Synthesis, Characterization and Visible-Light Photocatalytic Activity of Solid and Tio2-Supported Uranium Oxycompounds

nanomaterials

Article Synthesis, Characterization and Visible-Light Photocatalytic Activity of Solid and TiO2-Supported Uranium Oxycompounds

Mikhail Lyulyukin 1 , Tikhon Filippov 2, Svetlana Cherepanova 3, Maria Solovyeva 1, Igor Prosvirin 4, Andrey Bukhtiyarov 4 , Denis Kozlov 1 and Dmitry Selishchev 1,*

1 Department of Unconventional Catalytic Processes, Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia; [email protected] (M.L.); [email protected] (M.S.); [email protected] (D.K.) 2 Schulich Faculty of Chemistry, Technion–Israel Institute of Technology, Haifa 32000, Israel; [email protected] 3 Department of Catalysts Study, Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia; [email protected] 4 Department of Physicochemical Methods of Research, Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia; [email protected] (I.P.); [email protected] (A.B.) * Correspondence: [email protected]; Tel.: +7-3833269429

Abstract: In this study, various solid uranium oxycompounds and TiO2-supported materials based on nanocrystalline anatase TiO2 are synthesized using uranyl nitrate hexahydrate as a precursor. All uranium-contained samples are characterized using N2 adsorption, XRD, UV–vis, Raman, TEM, XPS and tested in the oxidation of a volatile organic compound under visible light of the blue region to ﬁnd correlations between their physicochemical characteristics and photocatalytic activity.

Citation: Lyulyukin, M.; Filippov, T.; Both uranium oxycompounds and TiO2-supported materials are photocatalytically active and are Cherepanova, S.; Solovyeva, M.; able to completely oxidize gaseous organic compounds under visible light. If compared to the Prosvirin, I.; Bukhtiyarov, A.; Kozlov, ® commercial visible-light TiO2 KRONOS vlp 7000 photocatalyst used as a benchmark, solid uranium D.; Selishchev, D. Synthesis, oxycompounds exhibit lower or comparable photocatalytic activity under blue light. At the same Characterization and Visible-Light time, uranium compounds contained uranyl ion with a uranium charge state of 6+, exhibiting much Photocatalytic Activity of Solid and higher activity than other compounds with a lower charge state of uranium. Immobilization of uranyl TiO2-Supported Uranium ions on the surface of nanocrystalline anatase TiO allows for substantial increase in visible-light Oxycompounds. Nanomaterials 2021, 2 11, 1036. https://doi.org/10.3390/ activity. The photonic efficiency of reaction over uranyl-grafted TiO2, 12.2%, is 17 times higher than nano11041036 the efficiency for commercial vlp 7000 photocatalyst. Uranyl-grafted TiO2 has the potential as a visible-light photocatalyst for special areas of application where there is no strict control for use of Academic Editor: Imre uranium compounds (e.g., in spaceships or submarines). Miklós Szilágyi Keywords: uranyl nitrate; titanium dioxide; photocatalysis; VOC oxidation; visible light; LED Received: 20 March 2021 Accepted: 16 April 2021 Published: 19 April 2021 1. Introduction Publisher’s Note: MDPI stays neutral Semiconductor photocatalysis has the potential for efficient utilization of light energy with regard to jurisdictional claims in reaching the Earth from the Sun [1]. Commonly, semiconducting materials can be used for published maps and institutional affil- utilization of solar radiation via a number of routes: (i) to convert the energy of light directly iations. into electricity by the photovoltaic effect using various silicon, cadmium telluride, copper indium gallium selenide, dye-sensitized (also well-known as Grätzel cells), perovskite, and other solar cells [2,3]; (ii) to store the energy in a form of chemical bond energy in fuels via photocatalytic or photoelectrochemical hydrogen evolution [4–7] and reduction of carbon Copyright: © 2021 by the authors. dioxide [8–10]; (iii) to carry out useful chemical reactions, for instance, to synthesize high- Licensee MDPI, Basel, Switzerland. value products via partial selective photocatalytic or photoelectrochemical oxidation [11,12] This article is an open access article and to purify water or air via photocatalytic degradation and complete oxidation of distributed under the terms and pollutants [13,14]. Therefore, the photocatalytic processes can solve both energy and conditions of the Creative Commons environmental issues. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Photocatalytic oxidation is one of main fields of photocatalysis and has the highest 4.0/). potential in the degradation of chemical and biological contaminants, which pollute air,

Nanomaterials 2021, 11, 1036. https://doi.org/10.3390/nano11041036 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1036 2 of 17

water, and surfaces [15–17]. Wide-bandgap metal oxide semiconductors, namely TiO2, ZnO, WO3, exhibit high activities in this type of photocatalytic reaction and can be applied in a form of powder, thin film, or coating. TiO2 is the most commonly known and used photocatalyst for the degradation of air and water pollutants due to its chemical stability, simple preparation from minerals and other precursors, low cost, and high activity [18–22]. However, the action spectrum of TiO2 is restricted to the UV region due to its wide band gap that commonly has a value of 3.0–3.2 eV [23–25]. According to ASTM G-173-03 [26], a fraction of UV light in the solar radiation does not exceed 5%. Therefore, conventional TiO2 photocatalysts have a high performance only under radiation from artificial UV-light sources (e.g., mercury lamps, UV light-emitting diodes). Search and development of new materials is an important task for efficient utilization of solar radiation. Current status and challenges in the development of materials for photocatalytic oxidation under visible light can be found in the recently published review [17]. Despite the other discovered materials (e.g., bismuth oxyhalides, graphitic carbon nitride), which can absorb visible light and have photocatalytic activity in this region, the modification of TiO2 to extend its action spectrum to the visible region still remains the main approach to develop an efficient photocatalyst for visible light. Sensitization with dyes, doping with metals or non-metals, metal loading, combination with narrow-bandgap semiconductors are possible methods for modification of TiO2 that provide its activation under visible light [17,27]. In addition to these methods, we have previously shown that the surface 2+ modification of TiO2 with uranyl ion (UO2 ) results in a photocatalyst that has great activity under visible light and can completely oxidize vapor of organic compounds at a high rate [28–30]. The photocatalytic activity of this system under visible light is due to the excitation of uranyl ion and reverse redox transformations between U6+ and U4+ charge states during the interactions with organic compounds and oxygen. Many studies describe the photochemical and photocatalytic properties of uranyl ion itself in homogeneous solutions [31–35] or in heterogenous systems where it is grafted on the surface of porous supports [36–40]. Less information can be found on visible-light photocatalytic activity of other uranium compounds, especially solid materials, which have a benefit from the practical point of view compared to homogeneous systems. Uranium oxides are known to be semiconductors with a band gap less than 3 eV [41] and can potentially have photocatalytic activity in the oxidation of organic pollutants under visible light. It was our motivation to perform a mechanistic study for examining photocatalytic activity of various solid and supported uranium compounds. In this study, we synthesize several uranium oxycompounds using a UO2(NO3)2 precursor, characterize them using physical methods, and evaluate their photocatalytic activity in a test reaction of acetone oxidation under blue light in a continuous-flow setup. A correlation between physicochemical properties of materials and their photocatalytic activities is discussed. The values of photonic efficiency are shown for all uranium samples, ® as well as for the commercial visible-light-active photocatalyst TiO2 KRONOS vlp 7000, tested under the same conditions, to give other researchers references for comparison of photocatalytic activity.

2. Materials and Methods 2.1. Materials

Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) from Isotope JSC (Moscow, Russia) was used as a uranium-containing precursor for the synthesis of uranium compounds and for the modification of a TiO2 photocatalyst. High purity or reagent grade chemicals, namely ethylenediamine (NH2CH2CH2NH2, 99.5%), ethanol (C2H5OH, 99.5%), am- monium hydroxide solution (NH4OH, 28%), sodium borohydride (NaBH4, 98%), and hydrazine hydrate (N2H4·xH2O, 64%) were used during the synthesis of samples as received from Sigma-Aldrich Corp. (St. Louis, MO, USA) without further purification. Commercial UV-active photocatalyst TiO2 Hombifine N from Sachtleben Chemie GmbH (Duisburg, Germany) was modified with uranyl nitrate to provide photocatalytic activ- Nanomaterials 2021, 11, 1036 3 of 17

® ity under visible light [29,30]. Additionally, commercial photocatalyst TiO2 KRONOS 2 –1 vlp 7000 (95% anatase, as,BET = 250 m g ) from KRONOS TITAN GmbH (Leverkusen, Germany), which has activity both under UV and visible light, was used as a reference sample for benchmarking. High purity grade acetone (CH3COCH3, 99.5%) from AO REAHIM Inc. (Moscow, Russia) was used as a test organic compound for oxidation during the photocatalytic experiments.

