Enhanced Visible-Light-Driven Photocatalytic Activity of ZnAl Layered Double Hydroxide by Incorporation of Co2+

Co-doped ZnAl layered double hydroxides (LDH) were papered by coprecipitation. The prepared samples were characterized by multiple techniques including X-ray Diffraction (XRD), Brunauer−Emmett−Teller (BET) surface area, Scanning Electronic Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and UV−Vis Diffuse-Reflectance Spectroscopy (UV−Vis DRS). The incorporation of Co2+ into the ZnAl LDH sheets as CrO6 octahedron forms a new energy level which contributes for the excitation of electrons under visible light. The doped Co2+ at a reasonable content also serves as photo-generated charges separator and improves the visible light photocatalytic activity of ZnAl LDH. A degradation mechanism based on the hydroxyl radical as the active species was proposed. Copyright © 2018 BCREC Group. All rights reserved

The quantities of the layer metal cations and the anions between the layers are adjustable to bring changes to the layer charge density and other chemical properties, therefore, it could generate useful novel catalytic properties for the LDHs materials, such as catalysis, electrocatalysis, energy storage, photocatalysis, adsorption, and films/coatings for protection [4,5]. Recently LDHs has been used as an efficient and effective photocatalyst on the organic pollutant degradation reaction, as they are nontoxic, cheap and easily to prepare [6,7]. The pioneer work may be reported by Silva and coauthors [8]. They prepared ZnTi, ZnCe and ZnCr LDHs photocatalysts for generation of oxygen through photocatalytic water splitting under visible light irradiation. These photocatalysts showed good visible light oxygen generation activities. After that numbers of LDH materials, like MgAl LDH [9], CuFe LDH [10], ZnM LDH (M = Al, Fe, Ti, and Fe/Ti) [11], ZnCr LDH [12], CoFe LDH [7] LDH, MnCr LDH [6], etc., were developed as photocatalysts for degradation of organic contaminants. Among these photocatalysts, Zn-based LDHs photocatalysts found an increasing interest in the past few years [11][12][13][14][15]. Most of the reported photoactive Zn-based LDH materials have a Zn/Al nominal molar ratio close to 2, however, the bigger band gap energy limits the ZnAl LDH photocatalyst to be driven by UV light.
Generally, photocatalysts acquire high energy by absorbing suitable wavelength of photons. This high energy is used to shift the electrons from valence band (VB) to the conduction band (CB) which causes a redox reaction. This kind of shifting of electrons can be made more feasible by the following three methods: (i) by doping or coating the native compounds with suitable metal compounds. (ii) by changing their morphologies with the change in chemical compositions. (iii) by altering the synthesis methodologies [16]. It have been reported that modification of ZnAl LDH catalysts by doping metal ions or oxides can improve their photocatalytic activities under visible light [17][18][19][20][21]. Morales-Mendoza et al. [18,19] studied Mndoped Zn/Al LDH with Mn 0.5-3.0 % mol respect to Zn content with improved photocatalytic degradation of 4-chlorophenol (4Clphenol). They suggested the proposed mechanism is that Mn enhances the charge separation acting as electron e − (Mn 3+ ; Mn 4+ ) or hole h + (Mn 2+ ; Mn 3+ ) traps according to its oxidation state. The results reported by Parida et al. [22] showed that the substitution of Fe 3+ into Mg/Al-CO3 LDH can make it visible light active in water decomposition. The presence of iron favours the migration of photoproduced electrons to the conduction band and Fe 3+ ions trap electrons, thus improving the electron hole separation.
We prepared Ni-doped ZnAl LDH and investigated the visible light driven photocatalytic activity for degradation of Rhodamine B (RhB) [23]. The results showed that the doped Ni 2+ served as photo-generated charges separator which promoted the separation of electron and hole. It was reported that Co doping can be used to decorate the electron structure of metal oxides and achieve visible-light absorption [24]. In this present work, we prepared cobalt doped ZnAl LDH and investigated its photocatalytic activity under visible light irradiation. The role of Co 2+ was discussed and the degradation mechanism under visible light irradiation for RhB was proposed.

