Layered Perovskite Pb2Bi4Ti5O18 for Excellent Visible Light-Driven

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Layered Perovskite Pb2Bi4Ti5O18 for Excellent Visible Light-Driven Photocatalytic NO Removal Reshalaiti Hailili, Guohui Dong, Yichi Ma, Si Jin, Chuanyi Wang, and Tao Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04706 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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Industrial & Engineering Chemistry Research

Layered Perovskite Pb2Bi4Ti5O18 for Excellent Visible Light-Driven Photocatalytic NO Removal Reshalaiti·Haililiab, Guohui Donga, Yichi Mac, Si Jina, Chuanyi Wanga*, Tao Xuc* a

Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics &

Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China b

The Graduate School of Chinese Academy of Science, Beijing 100049, China

c

Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb Illinois 60115,

United States

1

ACS Paragon Plus Environment


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ABSTRACT: A grand challenge in photocatalysis is to seek chemically stable photcatalysts with minimized recombination of photoinduced charges in order to maximize the efficiency of the subsequent redox reactions. Due to the structural noncentrosymmetry, TiO2-based and lead-containing perovskite-type photocatalysts exhibit excellence in both stability and ferroelectricity to facilitate charge separation state. To extend their visible light activity, we use molten salts synthesis methodology to prepare Pb2Bi4Ti5O18 perovskites with various nanoscale morphologies and evaluated their visible light-driven photocatalytic activity for NO removal. The results show that perovskite Pb2Bi4Ti5O18 samples exhibit excellent stability as well as display over 50% NO removal efficiency under visible light, in contrast to no more than 15% for commercial P25. A set of radical scavengers were used to probe the reaction mechanism. A bond-valence method is used to calculate the direction and magnitude of the dipole moments of the asymmetric unit in the structure. We revealed that the photocatalytic Pb2Bi4Ti5O18 possesses distorted units, in which the dipole-induced internal fields promote the charge separation, leading to the enhanced photocatalytic activity. Since this novel photocatalytic NO removal is a solid-gas reaction, in which the lead content is secured in solid phase, this work finds a suitable application for Pb-containing perovskite photocatalysts with great industrial interests and add a new fundamental coordination for better photocatalysts-by-designing. Keywords: Perovskite; Photocatalytic; NO Removal; Visible Light

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1. INTRODUCTION Pioneered with the first milestone work of photolysis of water on TiO2 electrodes, semiconductor-based photocatalysts have attracted enormous research effort due to their splendid energy and environmental applications such as photoelectrochemical water splitting, photocatalytic degradation of organic pollutants, industrialized self-cleaning coating technology and so on.1-3 Yet despite the aggressive progress in developing new photocatalysts, TiO2 still champions among its peers as the most stable and environmentally benign candidate. However, pristine TiO2 also suffers from its low yield of the photocatalytic reactions owing to the fast recombination of photon-induced carriers and the limited absorption in solar spectrum, which consists of 5% UV (300-400 nm), 43% visible (400-700 nm), and 52% infrared (700-2500 nm).4-9 Therefore, TiO2-based photocatalysts that are spectrally active in visible region and possesses crystal structures capable of mitigating the recombination of photoinduced electron-hole pairs will be exceptional desired. Thus, it is critical to understand the interplay between structure and photoelectronic properties in TiO2-based photocatalysts so as to promote industrial level applications. Studies show that the highly symmetric surface octahedral Ti-O configuration in TiO2 is less photoactive than the low symmetric and opened tetrahedral Ti4+ sites in bulk TiO2, because the high symmetry impedes the photoinduced carrier separation and their subsequent transfer.10 In fact, electron paramagnetic resonance (EPR) spectroscopy revealed the existence of highly distorted tetrahedral Ti4+ sites in Degussa P25, which attributes to the observed two-folds enhancement in photocatalytic degradation of phenol and methyl blue under UV over the anatase with octahedral Ti4+.10 Furthermore, Raman spectroscopy and plane-wave density function theory (DFT) calculation reveal that the actual photoactive sites in metal oxide are those distorted MO6 octahedra with a dipole moment.11 Thus, the critical clue we can gain herein is that the dipole moment, namely, a built-in internal electric fields can promote the charge separation.12 Quite coincidentally, many lead-containing perovskite-structured semiconductors that have recently attracted tremendous attentions from researchers in various fields including photovoltaics, photocatalysis, ferroelectricity, piezoelectricity, and second order nonlinear optics, share a similar key property as the photoactive sites in TiO2, namely the 3