2.2. Characterization Techniques The content of uranium in the samples was measured by X-ray fluorescence (XRF) analysis using an ARL ADVANT’X device (Thermo Fisher Scientific Inc., Waltham, MA, USA) equipped with a Rh anode of the X-ray tube. The phase composition was analyzed by X-ray powder diffraction (XRD) using a D8 Advance diffractometer (Bruker, Billerica, MA, USA) under CuKα radiation. The scanning procedure was performed using a linear detector in the 2θ range of 5–90◦ with a step of 0.05◦. Thermal analysis was performed in the temperature range of 20–1200 ◦C using an STA 449C instrument from NETZSCH-Gerätebau GmbH (Selb, Germany). The heating rate during the analysis was 10 ◦C g–1. The textural properties were investigated by N2 adsorption at 77 K using an ASAP 2400 instrument (Micromeritics Instrument Corp., Norcross, GA, USA). The surface area was calculated by BET analysis of N2 adsorption/desorption isotherms. The morphology was investigated by transmission electron microscopy (TEM) using a JEOL-2010 microscope (JEOL Ltd., Tokyo, Japan). TEM micrographs were received at an accelerating voltage of 200 kV and a resolution of 0.14 nm. The surface composition was investigated by X-ray photoelectron spectroscopy (XPS) using a SPECS photoelectron spectrometer (SPECS Surface Nano Analysis GmbH, Berlin, Germany) equipped with a PHOIBOS-150 hemispherical energy analyzer and an AlKα radiation source (hν = 1486.6 eV, 150 W). The binding energy (BE) scale was pre-calibrated using the positions of Au4f7/2 (84.0 eV) and Cu2p3/2 (932.67 eV) photoelectron lines from metallic gold and copper foils. The background pressure at the analysis chamber did not exceed 8 × 10–7 Pa. The detailed Ti2p, C1s, and U4f spectral regions were recorded with a step of 0.1 eV. The C1s peak at 284.8 eV, attributed to carbon impurities, was used as an internal standard for the calibration of spectra. Experimental spectra were fitted using an approximation function based on a combination of the Gaussian and Lorentzian functions with subtraction of a Shirley-type background [42]. The data processing and peak fitting were performed using XPSPeak 4.1 software. The ratio between charge states of uranium was calculated using the areas of corresponding fitted peaks. The optical properties were investigated by UV–vis spectroscopy. The diffuse reflectance spectra (DRS) were recorded at room temperature in the range of 250–850 nm with a resolution of 1 nm using a Cary 300 UV–vis spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a DRA-30I diffuse reflectance accessory. Special pre-packed polytetrafluoroethylene (PTFE) from Agilent was used as the reflectance standard. FT-Raman spectra were collected using a Bruker RFS 100/S spectrometer at 180◦ geometry in the range of 3700−100 cm−1 with a resolution of 4 cm−1. Excitation of the 1064-nm line was provided by a Nd-YAG laser with output power of 100 mW.

2.3. Photocatalytic Experiments A continuous-flow setup was used for the evaluation of photocatalytic activity of synthesized and reference samples in a test reaction of acetone oxidation under blue light [43]. Each sample was uniformly deposited on a round glass plate with an area of 9.1 cm2 to obtain a layer with an area density of 2 mg cm−2 and was placed into the photoreactor. The temperature of photoreactor was 40.0 ± 0.1 ◦C. The mass flow controllers were adjusted to obtain the inlet humidified air with relative humidity of 20 ± 1% and volume flow rate of 0.069 ± 0.001 L min−1. The plate with sample was irradiated using a 100-W light-emitting diode (LED) with a maximum of emission at 450 nm (see Supplementary for details). The specific total irradiance of the sample measured using an ILT 950 spectroradiometer from International Light Technologies Inc. (Peabody, MA, Nanomaterials 2021, 11, 1036 4 of 17

USA) was 93.5 mW cm−2. Acetone was selected as a test organic compound for oxidation, and its inlet concentration in the gas phase was 29 ± 2 µmol L−1. The inlet and outlet gas mixtures were periodically analyzed using an FT-IR spectrometer FT-801 from Simex LLC (Novosibirsk, Russia) equipped with an IR long-path gas cell from Infrared Analysis Inc. (Anaheim, CA, USA). The quantitative analysis was performed based on the Beer–Lambert law via the integration of collected IR spectra in the range of 1160–1265 cm−1 for the −1 evaluation of acetone concentration and of 2200–2400 cm for the evaluation of CO2 concentration. The former range (i.e., 1160–1265 cm−1) is attributed to the absorption band due to vibration of C–C bond (νC–C ) in acetone molecule, and the latter is attributed to the absorption bands due to vibrations in CO2 molecule. The corresponding attenuation coefficients for acetone and CO2 were estimated from the preliminary calibration data. Steady-state rate of CO2 formation during the acetone oxidation, evaluated as the product of the volume flow rate and the difference between outlet and inlet concentrations of CO2, was used as the photocatalytic activity of the samples. The photonic efficiency of ) CO2 formation (ξCO2 was estimated to evaluate the efficiency of light utilization as follows: W = CO2 × ξCO2 0 100% qn,p

µ –1 0 where WCO2 is the measured steady-state rate of CO2 formation ( mol min ), qn,p is the 0 –1 total incident photon ﬂux (qn,p = 32.4 µmol min ).

3. Results and Discussion The photocatalytic activity was expected to strongly depend on the charge state of uranium and the composition of its compound. A lot of effort in this study was spent on the synthesis and characterization of different uranium compounds. The results are shown below sequentially from the synthesis and characterization of samples to evaluation of their photocatalytic activity and discussion of correlations with physicochemical characteristics.

3.1. Synthesis and Characterization Data One of the simple methods for the synthesis of uranium oxides is the thermal decomposition of uranyl nitrate that can lead to the formation of UO3 and U3O8 oxides. The thermogravimetric analysis of initial UO2(NO3)2·6H2O precursor was performed to select the temperature range for the preparation of corresponding oxide. The peaks in DTG and DTA curves at temperatures below 300 ◦C correspond to the loss of crystallization water and nitrate by UO2(NO3)2·6H2O (Figure1). Uranium oxides form at higher temperatures. ◦ The formation of uranium(VI) oxide (i.e., UO3) starts at a temperature of 340 C. A further ◦ increase in the calcination temperature above 570 C leads to a loss of oxygen by UO3 and Nanomaterials 2021, 11, x FOR PEER REVIEWa partial reduction of uranium with the formation of a mixed U3O8 oxide. These results5 of 17

agree with the published data [44].

Figure 1. Thermogravimetric analysis of UO2(NO(NO3))22·6H·6H2O2O precursor. precursor.

According to XPS analysis (Figure 2a), the sample prepared via the calcination of UO2(NO3)2·6H2O at 400 °C contains uranium with the charge state of 6+. This result in combination with XRD data (Figure 2b) confirms the formation of UO3 oxide at this temperature. In addition to an amorphous form, UO3 commonly has seven polymorphic mod- ifications from α to η [45]. Although the literature data mainly state that the calcination of uranyl nitrate in air at temperatures of 400–600 °C results in the formation of γ-UO3 [46], the XRD pattern of the sample prepared at 400 °C shows the best coincidence with β modification of UO3 (Figure 2b). The synthesized sample is referred in the paper as UO3.

Figure 2. Photoelectron U4f spectral region (a) and XRD patterns (b,c) for the samples prepared via the calcination of uranyl nitrate.

