Next Article in Journal
Experimental and Theoretical Investigation of Thiazolyl Blue as a Corrosion Inhibitor for Copper in Neutral Sodium Chloride Solution
Next Article in Special Issue
Facile Fabrication of Dumbbell-Like β-Bi2O3/Graphene Nanocomposites and Their Highly Efficient Photocatalytic Activity
Previous Article in Journal
Influence of Microstructure and Shot Peening Treatment on Corrosion Resistance of AISI F55-UNS S32760 Super Duplex Stainless Steel
Previous Article in Special Issue
Synthesis of Various TiO2 Micro-/Nano-Structures and Their Photocatalytic Performance
Open AccessArticle

Elucidating the Photocatalytic Behavior of TiO2-SnS2 Composites Based on Their Energy Band Structure

Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, Zagreb 10000, Croatia
*
Authors to whom correspondence should be addressed.
Materials 2018, 11(6), 1041; https://doi.org/10.3390/ma11061041
Received: 29 May 2018 / Revised: 12 June 2018 / Accepted: 14 June 2018 / Published: 19 June 2018
(This article belongs to the Special Issue Photocatalysis for Wastewater Treatment)

Abstract

TiO2-SnS2 composite semiconducting photocatalysts with different building component ratios were prepared by hydrothermal synthesis (TiO2-SnS2-HT) and by immobilization of commercial TiO2 and SnS2 particles (TiO2-SnS2-COMM). The band gap values, which determine the catalysts’ photoactivity, were examined by diffuse reflectance spectroscopy and Kubelka–Munk transformations. The catalysts’ surface properties: specific surface area, charge and adsorption capacitance at the solid–solution interface were characterized using BET analysis, potentiometric titration and electrochemical impedance spectroscopy, respectively. The electronic band structure of TiO2-SnS2 photocatalyst, as the key property for the solar-driven photocatalysis, was deduced from the thermodynamic data and the semiconducting parameters (type of semiconductivity, concentration of the charge carriers, flat band potential) obtained by Mott–Schottky analysis. The photoactivity of both composites was studied in photocatalytic treatment of diclofenac (DCF) under simulated solar irradiation and was compared to the benchmark photocatalyst (TiO2 P25) activity. The influence of process parameters, such as pH, H2O2, and composite formulation on the effectiveness of DCF removal and conversion was investigated and discussed by employing response surface modeling (RSM) approach. The photocatalytic efficiency of both composite materials was discussed on the basis of the hetereojunction formation that facilitated the photoelectron transfer, promoting more efficient photocatalytic degradation of DCF.
Keywords: TiO2-SnS2 composite catalysts; semiconducting parameters; energy band diagram construction; solar photocatalytic water treatment; diclofenac TiO2-SnS2 composite catalysts; semiconducting parameters; energy band diagram construction; solar photocatalytic water treatment; diclofenac

1. Introduction

Semiconductor materials have become of great significance owing to their wide potential applications in important fields of modern society, such as electrochemical sensors, organic synthesis, self-cleaning surfaces, solar driven hydrogen production, and wastewater treatment [1,2,3,4,5]. Current wastewater treatment technologies can be assisted by semiconductor photocatalysis in overcoming inadequate removal of emerging contaminants, including pharmaceuticals [5,6,7].
TiO2 is de facto the gold standard in photocatalytic wastewater treatment, due to its low cost, chemical stability, high activity, and low toxicity. A widespread and cost-effective application of TiO2 is hindered by its prohibitively wide band gap (Eg ≈ 3.0−3.2 eV), limiting photoactivation only to UV part of the solar spectrum [8]. Transition metal sulfides, on the other hand, have significantly narrow band gaps (Eg < 2 eV) [9,10,11] and are photoactive under visible light. An increase in cost-effectiveness from narrow band gaps is easy to comprehend, considering that only 5% of solar irradiation belongs to UV [12]. A major hurdle for broader sulfide photocatalyst application is their tendency towards photocorrosion [13,14].
The combination of different semiconductor materials may have complementary properties and create new materials with optimal performances in visible-light-driven photocatalyst water treatment systems. For instance, forming unique semiconductor–semiconductor junctions, depending on the semiconductors’ band-structure, can extend the spectral range for light absorption and enhance electron–hole separation [15]. A typical heterojunction photocatalyst is CdS-TiO2 composite, which can be activated by visible light owing to the narrow band gap of CdS component. Composite catalyst exhibits high quantum efficiency since the photoelectrons generated in CdS can easily migrate into TiO2, leading to the depression of photoelectron–hole recombination in TiO2 [16]. However, CdS-TiO2 composite displays poor durability due to CdS photocorrosion [17,18], whilst also leaching toxic Cd2+ ions, thereby causing adverse effects in water treatment purposes [19]. The SnS2 semiconductor is considered to be a viable option, due to its narrow band gap, low toxicity, and better photostability in comparison to CdS [4,20,21].
The aim of this study was to determine the properties of TiO2-SnS2 composite materials prepared by different methods. Barrier properties (electronic conductivity, corrosion resistance) and electronic structure (semiconducting properties) of TiO2-SnS2 composites were investigated in situ using electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis [22,23,24]. Thermodynamic stability of TiO2-SnS2 hybrid systems against cathodic and anodic decomposition was discussed. Within this study, the surface and optical properties of the composites were also characterized using BET method (specific surface area), potentiometric titration (point of zero charge, pHPZC), and diffuse reflectance spectroscopy (DRS) (light absorption capacity and corresponding band gap energy calculation). The activity of TiO2-SnS2 composites under solar irradiation was studied towards diclofenac (DCF) removal and conversion. DCF is one of the most commonly used analgesics, recently included in the “watch list” of priority substances in the EU Water Framework Directive [25]. Its presence in natural recipients due to inadequate removal by current wastewater treatment technologies, and the potential to cause adverse effects in aquatic ecosystems is documented [26,27]. The DCF removal and conversion extents during solar-driven treatment using TiO2-SnS2 composites were evaluated in correlation with process parameters studied: initial pH, composite formulation, and concentration of oxidant (H2O2) using a statistical/empirical approach employing response surface modeling (RSM). The obtained results were correlated with determined surface and semiconducting properties of applied photocatalytic composite materials.

2. Materials and Methods

2.1. Chemicals

Titanium (IV) butoxide (TBO) (97%, Acros Organics, Geel, Belgium), tin (IV) chloride (98%, Sigma-Aldrich, Taufkirchen, Germany ), thioacetamide (≥99.0%, Sigma-Aldrich, Taufkirchen, Germany), ethanol (abs., Gram-mol, Zagreb, Croatia), and acetic acid (≥99.7%, Carlo Erba Reagents, Val-de-Reuil, France), were used in synthesis of TiO2, SnS2 and TiO2-SnS2 composite catalysts, denoted as TiO2-SnS2-HT. Commercial TiO2 (AEROXIDE P25, Evonik, Essen, Germany) and SnS2 particles (MKN-900, MKnano, MK Impex Corp., Ontario, Canada) were used for the preparation of composite denoted as TiO2-SnS2-COMM. Titanium tetraisopropoxide (TTIP) (97%, Sigma-Aldrich), ethanol, perchloric acid (70%, Kemika, Zagreb, Croatia), tetraethyl orthosilicate (TEOS) (99% GC grade, Sigma-Aldrich), hydrochloric acid (36.5%, Gram-mol, Zagreb, Croatia), and Levasil® 200/30 (Obermeier, Bad-Berleburg, Germany) were used for immobilization of thin films. Spectroscopically pure titanium foil (Ti, 99.9%, Alfa Aesar, Karlsruhe, Germany) was used as the substrate for working electrodes in electrochemical measurements, while electrolyte contained sodium chloride (p.a., Kemika, Zagreb, Croatia). Diclofenac sodium salt (DCF) (p.a., Sigma-Aldrich, Taufkirchen, Germany) was used as a model water pollutant. Following auxiliary chemicals were used as well: hydrogen peroxide (w = 30%, Gram-mol, Zagreb, Croatia), sodium hydroxide (p.a., Kemika, Zagreb, Croatia), sulfuric acid (>96%, Kemika, Zagreb, Croatia), ammonium metavanadate (p.a., Kemika, Zagreb, Croatia), methanol (HPLC grade, Sigma-Aldrich, Taufkirchen, Germany), orthophosphoric acid (w ≈ 85%, Fluka, Bucharest, Romania), potassium nitrate (p.a., Kemika, Zagreb, Croatia), potassium hydroxide (p.a., Kemika, Zagreb, Croatia), and nitric acid (≥90%, Sigma Aldrich, Taufkirchen, Germany).

