One-Pot Synthesis of Ru-Doped ZnO Oxides for Photodegradation of 4-Chlorophenol

ESIQIE, Instituto Politécnico Nacional, Av. Instituto Politécnico Nacional s/n, Col. Zacatenco, 07738 Mexico City, Mexico Departamento de Ciencias Básicas, Universidad Autónoma Metropolitana-Azcapotzalco, Av. San Pablo 180, Col. Reynosa-Tamaulipas, 02200 Mexico City, Mexico Departamento Química, Universidad Autónoma Metropolitana-Iztapalapa, San Rafael Atlixco 186, 09340 Mexico City, Mexico GDMyPQ, Eje Lázaro Cárdenas 152, Instituto Mexicano del Petróleo, 07730 Mexico City, Mexico College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang, China


Introduction
Although most water effluents are effectively treated by traditional biological and/or physicochemical methods, the presence of chlorinated compounds represents an increasing problem, since these compounds, for example, chlorinated phenol derivatives, are often mutagenic or carcinogenic. 4-Chlorophenol is one of the most dangerous substances as far as toxicity is concerned. Therefore, it is highly necessary to develop more effective processes for the destruction of such contaminants. Actually, the expanding demand for efficient and economical techniques for wastewater treatment is the origin of a wide spectrum of investigations and technological innovation [1][2][3][4][5]. Some new technologies such as the advanced oxidation process (AOP) have been applied for the elimination of organic compounds in water systems. In AOPs, the oxidation of organic compounds by the very reactive free radicals takes place on the surface of heterogeneous catalysts which has been proven to be of interest due to the efficient degradation of recalcitrant organic compounds in water [6][7][8].
Developed in the 1970s, photocatalytic oxidation has attracted considerable attention, and in recent years, numerous studies have been carried out on the applications of photocatalytic oxidation in the presence of a semiconductor oxide [9][10][11][12][13]. As a good candidate of photocatalyst, ZnO has a band gap energy (E g ) of 3.37 eV, which corresponds to light absorption in the UV region [14]; this E g value indicates that its photocatalytic properties are almost comparable with TiO 2 . Li et al. reported that the total photodegradation of chlorophenol and dichlorobenzene can be realized by using ZnO nanoparticles [15]. On the other hand, RuO 2 has been studied as a photocatalyst showing strong oxidation properties [16]. The catalytic role in the N 2 O decomposition over the Ru-doped FAU zeolite was also investigated [17]. Zhuiykova et al. reported that the electrocatalytic activity of ZnO-RuO 2 catalysts appears to be governed by the redox behavior of the active surface sites [18].
In the present work, we report the one-pot homogeneous coprecipitation method for the synthesis of a set of Ru-doped ZnO oxides with different Ru contents (0.5, 1, and 3 wt% of RuO 2 ). These catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron, and UV-Vis spectroscopic techniques. The textural properties including the surface area and pore volume were determined by the N 2 adsorption-desorption isotherms method. The crystalline structure of the samples was refined by the Rietveld refinement method, and the surface chemical composition was analyzed by the electron dispersive spectroscopic (EDS) technique. The formation of the surface • OH radicals was monitored by using fluorescence emission spectroscopy. The photocatalytic performance of the RuO x -ZnO catalysts with various RuO 2 contents for the degradation of 4chlorophenol was investigated, and the surface photoreaction mechanism was discussed.

Sample Preparation.
The RuO x -ZnO semiconductors were synthesized by the one-pot homogeneous coprecipitation method as follows: appropriate amounts of Zn(CH 3 COO) 2 (Baker, 99.99%) and RuCl 3 ·3H 2 O (Aldrich, 99.9%) solutions were mixed at a pH of~8.5 adjusted with an ammonia solution. The amount of RuCl 3 ·3H 2 O was calculated in order to obtain final solids with 0, 0.5, 1.0, and 3.0 wt% of RuO 2 . Afterwards, the mixture containing both metallic precursors was heated at 80°C under constant stirring for 2 h. The resultant precipitate was collected by filtration and washed with deionized water until a neutral pH in the effluent was reached, and finally, the wet solid was dried at 100°C in air for 6 h and calcined at 400°C in a muffle furnace for 12 h. These solids will be hereinafter referred to as xwt% RuO 2 -ZnO, where x = 0, 0.5, 1, and 3.

