Mesoporous Ni-Doped Î´-Bi2O3 Microspheres for Enhanced Solar

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Mesoporous Ni-Doped #-BiO Microspheres for Enhanced Solar-Driven Photocatalysis: A Combined Experimental and Theoretical Investigation Shijin Zhu, Lilin Lu, Zai Wang Zhao, Tian Wang, Xiao Ying Liu, Haijun Zhang, Fan Dong, and Yuxin Zhang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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The Journal of Physical Chemistry

Mesoporous Ni-doped δ-Bi2O3 Microspheres for Enhanced Solar-driven Photocatalysis: A Combined Experimental and Theoretical Tnvestigation Shijin Zhu†, Lilin Lu‡ *, Zaiwang Zhao††, Tian Wang†, Xiaoying Liu†, Haijun Zhang‡, Fan Dong††*, Yuxin Zhang†,* †

State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China.

‡

State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, P.R. China.

††

Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, P. R. China.

ABSTRACT Ni-doped Bi2O3 (Ni-Bi2O3) microspheres have been synthesized by a one-pot solvothermal route. The morphology and structure of Ni-Bi2O3 microspheres can be well controlled by tailoring the preparative parameters (e.g., the content of Ni and reaction time). The resulted Ni-Bi2O3 microspheres are formed via a dissolution-recrystallization process resulting in mesoporous structure. Furthermore, when using as photocatalyst for removal of NO in air, it exhibits an enhanced performance in comparison with bare Bi2O3 under simulated solar light. First-principles calculations reveal that the enhanced activity of the catalyst could be attributed to the modification of geometric and electronic structure resulted from the dopant of Ni. The present work could provide a new approach for synthesizing doped mesoporous Bi2O3 nanostructures with controlled morphology.

1. INTRODUCTION In the past decades, photocatalysis technology has gained much attention as a promising solution to the environmental pollution globally.1-3 An efficient 1

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photocatalyst should possess a band gap structure with efficient separation of electron–hole pairs.4-5 Light absorption and the consequent photoexcitation of electron–hole pairs occur when the energy of the incident photons matches the band gap, and then the excited electrons and holes migrate to the surface to initiate chemical reactions. Conventional photocatalyst (TiO2), on account of its unique properties of chemical inertness, resistance to photocorrosion, nontoxicity and low cost, has attracted an increasing attention for decades. However, as a semiconductor with a wide-band gap (3.2 eV for anatase), TiO2 can only absorb 3–5 % of sunlight located in the ultraviolet region, thus limits its practical use.6 Thus, the development of photocatalysts with wide light absorption region has become a very vital theme. Bismuth with unusual electronic properties (e.g., small effective electron mass of ~0.001 m0 along with the trigonal direction and large electron Fermi wavelength of about 40 nm) is considered to be the most potential materials for photocatalysis for its order of magnitude larger than that for a metal.7 Very recently, semimetal bismuth element has been found to be a plasmonic photocatalyst with a catalytic ‘‘memory’’ capability following illumination.8 The only fly in the ointment could be the poor properties in visible light region. More broadly, various ramifications of bismuth, including BiOX (X = Cl, Br, I), Bi2O3, BiVO4, BiFeO3, Bi2Ti2O7, Bi2MoO6, and Bi2WO6, exhibited good photocatalytic property.9-15 Among them, bismuth oxide (Bi2O3) withα or β phase is a promising material because of its good

