Enhanced Photocatalytic Performance and Mechanism of Au@CaTiO3 Composites with Au Nanoparticles Assembled on CaTiO3 Nanocuboids

Using P25 as the titanium source and based on a hydrothermal route, we have synthesized CaTiO3 nanocuboids (NCs) with the width of 0.3–0.5 μm and length of 0.8–1.1 μm, and systematically investigated their growth process. Au nanoparticles (NPs) of 3–7 nm in size were assembled on the surface of CaTiO3 NCs via a photocatalytic reduction method to achieve excellent Au@CaTiO3 composite photocatalysts. Various techniques were used to characterize the as-prepared samples, including X-ray powder diffraction (XRD), scanning/transmission electron microscopy (SEM/TEM), diffuse reflectance spectroscopy (UV-vis DRS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Rhodamine B (RhB) in aqueous solution was chosen as the model pollutant to assess the photocatalytic performance of the samples separately under simulated-sunlight, ultraviolet (UV) and visible-light irradiation. Under irradiation of all kinds of light sources, the Au@CaTiO3 composites, particularly the 4.3%Au@CaTiO3 composite, exhibit greatly enhanced photocatalytic performance when compared with bare CaTiO3 NCs. The main roles of Au NPs in the enhanced photocatalytic mechanism of the Au@CaTiO3 composites manifest in the following aspects: (1) Au NPs act as excellent electron sinks to capture the photoexcited electrons in CaTiO3, thus leading to an efficient separation of photoexcited electron/hole pairs in CaTiO3; (2) the electromagnetic field caused by localized surface plasmon resonance (LSPR) of Au NPs could facilitate the generation and separation of electron/hole pairs in CaTiO3; and (3) the LSPR-induced electrons in Au NPs could take part in the photocatalytic reactions.


Introduction
The rapid social development has raised two big issues facing mankind, i.e., environmental pollution and energy shortage. In particular, water resources are seriously and increasingly becoming polluted by the wastewater discharged from chemical industries, which poses a great threat to human health and survival. Organic dyes and pigments, most commonly existing in the industrial wastewater, are carcinogenic to humans and hardly self-decomposed [1,2]. It is imperative to remove the organic pollutants and purify water resources via a simple, low-cost, green and non-fossil-consumptive technology. In this sense, semiconductor-based photocatalysis has sparked a great interest due to its potential applications in wastewater treatment [3][4][5][6][7][8]. This technology allows the use of solar light-a sustainable, inexhaustible and economically attractive energy source-as the power source to degrade

Synthesis of CaTiO 3 NCs
CaTiO 3 NCs were synthesized via a hydrothermal route at 200 • C as described in the literature [55]. To unveil the growth process of CaTiO 3 NCs, different hydrothermal reaction times (0.5, 1, 5, 10 and 24 h) were performed.

Assembly of Au NPs on CaTiO 3 NCs
A photocatalytic reduction method was employed to hybridize Au NPs on the surface of CaTiO 3 NCs synthesized at 200 • C for 24 h. 0.1 g of the as-synthesized CaTiO 3 NCs and 0.025 g of ammonium oxalate (AO) were successively added in 80 mL of deionized water. The suspension was ultrasonically treated for 30 min and then magnetically stirred for another 30 min to make CaTiO 3 particles uniformly disperse. 0.8 mL of HAuCl 4 solution (0.029 mol·L −1 , M) was dropped in the CaTiO 3 suspension. The resultant mixture was magnetically stirred for 60 min, and then irradiated with ultraviolet (UV) light, which was emitted from a 15 W low-pressure mercury lamp, for 30 min under mild stirring. During the irradiation process, Au 3+ ions were reduced by the photogenerated electrons in CaTiO 3 to form Au NPs, which were simultaneously assembled on the surface of CaTiO 3 NCs. The product was collected by centrifugation. After washing several times with deionized water and ethanol, and then drying at 60 • C for 12 h, the final product was obtained as the 4.3%Au@CaTiO 3 composite with an Au mass fraction of 4.3%. By adding different volumes of HAuCl 4 solution (0.2, 0.5, 1.1 and 1.4 mL), several other composite samples 1.1%Au@CaTiO 3 , 2.7%Au@CaTiO 3 , 5.9%Au@CaTiO 3 and 7.4%Au@CaTiO 3 were also prepared.