2.2. Photocatalyst Synthesis and Immobilization

Hydrothermal method, according to procedure adopted from Zhang et al. [9], was applied to prepare TiO2, SnS2 and their composites denoted as TiO2-SnS2-HT. TiO2 was synthesized by dissolving an aliquot of TBO precursor in a 5% v/v solution of acetic acid in ethanol with a constant stirring in Teflon reaction vessel, which was then transferred to a stainless steel autoclave, where treated for 12 h at 180 °C. After cooling naturally to room temperature, the obtained suspension was rinsed with distilled water, centrifuged (3500 rpm for 3 min), dried in a vacuum (3 h at 60 °C), and then homogenized with a porcelain pestle and mortar. The same procedure was applied for SnS2 and TiO2-SnS2-HT composite synthesis, using corresponding precursors (tin(IV) chloride, thioacetamide, and TBO in later case), while their stoichiometric were varied to obtain the composite with different SnS2 content (wt % of 5, 27.5, 50, 72.5). Composites denoted as TiO2-SnS2-COMM were prepared from the commercial TiO2 and SnS2 particles during immobilization procedure, whereas their weights were varied to obtain abovementioned ratios.
The immobilization procedure included the addition of as-prepared composite or a mixture of commercial particles into titania/silica binder sol (TSB) to obtain TiO2-SnS2-HT and TiO2-SnS2-COMM, respectively. TSB was made by mixing two as-prepared sols (nanocrystaline titania sol synthetized from TTIP and silica sol synthetized from TEOS), colloidal SiO2 and ethanol [28]. The suspended particles within TSB sol were firstly homogenized using ultrasonic bath, and then the final sol suspension was immobilized on (i) round titanium discs (d = 1 mm, r = 12 mm) in three layers, and (ii) round glass plates (r = 35.5 mm) in one layer using spin coating (1500 rpm) technique with KW-4A Spin Coater (Chemat Technology, Northridge, CA, USA). The final fixation step was performed by heating the coated titanium discs or glass plates in a laboratory oven (2 h at 200 °C).

2.3. Photocatalyst Characterization

Diffuse reflectance spectra (DRS) of immobilized photocatalysts, were measured using UV–vis spectrophotometer equipped with an integrating sphere, Lambda 650S (Perkin Elmer, Waltham, MA, USA). The acquired reflectance vs. wavelength spectra were transformed into the Kubelka–Munk function (KM) vs. photon energy () in order to obtain the band gap values [29,30,31]. Nitrogen adsorption analysis, performed using Gemini 2380 instrument (Micromeritics, Norcross, GA, USA), was used to determine the specific surface area of composites and their pure components. Brunauer–Emmett–Teller (BET) surface area was calculated from BET plot. The samples of immobilized composites were obtained by carefully scratching the films and collecting the corresponding powder, which was then grounded in an agate mortar. The same procedure for obtaining such powdered samples of composites was applied prior determination of point of zero charge (pHPCZ) values. Handylab pH/LF portable pH meter (Schott Instruments GmbH, Mainz, Germany) was used for pH monitoring during potentiometric titration applied for the determination of pHPCZ of TiO2-SnS2 composites. The pHPCZ values were determined according to the modified procedure of Uppal et al. [32], while detail description of applied methodology is provided in the previous study [33].
Electrochemical characterization of photocatalysts immobilized on titanium substrates was performed using Solartron potentiostat/galvanostat 1287 with FRA 1260 in a conventional three-electrode cell: Ti-coated disc was working electrode, the counter electrode was Pt and the reference electrode was Ag|AgCl (E = 0.208 V vs. SHE). The electrolyte was 3% NaCl solution, pure or spiked with 0.1 mM DCF. The structure of catalyst films–electrolyte interface was investigated at the open circuit potential (Eocp) using electrochemical impedance spectroscopy (EIS); frequency ranged from 100 kHz to 5 mHz at an ac voltage amplitude of ±5 mV. The experimental data were fitted using the complex non-linear least squares fit analysis software [34] and values of elements of proposed electric equivalent circuit (EEC) were derived with χ2 values <5 × 10−3. The electronic-semiconducting properties of catalyst films were investigated by Mott–Schottky analysis [22,23,24]. The interfacial capacitance values were obtained from EIS measurements as a function of both frequency and applied potential in a rapid polarization scan. Detailed description is given in Supplementary Material.

2.4. Photocatalytic Activity under Solar Irradiation

The photocatalytic experiments were performed in a water-jacket batch reactor (V = 0.09 L and T = 25.0 ± 0.2 °C). The source of simulated solar irradiation was a 450 W Xenon arc lamp (Osram, Munich, Germany) situated in Oriel/Newport, USA housing with collimating optics. An Oriel AM1.5 G air mass filter was situated in the path of the collimated beam, mimicking solar spectral characteristics when the Sun is at a zenith angle of 48.2°. The light intensity, 124.78 ± 0.11 mW cm−2, was determined by pyranometer CMP21 (Kipp & Zonen, Delft, The Netherlands), while the intensity of UV-A irradiation emitted by solar simulator was found to be 2.05 ± 0.07 mW cm−2, determined using UVX radiometer equipped with UVX-36 longwave sensor (both UVP, Cambridge, UK). Aqueous solution of DCF (c0 = 0.1 mM) was treated by solar driven photocatalytic processes using TiO2-SnS2-COMM or TiO2-SnS2-HT in the presence and absence of H2O2, whereas TiO2 P25 was used as a benchmark material. The DCF solution with adjusted pH was then spiked with H2O2 (where applicable) according to full factorial (FFD) or Box–Behnken designs (BBD) (Table 1 and Tables S1–S4, Supplementary Material), which was followed by the immersion of the glass plates into the reactor placed on an orbital shaker DOS-20 (90 rpm, neoLab, Heidelberg, Germany). Adsorption equilibrium was reached within 30 min in a dark, and thereafter the reaction solution was exposed to simulated solar irradiation. The samples (500 μL aliquots) were taken during experiments at −30 (30 min prior to irradiation), 0 (the start of irradiation), 15, 30, 45, and 60 min, filtered using Chromafil XTRA RC (25 mm, 0.45 μm, Macherey Nagel, Düren, Germany), quenched with CH3OH and submitted to HPLC analysis. All experiments were repeated at least three times and averages are reported; the reproducibility was >97.3%. DCF concentration was monitored by HPLC analysis, using Series 10 apparatus equipped with UV-DAD, SPD-M10AVP (Shimadzu, Japan), Nucleosil C18 column (5 μm, 25.0 cm × 4.6 mm, Macherey Nagel, Düren, Germany), and mobile phase CH3OH 0.1% formic acid operating at 1.0 mL min−1 flow. UV–vis spectrophotometer, Lambda EZ 201, Perkin Elmer (USA) was used for the monitoring of H2O2 by metavanadate method [35]. In desorption tests, the plates used in photocatalytic experiments were immersed in NaOH solution (90 mL, pH 8.00 ± 0.05) and placed on the shaker for 30 min. The solution was analyzed according to the above procedure. The influence of initial pH, H2O2, and SnS2 wt % within TiO2-SnS2 composites, on DCF removal and conversion by solar driven photocatalytic treatment was correlated by means of RSM. Related calculations, procedure, and analysis are described in Supplementary Material.

