Controlled synthesis of CaTiO₃:Ln³⁺ nanocrystals for luminescence and photocatalytic hydrogen production

Abstract

Bifunctional CaTiO₃:Ln³⁺ (Ln = Eu and Er) nanocrystals were prepared via a facile method followed by calcination in air. The as-prepared CaTiO₃:Ln³⁺ nanocrystals exhibited bifunctional performance for both photoluminescence and photocatalytic hydrogen production. As a phosphor powder, the luminescence properties of CaTiO₃:Ln³⁺ nanocrystals could be controlled by doping with different Ln³⁺ ions. They showed very stable luminescence properties and a much higher quenching concentration due to the scheelite related structure of CaTiO₃, which was up to 17% (Eu doping). As a photocatalyst, the CaTiO₃:Ln³⁺ nanocrystals exhibited a higher activity for hydrogen production under ultraviolet light irradiation. The CaTiO₃:Er³⁺ nanocrystals displayed the highest photocatalytic activity of up to 461.25 μmol h⁻¹, which was higher than that of CaTiO₃:Eu³⁺ nanocrystals and pure CaTiO₃. The results indicated that incorporation of Ln³⁺ ions benefits electron transfer as well as the reduction of the band gap of the CaTiO₃ photocatalyst.

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1. Introduction

The special spectroscopic properties of rare-earth (RE) ions in different host lattices are applied in many aspects, including lamp phosphors, radiation monitoring, lasers, and white light-emitting diodes.^1–6 These applications depend strongly upon the luminescence properties, which are relative to the morphology and composition of the materials. Although the transitions of RE ions are substantially shielded, the luminescent properties of their nanocrystals are also affected by RE ion size, shape, crystal structure, and chemical composition of the materials.^7–11 In recent years, the controllable synthesis of nanocrystals has attracted considerable interest due to their significance in basic scientific research and potential technological applications, based on their specific geometries and distinct properties.^12–14 A limited amount of precursor was used in the synthesis process to control the growth and achieve nanostructures with clean surfaces, which is required for high-performance electric and optical applications.^15–17

CaTiO₃ is one of the alkaline earth titanates with a perovskite structure and interesting electronic, optical, magnetic and catalytic properties, due to its excellent resistance against photocorrosion, high thermal stability and structure stability when doped with metal ions to alter the opto-electrical properties. RE ion doped CaTiO₃ as optical materials have been studied by some researchers because of the well-known high chemical durability and thermal stability of CaTiO₃ nanocrystals in this field, for example, a high color rendering index, high luminescence efficiency, long lifetime, low power consumption, and environmental friendliness.^18,19

Moreover, photocatalytic hydrogen production from water using a semiconductor nanomaterial has attracted a tremendous amount of interest.^20–23 Over the past several decades, many photocatalysts have been found to have photocatalytic activities for photocatalytic hydrogen production.^24–27 Thermodynamically, water splitting into H₂ and O₂ is an uphill reaction, accompanied by a large positive change in the Gibbs free energy.²⁸ Thus, a suitable semiconductor is urgently needed. CaTiO₃ is also a wide band gap (∼3.5 eV) semiconductor, but its conduction band is very negative (−0.86 eV vs. NHE), which is efficient to reduce proton to hydrogen.^29,30 If the band gap is reduced by doping with RE metals, it would be one of the most efficient methods to improve its photocatalytic activity, related to the vacant f orbitals of the rare earth metal ions that allow for intermediate energy states, reducing the band gap size and thus enhancing the photoactivity.³¹

Based on the considerations mentioned above, we report a bifunctional material of CaTiO₃:Ln³⁺ (Ln = Eu and Er) with excellent luminescence properties and photocatalytic hydrogen production activity. Importantly, the concomitant impurities of CaCO₃ were overcome and pure phase CaTiO₃ nanocrystals were successfully prepared by a sol–gel method. Compared with a hydrothermal method, the sol–gel method could ensure the combination of Ca and Ti in a stoichiometry with out dissociating Ca²⁺ to form CaCO₃. Novel luminescence properties and higher activity for photocatalytic hydrogen production were displayed due to the unique perovskite structure of CaTiO₃ and RE ion doping, which increased the BET surface areas as well as reduced the band gaps and improved the charge separation, which was proven by electrochemical measurements.