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electroconductivity, fine thermal properties, and narrow band-gap (2.8 eV).16 Recently, many research efforts have focused on developing various morphologies of Bi2O3 and researching its optical properties.17-18 Additionally, doped-Bi2O3 with α or β phase has displayed an enhanced optical properties due to the defects caused by different metal atoms.19-20 However, due to the impact of doping metal atoms, the morphologies are often influenced due to uncontrollable interference. As a key factor for photocatalysis, the special morphological structure with respect to large surface area and porous structure should be considered. Thus, rational design of materials should be a feasible route for enhancing photocatalysis. To date, there are few reports on δ-Bi2O3 and Ni-doped δ-Bi2O3 with well-designed morphology for photocatalytic applications. Herein, we report the first δ-Bi2O3 and Ni-doped δ-Bi2O3 microspheres prepared by a facile route. Particularly, their morphology can be well tailored by adjusting the content of Ni. Time-dependent experiment reveals that the Ni-doped δ-Bi2O3 microspheres are developed from a layered platelet precursor to subsequent growth of microspheres and consumption of precursor simultaneously. Theoretical calculation was carried out to investigate the effects of Ni-doping on the geometric and electronic structure of δ-Bi2O3. Moreover, the solar driven photocatalytic application for removal of NO in air is carried out. The as-prepared Ni-doped δ-Bi2O3 microspheres exhibit an enhanced photocatalytic activity due to Ni doping, the optimized micro and electronic structure.

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2.EXPERIMENTAL SECTION 2.1 Preparation of Ni-Bi2O3 microspheres. Ni-Bi2O3 microspheres were synthesized via a modified solvothermal route.21 In a typical process, Bi(NO3)3·5H2O (1 mmol) was dissolved in a solution that included ethylene glycol (8 mL) and ethanol (32 mL) with vigorous stirring till dissolution. Then, nickel powder (5 mg) was added into the above mixed solution, and immediately sealed in a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was maintained at 160 oC for 8 h. After cooling, the products were separated, washed by deionized water and ethanol several times, and dried in vacuum at 60 oC for 24 h. For comparison, a series of experiments were carried out by tuning the reaction time and the content of nickel powder. The solid samples were designated as undoped Bi2O3, Ni-Bi2O3-2, and Ni-Bi2O3-5, implying the mass of Ni powder added in the reaction of 0, 2, and 5 mg. 2.2 Computational Methods. All first-principle calculations are performed based on the plane-wave pseudopotential (PW-PP) approach using CASTEP code

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in the Material Studio 8.0 package. The electron-ion interactions are represented by ultrasoft pseudopotentials (USP),23 the electron-electron interactions are calculated by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional.24 The equilibrium geometries of Bi2O3 and Ni doped Bi2O3 were

obtained by performing geometry optimization

Broyden-Fletcher-Goldfarb-Shannon (BFGS) minimization method,

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using

the energy

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cutoff for the plane wave basis set was set as 500 eV, and the Brillouin zone was sampled at 6×6×6 Monkhorst-Pack k-points. The applied convergence criteria for geometry optimization were 2.0×10-5 eV/atom for energy, 0.001 eV/Å for force and 0.002 Å for displacement. Density of states (DOS), including the total density of state (TDOS) and partial density of states (PDOS), are evaluated by a simplified linear interpolation scheme26. Electronic eigenvalues on the same Monkhorst-Pack grid as in geometry optimization are calculated non-self-consistently for both valence and conduction bands, using electronic charge densities and potentials generated during the geometry optimization. First-principles constant-temperature molecular dynamics (MD) simulations using the Nose-Hoover method were performed to investigate the diffusion properties of NO in Bi2O3 and Ni doped Bi2O3. The Nose thermostat was set to keep the temperature at 298 K, time steps of 1 fs are used to solve the Nose-Hoover equations of motion. All simulations were run for unit cell with one NO molecule in the channel of Bi2O3 and Ni doped Bi2O3 with the canonical ensemble (NVT) over a period of 2000 time steps. Energy cutoff of 500 eV and 6×6×6 k-point mesh were used, and period boundary conditions were adopted. Diffusion was determined by linear fitting the slope of the mean squared displacement using the Einstein equation.27 2.3 Characterization.

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As-prepared materials were characterized via XRD, SEM, HRTEM, FT-IR spectra, UV−vis and BET methods. The details of these characterizations can be seen in SI-1. 2.4 Evaluation of photocatalytic activity. The photocatalytic activity was investigated by removing the NO at ppb levels in a continuous flow reactor at ambient temperature (SI-2).

