrochemical hydrogen storage properties†

Yongqian Lei,a,b Guanhua Wang,a,b Shuyan Song,a,b Weiqiang Fan,a,b Min Pang,a,b Jinkui Tanga andHongjie Zhang*a

Received 22nd October 2009, Accepted 14th January 2010First published as an Advance Article on the web 12th February 2010DOI: 10.1039/b922126c

Three-dimensional (3D) flower-like BiOI hierarchical structures have been successfully synthesized by asolution route at room temperature. The obtained sample was systematically studied by powder X-raydiffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electronmicroscopy (TEM). It is noteworthy that no surfactant or assisted reagent was needed during theformation of this uniform flower-like structure. It was proposed that primary nanoflakes of Bi-EtOHcomplexes were initially formed in the solution, and then anisotropic growth led to the development of3D flowerlike structures. In addition, the photocatalytic property of the sample under visible-light wasinvestigated with three types of dye: heteropolyaromatic (Methylene Blue (MB)), azoic (Methyl Orange(MO)) and phenol (rhodamine B (RhB)). High photocatalytic activity was observed for the phenol dyeRhB. The electrochemical hydrogen storage properties were also investigated.

Introduction

Synthesis of highly ordered inorganic materials with specific sizes,shapes and hierarchical structures has received much attentionin the potential design of new materials and devices in vari-ous fields.1 Hierarchical micro/nanostructures, assemblies usingnanoparticles,2 nanoplates3 and nanorods4 as building blocks, andcomplex nanocrystals with well-defined shape and inner structurehave received great interest5 and have been obtained throughdifferent strategies.6 Recently, bismuth and related compoundsespecially bismuth oxyhalides have received more attention dueto their high photocatalytic activity.7 Generally in BiOX (Cl,Br, I), the valence band width decreases with the rising atomicnumber of X. The Bi 5d states reduce the valence band widthsand increase the conduction band widths. Among the compoundsof BiOX, BiOI seems to be the most desirable compound asa photocatalyst due to its excellent indirect nature. Generally,the indirect feature is favourable for photocatalytic reactions byhindering the recombination of the excited electrons and holes.8

In the BiOI structure, the dispersed bottom of conduction bandsand top of valence band favours the excited electrons travellinglonger distances and reduces the probability of electron-holerecombination.9 Indeed, BiOI micro/nanostructured materialshave recently attracted intensive research interest due to theirlow band gaps, which have potential applications in catalysis,photochromic devices10 and solar cells.11

aState Key Laboratory of Rare Earth Resource Utilization, ChangchunInstitute of Applied Chemistry, Chinese Academy of Sciences, Changchun,130022, Jilin, China. E-mail: hongjie@ciac.jl.cn; Fax: +86-431-85698041;Tel: +86-431-85262127bGraduate School of the Chinese Academy of Sciences, Beijing, 100039,P. R. China† Electronic supplementary information (ESI) available: EDX spectrum,UV-vis absorption spectra of the RhB and electrochemical hydrogenstorage charge and discharge curve. See DOI: 10.1039/b922126c

Until now, several methods have been reported for the prepa-ration of BiOX micro/nanostructures, including hydrothermal(solvothermal) at high temperature,12 the chemical vapour deposi-tion (CVD) route,13 the reverse microemulsions,14 and the hydroly-sis of Bi salts with additional reagents under long reaction times.15

However, low energy and environmentally friendly preparationsof BiOX are still rarely reported, especially for BiOI. The useof surfactants or directing agents may introduce heterogeneousimpurities. The preparation of templates is tedious and thepost-synthetic treatments to remove them from the productsmight destroy the prepared hierarchical nanostructured materials.Therefore it is essential to introduce a simple and rapid methodto produce a uniform hierarchical structure of BiOI to meet thepotential applications. In this paper, we report a low-cost roomtemperature route to prepare the well-defined hierarchical flower-like BiOI structures, using nanoflakes as building blocks, withoutusing any template or surfactant. A possible growth mechanism forthe hierarchical structure was proposed. The synthesized sampleshows high photocatalytic activity under visible-light. In addition,the electrochemical hydrogen storage property was investigatedpreliminarily.

