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Rational Design of Charge Shunt: Modification upon Crystal Facet Engineering of Semiconductor Photocatalysts
Wenhui Feng, Sunxian Weng, Zuyang Zheng, Zhibin Fang and Ping Liu* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Another excellent carrier shunt is artificially bridged over different facets exposed of semiconductor, to achieve a unique architecture with superior photoactivity, which is mainly attributed to the facet-driven charge dual-selectivity-channel separation mechanism. This novel concept may break new ground towards rational modification of crystal facet engineering of semiconductor. Nowadays, the worsening environmental pollution and energy crisis have become the stumbling block to the development of human society. It is urgent need to develop an efficient means of environmental remediation and pursue clean and renewable energy resources.1-3 During the past decades, numerous researches have proven that photocatalysis is a promising approach to meet the requirement of environmental and energy in sustainable processes.4-10 As known, charge separation plays a crucial role in determining solar energy conversion efficiency of photocatalysis. Therefore, it is of great significance to optimize the charge separation for semiconductor-based photocatalysts.10-17 Recent investigations on crystal facet engineering of semiconductors have demonstrated that photo-excited electrons and holes may be driven to different crystal facets for various semiconductors (for instance, TiO2,18-22 BiVO4,23 BiOCl,24 ZnO,25 Fe2O326 and WO327). Which offers a new opportunity to overcome limited charge separation efficiency of semiconductorbased photocatalysts. Due to the differences of the energy levels in the conduction bands (CB) and valence bands (VB) between the different facets of the semiconductor, the photogenerated electrons from a facets with more negative potential on CB to another facets with less negative potential on CB, while holes transfer in the opposite direction between the facets.23, 28 Indeed, the spatial charge separation between different facets prolongs the lifetime of charge carriers by reducing the probability for their recombination, enhancing the photocatalytic property.29-33 However, during the spontaneous counter-transitions of electrons and holes in the same channel (the internal structure of semiconductor), the carrier recombination should be unavoidable, which has discounted the separation efficiency of charges. Fabricating rationally a novel and more effective path for carrier separation, therefore, would be a promising strategy to further optimize facet-dependent photocatalytic performance of semiconductor-based photocatalysts. Based on the above guidance, a possible way to achieve efficient charge migration is to build another new charge shunt across the different facets, through which only one kind of carrier is permitted to pass, achieving dual-selectivity-channel carrier separation. If so, the collision probability between photoexcited holes and electrons will be lowered greatly during the charge This journal is © The Royal Society of Chemistry [year]
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migration. Nevertheless, it is indeed difficulties that all the necessary conditions should be met simultaneously, which is listed as follow: First, either electrons or holes pass the new charge shunt. Second, a driving force between different facets is need to kept in the directionality of carriers’ migration. Third, it is also important that the interface between channel and each facet should be as large as possible in order to assure maximal flux of charge transferring across the interface. Given comprehensive consideration to the above factors, graphene is a good case in point. Graphene is a two-dimensional, one-atom-thick layer of sp2-hybridized carbon atoms, and regular honeycomb structure. Its unique nanostructure and superior physicochemical properties, including good transparency, large extremely specific surface area, outstanding mechanical flexibility, excellent chemical stability and unique high electron conductivity,34, 35 certify for it competence as a good charge transport channel candidate. Combining the nature of graphene and architectural engineering of semiconductor/graphene composite, we propose a configuration of thin graphene layer completely covering semiconductor with different facets exposed. Our design basis is that, graphene layer acts as the electronic dedicated channel on the surface of semiconductor. Potential differences of CB and VB on the various facets is the power source of directional motivation for carriers. And the totally wrapped structure maximizes the contact area between graphene and various facets. Such a predictive model happens to meet the above three conditions and is expected to possess better charge separation efficiency. Furthermore, this design concept may provide a new dimension to modify the facet engineering of semiconductor-based photocatalysts, which may show a good prospect. As a proof of this concept, monoclinic bismuth vanadate (BiVO4) is deemed to be an ideal prototype semiconductor for the following reasons. BiVO4, with a suitable band-gap energy of 2.4 eV, is a promising visible-light-driven photocatalyst and has been extensively used in photodegradation of organic pollutants and photocatalytic O2 evolution. The BiVO4 is also an anti-corrosion and nontoxic photocatalyst. BiVO4 can be readily obtained, even it have large crystalline size, with smooth and tunable facets exposed.9, 36-40 Besides, the previous investigations on crystal facet engineering of BiVO4 lay the theoretic and experimental foundation of well optimization of crystal facet engineering.19, 20, 23, 36, 39, 41 On the other hand, poor charge transport properties and excessive hole-electron recombination for pure BiVO4 have restricted its wider application.37, 40, 42 Therefore, rationally design a novel charge migration route of BiVO4 to realize high-efficient separation of photoexcited carriers and achieve superior photocatalytic performance, is also a promising strategy for developing efficient BiVO4-based photocatalysts. [journal], [year], [vol], 00–00 | 1
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For implementing aforementioned concepts, we synthesize BiVO4 with {010} and {110} facets exposed by a hydrothermal method.23, 39 Subsequently, the BiVO4/RGO composites were constructed through an evaporation-induced self-assembly process and a mild photoreduction process.43 The detailed experimental processes is presented in the Supporting Information.
