Role of Surface Oxygen Vacancies and Lanthanide Contraction

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Article Cite This: ACS Omega 2018, 3, 6267−6278

Role of Surface Oxygen Vacancies and Lanthanide Contraction Phenomenon of Ln(OH)3 (Ln = La, Pr, and Nd) in Sulfide-Mediated Photoelectrochemical Water Splitting Suresh Reddy Sanivarapu,† John Berchmans Lawrence,‡ and Gosipathala Sreedhar*,‡ †

Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamil Nadu, India ‡ Electropyro Metallurgy Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamil Nadu, India S Supporting Information *

ABSTRACT: Herein, we report the role of surface oxygen vacancies and lanthanide contraction phenomenon on HS− anion adsorption and desorption in the sulfide-mediated photoelectrochemical water splitting of Ln(OH)3 (Ln = La, Pr, and Nd). The Ln(OH)3 were synthesized via a solvothermal route using ethylenediamine as the solvent. The surface defects are characterized by Raman, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and highresolution transmission electron microscopy analyses. The photoelectrochemical water-splitting behavior of Ln(OH)3 enriched with surface oxygen vacancies has been examined in a 1 M Na2S solution under illumination conditions. La(OH)3 exhibited a highly stable and saturated current density of ∼26 mA/cm2 at 0.8 V (vs Ag/AgCl). Similarly, the hydroxides of Pr and Nd demonstrated current densities of 18 and 14 mA/cm2, respectively, at 0.8 V (vs Ag/AgCl). A reduction trend in the saturated current densities from La to Nd indicates the lanthanide contraction phenomenon, where the basicity decreases in the same order. The results also demonstrate that the surface adsorption of the HS− anion in the active sites of the surface oxygen vacancies played a vital role in enhancing the photoelectrochemical water-splitting behavior of Ln(OH)3. The stability of Ln(OH)3 was examined after 4 h of chronoamperometry studies at 0.8 V (vs Ag/AgCl) and analyzed using X-ray diffraction, Fourier transform infrared, Raman, and EPR and XPS analyses. The results show that the Ln(OH)3 exhibited excellent stability by demonstrating their phase purity after photoelectrochemical water splitting. We propose Ln(OH)3 as highly stable photoelectrochemical water-splitting catalysts in highly concentrated sulfide-based electrolytes and anticipate Ln(OH)3 systems to be explored in a major scale for the production of H2 as an ecofriendly process.

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Wang et al.9 demonstrated an enhanced photocatalytic degradation of Congo red dye using Ln (Ln = Sm, Er, Gd, Dy, and Eu)-doped La(OH)3 in the presence of lattice defects which influenced the electron−hole separation. In addition to this, Dong et al.10 articulated that the surface oxygen vacancies in La(OH)3 drive excellent photocatalytic activity in NO removal because of the extended ultraviolet (UV) light capture. Though the surface oxygen vacancies in Ln(OH)3 are extensively studied,10 the use of rare-earth hydroxides as catalysts in photoelectrochemical water splitting is extremely sparse. In this work, we have synthesized the rare-earth hydroxides of lanthanum (La), praseodymium (Pr), and neodymium (Nd) by a simple solvothermal method using ethylenediamine (ED)

INTRODUCTION The utilization of solar energy (1.3 × 105 TW irradiates on the surface of earth) as an ecofriendly and renewable energy source to address the global warming issues is considered as an important field in the recent research and development across the world.1−3 The discovery of visible light-driven photochemical and electrochemical processes to produce hydrogen as a clean fuel is one of the promising routes to hinder the utilization of fossil fuels. In this regard, in 1972, Fujishima and Honda developed the TiO2 catalyst and demonstrated the photoelectrochemical water-splitting process as a highly efficient direction to produce hydrogen from water,4 and since then much research has been focused on developing photoactive semiconductor-based catalysts. Recently, rare-earth hydroxides Ln(OH)3 are attracting greater interest because of their unique electronic configuration in 4f shells, and owing to this advantage, rare-earth hydroxides are being explored as sensors,5 capacitors,6 catalysts,7 and sorbents.8 To highlight, © 2018 American Chemical Society

Received: March 7, 2018 Accepted: June 1, 2018 Published: June 11, 2018 6267

DOI: 10.1021/acsomega.8b00429 ACS Omega 2018, 3, 6267−6278

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Figure 1. (a) X-ray powder diffraction patterns for La(OH)3, Pr(OH)3, and Nd(OH)3; (b) variation of lattice parameter “a” from La to Nd hydroxides; and (c) variation in the crystallite size from La to Nd hydroxides.

Figure 2. HR-TEM image of (a) La(OH)3 NPs, (b) FFT pattern of La(OH)3 NPs, and (c) SAED pattern of La(OH)3 NPs. HR-TEM image of (d) Pr(OH)3 NPs, (e) FFT pattern of Pr(OH)3 NPs, and (f) SAED pattern of Pr(OH)3 NPs. HR-TEM image of (g) Nd(OH)3 nanorods, (h) FFT pattern of Nd(OH)3 nanorods, and (i) SAED pattern of Nd(OH)3 nanorods.

We have also carried out a 12 h chronoamperometry test for La(OH)3 and verified the stability issues in the long run. The photocurrent densities that tend to decrease from La to Nd reveal the lanthanide contraction phenomenon, where the basicity decreases in the same order. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), Raman, and Fourier transform infrared (FTIR) results before and after the photoelectrochemical tests demonstrated the excellent stability of the Ln(OH)3 systems. Our results pave the pathway of stable and prolonged activity for the Ln(OH)3 systems and can be anticipated on a large-scale photoelectrochemical hydrogen generation. This

as the solvent and explored them as photoelectrocatalysts in 1 M Na2S electrolyte. The results demonstrated that the Ln(OH)3 systems exhibited highly stable and saturated photocurrent densities because of the presence of surface oxygen vacancies, as active sites, for the surface adsorption of HS− anion, and promoted the extensive hole (h+) utilization and subsequently enhanced the hydrogen generation under illumination in the sulfide-based electrolytes. Our results also reveal the surface adsorption of the HS− anion is more significant than that of the OH− anion in an enhanced photoelectrochemical water-splitting activity of Ln(OH)3. The chronoamperometry tests of Ln(OH)3 demonstrated the highly stable and saturated photocurrent densities for a period of 4 h. 6268


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Figure 3. UV-DRS of (a) La(OH)3, (b) Pr(OH)3, and (c) Nd(OH)3.

would be the first report on Ln(OH)3 in photoelectrochemical water splitting.