Experimental section

All chemical reagents used in this experimental procedure wereanalytical grade and used without further purification. In a typicalexperiment, a 10 mL aqueous solution containing 0.02 g KI wasadded dropwise to 0.05 g Bi(NO3)3 dissolved in 10 mL ethanol. Theresultant suspension was stirred intensively and the color of thesuspension turned from yellow to red gradually. The final productswere collected after stirring for 20 min and washed with anhydrousethanol several times and then dried in air at 60 ◦C for 2 h beforefurther characterizations.

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Fig. 1 (a) SEM image of the obtained sample, (b) EDX pattern of the synthesized sample, (c) the individual flower-like structure of the sample, (d) theXRD pattern of the sample.

Characterization

The crystalline and phase purity of the products were examinedby powder XRD. The measurements were performed on a RigakuX-ray diffractometer with Cu-Ka radiation with the acceleratingvoltage and applied current of 40 kV and 40 mA, respectively. Thesize, general morphology, energy-dispersive X-ray spectroscopy(EDX) and structure of the synthesized samples were character-ized using Field-Emission Scanning Electron Microscopy (HitachiS4800) at an accelerating voltage of 10 kV and Hitachi 8100Transmission Electron Microscope (TEM) at an operation voltageof 200 kV.

The specific surface areas of the powders were determinedusing a Micromeritics ASAP 2020 specific surface area andporosity analyzer, using the method of Brunauer-Emmett-Teller(BET). The sample was degassed at 373 K overnight before BETmeasurements. UV-vis diffuse reflectance spectrum was measuredon a SHIMADZU UV3600 spectrometer. The electrochemicalmeasurements were carried out following the reported method:16 amixture of synthesized sample and hydroxyl-nickel powder (0.15 gsample and 0.75 g nickel hydroxide) was pressed at 50 MPa to forma test cell. The prepared sample was tested as the working electrodeand Ni(OH)2(NiOOH) was used as the counter electrode. All ofthe experiments were performed in a 6 M KOH solution at roomtemperature.

Photocatalytic Activity Test. The photocatalytic measurementswere carried out in an aqueous solution at ambient temperatureand performed in a three-necked column container. 0.1 g samplewas suspended in a 200 mL aqueous solution of 0.01 mM dye. Priorto irradiation, the suspension was magnetically stirred in the darkfor 30 min to establish an absorption-desorption equilibrium. Thesamples were evaluated by degradation of the dyes in an aqueoussolution under a 500 W Xe lamp with a 420 nm UV light cut

filter. The concentration of the dyes during the degradation wasmonitored by UV-vis spectroscopy of aliquots of solution removedat regular time intervals. The residue of collected solution wasremoved by a centrifuge before UV-vis characterization.

Results and discussion

The typical overall SEM image (Fig. 1a) shows that the obtainedproducts are uniform flower-like spheres. These spheres have anaverage size of 500 nm in diameter. The corresponding EDXpattern (Fig. 1b) indicates that the sample contains only Bi, Oand I, and no impurity peaks such as N and K were observed (thepeak near 8 keV corresponds to the element of Cu, which comesfrom the substrate). The enlarged image (Fig. 1c) also shows theclear structure of the individual sphere. It can be seen that thesphere is constructed with cross nanoflakes as building blocks.The sharp peaks of the XRD pattern indicate that the product waswell crystallized. All the diffraction peaks are in good agreementwith the standard card (JCPDS card no. 10-0445). Additionally,the 002 and 004 peaks are stronger than the standard card whichmay be attributed to the strong oriented growth in the flower-likehierarchical structure.

The detailed morphology was also studied by TEM and highresolution TEM (HRTEM). The image (Fig. 2a) shows that theindividual flower-like structure consists of aggregated nanoflakes.These nanoflakes, with a random array, are connected to eachother to build a flower-like architecture. The dark center of theflower indicates that the nanoflakes are cumulated tightly. Thedetailed structure was also characterized by HRTEM (Fig. 2c)and the corresponding live fast Fourier transform (FFT) pattern(Fig. 2b). The bright spots in the pattern can be well indexed to the(200) and (110) planes. The clear lattice fringes of the interplane

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Fig. 2 (a) TEM image of the flower-like sample; (b) live fast Fourier transform (FFT) pattern of an individual nanoflake; (c) high resolution TEM imageof an individual nanoflake.

are 0.30 nm in the HRTEM image, which is in accordance withthe [102] direction.

