Effect of Co2+ Substitution in the Framework of Carbonate Intercalated

Sep 19, 2012 - Stefan Berner , Paulo Araya , Joseph Govan , Humberto Palza. Journal of Industrial and Engineering Chemistry 2018 59, 134-140 ...
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Effect of Co2+ Substitution in the Framework of Carbonate Intercalated Cu/Cr LDH on Structural, Electronic, Optical, and Photocatalytic Properties Kulamani Parida,* Lagnamayee Mohapatra, and Niranjan Baliarsingh Colloids & Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar-751013, Odissa, India S Supporting Information *

ABSTRACT: In the present work, a series of Cu−Co/Cr ternary LDHs containing CO32− in the interlayer was prepared by coprecipitation method. To investigate the effect of divalent metal ions on the catalytic activity, we vaired Cu/Co atomic ratios, keeping constant the atomic ratio of Cu+Co/Cr (2:1). Several characterization tools, such as powder X-ray diffraction (PXRD), Brunauer−Emmett−Teller surface area, Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy, and UV−vis diffuse reflectance spectroscopy, were employed to study the phase structures, textural, and optical properties of the samples. The PXRD of all samples showed the characteristic pattern of the hydrotalcite without any detectable impurity phases. The expected cell parameter variation was calculated assuming the Vegard’s law and proved the ideal atomic arrangement for the cations in the brucite layer. The shifting of the diffraction plane “d110” toward lower angle clearly indicates that Co2+ is substituted in the brucite layer. The formation of the highest amount of hydroxyl radicals (OH•) on the surface of visible-light illuminated LDHs detected by the luminescence technique using terephthalic acid as probe molecules supports the highest activity LDH-4 with Cu/ Co atomic ratio 0.033 + 0.1 (i.e., 1:3) toward MG degradation. The degradation of malachite green (MG) followed pseudo-firstorder kinetics. The highest photocatalytic activity of LDH4 ascribed to the oxo-bridged system was explained by UV−vis DRS and EPR study.The degradation of MG followed pseudo-first-order kinetics, and the photocatalytic degradation mechanism was also explained in detail.

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visible irradiation over an iron-loaded cationic resin, 6 mesoporous TiO2−xNx,7 N- and S-incorporated nanocrystalline TiO2,8 and also the photocatalytic activity of ZnO toward the photo-oxidation of phenol under solar radiation.9 Layered double hydroxides (LDHs), also known as hydrotalcite like compounds, have gained considerable attention in the past years because of their cation-exchange ability of the brucite layer, the anion-exchange ability of the interlayer, its adjustable surface basicity, and adsorption capacity. They can be economically synthesized under laboratory conditions, engineered for specific application, and also regenerated for reuse.10 LDHs consist of a flat 2-D structural network composed of brucite-like layers [Mg (OH) 2] containing bivalent and trivalent metal cations. The partial replacement of bivalent cations with trivalent metal cations generates an excess of positive charge, which is compensated by gallery anions, located in the interlayer domains along with the water molecule.11−13These materials are characterized by the general

INTRODUCTION Malachite green (MG), a triphenylmethane dye, is worldwide used as a biocide in the aquaculture industry because of its efficiency against important protozoa and fungal organisms.1 This chemical can also be used as a food coloring agent, food additive, medical disinfectant, and anthelminthic as well as dye in the silk, wool, jute, and cotton, paper, and acrylic industry.2 Despite its extensive use, MG is a highly controversial material due to its toxic properties, which are known to cause carcinogenesis, mutagenesis, teratogenecity, and respiratory toxicity.3 Moreover, it is poorly biodegradable under environmental conditions due to the complex structure and xenobiotic properties. Hence, its removal from the effluents is necessary. Therefore, various attempts were made for its removal. Unlike the conventional biodegradable or activated carbon adsorption method, TiO2-based photocatalysis offered powerful oxidation method in the treatment of bioresistant organic contaminants such as dye wastewater by converting them into CO2.4 Semiconductor photocatalysis was extensively studied for the degradation of toxic organic pollutants. Recent studies have dealt with the light-induced degradation of organic pollutants by means of UV-A irradiation in TiO2 aqueous suspensions,5 © 2012 American Chemical Society