Preparation of Co-doped ZnAl LDHs
All the reagents were of chemical reagent grade and purchased from Sinopharm Chemical Reagent Company, Ltd., China. They were used without further purification. Doubleddistilled water was employed for all experiments. Co-doped ZnAl LDHs were prepared by a coprecipitation process at room temperature under air. Aqueous solutions (250 mL) with the desired amounts of Zn(NO3)2·6H2O, Co(NO3)2·6H2O and Al(NO3)3·9H2O, the total cation concentration is 1 mol/L, as precursors, and aqueous solutions with NaOH and Na2CO3 , as precipitants, were prepared. The Co/Zn/Al atomic ratios in the precursor solutions were set as 0/2/1, 0.1/2/1, 2/2/1, and 4/2/1. The precursor and precipitant solutions were added dropwise simultaneously with vigorous stirring into deionized water (300 mL) in a beaker glass at room temperature, keeping the pH at approximately 10 by controlling the addition speeds. The resulting mixtures were aged for 24 h at 65 °C, then filtered and washed with a large amount of water until pH was around 7. The resulting products were dried at 80 °C in an air oven overnight and Co-doped ZnAl LDHs were obtained. The resulted samples were labeled as Co-0, Co-0.1, Co-1.0, Co-2.0, and Co-4.0 according to the Co ratios in the samples.

Characterization
X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax 2000 diffractometer with Cu-Kα irradiation source, ranging from 5° to 70°. Scanning electron microscope (SEM) images were taken by JSM-6360LV (JEOL, Japan) at 10.0 kV. TEM micrographs were performed on FEI TECNAI-10. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM 800. A Mg-Kα X-ray source was used at 12 kV and 12 mA. The X-ray photoelectron spectra were referenced to the C1s peak (BE = 284.80 eV) resulting from adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. UV-vis diffuse reflectance spectra were recorded on a UV-vis spectrophotometer (UV-2550, Shimadzu) equipped with an integrating sphere, and BaSO4 was used as the reference. A V-SORB 2800P specific surface and pore size analyzer (Gold APP, China) was used to estimate the specific surface area and pore size of as-prepared samples. The samples were evacuated 3 h at 523 K, then cooled to 77 K using liquid N2 at which point N2 adsorption was measured.

Evaluation of the Photocatalytic Activity
The irradiation source was a 300 W Xe lamp purchased from Hefei Haibilan Scientific Instruments Co,. Ltd., China. Light was passed through a cutoff filter ( > 420 nm for visiblelight illumination) and then focused on the reactor. The lamps was located on the top of the solution with the distance of 50 mm. The initial concentration of Rhodamine-B (RhB) solution was 5 mg/L. 0.1 g of the prepared photocatalyst was added into 100 mL RhB solution. The degradation reaction was performed at room temperature. After regular intervals, the samples were removed and centrifuged to separate the photocatalyst for analysis. The concentration of RhB was measured by a T6 spectrophotometer (Purkinje General Company, China) at 554 nm and analyzed by Lambert-Beer law.

