Rajendra P.
Panmand
a,
Yogesh A.
Sethi
a,
Sunil R.
Kadam
a,
Mohaseen S.
Tamboli
a,
Latesh K.
Nikam
a,
Jalinder D.
Ambekar
a,
Chan-Jin
Park
*b and
Bharat B.
Kale
*a
aCentre for Materials for Electronics Technology (C-MET), Panchwati off Pashan Road, Pune-411 008, India. E-mail: bbkale@cmet.gov.in
bDepartment of Materials Science and Engineering, Chonnam National University, 77, Yongbongro, Bukgu, Gwangju 500-757, Korea. E-mail: parkcj@jnu.ac.kr
First published on 10th November 2014
Three dimensional (3D) hierarchical nanostructures of orthorhombic Bi2WO6 with unique morphologies were successfully synthesized by a solvothermal method. The precursor concentration plays a key role in the architecture of the hierarchical nanostructures. A peony flower-like morphology was obtained at higher precursor concentrations, and a red blood cell (RBC)-like morphology with average diameter of 1.5 μm was obtained at lower concentrations. These hierarchical nanostructures were assembled by self-alignment of 20 nm nanoplates. As their band gap is in the visible region, the photocatalytic activity of the Bi2WO6 hierarchical nanostructures for the production of hydrogen from glycerol, and the degradation of rhodamine B (RhB) and methylene blue (MB) under ambient conditions in the presence of solar light was investigated. The Bi2WO6 with peony flower morphology was observed to be the most efficient photocatalyst (H2: 7.40 mmol h−1 g−1, kRhB: 0.240 and kMB: 0.100) of the reported nanostructures. The higher activity of the peony flowers was due to their porous nature, high surface area and lower band gap. Such unique 3D nanostructures of Bi2WO6 have been fabricated for the first time, and their use as photocatalysts in the production of hydrogen from glycerol has hitherto not been attempted. These nanostructures may have potential in ferroelectric, piezoelectric, pyroelectric and nonlinear dielectric applications.
Oxide semiconductors with Aurivillius structures are of immense importance due to their layered structures and unique properties.8 Among these semiconductors, Bi2WO6 is an n-type semiconductor, and is significant because of its excellent intrinsic physico-chemical properties. Bi2WO6 has been used as a photocatalyst for rhodamine B dye degradation by a few researchers.9,10,11a–c However, very limited studies on shape controlled Bi2WO6 and its associated photocatalytic activity have been performed, and 2D and 3D assemblies of Bi2WO6 have not been used in hydrogen production.
In the last few years, there has been exhaustive research toward the development of novel hydrogen production technologies based on renewable and natural assets, such as water and biomass.12–14 Our group has demonstrated hydrogen production from waste H2S.15–19 Among the various biomass resources for hydrogen production, glycerol (C3H8O3) is significant because huge amounts are produced as waste in the processing of vegetable oil.20 The world demand for glycerol is limited, and in response to the rapid increase in global biodiesel production, crude glycerol is rapidly becoming a uneconomical waste product because it is expensive to dispose of. Thus, current research is focused on developing new technologies for the utilization of glycerol. One promising possibility is to use glycerol as a renewable source of hydrogen. This is possible with the use of several methods, including photoinduced reforming of glycerol in the liquid phase.21,22 There are limited reports on hydrogen generation from the decomposition of glycerol. Photocatalytic reforming of aqueous solutions of glycerol under ambient conditions has been investigated, using Pt/TiO2, CuO/TiO2 and other photocatalysts under a simulated solar light source.23–26
In this context, we have fabricated hierarchical peony flower/bowl/red blood cell (RBC)-like Bi2WO6 nanostructures via a simple template-free solvothermal method for the first time. These nanostructures have been thoroughly characterized by XRD, FESEM, HRTEM, and UV-Vis spectroscopy. The formation mechanism and the effects of reaction time and precursor concentrations were also investigated. The nanostructures of Bi2WO6 were used as photocatalysts for the production of hydrogen from glycerol, which has hitherto not been attempted. Additionally, their performance in rhodamine B dye degradation under visible light irradiation was studied.
