Facile preparation and characterization of a novel visible-light-responsive Rb2HgI4 nanostructure photocatalyst

Visible photocatalytic procedures exhibit encouraging potential in water purification by increasing the photocatalytic performance. Therefore, the improvement of low-cost and efficient photocatalysts for environmental remediation is an increasing demand, and photocatalysts based on semiconductors have gained considerable attention due to their superior stability and activity. In the current study, novel Rb2HgI4 nanostructures were prepared via a simple, low-cost, and low-temperature solid-state method. The effects of different parameters such as type of surfactants, reaction temperature, and reaction time were studied on the structure, crystallinity, particle size, and shape of nanostructures. This new compound has a suitable band gap (2.6 eV) in the visible region. The photocatalytic performance of Rb2HgI4 was examined for the removal of coloring agents under visible light irradiation and it was found that this compound could degrade and eliminate acid black 1 by about 72.1%.


Introduction
Nowadays, the primary requirement for freshwater reservoirs has drawn worldwide attention due to fast development in industrialization, urbanization, and massive population growth. These accelerated developments have led to ecological issues, including polluted groundwater and air along with hazardous wastes. It is predicted that the growing demand for freshwater will worsen due to the continuous release of contaminants and pollutants in natural water sources. The reusing and recycling of sewage are essential to enhance the inadequate supply of freshwater. 1,2 Numerous strategies, including biological treatment, 3 adsorption, 4 chemical treatment, 5 and membrane-based separations 6 have been studied to establish specic water purication. Biological approaches traditionally developed to efficiently eliminate the multiple varieties of pollutants from water nally led to the generation of secondary contaminants, including healththreatening bacteria and soluble refractory organic compounds, which are challenging to eliminate. 7 Hence, the improvement of sustainable, nondestructive, and green technology for wastewater/water purication is of highest concern. Semiconductor-based photocatalysts are one of the most successful strategies for wastewater/water treatment owing to their high potential and high efficiency in eliminating toxic organic pollutants utilizing solar light. [8][9][10][11] The structural, electronic, and optical properties of the material must be carefully studied to promote efficient semiconductor-based photocatalysts for the photodegradation of contaminants in wastewater/water. Commonly, various characteristics, for instance the selection of semiconductor materials, morphological architecture, and surface features, should be considered when designing efficient and stable visible-light-sensitive photocatalysts. 12 To design more effective photocatalysts based on semiconductors, strategies based on fundamental principles have been a benecial instrument in presenting a broad comprehension of photocatalysis, describing experimental data, and predicting innovative semiconductor photocatalyst materials with excellent performance.
Semiconductors, including CdS, ZnS, Fe 2 O 3 , ZnO, and TiO 2 , can be used as sensitizers for photoinduced redox reactions because of the electronic conguration of the metal atoms in the chemical composition, which is described with an empty conduction band (CB) and a lled valence band (VB). 13,14 By radiation, VB electrons (eÀ) are directed to the CB producing holes (h + ). These e À -h + pairs can either interact separately with other molecules or can recombine. The holes might react with hydroxide ions or with electron donors in the solution to create strong oxidizing agents such as superoxide (O 2 À c) or hydroxyl (cOH) radicals. [15][16][17] Alternatively, semiconductors are substances whose VB and CB are separated by band gap or energy gap. When a semiconductor receives photons with energy equivalent to or higher than its band gap, electrons in the VB can get excited and jump up into the CB, therefore producing charge carriers. The recombination of e À -h + needs to be restricted as much as possible aer the rst charge separation to have an efficient photocatalytic reaction. 18 Several studies have been conducted on halide ferroelectrics having the general formula A 2 BX 4 (A ¼ Rb, Cs, k, Tl; B ¼ Hg, Cd, Zn, Co; X ¼ Cl, Br, I). [19][20][21][22] They are categorized into two groups with various structures. One group is ferroelectrics possessing an orthorhombic b-K 2 SO 4 structure with the Pmcn space group, and another has a monoclinic Sr 2 GeS 4 structure with the P2 1 /m space group. 23 Rubidium tetraiodomercurate(II) (Rb 2 HgI 4 ) is one of these compounds that is categorized into two classes: superionic materials identied as solid electrolytes, and thermochromic materials, which are a branch of smart materials. 24 Solid electrolytes provide the movement of ions without the need of a liquid. The cations in the compounds induce electrical conductivity with their motions. 25 Superionic materials are intermediates of solid crystals with organized structure and liquid crystals without organized structure that possesses mobile ions. These compounds are employed as solid batteries, fuel cells, multiple chemical sensors, and supercapacitors. Today, ecological contamination has become a worldwide disaster; therefore, new energy reservoirs, including fuel cells and solar cells have drawn considerable attention. Photodegradation is also one of the most important technologies used in the disposal of pollutants in industrial wastewater. Researchers want to use natural resources available to obtain the energy needed for the degradation of dyes in industrial wastewater, and the most important source of natural energy is sunlight that consists of about 5-7% of UV light, 46% of visible light and 47% of infrared radiation. 26,27 The photocatalytic oxidation of numerous harmful organic dyes in industrial wastewater has been carried out over different semiconductor photocatalysts under UV light irradiation. Research is now focused on achieving high photocatalytic efficiency with new photocatalysts, particularly with sunlight. Due to the suitable band gap of Rb 2 HgI 4 (2.6 eV), we decided to study its photocatalytic activity under visible light for the rst time. In this research, we have applied the solid-state method to produce Rb 2 HgI 4 nanostructures. Our main purposes are noted below: (1) Fabrication of Rb 2 HgI 4 nanostructures by a facile route.
(2) Investigation of the photocatalytic activity of this compound for the rst time.
Rb 2 HgI 4 nanostructures were constructed via a simple, lowcost solid-state method at low temperatures to achieve the rst aim. The morphology of the nanostructures was homogeneous, and we selected this pathway to produce the nanostructures and studied the inuence of different types of surfactants on the morphology of the obtained products. So far, nano-Rb 2 HgI 4 has not been reported, and this is the rst time that Rb 2 HgI 4 nanostructures are synthesized.
The as-synthesized Rb 2 HgI 4 nanostructures were applied as photocatalysts for the rst time to fulll the second aim. In this study, we have used Rb 2 HgI 4 nanostructures as a novel and efficient catalyst for the photodegradation of organic dyes.