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pattern. Figure 2a represents the La(OH)3 nanoparticles (NPs) and elucidates the uniform size distribution in the range of 5− 20 nm in diameter, which were derived from the solvothermal synthesis. The nanosize nature of particles is also supported from the broadness of the XRD pattern for La(OH)3. Figure 2b depicts the fast Fourier transform (FFT) patterns for the La(OH)3 NPs and elucidates clear lattice fringes with interplanar distances of 0.318 and 0.228 nm that correspond to the (101) and (201) planes of the hexagonal system with the P63/m space group, respectively. The SAED pattern of the La(OH)3 system is depicted in Figure 2c and reveals the polycrystallinity. The indexed circles (bright spot array) represent the (101), (201), and (102) planes, which are consistent with the XRD patterns of La(OH)3. Further, Figure 2d represents the morphology of the Pr(OH)3 particles, and it is observed that the formation of NPs in the size range of 10− 40 nm is achieved via solvothermal synthesis. Figure 2e demonstrates the lattice fringes of the Pr(OH)3 NPs with interplanar distances of 0.314 and 0.225 nm. The corresponding SAED patterns shown in Figure 2e and the indexed planes of (101), (201), and (211) correspond to the hexagonal phase of Pr(OH)3 with the P63/m space group with polycrystallinity. Likewise, the morphology of Nd(OH)3 is shown in Figure 2g. Interestingly, Nd(OH)3 demonstrates the one-dimensional nanorod formation with the size in the range of 100−300 nm in length and 30−50 nm in width. Figure 2h represents the lattice fringes of the Nd(OH)3 nanorods with interplanar distances of 0.310 and 0.223 nm. The corresponding SAED patterns are shown in Figure 2i, with the indexed planes of (100), (101), and (201) that correspond to the hexagonal phase of Nd(OH)3 with the P63/m space group. The size of the rare-earth hydroxides is increased from La(OH)3 to Nd(OH)3 because of the nature of the rare-earth elements, chemical potential of the reaction solution, and the capping agents. However, in the presence of surfactants or capping agents, the growth mechanism can be varied. In the current work, ED acts as both a surfactant and a capping agent.13 As a basic solvent, it triggers the controlled oxidation of the rare-earth nitrate salt into hydroxides. By covering the surface of the nanocrystals as a layer, ED further prevents the inward diffusion of the elements, thus restricting the grain growth. The change in the size from La to Nd hydroxides may be due to the amount of ED covering, which is based on the positive charge of the crystallites. As the covalent nature of Ln-(OH)3 increases from La to Nd, the electrostatic forces between Ln-(OH)3 and ED will be suppressed, driving the lower ionic radii containing element “Nd” to grow anisotropically. In most of the studies,10,14 Ln(OH)3 synthesized without surfactants and templates reveals

RESULTS AND DISCUSSION

X-ray Diffraction Analysis. The crystallinity and phase purity of the synthesized La(OH)3, Pr(OH)3, and Nd(OH)3 were analyzed using XRD (Figure 1a). The XRD spectrum for La(OH)3 exhibits the planes (100), (101), (200), (201), (300), (112), (311), (302), and (410) at 2θ of 15.66°, 27.97°, 31.62°, 39.47°, 48.26°, 55.26°, 64.02°, 69.70°, and 77.27°, respectively. The planes indicate the high purity and crystallinity of La(OH)3. The broadened peaks elucidate the “nano” structure of La(OH)3 particles. All the reflected planes are well-matched with the standard JCPDS file no. 36-1481 (a = b = 6.528 Å and c = 3.858 Å), which demonstrates a hexagonal phase for La(OH)3 with the P63/m space group. The XRD spectrum for Pr(OH)3, exhibited the planes (100), (101), (201), (211), and (220), at 2θ of 15.9°, 28.7°, 40.44°, 49.56°, and 56.37°, respectively, indicating high crystallinity and high phase purity. The obtained planes are well-matched with the standard JCPDS file no. 45-0086 (a = b = 6.524 Å and c = 3.780 Å) and demonstrates a hexagonal phase for Pr(OH)3 with the P63/m space group. Further, the XRD spectrum for Nd(OH)3 exhibited the planes (100), (101), (200), (201), (211), (220), (131), (302), and (410) at 2θ of 16.66°, 28.87°, 32.27°, 40.47°, 49.77°, 57.14°, 65.58°, 71.73°, and 78.77°, respectively, indicating high crystallinity and high phase purity. These patterns are well-matched with the standard JCPDS file no. 70-0215 (a = b = 6.422 Å and c = 3.742 Å) and demonstrates a hexagonal phase for Nd(OH)3 which also belongs to the P63/m space group. The lattice parameter “a” is decreased from La to Nd because of the lanthanide contraction phenomenon (shrinking of the unit cell volume) (Figure 1b). As the ionic size of the lanthanides in Ln3+ decreases from La to Nd, the covalent character of Ln-(OH)3 increases and hence the basicity decreases.11 Thus, La(OH)3 is more basic, whereas Nd(OH)3 is less basic. The addition of electrons in the 4f shell increases the nuclear charge and the outer electrons become constricted (changes in the crystallite size, Figure 1c). The broadness or the full width at half-maximum of the planes for Ln(OH)3 decreased from La to Nd, which is again also realized from the lanthanide contraction phenomenon.12 High-Resolution Transmission Electron Microscopy Analysis. Figure 2 illustrates the high-resolution transmission electron microscopy (HR-TEM) images of La(OH)3, Pr(OH)3, and Nd(OH)3, giving insight into the morphology, lattice fringes, and the selected area electron diffraction (SAED) 6269


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Figure 4. PL spectra of (a) La(OH)3, (b) Pr(OH)3, and (c) Nd(OH)3.

Figure 5. (a) EPR spectra of La(OH)3, Pr(OH)3, and Nd(OH)3; (b) g factor value variation from La(OH)3 to Nd(OH)3; and (c) EPR signal intensity variation from La(OH)3 to Nd(OH)3.

The absorption bands beyond 600 nm, found at 682, 746, 800, and 872 nm, are due to the absorption from 4I9/2 to the 4F9/2, 4 F7/2 + 4S3/2, 4F5/2 + 2H9/2, and 4F3/2 transitions of Nd3+ ions, respectively.16,17 Photoluminescence Analysis. Figure 4 represents the emission bands ranging from 400 to 500 nm because of the crystal defects arising from the 4f to O 2p bands of the rareearth metals.18 According to the literature, surface oxygen vacancies play a vital role in creating trap-state emissions which enhance the photoluminescence of the rare-earth hydroxides.19 The room-temperature emission photoluminescence spectra of the hydroxides of La, Pr, and Nd powders with an excitation wavelength at 230 nm are reported in Figure 4. Because of the vacant f-orbital in La(OH)3, the La3+ ionic center is not regarded as an emission center and does not emit light from the inner 4f atomic orbital. The oxygen vacancies that exist in the La(OH)3 lattice act as the trap states for photoinduced electrons. A strong blue-emission band is observed at 469 nm and other small bands appear because of the trap-state emission from a deep-level charge transfer of the La(OH)3 crystal lattice.19 The photoluminescence (PL) spectra of Pr(OH)3 demonstrated peaks at 434, 464, and 482 nm. These bands are obtained from the f−f transitions of the ground-state 3H4 absorption to the next higher orbital (3H4 → 3P2, 3H4 → 3P1, and 3H4 → 3P0).20 The PL spectra of Nd(OH)3 show peaks at 428, 451, and 469 nm. The blue band is obtained at around 469

that the anisotropic growth is mainly directed by the inherent nature of the rare-earth elements and the chemical potential of the reaction solution.14 Here, the surface morphologies of La, Pr, and Nd hydroxides are found to be dissimilar in comparison with the reported literature, which is attributed to the role of the surfactant and the covalent nature of Ln-(OH)3. UV−Vis Analysis. Figure 3 represents the UV−visible diffuse reflectance spectra (UV-DRS) of the powdered samples of La(OH)3, Pr(OH)3, and Nd(OH)3. La(OH)3 shows the absorption band at 275 nm (Figure 3a), which can be assigned to the intrinsic band absorption, and further, the broad absorption in the range of 280−350 nm elucidates the surface oxygen vacancies.10 The direct band gap of La(OH)3 is calculated and found to be 4.5 eV. Similarly, the absorption spectra of Pr(OH)315 illustrate well-defined and sharp absorption bands at 450, 475, 488, and 595/600 nm. The first three bands are called blue bands and are attributed to the absorption from the ground-state term (3H4) to 3P2, 1l6 + 3P1, and 3P0. A red band is observed at 595/600 nm because of the transition from the ground-state term (3H4) to 1D2 and 1G4. This band appeared because of the water molecules attached to the Pr3+ ion.16 Further, in the case of Nd(OH)3, we have noticed well-defined bands at 358 nm in the UV region and at 430, 470, 524, and 578 nm in the visible region, attributed to the absorption from the ground-state term (4I9/2) to 2H11/2, 2 G9/2, 4G7/2 + 4G9/2 + 2K13/2, and 4G5/2 + 2G7/2, respectively. 6270


