The influence of noble metals on photocatalytic activity of ZnO for Congo red degradation

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The influence of noble metals on photocatalytic activity of ZnO for Congo red degradation a,b,* € Nuray Gu¨y a, Mahmut Ozacar a

Department of Chemistry, Science & Arts Faculty, Sakarya University, Sakarya 54187, Turkey Biomedical, Magnetic and Semiconductor Materials Research Center (BIMAS-RC), Sakarya University, Sakarya 54187, Turkey

b

article info

abstract

Article history:

The most suitable method to improve the photocatalytic activity of semiconductors is

Received 30 April 2016

doping of noble metallic nanoparticles on them. In this research, Au, Ag and Pd noble

Received in revised form

metals were separately doped on ZnO nano photocatalysts by borohydride reduction

8 July 2016

method. The as-prepared doped on ZnO nano photocatalysts were characterized by X-ray

Accepted 10 July 2016

diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), ferromagnetic resonance (FMR) and UVeVis diffuse reflectance/absorbance spectroscopy

Keywords:

(DRS). The photocatalytic activities of nano photocatalysts were evaluated by the degra-

Photodegradation

dation of Congo red (CR) dye under UV irradiation. It was found that noble metals doped

Noble metals doped ZnO

ZnO nano photocatalysts could significantly increase the photocatalytic activity of ZnO.

WilliamsoneHall

When we compared the enhancing effects of Au, Ag and Pd, it was found that Pd was more

Borohydride reduction

effective than others. The differences in activities of various noble metals doped ZnO may

Congo red

be related differences in the amount of reducing noble metals, in their work functions and in their heights of Schottky barriers. The results of these studies demonstrate that doping of noble metals on ZnO can delay significantly the recombination process of the electron ehole pairs generated by the photon absorption and consequently improve the photocatalytic activity of the ZnO. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The dyestuffs from the textile and dye industries are the major sources of environmental pollution [1]. The presence of these pollutants in water is highly toxic and hazardous for the environment and living organisms. Many different techniques have been applied for water treatment such as coagulation, flocculation, membrane filtration, adsorption and specially the photocatalysis [2].

Among these techniques, photocatalysis is an effective and applicable method to degrade the organic contaminants dyes in wastewater. Therefore, nano-sized metal oxide semiconductors have shown good photocatalytic activity for water purification under irradiation due to their special optical and electrical properties [3]. ZnO is one of the technologically important semiconductors and has been widely used as a photocatalyst because of its large area-to-volume ratio, direct wide band gap (3.37 eV), high photosensitivity, low cost and

* Corresponding author. Department of Chemistry, Science & Arts Faculty, Sakarya University, Sakarya 54187, Turkey. Fax: þ90 264 295 59 50. € E-mail address: [email protected] (M. Ozacar). http://dx.doi.org/10.1016/j.ijhydene.2016.07.063 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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high chemical stability [4,5]. One of the major disadvantages of ZnO as a photocatalyst is the rapid electron/hole recombination, which is faster than the surface redox reactions and limits the photodegradation reaction under normal conditions [5e7]. Many studies have been reported to prevent recombination of electron/hole pairs and improve photocatalytic activity of ZnO, such as using ZnO with noble metals (e.g., Au, Ag, Pt, or Pd) [8e11], metals (Fe, Mg, Ca and Al) [12], metal oxide [13,14] and carbon materials (graphene and graphene oxide) [15]. As a principle, doping ZnO nanomaterials by noble metals prevents the recombination of photoinduced electrons and holes, broadens the absorption spectrum and facilitates some specific reactions on the surface of catalysts. The noble metals such as palladium and gold were used for the ZnOemetal formation because they have high electron affinity behavior [16,17] and produce the highest Schottky barrier when between metal and semiconductor. Fermi levels of Au (EF,Au), Ag (EF,Ag) and Pd (EF,Pd) are lower than the conduction band edge of ZnO (4.2 eV), so Au, Ag and Pd accumulate photogenerated electrons during photoexcitation of ZnO and lead to increase the photocatalytic efficiency [18e20]. Herein, ZnO nanoplates were successfully fabricated by microwave-hydrothermal method. The Ag, Au and Pd noble metals were separately doped on ZnO nano photocatalysts by borohydride reduction method. We have compared effect of Au, Ag, and Pd doped ZnO photocatalysts on the photocatalytic degradation of Congo red (CR) in the presence of UV irradiation. Congo red (C32H22N6Na2S2O6) is a benzidine-based anionic diazo dye prepared by coupling tetrazotised benzidine with two molecules of naphthionic acid [21]. Benzidine is a toxic metabolite of CR, a known human carcinogen. Therefore, the CR removal from wastewater is crucial important to prevent its toxicity for aquatic life. Improved photocatalytic activity of ZnO by doping noble metals may be attributed to dispersion of high metal nanoparticles on ZnO surfaces or incorporation in ZnO lattice, enhancement of light absorption of ZnO and the differences of work functions of noble metalsZnO systems.

Materials and methods Materials Zinc chloride (ZnCl2, Merck), sodium hydroxide (NaOH, Merck), palladium chloride (PdCl2, Alfa-Aesar), Au solution (HAuCl4, Merck), silver nitrate (AgNO3, Carlo Erba), sodium borohydride (NaBH4, Merck), CR (commercial grade), and ethanol (Merck) were purchased and used without further purification. All compounds except CR were of reagent grade. Deionized water was used throughout the entire experiments.

Preparation of photocatalysts The ZnO powder was prepared by microwave-assisted hydrothermal process. The preparation method of undoped ZnO powders is as follows: 0.50 g of ZnCl2 was dissolved in 10 mL of distilled water and then 10 mL 0.80 g of NaOH solution was added dropwise into the solution and stirred for 1 h to obtain a milky solution. The resulting solutions were transferred to

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and sealed in a 100 mL Teflon-lined autoclave, placed into a reactor, and then heated to160  C in a microwave oven (CEM Mars 5 model) with a controlled power of 700 W for 5 min and then cooled at room temperature naturally. The precipitate was centrifuged at 8000 rpm for 5 min, washed with distilled water and absolute ethanol several times, and then dried in an oven at 60  C for 24 h. Noble metal-doped photocatalysts were prepared by borohydride reduction method. Required amount of the salt solution of the metal for doping was added to 100 mg of ZnO and dispersed in 40 mL of distilled water. The weight ratio of Au, Ag and Pd to ZnO in this representative reaction was 5%.Then 0.0175 M 20 mL sodium borohydride solution as a reducing agent was added dropwise to the mixture and stirred for 1 h at room temperature to reduce adsorbed metals ions to metallic nanoparticles onto ZnO surface. The precipitate was collected by centrifugation at 8000 rpm for 5 min and washed distilled water and absolute ethanol several times, and then dried in an oven at 60  C for 24 h.