Sample Characterization
The crystal structures, morphologies and microstructures of the samples were characterized by X-ray powder diffraction (XRD) and scanning/transmission electron microscopy (SEM/TEM). The used apparatuses were a D8 Advance X-ray diffractometer (Bruker AXS, Karlsruhe, Germany), a JSM-6701F field-emission scanning electron microscope (JEOL Ltd., Tokyo, Japan) and a JEM-1200EX field-emission transmission electron microscope (JEOL Ltd., Tokyo, Japan). A PHI-5702 multi-functional X-ray photoelectron spectrometer (Physical Electronics, hanhassen, MN, USA) was employed for the X-ray photoelectron spectroscopy (XPS) analysis. Ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy (DRS) measurements were performed on a TU-1901 double beam UV-vis spectrophotometer (Beijing Purkinje General Instrument Co. Ltd., Beijing, China). A Spectrum Two Fourier transform infrared (FTIR) spectrophotometer (PerkinElmer, Waltham, MA, USA) was used for the FTIR spectroscopy analysis of the samples.

Photocatalytic Test
To assess the photocatalytic performances of the samples, RhB in aqueous solution (5 mg·L −1 ) was used as the model pollutant. Simulated sunlight (emitted from a 200 W xenon lamp, 300 < λ < 2500 nm), UV light (emitted from a 30 W low-pressure mercury lamp, λ = 254 nm) and visible light (generated by a 200 W halogen-tungsten lamp, λ > 400 nm) were separately used as the light source. The reaction solution was composed of 100 mL of RhB solution and 0.1 g of the photocatalyst. The adsorption of RhB on the photocatalyst surface was examined by magnetically stirring the mixture in the dark for 30 min. The RhB concentration was monitored by measuring the absorbance of the reaction solution. 2.5 mL of the reaction solution was sampled from the photoreactor and centrifuged to remove the photocatalyst. The absorbance measurement was performed on a UV-vis spectrophotometer at λ = 554 nm. The percentage degradation of RhB (D%) was obtained according to D% = (C 0 − C t )/C 0 × 100% (C 0 = initial RhB concentration; C t = residual RhB concentration).

Synthesis and Growth Process of CaTiO 3 NCs
The crystal structures of precursor P25 and the samples prepared at 200 • C with different reaction times (0.5, 1, 5, 10 and 24 h) were determined by XRD patterns, as shown in Figure 1. It is seen that at 0.5 h reaction time, the obtained sample maintains a crystal structure identical to that of P25 without the formation of CaTiO 3 phase. At 1 h reaction time, CaTiO 3 phase is largely crystallized and only minor TiO 2 is observed in the resultant sample. When the reaction time is increased up to 5 h, single CaTiO 3 phase is obtained. All the diffraction peaks of the sample can be indexed to the CaTiO 3 orthorhombic phase (JCPDS#42-0423). With further prolonging the reaction time, the diffraction peaks of the resultant samples undergo no change, indicating no structural change of CaTiO 3 crystals.
Micromachines 2018, 9, x 4 of 15 seen that at 0.5 h reaction time, the obtained sample maintains a crystal structure identical to that of P25 without the formation of CaTiO3 phase. At 1 h reaction time, CaTiO3 phase is largely crystallized and only minor TiO2 is observed in the resultant sample. When the reaction time is increased up to 5 h, single CaTiO3 phase is obtained. All the diffraction peaks of the sample can be indexed to the CaTiO3 orthorhombic phase (JCPDS#42-0423). With further prolonging the reaction time, the diffraction peaks of the resultant samples undergo no change, indicating no structural change of CaTiO3 crystals.  The growth process and mechanism of CaTiO3 NCs is schematically illustrated in Figure 3. In the concentrated NaOH solution, polycrystalline TiO2 nanoparticles are expected to react with NaOH to form Na2Ti2O4(OH)2 particles [59]. Simultaneously, in the strong alkaline environment and high temperature-high pressure conditions, Na2Ti2O4(OH)2 dissociates to form Ti(OH)6 2− ion groups [60]. Ca 2+ ions in the precursor solution are adsorbed onto the surface of Ti(OH)6 2− ions and further penetrate into the interior. During this process, a series of chemical reactions will take place, including the breaking of Ti-O-Ti bonds, dehydroxylation and nucleation of CaTiO3 crystals. To reduce the overall surface energy, the nucleated CaTiO3 crystals grow into nanocuboids finally. The dominant chemical reactions involved can be briefly described as follows: Na2Ti2O4(OH)2 + 2OH − + 4H2O → 2Ti(OH)6 2− + 2Na +  The growth process and mechanism of CaTiO 3 NCs is schematically illustrated in Figure 3. In the concentrated NaOH solution, polycrystalline TiO 2 nanoparticles are expected to react with NaOH to form Na 2 Ti 2 O 4 (OH) 2 particles [59]. Simultaneously, in the strong alkaline environment and high temperature-high pressure conditions, Na 2 Ti 2 O 4 (OH) 2 dissociates to form Ti(OH) 6 2− ion groups [60].
