Synthesis and Characterization of Rhodium Doped on TiO 2 / HCP for Enhanced Photocatalytic Performance on Pentachlorophenol

Visible-light-responsive material based on Rhodium doped on titanium dispersed on dealuminated clinoptilolite (TiO 2 /HCP) was synthesized via a combination of the sol-gel method and photoreductive deposition technique. The photocatalyst surface characterization, structural and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), and UV-visible spectra (UV-VIS). Doping TiO 2 /HCP with Rh imparts a red shifting of the absorption band into the visible light region according to UV-VIS. The prepared composite materials were evaluated for their photocatalytic activities on pentachlorophenol (PCP) degradation under sunlight irradiation. The Rhodium doped TiO 2 /HCP exhibited enhanced photocatalytic activity and can be considered as a potential photocatalyst in wastewater treatment.


Introduction
Metal oxide semiconductor based on heterogeneous photocatalysis has been receiving wide attention under "advanced oxidation processes" (AOPs) for removal of organic pollutant in wastewater [1][2][3].TiO 2 (with band gap energy of 3.2 eV) is the most employed metal oxide semiconductor in the area of heterogeneous photocatalysis and this is a result of its low toxicity as well as its efficient capability as a photocatalyst for the treatment of wastewater containing organic micropollutant [4][5][6][7].TiO 2 exists in 3 crystal forms, namely, anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) [8].Anatase TiO 2 is vastly used as compared to other crystal forms, which is ascribed to its low recombination rate of the photogenerated electronhole pairs.However, challenges such as rapid recombination of electron-hole pairs due to TiO 2 wide band gap, poor recycling, and low adsorptive power, limit its activity towards the degradation of organic contaminants.In addition, solar light utilization by TiO 2 is less than 5%, which overall limits TiO 2 efficient application in the visible region of the solar spectrum.
These challenges led to the development of photocatalyst material that display photoresponses in the visible region and can also be separated from aqueous solutions after degradation activities.In view of this, studies revealed that doping or codoping of TiO 2 with noble metals, transition metals, nonmetals, and rare earth metals does enhance TiO 2 photocatalytic activities and extends TiO 2 photoresponse into the visible light region of the solar spectrum [9][10][11][12].Doping with other metal oxide semiconductors with narrow band gap [13] and with dye sensitized materials [14] has also influenced the enhancement of TiO 2 photocatalytic strength.Doping reduces the recombination rate via separation of electron-hole pairs (e − and h + ) and also acts as charge trapping sites, which further improves overall photocatalytic performance of the photocatalyst material.However, doping with transitional metals can result in the formation of defect states, which in turn reduces the mobility of photogenerated electron and holes and invariably limits the photocatalytic rate.The application of platinum group metals (Pt, Os, Pd, and Rh) however inhibits the recombination rate effectively and they are capable of enhancing photoactivity of composite material by increasing the lifetime of the holes.
Immobilization of TiO 2 on porous support materials enhances the photocatalytic activity of TiO 2 and also solves the issue of poor recycling of titania nanoparticles in solution.Several studies giving rise to positive conclusions have been conducted involving the immobilization of TiO 2 on different support materials like carbon, clay, magnetite core, silica, pumice stone, and zeolites [15][16][17][18][19][20][21].However, some of these support materials are less photoactive for organic pollutant removal as compared to TiO 2 alone [22].Zeolite (threedimensional aluminosilicates material) ranks best as compared to other support materials [23] applied in photocatalytic degradation of organic pollutants.Their large surface area [24][25][26] and crystallize size promote mass transfer of photogenerated electrons on the photocatalyst surface, which invariably assists in fast charge kinetics.Zeolites with their intrinsic properties possess cages and channels which assist in confining organic micropollutant molecules on their surface hence aiding enhanced photocatalytic activities.Zeolites ability to act as an electron donor or acceptor depends on their adsorption sites, an intrinsic ability that helps in the reduction of electron-hole pair recombination rates when TiO 2 is encapsulated on their surface [27,28].
Zeolites as porous material can be natural or synthetic.Natural zeolites possess vital properties such as smaller channels and porous structure and are also cost-effective as compared to the synthetic zeolites [29,30].For natural zeolites to be at par with synthetic ones, their physicochemical properties have to be fine-tuned via ion exchange with simple inorganic salts and alkali bases, acid leaching, steaming, and high temperature calcination [31].Dealumination or decationisation of natural zeolites influences exchange and removal of trace amount of Mg 2+ , Na + , Ca 2+ , and K + embedded in the cavities of natural zeolites.Dealumination enhances reformation of pore structure, increases the surface area of natural zeolites, and also contributes significantly to the removal of amorphous aluminum species [32].Though reports on dispersion of TiO 2 on dealuminated clinoptilolite are few, several studies have been conducted by doping of a noble or transition metal on clinoptilolite with TiO 2 .The photoactivity of Pt-modified TiO 2 loaded on a natural zeolite on methyl orange (MO) was conducted [33].Lazau et al. created a hybrid material based on clinoptilolite [34] via solid state reaction for degradation and mineralization of humic acid under ultraviolet and visible irradiation.Wang et al. synthesized natural zeolite supported Cr-doped TiO 2 photocatalyst for MO degradation [35].
Clinoptilolite (CP), the most abundant zeolite in nature, isostructural with heulandite, is employed as sample zeolite in this study.Rhodium (noble metal) doped on titania, dispersed on dealuminated clinoptilolite (TiO 2 /HCP), is synthesized via a combination of the sol-gel method and the photoreductive deposition technique.The composite material is further characterized using X-ray diffraction, FTIR spectroscopy, UV-VIS spectroscopy, SEM/EDAX, and thermal gravimetric analysis.The photocatalytic activities of prepared composite material were carried out on pentachlorophenol (organic contaminant).