Received: July 25, 2012 Revised: September 19, 2012 Published: September 19, 2012 22417

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formula [MII1−x MIIIx (OH)2][An−]x/n·mH2O, where M II includes divalent cations like Mg2+, Co2+, Ni2+, Cu2+, Zn2+, and so on. MIII may be Al3+, Fe3+, Cr3+, and so on, and An− might be organic or inorganic anions.14−16 The schematic diagram of LDH is presented in Figure 1. They are rare in

(LDH-2), 0.067 + 0.067/0.067 (LDH-3), 0.033 + 0.1/0.067 (LDH-4), and 0 + 0.133/0.067 (LDH-5). The preparation was followed as per literature27 using the coprecipitation method by adding 2 M solution of NaOH and Na2CO3 dropwise to a solution containing mixed salt of divalent and trivalent metal ions at constant pH 9.5 at room temperature with constant stirring for 6 h. The resulting suspension was filtered with 0.2 μm membranes, washed with deionized water, and then dried at 80 °C for overnight. Photocatalytic Decolorization Process. The photocatalytic decolorization of MG was performed by taking 20 mL of 100 ppm solution in 100 mL Pyrex flask containing 0.02 g of catalyst. The solution was exposed to sunlight and agitated with a magnetic stirrer. All experiments were performed in duplicate during the first half of April 2012 (sunny days), 10:00 a.m. to 2:00 p.m. The average light intensity was around 104 ,100 Lx measured by using LT lutron Lx-101A digital light meter, which was nearly constant during the experiments. After irradiation, the suspension was centrifuged, and the content of MG was analyzed quantitatively by using a Varian Cary UV−vis spectrophotometer (model EL 96043181) at a wavelength 614 nm. The decolorization of MG due to adsorption was also measured by carrying similar experiments in dark. For determining chemical oxygen demand (COD) of different materials, aliquots were collected and taken for analysis by following a standard method.28 On the basis of the COD results, the photocatalytic degradation efficiency was calculated by following eq 1.

Figure 1. Schematic diagram of LDH.

nature, however, and can be easily synthesized by the coprecipitation method under laboratory conditions.17Many ternary LDHs involving mixture of different divalent or trivalent metal ions were also prepared.18 Work on ternary LDHs with the variation of ternary cations has extended to certain applications like in the field of organic transformation and fluorescence sensitivity and photocatalytic CO2 reduction.19−23 A recent report about the high photocatalytic activity of the Zn-M (M = Cr, Ti, Ce) LDH for photocatalytic water oxidation is reported by Silva et.al.24 LDHs with a cation doping capacity at the octahedral sites of the brucite layers reflected the properties of a doped semiconductor toward photocatalysis.25 Recently, our group has successfully demonstrated the effect on textural properties and photocatalytic activity of Mg/Al+Fe LDH by changing the atomic ratios of Al3+/Fe3+ (ternary cations).26 In the present work, we fabricated Cu+Co/Cr LDH by changing the atomic ratio of binary cations (Cu2+/Co2+) and the influence of Co2+ on structural, electronic, optical, and photocatalytic properties of Cu/Cr LDHs. Although scattered information on Cu/Cr LDH is available, a systematic investigation of doping of Co2+ in the framework of Cu/Cr LDH precursor and its effect on the textural and photocatalytic properties is still lacking. The possible photocatalytic degradation mechanism of MG was also briefly discussed.

photodegradation efficiency = (initial COD − final COD/initial COD) × 100

(1)