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Figure 1 XRD patterns of pure BiVO4 and BiVO4/RGO composites. 40
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Figure 2 Typical SEM images of the primary BiVO4 (a), BiVO4/RGO (b), and (c, d) TEM and HRTEM images of BiVO4/RGO hybrid. 60
The crystallographic structure and phase purity of the asprepared samples are characterized by powder X-ray diffraction (XRD) analysis (Fig. 1). All the diffraction peaks for all samples well-match with pure monoclinic phase of BiVO4 corresponding to the JCPDS No. 75-1867. No obvious characteristic peaks of RGO are detected. This phenomenon mainly results from the little RGO.44 The structure and morphology of the as-obtained pristine BiVO4 and BiVO4/RGO composite are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the pristine BiVO4 with {010} and {110} facets exposed is obtained successfully, and the facets exposed is smooth. The SEM image of BiVO4/RGO composite is shown in Fig. 2b, which displays that the BiVO4 is wrapped by RGO nanosheets uniformly. The TEM and HRTEM images further reveal that RGO nanosheets adhere tightly to the BiVO4 surfaces, and the thickness of the RGO layer is ~5 nm (as Fig. 2c-d). From the Fig. 2d, the lattice spacing of 0.47 nm is well agree with the (011) plane of the monoclinic BiVO4 phase.45 These results indicate that the evaporation and photo-reduction processes force effectively RGO layer with the thickness of ~5 nm to coat on BiVO4 closely and uniformly, without any changes in crystallographic structure and morphology of BiVO4. In order to further demonstrate the effective reduction of GO to RGO in the photo-reduction process, XPS analysis of BiVO4/GO and BiVO4/RGO is done. The C 1s spectra of BiVO4/GO and BiVO4/RGO are exhibited in Fig. 3. Apparently, each spectrum of C 1s of BiVO4/GO and BiVO4/RGO can be deconvoluted into three peaks at 284.6 eV, 286.7 eV and 288.6 eV, those are ascribed to the non-oxygenated ring carbon species, C-O species and C=O species, respectively. It is clearly seen that the peaks corresponding to C-O and C=O groups significantly degraded upon the photo-reduction, indicating that GO has been well reduced to RGO.46 Besides, the UV-Vis diffuse reflectance spectra (DRS) is employed to investigate the optical properties of BiVO4 and BiVO4/RGO. As presented in Fig. 4, BiVO4/ RGO composite have a better light absorption ability in the range of 200-800 nm due to the wideband absorption of RGO.43, 47 And the band gaps of bare BiVO4 and BiVO4/RGO composites are estimated to be 2.40 and 2.33 eV, respectively. The slight redshift of the absorption onset should be attributed to the interfacial interaction. It further testifies the tight contact between RGO and BiVO4.48 (a)
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Figure 3 XPS spectra of C 1s of the BiVO4/GO and BiVO4/RGO.
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Figure 5 (a) Photodegradation curves of RhB over BiVO4 and BiVO4/RGO under light (λ=415 15 nm) irradiation. (b) The linear fitting pseudo-first-order reaction rate (k) of BiVO4 and BiVO4/RGO .