2.2.
Characterization. X-ray diffraction (XRD) patterns of the RuO 2 -ZnO solids were obtained by an Empyrean multipurpose research X-ray diffractometer (PANalytical) with a Cu-Kα radiation (1.5418 Å) source. Diffraction intensity was measured between 4°and 70°with a 2θ step of 0.02°. The crystalline structures of the catalysts were refined using the Rietveld method. The JAVA-based software named Materials Analysis Using Diffraction (MAUD) was employed for refining each XRD pattern [19]. The atomic fraction coordinates for ZnO are reported in Table 1.
The textural properties of the samples were obtained by nitrogen adsorption-desorption isotherms at −196°C in a Quantachrome Autosorb-1C instrument. The surface area was calculated by the Brunauer-Emmett-Teller (BET) equation. All samples were degassed at 300°C for 6 h before N 2 adsorption.
Raman spectra were recorded at room temperature on previously calcined samples using an ISA Labram micro-Raman apparatus. The excitation line of a He-Ne laser was 632 nm. To avoid thermal effects, the laser power on the samples was kept at~1 nW.
X-ray photoelectron spectroscopy (XPS) was used to analyze the surface metal oxidation state of the samples. The XPS spectra were acquired with a THERMO Scientific K-Alpha spectrometer, equipped with a hemispherical electron analyzer and Al Kα X-ray source (1486.6 eV).
Scanning electron microscopy analysis (SEM) was carried out on a Quanta 3D FEG microscope. The microscope has detectors for surface chemical analysis by X-ray energy dispersive spectroscopy (EDS).
The band gap energy (E g ) of the catalysts was measured using a Cary 100 spectrophotometer with an integration sphere, and MgO was used as 100% reflectance patron. The band gap energy values were determined using the Kubelka-Munk function by the method.
Fluorescence emission spectra of 2-hydroxyterephthalic acid were measured on a SCINCO fluorescence spectrometer FS-2. The • OH radicals generated by the semiconductor material in the absence of 4-chlorophenol were measured with the following procedure: terephthalic acid (TA) (5 × 10-4 M) was dissolved in a water/NaOH solution (2 × 10 −3 M). Then 200 mg of the photocatalysts was added and the suspension was stirred for 60 min under dark conditions. Afterwards, it was irradiated by a UV light Pen-Ray lamp with a wavelength of 254 nm (I0 = 4.4 mW cm −2 ), during 60 min, and aliquots were taken each 5 min. The fluorescence emission spectra of the irradiated solution were analyzed by PL (excited at 320 nm).
2.3. Photocatalytic Activity. The photocatalytic degradation of 4-chlorophenol in water was performed at room temperature (~25°C) using a solution of 4-chlorophenol in water (80 ppm) employing a Pyrex reactor (500 mL). 200 mg of the catalyst with particle size 60-80 meshes was used for the photodegradation reaction. The mixture was under stirring and bubbling with air (1 mL/s) generated by a magnetic stirring and electric pump at room temperature (approximately 30°C). The mixture prepared was exposed to UV radiation for 6 h. The irradiation source was a UV Pen-Ray lamp (λ = 254 nm and intensity = 4400 μW/cm 2 ), protected with a International Journal of Photoenergy quartz tube whose outer and inner diameters were 0.5 and 0.35 in nm, respectively. The 4-chlorophenol photooxidation reaction procedure was monitored via following the variation of the characteristic absorption band at 221 nm using the UV-Vis spectrophotometer (Varian Cary 100 UV-Vis). The 4-chlorophenol concentration was monitored by taking aliquots of 3 mL every 15 min of reaction. The aliquots were filtered with a 0.45 μm nylon filter to eliminate any suspended solids.