The content of uranium in the samples prepared via the calcination of uranyl nitrate at 600 and 900 °C is in the range of 86.4–87.8 wt.% which is close to the theoretically estimated value for the U3O8 compound (ωU = 84.8 wt.%). XRD analysis also confirms that both samples have the crystal phase of U3O8 (Figure 2c). The sample prepared at 900 °C (i.e., U3O8-T900) exhibits a better separation of peaks in the XRD pattern and their higher intensity if compared with the sample prepared at 600 °C (i.e., U3O8-T600). This result indicates a higher crystallinity of the U3O8-T900 sample and a larger size of its crystallites

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Based on the data described above, the temperatures of 400, 600, and 900 ◦C were selected to prepare uranium oxides. The calcination of uranyl nitrate at 400 ◦C was ◦ expectedFigure 1. Thermogravimetric to result in the formation analysis of of UO UO2(NO3, whereas3)2·6H2O precursor. the temperatures of 600 and 900 C were employed to obtain U3O8 with different textural characteristics. AccordingAccording toto XPSXPS analysisanalysis (Figure(Figure2 a),2a), the the sample sample prepared prepared via via the the calcination calcination of of ◦ UOUO22(NO(NO3))2·6H·6H22OO at at 400 400 °CC contains contains uranium uranium with with the the charge charge state state of of 6+. 6+. This This result result in incombination combination with with XRD XRD data data (Figure (Figure 2b)2 b)confirms confirms the the formation formation of UO of UO3 oxide3 oxide at this at tem- this temperature.perature. In addition In addition to an to amorphous an amorphous form, form, UO3 UOcommonly3 commonly has seve hasn seven polymorphic polymorphic mod- modificationsifications from from α toα ηto [45].η [45 Although]. Although the theliterature literature data data mainly mainly state state that that the the calcination calcination of of uranyl nitrate in air at temperatures of 400–600 ◦C results in the formation of γ-UO [46], uranyl nitrate in air at temperatures of 400–600 °C results in the formation of γ-UO33 [46], ◦ thethe XRDXRD pattern of of the the sample sample prepared prepared at at400 400 °C showsC shows the thebest best coincidence coincidence with withβ mod-β modification of UO (Figure2b). The synthesized sample is referred in the paper as UO . ification of UO3 (Figure3 2b). The synthesized sample is referred in the paper as UO3. 3

FigureFigure 2.2. PhotoelectronPhotoelectron U4fU4f spectralspectral regionregion (a(a)) and and XRD XRD patterns patterns ( b(b,c,c)) for for the the samples samples prepared prepared via thevia calcinationthe calcination of uranyl of uranyl nitrate. nitrate.

TheThe contentcontent ofof uraniumuranium inin thethe samplessamples preparedprepared viavia thethe calcinationcalcination ofof uranyluranyl nitratenitrate atat 600600 and 900 °C◦C is is in in the the range range of of86.4–87.8 86.4–87.8 wt.% wt.% which which is close is close to the to thetheoretically theoretically esti- estimatedmated value value for forthe the U3O U83 Ocompound8 compound (ωU ( ω= U84.8=84.8 wt.%). wt.%). XRD XRD analysis analysis also alsoconfirms confirms that thatboth both samples samples have have the crystal the crystal phase phase of U3 ofO8 U(Figure3O8 (Figure 2c). The2c). sample The sample prepared prepared at 900at °C ◦ 900(i.e., CU3 (i.e.,O8-T900U3O)8 exhibits-T900) exhibits a better a separation better separation of peaks of in peaks the XRD in the pattern XRD pattern and their and higher their ◦ higherintensity intensity if compared if compared with the with sample the sample prepared prepared at 600 at °C 600 (i.e.,C U (i.e.,3O8-T600U3O8).-T600 This). result This resultindicates indicates a higher a highercrystallinity crystallinity of the U of3O the8-T900U3O sample8-T900 andsample a larger and size a larger of its crystallites size of its crystallites due to sintering. As has been shown in our previously published paper [29], 6+ 4+ the U3O8-T900 sample contains uranium in the charge states of U and U , and a ratio between these states is close to 2. The hydrothermal method was employed in an attempt to synthesize a single uranium(IV) oxide (i.e., UO2). For this purpose, UO2(NO3)2·6H2O was dissolved in deionized water. Ethylenediamine was added to the solution until a 150-times molar excess compared to uranyl nitrate was reached. A Teflon lined autoclave was filled with the prepared solution, tightly closed, and stored in an electric oven at 160 ◦C for 72 h. The synthesized sample was thoroughly washed with deionized water and dried in air at 70 ◦C. The XRD pattern Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 17

due to sintering. As has been shown in our previously published paper [29], the U3O8- T900 sample contains uranium in the charge states of U6+ and U4+, and a ratio between these states is close to 2. The hydrothermal method was employed in an attempt to synthesize a single uranium(IV) oxide (i.e., UO2). For this purpose, UO2(NO3)2·6H2O was dissolved in deionized water. Ethylenediamine was added to the solution until a 150-times molar excess com- Nanomaterials 2021 11 , , 1036 pared to uranyl nitrate was reached. A Teflon lined autoclave was filled with the prepared6 of 17 solution, tightly closed, and stored in an electric oven at 160 °C for 72 h. The synthesized sample was thoroughly washed with deionized water and dried in air at 70 °C. The XRD ofpattern this sample of this shows sample a veryshows broad a very peak broad at ca. peak 17◦, at and ca. narrow 17°, and peaks, narrow which peaks, correspond which cor- to therespond crystalline to the phase crystalline of UO 2phase(Figure of 3UOa).2 There (Figure are 3a). no otherThere broad are no peaks other in broad the pattern, peaks in and the thepattern, observed and peakthe observed at ca. 17◦ canpeak be at attributed ca. 17° can to anbe amorphousattributed phase.to an amorphous It should be phase. noticed It thatshould diffraction be noticed peaks that attributed diffraction to peaks the crystalline attributed phase to the are crystalline shifted to phase high-angle are shifted region to relativehigh-angle to the region positions relative of UOto the2 peaks. positions This of shift UO indicates2 peaks. This modiﬁed shift indicates parameters modified of crystal pa- latticerameters in theof crystal prepared lattice sample in the compared prepared to sample the single compared crystal to of the UO 2singlepossibly crystal due of to UO an 2 excesspossibly of oxygendue to inan interstitial excess of positionsoxygen in [47 inte,48].rstitial Formation positions of nonstoichiometric, [47,48]. Formation as wellof non- as mixedstoichiometric, oxides is as a commonwell as mixed case foroxides uranium is a common [49,50]. case The formulafor uranium of the [49,50]. prepared The sampleformula canof the be expressedprepared sample as UO2+x canwith be expressed x value of 0.12as UO estimated2+x with x from value the of XRD 0.12 dataestimated (see Figure from the S1 inXRD Supplementary). data (see Figure It isS1 importantin Supplementary). to note that It is this important expression to note does that not this correspond expression to otherdoes not stoichiometric correspond uraniumto other stoich oxides,iometric namely uranium U4O9 (i.e., oxides, UO namely2.25) and U U4O3O9 (i.e.,7 (i.e., UO UO2.25)2.33 and), becauseU3O7 (i.e., the UO estimated2.33), because x value the estimated is much lower. x value The is much formula lower. of UO The2.12 formulagives a of value UO2.12 of gives 4.2+ asa value a formal of 4.2+ charge as a offormal uranium. charge The of datauranium. of XPS The analysis data of show XPS analysis the charge show states the ofcharge U4+ andstates U5+ offor U4+ this and sample U5+ forwhich this sample also conﬁrms which also a nonstoichiometry confirms a nonstoichiometry (Figure3c). The (Figure surface 3c). compositionThe surface composition of the sample of estimated the sample using estimated XPS gives using a XPS value gives of 4.8+ a value as a of formal 4.8+ as charge a for- ofmal uranium. charge of This uranium. result indicatesThis result that indicates the surface that the and surface bulk compositions and bulk compositions of the prepared of the sampleprepared may sample substantially may substantially differ. An differ. amorphous An amorphous phase mentioned phase mentioned above for above this sample for this maysample be amay reason be a for reason that. Therefore,for that. Therefore, the sample the prepared sample prepared via the hydrothermal via the hydrothermal thermal treatmentthermal treatment of uranyl of nitrate uranyl with nitrate ethylenediamine with ethylenediamine is referred is referred in the paper in the as paperUO2+x as. UO2+x.

FigureFigure 3.3.XRD XRD patternspatterns ( a(a,b,b)) and and photoelectron photoelectron U4f U4f spectral spectral regions regions (c ()c for) for the the samples samples prepared prepared via thevia hydrothermalthe hydrothermal method. method.

