A visible light-sensitive argentous oxide/calcium aluminate nanocomposite photocatalyst

A photo-deposition method was developed to synthesise argentous oxide/calcium aluminate nanocomposites (Ag2O-CAONCS) with orthorhombic Ca5Al6O14 and cubic Ag2O phases. Micro-morphology, micro-structure, optical, and visible-light catalytic properties were investigated via different techniques. Ag2O enhances visible-light absorption performance and decreases the band gap of the Ag2O-CAONCS. About 20 mL crystal violet (CV) dye (10 mgL–1) is totally degraded using 20 mg Ag2O-CAONCS under sunlight irradiation for 20 min. The enhanced photocatalytic activity of the Ag2O-CAONCS towards CV under sunlight irradiation is ascribed to the reduction of the band gap, improvement in visible-light absorption and inhibitation role of carriers recombination by Ag2O. The high, stable visible-light catalytic activity makes Ag2O-CAONCS promising photocatalysts for removing organic pollutants.


Introduction
Calcium aluminate, as an inorganic compound in Al 2 O 3 -CaO binary system, exhibits great application promising in the fields of high alumina cement, refractory, photocatalysts, optical devices, flame detectors, dental cements and biomaterials [1][2][3][4][5].Calcium aluminate nanoparticles with an average crystalline size of 8.34 nm exhibited good photocatalytic performance towards Coralene Dark Red 2B azo under solar irradiation [6].The photocatalytic efficiency reached 86.87% using 1500 mg calcium aluminate nanoparticles in 300 mL 30 mg/L of Coralene Dark Red 2B dye in a neutral solution.Calcium aluminate paste showed good flowability and rigidity owing to the lubricating role between the calcium aluminate particles exhibiting good application potential in the inorganic binder field [7].La, Te-doped calcium aluminate nanophosphors exhibited strong PL emission at 395 and 535 nm corresponding to blue light and green light, respectively [8].Calcium aluminate@reduced graphene oxide (rGO) composite electrode showed good electrochemical double-layer capacitive behaviour with the specific capacitance of 79.33 Fg -1 at 0.2 Ag -1 due to synergistic role of free electron transfer and diffusion between the interface of the calcium aluminate and rGO nanosheets [9].However, calcium aluminate possesses a large band gap which limits the practical application for wastewater treatment as the photocatalyst.
Visible-light-sensitive semiconductor photocatalysts are effective to remove organic pollutants in wastewater under visible-light illumination which is beneficial for the practical application of the photocatalysts owing to the low cost and facile treatment process.Therefore, it is important to modify calcium aluminate nanomaterials for practical wastewater treatment under visible-light irradiation.
It is an effective method to prepare inorganic nanocomposite photocatalysts for promoting the migration of the photo-generated carriers, visiblelight catalytic activity [10,11].Argentous oxide (Ag 2 O) belongs to a visible-light-driven photocatalyst with a narrow band gap (1.2 eV) [12][13][14].However, the application of Ag 2 O is limited owing to the high cost and instability for the decomposition into metallic Ag during the photocatalytic degradation of the inorganic pollutants [15].Nanocomposites formed from semiconductors and Ag 2 O can work as efficient and stable visible-light photocatalysts [16,17].Hierarchical Ag 2 O/TiO 2 heterojunction-loaded CuC 2 O 4 nanosheet-modified Cu mesh (Ag 2 O/TiO 2 @CuC 2 O 4 CM) showed superior photocatalytic ability which greatly enhanced the anti-pollution ability of the substrate Cu mesh [18].Ag 2 O/TiO 2 @CuC 2 O 4 CM exhibited a high photo-degradation efficiency towards methylene blue (MB) under visible light for 60 min and self-cleaning ability for regenerating oil-contaminated mesh.Ag 2 O/TiO 2 nanocomposites exhibited good photocatalytic activity towards MB and phenol which was 8.9 and 2.9 times superior to that of TiO 2 , respectively [19].The enhanced photocatalytic activity was ascribed to improved light absorption and reduced recombination of electron-hole pairs.ZnO/Ag 0 /Ag 2 O heterojunction structures exhibited enhanced photocatalytic activity towards MB dye with a degradation ratio of 96.24% within 30 min visible-light irradiation which was 26.75 times higher than that of ZnO [20].Ag 2 O can be used as an efficient electron trapping agent to improve the visible-light catalytic activity of semiconductor photocatalysts.Thus, a combination of Ag 2 O and calcium aluminate nanomaterials, such as calcium aluminate nanosheets (CaAlONS), will improve the catalytic performance of CaAlONS due to the stepwise structure of band-edge levels in the Ag 2 O/CaAlONS.To date, the research on the synthesis and photocatalytic activity of the argentous oxide/calcium aluminate nanocomposites (Ag 2 O-CAONCS) has not been reported.
In this work, Ag 2 O-CAONCS were obtained through a photo-deposition approach.The as-synthesised Ag 2 O-CAONCS show improved catalytic activity in crystal violet (CV) removal under natural sunlight compared with the CaAlONS.Furthermore, photo-stability and photocatalytic mechanisms for CV degradation using the Ag 2 O-CAONCS were also researched.

