Fabrication of ILs-Assisted AgTaO3 Nanoparticles for the Water Splitting Reaction: The Effect of ILs on Morphology and Photoactivity

The design of an active, stable and efficient photocatalyst that is able to be used for hydrogen production is of great interest nowadays. Therefore, four methods of AgTaO3 perovskite synthesis, such as hydrothermal, solvothermal, sol-gel and solid state reactions, were proposed in this study to identify the one with the highest hydrogen generation efficiency by the water splitting reaction. The comprehensive results clearly show that the solid state reaction (SSR) led to the obtainment of a sample with an almost seven times higher photocatalytic activity than the other methods. Furthermore, four ionic liquids, all possessing nitrogen in the form of organic cations (two imidazoliums with different anions, ammonium and tetrazolium), were used for the first time to prepare composites consisting of AgTaO3 modified with IL and Pt, simultaneously. The effect of the ionic liquids (ILs) and Pt nanoparticles’ presence on the structure, morphology, optical properties, elemental composition and the effectiveness of the hydrogen generation was investigated and discussed. The morphology investigation revealed that the AgTaO3 photocatalysts with the application of [OMIM]-cation based ILs created smaller granules (<500 nm), whereas [TBA] [Cl] and [TPTZ] [Cl] ILs caused the formation of larger particles (up to 2 μm). We found that various ILs used for the synthesis did not improve the photocatalytic activity of the obtained samples in comparison with pristine AgTaO3. It was detected that the compound with the highest ability for hydrogen generation under UV-Vis irradiation was the AgTaO3_0.2% Pt (248.5 μmol∙g−1), having an almost 13 times higher efficiency in comparison with the non-modified pristine sample. It is evidenced that the enhanced photocatalytic activity of modified composites originated mainly from the presence of the platinum particles. The mechanism of photocatalytic H2 production under UV-Vis light irradiation in the presence of an AgTaO3_IL_Pt composite in the water splitting reaction was also proposed.


