Highly Efficient, Solar Active, and Reusable Photocatalyst: Zr-Loaded

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Highly efficient, solar active and reusable photocatalyst, Zr loaded Ag-ZnO for Reactive Red 120 dye degradation with synergistic effect and dye sensitized mechanism B. Subash, Balu Krishnakumar, Meenakshisundaram Swaminathan, and M. Shanthi Langmuir, Just Accepted Manuscript • DOI: 10.1021/la303842c • Publication Date (Web): 12 Dec 2012 Downloaded from http://pubs.acs.org on December 17, 2012

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Highly efficient, solar active and reusable photocatalyst, Zr loaded Ag-ZnO for Reactive Red 120 dye degradation with synergistic effect and dye sensitized mechanism B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi* Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India.

*Corresponding author: Address Dr. M. Shanthi Professor Photocatalysis Laboratory Department of Chemistry Annamalai University Annamalainagar- 608 002 Tamil Nadu India Tel/Fax +91 4144 237386 E-mail: [email protected]

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Highly efficient, solar active and reusable photocatalyst, Zr loaded Ag-ZnO for Reactive Red 120 dye degradation with synergistic effect and dye sensitized mechanism B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi* Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India.

The different wt% of Zr co-doped Ag-ZnO catalysts were prepared by simple precipitation–thermal decomposition method and used for the degradation of anionic azo dye Reactive Red 120 (RR 120) under natural sun light. Highly efficient 4 wt% of Zr codoped Ag-ZnO was characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HR-TEM) images, field emission scanning electron microscope (FESEM) images, energy dispersive spectra (EDS), diffuse reflectance spectra (DRS), photoluminescence spectra (PL), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and BET surface area measurements. Metal co-dopants increase the absorbance of ZnO to entire visible region. XRD and XPS reveal that Ag in the form of Ag0 and Zr in the form of Zr4+. The photocatalytic activity of 4 wt% Zr co-doped Ag-ZnO was compared with other single metal doped, undoped and commercial catalysts. The quantum yields of all the process were determined and analyzed. Zr-Ag-ZnO was found to be more efficient than Ag-ZnO, Zr-ZnO, commercial ZnO, prepared ZnO, TiO2-P25 and TiO2 at neutral pH for the mineralization of RR 120 under solar light. To the best of our knowledge this is the first report on the synthesis of Zr co-doped Ag-ZnO and its use in the degradation of RR 120 dye under natural sun light illuminatioin. The influences of operational parameters such as the amount of photocatalyst, dye concentration, initial pH on photo mineralization of RR 120 have been analyzed. The mineralization of RR 120 has been confirmed by Chemical Oxygen Demand (COD) measurements. A dual mechanism


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has been proposed for the higher efficiency of Zr-Ag-ZnO at neutral pH under solar light. This catalyst is found to be reusable.

1. Introduction The textile industry utilizes about 10000 dyes and pigments. Azo dyes constitute the largest and the most important class of commercial dyes.1 Nowadays, pollution has become one of the most serious environmental problems. Because of their electronic structure, semiconductor photocatalysts, titanium oxide (TiO2) and zinc oxide (ZnO), have been applied to a variety of environmental processes such as remediation of organic contaminants and destruction of microorganisms.2–5 The choice of ZnO as photocatalyst is primarily motivated by its non-toxicity and abundance. ZnO is a wide band gap semiconductor (≈3.3 eV) with a high exciton binding energy of ≈ 60 meV, wurtzite crystal structure, and piezoelectric properties. 6, 7 The band gap energy of ZnO is close to that of TiO2, the most used and benchmark photocatalytic material, and so it has the same photocatalytic ability as TiO2. Hoffman et al. have observed the production of H2O2 on ZnO as early as 1994.8 The advantage of lower cost of ZnO than TiO2 implies that ZnO should be a promising alternative to TiO2 in photocatalysis.9 However, the application of ZnO has been limited due to its photocorrosion and fast electron-hole recombination.10 To meet the need for practical application, ZnO photocatalyst should be modified to have high visible light photocatalytic activity and less photocorrosion. Therefore, in order to shift the optical absorption of ZnO or TiO2 into the visible region and to prevent electron-hole recombination on the photocatalysts surface, various attempts have been made. For example, it can be achieved by doping transitional metal ions, such as Au, Ni, Mg, V, Ag and Fe, or by doping non metals such as N and S.


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Nowadays doping of two kinds of atoms into ZnO or TiO2 materials has attracted considerable interest, since it could result in a higher photocatalytic activity and improved characteristics compared with single element doping into semiconductor oxides. Several studies are reported on co-doped materials on ZnO with Cd and Al,19 Pd and N,20 metallic silver and V,21

Ga/Al/Co.22

Additionally, there are few reports on the enhanced

synergistic effects of the co-doped TiO2 or ZnO photocatalysts.23–26 As a transition metal, the addition of zirconium to ZnO, TiO2 could lead to enhanced phase stability, smaller particles, suppression of electron-hole recombination and increased surface area, ultimately enhancing photocatalytic activity.27-29 Wang et al. reported that incorporation of Zr4+ ion into TiO2 led to small grain size, high surface area, large lattice deformation and formation of capture traps, all of which contribute to higher separation efficiency of the photogenerated carriers.30 The improved photocatalytic performance of Zr4+ doped TiO2 is attributed to the strong reduction potential of the photogenerated electrons, resulting from the elevation of conduction band.31 Venkatachalam et al. reported that doping of Zr4+ in nano TiO2 decreased the particle size and enhanced the adsorption of 4-chlorophenol on the catalyst surface.32 remediation,33,

