Combined Experimental and Theoretical Insights into the Synergistic

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C: Energy Conversion and Storage; Energy and Charge Transport

A Combined Experimental and Theoretical Insights into the Synergistic Effect of Cerium Doping and Oxygen Vacancies into BaZrO Hollow Nanospheres for Efficient Photocatalytic Hydrogen Production 3-#

Anindya S. Patra, Manendra S. Chauhan, Sam Keene, Gaurangi Gogoi, K. Anki Reddy, Shane Ardo, Dasari L. V. K. Prasad, and Mohammad Qureshi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10626 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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The Journal of Physical Chemistry

A Combined Experimental and Theoretical Insights into the Synergistic Effect of Cerium Doping and Oxygen

Vacancies

into

BaZrO3-δ

Hollow

Nanospheres for Efficient Photocatalytic Hydrogen Production Anindya S. Patra†, Manendra S. Chauhan§, Sam Keene¶, Gaurangi Gogoi†, K. Anki Reddy$, Shane Ardo‡#*, Dasari L. V. K. Prasad§*, and Mohammad Qureshi†* †Department

of Chemistry and $Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Guwahati – 781039, Assam, India.

§Department

¶Department

of Chemistry, Indian Institute of Technology, Kanpur 208016, India

of Physics, ‡Department of Chemistry, #Department of Chemical Engineering and

Materials Science, University of California, Irvine, Irvine, CA – 92697, USA.

ACS Paragon Plus Environment

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ABSTRACT:

The long-standing debate over the influence of oxygen vacancies and various dopants has been the center point in perovskite-based compounds for their photocatalytic applications. Hydrothermally synthesized Cerium doped BaZrO3 (BZO) hollow nanospheres has been systematically studied by experimental and theoretical calculations to understand the effect of Cerium doping and oxygen vacancies on the photocatalytic properties. Compounds synthesized by a template-free route were composed of hollow nanospheres generated by Ostwald ripening of spherical nanospheres, which were formed by agglomeration of nanoparticles. The high alkaline condition and high temperature during the hydrothermal condition may lead to the formation of local disorders and oxygen vacancies in the compounds, confirmed by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analysis and density functional theoretical (DFT) calculations. Combination of oxygen vacancies and progressive doping of Ce onto BZO, BaZr1–xCexO3 (x = 0.00 – 0.04), creates additional energy levels stipulated by vacancy defects and Ce mixed valance states within the band gap of BZO thereby reducing its band gap. The photocatalytic efficacy of the compounds has been examined by photo-driven H2 generation concomitant with oxidation of a sacrificial donor. In this study, BaZr0.97Ce0.03O3 shows the highest efficiency (823 µmol h-1 g-1) with an apparent quantum yield (AQY) of 6% in photocatalytic H2 production among all five synthesized samples. The data obtained from the UV–Vis DRS, XPS, ESR analysis and DFT calculations, the synergistic effect of decreasing the band gap due to Ce doping and the presence of Ce (III)/Ce (IV) pairs along with oxygen vacancies and lattice distortions could be the reasons behind the enhanced photocatalytic efficacy of BaZr1–xCexO3 (x = 0.00 – 0.04) under UV–Visible light.


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INTRODUCTION One of the promising ways to meet the rising demand for a clean, economical, renewable, sustainable and inexpensive energy is the solar water splitting using nanoparticle photocatalysts. Solar water splitting generates molecular hydrogen and oxygen from water in the presence of a suitable photocatalyst and a light source.1,

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H2 is a favorable next-generation energy carrier

because of its high energy density with only water as a combustion by-product and is carbon neutral when derived from water. After the seminal work by Honda and Fujishima in photoelectrochemically splitting water using a TiO2 anode coupled with a platinum cathode, lot of research has been conducted in this area to explore possibilities to achieve stable and efficient heterogeneous photocatalysts.3 A photocatalytic water splitting reaction starts with the absorption of light of suitable energy through an electronic transition from the valence band to the conduction band of a photocatalyst to generate an electron–hole pair. Subsequently, these photogenerated charge carriers with sufficiently high lifetime, diffuse to the photocatalyst surface to initiate a redox reaction and produce hydrogen and oxygen at the respective electrodes. For hydrogen generation, redox potential of the conduction band minima needs to be more negative than the redox potential of H+/H2 (0 V vs. RHE) and for oxygen, the redox potential of the valence band maxima needs to be more positive than the redox potential of H2O/O2 (1.23 V vs. RHE).4 Therefore, band engineering is a key strategy to synthesize and develop new photocatalysts for efficient water splitting.5 In this process, many materials have been synthesized and investigated by various experimental techniques coupled with electronic structure calculations to design stable and suitable band gap materials for the photocatalytic water splitting reaction.6-8 Among others, metal oxides are considered to be potential candidates for tuning the electronic structure, as they can act as a versatile host for doping and therefore to alter the band gap, which


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can yield efficient, tunable and stable catalysts for water splitting .9, 10 One popular class of metal oxides are inorganic perovskite materials with the general formula ABO3, where A is a larger lanthanide, alkaline, or rare-earth metal ion and B is a relatively smaller transition metal ion. Apart from the chemical and thermal stability, electronic properties of perovskites can also be fine-tuned by incorporation of suitable foreign atoms owing to their structural flexibility.11 The doping concertation and selection of dopants with reference to their ionic radii can be quantified by the tolerance factor as proposed by Goldsmith rule.12 The ideal perovskite oxides have the cubic phase with Pm3m space group. These materials form a three-dimensional structure, in which the larger cation A occupy the vertices of the cubic unit cell which are the sites of 12-fold cuboctahedral geometry and the smaller cation B resides at the center of the octahedron to form a BO6 unit with face-sharing O atoms as shown in Figure 1. Furthermore, owing to their structural tolerance, perovskite oxides can withstand a significant amount of impurities or doping, including substoichiometric oxygen in their crystal lattice, which in turn can modify their optical property and catalytic effectiveness.11 A range of such oxide materials can be designed with structural formula A1 ― xA′xBO3, AB1 ― xB′xO3 or A1 ― xA′xB1 ― yB′yO3, where, A′, B′ are foreign elements and x, y are the amount of substitutions that still satisfy the tolerance factor.12 Typically, in ABO3, the valence band around the Fermi energy level (EF) is derived from the 2p orbitals of oxygen atom and conduction band is derived from the d orbitals of B atom and the extent of hybridization of B‒O depends on the electronegativity of the B atom. Hence, the nature of the B- site element essentially determines the photocatalytic activity of the perovskite materials.


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Figure 1. Crystal structure of an ideal ABO3 perovskite with space group Pm3m (No:221) in which (a) the cation A (green) occupy the vertex sites of 12-fold cuboctahedral geometry of the cubic lattice and (b) the cation B (gray) forms an extended three-dimensional network of corner sharing BO6 octahedral units, where the oxygen atoms are shown red in color. Among several studied perovskite oxide photocatalysts, 13-16 barium zirconate, BaZrO3, a typical cubic perovskite oxide, is a promising material with an ample range of technological applications in modern industries.17-19 BaZrO3 has been studied extensively due to its high conductivity and ability to host wide range of dopants into it. In recent times, Yuan et al. have reported BaZrO3 as an efficient photocatalyst for photocatalytic water splitting without using any co-catalyst.20 In spite of having decent efficiency in photocatalytic hydrogen production, BaZrO3 has an absorption onset only in the ultraviolet regime of light spectrum due to its large band gap. In order to enhance the visible-light absorption of such wide band gap materials, several attempts have been made to modify their band positions by incorporating metal or anion dopants and creating oxygen vacancies into the lattice.5 Surface oxygen vacancies act as trap states for photo-induced charge carriers reducing the probability of electron–hole recombination, whereas, bulk oxygen vacancies acts as