Sample | Reaction parameters of the solvothermal system | Concentration of Bi3+ and (WO4)2− |
---|---|---|
BWO-1 | Temp: 150 °C | 0.150 and 0.075 M |
Time: 5 h | ||
BWO-2 | Temp: 150 °C | 0.0375 and 0.019 M |
Time: 18 h | ||
BWO-3 | Temp: 150 °C | 0.075 and 0.038 M |
Time: 18 h | ||
BWO-4 | Temp: 150 °C | 0.150 and 0.075 M |
Time: 18 h |
The apparatus used for the photocatalytic experiments consists of a simulated solar light source, a quartz photoreactor and an online analysis system. The lamp housing (Oriel) is furnished with a Xe arc lamp (Osram XBO 300 W), a set of lenses for light collection and focusing, and a water filter, which serves to eliminate infrared radiation. The photoreactor has a cylindrical shape, and its top cover has provisions for measuring solution pH and temperature, as well as connections for the carrier gas (argon) inlet/outlet. The gas outlet is equipped with a water-cooled condenser, which does not allow vapors to escape from the reactor. The outlet of the reactor is connected to a measuring cylinder in order to collect the gas, which is further used for analysis (gas chromatography).
The photocatalytic activities of the samples were evaluated for the degradation of rhodamine B (RhB) and methylene blue (MB) under visible light irradiation using the same lamp as mentioned above. In each experiment, 0.1 g of the photocatalyst was added to 600 mL of RhB and MB solution (1 × 10−5 and 1.6 × 10−5 M, respectively). Before illumination, the suspension was vigorously stirred in the dark for 1 h to ensure the establishment of an adsorption–desorption equilibrium between the photocatalyst and the dye. Then, the solution was exposed to visible light irradiation. At certain intervals, a 10 mL solution was sampled and centrifuged to remove the remnants of photocatalyst. Finally, the absorption UV-Vis spectrum of the filtrate was recorded using a λ-950 (Perkin Elmer) spectrophotometer.
Considering the formation of a unique flower-like morphology after 18 h at 150 °C, further experiments were performed at various precursor concentrations under the same solvothermal conditions. Surprisingly, different morphologies were obtained on varying the precursor concentrations.
Fig. 3 shows the FESEM images of Bi2WO4 obtained using different Bi3+ and (WO4)2− precursor concentrations in a water–acetic acid (1:1) solution system autoclaved at 150 °C for 18 h. Fig. 3a and b show FESEM images of uniform bowl-shaped Bi2WO6 nanostructures of size 1.5 μm obtained with Bi(NO3)3·5H2O and Na2WO4·2H2O precursor concentrations of 0.075 and 0.038 M, respectively. Here, nanoplates (width × length of 60 × 80 nm and thickness of 25 nm) form nanosheets by oriented attachment. These nanosheets are much smaller compared to the previously discussed peony flower-like structures. In this case, the nanosheets are not curved, due to their smaller size. Hence, there may be seeding at the centre and peripheral growth due to oriented attachment of the building blocks. The growth is slower at the centre than at the periphery, which ultimately creates a cavity at the centre. So, uniform size bowls (1.5 μm) are seen in the FESEM image (Fig. 3a), and the magnified image shows a single bowl with a cavity (Fig. 3b). Fig. 3c and d show FESEM images of uniform RBC-shaped Bi2WO6 nanostructures of size 1–1.5 μm obtained with Bi(NO3)3·5H2O and Na2WO4·2H2O precursor concentrations of 0.038 and 0.019 M, respectively. Surprisingly, at these concentrations, the FESEM image shows RBC-like nanostructures of Bi2WO6. As discussed above, the cells of 1.5 μm diameter are composed of a large quantity of two dimensional nanosheets created from rectangular nanoplates of 40 × 50 nm size. Again, the growth of these nanosheets originates at the centre and further growth occurs at the periphery. Hence, the growth is slightly slower at the centre, resulting in a dimple in the middle which looks like a red blood cell, as shown in the magnified FESEM image (Fig. 3d). Again, the nanosheets are stacked by oriented attachment, which is clearly revealed in the FESEM image (Fig. 3d).
Fig. 3 FESEM images of the Bi2WO6 nanostructures obtained at different Bi3+ and (WO4)2− precursor concentrations: (a, b) 0.075 and 0.0375 M respectively; (c, d) 0.0375 and 0.01875 M respectively. |
The red blood cell-like structure of Bi2WO6 was further investigated by TEM. Fig. 4 shows low and high magnification high resolution transmission electron microscopy (HRTEM) images. Fig. 4a presents the HRTEM image of an individual red blood cell. The light colour at the centre indicates that the Bi2WO6 disc has a pit in the middle, which is in accordance with the FESEM images (Fig. 3d). Fig. 4b shows the corresponding selected area electron diffraction (SAED) pattern of the red blood cell-like structure of Bi2WO6, which reveals that its structure is polycrystalline rather than a well-defined single crystal.