Ca 2+ ions in the precursor solution are adsorbed onto the surface of Ti(OH) 6 2− ions and further penetrate into the interior. During this process, a series of chemical reactions will take place, including the breaking of Ti-O-Ti bonds, dehydroxylation and nucleation of CaTiO 3 crystals. To reduce the overall surface energy, the nucleated CaTiO 3 crystals grow into nanocuboids finally. The dominant chemical reactions involved can be briefly described as follows: 2NaOH + 2TiO 2 → Na 2 Ti 2 O 4 (OH) 2 (1) Ca 2+ + Ti(OH) 6

Au NPs Modified CaTiO3 NCs
CaTiO3 NCs synthesized at 200 °C for 24 h was chosen to be modified with Au NPs with the aim of enhancing their photocatalytic performance. Figure 4 shows the XRD patterns of bare CaTiO3 NCs and 4.3%Au@CaTiO3 composite. The dominant diffraction peaks of 4.3%Au@CaTiO3 are similar to those of bare CaTiO3, indicating that the CaTiO3 orthorhombic structure undergoes no change on the decoration of Au NPs. However, additional weak diffraction peaks characterized as the Au cubic structure (JCPDS#04-0784) are clearly detected on the XRD pattern of the composite, which confirms the formation of Au NPs onto CaTiO3 NCs. TEM investigation was further carried out to unveil the microstructure of the 4.3%Au@CaTiO3 composite. Figure 5a shows the TEM image of the composite, from which one can see that CaTiO3 presents a regular morphology of NCs with width of 0.3-0.5 μm and length of 0.8-1.1 μm. The CaTiO3 morphology obtained by TEM is in accordance with the SEM observation result. Moreover, small-sized Au NPs are seen to be decorated on the surface of CaTiO3 NCs. The high-resolution TEM (HRTEM) image depicted in Figure 5b further confirms the good assembly of Au NPs on the surface of CaTiO3 NCs. The decorated Au NPs are shaped like spheres and have a size distribution of 3-7 nm. Energy-dispersive x-ray elemental mapping was used to investigate the elemental distribution of the 4.3%Au@CaTiO3 composite. Figure 5c illustrates the dark-field scanning TEM (DF-STEM) image of the composite, and the corresponding elemental mapping images are given in Figure 5d-g. It is observed that the NCs present uniformly-distributed elements of Ca, Ti and O.

Au NPs Modified CaTiO 3 NCs
CaTiO 3 NCs synthesized at 200 • C for 24 h was chosen to be modified with Au NPs with the aim of enhancing their photocatalytic performance. Figure 4 shows the XRD patterns of bare CaTiO 3 NCs and 4.3%Au@CaTiO 3 composite. The dominant diffraction peaks of 4.3%Au@CaTiO 3 are similar to those of bare CaTiO 3 , indicating that the CaTiO 3 orthorhombic structure undergoes no change on the decoration of Au NPs. However, additional weak diffraction peaks characterized as the Au cubic structure (JCPDS#04-0784) are clearly detected on the XRD pattern of the composite, which confirms the formation of Au NPs onto CaTiO 3 NCs.

Au NPs Modified CaTiO3 NCs
CaTiO3 NCs synthesized at 200 °C for 24 h was chosen to be modified with Au NPs with the aim of enhancing their photocatalytic performance. Figure 4 shows the XRD patterns of bare CaTiO3 NCs and 4.3%Au@CaTiO3 composite. The dominant diffraction peaks of 4.3%Au@CaTiO3 are similar to those of bare CaTiO3, indicating that the CaTiO3 orthorhombic structure undergoes no change on the decoration of Au NPs. However, additional weak diffraction peaks characterized as the Au cubic structure (JCPDS#04-0784) are clearly detected on the XRD pattern of the composite, which confirms the formation of Au NPs onto CaTiO3 NCs. TEM investigation was further carried out to unveil the microstructure of the 4.3%Au@CaTiO3 composite. Figure 5a shows the TEM image of the composite, from which one can see that CaTiO3 presents a regular morphology of NCs with width of 0.3-0.5 μm and length of 0.8-1.1 μm. The CaTiO3 morphology obtained by TEM is in accordance with the SEM observation result. Moreover, small-sized Au NPs are seen to be decorated on the surface of CaTiO3 NCs. The high-resolution TEM (HRTEM) image depicted in Figure 5b further confirms the good assembly of Au NPs on the surface of CaTiO3 NCs. The decorated Au NPs are shaped like spheres and have a size distribution of 3-7 nm. Energy-dispersive x-ray elemental mapping was used to investigate the elemental distribution of the 4.3%Au@CaTiO3 composite. Figure 5c illustrates the dark-field scanning TEM (DF-STEM) image of the composite, and the corresponding elemental mapping images are given in Figure 5d-g. It is observed that the NCs present uniformly-distributed elements of Ca, Ti and O. TEM investigation was further carried out to unveil the microstructure of the 4.3%Au@CaTiO 3 composite. Figure 5a shows the TEM image of the composite, from which one can see that CaTiO 3 presents a regular morphology of NCs with width of 0.3-0.5 µm and length of 0.8-1.1 µm. The CaTiO 3 morphology obtained by TEM is in accordance with the SEM observation result. Moreover, small-sized Au NPs are seen to be decorated on the surface of CaTiO 3 NCs. The