Characterization of photocatalysts The obtained samples were confirmed by powder X-ray diffraction (XRD, RIGAKU D max 2200 X-ray diffractometer with Cu KR (l) 0.154 nm radiation) in the 2q angles ranging from 10 to 90. The morphologies of Au/ZnO, Ag/ZnO, Pd/ZnO and undoped ZnO nano photocatalysts were characterized by using a field emission scanning electron microscopy (FESEM, FEI QUANTA FEG 450). The surface compositions of the samples were identified by energy dispersive spectroscopy (EDS). The Au, Ag and Pd contents of the photocatalysts were determined by an inductively coupled plasma-optical emission spectrometer (ICP-OES, Spectro Analytical Instruments, Kleve, Germany). FMR measurements have been carried out at room temperature using a commercial EMX X-Band (9.51 GHz) spectrometer, which is equipped with pole-cap providing a dc magnetic field up to the 22 kG magnetic field. The UVeVis absorption spectra of the CR solution and photocatalysts were obtained by using a UVevisible spectrophotometer (UVeVis, Shimadzu UV-2600PC). The diffuse reflectance of the photocatalysts was measured by using a UVevisible spectrophotometer fitted with a diffuse reflectance attachment. The band gap energies of the nano photocatalysts were determined by the KubelkaeMunk function, F(R) and by extrapolating the [F(R)hv]1/2 versus photon energy (hn).

Photocatalytic testing Photocatalytic activities of as-prepared photocatalysts were evaluated by degradation of CR in water under UV irradiation of a 100 W UV light (the strongest emission at 365 nm). For each experiment, 50 mg of photocatalyst was dispersed in 100 mL of 16 mg/L of the CR aqueous solution. Prior to UV irradiation, the suspensions were stirred magnetically for 30 min in the dark conditions to ensure establishment of adsorption/desorption equilibrium of CR on surfaces of the photocatalysts in the aqueous solutions. 5 mL of the aliquots were sampled at predetermined time intervals, centrifuged and analyzed by recording variations in the absorption band

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(498 nm) in the UVeVis spectra of CR. The percentage of degradation was calculated by using Eqs. (1) and (2) [22]: degradation ð%Þ ¼

¼

C0  C  100 C0

A0  A  100 A0

(1)

(2)

where C0 represents the initial concentration after the equilibrium adsorption, C represents the reaction concentration of CR, A0 represents the initial absorbance, and A represents the changed absorbance of the CR at the characteristic absorption wavelength of 500 nm.

Au, Ag and Pd doped ZnO photocatalysts were prepared by chemical reduction of noble metal ions on ZnO in presence of NaBH4. NaBH4 is a metal hydride with strong reduction capacity and high hydrogen content. It releases its hydrogen as a result of hydrolysis. The aqueous reduction of Au3þ, Agþ and Pd2þ with borohydride is thought to be a combination of three independent reactions [23].  BH 4 þ 2H2 O/BO2 þ 4H2 [

(6)

3þ þ BH þ 2H2 O þ ZnO/Au=ZnO þ BO 4 þ Au 2 þ 3H þ 5=2H2 [

(7)  BH 4 þ H2 O/B þ OH þ 2:5H2 [

Adsorption experiments

(8)

 þ þ BH 4 þ Ag þ 2H2 O þ ZnO/Ag=ZnO þ BO2 þ H þ 7=2H2 [

Adsorption experiments were performed in 250 mL flasks with 100 mL of different concentrations (8e128 mg/L) of CR solution, and 20 mg Pd/ZnO photocatalysts were added to the flasks at room temperature. The suspension was shaken on a horizontal bench shaker (Biosan PSU-10i) in the dark for 3 h to ensure that between CR adsorbed and CR in solution has reached equilibrium. At the end of the adsorption period, the solution was centrifuged for 15 min at 5000 rpm. All the concentrations were measured at the wavelength corresponding to max. absorbance, lmax ¼ 498 nm, using a spectrometer (Shimadzu UV-2600PC). Then the concentrations of the samples were determined by using a standard calibration graph. The amounts of CR adsorbed onto Pd/ZnO photocatalysts were calculated from the concentrations in solutions before and after the adsorption process.

(9) BH 4 þ 2Pd

2þ

þ þ 2H2 O þ 2ZnO/2Pd=ZnO þ BO 2 þ 4H þ 2H2 [

(10) Experimental conditions such as borohydride and metal concentrations, pH value, temperature and other components in the reaction mixture, play a significant role in determining the extent to which each reaction participates during the reduction process. In this study, reactions (7), (9) and (10) are important because they are leading to the reduction of the Au3þ, Agþ and Pd2þ ions and the incorporation/deposition of Au0, Ag0 and Pd0 in/on ZnO nanoplates during Au3þ, Agþ and Pd2þ reduction.

Characterization of photocatalysts

Results and discussion Production mechanisms of ZnO nanoplate and noble metals doped ZnO The ZnO nanoplates were synthesized by microwave hydrothermal method. Hydroxyl anions were provided by NaOH solution. The addition of NaOH to ZnCl2 controlled the ZnO nanoparticles formation. The possible reactions involved in the formation of ZnO nanoplates can be represented as Eqs. (3), (4) and (5): stirring at 25 C

ZnCl2ðaqÞ þ 2NaOHðaqÞ ƒƒƒƒƒƒƒƒ! ZnðOHÞ2ðsÞ Y þ 2NaClðaqÞ MW irradiation

ZnðOHÞ2 þ 2OH ƒƒƒƒƒƒƒƒ! ZnðOHÞ2 4 MW irradiation

 ZnðOHÞ2 4 ƒƒƒƒƒƒƒ! ZnO þ H2 O þ 2OH

The crystallinity, phase, and purity of the as-prepared samples were determined by X-ray diffraction (XRD). Fig. 1 presents X-ray diffraction (XRD) spectra of as-synthesized ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO. Obviously, Au/ZnO, Ag/ZnO and Pd/ZnO samples exhibit the usual wurtzite, just like original ZnO nanoplates, with similar peak intensities and shapes,

(3)

(4) (5)

The zinc cations react with hydroxyl anions to form stable complexes, which act as the growth tetrahedral ZnðOHÞ2 4 units of ZnO nanostructures. They are then decomposed by the effect of the electric field of microwave radiation to form ZnO crystal nuclei acting as dipoles in the mixture solution. After a while, ZnO nuclei grow into nanoplates due to the surface energy difference of crystallographic planes under microwave power (750 W).

Fig. 1 e XRD patterns of ZnO, Pd/ZnO, Ag/ZnO and Au/ZnO nano photocatalysts.