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Figure 6. Dislocation images of (a) La(OH)3, (b) Pr(OH)3, and (c) Nd(OH)3.

nm from the 4D3/2 → 4I15/2 or 2P3/2 → 4I13/2 transition.21 The bands at 425 and 451 nm are obtained from the 2P1/2 → 4I9/2 and 4G11/2 → 4I9/2 transitions, respectively.22 The UV-DRS and PL spectra of Ln(OH)3 indicate that the synthesized compounds do possess the surface defects and oxygen vacancies. These observations are further analyzed by EPR. EPR Analysis. The EPR analysis was employed to detect the nature of defect on the La(OH)3, Pr(OH)3, and Nd(OH)3 systems. The EPR experiments were conducted at 100 K. The EPR spectra of the rare-earth hydroxides of La(OH)3, Pr(OH)3, and Nd(OH)3 are reported in Figure 5a. The rareearth hydroxides exhibited a dominant signal with a g factor of 2.02788, 2.01488, and 2.0006 for La(OH)3, Pr(OH)3, and Nd(OH)3, respectively. The demonstrated g values are not equal to the free-electron g-value, 2.0023.23 As reported recently by Kakazey et al., the g value for the oxygen vacancies was found to be ∼2.003 because of the chemisorbed O2−.24 To highlight, Zhu et al.25 acquired a dominant EPR signal at a g factor of ∼2.0190 for the surface oxygen vacancies in ZnO. Likewise, Jing et al.26 demonstrate that the O2− (oxygen vacancies) gave signals at g ≈ 2.0106 and is confirmed in the ZnO crystals. On the basis of these reported works, the elucidated g factors for La(OH)3, Pr(OH)3, and Nd(OH)3 clearly authenticated the presence of the surface oxygen vacancies. Figure 5b shows that the surface oxygen vacancies are more for La(OH)3 than that for Pr and Nd hydroxides. These oxygen vacancies can serve as photoinduced charge traps10 (as observed from the UV and PL studies) and can be anticipated to demonstrate the enhanced photoelectrocatalytic activity of the rare-earth hydroxides of the La(OH)3, Pr(OH)3, and Nd(OH)3 crystals. The intensity of signals decreased from La(OH)3 to Nd(OH)3, elucidating the decrease in the surface oxygen vacancies (Figure 5c). The formation of oxygen vacancies depends on the conditions favorable for the crystal growth of these rare-earth hydroxides. As discussed earlier, the surfactant ED suppresses the OH bonding with the rare-earth metal node. As the covalent nature of Ln(OH)3 increases from La to Nd, the role of the surfactant on Nd(OH)3 will decrease. Further, the presence of oxygen vacancies is confirmed by the HR-TEM images of these rare-earth hydroxides of the La(OH)3, Pr(OH)3, and Nd(OH)3 crystals and is represented in Figure 6. Figure 6 clearly indicates the formation of surface defects in the HR-TEM images on the surfaces of La(OH)3, Pr(OH)3, and Nd(OH)3, and similar lattice defects were also reported by Dong et al. in an earlier work.10 In Ln(OH)3, along with the surface oxygen vacancies, defects such as edge dislocations and screw dislocations are observed. Raman Spectra. The Raman spectra of La(OH)3, Pr(OH)3, and Nd(OH)3 obtained with an excitation wavelength of 360 nm are depicted in Figure 7. By the factor group analysis on the space group P63/m, the hexagonal Ln(OH)3 structures

Figure 7. Raman spectra of La(OH)3, Pr(OH)3, and Nd(OH)3.

are realized as the irreducible representations of the Ramanactive vibrations A1g, E2g, and E1g.27 The Raman-active modes E2g and A1g are obtained from the anion translation behavior, whereas the Raman-active mode E1g is due to liberation mode of −OH dipole.27 The Raman spectrum of La(OH)3 that reveals the bands at 282, 340, and 451 cm−1 corresponding to the irreducible representations of A1g, E2g, and E1g clearly indicates the Raman-active modes of La(OH)3,28 as reported by Hoang et al., Similarly, the Raman bands obtained at 292, 354, and 466 cm−1 corresponding to the irreducible representations of A1g, E2g, and E1g designate the Raman-active modes of Pr(OH)3.29 Further, the Raman bands observed at 294, 365, and 472 cm−1 corresponding to the irreducible representations A1g, E2g, and E1g signify the Raman-active modes of Nd(OH)3.30 These Raman-active modes (A1g, E2g, and E1g) are found to shift to a higher wavenumber from La to Nd, which specifies the decrease in the cation to anion distance or, in other words, Ln to OH distance, also known as the lanthanide contraction phenomenon. However, a sharp band at around 3601 cm−1 has been identified for all Ln(OH)3 and can be assigned to the surface OH groups.31 In addition to this, this band also demonstrates the crystal defects and impurities.32 As the covalent nature increases from La to Nd, it is anticipated that more OH groups attach to Nd (see the discussion in the TEM analysis for size variation and surface oxygen vacancies). In contrast, the relative intensities of the Raman vibration modes at around 3601 cm−1 reveal the reduction from La to Nd, and based on this observation, the vibration mode may be attributed to the surface defects, rather than the surface OH group alone. This trend indirectly demonstrates that the surface defects are more for La(OH)3 and less for Nd(OH)3. 6271


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Figure 8. XPS (a) 3d spectra of La(OH)3, (b) O 1s spectrum of La(OH)3, (c) 3d spectra of Pr(OH)3, (d) O 1s spectrum of Pr(OH)3, (e) 3d spectra of Nd(OH)3, and (f) O 1s spectrum of the Nd(OH)3 electrodes.

XPS Analysis. The chemical speciation of the rare-earth hydroxides (Ln(OH)3) is examined by XPS. The full survey of the La(OH)3 electrodes shows the presence of La, O, C, F, and Sn elements (Figure S1a). The elements F and Sn are acquired from the base fluorine-doped tin oxide (FTO) plate. The photoelectron spectra of La 3d and O 1s were recorded and depicted correspondingly in Figure 8a,b. Typically, the La 3d spectrum exhibits multiplet split for the binding energies of the 3d5/2 and 3d3/2 regions with four Gaussian components. The peaks at 834 and 851 eV are attributed to the corresponding two spin−orbits of 3d5/2 and 3d3/2, and the same is also authenticated from the reported literature.33−35 Further, the peaks at 837 and 854 eV can be attributed to the satellite peaks of 3d5/2 and 3d3/2 spin−orbits, respectively. As stated by Mullica et al.,35 the electronic state configurations of [3d9]hole 4f0[L] (a core hole) and [3d9]hole 4f1[L]hole (a core hole with an electron transferred from L’s valence band to an empty 4f orbital) (L stands for the oxygen ligand) are responsible for the spin−orbits in La(OH)3, where f0 and f1 are acquired at low and high binding energies, respectively.36−39 The binding energy separation (ΔE) between f1 and f0 is found to be 3.9 eV, and ratio of f0/f1 is more than 1, which confirms the La3+ state of La(OH)3 and matches well with the earlier reports.33,35 The La 4p spectrum (Figure S1b) also reveals the La(OH)3 system based on the main peak (195.3 eV) and the satellite peak (199.3 eV) positions.33 The O 1s spectrum demonstrated a fine deconvolution with the two peaks. The peak at 530.1 eV is designated to the metallic oxygen (La−O) and the other peak at a higher binding energy of 532.1 eV can be assigned to the hydroxyl group.33,40−42 Similarly, XPS analysis is also carried out on the Pr(OH)3 electrode, and the full survey is presented in Figure S2. The full survey of the Pr(OH)3 electrodes shows the presence of the Pr, O, C, F, and Sn elements. The elements F and Sn are acquired from the base FTO conducting glass plate. The photoelectron spectra of Pr 3d and O 1s were