TheThe hydrothermalhydrothermal treatmenttreatment waswas alsoalso usedused forfor thethe preparationpreparation ofof aa uraniumuranium com-com- poundpound withwith ammonia.ammonia. ForFor thisthis purpose,purpose, uranyluranyl nitratenitrate waswas similarlysimilarly autoclavedautoclaved inin anan aqueousaqueous solutionsolution ofof ammoniaammonia andand ethanol,ethanol, whichwhich werewere bothboth takentaken inin aa 10-times10-times molarmolar excess.excess. FigureFigure3 3bb showsshows thethe resultsresults ofof XRDXRD analysisanalysis ofof thethe samplesample synthesizedsynthesized viavia thisthis method.method. ExceptExcept aa smallsmall peakpeak atat 22θθ= = 1313°,◦, thethe XRDXRD patternpattern ofof thisthis sample sample showsshows a a good good coincidence with the phase of U2(NH3)O6·3H2O. This sample is referred in the paper as U2(NH3)O6·3H2O. XPS analysis of this sample shows the uranium charge states of U5+ and U6+ (Figure3c) . According to a ratio of peak areas, the surface content of U5+ and U6+ states are 34% and 66%, respectively. This ratio gives a value of 5.7+ as a formal charge of uranium, which is less than the value estimated from the formula of U2(NH3)O6·3H2O (i.e., 6+). An additional phase, which gives a small signal in the XRD pattern at 2θ = 13◦, may be present in the sample and may contain uranium with the charge state of 5+ which reduces the total value of formal charge. A partial reduction of U6+ to U5+ may also occur in a spectrometer chamber under vacuum conditions and exposure to X-rays. The results of Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 17

coincidence with the phase of U2(NH3)O6·3H2O. This sample is referred in the paper as U2(NH3)O6·3H2O. XPS analysis of this sample shows the uranium charge states of U5+ and U6+ (Figure 3c). According to a ratio of peak areas, the surface content of U5+ and U6+ states are 34% and 66%, respectively. This ratio gives a value of 5.7+ as a formal charge of uranium, which is less than the value estimated from the formula of U2(NH3)O6·3H2O (i.e., 6+). An additional phase, which gives a small signal in the XRD pattern at 2θ = 13°, may be present in Nanomaterials 2021, 11, 1036 the sample and may contain uranium with the charge state of 5+ which reduces the7 total of 17 value of formal charge. A partial reduction of U6+ to U5+ may also occur in a spectrometer chamber under vacuum conditions and exposure to X-rays. The results of the physical themethods physical illustrate methods that, illustrate in contrast that, to in ethy contrastlenediamine, to ethylenediamine, ammonia does ammonia not lead doesto a sub- not leadstantial to areduction substantial of reductionuranium in of uranyl uranium ion in under uranyl hydrothermal ion under hydrothermal treatment and treatment mainly andaffects mainly the coordination affects the coordination sphere. sphere. TheThe thirdthird methodmethod usedused forfor thethe preparation ofof uraniumuranium compoundscompounds waswas thethe reductionreduction ofof uranyl nitrate with with sodium sodium borohydride borohydride or or hydrazine. hydrazine. These These reducing reducing agents agents were were se- selectedlected based based on on the the values values of of standard standard redox redox potentials, potentials, which which should be high enoughenough forfor reductionreduction ofof uranyluranyl ionion toto uranium(IV)uranium(IV) oxide.oxide. Concerning the results of physical analyses, itit is difficult difficult to to make make clear clear conclusions conclusions on on the the chemical chemical and and phase phase composition composition of the of sam- the samplesples prepared prepared via viathe thechemical chemical reduction reduction of uranyl of uranyl nitrate. nitrate. InIn thethe case case of of sodium sodium borohydride, borohydride, the the crystal crystal data data of sodium of sodium polyuranates polyuranates sufficiently suffi- fitciently the results fit the of results XRD analysis.of XRD analysis. The crystal The phase crystal of Naphase2U7 ofO22 Napolyuranate2U7O22 polyuranate shows the shows best coincidencethe best coincidence with the XRDwith patternthe XRD of pattern the reduced of the sample reduced (Figure sample4a). (Figure A broad 4a). peak A inbroad the rangepeak in of the 22–30 range◦ in theof 22–30° XRD pattern in the XRD indicates pattern an amorphous indicates an phase amorphous in the sample. phase in A faintthe sam- but distinctple. A faint peak but at 1071.6distinct eV peak in the at photoelectron1071.6 eV in the Na1s photoelectron spectral region Na1s confirms spectral the region presence con- offirms sodium the presence in the prepared of sodium sample in the (see prepared the inset sample in Figure (see4c). the The inset XPS in dataFigure also 4c). show The UXPS5+ 6+ anddata U alsocharge show statesU5+ and with U6+ the charge surface states contents with the of31 surface and 69%, contents respectively of 31 and (Figure 69%,4 respec-c). On thetively other (Figure hand, 4c). a mass On the fraction other ofhand, uranium a mass in fraction the prepared of uranium sample in measured the prepared using sample XRF analysismeasured is 71.2using wt.%. XRF Thisanalysis value is does71.2 notwt.%. correspond This value to does the value not correspond theoretically toestimated the value fortheoretically sodium polyuranate estimated for of Nasodium2U7O polyuranate22 (i.e., ωU = of 82.6 Na wt.%),2U7O22 as(i.e., well ωU as = 82.6 for main wt.%), uranium as well oxidesas for main (88.2, 84.8,uranium and 83.2oxides wt.% (88.2, for UO84.8,2,U and3O 883.2, and wt.% UO3 ,for respectively). UO2, U3O8, Sodiumand UO diuranate3, respec- ω (i.e.,tively). Na Sodium2U2O7) withdiuranateU = (i.e., 75.1 Na wt.%2U2O more7) with correlates ωU = 75.1to wt.% the more measured correlates mass to fraction the meas- of uranium.ured mass This fraction indicates of uranium. that the chemicalThis indicates composition that the of chemical the prepared composition sample mayof the differ pre- frompared Na sample2U7O22 maypolyuranate, differ from which Na2U is7O expected22 polyuranate, from XRD which analysis. is expected An amorphous from XRD phaseanaly- insis. the An sample amorphous may also phase be ain reason the sample for the may reduced also valuebe a reason of uranium for the content. reduced Therefore, value of this sample is referred in the paper as NaU O . uranium content. Therefore, this sample isx referredy in the paper as NaUxOy.

FigureFigure 4.4. XRD patternspatterns ((aa,,bb)) andand photoelectronphotoelectron U4f U4f spectral spectral regions regions ( c()c) for for the the samples samples prepared prepared via thevia chemicalthe chemical reduction. reduction.

In the case of hydrazine, XPS analysis (Figure4c) shows only the U 6+ charge state of uranium in the prepared sample. XRF analysis gives a value of 80.1 wt.% as a mass fraction of uranium, and this value is lower than the fraction corresponding to the stoichiometric UO3 oxide (see above). Actually, the XRD pattern of the sample prepared using hydrazine does not correspond to the crystal data of the UO3 oxide while the data for crystalline UO3·H2O hydrate (or UO2(OH)2) shows the best coincidence (Figure4b). Therefore, this sample is referred in the paper as UO3·H2O. On the contrary to our expectations, both sodium borohydride and hydrazine as reducing agents do not provide a strong reduction of uranium during interaction with uranyl nitrate. Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 17

In the case of hydrazine, XPS analysis (Figure 4c) shows only the U6+ charge state of uranium in the prepared sample. XRF analysis gives a value of 80.1 wt.% as a mass fraction of uranium, and this value is lower than the fraction corresponding to the stoichiometric UO3 oxide (see above). Actually, the XRD pattern of the sample prepared using hydrazine does not correspond to the crystal data of the UO3 oxide while the data for crystalline UO3·H2O hydrate (or UO2(OH)2) shows the best coincidence (Figure 4b). There- Nanomaterials 2021, 11, 1036 8 of 17 fore, this sample is referred in the paper as UO3·H2O. On the contrary to our expectations, both sodium borohydride and hydrazine as reducing agents do not provide a strong reduction of uranium during interaction with uranyl nitrate. AlongAlong with with solid solid uranium uranium oxycompounds, oxycompounds, we we investigate investigate supported supported materials materials based based onon nanocrystallinenanocrystalline TiO 2.. For For this this purpose, purpose, a superfine a superfine anatase anatase TiO TiO2 was2 was impregnated impregnated with withaqueous aqueous solution solution of uranyl of uranyl nitrate nitrate followed followed by drying by drying at 160 at °C. 160 A◦C. mass A mass loading load- of ingUO of2(NO UO3)22 (NOin the3)2 samplein the was sample 5%. wasDetailed 5%. information Detailed information on the synthesis on the can synthesis be found can in beour found previously in our previously published published paper [29]. paper This [29 ].sample This sample is referred is referred in inthe the paper asas UOUO22(NO(NO33))22/TiO2.. TheThe modification modification of of TiO TiO2 2with withuranyl uranyl nitrate nitrate changes changes its its optical optical properties properties and and results results inin appearance appearance of of light light absorption absorption in in the the range range of of 400–530 400–530 nm nm (Figure (Figure5 a).5a). This This absorption absorption correspondscorresponds to to the the spectrum spectrum of of uranyl uranyl nitrate nitrate with with differences differences as as follows: follows: (i) (i) fine fine structure structure ofof thethe absorptionabsorption isis notnot observed in the supported sample; sample; (ii) (ii) maximum maximum of of absorption absorption is isshifted shifted to to the the long-wave long-wave region. region. Other Other physical physical methods methods indicate indicate a achemisorbed chemisorbed state state of ofuranyl uranyl ion ion on on the the TiO TiO2 2surface.surface. According According to to TEM TEM and XRD analysesanalyses (Figure(Figure5 b,c),5b,c), no no crystallinecrystalline particlesparticles of UO 2(NO(NO33)2) 2areare present present on on the the TiO TiO2 surface.2 surface. An Anaveraged averaged sizesize of TiO of 2 TiOcrystallites,2 crystallites, 9 nm, 9 nm,does does not change not change after after surface surface modification modification with with uranyl uranyl nitrate. nitrate.

