Photodecoloration of Methyl Orange Solution Assisted by ZrS3 Powders

Article history: Received: 20 November, 2018 Accepted: 01 February, 2019 Online : 07 February, 2019 Zirconium trisulfide ZrS3 is a representative of transition metal polysulfides containing sulfur as S in polysulfide, usually disulfide S2 groups. Semiconductive zirconium trisulfide which absorbs visible light near UV edge was considered as a possible photocatalyst. We experimentally studied photodecoloration of methyl orange in presence of ZrS3. It was shown for the first time that crystalline ZrS3 strongly deepens photodegradation reaction, and in one case the methylene orange conversion reached almost 100%. The rates of degradation curves were associated with the ZrS3 samples morphology; the best result revealed for microribbons ZrS3 synthesized at 650°C.

Zirconium trisulfide ZrS3 is a typical representative of layered transition metal trisulfides. The crystal structure of ZrS3 (attributed to ZrSe3 structural type [8]) (Fig. 1) comprise double layers whose surfaces are produced by disulfide groups (S2) 2-. Particularly, a basic structural fragment prism {ZrS6/2}, is constructed from two disulfide (S-S) 2groups as well as two sulfide groups S 2-, the Zr atom is situated near the center of the prism (Fig. 1). Such prisms {ZrS6/2} with metal atoms situated close to the prism centers are connected to each other via common triangle bases to form infinite columns which are oriented along b axis. Additionally, these columns are connected to the neighboring ones forming layers two prisms thick. The infinite layers are bind to the neighboring via van der Waals S…S contacts. Hence the layered zirconium trisulfide may be described by ionic model Zr 4+ (S2) 2-S 2-. Presence of the disulfide group introduces new chemical properties which could be useful in known physico-chemical processes required in modern industry. Sulfur in the disulfide group is charged as -1, so it can be oxidized or reduced [11] in 1electron ox-red processes, so these systems can be considered as ASTESJ ISSN: 2415-6698 electron reservoirs [12]. This property can meet an application in high capacity alkaline batteries [4], [13]. In the ionic model approximation, trisulfides MS3 (M = Ti, Zr, Hf) are d 0 -complexes, a diamagnetic semiconductors. The semiconductor band gaps of the trisulfides MS3 (M = Ti, Zr, Hf) were optically defined (1.8, 2.0, and 2.2 eV, respectively), and were recently examined as cathodes in hydrogen evaluation reaction [14]. In addition, ZrS3 nanomaterial was used as efficient anode for catalytic oxygen evaluation reaction in both alkaline and neutral aqueous solutions [15].
Photocatalytic degradation of organic molecules is an actual theme now, and it becomes more popular once renewable energy source is used (solar irradiation); the nature of substrates is dictated by polluting agents in natural waters and soils. Semiconductor photocatalysis is an important option for solar energy conversion and pollutant degradation according to the idea of green chemistry. For purposes of degradation of the pollutants, a known set of catalysts is already designed [16]: double layered hydroxides [17], titanium dioxide [18], composites based on TiO2 [19], other oxides (ZnO [20], WO3 [21]), composites with oxides [22-24, 25, 26], as well as composites with MoS2 [27][28][29].
Many transition metal di-and polysulfides are dark or metal black in color. A pair of "colored exceptions" can be distinguished here, ZrS3 and HfS3, which are orange and dark red, respectively. According to study [30], these compounds absorb visible light in near UV region, so we decided to examine ZrS3 compound as photocatalyst in a model reaction of methyl orange degradation. Shifting from transition metal disulfides (MoS2, WS2) to transition metal trisulfides as photocatalyst is something new in the present work.

Syntheses of zirconium trisulfide at different temperatures
ZrS3 was obtained by heating a stoichiometric mixture of Zr and S. Mixtures of powder zirconium (4.868 g 0.05336 mol) and crystalline sulfur (5.132 g 0.1601 mol) were evacuated in three quartz ampules (about 20 ml volume) and sealed. The syntheses were carried out at temperatures 350, 500, and 650°C for 150 hours (up to these temperatures, the ampoules were heated for 15 h). The ampules were cooled down with the furnace. The products were thin powders different shades of red-orange. The products were heated in dynamic vacuum at 200°C during 1 h in order to remove elementary sulfur after syntheses.