Results and Discussion
3.1. Crystalline Structure and Rietveld Refinements. The XRD patterns of the samples calcined at 400°C are shown in Figure 1. All the diffraction peaks can be indexed to the hexagonal Wurtzite structure of ZnO (JCPDS card 36-1451) [20,21]. XRD peaks corresponding to RuO 2 and changes in the crystalline structure of ZnO are not observed; thus, it can be assumed that the ruthenium oxide like RuO 2 is not detectable since its quantity is very small; perhaps, it is dispersed uniformly on the surface of ZnO with a crystallite size smaller than 4 nm. The crystalline structures of the solids were refined with the Rietveld refinement technique. A plot of the Rietveld refinement for the 3 wt% RuO 2 -ZnO catalyst is shown in Figure 2. The crystallite size, lattice cell parameters, and R wp are reported in Table 2. The crystallite sizes determined by Scherrer's equation and obtained by the Rietveld refinement were 60.6, 75.9, 69.5, and 61.8 nm for the 0RuO 2 -ZnO, 0.5RuO 2 -ZnO, 1RuO 2 -ZnO, and 3RuO 2 -ZnO samples, respectively. It seems that Ru addition led to a slightly greater ZnO crystallite size, even if this effect becomes weak for the higher Ru content sample. Ru doping probably promotes the formation of surface hydroxyls which may gain linkage degree with the hydroxide units during the synthesis, leading to a slight enlargement of the crystallite size. The lattice cell parameters of ZnO with a hexagonal wurtzite structure varied: a = b from 3.2903 to 3.4098 Å and c from 5.2219 to 5.4039 Å, depending on the Ru content.
3.2. Raman Spectroscopy. The Raman spectra of the samples were obtained in the wavelength interval 300-550 cm −1 , and they are shown in Figure 3(a). The Raman bands at 330, 380, and 437 cm −1 are assigned to ZnO with a wurtzite (hexagonal) structure belonging to the P63mc space group. The dominant and sharp peak at 332 cm −1 is due to the secondorder Raman spectrum, and the peak labeled as E 2 at 437 cm −1 is known as the Raman active phonon mode, which is a characteristic of the Wurtzite hexagonal phase of ZnO.
In Figure 3(b), the Raman modes of E g , A 1g , and B 2g at 528, 646, and 716 cm −1 , respectively, are shown, corresponding to RuO 2 in 1RuO 2 -ZnO and 3RuO 2 -ZnO samples. The intensity of Raman peaks increases with the increasing of the RuO 2 content [22,23]. Although Ru oxides were not detectable by XRD analysis, Ru-O bonds are clearly present in the surface of the catalysts. Therefore, the Raman spectroscopy is more sensitive for identifying the surface Ru-O bonds.

XPS Characterization.
The chemical oxidation states of Ru and Zn in the Ru-doped ZnO samples were analyzed by the XPS technique. The Ru 3d core levels in the XPS spectra of all samples are presented in Figure 4. By deconvoluting with software (XPS PEAK 4.1), two doublets in XPS spectra were determined: the peaks at 280.9 and 285.0 eV are assigned to Ru 3d 5/2 and Ru 3d 3/2, respectively, indicating that Ru is in the oxidation state Ru 4+ (RuO 2 ) [24]. The other two small peaks at 282.5 and 286.3 eV correspond to Ru 6+ (ReO 3 ). The intensity of these peaks increases with increasing content of RuO 2 .
The XPS spectra of Zn 2p core levels are shown in Figure 5, and they exhibited two symmetric peaks centered at 1022.0 and 1044.7 eV, which are attributed to Zn 2p 3/2 and Zn 2p 1/2, respectively, indicating the Zn 2+ oxidation state [25]. Figure 6 shows the asymmetric O1s spectra on the surface of 0RuO 2 -ZnO, 0.5RuO 2 -ZnO, 1RuO 2 -ZnO, and 3RuO 2 -ZnO samples. For the pure ZnO sample, the O1s signal was deconvoluted into two peaks at 530.6 eV and 532.6 eV, corresponding to the lattice O 2− in the Zn-O bond and surface oxygen in hydroxyls (OH) [26]. The surface atomic concentrations were obtained from the XPS quantitative analysis and are reported in Table 3. The surface Ru/Zn atomic ratio increases from 0.081 to 0.093 and 0.207 as the RuO 2 nominal content increases from 0.5 wt% to 1 and 3 wt%. The O/(Zn + Ru) atomic ratio varies from 1.12 for 0.5RuZnO to 1.40 for 1RuZnO and 2.84 for 3RuZnO. Therefore, the surface oxygen concentration clearly gains as the Ru content increases. Ru doping led to oxygen species rich in the surface of the RuO 2 -ZnO solids. Figure 7 shows the SEM images and the selective EDS spectra for ZnO and RuO 2 -ZnO solids. For the pure ZnO, nanorods with uniform size were obtained (Figure 7(a)). These nanorods did not present a preferential alignment but had a random distribution. The length of the rods varied from 300 to 500 nm. In the sample with 0.5 wt% of RuO 2 (Figure 7(c)), the morphology is slightly modified as the nanorods become thinner and shorter. When the RuO 2 content increases to 1 wt% (Figure 7(e)), the length of nanorods ranges between 100 and 500 nm in length. Finally, with a greater amount of RuO 2 (Figure 7(g)), the morphology was more homogeneous and the nanorods had a length of approximately 100 nm. So doping with a proper amount of Ru may lead to the formation of short and homogeneous ZnO nanorods.

