Characterization of Newly Synthesized ZrFe₂O₅ Nanomaterial and Investigations of Its Tremendous Photocatalytic Properties under Visible Light Irradiation

Abstract

High functional ZrFe₂O₅ nanoparticles were synthesized using coprecipitation technique. The chemical composition of nanomaterials was studied by energy-dispersive X-ray (EDX). To observe the morphology, field emission scanning electron microscopy (FE-SEM) was used. X-ray diffraction (XRD) technique was utilized to appraise the structure of the synthesized material. The photocatalytic behavior of ZrFe₂O₅ nano-particles was investigated by measuring the degradation rate of toluidine blue O (TBO) dye in aqueous solution in the presence of ZrFe₂O₅ nano-particles under visible light irradiation. A steady decrease in absorption peak under visible light irradiation was observed by increasing exposure time. The degradation efficiency was observed as 92% after 140 min of exposure to visible light. Besides, ZrFe₂O₅ nanophotocatalyst could be recovered and recycled easily. The rate of TBO and total organic carbon (TOC) removal under visible light irradiation decreased by only 5% and 10%, respectively, after seven cycles of use, demonstrating the high photostability of the synthesized nano-photocatalyst material.

1. Introduction

Over the past few decades, environmental and water decontamination issues have become the foremost area of the scientific research [1–3]. To cope with such issues, there has been great interest among scientists in developing semiconductor photocatalysts with great prospective for environment protection applications such as air purification and water disinfection [1–7].

Among various photo-catalysts, transition metal oxides, such as TiO₂ [6] and NaNbO₃ [7], constitute a fascinating and promising class of semiconducting photocatalyst materials that have been widely studied for their photocatalytic activities under UV/Visible light. However, their industrial use in waste water treatment is limited due to their poor visible light absorption capability, reclaiming, and low quantum yield due to fast recombination of charge carriers generated by visible light irradiation [8]. In general, to utilize the visible part of the electromagnetic spectrum (𝜆>400 nm), the bandgap of a photo-catalyst material must be narrow (up to 3.0 eV), and the preferred range of ionic character is between 20 and 30% [9–12].

Searching for new types of potential photo-catalytic materials that can be exploited by solar irradiation particularly under visible light (𝜆>400 nm) along with providing better stability by separating the electron-hole pairs more effectively has become an imperative issue in current photocatalysis and environmental research areas [1–10]. In this regard, better photo-catalytic activity has been observed on a few semiconductor composites combining ZrFe₂O₅ with secondary semiconductor, such as TiO₂ under visible light irradiation [13].

Therefore, we were motivated to prepare ZrFe₂O₅ with the expectation of improved catalytic performance. Furthermore, to the best knowledge of authors, ZrFe₂O₅ has neither been synthesized, nor its photocatalytic behavior has been reported in the literature. Hence, bridging the research gap in photocatalysis using novel ZrFe₂O₅ nanomaterial is imperative. Hence, in the present work, we report synthesis of ZrFe₂O₅ nanoparticles by coprecipitation technique and the investigation of their photo-catalytic properties under visible light irradiation for the degradation of toluidine blue O dye.

2. Experimental

2.1. Materials

All the chemicals (ZrOCl₂·8H₂O, FeCl₃·6H₂O, NH₄OH, and toluidine blue O 85% dye contents) used in the synthesis were purchased from Sigma Chemical Co. (St. Louis, MO, USA) and were used without further purification.

2.2. Synthesis of ZrFe₂O₅ Nanoparticles

The ZrFe₂O₅ nanoparticles were synthesized by chemical co-precipitation technique [14], in which 50 mL solution A of ZrOCl₂·8H₂O was prepared by dissolving “a” grams in deionized water FeCl₃·6H₂O and solution B was prepared by dissolving “b” grams in 50 mL as shown in Table 1. Solutions A and B were mixed slowly and stirred for 30 min at 65°C. To precipitate chloride precursors, the pH of the solution was raised to 10 by adding 3.5 M NH₄OH dropwise with continuous stirring; this process took about 120 min. The resulting mixture was left stirred for another 60 min. The precipitate of ZrFe₂O₅ was filtered and washed with de-ionized water till chloride became free. The precipitate obtained was dried at 100°C for 90 min in an oven and calcined at 400°C for 4 hours in a muffle furnace.

Table 1 

Amounts of ZrOCl₂ · 8H₂O and FeCl₃ · 6H₂O in solutions A and B.