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noncentrosymmetric (NCS) structure.13-16 For example, the asymmetric polar cationic organic rotors plays a decisive role in stabilizing the charge separation state i.e. long carrier lifetime in organic-inorganic lead halide hybrid perovskite photovoltaics;17-19 the ns2 lone pair electrons in Pb2+, or Bi3+-containing perovskite oxides exhibits high photoactivity because the high polarizability of the polyhedra units provides an internal ferroelectric field that remarkably facilitates the photoinduced charge separation.20-22 For instance, Aurivillius-phase perovskite PbBi2Nb2O9 as an efficient photocatalyst for water splitting, and photodegradation of isopropanol to CO2 under visible light irradiation.23,24 PbTiO3, PbBi4Ti4O15, PbZrO3, etc, have been studied for dye degradation and water splitting under visible light irradiation.25-32 However, the use of Pb-containing materials in these wet chemistry applications inevitably raises environmental concerns. Therefore, it would, practically, be more attractive to seek an application of great industrial interest that can utilize these highly photoactive Pb-based perovskite catalysts in an unleakable setting. NOx is recognized as one of the major gaseous pollutant released from modern industry and exhibits serious harmful effect on environment and health-related issues.33,34 Since NOx is miscible with air, the removal of NOx becomes a formidable challenge in industry. For all these regards, we aim to combine the high stability of TiO2-based photocatalysts with the strong internal field induced by Pb-based perovskite structures and the visible light activity brought by Bi, and develop Pb2Bi4Ti5O18 as a novel photocatalyst for NO removal for the following reasons: (1) NO removal is a solid-gas reaction, in which the Pb components are immobilized in solid phase so as to avoid any secondary contamination; (2) Bi in the structure can extend the absorption band edge to the visible region because of the formation of a hybridized valence band comprising of Bi 6s and O 2p states; (3) Pb and Bi have similar inert pair stereochemically active 6s2 lone pair electronic configuration of the Pb2+ and Bi3+ cations on the A site and these units exhibit high polarization that can lead to internal field variation, which is beneficial for effective photogenerated charge separation; (4) Pb2Bi4Ti5O18 forms a unique layered structure (Figure 1), consisting of [Bi2O2]2+ chains, which acts as fast charge transport pathways.

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[Bi2O2]2+

[Pb Pb 2Bi2Ti5O16]2-

Pb Bi Ti O

Figure 1. Crystal structure of Pb2Bi4Ti5O18 with amplified [Pb2Bi2Ti5O16]2- and [Bi2O2]2+ units. Moreover, since morphology and surface area also exhibit strong influence on the photocatalytic activity, we adopt molten salt synthesis (MSS) method to obtain Pb2Bi4Ti5O18 and tailor the morphologies by alternating the compositions and molar ratio of the salts, the reaction time and temperatures, as well as other synthetic parameters.35,36 Indeed, photocatalytic NO removal requires several photoinduced electron transfer steps as oxidation of NO to NO2 and further to NO3-. To the best of our knowledge, photocatalytic NO removal over Pb-based perovskite has not been reported yet.

2. EXPERIMENTAL SECTION 2.1 Synthesis of Pb2Bi4Ti5O18. All the reagents are of analytical grade and were purchased from Aldrich without further purification. The samples were synthesized by MSS method. In a typical synthesis, the stoichiometric mixture of PbO, Bi2O3, and TiO2 were added to the different salts system in the mole ratio of Pb2Bi4Ti5O18: M: M = 1:8:8, which were ball-milled with small amount of ethanol for 2 hours. Then, the dried mixture was heated in the corundum crucible at 900 oC for 15 h, and then cooled to room temperature naturally. The as-obtained samples were washed with water to remove the chlorides, which were further examined by AgNO3 solution. Then, the samples were dried at 60 oC for 12 h for other general characterizations. 5



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2.2 General Remarks X-ray Powder Diffraction. X-ray powder diffraction analysis of Pb2Bi4Ti5O18 was performed at room temperature in the angular range of 2 θ = 5° ~ 80° with a scan step width of 0.02° and a fixed counting time of 1 s/step using an automated Bruker D8 ADVANCE X-ray diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.5418 Å). Inductively Coupled Plasma Emission Spectrometry (ICP) Analysis. The amounts of the metals were determined by inductively coupled plasma optical emission spectrometry (ICP; VISTA-PRO). Before the measurement, the sample was dissolved in HNO3 and undergoes hydrothermally treatment at 180 oC for 3 h. UV-visible Diffuse Reflectance Spectra (UV-vis DRS). Optical diffuse reflectance spectral