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Figure 4 (a) The UV-vis diffuse reflectance spectrum of pristine BiVO4 and BiVO4/RGO; (b) Transformed Kubelka-Munk function ((αhυ) 2) versus light energy.
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Since the designed configuration of thin graphene completely covering semiconductor with different facets exposed has been established ideally, how about its photocatalytic property? To demonstrate the photoactivity of as-synthesized samples, photodegradation of Rhodamin B (RhB) by BiVO4 and BiVO4/RGO in the aqueous phase under visible-light irradiation is used as evaluation system. And to avoid interfering by the photosensitization of RhB, the particular light λ 415 15 nm , which is little absorbed by RhB (†Fig. S1), is chosen to This journal is © The Royal Society of Chemistry [year]
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drive the photocatalytic process. Experimental results are exhibited in Fig. 5. This experimental details in Supporting Information. As we can see from †Fig. S2, the dark equilibrium adsorption states of the pristine BiVO4 and BiVO4/RGO are accomplished for 60 min. The absorptivity of BiVO4/RGO hybrid outperforms that of BiVO4 due to the synergy of the noncovalent intermolecular π-π interactions between the RGO layer and RhB molecules and the enlargement of specific surface area. It can be noticed that in Fig. 5a, the photocatalytic performance of BiVO4/RGO composite has been improved dramatically as compared to the primary BiVO4. The kinetics of the photocatalytic reactions can be regarded as a pseudo-first-order reaction mode, and k values are calculated from the linear fit of ln(C/C0) vs irradiation time (t) (Fig. 5b). The k value of BiVO4/RGO composite is about 5 times as high as that of the pure BiVO4. The remarkable enhanced photocatalytic performance for BiVO4/RGO may be mainly attributed to the optimized charge transport and separation properties and improved adsorbability for RhB. To describe the photocatalytic performance more exactly, the reaction rates of all samples for photocatalytic degradation are normalizated with the surface areas (the measured BET surface area is ~1 m2•g-1 for BiVO4 and ~2 m2•g-1 for BiVO4/RGO). After normalization with the surface area, BiVO4/RGO still exhibits a remarkably higher photocatalytic activity compared to the bare BiVO4 (see in †Fig. S3), which convincingly demonstrates the advantage of charge separation properties of BiVO4/RGO. (a)
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Scheme 1 Schematic diagrams illustrating the process of photodegradation of RhB on BiVO4/RGO.
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Figure 6 (a) PL spectra of pure RGO, BiVO4/GO and BiVO4/RGO composites. (b) Schematic diagram of recombination mechanism free excitation on the {010} and {110} facets of BiVO4 in BiVO4/RGO.
To further reveal the migration path of the photo-induced carriers in BiVO4/RGO composite and explicit the contribution originating from the optimized charge transport and separation properties to the superior photocatalytic performance, roomtemperature photoluminescence (PL) measurement is carried out. Fig. 6a presents the PL emission spectra of RGO, BiVO4 and BiVO4/RGO hybrid, operated at an excitation wavelength of 360 nm. As displayed in Fig. 6a, for the pure RGO, no obvious PL peak comes out. And the bare BiVO4 shows a broad photoluminescence peak centered around at 560 nm, which is derived from the recombination of the band-gap photo-excited electron-hole pairs.49 While the PL emission spectrum of BiVO4/RGO can be deconvoluted into two peaks at ca. 560 nm and 440 nm. Compared with the PL spectrum of BiVO4, the intensity of the emission peak at ca. 560 nm of BiVO4/RGO, is significantly weaker than that of bare BiVO4, while the curve shapes and positions of the emission peaks are similar. It is amazing to note that a obvious peak at ca. 440 nm is also observed. This fancy phenomenon implies that the BiVO4/RGO hybrid with preconceived architecture is indeed able to induce a new recombination. Carefully analyzing the surface band structures of {110} and {010} facets for BiVO4, we find that the difference of the energy level in the VB is calculated at ca. 0.42 eV as depicted in Fig. 6b.23 In addition, RGO layer, an excellent electron conductor, can transmit electrons from the CB of {110} This journal is © The Royal Society of Chemistry [year]
facets to the CB of {010} facets in the unique configuration. During the process, transferred electrons through the RGO layer possibly are captured by the photo-excited holes in VB on {010} facets, resulting the corresponding PL emission. Indeed, the calculated difference of the energy level between the CB on the {110} facets and the VB on the {010} facets is ca. 2.82 eV (λ≈440 nm), which fits well with the peculiar phenomenon that the emerging emission peaks at ca. 440 nm. The reduction of the intensity of the emission peak centered around 560 nm and the emergence of the emission peak at ca. 440 nm confirm the existence of the shunt-channel for carrier separation, and the radiative recombination process of self-trapped excitation could be inhibited effectively in BiVO4/RGO composite.50 Hence, this optimized charge separation properties would be responsible for the remarkable enhancement of photocatalytic properties.