3. RESULTS AND DISCUSSION 3.1 Structures and Morphologies. The XRD patterns of as-prepared pure Bi2O3 and Ni-Bi2O3 microspheres with broad and sluggish diffraction peaks (Figure 1a) reveal that the obtained products are composed of nanocrystallines. Four major reflections located at 27.8°, 32.3°, 46.3° and 54.6° correspond to the (111), (200), (220) and (311) planes of δ-Bi2O3 crystal with a cubic phase (JCPDS card no. 27-0052).28 No other impurity peak is detected, suggesting the high purity of the products. There is no difference in the position of (111) peak of the samples. However, the peaks indexing to (200) plane reveal a small shift to higher angle with the increasing of Ni content, indicating the diminution of interplanar spacing of Ni-doped Bi2O3. The results suggest that the Bi2O3 can be obtained via a simple solvothermal method with the existence of Ni. As exhibited clearly in Figure 2a, the undoped Bi2O3 samples are assembled by many nanosheets,21, 28 resembling into unite flowers. In such assembled flowers, the double-faces of nanosheets are well exposed and the pores are interconnected,

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resulting in enlarged active areas. The diameter of a single flower is around 1 µm and the thickness of the nanosheets ranges from 30 to 60 nm. However, with the increasing of the content of Ni powder to 2 mg, the surfaces of the obtained samples gradually become flat and several sheets aggregate together to form spheres randomly (Figure 2b). These spheres are hugging together to form a dividing stem cell with a diameter ranged from 300 to 1000 nm. Nevertheless, the surfaces of these cells are not exactly smooth. The surfaces made from a large amount of ultrathin interconnected nanosheets results in the formation a porous structure (inset of Figure 2b). With the Ni content increased to 5 mg, uniform Bi2O3 microspheres are obtained with average diameter of 350 nm. Similarly, the surfaces of these monodisperse microspheres are not close-grained. To the best of our knowledge, porous sections are often formed at the surfaces of spheres with dense inner structures. The porous section formed by ultrafine nanofibers exists in both inside and outside of the microspheres, which enlarges the specific surface area of the samples for an expected enhanced photocatalysis (SI, Figure S1). The status of Ni also plays an important role in modifying the morphology of the Bi2O3 microspheres. The morphology of the product obtained by using equidensity of Ni(NO3)2 has been exhibited in Figure S2. To further the effect of Ni element on the morphology of the samples, the mapping analysis under the FIB/SEM is shown in Figure 2d-2g. The specific structure confirms the as-prepared microspheres unambiguously. The distributions of Bi and O elements clearly reveal their

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positions, which indicate the existence of Bi2O3 combined with XRD pattern. Note that, the Ni element do not appear in the section highlighted in mappings indicating the Ni element only existed in the microspheres instead of pure Bi2O3 nanosheets, which further demonstrates the importance of Ni element on the morphology of the Bi2O3 microspheres. The XPS survey spectrum (Figure 3a) shows the existence of element Bi, Ni and O, together with C in the samples. The binding energy for the C (1s) peak (284.6 eV) is utilized as an internal reference.29 The Bi 4f peak is investigated by high-resolution XPS (Figure 3b). The peaks occurred at 158.7 and 164.1 eV agree well with the literature data, which can be assigned to Bi2O3.30, 31 Furthermore, The Ni 2p XPS spectra was fitted with four peaks at 855.58, 861.36, 873.39 and 879.38 eV, respectively. The peaks occurred at around 855.58 and 861.36 eV can be in accordance with the binding energies of Ni 2p3/2 and the peaks at 873.39 and 880.93 eV can be assigned to Ni 2p1/2, referring the existence of Ni-O bond.32, 33 Additionally, the high-resolution spectra for the O 1s region (Figure 3d) show four oxygen contributions, labeled as O1, O2, O3 and O4. The peak at 529.8 eV is characteristic of Bi/Ni-O binding energy.34, 35 The fitting peak of O2 at 530.6 eV is usually matched with oxygen in OH- groups and the existence of this contribution in the O 1s spectrum. The component O3 located at 531.4 eV corresponds to plenty of defect sites with low oxygen coordination, which can be observed usually in materials with small particles.36 Moreover, the component O4 at 532.33 eV can be