recorded and depicted in Figure 8c,d. The Pr 3d spectrum shows two prominent peaks at 934 and 954 eV, which can be designated to the spin−orbits of Pr 3d5/2 and Pr 3d3/2, respectively, with a separation of 20 eV. The shoulder peaks at 928 and 949 eV are attributed to the well-screened [3d9] 4f3 satellite peaks. The main peaks and satellite peaks indicate the trivalent Pr in the Pr(OH)3 system and is well-matched with the earlier reports.15,43−45 Figure 8d represents the O 1s spectrum of Pr(OH)3 that is deconvoluted into two peaks at 529.5 and 531.6 eV, which correspond to the oxygen with Pr (Pr−O) and the oxygen in the hydroxyl groups, respectively. Further, XPS analysis was also carried out on the Nd(OH)3 electrode, and the full survey is illustrated in Figure S3. The full survey of the Nd(OH)3 electrodes shows the presence of the elements Nd, O, C, F, and Sn. The elements F and Sn are acquired from the base FTO conducting glass plate. The photoelectron spectra of Nd 3d and O 1s were recorded and depicted in Figure 8e,f. The O KLL auger electron signals appear in the same range with Nd 3d and approximated with the clean KLL signals of La(OH)3 and Pr(OH)3 and subtracted by constructing the average oxygen KLL spectrum. In the photoelectron spectrum of Nd 3d, two prominent peaks are observed at 982 and 1006 eV, which are attributed to the spin− orbits of 3d5/2 and 3d3/2 with the [Nd4+ (3d5/2)1 4f3, 9OH−] final state I distribution46 and state II distribution for satellite peaks [Nd3+ (3d5/2)1 4f3, 9OH− − e−].47 Figure 8f represents the O 1s spectrum of Nd(OH)3 that is deconvoluted into three peaks at 529.5, 531.5, and 534.2 eV, which correspond to the oxygen with Nd (Nd−O), the oxygen in the hydroxyl groups, and that in carbonate (CO32−), respectively. An extra deconvoluted peak is obtained at 534.2 eV, which may be because of the consumption of CO2 by the surface rare-earth hydroxides. The molar ratios of O 1s/Ln 3d were found to be less than 3, which indicate the oxygen vacancies in all the three compounds. 6272


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Figure 9. Photoelectrochemical water-splitting behavior of the La(OH)3, Pr(OH)3, and Nd(OH)3 electrodes. (a) LSV curves, (b) chronoamperometry studies for a period of 1000 s, (c) chronoamperometry studies for a period of 4 h, and (d) amount of hydrogen evolution for the duration of 4 h for each electrode.

Photoelectrochemical Studies. Photoelectrochemical tests were conducted in a typical three-electrode system, in which platinum electrode was used as a counter electrode, Ag/ AgCl as a reference electrode, and the rare-earth Ln(OH)3 coated on FTO served as a working electrode. All these tests were conducted in 1 M Na2S solution as a sacrificial electrolyte, under the light illumination of 100 mW/cm2. Figure 9a represents the linear sweep voltammogram (LSV) curves of La(OH)3, Pr(OH)3, and Nd(OH)3, demonstrating the current densities of 19.5, 17, and 14 mA/cm2, respectively, at 0.8 V (Ag/AgCl) with a scan rate of 5 mV/s. Figure 9b represents the photoactivities of La(OH)3, Pr(OH)3, and Nd(OH)3 which were found to be 2, 1.5, and 1.0 mA/cm2 at 0.8 V (Ag/AgCl), respectively. Figure 9c represents the chronoamperometry of La(OH)3, Pr(OH)3, and Nd(OH)3 for a period of 4 h. The La(OH)3 electrode shows a saturated photocurrent density of 26.5 mA/cm2, and this saturation in current density was obtained after 1.3 h of the chronoamperometry test. The gradual increase in the current density of La(OH)3 clearly indicates that the surface reactions might take place on the electrode, which are time-dependent. This is due to the sluggish adsorption kinetics of the anionic species HO− and HS− present in the electrolyte and to the desorption of the SO42− species, as mentioned in the mechanism (eqs 1−14). In case of Pr(OH)3 and Nd(OH)3, the saturation photocurrent densities of 17.5 and 14 mA/cm2 are obtained in an early stage of 0.5 h of chronoamperometry. In case of La(OH)3, we have noticed a 2-fold increase in the photocurrent density when compared

with Nd(OH)3. This is due to the fact that the basicity of La(OH)3 is more and reactivity is high when compared with the other rare-earths hydroxides. Figure 9d indicates a pictorial representation of the hydrogen evolution diagram of La(OH)3, Pr(OH)3, and Nd(OH)3 for a period of 4 h. The La(OH)3 photoelectrode exhibited 1.1 mmol for 4 h, whereas Pr(OH)3 and Nd(OH)3 photoelectrodes exhibited 0.5 and 0.38 mmol, respectively. As mentioned above, the reduction in the basicity from La to Nd would decrease the activity. The adsorption and desorption processes on the rareearth electrodes in the Na2S electrolyte played a vital role, where the sulfide ions are electrochemically very active components that are adsorbed on the anode surface and donate electrons to undergo oxidation to produce sulfate as the end product. The mechanism48−50 for hydrogen production in a sulfide medium involves various reactions as described below. Irradiation with an electromagnetic radiation and generation of an electron−hole pair Ln(OH)3 + hv → Ln(OH)3 + h+ + e− (Ln = La, Pr and Nd)

Reversible reactions in an electrolyte

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Na 2S ↔ 2Na + + S2 −

(1)

S2 − + H 2O ↔ HS− + OH−

(2)

X + H 2O ↔ X−OH− + H+ + e− (X = Ln(OH)3 )

(3)


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(4)

Adsorption of active species on a catalyst surface X + HS− → X−HS− −

X + OH → X−OH

(5) −

X + S2 − → X−S2 −

(6) (7)

Oxidation of bisulfide and sulfide X−S2 − + 4H 2O → X−SO4 2 − + 8H+ + 8e−

(8)

X−HS− + h+ → X−HS•

(9)

X−HS• + X−OH− → X + X−S•− + H 2O

(10)

X−HS• + X−S•− → X + X−S2 H−

(11)

Figure 10. Chronoamperometry study of the La(OH)3 electrode in 1 M Na2S electrolyte for the period of 12 h.

(12)

water splitting. Also, from the photoelectrochemical studies of the Na2SO4 electrolytes (Figure S5), it is understood that the concentration of HS− is important for enhanced photoelectrochemical water splitting, and the HS− concentration is higher in Na2S than in the Na2SO4 electrolyte. After the photoelectrochemical water-splitting tests, we have performed a series of characterizations on the Ln(OH)3 electrodes using XRD, FTIR, Raman, and XPS analyses to demonstrate the extended stability in the sulfide media. XPS is employed on the surface of the electrodes to discover the changes in the chemical potentials of the rare-earth hydroxides. The full surveys of the Ln(OH)3 electrodes after the test are presented in Figures S1−S3. The La(OH)3 electrode exhibited the presence of extra elements of Na and S when compared before and after the test and is deconvoluted, as shown in Figure S6. The La 3d spectrum reveals a good match with the condition before the test and confirms that the La3+ state corresponds to that of La(OH)3. The two spin−orbit peaks at 834 and 851 eV and the satellite peaks at 837 and 854 eV of 3d5/2 and 3d3/2 exactly fit with the condition before the test. This observation affirms that the chemical potential of the La(OH)3 system has not been altered. Further, the O 1s spectrum is deconvoluted into two peaks, which are assigned to the metallic oxygen at 530.1 eV and the hydroxyl groups at 532.1 eV. Interestingly, the ratio of the peak areas of the metallic oxygen to the hydroxyl group is found to be lower than that before the test condition, which indicates that the surface is enriched with more hydroxyl groups, attributed to the surface adsorption of the OH− species in the oxygen vacancy sites on the electrode during the photoelectrochemical water splitting. The scans were also extended to the Na 1s and S 2p regions. The XPS spectrum of Na 1s is presented in Figure S6c and is deconvoluted into a single peak at 1071.6 eV, which indicates that Na+ is in the form of either sulfate or polysulfide.52 This observation is further supported by the S 2p spectrum (Figure S6d), which demonstrated two doublet peaks at 161.7 and 163.3 eV and another peak at a higher binding energy of 171.1 eV. As per the literature,52 the peak at 161.7 eV is assigned to the terminal polysulfide and the peak at 163.3 eV is assigned to the central polysulfide. The peak at the higher binding energy of 171.1 eV is assigned to the sulfates. Though the central polysulfides exhibit a peak at 160.8 eV for the Na2S salts reported by Sartz et al.53 in 1971, in the recent work, the peak at the binding energy of 163.3 eV is assigned to the central polysulfides,52 and the same is followed in this work. From