Material Characterizations. Powder X-ray diffraction (PXRD) measurements were performed. The PXRD pattern of all samples was recorded on a Rigaku Miniflex (set at 30 kV and 15 mA) powder diffractometer using Cu Kα radiation within the 2q range from 5 to 75° at a rate of 2θ min−1 in steps of 0.01°. Brunauer−Emmett−Teller (BET) surface area of the samples was analyzed by the multipoint N2 adsorption− desorption method at liquid nitrogen temperature (−196 °C) by an ASAP 2020 (Micromeritics) instrument. The Fourier transform infrared (FT-IR) spectra were obtained from Varian FTIR-800 spectrophotometer on KBr matrix in the range of 4000−400 cm−1. The thermogravimetric analysis (TG-DTA) thermograms were recorded on a thermal analyzer (NETZSCH, STA 449F3 Jupiter) in the temperature range from 30 to 800 °C at a heating rate of 10 °C min−1 in a nitrogen atmosphere at room temperature. Transmission electron microscopic (TEM) images were obtained on Philips TECHNAI G2 operated at 200 kV. The samples were prepared by dispersing the powdered samples in ethanol by sonication for 15 min and then drop-drying on a copper grid coated with carbon film. The diffused reflectance UV−vis (DRUV−vis) spectra of samples in the pallet form were measured in a Varian Cary UV−vis spectrophotometer equipped with a diffuse reflectance accessory in the region 200−800 nm with an integrating sphere of 150 mm. An appropriate amount of boric acid required for making 2 mm thickness and 40 mm diameter pallet was taken in the die, and ∼0.05 g of catalyst was put in the center. The power was pressed by 3600 ton pressure. Here boric acid acts as reference because it is not optically active and no absorption maxima are present between 200 to 800 nm. The relative humidity was acquired by using automatic weather

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EXPERIMENTAL SECTION Sample Preparation. All chemicals used in this study were commercially available (from Merck) and were used without further purification. To assess the effect of Cu2+/Co2+ on the properties of (Cu + Co/Cr) LDH, we prepared a series of samples by varying the concentration of the two metal ions mixed salt solutions of Cu 2+, Co2+, and Cr3+ with a (Cu2++Co2+)/Cr3+ ratio of 2:1 and by changing the Cu2+/ Co2+ concentration in a proportion that the Cu2++Co2+/Cr3+ ratios were 0.133 + 0/0.067 (LDH-1), 0.1 + 0.033/0.067 22418

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station (Rainwise, model: CC3000). For OH• radical detection, the experimental procedure is the same as the measurement of photocatalytic activity, except for replacing the aqueous solution of MG by 5× 10−4 M terephthalic acid (TA) with a concentration of 2 × 10−3 M NaOH. The PL spectra were measured on a FLUOROMAX-4 spectrophotometer. The reaction solution emits fluorescence spectra at around ∼425 nm on the excitation of 315 nm light. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMX Xband spectrometer with 100 kHz field modulation. The microwave frequency was calibrated using a frequency counter of the microwave bridge ER 041 XG-D. DPPH was used as a field marker (g = 2.0036). The irradiation light sources for EPR study is HPA 400/30S lamp (400 W, Philips). The magnetic field values of radicals were determined by employing the formula, hν = gßH.

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RESULTS AND DISCUSSION All of the samples were systematically analyzed by PXRD technique to examine the phase composition and the possible structural changes by the incorporation of Co2+ in the framework of Cu−Cr LDH. PXRD patterns of all of the samples are shown in Figure 2a. Sharp, intense peaks at low diffraction angles (peaks close to 2θ = 11, 24, and 35°; known to diffraction by basal planes (003), (006), and (009), respectively) and broad, less intense peaks at higher angles (peaks close to 2θ = 38, 46, and 60° ascribed to diffraction by (105), (108), and (110) planes) confirm the presence of hydrotalcite (JCPDS: 41-1428) with 3R packing of layers. The position of the (110) reflection at high angle (near 2θ = 60° for Cu Kα radiation) allows the value of the lattice parameter a. So, the parameter a = 2d(110)29,30 is a function of the average distance of metal ions within the layers, and according to the stacking mode sequence (3R), c is related to three times the distance given by d003 as a function of the average charge of the metal cations, the nature of the interlayer anion, and the water content.31 The thickness of the brucite-like layer and the interlayer distance is estimated from the c parameter. The lattice parameters a and c are calculated and summarized in Table 1. It appears that the lattice parameter a increases with increasing Co2+ substitution in the brucite layer (in Figure 2b), as expected with the difference in size between the two cations so as to the larger ionic radius of Co2+ (0.74 Å) with respect to Cu2+ (0.69 Å) in an octahedral environment. On plotting the value of a versus cobalt content in salt solution for all of the asprepared LDHs, a straight line is obtained, which is in good agreement with Vegard’s law32 (Figure 2c). According to the ideal model of perfect edge-sharing octahedron in LDH, the size of the octahedral sharing edges is related to the metal− oxygen distance by the relation a = 21/2d(M−O)