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assigned to a multiplicity of chemically and physically bonded water near and within the surface.37 The XPS data clearly demonstrates that the as-prepared Ni-Bi2O3 microspheres have a polybasic composition with Ni doping Bi2O3 structures. To demonstrate the homogeneous doping of Ni element, an etching XPS is curried out using argon ion as the graver (SI, Figure S3). After etching 600 s and 1200 s, the contents of Ni element are almost invariant, indicating the homogeneous distribution in Ni-Bi2O3 microspheres. The existence of Bi(0) and Ni(0) can be imputed to the reductive argon ion. To further reveal the formation process of the Ni-Bi2O3 microspheres, experiments with different reaction time were carried out and the as-prepared products are analyzed by FIB/SEM (Figure 4). After the Bi(NO3)3 is added in CH2OH-CH2OH, the pH value of the resulting mixture decreases due to the following reactions: CH2OH-CH2OH + Bi3+→CH2O-CH2O-Bi+ + 2H+

(1)

After the addition of Ni powder, the chemical reaction in the process to obtain Ni-Bi2O3 microspheres could be formulated as follows: Ni + 2H+→ Ni2+ + H2

(2)

A lot of large planks with smooth surfaces start to form in initial stage (Figure 4a). The planks are layer structure, detected by FIB/SEM and XRD pattern (SI, Figure S4). With the reaction time increasing, a large amount of nanoparticles adhere to the surface of the planks forming a sandwich structure (Figure 4b). Then, the

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nanoparticles start to growing on the surfaces of the planks with the diameter of the particles increases (Figure 4c and its inset). When the reaction time is prolonged to 6 h, some closely packed microspheres with rough surfaces form a plane which remain the shapes of the plank, thereby indicating the using out of the precursor (Figure 4d), following by the collective microspheres separated to form a single sphere and shrink their volume (Figure 4e). Finally, the uniform microspheres with smooth surfaces form (Figure 4f). Thus, a new possible formation process is schematically illustrated and displayed in Figure 5. First, the EG in solution adsorbs Bi3+ to form a relatively stable complex, Bi2(OCH2CH2O)3, because of its strong coordination with Bi3+ resulting in a acid solution. When the Ni powder is added, the Ni2+ can also be adsorbed by EG leading to the accrete cluster. Second, the planks-shape precursor obtained by means of the hydrolyzation of accrete cluster in solution. Then these precursors grow with the direction of 2 dimension, thus leading to the formation of planks with the excrescent Bi3+ forming the pure Bi2O3 nanoparticles. Third, the planks are not a stable product since the discrepant chemical properties of Bi and Ni resulting in the tendency of disassembly of the planks. As the mass diffusion and Ostwald ripening process proceed, thousands of packed Ni doped Bi2O3 microspheres with rough surfaces are obtained. Finally, the packed microspheres contractile wings to reduce their surface free energy. Then, the monodisperse Ni doped Bi2O3 microspheres are obtained.

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To further confirm the microstructure of Ni-Bi2O3 microspheres, the TEM results are investigated. As seen in Figure 6a, the Ni-Bi2O3 microspheres with a uniform size of about 400 nm are displayed. As shown clearly, the monodispersed sphere with a regulatory surface indicates the benign growth of Bi2O3 without any interference during the hydrothermal treatment. The unsmooth surface indicates the composition of loose substance, which agrees well with the FIB/SEM results (Figure 6b). Similar to other spherical materials, the inner structure cannot be differentiated due to the massy microspheres. However, the loose section obtained at the edges demonstrates that the spheres are consisted of thousands of holes formed by interconnected units, thus resulting in a mesopouous structure (Figure 6c). Additionally, the rings (Figure 6d) indicate the polycrystalline nature of Ni-Bi2O3 microspheres. The porous structure and enhanced specific surface area are beneficial to photocatalysis due to more active sites (SI, Figure S5). 3.2 Photocatalytic Activities. The UV–vis DRS spectra of the samples mentioned above can be seen in Figure 7a. The band gaps (Eg) of pure Bi2O3, Ni-Bi2O3-2 and Ni-Bi2O3-5 are estimated to be ca. 2.87, 2.58 and 2.49 eV, respectively. Here, Eg is derived from Eg= 1239.8 λg, where λg is the absorption edge in the UV–vis spectra.38 Compared with undoped Bi2O3, the absorption edges of Ni-doped Bi2O3 samples show a small red shift. The shift in absorption edge can also be reflected by the change in color of the samples from faint yellow to prasinous owning to the existence of Ni2+. Besides, the sample