Introduction
All across the world, people are faced everyday with many forms of environmental pollution, such as: water, air and land pollution. These environmental problems affect every human, animal and plant [1][2][3][4]. The best solution would be to reduce or even remove the input of pollutants; however, this is impossible. Another very important problem that the world has to face today is the demand for energy [5][6][7]. Therefore, alternative solutions aimed at removing harmful substances from the air, water and soil, as well as ways for the acquisition of clean energy are being searched for [8][9][10][11]. One such solution is the application of photoactive material able to remove pollution and/or generate hydrogen in the presence of light with specific radiation. A challenge in the field of heterogeneous photocatalysis is to develop a new type of photoactive materials activated by low-powered and low-cost irradiation sources (also sunlight) [12][13][14]. Currently, hydrogen is mainly produced from carbon monoxide and natural gas (from fossil fuels) through a steam reforming reaction [15]. However, the risk of fossil fuel depletion, as well as the serious environmental problems associated with CO 2 generation, has forced researchers to look for alternative solutions [16]. Recently, great interest has been focused on hydrogen production with a photocatalytic water-splitting reaction in the presence of semiconductor nanoparticles and UV-Vis or solar irradiation [17][18][19][20][21][22]. The basic requirements for developing photocatalysts for overall water splitting are: (i) sustainable conduction band (CB) and valence band (VB) edge potentials for overall water splitting, (ii) band-gap energy lower than 3 eV for visible-light harvesting, and (iii) photostability in time [23,24].
In this regard, the wide-bandgap semiconductors with d0 and d10 configuration such as Ti 4+ , Nb 5+ and Ta 5+ are used as photocatalysts for the degradation of pollutants and for hydrogen generation in the reaction of water-splitting [25,26]. Until now, the most commonly used materials were titanates (Ti 3d) because of their high ability to reduce water for H 2 production. However, it was found that tantalate photocatalysts could be a better candidate since (i) the Ta 5+ ion possesses higher reduction potential for hydrogen generation than most of the studied d0 elements [27] and (ii) the bond angle of Ta-O-Ta is close to 180 degrees, providing a high degree of delocalization and excellent mobility [28]. One of the very promising wide-bandgap semiconductors among tantaletes such as LiTaO 3 [29], NaTaO 3 [30], KTaO 3 [31], CsTaO 3 [32] is silver tantalate (AgTaO 3 ) with a perovskite structure. However, to date, the practical applications of AgTaO 3 are limited. It was reported that the AgTaO 3 band gap of about 3.4 eV determined the ability to absorb only UV irradiation [33]. An additional problem is the low quantum yield and high recombination rate of the photogenerated charge carriers [34]. The density functional theory (DFT) calculations demonstrated that appropriate N/F co-doping could narrow the band gap of AgTaO 3 to 2.9 eV while increasing the charge carrier mobility and the reductive strength towards hydrogen production [34]. Therefore, the following methods for increasing the photocatalytic performance of AgTaO 3 were investigated: (i) co-doping [34], (ii) application of semiconductor composites [35] and (iii) modification with noble metals [36]. Among them, enhanced photocatalytic activity for hydrogen generation by the use of a co-catalyst was the most frequently investigated. As a result of the synergy of the interaction between the photocatalyst and the co-catalyst, effective separation of the photogenerated charge carriers occurs, due to changes in the semiconductor electronic structure, such as the band gap width or the position of the valence and the conduction bands. Recently, platinum was found to be the most effective co-catalyst owing to its largest work function and lowest overpotential for H 2 evolution [37]. However, up to now, the research has focused mainly on the use of Ag nanoparticles on the AgTaO 3 surface to enhance photocatalytic activity for hydrogen generation. For instance, Ag nanoparticles deposited on the AgTaO 3 surface by means of a simple one-step chemical reduction treatment using ethylene glycol as a reducing agent allowed the preparation of the photocatalyst with a four-time increment for hydrogen production [38]. Yu et al. described the growth of Ag nanoparticles onto a AgTaO 3 /SrTiO 3 solid solution using an in situ exolution procedure with ethylene glycol [39]. The presence of a co-catalyst led to the enhancement of hydrogen generation by nearly 45% due to the localized surface plasmon effect. Photodeposition of Ag and Cu nanoparticles onto AgTaO 3 perovskite for improved photocatalytic hydrogen evolution was also reported [36]. According to our knowledge, there are no reports regarding the use of AgTaO 3 decoration using Pt nanoparticles for hydrogen evolution.
Another, actually surprising, way to improve the wide-bandgap semiconductor photoactivity under visible light is the use of ionic liquids (ILs) for photocatalyst preparation [40]. Semiconductor modification with ionic liquids is a new, effective approach, but the mechanism of their action is not yet fully explained. It is known that ionic liquids form a protective layer on the semiconductor particle surface, thus electrosteric solvation and viscous stabilization of the growing particles occurs [41]. The presence of ionic liquids in the reaction system can also promote the formation of oxygen vacancies, which can be a source of the electronic charge required for O 2 adsorption and intermediate energy level [42]. Additionally, ILs as organic compounds possess HOMO-LUMO levels. In this regard, between TiO 2 , being an n-type semiconductor, and the halogen anion of an IL (where the HOMO orbital is located), new energy levels can be formed [43]. The literature reports that ILs introduced during the preparation of a broadband photocatalyst may increase its activity under visible light due to: (i) doping of non-metal elements (e.g., N, B, F) derived from the IL structure, inducing a narrower band gap and improving the separation efficiency of the photogenerated electron/hole pairs [44]; (ii) it favoring oxygen vacancies [42]; (iii)surface complex charge transfer [45] and (iv) it affecting transport of photogenerated charges [46]. As far as we know, no one has investigated the photocatalytic activity of IL_AgTaO 3 loaded with Pt nanoparticles towards hydrogen generation.
Although AgTaO 3 has been studied for different applications, no one has reported the comparisons of four different synthesis methods, such as solvothermal (SS), sol-gel (SG), hydrothermal (HS) and solid-state reactions (SSR), to synthesize the photocatalyst with a desirable structure, morphology and enhanced photocatalytic activity using the water splitting reaction. Moreover, for the first time, the effect of ILs differing in structure, namely 2,3,5-triphenyltetrazolium chloride [ Figure 1) and Pt nanoparticles' presence on the morphology and photoactivity of AgTaO 3 has been investigated. knowledge, there are no reports regarding the use of AgTaO3 decoration using Pt nanoparticles for hydrogen evolution. Another, actually surprising, way to improve the wide-bandgap semiconductor photoactivity under visible light is the use of ionic liquids (ILs) for photocatalyst preparation [40]. Semiconductor modification with ionic liquids is a new, effective approach, but the mechanism of their action is not yet fully explained. It is known that ionic liquids form a protective layer on the semiconductor particle surface, thus electrosteric solvation and viscous stabilization of the growing particles occurs [41]. The presence of ionic liquids in the reaction system can also promote the formation of oxygen vacancies, which can be a source of the electronic charge required for O2 adsorption and intermediate energy level [42]. Additionally, ILs as organic compounds possess HOMO-LUMO levels. In this regard, between TiO2, being an n-type semiconductor, and the halogen anion of an IL (where the HOMO orbital is located), new energy levels can be formed [43]. The literature reports that ILs introduced during the preparation of a broadband photocatalyst may increase its activity under visible light due to: (i) doping of non-metal elements (e.g., N, B, F) derived from the IL structure, inducing a narrower band gap and improving the separation efficiency of the photogenerated electron/hole pairs [44]; (ii) it favoring oxygen vacancies [42]; (iii) surface complex charge transfer [45] and (iv) it affecting transport of photogenerated charges [46]. As far as we know, no one has investigated the photocatalytic activity of IL_AgTaO3 loaded with Pt nanoparticles towards hydrogen generation.
Although AgTaO3 has been studied for different applications, no one has reported the comparisons of four different synthesis methods, such as solvothermal (SS), sol-gel (SG), hydrothermal (HS) and solid-state reactions (SSR), to synthesize the photocatalyst with a desirable structure, morphology and enhanced photocatalytic activity using the water splitting reaction. Moreover, for the first time, the effect of ILs differing in structure, namely 2,3,5-triphenyltetrazolium chloride [ Figure 1) and Pt nanoparticles' presence on the morphology and photoactivity of AgTaO3 has been investigated.