X−S2 H− + H 2O + 2e− → X−HS− + HS− + OH−

Desorption of sulfate X−SO4 2 − → X + SO4 2 −

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Production of hydrogen n nH+ + ne− → H 2 (here n = number of reactive species) 2 (14)

By the absorption of photons from light, the catalyst produces electron−hole pairs. Because of the presence of the oxygen vacancies on the surface of the photoanode, the electrons trapped on those catalytic sites and the Na2S sacrificial electrolyte consume the photogenerated holes and reduces the recombination of the electron−hole pairs. The surface adsorption of the sulfide, bisulfide, and hydroxyl ions is greatly influenced by the surface oxygen vacancies, which provide space for the adsorption sites. The sulfide and bisulfide species are very active species compared with the hydroxyl ions and thus greatly enhance the photoelectrochemical activity. As shown in eq 8, the sulfide ions are converted into sulfate ions directly or indirectly through a number of intermediate products such as polysulfides, sulfites and thiosulfates.51 Thus, the ultimate formation of sulfate ions taken place and go back to the electrolytic solution through the process of desorption. The bisulfide ions, as shown in eq 9, act as hole scavengers, which are responsible for the higher photocatalytic activity of the Na2S solution. Thus, the formed oxygen is consumed by the Na2S sacrificial electrolyte and eventually converted into hydroxide ions (eq 12) and finally converted into sulfate ions. The photogenerated electrons are consumed by the hydrogen ions and are converted into hydrogen gas at the cathode. The long-term chronoamperometry study of the La(OH)3 electrode is depicted in Figure 10. The chronoamperometry studies of the La(OH)3 electrode reveal the stability of the photoanode for a 12 h period. We have obtained a stable current density of 26 mA/cm2, which indicates the stability of the rare-earth hydroxides in the case of photoelectrochemical water splitting. In the photoelectrochemical studies of these rare-earth hydroxides in other electrolytes like KOH and Na2SO4, the electrodes exhibited a poor performance (0.45 mA/cm2 at 0.8 V Ag/AgCl) (Figure S4). In the KOH electrolyte, the surface adsorption and desorption of OH− are not realized as the major contributing reactions for enhanced 6274


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in the other electrodes. These observations are further supported by the FTIR analysis before and after the tests of the rare-earth hydroxide electrodes. The FTIR analysis is carried out on Ln(OH)3 after long-term photoelectrochemical tests and is reported in Figure S9. La(OH)3 shows the bands at 649, 1388, 1496, 3425, and 3612 cm−1 before the test (Figure S9a). The vibration mode at 649 cm−1 is attributed to the characteristic La−O−H bending vibrations; the vibrations at 1388 and 1496 cm−1 are due to the N−H stretching of the amine group of the ED surfactant; and that at 3425 cm−1 is attributed to the O−H vibrations of the surface-adsorbed water molecules.54 The vibration at 3612 cm−1 is attained as a result of the O−H tension in the hydroxyl group. Interestingly, after the test, La(OH)3 shows the presence of extra vibrations at 1002 and 1136 cm−1 along with the vibrations that were present before the test. The vibration bands at 1002 and 1136 cm−1 are the characteristic peaks of the inorganic ions and indicate the resonance interaction attributed to the polysulfides and sulfates.55,56 To highlight, Moraes et al.56 describes that the vibration mode around 1150 cm−1 is due to the adsorbed SO42−, and this observation is further supported by Peak et al.57 Further, the intensity of the vibration mode at 3425 cm−1 increased when compared before and after the test, which indicates the extra OH surface adsorption during photoelectrochemical water splitting. Similarly, Pr(OH)3 shows the bands at 665, 1382, 1506, 3427, and 3608 cm−1 before the test (Figure S9b). The vibration mode at 665 cm−1 is attributed to the Pr−O−H bending vibrations;15 the 1382 and 1506 cm−1 vibrations are due to the N−H stretch of the amine group of the ED surfactant; and that at 3427 cm−1 is attributed to the O−H vibrations of the surface-adsorbed water molecules.54 The vibration at 3608 cm−1 is attained as a result of the O−H tension in the adsorbed hydroxyl group. Interestingly, after the test, Pr(OH)3 shows the presence of extra vibrations at 1002 and 1132 cm−1 along with the other vibrations that were present before the test. The vibration bands at 1002 and 1132 cm−1 are the characteristic peaks of inorganic ions and indicate the resonance interaction attributed to the polysulfides and sulfates.55,56 The intensity of the vibration mode at 3427 cm−1 increased when compared before and after the test, which indicates the OH surface adsorption during photoelectrochemical water splitting. Further, Nd(OH)3 shows the bands at 675, 1390, 1514, 3425, and 3608 cm−1 before the test (Figure S9c). The vibration mode at 675 cm−1 is attributed to the Nd−O−H bending vibrations;58 the 1382 and 1506 cm−1 vibrations are due to the N−H stretch of the amine group of the ED surfactant; and the 3425 cm−1 vibration is attributed to the O− H vibrations of the surface-adsorbed water molecules.54 The vibration at 3608 cm−1 is attained as a result of the O−H tension in the adsorbed hydroxyl group. Interestingly, after the test, Nd(OH)3 shows the presence of extra vibrations at 1004 and 1136 cm−1 along with the other vibrations that were present before the test. The vibration bands at 1004 and 1136 cm−1 are the characteristic peaks of the inorganic ions and indicate the resonance interaction attributed to the polysulfides and sulfates.55,56 The intensity of the vibration mode at 3425 cm−1 increased when compared before and after the test, which indicates the OH surface adsorption during photoelectrochemical water splitting. The Ln−O−H stretching is found to shift to a higher frequency from La to Nd hydroxides because of the increase in the covalent bonding nature (before the test