2.3. Characterization

The structural analysis of ZrFe₂O₅ nano-photocatalyst was performed using field emission scanning electron microscope (JEOL JSM 7401 F), energy-dispersive X-ray analysis, and X-ray diffractometer (D8 FOCUS 2220 Bruker AXS) with Cu Kα radiation (𝜆=1.5418 A°). Photocatalytic degradation studies were performed by using UV/Vis spectrophotometer (Shimadzu-3600, Japan).

2.4. BET Specific Surface Area Determination

BET specific surface area was determined by N₂ adsorption at (77 K) with a Micromeritics ASAP 2000 system following the overnight treatment to degas the sample in vacuum at 130°C.

2.5. Photocatalytic Activity Test

The reaction mixture was prepared by adding the ZrFe₂O₅ nanopowder catalyst (6 mg/L) to TBO dye solution having an initial concentration (𝐶_o) of 0.05 mM. The mixture was then shifted into a glass reactor, where it was stirred for 30 min in dark to attain the adsorption equilibrium [10]. After the adsorption-desorption process, the reaction mixture was irradiated with visible light under constant stirring in oxygen atmosphere. In the photoreaction, the mixture was exposed to visible light (𝜆≥510 nm) using a visible lamp (150 W) and a UV cut-off filter. After starting the irradiation process, 3 mL suspension was taken out (with 20 min time intervals) from irradiated mixture and immediately centrifuged at 4000 rpm for 5 min to separate the catalyst nano-particles from the suspension. Absorption of clear solution was taken using UV-Vis spectrophotometer (Shimadzu 3600, Japan) at 30°C and ambient pressure.

The percentage of degradation 𝐷% was calculated using equation [10]: 𝐷%=^∘𝐴−𝐴_∘𝐴×100, (1) where 𝐴^∘ and 𝐴 are the absorbance of the mixture before and after degradation, respectively.

2.6. Stability and Reusability Test

Photo-catalyst activity of same ZrFe₂O₅ nanomaterial was tested repeatedly for seven (7) times. After each use, photo-catalyst was separated, washed, dried and 3 mg of fresh photo-catalyst was added at the end of each cycle to compensate for the loss of catalyst during use.

2.7. Total Organic Carbon (TOC) Removal Test

TOC removal was estimated to check mineralization of TBO with ZrFe₂O₅ (Figure 6). The values of TOC were determined from illuminated mixture of 0.05 mM dye concentration and catalyst load of 6 mgL⁻¹ using TOC analyzer (Thornton 770 Max with 5000TOC sensor).

The degree of TBO mineralization was estimated by determining the decrease in TOC in the reaction solution [10].

3. Results and Discussion

3.1. Characterization of ZrFe₂O₅

FE-SEM images of the as-synthesized ZrFe₂O₅ nano-particles (ZrO₂ : Fe₂O₃ = 1 : 1) are shown in Figure 1. FE-SEM analysis showed that ZrFe₂O₅ nano-particles comprise a mean diameter of 30 nm. The chemical composition of the ZrFe₂O₅ nano-particles was appraised by energy dispersive X-ray analysis (EDX) as shown in Figure 2. It is clear from Figure 2(b) that for the 1 : 1 molar ratio, the average composition of Zr/Fe is 68 : 32.

Figure 1 

FE-SEM images of ZrFe2O5 sample S1 after annealing at 400°C for 4 hours.

(a)

(b)

Figure 2 

(a) EDX elemental analysis of ZrFe2O5 nanoparticles and (b) chemical composition of ZrFe2O5 nano-particles determined by EDX.

The phase and crystallinity of ZrFe₂O₅ nano-particles before and after the calcinations were examined by X-ray diffraction technique on an MX Labo powder diffractometer using Cu Ka radiation (40 kV, 20 mA), at the rate of 2°/min over the range of 20–80°. The XRD patterns showed that ZrFe₂O₅ nano-particles were amorphous prior to calcinations (Figure 3(a)). However, after calcinations at 400°C for 4 hours, the nano-particles had transformed into a crystalline ZrFe₂O₅ phase (Figure 3(b)).

(a)

(b)

Figure 3 

XRD pattern of (a) as-grown ZrFe2O5 nano-particles by co-precipitation at room temperature and (b) ZrFe2O5 nano-particles after annealing at 400°C for 4 hours.