The growth process of the sample at different reaction stageswas also investigated for the formation of the flower-like structure.Fig. 3 shows the morphology of the sample at different reactionstages. In the initial step, when the Bi(NO3)3 was added inethanol to form the suspension solution, the obtained productsare small particles with an average diameter of 10 nm (Fig. 3a).Low and weak crystallographic reflections (Fig. 3d, line a) wereobserved by XRD characterization, suggesting that the precursormay contain a complex of Bi and ethanol (JCPDS 38-0548).The EDX pattern (Fig. S1†) further confirms that the freshlyprepared precursor only contains Bi, C, O and no peak of N wasobserved. In our experiment, the formation of BiOI is stronglycontrolled by KI. When 1 mL KI aqueous solution was initiallydropped into the above precursor, the color changed from whiteto orange instantly. The corresponding morphology and phasestructure is shown in Fig. 3b and 3d. The flower-like structureswere finally formed by complete addition of the stoichiometricKI solution with long stirring to promote the anisotropic growthof the connected nanoflakes (Fig. 3c). The color changed tored and no obvious morphology changes of the final productswere observed upon further prolonging the reaction time in ourexperiment.

From the above results, it can be concluded that the growingprocess is consistent with the previous report of 3D MnWO4

flower-like structures.17 It is a two-stage growth process, whichinvolves a fast nucleation of primary particle precursors followedby a slow aggregation and crystallization of the building blocks.A possible growth mechanism can be summarized. Firstly, theparticles of Bi-EtOH were formed, for Bi3+ can be hydrolyticintensively when it was added in the ethanol solution. But thiscomplex is unstable when KI solution is added. The color changeof the solution indicates that BiOI was formed immediately. The

particles tend to form the plate-like structure due to the innerlaminar structure of BiOI, and the further addition of KI solutionaccelerates the growth of the nanoflake. When two nanoplateswith opposite growth direction meet each other, these nanoflakesbegin to cross link, which leads to the final flower-like structure(Fig. 3e). This phenomenon is in accordance with the formationof SnS2 flower-like structures.18

As a narrow band gap semiconductor, BiOI shows wideabsorption in the UV-vis light region. Fig. 4 shows the absorptionspectrum and calculated band gap of the obtained sample. It canbe seen that the absorption band ranges from 300 nm to 600 nm,with high absorption. The obvious absorption edge is located atabout 640 nm in the visible light region. The light absorptivityand the migration of the light-induced electrons and holes are thekey factors for photocatalytic properties, which rely on the elec-tronic structure characteristics of the materials.19 As a crystallinesemiconductor of indirect transition, the optical absorption nearthe band edge follows the formula ahv = A(hv - Eg)2, where a, v,Eg and A are the absorption coefficient, light frequency, band gapenergy, and a constant, respectively.20 The band gap energy (Eg)of BiOI is estimated to be 1.83 eV, which is close to the literature.21

The Brunauer-Emmett-Teller (BET) specific surface area of thesample was studied by nitrogen adsorption. Fig. 5 shows the typeII adsorption-desorption isotherm of the BiOI sample, in whichthe weak adsorption-desorption hysteresis indicated monolayerabsorption.22 However, the BET surface area is poor at 8.7 m2 g-1,which is in accordance with the reported data.15

The photocatalytic property of the sample was also testedby decomposition of three different dyes: methylene blue (MB),methyl orange (MO) and rhodamine B (RhB) under visible-lightirradiation. The decrease in concentration of the dyes versusthe irradiation time are plotted. Fig. 6 shows the curves of theconcentration change with irradiation time for the three dyes. Itcan be seen that the obtained sample shows higher photocatalytic

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Fig. 3 SEM images of the sample at different reaction stages: (a) no KI added, (b) 1 mL aqueous KI added, (c) after addition of stoichiometric KI. (d)XRD pattern of the above sample at different reaction stages. (e) The proposed growth scheme of the flower-like structures.