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With the purpose of more intuitively understanding of the concept, we propose a feasible photocatalytic mechanism, elucidated schematically in Scheme 1, based on the above the theory analysis and experimental results. We suggest that the dual-selectivity-channel carrier separation effects can give rise to significant enhancement of photocatalytic performance in the BiVO4/RGO composite. As illustrated in Scheme 1, the electrons in VB are photoexcited to CB on both {110} facets and {010} facets under the visible light irradiation. It is reported that the potential of CB on {110} facets is more negative than that of CB on {010} facets and the potential of VB on {010} facets is more positive than that of VB on {110} facets.23 The differences of the energy levels drive the few photoinduced electron transfer from {110} to {010} facets, and the holes from {010} to {110} facets through the internal structure of BiVO4. Meanwhile, most photogenerated electrons on {110} facets migrate to {010} facets through the external channel (RGO). In effect, the transferred electrons of passing graphene layer may be scavenged by the adsorbed oxygen (O2) and yield superoxide radicals (•O2-) to oxidize the dyes absorbed on the RGO layer in transit or upon arrival at {010} facets (†Fig. S4). And the holes on both {110}facets and {010} facets can be captured by the ubiquitous H2O molecules to produce hydroxyl radical (•OH), which can oxidize the dyes as well. Thus, the unique architecture ensures high separation efficiency of photogenerated carriers and enlarges the photocatalytic reaction space. Besides, the effect of adsorption of RGO layers also contributes to the improved photoactivity.
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In summary, we come up with a ideal model system of RGO layer completely coating semiconductor with different facets exposed. In the unique configuration, RGO layer carves out a new electrical channel, and the potential differences of CB and Journal Name, [year], [vol], 00–00 | 3
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VB on the various facets exposed direct the motivation for charge in both tunnels. As a proof of concept, BiVO4 is taken as a prototype semiconductor, and RGO is chosen as an conducting net. The configuration of totally close-coating BiVO4 with {010} and {110} facets exposed by thin RGO layer, which maximizes the contact area, has been fabricated successfully. Indeed, the designed facet-driven dual-selectivity-channel carrier separation is beneficial for charge migration both in the bulk and on the surface, and the BiVO4/RGO composite with such architecture shows about 5 times photocatalytic degradation rate of RhB as high as the pristine BiVO4. Furthermore, this design philosophy may serve as a guideline for design of high-performance photocatalyst, and break new ground towards rational modification of crystal facet engineering of semiconductor.
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Notes and references
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Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, P. R. China. Fax: +86-591-8377-9239; Tel:+86-591-8377-9239; E-mail: [email protected]. †Electronic Supplementary Information (ESI) available: [details of experimental procedures and characterization; Dark absorption curves of aqueous solutions of RhB in the presence of bare BiVO4 and BiVO4/RGO composite; The normalized reaction rate contants with surface area for photocatalytic degradation of RhB over samples over BiVO4 and BiVO4/RGO; Transmission spectrum of visible source with 420 nm pass filter RhB absorption spectrum (arbitrary scale); ESR spectra of radical adduct trapped by 3,4-dihydro-2,3-dimethyl-2H-pyrrole-1-oxide (DMPO) (DMPO-•O2-) over the BiVO4 or BiVO4/RGO suspension in the methanol solution without or with the light and the corresponding spectra elucidation.]. See DOI: 10.1039/b000000x/
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The support from National Natural Science Foundation of China (21173046, 21473031 and 21273035), Science & Technology Plan Project of Fujian Province (2014Y2003) and the funding under National Basic Research Program of China (973 Program: 2013CB632405). 1.