Preparation of AgTaO 3
At first, we decided to use four different methods, namely solvothermal, sol-gel, hydrothermal and solid state reactions to prepare the perovskite. The applied synthesis procedures were as follows: • Preparation of AgTaO 3 by the solvothermal method. The AgTaO 3 powder was obtained as follows: 0.95 g AgNO 3 was dissolved in 120 mL of ethylene glycol and then 2.02 g TaCl  It is known that silver-based materials suffer a loss of silver at high calcination temperature. Therefore, to overcome this drawback, 3.0 wt% of Ag 2 O was added in excess to maintain the required stoichiometry [33,47]. The mixture was calcinated in air at 900 • C for 24 h, with a heating rate of 1 • C·min −1 . After this process, the sample was naturally cooled down in a furnace to the ambient temperature.

Modification of AgTaO 3 with IL and Co-Catalyst Pt by Using the Photodeposition Method
The IL-modified AgTaO 3 powders were successfully prepared via a solid state reaction by homogenization Ag 2 O, Ta 2 O 5 and IL in mortar (molar ratio of Ag 2 O to IL was constant and equaled 1:2) and calcinated at 900 • C for 24 h with a heating rate of 1 • C·min −1 (see Preparation of AgTaO 3 by Solid State Reaction).
A suspension containing AgTaO 3 or AgTaO 3 _IL (2 g), 70 mL of ethanol solution and the platinum precursor K 2 PtCl 4 (0.2 wt% of Pt) was placed in a quartz reactor and sonicated for 10 min. Then, Materials 2020, 13, 4055 5 of 18 the solution was degassed with nitrogen (8 dm 3 ·h −1 ) and stirred in the dark for 30 min. The as-prepared suspension was irradiated with an Xe lamp (250 W, Heraeus Noblelight GmbH, Cambridge, UK) used as an irradiation source of UV for 1 h. The obtained samples were separated by centrifugation, sequentially rinsed with deionized water, and dried at 60 • C for 12 h. The specific concentration of platinum in the suspension was selected based on our previous research [48].

Characterization of Materials
The crystal structure of the samples obtained was characterized by the X-ray powder diffraction method (XRD, Rigaku MiniFlex 600, Rigaku, The Woodlands, TX, USA) measured in the 2θ range of 20-80 • with the target Cu Kα irradiation. The mean crystallite size from the Scherrer equation was also estimated. The shape and size of the particles were observed by scanning electron microscopy (SEM, JEOL JSM-7610F, Jeol Ltd., Tokyo, Japan). The surface content of the samples was determined by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe TM , ULVAC-PHI, Chigasaki, Japan) with a source of monochromatic Al Kα irradiation (hν = 1486.6 eV). High-resolution spectra (HR-XPS) were measured using a hemispherical analyzer (transition energy 23.5 eV, energy step size 0.1 eV). The recorded C1s spectrum of carbon was used as reference for binding energy (284.8 eV). The BET (Gemini V (model 2365)) surface area was determined by a multipoint method with the use of adsorption data in the relative pressure (P/P0) range of 0.05-0.3 after degassing the samples at 200 • C. The diffuse reflectance spectra (UV-Vis) were recorded with a spectrophotometer (Evolution 220, Thermo Fisher Scientific, Waltham, MA, USA) in the scanning range of 200-900 nm. The spectrophotometer was equipped with an integrating sphere accessory for diffuse reflection with the baseline performed using barium sulphate. Fourier transformed infrared spectra (FTIR) were obtained with a Nicolet iS10 FTIR spectrometer in a scanning range of 500-4000 cm −1 with a resolution of 4 cm −1 . Before analysis, the samples were prepared by diluting in KBr 5% of the photocatalysts. Raman spectra were recorded a DXR Smart Raman on spectrometer. A laser emitting irradiation with a wavelength of 532 nm was used as the excitation source.