Materials and Physical Measurements.
Reagents and solvents were of analytical grade and were used as purchased from Sigma Aldrich without further purification.The crystalline structure of the synthesized photocatalyst was characterized using Bruker diffractometer AXS with Cuk source.The surface morphology of the samples was examined using scanning electron microscopy equipped with an EDAX system for energy-dispersive spectroscopic analysis, which was coated with a thin layer of gold film to avoid charging.The bond vibrations were analyzed on Fourier transform infrared (FTIR) spectrophotometer (Perkin Elmer spectrum 400) and the FTIR spectra were in the range of 600-4000 cm −1 .UV-visible reflectance spectroscopy (UV-VIS) of prepared photocatalyst was recorded on the Perkin Elmer spectrometer in the spectral range of 200-800 nm to show its optical properties.Thermal gravimetric analysis (TGA) was carried out using Perkin Elmer STA 6000 thermal simultaneous analyzer at a heating rate of 10 ∘ C/min from 30 to 900 ∘ C under 19.8 mL/min in nitrogen purge stream.

Preparation of HCP and TiO
2 /HCP.Natural clinoptilolite (CP) was crushed in an agate mortar and sieved to less than 100 m particle size fractions.For dealumination experiment, sieved CP was added to 1 M NH 4 Cl solution which was continuously stirred at 60-90 ∘ C for 24 h to achieve the desired ion-exchange process [36].The resultant mixture was filtered and washed well with deionized water until the supernatant was neutral and free of chloride ions using silver nitrate.The obtained sample was air dried at 120-150 ∘ C for 8 h, ground to fine powder, and further calcined in the muffle oven at 550 ∘ C for 8 h, required for activation of the dealuminated clinoptilolite (HCP).TiO 2 /HCP was synthesized via a sol-gel method.Titanium-n-butoxide was added slowly to HCP suspended in ethanol solution, with vigorous stirring at room temperature for 3 h.A mixture of ethanol and water solution (1 : 1) was further added to hydrolyze titanium (IV)-n-butoxide adsorbed on HCP under continuous stirring for 3 h.The resultant mixture was filtered and washed with deionized water until the supernatant was neutral.The gel material was air dried in the oven at 110-120 ∘ C overnight.The obtained solid was crushed to fine material and further calcined in a muffle oven at 550 ∘ C for 4 h to achieve TiO 2 /HCP (5%), which was stored in the dark before being structurally characterized using analytical methods.