Synthesis of Ag 2 O-CAONCS
About 0.2 g of sodium aluminate and 0.226 g of calcium chloride were used to synthesise CaAlONS in an autoclave at 180 °C for 24 h by hydrothermal process.Ag 2 O-CAONCS were prepared by the photo-deposition process as follows: A certain amount of Ag acetate and 60 mg of CaAlONS were mixed in 15 mL of distilled water.The uniform dispersion was obtained by sonicating for 15 min; 5 mL methanol was added to above solution stirring for 15 min.The mixed solution with the CaAlONS and Ag acetate was illuminated for 0.5 h under a 200 W Hg lamp.White Ag 2 O-CAONCS samples were isolated and obtained by the filtration process.The Ag 2 O-CAONCS samples were named 0.05-Ag 2 O-CAONCS, 0.1-Ag 2 O-CAONCS corresponding to the Ag 2 O content of 0.05, 0.1 mmol, respectively.

Characterisation techniques
The crystal structures of the as-grown Ag 2 O-CAONCS samples were realised from X-ray diffraction (XRD) patterns with graphite monochromatised Cu-K α radiation (Bruker AXS D8).The microscopic morphologies and structures of the Ag 2 O-CAONCS samples were recorded using scanning electron microscopy (SEM, JEOL JSM-6490LV) and transmission electron microscope (TEM, JEOL JEM-2100).Ag 2 O-CAONCS samples were dispersed into distilled water with the supersonic wave sonication for 10 min.Then, several drops of solution with Ag 2 O-CAONCS were added to the copper grid and dried to get TEM sample.Element mapping was obtained using energy dispersive spectroscopy (EDS) attached to SEM equipment.Solid ultraviolet-visible (UV-vis) diffusion reflectance spectra (DRS) of Ag 2 O-CAONCS were obtained via a UV3600 UV-vis spectrometer with an integrating sphere attachment.The elemental compositions of the samples were measured by X-ray photoelectron spectroscope (XPS, Thermo Fisher Scientific).Raman spectra of the samples were measured by a Renishaw Invia model confocal Raman spectrometer with a He/Ne light source (10 mW), excitation wavelength of 532 nm and exposure time of 10 s.Photoluminescence (PL) spectra were recorded using FLS920 model fluorescence spectrophotometer with an excitation wavelength of 330 nm from an Xe-laser.Electrochemical impedance spectroscope (EIS) was analysed in 10 mg•L −1 CV solution in the air using the CHI660E model workstation.

Evaluation of the photocatalytic performance of Ag 2 O-CAONCS
Photocatalytic experiments were performed as follows: The Ag 2 O-CAONCS (20 mg) were added to 20 mL of CV dye (10 mg•L -1 ).Before sunlight irradiation, the mixed suspension was stirred in the dark for 20 min to obtain equilibrium between CV molecules and Ag 2 O-CAONCS.At an interval of 10 min, 2 mL of suspension was collected to remove remnant Ag 2 O-CAONCS.CV concentration in the supernatant was obtained by analysing the intensity of the UV-vis absorption peak on a Youke UV756 model UV -vis spectrophotometer.For the comparison, the photocatalytic activity of the CaAlONS was also determined under identical conditions.About 20 mg of 0.1-Ag 2 O-CAONCS sample, 10 mL of CV solution, and 10 mL scavenger solution (10 mg•L −1 ) were applied for scavenger experiments to determine the reaction active species; 200 mg of 0.1-Ag 2 O-CAONCS and 200 mL CV solution (10 mg•L −1 ) were applied for investigating the re-usability of the Ag 2 O-CAONCS under sunlight irradiation.The 0.1-Ag 2 O-CAONCS were washed with distilled water and ethanol, respectively, and dried after each photocatalytic experiment.