Preparation of precursors
The RbI precursor was fabricated by a facile co-precipitation method from Rb 2 SO 4 and LiI. First, a certain amount of Scheme 1 Schematic of the fabrication of Rb 2 HgI 4 nanostructures. Rb 2 SO 4 , LiI, and different surfactants (such as EDTA, SDS, NaHSal, and PVP) was liqueed in distilled water in separate beakers, and the solutions were combined with each other. The resultant solution was allowed to stand for crystallizing the white RbI precipitate. An HgI 2 precursor was synthesized by adding a LiI solution to Hg(O 2 CCH 3 ) 2 solution to obtain an orange precipitate.

Fabrication of Rb 2 HgI 4
HgI 2 and RbI were mixed in a certain ratio and ground in a mortar. The resulting powder was heated at different temperatures (180-220 C) in air for 9-12 h. Before the temperature reaches 220 C, the average temperature rise is 5 C per minute. Aer continuing the thermal treatment at 220 C for a specied time, it was allowed to cool naturally to room temperature (Scheme 1). Table 1 indicates the different conditions for the fabrication of Rb 2 HgI 4 to obtain the best conditions.

Photocatalytic performance
The photocatalytic activity of Rb 2 HgI 4 was examined via its potential for the degradation of different organic colorants under visible radiation. An Osram light (150 W) was employed as the radiation source, containing a wavelength range of 400-780 nm for the photocatalytic process. The experiments were conducted without catalyst and light, and almost no dye was degraded aer 90 min. 100 mg Rb 2 HgI 4 was added to 100 mL 10 ppm of dye solution in each experiment. The suspension was mixed in dark for 0.5 h before turning on the visible light. 5 mL sample was removed from the suspension every 15 min during irradiation and centrifuged at 10 000 rpm for 4 min. The buoyant was collected, separated, and observed with a UV-Vis spectrophotometer.

Characterization
The XRD patterns are ngerprints that can help us understand what is in compounds. The XRD pattern of a compound is a simple sum of the diffraction peaks of every phase. Fig. 1a indicates the XRD pattern of sample 1 with a molar ratio of 2 : 1 for RbI to HgI 2 formed from rubidium iodide (JCPDS no. 01-073-0385) and mercury iodide (JCPDS no. 01-072-1615) and an amount of unknown phase. Fig. 1b-e presents the XRD patterns of samples in the presence of different surfactants at 220 C for 12 h. A large amount of unknown phase is observed in these patterns in addition to HgI 2 (tetragonal structure) and RbI (cubic structure). The remarkable feature in all XRD patterns is the presence of an unknown phase at different 2q values that does not match with any of the combinations reported in the database. We aimed to fabricate Rb 2 HgI 4 nanostructures; however, since the reference code of Rb 2 HgI 4 has not been reported, we believe that the general pattern of Rb 2 HgI 4 is related to these XRD patterns. Fig. 1f-i show the effects of reaction time and temperature on the purity of products. Reducing the time of the reaction to 9 h decreased the unknown phase (Fig. 1f). Alternatively, this phase did not have enough time to form. In contrast, increasing the time to 15 h caused an increase in the unknown phase (Fig. 1g). Decreasing the temperature to 200 C and 180 C reduces crystallinity and the unknown phase. According to the XRD patterns, the optimum condition was selected at 220 C for 12 h in the presence of SDS as a capping agent. The crystallite size was determined by the Scherrer equation: D ¼ Kl/b cos q (ref. 28) was found to be between 32 and 39 nm. The FESEM images of samples were obtained for studying the morphology, uniformity, shape, and particle size (Fig. 2).   