2. Experimental section

2.1 Sample preparation

All the chemicals used in this paper were of analytical grade and used without further purification. A typical synthesis of CaTiO₃ nanocrystals was as follows: 1.58 g calcium acetate (Ca(CH₃COO)₂·2H₂O) and 3.4 mL tetrabutyl titanate (Ti(OC₄H₉)₄) were dissolved in 30 mL ethylene glycol, then the solution was stirred at room temperature for about 30 min. Then, the milky suspensions was dried in air at 180 °C for 24 h and sintered at 600 °C for 2 h. For comparison, the abovementioned milky suspensions were hydrothermally treated at 180 °C for 24 h and sintered at 600 °C for 2 h.

CaTiO₃:Ln³⁺ (Ln = Eu and Er) nanocrystals were prepared by the same procedure, except for adding additional Ln(NO₃)₃ into the solution of ethylene glycol at the initial stage.

2.2 Characterization

The crystal structure was analyzed by a Rigaku (Japan) D/MAX-rA X-ray diffractometer (XRD) equipped with graphite monochromatized Cu Kα radiation (γ = 1.541874 Å), keeping the operating voltage and current at 40 kV and 40 mA, respectively. The size and morphology of the final products were determined using a JSM-6301F scanning electron microscope (SEM, Tokyo, Japan) and JEM-2010F transmission electron microscope (TEM, JEOL, Tokyo, Japan) operating at 200 kV. Nitrogen adsorption–desorption isotherms were collected using an Autosorb-1 (Quantachrome Instruments, Boynton Beach, FL) nitrogen adsorption apparatus at 77 K. The pore size distribution plots were obtained by the Barrett–Joyner–Halenda (BJH) model. Ultraviolet-visible (UV-Vis) absorption spectra were obtained by a UV-Vis spectrophotometer (Shimadzu UV-2550, Tokyo, Japan).

The photocatalytic H₂ evolution from water was conducted in an online photocatalytic hydrogen production system (AuLight, Beijing, China, CEL-SPH2N). A powder sample of the catalyst (0.1 g) was suspended in a mixture of 80 mL distilled water and 20 mL methanol in the cell using a magnetic stirrer. Pt-loaded photocatalysts were prepared by a known standard method of in situ photo-deposition. Before the reaction, the mixture was deaerated by evacuation to remove O₂ and CO₂ dissolved in water. The reaction was carried out by irradiating the mixture with UV light from a 300 W Xe lamp with a 320–390 nm reflection filter, which means the wavelength of light was approximately 320–390 nm. Gas evolution was observed only under photo-irradiation, being analyzed by an online gas chromatograph (SP7800, thermal conductivity detector, molecular sieve 5 Å, N₂ carrier, Beijing Keruida Limited, Beijing, China).

3. Results and discussion

3.1 Crystal structures and morphologies

Controlling the generated CaCO₃ impurity of CaCO₃ is important when preparing CaTiO₃. Fig. S1a† shows the XRD patterns of pure CaTiO₃ nanocrystals without doping. No peaks corresponding to any other phases or impurities were detected, indicating the high purity. SEM, TEM and HRTEM (Fig. S1b and c†) show that the particle size was approximately 50 nm. The 0.27 nm interplanar spacing corresponds to the distance of the {200} planes of the CaTiO₃ orthorhombic phase. In addition, XRD patterns of samples prepared at different conditions, including the solvent, concentration and reaction manners, in Fig. S2† indicate that CaCO₃ appears in these conditions. This means that CaTiO₃ nanoparticles without impurities could be controllably prepared by adjusting the reaction conditions. Fig. S3† shows the Mott–Schottky plots of CaTiO₃ nanocrystals that show positive slopes, which implies CaTiO₃ is an n-type semiconductor.³² Flat band potential of the CaTiO₃ nanocrystals was found to be −1.5 V (versus Ag/AgCl), indicating the good hydrogen production ability of CaTiO₃.