34

In continuation of our ongoing research on environmental

the present work is focused on the synthesis, characterization and

photocatalytic activity of the Zr co-doped Ag-ZnO photocatalyst using RR 120 dye under solar light irradiation. 2. EXPERIMENTAL

2.1. Materials .The commercial azo dye Reactive Red 120 (Figure S1, see Supplementary data) from Balaji Colour Company, Dyes and Auxiliaries (Chennai) was used as such. Oxalic acid dihydrate (99%) and zinc nitrate hexahydrate (99%) were obtained from Himedia


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chemicals. AgNO3 and ZrOCl2.8H2O were obtained from sigma Aldrich. ZnO (Himedia), TiO2 (Merck) were used as received. A gift sample of Degussa TiO2-P25 was obtained from Evonik (Germany). It is a 80:20 mixture of anatase and rutile with the particle size of 30 nm and BET surface area of 50 m2 g−1. K2Cr2O7 (s.d.fine), Ag2SO4 (s.d.fine), HgSO4 and FeSO4.7H2O (Qualigens) were used as received. The double distilled water was used to prepare experimental solution. The pH of the solution before irradiation was adjusted using H2SO4 or NaOH. 2.2. Preparation of Zr loaded Ag-ZnO. Zr loaded Ag-ZnO was prepared by precipitationthermal decomposition method (Scheme 1). Aqueous solutions of 100 mL of 0.4 M zinc nitrate hexahydrate and 100 mL of 0.6 M oxalic acid in deionized water were heated 90° C separately. 5 mL of 0.128 g (7.5 × 10–4M) of AgNO3 was added to zinc nitrate solution. To this solution, oxalic acid solution was added. Zinc oxalate with Ag was precipitated (2 wt% Ag with related to ZnO). A solution of 0.372g (11.8 ×10–4M) of ZrOCl2 .8H2O in 5ml of water was added to this mixture heated to 60-70 °C and stirred for one hour. The solution was cooled to room temperature. The Zr with Ag-zinc oxalate crystals were washed several times with distilled water, air-dried overnight and dried at 100°C for 5 h. It was calcined in the muffle furnace at the rate of 20°C min−1 to reach the decomposition temperature of zinc oxalate 450°C. After 12 h, the furnace was allowed to cool down to room temperature. The Zr loaded Ag-ZnO catalyst was collected and used for further analysis. This catalyst contained 4 wt% of Zr. Catalysts with 1, 2, 3 and 5 wt% of Zr were prepared by using the same procedure with appropriate amounts of ZrOCl2.8H2O. The bare ZnO was prepared without addition of AgNO3 and ZrOCl2.8H2O. Ag-ZnO and Zr-ZnO were prepared by the same procedure with respective precursors.


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2.3. Analytical methods. Powder X-ray diffraction patterns were obtained using X’Per PRO diffractometer equipped with a CuKα radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identity the crystalline phase. The grids were dried under natural conditions and examined using a HR-TEM Hitachi H-7500. The morphology of catalyst was examined using a JEOL JSM-6701F cold field emission scanning electron microscope (FE-SEM). Before FE-SEM measurements, the samples were mounted on a gold platform placed in the scanning electron microscope for subsequent analysis at various magnifications. Diffuse reflectance spectra were recorded using Shimadzu UV-2450. Photoluminescence (PL) spectra at room temperature were recorded using a Perkin Elmer LS 55 fluorescence spectrometer. The nanoparticles were dispersed in carbon tetrachloride and excited using light of wavelength 300 nm. Cyclic voltammetry (CV) measurements were carried out using CHI 60 AC electrochemical analyzer (CHI Instruments Inc. USA). X-Ray photoelectron spectra of the catalysts were recorded in an ESCA-3 Mark II spectrometer (VG scientific Ltd., England) using Al Kα (1486.6 eV) radiation as the source. The spectra were referenced to the binding energy of C1s (285 eV). The specific surface areas of the samples were determined through nitrogen adsorption at 77 K on the basis of BET equation using a Micromeritics ASAP 2020 V3.00 H.

A Shimadzu (Japan) AA6300 spectra Atomic

Absorption spectrometer was used to measure the concentration of Zn2+ ions.

UV spectral

measurements were done using Hitachi-U-2001 spectrometer. 2.4. Photodegradation experiments. Solar photocatalytic degradation experiments were carried out under similar conditions on sunny days between 11 am to 2 pm. An open borosilicate glass tube of 50 mL capacity, 40 cm height and 20 mm diameter was used as the reaction vessel. Fifty milliliters of RR 120 (2 × 10–4M) with the appropriate amount of catalyst


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was stirred for 30 min in dark prior to illumination in order to achieve maximum adsorption of dye onto the semiconductor surface. Irradiation was carried out in the open air with continuous aeration by a pump to provide oxygen and for the complete mixing of reaction solution. During the illumination time no volatility of the solvent was observed. The temperature of the experimental solution is at 30°C. In all cases, 50 mL of reaction mixture was irradiated. At specific time intervals, 2–3 mL of the sample was withdrawn and centrifuged to separate the catalyst. One milliliter of the sample was suitably diluted and dye concentration was determined from the absorbance at the analytical wavelength (RR 120– 285 nm). 4-nitrophenol degradation was carried out under both UV and solar sources. For the degradation of 4nitrophenol by UV-A light (365 nm), a Heber Multilamp-photoreactor HML MP 88 was used (Figure S2, see Supplementary data).