3. Results and Discussion

3.1. Semiconducting and Surface Properties of TiO2-SnS2 Composite Materials

DRS analysis of TiO2-SnS2-COMM and TiO2-SnS2-HT composites, with different SnS2 content ranging from 5 to 72.5 wt %, was performed to determine the band gap values, Eg of composite materials studied. DRS spectra of the pure components TiO2-HT, SnS2-HT and SnS2 MKN-900 were also recorded. The spectra obtained (Figure S1A,B, Supplementary Material) were transformed using Kubelka–Munk function, Figure 1A,B. The Eg values determined are given in Table 2. While F(R)1/2 vs. plots of TiO2-SnS2-HT are characterized with only one flat section, the existence of two flat sections in plots of TiO2-SnS2-COMM, more emphasized with SnS2 wt % increase, point to the segregation of TiO2 and SnS2 composite phases. Such results are in accordance with the established morphology revealed by SEM/EDX analysis showing phase separation in the COMM composite while the HT composite consisted of TiO2 matrix with embedded and homogenously dispersed SnS2 small proto-platelets [33,36]. The described morphology characteristics, i.e., phase coupling degree of composite components, enable/disable charge carries transfer through both materials present within. The band gap values of both composites decreased with SnS2 wt % increase. However, this effect became less significant upon exceeding SnS2 wt % over 50%, Table 2. Based on the results obtained, the composites possessing SnS2 wt % in range 5–50% were set as preferable to be studied in the further tests. Accordingly, the formulation of 72.5 wt % of TiO2 and 27.5 wt % of SnS2 presents the middle point of such set range.
Semiconducting properties of TiO2-SnS2-COMM and TiO2-SnS2-HT films on titanium substrate were further probed by Mott–Schottky analysis. This analysis was performed taking the middle point formulation (27.5 wt % of SnS2) for both composites. Mott–Schottky plots for composites (Figure 2) show linear dependence of space charge capacitance ( C SC 2 ) against applied potential (E) with a positive slope suggesting n-type semiconducting behavior for both composite films. Under depletion conditions, Mott–Schottky approximation of the capacitance-voltage relationship for n-type semiconductor has the form [37]
  1 C SC 2 = [ 2 ( ε ε 0 e N D ) ] ( E E FB k T e )
where ε0 is vacuum permittivity, ε is dielectric constant of composite film, e is electron charge, ND is donor concentration, EFB is flat band potential, k is Boltzmann constant, and T is thermodynamic temperature. From the slope values (Figure 2), ND values equal to 3.6 × 1020 cm−3 and 3.9 × 1019 cm−3 were determined for TiO2-SnS2-COMM and TiO2-SnS2-HT films, respectively. The lower ND values for TiO2-SnS2-HT in comparison to TiO2-SnS2-COMM suggests the formation of more crystalline film, as was corroborated by XRD and SEM/EDX results in the previous studies [33,36]. From the slopes and intercepts of Mott–Schottky plots (Figure 2, Equation (1)), EFB values equal to −0.81 V and −0.77 V were determined for TiO2-SnS2-COMM and TiO2-SnS2-HT films, respectively. The pH of electrolyte solution has strong influence on EFB in accordance with Nernstian expression for EFB of a semiconductor [38,39]
E FB = E F 0 q + Δ ϕ H
Δ ϕ H = 0.059 V ( pH pzc pH )
where pHpzc is the point of zero charge, E F 0 is Fermi level of semiconductor at pHpzc, and Δ ϕ H is the potential drop in Helmholtz double layer at the solid|electrolyte solution interface. At a certain potential, the Fermi level lies at the same energy as the electrolyte solution potential. There is no net transfer of charge, and hence there is no band bending. This potential is therefore referred to as the flat band potential, EFB.
The specific surface area of studied TiO2-SnS2 composites and their pure components (TiO2 P25, SnS2 MKN-900, TiO2-HT, and SnS2-HT) was determined using BET analysis. The surface charge of studied composites was evaluated by means of their pHPZC values, determined by potentiometric titration. The results are summarized in Table 3 (for composites) and Table S5 (Supplementary Material) (for their pure Components), and presented graphically for pHPZC of both composites (Figure S2, Supplementary material). BET surface areas of studied composites differ significantly (Table 3). Specific area of TiO2-SnS2-HT is almost twice as large as that of TiO2-SnS2-COMM. A similar trend in the specific surface area values observed for their pure components was assigned to the particle size and crystalline phase types (Table S5). TiO2-HT is characterized by smaller particles comparing to TiO2 P25 and exclusively anatase crystalline phase [33,36]. Both SnS2 materials possess the same crystalline phase (berndtite), exhibiting rather different morphologies with SnS2-HT comprised of significantly smaller structures in comparison to SnS2 MKN-900 [33]. Taking into account the pHPZC values determined, it can be concluded that as-synthetized particles have significantly lower pHPZC values than commercial ones (Table 3), which might be reflected in their photocatalytic activity, as investigated further in the study.
The further characterization of composites’ surface properties was conducted by EIS measurements, performed at Eocp representing real conditions of potential catalyst application. The impedance spectra of both composite films on Ti plates with and without presence of the organic pollutant DCF (Figure 3), were fitted using EEC (insert in Figure 3); corresponding EEC parameter values are summarized in Table 4. R corresponds to the ohmic (electrolyte) resistance. The Rct and Cdl parameters can be linked to the charge transfer processes that occur in an outer porous part of the composite catalyst film at the film–electrolyte interface. Thus, Rct is a charge transfer resistance and Cdl represents the interfacial (double layer) capacitance. The impedance parameters Rf and Cf correspond to the resistance and capacitance of the inner part of the barrier composite film. Cdl of TiO2-SnS2-HT is several times larger when DCF is present in the electrolyte, indicating a strong and large adsorption extent on the film’s surface. The changes in Cf and Rf values are ascribed to different dielectric properties upon formation of the thin adsorption layer. On the other hand, the presence of DCF molecules in the electrolyte influenced R and Rct values of TiO2-SnS2-COMM film–electrolyte interface as can be seen in high frequency part of EIS magnitude plots (Figure 3). The impact on the film’s inner part, according to Rf and Cf values was negligible also confirming the less degree of DCF adsorption on TiO2-SnS2-COMM film.