Methods and apparatus
X-ray powder diffraction patterns for solids including exfoliated samples were collected with a Philips PW 1830/1710 automated diffractometer (Cu Kα radiation, graphite monochromator, silicon plate as an external standard). Raman spectra were recorded with a LabRam HR Revolution (Horiba Scientific) instrument at wavelength 488nm. UV spectra were recorded with an Agilent Cary 60 Spectrophotometer in the range of 200 -800 nm. Eppendorf Centrifuge 5430 equipped with container for 15 and 50 ml tubes used for centrifugation of the reactive mixtures. Scanning electron microscopy (SEM) images were collected with Hitachi S3400N instrument.
X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Specs Phoibos-150 spectrometer with an Al Kα monochromatic excitation. The pass energy of an electron analyzer was set at 20 eV. For the compensation of a charging effect from non-conductive samples, a low energy electron beam was applied. Binding energies were measured from the C1s level (285.0 eV) of surface hydrocarbon contaminations. Relative atomic concentrations are calculated from the measured areas of spectra taking into account photoionization cross-sections, inelastic mean free paths and the transmission function of the spectrometer.
The Brunauer-Emmett-Teller (BET) surface areas were determined by N2 adsorption 5-point measurements in pressure range p/p0 = 0.05-0.25 using SORBTOMETER-M surface area analyzer. The ZrS3 powder samples were preliminary heated at 200°C during 1 hour in nitrogen flow.

Photodecoloration experiment
The photocatalytic activity of ZrS3 powders was evaluated by standard decoloration reaction of dye methyl orange (MO). The reactor for the decoloration experiments consisted of a cylindrical beaker on a magnetic stirrer, with a common LH9-U/BLB/G23 black light lamp (Camelion). Experimental conditions for methyl orange (MO) decoloration on powders were: aqueous solution of MO with initial concentration 0.022 mM, amount of ZrS3 powder 0.175 g/L (14 mg per each experiment), and UV irradiation time was kept constant for 2-3 hours. The aqueous suspensions were stirred throughout the experiment. In the experiment, to 80 ml of aqueous solution of MO (C = 0.022 mM) 14 mg of ZrS3 powder was added, and mixed during 3 hours excluding irradiation of any light (for dark sorption). At the stage of dark sorption several points were documented in the following way: 1 ml of suspension was collected from the solution with Pasteur pipette, centrifuged with acceleration 3000 g for 10 min for removal of ZrS3 solid, the transparent solution decanted, and the UV-Vis spectrum in region 200-800 nm written for the solution. After 3 hours of dark sorption a lamp was turned on above the tested mixture and every 15 min points were examined in the same way. Three series of experiments were carried out, each series three times: using powder samples of ZrS3 synthesized at 350 (ZrS3-350), 500 (ZrS3-500), and 650°C (ZrS3-650). Graphics of MO absorbance vs time of irradiation were built for comparison.

Photodecoloration experiment with Degussa P25
The experiment with Degussa P25 ® was carried out in similar way, using the same experimental setup. The portion of powder Degussa P25 was 14 mg per every degradation experiment.
After the experiments the photocatalysts were washed with 20 ml of water, centrifuged and decanted, and then washed with 20 ml of ethanol. After these operations the solids were dried in air. They were examined by XRD and Raman spectroscopy, as well as XPS for ZrS3-650.