Measurements of Photocatalytic Activity in Water-Splitting Reaction
The photocatalytic hydrogen evolution experiments were carried out in a tightly closed cylindrical quartz reactor. In a typical experiment, the photocatalyst (0.1 g) was dispersed with continuous stirring (700 rpm) in an aqueous methanol solution (80 mL, C = 10%), which was used as a sacrificial reagent for holes (h + ). The process was carried out at a constant temperature of 10 • C set by a thermostatically controlled water bath. The space above the suspension was purged with nitrogen for 30 min to remove residual oxygen, and then the system was irradiated with a 1000 W Xe lamp (Oriel Instruments, Stratford, CT, USA) which emitted UV-Vis irradiation. The evolved gas (200 µL) was collected through the septum at regular time intervals every 60 min using a gas-tight syringe. The total exposure time of the sample was 240 min (in the case of testing, the exposure time of the most photoactive composite was 20 h). The amount of hydrogen generated in the tested samples was analyzed using a gas chromatograph (Trace 1300, Thermo Fisher Scientific, Waltham, MA, USA) equipped with a thermal conductivity detector (TCD) with N 2 as the carrier gas and with a column (HayeSep Q (80/100)). Hydrogen generation was determined by a blank test in the absence of a photocatalyst, where evolution of H 2 was not observed. The specific conditions for conducting the hydrogen generation process (type and concentration of sacrificial agent (10% methanol) as well as the amount of the photocatalyst (0.1 g)) were established based on our previous research [48]. Additionally, the measurement with a glass filter (GG420, Optel, Opole, Poland) cutting off wavelengths shorter than 420 nm revealed no hydrogen generation.

Results and Discussion
Firstly, the preparation routes of the AgTaO 3 synthesis was taken into consideration. Four different methods were applied, and based on the obtained results, including crystallite size (Table S1) and hydrogen evolution in the water splitting reaction (see Table S1, Figure 2), it was concluded that the technique which led to the obtainment of AgTaO 3 with the smallest crystallite size, and thus with the highest ability to generate hydrogen, was SSR in comparison with the other methods-SS, HS, SSR and SG. Therefore, we decided to select SSR for the preparation of the IL-modified samples followed by surface decoration with Pt particles using the photodeposition method.

Results and Discussion
Firstly, the preparation routes of the AgTaO3 synthesis was taken into consideration. Four different methods were applied, and based on the obtained results, including crystallite size (Table  S1) and hydrogen evolution in the water splitting reaction (see Table S1, Figure 2), it was concluded that the technique which led to the obtainment of AgTaO3 with the smallest crystallite size, and thus with the highest ability to generate hydrogen, was SSR in comparison with the other methods-SS, HS, SSR and SG. Therefore, we decided to select SSR for the preparation of the IL-modified samples followed by surface decoration with Pt particles using the photodeposition method.

Morphology
The microstructures of as-prepared powders were inspected under electron microscopy conditions. Typical SEM images of the following samples

Morphology
The microstructures of as-prepared powders were inspected under electron microscopy conditions. Typical SEM images of the following samples  (Figure 3d,e). What is more, the formation of asymmetrical cubes for those samples was observed.

The XRD and BET Analyses
The XRD patterns of the as-prepared samples are shown in Figure 4. The peaks near 22.8°, 32.6°, 46.3°, 52.2°, 57.7°, 72.4° and 76.9° corresponded to a pure phase of AgTaO3. Calcination of these samples at 900 °C for 24 h led to the formation of AgTaO3 nanoparticles with a rhombohedral perovskite type structure with R3c space group. The refined lattice parameters a, b and c, unit cell volume, and average crystallite size are gathered in Table 1 [Cl] possessed additional peaks which could originate from the ILs residual impurities. Decoration with Pt nanoparticles did not have any influence on the peak position, which indicated that Pt was deposited on the surface instead of being inserted in the crystal lattice of AgTaO3. Furthermore, no peaks derived from Pt were observed. This is probably due to their high dispersion and low content on the AgTaO3 photocatalyst. The average crystallite size was estimated based on the Scherer equation. The discrepancies in the crystallite sizes of the modified samples in comparison with the reference AgTaO3 are thought to originate from the presence of different ILs structures and the results were collected in Table 1 ), whereas for the rest, BET surface area decreased (see Table 1).

The XRD and BET Analyses
The XRD patterns of the as-prepared samples are shown in [Cl] possessed additional peaks which could originate from the ILs residual impurities. Decoration with Pt nanoparticles did not have any influence on the peak position, which indicated that Pt was deposited on the surface instead of being inserted in the crystal lattice of AgTaO 3 . Furthermore, no peaks derived from Pt were observed. This is probably due to their high dispersion and low content on the AgTaO 3 photocatalyst. The average crystallite size was estimated based on the Scherer equation. The discrepancies in the crystallite sizes of the modified samples in comparison with the reference AgTaO 3 are thought to originate from the presence of different ILs structures and the results were collected in Table 1 Table 1).

The XPS Analyses
The elemental composition in the surface region of pristine AgTaO 3 and the IL-modified AgTaO 3 _0.2% Pt composites was determined by XPS and collected in Table 2. The HR spectra of Ag 3d, Ta 4f and Pt 4f, presented in Figure 5, identify well Ag, Ta and Pt as main elements of these samples [49].