these observations, it is confirmed that Na is chemically bound with S and forms sodium sulfide and sodium sulfate. These results also confirm that sulfides and sulfates are formed over the electrode during photoelectrochemical water splitting.51 Further, the Pr(OH)3 electrode also exhibited the presence of extra elements of Na and S when compared before and after the test and is deconvoluted, as presented in Figure S7. The Pr 3d spectrum divulges a perfect match with the condition before the test and confirms the Pr3+ state corresponds to Pr(OH)3. The two spin−orbit peaks at 934 and 954 eV and the satellite peaks at 928 and 949 eV of 3d5/2 and 3d3/2 exactly fit with the same peaks with before condition. This observation asserts that the chemical potential of the Pr(OH)3 system has not been altered after the photoelectrochemical water-splitting test. The O 1s spectrum is deconvoluted into two peaks which correspond to the metallic oxygen at 530.1 eV and the hydroxyl groups at 532.1 eV. The XPS spectrum for Na 1s (Figure S7c) is deconvoluted into a single peak at 1071.6 eV and indicates that Na+ is in the form of either sulfates or polysulfides.52 Again, this observation is further supported by the S 2p spectrum (Figure S7d), which demonstrated two doublet peaks at 161.7 and 163.3 eV and another peak at a higher binding energy of 171.1 eV. As per the literature,52 the peak at 161.7 eV is assigned to the terminal polysulfide and the peak at 163.3 eV is assigned to the central polysulfides. The peak at the higher binding energy of 171.1 eV is assigned to the sulfates. From these observations, it is confirmed that Na is chemically bound with S and forms sodium sulfide and sodium sulfate. These results also confirm that sulfides are adsorbed and sulfates are formed over the electrode during photoelectrochemical water splitting.51 Similarly, the Nd(OH)3 electrode also exhibited the presence of extra elements of Na and S, and the Nd 3d spectrum notifies a perfect match with the condition before the test (Figure S8), confirming the Nd3+ state corresponds to Nd(OH)3. This observation declares that the chemical potential of the Nd(OH)3 system has not been altered. The O 1s spectrum is deconvoluted into three peaks which correspond to the metallic oxygen at 530.1 eV, the hydroxyl groups at 532.1 eV, and the carbonate at 534.2 eV. The XPS spectrum of Na 1s (Figure S8c) is deconvoluted into a single peak at 1071.6 eV and indicates that Na+ is in the form of either sulfates or polysulfides.52 The S 2p spectrum demonstrated two doublet peaks at 161.7 and 163.3 eV and another peak at a higher binding energy of 171.1 eV. Again, based on the literature,52 the peak at 161.7 eV is assigned to the terminal polysulfide and that at 163.3 eV is assigned to the central polysulfides. The peak at the higher binding energy of 171.1 eV is assigned to the sulfates. For all the three electrodes of the rare-earth hydroxides, before the test, the ratio of O/Ln was found to be lower than 3, which indicates the existence of surface oxygen vacancies on the electrodes. After the test, the ratio of O/Ln was found to more than 3, which further reveals the extra hydroxylation that induced more surface-adsorbed OH groups over the surface of the electrodes. Interestingly, the sulfate formation over the electrodes is decreased from La to Nd hydroxides during the 4 h photoelectrochemical tests. Similarly, the central and terminal polysulfides also follow the same trend from La to Nd hydroxides. The sulfate and polysulfide formation rate depends on the photoelectrochemical activity; as a higher activity is attained for the La(OH)3 electrodes, the formation of sulfate and polysulfides is higher in the La(OH)3 electrodes than that 6275


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CONCLUSIONS We have successfully demonstrated Ln(OH)3 as efficient and stable catalysts in sulfide-mediated photoelectrochemical water splitting and also provided clear insights into the role of the surface oxygen vacancies and lanthanide contraction phenomenon for rare-earth hydroxides in solar water splitting. The excellent stability demonstrated for the La(OH)3 electrode for 12 h duration elucidates the immensity of Ln(OH)3. We propose Ln(OH)3 as highly stable photoelectrochemical watersplitting catalysts in highly concentrated sulfide-based electrolytes and anticipate the Ln(OH)3 systems to be explored in a major scale for the production of hydrogen.

condition) and reveals the lanthanide contraction phenomenon, which is also supported by Raman spectroscopy. The Raman spectroscopy results of the electrodes after the tests are presented in Figure S10. The Raman spectrum of La(OH)3 reveals the bands at 273, 334, and 456 cm−1, corresponding to the irreducible representations of the A1g, E2g, and E1g active modes. Similarly, the Raman bands are obtained at 281, 349, and 452 cm−1 corresponding to the A1g, E2g, and E1g modes of Pr(OH)3. Further, the Raman bands observed at 287, 354, and 453 cm−1 corresponding to the A1g, E2g, and E1g signifies the Raman-active modes of Nd(OH)3. These Ramanactive modes (A1g, E2g, and E1g) are found to shift to a higher wavenumber from La to Nd, which specifies the decrease in the cation to anion distance or, in other words, Ln to OH distance, also known as the lanthanide contraction phenomenon. In addition to this, a sharp band at around 3601 cm−1 has been identified in all Ln(OH)3 and can be assigned to the surface OH groups31 and surface defects.32 In contrast, for each rareearth hydroxide, compared before and after tests, the respective values for A1g, E2g, and E1g were found to shift to a lower wavenumber, which might be due to the extra adsorbed OH group-induced relaxation in the electrostatic forces between the cation and anion in the unit cell. Further, the vibration peak at around 3601 cm−1 does not exhibit an appreciable change when compared before and after the tests for all the rare-earth hydroxide electrodes. However, these vibration mode peak intensities are decreased for all hydroxides when compared with those before the tests. The reduction percentages are found to be 93.4, 80.9, and 26.4 for La(OH)3, Pr(OH)3, and Nd(OH)3, respectively. This could be explained based on the reduction of the surface defects because of the adsorption of the anionic species over the surfaces of the electrodes. According to the photoelectrochemical water-splitting studies, SH− would be the rate-limiting anionic species than OH− in Na2S solutions.48 As the SH− anionic species is more active, according to eq 8, the oxidation reaction forms X−SO42− (as Na2SO4) on the surface. At higher photoelectrochemical current densities, the formation rate is higher over the electrodes, and hence the XPS results demonstrated a higher quantity of sulfides and sulfates on the La(OH)3 surface, which decreased gradually to Nd(OH)3. The EPR analyses on the after-test rare-earth hydroxide materials reveal the reduction in the intensities (Figure S11) and confirm the reduction in the surface defects by the strong adsorption of OH− ions, which is supported by XPS, Raman, and FTIR. Even though the XRD results (Figure S12) before and after the photoelectrochemical water-splitting tests did not reveal any of the sulfate-based compounds, indicating the negligible quantity for identification, it is strongly demonstrated that Ln(OH)3 remains in the same phase and purity (Figure S12). From this, it can be concluded that the rare-earth hydroxides have a strong stability in the Na2S electrolyte; adsorption and desorption are realized as the major mechanisms; and surface defects play a vital role in enhancing the photoelectrochemical water-splitting behavior of Ln(OH)3. On the basis of these findings, this work eradicates the ambiguity of the rare-earth hydroxides in the solar watersplitting application as photoelectrocatalysts. As the other catalysts can degrade or exhibit photocorrosion in high alkaline mediums, the proposed rare-earth hydroxides stand first in elucidating their stability in the long run and pave the pathway for their application in solar water splitting.

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EXPERIMENTAL SECTION Synthesis of Ln(OH)3. One gram of rare-earth nitrates (La, Pr, and Nd) are added to 30 mL of ED under stirring and transferred to 50 mL of autoclave and kept in a hot air oven at 180 °C for 8 h. Then, the autoclave is cooled to room temperature, and the precipitate is washed several times with ethanol−water by centrifugation and dried in vacuum at 65 °C. Preparation of Photoanode. The Ln(OH)3 powders of 3 mg each were mixed with 5−6 μL of Nafion and 50 μL ethanol solution until it becomes a paste. For conductivity purpose, 0.15 mg of graphite powder is added to the above paste and ground well. Then, the paste is applied as a thin layer on a PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)-coated FTO glass in a 1 cm2 area, dried, and used in photoelectrochemical studies. Photoelectrochemical Water Splitting. The photoelectrochemical water-splitting activities of Ln(OH)3 were recorded using a typical three-electrode potentiostat system (Autolab, PGSTAT 302N, Metrohm, model: AUT 83515 and Nova 1.8 software) where a Pt counter electrode was used as the cathode, Ag/AgCl was the reference electrode, and a photocatalyst material-coated FTO plate was used as the photoanode (with 1 cm2 exposure area). The exposure area of the photoanode was illuminated with a solar-simulated light source with a power intensity of 100 mW/cm2 generated from a 300 W Xe lamp. The LSVs were recorded between −0.8 and 0.8 V potential with a scan rate of 5 mV/s. The nitrogen gas was bubbled in the 1 M Na2S (pH = 13) electrolyte to remove the dissolved oxygen effect in photoelectrochemical studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00429. Chemicals; characterization; methods; XPS full survey spectra for Ln(OH)3 electrodes, before and after photoelectrochemical water splitting; LSV of Ln(OH)3 electrodes in 1 M KOH electrolyte under light condition; LSV of Ln(OH)3 electrodes in 0.5 M Na2SO4 electrolyte under light condition; FTIR, Raman, and EPR spectra of Ln(OH)3 before and after PEC tests; and XRD patterns for Ln(OH)3 before and after PEC tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.S.). ORCID