measurements

were

conducted

at

room

temperature

with

Shimadzu

SolidSpec-3700DUV spectrophotometer. BaSO4 powder used as 100% reflectance reference and data were collected in the wavelength range 200-800 nm, and reflectance was converted to absorbance with the Kubelka-Munk function.37,38 BET Measurement. The Brunauer-Emmett-Teller (BET) surface areas of the samples were determined from the N2 adsorption-desorption isotherms recorded at 77 K on a Quantachrome Instrument (QUADRASORB IQ). Vibrational Spectroscopy. Infrared spectra were recorded on Bruker Optics TENSOR 27 Fourier transform infrared spectrometer in the range from 400 to 4000 cm-1 with resolution of 2 cm-1. Photocatalytic Activity Evaluation. Photocatalytic activities of as-obtained samples were evaluated by NO removal (500 ppb level) at the ambient conditions in the successive flow reactor in a cylindrical reactor. The reactor was irradiated with a 300 W Xe lamp (PLS-SXE300, Perfect Light, Beijing, China), which is equipped with a cut-off filter (λ ≥ 420 nm). In a typical procedure, 50 mg of catalyst was dispersed in water, then the aqueous suspension was deposited onto the glass dish with a diameter of 5.4 cm, followed by 20 min ultrasonication, and then dried at 60 oC. After that, glass dish was placed in the middle of the reactor. The NO gas was obtained from a compressed gas cylinder from National Institute of Standards and Technology specifications. The initial concentration of NO was diluted to 6


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about 500 ppb by the air stream supplied by a zero air generator. The gas streams were completely premixed by a gas blender, and the flow rate was at 1 L/min by a mass flow controller. For all experiments, light irradiation was started after the adsorption-desorption balance. The effluent NO and NOx (NO + NO2) concentrations were continuously measured using a chemiluminescence detector. The specific concentration was collected on NO analyzer (Teledyne, NOx analyzer, Model 42i), and the NO2 concentration was determined by the concentration difference between NOx and NO. The NO removal percentage was obtained by following equation:39,40 NO removal (%) =

Co - Ct Co

where Co is initial NO in ppb and Ct is varied NO at given irradiation time t in ppb level. Catalyst Stability and Photocatalytic Cycle Test. To examine the stability and efficiency of the catalyst for NO removal reaction, the whole process was investigated for five successive cycles with the same catalyst exactly under the same reaction conditions. 50 mg of catalyst was dispersed in water and then dried at 60 oC for recycle activity test. After the fifth cycle, the photocatalyst was collected and dried at 60 oC for PXRD measurement to confirm the phase stability. Detection of Active Species. Hydroxyl radicals ( ⋅ OH), superoxide radical anions ( ⋅ O2-) and electron/holes (e-/h+) are the main active species involved in the photocatalytic processes. To determine and reveal the role of these intermediates and reaction mechanism, a series of scavenger test experiments were conducted. Herein, isopropyl alcohol (IPA), potassium iodide (KI), P-benzoquinone (PBQ) and AgNO3 were used as ⋅ OH-, h+, ⋅ O2- and escavenger, respectively. The Calculation Method of Dipole Moment. For the dipole moment calculations, the Debye equation, µ = neR (µ is the net dipole moment in Debye (10-18 esu cm), n is representative total electron numbers, e is the charge on an electron, -4.8 × 10-10 esu, and R is the difference in cm (between the “centroids” of positive and negative charge)) was used to calculate the dipole moment.41,42 Distribution of the electrons on the Pb/Bi/O atoms was estimated using the bond valence theory (Si = exp[(Ro - Ri)/B]; where Ro is an empirical constant, Ri is the length of the bond “i” in Å, and B = 0.37). 7



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3. RESULTS AND DISCUSSION Morphology and BET Analysis of Samples. Figure 2 shows typical morphologies of the as-prepared samples via MSS, in which the composition of salt can be respectively selected, and mixture of the NaCl, KCl, Na2SO4 and NaH2PO4 were applied as a flux agent. As clearly illustrated in Figure 2, the morphologies of the samples vary significantly by altering the nature of the salts. Figure 2a exhibits nanospherical morphology, denoted as sample 1, which was obtained with the assistance of flux agent NaCl-NaH2PO4. The magnified image in the inset indicates that the spherical morphology is composed of numerous nanoplates. However, Figure 2b shows the sample obtained in NaCl-KCl solution, denoted as sample 2, which consists of very uniform and small nanoparticles with size range of 20-40 nm (see the Inset image). Sample 3 was obtained in NaCl-Na2SO4, and exhibits rectangular shaped morphology (Figure 2c). Sample 4 appears as thin nanosheets as depicted in Figure 2d, which was obtained in molten KCl-Na2SO4 system. These results suggests that the morphology of Pb2Bi4Ti5O18 can be readily tuned by altering the MSS systems. The formation mechanism of the diverse morphology is rather complicated and during the MSS, the first step is the ionization of the salts solvent at its molten point, forming ions that are responsible for the various morphologies. For instance, during the synthesis of sample 1 via MSS, it acquires relatively high surface energy for Cl- and H2PO4- ions to absorb on the edges, corners or specific index surface of the Pb2Bi4Ti5O18 crystals, leading to the formation of nanoplates morphology. The

specific surface areas of the

samples were measured with nitrogen

adsorption-desorption method and Figure 2e, indicates the specific surface area of sample 1 = 41.10 m2/g, sample 2 = 6.153 m2/g, sample 3 = 4.879 m2/g and sample 4 = 0.071 m2/g.