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which shows 11 peaks at 31.7 , 34.4 , 36.2 , 47.5 , 56.6 , 62.9 , 66.4 , 68.0 , 69.1 , 72.6 , and 77.0 , indexed to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes of the ZnO crystal, respectively. The XRD patterns of samples exhibit peaks corresponding to the (111) and (200) facets of Au, Ag and Pd (JCPDS cards file no.; ZnO:36-1451, Au:04-0784, Ag:04-0783, Pd:05-0681), which demonstrate that Au, Ag and Pd are all composed of pure crystalline with the face-centered cubic (fcc) structure [24]. Comparing with the diffraction peaks of pure ZnO, no characteristics peaks of impurities and other phases such as Zn(OH)2 and metal oxides were observed. Additionally, a characteristic peak of Ag (220) appeared by indicating the formation of crystalline Ag nanoparticles. The formation of noble metal nanoparticles on ZnO surface is confirmed by all these XRD findings [25]. The d space between adjacent (hkl) planes can be calculated from Bragg equation ðl ¼ 2dsinqÞ [21e23]. Lattice constants and unit cell volumes are calculated using unitcellwin software [24] and the Lattice geometry equations presented below [26,27]. Calculated lattice parameters of prepared samples are summarized in Table 1. 1 d

2

¼

V¼

 2 2 2 4 h þ hk þ l l þ 2 2 a c 3

(11)

√3a2 c ¼ 0:866a2 c 2

(12)

Compared to ZnO, the XRD patterns of ZnO doped Au/ZnO, Ag/ZnO and Pd/ZnO are similar, it suggests that considerable amount of metals are not incorporated into the ZnO lattice. As shown in Table 1, doping increases lattice parameters of ZnO. This can be related to the significantly larger ionic radius of Au3þ (99 pm), Agþ (126 pm) and Pd2þ (80 pm) ions which substitute with Zn2þ ions (74 pm) in ZnO crystal [28,29]. Consequently, these results indicate indirect evidence of which partial metallic nanoparticles are formed successfully on the surface and the rests are possibly doped in the lattice of ZnO. In addition, the XRD peaks of Au, Ag and Pd(111) are comparatively weak, as shown in Fig. 1, proving a small size and well dispersed metallic nanoparticles at the surface of ZnO, which confirms FESEM images [5,10,30,31]. The average crystallite sizes of the ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO were calculated using both the Scherrer equation (Eq. (13)) and WilliamsoneHall (WeH) (Eq. (14)), namely uniform deformation model (UDM), equation as given below:

Fig. 2 e WilliamsoneHall plot for a)ZnO, b)Au/ZnO, c)Ag/ ZnO and d)Pd/ZnO.

D¼

0:9l b cosq

b cosq ¼

(13)

0:9l þ 4sinq D

(14)

where l is the wavelength of the radiation, q is the diffraction angle, and b is the corrected half width of the diffraction peak. When plotting the WilliamsoneHall equation between bcosq vs 4sinq, the slope of the plot gives the strain ε (Fig. 2). Plotted data met a linear fit with regression coefficients of >0.98. The WeH plots show a negative strain for undoped and noble metals doped ZnO nanoplates which are an indication of compressive strain and shrinkage lattice [32,33]. The strains and crystal sizes are given in Table 1. As shown in Table 1, the

Table 1 e The structure parameters of undoped and noble metals (Au, Ag, Pd) doped ZnO nanoplates. Compound Scherer WeH D (nm) 2q ± 0.01 D (nm)

ZnO

28.89

23.91

Au/ZnO

30.45

27.19

Ag/ZnO

31.00

27.73

Pd/ZnO

34.43

32.24

31.76 34.42 31.73 34.37 31.77 34.43 31.64 34.30

hkl

dhkl ( A) Structure

Unitcellwin Lattice V ( A)3  parameter (A)

(100) (002) (100) (002) (100) (002) (100) (002)

0.2815 0.2603 0.2817 0.2606 0.2817 0.2605 0.2826 0.2613

Hexagonal Hexagonal Hexagonal Hexagonal

a ¼ 3.250 c ¼ 5.207 a ¼ 3.253 c ¼ 5.211 a ¼ 3.254 c ¼ 5.220 a ¼ 3.256 c ¼ 5.215

47.63 47.75 47.85 47.86

Lattice geometry equation Lattice parameter ( A) a ¼ 3.251 c ¼ 5.207 a ¼ 3.255 c ¼ 5.216 a ¼ 3.253 c ¼ 5.210 a ¼ 3.263 c ¼ 5.224

V ( A)3 47.74 47.86 47.63 48.17

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compressive strain decreases in the following order ZnO < Au/ ZnO < Ag/ZnO < Pd/ZnO. This could be attributed to the differences in the ionic radius of Zn and noble metals and in the amounts of noble metals ions which incorporated [26,34]. Even though the ionic radius of Pd2þ is smaller than the ionic radius of the Agþ and Au3þ, the lattice expands more and compressive strain decreases more than Agþ and Au3þ due to the greater incorporated amounts of Pd2þ ions than the other ions. The lower intercept and slope values indicates a large crystallite size and a low strain [35]. The lattice parameters calculated by both unit cell and lattice geometry equation confirm these results. From Table 1, it is observed that the crystal sizes of all the samples determined by both methods are almost in agreement with each other, and this confirms the reliability of these values. Difference between the samples of crystal sizes can be due to the amount of reducing and covering of small metallic nanoparticles at the surface of ZnO. When the crystal sizes of samples are compared, it is seen that the crystal sizes of Pd/ZnO is a bit larger than the sizes of undoped ZnO, Ag/ZnO and Au/ZnO. It may be due to much more palladium reduced with borohydride method. EDS and FESEM results also confirm the higher amount of palladium doped or cover the surface. At the noble metal doped ZnO photocatalysts, ZnO has larger crystal sizes because Zn2þ ions substitute with noble metal ions and so it promotes the growth of the particle size. Before the photodegradation, adsorption mainly takes places on the surface of the photocatalyst. So, first the crystal size influences the adsorption and then it can also influence the photocatalytic activity [10,30]. But in accordance with our results are consistent with those of Zhong et al. [36] and Khataee et al. [5], crystal sizes of noble metals doped ZnO nano photocatalysts caused no effect on the photocatalytic activity. The morphological analysis of the as-prepared samples was carried out using an FESEM, which is shown in Fig. 3. ZnO shows an irregular nanoplate like structure Fig. 3a. At a higher magnification of 200 nm, hexagonal structure of ZnO is clearly seen (indicated by arrow marks). As can be seen in Fig. 3aed Au, Ag and Pd doping in ZnO does not change the plate