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photogenerated charge recombination centers and thus playing a crucial role in photocatalysis.21, 22 Moreover, trap states induce mid-gap energy levels in the material and thereby allows fine tuning

of the electronic structure.23 Efficient utilization of incident sunlight by the semiconductor could be one effective way to enhance the catalytic activity of a photocatalyst material. Literature reports have shown that structures with suitable inner voids can contribute to the incident light absorption by multiple reflections and scattering inside the voids.24 For example in case of TiO2, a hollow sphere is known to exhibit a superior photocatalytic effect compared to its dense counterpart.25 Thus, morphological modification to generate hollow nanospheres can contribute to enhanced light absorption. Zou et al. have reported that Sn (IV)-doped BaZrO3 shows enhanced photocatalytic hydrogen production because of the reduction in band gap energy, thereby altering the density of states in the vicinity of the conduction band, changing the charge-carrier excitation process in BaZrO3.26 Similarly, Díaz-Torres et al. have reported enhanced photocatalytic activity from Bi-doped BaZrO3 due to the shift in absorption onset toward visible light.27 Furthermore, lanthanide elements can be used as electron acceptor dopants in wide band gap semiconductors, which results in a shift in the absorption onset toward the visible spectral region.28 Among the lanthanide elements, Cerium is one of the most favorable choices due to its low cost and formation of a stable Ce (III)/Ce (IV) redox couple in oxidizing and reducing conditions. It is reported that the Ce (III)/Ce (IV) couple whose different electronic configurations of 4f15d0/4f05d0 can introduce diverse optical properties and improve electron-hole pair separation in doped photocatalysts.29 Additionally, these combination of ions result in an increased oxygen vacancies in compounds which could be beneficial for photocatalytic efficiency of the material.30 These advantages motivated us to synthesize and characterize hollow nanospheres of BaZr1−xCexO3 with various experimental and


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theoretical calculations, where x = 0.00 ‒ 0.04 denotes the partial degree of Ce substitution, to gain an understanding on the effect of Ce on the crystal structure, electronic structure, band gaps, optical and photocatalytic properties of BZO. To the best of our knowledge, these combined experimental and computational work is the first report on photocatalytic H2 production by Ce-doped BZO. METHODS: EXPERIMENT AND COMPUTATION Materials All the reagents and chemicals purchased for the experiments were of analytical grade and used without further purification. Zirconium oxychloride octahydrate (Sigma Aldrich), ceric ammonium nitrate (Merck), barium chloride dihydrate (Merck), potassium hydroxide pellets (Merck), sodium sulfide (Sigma Aldrich), sodium sulfite (Sigma Aldrich) and glacial acetic acid (Merck) were used as received. Milli-Q water (18.2 MΩ cm) was used in all the experiments. Preparation of BaZr1−xCexO3 (x = 0.00 – 0.04) Cerium doped barium zirconate compounds (BZCO) were prepared following a modified hydrothermal method.31 The reactions were carried out in a stainless-steel autoclave with Teflon liner at 200 °C and autogenous pressure. Briefly, a stoichiometric amount of BaCl2·2H2O, ZrOCl2·8H2O and (NH4)2Ce(NO3)6 were weighed according to the stoichiometry of BaZr1−xCexO3 (x = 0.00 – 0.04) and transferred to a Teflon made autoclave reactor. A 20 M aqueous KOH solution was prepared in a round bottomed flask by dissolving a calculated amount of KOH pellets into milli-Q water. To avoid any inhomogeneous mixing of the reagents, the as-prepared KOH solution was added to the previously taken metal salts and filled the reactor up to 70% of its volume and vigorously stirred for 1 h. Thereafter, these Teflon beakers were sealed inside a stainless steel jacket and kept in a pre-heated electric oven for 24 h at 200 °C. After completion of the reaction, autoclaves were allowed to cool to room temperature. The white precipitate was centrifuged and


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washed with water, dilute acetic acid and ethanol several times to remove the impurities. Finally, the obtained purified products were dried at 100 °C overnight. Photocatalytic H2 Production A 100 mL two-neck double walled quartz round bottomed flask was used for all the photocatalytic H2 evolution reactions. The reactor was sealed with rubber septum to prevent gas leakage. To conduct the photocatalytic reaction, a 300 W tungsten halogen lamp (Osram, USA) was used as the light source with emission profile in between the wavelength ranges 195 – 1100 nm. In every photocatalytic experiment, typically 25 mg of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixture as a sacrificial reagent for hole scavenging was dispersed in 50 mL of water using a magnetic stirrer. To remove the dissolved oxygen and other gases within the reactor, the reactor was first purged with nitrogen gas for 15 minutes at a flow rate of 0.2 liter per minute and subsequently the reactor was evacuated with the help of a vacuum pump. In order to ensure the complete removal of the unwanted gases from the reactor, this process was repeated several times before irradiating the reactor by the lamp placed 15 cm away from the reactor. During the photocatalytic experiments, continuous water flow was circulated through the outer jacket of the reactor to maintain the constant reaction temperature. The catalyst suspension was stirred throughout the experiment to ensure homogeneous light exposure and to avoid sedimentation of the catalyst. During the experiment, gas from the headspace of the reactor was analyzed by taking out 1 mL of the gas in every 15 min up to 1 h by a gastight syringe and was analyzed by gas chromatography (Agilent 7890A gas chromatograph), with a Molesieve column with nitrogen as the carrier gas, and using a thermal conductivity detector (TCD). In the absence of either photocatalyst or light irradiation, no H2 gas was detected which conﬁrms the role of the


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photocatalyst in H2 gas production. Under the same reaction condition, by using the following equation, the apparent quantum yield (AQY) of the photocatalysts were calculated: 4

𝐀𝐐𝐘 =

=

𝐍𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐫𝐞𝐚𝐜𝐭𝐞𝐝 𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐧𝐬 × 𝟏𝟎𝟎% 𝐍𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐢𝐧𝐜𝐢𝐝𝐞𝐧𝐭 𝐩𝐡𝐨𝐭𝐨𝐧𝐬

Number of hydrogen molecules produced in 1 hour × 100% Number of incident photons in 1 hour

Characterization Powder X-ray diffraction (PXRD) patterns of the compounds were recorded to analyze the crystal structure and chemical purity of the above synthesized samples using a Rigaku X-ray diffractometer (model TTRAX III) with Cu-Kα1 (λ=1.54056 Å) as the radiation source at an operating voltage of 50 kV and an operating current of 100 mA. The scan range was set to 20° – 80° with a step size of 0.02 °/s. The phase purity and crystallographic parameters of each compound was assessed by Rietveld refinement on the PXRD patterns using the FullProf Suite.32 The background of the instrument and the peak shape of the PXRD patterns were refined by using a sixth-order polynomial and pseudo-Voigt function, respectively. The scale factor, zero correction parameters, lattice parameters, full width half maximum (FWHM), atomic fractional position coordinates and thermal parameters were refined in this Rietveld refinement method. The microstructural morphology and the compositional uniformity of the photocatalysts were studied by field emission scanning electron microscope (FESEM) using a Zeiss Sigma instrument at an operating voltage of 1 - 5 kV and energy dispersive X-ray spectroscopy (EDX) at an operating voltage of 20 kV. Field emission transmission electron microscopy (FETEM) analysis of the compounds were carried out in a JEOL JEM 2100 microscope at 200 kV operating voltage. JASCO V-650 spectrophotometer with an integrating sphere of 150 mm and BaSO4 as an internal


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reflectance standard was used to measure the ultraviolet–visible diffuse reflectance spectra (UVVis DRS) of the photocatalysts over a wavelength range of 200 ‒ 800 nm. Electron spin resonance (ESR) spectra were measured on X-band Microwave Unit, JES-FA200 ESR spectrometer at 100 G amplitude (χ), 9.444 GHz microwave frequency and 100 kHz modulation frequencies at room temperature. X-ray photoelectron spectroscopy (XPS) were carried out using a Kratos AXIS Supra photoelectron spectrometer with a monochromatized X-ray source of Al-Kα (hν = 1486.6 eV) at a residual gas pressure of 10-6 Pa. Pass energies of 160 eV and 20 eV were used for collecting the survey and high-resolution core level spectra, respectively. To compensate the surface charging effect, here in this analysis all the peaks were referenced with respect to C 1s spectrum (284.77 eV). All the XPS core level spectral data were analyzed and quantified by using XPSPEAK 4.1 software. The background of all XPS spectra was subtracted by Tougaard background method. Computational Methodology Spin and nonspin-polarized electronic structure calculations were carried out using plane wave density functional theoretical (DFT) approach encoded in Vienna Ab initio Simulation Package (VASP).33 The Perdew, Burke, and Ernzerhof (PBE)

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parameterization of the generalized

gradient approximation (GGA) was employed for the exchange-correlation energy. The valance electron configurations for the constituent elements in the titled compounds were considered as Ba (5s25p66s2), Zr (4s24p65s24d2), Ce (5s25p64f15d16s2), and O (2s22p4) and the valence electron–ion core interactions were described by pseudopotentials built within the scalar relativistic all-electron Blöch’s projector augmented wave (PAW) method.35, 36 A plane wave energy cutoff of 420 eV was set throughout all the calculations. The sampling of the Brillouin-zone (BZ) integrations was done using a k-mesh with spacing 2π x 0.0125 Å-1 for conventional cubic BaZrO3 (BZO)/BaCeO3 (BCO) unit cells and gamma-point for Ce doped BZO (BZCO) BaZr1-xCexO3-δ (x = 0.00 – 0.04