Solar intensity (1250×100Lux ±100) was almost

constant during the experiments. 3. RESULTS AND DISCUSSION

3.1. Characterization of catalyst. Primary analysis of photocatalytic degradation of RR 120 with different wt % of Zr-Ag-ZnO catalysts was carried out. Pseudo-first order rate constants determined for 1, 2, 3, 4 and 5 wt% of Zr loading were 0.0672, 0.0843, 0.0951, 0.1090 and 0.0915 min–1, respectively. The catalyst loaded with 4 wt% of Zr was found to be the most efficient. Hence, 4 wt% of Zr was taken as optimum concentration of Zr on Ag-ZnO and this catalyst was characterized by XRD, HR-TEM, FE-SEM, EDS, DRS, PL, CV, XPS and BET surface area measurements. XRD patterns of the bare ZnO, Zr-Ag-ZnO and Ag-ZnO are shown in Figure 1. The diffraction peaks of bare ZnO (Figure 1a) at 31.68°, 34.36°, 36.18° and 56.56°, correspond to (100), (002), (101) and (110) planes of wurtzite ZnO (JCPDS 89-0511). The diffraction pattern of Zr-Ag -ZnO is different from that of ZnO as shown in Figure 1b. In the


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Zr-Ag-ZnO system, there are three new peaks with 2θ values of 27.8°, 46.2° and 54.8° corresponding to Zr35 (JCPDS 37-1484). This confirms the loading of Zr on Ag-ZnO. Due to very low concentration ‘Ag’ could not be detected by XRD.36 To confirm the loading of Ag, wt% Ag is increased from 2 to 10 wt% and its XRD is given as Figure 1d. A new peak with 2θ value at 38.2° (JCPDS 03-092) confirms the presence of metallic silver (Ag0) in the catalyst.37 However, EDS shows the presence of Ag even with 2 wt% Ag in the catalyst. If the silver and zirconium are substituted in place of Zn, a corresponding peak shift is expected in XRD. Lack of such shifts in the XRD of Zr-Ag-ZnO indicates the presence of Ag and Zr on the surface of ZnO. In addition, the doping possibility of both metals is unlikely because of the difference in ionic radii between Zn2+ (0.72 Å), Ag+ (1.22 Å) and Zr4+(0.79 Å). Broadening of Zr-Ag-ZnO peaks indicate that the reduction of size of the particle when compared to bare ZnO. The crystallite sizes of bare ZnO and Zr–Ag–ZnO, determined using Debye-Scherrer equation are found to be 30.1 and 5.3 nm, respectively. The crystallite size of Zr-Ag-ZnO (5.3 nm) is lower than bare ZnO (30.1 nm). The surface morphology of Zr-Ag-ZnO has been analyzed by HR-TEM and FE-SEM images. Figure 2 shows the HR-TEM images of Zr-Ag-ZnO at different magnifications (Figure 2(a)-(d)). At higher magnifications, the hexagonal structure of ZnO particles are clearly seen (Figure 2a- 2d). It can be seen that the Zr-Ag-ZnO particle sizes are in the range from 5 to 35 nm. The size distribution of Zr-Ag-ZnO samples is found to be a narrow one. The FE-SEM images at two different magnifications with different locations are given in Figure 3a-d. At higher magnification of 20 nm “hexagonal” structure of ZnO is clearly seen (indicated by circle) (Figure 3a). Lower magnification of 100 nm in three different locations (Figure 3b-3d) clearly indicate the presence of Zr (indicated by arrow marks). Zr particles are highly dispersed on the surface of ZnO


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(Figure 3b-3d).

Moreover morphology of the particles is roughly “hexagonal” as well as

“spherical” shape. In addition, the aggregates of Zr to Ag-ZnO exhibit varying sizes. Single aggregate is not advantageous, whereas highly dispersed aggregates have maximum photocatalytic activity. The EDS of Zr-Ag-ZnO shown in Figure 4 reveals the presence of Zr, Ag, Zn and O. The diffuse reflectance spectra of pure and Zr4+ doped Ag–ZnO are depicted in Figure 5a and 5b. Zr4+ might covalently interact with ZnO and reduce its band gap. It is also reported earlier that band gap of Zr4+ doped M–TiO2 is shifted towards longer wavelength compared to pure M–TiO2.38 Ag and Zr co-doping in ZnO causes a redshift in the absorption edge from 400 to 422 nm. The redshifted absorption spectrum is interpreted as a possible evidence for good interaction between ZnO, Ag and Zr species. Metal doping produces some energy levels between CB and VB of ZnO.39 Therefore, the red shift of Zr-Ag-ZnO photocatalyst can be ascribed to the charge transfer between ZnO valance or conduction band and the energy levels formed by Ag and Zr. Similar result has been reported from our laboratory for Ce-Ag-ZnO.33 In addition UV-vis spectra in the diffuse reflectance mode (R) were transformed to the KubelkaMunk function F(R) to separate the extent of light absorption from scattering. The band gap energy was obtained from the plot of the modified Kubelka-Munk function (F(R)E)1/2 versus the energy of the absorbed light E ( eqn 1) (Figure 6). 1/2

F (R) E

1/2

=

(1-R)2 x hv 2R

…

(1)

The band gap of bare ZnO and Zr-Ag-ZnO are found to be 3.15 and 2.90 eV respectively. Photoluminescence spectra of bare ZnO and Zr-Ag-ZnO are shown in Figure 7a and 7b,