The XRD patterns of CaTiO₃:Eu³⁺ and CaTiO₃:Er³⁺ with different doping concentrations are shown in Fig. S4 and S5.† It could be observed that after doping with RE ions of Eu and Er, even with different amounts, the crystal structure was maintained. The morphology of the nanoparticles after doping was studied by TEM and HRTEM, which are shown in Fig. 1. The high-resolution HRTEM analysis showed an interplanar spacing of 0.27 nm corresponding to the {200} planes of the orthorhombic phase of CaTiO₃, which was not changed as well. In order to study the doping elements, energy-dispersive X-ray (EDX) analysis is performed and the results are shown in Fig. S6.† It can be seen that the samples are composed of Ca, Ti and O for pure CaTiO₃ and Eu and Er for the doping ones, respectively. These results give the evidence that CaTiO₃:Ln³⁺ (Ln = Eu and Er) nanocrystals were prepared. The photophysics properties are also measured to make the bifunctional mechanism clear. According to the abovementioned analysis, Ln³⁺ (Eu and Er) doping pure CaTiO₃ nanocrystals were successfully prepared. The RE ion doping did not change the crystal structure and the crystallinity of the pure CaTiO₃ nanocrystals.

Fig. 1 TEM and HRTEM images of (a and b) CaTiO₃:Eu³⁺(0.5%) and (c and d) CaTiO₃:Er³⁺(0.5%) nanocrystals.

3.2 Luminescence spectra of CaTiO₃:Eu³⁺ nanocrystals

The doping concentration of RE ion could affect the luminescence properties.³³ In general, the higher the doping concentration, the better the luminescence properties. However, a contradiction that more doping may lead to the lattice deformation and further influence the properties was observed. Perovskite structure oxides are very stable, and thus they are suitable as a phosphor substance. The emission spectra of the CaTiO₃:Eu³⁺ nanocrystals excited at 397 nm were studied and shown in Fig. 2a. Obviously, the spectral configurations of CaTiO₃:Eu³⁺ nanocrystals were unchanged with the Eu³⁺ content. In addition, the maximum Eu³⁺ concentrations was 17% mol% of the ⁵D₀ → ⁷F₂ transition. The excitation spectra of the CaTiO₃:Eu³⁺ nanocrystals prepared with different Eu(NO₃)₃ content were monitored at 619 nm and are shown Fig. 2b.

Fig. 2 Emission (left, λ_ex = 397 nm) and excitation (right, λ_em = 619 nm) spectra of (a) CaTiO₃:Eu³⁺(0.5%), (b) CaTiO₃:Eu³⁺(5%), (c) CaTiO₃:Eu³⁺(7%), (d) CaTiO₃:Eu³⁺(10%), (e) CaTiO₃:Eu³⁺(13%), (f) CaTiO₃:Eu³⁺(15%), (g) CaTiO₃:Eu³⁺(17%) and (h) CaTiO₃:Eu³⁺(20%).

Fig. 3a shows the emission spectra of the CaTiO₃:Eu³⁺ (17%) nanocrystals excited at different excitation wavelengths. The ⁵D₀ → ⁷F₁ (589–602 nm), ⁵D₀ → ⁷F₂ (615–633 nm), ⁵D₀ → ⁷F₃ (∼654 nm), and ⁵D₀ → ⁷F₄ (∼713 nm) transitions of Eu³⁺ were observed. The emission intensity was the strongest when the excitation was performed at 397 nm. Because the 4f energy levels of Eu³⁺ are hardly affected by the crystal field, there was no notable shift in the positions of the emission peaks compared to other Eu³⁺-doped systems.³⁶ The ⁵D₀ → ⁷F₁ transition is magnetic-dipole-allowed and its intensity is almost independent of the local environment around the Eu³⁺ ions. The ⁵D₀ → ⁷F₂ transition is electric-dipole-allowed due to an admixture of opposite parity 4fⁿ⁻¹5d states by an odd parity crystal-field component. Therefore, its intensity is sensitive to the local structure around the Eu³⁺ ions. The ⁵D₀ → ⁷F₃ transition exhibits a mixed magnetic dipole and electric dipole character. The ⁵D₀ → ⁷F₄ is an electric dipole transition. The ⁵D₀ → ⁷F₁ is dominating in a site with inversion symmetry, whereas the ⁵D₀ → ⁷F₂ is the strongest in a site without inversion symmetry. Fig. 3b shows the excitation spectra of the CaTiO₃:Eu³⁺(17%) nanocrystals monitored at 597, 619 and 658 nm. The positions of the excitation peaks are practically identical to the characteristic absorption bands for f–f intra-configuration transitions in trivalent europium.¹⁴

Fig. 3 (a) Emission spectra of CaTiO₃:Eu³⁺(17%) nanocrystals excited at different wavelengths. (b) Excitation spectra of CaTiO₃:Eu³⁺(17%) monitored at different emission wavelengths.