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and δ = 0, 0.01) 5×5×4 supercells consisting of 500 (δ = 0) or 499 (δ = 0.01 i.e. 0.33% of oxygen vacancies i.e. 1 out of 300 oxygen) atoms per cell, respectively. Modeling of doping Ce (IV) and Ce (III)/Ce (IV) in addition with oxygen vacancies are discussed in detailed below in results and discussion. A BZO 5×5×4 supercell (BaZr1-xCexO3-δ where x = δ = 0) was verified for energy convergence with respect to a k-mesh spacing 2π x 0.025 Å-1. We found that, between gamma-point and k spacing 2π x 0.025 Å-1, the total electronic energy is only differed by less than 1 meV/supercell and there were barely any noticeable changes in the electronic band gaps and band alignments. Therefore, modeling of Ce doped BZO was safely undertaken by adopting supercells whose BZ integrations were done at gamma-point for electronic structure analysis. These computational thresholds were used for the primitive and supercell geometry total optimizations where the ions, lattice vectors, and unit cell volumes are fully relaxed until the forces on ions converge to 10-3 eV/Å. In order to analyze the electronic structure of the subject line materials the interaction of the orbitals of the constituent elements have been analyzed by calculating the total and projected electronic density of states (DOS). The atom and orbital angular-moment projections have been calculated using the projection scheme implemented in VASP, wherein the Wigner-Seitz radii were chosen from the respective PAW potentials for the projected density of states. RESULTS AND DISCUSSION Structural Analysis Structural analysis of the as-synthesized BaZr1-xCexO3 (x = 0.00 ‒ 0.04) compounds was systematically carried-out by powder X-ray diffraction, Rietveld refinement, and crystal structure geometry-optimizations by DFT calculations. PXRD Patterns


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Phase purity and crystal structure of the as-synthesized materials were determined by PXRD. Well-indexed PXRD patterns in the 2θ range of 20° – 80° for BaZr1-xCexO3 (x = 0.00 – 0.04) photocatalysts are shown in Figure 2A. From the diffractogram patterns it is verified that all the compounds belong to the cubic phase with Pm3m space group (no. 221) [JCPDS file No.06-0399]. The PXRD patterns of the parent compound BZO and Ce doped BZO are free from any discernable impurity peaks or peaks for CeO2 (2θ = 28.5°, 47.7°, 56.6°), which were not observed in the Cedoped BZO compositions and supports the successful doping of Ce in BZO. Figure 2B shows a gradual shifting of (110) diffraction peaks toward lower values in the 2θ range of 29.6° – 30.6°. This can be explained based on the ionic radii of Ce(III) / Ce(IV) (101 pm/87 pm), which are larger than that of Zr (IV) (72 pm) in an octahedral environment.37 Hence, doping of Ce at Zr sites expands the unit cell volume and shifts the 2θ value in the PXRD patterns toward lower values. In order to understand the effect of Ce doping on the BZO crystal structure, lattice parameters,

(A)

30

40

50

60

2 (Degree)

BaZr0.97Ce0.03O3

BaZr0.98Ce0.02O3 BaZr0.99Ce0.01O3

70

BaZrO3

222

310

BaZrO3

311

220

211

210

200

111

110

BaZr0.99Ce0.01O3

BaZr0.96Ce0.04O3

Intensity (a. u.)

Intensity (a. u.)

BaZr0.98Ce0.02O3

20

(B)

BaZr0.96Ce0.04O3

BaZr0.97Ce0.03O3

100

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80 29.6

29.8

30.0

30.2

2 (Degree)

30.4

30.6

Figure 2. Powder X-ray diffraction patterns of BaZr1−xCexO3 (x = 0.00 – 0.04). (A) Full scale, (B) enlarged within 2θ = 29.6° – 30.6°.


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unit cell volume, and local structural distortions, we have performed (1) Rietveld refinement and (2) density functional theoretical calculations. Rietveld refinements of the PXRD diffraction patterns of BaZr1-xCexO3 (x = 0.00 ‒ 0.04) compounds were done using the FullProf program and DFT modeling and crystal structure optimizations of Ce doped BZO were carried out by VASP.33 Rietveld Refinement In the cubic BZO and Ce-doped BZO perovskite structures, the Ba atoms are in the Wyckoff position 1b (0.5, 0.5, 0.5), Zr/Ce in 1a (0, 0, 0) and O in 3d (0.5, 0, 0). Figure 3A shows the Rietveld refined PXRD pattern of the best performing catalyst, BaZr0.97Ce0.03O3, while the Rietveld refined PXRD pattern of BaZrO3, BaZr0.99Ce0.01O3, BaZr0.98Ce0.02O3 and BaZr0.96Ce0.04O3 compounds are shown in Figure S1 (ESI). Figure 3B displays the Rietveld and DFT results of the variation of lattice parameters and cell volume with Ce doping concentration in BZO. The datum indicates that the compounds crystallize in the cubic phase and with increase in Ce doping in BZO, the cell parameters and unit cell volume increase linearly which is in excellent agreement with the results of DFT calculations. As mentioned earlier, the qualitative comparison due to the size difference between cerium and zirconium, and the quantitative DFT volume trends together indicate that, the unit cell volume of doped compounds will increase compared to the parent BZO. Hence, it is proved from Figure 3B that we have successfully prepared Ce doped BZO samples with different atom% of Ce. In Table S1 (ESI), the comprehensive values of the refined lattice parameters, unit cell volume, goodness of fit (χ2), Rp, Rwp, Rexp and atomic positions of Ba, Zr/Ce and O of BaZr1- xCexO3 (x = 0.00 ‒ 0.04) are tabulated. The DFT calculated crystal coordinates are reported as appendix to the ESI.


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(B)

20

30

40

50

2 (Degree)

60

70

80

75.3

Lattice Parameter_Experimental Lattice Parameter_DFT Cell Volume_Experimental Cell Volume_DFT

4.220 4.218

75.2 75.1

3

Yobs Ycalc Yobs-Ycalc Bragg_position

BaZr0.97Ce0.03O3

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Cell volume (Å )

(A) 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

Lattice parameter [a = b= c] (Å)


4.216

75.0

4.214

74.9

4.212

74.8

4.210

74.7 74.6

4.208

74.5

4.206

74.4

4.204

74.3 0.00

0.01

0.02

0.03

Ce (degree of subsitution, x)

0.04

Figure 3. (A) Rietveld refined PXRD profile of the as-synthesized BaZr0.97Ce0.03O3. Blue line in the bottom shows the difference between observed and calculated values and vertical sticks in olive color denote calculated Bragg positions for cubic phases in the Rietveld refined plot. (B) Variation of lattice parameters and cell volume with Ce doping concentration in BZO. It is well known that the presence of oxygen vacancies are inevitable in the hydrothermal synthesis of Ce doped BZO compounds, although the bulk crystal structures of Ce doped BZO were arrived unambiguously by the Rietveld refinement process, fixing the oxygen vacancy fraction cannot be determined with the PXRD data. Therefore, in an effort to pursue the influence of oxygen vacancies and the ensuing Ce mixed valence states 4f0 (Ce (IV)) and 4f1 (Ce (III)) on the crystal lattice and electronic structure of the true Ce doped BZO, BaZr1-xCexO3-δ (x = 0.00 ‒ 0.04), DFT calculations and various photo-excitation experiments have been performed, and these are discussed below in respective sub sections. DFT Modeling of Ce Doping and Oxygen Vacancies in BZO Modeling of Ce doped BZO was conceived and studied in two steps. In first, doping of Ce (IV) and in the second step, Ce (III)/Ce (IV) in conjunction with the creation of various oxygen vacancies. Consequently, a systematic study has been done to understand the effect of Ce doping


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and oxygen vacancies on the structure, electronic properties, spin states, band gaps and hydrogen production efficiency of BZCO compounds. While the structures of BZCO involved in step 2 are more realistic with respect to the experimental observations presented here, the results of the calculations derived in step 1 by doping only Ce (IV) without oxygen vacancies would stand as a good reference for comparison with the Ce (III)/Ce (IV) mixed valence states. All the structures studied here, including the structures with oxygen vacancies are fully optimized. Ce (IV) Doping To begin with, crystalline BaZr1-xCexO3-δ (x = 0.00 – 0.04 and δ = 0) compositions were modeled by constructing a 5×5×4 supercell from the parent cubic BaZrO3 (Pm3m and Z = 1) unit cell and thereby Zr Wyckoff sites are selectively substituted by Ce. Thus obtained Ce substituted BZO compositions are then fully optimized as described in computational methodology. Undoped BZO and the 3% Ce doped BZO optimized 5×5×4 crystal structures are shown in Figure 4. The alternate plausible sites for Ce substitution are huge in number as there are 100 Zr sites per 5×5×4 supercell; in particular, when the Ce doping is regarded in dilute concentrations. For example, for 2% Ce doped case, there can be as many as 4950 combinations would deemed to be applicable for Zr site occupancy. But the presence of highly symmetric ZrO6 octahedral coordination environment throughout the BZO crystal lattice would drastically minimize the alternate site occupancies for Ce doping. We have modeled a few configurations for Ce site occupancies by keeping Ce adjacent to far from each other and what we found is that the configurations are nearly energetically degenerate with the energy differences less than that of 0.1 meV/atom (see ESI, Table S2). To obtain accurate final energies within the cut-offs, all supercell relaxations were followed by a final static calculation.