high-resolution TEM (HRTEM) image depicted in Figure 5b further confirms the good assembly of Au NPs on the surface of CaTiO 3 NCs. The decorated Au NPs are shaped like spheres and have a size distribution of 3-7 nm. Energy-dispersive x-ray elemental mapping was used to investigate the elemental distribution of the 4.3%Au@CaTiO 3 composite. Figure 5c illustrates the dark-field scanning TEM (DF-STEM) image of the composite, and the corresponding elemental mapping images are given in Figure 5d-g. It is observed that the NCs present uniformly-distributed elements of Ca, Ti and O. Moreover, Au element is also seen to be uniformly distributed throughout the NCs, indicating that CaTiO 3 NCs are uniformly decorated with Au NPs. Energy-dispersive X-ray spectroscopy (EDS) spectrum was further used to examine the chemical composition of the 4.3%Au@CaTiO 3 composite. As shown in Figure 5h, the composite sample is composed of Ca, Ti, O, and Au elements. Additional Cu and C signals detected on the EDS spectrum could come from the TEM microgrid holder [61]. The obtained Au content from the EDS spectrum is 4.1%, which is basically in agreement with the stoichiometric composition of the 4.3%Au@CaTiO 3 composite. Moreover, Au element is also seen to be uniformly distributed throughout the NCs, indicating that CaTiO3 NCs are uniformly decorated with Au NPs. Energy-dispersive X-ray spectroscopy (EDS) spectrum was further used to examine the chemical composition of the 4.3%Au@CaTiO3 composite. As shown in Figure 5h, the composite sample is composed of Ca, Ti, O, and Au elements. Additional Cu and C signals detected on the EDS spectrum could come from the TEM microgrid holder [61]. The obtained Au content from the EDS spectrum is 4.1%, The XPS analyses were performed on 4.3%Au@CaTiO3 to elucidate the chemical states of elements. Figure 6a shows the survey XPS spectrum, revealing that the composite is composed of the elements Ca, Ti, O and Au. The observed C signal comes from adventitious carbon, which is used for the calibration of binding energy (C 1s binding energy: 284.8 eV). On the high-resolution XPS spectrum of Ca 2p core level (Figure 6b), the observed peaks at 346.7 and 350.2 eV account for Ca 2p3/2 and Ca 2p1/2, respectively. On the Ti 2p XPS spectrum (Figure 6c), the Ti 2p3/2 and Ti 2p1/2 binding energies are observed at 458.8 and 464.4 eV, respectively. The binding energy positions suggest that the Ca and Ti species behave as Ca 2+ and Ti 4+ oxidation states, respectively [13]. Two peaks at 529.9 and 532.1 eV are detected on the O 1s XPS spectrum (Figure 6d), which are assigned to the crystal lattice oxygen in CaTiO3 and chemisorbed oxygen species [13,62]. On the Au 4f XPS spectrum (Figure 6e), the peak at 83.6 is attributed to Au 4f7/2 and the peak at 87.2 eV corresponds to Au 4f5/2. This implies the Au species exists in the metallic state [63]. The XPS analyses were performed on 4.3%Au@CaTiO 3 to elucidate the chemical states of elements. Figure 6a shows the survey XPS spectrum, revealing that the composite is composed of the elements Ca, Ti, O and Au. The observed C signal comes from adventitious carbon, which is used for the calibration of binding energy (C 1s binding energy: 284.8 eV). On the high-resolution XPS spectrum of Ca 2p core level (Figure 6b), the observed peaks at 346.7 and 350.2 eV account for Ca 2p 3/2 and Ca 2p 1/2 , respectively. On the Ti 2p XPS spectrum (Figure 6c), the Ti 2p 3/2 and Ti 2p 1/2 binding energies are observed at 458.8 and 464.4 eV, respectively. The binding energy positions suggest that the Ca and Ti species behave as Ca 2+ and Ti 4+ oxidation states, respectively [13]. Two peaks at 529.9 and 532.1 eV are detected on the O 1s XPS spectrum (Figure 6d), which are assigned to the crystal lattice oxygen in CaTiO 3 and chemisorbed oxygen species [13,62]. On the Au 4f XPS spectrum (Figure 6e), the peak at 83.6 is attributed to Au 4f 7/2 and the peak at 87.2 eV corresponds to Au 4f 5/2 . This implies the Au species exists in the metallic state [63].  Figure 7a,b depict the UV-vis DRS spectra of CaTiO3 and 4.3%Au@CaTiO3, respectively, along with the corresponding first derivative curves of the UV-vis DRS spectra and the digital images of the samples (insets). It is seen that the decoration of Au NPs onto CaTiO3 NCs obviously enhances the visible-light absorption. This is further confirmed by the deepening of the apparent color for the 4.3%Au@CaTiO3 composite (dark gray), as compared to bare CaTiO3 NCs (cream white). The enhanced visible-light absorption implies that the Au@CaTiO3 composite photocatalyst can utilize photons more effectively. From the first derivative spectra, the absorption edge of CaTiO3 and 4.3%Au@CaTiO3 is obtained as 369.3 and 355.0 nm, respectively [64]. This suggests that bare CaTiO3 has a bandgap energy (Eg) of 3.36 eV and 4.3%Au@CaTiO3 exhibits an Eg of 3.49 eV. The slight increase in the Eg of the composite could be due to the interaction between CaTiO3 NCs and Au NPs.