SEM Observation and EDS Analysis.
The surface elemental composition analysis of each sample was performed by the EDS technique. Signals belonging 3 International Journal of Photoenergy to Zn, Ru, C, and O elements are observed (see Figures 7(b), 7(d), 7(f), and 7(h)). Figure 7(b) only shows the signals of Zn and O with carbon traces. Carbon was probably formed from the acetate precursor used in the ZnO synthesis. Figures 7(d), 7(f), and 7(h) show the chemical composition of the xRuO 2 -ZnO. A small peak for Ru appears which increases in intensity as the content of RuO 2 increases. It is also noted that as the RuO 2 content increases, the peak corresponding to oxygen increases in agreement with the results of XPS analysis. The increment of the surface oxygen content is important as the oxygen species usually take part in the oxidation reactions.     International Journal of Photoenergy 11~38 m 2 /g. When the RuO 2 content is greater, the specific surface area is bigger. The 3 wt% RuO 2 -ZnO sample shows the biggest specific surface area, three times greater than that obtained for pure ZnO. According to the above results, we can assume that the variations on the BET surface area are due to the changes of the structure resulted from the modification of RuO 2 doping.
3.6. Band-Gap Energy Measurement. Figure 8 shows the plots of the Kubelka-Munk function ( F R 1 E 1/2 ) against the absorbed light energy (E). The optical band gap energies (E g ) of each sample can be estimated by extrapolating the slope to cut the abscissa axis [28][29][30]. The presence of RuO 2 (0.5, 1, and 3 wt %) produces to a red-shift of the absorption band towards the visible region. The band gap energy for pure ZnO was approximately 3.20 eV, which is close to report in the literature [31,32]. Several previous investigations suggest that the visible emission is due to the surface defects and oxygen vacancies in the crystalline structure of the ZnO nanomaterials [33,34]. These RuO 2 -ZnO catalysts show a continuous absorption in the visible range and the absorption is stronger when the RuO 2 content is greater. The estimated band gap energies decrease from 3.20 eV for pure ZnO to 3.07 eV for 3 wt% RuO 2 -ZnO. The E g data obtained from Figure 8 are reported in Table 4.

Photocatalytic Activity Test.
To investigate the photocatalytic activity, the RuO 2 -ZnO samples were evaluated in the photocatalytic degradation of 4-chlorophenol in aqueous solution. Figure 9 illustrates a set of UV-Vis absorption spectra of 4-chlorophenol molecule at different intervals of reaction time for the 3 wt% RuO 2 -ZnO catalyst. The maximum absorption bands of 4-chlorophenol appear at 221 and 277 nm under UV irradiation. These absorption bands gradually decrease in intensity within 150 min, indicating that 4chlorophenol was photodegradated. The influence of the content of RuO 2 on the photodegradation efficiency (noted as C/C 0 ) of 4-chlorophenol is presented in Figure 10. The Ru-doped ZnO catalysts exhibit an increased photocatalytic activity compared to pure ZnO. After 225 min of reaction time, the photodegradation efficiency of 4-chlorophenol pollutant is about 46%, 61%, 70%, and 92% for 0RuO 2 -ZnO, 0.5RuO 2 -ZnO, 1RuO 2 -ZnO, and 3RuO 2 -ZnO, respectively.