morphology. But small nanoparticles take place on ZnO surface. Such results suggest that most of the Au, Ag and Pd atoms were not incorporated in the ZnO lattice or only to a very small extent. The images also suggest that the distribution of metallic Pd on ZnO is much greater than Ag/ZnO and Au/ZnO. From the EDS analysis, the atomic percentages of elemental Au, Ag and Pd on the ZnO were determined as 0.96, 1.54 and 3.34 respectively. As shown in Table 2, when comparing the atomic percentage of doping in the samples, it was observed that there was a greater amount of elemental palladium presence in the catalyst that was modified with same amount Au and Ag. Further, the estimated percentages of noble metals in the doped ZnO nanoparticles obtained by ICP-OES analysis show that the amount of metals incorporated into the Zn2þ host lattice is less than the amount used during the synthesis. This may be due to the less solubilities of metals in ZnO. FESEM, ICP-OES and EDS results clearly indicate a higher amount of metal nanoparticles covered on the Pd/ZnO photocatalyst compared to the Ag/ZnO and Au/ZnO [31,37,38]. The FMR technique is sensitive and useful as to determine magnetic properties of the samples. Moreover, this technique also provides information about the lattice site in which a ferromagnetic dopant ion is located [39]. Fig. 4 shows the FMR spectrum of undoped ZnO and noble metals doped nanoplates. The FMR spectra of the samples are very similar. The observed resonance signals at g y 2.0024 are commonly attributed to chemisorbed oxygen or Zn vacancy. Another signals at g y 1.9960 and g y 1.9580 are attributed to be shallow donor caused by surface O vacancy and interstitial Zn. So the super paramagnetic behavior in ZnO nanoplates can be contributed because the Zn vacancy or oxygen vacancies formed at the surface [40,41]. No FMR spectrum was observed for doped noble metals due to a small amount of reducing and covering of small metallic nanoparticles at the surface of ZnO. UVeVis absorption spectra of ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO are shown in Fig. 5. ZnO exhibits a sharp band at 377 nm, which corresponds to the ZnO nanoplates [42,43]. Comparison of the spectra of samples indicates no notable differences in the UV region, with the strong, broad absorption observed below 400 nm attributable to charge-transfer from the valence to the conduction band of the ZnO. On the other hand, in the visible region of the spectra, the noble metal doped ZnO nanoplates show characteristic absorption patterns [38]. The absorption maximum at 378 nm of Pd/ZnO corresponds to the presence of ZnO and two small peaks

Table 2 e The EDS and ICP-OES analysis of undoped and noble metals (Au, Ag and Pd) doped ZnO nanoplates.

Fig. 3 e FESEM images of: (a) undoped ZnO, (b) Au/ZnO, (c) Ag/ZnO, (d) Pd/ZnO.

Elements/(at.%) (EDS)

ZnO

Au/ZnO

Ag/ZnO

Pd/ZnO

O Zn Au Ag Pd Total Elements/(w.%) (ICP-OES)

81.52 18.48 0 0 0 100.0 e

91.69 7.252 0.958 0 0 100.0 1.2

91.23 7.233 0 1.537 0 100.0 1.583

87.71 8.950 0 0 3.340 100.0 3.440

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metal cluster. So the diffused reflectance spectra of Pd-doped ZnO show constant absorption in the range 450e750 nm. Consequently, multiple signals corresponding to the various permitted electronic transitions are observed in the visible region because of the variable size of metal deposits [20]. But Au/ZnO nanoplates show an only broad absorption near 550 nm that corresponds to the surface plasmon resonance band of gold nanoparticles. The surface plasmon resonance is a unique feature of noble metals such as gold and silver and originated from the collective oscillations of the free conduction band electrons. Similarly, broad peak at 450 nm is observed in the absorbance spectra of the Ag/ZnO representing the formation of Ag nanoparticles [45,46]. The diffuse reflectance spectroscopic measurements were done to examine the optical properties of nano photocatalysts. All the reflectance spectra show a similar shape. Fig. 5 shows the KubelkaeMunk transformed reflectance spectra for nano photocatalysts. The band gap energy can be derived from the optical reflectance spectrum of samples by using KubelkaeMunk function: FðRÞ ¼

Fig. 4 e FMR spectrum of: (a) undoped ZnO, (b) Au/ZnO, (c) Ag/ZnO, (d) Pd/ZnO.

ð1  RÞ2 2R

(15)

where R is the observed reflectance in UVeVis spectra, h is Planck's constant, and n is the frequency of light. A graph is plotted between [F(R)hn]2 and hn, and the intercept which is obtained presents the band gap energy (Fig. 6) [46]. As using this method, the estimated band-gaps of the undoped ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO nano photocatalysts are found to be 3.25, 3.29, 3.28 and 3.3 eV respectively, corresponding to the violeteblue region of the electromagnetic spectrum. The experimental results indicate that the band gap energies of the Au/ZnO, Ag/ZnO and Pd/ZnO nano photocatalysts do not change significantly with doping noble metals. But the presence of visible absorbance peaks in UVeVis absorption spectra for Au/ZnO, Ag/ZnO and Pd/ZnO represents the formation metal nanoparticles on ZnO surface [38,44,45,47]. This is also confirmed by both FESEM and EDS results.

Fig. 5 e UVeVis absorption spectra of ZnO, Au/ZnO, Ag/ ZnO and Pd/ZnO.

observed at 470 and 530 nm can be attributed to the Pd nanoparticles. These results are in good agreement with the values reported in the literature [44,45]. The absorption of visible light by metal doped samples can be explained to lowenergy transitions between the valence band of ZnO and localized energy levels in the band-gap by doped metal nanoparticles. Pd nanoparticles give rise to localized energy levels in the band gap of ZnO into which valence band electrons of ZnO are excited at wavelength longer than 378 nm. If equal sized metal clusters are formed, it will lead to constant absorption in the visible region corresponding to the excitation from the valence band of ZnO to the unoccupied level of

Fig. 6 e The plot of the transformed KubelkaeMunk function versus the gap energy of ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO.

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Photocatalytic activity mechanism The photodegradation of CR was determined to evaluate the photocatalytic activity of ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO nanoplates under UV irradiation. Fig. 7aed shows a series of absorption spectra of the aqueous solution of CR with 50 mg of ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO nano photocatalysts. The absorption characteristics of CR at about 498 nm decrease gradually over time for all the samples, and almost complete degradation of the dye within 60e180 min by using undoped and noble metals doped ZnO as nano photocatalysts. The comparison of the time profiles of the decrease in CR concentration in the presence of different photocatalysts is shown in Fig. 8a. As illustrated in Fig. 8a, the photocatalytic activities of nano photocatalysts are found as following the order: Pd/ZnO > Ag/ZnO > Au/ZnO > ZnO. Pd/ZnO nanophotocatalyst exhibits the fastest adsorption following photodegradation and photo-assisted decomposition of 98% of CR in the 1 h Fig. 8b shows the pseudo-first order kinetics of the CR degradation of the noble metals doped ZnO photocatalysts. Pseudo-first-order reaction kinetics is given by the following equation [48]: lnðC0 =Ct Þ ¼ kt