15


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Figure 4. DFT optimized crystal structures of BaZrO3 and 3% Ce-doped BaZrO3 5×5×4 supercells: Perspective views of (a) BaZrO3 and (b) 3% Ce-doped BaZrO3 with zero oxygen vacancy (denoted by [OO× ]) that is BaZr1-xCexO3-δ, where x = 0.03 and δ = 0, and (c) 3% Ce-doped BaZrO3 with inclusion of [VO] monoanionic oxygen vacancy, (BaZr1-xCexO3-δ, where x = 0.03 and δ = 0.01). The structures are shown along ab lattice vectors. The green, gray, and red spheres represent Ba, Zr, and O atoms, respectively. The coordination polyhedra (Oh) around a Zr in BaZrO3 and all 3%Ce are shown in polyhedral representation. The Olive polyhedron is CeO6 and CeO5. Ce (III)/Ce (IV) Doping and Oxygen Vacancies In order to study the influence of Ce mixed valence states 4f0 (Ce (IV)) and 4f1 (Ce (III)) on the crystal and electronic structure of BaZr1-xCexO3-δ, the creation of oxygen vacancies is necessary as the solid should be charge balanced. In general, three probable oxygen vacancies are reported in literature and these are popularly represented in perovskite family of compounds by Kröger-Vink notation as [ZrO5VO× ], [ZrO5VO], and [ZrO5V O ] which represents a penta-coordinated Zr (it could be Ce as well in Ce doped BZO) and an oxygen vacancy with two, one and zero electrons respectively. Structures with zero oxygen vacancies, for example pristine BZO, in where the metal Zr is six coordinated (ZrO6) and whose notation is represented by [ZrO6 OO× ]. In modeling of the


16

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vacancies in computations, the three types of oxygen vacancies have been created by chipping a neutral oxygen (O), monoanion (O1 ― ), and dianion (O2 ― ) from the 5×5×4 Ce doped BZO supercells by maintaining the charge neutrality. This would create occurrences of Ce (III) and Ce (IV) states. The mixed Ce valence states have been explicitly verified both in calculations and experiments presented here, such as the computational study of BZCO spin states and electronic density of states analysis and the EPR and XPS measurements respectively. The oxygen which is common to Ce in the respective supercells has been chosen to create all the three types of the possible oxygen vacancies. Such an oxygen site is found to be thermodynamically favorable among other possibilities. See ESI (Table S3) for more details on oxygen site preferences. The optimized crystal structure of 3% Ce doped BZO with [VO] oxygen vacancy is shown in Figure 4C. Creation of more than an oxygen vacancy is possible, however, due to the demand of huge computer time in performing such computationally intensive DFT supercell calculations, we have restricted ourselves in generating one oxygen vacancy of all the three types, [VO× ], [VO], and [V O ]. For example, 1 to 4 percent Ce doped BZO with one (0.33%) oxygen

vacancy

BaZr0.99Ce0.01O2.99,

BaZr0.98Ce0.02O2.99,

BaZr0.97Ce0.03O2.99,

and

BaZr0.96Ce0.04O2.99, respectively have been modelled, wherein all the three types of oxygen vacancies are studied for each composition. We also have considered the oxygen vacancies for the undoped BZO, whose formula reads as BaZrO2.99. BZCO – Crystal Structural Aspects The calculated BaZr1-xCexO3-δ (BZCO, where x = 0.00 – 0.04 and δ = 0.00 and 0.01) crystal structure data – lattice parameters, unit cell volumes, distances and angles with respect to the %Ce doping and various oxygen vacancies are presented in Figure 5 and Table S4. The results are in


17


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good agreement with the experimental results presented here and also with the other available experimental and theoretical values reported in the literature.38 ‒ 46 The lattice parameters and the corresponding unit cell volumes of the Ce doped BZO compounds are elongated and expanded, respectively in comparison to the undoped BZO. As Ce (IV/III) is larger than Zr (IV) 37 one would anticipate that the Ce-doped BZO crystals stabilize in somewhat higher unit cell volumes than the BZO, as also observed in X-ray diffraction studies (see Figure 3). The raise in the volume reflected in as much as the %Ce (dilute concentrations: 1% to 4%) is substituted for Zr. For example, the increment in volume is about 0.5% in calculations from undoped BZO to 4% Ce doped BZO, which is comparable to the experimental value 0.3% increase, reported from the Rietveld data. As the BZCO crystals are cubic, the trends in lattice parameters remained similar to the volume expansions (see Figure 5A and 5B). It is interesting to note that the volumes of the BZCO unit cells with neutral oxygen vacancies [VO× ] are similar to BZCO with zero oxygen vacancies [OO× ]. However, the volumes of the BZCO are still higher side by a fraction even in the presence of oxygen vacancies when compared to the undoped BZO unit cell volumes. Therefore, the volume expansions can be predominantly attributed to the presence of Ce in BZO rather than oxygen vacancies. Otherwise, it can be also seen from Figure 5B that for a given %Ce doped BZO structure, the unit cell volumes decreases from [OO× ] to [V O ]. It is due to the local structural distortions induced by the presence of oxygen vacancies as reflected in Zr-O and Ce-O distances and O-Zr-O and O-Ce-O angles (Figure 5C to 5F). The crystal coordinates of all the geometries discussed here are given in ESI (appendix).


18

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Figure 5. Evolution of the DFT optimized (A) lattice parameters, (B) volumes, and (C) Zr-O, (D) Ce-O distances, and (E) O-Zr-O (F) O-Ce-O angles of BaZr1-xCexO3-δ (where x = 0.00 – 0.04 and δ = 0.00 and 0.01) with no oxygen vacancy denoted by [OO× ], and with inclusion of all the three possible oxygen vacancy types, [VO× ], [VO], and [V O ]. See also ESI, Table S4.


19


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Figure 6. DFT optimized crystal structures of BZO and BZCO [VO× ], [VO], and [V O ] 5×5×4 supercells: Shown are the partial supercells of (A) BaZrO3 with no oxygen vacancies; (B), (C), and (D) are 3% Ce-doped BaZrO3 with [VO× ], [VO], and [V O ] oxygen vacancies (BaZr1-xCexO3-δ, where x = 0.03 and δ = 0.01), respectively. The structures are shown along c-axis, top view. For clarity, the Ba atoms are omitted. The gray and red spheres represent Zr and O atoms, respectively. The coordination environment around Ce is shown in polyhedral representation. The Olive polyhedron is CeO6 and CeO5. While there are sizable local ZrO5 and CeO5 polyhedral distortions due to the oxygen vacancies, the structural distortions are found to be negligible in octahedral units of ZrO6 and CeO6 that are far from these vacancies. The calculated nearest neighbor distances, Zr-O (2.107 Å) and Ce-O (2.223 Å) in cubic BaZrO3 and BaCeO3 (BCO) are in excellent agreement with the experimental values 2.097 Å and 2.222 Å, respectively (Figure 5C, 5D and Table S4).38

‒ 40, 45

The Ce-O

distances in Ce-doped BZO3-δ (δ = 0.0) are in the range of 2.204 to 2.207 Å, closer to the Ce-O distance (2.222 Å) in pristine BCO. The Zr-O distances in Ce-doped BZO3-δ (δ = 0.0) are not much deviated from the undoped pristine BZO. However, the oxygen vacancies lead to finite local polyhedral distortions in terms of tilting ZrO5 or CeO5 polyhedral units in BaZr1-xCexO3-δ (x = 0.00 – 0.04 and δ = 0.01). In the distorted local polyhedral units, the basal plane perpendicular to the oxygen vacancy is skewed; it is spread from 180 to 157 degrees along O-Zr-O and O-Ce-O (Figure