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DOI: 10.1039/C5CC02700D
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Rational Design of Charge Shunt: Modification upon Crystal Facet Engineering of Semiconductor Photocatalysts
Wenhui Feng, Sunxian Weng, Zuyang Zheng, Zhibin Fang and Ping Liu* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Another excellent carrier shunt is artificially bridged over different facets exposed of semiconductor, to achieve a unique architecture with superior photoactivity, which is mainly attributed to the facet-driven charge dual-selectivity-channel separation mechanism. This novel concept may break new ground towards rational modification of crystal facet engineering of semiconductor. Nowadays, the worsening environmental pollution and energy crisis have become the stumbling block to the development of human society. It is urgent need to develop an efficient means of environmental remediation and pursue clean and renewable energy resources.1-3 During the past decades, numerous researches have proven that photocatalysis is a promising approach to meet the requirement of environmental and energy in sustainable processes.4-10 As known, charge separation plays a crucial role in determining solar energy conversion efficiency of photocatalysis. Therefore, it is of great significance to optimize the charge separation for semiconductor-based photocatalysts.10-17 Recent investigations on crystal facet engineering of semiconductors have demonstrated that photo-excited electrons and holes may be driven to different crystal facets for various semiconductors (for instance, TiO2,18-22 BiVO4,23 BiOCl,24 ZnO,25 Fe2O326 and WO327). Which offers a new opportunity to overcome limited charge separation efficiency of semiconductorbased photocatalysts. Due to the differences of the energy levels in the conduction bands (CB) and valence bands (VB) between the different facets of the semiconductor, the photogenerated electrons from a facets with more negative potential on CB to another facets with less negative potential on CB, while holes transfer in the opposite direction between the facets.23, 28 Indeed, the spatial charge separation between different facets prolongs the lifetime of charge carriers by reducing the probability for their recombination, enhancing the photocatalytic property.29-33 However, during the spontaneous counter-transitions of electrons and holes in the same channel (the internal structure of semiconductor), the carrier recombination should be unavoidable, which has discounted the separation efficiency of charges. Fabricating rationally a novel and more effective path for carrier separation, therefore, would be a promising strategy to further optimize facet-dependent photocatalytic performance of semiconductor-based photocatalysts. Based on the above guidance, a possible way to achieve efficient charge migration is to build another new charge shunt across the different facets, through which only one kind of carrier is permitted to pass, achieving dual-selectivity-channel carrier separation. If so, the collision probability between photoexcited holes and electrons will be lowered greatly during the charge This journal is © The Royal Society of Chemistry [year]
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migration. Nevertheless, it is indeed difficulties that all the necessary conditions should be met simultaneously, which is listed as follow: First, either electrons or holes pass the new charge shunt. Second, a driving force between different facets is need to kept in the directionality of carriers’ migration. Third, it is also important that the interface between channel and each facet should be as large as possible in order to assure maximal flux of charge transferring across the interface. Given comprehensive consideration to the above factors, graphene is a good case in point. Graphene is a two-dimensional, one-atom-thick layer of sp2-hybridized carbon atoms, and regular honeycomb structure. Its unique nanostructure and superior physicochemical properties, including good transparency, large extremely specific surface area, outstanding mechanical flexibility, excellent chemical stability and unique high electron conductivity,34, 35 certify for it competence as a good charge transport channel candidate. Combining the nature of graphene and architectural engineering of semiconductor/graphene composite, we propose a configuration of thin graphene layer completely covering semiconductor with different facets exposed. Our design basis is that, graphene layer acts as the electronic dedicated channel on the surface of semiconductor. Potential differences of CB and VB on the various facets is the power source of directional motivation for carriers. And the totally wrapped structure maximizes the contact area between graphene and various facets. Such a predictive model happens to meet the above three conditions and is expected to possess better charge separation efficiency. Furthermore, this design concept may provide a new dimension to modify the facet engineering of semiconductor-based photocatalysts, which may show a good prospect. As a proof of this concept, monoclinic bismuth vanadate (BiVO4) is deemed to be an ideal prototype semiconductor for the following reasons. BiVO4, with a suitable band-gap energy of 2.4 eV, is a promising visible-light-driven photocatalyst and has been extensively used in photodegradation of organic pollutants and photocatalytic O2 evolution. The BiVO4 is also an anti-corrosion and nontoxic photocatalyst. BiVO4 can be readily obtained, even it have large crystalline size, with smooth and tunable facets exposed.9, 36-40 Besides, the previous investigations on crystal facet engineering of BiVO4 lay the theoretic and experimental foundation of well optimization of crystal facet engineering.19, 20, 23, 36, 39, 41 On the other hand, poor charge transport properties and excessive hole-electron recombination for pure BiVO4 have restricted its wider application.37, 40, 42 Therefore, rationally design a novel charge migration route of BiVO4 to realize high-efficient separation of photoexcited carriers and achieve superior photocatalytic performance, is also a promising strategy for developing efficient BiVO4-based photocatalysts. [journal], [year], [vol], 00–00 | 1
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For implementing aforementioned concepts, we synthesize BiVO4 with {010} and {110} facets exposed by a hydrothermal method.23, 39 Subsequently, the BiVO4/RGO composites were constructed through an evaporation-induced self-assembly process and a mild photoreduction process.43 The detailed experimental processes is presented in the Supporting Information.