FigureFigure 5. 5.UV–vis UV–vis ( a(a),), TEM TEM ( b(b),), and and XRD XRD ( c()c) data data for for TiO TiO2 2modiﬁed modified with with uranyl uranyl nitrate. nitrate.

–1 –1 –1 FigureFigure6 6shows shows Raman Raman spectra. spectra. TheThe peakspeaks atat 150150 cmcm–1 (E(Eg),g), 397 397 cm cm–1 (B(B1g),1g 514), 514 cm cm–1 (B1g –1 (B+ 1gA1g+), Aand1g), 638 and cm 638–1 (E cmg), that(E gare), that specific are specificfor anatase, for anatase,are observed are observedfor pristine for and pristine modi- -1 –1 andfiedmodified TiO2 samples. TiO2 samples.As shown As in shownliterature in literature[51], the Raman [51], the peak Raman at 150 peak cm at is 150 extremely cm is extremelysensitive to sensitive the impurities to the impurities of uranium of uraniuminto the crystal into the lattice crystal of latticeanatase of because anatase these because im- thesepurities impurities change the change shape the of the shape peak of and the posi peaktion and of its position maximum. of its In maximum. our case, the In shape our case,and position the shape are and similar position which are indicates similar which the absence indicates of new the impurities absence of in new the impurities crystal lattice in theafter crystal modification. lattice after A slight modification. decrease in A the slight intensity decrease of the in thesignal intensity for the ofUO the2(NO signal3)2/TiO for 2 thesampleUO2 (NOcompared3)2/TiO to2 thesample pristine compared TiO2 is due to the to pristinesurface shielding TiO2 is due by touranyl surface ions. shielding by uranyl ions. –1 Absorption bands at 832 and 1049 cm for the uranyl-modified TiO2 sample (Figure6b) correspond to symmetric valence oscillations of uranyl ion and the nitrate group, respectively [52,53]. An increase in the loading of uranyl nitrate from 5 to 10 wt.% results in

double increase in the intensity of these bands (Figure6c). The positions of the bands substantially differ from the signals for initial crystalline uranyl nitrate (Figure6a). The absence of signals at 874 and 1038 cm–1, attributed to crystalline uranyl nitrate, in the spectra of uranyl-modiﬁed samples (Figure6b,c), indicates a chemisorbed state of uranyl –1 ions on the TiO2 surface. The position of the signal for the nitrate group at 1049 cm , Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 17

Absorption bands at 832 and 1049 cm–1 for the uranyl-modified TiO2 sample (Figure 6b) correspond to symmetric valence oscillations of uranyl ion and the nitrate group, respectively [52,53]. An increase in the loading of uranyl nitrate from 5 to 10 wt.% results in double increase in the intensity of these bands (Figure 6c). The positions of the bands sub-

Nanomaterials 2021, 11, 1036 stantially differ from the signals for initial crystalline uranyl nitrate (Figure 6a). The9 of ab- 17 sence of signals at 874 and 1038 cm–1, attributed to crystalline uranyl nitrate, in the spectra of uranyl-modified samples (Figure 6b,c), indicates a chemisorbed state of uranyl ions on the TiO2 surface. The position of the signal for the nitrate group at 1049 cm–1, which is whichcommonly is commonly attributed attributed to isolated to isolatedgroups on groups the TiO on2 thesurface TiO2 [54],surface also [ 54supports], also supports this con- thisclusion. conclusion.

FigureFigure 6.6.Raman Raman spectraspectra ofof UOUO22(NO(NO33)2,, TiO TiO22,, and and UO UO22(NO(NO3)32)/TiO2/TiO2 (2a).(a ().b, (cb) ,showc) show corresponding corresponding spectraspectra with with a a higher higher magniﬁcation magnification in in the the range range of of 1200–700 1200–700 cm cm–1–1..

6+ XPSXPS analysis analysis shows shows the the U U6+charge charge state state in in uranyl uranyl ion ion chemisorbed chemisorbed on theon TiOthe 2TiOsurface2 sur- (Figureface (Figure7a). It 7a). is importantIt is important to note to note that that TiO TiO2 as2 as a supporta support for for uranyl uranyl ion ion promotes promotes fast fast 6+ 4+ andand reversiblereversible transformations between between U U6+ andand U U4+ chargecharge states states [29]. [29 Under]. Under vacuum vacuum con- Nanomaterials 2021, 11, x FOR PEER REVIEW 6+ 4+ 10 of 17 conditionsditions and and exposure exposure to X-rays, to X-rays, a partial a partial reduction reduction of U of6+ Uto Uto4+ occurs U occurs but oxygen but oxygen treat- 6+ treatmentment results results in a infast a fastreoxidation reoxidation back back to U to6+. U . Hydrothermal treatment of UO2(NO3)2/TiO2 in water solution with ethanol at 160 °C for 48 h (i.e., UO2(NO3)2/TiO2-HT) and thermal treatment of UO2(NO3)2/TiO2 in air at 500 °C for 3 h (i.e., UO2(NO3)2/TiO2-T500) were employed to change the chemical state of uranyl ion on the TiO2 surface. No uranium-contained crystal phases are detected in the samples after these treatments using XRD analysis. This result indicates that, similarly to the UO2(NO3)2/TiO2 sample, uranium on the surface of treated samples presents in a form of chemisorbed species or small clusters and does not form large crystallites. XRD patterns of the treated samples show only peaks of anatase (Figure 7b). A substantial change in both cases is that the treated samples have higher size of crystallites, namely 9 nm for pristine TiO2 and 21–22 nm for the treated samples. These data correlate with the data on the specific surface area of initial UO2(NO3)2/TiO2 and treated samples (Table 1). Accord- ing to DRIFT analysis (Figure S2 in Supplementary), both treatments result in the removal of nitrate groups from the surface of TiO2.

FigureFigure 7. 7.Photoelectron Photoelectron U4f U4f spectral spectral regions regions (a )(a and) and XRD XRD patterns patterns (b )(b for) for the the uranium-contained uranium-contained

TiOTiO2 2samples. samples. ◦ HydrothermalIn contrast to treatment UO2(NO of3)2UO/TiO2(NO2, XPS3)2/TiO analysis2 in water shows solution three withcharge ethanol states at of 160 uraniumC forfor 48 the h (i.e.,autoclavedUO2(NO sample3)2/TiO while2-HT) only and thermalthe U4+ charge treatment state of forUO the2(NO sample3)2/TiO calcined2 in air at in air. We expected that the calcination of UO2(NO3)2/TiO2 at 500 °C would result in the formation of solid uranium oxide on the surface of TiO2 but as mentioned above, no corresponding peaks are observed in the XRD pattern. The treatment at a higher temperature was not employed to avoid the phase transformation of TiO2 from anatase to rutile.

Table 1. List of prepared uranium-contained samples and their characteristics.