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5E and 5F). For example, the CeO5 distortions in 3% Ce doped BZCO due to oxygen vacancies are shown in Figure 6, which influence the electronic structure and in turn the band gaps and the photocatalytic properties of BZCO3-δ compounds. One common trend among all the BZCO3-δ structure studied here is that the lattice parameters, volumes, and various distances and angles consistently decreased with the inclusion of oxygen vacancies, from [OO× ] type to [V O ] as shown in Figure 5. Morphological Analysis Figure 7A shows FESEM images of BaZr0.97Ce0.03O3 at different magnifications. The FESEM images reveal that as-synthesized BaZr0.97Ce0.03O3 particles are more or less spherical and a higher magnified image [inset to Figure 7A] shows the presence of a hollow structure. Figure 7B shows TEM images at different magnifications. Particle diameters are found to be ~150 – 200 nm. The hollow nature of the particles can be evidenced from the inset of Figure 7B, which is the TEM image of an individual particle and shows a sharp contrast between the dark edge and the bright region. The HRTEM image of BaZr0.97Ce0.03O3 (Figure 7C) shows the repeated uniform lattice fringes. Figure 7D shows the IFFT of the HRTEM image where the obtained spacing of the lattice fringes (d-spacing) is ~0.29 nm and can be assigned to the (110) lattice plane of the cubic phase of BaZr0.97Ce0.03O3 consistent with the fact that cubic BZO usually exposes its [110] facet due to that being a lower energy surface.47 The inset to Figure 7D shows the SAED patterns of the BaZr0.97Ce0.03O3 hollow nanospheres. The TEM as well as FESEM images prove the hollow nature of the product. The formation of hollow nanospheres in hydrothermal synthesis could be explained in three different steps, viz. hydrolysis of precursors, nucleation, and growth process.48 All steps are controlled by the course of the reaction and reaction conditions. Owing to the highly alkaline


21


(A)

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(B) 200 nm

20 nm

200 nm

50 nm

(C)

(D)

110

5 1/nm

5 nm

Figure 7. (A) Field-emission scanning electron microscopic (FESEM) images, inset to trace (A) shows higher magnified FESEM image, (B) transmission electron microscopic (TEM) images, inset to trace (B) shows TEM image of a single hollow nanospheres, (C) high-resolution TEM (HRTEM) image of BaZr0.97Ce0.03O3 and (D) inverse fast Fourier transformed (IFFT) of the HRTEM image of BaZr0.97Ce0.03O3. Inset to trace (D) shows the selected area energy diffraction (SAED) patterns of BaZr0.97Ce0.03O3 hollow nanospheres.


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synthesis conditions (pH ≈ 14), barium, zirconium and cerium salts undergo hydrolysis to form corresponding hydroxides in the reaction medium. Here, KOH acts as a mineralizer and these metal hydroxides act as monomers of the desired product. During the course of reaction, solubility of the salts increases and they react to form a supersaturated sol-like suspension, which favors nucleation to generate very small product particulates by suppressing the grain-growth process. It is known that the pH of the solution, temperature of the system and time of the reaction catalyzes the rate of nucleation.49 Because the nanoparticles have high surface energy, they have the tendency to form agglomerates under the influence of van der Waals forces. This growth process of BZO follows hydrolysis or decomposition of metal hydroxide monomers. This process avoids the high surface energy and proceeds until an electrostatic barrier layer is established.49 In the course of 24 h, hollow nanospheres are created by an Ostwald ripening process. Ostwald ripening has been widely used to synthesize many hollow inorganic nanostructures by wet chemical routes.50, 51 Ostwald ripening is a well-known physical phenomenon in the field of crystal growth and it is related to the recrystallization process in solution phase. Like in the nucleation rate, the reaction temperature and the concentration of the alkaline solution plays a vital role in this final step. As reported by Dong et al. and Zou et al., in the low reaction temperature and less alkaline conditions, only partially hollow or dense nanoparticles are forms.31, 52 Here the formation of a typical spherical structure of BZO can be attributed to absorbed hydroxyl groups on the surface of the particles under the highly alkaline condition.53 Scheme 1 illustrates step-wise formation mechanism for BZO hollow nanospheres. The decomposition or dehydration pathway can be explained by the following plausible reaction mechanism -

[Ba2 + (OH)a― ]m + [Zr4 + (OH)b― ]n = BaZrO3 + 3H2O


23


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[Ba2 + (OH)a― ]m + [Zr4 + (OH)b― ]n1 ― x + [Ce4 + (OH)c― ]ox = BaZr1 ― xCexO3 + 3H2O The above mentioned dehydration process is slow in a superheated and supersaturated solution and may withhold a fraction of OH– and H2O in BZO and form a defective crystal.54 During the dehydration process some of the OH– occupying the surface oxygen sites in BZO are released which leads to the formation of defective crystals with oxygen vacancies. EDX mapping was performed to examine the elemental distribution in the synthesized compounds. Figure S3A (ESI) shows the elemental distribution of the best performing catalyst, BaZr0.97Ce0.03O3 and Figure S3B−E (ESI) shows the distribution of Ba, Zr, O and Ce, respectively in the scan area from panel A. It can be noticed that all the elements are uniformly distributed all over the sample. EDX mapping of BZO was also examined, which shows the uniform distribution of Ba, Zr and O (Figure S2, ESI) in it. Scheme 1. Schematic representation of the formation of BZO hollow nanospheres in the hydrothermal process, showing stepwise formation of hollow nanospheres through the Ostwald ripening process.

pH~14


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Ultraviolet-visible Diffuse Reflectance Spectra Analysis and Band gap Calculation

Figure 8. Diffuse reflectance spectra of as-synthesized BaZr1−xCexO3 (x = 0.00 – 0.04) catalysts: The graphical image in the inset demonstrates the different phenomena responsible for the absorbance in different regions of the electromagnetic spectrum. The photocatalytic effectiveness of a material is predominantly dependent on its light absorbing capability and therefore its band gap structure. The UV–Vis DRS of the as synthesized catalysts are shown in Figure 8 within the wavelength range of 200 – 800 nm. A major peak at around 230 nm is observed which can be assigned to a band-to-band transition and a band tail with an absorption extended beyond 400 nm is observed which is reasonably assigned to the presence of lattice defect, which are known to give rise to a band tail in the absorption spectrum, as we have stated before that in BZO oxygen vacancies and different types of structural and electronic disorders are created due to poor oxygen atmosphere or by the removal of oxygen atoms by de-


25


hydroxylation in BZO,

55 ‒ 57

Page 26 of 55

as is also systematically verified through the DFT studies on the

BZCO3-δ lattice distortions. In the Ce-doped materials, it is observed that the intensity of band tail absorption peak increased with doping percentage. With increase in Ce doping concentration from x = 0.0 – 0.04, the absorption onset of the doped compounds are slightly red shifted. We have calculated the band gap of all the compounds by Tauc method and is shown in Figure S4 (ESI).58 From Figure S4 (ESI), we observe that the band gap of parent BZO is 2.37 eV and with Ce doping the band gap of the materials decreases linearly. X-ray Photoelectron Spectroscopic Analysis To gain a better understanding about the core-level electronic structure of the constituent elements as well as the surface properties of as-synthesized compounds, XPS analyses were performed. A typical survey spectrum of BaZr0.97Ce0.03O3 is presented in Figure 9C and shows the presence of all elements, viz., Ba, Zr, O and Ce. Figure 9A shows the high-resolution XPS spectra of Ba 3d of BZO and BaZr0.97Ce0.03O3, respectively. The Ba 3d core level XPS spectra of BZO appears at a binding energy (B. E.) of 781.18 eV and 796.48 eV, which corresponds to the Ba 3d5/2 and Ba 3d3/2 respectively. Ba 3d core level XPS spectra of BaZr0.97Ce0.03O3 appears at 780.38 eV and 795.68 eV, corresponding to Ba 3d5/2 and Ba 3d3/2 respectively. Although the peak positions are little deviated from the previously reported values, the peak separation energy between the two peaks in the Ba 3d doublet (ΔEB. E. = B. E. 3d3/2 – B. E. 3d5/2) is found to be ~15.3 eV, which supports our peak assignment.59 Zr 3d XPS spectra of BaZrO3 and BaZr0.97Ce0.03O3 are shown in Figure 9B. This peak can be deconvoluted into three peaks with binding energies of 179.88 eV, 183.48 eV and 185.82 eV for BaZrO3 and 178.88 eV, 182.57 eV and 184.89 eV for BaZr0.97Ce0.03O3 corresponding to Ba 4p3/2, Zr 3d5/2 and Zr 3d3/2, respectively.60 Although the peak


26

Page 27 of 55

Intensity (a. u.)