(16)

where k is the degradation rate constant and C0 and C are the initial concentrations of dye and at the reaction time t, respectively. The pseudo-first order rate constants of the ZnO, Au/ZnO, Ag/ZnO and Pd/ZnO are shown in Table 3. The results clearly exhibit that the Pd doped ZnO photocatalyst with its

high Pd nanoparticles dispersion and highest UVeVis absorption proves to have the fastest photodegradation rate among all the nano photocatalysts tested. During the photocatalysis experiments, the excellent adsorption is contributed to the improvement of photocatalytic activity. Prior to the photocatalytic removal under the UV light irradiation, the dark adsorption of CR on Pd/ZnO photocatalyst was evaluated and the corresponding adsorption isotherms were recorded. Equilibrium data, commonly known as adsorption isotherms, are important for the description of how molecules or ions of adsorbate interact with adsorbent surface sites and also, are critical in optimizing the use of adsorbent. These data provide information on the capacity of the adsorbent or the amount required to remove a unit mass of pollutant under the system conditions. Hence, the correlation of equilibrium data using either a theoretical or empirical equation is essential for the adsorption interpretation and prediction of the extent of adsorption. Therefore, the equilibrium experimental data for adsorbed data CR on Pd/ZnO photocatalyst were analyzed using the Langmuir and Freundlich isotherms in the present study. Ce 1 aL ¼ þ Ce qe KL KL

(17)

1 log qe ¼ log KF þ logCe n

(18)

where qe (mg/g) and Ce (mg/L) are the amount of adsorbed adsorbate per unit weight of adsorbent and unadsorbed

Fig. 7 e UVeVis absorption spectra of the aqueous solutions of CR (16 ppm, 250 mL), in the presence of a) ZnO, b) Au/ZnO, c) Ag/ZnO and d) Pd/ZnO.

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Fig. 8 e a) The photodegradation of CR in presence of various photocatalysts. b) Pseudo-first order kinetics for CR dye.

Table 3 e Kinetics data of CR photodegradation in the presence of undoped and noble metals (Au, Ag and Pd) doped ZnO photocatalysts. Samples

Rate constants, k (min1)

ZnO Au/ZnO Ag/ZnO Pd/ZnO

0.62 1.96 2.26 5.76

   

Photodegradation ratios after 1 h irradiation (%)

102 102 102 102

53.1 77.2 81.6 98.2

adsorbate concentration in solution at equilibrium, respectively. The constant KL (L/g) and aL (L/mg) are the Langmuir equilibrium constants and the KL/aL gives the theoretical monolayer saturation capacity, Q0. KF (mg11/nL1/ng1) is the Freundlich constant and n (g/L) is the Freundlich exponent. Therefore, a plot of Ce/qe versus Ce gives a straight line of slope aL/KL and intercepts 1/KL and a plot of log qe versus log Ce enables the constant KF and exponent n to be determined. The adsorption data were analyzed according to the linear forms of the Langmuir and the Freundlich isotherms (Eqs. (17) and (18)). The isotherm data were calculated from the least squares methods. Table 4 shows the values of the parameters of two isotherms and the related correlation coefficients. A comparison is also made between two isotherms plotted in Fig. 9, which shows the experimental data points and the two theoretical isotherms plotted on the same graph. As seen from Table 4 and Fig. 9, the Langmuir isotherm shows a better fit to adsorption data than the Freundlich isotherm. The fact that Langmuir isotherm fits the experimental data very well confirms the monolayer coverage of CR onto Pd/ZnO composite and also the homogeneous distribution of active sites on the photocatalyst, since the Langmuir equation assumes that the surface is homogeneous. The monolayer saturation capacity

Table 4 e Adsorption isotherm constants for CR adsorption on Pd/ZnO photocatalyst. Langmuir

Freundlich

KL aL Q0 (L/g) (L/mg) (mg/g) 6.173

0.031

199.1

r2 0.999

n KF (mg1(1/n)L1/ng1) (g/L) 11.02

1.724

r2 0.970

Fig. 9 e Adsorption isotherms of CR on Pd/ZnO.

of Pd/ZnO composite, Q0, was found to 199.1 mg/g. Therefore, the Langmuir equation best describes the equilibrium data for the CR-Pd/ZnO system over the concentration range used in this investigation [49e51]. The Au/ZnO, Ag/ZnO and Pd/ZnO show a remarkable enhancement in the photocatalytic activity under UV-light irradiation compared to the undoped ZnO. The enhancement could be related to contributing factors such as particle size and work function of heterostructures play important roles in the highly efficient photocatalytic activity. Noble metal nanoparticles doped on the ZnO surface are known to act as effective traps for photogenerated electrons due to the formation of a Schottky barrier at the metal-semiconductor interface. Noble metals prevent the photo excited electrons to return the ZnO surface and prolong the electronehole recombination time [27]. The charge trapping of metals (Au, Ag and Pd) and oxygen reduction in the degradation of CR can be depicted by the following equations:   þ ZnO þ hv/ZnO e CB þ hVB

(19)



ZnO e þ MðAu; Ag; PdÞ/ZnOeM e

(20)

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 ZnOeM e þ O2 / ZnOeM þ O 2

(21)

þ   O 2 þ Haq /HO2

(22)

2HO2 /H2 O2 þ O2

(23)

  H2 O2 þO2 þe CB /O2 þHO þOH

(24)

þ

OH=hVB þ pollutant/degradation products



(25)

A possible mechanism of the charge separation and photocatalytic reaction for noble metal doped photocatalyst is shown in Scheme 1. When semiconductor is illuminated by UV irradiation, a valence band electron (VB) transfer to the conduction band (CB), leaving a hole in the valence band. The presence of noble metals trapping the electron from CB of ZnO inhibits the electronhole recombination. The noble metal, acting as a trap  produce more superoxide radical anion ðO 2 Þ and at the same time VB holes of ZnO react with water to produce highly reactive hydroxyl (OH) radical. As a result, the superoxide radical anion and hydroxyl radical are used for degradation of CR.

According to experimental results, the photocatalytic activity of Pd/ZnO nano photocatalysts is the highest among the other noble metals doped samples. It is reported that noble metals doped ZnO nanocomposites can enhance reactive oxygen species and photocatalytic activities of ZnO [18]. When the Au, Ag or Pd nanoparticles contact with n-type semiconductor, ZnO, they will induce bending of the energy band of the ZnO at the interface and a Schottky barrier which facilities electron capture, will be formed at the junction and then the free electrons will transfer betweendAg and ZnO or Au and ZnO or Pd and ZnOddue to differences of their work functions. Pd has the highest work function (ɸm)of 5.2 eV [52], followed by Au which is 5.1 eV [17,19,53] of ZnO (ɸs) 4.3 eV [18,54] and Ag has the lowest value of 4.2 eV [18,52]. This facilitates electron transfer from the conduction band of ZnO to metals, so it prolongs the lifetime of charge carriers and generation of reactive oxygen species. Apparently, Pd is more effective than Ag and Au in enhancing the separation of charge carriers because of its higher work functions. Besides, Schottky barrier formed at the interface leads to an upward bending of the energy band of ZnO (Scheme 1a,b.), because the

Scheme 1 e The schematics of energy band mechanisms of noble metals (Pd, Au, Ag) and ZnO in contact before and after equilibrium. EVAC, vacuum energy; EF,Pd, EF,Au and EF,Ag, the Fermi levels of Pd, Au and Ag; EC, ZnO conduction band energy; EV, ZnO valence band energy; ɸm, noble metal (Pd, Au or Ag) work functions; ɸm,ZnO, ZnO work function; cZnO, ZnO electron affinity; HSB, Height of Schottky barrier; VBB, band bending.