We measured the FTIR spectra of CaTiO3 and 4.3%Au@CaTiO3 to elucidate their functional groups, as shown in Figure 8. The absorption peaks at 437 and 560 cm −1 are characterized as the  Figure 7a,b depict the UV-vis DRS spectra of CaTiO 3 and 4.3%Au@CaTiO 3 , respectively, along with the corresponding first derivative curves of the UV-vis DRS spectra and the digital images of the samples (insets). It is seen that the decoration of Au NPs onto CaTiO 3 NCs obviously enhances the visible-light absorption. This is further confirmed by the deepening of the apparent color for the 4.3%Au@CaTiO 3 composite (dark gray), as compared to bare CaTiO 3 NCs (cream white). The enhanced visible-light absorption implies that the Au@CaTiO 3 composite photocatalyst can utilize photons more effectively. From the first derivative spectra, the absorption edge of CaTiO 3 and 4.3%Au@CaTiO 3 is obtained as 369.3 and 355.0 nm, respectively [64]. This suggests that bare CaTiO 3 has a bandgap energy (E g ) of 3.36 eV and 4.3%Au@CaTiO 3 exhibits an E g of 3.49 eV. The slight increase in the E g of the composite could be due to the interaction between CaTiO 3 NCs and Au NPs.
We measured the FTIR spectra of CaTiO 3 and 4.3%Au@CaTiO 3 to elucidate their functional groups, as shown in Figure 8. The absorption peaks at 437 and 560 cm −1 are characterized as the Ti-O stretching vibration and Ti-O-Ti bridging stretching mode [65,66]. This implies the existence of TiO 6 octahedra and the formation of CaTiO 3 perovskite-type structure. The observed broad bands at 3430 (H 2 O stretching vibration) and 1637 cm −1 (H 2 O bending vibration) suggest the presence of water molecules absorbed on the surface of the samples [67]. The presence of NH 3+ group can be confirmed by the N-H stretching vibration located at around 3146 cm −1 [68]. The peaks detected at 1395 and 1102 cm −1 are attributed to the O-H in-plane deformation and C-OH stretching vibrations of alcohols [34]. This indicates that alcohols could be anchored on the samples during their washing process. In addition, no characteristic peaks assignable to Au oxides are observed for the 4.3%Au@CaTiO 3 composite, implying Au species exists in the metallic state. Ti-O stretching vibration and Ti-O-Ti bridging stretching mode [65,66]. This implies the existence of TiO6 octahedra and the formation of CaTiO3 perovskite-type structure. The observed broad bands at 3430 (H2O stretching vibration) and 1637 cm −1 (H2O bending vibration) suggest the presence of water molecules absorbed on the surface of the samples [67]. The presence of NH 3+ group can be confirmed by the N-H stretching vibration located at around 3146 cm −1 [68]. The peaks detected at 1395 and 1102 cm −1 are attributed to the O-H in-plane deformation and C-OH stretching vibrations of alcohols [34]. This indicates that alcohols could be anchored on the samples during their washing process. In addition, no characteristic peaks assignable to Au oxides are observed for the 4.3%Au@CaTiO3 composite, implying Au species exists in the metallic state.  To assess the photocatalytic degradation of RhB over the samples, three types of light source were separately used, i.e., simulated sunlight (300 < λ < 2500 nm), UV light (λ = 254 nm) and visible light (λ > 400 nm). Figure 9a shows the simulated-sunlight degradation of RhB photocatalyzed by CaTiO3 and Au@CaTiO3 composites. It is demonstrated that the Au@CaTiO3 composites manifests a photocatalytic activity much superior to that of bare CaTiO3 NCs. The content of decorated Au NPs has an obvious effect on the photocatalytic activity of the composite samples. With increasing the Au content, the photocatalytic activity of the samples gradually increases and reaches the highest level for 4.3%Au@CaTiO3. However, further increasing the Au content gives rise to a decrease in the photocatalytic activity. This could be explained by the fact that excessive decoration of Au NPs on the surface of CaTiO3 NCs could reduce the light absorption of CaTiO3. The degradation percentage of RhB after 120 min of photocatalytic reaction is inserted in Figure 9a. For the optimal composite sample-4.3%Au@CaTiO3, the dye degradation reaches 99.6%, which is increased by 23.2% when compared with that for bare CaTiO3 (76.4%). We also carried out the kinetic analysis of the dye degradation over the samples, as illustrated in Figure 9b. One can see that the plots of Ln(Ct/C0) vs t Ti-O stretching vibration and Ti-O-Ti bridging stretching mode [65,66]. This implies the existence