Synthesis and Facet-Dependent Photoreactivity of BiOCl Single-Crystalline Nanosheets
Jing Jiang,Kun Zhao,Xiaoyi Xiao,and Lizhi Zhang *
Key Laboratory of Pesticide &Chemical Biology of the Ministry of Education,College of Chemistry,Central China Normal University,Wuhan 430079,P.R.China
*Supporting Information
performance of a single-crystalline material.Usually,different facets of a single-crystalline material possess different geometric and electronic structures,thus endowing them with distinctive properties.1−4Recently,considerable attention has been paid to the facet-controlled fabrication of single-crystalline semiconductors with well-defined morphologies because of their facet-dependent photocatalytic,photoelectric,and other surface-related properties.5−8Especially,surface properties are vital to a semiconductor ’s photocatalytic performance,which sensitively depends on its exposed surfaces with distinct crystal facets.9−14For example,recent progress made by Lu and co-workers on the synthesis of anatase single crystals with highly reactive {001}facets have paved a new way for the enhancement of photocatalytic performance.15−17They dem-onstrated theoretically and experimentally that the {001}facets of anatase are much more reactive than the thermodynamically more stable {101}facets.Undoubtedly,facet engineering not only is an exciting direction to pursue for highly active new-generation photocatalysts but also offers opportunities to investigate the relationship between the surface properties and the photocatalytic properties.In this communication,we first report on the facet-controllable synthesis of BiOCl single-crystalline nanosheets (BOC SCNSs)with exposed {001}and {010}facets and their facet-dependent photoreactivity for photodegradation of methyl orange (MO)via direct semiconductor photoexcitation under UV light and indirect dye photosensitization under visible light.Structural insights into the interesting differences are provided on the basis of detailed experimental results.BOC SCNSs were synthesized by reacting Bi(NO 3)3·5H 2O
and KCl in distilled water via a facile hydrothermal route at 160°C.The facet control was realized by adjusting the pH of the solution by addition of NaOH.The samples obtained in the absence and presence of NaOH were denoted as BOC-001and
BOC-010,respectively.Scanning electron microscopy (SEM)examination revealed that the BOC-001sample consisted of large-scale sheet-shaped structures with widths of 1−3μm and thicknesses of 80−200nm (Figure S1in the Supporting Information).Transmission electron microscopy (TEM)images of an individual nanosheet (Figure 1a and Figure S2)further confirmed its sheet-shaped structure.The correspond-ing selected-area electron diffraction (SAED)pattern (Figure 1b)indicated the single-crystalline characteristic of the BOC-001sample.The angle labeled in the SAED pattern is 45°,which is in agreement with the theoretical value of the angle between the (110)and (200)planes.The set of diffraction spots can be indexed as the [001]zone axis of tetragonal BiOCl.High-resolution TEM (HRTEM)(Figure 1c)revealed the highly crystalline nature of the nanosheets.The clear lattice fringes with an interplanar lattice spacing of 0.275nm correspond to the (110)atomic planes.On the basis of the
above results and the symmetries of tetragonal BiOCl,the Received:November 8,
2011
Figure
1.(a)TEM image,(b)SAED pattern,and (c)HRTEM image
of the BOC-001SCNSs.(d)Schematic illustration of the crystal orientation
of the nanosheet.(e,f)Atomic structure of the {001}facets:(e)side view;(f)top view.