Initial powder ZrS3 samples
Zirconium trisulfide samples were synthesized from the elements in evacuated quartz ampules at different temperatures according to reaction which is rather common for synthesis of transition metal trichalcogenides [31,32]: Zr + 3S → ZrS3 (T = 350, 500, 650°C).
The yields were quantitative or near-quantitative. Preliminary EDS tests revealed a quantity of elementary sulfur in the products, so we heated the ZrS3 samples in dynamic vacuum in order to remove elementary sulfur. The products were thin powders, redorange in color (red for ZrS3-350, yellow-orange for ZrS3-500 and ZrS3-650). Powder diffraction patterns (XRD) revealed only monoclinic ZrS3 phase (ICSD 42-073) as crystalline product of reaction in the cases of temperatures 500 and 650°C (Fig. 2). Positions of some observable reflections were as follows:  EDS data for the prepared ZrS3 samples showed rather low content of sulfur (Zr : S ratios were from 2.2 to 2.4), so we considered that the materials demonstrated mainly surface stoichiometry which could include some deal of oxides also. These data contradicted with the XRD data where clear crystalline monoclinic ZrS3 phase occurred. In order to define the exact Zr : S ratios in these samples, we carried out gravimetric experiments: carefully weighed ZrS3 samples were put into pre-weighed crucibles and heated in a muffle furnace at 600°C during 20 hours. The products of calcination in the crucibles were weighed, and the quantity of sulfur was calculated using mass difference according to the equation: ZrSx + (1+x)O2 = ZrO2 + xSO2.
After weighing the calcination products, they were examined by X-ray powder diffraction in order to make sure that ZrO2 (gravimetric form) were the only calcination product. It was found that the bulk material ZrS3-350 was ZrS2.56, ZrS3-500 was ZrS2.76, and ZrS3-650 was ZrS2.76. So we concluded from this data that the ZrS3 samples synthesized from stoichiometric mixtures of Zr and S are monoclinic ZrS3 albeit possess remarkable deficiency in sulfur.
Probably our ZrS3 samples have such a sulfur deficiency due to synthetic procedure: only 3 equivalents of S were taken for reaction per one equivalent of Zr. Crystals of zirconium trisulfide with formulae very close to ZrS3 are synthesized in gas-transport reactions with some excess of sulfur [35], longer heating, or the stoichiometric mixture which was heated at higher temperature (1000°C) [35], or applied transport species (for example, bromine) [36].
SEM images of the powder ZrS3 samples revealed different morphologies (Fig. 3). ZrS3-350 sample is aggregates of lamellar slabs about 5μm size; ZrS3-500 sample is a mixture of particles of two types: aggregates of lamellar slabs ca. 5μm size and fibers up to 20 μm long; ZrS3-650 sample is ribbons 0.5-1 μm width and ca. 20μm long. Visible texturing is observed for ZrS3-650 sample (Fig.  2): apparently, the preferred orientation of the ribbons is layering in parallel to their wide side. Strong dependence of the BET surface areas on the ZrS3 morphology (min BET 9 m 2 /g for ZrS3-650, Table 1) indicates that the lamellar slab aggregates make the main contribution to the surface area: BET of the samples ZrS3-350 and ZrS3-500 having components of such morphology, are noticeably higher (49 and 24 m 2 /g, respectively). Degree of dark sorption correlated with the surface area values: the percentage of absorbed MO on ZrS3 powders increased while increasing their BET in series ZrS3-650 < ZrS3-500 < ZrS3-350. The minimal dark adsorption of ZrS3-650 is in agreement with the maximal CSDS value, 77 Å, and the minimal BET surface.