of TiO6 octahedra and the formation of CaTiO3 perovskite-type structure. The observed broad bands at 3430 (H2O stretching vibration) and 1637 cm −1 (H2O bending vibration) suggest the presence of water molecules absorbed on the surface of the samples [67]. The presence of NH 3+ group can be confirmed by the N-H stretching vibration located at around 3146 cm −1 [68]. The peaks detected at 1395 and 1102 cm −1 are attributed to the O-H in-plane deformation and C-OH stretching vibrations of alcohols [34]. This indicates that alcohols could be anchored on the samples during their washing process. In addition, no characteristic peaks assignable to Au oxides are observed for the 4.3%Au@CaTiO3 composite, implying Au species exists in the metallic state.  To assess the photocatalytic degradation of RhB over the samples, three types of light source were separately used, i.e., simulated sunlight (300 < λ < 2500 nm), UV light (λ = 254 nm) and visible light (λ > 400 nm). Figure 9a shows the simulated-sunlight degradation of RhB photocatalyzed by CaTiO3 and Au@CaTiO3 composites. It is demonstrated that the Au@CaTiO3 composites manifests a photocatalytic activity much superior to that of bare CaTiO3 NCs. The content of decorated Au NPs has an obvious effect on the photocatalytic activity of the composite samples. With increasing the Au content, the photocatalytic activity of the samples gradually increases and reaches the highest level for 4.3%Au@CaTiO3. However, further increasing the Au content gives rise to a decrease in the photocatalytic activity. This could be explained by the fact that excessive decoration of Au NPs on the surface of CaTiO3 NCs could reduce the light absorption of CaTiO3. The degradation percentage of RhB after 120 min of photocatalytic reaction is inserted in Figure 9a. For the optimal composite sample-4.3%Au@CaTiO3, the dye degradation reaches 99.6%, which is increased by 23.2% when compared with that for bare CaTiO3 (76.4%). We also carried out the kinetic analysis of the dye degradation over the samples, as illustrated in Figure 9b. One can see that the plots of Ln(Ct/C0) vs t To assess the photocatalytic degradation of RhB over the samples, three types of light source were separately used, i.e., simulated sunlight (300 < λ < 2500 nm), UV light (λ = 254 nm) and visible light (λ > 400 nm). Figure 9a shows the simulated-sunlight degradation of RhB photocatalyzed by CaTiO 3 and Au@CaTiO 3 composites. It is demonstrated that the Au@CaTiO 3 composites manifests a photocatalytic activity much superior to that of bare CaTiO 3 NCs. The content of decorated Au NPs has an obvious effect on the photocatalytic activity of the composite samples. With increasing the Au content, the photocatalytic activity of the samples gradually increases and reaches the highest level for 4.3%Au@CaTiO 3 . However, further increasing the Au content gives rise to a decrease in the photocatalytic activity. This could be explained by the fact that excessive decoration of Au NPs on the surface of CaTiO 3 NCs could reduce the light absorption of CaTiO 3 . The degradation percentage of RhB after 120 min of photocatalytic reaction is inserted in Figure 9a. For the optimal composite sample-4.3%Au@CaTiO 3 , the dye degradation reaches 99.6%, which is increased by 23.2% when compared with that for bare CaTiO 3 (76.4%). We also carried out the kinetic analysis of the dye degradation over the samples, as illustrated in Figure 9b. One can see that the plots of Ln(C t /C 0 ) vs. t present a good linear relationship and can be perfectly described using the pseudo-first-order kinetic equation: Ln(C t /C 0 ) = −k app t [69,70]. Based on the linear-regression fitting, the apparent first-order reaction rate constant k app is obtained, as inserted in Figure 9b. It is concluded from the reaction rate constants (k app (CaTiO 3 ) = 0.01195 min −1 ; k app (4.3%Au@CaTiO 3 ) = 0.04701 min −1 ) that the photocatalytic activity of 4.3%Au@CaTiO 3 is~3.9 times as large as that of bare CaTiO 3 . Further comparison of the UV and visible-light photocatalytic performance between CaTiO 3 and 4.3%Au@CaTiO 3 was carried out. As shown in Figure 9c, CaTiO 3 exhibits an important UV photocatalytic activity toward the degradation of RhB. Moreover, a significantly enhanced UV photocatalytic activity is observed for the 4.3%Au@CaTiO 3 composite photocatalyst, which is ca. 3.0 times larger than that of bare CaTiO 3 . The visible-light photocatalytic degradation of RhB over CaTiO 3 and 4.3%Au@CaTiO 3 is