bottom and top surfaces of the BOC-001sample are identified as {001}facets,while the four lateral surfaces are {110}facets (Figure 1d).The percentage of {001}facets was estimated from the geometric calculation to be ca.80%for the BOC-001sample.The atomic structure of the {001}facets is characterized by the high density of oxygen atoms (Figure 1e,f).The electron microscopy observations showed that the BOC-010product also presents well-defined sheet-shaped structures (Figure 2a and Figures S3−S5).The width and thickness of the sheets were estimated to be 150−600and 10−60nm,respectively.The corresponding SAED pattern,indexed as the [010]zone,displays (002)and (102)planes with an interfacial angle of 43.4°(Figure 2b),which is identical to the theoretical value.A typical HRTEM image of a BOC-010nanosheet (Figure 2c)shows clear and continuous lattice fringes.The (002)atomic planes with a lattice spacing of 0.37nm can be obviously observed.Furthermore,the HRTEM image of vertical nanosheets (Figure 2d)shows the lattice fringe sp
acing of 0.385nm,which is assigned to the (010)planes of BiOCl.Hence,these BOC SCNSs are enclosed by dominant {010}facets (Figure 2e).The percentage of {010}facets exposed on the surface of the BOC-010SCNSs was estimated to be ca.73%.The atomic structure of the {010}facets (Figure 2f,g)indicates its open channel feature.As evidenced by X-ray diffraction (XRD)characterization (Figure S6),the intensity ratios of the (002)and (200)peaks were 5.88and 1.07for BOC-001and BOC-010,respectively.This difference in the intensity ratios reflects the difference between the surfaces of the two samples,which is in good agreement with the HRTEM and SAED results.UV −vis diffuse-reflectance spectroscopy (DRS)(Figure S7)showed that two BOC SCNSs samples have very close absorption edges in the UV region and very similar band-gap energies,which were estimated to be 3.31and 3.29eV,respectively.The success in controlling the facet exposure for the BOC SCNSs allowed us to investigate their facet-dependent photocatalytic performance systematically.It is well-established that the photocatalytic degradation of dyes on semiconductors can originate from two kinds of processes,namely,direct semiconductor photoexcitation and indirect dye photosensiti-zation.To this end,the photocatalytic performances of the BOC SCNSs were evaluated using MO as the probe molecule under UV (λ=254nm)or visible (λ>420nm)light irradiation,respectively.We first investigated the photocatalytic abilities of the BOC SCNSs to degrade MO under UV light.We found MO was slightly self-degraded (13%in 45min)under UV light in the absence of photocatalyst (Figure S
8).However,the presence of the BOC SCNSs resulted in more obvious degradation of MO under UV light.After UV light irradiation for 45min,the photodegradation efficiencies of MO over BOC-001and BOC-010were ca.99%and 59%,respectively.BOC-001exhibits a higher apparent reaction rate constant (k )than BOC-010(Figure 3).Since BOC-001has a smaller specific surface area than BOC-010(Figure S9),its apparent reaction rate constant normalized by the surface area (k ′)is also higher than that of BOC-010(Figure 3),suggesting that the direct semiconductor photocatalysis of the BOC SCNSs is related to surface structure rather than surface area.Under visible-light irradiation,the photodegradation of MO would take place via an indirect dye photosensitization process,as these large-band-gap BOC SCNSs could not be excited under visible light (Figure S10).We further studied the photocatalytic degradation of MO over the BOC SCNSs under visible-light irradiation to gain insight into the surface-related photosensitization process.Interestingly,similar to the direct semiconductor photocatalysis,the dye photosensitization process also depends on the photocatalyst ’s surface,although the MO photodegradation over BOC SCNSs must be initiated with the help of MO photoexcitation.The MO degradation efficiencies over BOC-001and BOC-010were ca.10%and 33%,respectively,under visible-light irradiation for 180min (Figure S10b).Obviously,BOC-010exhibits a much higher photosensitization degradation activity than BOC-001,in contrast to the case of the semiconductor photocatalysis process.This suggests that BOC SCNSs with different exposed facets p
ossess different light-dependent photocatalytic proper-ties.More importantly,the k value for BOC-010significantly decreased upon normalization by the surface area,and the normalized k ′values of BOC-010and BOC-001became close (Figure 3),clearly suggesting that the photosensitization degradation process is strongly associated with the surface area.The stability of the BOC SCNSs during photocatalytic reaction was further explored by recycling tests and XRD measurements (Figures S6and S11).No significant change in the photocatalytic activity and crystalline structure
was
Figure 2.(a)TEM image,(b)SAED pattern,and (c,d)HRTEM images of the BOC-010SCNSs.(e)Schematic illustration of the crystal orientation of the nanosheet.(f,g)Atomic structure of the {010}facets:(f)side view;(g)top
view.