Characterization of Samples
The XRD patterns of the prepared samples are presented in Figure 1. The diffraction patterns of all the samples match well with those of ZnAl LDHs with Zn/Al ratio of 2 (JCPDS file no. 48-1023). Other peaks were not found indicating pure phase synthesis of ZnAl LDHs. The intensity of diffraction peaks decreases with the increase in Co 2+ content owing to the introduction of Co ions into the layers. Further ob-servation shows that, with the increase in Co 2+ content, the peak of plane (003) moves to lower angle, while that of plane (110) moves to higher angle. Considering that the parameter a = 2 × d(110) corresponds to the cation-cation distance within the Brucite layer, and the parameter c = 3 × d(003) to its thickness, both parameters generally increase when Co 2+ cations are inserted into the lattice of the LDH structure due to a partial substitution of Al 3+ by Co 2+ . Because the ionic radius of Co 2+ is bigger than that of Al 3+ (ionic radius 0.065 nm for Co 2+ , and 0.054 nm for Al 3+ ) [25], the replacement of Al 3+ by Co 2+ causes the increase in cation-cation distance and reduces the electrostatic interaction and increase of interlayer distance [26]. Co-0 Co-4.0 The morphology of selected sample Co-1.0 was observed by SEM and TEM techniques, the results are shown in Figure 2. It can be seen from SEM image that Co-doped ZnAl-LDH nanoparticles display obvious lamellar crystal with layered structure, but the material layers pile up together. TEM image shows the laminar hexagonally-shaped morphology, which is typical of LDHs.
The surface area and pore diameter distribution of samples are determined by nitrogen physical adsorption method. Figure 3(a) shows the nitrogen adsorption-desorption isotherms of ZnAl-LDHs and CoZnAl-LDHs with different Co doping contents. For all the samples, typical type IV isotherms with a distinct hysteresis loops are observed in the range of 0.2-1.0 p/p0, which is characteristic of porous materials, suggesting well the presence of mesopores in the nanoplates. This is derived mainly from the platelet stacking of ZnAl-LDHs or CoZnAl-LDHs nanoparticles. The isotherm also shows adsorption at relative pressures (p/p0) approaching 1.0, suggesting the formation of large macropores [27]. The pore diameter distributions of all samples are shown in the insets of Figure 3(b). It can be found that all samples have intense pore distribution in mesopore (2-10 nm) and broad distribution in macropore (10-80 nm), indicating coexist of mesopore and macropore. This hierarchical structure with meso and macropores can serve as efficient transport paths for reactants and products in photocatalytic reactions. Moreover, the BET specific surface area increases with the Co doping amount increasing, except for Co-4.0. The observed specific surface areas are 22.92, 27.98, 57.24, 64.57, and 56.62 m 2 /g for samples Co-0, Co-0.1, Co-1.0, Co-2.0 and Co-4.0, respectively.
X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical component and element valence states of the sample, the results are shown in Figure 4. The survey spectrum (Figure 3a) indicates that the sample is mainly composed of elements of Zn, Co, Al, O, and C. High-resolution XPS spectrum of Co-2p is shown in Figure 4b. The Co-2p spectra exhibited the expected spin-orbit split doublet, with Co-2p3/2 and Co-2p1/2 components at 780.2 and 796.1 eV binding energy, respectively, and satellites at 784.9 and 802.1 eV indicative of high-spin divalent Co 2+ species within the CoAl-LDH layers [28]. Two binding energies at 1021.2 eV and 1044.2 eV correspond to Zn-2p3/2 and Zn-2p1/2, indicating the presence of Zn 2+ species bonding with oxygen [21,29]. The binding energy for Al-2p at 74.2 eV is associated to an aluminum hydroxide species Al(OH)n [20,29]. The element composition estimated from XPS was 6.09 %, 12.66 %, and 5.58 % for Co, Zn, and Al, respectively, showing the Co/Zn/Al ratio was 1.1/2.27/1. Compared with the designed Co/Zn/Al ratio of 1/2/1, the Al content is slightly less, perhaps attributed to the incompletely precipitation of Al element during the preparation at pH of 10. Figure 5 presents the UV-vis diffuse reflectance spectra of all samples. Here F(R) represents the Kubelka-Munk function (K-M), which is expressed as Equation (1), where, R is reflectance and F(R) is K-M function. (1) The UV-Vis diffuse reflectance spectrum for the ZnAl LDH shows a weak absorption peak in the spectral region of 200-400 nm, which is due to the electronic transition of ZnAl LDH from the O-2p state to the metal ns or np levels (n = 4 for Zn and n = 3 for Al) [20]. The low absorption ability of ZnAl LDH indicates the low photocatalytic activity, which was also observed by other authors [30,31]. The CoZnAl LDH samples display two distinct absorption features, a strong absorption band in the ultraviolet light range and a broad band in the visible region centered around 500 nm attributable to d-d transitions of octahedral Co 2+ within the CoZnAl LDH layer [28,32]. It is observed that, with the Co doping amount increasing, the absorption band shifts to higher wavelength and the intensity of the absorption peak around 500 nm increases, indicating the increase in