Figure 2. (a) PXRD patterns of all of the samples. (b) High-angle 2θ. (c) Plot of lattice parameter a versus cobalt content in salt solution.

(2)

where M−O is the metal−oxygen distance. The average radius of the cation can be calculated from the chemical composition according to the following formula

in the layer structure of the hydrotalcite and that the substitution is taking place in the sheets. Improvement in the orderliness of the layer was noted with decreasing copper content, as indicated by both the increase in intensity and sharpness of (110) and (113) reflections observed around 60 and 62°. This is expected because of Jahn−Teller distortion of Cu2+ (d9) ion, leading to poor long-range ordering; however, it is reported in the literature that the Cu2+ is stabilized by a Jahn−Teller distorted octahedral coordination environment.33 In the case of Cu−Cr LDH, the XRD pattern confirmed that

r = (0.133 − x) ·r Cu + x·r Co + 0.067·r Cr = 0.133·r Co + x(r Cu − r Co) + 0.067·r Cr

(3)

So, a linear dependence of a versus cobalt amount is consistent with the variation of cation size, and the obtained slope is 0.697 Å, which is in agreement with the theoretical value 0.707 Å. This confirms the presence of the three cations 22419

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Table 1. Cell Parameters, BET Surface Area, Pore Volume, Pore Diameter, Band Gap and Energy for Different LDHs lattice parameters (Å) catalysts

d003 (Å)a

a

c

band gap energy (eV)b

BET surface area (m2/g)c

LDH1 LDH2 LDH3 LDH4 LDH5

7.6 7.6 7.6 7.6 7.6

3.50 3.52 3.54 3.57 3.58

22.8 22.8 22.8 22.8 22.8

1.73 1.74 1.75 2. 32 2.48

117 104 110 100 104

a

Calculated from XRD patterns. bUV−vis DRS and measured from N2 adsorption−desorption isotherms. cMeasured from N2 adsorptiondesorption isotherms.

the structure has rhombohedral symmetry, and the average Cu−O bond is unaffected as if Cu (OH)6 octahedral is locally distorted. Because the Cu + Co/Cr ratio in all five LDHs is the same and all have CO32− as the interlayer anion, no variation in the c parameter is expected; however, it is lower than the typical Mg/Al-CO32− LDH (23.2 Å), as reported by Cavani et al.34 The porosity and specific surface area of the Cu−Cr−LDH and Co doped Cu−Cr LDHs sample were further studied by nitrogen sorption measurements. The nitrogen sorption isotherms of the entire as-prepared LDH sample are shown in Figure 3 and Table 1. LDH1, LDH2, and LDH3 have the

Figure 4. FTIR spectra of all as-prepared LDHs.