Fig. 4 (a) The UV-vis absorption spectrum. (b) Calculated band gap of the sample.

activity towards the dye of RhB than the other dyes of MB andMO. During the irradiation time, the color of the RhB solutionchanged gradually from red to yellow. There were no more changesafter 1.5 h. The absorption curve (Fig. S2†) of the RhB shows thatthe intensity decreased gradually in the initial 1 h. However, themaximum peak has an obvious hypsochromic shift from 554 nmto 503 nm. This can be ascribed to the formation of N-de-ethylated rhodamine, which was observed in the polyoxometalatedegradation of dye.23 The disappearance of the maximum peak at554 nm indicated that the dye RhB had completely degraded.Compared with RhB, the activity of MB and MO is lower,especially for MB. After 3 h irradiation with visible-light the total

degradation ratio of MB is about 30% and MO is about 60%. Thesedifferent degradation ratios may be attributed to the differentmolecular structures of the three types of dye, which result indifferent degradation mechanisms.24 The general dye degradationprocess is initiated by the generation of an electron-hole by lightirradiation of the photocatalyst, which involves photocatalytic andphotosensitization pathways.12,25 The electron-hole pair is unstableand can induce the generation of radicals in aqueous solution ifit is not recombined. Decomposition reactions begin to occurbetween molecules of dye and highly oxidizing radicals.26 Thehigh photocatalytic activity with RhB may be attributed to thede-ethylation reaction which accelerates the degradation process.

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Fig. 5 Nitrogen adsorption-desorption isotherm of the BiOI sample.

Fig. 6 The photocatalytic degradation curve of the sample with threedifferent dyes: methylene blue (MB), methyl orange (MO) and rhodamineB (RhB).

The absolute de-ethylated products were eventually formed.23 Forthe azo-dye of MO, it contains N=N double bond which is moreactive than MB. Additionally, the sulfonyl group located on thep-benzene ring in MO molecule, which is more reactive than sulfurinvolved in =S+- aromatic links in MB molecule during the radicalsreactions.27

It is observed that laminar structures usually show goodelectrochemical hydrogen storage properties at room temperature.The electrochemical hydrogen storage properties of the obtainedsample were also investigated. Fig. S3† shows the charge-dischargevoltage changes of the sample. There is an obvious plateau ofpotential at 80 mA h g-1. A weak but perceptible voltage plateauis also observed in low electrochemical capacity of 20 mA h g-1.It can be seen that there are two different electrochemical stepsin the charge process. This phenomenon may be attributed tothe two-step charge process in the flower-like structure.28 It wasassumed that hydrogen was firstly adsorbed on the interstitialsites between BiOI nanoflakes and then partly entered into theinterlayers of BiOI nanoflakes. The discharge curve was also

measured (see Fig. S3†), however, the discharging capacity islow at 40 mA h g-1. It can be seen that the discharge potentialdecreases intensely and results in the low discharge capacity. Thislow discharge capacity may be partly due to the poor BET surfacearea. Although the discharge capacity of this compound is notideal, the investigation on the electrochemical hydrogen storageprovides useful knowledge of the electrochemical processes of thiscompound.

Conclusion

In summary, we have successfully synthesized BiOI hierarchicalstructures via a template-free route at room temperature. The ob-tained product, with uniform structure, is characterized by variousmethods and the mechanical experiments indicate a two stagegrowth process in the formation of the flower-like structure. Pho-tocatalytic experiments indicate that the samples have high visible-light photocatalytic activity towards dye RhB. Additionally, theelectrochemical hydrogen properties reveal that this flower-likestructure has two electrochemical steps in the electrochemicalcharge process.

Acknowledgements

The authors are grateful for the financial aid from the NationalNatural Science Foundation of China (Grant No: 20631040,20771099) and the MOST of China (Grant No: 2006CB601103,2006DFA42610).

Notes and references

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