Ba 3d5/2

Raw Fitting Background

Ba 3d3/2

(B)


Zr 3d5/2

BaZr0.97Ce0.03O3 Zr 3d3/2

Intensity (a. u.)

(A)

BaZr0.97Ce0.03O3

BaZrO3

BaZrO3

790

785

780

Ba 3d

(C)

188

775

Binding energy (eV)

Ba 4d

Zr 3p

Zr 3d

Ce 3d

O 1s

186

184

182

1000

800

600

400

Binding energy (eV)

200

178


U''' U'' U' U V'''

1200

180

Binding energy (eV)

(D)

BaZr0.97Ce0.03O3

Intensity (a. u.)

795

C 1s

800

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


0

920

910

900

V'' 890

V'

Binding energy (eV)

V 880

Figure 9. XPS spectra of (A) Ba 3d of BaZrO3 and BaZr0.97Ce0.03O3, (B) Zr 3d of BaZrO3 and BaZr0.97Ce0.03O3, (C) survey spectrum of BaZr0.97Ce0.03O3 and (D) Ce 3d core levels of BaZr0.97Ce0.03O3. positions are little deviated from the previously reported values, the peak separation energy between the two peaks in the Zr 3d doublet (ΔEB. E. = B. E. 3d3/2 – B. E. 3d5/2) is found to be ~2.3 eV, which supports our peak assignment.59 The XPS spectra of Ce 3d of BaZr0.97Ce0.03O3 is shown in Figure 9D. Due to very low concentration of Ce in BaZr0.97Ce0.03O3, the shape of the spectra is not as distinct as for pure CeO2. The Ce 3d XPS spectral analysis is complicated due to spin-orbit coupling between Ce 4f and O 2p electrons, leading to the structured peaks.61 Many reports have shown that Ce 3d spectra can consist of eight peaks with four pairs of spin-orbit coupling


27


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contributions, viz. u/v, u'/v', u''/v'' and u'''/v''', where u and v symbolize two sets of spin-orbital multiplets, viz. Ce 3d5/2 and Ce 3d3/2 respectively.61, 62 From the preceding reports, the u'/v' bands arise in the Ce 3d spectrum for Ce (III) 3d5/2 and Ce (III) 3d3/2, respectively. The other three pairs of peaks are generated from spin-orbit coupling for Ce (IV) 3d5/2 and Ce (IV) 3d3/2. In this spectra, the fraction of Ce (III) present can be evaluated by dividing the total integrated peak intensity for Ce (III) by that due to both Ce (IV) and Ce (III).63 Here, we found that the fraction of Ce (III) in BaZr0.97Ce0.03O3 is 20%. From this analysis and also evidenced in DFT spin states and electronic densities of states of BZCO3-δ (discussed below), it is clear that in Ce-doped BZO samples, Ce exist in a mixed valence state of Ce (III) and Ce (IV). Consequently, to neutralize the charge imbalance in doped compounds due to the formation of Ce (III), additional oxygen vacancies are created.64, 65 Progressive increase in the intensity of the absorption band tail in the UV-Vis DRS is also an evidence that the cerium doping has a direct impact on increase in oxygen vacancies. To investigate the presence of oxygen vacancies we analyzed the high-resolution XPS spectra of O 1s as shown in Figure 10A. Deconvolution of an asymmetric shaped O 1s peak of BZO shows two peaks positioned at 531.71 eV and 533.54 eV corresponding to lattice oxygen (Olatt.) and adsorbed oxygen (Oads.) in the oxygen deficient region within the compound matrix and surface H2O, respectively.66 The two fitted deconvoluted bands of O 1s peak for BaZr0.97Ce0.03O3 appear at 530.61 eV and 532.73 eV. The area under the fitted band of non-lattice oxygen (Oads.) in BaZr0.97Ce0.03O3 is larger than that of BaZrO3 suggesting that with Ce doping, the number of oxygen vacancy increases in the doped BaZrO3, complimented by the UV-Vis DRS data of the as synthesized compounds in Figure 8. The Ba 3d, O 1s, and Zr 3d peaks are all shifted to lower energy in the Ce-doped compounds compared to the undoped BZO. The spectral shifts in XPS are


28

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(A)


BaZrO3

536

534

532

530

Binding energy (eV)

528

(B)

BaZr0.97Ce0.03O3

Intensity (a. u.)

BaZr0.97Ce0.03O3

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



BaZrO3

536

534

532

530

Binding energy (eV)

528

Figure 10. O 1s core level XPS spectra of BaZrO3 and BaZr0.97Ce0.03O3, (A) before and (B) after annealing in air at 700 °C for 2 h. attributed to the change in chemical environment around the Ba, Zr, O in the Ce doped compounds due the larger size of Ce (IV) and its decreased electronegativity as compared to Zr (IV).26, 67 We calcined all compounds at 700 °C in a box furnace under air for 2h to further support our claim of oxygen vacancies. It is known that oxygen vacancies of a compound could be moderately reduced by calcining at an elevated temperature under air or oxygen atmosphere.68 The highresolution O 1s core level XPS spectra of the calcined products are shown in Figure 10B. It is clear that after calcination, the intensity of the peak corresponding to the oxygen vacancy reduces due to the partial filling up of oxygen vacancies by atmospheric oxygen. Electron Spin Resonance Study To check the presence of oxygen vacancies in as-synthesized compounds and calcined photocatalysts, we performed ESR analysis on all compounds as the singly ionized oxygen vacancy has only one unpaired electron, it gives rise to a strong ESR signal. From Figure 11, we observe that the as-synthesized BZO and Ce-doped BZO compositions show a peak at around gtensor value of 2.005, which is due to the singly ionized paramagnetic oxygen vacancies (V•O).69 In


29


Page 30 of 55

Figure 11. ESR spectra of as-prepared BaZr1–xCexO3 (x = 0.00 – 0.04) catalysts at room temperature. case of calcined compounds, we have also observed a sharp peak at around g-tensor value of ~2.005, corresponding to the singly ionized paramagnetic oxygen vacancies (Figure S5, ESI). These results from XPS and ESR indicate that all the photocatalysts retain oxygen vacancies in their structure even after calcination at 700 °C for 2 h under air. Hence, from these above findings we can say that all the compounds in this study are suffered from ordered-disordered type of lattice defects with considerable amount of oxygen vacancies. The presence of these defects could be represented by Kröger-Vink notation,70

[ZrO6]x + [ZrO5. VxO]⟶[ZrO6]′ + [ZrO5. V•O]


30

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[𝐙𝐫𝐎𝟔]𝐱 + [𝐙𝐫𝐎𝟓. 𝐕•𝐎]⟶[𝐙𝐫𝐎𝟔]′ + [𝐙𝐫𝐎𝟓. 𝐕•• 𝐎]

[𝐙𝐫𝐎𝟓. 𝐕•𝐎] +

𝟏 𝐎 ⟶[𝐙𝐫𝐎𝟔] 𝟐 𝟐

[𝐙𝐫𝐎𝟓. 𝐕•𝐎] +

𝟏 𝐎 ⟶[𝐙𝐫𝐎𝟔] 𝟐 𝟐

Experimentally the position of valence band maxima of a material can be derived by fitting the XPS valence band spectra. The XPS valence band spectra of BaZrO3 and BaZr0.97Ce0.03O3 are shown in Figure 12, where we see that with doping, the position of valence band is shifted towards lower binding energy. This shift in valence band maxima in Ce-doped materials clearly indicates the formation of a defect/impurity band next to the valence band. The band tail in both the spectra

Figure 12. XPS valance band spectra of BaZrO3 and BaZr0.97Ce0.03O3.