3470 cm−1 is attributed to the stretching vibrations of surface hydroxyl groups and interlayer water molecules,37 which were found at lower frequency in the LDH compared with the O−H stretching vibration in free water at 3600 cm−1.38 The broadening arises from extended hydrogen bonding. The broadness of the band indicates that hydrogen bonds with a wide range of strength exist. A weaker band at 1645 cm−1 was due to the bending mode of water molecules.39 The absorption band at 1360 cm−1 is assigned to the presence of carbonate ion.40 The bands observed in the low-frequency region of the spectrum (below 1000 cm−1) are interpreted as the lattice vibration modes, such as the M−O−H vibration centered at 667 cm−1 and the O−M−O vibration around 500 cm−1.41To understand the decomposition procedure of the Cu−Co/Cr LDHs, we show TGA plots in Figure 5a. Two weight-loss stages were observed for all samples, demonstrating that the decomposition proceeds in two steps. The first weight loss at low temperature (200−230 °C) is due to the removal of weakly bonded water molecules and surface water. The second step almost completed at 450 °C corresponds to the dehydroxylation of the brucite-like layer as well as the interlayer carbonate anions. Differential thermal analysis curves for the prepared samples shown in Figure 5b assigned the endothermic weight loss. The first endothermic peak at 200 °C is associated with the removal of interlayer water molecules, and the second peaks at 400 °C are due to removal of layer hydroxyls and interlayer carbonates. The shifting of the peak at 400 °C toward the higher temperature was noted from LDH-1 to LDH-5, indicating the increase in thermal stability. This was ascribed to the increase in Co2+ substitution in the Cu−Cr LDH brucite layer.42 Figure S1 of the Supporting Information shows the TEM of the LDH4 sample. They are small and irregular flakes with well-dispersed particles. The selected area electron diffraction (SAED) pattern taken from these nanocrystalline particles shows the presence of concentric diffraction rings, revealing the pure LDH phases. Optical absorption behavior of the catalyst is indispensable for visible-light-driven photocatalytic activity. The UV−vis DRS spectra of all as-synthesized LDHs were measured and are shown in Figure 6. All LDHs showed optical absorption at about 200−300 nm exclusively due to ligand-to-metal charge-transfer (LMCT) excitations occurring in the MO6 octahedral of layered structure.43,44 The red shift gradually increased with increase in Cu2+ content in

Figure 3. Nitrogen sorption isotherm of all as-prepared LDHs.

isotherms of type IV and with distinct H3-type hysteresis loops that are characteristic of mesoporous material aggregates of plate-like particles, giving rising to slit-like pores with nonuniform size. In the case of LDH5, the adsorption− desorption isotherm is of type III with H3 hysteresis loop characterized by a small adsorbate−adsorbent interaction and also slit-like pores,35 but the isotherms obtained in LDH4 are between type II and IV, consistent with the presence of mesoporosity, whereas the hysteresis loop is H2, which indicates that the material consists of ink-bottle pores with uniform pore size distribution.36 Consequently the well-defined pore size distribution in LDH4 improved its photocatalytic activity. The FT-IR spectra were used to determine the actual bonding type, the interlayer anion, and the cations in the brucite-like layer. All samples possessed rather similar spectra shown in Figure 4. A broad strong absorption band centered at 22420

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(without cobalt and copper-rich material) clearly showed a broad band in visible centered at 750 cm−1 of the 2Eg(D) → 2 T2g(D), which is the expected transitions for Cu2+ in a distorted octahedral coordination in the LDH framework due to Jahn−Teller effect.45 The broadness of the band was due to the dz2→ dx2−y2, dxz,yz → dx2−y2, dxy→ dx2−y2 transitions owing to distorted Cu2+ octahedron. By increasing the cobalt amount in the brucite layer, blue shift of absorbance took place. The material LDH4 exhibited broad bands at ∼520 nm, indicating the existence of both cations in the brucite layer because octahedral coordination gave 4T1g→4T2g in addition to dz2→ dx2−y2 transitions. LDH5 exhibited a broad band at ∼500 nm, which is in good agreement with the literature that Co2+ is octahedral coordinated with the weak field ligands of [Co(H2O)6]2+ and exists in the LDH framework.46 For both LDH2 and LDH3, the broad band at ∼600 nm originated from the d−d transition of octahedrally coordinated CrIII (spin−orbit components 4A2g→4T1g) and CuII and CoII (4T1g→4T2g and dz2→ dx2−y2), respectively.47 Under the same condition, we have taken P25, a UV-active photocatalyst, to compare with all asprepared LDHS (Figure S2 of the Supporting Information). The band gap energy (Eg) of LDHs can be calculated by the formula given in eq 4, and the values are summarized in Table 1. Eg = 1240/λ (nm)

(4)