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work functions of Pd and Au are higher than work function of ZnO (ɸm>ɸs) for Pd/ZnO and Au/ZnO. On the contrary, the energy band for Ag/ZnO bends downward due to ɸm<ɸs (Scheme 1c) [18,55]. The Schottky barrier height can be determined by the equation: HSB ¼ fm ; XZnO

(26)

where HSB, is the Schottky barrier height, and XZnO (4.2 eV) is electron affinity of ZnO [18,55]. According to Eq. (24), HSB, Pd ¼ 1 eV for Pd and HSB, Au ¼ 0.9 eV for Au are calculated [18,19]. Higher band bending and Schottky barrier height decelerate the electrons' transfer from ZnO to Pd at equilibrium for Pd/ZnO. In this case, more photogenerated holes may accumulate at the interface and this situation enhances oxidizing ability and the formation of hydroxyl radicals. So, Pd/ZnO exhibits a higher photocatalytic activity than Ag/ZnO and Au/ZnO. This results are consistent with reported studies for Pd/ZnO [18,19]. In fact, higher work function value of Au when compared to Ag makes Au/ZnO more-effective for electron capture under the same conditions, Au/ZnO is moreeffective than Ag/ZnO for electron acceptor because to the work function value of Au is higher than Ag. However, in this study Ag/ZnO shows a significant improvement in the degradation of CR in comparison to Au/ZnO under UV light illumination due to the atomic percentage of elemental Ag is higher than Au in ZnO. Until today there have not been any report about the use and comparison of Au, Ag and Pd doped ZnO photocatalysts for the degradation of CR dye under the UV-light. Doping of ZnO nanoplates through Au, Ag and Pd is increasingly being considered for enhancing its photocatalytic activity. Table 5 presents Au, Ag and Pd noble metals influence on the photodegradation of various dyes. In comparison of earlier reported works including Au, Ag and Pd doped ZnO photocatalysts, it is seen that Pd/ZnO photocatalyst has the highest photocatalytic activity. According to Table 5, Pd/ZnO photocatalyst may be a good option for wastewater treatment. The photocatalytic activities of the different amounts of palladium doped ZnO photocatalysts are shown in Fig. 10. As shown in Fig. 10, the different contents of palladium enhance the degradation rate to a different degree. The degradation rate increases with the increase of Pd content up to 5 at.% (5 at.% palladium doping with sodium borohydride method corresponds to approximately 3 at.% palladium). Then, the

Fig. 10 e The photodegradation of CR in the presence of photocatalysts including different amount of Pd.

degradation rate decreases with the further increase of Pd content. So, the optimal value of palladium is 5 at.%. When the content of the Pd of Pd/ZnO composites is below the optimal value, the poor photocatalytic performance is due to the decreased heterojunctions and low oxygen defects concentration. The Pd/ZnO composites show the better charge separation than that of ZnO. Furthermore, the deposited silver increases the rate of electron transfer to dissolved oxygen. On the other hand, Pd nanoparticles act as charge carrier recombination centers after the Pd content exceeds the optimal value, the over accumulations of electron on metal deposits could attract the photogenerated holes to the metal sites. This may encourage the recombination of charge carriers and the metal deposits reversely behave as recombinant centers [28,43,61]. In addition, higher surface loadings of metal deposits may reduce the catalytic efficiency of the semiconductor due to the decreasing the active sites on the semiconductor surface for light absorption and pollutant adsorption [61].

The stability and recyclability of photocatalyst To investigate the stability and reusability of photocatalyst, the Pd/ZnO composite was evaluated in five successive cycles

Table 5 e Summary of the literature on the photocatalytic degradation efficiencies of the Au, Ag and Pd doped ZnO photocatalysts and the photocatalysts prepared in this work. Catalyst Au/ZnO Au/ZnO Au/ZnO Ag/ZnO Ag/ZnO Ag/ZnO Pd/ZnO Pd/ZnO Au/ZnO Ag/ZnO Pd/ZnO

Dye

Initial dye con.

Light source

Time (min)

Deg. (%) 49 99 80 75 99 100 100 48.2 77 81 98.2

Methyl orange Methylene blue Methyl orange

20 ppm 5  105 M e

300 W Hg lamp 365 nm UV lamp 365 nm UV lamp

210 30 180

Rhodamine B Methylene blue Rhodamine B Methyl orange Congo red

5  105 M 20 ppm 1.0  105 mol/L 10 ppm 16 ppm

300 300 300 300 100

100 120 25 60 60

W Hg lamp W Hg lamp W Hg lamp W Hg lamp W Hg lamp (365 nm)

Reference [56] [25] [57] [58] [59] [60] [36] This study

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Acknowledgments € N.G. thanks the Teaching Staff Training Program (OYP). M.O. acknowledges partial support from the Turkish Academy of Sciences (TUBA).

references

Fig. 11 e The recycling tests of Pd/ZnO photocatalyst for photodegradation of CR photocatalytic under UV light irradiation.

for the degradation of 16 mg/L CR under UV light irradiation. After each run, the Pd/ZnO composite was recovered by centrifugation (5000 rpm for 15 min) and redispersed in the CR solution without any washing or drying. As shown in Fig. 11, the photocatalytic efficiency decreased only slightly compared to the as-synthesized Pd/ZnO photocatalyst after five cycles. As a result, the Pd/ZnO composite exhibits high stability for the photocatalytic degradation of CR, which is important for its practical application.

Conclusion In this work, different noble metals (Au, Ag and Pd) doped ZnO nano photocatalysts were synthesized by borohydride reduction method and exhibited enhanced photocatalytic activities that undoped ZnO in the photocatalytic degradation of CR under UV irradiation. The cause of the enhancement is an inhibition of the charge carrier recombination by noble metal particles which capture photogenerated electrons. When the photocatalytic activities of prepared photocatalysts were compared, it was found that Pd/ZnO had the best performance. The superior photocatalytic behavior may be correlated to the higher concentration and dispersities of metallic Pd nanoparticles in Pd/ZnO photocatalyst. In addition, upward band bending occurs when the Pd metals are doped ZnO and the results in the formation of the higher Schottky barrier height increasing the photocatalytic activity of ZnO. On the basis of the experimental findings, this work using noble metal doped metal oxide photocatalysts may be more useful for wastewater treatment due to their advantage of their high performance and simplicity. To develop the new photocatalyst and to apply in the degradation processes, we still study on the synthesizing different photocatalysts and will use these photocatalysts for the different applications such as colorless aqueous or gaseous pollutants as probe, and we will report the next investigation in the future.