3.2. Solar-Driven Photocatalytic Treatment of DCF Using TiO2-SnS2 Catalysts

The photoactivity of TiO2-SnS2-COMM and TiO2-SnS2-HT composites under simulated solar irradiation was studied using DCF as a model organic pollutant. The influence of key process parameters; initial pH, composite formulation and concentration of H2O2 oxidant, on the DCF removal/conversion yield was evaluated. The yield results were compared to that obtained using benchmark material, TiO2 P25. The determined kinetic profiles of DCF removal by solar/TiO2, solar/TiO2-SnS2-COMM, solar/TiO2-SnS2-COMM/H2O2, solar/TiO2-SnS2-HT, and solar/TiO2-SnS2-HT/H2O2 processes are given in Figure 4 and Figures S3–S6 (Supplementary Material), respectively. Two common effects can be observed; (i) the DCF adsorption is closely related to initial pH and (ii) kinetic profiles of DCF removal upon exposure to solar irradiation obeyed first-order kinetics regardless the catalyst material type and formulation, as well as conditions applied. The latter is in good agreement with the literature [40].
The graphical representations showing only end treatment points, i.e., removals during initial dark period and total, obtained by solar/TiO2-SnS2-COMM and solar/TiO2-SnS2-HT (Figure 5A), and solar/TiO2-SnS2-COMM/H2O2 and solar/TiO2-SnS2-HT/H2O2 processes (Figure 5B), are constructed for experiments conducted according to FFD and BBD (Tables S1–S4, Supplementary Material). The corresponding graphs presenting recorded DCF conversions during the same treatments, performed at the same conditions are given in Figure S7 (Supplementary Material). As can be seen, the adsorption affinity of applied materials, including TiO2 P25 (Figure 4), toward DCF in a dark strongly increases by lowering initial pH values. Such effect is related to the surface charge and specific surface area of material, as well as the dissociation constant of DCF. TiO2-SnS2-COMM has pHPZC value of 6.31, while pHPZC of TiO2-SnS2-HT is lower (4.61) (Table 3). Furthermore, TiO2-SnS2-HT has almost double larger specific surface area than TiO2-SnS2-COMM (Table 2). EIS measurements clearly demonstrated that Cdl of TiO2-SnS2-HT is several times larger when DCF is present in the electrolyte (230.6 >> 61.5 μF cm−2, Table 4), indicating a strong and large adsorption extent on the film’s surface, which was not the case with TiO2-SnS2-COMM (63.8 > 45.0 μF cm−2). Trovo and Nogueira [41] stated that pKa of DCF is 4.15, meaning that at pH > pKa DCF would be present mostly in its deprotonated form and vice versa. These facts mean that the adsorption of DCF (in its deprotonated form) would be promoted at pH < pHPZC of studied materials. The strongest adsorptions in our case, i.e., DCF removal extents during the initial dark period, were obtained at the lowest studied pH (4) (Figure 5), especially in the case of TiO2-SnS2-COMM. This material exhibited rather limited adsorption capacity of DCF at other studied pH values (5.5 and 7, Figure 5). Similar results are obtained by benchmark material TiO2 P25 (Figure 4), which is built into TiO2-SnS2-COMM. On the other hand, TiO2-SnS2-HT showed rather high affinity toward DCF adsorption during initial dark period at pH 5.5 as well (Figure 5). Such results are the consequence of abovementioned differences in BET surface area (Table 2), but also the permeability of immobilized films, as demonstrated by EIS measurements (Figure 3, Table 4). It should be also noted that we have studied the initial pH of DCF solution, thus the immersion of glass plate with immobilized TiO2-SnS2 composite resulted with the certain decrease of adjusted initial pH toward more acidic pH values (e.g., from 5.5 to 4.65 in the case of TiO2-SnS2-HT). As can be seen from the results showed in Figure 5, and DCF removal by TiO2 P25 (Figure 4), the significant overall DCF removal (≥70%) was accomplished when rather large portion of DCF (≥40%) was adsorbed during initial dark period. Such an effect is particularly pronounced in the case of processes using TiO2-SnS2-COMM composite. Usually such high removals were followed with rather high DCF conversions as well (Figure 4 and Figure S7, Supplementary Material). The importance of adsorption during dark period and its relationship with high overall removal/conversion upon exposure to solar irradiation can be related with the mechanism of reactive species generation within photocatalytic process. Upon illumination of semiconducting material with the sufficient high energy, hν ≥ Eg, electron and hole pairs (e/h+) formed at catalyst surface enable the generation of radical species or direct oxidation/reduction of adsorbed organics [42]. Hence, effective adsorption of organics during dark period enables their direct degradation at surface either at e/h+ or by formed radicals, overcoming transfer limitations related to their diffusion into the solution where bulk organics can react with them [40,43]. Improved photocatalytic activity of TiO2-SnS2-HT composite over TiO2-SnS2-COMM can be attributed to (i) higher number of free charge carriers in TiO2-SnS2-HT composite (Figure 2) and (ii) lower Rct values pointing to facilitated charge transfer across the film (Table 4). A decrease in Rct value, when DCF is present, could indicate charge transfer between TiO2 and SnS2 hetero-junctions mediated by DCF. It is also noteworthy to stress that both composite films possess the excellent corrosion resistance (Rcorr), which is a key factor for their application as catalysts in photocatalytic processes for water purification purposes. Rcorr equals to the sum of the resistance components Rct and Rf (Table 4) and for both film is in order of MΩ cm2. This result also displays the durability of TiO2-SnS2 interaction to the inhibition of SnS2 leaching and in turn materials’ (catalysts’) photostability. In conclusion, EIS as non-destructive in situ method clearly showed that TiO2-SnS2-HT composite film exhibits high DCF adsorption degree and offers good electric and charge transfer properties which are key-governing factor for the enhanced efficient photocatalytic activity in the solar driven water treatment. Comparing the effectiveness of processes using composites (Figure 5 and Figure S7, Supplementary Material) to that with benchmark material (Figure 4.), following can be concluded. The adsorption of DCF during initial dark period was similar at the lowest initial pH studied (~50% of DCF was removed), regardless the material used. The increase in SnS2 wt % yielded an increase in overall DCF removal and conversion comparing to process using benchmark material, presumably due to higher photoactivity of composites under induced simulated solar irradiation.
In order to provide deeper insight into effects occurring in TiO2-SnS2 composites that are responsible for its activity within solar driven water treatment, the semiconducting properties of pure components TiO2-HT and SnS2-HT were also investigated using Mott–Schottky analysis (Figure 6A). Both catalyst films exhibit positive slope of C SC 2 against E dependence, suggesting n-type semiconducting behavior. From the slope and the intercepts values of Mott–Schottky plots (Figure 6A), ND and EFB were calculated using Equation (1): ND values are determined as 1.6 × 1018 cm−3 and 2.6 × 1020 cm−3, and EFB as −0.61 V and −0.77 for TiO2 and SnS2, respectively.
Taking into account the semiconducting parameters; EFB values and Eg values (Table 2), the energy band diagram was constructed (Figure 6B). The conduction band (EC) and valence band (EV) potentials of SnS2 are more negative than those of TiO2. This thermodynamically allows the photogenerated electrons to transfer from the SnS2 conduction band to TiO2 conduction band under visible-light irradiation (λ > 420 nm) that enhances the separation of photogenerated electron and holes in SnS2 and bring about the sensitization of TiO2. Additionally, the positions of the decomposition potentials of the semiconductor in competition with other redox reactions in the solution were calculated from the thermodynamic data [44,45,46].
According to Gerischer’s approach developed for the binary compound semiconductors [38,47], the reductive decomposition potentials (nEdecomp) of both sulfide film and oxide film lie outside of the band edges, hence both semiconductor components are stable towards theirs reductive decomposition processes, which take place by electrons as major charge carriers in n-type semiconductor
TiO 2 + 4 H + + 4 e Ti + 2 H 2 O
SnS 2 + 2 H + + 4 e Sn + 2 HS
At first glance, the positions of the anodic decomposition potential values (pEdecomp) may suggest the susceptibility of both semiconductor materials to the anodic decomposition; however, when discussing the materials’ stability all competitive redox process at the film–electrolyte interface must be taken into consideration. If redox potentials of solvent are located above pEdecomp and if these reactions are fast enough, decomposition of the semiconductor would be stabilized in the dark and during solar irradiation conditions [47]. As can be seen from the energy band diagram constructed, based on the redox potential (Fermi level position), the most thermodynamically favorable reaction is the formation of hydroxyl radical, HO• via reaction
H 2 O 2 + e HO + OH
that is desirable for DCF conversion process. The position of calculated energy levels for the anodic decomposition potential (pEdecomp) of SnS2
SnS 2 + 4 H 2 O + 2 O 2   +   8 h VB + Sn 4 + + 2 SO 4 2 + 8 H +
and for reaction
H 2 O 2 + H + + e HO + H 2 O
suggest these are competitive process, thus latter reaction is kinetically more favorable, i.e., formation of HO• is more favorable than SnS2 oxidative decomposition. Likewise, the oxidative decomposition of TiO2
TiO 2 + 4 Cl +   4 h VB + TiCl 4 ( aq ) + O 2
and O2 evolution reaction
2 H 2 O +   4 h VB + 4 H + + O 2
are also competitive processes with the latter being thermodynamically and kinetically favorable. In summary, it should be stressed out that even though if conditions of solar irradiation lead to SnS2 oxidative decomposition, the composite catalyst material would remain stable and photo-catalytically active due to stable TiO2 phase/component since pEdecomp of TiO2 in the presence of chloride ions is below pEdecomp of SnS2. Additionally, reactions (7) and (9) take place using holes as minor charge carriers in the n-type semiconductor.
Since no clear pattern of behavior and influence of studied process parameters (initial pH, composite formulation, and presence and concentration of H2O2) can be observed neither from the kinetic profiles (Figures S3–S6, Supplementary Material) nor from the end treatment points (providing insight into pH influence only) (Figure 5 and Figure S7, Supplementary Material), the RSM approach was employed. The statistical part on derived RSM models is summarized below. RSM models (M1–M8) were derived (Table S6, Supplementary Material) by applying the multiple regression analysis on FFD or BBD matrices and DCF removal and conversion extents after 60 min treatment (Tables S1–S4, Supplementary Material). The models are characterized for their accuracy, significance, and predictivity by ANOVA (0.982 < R2 < 0.998 and 0.0001 < p < 0.0054; Tables S7–S10, Supplementary Material) and RD tools (example provided in Figure S8, Supplementary Material). According to obtained results, models can be used hereinafter as tools to enlighten the influence of studied parameters of on DCF removal and conversion, correlating determined semiconducting and barrier properties of applied semiconducting materials and occurring mechanisms within the solar driven photocatalytic treatment.
The mutual effects of studied process parameters on DCF removal and conversion are presented through 3D surface and contour plots. As discussed above, the pH has significant influence on water treatment by studied processes, particularly on the adsorption during initial dark period and consequently on the overall yield. This effect can be observed from 3D surface and contour plots describing overall DCF removal and conversion by solar/TiO2-SnS2-COMM (Figure 7), solar/TiO2-SnS2-COMM/H2O2 (Figure 8), solar/TiO2-SnS2-HT (Figure 9), and solar/TiO2-SnS2-HT/H2O2 (Figure 10). ANOVA indicated the same trend as well (Tables S7–S10, Supplementary Material). ANOVA also revealed that both H2O2 (where applicable) and SnS2 wt % are significant process parameters. The importance of SnS2 wt % in composite can be related to the decrease in band gap values by increasing SnS2 content within composite (Table 2). The composites with the lowest SnS2 content (5 wt %) exhibited similar effectiveness as TiO2 P25 when treatments were conducted in the absence of H2O2 (Figure 4, Figure 7 and Figure 9). DCF removal and conversion extents obtained at pH 4 were ~70% and ~60%, respectively. However, the increase in SnS2 content increased effectiveness of process using TiO2-SnS2-COMM almost linearly reaching 81% and 67% of DCF removal and conversion, respectively, at pH 4 and 50 wt % of SnS2 (Figure 7). TiO2-SnS2-HT showed even higher effectiveness at comparable conditions (93% and 77% of DCF removal and conversion, respectively) (Figure 9). Its effectiveness increase follows the exponential trend, similar as noticed in the decrease of band gaps values (Table 2). The addition of H2O2 into the system improved its performance for approximately 10% in both DCF removal and conversion, regardless the composite preparation method (Figure 7, Figure 8, Figure 9 and Figure 10). According to ANOVA (Tables S8 and S10, Supplementary Material) [H2O2] is a significant process parameter, while the constructed energy–band diagram (Figure 6B) clearly shows favorability of HO• generation through Equations (6) and (8). Due to the fact that no changes in H2O2 during the treatment were observed (results not showed), one can assume that either (i) H2O2 was not consumed or (ii) H2O2 was simultaneously consumed and produced within the system. In the former case, the observed positive effect of H2O2 (Figure 7, Figure 8, Figure 9 and Figure 10) can be explained by playing a role as a weak acid, thus lowering pH and consequently promoting favorable conditions for the adsorption during the initial dark period. In turn, subsequent conversion at the catalyst surface during the irradiation period is improved as well. However, the latter assumption on simultaneous consumption/generation is more likely to occur as it is supported by the energy band diagram (Figure 6B). The generation of HO• through reactions involving H2O2 depletion by photogenerated electrons are the most thermodynamically and kinetically favorable anodic reactions (Figure 6B). The optimal wt % of SnS2 is also influenced by H2O2. A lower wt % of SnS2 is required to achieve maximum DCF removal and conversion in the presence of H2O2. In the case of solar/TiO2-SnS2-HT/H2O2 36.1 wt % SnS2 was found to be optimal, while 46.5 wt % was deemed optimal for solar/TiO2-SnS2-HT, according to Figure 9 and Figure 10, respectively. The plausible explanation for such effect can be found in the competitive reactions occurring at anode (7) and (8), as shown through constructed energy band diagram (Figure 6B). Hence, when oxidant is absent, more SnS2 would be exposed to decomposition by photogenerated h+ (7), thus this negative effect should be compensated with higher SnS2 content within the composite. On the other hand, SnS2 decomposition is suppressed in the presence of H2O2 due its favorable depletion by photogenerated e (8).