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Ni-Bi2O3-5 displays an enhanced absorption in visible light region, which can be ascribed to the high content of Ni. As shown in Figure 7b, the band gap energies of Ni-Bi2O3-2 and Ni-Bi2O3-5 samples are estimated to be 2.58 and 2.49 eV, respectively, which are lower than that of undoped Bi2O3 (2.87 eV). The decrease in the band gap can be explained as the doped Ni in Bi2O3, which enables the samples with enhanced absorption in visible light region. Although the band gaps of Ni-doped Bi2O3 samples show unconspicuous changes, the absorption intensities in both ultraviolet light region and visible light region are increased after doping Ni. To reveal the migration, transfer and separation of photogenerated charge carriers reflecting the recombination of free charge carriers, the PL spectra of the pure Bi2O3, Ni-Bi2O3-2 and Ni-Bi2O3-5 samples with an excitation wavelength of 280 nm are exhibited in Figure 7c. Clearly, the fluorescence emission peaks are mainly centered at 325–425 nm and 450–500 nm, which represents UV and visible light region, respectively. Further observation implies that the peak intensity of Ni-Bi2O3-5 sample is the lowest in both UV and visible light region. This finding suggests that the recombination rate of electron–hole pairs on Ni-Bi2O3-5 is the lowest. Therefore, we can infer that Ni-Bi2O3-5 sample may exhibit the most efficient photocatalytic activity. The photocatalytic performances of all the samples are measured for removal of NO in air under simulated solar light from visible light up to near-infrared range (>800 nm) irradiation in a continuous reactor. As previous reported, only when a

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photocatalyst and illumination exist at the same time can NO be photolyzed. In the presence of the as-prepared samples, NO reacts with the photogenerated reactive radicals to produce HNO2 and HNO3.33,

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Figure 8a shows the curve of NO

concentration (C/C0%) with irradiation time of Ni-Bi2O3-5 sample. Here, C0 and C represent the concentration of NO at the time of 0 and t The Ni-Bi2O3-2 and Ni-Bi2O3-5 display enhanced photocatalytic performance compared with undoped Bi2O3, suggesting the importance the Ni element. Additionally, the sluggish plot indicates that the poor catalytic rate agrees well with the UV–vis and PL spectra. The Ni-Bi2O3-5 sample exhibits outstanding photocatalytic activity with a NO removal ratio of 52.2%, much higher than that of undoped Bi2O3 (31.1%), which can be attributed to the well-controlled morphology with mesopouous structure and the doping of Ni element. As the mesoporous materials with a large specific surface area could provide abundant active sites for photocatalysis. Additionally, the pores can be used as the transfer path for HNO2 and HNO3, which prevent the accumulation of the reaction product. Furthermore, Ni doping of Bi2O3 facilitates the absorption of visible light and charge separation. Thus, the photocatalytic performances of the Ni-doped Bi2O3 microspheres are significantly enhanced. To determine the stability of the Ni-Bi2O3-5 sample as solar-driven photocatalyst, we carried out repeated consecutive experiments (Figure 8b). It can be seen that the Ni-Bi2O3-5 sample shows a high NO removal ratio under repeated irradiation.