Doping of Rhodium on
TiO 2 /HCP.Rh doped on TiO 2 / HCP was conducted in accordance with the photoreduction process [37] which proceeds with the addition of Rhodium (III) chloride to a mixture of ethanol.The resultant solution was added to a stirred solution of TiO 2 /HCP dissolved in ethanol.The mixture was further irradiated with UV-visible light radiation within 1 h for the photoreduction process, which enhances the penetration of Rh metal within the titania matrices.The product was washed well with ethanol and water and air dried in the oven overnight.The dried material was further pulverized to a fine powder and calcined in a muffled oven at 400 ∘ C to obtain the final product, which was stored in the dark before the elucidation of structural properties.The percentages of Rh doped on TiO 2 /HCP were 0.5%, 1.5%, and 2.5%, with the samples labeled as 0.5% Rh-TiO 2 /HCP, 1.5% Rh-TiO 2 /HCP, and 2.5% Rh-TiO 2 /HCP, respectively.

Results and Discussion
3.1.XRD Analysis.The crystalline structure of the prepared photocatalyst which transforms from an amorphous to anatase or rutile phase after calcination was determined by means of X-ray powder diffraction to depict the effect of titania and Rh loadings on the structure of dealuminated clinoptilolite HCP.A characteristic XRD pattern of prepared composite material (5% TiO 2 /HCP, 0.5% Rh-TiO 2 /HCP, 1.5% Rh-TiO 2 /HCP, and 2.5% Rh-TiO 2 /HCP) is shown in Figure 1(a).The distinctive crystalline peaks of clinoptilolite CP which exist at 10 ∘ , 11.17 ∘ , 13.08 ∘ , 17.3 ∘ , 23.17 ∘ , 30 ∘ , and 32.74 ∘ are attributed to distinctive thermal stabilization of natural CP after loading of TiO 2 and Rhodium on HCP surface [38,39].These peaks of CP were in good agreement with standard JCPDS file number 025-1349.However, few characteristic peaks of anatase TiO 2 exist at 25.3 ∘ , 36.94 ∘ , and 37.8 ∘ , which is in good agreement with standard JCPDS file number 021-1272, and also weak rutile peak exist at 27.53 ∘ .These peaks occur from the partial dispersion of TiO 2 on the surface of HCP after calcination of photocatalyst in air.
The XRD result shows that doping of Rh into the TiO 2 /HCP matrices did not affect the crystalline structure of HCP, due to lower concentration of Rh metal doped in the composite material.Figure 1(b) depicts the XRD patterns of the composite materials calcined at different temperatures from 400 to 700 ∘ C. Though, studies reveal that calcination temperature improves crystallization of TiO 2 and accelerates formation of anatase and rutile phase [34].With increasing calcination temperature, the anatase phase at 101 reflection plane (25.3 ∘ ) becomes narrower which is ascribed to accelerated growth of anatase crystallites.However as the calcination temperature increases from 600 to 700 ∘ C, a rutile peak at 43.68 ∘ is detected and is in good agreement with standard JCPDS file number 021-1276.However, as the magnification of SEM images increases for 1.5% Rh-TiO 2 /HCP, the images become cloudy and with irregular distribution of titania nanoparticles and Rhodium metal on the surface of dealuminated clinoptilolite as shown in Figures 3(a)-3(d).
EDAX microprobe through SEM image gave an elemental analysis of the composite material, showing relative distribution of titania nanoparticles and Rh metal that exists on the composite material surface and confirms the high purity of the photocatalyst.The presence of the main elements without unexpected elements being detected shows that the composite material is to a great extent in its pure form, free of contaminated elements.From Figure 4, a successful doping of Rh on TiO 2 /HCP surface via photochemical reduction process is observed and it is due to the sol-gel method employed.This also justifies the presence of titania on HCP surface.