4. Conclusions

The TiO2-SnS2 composite catalysts were prepared by hydrothermal synthesis (TiO2-SnS2-HT) and by immobilization of the commercial TiO2 and SnS2 particles (TiO2-SnS2-COMM) and applied for the solar-driven DCF degradation.
The first property relevant to the photocatalytic activity of TiO2-SnS2 photocatalyst is its energy band configuration. Energy band diagram was constructed using semiconducting parameters (results of DRS and Mott–Schottky analysis) as well as the thermodynamic data for the determination of the energy levels for the catalysts’ oxidative and reductive decomposition and competitive redox reactions within the band gap. Based on these results, the catalysts’ photocorrosion stability and kinetically preferable redox reactions were discussed in the terms of the solar-driven DCF degradation.
The difference in the conduction band edges of the SnS2 and TiO2 composite components of approx. 160 mV enabled the transfer of photogenerated electrons from the conduction band of the sensitized narrow-band gap semiconductor SnS2 to the conduction band of the wide-band gap TiO2 semiconductor. Thus, SnS2-TiO2 photocatalysts with the heterojunction structure satisfy the light adsorption and charge separation criteria simultaneously, resulting in an efficient photoactivity.
Another key issue influencing the photocatalytic capability of SnS2-TiO2 photocatalyst is a nature of the surface–interface chemistry. The increased values of the specific area surface (BET analysis) and the adsorption capacity at the solid–solution interfaces (EIS results) and the lower pHpzc values (potentiometric titration) revealed that composite catalyst prepared by HT synthesis shows remarkable improvement in the effectiveness of the DCF removal and conversion during solar-driven water treatment. More SnS2-TiO2 heterojunctions facilitated the effective photoelectron transfer, which promoted photocatalytic DCF degradation.
Furthermore, thermodynamically favored reactions in the energy band diagrams elucidated the observed effects during the photocatalytic DCF treatment. In the presence of H2O2 less wt % SnS2 in the composite material is required to achieve the highest DCF removal and conversion degree, which is contributed to competitive depletion of H2O2 by the photogenerated electron. Although the effectiveness of TiO2-SnS2-COMM is fairly limited at circumneutral conditions, both composites are effective for DCF removal and conversion at acidic pHs and showed higher activity at comparable conditions under solar irradiation than benchmark material TiO2 P25. This is particularly valid for TiO2-SnS2-HT which was more active than TiO2 P25 through the entire range of parameters studied.