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In order to further reveal the mechanism for the photo-oxidation of NO over Ni-Bi2O3-5, the spin trapping ESR measurements are carried to determine the main active species both under visible-light irradiation and in the dark, utilizing DMPO as spin trapping chemicals (Figure. S6). Figure. S6 showed that both the DMPO-•OH and DMPO-•O2- signals are absent in the dark, while the signal intensity obviously enhanced as the irradiated time prolonged from 2, 4 to 6 mins. These results demonstrate that the •OH and •O2- radicals were the main active radicals of the Bi2O3-5 photocatalysts, which are responsible for oxidizing the NO pollutant. 3.3 First principle Studies. It is accepted that δ-Bi2O3 possesses a defective fluorite structure with two oxygen vacancies per unit cell. Walsh et al. reported three different configurations of δ-Bi2O3 with oxygen vacancies aligned respectively along with the , , and directions, and they found that the configuration in which oxygen vacancies aligning along was more stable than that of and configurations.40 Thus, δ-Bi2O3 with the configuration of oxygen vacancies was taken into account to investigate the effect of doped Ni on the geometric and electronic structure of the δ-Bi2O3 in the present paper. Our XPS experimental result indicated that about 20 atom% Ni presents in the Ni doped δ-Bi2O3 (Ni-Bi2O3-5 sample), indicating that about half of Bi atoms were substituted by Ni atoms. Based on this result, four possible Ni doped δ-Bi2O3 configurations (SI, Figure S8) were studied by using first-principle

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calculation at the GGA-PBE/USP level of theory, the calculated total energies show the structureⅠ is more stable than the structures Ⅱ, Ⅲ, Ⅳ by 1.01, 0.16 and 4.57 eV, respectively. The most stable configuration (structureⅠ) contains one penta- and one hexa- coordinated Bi atoms and two tetra- coordinated Ni atoms, and both the Bi and Ni atoms were surrounded by neighbor oxygen atoms. The calculation results also showed that the doped Ni atoms prefer to deviate from the face center site and tend to approach the coordinated oxygen atoms, which finally results in the formation of a distorted structure of the Ni doped δ-Bi2O3. Moreover, a large channel was formed in the doped δ-Bi2O3, and the cross-section of the distorted channel increase remarkably from 16.02 Å2 of the parent δ-Bi2O3 to 35.16 Å2 of the doped δ-Bi2O3 (Figure 9). The formation of large channel would increase the diffusion rate of the NO molecule inside the catalyst, and then play a positively effect on the interaction between NO molecule and the photogenerated active species formed in the catalyst. To confirm this, the self-diffusion coefficients of NO molecules inside the cell of parent δ-Bi2O3 and the doped δ-Bi2O3 were calculated from first-principles MD trajectory, the results show that the self-diffusion coefficient of NO molecule increases from 3.06×10-9 m2 s-1 in the parent δ-Bi2O3 to 2.01×10-8 m2 s-1 in the doped δ-Bi2O3 at 298 K, we believed that the increasing NO diffusion rate in the cell of the doped δ-Bi2O3 will increase the collision of the reaction substrate with the photogenerated active species and then enhance the efficiency of the catalysis process.

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In addition, the effect of doped Ni on the electronic structure of the parent δ-Bi2O3 are also studied to investigate the mechanism of enhanced catalytic performance of the Ni doped δ-Bi2O3. Figure 10 shows the total density of states (TDOS) and partial density of states (PDOS) of parent δ-Bi2O3 and Ni doped δ-Bi2O3. The most remarkable feature is the formation of pesudogap at the doped δ-Bi2O3 in visible light region. Moreover, the number of states at the Fermi energy level of the Ni doped δ-Bi2O3 is significantly larger than that of parent δ-Bi2O3, this means the Ni doped δ-Bi2O3 will possess a relatively high conductivity. The high conductivity of the doped δ-Bi2O3 will favor the separation of photogenerated electrons and photogenerated holes, and then promote the reaction between NO and the photogenerated active species. As can be seen from Figure 10, the difference of the number of states at the Fermi energy level of the Ni doped δ-Bi2O3 from that parent δ-Bi2O3 can be ascribed to the d-orbital contribution of the doped Ni atoms to the TDOS, and the resulted modification of electronic structure is responsible for the enhancement of photo-catalytic activity of the doped δ-Bi2O3.