Evaluation of Photocatalytic Activity. To evaluate the photocatalytic ability of all the catalysts, we carried out the MG degradation under direct sunlight irradiation. Prior to visiblelight irradiation the adsorption of MG was examined in dark. The adsorption equilibrium of MG over all as-synthesized LDHs is established within 30 min, and the amount of MG that gets adsorbed is ∼20% over all samples. This implies that all LDHs have similar ability to adsorb MG in aqueous solution. The photocatalytic degradation of MG in aqueous solution over LDHs and P25 (as reference) under the same conditions is depicted in Figure 7a. Among all, the LDH-4 exhibits the highest photocatalytic activity. The activity of all catalysts followed the order as: LDH4 > LDH3 > LDH2 > LDH1 > LDH5 > P25. However, only ∼2% variations in MG degradation were found in the experimental periods due to change in the solar light intensity, which is given in Table S1 of the Supporting Information. Also, the variation of other factors like humidity and O2 was very small and did not have appreciable effect on MG decolorization. In analyzing the kinetic data of photodegradation mediate by all as-prepared LDHs, the data were analyzed to a simple rate expression of Langmuir−Hinshelwood (L−H) model as follows

Figure 5. (a) TGA plots. (b) Differential thermal analysis curves for all prepared LDHs.

ln C t /C0 = K aapt

(5)

where kapp is the apparent reaction rate constant and t is the reaction time. Rate constant kapp independent of used concentration can be determined from the slope of the curve obtained by plotting ln Ct/C0 versus t shown in Figure 7b. The linearity of the curves indicates that the kinetics for the photocatalytic decomposition of MG follows pseudo-first-order rate. The apparent decomposition rate constants as the kinetic evidence of the different samples were determined and found to be 0.175, 0.179, 0.23, 0.417, 0.166, and 0.156 for LDH1, LDH2, LDH3, LDH4, LDH 5, and P25, respectively. To confirm the decolorization of MG dyes in the photocatalysis reaction, we determined the percentage change

Figure 6. UV−vis DRS spectra of all as-synthesized LDHs.

the brucite layer of LDHs and followed the order, LDH-5 < LDH-4 < LDH-3 < LDH-2 < LDH-1. The material LDH1 22421

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Mechanistic Aspects. The experimental results are in good agreement with the material characterization studies. The possible explanation for the quick and effective photodegradation of MG over the LDHs can be explained with the help of following possible facts: (1) electron hole pair mechanism in MO6 octahedron and surface hydroxyl groups (2) trapping hydroxyl radicals by probing reagent and photoluminescence emission spectra (3) cooperative catalytic behavior of Cu and Co in Cu−Co/ Cr LDHs. (4) visible-light-induced MMCT in oxo-bridged bimetallic linkages The photocatalytic reaction starts with the activation of catalyst by radiation with energy higher than that of the band gap energy of catalyst. For a better photocatalyst, the Eg value lies between 2 and 3 eV. LDHs would be constituted by sheets of edge-shared MO6 octahedron. In this case, the excited 3d electrons of Co2+, Cu2+, and Cr3+ in MO6 octahedron were promoted from valence band to the conduction-band resulting holes and electrons. The photogenerated holes reacted with surface water to form hydroxyl radicals (OH•), and the photogenerated electrons were absorbed by oxygen on the surface of catalyst to form super oxide radicals (O2−•). These superoxide radicals again formed two molecules of HO2• species, which produced OH• radical in subsequent steps. The surface OH group of LDH also played a key role in the photocatalytic reaction of MG dye molecule, owing to its acceptance of photogenerated holes to form OH• radical, which was the principal reactive oxidant. The formation of O2−• on the LDH surface took part in the dye degradation, in addition to generating OH• radical. In this way the aqueous MG dye degraded by the chain free radical mechanism due to the action superoxide radicals, and hydroxyl radicals O2−• and OH• both were strong ROS for photocatalysis as follows Cu + Co/Cr LDH (MO6 octahedron) + hv Figure 7. (a) Photocatalytic degradation of MG in aqueous solution over all LDHs. (b) Pseudo-first-order kinetics of RhG for all LDHs.