[1] Farbod M, Khademalrasool M. Synthesis of TiO2 nanoparticles by a combined solegel ball milling method and investigation of nanoparticle size effect on their photocatalytic activities. Powder Technol 2011;214:344e8. [2] Senthilvelan S, Chandraboss VL, Karthikeyan B, Natanapatham L, Murugavelu M. TiO2, ZnO and nano bimetallic silica catalyzed photodegradation of methyl green. Mater Sci Semicond Process 2013;16:185e92. [3] Kajbafvala A, Ghorban H, Paravar A, Samberg JP, Kajbafvala E, Sadrnezhaad SK. Effects of morphology on photocatalytic performance of zinc oxide nanostructures synthesized by rapid microwave irradiation methods. Superlattice Microstruct 2012;51:512e22. [4] Pan L, Shen GQ, Zhang JW, Wei XC, Wang L, Zou JJ, et al. TiO2ZnO composite sphere decorated with ZnO clusters for effective charge isolation in photocatalysis. Ind Eng Chem Res 2015;54:7226e32. [5] Khataee A, Soltani RDC, Hanifehpour Y, Safarpour M, Ranjbar HG, Joo SW. Synthesis and characterization of dysprosium-doped ZnO nanoparticles for photocatalysis of a textile dye under visible light irradiation. Ind Eng Chem Res 2014;53:1924e32. [6] Subash B, Krishnakumar B, Swaminathan M, Shanthi M. Highly efficient, solar active, and reusable photocatalyst: ZrLoaded AgZnO for reactive red 120 dye degradation with synergistic effect and dye-sensitized mechanism. Langmuir 2013;29:939e49. [7] Kuriakose S, Satpati B, Mohapatra S. Enhanced photocatalytic activity of Co doped ZnO nanodisks and nanorods prepared by a facile wet chemical method. Phys Chem Chem Phys 2014;16:12741e9. [8] Yu C, Yang K, Zhou W, Fan Q, Wei L, Yu JC. Preparation, characterization and photocatalytic performance of noble metals (Ag, Pd, Pt, Rh) deposited on sponge-like ZnO microcuboids. J Phys Chem Solids 2013;74:1714e20. [9] Bouzid H, Faisal M, Harraza FA, Al-Sayari SA, Ismail AA. Synthesis of mesoporous Ag/ZnO nanocrystals with enhanced photocatalytic activity. Catal Today 2015;252:20e6. [10] Li P, Wei Z, Wu T, Peng Q, Li Y. Au-ZnO hybrid nanopyramids and their photocatalytic properties. J Am Chem Soc 2011;133:5660e3. [11] Liu Y, Wei S, Gao W. Ag/ZnO heterostructures and their photocatalytic activity under visible light: effect of reducing medium. J Hazard Mater 2015;287:59e68. [12] Wu ZJ, Huang W, Cui KK, Gao ZF, Wang P. Sustainable synthesis of metals-doped ZnO nanoparticles from zincbearing dust for photodegradation of phenol. J Hazard Mater 2014;278:91e9.   M, Tolasz J, Zetkova  K. ZnO/Bi2O3 [13] Stengl V, Henych J, Slu sn a nanowire composites as a new family of photocatalysts. Powder Technol 2015;270:83e91. [14] Lamba R, Umar Ah, Mehta SK, Kansal SK. ZnO doped SnO2 nanoparticles heterojunction photo-catalyst for environmental remediation. J Alloys Compd 2015;653:327e33.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 2 0 1 0 0 e2 0 1 1 2

[15] Hossain MM, Kub BC, Hahna JR. Synthesis of an efficient white-light photocatalyst composite of graphene and ZnO nanoparticles: application to methylene blue dye decomposition. Appl Surf Sci 2015;354:55e65. [16] Li FB, Li XZ. The enhancement of photodegradation efficiency using PteTiO2 catalyst. Chemosphere 2002;48:1103e11. [17] Pawinrat P, Mekasuwandumrong O, Panpranot J. Synthesis of AueZnO and PteZnO nanocomposites by one-step flame spray pyrolysis and its application for photocatalytic degradation of dyes. Catal Commun 2009;10:1380e5. [18] He W, Wu H, Wamer WG, Kim HK, Zheng J, Jia H, et al. Unraveling the enhanced photocatalytic activity and phototoxicity of ZnO/metal hybrid nanostructures from generation of reactive oxygen species and charge carriers. ACS Appl Mater Interfaces 2014;6:15527e35. [19] Misra M, Kapur P, Singla ML. Surface plasmon quenched of near band edge emission and enhanced visible photocatalytic activity of Au@ZnO core-shell nanostructure. Appl Catal B Environ 2014;150e151:605e11. [20] Sakthivel S, Shankar MV, Palanichamy M, Arabindoo B, Bahnemann DW, Murugesan V. Enhancement of photocatalytic activity by metal deposition: characterization and photonic efficiency of Pt, Au and Pd deposited on TiO2 catalyst. Water Res 2004;38:3001e8. [21] Bagheri M, Mahjoub AR, Mehri B. Enhanced photocatalytic degradation of Congo red by solvothermally synthesized CuInSe2eZnO nanocomposites. RSC Adv 2014;4:21757e64. [22] Jeyasubramanian K, Hikku GS, Sharma RK. Photocatalytic degradation of methyl violet dye using zinc oxide nanoparticles prepared by a novel precipitation method and its anti-bacterial activities. J Water Process Eng 2015;8:35e44. [23] Lu J, Dreisinger DB, Cooper WC. Cobalt precipitation by reduction with sodium borohydride. Hydrometallurgy 1997;45:305e22. [24] Gu H, Yang Y, Tian J, Shi G. Photochemical synthesis of noble metal (Ag, Pd, Au, Pt) on graphene/ZnO multi hybrid nanoarchitectures as electrocatalysis for H2O2 reduction. ACS Appl Mater Interfaces 2013;5:6762e8. [25] Fageria P, Gangopadhyay S, Pande S. Synthesis of ZnO/Au and ZnO/Ag nanoparticles and their photocatalytic application using UV and visible light. RSC Adv 2014;4:24962e72. [26] Khorrami GhH, Zak AK, Kompany A, Yousefi R. Optical and structural properties of X-doped (X ¼ Mn, Mg, and Zn) PZT nanoparticles by KramerseKronig and size strain plot methods. Ceram Int 2012;38:5683e90. [27] Zak AK, Yousefi R, Majid WHAbd, Muhamad MR. Facile synthesis and X-ray peak broadening studies of Zn1-xMgxO nanoparticles. Ceram Int 2012;38:2059e64. [28] Chang Y, Xu J, Zhang Y, Ma S, Xin L, Zhu L, et al. Optical properties and photocatalytic performances of Pd modified ZnO samples. J Phys Chem C 2009;113:18761e7. [29] http://www.saylor.org/site/wp-content/uploads/2011/06/ Ionic-Radius.pdf; 15 January 2016. [30] Krishnakumar T, Jayaprakash R, Pinna N, Singh VN, Mehta BR, Phani AR. Microwave-assisted synthesis and characterization of flower shaped zinc oxide nanostructures. Mater Lett 2009;63:242e5. [31] Ansari SA, Khan MM, Ansari MO, Lee J, Cho MH. Biogenic synthesis, photocatalytic, and photoelectrochemical performance of AgZnO nanocomposite. J Phys Chem C 2013;117:27023e30. [32] Zak AK, Majid WHAbd, Abrishami ME, Yousefi R. X-ray analysis of ZnO nanoparticles by WilliamsoneHall and sizestrain plot methods. Solid State Sci 2011;13:251e6. [33] Kaur J, Kumar P, Sathiaraj TS, Thangaraj R. Structural, optical and fluorescence properties of wet chemically synthesized ZnO: Pd2þ nanocrystals. Int Nano Lett 2013;3e4:1e7.