31


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reveals the presence of lattice disorder in the compounds.71 The enhanced light absorption of BaZr1−xCexO3 (x = 0.00 – 0.04) compounds can be explained by the electronic transition from the valence band tail to oxygen vacancies to the conduction band or to the impurity level due to cerium doping beneath the conduction band. We have calculated the valence band and conduction band positions of all the compounds (Table S5. ESI) and found that the band positions are also changing with Ce doping as we have found in XPS valence band spectra. DFT Spin States and DOS As observed in the electron spin resonance experiments, the undoped BZO and Ce doped BZO were found to be ESR active, which essentially an indication of the presence of [ZrO5• V•O ] and [CeO5• V•O] vacancy defect states. Indeed in calculations of Ce doped BZO with the inclusion of various oxygen defect states, predominantly the [ZrO5• V•O ] state is found to be spin polarized (~1.0 µB/vacancy). Intuitively, one would anticipate the monoanionic oxygen vacancy is of paramagnetic in nature. However, the neutral oxygen vacancy [VO× ] may get spin induced by Ce as it can vary in mixed valance states, Ce (IV) and Ce (III). In calculations, for Ce more than 2% in BZO, the vacancy state [CeO5• VO× ] is found to be spin polarized. The spin-polarized state of an oxygen vacancy defect state [CeO5• VO× ] can only be argued or explained for its occurrence when one of the doped Ce readily get reduced by donating only one electron rather than two electrons to the vacancy site. In fact, the magnetic moment gradually increased with increase of the percentage of Ce doping, from 0.4 to 1.3 µB/vacancy in structures consisting of [VO× ] oxygen vacancies. It is therefore, effectively signifying the presence of Ce (III) state. For example, we present here the case of 3% Ce doped BZO with no vacancy (BaZr0.97Ce0.03O3) and with all the possible three × • oxygen vacancy types (BaZr0.97Ce0.03O3), [VO× ], [V•O], and [V•• O ]. It is found that the [VO ] and [VO]


32

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Figure 13. Calculated spin polarized charge density isosurfaces of 3% Ce doped BZO (BaZr0.97Ce0.03O2.99) with the inclusion of (A) [VO× ] and (B) [VO] oxygen vacancy states. Ba sublattice is not shown for clarity. The atoms are colored with the same coloring scheme as in Figure 4. The spin polarized charge density isosurfaces are shown at the isosurface value of 0.002 (eV/Å3). For a somewhat better view of oxygen spin charge density, a zoom-in version of a part of Figure 13 is shown in Figure S6, where the isosurfaces around the vacancy region are shown at 0.001 (eV/Å3). are spin polarized in their ground states. The calculated spin charge density for these spins states are shown Figure 13. It is evident from the spin polarized charge density isosurfaces that the spin is mostly localized in the region of vacancy and as well as on Ce (III) lattice site (designated as Ce1). Interestingly, the Ce2 and Ce3 sites that are located diagonally farther from vacancy by 7.56 Å and 19.87 Å are


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residually also reduced to Ce (III) state. The oxygen atoms around the vacancy are partially oxidized as well (see Figure 13 and also S6, ESI). Note the 4f-like orbitals topology (l = 3, m = ± 2 and 0) in Ce (III) spin-polarized charge density and the residually spin-polarized oxygen 2p-like charge density states (l = 1, m = 0 and ± 1) around the vacancy connected to the nearest Ce/Zrmetal centers. All in all, the vacancy and geometric distortions around it synergistically triggered the formation of reduced Ce (III) sites. Further, it is also evident from the calculations of electronic density of states (as detailed below) that the Ce is indeed reduced to Ce (III) state in the Ce doped BZO3-δ (δ = 0.01). In calculations, the vacancy [ZrO5• V•• O ] is never found to be spin-active, irrespective of the percentage of Ce doping undertook in this study. However, the vacancy and Ce spin state is largely controlled by the fraction of oxygen vacancies verses the Ce concentrations in BZO. The electronic structure of BZCO3-δ compounds has been further analyzed by calculating the electronic density of states (DOS). The calculated DOS for the undoped and the 3% Ce doped BZO3-δ (δ = 0.01) at respective oxygen vacancies are shown in Figure 14. For reference the cubic phase of BaCeO3 is also calculated and compared with Ce doped BZO compounds. The DOS of BZO, 1%, 2% and 4% Ce doped BZO with all the oxygen vacancy types are shown in SI, Figure S7. It is well known that the conduction band of BZO is predominantly formed by 4d orbitals of Zr whereas the valence band is made of O 2p orbitals.26 The presence of oxygen vacancies and different disordered states can form additional mid-gap energy levels in the BaZr1–xCexO3-δ (x = 0.00 – 0.04) lattice. From preceding reports we know that surface oxygen vacancies with a pair of free trapped electrons form an additional mid-gap adjacent to the valence band whereas oxygen vacancies with zero trapped electrons form an additional mid-gap state adjacent to the conduction


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band of the material and paramagnetic oxygen vacancies with one unpaired trapped electron form mid-gap states with in the band gap of the material.72 Owing to the presence of Ce in Ce (III)/Ce (IV) mixed oxidation states, it creates an additional impurity level within the band gap of BZO which act as electron acceptor/trap centers leading to enhanced light absorption and charge separation.73 To that effect, the calculated DOS presented in Figure 14 indicate that the electronic states just below the Fermi (EF), between 0 to -4 eV are composed of mainly O(2p) states with some overlap from Zr(4d) and Ba(5p) orbitals. The conduction band minima is mainly contributed from Zr(4d)

Figure 14. Calculated DFT-PBE density of states (DOS) of BaZrO3, and 3% Ce doped BaZrO3 without oxygen vacancy denoted by [OO× ] and with the inclusion of all the three oxygen vacancy types, [VO× ] [V•O], and [V•• O ]. For reference, the DOS of cubic BaCeO3 is also shown here.


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states in undoped BZO and Ce(4f) states in all the Ce doped BZO and BCO compounds. As the amount of doped Ce is very minimal (1 to 4) % relatively to Zr in BZO the projection of Ce states are subsided near the conduction band minima (CBM) in Figure 14, where the DOS is shown per states/eV/electron. When the DOS is constructed per states/eV/atom-type and the total wave function around the Fermi is decomposed for the charge density, both the DOS and the projected charge density isosurfaces pointed that the CBM in Ce doped BZO is clearly composed of Ce (4f) states. These are shown in Figure S7a-S7g, respectively. As shown in Figure 14C and 14D, the [VO× ] and [V•O] states are spin polarized; the vacancy defect states are below and around the EF in the spin-polarized channels, respectively. Integrating the vacancy states in [VO× ] and [V•O] DOS resulted in total 1 electron, in each. It vindicate that the neutral [CeO5• VO× ] and anionic [CeO5• V•O] vacancy states in 3% Ce doped BZO certainly consists of reduced Ce (III) ions, as is also seen in the spin projected charge density maps. It is to be noted from the Figure 14 that the defect mid-gap states and the Ce (4f) states at CBM reduces the band gaps of BZCO3-δ in comparison to the undoped BZO. The calculated band gap for the undoped BZO is about 3.0 eV which is comparable to the previously reported DFT-PBE calculations,43, 44 but smaller than the experimental value 4.8 to 5.3 eV.20, 69, 75 The band gaps predicted by PBE calculations are usually underestimated; it is an artifact of the PBE semi-local exchange correlation functional. Upon doping Ce in BZO, the band gaps reduced to ~2 eV. Various oxygen vacancies in BZCO produced different band gaps ranging from 2.9 to 1.6 eV. All the calculated band gaps for BaZr1-xCexO3-δ, (where x = 0.00 ‒ 0.04 and δ = 0.00 and 0.01) are reported in Figure 15. The results are compared with the observed experimental values tabulated in Table S5. While the observed experimental values are for defect BZO and


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Figure 15. Calculated DFT-PBE band gaps of BaZr1-xCexO3-δ, (where x = 0.00 ‒ 0.04 and δ = 0.00 and 0.01) without oxygen vacancy denoted by [OO× ], and with the inclusion of all the three possible oxygen vacancy types, [VO× ], [VO], and [V O ]. Some of the BZO and BZCO structures studied here are found to be metallic/semi-metallic (zero or close to zero band gap) either in non-spin-polarized states (for example, zero band gap in BaZrO2.99 and BaZr0.99Ce0.01O2.99 with oxygen vacancy type [VO× ], shown above) or spin-polarized channels (not shown here, but referred to Figure 14 and Figure S7). BZCO structures, the trends in the band gaps are comparable between experiments and theory, in particular for the defect states of [VO× ] and [V•O]. Photocatalytic Hydrogen Production Ultraviolet-visible radiation assisted H2 evolution from water by BaZr1−xCexO3 (x = 0.00 – 0.04) photocatalysts were analyzed in the presence of 0.25 M Na2SO3/0.35 M Na2S mixture as the