Fig. 4 HRTEM images of the Bi2WO6 nanostructures obtained at Bi3+ and (WO4)2− precursor concentrations of 0.0375 and 0.01875 M, respectively. (a) Low magnification image, (b) SAED pattern, (c) HRTEM image, and (d) enlarged image of the area marked by a red rectangle in Fig. 4c. |
Fig. 4c shows a typical HRTEM image of the edge of an individual RBC-like structure of Bi2WO6. Fig. 4d is a magnified image of the area marked by a red rectangle in Fig. 4c. Fig. 4d clearly reveals resolved lattice spacings of 0.272 and 0.194 nm, which correspond to the (020) and (220) planes of orthorhombic Bi2WO6, respectively. The observed SAED pattern is in good agreement with the XRD analysis.
From the morphological studies using SEM and TEM analysis, it can be concluded that the formation of such intricate nanostructures is achieved via the first mechanism, i.e. a hierarchical assembly process, as illustrated in Fig. 5. Tiny Bi2WO6 nanoplates are formed in the early stages (Fig. 5B), followed by the oriented attachment of these building blocks into 2D nanosheets (Fig. 5C). Finally, self-assembly of the sheets gives rise to 3D hierarchical nanostructures (Fig. 5D). In our study, the formation of the primary Bi2WO6 nanoplates is a typical Ostwald ripening process. Initially, tiny nuclei are formed in a supersaturated solution, and then nanoparticles are formed by a well known crystal growth phenomenon. The larger particles grow at the cost of the smaller ones, as per the Gibbs–Thomson law.31 Further crystal growth resulting in the formation of 2D nanostructures is strongly related to the intrinsic crystal structure of Bi2WO6. It has been reported that orthorhombic Bi2WO6 is constructed from corner-sharing octahedral WO6 layers and [Bi2O2]2+ layers sandwiched between the octahedral WO6 layers.32 The layers are parallel to the (001) facets. On the basis of this structure, there should be equal numbers of octahedral W chains along the a- and b-axes, which indicates that the (200) and (020) facets have much higher chemical potentials in comparison with the other facets.33 As discussed previously,28 this feature leads to faster growth rates of the (200) and (020) facets, which exhibit preferential growth along the layers. As discussed above, rectangular Bi2WO6 nanoplates are formed after 5 h of reaction, and 3D nanostructures are formed after prolonged reaction times, i.e. 18 h (see Fig. 2).
During the formation of the hierarchical nanostructures via the assembly process, PVP plays an important role in the formation of integrated nanosheets and multilayered structures. It is generally believed that the building blocks for oriented aggregation/attachment are usually nanoparticles with surfaces stabilized by an organic coating, and weakly protected nanoparticles often undergo entropy-driven random aggregation.34,35 In the present work, it is believed that the selective adsorption of PVP onto various crystallographic planes of the Bi2WO6 nanoplates is significant in the initial growth stage. As evidenced by the TEM and SAED results, the assembled nanosheets/layers were constructed from tiny nanoplates, with their top and lateral surfaces enclosed by the (001), (020) and (220) planes, respectively. Thus, PVP should have stronger interactions with the (001) planes. In contrast, there is comparatively less adsorption on the (220) planes, and the PVP can be easily removed from these crystal planes. This results in preferential growth along the layers (a × b layer plane) rather than along the c-axis. In other words, the gradual enlargement of the 2D surface areas originates from the edge-to-edge assembly of the primary plates. Well-defined nanosheets are conferred on further organization of adjacent nanoplates to share the same 2D crystallographic orientation and subsequent coalescence of these building blocks. For the formation of 3D nanostructures, layer-by-layer growth may be favoured (Fig. 5). FESEM clearly shows the formation of flat multi-layered nanosheets by the stacking of in situ formed nanosheets along the [001] direction (Fig. 2a & b). This process could further reduce the surface energy of the nanosheets, and hence further stacking is facilitated by the special 2D geometrical shape. With a prolonged reaction time, instead of further growth of the nanosheets, organization of the existing nanosheets, originating from the centre, was observed. The centre develops a wrinkle, which may act as a template for further fabrication of hierarchical nanostructures. This specific growth may be due to lattice tension or surface interactions at the edges of the nanosheets.36 This type of growth, in which concaveness is observed along with stacking of the nanosheets, occurs in the case of the RBC structures. More new tilted nanosheets are added to the uneven nanosheets (acting as a template), which further enhances peripheral growth and restricts growth at the centre. Considering the above mechanism, the same phenomenon is applicable in the case of the growth of Bi2WO6 nanostructures (2D flat rectangular plates, RBC-like, 3D bowl-like, and hierarchical peony flower-like).