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respectively. As the photoluminescence occurs due to electron-hole recombination, its intensity is directly proportional to the rate of electron-hole recombination. The bare ZnO gave two emissions at 420 and 480nm. The doping of Zr and Ag with ZnO do not shift the emission of ZnO but the intensity of PL emission is less when compared to bare ZnO. This is because of suppression of recombination of electron-hole pairs by Ag and Zr, which enhanced the photocatalytic activity of the catalyst. Cyclic Voltammetry measurements were carried out to prove the presence of metal ion in the catalyst. Both bare ZnO and Zr-Ag-ZnO were used in the redox reaction of potassium ferrocyanide (3mM). This measurement is used to find out the electrical conductivity of the catalyst. A predetermined amount of catalyst was dispersed in a 0.1% nafion in ethanol solution for 1h in an ultrasonic bath to form a stable suspension. The catalyst was deposited on the glassy carbon electrode by droplet evaporation for 15 min and then drying in nitrogen atmosphere for 20 min. Bare ZnO does not give any anodic potential and current (Figure 8a). But with Zr-Ag-ZnO, the anodic potential and current are E = 0.698 V and I = 5.626 × 10–6A, respectively (Figure 8b). This increase in current indicates presence of metal ion in the catalyst.40 In order to know the chemical state of Ag and Zr present in this catalyst, the XPS of this sample was taken. The survey spectrum (Figure 9a) of the Zr-Ag-ZnO indicates the peaks of elements Zn, O, Ag, and Zr. The carbon peak is attributed to the residual carbon from the sample and adventitious hydrocarbon from XPS instrument itself and its not indicated. Figs 9b, 9c and 9d show the binding energy peaks of Ag, O and Zn, respectively. Binding energy peak of Zr is given in inset of Figure 9a. The binding energy peaks of Zr at 181.7 and 184.2 eVcorrespond to 3d5/2 and 3d3/2 of Zr4+ respectively.41 Binding energy peaks of Ag, 3d5/2 and 3d3/2 were observed at 374.4 and 368.4 eV, respectively (Figure 9b). According to Zhang et al., 42 the peaks at 373.96


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and 368.11 eV can be attributed to metallic silver (Ag0), in the catalyst. In Figure 9c, the O1s profile is asymmetric and can be fitted to two symmetrical peaks (α and β locating at 530.6 and 532.3 eV, respectively), indicating two different kinds of O species in the sample. The peaks α and β should be associated with the lattice oxygen (OL) of ZnO and chemisorbed oxygen (OH) caused by the surface hydroxyl,43 respectively. Zn2p occurred at 1022.6 and 1045.7 eV (Figure 9c), which confirms the presence of Zn2+ in the catalyst. In general the surface area of the catalyst is the most important factor influencing the catalytic activity. The surface area of Zr-Ag-ZnO was determined using the nitrogen gas adsorption method. N2 adsorption–desorption isotherms of bare ZnO and Zr–Ag–ZnO are presented in Figure 10a –10b, respectively. The isotherms of bare ZnO and Zr–Ag–ZnO reveal type II hysteresis loop. The pore size distribution of the bare ZnO and Zr-Ag-ZnO are given in inset of Figure 10a and b, respectively. The BET surface and pore volume of bare ZnO and ZrAg-ZnO are given in Table 1. BET surface area of Zr-Ag-ZnO (18.3 m2 g–1) is higher than bare ZnO (11.5 m2 g–1). 3.2. Photodegradability of RR 120. The photodegradability of RR 120 with different photocatalysts under solar light irradiation is shown in Figure 11.

Almost complete

degradation of the dye takes place at the time of 30 min with Zr-Ag-ZnO (curve a) under solar light. 50.0% decrease in dye concentration occurred for the same experiment performed with Zr-Ag-ZnO in the absence of solar light (curve b). This may be due to adsorption of the dye on the surface of the catalyst. Negligible degradation (0.2 %) was observed when the reaction was allowed to occur in the presence of solar light without any catalyst. By these observations, we can say that both solar light and catalyst are needed for effective degradation of the RR 120. When Ag-ZnO, Zr-ZnO, bare ZnO, commercial ZnO, TiO2-P25


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and TiO2 (Merck) were used under same conditions 70.6 (curve c), 71.1 (curve d), 71.1 (curve e), 69.9 (curve f), 71.1 (curve g) and 59.9 (curve h) percentages of degradation occurred, respectively. This shows that Zr-Ag-ZnO is more efficient in RR 120 degradation than other catalysts. We had tested the efficiency of this catalyst with Reactive Orange 4 (RO 4) and Reactive Yellow 84 (RY 84) dyes degradation.

As shown in Figures S3-S6, (see

Supplementary data) this catalyst was found to be most efficient in the degradation of these both dyes also. Since the degradation was highly effective with Zr-Ag-ZnO, the influence of operational parameters had been carried out to find out the optimum conditions. The photocatalytic degradation of RR 120 dye containing Zr-Ag-ZnO obeys pseudo-first order kinetics. At low initial dye concentration the rate expression is given by d[C]/dt = k′[C]

… (2)

where k′ is the pseudo-first order rate constant. The dye is adsorbed onto the Zr-Ag-ZnO surface and the adsorption-desorption equilibrium is reached in 30 min.

After adsorption, the

equilibrium concentration of the dye solution is determined and it is taken as the initial dye concentration for kinetic analysis. Integration of eqn. 2 (with the limit of C = C0 at t = 0 with C0 being the equilibrium concentration of the bulk solution) gives eqn. 3,

ln (C0/C) = k′t

… (3)

where C0 is the equilibrium concentration of dye and C is the concentration at time t.


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Photonic efficiency under optimum conditions for RR 120 dye degradation by Zr-Ag-ZnO was calculated using the reported method.44 Quantum yield of a photocatalytic reaction is defined as the number of RR 120 molecules being decomposed (degraded) per photon absorbed. (Eq.(4)).

Φ

=

Number of molecules decomposed

… (4)

Number of photons of light absorbed The photo degradation rate constants (k’) of RR 120 dyes under the monochromatic light source can also be used for the calculation of its reaction quantum yield using (Eq (4)).