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sacrificial reagent for photogenerated holes. The data are shown in Figure 16, and clearly indicate that photocatalytic H2 production efficiency in Ce doped BZO is higher than that of the BZO and BaZr0.97Ce0.03O3 shows highest photocatalytic efficacy among all the catalysts. The valence band maxima of the photocatalysts is more positive than the H2O/O2 reduction potential and the conduction band minima is more negative than the H2/H+ redox potential.20,

26

Upon photo-

excitation of the semiconductor by light of suitable energy, photo-generated electrons and holes are produced. These photogenerated charge carriers may diffuse to the surface of the catalyst where they react with the reactants. It is well known that during photocatalytic H2 evolution, the reduction of photogenerated holes by using a sacrificial reagent can enhance its efficiency as the sacrificial reagent (Na2SO3) can consume the photogenerated holes in the valence band of the semiconductor and leaves the photogenerated electrons in the conduction band, which in turn reduces the charge carrier recombination and thereby enhances the charge separation.76, 77 In this work, the amount of H2 gas produced by as-synthesized BZO is 316 µmol per hour per gram. Compared to parent BZO, the Ce doped BZO photocatalysts exhibit superior catalytic activity, inferring the beneficial role of Ce-doping in BZO. The improved H2 production efficacy can be attributed to the change in electronic structure and enhanced light absorption of the catalysts in the visible light regime after Ce doping. From Figure 16 we note that in as-synthesized compounds, 3 atom% is the optimal Ce doping concentration for the highest amount of H2 gas evolution (545 µmol/h/g), whereas, BaZr0.99Ce0.01O3, BaZr0.98Ce0.02O3 and BaZr0.96Ce0.04O3 produces 349 µmol/h/g, 412 µmol/h/g and 456 µmol/h/g of H2 gas respectively. The calculated AQY values of H2 production of as-synthesized BZO, BaZr0.99Ce0.01O3, BaZr0.98Ce0.02O3, BaZr0.97Ce0.03O3 and BaZr0.96Ce0.04O3 are 2.3%, 2.6%, 3%, 4% and 3.3% respectively. It is observed that beyond 3 atom % Ce doping, the H2 gas production efficacy decreases which can be


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Hydrogen production (µmol/h/g) 823 684 482

405 316

BaZrO3

570

349

545 412

456

BaZr0.99Ce 0.01O3 BaZr0.98Ce 0.02O3 BaZr0.97Ce 0.03O3 BaZr0.96Ce 0.04O3

As-synthesized photocatalysts

Calcined photocatalysts

Figure 16. Rate of photocatalytic hydrogen gas evolution of BaZr1−xCexO3 (x = 0.00 – 0.04) catalysts under UV-visible light irradiation with 0.25 M Na2SO3/0.35 M Na2S mixture as sacrificial reagent. explained by the formation of defect states which act as a charge carrier recombination site.78 After calcination at 700 °C for 2 h under air the efficiency of all the catalysts increased to 405 µmol/h/g (BZO), 482 µmol/h/g (BaZr0.99Ce0.01O3), 570 µmol/h/g (BaZr0.98Ce0.02O3), 823 µmol/h/g (BaZr0.97Ce0.03O3) and 684 µmol/h/g (BaZr0.96Ce0.04O3). The calculated AQY values of H2 production of calcined BZO, BaZr0.99Ce0.01O3, BaZr0.98Ce0.02O3, BaZr0.97Ce0.03O3 and BaZr0.96Ce0.04O3 are 3%, 3.5%, 4.2%, 6% and 5% respectively. From the O 1s XPS spectra and ESR analysis, we have found that even after calcination the catalysts possess certain amount of oxygen vacancies in their lattice. Therefore, the increase in photocatalytic efficiency in these catalysts after calcination could be due to the reduction of crystal defects and increase in crystallinity.


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CONCLUSIONS In summary, barium zirconate and cerium doped barium zirconate hollow spheres have been successfully synthesized using low temperature template free hydrothermal method. All the compounds show cubic phase with high range of crystallinity. It is demonstrated through systematic DFT calculations and photo- and spin-resonance spectral analysis that the Ce exists clearly in mixed valance states (III/IV) stipulated by the presence of various oxygen defect states. The as-synthesized compounds produced hydrogen upon illumination without any co-catalysts but in the presence of sacrificial donor molecules. As found from UV-Vis DRS, XPS, ESR and DFT analysis, the presence of disordered lattice, oxygen vacancies and cerium doping plays a critical role in enhancing the photocatalytic activity in the ultraviolet-visible region. BaZr0.97Ce0.03O3 among all the as-synthesized compounds, BaZr1−xCexO3 (x = 0.00 – 0.04), shows highest efficiency in hydrogen gas production concomitant with oxidation of a sacrificial donor. ASSOCIATED CONTENT Supporting Information. Rietveld refined PXRD profiles of the synthesized samples with nominal compositions corresponding to the following formulas: (A) BaZrO3, (B) BaZr0.99Ce0.01O3, (C) BaZr0.98Ce0.02O3 and (D) BaZr0.96Ce0.04O3; Structural parameters obtained by Rietveld refinement of the powder Xray diffraction data of BaZr1−xCexO3 (x = 0.00 – 0.04) catalysts; EDX analysis of BaZrO3; EDX analysis of BaZr0.97Ce0.03O3; Tauc plot of as-synthesized BaZr1−xCexO3 (x = 0.00 – 0.04) photocatalysts; Room temperature ESR spectra of BaZr1−xCexO3 (x = 0.00 – 0.04) catalysts calcined at 700 °C for 2h under air; band position calculations of the catalysts; DFT optimized crystal coordinates and structural parameters like distances etc; total electronic energy


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comparisons, spin-charge density and band decomposed charge density isosurfaces; DOS of BaZr1-xCexO3-δ, (where x = 0.00 to 0.04 and δ = 0.00 and 0.01). AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected] (M.Q.).

*E-mail:

[email protected] (D.L.V.K.P.).

*E-mail:

[email protected] (S.A.).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style). ACKNOWLEDGMENT We thank the Department of Science and Technology for financial support (project no. DST/EMR/2016/005123). Instrumentation help from the Indian Institute of Technology (IIT) Guwahati and Central Instruments Facility; IIT Guwahati are acknowledged. XPS studies were performed at the UC Irvine Materials Research Institute (IMRI) using instrumentation funded in part by the National Science Foundation Major Research Instrumentation Program under grant no. CHE-1338173. ASP acknowledges Prasenjit Sarkar, Bibhash Bhunia for their help in ESR and EDX measurements. MQ acknowledges his Fulbright fellowship grant: 2205/F-NAPE/2016 to


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visit University of California Irvine with Professor Shane Ardo. MSC thanks the CSIR (India) for a SRF research fellowship. DLVKP acknowledge the Computer Center High Performance Computing facility at IIT Kanpur and HPC/RNJJ (through Initiation Grant No. IITK/CHM/20130116) for computational resources. REFERENCES (1) Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A. Z-Schematic Water Splitting into H2 and O2 Using Metal Sulfide as a Hydrogen-Evolving Photocatalyst and Reduced Graphene Oxide as a Solid-State Electron Mediator. J. Am. Chem. Soc. 2015, 137 (2), 604– 607. (2) Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. J. Am. Chem. Soc. 2011, 133 (29), 11054–11057. (3) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37‒38. (4) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38 (1), 253–278. (5) Navarro Yerga, R. M.; Álvarez Galván, M. C.; del Valle, F.; de la Mano, J. A.; Fierro, J. L. G. Water Splitting on Semiconductor Catalysts under Visible-Light Irradiation. ChemSusChem 2009, 2 (6), 471–485.


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Ba

536

Zr/Ce


BaZrO3

534

532

530

O

528

Binding energy (eV)


BaZr0.96Ce0.04O3

Hydrogen production (µmol/h/g)

BaZr0.97Ce0.03O3

823 684

U''' U'

316

U V'''

V''

V'

910

900

890

Binding energy (eV)

349

545 412

456

BaZr0.98Ce0.02O3 BaZr0.99Ce0.01O3 BaZrO3

V BaZrO 3

920

570

482

405

U''

Intensity (a. u.)

Intensity (a. u.)

BaZr0.97Ce0.03O3

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


880

BaZr0.99Ce0.01O 3 BaZr0.98Ce0.02O 3 BaZr0.97Ce0.03O 3

As-synthesized photocatalysts

BaZr0.96Ce0.04O 3

Calcined photocatalysts

320

340

360

380

Magnetic field (mT)

400


55

Combined Experimental and Theoretical Insights into the Synergistic

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