4. CONCLUSIONS In summary, a novel mesopouous Ni-doped δ-Bi2O3 photocatalyst was prepared by a simple solvothermal reaction. The Ni ions played a key role in the growing process of the mesoporous microspheres leading to the merging of growth of microspheres and consumption of precursor simultaneously. Theoretical calculation

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implied that Ni-doping induced high conductivity and the formation of large channel in the crystal. The enhanced photocatalytic ability of the as-prepared samples in the photodegradation of NO could be attributed to the mesoporous structure and the modification of geometric and electronic structure resulted from the dopant of Ni, the enhanced transfer of photogenerated carriers and the increased photo-absorption. The present work has demonstrated a new approach for synthesizing doped mesporous nanostructures (e.g. Fe doped Bi2O3 and Zn doped Bi2O3).

ASSOCIATED CONTENT Supporting Information. SEM images, XRD pattern, Nitrogen adsorption and desorption isotherms, Etching XPS spectrum, DMPO spin-trapping ESR spectra of the materials. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Tel.: +86-023-65104131. E-mail: [email protected]; [email protected]; [email protected] .

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 21576034, 51421001, and 21671154), the Innovative Research Team of Chongqing (CXTDG201602014) and State Education Ministry and Fundamental Research Funds for the Central Universities (CDJZR12130038 and 106112016CDJZR135506). The authors also thank Electron Microscopy Center of Chongqing University for materials.

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(17) Qiu, Y.; Liu, D.; Yang, J.; Yang, S. Controlled Synthesis of Bismuth Oxide Nanowires by An Oxidative Metal Vapor Transport Deposition Tchnique. Adv. Mater. 2006, 18, 2604-2608. (18) Shang, M.; Wang, W.; Zhang, L.; Sun, S.; Wang, L.; Zhou, L. 3d Bi2WO6/TiO2 Hierarchical Heterostructure: Controllable Synthesis and Enhanced Visible Photocatalytic Degradation Performances. J. Phys. Chem. C 2009, 113, 14727-14731. (19) Wang, Y.; Wen, Y.; Ding, H.; Shan, Y. Improved Structural Stability of Titanium-Doped β-Bi2O3 During Visible-Light-Activated Photocatalytic Processes. J. Mater. Sci. 2010, 45, 1385-1392. (20) Krishna Reddy, J.; Srinivas, B.; Durga Kumari, V.; Subrahmanyam, M. Sm3+-Doped Bi2O3 Photocatalyst Prepared by Hydrothermal Synthesis. ChemCatChem 2009, 1, 492-496. (21) Wang, Y.; Li, S.; Xing, X.; Huang, F.; Shen, Y.; Xie, A.; Wang, X.; Zhang, J. Self-Assembled 3d Flowerlike

Hierarchical

Fe3O4@Bi2O3

Core-Shell

Architectures

and

Their

Enhanced

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List of Figures Figure 1. XRD pattern of as-prepared Ni-Bi2O3. Figure 2. FIB/SEM images of undoped Bi2O3 (a); Ni-Bi2O3-2 (b); Ni-Bi2O3-5 (c) and EDS mapping of Ni-Bi2O3-5(d-g).

Figure 3. Survey XPS spectrum of Ni-Bi2O3-5 (a) and of Bi 4f (b); Ni 2p (c) and O 1s (d) of Ni-Bi2O3-5.

high resolution XPS spectra

Figure 4. FIB/SEM images of the obtained Ni-Bi2O3-5 microspheres at various reaction stages: 1 h (a), 1.5 h (b), 2.5 h (c), 4 h (d), 6 h (e) and 8 h (f). Figure 5. Schematic illustration of the proposed formation mechanism of Ni-Bi2O3 microspheres. Figure 6. TEM images of Ni-Bi2O3-5. Figure 7. UV–vis DRS (a), the plot of (αhv)1/2 vs. photon energy (b) and PL spectra of Ni-Bi2O3-5 (c).

Figure 8. Photocatalytic performance of undoped Bi2O3, Ni-Bi2O3-2, Ni-Bi2O3-5 (a); multiple photocatalytic reaction over Ni-Bi2O3-5 (b). Figure 9. The GGA-PBE/USP optimized geometries and the cross-section of channel of Bi2O3 (a) and Ni doped Bi2O3 (b). Figure 10. The total density of states (TDOS) and partial density of states (PDOS) of Bi2O3 (a) and Ni doped Bi2O3 (b).