→ hVB+ + eCB−

of COD, which reflects that the extent of degradation or mineralization of organic species was determined over different as-prepared materials under visible light. If the dye was not degraded completely, then the residual organic molecules could be oxidized by K2Cr2O7; thus, the oxygen demand could be more. In other words, the COD value would be higher than that of the completely mineralized dye. The initial COD concentration of the 100 ppm MG solution is 286.2 mg L −1, and after 4 h of visible-light irradiation, the COD concentration decreased to 152 mg/L for LDH1, 97 mg/L for LDH2, 72 mg/ L for LDH3, 27.0 mg/L for LDH4, and 196 mg/L for LDH5; in addition, the % efficiency was 46.0, 66.0, 74.0, 90.0, and 31.0% for LDH1, LDH2, LDH3, LDH4 and LDH5, respectively, which indicated that the mineralization of dyes is faster in LDH4 than others shown in Figure S3a,b of the Supporting Information. For investigating the long-term stability of LDH materials under visible-light irradiation, we carried out recycling experiments using LDH4 catalyst. For each new cycle, the sample was collected and dried at 100 °C for 2 h by keeping other reaction conditions constant. As shown in Figure S4 of the Supporting Information, it is apparent that the photocatalytic degradation rates of MG are the same for all cycles.

(6)

hVB+ + H 2O → 2H+ + 2OH•

(7)

eCB− + O2 → O2−•

(8)

O2−• + H+ → HO2•

(9)

HO2• + HO2• → H 2O2 + O2

(10)

H 2O2 → 2OH•

(11)

dye + OH• → dyeoxidation

(12)

O2

−•

+ dye → dye reduction

(13)

•

The formation of OH radical for all as-prepared samples was detected by PL technique using terephthalic acid (TA) as a probe reagent. TA reacts with OH• readily to produce a highly fluorescent product, 2-Hydroxyterephthalic acid, whose PL peak intensity is directly proportional to the amount of OH• radicals produced in water. The greater the formation of OH• radicals, the higher the separation of e− and h+ pairs, so the photocatalytic activity had positive correlation to the formation of radicals.49 OH• + TA → TAOH 22422

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LDH4 resulted in the production of superoxide anion radical, which is the main oxidative species for MG degradation. The presence of the MMCT of CrIII−O−CuII and CrIII−O−CoII was more clearly revealed by the optical difference spectroscopy following an exposure of the LDH4 material to 420 nm irradiation for 5 h in the presence of O2, as shown in Figure 9.

Figure 8 shows unique signal originated from TAOH, indicating the formation of hydroxyl radicals. By comparing the

Figure 8. PL spectral changes with visible-light irradiation time over all as-prepared samples in a 5 × 10−5 M basic solution of terephthalic acid.

intensity of fluorescence signal after 120 min irradiation, it was found that the amount of hydroxyl radicals for all as-prepared LDHs is as follows: LDH4 > LDH3 > LDH2 > LDH1 > LDH5 > P25, which agrees well with the photocatalytic activity results. Additionally, the OH• was not detected in the blank test (no photocatalyst) under visible-light irradiation. In CuO6, the local arrangement around 3d9 copper atoms corresponds to an octahedral distortion related to the Jahn− Teller effect. The octahedral site of CuO6, a partially filled d level’s configuration of Cu2+, is 4s0 3d9, which will affect particularly the eg orbital. The degeneracy splitting would then lead ligands to get closer in the xy plane but would favor those in the z direction to keep away from each other, as calculated in an EXAFS study by Roussel.49 Then, the arrangements of copper in distorted octahedrons were more regular and wellshaped with chromium in brucite layer in different planes. Because a Cu2+ ion has an unfilled 3d shell with t2g and eg configuration and the decrease in the symmetry would allow splitting the degeneracy and to enhance the partially occupied d orbital’s stability,51 the reduction of Cu2+ is thermodynamically feasible and so enhanced the photocatalytic activity. To investigate the correlation of copper and cobalt in the photocatalytic activities over different LDHs, we considered that LDH4 (greater amount of cobalt in brucite sheet) possesses more electron-transfer capability because the reduction potential of Co (E0Co2+/Co = −0.28 V) is at higher level than that of Cu (E0Co2+/Cu = +0.34 V) and cobalt is also slightly close to the oxygen reduction potential (E0 = −0.13 V), which can transfer more electrons to the surface-absorbed oxygen. Consequently, a greater amount of cobalt in the brucite sheet (LDH4) transferred more amounts of electrons to the surface-absorbed oxygen to form large number of superoxide radicals. Despite Jahn−Teller distortion shown by Cu2+, its presence in LDH is highly essential because it helps in forming more and more hydroxyl radicals in aqueous solution; consequently, the photocatalytic activity increases. In addition to the above, the oxo-bridged binuclear CrIII−O− CuII (CoII) linkages constructed in LDHs are supposed to function as a visible-light-induced catalytic redox center, and the visible-light-induced MMCT transition of CrIII−O−CuII → CrIV−O−CuI and CrIII−O−CoII → CrIV−O−CoI over the