The average particle size was measured by the Digimizer soware. Fig. 2a indicates that without surfactant, nanoparticles with an average particle size of 24 nm are formed at 220 C for 12 h (sample 1). Fig. 2b-e show the effects of different types of surfactants. It can be seen that homogenous nanoparticles are formed by using all the surfactant types. Reducing the time of reaction to 9 h caused irregular rod-like structures and microstructures (Fig. 2f). Nanoparticles with an average size of 9 nm were formed on the microstructures by increasing the time of reaction to 15 h (Fig. 2g). Fig. 2h and i show that decreasing the reaction temperature to 200 C and 180 C resulted in bulk structures. Therefore, the optimum condition was selected in the presence of SDS as an anionic capping agent at 220 C for 12 h. Fig. 3 displays the histogram size distribution of samples 1-5 obtained using the Digimizer soware, indicating that most particles are between 20 and 30 nm.
Energy-dispersive X-ray spectroscopy (EDX) is an analytical method utilized for chemical characterization or elemental analysis of a sample. Fig. 4 demonstrates the EDX spectra of Rb 2 HgI 4 , which indicate that all the peaks are assigned to Rb, Hg, and I elements. Consequently, the products are perfectly puried and related to the XRD outcomes. Besides, the EDX result conrmed the uniform dispersion of the elements on the samples. Fig. 5 shows the TEM photographs of Rb 2 HgI 4 nanostructures (sample 2) in different scales of 80, 40, and 20 nm. Uniform nanoparticles with an average size of 28 nm are observed in this gure, which corresponds to the SEM and XRD outcomes.
The BET surface area analysis is a standard instrument to calculate the specic surface area and pore volume of samples. The morphology of samples prepared with different surfactant types is similar, and there is no need to analyze the BET surface area of all samples. The adsorption-desorption isotherms of nitrogen for samples 2 and 7 are depicted in Fig. 6a and d, respectively. Based on the IUPAC category, sample 2 exhibits the isotherm type III with the H4-type hysteresis loop (Fig. 6a), which is ascribed to the mesoporous materials (Fig. 6b). The specic BET surface area was calculated to be 36.6 m 2 g À1 , and the average pore diameter was 3.16 nm. From the BJH plot, the total pore volume and pore diameters were 0.0289 cm 3 g À1 and 1.21 nm, respectively (Fig. 6c). The isotherm of sample 7 is type III with an H3-type hysteresis loop, indicating microporous materials (Fig. 6d). The specic surface area was calculated to be 29.2 m 2 g À1 , and the average pore diameter was 3.72 nm. From the BJH plot, the average pore volume and pore diameters are 0.027 cm 3 g À1 and 1.21 nm, respectively (Fig. 6f). The specic surface area of sample 2 (S BET ¼ 36.6 m 2 g À1 ) is greater than that of sample 7 (S BET ¼ 29.2 m 2 g À1 ). This information is well corroborated with the SEM and TEM results. Table 2 compared the BET data of this nanostructure with other iodide nanostructures. Rb 2 HgI 4 nanostructures have a large specic surface area compared with CuPbI 3 , Tl 4 HgI 6 , and Tl 4 CdI 6 and possess a small average pore diameter.