Photodecoloration of MO in presence of ZrS3 powders
ZrS3 powders were used as photocatalysts for decomposing of dye molecule MO for the first time (Fig. 4). ZrS3 powders were used as synthesized. Photocatalytic activity of ZrS3 was evaluated in terms of the degradation of MO aqueous solution under UV irradiation. The efficiency of the dye degradation was calculated based on the changes in absorption of MO in UV-Vis spectra. At the lamp irradiation, MO didn't decolorate in absence of ZrS3. From the UV-Vis spectroscopy data (Fig. 4, inset) one can see that MO absorbs on ZrS3 particles in the cases ZrS3-350 and ZrS3-500 in the dark phase whereas presence of ZrS3-650 decreases MO concentration to a lesser degree. After start of the irradiation, the concentration of MO in the reaction mixture was decreased noticeably, that indicated start of MO photodegradation.
The decoloration dependencies vs time of irradiation are averaged from at least three experiments, and the standard deviations of the data points not exceed 5% from absorbance values at time starting UV treatment (t = 0 min). At the initial concentration of МО in aqueous solution 0.022 М, photodegradation time was confined by two hours. The longest MO degradation process concerns to ZrS3-350 sample, 140 min with 90% of the initial MO amount. The photodegradation dependences have linear character at least for the first 60 min, for ZrS3-500 and ZrS3-650 they are very close while the line for ZrS3-350 has gentler slope, and doesn't demonstrate complete degradation of MO in 140 min of the experiment.
One can see ZrS3-650 is the most efficient sample from the three. The reason of this may be the minimal dark sorption, so the sample surface is better accessible for dissolved oxygen; ZrS3-650 also possesses larger size of coherent scattering domain (77Å) that allows hoping to have more regular surface of the particles. The MO degradation results obtained are comparable with MO degradation results in the presence of MoS2 of two morphologies: nanorods and nanoflowers [37].
The analysis of XPS data before and after the photocatalytic experiments for ZrS3-650 as critical sample, revealed certain differences. In the initial powder sample ZrS3-650 (Fig. 6), the atomic concentration ratio S/Zr was 2.78. S2p spectrum contained two doublets from S2 2and S 2with positions of S2p3/2 components at 162.6 and 161.6 eV, respectively; the ratio of their intensities was 1.78. The values of bond energies were well agreed with the literature data [40]. Spectrum Zr3d5/2, 3/2 also contains two doublets with spin-orbital splitting 2.4 eV. The Zr3d5/2 components were situated at 181.2 and 182.9 eV with intensity ratio 9:1. The first component belonged to zirconium in ZrS3 while the second might be associated with an oxide phase on surface of the ZrS3 particles [41]. The composition of the surface sulfide phase may be thereby formulated as Zr0.9(S2)0.89S. In the sample ZrS3-650 after the photodegradation experiment, weak peaks in the region 168.0 eV appeared in S2p3/2 spectrum that corresponds to sulfate groups on the surface. Component corresponding to an oxide phase, strongly arose in the Zr3d5/2, 3/2 spectrum. These data indicate the oxidation of the ZrS3 particles surface after photocatalysis experiment.

Comparison with Degussa P25
We carried out photocatalytic experiment with Degussa P25 using the same experimental setup in order to compare photocatalytic ability of ZrS3 with the international reference TiO2 (TiO2, 80% anatase and 20% rutile). 14 mg of Degussa P25 in 80 ml of 0.022 mM MO solution was used for the experiment. As a result of experiment in our setup, MO decolorated in presence of Degussa P25 at 50% in 138 min. Although kinetics of MO decoloration was faster for ZrS3-650 than for Degussa P25 here, we discussed here only the first cycle of decoloration. Additional studies are required to test ZrS3 powders in series of decoloration cycles, and examination of the catalysts after the experiments.
The preliminary experiment on photodegradation of MO in aqueous solution in presence of powder ZrS3 opens way to design of new light-sensitive materials for photocatalysis and other applications. Some additional studies are to carry out in order to realize the mechanism and other characteristics of the photodegradation observed. Authors believe that photosensitive properties of ZrS3 (and probably some another transition metal trisulfides) may be enhanced by different ways (crystal or surface modification, assembly in composites) to obtain new materials for photocatalysis.

Conclusion
Zirconium trisulfide revealed appreciable photocatalytic ability in dye molecules decoloration, on the example of methyl orange decoloration for the first time. The surface of the photocatalyst increases the content of oxide phase after the degradation reaction. It was ascertained that the photocatalytic behavior of ZrS3 is dependent on its morphology, which in turn is dependent on temperature of synthesis of ZrS3. Based on qualitative level results, ZrS3-containing species are considered as perspective materials for photocatalysis.