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(b)

(a)

1 µm

1 µm (c)

(d)

1 µm

1 µm

(e) 100

3

Adsorbed Volume (cm /g)

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80 60 40

Sample 1 Sample 2 Sample 3 Sample 4

20 0

0.0

0.2 0.4 0.6 0.8 Relative Pressure (P/Po)

1.0

Figure 2. (a) Nanospherical Pb2Bi4Ti5O18 (sample 1) obtained in molten NaCl-NaH2PO4 solution. The inset is the magnified image showing that the spherical morphology is composed of numerous nanoplates; (b) Nanoparticle Pb2Bi4Ti5O18 (sample 2) obtained in molten NaCl-KCl solution. The Inset is the magnified image showing detailed small nanoparticles; (c) Rectangular Pb2Bi4Ti5O18 (sample 3) obtained in molten NaCl-Na2SO4 solution. The Inset is the magnified image showing detailed rectangles; (d) Nanosheet Pb2Bi4Ti5O18 (sample 4), obtained in molten KCl-Na2SO4 solution. The inset is the magnified image showing detailed nanosheet morphology; (e) BET analysis of sample 1-sample 4. Phase Identification. As shown in Figure 3a, all the peaks of the samples can be indexed to the standard phase of Pb2Bi4Ti5O18 (Orthorhombic, Aba2 (41) space group, lattice constant a = 5.4700 Å b = 5.4575 Å c = 49.6434 Å). No peaks of impurities are presented, indicating the high phase purity and high crystallinity of the as-prepared Pb2Bi4Ti5O18 samples. Moreover, energy dispersive x-ray (EDX) analysis on sample 1 as shown in Figure 9



3b confirms that only Pb, Bi, O and Ti are presented. To obtain the exact element composition in samples, the ICP analysis was carried out over sample 1, and the corresponding atomic ratio of Pb, Bi and Ti was determined to be 1.98, 3.96, and 4.86, respectively, thus, the

Sample 4

Bi

Counts (a.u.)

(1,3,13)

(2,0,0) (1,0,23) (1,1,20)

(b) (1,1,12) (0,0,22)

(0,0,10)

(a)

(0,0,12) (1,0,1) (1,0,3) (1,0,9) (1,0,11) (1,1,0)

chemical formula of the sample 1 agrees well with Pb2Bi4Ti5O18.

Intensity (a.u.)

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Sample 3

Sample 2

O

Ti Ti

Bi

Sample 1

10

20

30 40 50 60 o 2 Theta degree ( )

70

80

0

5

Pb

10 15 Energy (keV)

20

Figure 3. (a) XRD patterns of the as-prepared Pb2Bi4Ti5O18 samples; (b) EDX of sample 1. Optical Properties. The optical absorption of the samples were analyzed by UV-vis diffuse reflectance spectroscopy. As shown in Figure 4, the band gap of samples can be calculated by the formula: αhν = A (hν-Eg)1/2 (where α, h, ν, A, and Eg are absorption coefficient, Planck’s constant, light frequency, a constant, and band gap energy, respectively),37,38 and estimated to be 2.83, 2.95, 3.02 and 3.03 eV, for sample 1 ~ 4, respectively. Compared to the bandgap of prestine TiO2, these narrow bandgaps of our samples suggest the extension of their absorption in visible region. Previous first-principle calculation on the electronic band structures of Pb-based compounds by density functional (DFT) theory demonstrates that, in the lead free compounds, such as CaBi4Ti4O15 and Sr3Ti2O7, the conduction and valence bands are mainly composed of Ti 3d and occupied O 2p orbital, respectively.24,25 However, in the presence of Bi and Pb, the newly formed hybridized Pb 6s and the occupied O 2p would push up the position of valence band in Pb2Bi4Ti5O18, display smaller band gap than other Pb-free contained compounds like CaBi4Ti4O15 (3.36 eV). The 6s orbital of Bi also form new hybridized orbital comprised of Bi 6s and O 2p, which located below the top of valence band. The same observations were found in PbBiNb5O15, the hybridized orbital composed of O 2p and Pb 6s, which leads to narrower band gap.25 10


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(a) 4

(b) 4 Kubella-Munk (a.u.)