Photocatalytic Activities
The catalytic activity of the as-synthesized catalysts was evaluated by the degradation of RhB aqueous solution under visible light irradiation. The evolution of the relative concentration of RhB aqueous solution as a function of irradiation time over different photocatalysts is shown in Figure 6. As shown in Figure 6, the adsorption percentage of RhB on the catalyst is about 32 % (shown as blank in Figure 6), while the self-degradation of RhB in the absence of catalyst is about 30 %. The photodegradation percentage of RhB over bulk ZnAl LDH (sample Co-0) reaches 48 % after visible light irradiation for 180 min. For ZnAl LDH, the absorption band energy is so high (3.21 eV) that the electrons cannot be excited under the irradiation of visible light. Thus, the photodragation percentage of RhB over ZnAl LDH is attributed to the strongly photosensitization of RhB [33]. When ZnAl LDH is doped by Co 2+ , the degradation percentage over Co-doped ZnAl LDHs can reach 75-87 % after visible light irradiation for 180 min, indicating the    doping of Co into ZnAl LDH improve the photocatalytic activity of ZnAl LDH. With Co doping amount increasing, the photocatalytic activity increases to a maximum value, then diminishes at higher Co content. The photocatalytic activities were observed in the order as follows Co-0 < Co-4.0 < Co-0.1 ≈ Co-2.0 < Co-1.0 with photocatalytic degradation values of 48, 71, 81, 82, and 87 %, respectively. These results suggest that an optimum in Co content.
The Langmuir-Hinshelwood model is commonly used to describe the kinetics of photocatalytic reactions of organic compounds in aqueous solutions [34]. It relates the degradation rate r and the concentration of organic compound C, and is expressed as follows: (2) where kr is the intrinsic rate constant and Kad is the adsorption equilibrium constant. When the adsorption is relatively weak and the concentration of organic compounds is low, the factor KadC is insignificant, and the equation can be simplified to the first-order kinetics with an apparent rate constant (kapp = k r K ad ), which gives, after integration in the interval [C,C0]: Plotting of ln(C0/C) versus reaction time t yields a straight line, where the slope is the apparent rate constant. Figure 7 shows the lineal plot of RhB photodegradation, which adjusts well to a pseudo-first-order kinetic behavior, as most photocatalytic degradation reactions. Apparent rate constant k app and the linearization coefficient r 2 are summarized in Table 2. The apparent rate values obtained for the Co-doped ZnAl LDHs are 1.86-3.11 times higher than ZnAl LDH.