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Figure 1 XRD patterns of pure BiVO4 and BiVO4/RGO composites. 40
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Figure 2 Typical SEM images of the primary BiVO4 (a), BiVO4/RGO (b), and (c, d) TEM and HRTEM images of BiVO4/RGO hybrid. 60
The crystallographic structure and phase purity of the asprepared samples are characterized by powder X-ray diffraction (XRD) analysis (Fig. 1). All the diffraction peaks for all samples well-match with pure monoclinic phase of BiVO4 corresponding to the JCPDS No. 75-1867. No obvious characteristic peaks of RGO are detected. This phenomenon mainly results from the little RGO.44 The structure and morphology of the as-obtained pristine BiVO4 and BiVO4/RGO composite are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the pristine BiVO4 with {010} and {110} facets exposed is obtained successfully, and the facets exposed is smooth. The SEM image of BiVO4/RGO composite is shown in Fig. 2b, which displays that the BiVO4 is wrapped by RGO nanosheets uniformly. The TEM and HRTEM images further reveal that RGO nanosheets adhere tightly to the BiVO4 surfaces, and the thickness of the RGO layer is ~5 nm (as Fig. 2c-d). From the Fig. 2d, the lattice spacing of 0.47 nm is well agree with the (011) plane of the monoclinic BiVO4 phase.45 These results indicate that the evaporation and photo-reduction processes force effectively RGO layer with the thickness of ~5 nm to coat on BiVO4 closely and uniformly, without any changes in crystallographic structure and morphology of BiVO4. In order to further demonstrate the effective reduction of GO to RGO in the photo-reduction process, XPS analysis of BiVO4/GO and BiVO4/RGO is done. The C 1s spectra of BiVO4/GO and BiVO4/RGO are exhibited in Fig. 3. Apparently, each spectrum of C 1s of BiVO4/GO and BiVO4/RGO can be deconvoluted into three peaks at 284.6 eV, 286.7 eV and 288.6 eV, those are ascribed to the non-oxygenated ring carbon species, C-O species and C=O species, respectively. It is clearly seen that the peaks corresponding to C-O and C=O groups significantly degraded upon the photo-reduction, indicating that GO has been well reduced to RGO.46 Besides, the UV-Vis diffuse reflectance spectra (DRS) is employed to investigate the optical properties of BiVO4 and BiVO4/RGO. As presented in Fig. 4, BiVO4/ RGO composite have a better light absorption ability in the range of 200-800 nm due to the wideband absorption of RGO.43, 47 And the band gaps of bare BiVO4 and BiVO4/RGO composites are estimated to be 2.40 and 2.33 eV, respectively. The slight redshift of the absorption onset should be attributed to the interfacial interaction. It further testifies the tight contact between RGO and BiVO4.48 (a)
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Figure 3 XPS spectra of C 1s of the BiVO4/GO and BiVO4/RGO.
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Figure 5 (a) Photodegradation curves of RhB over BiVO4 and BiVO4/RGO under light (λ=415 15 nm) irradiation. (b) The linear fitting pseudo-first-order reaction rate (k) of BiVO4 and BiVO4/RGO .