Supplementary Materials

The following are available online at https://www.mdpi.com/1996-1944/11/6/1041/s1, Figure S1: Diffuse reflectance spectra of immobilized TiO2-SnS2 composites with different SnS2 wt %; commercial (COMM) (A) and hydrothermal (HT) (B), Figure S2: Determination of pHPZC values TiO2-SnS2-COMM (A) and TiO2-SnS2-HT (B) composites, Figure S3: Kinetics of DCF removal by solar/TiO2-SnS2-COMM process; TiO2-SnS2-COMM prepared by immobilization using AEROXIDE TIO2 P25 and SnS2 MKN-900 (Experimental conditions listed in Table 1, and experimental matrix provided by FFD, Table S1, Supplementary material), Figure S4: Kinetics of DCF removal by solar/TiO2-SnS2-COMM/H2O2 process; TiO2-SnS2-COMM prepared by immobilization using AEROXIDE TIO2 P25 and SnS2 MKN-900 (Experimental conditions listed in Table 1, and experimental matrix provided by BBD, Table S2, Supplementary material), Figure S5: Kinetics of DCF removal by solar/TiO2-SnS2-HT process; TiO2-SnS2-HT prepared by hydrothermal method (Experimental conditions listed in Table 1, and experimental matrix provided by FFD, Table S3, Supplementary material), Figure S6: Kinetics of DCF removal by solar/TiO2-SnS2-HT/H2O2 process; TiO2-SnS2-HT prepared by hydrothermal method (Experimental conditions listed in Table 1, and experimental matrix provided by BBD, Table S4, Supplementary material), Figure S7: Comparison of DCF conversion using TiO2-SnS2-COMM and TiO2-SnS2-HT without H2O2 (A) and with H2O2 addition (B) under solar radiation at conditions set by FFD (Tables 1, and S1 and S3, Supplementary material) and BBD (Tables 1, and S2 and S4, Supplementary material), respectively, Figure S8: Residual diagnostics of model M6 for the prediction of the conversion of DCF by solar/TiO2-SnS2-HT/H2O2 process: (A) observed vs. predicted plot, (B) normal probability plot, and (C) internally studentized residuals vs. predicted values plot, Table S1: FFD matrix for removal (M1) and conversion (M2) of diclofenac by solar/TiO2-SnS2-COMM process after 60 min exposure, Table S2: BBD matrix for removal (M3) and conversion (M4) of diclofenac by solar/TiO2-SnS2-COMM/H2O2 process after 60 min exposure, Table S3: FFD matrix for removal (M4) and conversion (M5) of diclofenac by solar/TiO2-SnS2-HT process after 60 min exposure, Table S4: BBD matrix for removal (M7) and conversion (M8) of diclofenac by solar/TiO2-SnS2-HT/H2O2 process after 60 min exposure, Table S5: Specific surface area of constituents of studied TiO2-SnS2 composites, Table S6: Model equations of derived RSM models for DCF removal and conversion by solar/TiO2-SnS2 without and with an oxidant H2O2, Table S7: Analysis of variance (ANOVA) of RSM models M1 and M2 predicting removal and conversion of diclofenac by solar/TiO2-SnS2-COMM process after 60 min exposure (transformed and non-transformed response values), Table S8: Analysis of variance (ANOVA) of RSM models M3 and M4 predicting removal and conversion of diclofenac by solar/TiO2-SnS2-COMM/H2O2 process after 60 min exposure (transformed and non-transformed response values), Table S9: Analysis of variance (ANOVA) of RSM models M5 and M6 predicting removal and conversion of diclofenac by solar/TiO2-SnS2-HT process after 60 min exposure, Table S10: Analysis of variance (ANOVA) of RSM models M7 and M8 predicting removal and conversion of diclofenac by solar/TiO2-SnS2-HT/H2O2 process after 60 min exposure.

Author Contributions

Conceptualization, H.K., M.M.H.; Methodology, M.K., J.K.; Software, H.K, J.K.; Validation, H.K., J.K.; Formal Analysis, M.K., J.K.; Investigation, M.K., J.K., A.L.B.; Resources, H.K.; Data Curation, A.L.B., M.M.H.; Writing-Original Draft Preparation, M.K., J.K., H.K.; Writing-Review & Editing, H.K., A.L.B., M.M.H.; Visualization, H.K., J.K., M.M.H.; Supervision, H.K., A.L.B., M.M.H.; Project Administration, H.K.; Funding Acquisition, H.K.

Funding

This research was funded by Croatian Science Foundation (UIP-11-2013-7900) and joint Croatian-French project within COGITO Hubert Curien Partnership

Acknowledgments

The research is financed through projects Environmental Implications of the Application of Nanomaterials in Water Purification Technologies (NanoWaP) (UIP-11-2013-7900, Croatian Science Foundation) and Novel Catalysts for Solar driven Water Treatment; Quantum-Chemical and Experimental Approach (joint Croatian-French project within COGITO Hubert Curien Partnership).