Structures, morphologies and compositions of Ag 2 O-CAONCS
The XRD data of CaAlONS and Ag 2 O-CAONCS are shown in Figure 1.CaAlONS sample is composed    are caused by Ca 2p 3/2 and Ca 2p 1/2 , respectively (Figure 4(d)) [23,24].Ag 3d XPS peaks (Figure 4(e)) demonstrate spin-orbit split lines of Ag 3d 5/2 and Ag 3d 3/2 at 366.8, 372.1 eV, which indicates the existence of Ag + and formation of the Ag 2 O in the nanocomposites [25].The binding energies of the elements Ca, Al, O and Ag in the Ag 2 O-CAONCS sample are similar to those of the undoped CaAlONS (supplementary Figure S2) and Ag 2 O CaAlONS (supplementary Figure S3).

Optical performance of the Ag 2 O-CAONCS
The photochemical activity of the semiconductor photocatalysts is relevant to the optical performance.Therefore, the optical performance of the Ag 2 O-CAONCS was analysed by DRS, PL spectra of the CaAlONS and Ag 2 O-CAONCS.As shown in Figure PL Emission can be used to analyse the separation capability of the carriers in the semiconductor photocatalysts.It is well known that the lower intensity of the PL emission peak implies lower recombination efficiency of photo-generated carriers [30][31][32][33].As shown in Figure 6, the broad emission band at 640-680 nm with the emission peak at 658.4 nm is assigned to the charge transfer transition of oxygen vacancy trapped electrons of the Ca 5 Al 6 O 14 .When

Photocatalytic activity of the Ag 2 O-CAONCS
CV was chosen as model pollutant for analysing catalytic activity of the Ag 2 O-CAONCS under sunlight irradiation compared with the CaAlONS.About 20 mg photocatalysts and 20 mL 10 mgL -1 CV solution were used for photocatalytic degradation experiments.Figure 7(a) shows the photocatalytic activities of the Kinetic analysis for CV degradation can be conducted through the fitting of the experimental data using the first-order kinetic formula as the following: -ln(C/C 0 ) = kt [36], where k is the apparent first-order reaction rate constant.Raman spectra of 0.1-Ag 2 O-CAONCS irradiated by sunlight for different times in 10 mg•L -1 CV solution were further analysed for the CV degradation (Figure 8).Observed from the Raman spectrum of fresh 0.1-Ag 2 O-CAONCS, a Raman peak is located at 1573.1 cm -1 .After the CV solution with 0.1-Ag 2 O-CAONCS was irradiated under sunlight for 7 min, the Raman peak of 0.1-Ag 2 O-CAONCS shifted to 1590.9 cm -1 .Some Raman peaks located at 1445.1, 1387.7,1179.7 and 1140.1 cm -1 are observed which corresponds to ring C-C stretching, N-phenyl stretching, ring C-H bend (II) and ring C-H bend (II), respectively [37,38].With increasing sunlight irradiation time to 14 min, the intensity of Raman peaks at 1440.4,1175.1 and 1135.3 cm -1 sharply decreases showing the catalytic degradation of CV molecules.After 20 min sunlight irradiation, the Raman spectrum of 0.1-Ag 2 O-CAONCS is similar to that of fresh sample further demonstrating that CV solution can be entirely degraded by 0.1-Ag 2 O-CAONCS with sunlight irradiation for 20 min.
Metallic cations usually exist in wastewater, and it is important to analyse the role of metallic cations on the catalytic activity of Ag 2 O-CAONCS for CV degradation under sunlight.CV degradation efficiency using 0.1-Ag 2 O-CAONCS decreases to 78.6%, 82.4% and 75.3% when Mn 2+ , Cu 2+ and Zn 2+ are introduced into CV solution, respectively (supplementary Figure S4).It is obvious that these metallic ions inhibit CV catalytic degradation which is caused by the adsorption of metallic cations on Ag 2 O-CAONCS decreasing reaction active sites [39,40].
To analyse the stability of visible-light catalytic property, the as-synthesised 0.

Photocatalytic mechanism of the Ag 2 O-CAONCS
The interfacial charge transfer ability of the CaAlONS and 0.1-Ag 2 O-CAONCS was analysed using EIS (Figure 9).Nyquist plots of EIS show an arc part of the semi-circle and a linear part.The radius of the semi-circular arc part of 0.1-Ag 2 O-CAONCS is far smaller than that of the CaAlONS and 0.05-Ag 2 O-CAONCS showing a lower interfacial charge transfer resistance [41].Using Zview3.0 software, the interfacial charge transfer resistance was calculated by fitting the EIS data via an equivalent circuit (inset During the catalytic degradation of the organic pollutants, reaction active substances including • OH, • O 2 − and h + play essential roles in the photocatalytic reaction [44][45][46].For this reason, the reaction active substances trapping experiments were carried out