shown in Figure 9d. It is observed that bare CaTiO 3 photocatalyzes only 15.1% degradation of the dye after 120 min of photocatalysis, which could be ascribed to the dye-photosensitized degradation. This is indicative of a poor photocatalytic activity of CaTiO 3 . Whereas 46.1% of RhB is observed to be degraded over 4.3%Au@CaTiO 3 , implying a greatly enhanced visible-light photocatalytic activity of the composite. present a good linear relationship and can be perfectly described using the pseudo-first-order kinetic equation: Ln(Ct/C0) = −kappt [69,70]. Based on the linear-regression fitting, the apparent first-order reaction rate constant kapp is obtained, as inserted in Figure 9b. It is concluded from the reaction rate constants (kapp(CaTiO3) = 0.01195 min −1 ; kapp(4.3%Au@CaTiO3) = 0.04701 min −1 ) that the photocatalytic activity of 4.3%Au@CaTiO3 is ~3.9 times as large as that of bare CaTiO3. Further comparison of the UV and visible-light photocatalytic performance between CaTiO3 and 4.3%Au@CaTiO3 was carried out. As shown in Figure 9c, CaTiO3 exhibits an important UV photocatalytic activity toward the degradation of RhB. Moreover, a significantly enhanced UV photocatalytic activity is observed for the 4.3%Au@CaTiO3 composite photocatalyst, which is ca. 3.0 times larger than that of bare CaTiO3. The visible-light photocatalytic degradation of RhB over CaTiO3 and 4.3%Au@CaTiO3 is shown in Figure 9d. It is observed that bare CaTiO3 photocatalyzes only 15.1% degradation of the dye after 120 min of photocatalysis, which could be ascribed to the dye-photosensitized degradation. This is indicative of a poor photocatalytic activity of CaTiO3. Whereas 46.1% of RhB is observed to be degraded over 4.3%Au@CaTiO3, implying a greatly enhanced visible-light photocatalytic activity of the composite. Based on the above experimental results, we propose a possible mechanism to elucidate the enhanced photocatalytic performance of the Au NPs modified CaTiO3 NCs, as schematically depicted Figure 10. Under UV irradiation, CaTiO3 is excited to produce electrons in the CB and holes in the valence band (VB) of the semiconductor. Au NPs cannot be excited under UV irradiation, but they can act as excellent electron sinks to capture the photogenerated electrons in CaTiO3. This is because the Fermi level of Au (+0.45 V vs normal hydrogen electrode (NHE) [71,72]) is more positive Based on the above experimental results, we propose a possible mechanism to elucidate the enhanced photocatalytic performance of the Au NPs modified CaTiO 3 NCs, as schematically depicted Figure 10. Under UV irradiation, CaTiO 3 is excited to produce electrons in the CB and holes in the valence band (VB) of the semiconductor. Au NPs cannot be excited under UV irradiation, but they can act as excellent electron sinks to capture the photogenerated electrons in CaTiO 3 . This is because the Fermi level of Au (+0.45 V vs. normal hydrogen electrode (NHE) [71,72]) is more positive than the CB potential of CaTiO 3 (−0.43 V vs. NHE [55]). The electron transfer process from the CB of CaTiO 3 to Au NPs promotes the spatial separation of the electron/hole pairs in CaTiO 3 . This is supported by photoluminescence spectra (Figure S1), photocurrent response curves ( Figure S2a) and electrochemical impedance spectroscopy (EIS) spectra ( Figure S2b). As a result, more holes in the VB of CaTiO 3 are available for taking part in the photocatalytic reactions. Under visible-light irradiation, CaTiO 3 cannot be directly excited to produce electron/hole pairs, instead LSPR of Au NPs is induced by the visible-light absorption [39,40]. The LSPR-induced electrons in Au NPs could take part in the photocatalytic reactions, and moreover, the electromagnetic field caused by LSPR could stimulate the generation of electron/hole pairs in CaTiO 3 . This is why the Au@CaTiO 3 composites also manifest enhanced visible-light photocatalytic degradation of RhB. When simulated sunlight is used as the light source, Au NPs simultaneously act as electron sinks and behave as LSPR effect in the Au@CaTiO 3 composites. The two mechanisms collectively result in the enhanced photocatalytic performance of the composites under simulated sunlight irradiation.
Hydroxyl (•OH), superoxide (•O 2 − ) and h + are generally considered to be the main active species in most of photocatalytic systems [73]. The reactive species trapping experiments ( Figure S3) Figure S4) is indicative of a good recycling stability of the Au@CaTiO 3 composite photocatalyst.