Detection of fluorine (F1s spectrum) and boron (B1s spectrum) in AgTaO 3 _[OMIM][BF 4 ] and fluorine and sulphur (S2p spectrum) in AgTaO 3 _[OMIM] [Tf 2 N] evidences the successful modification of AgTaO 3 _Pt samples by [OMIM] [BF 4 ] and [OMIM] [Tf 2 N] ionic liquids, respectively. The Cl 2p spectra recorded on AgTaO 3 _[TPTZ] [Cl] and AgTaO 3 _[TBA] [Cl] samples confirm the successful modification of AgTaO 3 with [TPTZ] [Cl] and [TBA]
[Cl] IL, respectively. However, nitrogen, originated from all IL dopants, was detected in the BE region of N 1s overlapped by intensive Ta 4p 3/2 signals. Thus, the deconvolution of these complex spectra was necessary to evaluate the nitrogen content in all samples (Table 2). Similarly, the Pt 4f spectra were partially overlapped by the Ta 5s signals. However, after deconvolution, three Pt states were identified, represented by Pt 4f 7/2 signals, located at BE of 69.9-70.2, 70.8-71.4 and 71.8-72.7 eV (see Pt 4f 7/2 fractions named as Pt1, Pt2 and Pt3, respectively in Table 2). The first Pt state (Pt1) is addressed to Pt-Ag bonds formed as a result of the Pt interaction with AgTaO 3 [49], the second (Pt2) can be attributed to Pt(0) and Pt-CO adsorbate and the last one (Pt3) we assign to Pt bound formed by CxHy and IL surface species interacting with AgTaO 3 [49]. The Pt1 state is a dominant fraction of Pt compounds in the surface region of all samples. It is interesting to note that both chloride composites, namely AgTaO 3

_[TBA] [Cl] and AgTaO 3 _[TPTZ] [Cl]
, exhibit a larger platinum content than the other samples (Table 2), which suggests the segregation of Pt to the surface region of these samples. This supposition is supported by the Pt/Ag ratios of both samples, being about two times higher than the other ones ( Table 2). The larger surface amount of Pt in these samples is accompanied by a larger amount of carbon species (see C/Ag ratios of both samples in Table 2), which indicates a larger concentration of IL at the surface. The increased amount of IL adsorbate is also detectable in the Pt 4f spectra of both samples. We observed a relative decrease in Pt1 and an increase in Pt3 fractions contributing to the Pt 4f spectra ( Table 2).
intensive Ta 4p3/2 signals. Thus, the deconvolution of these complex spectra was necessary to evaluate the nitrogen content in all samples (Table 2). Similarly, the Pt 4f spectra were partially overlapped by the Ta 5s signals. However, after deconvolution, three Pt states were identified, represented by Pt 4f7/2 signals, located at BE of 69.9-70.2, 70.8-71.4 and 71.8-72.7 eV (see Pt 4f7/2 fractions named as Pt1, Pt2 and Pt3, respectively in Table 2). The first Pt state (Pt1) is addressed to Pt-Ag bonds formed as a result of the Pt interaction with AgTaO3 [49], the second (Pt2) can be attributed to Pt(0) and Pt-CO adsorbate and the last one (Pt3) we assign to Pt bound formed by CxHy and IL surface species interacting with AgTaO3 [49]. The Pt1 state is a dominant fraction of Pt compounds in the surface region of all samples. It is interesting to note that both chloride composites, namely AgTaO3_[TBA] [Cl] and AgTaO3_[TPTZ] [Cl], exhibit a larger platinum content than the other samples (Table 2), which suggests the segregation of Pt to the surface region of these samples. This supposition is supported by the Pt/Ag ratios of both samples, being about two times higher than the other ones ( Table 2). The larger surface amount of Pt in these samples is accompanied by a larger amount of carbon species (see C/Ag ratios of both samples in Table 2), which indicates a larger concentration of IL at the surface. The increased amount of IL adsorbate is also detectable in the Pt 4f spectra of both samples. We observed a relative decrease in Pt1 and an increase in Pt3 fractions contributing to the Pt 4f spectra ( Table 2).

The FTIR and Raman Analyses of Lattice Vibration Modes
The FTIR and Raman analyses carried out confirmed the obtainment of the AgTaO 3 structure. The FTIR spectra of pristine and IL-modified AgTaO 3 are shown in Figure S1a

Optical Properties
The UV-Vis absorption spectra of the pristine and IL-modified AgTaO 3 perovskite loaded with 0.2 wt% Pt are presented in Figure S2. It can be noted that all of the obtained samples absorbed radiation mainly in the UV-Vis region. The application of ILs did not practically influence the results. However, in the case of the IL-modified samples, the absorption intensity was higher when a co-catalyst was deposited. Furthermore, the absorption band related to AgTaO 3 from 300 to 370 nm represented the co-catalyst-decorated samples. It was observed that the absorption band of the perovskite modified with ILs and the Pt particles in visible light increased in intensity, whereas the red shift was negligible. It might suggest that the ability to absorb the higher wavelength mainly came from Pt co-catalyst particles deposited on the surface of the AgTaO 3 . Platinum particles were not observed on the spectrum, probably as a result of the overlap with the absorption spectrum of AgTaO 3 . Similar results were observed for AgMO 3 (M = V, Nb, Ta) perovskite materials. The absorption band related to AgTaO 3 from 200 to 350 nm was found in regard to the co-catalyst surface-loaded samples [33].
AgTaO 3 belongs to the type of semiconductors with an indirect band gap, therefore its width was determined on the basis of the tangent lines in the plots of the square root of the Kubelka-Munk function vs. photon energy, as shown in Figure S3. It has been reported that the valence band of AgTaO 3 perovskites is generally composed by O 2p states, which can be hybridized with Ag 4d states [50]. The tangent lines, which are extrapolated to (hνα) 1