3.3 Luminescence spectra of CaTiO₃:Er³⁺ nanocrystals

Fig. 4 shows the upconversion (UC) luminescence spectra of CaTiO₃ nanocrystals with different Er³⁺ concentrations. The spectral peaks correspond to the following transitions: ²H_11/2 → ⁴I_15/2 (~526 nm), ⁴S_3/2 → ⁴I_15/2 (~544 nm) and ⁴F_9/2 → ⁴I_15/2 (~662 nm). It was observed that the dominant emissions were located in the green luminescence range for the CaTiO₃:Er³⁺ nanocrystals. The relative intensity of ⁴F_9/2 → ⁴I_15/2 to ²H_11/2/⁴S_3/2 → ⁴I_15/2 increases with increasing the Er³⁺ content. When Er³⁺ concentration was 10%, the luminescence almost vanished due to luminescence quenching. The CIE coordinates of the UC luminescence were (0.309, 0.511), (0.313, 0.496) and (0.318, 0.45) for the CaTiO₃:Er³⁺(0.5%), CaTiO₃:Er³⁺(3%) and CaTiO₃:Er³⁺(5%), respectively. Obviously, the CIE coordinates changed with the different Er³⁺ concentrations.

Fig. 4 UC luminescence spectra and corresponding CIE 1931 chromaticity diagram of (a) CaTiO₃:Er³⁺(0.5%), (b) CaTiO₃:Er³⁺(3%), (c) CaTiO₃:Er³⁺(5%) and (d) CaTiO₃:Er³⁺(10%) nanocrystals.

Dependence of the UC luminescence intensity on pump power was investigated to obtain a better understanding of the UC processes. For an unsaturated UC process, the emission intensity (I_f) will be proportional to power (n) of the infrared excitation (P) power: I_f & Pⁿ, where n is the number of infrared photons absorbed per visible photon emitted. Fig. 5 shows the double logarithmic plots of the emission intensity as a function of excitation power for the ⁴S_3/2/²H_11/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 emissions. For the red mission, the values of n were separately determined to be 1.26, 1.37 and 1.46 for the CaTiO₃:Er³⁺(0.5%), CaTiO₃:Er³⁺(3%) and CaTiO₃:Er³⁺(5%), suggesting that a two-photon process should be involved for populating the red levels. For the green transitions, the values of n were separately determined to be 1.95, 1.63 and 1.58 for the CaTiO₃:Er³⁺(0.5%), CaTiO₃:Er³⁺(3%) and CaTiO₃:Er³⁺(5%), respectively, suggesting that a two-photon process should be involved for populating the green levels.

Fig. 5 Dependence of the UC emission intensities on the excitation power in the CaTiO₃ nanocrystals with different Er³⁺ concentration nanocrystals.

3.4 Photocatalytic activity of CaTiO₃:Ln³⁺ nanocrystals

A metal ion doped wide band gap semiconductor is an effective strategy to improve the light absorption and charge separation, which further improve photocatalytic performance.^34,35 Although the conduction band of CaTiO₃ is very negative to reduce proton, the band gap of it is very wide, which is unfavourable to light absorption. The prepared CaTiO₃:Ln³⁺ (Ln = Eu and Er) nanocrystals are thus believed to be excellent catalysts for photocatalytic hydrogen production. According to the luminescence property, CaTiO₃:Er³⁺(0.5%) and CaTiO₃:Eu³⁺(0.5%) nanocrystals were utilized as photocatalysts for hydrogen production. The optical absorptions of the samples were conducted with a UV-Vis absorption spectrometer, as shown in Fig. 6. All the samples show the absorption band edge at in the UV light region (λ < 400 nm), implying these are wide band semiconductors. The band gap (E_g) of the CaTiO₃:Er³⁺(0.5%) and CaTiO₃:Eu³⁺(0.5%) were calculated to be about 3.3 eV from the onset of the absorption edge (inset of Fig. 6). However, compare to CaTiO₃:Eu³⁺(0.5%), CaTiO₃:Er³⁺(0.5%) showed little enhanced light absorption from 400 to 700 nm, as shown in Fig. 6. This interesting phenomenon may be attributed to the special molecular orbital structure of Er. In addition, the absorption peaks of CaTiO₃:Er³⁺ correspond to the f–f transitions of Er³⁺ ions: ⁴I_15/2 → ²H_11/2, ⁴I_15/2 → ⁴S_3/2 and ⁴I_15/2 → ⁴F_9/2.³⁶

Fig. 6 UV-vis absorption spectra of CaTiO₃, CaTiO₃:Er³⁺(0.5%) and CaTiO₃:Eu³⁺(0.5%) nanocrystals.