The concentration gradient (at a particular precursor concentration) under solvothermal conditions also plays an important role in the growth of hierarchical nano-microstructures.36,37a–c As discussed above, at low precursor concentration, RBC-like structures of Bi2WO6 were observed. At low concentration, the mobility of Bi3+ and (WO4)2− is low, which restricts the growth of nuclei. Hence, very thin nanosheets are formed during the reaction, even after prolonged reaction time. These nanosheets are self-organized and exhibit peripheral growth to create RBC-like structures. The oriented attachment of nanoplates is initiated by nanosheet seeds, which are situated at the centre, and hence restrict growth at the centre and favour growth at the periphery. Therefore, we observed a dimple at the centre of the RBC-like structure. At slightly higher precursor concentration, a bowl-like morphology is observed. Due to the higher concentration of precursors, the mobility of the ions increases, which favours the growth of the nanoplates. As discussed for the RBC-like structure, the peripheral growth increases further, leaving slow growth at the centre. Hence, bowl-like structures are observed. At even higher concentration, central as well as peripheral growth is observed. In this case, long nanosheets are formed due to the faster growth rate. However, after prolonged reaction time, curved nanosheets are formed due to the development of strain in the long nanosheets, which self-organize further to form peony flower-like structures.
Fig. 6 UV-Vis absorption spectra of peony flower, bowl and red blood cell-like nanostructures of Bi2WO6. Inset shows the Tauc plots of the corresponding absorption spectra. |
Photoluminescence studies are useful for investigation of the migration, transfer and recombination processes of photoinduced electron–hole pairs in a semiconductor because photoluminescence (PL) emission is mainly due to the recombination of these electron–hole pairs. The probability of charge carrier recombination increases with increasing PL intensity.39,40 A comparison of the PL spectra (excited at 350 nm) of peony flower, bowl and RBC-like Bi2WO6 at room temperature is shown in Fig. 7. The Bi2WO6 hierarchical nanostructures show a broad blue-green emission peak. The intense blue emission peak appearing at 450–470 nm is associated with the intrinsic luminescence of Bi2WO6. This is due to charge-transfer transitions between the Bi 6s and O 2p hybrid orbital (VB) to the empty W 5d orbital (CB) in WO62−.41a It is well known that doping, surface area, crystallite size and morphology affect the PL characteristics.41b,cFig. 7 reveals that the photoluminescence intensity of the peony flower-like nanostructures was quenched as compared with that of bowl and RBC-like Bi2WO6, which clearly indicates that the recombination of photogenerated charge carriers between the Bi 6s and O 2p hybrid orbital (VB) and the empty W 5d orbital is greatly affected by the different nanostructures. From the photoluminescence study, it is confirmed that the PL intensity decreases with increasing precursor concentration, which ultimately shows the suppression of charge carrier recombination. This could be due to the formation of a morphology with high surface area, which increases the number of surface defects. Hence, the quenching of the PL intensity observed in the case of the peony flower-like hierarchical structure is quite justified.