Kubella-Munk (a.u.)

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3 2 Sample 1 Sample 2 Sample 3 Sample 4

1 0

300

400 500 600 700 Wavelength (nm)

800

3 2 1 0 1.5

Sample 1 Sample 2 Sample 3 Sample 4

Eg=2.83 eV Eg=2.95 eV Eg=3.02 eV Eg=3.03 eV

2.0 2.5 3.0 3.5 Photon energy (eV)

Figure 4. (a) Room-temperature UV-vis absorption spectra of as-prepared Pb2Bi4Ti5O18 samples; (b) the plot of (ahv)2 versus photon energy hv. As DRS depicted, the band gap of Pb2Bi4Ti5O18 is smaller compared to its lead-free counterparts. Importantly, the hybridized orbital of O 2p and Pb 6s lift the valence band, thus extends the absorption to visible light region and consequently promote visible light photoactivity of the Pb2Bi4Ti5O18 sample. Table 1 summarizes the detailed synthetic conditions, band gaps, BET surface areas and the morphologies of the samples. Table 1. Synthesis condition, morphology, band gap and BET dates of Pb2Bi4Ti5O18 samples Sample No.

Salt Composition

Reaction Conditions (oC, h)

Morphology

Band Gap (eV)

BET (m2/g)

1

NaCl-NaH2PO4

900, 15

nanosphere

2.83

41.1

2

NaCl-KCl

900, 15

nanoparticle

2.95

6.15

3

NaCl-Na2SO4

900, 15

rectangular

3.02

4.88

4

KCl-Na2SO4

900, 15

nanosheet

3.03

0.07

Photocatalytic Activities. The photocatalytic performances of as-obtained samples were evaluated by photo-conversion of NO with visible light (λ > 420 nm) irradiation under room temperature. As shown in Figure 5a, a series of blank experiments and comparison experiments were conducted and the commercially available P25 used as the reference sample. Without light and photocatalyst, there is no obvious concentration change observed over samples, indicating that both of the light and photocatalyst are essential during whole process. All synthesized samples display better photocatalytic activities for NO removal than 11



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P25, among which sample 1 shows the highest efficiency. After 25 min visible light irradiation, the NO removal percentage follows this sequences: sample 1 (50.30%) > sample 2 (44.81%) > sample 3 (18.95%) > sample 4 (15.84%) > P25 (11.63%). Sample 1 displays high efficiency for NO removal, as evidence by the least post-treatment NO2 concentration (6 ppb), in comparison to the 13 ppb for P25, as shown in Figure 5b. The stability test was also carried out over sample 1 as shown in Figure 5c for five successive experiments. After fifth runs, the efficiency only underwent slight deterioration, which is due to the occupied surface sites by NO3- formed during the reaction. The stability of sample 1 can be evidenced by the nearly unchanged XRD patterns before the test and after the fifth cycle as shown in Figure 5d. From the view point of practical application of Pb-containing compound, the stability should be seriously considered. To clarify and confirm whether the Pb leaching from the perovskite after fifth run measurements, we applied ICP analysis for the accurate compositions of synthesized samples before and after the reaction. It was observed from the ICP analysis that, the ratio of Pb, Bi and Ti in the sample 1 was determined to be 1.98 : 3.96 : 4.86, which were compared with the element composition after fifth run test with the accurate ratio of Pb : Bi : Ti = 1.97 : 3.94 : 4.85. On the basis of successful fifth run tests, the nearly unchanged XRD and composition of sample 1, it can be concluded that the sample was stable, no apparent exhaustion and photocorrosion occurring during photocatalytic process. More importantly, the lead did not undergo leaching from the structure. Attempts have been devoted to enhanced NOx removal efficiency and stability by visible light driven photocatalysts, such as BiOI, PI-g-C3N4, Bi2MoO6, BiOBr, Ag-SrTiO3, plasmonic Bi/ZnWO4 and Bi2Sn2O7, etc,40,43-47 and among which the best reported removal efficiency was achieved by PI-g-C3N4 and BiOI with approximately 47% and 60% removal efficiency, respectively. Although, g-C3N4 displays high efficiency towards NO removal, it produces more toxic gas NO2 and suffers from fast recombination of photogenerated carries. On the other hand, Ti-based perovskite photocatalyst, Ag-SrTiO3 produced less toxic NO2 molecules (6.3%) at a mixture gas with concentration of 400 ppb, however, it exhibits only 30% efficiency in photocatalytic NOx removal.45 Hence, compared to the previous work that have to use precious metal such as silver or unstable materials (sublimation of iodine content), or producing more toxic NO2 (in the case of g-C3N4), our Pb2Bi4Ti5O18 simultaneously 12