Determination of • OH Radical Generation and Reaction
Mechanism. In order to understand the mechanism of the photodegradation of 4-chlorophenol, it is important to obtain information about the formation of free radicals, especially the formation of • OH radicals, which have a high oxidative power and great capacity for degradation of organic contaminants in an aqueous medium. The • OH radicals can be monitored by the fluorescence spectrum resulting from the formation of 2-hydroxy terephthalic acid by the • OH radicals reacting with terephthalic acid when the solid was irradiated with ultraviolet light [35,36]. The most active catalyst 3 wt%RuO 2 was measured by comparison with TiO 2 -P25 in the absence of 4-chlorophenol. The  5 International Journal of Photoenergy photolysis performance was also obtained in the absence of any catalyst. Figure 11(a) shows the fluorescence spectra obtained with the 3RuO 2 -ZnO and TiO 2 P25 catalysts. The peak at 454 nm indicated the amount of produced 2-hydroxy terephthalic acid or the amount of • OH radicals produced. The peak intensity clearly increases as a function of irradiation time. Both 3RuO 2 -ZnO and TiO 2 P25 show almost the linear relationship between the band intensity and the irradiation time, but 3RuO 2 -ZnO exhibits a greater concentration of • OH radicals on the surface in comparison with TiO 2 P25 during the irradiation procedure. Without the catalyst, the photolysis produces a rather low concentration of hydroxyl terephthalic acid, indicating the low concentration of • OH radicals.
The reaction mechanism considered for heterogeneous photocatalysis usually consists of two steps: the fast adsorption of the reactants on the photocatalyst surface, followed by the slow surface reaction of the adsorbed reactants with photo-generated radicals (e.g., O 2 − , • OH) to generate the The surface of RuO 2 -ZnO solids presents a high degree of surface hydroxylation. For instance, the catalyst with 3 wt%RuO 2 presents a high concentration of surface hydroxyl species as evidenced by the XPS spectra in Figure 6 and by the fluorescence spectroscopy in Figure 11, and therefore, it shows the highest concentration of surface active oxygen     International Journal of Photoenergy species and thus the best photoactivity. All catalysts with RuO 2 exhibit higher reaction rates than the pure ZnO. Perhaps, there is a synergistic interaction between RuO 2 and ZnO that favors the improvement of photocatalytic activity. It is assumed that a heterojunction structure was created in the interface of the ZnO and RuO 2 nanoparticles. Ru doping strongly affects the surface properties of semiconductors by generating a "barrier" in the ZnO which acts as an electron trap of the Ru metal in contact with the ZnO semiconductor surface. The electrons in the Ru conduction band (CB) transferred to the ZnO CB under the force of the electronic field of the heterojunction structure [37]. Thus, the surface of the ZnO was more negative and favors the formation of more hydroradicals. Therefore, Ru doping promotes the active surface oxygen formation via electron transfer. All these may inhibit the recombination rate of the electronholes and benefit the photodegradation of 4-chlorophenol. A proposed reaction mechanism is illustrated in Scheme 1.

Conclusions
This work showed that Ru doping for ZnO reduced the band gap energy of ZnO solid, generated more surface active oxygen species such as hydroxyls, O 2 2− , and O 2 − , and increased the surface area by modification of the crystalline structure and morphological features. In the photodegradation of 4chlorophenol solution, the RuO x -ZnO catalysts could effectively photodegrade the 4-chlorophenol contaminant with a much faster reaction rate than the pure ZnO. This can be explained by rich active oxygen species and high concentration of free radicals on the surface of the Ru-doped catalyst that may inhibit the electron-hole recombination rate and promotes the Ru 6+ /Ru 4+ ox-red cycles due to the electron transfer from the RuO x (RuO 3 and RuO 2 ) nanoparticles to the Zn-O bond. These results show that the one-pot homogeneous coprecipitation synthesis reported herein is a simple but very effective method for obtaining RuO 2 -ZnO nanocatalysts with high photocatalyic activity in the photodegradation of 4-chlorophenol.