Morphology | Surface area (m2 g−1) by BET | Pore size (nm) by BJH | Pore density (g−1) × 109 |
---|---|---|---|
Peony flower | 37.7 | 13.6 | 2.92 |
Bowl | 33.4 | 14.3 | 2.30 |
Red blood cell | 32.1 | 16.3 | 1.96 |
Bi2WO6 Catalyst | Rate constant (k) | |
---|---|---|
MB (kMB) | RhB (kRhB) | |
RBC structure | 0.030 | 0.113 |
Bowl structure | 0.036 | 0.135 |
Peony flower structure | 0.100 | 0.240 |
The rate constants obtained for the different Bi2WO6 nanostructures indicate that their photocatalytic performances are strongly dependent on shape, size, and structure. Peony flower-like Bi2WO6 showed higher photocatalytic activity for the degradation of MB and RhB than bowl-like and red blood cell-like Bi2WO6, as shown in Fig. 9 and Table 3. The enhanced photocatalytic activity of hierarchical peony flower-like Bi2WO6 can be attributed to the combined effects of several factors: (1) the moderately high surface area of flower-like Bi2WO6 structures, which ultimately enhances light absorption and dye adsorption; (2) the presence of a larger number of pores, which can be considered as transport paths for the dye molecules on the framework walls;44 and (3) the porous peony flowers allow multiple reflections of visible light within the interior cavity, which facilitates more efficient use of the light.45
Fig. SII (ESI†) reveals the temporal evolution of the absorption spectra of aqueous solutions of MB and RhB in the presence of hierarchical Bi2WO6 under visible light irradiation (λ > 400 nm). Bi2WO6 is a strongly acidic oxide, which has great significance in photocatalytic dye degradation.46 The lowering of the valance band maxima indicates that Bi2WO6 has strong oxidizing power. The surface holes, hvb+, generated in the valence band will also have strong oxidizing power. Thus, thermodynamically, OH− ions (or H2O) in the solution can easily accept surface hvb+ and form OH˙ radicals (reaction 1), which ultimately aids the direct oxidation of MB in the bulk solution. Hence, the higher photocatalytic oxidation rate in the presence of Bi2WO6 has been ascribed to the generation of OH˙ radicals.
OH− + hvb+ → OH˙ + H+ | (1) |
The strong RhB absorption band located at 554 nm is due to the presence of four ethylated groups (–C2H5) attached to the heterocyclic ring structure.47 A regular decrease in the RhB absorption at 554 nm under visible light irradiation is observed, accompanied by a shift of the absorption peak to lower wavelength. This shift may be attributed to the dye's de-ethylation, i.e. conversion of N,N,N′,N′-tetraethylated rhodamine to rhodamine. The intermediates formed successively presented different maximum absorption bands, i.e. N,N,N-triethylated rhodamine: 539 nm; N,N-diethylated rhodamine: 522 nm; N-ethylated rhodamine: 510 nm; and rhodamine: 498 nm.48 Also, the colour of the dye solution changed from red to a light greenish yellow and then to colourless, which can be visualised easily, indicating the complete photocatalytic degradation of the aqueous RhB solution during the reaction. It has been reported that the mechanism of degradation of RhB involves a photocatalytic process, as well as a photosensitized process. During the degradation process, Bi2WO6 acts as an active photocatalyst, and RhB is degraded by direct interaction with a strong oxidizing hole originating from the hybridization of the Bi 6s and O 2p orbitals.49,50 The RhB molecules are attacked and then degraded via the destruction of the conjugated structure. At the same time, during the photosensitized process, the RhB dye can absorb visible light, which is attributed to the ground state and excited state of the dye. The dye mineralization originates from the active oxygen species, O2−, and the radical cations, dye+. The photosensitized degradation of RhB is commonly accomplished via the N-demethylation process.50,51
The photocatalytic production of hydrogen from a glycerol–water mixture involves two distinct mechanisms, that is, photo-splitting of water and photo-reforming of glycerol.52 First, glycerol is oxidized, and then several intermediate compounds are produced, followed by the generation of hydrogen as the product. Second, glycerol acts as a sacrificial agent in photocatalytic water splitting.53 Glycerol, which acts as a sacrificial electron donor, is able to rapidly remove the photo-generated holes (hydroxyl radicals) and/or photo-generated oxygen in an irreversible fashion, thereby suppressing electron–hole recombination and/or the reverse reaction of H2 and O2. By doing so, glycerol is progressively oxidized to CO2, with the intermediate formation of partially oxidized products. When complete oxidation of glycerol (and the reaction intermediates) is achieved, oxygen can no longer be removed from the photocatalyst surface, and the rate of hydrogen production drops to a steady-state value comparable to that observed in solution in the absence of glycerol.
The overall process, which may be described as photo-induced reforming of glycerol at room temperature, can be expressed by the following equation:
C3H8O3 + 3H2O → 3CO2 + 7H2 | (2) |
As per this reaction, H2 and CO2 are formed in the degradation of glycerol, which shows that efficient production of hydrogen is possible. Overall, the peony flower-like nanostructure of Bi2WO6 showed excellent photocatalytic activity.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ce01968g |
This journal is © The Royal Society of Chemistry 2015 |