The Proposed Mechanism
The UV-visible spectral changes during the oxidation of RhB in the presence of Co-doped ZnAl LDH was spectrophotometrically monitored ( Figure 8). There were two main characteristic absorption bands of RhB at UV region (259 nm) and visible light region (554 nm), respectively. The visible band absorbance at 554 nm, which is used to monitor residual RhB, is responsible for RhB coloration because of the conjugated structure of C=N and C=O groups. The UV absorbance at 259 nm corresponds to the existence of aromatic rings in RhB [35]. It has been reported that RhB decolorization takes place via two competitive pathways: N-deethylation or cleavage of the chromophore structure [36]. The signal at 554 nm was observed diminishing very quickly within the first 1 min and then slowly until 60 min, following the decolorization of the RhB solution. However, no band shift was observed during the spectra evolution, indicating the destruction of C=N and C=O groups. Moreover, the decrease in absorbance at 259 nm indicated the destruction of the aromatic part, suggesting that a broken conjugated structure was predominant decolorization mechanism.
To reveal the predominant reactive species in the degradation of RhB over CoZnAl LDH
photocatalysts, benzoquinone and tert-butyl alcohol (TBA) were chosen as scavengers for a superoxide radical (•O2 − ) and a hydroxyl radical (•OH), respectively [13,33]. As shown in Figure  9, the introduction of benzoquinone into the photodegradation system led to a slight reduction in the degradation of RhB after 180 min irradiation. The significant reduction of RhB degradation was observed due to the introduction of TBA. The above results indicated that the •OH radical was the main active oxidizing species rather than the •O2 − radical in the photogradation process.
On the other hand, the assistance of the dye sensitization further facilitates the degradation of RhB. It was reported that the essence of the photosensitization was to be the increasing of H2O2 in the reaction system [33] and then H2O2 served as electron capture to improve the effective separation of e − -H + pairs, resulting in a higher photocatalytic degradation efficiency of dye [37,38]. In this work, we also investigated the influence of H2O2 on the degradation of RhB, the result is shown in Figure 9. After adding H2O2 into the reaction system, the RhB concentration declined rapidly and completely after irradiation of 90 min. This result suggests the promotion of H2O2 on the RhB degradation.
Furthermore, the degradation percentages of RhB over Co-doped ZnAl LDHs with different Co doping levels showed that an optimum in the Co content. Here it is proposed that Co 2+ acts as a dopant into the LDH layer sheets, which can capture the photogenerated holes and electrons and suppress their recombination, increasing the life of excited charge carri-  er in the optimum amount. However, in a higher concentration, Co species do not act as trapping sites but electron-hole recombination centers [21]. Thus a higher degradation percentage of RhB was obtained over Co-1.0 sample in this work.
Based on the results, we propose a mechanism based on the •OH as the main active species and Co 2+ as the charge separator ( Figure  10). When Co-doped ZnAl LDH is exposed to visible light, electrons are generated from CoO6 octahedrons in the layer sheets. The photogenerated holes move to the surface of the catalyst and react with water and OH − ions to yield •OH radicals. The electrons may react with adsorbed oxygen on the catalyst surface to produce superoxide (•O2 − ). Superoxide (•O2 − ) can further react with adsorbed H + to produce peroxide radicals (•OOH), further yielding H2O2 which further generate hydroxyl radicals (•OH). Hydroxyl (•OH) is highly reactive species that will oxidize/degrade RhB into simple molecules. On the other hand, under irradiation of visible light, the photo-generated electrons reduce the Co 3+ species present on the catalyst surface (electron trap), favoring the charge separation and producing an accumulation of h + which can move toward OH − adsorbed and produce •OH [18,19,21]. To recover its oxidation estate Co 3+ , the capture e − are transfer toward H + adsorbed on the surface and leading to the formation of H2, analogously as when is employed an organic pollutant as sacrificial molecule for the H2 evolution [18]. In addition, the absorption of the visible light over RhB could excites the RhB to form electrons to inject into the conduction band of Co-doped ZnAl LDH, which is called dye sensitization producing •O2 -or H2O2 can facilitate the degradation of the RhB molecule [33]. The proposed the mechanism involving the sequence of reactions is illustrated as follows: CoZnAl LDH + hν → CoZnAl LDH (e -+ h + ) (4) h + + H2O → •OH + H + h + + OH -→ •OH RhB + hν → RhB* RhB* → e -+ RhB + (8) e -+O2 → •O2 - •O2 -+ H + → •HOO (10) •HOO + •HOO → H2O2 + O2 H2O2 + H + + e − → •OH •OH+RhB(RhB + ) → Degradation products (13)

Conclusions
The high crystallinity Co-doped ZnAl LDHs based on Co/Zn/Al molar ratios ranging from 0.1:2:1 to 4:2:1 were prepared by coprecipitation method. The doping of Co 2+ into the LDH sheets increases the light absorption capacity of ZnAl LDH materials. The formed CoO6 octahedrons contributes the excitation of electrons under visible light. The doped cobalt ions server as the photo-generated charges separator which effectively separates the excited electrons and holes, resulting in the higher RhB photocatalytic degradation efficiency. The process of the RhB degradation has been found to fit the first-order kinetics according to the Langmuir-Hinshelwood model. A RhB degradation mechanism based on the hydroxyl radicals was supposed. The approach which improves the visible light photocatalytic activity of LDH materials by doping transition metals into LDH sheets can be extended to prepare visible-light-driven LDHs.