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Wang, F.; Hu, S. Electrochemical sensors based on metal and semiconductor nanoparticles. Microchim. Acta 2009, 165, 1–22. [Google Scholar] [CrossRef]
  2. Oi, L.E.; Choo, M.Y.; Lee, H.V.; Ong, H.C.; Hamid, S.B.A.; Juan, J.C. Recent advances of titanium dioxide (TiO2) for green organic synthesis. RSC Adv. 2016, 6, 108741–108754. [Google Scholar] [CrossRef]
  3. Banerjee, S.; Dionysiou, D.D.; Pillai, S.C. Self-cleaning applications of TiO2 by photo-induced hydrophilicity and photocatalysis. Appl. Catal. B 2015, 176–177, 396–428. [Google Scholar] [CrossRef]
  4. Ni, M.; Leung, M.K.H.; Leung, D.Y.C.; Sumathy, K. A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew. Sustain. Energy Rev. 2007, 11, 401–425. [Google Scholar] [CrossRef]
  5. Kovacic, M.; Salaeh, S.; Kusic, H.; Suligoj, A.; Fanetti, M.; Stangar, U.L.; Dionysiou, D.D.; Bozic, A.L. Solar-driven photocatalytic treatment of diclofenac using immobilized TiO2-based zeolite composites. Environ. Sci. Pollut. Res. 2016, 23, 17982–17994. [Google Scholar] [CrossRef] [PubMed]
  6. Yang, W.; Zhou, H.; Cicek, N. Treatment of Organic Micropollutants in Water and Wastewater by UV-Based Processes: A Literature Review. Crit. Rev. Environ. Sci. Technol. 2014, 44, 1443–1476. [Google Scholar] [CrossRef]
  7. Méndez, E.; González-Fuentes, M.A.; Rebollar-Perez, G.; Méndez-Abores, A.; Torres, E. Emerging pollutant treatments in wastewater: Cases of antibiotics and hormones. J. Environ. Sci. Health Part A 2017, 52, 235–253. [Google Scholar] [CrossRef] [PubMed]
  8. Landmann, M.; Raus, E.; Schmidt, W.G. The electronic structure and optical response of rutile, anatase and brookite TiO2. J. Phys. Condens. Matter 2012, 24, 195503. [Google Scholar] [CrossRef] [PubMed]
  9. Zhang, Y.C.; Li, J.; Xu, H.Y. One-step in situ solvothermal synthesis of SnS2/TiO2 nanocomposites with high performance in visible light-driven photocatalytic reduction of aqueous Cr(VI). Appl. Catal. B 2012, 123–124, 18–26. [Google Scholar] [CrossRef]
  10. Burton, L.A.; Colombara, D.; Abellon, R.D.; Grozema, F.C.; Peter, L.M.; Savenije, T.J.; Dennler, G.; Walsh, A. Synthesis, characterization, and electronic structure of single-crystal SnS, Sn2S3, and SnS2. Chem. Mater. 2013, 25, 4908–4916. [Google Scholar] [CrossRef]
  11. Vogel, R.; Hoyer, P.; Weller, H. Quantum-sized PbS, CdS, Ag2S, SbS3, and Bi2S3 particles as sensitizers for various nanoporous wide- bandgap semiconductors. J. Phys. Chem. 1994, 98, 3183–3188. [Google Scholar] [CrossRef]
  12. Velasco, M.V.R.; Sarruf, F.D.; Salgado-Santos, I.M.N.; Haroutiounian-Filho, C.A.; Kaneko, T.M.; Baby, A.R. Broad spectrum bioactive sunscreens. Int. J. Pharm. 2008, 363, 50–57. [Google Scholar] [CrossRef] [PubMed]
  13. Meissner, D.; Benndorf, C.; Memming, R. Photocorrosion of cadmium sulfide: Analysis by photoelectron spectroscopy. Appl. Surf. Sci. 1987, 38, 423–436. [Google Scholar] [CrossRef]
  14. Ohko, Y.; Setani, M.; Sakata, T.; Mori, H.; Yoneyama, H. Preparation of monodisperse ZnS Nanoparticles by size selective photocorrosion. Chem. Lett. 1999, 7, 663–664. [Google Scholar] [CrossRef]
  15. Nguyen, C.C.; Vu, N.N.; Do, T.O. Recent advances in the development of sunlight-driven hollow structure photocatalysts and their application. J. Mater. Chem. A 2015, 3, 18345–18359. [Google Scholar] [CrossRef]
  16. Tachibana, Y.; Umekita, K.; Otsuka, Y.; Kuwabata, S. Charge Recombination Kinetics at an in Situ Chemical Bath-Deposited CdS/Nanocrystalline TiO2 Interface. J. Phys. Chem. C 2009, 113, 6852–6858. [Google Scholar] [CrossRef]
  17. Chi, Y.J.; Fu, H.G.; Qi, L.H.; Shi, K.Y.; Zhang, H.B.; Yu, H.T. Preparation and photoelectric performance of ITO/TiO2/CdS composite thin films. J. Photochem. Photobiol. A 2008, 195, 357–363. [Google Scholar] [CrossRef]
  18. Li, X.; Zhu, J.; Li, H.X. Comparative study on the mechanism in photocatalytic degradation of different-type organic dyes on SnS2 and CdS. Appl. Catal. B 2012, 123–124, 174–181. [Google Scholar] [CrossRef]
  19. Kusic, H.; Leszczynska, D. Altered toxicity of organic pollutants in water originated from simultaneous exposure to UV photolysis and CdSe/ZnS quantum dots. Chemosphere 2012, 89, 900–906. [Google Scholar] [CrossRef] [PubMed]
  20. Burton, L.A.; Whittles, T.J.; Hesp, D.; Linhart, W.M.; Skelton, J.M.; Hou, B.; Webster, R.F.; O’Dowd, G.; Reece, C.; Cherns, D.; et al. Electronic and optical properties of single crystal SnS2: An earth-abundant disulfide photocatalyst. J. Mater. Chem. A 2016, 4, 1312–1318. [Google Scholar] [CrossRef]
  21. Hinojosa-Reyes, L.; Guzmán-Mar, J.L.; Villanueva-Rodríguez, M. Semiconductor Materials for Photocatalytic Oxidation of Organic Pollutants in Wastewater. In Photocatalytic Semiconductors: Synthesis, Characterization, and Environmental Applications; Hernández-Ramírez, A., Medina-Ramírez, I., Eds.; Springer International Publishing: Basel, Switzerland, 2015. [Google Scholar]
  22. Orazem, M.E.; Tribollet, B. Electrochemical Impedance Spectroscopy; John Wiley & Sons: New York, NY, USA, 2008. [Google Scholar]
  23. Katić, J.; Metikoš-Huković, M.; Šarić, I.; Petravić, M. Semiconducting properties of the oxide films formed on tin: Capacitive and XPS studies. J. Electrochem. Soc. 2016, 163, C221–C227. [Google Scholar] [CrossRef]
  24. Katić, J.; Metikoš-Huković, M.; Šarić, I.; Petravić, M. Electronic structure and redox behavior of tin sulfide films potentiostatically formed on tin. J. Electrochem. Soc. 2017, 164, C383–C389. [Google Scholar] [CrossRef]
  25. EU. Directive 2013/39/EU of the European Parliament and of the Council amending Directives 2000/60/EC and 2008/105/EC as regards priority substances in the field of water policy. Off. J. Eur. Commun. 2013, 226, 1–17. [Google Scholar]
  26. Postigo, C.; Barceló, D. Synthetic organic compounds and their transformation products in groundwater: Occurrence, fate and mitigation. Sci. Total Environ. 2015, 503–504, 32–47. [Google Scholar] [CrossRef] [PubMed]
  27. Stülten, D.; Zühlke, S.; Lamshöft, M.; Spiteller, M. Occurrence of diclofenac and selected metabolites in sewage effluents. Sci. Total Environ. 2008, 405, 310–316. [Google Scholar] [CrossRef] [PubMed]
  28. Kete, M.; Pavlica, E.; Fresno, F.; Bratina, G.; Stangar, U.L. Highly active photocatalytic coatings prepared by a low-temperature method. Environ. Sci. Pollut. Res. 2014, 21, 11238–11249. [Google Scholar] [CrossRef] [PubMed]
  29. Lopez, R.; Gomez, R. Band-gap energy estimation from diffuse reflectance measurements on sol–gel and commercial TiO2: A comparative study. J. Sol-Gel Sci. Technol. 2012, 61, 1–7. [Google Scholar] [CrossRef]
  30. Koci, K.; Obalova, L.; Matejova, L.; Placha, D.; Lacny, Z.; Jirkovsky, J.; Solcova, O. Effect of TiO2 particle size on the photocatalytic reduction of CO2. Appl. Catal. B 2009, 89, 494–502. [Google Scholar] [CrossRef]
  31. D’Amato, C.A.; Giovannetti, R.; Zannotti, M.; Rommozzi, E.; Ferraro, S.; Seghetti, C.; Minicucci, M.; Gunnella, R.; Di Cicco, A. Enhancement of visible-light photoactivity by polypropylene coated plasmonic Au/TiO2 for dye degradation in water solution. Appl. Surf. Sci. 2018, 441, 575–587. [Google Scholar] [CrossRef]
  32. Uppal, H.; Tawale, J.; Singh, N. Zinc peroxide functionalized synthetic graphite: An economical and efficient adsorbent for adsorption of arsenic (III) and (V). J. Environ. Chem. Eng. 2016, 4, 2964–2975. [Google Scholar] [CrossRef]
  33. Kovacic, M.; Kusic, H.; Fanetti, M.; Stangar, U.L.; Valant, M.; Dionysiou, D.D.; Bozic, A.L. TiO2-SnS2 nanocomposites; solar active photocatalytic materials for water treatment. Environ. Sci. Pollut. Res. 2017, 24, 19965–19979. [Google Scholar] [CrossRef] [PubMed]
  34. Boukamp, A. A nonlinear least squares fit procedure for analysis of immittance data of electrochemical systems. Solid State Ion. 1986, 20, 31–44. [Google Scholar] [CrossRef]
  35. Nogueira, F.R.; Oliveira, M.C.; Paterlini, W.C. Simple and fast spectrophotometric determination of H2O2 in photo-Fenton reactions using metavanadate. Talanta 2005, 66, 86–91. [Google Scholar] [CrossRef] [PubMed]
  36. Kovacic, M.; Kopcic, N.; Kusic, H.; Bozic, A.L. Solar driven degradation of 17β-estradiol using composite photocatalytic materials and artificial irradiation source; influence of process and water matrix parameters. J. Photochem. Photobiol. A 2018, 361, 48–61. [Google Scholar] [CrossRef]