Structural Properties.
FTIR spectra of prepared photocatalyst are shown in Figure 5.The FTIR data gives us an idea of the functional groups present in the composite material and how the effect of Rh loading rate on TiO 2 /HCP affects its composition.The characteristic intensity peak of OH group and molecular water of zeolite in the prepared photocatalyst are observed between 3400 and 3600 cm −1 [40].There was a considerable increase in intensity of OH peaks when Rh loading rate increases from 0.5 to 2.5%, an indication of an increase in surface OH groups.The distorted OH bending at 1640-1670 cm −1 (Lewis sites) due to the zeolite water is also present in the prepared photocatalyst material [41] and this surface hydroxyl group on the composite material surface does enhance the photocatalytic activity on organic pollutants.
A characteristic stretching vibrational peak at 1040 cm −1 , ascribed to a band as a result of the Si-O-Si present in the tetrahedral natural zeolite framework, is an evidence of the composite materials.Strong peaks at 790 cm −1 are attributed to the vibration of Ti-O and the Ti-O-Ti bridging stretching mode.The intensity of these bands increases as loading rate of Rh increases and is indicative of the spread of titania on the surface and on the channels of HCP but not within the pores of HCP during sol-gel process.No strong peak of the TiO 2 bounded to the dealuminated clinoptilolite was observable at 900-960 cm −1 for the composite materials unfortunately.There was, however, an impartial influence of Rh doping on TiO 2 /HCP as loading rate increases within the composite material proposed, as much as no characteristic peak of Rh was detected in the FTIR spectra (Figure 5).

Thermal Gravimetric Analysis.
Thermal gravimetric analysis (TGA) of prepared photocatalyst is displayed in Figure 6 and gives an ideaof the thermal stability and to an extent the purity of the composite material.The material undergoes simultaneous weight loss in the range of 30-800 ∘ C in accordance with similar observation in the literature [42].The total weight loss observed up to 800 ∘ C for TiO 2 /HCP and 1.5% Rh-TiO 2 /HCP was 32.73 and 35.38%, respectively.The weight loss in both materials occurred below 200 ∘ C which is ascribed to loss of inter layered bounded water molecules in the composite materials, which corresponds to the broad endothermic peak in the DTA curve.The second weight loss is observed below 500 ∘ C, which is as a result of the loss of coordinated water molecules from the parent material and decomposition of organic chemical species present within the composite material.However, at temperature above 600 ∘ C, the corresponding weight loss for TiO 2 /HCP and 1.5% Rh-TiO 2 /HCP is ascribed to phase transformation for the former, while decomposition by chloride of the Rhodium metal is ascribed to weight loss for the latter.The same pattern is observable in the DTA curve for TiO 2 /HCP in Figure 6(b), while complete disappearance after 200-400 ∘ C for 1.5% Rh-TiO 2 /HCP is observable.The composite materials possess similar thermal properties which may be due to the good synergy interaction of Rh metal and TiO 2 /HCP, as this improves overall thermal stability of the photocatalyst.

Optical Properties. Optical properties and absorption
wavelength with respect to the band gap of prepared photocatalyst were determined with UV-VIS spectroscopy.Figure 7 shows the UV-VIS absorption spectra of prepared catalyst, with Rh-TiO 2 /HCP partly red shifted into the UVvisible region (405 nm) as compared to TiO 2 /HCP with absorption at 390 nm.This increased absorption band in the visible region is ascribed to excitation of 4d electrons of Rh ions incorporation inside matrices of TiO 2 /HCP [43], which in turn affects the electronic and crystal structure of  the composite material.Doping of Rh on TiO 2 /HCP and dispersion of titania on the mesoporous material further assist in overcoming fast recombination of electron-hole pair and facilitate movement of photogenerated electrons.The red shifting of absorption edge depicts a good interaction between Rh and TiO 2 /HCP and the band gap is calculated from   = 1240/ [44].The reduction of band gap for the composite materials (Table 1) as compared to the titania (3.20 eV) is obvious and this emphasizes that the doping of Rh on TiO 2 /HCP enhances absorption of the composite material and shows absorption into the visible light region.