Figure 3 (
a) indicates the TEM image of the 0.1-Ag 2 O-CAONCS sample; 0.1-Ag 2 O-CAONCS shows the best photocatalytic performance towards CV which is confirmed by the following photocatalytic experiments.Therefore, 0.1-Ag 2 O-CAONCS were used as the representative sample for TEM and HRTEM observation.It is clearly observed that the nanocomposites are constructed from the nanoflakes with randomly distributed nanoparticles; 0.1-Ag 2 O-CAONCS sample exhibits regular lattice fringes (Figure 3(b)).The interplanar distance is 0.79, 0.33 nm, which is ascribed to (110) plane of orthorhombic Ca 5 Al 6 O 14 , (110) plane of cubic Ag 2 O.The regional electron diffraction patterns of the corresponding crystal planes are shown in the insets of Figure 3(b) as the Fast Fourier transform (FFT) patterns show single crystalline nature.Combined with the XRD, SEM and TEM analyses, Ag 2 O-CAONCS are composed of orthorhombic Ca 5 Al 6 O 14 and cubic Ag 2 O phases.The surface chemical compositions of 0.1-Ag 2 O-CAONCS sample were characterised by XPS.

Figure 4 (
a) shows the survey spectrum displaying that the Ag 2 O-CAONCS sample consists of O, Ag Ca and Al.The peak of high-resolution O 1s at 528.9 eV (Figure 4(b)) can be contributed to Al-O or Ag-O peak of the Ag 2 O-CAONCS [21].Al 2p XPS peak at 71.4 eV (Figure 4(c)) is assigned to Al 2p 3/2 of the Ag 2 O-CAONCS [22].Ca 2p XPS peaks at 344.3 and 347.8 eV

Ag 2 O
is introduced into CaAlONS to form Ag 2 O-CAONCS, Ag 2 O-CAONCS exhibits a sharper decrease in the intensity of the PL peak than CaAlONS.Therefore, Ag 2 O-CAONCS exhibits fast charge transfer and enhanced separation of photo-generated carriers at the interface between Ca 5 Al 6 O 14 and Ag 2 O, which will decrease the recombination efficiency of carriers and enhance the catalytic performance[34,35].

Figure 7 (
b) shows the first-order kinetic data for CV degradation with Ag 2 O-CAONCS.The linear relationship between -ln(C/C 0 ) and irradiation time t indicates that CV degradation follows the pseudo-first-order kinetics.CV reaction rate constant (k) with CaAlONS, 0.05-Ag 2 O-CAONCS and 0.1-Ag 2 O-CAONCS is estimated to be 0.0004, 0.0847 and 0.1121 min -1 with the correlation coefficient of 0.997,

1 -
Ag 2 O-CAONCS was used to degrade CV dye in five repeated cycles.The corresponding photocatalytic results are shown in supplementary Figure S5.It is noted that the photocatalytic performance of the Ag 2 O-CAONCS shows effective photocatalytic stability under sunlight irradiation.The CV degradation efficiency reduces only by 6% after five cycles indicating good stability of Ag 2 O-CAONCS photocatalyst.

using 0. 1 -
Ag 2 O-CAONCS to identify the effect of the reaction active substances.The reaction active trapping agents including ascorbic acid (AA), disodium ethylenediaminetetraacetate (EDTA) and methanol were utilised to scavenge • O 2 − , h + and • OH radicals.As seen in Figure 10, methanol has no effect on the photocatalytic degradation efficiency compared with 0.1-Ag 2 O-CAONCS without scavengers under sunlight irradiation.Although • OH radical belongs to a crucial reaction active substance formed during the photocatalytic reaction, it doesn't act as the reaction active substance for CV degradation.However, in the presence of AA and EDTA, the CV photocatalytic degradation efficiency decreases to 21.3% and 9.5%, respectively, showing the inhibition role of the photocatalytic activity.The results indicate that • O 2 − and h + are the main

Figure 9 .
Figure 9. Experimental and fitting data of EIS of the CaAlONS and Ag 2 O-CAONCS, the inset in the upper-left part is electrical equivalent circuit.

Figure 10 .
Figure 10.CV Degradation ratio using 0.1-Ag 2 O-CAONCS in the CV solution with different scavengers.

Figure 11 . 2 −
Figure 11.CV Photocatalytic degradation schematics using Ag 2 O-CAONCS.The inset is the VB spectrum of the CaAlONS.