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Figure 4 (a) The UV-vis diffuse reflectance spectrum of pristine BiVO4 and BiVO4/RGO; (b) Transformed Kubelka-Munk function ((αhυ) 2) versus light energy.
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Since the designed configuration of thin graphene completely covering semiconductor with different facets exposed has been established ideally, how about its photocatalytic property? To demonstrate the photoactivity of as-synthesized samples, photodegradation of Rhodamin B (RhB) by BiVO4 and BiVO4/RGO in the aqueous phase under visible-light irradiation is used as evaluation system. And to avoid interfering by the photosensitization of RhB, the particular light λ 415 15 nm , which is little absorbed by RhB (†Fig. S1), is chosen to This journal is © The Royal Society of Chemistry [year]
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drive the photocatalytic process. Experimental results are exhibited in Fig. 5. This experimental details in Supporting Information. As we can see from †Fig. S2, the dark equilibrium adsorption states of the pristine BiVO4 and BiVO4/RGO are accomplished for 60 min. The absorptivity of BiVO4/RGO hybrid outperforms that of BiVO4 due to the synergy of the noncovalent intermolecular π-π interactions between the RGO layer and RhB molecules and the enlargement of specific surface area. It can be noticed that in Fig. 5a, the photocatalytic performance of BiVO4/RGO composite has been improved dramatically as compared to the primary BiVO4. The kinetics of the photocatalytic reactions can be regarded as a pseudo-first-order reaction mode, and k values are calculated from the linear fit of ln(C/C0) vs irradiation time (t) (Fig. 5b). The k value of BiVO4/RGO composite is about 5 times as high as that of the pure BiVO4. The remarkable enhanced photocatalytic performance for BiVO4/RGO may be mainly attributed to the optimized charge transport and separation properties and improved adsorbability for RhB. To describe the photocatalytic performance more exactly, the reaction rates of all samples for photocatalytic degradation are normalizated with the surface areas (the measured BET surface area is ~1 m2•g-1 for BiVO4 and ~2 m2•g-1 for BiVO4/RGO). After normalization with the surface area, BiVO4/RGO still exhibits a remarkably higher photocatalytic activity compared to the bare BiVO4 (see in †Fig. S3), which convincingly demonstrates the advantage of charge separation properties of BiVO4/RGO. (a)
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Scheme 1 Schematic diagrams illustrating the process of photodegradation of RhB on BiVO4/RGO.
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Figure 6 (a) PL spectra of pure RGO, BiVO4/GO and BiVO4/RGO composites. (b) Schematic diagram of recombination mechanism free excitation on the {010} and {110} facets of BiVO4 in BiVO4/RGO.
To further reveal the migration path of the photo-induced carriers in BiVO4/RGO composite and explicit the contribution originating from the optimized charge transport and separation properties to the superior photocatalytic performance, roomtemperature photoluminescence (PL) measurement is carried out. Fig. 6a presents the PL emission spectra of RGO, BiVO4 and BiVO4/RGO hybrid, operated at an excitation wavelength of 360 nm. As displayed in Fig. 6a, for the pure RGO, no obvious PL peak comes out. And the bare BiVO4 shows a broad photoluminescence peak centered around at 560 nm, which is derived from the recombination of the band-gap photo-excited electron-hole pairs.49 While the PL emission spectrum of BiVO4/RGO can be deconvoluted into two peaks at ca. 560 nm and 440 nm. Compared with the PL spectrum of BiVO4, the intensity of the emission peak at ca. 560 nm of BiVO4/RGO, is significantly weaker than that of bare BiVO4, while the curve shapes and positions of the emission peaks are similar. It is amazing to note that a obvious peak at ca. 440 nm is also observed. This fancy phenomenon implies that the BiVO4/RGO hybrid with preconceived architecture is indeed able to induce a new recombination. Carefully analyzing the surface band structures of {110} and {010} facets for BiVO4, we find that the difference of the energy level in the VB is calculated at ca. 0.42 eV as depicted in Fig. 6b.23 In addition, RGO layer, an excellent electron conductor, can transmit electrons from the CB of {110} This journal is © The Royal Society of Chemistry [year]
facets to the CB of {010} facets in the unique configuration. During the process, transferred electrons through the RGO layer possibly are captured by the photo-excited holes in VB on {010} facets, resulting the corresponding PL emission. Indeed, the calculated difference of the energy level between the CB on the {110} facets and the VB on the {010} facets is ca. 2.82 eV (λ≈440 nm), which fits well with the peculiar phenomenon that the emerging emission peaks at ca. 440 nm. The reduction of the intensity of the emission peak centered around 560 nm and the emergence of the emission peak at ca. 440 nm confirm the existence of the shunt-channel for carrier separation, and the radiative recombination process of self-trapped excitation could be inhibited effectively in BiVO4/RGO composite.50 Hence, this optimized charge separation properties would be responsible for the remarkable enhancement of photocatalytic properties.