Photocatalytic Activity on PCP.
The photocatalytic degradation efficiency of the composite materials, TiO 2 , and the solar light effect on the PCP solution under sunlight The improved photocatalytic activity of Rh-TiO 2 /HCP can be ascribed to intrinsic adsorbent properties of HCP and enhanced specific surface area of HCP by gathering PCP to its active surface.However, rapid generation of OH radicals on the photocatalyst surface via dispersion of TiO 2 on HCP aids in the adsorption of PCP and also the oxidation process of adsorbed PCP on the photocatalyst surface under solar light irradiation.The enhancement of the photocatalytic activity of Rh-TiO 2 /HCP on PCP can also be ascribed to influence of Rh intrinsic ability to capture the photoinduced electrons under solar light irradiation and inhibits their recombination with the holes.Rhodium metal facilitates movement of photogenerated electrons and separation of electron-hole (e − /h + ) pairs, and the electron from Rh metal is further transferred to adsorbed O 2 on TiO 2 surface to produce superoxide radical O 2 •− .HCP acts as a hole trap in reducing recombination activities of e − /h + pairs and assists in rapid transport of electron charge migration, which is corroborated with the electron acceptor and transporter properties of HCP.The hole reacts with water or hydroxyl to produce the hydroxide radical (OH • ).Both superoxide and hydroxyl radicals generated assist in decomposing PCP molecules into intermediate products [45].Rh metal induces great enhancement by the creation of Schottky junction between the metal and TiO 2 /HCP, which also improves charge separation and enhances the degradation of PCP effectively.

Conclusions
Rhodium metal doped on TiO 2 /HCP has being successfully synthesized via a combination of sol-gel method and photoreductive deposition technique.In accordance with XRD results, the photocatalyst was crystalline and was composed of anatase TiO 2 .The SEM reveals a uniform distribution of titania nanoparticles and Rhodium metal on the HCP surface.UV-VIS spectra confirmed that the photocatalyst absorption band was efficiently shifted into the visible light range and with reduction in band gap energy of the composite material.However, the HCP composite material large surface area and Rh doping enhance the photocatalytic activity on PCP, which further facilitate movement of photogenerated electrons and efficient separation of e − /h + pairs.The reported synthesized Rh-TiO 2 /HCP shows promising capability for degradation of pentachlorophenol under sunlight irradiation as compared to TiO 2 /HCP and sunlight irradiation.

3. 2 .
Surface Morphology and Elemental Properties.The micrograph structures of prepared photocatalyst are depicted in Figure 2, with uniform distribution of titania (Figure 2(a)) and Rh nanoparticles (Figure 2(b)) on the surface of HCP.There exists agglomeration (van der Waals interaction) of titania and Rh metal on the surface at a magnification of ×1000, although characteristic properties of HCP are unaffected, after loading of the TiO 2 and doping with Rh as shown by SEM image result.
Evaluation.The photocatalytic activities of prepared composites material were determined on the degradation of pentachlorophenol (PCP) in a simple cylindrical glass reactor (250 mL), conducted under direct sunlight radiation on top of the Chemistry Department Building, Vaal University of Technology (26 ∘ 69  S and 27 ∘ 84  E), by employing 0.1 g of photocatalyst suspended in 10 mg/L of PCP.The experiment was performed simultaneously with similar model pollutant under identical solar irradiance on a sunny day between the hours of 9.00 am and 3.00 pm.

Table 1 :
Wavelength and calculated band gap of prepared composite materials.