Fig. 7 shows the time depended H₂ evolution over the samples under UV light irradiation. Obviously, the photocatalytic performance of CaTiO₃:Er³⁺(0.5%) is better than CaTiO₃ and CaTiO₃:Eu³⁺(0.5%) nanocrystals. The average H₂ production yield is up to 461.25 μmol h⁻¹. This is in accordance with the light absorption that may improve the generation of photo-electrons that promote the photocatalytic activity. In addition to this, the surface area is significant to photocatalytic activity because it would produce more reaction sites and improve the surface catalysis to produce more hydrogen.

Fig. 7 H₂ production activity of the CaTiO₃, CaTiO₃:Er³⁺(0.5%) and CaTiO₃:Eu³⁺(0.5%) nanocrystals under UV light irradiation.

The N₂ adsorption–desorption isotherms and the corresponding BJH pore size distribution plots of the samples (Fig. S7†) show that the Brunauer–Emmett–Teller (BET) surface areas of the pure CaTiO₃, CaTiO₃:Er³⁺(0.5%) and CaTiO₃:Eu³⁺(0.5%) nanocrystals were 18.48, 19.58 and 17.57 m² g⁻¹, respectively. Obviously, the BET surface areas of CaTiO₃:Er³⁺ nanocrystals were larger that of pure CaTiO₃ and CaTiO₃:Er³⁺ nanocrystals on account of the radius of the Er³⁺ ions being smaller than that of Ca²⁺ and Eu³⁺ ions, which is very advantageous for photocatalytic hydrogen production; this demonstrated that surface area played an important role in this study.

The photoelectrochemical measurement could reflect the charge transport of semiconductors.^37,38 Fig. 8 shows the typical EIS Nyquist plots of the samples under UV light irradiation. The measurements show a bit smaller interfacial resistance for the CaTiO₃:Er³⁺(0.5%) than that of CaTiO₃ under UV light irradiation, indicating a more efficient charge separation and fast electron transport. The photoelectrochemical results show that the Er doping is acceptable for CaTiO₃ as photocatalyst for hydrogen production. Although the mechanism deep inside is not clear, such as what type of special molecular orbital affects the light absorption and the orbital interaction between Er and CaTiO₃, it is sure that the rare-earth ions doping is beneficial to photocatalytic hydrogen production.

Fig. 8 EIS Nyquist plots of (a) CaTiO₃ and (b) CaTiO₃:Er³⁺(0.5%), (c) CaTiO₃:Er³⁺(0.5%) nanocrystals (scanning the frequency from 1 MHz to 0.5 Hz at a bias of 0.5 V under UV light irradiation).

4. Conclusions

Bifunctional CaTiO₃:Ln³⁺ (Ln = Eu and Er) nanocrystals without any impurities was facilely prepared. The usual accompanying impurity CaCO₃ was overcome using ethylene glycol as a stabilizer. The CaTiO₃:Ln³⁺ (Ln = Eu and Er) nanocrystals exhibited both luminescence properties and photocatalytic hydrogen production activities. The luminescence results indicated pure CaTiO₃ promoted the RE luminescence properties and increased the quenching concentration, which was up to 17%. At the same time, doped RE ions improved light absorption as well as increased BET surface areas to enhance the photocatalytic hydrogen production activity, and CaTiO₃:Er³⁺ (0.5%) displayed the optimal activity that could be explained photoelectrochemically, which indicated a more efficient charge separation. This novel and high effective bifunctional material is believed to have potential application in the fields of photochemistry and photophysics.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21171052, 21471050, 21501052 and 21473051), the China Postdoctoral Science Foundation (2015M570304), the Program for Innovative Research Team in University (IRT-1237), and the Heilongjiang Province Natural Science Foundation (ZD201301, QC2015010).

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Footnote

† Electronic supplementary information (ESI) available: Table S1, Fig. S1–S7. See DOI: 10.1039/c5ra26250j

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