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exhibits high NO removal efficiency (~ 50%) and stability without using non-precious element. More importantly, the unique layered structure of our Pb2Bi4Ti5O18 acts as fast charge transport pathways to promote efficient production and separation of photoinduced carriers. Therefore, we think the Pb2Bi4Ti5O18 with nanospherical morphology should be very promising for practical application. (a) 100

(b) 18

80

Sample 1 Nanosphere Sample 2 Nanoparticle Sample 3 Rectangular Sample 4 Nanosheet P25 No Irradiation No Catalyst

70 60

CNO2 (ppb)

C/Co (%)

90

50 5

10 15 Time (min)

20

1 le mp a S

2 le mp a S

3 le mp Sa

le mp Sa

4

5 P2

(d) 1

2

3

4

5

80 70 60 0

6

Intensity (a.u.)

90

12

0

25

(c) 100

C/Co (%)

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


After

Before

20

40 60 80 Time (min)

100

10

20

30 40 50 60 o 2 Theta degree ( )

70

80

Figure 5. (a) Photocatalytic experiments of NO oxidation over Pb2Bi4Ti5O18 samples with various nanoscale morphologies under visible light irradiation; (b) NO2 generation over Pb2Bi4Ti5O18 and P25 samples; (c) The stability test of NO removal over sample 1; (d) XRD patterns of sample 1 before and after reaction. Mechanism and Structure-Property Relationship Analysis. To confirm which kinds of active species contributes to the observed photocatalytic removal of NO, a series of trapping experiments were carried out using various scavengers. Explicitly, isopropyl alcohol (IPA), potassium iodide (KI), P-benzoquinone (PBQ) and AgNO3 were used as trapping agents for ⋅ OH, h+, e- and ⋅ O2-, respectively. Figure 6a shows that upon the presence of AgNO3, the NO removal efficiency was greatly attenuated. Hence, the 13



photoinduced electrons are an active species. With the presence of KI, the reaction was also suppressed, thus, the photogenerated holes is also an active species. However, when IPA is added in the system as ⋅ OH trapper, the same reaction trends were obtained, implying ⋅ OH has very limited effect on NO removal reaction. Based on the above experimental findings, it is reasonable to deduce that the removal of NO is through its reaction with ⋅ O2- radicals which is formed by the reaction of surface adsorbed O2 with photoinduced electrons. To further confirm the role of superoxide radicals, PBQ is added in the reaction system as an ⋅ O2- scavenger. Indeed, the NO removal efficiency is drastically suppressed with addition of PBQ, indicating that ⋅ O2- is a contributing intermediate in the NO removal reaction. To identify the final products, infrared spectroscopic study was conducted before and after reactions as shown in Figure 6b. It can be seen from IR results that the peak at around 1387.2 cm-1 was formed during the reaction, which was not observed before, and this characteristic peak can be ascribed to the stretching vibration of NO3- groups.39,40

(a) 100

(b)

60 50 5

10 15 Time (min)

20

25

40 20 0 4000

845.1

60

Before Reaction After Reaction After Recycle After Trapping

537.7

AgNO3 IPA KI PBQ Sample 1 Nanosphere

70

80 1387.2

80

Transmittance (a.u.)

100 90 C/Co (%)

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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3000 2000 1000 -1 Wavenumber (cm )

Figure 6. (a) Photocatalytic trapping experiments of sample 1 over NO oxidation under visible light irradiation; (b) The IR spectra of sample 1 during reactions.

When the light with an energy hv larger or equal the band gap energy Eg of Pb2Bi4Ti5O18, the electron excited from its valence band to conduction band and leave same amount of holes to the form electron-hole pairs. These photogenerated electron-hole pairs migrate to the surface of Pb2Bi4Ti5O18 photocatalyst and react with electron donors and acceptors as they 14


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have strong reduction and oxidation ability. The first step of the NO removal process is the efficient adsorption of NO on the surface of photocatalyst to reach the adsorption-desorption equilibrium. When the as-obtained samples are irradiated by visible light, the surface adsorbed oxygen is activated (O*). Meanwhile, with the strong oxidation capability, the photoinduced holes, oxidize NO to into nitrite and or nitrate (NO*). The photoinduced electrons reduce surface-adsorbed oxygen to generate superoxide radicals, and these superoxide anions can oxidize NO, which further can react with O* to form NO2 and NO3-. Based on the above discussion, the NO removal process over Pb2Bi4Ti5O18 can be classified in to step reaction: (i) direct oxidization of NO into nitrite and or nitrate with the role of photoinduced holes and electrons; (ii) to generated NO3- from further oxidation of NO2 with the assistance of super oxide radicals. However, it can be seen from the photocatalytic activity tests that the formation of NO2 is not predominant, thus, the direct oxidation of NO to NO3- is the main step of NO removal in the presence of Pb2Bi4Ti5O18 samples. To this end, we can summarize the reaction mechanism by the following equations: hv Pb2Bi4Ti5O18 → Pb2Bi4Ti5O18 + h+ + e-