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Undoped Bi2O3

Undoped Bi2O3

(a)

Ni- Bi2O3-2

(b)

Ni- Bi2O3-2 Ni- Bi2O3-5

JCPDS no. 27-0052

JCPDS no. 27-0052

Intensity (a.u.)

Ni- Bi2O3-5

Intensity (a.u.)

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10

20

30

40

50

60

70

26

80

28

30 2 theta (degree)

2 theta (degree)

Figure 1. XRD pattern of as-prepared Ni-Bi2O3.

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Figure 2. FIB/SEM images of undoped Bi2O3 (a); Ni-Bi2O3-2 (b); Ni-Bi2O3-5 (c) and EDS mapping of Ni-Bi2O3-5(d-g).

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Bi 4f7/2

Intensity (a.u.)

Bi 4f Bi 5d

1000

800

600

400

200

0

164.1 eV

168

166

Binding energy (eV)

164

162

160

158

156

Binding energy (eV)

(c)

Ni 2p3/2

Ni 2p1/2

(b)

158.8 eV

Bi 4f5/2

C 1s

O 1s

Bi 4d

Ni LMM Bi 4p

Ni 2p

Intensity (a.u.)

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(a)

(d)

O 1s

855.58 eV

Intensity (a.u.)

879.38 eV 873.39 eV Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O KLL


861.36 eV

885

880

875

870

865

860

855

850

O3

O2

O4

536

534

O1

532

530

528

526

Binding energy (eV)

Binding energy (eV)

Figure 3. Survey XPS spectrum of Ni-Bi2O3-5 (a) and of Bi 4f (b); Ni 2p (c) and O 1s (d) of Ni-Bi2O3-5.

high resolution XPS spectra

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Figure 4. FIB/SEM images of the obtained Ni-Bi2O3-5 microspheres at various reaction stages: 1 h (a), 1.5 h (b), 2.5 h (c), 4 h (d), 6 h (e) and 8 h (f).

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Figure 5. Schematic illustration of the proposed formation mechanism of Ni-Bi2O3 microspheres.

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Figure 6. TEM images of Ni-Bi2O3-5.

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3.5

(a)

Ni-Bi2O3-2

(αhν)1/2

Ni-Bi2O3-5

Undoped Bi2O3

(b)

Undoped Bi2O3

3.0

Ni-Bi2O3-2

Ni-Bi2O3-2

2.5

Ni-Bi2O3-5

Ni-Bi2O3-5

(c)

Intensity (a.u.)

Undoped Bi2O3

Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0 1.5 1.0 0.5 0.0

200

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400

500

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600

700

2

3

4

5

6

300

band gap (eV)

350

400

450

500

Wavelength (nm)

Figure 7. UV–vis DRS (a), the plot of (αhv)1/2 vs. photon energy (b) and PL spectra of Ni-Bi2O3-5 (c).

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Undoped Bi2O3

100

(a)

100

Ni-Bi2O3-2

90

First run

90

Ni-Bi2O3-5

80

C/C0 (%)

C/C0 (%)

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70

Second Third run run

Forth run

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(b)

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0

5

10

15

20

25

30

Irradiation time (min)

0

20

60

80

120

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Irradiation time (min)

Figure 8. Photocatalytic performance of undoped Bi2O3, Ni-Bi2O3-2, Ni-Bi2O3-5 (a); multiple photocatalytic reaction over Ni-Bi2O3-5 (b).

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. The GGA-PBE/USP optimized geometries and the cross-section of channel of Bi2O3 (a) and Ni doped Bi2O3 (b).

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Figure 10. The total density of states (TDOS) and partial density of states (PDOS) of Bi2O3 (a) and Ni doped Bi2O3 (b).

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TOC Graphic

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Mesoporous Ni-Doped Î´-Bi2O3 Microspheres for Enhanced Solar

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