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With the purpose of more intuitively understanding of the concept, we propose a feasible photocatalytic mechanism, elucidated schematically in Scheme 1, based on the above the theory analysis and experimental results. We suggest that the dual-selectivity-channel carrier separation effects can give rise to significant enhancement of photocatalytic performance in the BiVO4/RGO composite. As illustrated in Scheme 1, the electrons in VB are photoexcited to CB on both {110} facets and {010} facets under the visible light irradiation. It is reported that the potential of CB on {110} facets is more negative than that of CB on {010} facets and the potential of VB on {010} facets is more positive than that of VB on {110} facets.23 The differences of the energy levels drive the few photoinduced electron transfer from {110} to {010} facets, and the holes from {010} to {110} facets through the internal structure of BiVO4. Meanwhile, most photogenerated electrons on {110} facets migrate to {010} facets through the external channel (RGO). In effect, the transferred electrons of passing graphene layer may be scavenged by the adsorbed oxygen (O2) and yield superoxide radicals (•O2-) to oxidize the dyes absorbed on the RGO layer in transit or upon arrival at {010} facets (†Fig. S4). And the holes on both {110}facets and {010} facets can be captured by the ubiquitous H2O molecules to produce hydroxyl radical (•OH), which can oxidize the dyes as well. Thus, the unique architecture ensures high separation efficiency of photogenerated carriers and enlarges the photocatalytic reaction space. Besides, the effect of adsorption of RGO layers also contributes to the improved photoactivity.
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In summary, we come up with a ideal model system of RGO layer completely coating semiconductor with different facets exposed. In the unique configuration, RGO layer carves out a new electrical channel, and the potential differences of CB and Journal Name, [year], [vol], 00–00 | 3
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VB on the various facets exposed direct the motivation for charge in both tunnels. As a proof of concept, BiVO4 is taken as a prototype semiconductor, and RGO is chosen as an conducting net. The configuration of totally close-coating BiVO4 with {010} and {110} facets exposed by thin RGO layer, which maximizes the contact area, has been fabricated successfully. Indeed, the designed facet-driven dual-selectivity-channel carrier separation is beneficial for charge migration both in the bulk and on the surface, and the BiVO4/RGO composite with such architecture shows about 5 times photocatalytic degradation rate of RhB as high as the pristine BiVO4. Furthermore, this design philosophy may serve as a guideline for design of high-performance photocatalyst, and break new ground towards rational modification of crystal facet engineering of semiconductor.
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Notes and references
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Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, P. R. China. Fax: +86-591-8377-9239; Tel:+86-591-8377-9239; E-mail: [email protected]. †Electronic Supplementary Information (ESI) available: [details of experimental procedures and characterization; Dark absorption curves of aqueous solutions of RhB in the presence of bare BiVO4 and BiVO4/RGO composite; The normalized reaction rate contants with surface area for photocatalytic degradation of RhB over samples over BiVO4 and BiVO4/RGO; Transmission spectrum of visible source with 420 nm pass filter RhB absorption spectrum (arbitrary scale); ESR spectra of radical adduct trapped by 3,4-dihydro-2,3-dimethyl-2H-pyrrole-1-oxide (DMPO) (DMPO-•O2-) over the BiVO4 or BiVO4/RGO suspension in the methanol solution without or with the light and the corresponding spectra elucidation.]. See DOI: 10.1039/b000000x/
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The support from National Natural Science Foundation of China (21173046, 21473031 and 21273035), Science & Technology Plan Project of Fujian Province (2014Y2003) and the funding under National Basic Research Program of China (973 Program: 2013CB632405). 1.
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ChemComm Accepted Manuscript
DOI: 10.1039/C5CC02700D