  37. Morrison, S.R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, NY, USA, 1980. [Google Scholar]
  38. Memming, R. Semiconductor Electrochemistry; Wiley-VCH: Weinheim, Germany, 2015. [Google Scholar]
  39. Bockris, J.O.M.; Khan, S.U.M. Surface Electrochemistry: A Molecular Level Approach; Plenum Press: New York, NY, USA, 1993. [Google Scholar]
  40. Chong, M.N.; Jin, B.; Chow, C.W.K.; Saint, C. Recent developments in photocatalytic water treatment technology: A review. Water Res. 2010, 44, 2997–3027. [Google Scholar] [CrossRef] [PubMed]
  41. Trovo, A.G.; Nogueira, R.F.P. Diclofenac abatement using modified solar photo-Fenton process with ammonium iron(III) citrate. J. Braz. Chem. Soc. 2011, 22, 1033–1039. [Google Scholar] [CrossRef]
  42. Pichat, P. Photocatalysis and Water Purification: From Fundamentals to Recent Applications; Wiley: Weinheim, Germany, 2013. [Google Scholar]
  43. Robertson, P.K.J.; Robertson, J.M.C.; Bahnemann, D.W. Removal of microorganisms and their chemical metabolites from water using semiconductor photocatalysis. J. Hazard. Mater. 2012, 211–212, 161–171. [Google Scholar] [CrossRef] [PubMed][Green Version]
  44. Bard, A.J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; IUPAC, Marcel Dekker Inc.: New York, NY, USA, 1985. [Google Scholar]
  45. Lide, D.R. (Ed.) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, USA, 1995. [Google Scholar]
  46. Perrone, J. (Ed.) Chemical Thermodynamics, Vol. 12 Chemical Thermodynamics of Tin; OECD Publications: Paris, France, 2012. [Google Scholar]
  47. Gerischer, H. The impact of semiconductors on the concepts of electrochemistry. Electrochim. Acta 1990, 35, 1677–1699. [Google Scholar] [CrossRef]
Figure 1. Plot of transformed Kubelka–Munk function vs. the light energy for immobilized TiO2-SnS2 composites with different SnS2 wt %: commercial (COMM) (A) and hydrothermal (HT) (B).
Figure 1. Plot of transformed Kubelka–Munk function vs. the light energy for immobilized TiO2-SnS2 composites with different SnS2 wt %: commercial (COMM) (A) and hydrothermal (HT) (B).
Materials 11 01041 g001
Figure 2. Mott–Schottky plots of the TiO2-SnS2-COMM and TiO2-SnS2-HT composite films recorded in NaCl solution.
Figure 2. Mott–Schottky plots of the TiO2-SnS2-COMM and TiO2-SnS2-HT composite films recorded in NaCl solution.
Materials 11 01041 g002
Figure 3. EIS plots of the TiO2-SnS2-COMM (A) and TiO2-SnS2-HT (B) composite films recorded in NaCl and NaCl + DCF solutions (The insert: EEC used to fit the EIS plots given).
Figure 3. EIS plots of the TiO2-SnS2-COMM (A) and TiO2-SnS2-HT (B) composite films recorded in NaCl and NaCl + DCF solutions (The insert: EEC used to fit the EIS plots given).
Materials 11 01041 g003
Figure 4. Kinetics of DCF removal by solar/TiO2 using reference photocatalytic material; AEROXIDE TiO2 P25.
Figure 4. Kinetics of DCF removal by solar/TiO2 using reference photocatalytic material; AEROXIDE TiO2 P25.
Materials 11 01041 g004
Figure 5. Comparison of DCF removal using TiO2-SnS2-COMM and TiO2-SnS2-HT without H2O2 (A) and with H2O2 addition (B) under solar radiation at conditions set by FFD (Table 1, Tables S1 and S3, Supplementary Material) and BBD (Table 1, Tables S2 and S4, Supplementary Material), respectively.
Figure 5. Comparison of DCF removal using TiO2-SnS2-COMM and TiO2-SnS2-HT without H2O2 (A) and with H2O2 addition (B) under solar radiation at conditions set by FFD (Table 1, Tables S1 and S3, Supplementary Material) and BBD (Table 1, Tables S2 and S4, Supplementary Material), respectively.
Materials 11 01041 g005
Figure 6. Mott–Schottky plots of the TiO2-HT and SnS2-HT films recorded in NaCl solution (A). The energy band diagram for TiO2 and SnS2 catalyst films (B).
Figure 6. Mott–Schottky plots of the TiO2-HT and SnS2-HT films recorded in NaCl solution (A). The energy band diagram for TiO2 and SnS2 catalyst films (B).
Materials 11 01041 g006
Figure 7. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and SnS2 wt % on DCF removal (A) and conversion (B) by solar/TiO2-SnS2-COMM.
Figure 7. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and SnS2 wt % on DCF removal (A) and conversion (B) by solar/TiO2-SnS2-COMM.
Materials 11 01041 g007
Figure 8. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and H2O2 (A,D), pH and SnS2 wt % (B,E), and H2O2 and SnS2 wt % (C,F) on the DCF removal (top row) and conversion (bottom row) by solar/TiO2-SnS2-COMM/H2O2 process (SnS2 wt %, H2O2 and pH were held at their respective center levels in (A,D), (B,E) and (C,F), respectively).
Figure 8. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and H2O2 (A,D), pH and SnS2 wt % (B,E), and H2O2 and SnS2 wt % (C,F) on the DCF removal (top row) and conversion (bottom row) by solar/TiO2-SnS2-COMM/H2O2 process (SnS2 wt %, H2O2 and pH were held at their respective center levels in (A,D), (B,E) and (C,F), respectively).
Materials 11 01041 g008
Figure 9. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and SnS2 wt % on the transformed values of DCF removal (A) and conversion (B) by solar/TiO2-SnS2-HT.
Figure 9. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and SnS2 wt % on the transformed values of DCF removal (A) and conversion (B) by solar/TiO2-SnS2-HT.
Materials 11 01041 g009
Figure 10. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and H2O2 (A,D), pH and SnS2 wt % (B,E), and H2O2 and SnS2 wt % (C,F) on the DCF removal (top row) and conversion (bottom row) by solar/TiO2-SnS2-HT/H2O2 process (SnS2 wt %, H2O2 and pH were held at their respective center levels in (A,D), (B,E) and (C,F), respectively).
Figure 10. 3D response surface and contour diagrams showing the effects of the mutual interactions of initial pH and H2O2 (A,D), pH and SnS2 wt % (B,E), and H2O2 and SnS2 wt % (C,F) on the DCF removal (top row) and conversion (bottom row) by solar/TiO2-SnS2-HT/H2O2 process (SnS2 wt %, H2O2 and pH were held at their respective center levels in (A,D), (B,E) and (C,F), respectively).
Materials 11 01041 g010
Table 1. Experimental range and levels of the independent variables for the solar driven photocatalytic treatment of DCF using immobilized TiO2-SnS2-COMM and TiO2-SnS2-HT in the presence and absence of H2O2
Table 1. Experimental range and levels of the independent variables for the solar driven photocatalytic treatment of DCF using immobilized TiO2-SnS2-COMM and TiO2-SnS2-HT in the presence and absence of H2O2
Process ParametersModel Variables/Coded ValuesLevel/Range
−101
pHX145.57
H2O2, mM *X20.52.755
SnS2, wt % **X3527.550
* where added. ** in the case of HT composites this is related to the stoichiometric ratio of chemicals in the synthesis.
Table 2. Band gap values of TiO2-SnS2-COMM and TiO2-SnS2-HT in the dependence to SnS2 wt % within composites
Table 2. Band gap values of TiO2-SnS2-COMM and TiO2-SnS2-HT in the dependence to SnS2 wt % within composites
TiO2-SnS2 Catalyst Band Gap; Eg, eV
TypeSnS2 wt %
0527.55072.5100
TiO2-SnS2-COMM3.05 *2.892.752.672.661.91
TiO2-SnS2-HT3.092.992.292.041.961.88
* Adopted from previous study [5].
Table 3. Surface properties (specific surface area and point of zero charge values) of studied TiO2-SnS2 composites (72.5:27.5 wt %).
Table 3. Surface properties (specific surface area and point of zero charge values) of studied TiO2-SnS2 composites (72.5:27.5 wt %).
MaterialBET Surface Area, m2 g−1pHPZC
TiO2-SnS2-COMM96.00 ± 0.786.31
TiO2-SnS2-HT160.58 ± 0.464.61
Table 4. Impedance parameters of TiO2-SnS2 catalyst films recorded in NaCl and NaCl + DCF solution (R, Cdl, Rct, Cf, and Rf stand for ohmic resistance, double layer capacitance, charge transfer resistance, capacitance and resistance of the composite film, respectively)
Table 4. Impedance parameters of TiO2-SnS2 catalyst films recorded in NaCl and NaCl + DCF solution (R, Cdl, Rct, Cf, and Rf stand for ohmic resistance, double layer capacitance, charge transfer resistance, capacitance and resistance of the composite film, respectively)
TiO2-SnS2-COMM
ElectrolyteR,
Ω cm2
Cdl,
μF cm−2
Rct,
kΩ cm2
Cf,
μF cm−2
Rf,
MΩ cm2
NaCl7.645.017.6349.81.21
NaCl + DCF63.363.84.5152.31.29
TiO2-SnS2-HT
NaCl4.561.50.9547.80.24
NaCl + DCF4.0230.60.1331.80.81
Back to TopTop