Photocatalytic Activity in the Water-Splitting Reaction
The photocatalytic activity of the obtained AgTaO 3 perovskite materials for hydrogen production via photocatalytic water splitting, where methanol was used as a hole scavenger, was investigated and the results are presented in Figure 6 and Table 1. The procedure was developed based on our previous experimental studies in the following system: sacrificial reagent-methanol; concentration of methanol-10%; amount of the photocatalyst-0.1 g [48]. Before the main photocatalytic process, control tests were performed. The experiments in the presence of 10% methanol but without the addition of the photocatalyst revealed no H 2 generation under UV-Vis irradiation. Moreover, under dark conditions, also no formation of hydrogen was detected.

Discussion of Photocatalytic Mechanism
The AgTaO3_0.2% Pt sample exhibited the highest hydrogen evolution under UV-Vis irradiation (248.5 μmol•g −1  , followed by decoration with Pt particles showed lower photoactivity in the water splitting reaction under the same conditions. The present paper aims to call into question: why does the addition of ILs suppress hydrogen evolution? Did the improvement in photoactivity originate only from the interaction between AgTaO3 and platinum? What was the role of ILs in the production of H2? To answer these questions, we need to carefully examine the above-mentioned results and the mechanisms of the photocatalytic reactions analyzed previously by our group. Based on the available literature and our own experience, it is commonly known that ILs play a significant role in the increase in photoactivity in the degradation of the aqueous phenol solution, MO and RhB solution, and photocatalytic hydrogen production [20,40,41,45,46,55,56]. Recently, we analyzed the effect of the various ILs used in the solvothermal reaction over TiO2 particles, and the results clearly show that the significant impact on the photocatalytic performance originated directly from interaction between photocatalyst particles and ILs [40,41,43,45]. The photoexcitation of the TiO2 samples modified with ILs occurred directly through the formation of a surface complex which resulted in the transfer of electrons from the LUMO orbit of the ionic liquid to the conductivity band TiO2. It was found that synthesis conditions of the solvothermal route allowed for the successful decomposition of the ILs, which resulted in the incorporation of nitrogen into the TiO2 structure and thereby significantly improved the photocatalytic activity under UV-Vis and Vis irradiation [41,43]. Qi et al. also investigated the effect of ILs on the photocatalytic activity of the TiO2 semiconductor. They realized that addition of an IL with a [Bmim] + cation slightly enhanced the photocatalytic degradation rate of MO due to enhanced trapping and transfer of the photogenerated electrons. On the other hand, presence of an IL suppressed the degradation rate of RhB on the TiO2 surface by restricting the diffusion of positively charged holes to the TiO2/solution interface [46]. On the other hand, investigation of the hydrogen evolution using IL-modified SrTiO3 perovskite followed by surface photodeposition of Pt nanoclusters did not reveal a direct correlation between the increase in H2 evolution due to the presence of the IL [48]. Furthermore, we found that the enhancement of the photoactivity originated mainly from the Pt loaded on the photocatalyst surface, not from the presence of the IL. In this research, we suppose that the addition of ILs during the synthesis stage (the ILs were grated with Ag2O and Ta2O5 in the molar ratio 1:2 vs. Ag2O: IL) could have resulted in the formation of a monolayer which stuck to the powder and decomposed during calcination in the The results indicate that the presence of the Pt particles on the AgTaO 3 surface significantly improved the photocatalytic activity under UV-Vis irradiation. Firstly, it was found that pristine AgTaO 3 exhibited photoactivity in hydrogen production even without any co-catalyst (20.4 µmol·g −1 ) hydrogen after 4 h of irradiation (see Figure 6a). This is because the photogenerated electrons could be transferred to Ag + via the interface to accelerate the charge separation and thus influence the photocatalytic efficiency [53]. Hydrogen generation from pristine AgMO 3 perovskite materials (M = V, Nb, Ta) was also observed by Moctezuma et al. [33]. As a result of the 3-h irradiation, they obtained 136 µmol g −1 of hydrogen in the presence of Na 2 SO 3 0.5 M as a sacrificial agent solution. Carrasco-Jaim et al. also received different hydrogen production efficiency (27 µmol after 3 h irradiation) [36]. Large differences in the efficiency of the conducted experiments could result from the different preparation route used to synthesize AgTaO 3 perovskite. Secondly, we noticed that the ILs modification did not influence the enhancement of the photocatalytic activity of AgTaO 3 composites compared to the reference sample. Moreover, the results show the opposite effect and the samples exhibited even lower photoactivity than the pristine AgTaO 3 (see Table 1). Only application of [OMIM] [Tf 2 N] caused slightly higher H 2 evolution (21.3 µmol g −1 ). In the next step, we analyzed the IL-modified samples with Pt loaded at the surfaces. The largest amount of H 2 was achieved for AgTaO 3 _0.2% Pt contributed to the highest H 2 evolution rate, 248.5 µmol·g −1 after 240 min under UV-Vis irradiation (almost 13 times higher than for the pristine sample). In each case, the amount of H 2 evaluated was slightly