Φ

=

k’

… (5)

2.303I0, εDλ, l where Φ is the reaction of quantum yield (dimensionless), Io is the light intensity of the incident light range at 400–800 nm (2.312 ×10–3 Einstein),

εDλ

is the molar absorptivity of

RR 120 at 512 nm (3.58 ×10 3 cm–1M–1), l is the path length of reaction tube and it is 0.24 m for 50 mL of irradiated solution. The results of degradation quantum yields by Zr-Ag-ZnO, Ag-ZnO, Zr-ZnO, bare ZnO, commercial ZnO, TiO2-P25 and TiO2 (Merck) are 0.0239, 0.0069, 0.0063, 0.0123, 0.0125, 0.0124 and 0.0096, respectively. These results indicate that the quantum yield of Zr-Ag-ZnO process is high when compared to other processes. The UV-vis spectra of RR 120 (2 ×10–4 M) solution at different irradiation times are shown in Figure 12, and the color of the corresponding solutions are shown as inset of Figure 12. There is no significant change in UV-visible spectra during irradiation but the intensities at 285


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and 512 nm decrease gradually during the degradation. This reveals that the intermediates do not absorb at the analytical wavelengths of 285 and 512 nm. 3.3. Influence of operational parameters 3.3.1. Effect of solution pH. The wastewater from textile industries usually has a wide range of pH.

pH of the solution not only plays an important role in the characteristics of

textile wastewater but also determines the surface charge properties of ZnO, the size of aggregates formed, the charge of dye molecules, adsorption of dyes onto Zr-Ag-ZnO surface and the concentration of hydroxyl radicals. Figure S7 (see Supplementary data) shows RR 120 degradation as a function of irradiation time under acidic and alkaline conditions. It is found that degradation strongly depends on the solution pH. The pseudo-first order rate constants for Zr-Ag-ZnO at pH 3, 5, 7, 9 and 11 are 0.0639, 0.0759, 0.1090, 0.0686 and 0.0658 min-1, respectively. It is observed that the increase in pH from 3 increases the removal efficiency of RR 120 up to pH 7, and then decreases. The optimum pH for efficient RR 120 removal on Zr-Ag-ZnO is 7. At acidic pH range the removal efficiency is less and it is due to the dissolution of ZnO. ZnO can react with acids to produce the corresponding salt at low acidic pH values. Degradation efficiency of a catalyst depends on the adsorption of dye molecules. Adsorption of dye molecule is influenced by ionic nature of the dye and pHzpc of the catalyst. Zero point of the catalyst Zr-Ag-ZnO was determined by reported method45 and shown in Figure S8 (see Supplementary data). The zero-point charge for ZrAg-ZnO is found to be 8.3 and below this value, ZnO surface is positively charged. Because the dye has six sulfonic acid groups in its structure it exists as negative ions at pH 7. The electrostatic attraction between positively charged Zr-Ag-ZnO and negatively charged dye solution may lead to strong adsorption of dye on Zr-Ag-ZnO surface. This facilitates the


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reaction at catalyst surface leading to higher degradation at this pH. An experiment to verify dark adsorption of RR 120 under different pH was carried out. The percentages of adsorption at pH 3, 5, 7, 9 and 11 were found to be 10.0, 41.1, 50.0, 40.8 and 38.7 after the attainment of adsorption equilibrium (30 min). As revealed by pHzpc, the adsorption is high at pH 7 and hence the degradation is most efficient at this pH. 3.3.2. Effect of catalyst loading. Catalyst loading in slurry photocatalytic processes is an important factor that can strongly influence the dye degradation. Experiments performed with different amounts of Zr-Ag-ZnO showed that the photodegradation efficiency increased with an increase in the amount up to 3 g L−1 and then slightly decrease as observed in Figure S9 (see Supplementary data). The pseudo-first order rate constants are 0.0963, 0.1090, 0.1086, 0.1081 and 0.1081 min-1 for Zr-Ag-ZnO at catalyst loading of 2, 3, 4, 5 and 6 g L–1, respectively. This observation can be explained in terms of availability of active sites on the catalyst surface and the penetration of visible light into the suspension. The total active surface area increases with increasing catalyst dosage.40 But with excess dosage there is a decrease in visible light penetration as a result of increased light scattering effect by catalyst particles.46 As a result, the photo activated volume of suspension decreases. Additionally, it is important to keep the treatment expenses low for industrial use. So we use 3 g L−1 as the optimal catalyst amount in our work. 3.3.3. Effect of initial dye concentration. It is important from an application point of view to study the dependence of degradation and adsorption on the initial concentration of dyes. Figure S10 (see Supplementary data) shows that the increase of dye concentration from 1 to 5 × 10−4 M decreases the rate constant from 0.1448 to 0.0294 min-1. The rate of degradation relates to the •OH (hydroxyl radical) formation on catalyst surface and probability of •OH reacting with


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dye molecule. As the initial concentration of the dye increases, the path length of the photons entering the solution decreases. Thus the photocatalytic degradation efficiency decreases,34,