2 O- + h+ → 2O* NO + h+ → NO* O2 + e- → ⋅ O2NO* + ⋅ O2- → NO32NO + 2O* → 2NO2 H2O + NO2 + h+ → NO3-+2H+

15



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Ti1O6

Pb/Bi1O5

Ti2O5

Bi2O4

Ti3O5

Pb/Bi3O4

Figure 7. Distorted units and directions of dipole moments of Pb2Bi4Ti5O18. To provide the insights on the structure-property relationship of Pb2Bi4Ti5O18, the nature of its geometric structure has been analyzed. The direction and magnitude of the distortion in Pb/BiOn, TiOn polyhedron and Bi2On have been determined by the local dipole moment calculation. The well-known Debye equation, µ = neR, is used to calculate the dipole moment.41,42 This method has been previously used to understand the distortion of Bi3O15 polyhedra and Nb1O6 octahedra in PbBiNb5O15 with -10.02/6.42/0 Debye (D) and 0/-2.44/-0.27 D along x/y/z direction and can well explain the experimental results.17 For charge on electrons, a bond-valence method was used to calculate the direction and magnitude of the dipole moments of the asymmetric units. The bond-valence (VBS) for elements was calculated to be Bi(1)/Pb (2.581), Bi2 (3.0180), Bi(3)/Pb (2.4810) Ti1 (4.1084), Ti2 (3.9959) and Ti3 (4.003), which are all very close to actual valence and within reasonable range. Here, for calculation clarity, the units such as Ti(1)O6, Ti(2)O5, Ti(3)O5, Bi(1)/Pb, Bi(3)/Pb and Bi(2)On in the cell of Pb2Bi4Ti5O18 are concerned. However, in the opposite direction in lattice, the dipole moments in the [Bi2O2]2+ units can be canceled out as shown in Figure 7. As a comparison, the 6-coordinated octahedral TiO6 octahedron of the anatase TiO2 (ICSD No.158779, tetragonal, I41/amd (141) space group) and TiO6 octahedron of rutile TiO2 (ICSD No. 51936, tetragonal, P42/mnm (136) space group) were calculated. Although the local dipole moment of distorted TiO6 octahedron of the anatase TiO2 is 7.860 D, the local 16


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dipole moment of the perfect TiO6 octahedron in the rutile TiO2 is zero. The polarizations of selected units in the Pb2Bi4Ti5O18 and the contributions from Pb/BiOn units and TiOn groups to the total polarization were shown in Table 2 and Figure 7. The local dipole moment of distorted Ti2O5, Pb/Bi1On and Pb/Bi3On were calculated as 9.273 D, 22.459 D, and 23.397 D. The analysis indicates that the local dipole moment of [TiOn] is higher than those of Ti1O6 units (anatase, 7.860 D). The magnitudes of the corner and face distortions are roughly equal. This result suggests that TiOn with higher distortion results in interfiled changes due to high dipole moment. It is worth to notice that the contribution of Pb/BiOn units cannot be neglected. Therefore, we can conclude that the enhanced photocatalytic activity of Pb2Bi4Ti5O18 mainly originates from the large distortions of TiOn and Pb/BiOn. Furthermore, the magnitudes of dipole moments along a, b-axis are cancelled and the net distortion for Pb2Bi4Ti5O18 approximately along [00-1] direction. These highly distorted units in the structure that cause internal field changes, and consequently facilitates the efficient separation of photoinduced electron-hole pairs, and thus enhance photoactivities in comparison to P25.