Micromachines 2018, 9, x 11 of 15 CaTiO3 to Au NPs promotes the spatial separation of the electron/hole pairs in CaTiO3. This is supported by photoluminescence spectra (Figure S1), photocurrent response curves ( Figure S2a) and electrochemical impedance spectroscopy (EIS) spectra ( Figure S2b). As a result, more holes in the VB of CaTiO3 are available for taking part in the photocatalytic reactions. Under visible-light irradiation, CaTiO3 cannot be directly excited to produce electron/hole pairs, instead LSPR of Au NPs is induced by the visible-light absorption [39,40]. The LSPR-induced electrons in Au NPs could take part in the photocatalytic reactions, and moreover, the electromagnetic field caused by LSPR could stimulate the generation of electron/hole pairs in CaTiO3. This is why the Au@CaTiO3 composites also manifest enhanced visible-light photocatalytic degradation of RhB. When simulated sunlight is used as the light source, Au NPs simultaneously act as electron sinks and behave as LSPR effect in the Au@CaTiO3 composites. The two mechanisms collectively result in the enhanced photocatalytic performance of the composites under simulated sunlight irradiation. Hydroxyl (•OH), superoxide (•O2 − ) and h + are generally considered to be the main active species in most of photocatalytic systems [73]. The reactive species trapping experiments ( Figure S3) reveal that the degradation of RhB is highly correlated with •OH and h + . However, the photogenerated h + could not directly oxide the dye, but rather reacts with OH − or H2O to produce another stronger oxidant •OH. Thermodynamically •OH is ready to be generated through the reaction between h + and OH -/H2O due to the sufficiently positive VB potential of CaTiO3 (+2.93 V vs NHE [55]) when compared with the redox potentials of H2O/•OH (+2.38 V vs NHE) and OH -/•OH (+1.99 V vs NHE) [74]. •O2 − plays only a slight role in the photocatalytic degradation of the dye. The sufficiently negative CB potential of CaTiO3 (−0.43 V vs NHE) suggests that the generation of •O2 − can be derived from the reaction of adsorbed O2 with the CB electrons of CaTiO3 (E 0 (O2/•O2 − = −0.13 V vs NHE) [74]. Moreover, the LSPR-induced electrons in Au NPs could also combine with O2 to produce •O2 − . Recycling photocatalytic experiment ( Figure S4) is indicative of a good recycling stability of the Au@CaTiO3 composite photocatalyst.

Conclusions
Based on a hydrothermal route, CaTiO3 NCs were synthesized using P25 as the titanium source, and their growth process was investigated by varying the reaction time. Au NPs were uniformly decorated on the CaTiO3 NCs surface by a photocatalytic reduction of HAuCl4 solution. Compared to bare CaTiO3 NCs, the as-derived Au@CaTiO3 composites manifest an increased visible-light absorption, increased photocurrent density, decreased charge-transfer resistance, decreased PL

Conclusions
Based on a hydrothermal route, CaTiO 3 NCs were synthesized using P25 as the titanium source, and their growth process was investigated by varying the reaction time. Au NPs were uniformly decorated on the CaTiO 3 NCs surface by a photocatalytic reduction of HAuCl 4 solution. Compared to bare CaTiO 3 NCs, the as-derived Au@CaTiO 3 composites manifest an increased visible-light absorption, increased photocurrent density, decreased charge-transfer resistance, decreased PL intensity, as well as enhanced photocatalytic performance for the degradation of RhB under irradiation of different light sources (simulated sunlight, UV light and visible light). The optimal composite sample, observed to be 4.3%Au@CaTiO 3 , has a photocatalytic activity 3.9 times as large as that of bare CaTiO 3 NCs, and photocatalyzes 99.6% degradation of RhB under simulated sunlight irradiation for 120 min. The enhanced photocatalytic mechanism of the Au@CaTiO 3 composites can be explained by (1) the efficient separation of photoexcited e − /h + pairs in CaTiO 3 due to the electron transfer from CaTiO 3 NCs to Au NPs, and (2) increased visible-light absorption due to the LSPR effect of Au NPs. The LSPR-caused electromagnetic field could stimulate the generation and separation of e − /h + pairs in CaTiO 3 , and moreover, the LSPR-induced electrons in Au NPs could be available for the photodegradation reactions. Reactive species trapping experiments reveal that the dye degradation is highly correlated with •OH radicals and photoexcited holes, whereas •O 2 − radicals play only a slight role in the photocatalysis.
The present Au@CaTiO 3 nanocomposite photocatalysts could offer promising applications in the design of micro/nano photocatalytic devices for the wastewater treatment.

Conflicts of Interest:
The authors declare no conflict of interest.