Uranium States aS,BET, Sample Synthesis Crystal Phase 1 at Surface 2 m2 g–1 UO2(NO3)2 Commercial UO2(NO3)2·6H2O UO2(NO3)2·6H2O U6+ (100%) [29] 2.5 ± 0.3 UO3 Calcination of UO2(NO3)2·6H2O at 400 °C for 3 h β-UO3 U6+ (100%) 0.6 ± 0.1 U3O8-T600 Calcination of UO2(NO3)2·6H2O at 600 °C for 3 h U3O8 5.3 ± 0.1 U6+ (63%) U3O8-T900 Calcination of UO2(NO3)2·6H2O at 900 °C for 3 h U3O8 1.5 ± 0.1 U4+ (37%) [29] Hydrothermal treatment of uranyl nitrate in water solution U5+ (79%) UO2+x U2.12 <0.5 3 with an excess of ethylenediamine at 160 °C for 72 h U4+ (21%) Hydrothermal treatment of uranyl nitrate in water solution U6+ (65%) U2(NH3)O6·3H2O U2(NH3)O6·3H2O 19.1 ± 0.2 with an excess of ethanol and ammonia at 160 °C for 72 h U5+ (35%) U6+ (69%) NaUxOy Chemical reduction of uranyl nitrate with sodium borohydride Na2U7O22 <2.3 3 U5+ (31%) UO3·H2O Chemical reduction of uranyl nitrate with hydrazine UO3·H2O U6+ (100%) 16.4 ± 0.4 TiO2 Commercial TiO2 Hombifine N Anatase 327 UO2(NO3)2/ Impregnation of TiO2 with water solution of uranyl nitrate fol- n.d. 4/anatase U6+ (100%) 5 298 TiO2 lowed by drying at 160 °C U6+ (24%) UO2(NO3)2/ Hydrothermal treatment of UO2(NO3)2/TiO2 in water suspen- n.d./anatase U5+ (25%) 84 TiO2-HT sion with an excess of ethanol at 160 °C for 48 h U4+ (51%) UO2(NO3)2/ Calcination of UO2(NO3)2/TiO2 at 500 °C for 3 h n.d./anatase U4+ (100%) 103 TiO2-T500 VLP7000 Commercial TiO2 KRONOS® vlp 7000 Anatase 250 1 Based on results of XRD analysis. 2 Based on results of XPS analysis. 3 The total value of sample’s surface area during measurement of N2 adsorption should be higher than 0.2 m2 for correct BET analysis; due to a small amount of sample, only an estimated value is presented. 4 The crystal phase for uranium compound is not detected (n.d.). 5 U4+ additionally forms under vacuum conditions and long-term exposure to X-rays.

Nanomaterials 2021, 11, 1036 10 of 17

◦ 500 C for 3 h (i.e., UO2(NO3)2/TiO2-T500) were employed to change the chemical state of uranyl ion on the TiO2 surface. No uranium-contained crystal phases are detected in the samples after these treatments using XRD analysis. This result indicates that, similarly to the UO2(NO3)2/TiO2 sample, uranium on the surface of treated samples presents in a form of chemisorbed species or small clusters and does not form large crystallites. XRD patterns of the treated samples show only peaks of anatase (Figure7b). A substantial change in both cases is that the treated samples have higher size of crystallites, namely 9 nm for pristine TiO2 and 21–22 nm for the treated samples. These data correlate with the data on the speciﬁc surface area of initial UO2(NO3)2/TiO2 and treated samples (Table1). According to DRIFT analysis (Figure S2 in Supplementary), both treatments result in the removal of nitrate groups from the surface of TiO2.

Table 1. List of prepared uranium-contained samples and their characteristics.

1 Uranium States as,BET, Sample Synthesis Crystal Phase 2 at Surface m2 g–1 6+ UO2(NO3)2 Commercial UO2(NO3)2·6H2O UO2(NO3)2·6H2O U (100%) [29] 2.5 ± 0.3 ◦ 6+ UO3 Calcination of UO2(NO3)2·6H2O at 400 C for 3 h β-UO3 U (100%) 0.6 ± 0.1 ◦ U3O8-T600 Calcination of UO2(NO3)2·6H2O at 600 C for 3 h U3O8 5.3 ± 0.1 6+ ◦ U (63%) U3O8-T900 Calcination of UO (NO ) ·6H O at 900 C for 3 h U O 1.5 ± 0.1 2 3 2 2 3 8 U4+ (37%) [29]

Hydrothermal treatment of uranyl nitrate in 5+ U (79%) 3 UO2+x water solution with an excess of ethylenediamine U2.12 4+ <0.5 at 160 ◦C for 72 h U (21%) Hydrothermal treatment of uranyl nitrate in U6+ (65%) U2(NH3)O6·3H2O water solution with an excess of ethanol and U2(NH3)O6·3H2O 5+ 19.1 ± 0.2 ammonia at 160 ◦C for 72 h U (35%) 6+ Chemical reduction of uranyl nitrate with U (69%) 3 NaUxOy Na U O <2.3 sodium borohydride 2 7 22 U5+ (31%) Chemical reduction of uranyl nitrate UO ·H O UO ·H O 6+ 16.4 ± 0.4 3 2 with hydrazine 3 2 U (100%)

TiO2 Commercial TiO2 Hombiﬁne N Anatase 327

UO2(NO3)2/ Impregnation of TiO2 with water solution of ◦ n.d. 4/anatase U6+ (100%) 5 298 TiO2 uranyl nitrate followed by drying at 160 C Hydrothermal treatment of UO (NO ) /TiO in U6+ (24%) UO (NO ) / 2 3 2 2 2 3 2 water suspension with an excess of ethanol at n.d./anatase U5+ (25%) 84 TiO2-HT ◦ 160 C for 48 h U4+ (51%)

UO2(NO3)2/ ◦ 4+ Calcination of UO2(NO3)2/TiO2 at 500 C for 3 h n.d./anatase U (100%) 103 TiO2-T500 ® VLP7000 Commercial TiO2 KRONOS vlp 7000 Anatase 250 1 Based on results of XRD analysis. 2 Based on results of XPS analysis. 3 The total value of sample’s surface area during measurement 2 of N2 adsorption should be higher than 0.2 m for correct BET analysis; due to a small amount of sample, only an estimated value is presented. 4 The crystal phase for uranium compound is not detected (n.d.). 5 U4+ additionally forms under vacuum conditions and long-term exposure to X-rays.

In contrast to UO2(NO3)2/TiO2, XPS analysis shows three charge states of uranium for the autoclaved sample while only the U4+ charge state for the sample calcined in air. We ◦ expected that the calcination of UO2(NO3)2/TiO2 at 500 C would result in the formation of solid uranium oxide on the surface of TiO2 but as mentioned above, no corresponding peaks are observed in the XRD pattern. The treatment at a higher temperature was not employed to avoid the phase transformation of TiO2 from anatase to rutile. As a result, we synthesize and characterize the main uranium oxides (i.e., UO3,U3O8, UO2), other uranium compounds (U2(NH3)O6·3H2O, polyuranate, UO3·H2O), and TiO2- supported materials contained uranium in a form of chemisorbed uranyl ions and species. Table1 summarizes the names of all prepared samples, short description of their synthesis, Nanomaterials 2021, 11, 1036 11 of 17

and physicochemical characteristics. The next section describes the photocatalytic activity of prepared materials under blue light.

3.2. Photocatalytic Activity

All the synthesized uranium compounds and TiO2-supported materials have a photocatalytic activity and are able to completely oxidize acetone vapor under blue light (450 nm, see Figure S3 in Supplementary). It indicates that the potentials of excited states or charge carriers photogenerated under blue light are high enough that they can be involved in redox interactions with organic molecules and oxygen. The photocatalytic oxidation of acetone to CO2 and water occurs over these samples without the formation of gaseous intermediates. The figures below show the data on steady-state rate and photonic efficiency of CO2 formation during the oxidation of acetone. The results are divided on the groups of solid uranium oxycompounds and TiO2-supported materials. As absorption of light is an essential step for photocatalytic reactions, UV–vis spectra of concerning samples are ® also shown in figures and discussed. Commercial photocatalyst TiO2 KRONOS vlp 7000, which has activity both under UV and visible light, plays in this study a role of benchmark to give other researchers possibility for comparison of the results.