20111

[34] Navale SC, Ravi V, Mulla IS. Investigations on Ru doped ZnO: strain calculations and gas sensing study. Sens Actuat BChem 2009;139:466e70.  n L, Morales J. A high-capacity anode for [35] Caballero A, Herna lithium batteries consisting of mesoporous NiO nanoplatelets. Energy Fuels 2013;27:5545e51. [36] Zhong JB, Li JZ, He XY, Zeng J, Lu Y, Hu W, et al. Improved photocatalytic performance of Pd-doped ZnO. Curr Appl Phys 2012;12:998e1001. [37] Kuo CL, Wang RC, Huang JL, Liu CP, Lai YF, Wang CY, et al. ZnO nanorods with two spatially distinct light emissions. Nanotechnology 2008;9:285703 (6pp). [38] Maicu M, Hidalgo MC, Colon G, Navio JA. Comparative study of the photodeposition of Pt, Au and Pd on presulphated TiO2 for the photocatalytic decomposition of phenol. J Photochem Photobiol A 2011;217:275e83.  n JJ, Barreroa CA, Punnoose A. Understanding the role [39] Beltra of iron in the magnetism of Fe doped ZnO nanoparticles. Phys Chem Chem Phys 2015;17:15284e96. [40] Jagannatha Reddy A, Kokila MK, Nagabhushana H, Chakradhar RPS, Shivakumara C, Rao JL, et al. Structural, optical and EPR studies on ZnO: Cu nanopowders prepared via low temperature solution combustion synthesis. J Alloys Compd 2011;509:5349e55. [41] Jagannatha Reddy A, Kokila MK, Nagabhushana H, Shivakumara C, Chakradhar RPS, Nagabhushana BM, et al. Luminescence studies and EPR investigation of solution combustion derived Eu doped ZnO. Spectrochim Acta A 2014;132:305e12. [42] Leong KH, Chu HY, Ibrahim S, Saravanan P. Palladium nanoparticles anchored to anatase TiO2 for enhanced surface plasmon resonance-stimulated, visible-light-driven photocatalytic activity. Beilstein J Nanotechnol 2015;6:428e37. [43] Mohapatra SK, Kondamudi N, Banerjee S, Misra M. Functionalization of self-organized TiO2 nanotubes with Pd nanoparticles for photocatalytic decomposition of dyes under solar light illumination. Langmuir 2008;24:11276e81. [44] Ong WL, Natarajan S, Kloostra B, Ho GW. Metal nanoparticleloaded hierarchically assembled ZnO nanoflakes for enhanced photocatalytic performance. Nanoscale 2013;5:5568e75. [45] Udawatte N, Lee M, Kim J, Lee D. Well-defined Au/ZnO nanoparticles composites exhibiting enhanced photocatalytic activities. ACS Appl Mater Interfaces 2011;3:4531e8.  pez R, Go  mez R. Band-gap energy estimation from diffuse [46] Lo reflectance measurements on solegel and commercial TiO2: a comparative study. J Sol-Gel Sci Technol 2012;61:1e7. [47] Lu J, Wang H, Dong S, Wang F, Dong Y. Effect of Ag shapes and surface compositions on the photocatalytic performance of Ag/ZnO nanorods. J Alloys Compd 2014;617:869e76. [48] Zhao W, Guo Y, Faiz Y, Yuan WT, Sun C, Wang SM, et al. Facile in-suit synthesis of Ag/AgVO3 one-dimensional hybrid nanoribbons with enhanced performance of plasmonic visible-light photocatalysis. Appl Catal B Environ 2015;163:288e97. € [49] Ozacar M. Equilibrium and kinetic modelling of adsorption of phosphorus on calcined alunite. Adsorption 2003;9:125e32. € [50] Ozacar M. Phosphate adsorption characteristics of alunite to be used as a cement additive. Cem Concr Res 2003;33:1583e7. € _ Adsorption of reactive dyes on calcined [51] Ozacar M, S‚engil IA. alunite from aqueous solutions. J Hazard Mater 2003;B98:211e24. [52] Tan LL, Ong WJ, Chai SP, Mohamed AR. Noble metal modified reduced graphene oxide/TiO2 ternary nanostructures for efficient visible-light-driven photoreduction of carbon

20112

[53] [54]

[55]

[56]

[57]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 2 0 1 0 0 e2 0 1 1 2

dioxide in to methane. Appl Catal B Environ 2015;166e167:251e9. Wu JJ, Tseng CH. Photocatalytic properties of nc-Au/ZnO nanorod composites. Appl Catal B Environ 2006;66:51e7. Zamiri R, Rebelo A, Zamiri G, Adnani A, Kuashal A, Belsley MS, et al. Far-infrared optical constants of ZnO and ZnO/Ag nanostructures. RSC Adv 2014;4:20902e8. Zhang Z, Yates Jr JT. Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces. Chem Rev 2012;112:5520e51. Zhao X, Wu Y, Hao X. Electrodeposition synthesis of Au-ZnO hybrid nanowires and their photocatalytic properties. Int J Electrochem Sci 2013;8:3349e56. Chiou JW, Ray SC, Tsai HM, Pao CW, Chien FZ, Pong WF, et al. Correlation between electronic structures and photocatalytic

[58] [59]

[60]

[61]

activities of nanocrystalline (Au, Ag, and Pt) particles on the surface of ZnO nanorods. J Phys Chem C 2011;115:2650e5. Xie J, Wu Q. One-pot synthesis of ZnO/Ag nanospheres with enhanced photocatalytic activity. Mater Lett 2010;64:389e92. Sun F, Tan F, Wang W, Qiao X, Qiu X. Facile synthesis of Ag/ ZnO heterostructure nanocrystals with enhanced photocatalytic performance. Mater Res Bull 2012;47:3357e61. Jin Y, Xi J, Zhang Z, Xiao J, Xiao F, Qian L, et al. An ultra-low Pd loading nanocatalyst with efficient catalytic activity. Nanoscale 2015;7:5510e5. Gao S, Jia X, Yang S, Li Z, Jiang K. Hierarchical Ag/ZnO micro/ nanostructure: green synthesis and enhanced photocatalytic performance. J Solid State Chem 2011;184:764e9.