different and could depend directly on the structure and properties of the ILs. Lower values of H 2 production were observed for AgTaO 3  µmol g −1 , respectively. Additionally, no other gases were detected during the process. These results indicate the possible transfer of the excited electrons and photogenerated holes from the valence band AgTaO 3 after the surface Pt deposition to the conduction band. According to the literature, the low Schottky barriers of metal semiconductor surfaces act as electron traps, facilitating electron-hole separation and catalyzing the proton reduction to H 2 molecules and thus the enhancement of their photoactivity [54]. , followed by decoration with Pt particles showed lower photoactivity in the water splitting reaction under the same conditions. The present paper aims to call into question: why does the addition of ILs suppress hydrogen evolution? Did the improvement in photoactivity originate only from the interaction between AgTaO 3 and platinum? What was the role of ILs in the production of H 2 ?
To answer these questions, we need to carefully examine the above-mentioned results and the mechanisms of the photocatalytic reactions analyzed previously by our group. Based on the available literature and our own experience, it is commonly known that ILs play a significant role in the increase in photoactivity in the degradation of the aqueous phenol solution, MO and RhB solution, and photocatalytic hydrogen production [20,40,41,45,46,55,56]. Recently, we analyzed the effect of the various ILs used in the solvothermal reaction over TiO 2 particles, and the results clearly show that the significant impact on the photocatalytic performance originated directly from interaction between photocatalyst particles and ILs [40,41,43,45]. The photoexcitation of the TiO 2 samples modified with ILs occurred directly through the formation of a surface complex which resulted in the transfer of electrons from the LUMO orbit of the ionic liquid to the conductivity band TiO 2 . It was found that synthesis conditions of the solvothermal route allowed for the successful decomposition of the ILs, which resulted in the incorporation of nitrogen into the TiO 2 structure and thereby significantly improved the photocatalytic activity under UV-Vis and Vis irradiation [41,43]. Qi et al. also investigated the effect of ILs on the photocatalytic activity of the TiO 2 semiconductor. They realized that addition of an IL with a [Bmim] + cation slightly enhanced the photocatalytic degradation rate of MO due to enhanced trapping and transfer of the photogenerated electrons. On the other hand, presence of an IL suppressed the degradation rate of RhB on the TiO 2 surface by restricting the diffusion of positively charged holes to the TiO 2 /solution interface [46]. On the other hand, investigation of the hydrogen evolution using IL-modified SrTiO 3 perovskite followed by surface photodeposition of Pt nanoclusters did not reveal a direct correlation between the increase in H 2 evolution due to the presence of the IL [48]. Furthermore, we found that the enhancement of the photoactivity originated mainly from the Pt loaded on the photocatalyst surface, not from the presence of the IL. In this research, we suppose that the addition of ILs during the synthesis stage (the ILs were grated with Ag 2 O and Ta 2 O 5 in the molar ratio 1:2 vs. Ag 2 O: IL) could have resulted in the formation of a monolayer which stuck to the powder and decomposed during calcination in the high temperature. The XPS analysis revealed the presence of the residual elements derived from the IL, namely fluorine and boron for AgTaO 3 Table 2 and Figure 5) The water splitting measurements were performed firstly for the non-modified and IL-modified samples. As expected, without the co-catalyst, the samples showed very poor performance. Only the AgTaO 3 _[OMIM] [Tf 2 N] showed slightly higher H 2 evolution in comparison with the non-modified AgTaO 3 sample (21.3 and 20.4 µmol·g −1 , respectively), whereas the other samples exhibited much lower photoactivity. As a result of the photodeposition of the Pt nanoparticles, enhanced H 2 generation in comparison with the non-modified pristine sample was found. Figure 7 shows the proposed mechanism of photocatalytic H 2 production under UV-Vis light irradiation in the presence of AgTaO 3 _IL_Pt. As a result of the absorption of UV-Vis irradiation, the photocatalyst was excited, generating pairs of charge carriers (e − -h + ). High-energy electrons from the conduction band were transferred to Pt particles, where they participated in the reduction of water to molecular hydrogen. Aldoni et al. achieved the same effect [57]. According to the literature, it was found that the noble metal nanoparticles adsorbed on the surface of photocatalysts provided additional reaction sites and act as effective electron traps for photogenerated electrons due to the formation of the Schottky barrier at the metal-semiconductor contact point and promotion of the charge carriers separation [58]. The highest H 2 production was observed for AgTaO 3 _0.2% Pt (248.5 µmol·g −1 ), whereas all the modified samples with both an IL and Pt possessed lower photoactivity (see Table 2 and Figure 6a) Table 2). It is evidenced that these two ILs could be adsorbed in a larger amount in the form of the residual elements derived from ILs at the composite surface, and suppressed the photoactivity in H 2 generation. Therefore, even higher Pt content adsorbed on the surface of those composites did not enhance the photocatalytic efficiency.