Table 2. Magnitude Debye of polyhedra in the asymmetric unit of Pb2Bi4Ti5O18 and TiO2 Species

symmetric code

Direction

dipole moment Magnitude Total(esu· Debye cm/Å3)

x (a)

y (b)

z (c)

-11.804

-6.553

17.9480

22.459

0.0607

-0.1212

0.0452

22.9032

22.904

0.0618

-0.531

-1.2857

23.36

23.397

0.064

Ti1O6 (x, y, z)

4.0804

1*10-14

7.5*10-14

4.0804

0.011

Ti2O5 (x, y, z)

2.1029

-0.5378

-0.9015

9.2730

0.025

Ti3O5 (x, y, z)

1.6816

-0.2923

-4.4813

4.795

0.013

TiO2 (Anatase)

Ti1O6 (x, y, z)

0

-7.8591

0

7.8591

0.0212

TiO2 (Rutile)

Ti1O6 (x, y, z)

0

0

0

0

Pb/Bi1O5 (x, y, z) Bi2O4 (x, y, z) Pb/Bi3O4 (x, y, Pb2Bi4Ti5O18 z)

0

17



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Furthermore, the layered structure in Pb2Bi4Ti5O18 provides a rapid charge transport channel for the photoinduced carriers on to the Pb2Bi4Ti5O18 surface, where the surface adsorbed oxygen can react with free electrons to form superoxide radicals. The formed superoxide and photogenerated holes can oxidize the NO, resulting in the removal of NO. Surface area of samples is another crucial factor. In the process of NO removal reaction, the first step should be the efficient adsorption of NO on the surface of photocatalyst because this gas-solid reaction must be preceded on the surface of the solid. Therefore, it is reasonable to conclude that the larger the surface area that the catalyst possesses, the more adsorbed reactive species are efficiently adsorbed on the surface, which would increase the adsorption of NO on the surface of samples, thus leading to higher photocatalytic activity. It can be seen from the BET results that with nanosphere morphology, sample 1 indeed acquires the largest surface area (41.1 m2/g) compared to its nanoparticles and nanosheets with the sample 2 (6.15 m2/g), sample 3 (4.87 m2/g) and sample 4 (0.07 m2/g), which is in consistence with the photocatalytic experiments. Thus, the surface property of diverse morphological Pb2Bi4Ti5O18 is prime factor in efficiency enhancement. CONCLUSIONS We have successfully synthesized visible light active perovskite Pb2Bi4Ti5O18 with diverse morphologies by one step, facile molten-salt synthetic approach. The results show that Pb2Bi4Ti5O18 samples with various morphology displays even better NO removal efficiency than commercially available star catalyst P25. The cyclic photocatalytic experiments and XRD results indicate good stability of samples. The IR measurement demonstrates the NO3- production during the process, which was produced by the reaction of NO with ⋅ O2-. The unique structure of layered, distorted polyhedral and high surface area play an concert role in promoting efficient charge separation, thus enhancing the overall photocatalytic activity. This work provides a practical application for Pb-based perovskites as photocatalysts in gaseous system so as to avoid the secondary contamination of Pb. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Phone: (86)991-3835879. Fax: (86)991-3838957. 18


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* E-mail: [email protected]. Phone: (815)753-6357 Notes The authors declare no competing financial interest. Yichi Ma is a high school volunteer researcher in Xu’s Lab from Saint Francis High School Mountain View, CA, USA. ACKNOWLEDGEMENTS The authors are grateful to the financial support by the National Nature Science Foundation of China (Grant Nos. 21473248 and 21428305), the CAS/SAFEA International Partnership Program for Creative Research Teams, the CAS “Western Action Plan” (KGZD-EW-502), and the High-Technology Research & Development Project of Xinjiang Uyghur Autonomous Region (201415110). TX also acknowledges the support from the U.S. National Science Foundation (NSF CBET-1150617). We all thank Dr. Hongwei Yu from University of Houston for discussions. REFERENCES 1. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at A Semiconductor Electrode. Nature 1972, 23837-23838. 2. Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen From Water. Nature 2006, 440, 295. 3. Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. 4. Chen, X. B.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. 5. Zhang, N.; Li, X. Y.; Ye, H. C.; Chen, S. M.; Ju, H. X.; Liu, D. B.; Lin, Y.; Ye, W.; Wang, C. M.; Xu, Q.; Zhu, J. F.; Song, L.; Jiang, J.; Xiong, Y. J. Oxide Defect Engineering Enables to Couple Solar Energy into Oxygen Activation. J. Am. Chem. Soc. 2016, 138, 8928-8935. 6. Liu, Q.; Zhou, Y.; Kou, J. H.; Chen, X. Y.; Tian, Z. P.; Gao, J.; Yan, S. C.; Zou, Z. G. High-yield Synthesis of Ultralong and Ultrathin Zn2GeO4 Nanoribbons Toward Improved Photocatalytic Reduction of CO2 into Renewable Hydrocarbon Fuel. J. Am. Chem. Soc. 2010, 132, 14385-14387. 7. Nuraje, N.; Asmatulu, R.; Kudaibergenov, S. Metal Oxide-based Functional Materials for Solar Energy Conversion: A Review. Curr. Inorg. Chem. 2012, 2, 124-126.

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Layered Perovskite Pb2Bi4Ti5O18 for Excellent Visible Light-Driven

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