3.2.1. Solid Uranium Oxycompounds Figure8 shows the data on photocatalytic activity of the prepared solid uranium oxycompounds and their UV–vis diffuse reﬂectance spectra. Uranyl nitrate is known to have a series of absorption bands between 360 and 500 nm, which form a broad absorption peak in this region (Figure8b). Pristine uranyl nitrate has photocatalytic activity under blue light (i.e., 450 nm). The steady-state rate of CO2 formation during the oxidation of acetone over solid uranyl nitrate is 0.021 µmol min–1 which corresponds to the photonic efﬁciency of 6.5 × 10–2%. These values are only 2 times lower than the corresponding ® –1 values for the visible-light photocatalyst TiO2 KRONOS vlp 7000: 0.043 µmol min and 13.3 × 10–2%. Despite an intense absorption of light in the corresponding region, uranium oxides prepared via the calcination method (i.e, UO3 and U3O8 samples) exhibit very low photocatalytic activity, (2.4–4.2) × 10–3 µmol min–1, which is only slightly higher than the detection limit for the used experimental setup (1.5 × 10–3 µmol min–1). A low surface area of the synthesized samples may be a reason for that because this parameter commonly affects the activity of heterogenous photocatalysts. As a support for this statement, an increase in the calcination temperature of uranyl nitrate from 600 to 900 ◦C leads to a 2 –1 decrease in the surface area of U3O8 oxide from 5.3 to 0.6 m g (Table1) and simultane- ous decrease in its activity about 2 times, despite an enhanced crystallinity of the sample calcined at higher temperature (Figure2c). On the contrary to our expectations, the UO2+x sample prepared via the hydrothermal treatment of uranyl nitrate in water solution with ethylenediamine exhibits an extremely low surface area but the photocatalytic activity of –3 –1 this sample (i.e., 3.3 × 10 µmol min ) is comparable to the activity of UO3 and U3O8 oxides. These results indicate that an extended surface area is not the only crucial parameter for the activity. Electrical conductivity of materials can also play an important role. According to previous works [55–58], electrical conductivity of uranium oxides increases similarly to the mass fraction of uranium in a sequence from UO3 to UO2. Additionally, hyperstoichiometric UO2 oxide is a p-type semiconductor while the UO3 and U3O8 oxides are n-type semiconductors [59]. Small values of observed photocatalytic activity of uranium compounds do not allow us to make a clear conclusion on the key parameter that affects the photocatalytic activity. Nanomaterials 2021, 11, x FOR PEERNanomaterials REVIEW2021 , 11, 1036 12 of 1712 of 17

Figure Figure8. Data 8. onData photocatalytic on photocatalytic activity activity ( a)) andand UV–vis UV–vis spectra spectra (b) of (b solid) of uraniumsolid uranium oxycompounds. oxycompounds. Other prepared uranium oxycompounds, which contain uranium in the charge state close to 6+, exhibit much higher photocatalytic activity. The U2(NH3)O6·3H2O sample Other preparedprepared uranium via oxycompounds, the hydrothermal treatment which contain of uranyl uranium nitrate in waterin the solution charge with state ethanol close to 6+, exhibit muchand ammonia higher exhibitsphotocatalytic the largest activity. specific surfaceThe U area2(NH among3)O6·3H all2 theO sample prepared pre- solid ura- pared via the hydrothermalnium oxycompounds treatment (i.e.,of uranyl 19.1 m2 nitrateg–1, Table in1 ).water The photocatalytic solution with activity ethanol of this and sample –1 ammonia exhibits theis 0.011 largestµmol specific min which surface is 2 times area loweramong than all the the activity prepared of pristine solid uranyl uranium nitrate. It is important to note that ammonia is known to undergo photocatalytic oxidation as well as oxycompounds (i.e., 19.1 m2 g–1, Table 1). The photocatalytic activity of this sample is 0.011 organic compounds. Therefore, NH3-groups in the composition of the U2(NH3)O6·3H2O μmol min–1 which issample 2 times may interactlower than with photogenerated the activity of holes, pristine suppressing uranyl the nitrate. pathway It of theiris im- interac- portant to note thattion ammonia with acetone is known and water to molecules, undergo consequently photocatalytic reducing oxidation the activity as of well the sample as in organic compounds.the Therefore, target process NH and3-groups playing ain negative the composition role. of the U2(NH3)O6·3H2O sample may interact withThe photogeneratedNaUxOy sample, which holes, is preparedsuppressing via the the chemical pathway reduction of their of uranyl inter- nitrate with sodium borohydride and has crystal phase of a polyuranate, exhibits substantially action with acetone higherand water photocatalytic molecules, activity conseq (0.018uentlyµmol reducing min–1), despite the activity a low value of the of specific sample surface in the target processarea and (Table playing1). It a is negative further proof role. that not only textural properties strongly affect the The NaUxOy sample,photocatalytic which activity. is prepared The highest via the activity chemical among reduction all the synthesized of uranyl solid nitrate uranium with sodium borohydrideoxycompounds and has is detectedcrystal inphas thee case of a of polyuranate, the UO3·H2O sample.exhibits The substantially rate and photonic efficiency of CO formation for this sample (0.065 µmol min–1 and 0.2%) are 1.5 times higher 2 –1 higher photocatalytic activity (0.018 μmol min ), despite a low value of specific surface® than the corresponding values for visible-light photocatalyst TiO2 KRONOS vlp 7000. area (Table 1). It is furtherTherefore, proof thethat chemical not only and textural phase compositions properties play strongly a key role affect in the the photocatalytic photocatalytic activity. activity.The highest Except theactivity uranium(VI) among oxide all preparedthe synthesized using a high-temperature solid uranium treatment, oxy- the compounds is detecteduranium in the compounds, case of whichthe UO contain3·H2O uranium sample. with The the rate charge an stated photonic close to 6+,effi- exhibit much higher activity compared to other compounds with lower charge state. The UO3·H2O ciency of CO2 formation for this sample (0.065 μmol min–1 and 0.2%) are 1.5 times higher compound can be expressed as uranyl hydroxide (i.e., UO2(OH)2). A high activity of the 2 ® than the correspondingUO3· Hvalues2O sample, for visible-light as well as initial photocatalyst uranyl nitrate, TiO additionally KRONOS supports vlp a specific7000. role of Therefore, the chemicaluranyl ion, and which phase provides compositio reactive excitedns play charge a key states role under in the radiation. photocatalytic It is important activity. Except the touranium(VI) note that the presenceoxide prepar of watered or using OH-groups a high-temperature in the crystal structure treatment, of the material, the as uranium compounds,in the which case of contain the UO3 ·uraniumH2O sample, wi mayth the be acharge reason forstate high close activity to of6+, this exhibit sample due to an enhanced formation of hydroxyl radicals. The pathway through interactions with much higher activityhydroxyl compared radicals commonlyto other playscompounds a key role with in the oxidationlower charge of organic state. compounds The in UO3·H2O compoundaqueous can be solutions expressed and humidifiedas uranyl air.hydroxide (i.e., UO2(OH)2). A high activity of the UO3·H2O sample, as well as initial uranyl nitrate, additionally supports a specific 3.2.2. TiO -Supported Materials role of uranyl ion, which provides2 reactive excited charge states under radiation. It is im- Uranium-contained samples based on TiO photocatalyst exhibit much higher activity portant to note that the presence of water or OH-groups2 in the crystal structure of the under blue light compared to the solid uranium oxycompounds. Figure9 shows the material, as in the case of the UO3·H2O sample, may be a reason for high activity of this corresponding values of steady-state rate and photonic efficiency of CO2 formation over ® sample due to an enhancedTiO2-supported formation samples, of initialhydroxyl precursors, radicals. and KRONOSThe pathwayvlp 7000 through reference. inter- UV–vis actions with hydroxylspectra radicals of all materialscommonly are alsoplays shown a key in role this figure.in the oxidation of organic compounds in aqueous solutions and humidified air.

3.2.2. TiO2-Supported Materials

Uranium-contained samples based on TiO2 photocatalyst exhibit much higher activity under blue light compared to the solid uranium oxycompounds. Figure 9 shows the corresponding values of steady-state rate and photonic efficiency of CO2 formation over TiO2-supported samples, initial precursors, and KRONOS® vlp 7000 reference. UV–vis spectra of all materials are also shown in this figure. TiO2 used as a support is 100% anatase with nanosized crystallites and has very high activity in the UV region corresponding to its bandgap absorption. Otherwise, its photocatalytic activity under blue light is extremely low (0.011 μmol min–1) and is mainly due to nitrogen impurities in its composition. In contrast to single TiO2 or solid uranyl nitrate,

Nanomaterials 2021, 11, x FOR PEER REVIEW 13 of 17

uranyl-grafted TiO2 (i.e., UO2(NO3)2/TiO2) exhibits the highest activity among all samples (0.73 μmol min–1 and 2.3%), which is 17 times higher than the activity of the commercial KRONOS® vlp 7000 visible-light photocatalyst. It is important to note that the mentioned value of photonic efficiency shows the efficiency of product formation (i.e., CO2). Many researchers estimate the efficiency for the reaction in whole. This estimation is commonly based on an assumption that one photon absorbed by (or incident on) the photocatalyst changes the oxidation state of carbon and oxygen into the oxidizing compound and molecular oxygen by 1. Taking into account the stoichiometry of acetone oxidation, the value of 2.3% corresponds to 12.2% as the photonic efficiency of this reaction (see Supplemen- Nanomaterials 2021, 11, 1036 13 of 17 tary for details). This is a very high value for the complete oxidation of organic compounds in the gas phase under visible light [60–62].