3.2. Bandgap Energy

Prior to investigating the photo-catalytic action, it is imperative to appraise the optical absorption of the ZrFe₂O₅ nano-particles for the motive that the UV Vis absorption edge is associated with energy band of the semiconductor catalyst [15]. The optical bandgap (Eg) of ZrFe₂O₅ nano-particles estimated from the Tauc plot is 2.4 eV, signifying that the synthesized nanomaterial can absorb visible light.

3.3. UV/Vis Absorption Spectra of TBO Degradation

UV/Vis absorption spectra of TBO degradation with ZrFe₂O₅ over a period of 120 min by absorbing visible light are shown in Figure 4.

Figure 4 

UV/Vis Absorption spectra of TBO degradation by S1 (ZrFe2O5).

3.4. Mechanism of Photocatalytic Reactions

The photo-catalysis using visible light/ZrFe₂O₅ is based on adsorption of photons with energy higher than 2.4 eV (𝜆≥510 nm), resulting in initiating excitation related to charge separation event. High-energy excited states of electron and hole pairs arise when semiconductors possessing wide band gap are subjected to irradiation higher than their band gap energy. The outcome is the promotion of an electron in the conductive band (e_CB⁻) and a positive hole formation in the valence band (h_VB⁺) as shown in (2). The h_VB⁺ and e_CB⁻ are powerful oxidizing and reducing agents, respectively. The h_VB⁺ reacts with TBO dye resulting in its oxidation. Consequently, CO₂ and H₂O are produced as end products (7). The h_VB⁺ can also oxidize organic compounds by reacting with water to generate ^•OH (8). Due to electron preferring nature of hydroxyl radical (^•OH), it can oxidize almost all electron rich organic dyes, ultimately converting them to CO₂ and water (9). The conductive band e_CB⁻ can react with O₂ forming an anion radical superoxide as shown in (3). Further reactions can lead to the formation of hydrogen peroxide which leads to the formation of ^•OH. Consider the following: ZrFe₂O₅+h𝜐(𝜆≥510nm)⟶ZrFe₂O₅e_CB⁻+h_VB⁺O_2ads+e_CB⁻⟶^•O₂⁻O₂⁻+H⁺⟶HO₂2HO₂⟶H₂O₂+O₂H₂O₂+h𝜐⟶2^•hOH_VB++TBO⟶intermediates⟶CO₂+H₂OH₂O+h_VB⁺⟶^•OH+H⁺_•OH+TBO⟶intermediates⟶CO₂+H₂O (2)(3)(4)(5)(6)(7)(8)(9)

During photo-catalytic degradation, the presence of dissolved oxygen restricts the recombination process on ZrFe₂O₅ (e_CB⁻/h_VB⁺) which results in maintaining the electroneutrality of the ZrFe₂O₅ particles. Hence, effective photo-catalytic degradation of TBO dye is achieved by the reduction process of oxygen and the oxidation of TBO simultaneously to avoid the accumulation of electron in the conduction band and thus reducing the rate of recombination of e_CB⁻ and h_VB⁺.

3.5. Evaluation of Photocatalytic Activity

TBO dye was taken as a test pollutant. The photo-catalytic activity was evaluated by studying the disintegration rates of TBO dye in the presence of samples S1, S2, S3, and S4 under visible light irradiation through a cut-off filter (𝜆≥510 nm). A 0.05 mM solution of TBO decomposed rapidly under visible light when sample S1 was used, highlighting their photo-catalytic activity. The dye solution was degraded by approximately 92% in 140 min under visible light for sample S1 (Figure 5). However, degradation of dye decreased with the increase in the amount of zirconium in the nano-particles which shows consistency with the optical absorbance results (Figure 4). For comparison we also studied the photo-catalytic behavior of bulk ZrFe₂O₅ as shown in Figure 5. The enhancement in the photo-catalytic behavior of ZrFe₂O₅ nano-particles can be ascribed to considerably higher specific surface area of nano-particles compared with the bulk ZrFe₂O₅ (Table 2).

Table 2 

Specific surface area of ZrFe₂O₅ nanoparticles of different molar ratios calcined at 400°C and % age degradation of TBO dye.

Figure 5 

Photo-catalytic degradation profile of the TBO mineralization concentration in the solution (50 mL) with samples S1, S2, S3, and S4 and bulk ZrFe2O5 versus the exposure time to visible light irradiation.

Figure 6 

The mineralization rate of contaminant as determined by measuring the disappearance of total organic carbon (TOC) during the photocatalytic degradation of TBO by photocatalysis with ZrFe2O5 nanoparticles under visible light irradiation.