Gosipathala Sreedhar: 0000-0001-8701-199X 6276


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1.06 μm Laser Applications. Spectrochim. Acta, Part A 2013, 115, 297− 304. (18) Islam, M. J.; Reddy, D. A.; Choi, J.; Kim, T. K. Surface Oxygen Vacancy Assisted Electron Transfer and Shuttling for Enhanced Photocatalytic Activity of a Z-Scheme CeO2-AgI Nanocomposite. RSC Adv. 2016, 6, 19341−19350. (19) Mu, Q.; Chen, T.; Wang, Y. Synthesis, Characterization and Photoluminescence of Lanthanum Hydroxide Nanorods by a Simple Route at Room Temperature. Nanotechnology 2009, 20, 345602. (20) Zhang, Q.; Sun, H.; Wang, X.; Zhang, Y.; Li, X. Strong Photoluminescence and Piezoelectricity Properties in Pr-Doped Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 Ceramics: Influence of Concentration and Microstructure. J. Eur. Ceram. Soc. 2014, 34, 1439−1444. (21) Que, W.; Hu, X.; Gan, L. H.; Deen, G. R. Preparation and Luminescence Properties of Neodymium(III) Oxide Nanocrystals Dispersed in Sol-Gel Titania/(γ-glycidoxypropyl) Trimethoxysilane Composite Thin Films. J. Mater. Res. 2002, 17, 1399−1405. (22) Liu, L.; Li, M.; Cai, S.; Yang, Y.; Mai, Y. Near-Infrared Quantum Cutting in Nd3+ and Yb3+ doped BaGd2ZnO5 Phosphors. Opt. Mater. Express 2015, 5, 756−763. (23) Taiwo, F. A. Electron Paramagnetic Resonance Spectroscopic Studies of Iron and Copper Proteins. Spectroscopy 2003, 17, 53−63. (24) Kakazey, N. G.; Sreckovic, T. V.; Ristic, M. M. Electronic Paramagnetic Resonance Investigation of the Evolution of Defects in Zinc Oxide During Tribophysical Activation. J. Mater. Sci. 1997, 32, 4619−4622. (25) Yu, B.; Zhu, C.; Gan, F.; Huang, Y. Electron Spin Resonance Properties of ZnO Microcrystallites. Mater. Lett. 1998, 33, 247−250. (26) Jing, L.; Xu, Z.; Shang, J.; Sun, X.; Cai, W.; Guo, H. The Preparation and Characterization of ZnO Ultrafine Particles. Mater. Sci. Eng., A 2002, 332, 356−361. (27) Zeng, Q. G.; Ding, Z. J.; Zhang, Z. M.; Sheng, Y. Q. Photoluminescence and Raman Spectroscopy Studies of Eu(OH)3 Rods at High Pressures. J. Phys. Chem. C 2010, 114, 4895−4900. (28) Hoang, D. L.; Dittmar, A.; Schneider, M.; Trunschke, A.; Lieske, H.; Brzezinka, K.-W.; Witke, K. Evidence of Lanthanum-Chromium Mixed Oxides Formed in CrOx/La2O3 Model Catalysts. Thermochim. Acta 2003, 400, 153−163. (29) Soares, J. C.; Siqueira, K. P. F.; Filho, P. C. d. S.; Moreira, R. L.; Dias, A. Thermal, Vibrational and Optical Properties of PrLuO3 Interlanthanides from Hydrothermally- Derived Precursors. Dalton Trans. 2017, 46, 825−835. (30) Cui, J.; Hope, G. A. Raman and Fluorescence Spectroscopy of CeO2, Er2O3, Nd2O3, Tm2O3, Yb2O3, La2O3, and Tb4O7. J. Spectrosc. 2015, 2015, 940172. (31) Walrafen, G. E.; Douglas, R. T. W. Raman Spectra from Very Concentrated Aquous NaOH and From Wet Dry, Solid, and Anhydrous Molten, LiOH, NaOH and KOH. J. Chem. Phys. 2006, 124, 114504. (32) Hall, D. S.; Lockwood, D. J.; Bock, C.; MacDougall, B. R. Nickel Hydroxides and Related Materials: A Review of Their Structures, Synthesis and Properties. Proc. R. Soc. A 2014, 471, 20140792. (33) Sunding, M. F.; Hadidi, K.; Diplas, S.; Løvvik, O. M.; Norby, T. E.; Gunnæs, A. E. XPS Characterization of In Situ Treated Lanthanum Oxide and Hydroxide Using Tailored Referencing and Peak Fitting Procedures. J. Electron Spectrosc. Relat. Phenom. 2011, 184, 399−409. (34) Ding, B.; Han, C.; Zheng, L.; Zhang, J.; Wang, R.; Tang, Z. Tuning Oxygen Vacancy Photoluminescence in Monoclinic Y2WO6 by Selectively Occupying Yttrium Sites Using Lanthanum. Sci. Rep. 2015, 5, 9443. (35) Mullica, D. F.; Lok, C. K. C.; Perkins, H. O.; Young, V. X-Ray Photoelectron Final-State Screening in La(OH)3: A Multiplet Structural Analysis. Phys. Rev. B: Condens. Matter Mater. Phys. 1985, 31, 4039−4042. (36) Berthou, H.; Jørgensen, C. K.; Bonnelle, C. Influence of the Ligands on 3d Photoelectron Spectra of the First Four Lanthanides. Chem. Phys. Lett. 1976, 38, 199−206. (37) Burroughs, P.; Hamnett, A.; Orchard, A. F.; Thornton, G. Satellite Structure in the X-Ray Photoelectron Spectra of Some Binary

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

S.R.S. thank CSIR, Government of India, for awarding CSIRJRF fellowship. G.S. would like to thank the Council of Scientific Industrial Research (CSIR), Government of India, for the financial support of the MULTIFUN project (no. CSC0101F).