47

while in low concentration the reverse effect is observed, thereby increasing the photon absorption by the catalyst. The large amount of adsorbed dye may also have a competing effect on the adsorption of oxygen and OH− onto the surface of catalyst. 3.3.4. Stability of the Catalyst. The catalysts lifetime is an important parameter of the photocatalytic process because its use for longer period of time leads to a significant cost reduction of the treatment. For this reason, the catalyst was recycled which showed a drop in efficiency from 100% (1st run) to 96.0 % (4th run) as shown in Figure 13. These results indicated that Zr-Ag-ZnO catalyst remained effective and reusable under solar light. The stability of Zr-Ag-ZnO was tested by its anti photocorrosion property. We analyzed photocorrosion of bare ZnO and Zr–Ag–ZnO. The concentration of Zn2+ ions in the solution after photoreaction was determined by atomic absorption spectrometer to examine the dissolution of ZnO by photocorrosion. The results presented in Table 2 reveals that bare ZnO suffers more dissolution by photocorrosion than the Zr–Ag–ZnO. The dissolution of Zr–Ag– ZnO is only 0.69 % whereas bare ZnO has the dissolution of 2.49 %, which is 3.6 times of the dissolution of Zr-Ag-ZnO. This reveals that Zr–Ag–ZnO is more antiphotocorrosive than bare ZnO. This may be the reason for its reusability and photostability. Earlier ZnO nanochain was reported to be more antiphotocorrosive than commercial ZnO.46 3.3.5. Chemical Oxygen Demand (COD) analysis. To confirm the mineralization of RR 120, the degradation was also analyzed by COD values. The percentage of COD reduction is given in Table 3. After 30 min irradiation with Zr-Ag-ZnO, 96.5 % of COD reduction is obtained. This indicates the mineralization of dye. Mineralization was also


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confirmed by the formation of CaCO3 when the evolved gas (carbon dioxide) during degradation was passed into Ca(OH)2 solution. 3.3.6. Mechanism of degradation. A mechanistic Scheme of the charge separation and photocatalytic reaction for Zr-Ag-ZnO photocatalyst is shown in Scheme 2. When semiconductor is illuminated by solar irradiation, a valance band electron (VB) goes to conductance band (CB) leaving a hole in valance band. Generally the recombination of electron-holes reduces the photocatalytic activity of semiconductors. But the presence of ‘Ag’ and ‘Zr’ trap the electron from CB of ZnO suppressing the electron-hole recombination. It is well established that ‘Ag’ traps the electrons from CB of ZnO. The ‘Zr’ doping also suppresses the recombination of electron and positive holes by electron trapping.29,30,48 It is shown that the photocatalytic activity of Zr-Ag-ZnO photocatalyst is higher than that of all other semiconductor photocatalysts. Similar result was reported from our laboratory for Ce-Ag-ZnO.33 The trapping nature of Ag and Zr produced more number of superoxide radical anion and at the same time VB holes of ZnO react with water to produce highly reactive hydroxyl (●OH) radical. The superoxide radical anion and hydroxyl radical are used for the degradation of dye. In addition to this, dye sensitized mechanism is also possible. We had carried out the degradation of RR 120 with 365 nm UV light (IUV =1.381×10-6 Einstein L-1 s–1) under the same conditions used for solar light. It was found that RR 120 underwent 73.7 % degradation with UV light (365 nm), but under the same conditions 94.4 % degradation occurred with solar light for 20 min. Higher efficiency of Zr-Ag-ZnO in solar light indicates the presence of dye sensitized mechanism in addition to ZnO sensitization. This occurs when more dye molecules are adsorbed on the semiconductor surface. The dark adsorption of Zr-Ag-ZnO (50.0%) is higher when compared to Zr-ZnO (14.0%) and Ag-ZnO (17.5 %). This enhances the photoexcited electron


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transfer from solar light sensitized dye molecule to the conduction band of ZnO and subsequently increases the electron transfer to the adsorbed oxygen (Eqns. 6, 7). The dye molecules are also degraded by the super oxide radicals produced by dye sensitization mechanism (Eqn. 8). Further to prove the dye sensitized mechanism, we had also carried out an experiment for the degradation of 4-nitrophenol by Zr-Ag-ZnO with UV and solar light. We found that the degradation of 4nitrophenol was more efficient in UV light(79.9 %) than in solar light( 34.4 %) in 60 minutes under the same condition, indicating presence of dye sensitized mechanism for the degradation of RR 120. Similar results have been reported for photodegradation of azo dyes.49

Dye* + Zr–Ag–ZnO ecb– + O2 Dye +● + O2/O2●–

Dye+● Zr–Ag–ZnO + ecb– O2●– degradation products

… (6) … (7) … (8)

4. CONCLUSION In summary, we have demonstrated cost-effective precipitation-thermal decomposition method for the production of Zr co-doped Ag-ZnO photocatalyst. The synthesis is simple, highyield and no special equipment is required. HR-TEM images reveal the presence of hexagonal wurtzite structure of ZnO. FE-SEM images reveal that the Zr particles are highly dispersed on the surface of ZnO. EDS shows the presence of Ag and Zr in the catalyst. Presence of Ag and Zr increase the absorption of ZnO to entire visible region. DRS spectra indicate the reduction of band gap of the Zr-Ag-ZnO, when compared to ZnO. The PL spectra show the suppression of recombination of the photogenerated electron-hole pairs by the Zr and Ag loading on ZnO. XPS reveal that the presence of Agº and Zr4+ in the catalyst. Zr-Ag-ZnO is found to be more efficient


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than Ag-ZnO, Zr-ZnO, bare ZnO, commercial ZnO, TiO2-P25 and TiO2 for degradation of RR 120 under solar light at neutral pH. The optimum pH and catalyst loading for efficient removal of dye are found to be 7 and 3 g L−1, respectively. COD measurements confirm the complete mineralization of RR 120 molecule. A dual mechanism involving dye sensitization has been proposed for the efficient dye degradation by Zr-Ag-ZnO under solar light. This catalyst was found to be reusable. Furthermore, prepared Zr-Ag-ZnO had superior photocatalytic activity towards degradation of RR 120. This process using Zr-Ag-ZnO photocatalytic material would be more useful for industrial effluent treatment, due to its advantage of its “simplicity”, “low cost”, “reusability” and “excellent performance.” SUPPORTING INFORMATIONS Figures S1 to S10 ACKNOWLEDGEMENTS One of the authors (M. Swaminathan) is thankful to CSIR, New Delhi, India for financial support through research Grant No. 21(0799)/10/EMR-II. One of the authors (M. Shanthi) is highly thankful to UGC, New Delhi, India for financial support through research project F.No 41-288/2012 (SR). One of the authors B. Subash is thankful to UGC, New Delhi, India for BSR Fellowship. One of the authors B. Krishnakumar is thankful to CSIR, New Delhi, India, for Senior Research Fellowship REFERENCES (1) Kirk-Othmer, Encyclopedia of Chemical Technology, John Wiley & Sons, New York, 1978, 3, 387–433. (2) Fujishima, A.; Honda, K.; Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 1972, 238, (5538), 37–38. (3) Guo, M.Y.; Ching Ng, A.M.; Liu, F.; Djurisic, A.B.; Chan, W. K.; Su, H.; Wong, K. S.;