Conclusions
Silver tantalate was successfully prepared via a solid-state reaction in the presence of four ILs differing in structure, namely, 2,3,5-triphenyltetrazolium chloride [ Tf2N], followed by surface platinum nanoparticle decoration using the photodeposition method. Morphology analysis revealed granules smaller than 500 nm in size when the samples were prepared in the presence of imidazolium ILs, and cubic shaped particles around 2 μm in size when ammonium and tetrazolium ILs were applied. We found that the various ILs used in the synthesis did not improve the photocatalytic activity of the obtained samples in comparison to pristine AgTaO3. The enhanced hydrogen generation came only from the presence of Pt nanoparticles on the photocatalyst's surface, not from the IL modification. Some literature reports associated higher H 2 generation with larger specific surface area and crystallinity, which promotes more active sites for gas evolution, improving the transfer process of photogenerated charge pairs and thus enhancing the photocatalytic activity [59,60]. Based on the previously discussed experimental data, a direct relation between the increase in the BET surface area and the improvement of photocatalytic H 2 production was observed. The AgTaO 3 _0.2% Pt sample possessed the largest BET surface area and also the highest hydrogen evolution compared to the other samples, 1.1408 m 2 ·g −1 and 248.5 µmol·g −1 , respectively. Hence, the samples with the smallest surface area, AgTaO 3 _[TPTZ][Cl]_0.2% Pt (0.7124 m 2 ·g −1 ) and AgTaO 3 _[TBA][Cl]_0.2% Pt (0.6989 m 2 ·g −1 ) generated much lower amounts of H 2 (55.4 and 25.1 µmol·g −1 , respectively). Evidently, the addition of ILs to the reaction medium reduced the specific BET surface area and suppressed the photocatalytic activity.

Conclusions
Silver tantalate was successfully prepared via a solid-state reaction in the presence of four ILs differing in structure, namely, 2,3,5-triphenyltetrazolium chloride [ 4 ], 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [OMIM][Tf 2 N], followed by surface platinum nanoparticle decoration using the photodeposition method. Morphology analysis revealed granules smaller than 500 nm in size when the samples were prepared in the presence of imidazolium ILs, and cubic shaped particles around 2 µm in size when ammonium and tetrazolium ILs were applied. We found that the various ILs used in the synthesis did not improve the photocatalytic activity of the obtained samples in comparison to pristine AgTaO 3 . The enhanced hydrogen generation came only from the presence of Pt nanoparticles on the photocatalyst's surface, not from the IL modification. Despite the confirmed interactions between IL and AgTaO 3 and its influence on the morphology, optical and photocatalytic properties, we suppose that ILs might block the sample surface and thus lower the photocatalytic activity. We assume that the reduced activity might result from the decomposition of the ionic liquid during the high calcination temperature, needed to obtain the final product AgTaO 3 [Cl]) and platinum content (1.28 at.% and 1.20 at.%, respectively) were characterized by the lowest H 2 evolution. Moreover, the addition of IL to the reaction environment reduced the BET specific surface area and suppressed H 2 generation. Among all the obtained samples, the compound with the highest ability to photocatalytically split water (248.5 µmol·g −1 ) was revealed to be AgTaO 3 _0.2% Pt (almost 13 times higher efficiency in comparison with the non-modified pristine sample), while among the modified ILs, AgTaO 3 _[OMIM] [Tf 2 N]_0.2% Pt (221.2 µmol·g −1 ). Interestingly, the Pt content for this sample was the lowest, and amounted to 0.74 at.%. Evidently, the enhanced H 2 generation came from the presence of Pt nanoparticles on the composite's surface and was suppressed due to use of ILs. The Pt nanoparticles promoted charge transfer from valence band to the AgTaO 3 conduction band, and inhibited the recombination probability of the photogenerated electrons and holes, which was beneficial for the improvement in the photocatalytic activity of the modified samples. As a consequence, the ILs were responsible for the decrease in photocatalytic activity in the water splitting reaction. The MeOH electrolyte stabilized this photocatalyst during the extended photocatalytic process.
Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/18/4055/s1, Figure S1: FTIR (a) and Raman (b) spectra of pristine and corresponding ILs and Pt modified AgTaO 3 samples. Figure S2: The diffusion reflection spectra of the pristine AgTaO 3 photocatalyst and the corresponding ILs and Pt modified materials. Figure S3: UV-Vis Kubelka-Munk absorption of the pristine AgTaO 3 photocatalyst and the corresponding ILs and Pt modified materials.