(1) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in Semiconductors for Photo catalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43, 7520−7535. (2) Li, X.; Yu, J.; Low, J.; Fang, Y.; Xiao, J.; Chen, X. Engineering Heterogeneous Semiconductors for Solar Water Splitting. J. Mater. Chem. A 2015, 3, 2485−2534. (3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (5) Xiang, Y.; Yu, X.-F.; He, D.-F.; Sun, Z.; Cao, Z.; Wang, Q.-Q. Synthesis of Highly Luminescent and Anion-Exchangeable CeriumDoped Layered Yttrium Hydroxidess for Sensing and Photofunctional Applications. Adv. Funct. Mater. 2011, 21, 4388−4396. (6) Chen, K.; Xue, D. Rare Earth and Transitional Metal Colloidal Supercapacitors. Sci. China: Technol. Sci. 2015, 58, 1768−1778. (7) Sun, Y.; Xiao, X.; Dong, X.; Dong, F.; Zhang, W. Heterostructured BiOI@La(OH)3 Nanorods with Enhanced Visible Light Photocatalytic NO Removal. Chin. J. Catal. 2017, 38, 217−226. (8) Zhu, L.; Zhang, L.; Li, J.; Zhang, D.; Chen, L.; Sheng, D.; Yang, S.; Xiao, C.; Wang, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. Selenium Sequestration in a Cationic Layered Rare Earth Hydroxide: A Combined Batch Experiments and EXAFS Investigation. Environ. Sci. Technol. 2017, 51, 8606−8615. (9) Wang, Y.; Liu, S.; Cai, Y.; Deng, S.; Han, B.; Han, R.; Li, Q.; Wang, Y. La(OH)3: Ln3+ (Ln= Sm, Er, Gd, Dy, and Eu) Nanorods Synthesized by a Facile Hydrothermal Method and their Enhanced Photocatalytic Degradation of Cango Red in the Aqueous Solution. Ceram. Int. 2014, 40, 5091−5095. (10) Dong, F.; Xiao, X.; Jiang, G.; Zhang, Y.; Cui, W.; Ma, J. Surface Oxygen-Vacancy Induced Photocatalytic Activity of La(OH)3 Nanorods Prepared by a Fast and Scalable Method. Phys. Chem. Chem. Phys. 2015, 17, 16058−16066. (11) Choudhary, V. R.; Rane, V. H. Acidity/Basicity of Rare-Earth Oxides and Their Catalytic Activity in Oxidative Coupling of Methane to C2-Hydrocarbons. J. Catal. 1991, 130, 411−422. (12) Hughes, I. D.; Däne, M.; Ernst, A.; Hergert, W.; Lüders, M.; Poulter, J.; Staunton, J. B.; Svane, A.; Szotek, Z.; Temmerman, W. M. Lanthanide Contraction and Magnetism in the Heavy Rare Earth Elements. Nature 2007, 446, 650−653. (13) Kar, S.; Patel, C.; Santra, S. Direct Room Temperature Synthesis of Valance State Engineered Ultra-Small Ceria Nanoparticles: Investigation on the Role of Ethylenediamine as a Capping Agent. J. Phys. Chem. C 2009, 113, 4862−4867. (14) Wang, X.; Li, Y. Synthesis and Characterization of Lanthanide Hydroxide Single-Crystal Nanowires. Angew. Chem., Int. Ed. 2002, 41, 4790−4793. (15) Kang, J.-G.; Min, B.-K.; Sohn, Y. Physicochemical Properties of Praseodymium Hydroxide and Oxide Nanorods. J. Alloys Compd. 2015, 619, 165−171. (16) Bell, J. T.; Thompson, C. C.; Helton, D. M. The HighTemperature Spectra Aqueous Transition Metal Salts. J. Phys. Chem. 1969, 73, 3338−3345. (17) Reddy, C. M.; Vijaya, N.; Raju, B. D. P. NIR Fluorescence Studies of Neodymium Ions Doped Sodium Fluoroborate Glasses for 6277


Article

ACS Omega and Mixed Oxides of Lanthanum and Cerium. J. Chem. Soc., Dalton Trans. 1976, 1686−1698. (38) Crecelius, G.; Wertheim, G. K.; Buchanan, D. N. E. Core-Hole Screening in Lanthanide Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1978, 18, 6519−6524. (39) Wertheim, G. K.; Campagna, M. Screening of 3d Holes in the Rare Earths. Solid State Commun. 1978, 26, 553−556. (40) Stoychev, D.; Valov, I.; Stefanov, P.; Atanasova, G.; Stoycheva, M.; Marinova, T. Electrochemical Growth of Thin La2O3 Films on Oxide and Metal Surfaces. Mater. Sci. Eng., C 2003, 23, 123−128. (41) Siegmann, H. C.; Schlapbach, L.; Brundle, C. R. Self-Restoring of the Active Surface in the Hydrogen Sponge LaNi5. Phys. Rev. Lett. 1978, 40, 972−975. (42) Mullica, D. F.; Perkins, H. O.; Lok, C. K. C.; Young, V. The XRay Photoemission Spectra of La(OH)3. J. Electron Spectrosc. Relat. Phenom. 1993, 61, 337−355. (43) Yan, L.; Yu, R.; Liu, G.; Xing, X. A Facile Template-Free Synthesis of Large-Scale Single Crystalline Pr(OH)3 and Pr6O11 Nanorods. Scr. Mater. 2008, 58, 707−710. ́ (44) Lütkehoff, S.; Neumann, M.; Slebarski, A. 3d and 4d X-RayPhotoelectron Spectra of Pr Under Gradual Oxidation. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 13808−13811. (45) Fuggle, J. C.; Campagna, M.; Zolnierek, Z.; Lässer, R. Observation of a Relationship Between Core-Level Line Shapes in Photoelectron Spectroscopy and the Localization of Screening Orbitals. Phys. Rev. Lett. 1980, 45, 1597−1600. (46) Mullica, D. F.; Lok, C. K. C.; Perkins, H. O.; Benesh, G. A.; Young, V. The X-Ray Photoemission Spectra of Nd(OH)3, Sm(OH)3, Eu(OH)3 and Gd(OH)3. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 1−20. (47) Spector, N.; Bonnelle, C.; Dufour, G.; Jørgensen, C. K.; Berthou, H. Photoelectron Spectra of Metallic Neodymium and Praseodymium Compared with Calculations of 3d94f3 and 3d94f2 in Intermediate Coupling. Chem. Phys. Lett. 1976, 41, 199−204. (48) Sabaté, J.; Cervera-March, S.; Simarro, R.; Giménez, J. Photocatalytic Production of Hydrogen from Sulphide and Sulphate Waste Streams: A Kinetic Model for Reactions Occurring in Illuminated Suspensions of CdS. Chem. Eng. Sci. 1990, 45, 3089−3096. (49) Tambago, H. M. G.; de Leon, R. L. Intrinsic Kinetic Modeling of Hydrogen Production by Photocatalytic Water Splitting Using Cadmium Zinc Sulfide Catalyst. Int. J. Chem. Eng. Appl. 2015, 6, 220− 227. (50) Ding, C.; Shi, J.; Wang, Z.; Li, C. Photoelectrocatalytic Water Splitting: Significance of Cocatalysts, Electrolyte, and Interfaces. ACS Catal. 2017, 7, 675−688. (51) Caliari, P. C.; Pacheco, M. J.; Ciríaco, L. F.; Lopes, A. M. C. Anodic Oxidation of Sulfide to Sulfate: Effect of Current Density on the Process Kinetics. J. Braz. Chem. Soc. 2017, 28, 557−566. (52) Fantauzzi, M.; Elsener, B.; Atzei, D.; Rigoldi, A.; Rossi, A. Exploiting XPS for the Identification of Sulfides and Polysulfides. RSC Adv. 2015, 5, 75953−75963. (53) Sartz, W. E.; Wynne, K. J., Jr.; Hercules, D. M. X-Ray Photoelectron Spectroscopic Investigation of Group VI-A Elements. Anal. Chem. 1971, 43, 1884−1887. (54) Jia, G.; Huang, Y.; Song, Y.; Yang, M.; Zhang, L.; You, H. Controllable Synthesis and Luminescence Properties of La(OH)3 and La(OH)3:Tb3+ Nanocrystals with Multiform Morphologies. Eur. J. Inorg. Chem. 2009, 3721−3726. (55) Devi, B. S. R.; Raveendran, R.; Vaidyan, A. V. Synthesis and Characterization of Mn2+-Doped ZnS Nanoparticles. J. Phys. 2007, 68, 679−687. (56) Moraes, I. R.; da Cunha, M. C. P. M.; Nart, F. C. Vibrational Spectroscopy of Adsorbed Sulfate and Nitrate Ions on Au(100) Electrodes. J. Braz. Chem. Soc. 1996, 7, 453−460. (57) Peak, D.; Ford, R. G.; Sparks, D. L. An In Situ ATR-FTIR Investigation of Sulfate Bonding Mechanisms on Goethite. J. Colloid Interface Sci. 1999, 218, 289−299.

(58) Kępiński, L.; Zawadzki, M.; Miśta, W. Hydrothermal Synthesis of Precursors of Neodymium Oxide Nanoparticles. Solid State Sci. 2004, 6, 1327−1336.

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Role of Surface Oxygen Vacancies and Lanthanide Contraction

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