By contrast, the TBO without ZrFe₂O₅ nano-particles as a catalyst under visible light was stable, and only 2% had degraded after 140 min. It is clearly seen from Figure 5 that under visible light, sample S1 of the ZrFe₂O₅ nano-particles shows 92% degree of mineralization after 140 min. This is the first report on photo-catalytic degradation of TBO with ZrFe₂O₅ nano-particles in a relatively shorter time with a high stability suggesting its reusability. Previously, Shakir et al. [10] observed degree of mineralization as 85% after 3 hours of degradation of TBO with Cu_0.33MoO₃ nanorods under visible light irradiation. In a new study reported by Ito et al. [16], zirconium ferrite particles were used for elimination of phosphate from water of sewage treatment plants to avert eutrophication of semienclosed bay of Tokyo. They further discovered good adsorbance of phosphate ions onto zirconium ferrite particles. Magnetic separation characteristic indicated that 90% of phosphate in the discharge water of sewage plants could be eliminated in 5 min [16]. Besides, the zirconium ferrite adsorbent could be used repeatedly.

3.6. Stability Evaluation of ZrFe₂O₅ Nanophotocatalyst

The photo-catalytic stability and long-term use of these ZrFe₂O₅ nano-particles were also evaluated by recycling the photo-catalyst for up to seven reaction cycles and measuring the degradation rates of TBO, as shown in Figure 7. After each photo-catalytic reaction, aqueous solution was centrifuged at 4000 rpm for 5 min to isolate the catalyst from aqueous solution and redispersed it in fresh TBO solution for another cycle. The ZrFe₂O₅ nanomaterial showed stable photo-catalytic behavior even after seven (7) cycles of reactions, demonstrating the high photostability of the ZrFe₂O₅ nano-particles. The total organic carbon (TOC) was also measured to evaluate the total destruction of TBO (Figure 6). The rate of TBO and TOC removal under visible light irradiation decreased by only 5% and 10%, respectively, after seven cycles, demonstrating the high photostability of the synthesized photo-catalyst against visible light.

Figure 7 

Dependence of the stability of photo-catalytic activity for the photodegradation of TBO under visible light irradiation by reusing the same ZrFe2O5 nano-particles over seven times under the same conditions.

One of the factors that improved the photo-catalytic performance of ZrFe₂O₅ to a great extent is the size shrinkage of ZrFe₂O₅, thereby enlarging its specific surface area, due to which it could adsorb more TBO to photodegrade on its surface. From the electronic structure point of view, the band potentials of ZrFe₂O₅ accomplish a straddling gap, which may make possible the transfer of charge carriers and retard the e⁻h⁺ recombination, ensuring the superior photo-catalytic performance [15, 17–19]. Furthermore, the superior reactivity of the ZrFe₂O₅ was observed on samples with appropriate 1 : 1 molar ratios of ZrO₂ : Fe₂O₃, suggesting that there is a critical ratio for such a positive synergistic effect. Above this critical ratio, excessive zirconium covers the active sites and hinders the visible light penetration in the sample to excite ZrFe₂O₅. This correspondingly deteriorates the photo-catalytic activity, as a consequence of increased recombination of the photogenerated charges on ZrFe₂O₅. However, optimum molar ratios of ZrO₂ : Fe₂O₃ became the cause for a quicker separation of electron-hole pair resulting in slower recombination.

4. Conclusions

A new type of photo-catalyst ZrFe₂O₅ nano-particles has been synthesized by co-precipitation technique, and its photo-catalytic properties were investigated. The photo-catalytic measurements showed that ZrFe₂O₅ nano-particles under visible light could be efficiently used for the photo-catalytic degradation of toluidine blue O dye. Photodegradation efficiency in the absence of ZrFe₂O₅ nano-particles showed no significant change in the absorption maximum of toluidine blue O. Although bulk ZrFe₂O₅ exhibits the photo-catalytic ability to decompose TBO dye under visible light irradiation, however, degradation with ZrFe₂O₅ nano-particles is tremendously more efficient than that of bulk ZrFe₂O₅.

Conflict of Interests

Authors have no direct financial relation with the commercial identities mentioned in the paper that might lead to a conflict of interests.

Acknowledgment

This project was supported by Deanship of Scientific Research, College of Science Research Center, King Saud University, Riyadh, Saudi Arabia.