Photocatalytic activity
The photocatalytic activity of Rb 2 HgI 4 nanostructures was studied by monitoring the degradation of anionic and cationic coloring agents as organic contaminants, such as acid black 1 (AB1), methyl orange (MO), methyl violet (MV), and rhodamine B (RhB) in an aqueous solution, under visible light (Osram light (150 W lamp produces 2600 lumens of light)) (Fig. 8). The spectral irradiance for the visible lamp was 400-780 nm according to the information provided by the manufacturer. Without Rb 2 HgI 4 or light, almost no dyes were degraded aer 90 min, revealing that the self-degradation part was insignicant. The degradation percentage (%D) was dened as follows: where A t and A 0 are the solution absorbance of the sample aer and before degradation, respectively. 32 The inuence of various coloring agents and catalyst dosage was measured to obtain the optimum performance. Fig. 8a (Fig. 8e).
Increasing the catalyst dosage is conducted in the space of the catalyst surface and enhances the dye adsorption on the catalyst surface. 33 Besides, Rb 2 HgI 4 nanostructures can destroy the anionic coloring agents better than the cationic ones. Hence, the highest performance was for AB1 (72.1%), and RhB (cationic dye) had the minimum yield. Table 3 shows the comparison of the photodegradation of different iodide compounds under visible and UV light. As demonstrated in this table, Rb 2 HgI 4 can compete with other iodide compounds as a photocatalyst. We can propose Rb 2 HgI 4 as a novel catalyst for the water purication process. Moreover, the possible reaction rate constants of the coloring agents were determined depending on the Langmuir-Hinshelwood mechanism. 28,34 where C 0 is the initial concentration of coloring agents; C is the concentration of coloring agents at t time; k is the pseudo-rst order rate constant (min À1 ). The rate constant (k) was determined from the linear plot of ln(C 0 /C) vs. reaction time. As displayed in Fig. 8b, d, and f, better photocatalytic performance was achieved with a higher reaction rate constant. Fig. 9a shows the decolorization of AB1 under visible light aer 120 min, and the percentage was obtained to be 75.3%. Increasing the time of the photocatalytic reaction improved the destruction by about 3%. The photocatalytic behavior of RbI and HgI 2 was studied over AB1 under visible light, indicating 65.5% and 67.0% degradation, respectively (Fig. 9b). The result showed that the performance of Rb 2 HgI 4 for the degradation of organic dye is higher than its precursors.