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Effect of Native Defects on Photocatalytic Properties of ZnO. J. Phys. Chem. C 2011, 115, (22) 11095–11101. (4) Li, p.; Wei, Z.; Wu, T.; Peng, O.; Li, Y.; Au-ZnO Hybrid Nanopyramids and Their Photocatalytic Properties. J. Am. Chem. Soc. 2011, 133, (15), 5660–5663. (5) Li, Y.; Zhou, X.; Hu, X.; Zhao, X.; Fang, P.; Formation of Surface Complex Leading to Efficient Visible Photocatalytic Activity and Improvement of Photostabilty of ZnO. J. Phys. Chem. C, 2009, 113, (36), 16188–1619. (6) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho S. J.; Morkoc¸ H. A comprehensive review of ZnO materials and devices, J. Appl. Phys., 2005, 98, (4), 041301–041404. (7) Xu, S.; Wang, Z. L.; One-dimensional ZnO nanostructures: Solution growth and functional properties, Nano Res., 2011, 4, (11), 1013–1098. (8) Hoffman, A. J.; Carraway E. R.; Hoffmann, M. R.; Photocatalytic production of H2O2 and organic peroxides on quantum-sized semiconductor colloids, Environ. Sci. Technol., 1994, 28, (5), 776–785. (9) Herna, M. D.; Alonso, N.; Fresno, F.; Suarez, S.; Coronado, J. M.; Development of alternative photocatalysts to TiO2: Challenges and Opportunities, Energy Environ. Sci., 2009, 2, (12), 1231–1257. (10) Dijken, A. V.; Janssen, A. H.; Smitsmans M. H. P.; Vanmaekelbergh D.; Meijerink, A.; Size-selective photoetching of nanocrystalline semiconductor particles, Chem. Mater., 1998, 10, (11), 3513–3522.


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Figure and Scheme captions Scheme 1 Schematic representation for preparation of Zr-Ag-ZnO. Scheme 2 Mechanism of degradation of RR 120 by Zr-Ag-ZnO Figure 1. XRD patterns of a) bare ZnO, b) Zr-Ag -ZnO c) 2 wt% Ag–ZnO and d) 10 wt% Ag-ZnO Figure 2 HR-TEM images of Zr-Ag-ZnO at different magnification (a, b, c and d) and particle size distribution (e). Figure 3. FE-SEM images a) Zr-Ag -ZnO (20 nm), b) Zr-Ag -ZnO (100 nm), c) Zr-AgZnO (100 nm) and d) Zr-Ag -ZnO (100 nm)

Figure 4. EDS of Zr-Ag-ZnO Figure 5. DRS of a) bare ZnO and b) Zr-Ag -ZnO Figure 6. Plot of transferred Kubelka-Munk versus energy of the light absorbed of the

a) bare ZnO and b)Zr-Ag-ZnO. Figure 7. Photoluminescence spectra of a) bare ZnO and b) Zr-Ag -ZnO Figure 8. Cyclic voltammograme of a) bare ZnO and b) Zr-Ag -ZnO Figure 9. XPS of Zr-Ag-ZnO a) survey spectrum b) Ag3d peak, c) O1s peak and d) Zn2p peak, Figure 10. N2 adsorption–desorption isotherms of a) bare ZnO and b) Zr-Ag-ZnO. Figure 11 Photodegradability of RR 120; [RR 120] = 2 × 10−4M, catalyst suspended = 3 g L−1, pH = 7, airflow rate = 8.1 mL s−1; Isolar= 1250×100±100 lux Figure 12. The changes in UV-vis spectra of RR 120 on irradiation with solar light in the presence Zr-Ag-ZnO: [RR 120] = 2× 10–4 M; pH = 7; catalyst suspended = 3 g L–1; airflow rate = 8.1 mL s–1; Isolar= 1250×100±100 lux; (a) 0 min, (b) 10 min, (c) 20 min and (d) 30 min. Figure 13. Catalyst reusability, [RR 120] = 2 × 10−4 M, 4 wt% Zr-Ag -ZnO suspended = 3 g L−1, airflow rate = 8.1 mL s−1, irradiation time = 30 min; Isolar= 1250×100±100 lux


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Table 1. Surface properties of the catalysts.

Properties

ZnO bare (Values)

Zr-Ag-ZnO (Values)

BET surface area

11.5 (m2 g−1)

18.3 (m2 g−1)

Total pore volume (single point)

0.07(cm3 g−1)

0.14 (cm3 g−1)


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Table 2. Concentration of Zn2+ ions mg L-1in the solution mixture after photoreaction.