Figure 3.Comparison of the apparent reaction rate constants for photocatalytic degradation of MO over the BOC SCNSs under (left)UV (λ=254nm)and (right)visible-light (λ>420nm)irradiation.
reaction rateobserved after three cycles,indicating that the BOC SCNSs were very stable during photocatalysis.To understand the origin of the facet-dependent photo-reactivity of the BOC SCNSs,we carefully examined the process of MO adsorption over BOC-001and BOC-010.The results (Figure S12)revealed that the BOC-010possesses a higher MO adsorption capacity than BOC-001,which might be associated with the active sites on the surface of the BOC SCNSs.The adsorption capacities of MO were estimated to be 0.2and 0.9mg g −1for BOC-001and BOC-010,respectively.The aforementioned structural characteristics reveal that the BOC-001surface atomic structure contains terminal oxygen atoms,and their (001)surfaces are expected to be more negatively charged than those of the BOC-010.The repulsive interaction between the negatively charged (001)surface and the anionic MO dye is responsible for the poor dye adsorption ability of BOC-001.On the other hand,the surface atomic structure of BOC-010presents the open channel characteristic,offering more active sites and larger accommodation space for the MO adsorption.For BOC-010,the adsorption capacity normalized by the surface area (0.19mg m −2)was significantly higher than that of BOC-001(0.12mg m −2),confirming that the higher adsorption capacity of BOC-010could be attributed not only to the surface area factor but also to its open channel characteristic.BiOCl has a unique layered structure (Figure 4a),characteriz
ed by [Bi 2O 2]slabs interleaved with double slabs of halogen atoms.This would induce the presence of internal static electric fields perpendicular to the [Bi 2O 2]slabs and halogen anionic slabs in BiOCl,18enabling the effective separation of the photoinduced electron −hole pairs along the [001]direction.As schematically illustrated in Figure 4b,the self-induced internal electric fields are perpendicular to the nanosheets of BOC-001but parallel to those of BOC-010.Therefore,the charge separation and transfer assisted by the internal electric fields are more favorable in BOC-001,with a shorter diffusion distance of photoinduced charge carriers than that in the BOC-010,as confirmed by the transient photo-current responses of the two BOC SCNS films (Figure 4c and Figure S13).Both electrodes were prompt in generating photocurrent with a reproducible response to on/off cycles,but the BOC-001film electrode exhibited a higher photocurrent than the BOC-010film electrode,indicating the more efficient photoinduced charge separation and transfer in BOC-001.Photoluminescence (PL)emission is a common and useful technique to survey the separation efficiency of the photo-generated charge carriers in a semiconductor because the recombination of excited electrons and holes gives rise to the PL emission signal.19A lower PL intensity is generally indicative of a lower recombination rate.As shown in Figure S14,the PL spectra of the BOC SCNSs display emission peaks in the 340−450nm range under the excitation at 320nm.20The weaker PL intensity of BOC-001confirms its better efficiency of separation of photogenerated electron −hole pairs.On the basis of t
he above results,we can analyze the origin of the facet-dependent photreactivity of the BOC SCNSs during direct semiconductor photoexcitation and indirect dye photo-sensitization MO degradation processes.The semiconductor photocatalysis process generally involves semiconductor photo-excitation,separation of the photoelectrons and photoholes,bulk diffusion,surface transfer of the photoinduced charge carriers,generation of active species,and pollutant degrada-tion,21,22which are cooperatively determined by the electronic structure and surface properties.In comparison with BOC-010,although BOC-001has a very similar absorption edge and smaller surface area,it exhibits higher photoreactivity during the direct semiconductor photoexcitation process,which should be attributed to surface structure rather than electronic structure.Meanwhile,the self-induced internal electric fields could induce more efficient photoinduced charge separation and transfer along the [001]direction in the nanosheets than along the [010]direction,as shown in Figure 4,which can inhibit the recombination of photogenerated electron −hole pairs to increase the photocatalytic degradation of MO.Therefore,a cooperative effect between the surface properties and suitable internal electric fields might account for the higher photoreactivity of BOC-001in the photodegradation of MO under UV light.On the other hand,the indirect dye photosensitization degradation process includes initial photoexcitation of the dye molecules,injection of photoexcited electrons into the conduction band of the semiconductor,and capture of the injected electrons by surface-adsorbed molecular oxygen to generate active species (e.