Figure 9. UV−vis diffuse reflectance spectra of LDH4 before and after exposure to 420 nm irradiation for 5 h in the presence of O2 (inset: Optical difference spectra of LDH4 material after 420 nm irradiation for 5 h in the presence of O2).

Frei and coworkers reported the MMCT of heterobimetallic assemblies, such as Ti IV−O−CuI and TiIV−O−SnII, a new class of visible-light-absorbing chromospheres.52 Recently, our group designed and fabricated the oxo-bridged Zn (II)−O−Y (III) LDH for photocatalytic applications.53 The EPR spectrum of LDH4 measured with O2 under visible irradiation (λ > 400 nm) shows a signal assigned to O2−• (gxx = 2.007, gyy = 2.010, gzz = 2.026), as shown in Figure S5 of the Supporting Information. The superoxide radicals (E0 = −0.13 V) are transferred to the catalyst surface because of the oxo-bridged MMCT and are responsible for MG dye degradation.

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CONCLUSIONS The incorporation of Co in the framework of Cu−Cr LDH was successfully calculated by Vegard’s law. The parameters a and c were found to be a linear function of the composition. The thermal stability of Cu−Cr LDH increased by increasing Co amount in the brucite layer, confirmed by DTA analysis. UV− vis diffuse reflection spectroscopy confirms that all LDHs showed broad absorption in the visible-light region (>400 nm). The visible-light-driven photocatalytic properties of Cu−Cr LDH and Co-doped Cu−Cr LDHs were successfully investigated. The LDH4 (Cu + Co/Cr = 0.033 + 0.1/0.067) photocatalyst exhibits high photocatalytic activity in the visible region due to the cooperative effect of binary cations and more electron-transfer capability of cobalt along with uniform pore size distribution, as shown in the N2 adsorption desorption curve and so on. Fluorescence spectra confirmed the formation of more hydroxyl radicals on the LDH4 surface. MMCT of oxobridged CrIII−O−CuII and CrIII−O−CoII LDH (proved by the optical difference spectra) and EPR spectra confirmed that the superoxide radical species is also an additive ROS for dye degradation. We expect that these findings obtained in the present study will provide fundamental information to design new visible-responsive photocatalysts. 22423

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ASSOCIATED CONTENT

S Supporting Information *

TEM images and SAED of all LDH4 material, UV−vis DRS spectra of P25, photocatalytic degradation of MG (100 ppm) with all prepared LDHs by COD (mg/L) plus photodegradation efficiency (%), recycling experiments over LDH4, EPR spectra of LDH4 sample, and the solar light intensity and % of MG degradation over all experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +91-674-2581637. Tel: +91(0674) 2581636-425. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. B. K. Mishra, Director, Institute of Minerals & Materials technology (IMMT), Bhubaneswar, for his constant encouragement and kind permission to publish this paper. The financial support from CSIR in the form of project (MLP-18) is highly appreciated. Lagnamayee Mohapatra is extremely grateful to CSIR New Delhi for awarding her an SRF.

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