Zr–Ag–ZnO

Bare ZnO

First run

2.0

2.1

Second run

5.0

16.2

Third run

9.9

31.4

Fourth run

10.8

49.7

Average

6.9

24.9

Initial ZnO

1000.0

1000.0

Dissolution (%)

0.69

2.49

[RR 120] = 2 × 10−4 M, catalyst suspended = 3 g L−1, pH = 7, airflow rate = 8.1 mL s−1, irradiation time = 30 min, Isolar= 1250×100±100 lux


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Table 3. COD measurements Time (min)

% COD reduction

10

29.8

20

61.5

30

96.5

[RR 120] = 2 × 10–4 M; 4 wt% Zr-Ag-ZnO suspended = 3 g L–1; pH = 7; airflow rate = 8.1 mL s–1; Isolar= 1250×100±100 lux


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(d) 0

(111)

* Ag * (c)

Counts

* Zr

(003)

(202)

(111)

(b)

(101)

0

10

20

4+

* *

*

(103) (200) (112) (201) (044) (202)

(110)

(a)

(102)

(100) (002)

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Figure 1


70

80

90

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(a)

(b)

(c)

(d)

12

(e)

10 Number

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10

15 20 Size (nm)

25

Figure 2


30

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(b) (a)

(c)

Figure 3


(d)

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Figure 4


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70

% of reflectance

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(a)

50

(b)

40 30 20 10 0 200

400 600 Wavelength (nm)

Figure 5


800

Page 35 of 45

35

3.5 3.0

K-M

2.5

(b)

2.0 1.5 1.0 0.5 0 0

1

2

3 4 Band gap (eV)

5

6

7

3.5 3.0

(a)

2.5

K-M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

2.0 1.5 1.0 0.5 0 0

1

2

3

4

5

Band gap (eV)

Figure 6


6

7

Langmuir

36

300 250 200 PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

150

(a)

100 (b) 50 0 250

300

350

400

450

Wavelength (nm)

Figure 7


500

300

Page 37 of 45

37

- 1.8 - 1.6 - 1.4 - 1.2 Current (1e-5A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(b)

- 1.0 - 0.8 - 0.6 - 0.4 - 0.2

(a)

0 0.2 0.4 0.6 0.8 0.8

0.6

0.4

0.2

- 0.2 0 Potential (V)

- 0.4

Figure 8


- 0.6

- 0.8

Langmuir

100

188

60

184

180

176

Binding Energy (eV)

Ag3d 374.4 Ag3d 368.4

O1S 532.3 O1S 530.6

80

Zr3d 181.7

Intensity (a.u)

Zn2P 1022.6

Intensity (cps)

120

(a) Zn2P 1045.7

140

38

Zr3d 184.2

160

Zr3d 184.2 Zr3d 181.7

40 20

1000

800

200

600 400 Binding Energy (eV)

0

(c) Intensity (a.u)

(b)

Intensity (a.u)

368.4 374.4 Ag3d Ag3d

380

372 376 368 Binding Energy (eV)

364

530.6 532.3 O1s O1s

548

544 540 536 532 Binding Energy (eV) 1022.6

(d)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

1045.7

Zn2p

Zn2p

1056

1048 1040 1032 Binding Energy (eV)

1024

ACS Paragon Plus Environment Figure 9

528

Page 39 of 45

39

Adsorption

Desorption

45

(a)

40 0.10

3

35

Pore volume (cm /g)

Quantity adsorbed (cm2/g STP)

0.12

30 25 20 15

0.08 0.06 0.04 0.02 0.00 0

10

100

400

300

200

Pore radius (A°)

5 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative pressure (P/Po) 90

Adsorption

60 50 40

3

70

Desorption

(b)

0.30

80 Pore volume (cm /g)

Quantity adsorbed (cm2/g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.25 0.20 0.15 0.10

30 0.00 20

0

100

10

200 300 ° Pore radius (A )

400

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Relative pressure (P/Po)

Figure 10


0.8

0.9

1.0

Langmuir

40

100 Concentration of RR 120 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

80

(c) (f)

(e)

60

(b) (d) (h)

40 (g) 20

(a) 0 0

5

10

15 20 Time (min)

25

Zr-Ag-ZnO (a) Ag-ZnO (c)

Zr-Ag-ZnO/dark (b) TiO2-P25 (g)

Commercial ZnO (f)

Bare ZnO (e)

Figure 11


30

35

Zr-ZnO (d) TiO2 Merck (h)

Page 41 of 45

41

1.0

(a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(b)

(c)

0.5

(d)

a d

0 200

400

600

Wavelength (nm)

Figure 12


800

Langmuir

42

70 60 % of RR 120 remaining

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

50 40 30 20 10 0 0

30 I Run

60 90 Time (min) II Run

III Run

Figure 13


120 IV Run

Page 43 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

43

Zn(NO3)2. 6H2O

AgNO3

(COOH)2. 2H2O

∆ ∆

Addition of Zr Zinc oxalate with Ag

∆

Stirring for 1 h at 60-70◦ C

Zr with Ag – Zinc oxalate

(i) Air oven 100◦C for 5 hr

(ii) Furnace at 450°C for 12 h

Zr loaded Ag-ZnO

Scheme 1


ZrOCl2. 8H2O

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 45

44

e

–

O2●–

O2

O2●–

Dye∗

Ag

e

Zr

–

hν

hν ZnO

Dye

Dye

+●

h+

H2O

●

OH

Dye / Dye* +

HO●

Mineral acids +

Dye / Dye* +

●–

Mineral acids +

O2

Scheme 2


CO2 + H2O CO2 + H2O

Page 45 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

45

Highly efficient, solar active and reusable photocatalyst, Zr loaded Ag-ZnO for Reactive Red 120 dye degradation with synergistic effect and dye sensitized mechanism B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi* Photocatalysis laboratory, Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India

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Highly Efficient, Solar Active, and Reusable Photocatalyst: Zr-Loaded

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