g.,O 2−•,OH •,etc.)for pollutant degradation.23,24Unlike the direct semiconductor photo-excitation degradation,the valence band of the semiconductor is not involved in the photoreaction during the indirect dye
photosensitization degradation process.Moreover,direct contact between the dye molecules and the semiconductor is a prerequisite for the injection of photoexcited electrons into the conduction band of the semiconductor.25Therefore,the photocatalytic performance of the BOC SCNSs in the photosensitization process intimately depends on the surface-
related interface properties between the BOC SCNSs and the dye molecules.Obviously,BOC-010with its larger surface area as well as the open channel characteristic could more easily adsorb MO molecules and provide more contact sites between the photocatalyst and dye molecules,facilitating more
efficient
Figure 4.(a)Crystal structure of BiOCl.(b)Model showing the direction of the internal electric field in each of the BOC SCNSs.(c)Photocurrent responses of the BOC SCNSs in 0.5M Na 2SO 4aqueous solutions under UV −vis irradiation.
electron injection from the photoexcited dye into the conduction band of the catalyst,resulting in more MO photodegradation over BOC-010than BOC-001during the indirect dye photosensitization process.
To confirm the above analyses,the two kinds of BOC SCNSs were further used to degrade colorless salicylic acid (SA)under UV and visible-light irradiation,respectively. Similar to the case of MO,BOC-001exhibited higher photoreactivity than BOC-010for degradation of SA via the direct semiconductor photoexcitation process under UV light (Figures S15and S16).As the SA cannot be excited by visible light,the indirect dye photosensitization degradation process could not take place.Therefore,we could not observe the degradation of SA over the two kinds of samples under visible-light irradiation(Figure S17).All of these results confirm that the BOC SCNSs with exposed{001}facets favor pollutant degradation under UV light because of their unique surface properties and more suitable internal electric fields,while BOC SCNSs with exposed{010}facets might be powerful for dye degradation under visible light because of their larger surface area and open channel characteristic.
In conclusion,we have synthesized BiOCl single-crystalline nanosheets with exposed{001}and{010}facets via a facile hydrothermal route.The resulting BiOCl single-crystalline nanosheets with exposed{001}facets exhibited higher activity for direct semiconductor photoexcitation pollutant degradation under UV light because of a cooperative effect between the surface atomic struct
ure and suitable internal electric fields. However,the counterpart with exposed{010}facets possessed superior activity for indirect dye photosensitization degradation under visible light because of its larger surface area and open channel characteristic.These findings could shed light on the deep understanding of facet-dependent photoreactivity of semiconductors and the fine manipulation of their photo-
reactivity as well as the development of new photocatalysts.■ASSOCIATED CONTENT
*Supporting Information
Experimental details,additional SEM images,XRD patterns, DRS spectra,a nitrogen adsorption−desorption isotherm,PL spectra,additional data,and the possible mechanism of facet-dependent synthesis.This material is available free of charge via
the Internet at
■AUTHOR INFORMATION
Corresponding Author
u.edu
Notes
The authors declare no competing financial interest.■ACKNOWLEDGMENTS
This work was supported by National Science Foundation of China(Grants21073069,91023010,21177048,and 21103141),the Program for Innovation Team of Hubei Province(Grant2009CDA048),and the Program for Changjiang Scholars and Innovative Research Team in
University(Grant IRT0953).
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