Figure 9. DataFigure on 9. photocatalyticData on photocatalytic activity activity (a) (anda) and UV–vis UV–vis spectra spectra ( b(b) for) for TiO TiO2-supported2-supported samples. samples.

TiO2 used as a support is 100% anatase with nanosized crystallites and has very The photocatalytichigh activity activity inof the uranyl-contained UV region corresponding materials to its under bandgap blue absorption. light is Otherwise,due to its the excitation of uranylphotocatalytic ion and activity reverse under redox blue light transformations is extremely low (0.011betweenµmol U min6+ –1and) and U is4+ mainly charges during the interactionsdue to nitrogen with impurities organic in compounds its composition. and In oxygen. contrast to If single compared TiO2 or to solid the uranyl crystalline form of UOnitrate,2(NO uranyl-grafted3)2, uranyl nitrate TiO2 (i.e., supportedUO2(NO3) 2on/TiO the2) exhibitssurface the of highest TiO2 exhibits activity among µ –1 extremely higher photocatalyticall samples (0.73 activity.mol min Twoand reasons 2.3%), whichmay explain is 17 times this higher synergistic than the activityeffect: of the commercial KRONOS® vlp 7000 visible-light photocatalyst. It is important to note that (i) TiO2, as well as otherthe mentioned porous supports, value of photonic increases efficiency the dispersion shows the efficiencyof uranyl of nitrate product and formation stabilizes a chemisorbed(i.e., COstate2). of Many uranyl researchers ion, as estimate has shown the efficiency above using for the characterization reaction in whole. This methods. This meansestimation that a higher is commonly number based of on U-sites an assumption are available that one photonfor interactions absorbed by with (or incident organic molecules andon) interfacial the photocatalyst charge changes transfer the oxidationunder irradiation. state of carbon This and statement oxygen into is the sup- oxidizing compound and molecular oxygen by 1. Taking into account the stoichiometry of acetone ported by our previouslyoxidation, publis the valuehed data of 2.3% [28] corresponds showing to a 12.2% linear as dependence the photonic efficiency of activity of this on reaction the content of uranyl(see nitrate Supplementary up to 10 for wt.%. details). (ii)This The is specific a very high role value of TiO for the2 as complete support oxidation for of uranyl ions. As we haveorganic shown compounds previously in the gas on phase comparison under visible with light silica [60–62 and]. alumina supports [28,29], just TiO2 providesThe photocatalytic fast redox activity transformations of uranyl-contained between materials U under6+ and blue U4+ light charge is due to the 6+ 4+ states that results in excitationa very high of uranyl photocatalytic ion and reverse activity redox of transformations this material. between U and U charges during the interactions with organic compounds and oxygen. If compared to the crystalline As has shown above, the chemical reduction of uranyl nitrate with hydrazine leads form of UO2(NO3)2, uranyl nitrate supported on the surface of TiO2 exhibits extremely to an increase in photocatalytichigher photocatalytic activity activity.of the prepared Two reasons sample may explain (i.e., UO this3 synergistic·H2O) compared effect: (i) TiO2, to initial uranyl nitrateas well (Figure as other 8a). porous In the supports, case of increases uranyl-grafted the dispersion TiO2 of, this uranyl treatment nitrate and con- stabilizes trary decreases the photocatalytica chemisorbed state activity. of uranyl It is ion, difficult as has shownto make above clear using conclusions characterization on the methods. This means that a higher number of U-sites are available for interactions with organic transformations of uranylmolecules ion and occurred interfacial on charge the TiO transfer2 surface under irradiation.during this This treatment. statement is There- supported by fore, the data for otherour previouslytreatments, published namely data calcination [28] showing and a linear hydrothermal dependence of treatment, activity on theare content discussed below to showof uranyl an nitrateimportant up to 10role wt.%. of (ii)uranyl The specific ion in role the of concerning TiO2 as support system. for uranyl Both ions. As employed thermal treatmentswe have shown lead previously to substantia on comparisonl decrease with in the silica activity and alumina of uranyl-grafted supports [28,29 ], just 6+ 4+ TiO2 provides fast redox transformations between U and U charge states that results in TiO2 (Figure 9a). The values of activity are 0.21 μmol min–1 (0.65%) and 0.10 μmol min–1 a very high photocatalytic activity of this material. (0.30%) for hydrothermallyAs has treated shown (i.e., above, UO the2(NO chemical3)2/TiO reduction2-HT) of and uranyl thermally nitrate with treated hydrazine (i.e., leads to UO2(NO3)2/TiO2-T500an) increasesamples, in photocatalyticrespectively. activity As mentioned of the prepared in the sample previous (i.e., UO section,3·H2O) comparedno uranium-contained crystalto initial phases uranyl are nitrate detected (Figure in8a). the In treated the case samples of uranyl-grafted using XRD TiO 2analysis., this treatment However, UV–vis spectracontrary for decreases these samples the photocatalytic differ from activity. the spectrum It is difficult of prepared to make clear uranyl- conclusions on the transformations of uranyl ion occurred on the TiO2 surface during this treatment. grafted TiO2 sample Therefore,because they the data exhibit for other much treatments, higher namelyabsorption calcination of light and in hydrothermal whole visible treatment, region (Figure 9b). Aare form discussed of these below spectra to show in an the important visible role region of uranyl corresponds ion in the concerning to the spectra system. Both employed thermal treatments lead to substantial decrease in the activity of uranyl-grafted –1 –1 TiO2 (Figure9a). The values of activity are 0.21 µmol min (0.65%) and 0.10 µmol min (0.30%) for hydrothermally treated (i.e., UO2(NO3)2/TiO2-HT) and thermally treated (i.e., UO2(NO3)2/TiO2-T500) samples, respectively. As mentioned in the previous section, no uranium-contained crystal phases are detected in the treated samples using XRD analysis.

Nanomaterials 2021, 11, 1036 14 of 17

However, UV–vis spectra for these samples differ from the spectrum of prepared uranyl- grafted TiO2 sample because they exhibit much higher absorption of light in whole visible region (Figure9b). A form of these spectra in the visible region corresponds to the spectra for solid U3O8 and UO2 oxides described above (Figure8b), which have a strong absorption of light in whole visible region and are dark brown or black in color. Therefore, according to UV–vis spectroscopy, both treatments lead to a change in the chemical state of uranyl ion on the TiO2 surface which is also supported with the data of XPS analysis (Figure7a ). Most probably, complete or partial transformation of uranyl ions to clusters or highly dispersed oxide particles occurs on the surface during the treatments. It is important to note that if these TiO2-supported materials are compared with solid uranium oxycompounds prepared from uranyl nitrate via thermal and hydrothermal treatments, they exhibit much higher activity. The suggestion mentioned above on an increased dispersion of uranium compounds on the TiO2 surface is also relevant for this case. The results of photocatalytic measurements indicate that only the state of uranyl ion with the charge of 6+ provides a high photocatalytic activity under blue light. Other states on the TiO2 surface with lower charge of uranium exhibit much lower activity. In 5+ the case of the UO2(NO3)2/TiO2-HT sample, an intermediate U charge state (Figure7a) interrupts a fast transition U6+↔U4+ and suppresses the photocatalytic activity. In the case of UO2(NO3)2/TiO2-T500, the equilibrium between uranium charge states is totally shifted to U4+ (Figure7a), and the activity is limited by the number of uranium species able to change their oxidation state and participate in the transfer of the charge carrier for the occurring photocatalytic reaction. It should be noted that substantial decrease in the activity of treated samples can also be due to a strong decrease in the surface area of TiO2 support after these treatments (Table1). Therefore, this study shows that solid uranium oxycompounds have a low photocatalytic activity under blue light. Otherwise, uranium species immobilized on TiO2 support exhibit a very high activity. Uranyl-grafted TiO2 has the potential as visible-light photocatalyst for special areas of application where there is no strict control for use of uranium compounds (e.g., in spaceships or submarines). It also can be investigated in the reactions of photocatalytic reduction under visible light.

4. Conclusions

We successfully synthesize and characterize the main uranium oxides (i.e., UO3,U3O8, UO2), other uranium compounds (U2(NH3)O6·3H2O, polyuranate, UO3·H2O), and TiO2- supported materials containing uranium in a state of chemisorbed uranyl ions and species. All the synthesized solid uranium oxycompounds and TiO2-supported materials have a photocatalytic activity in oxidation reactions and are able to completely oxidize acetone in the gas-phase under blue (450 nm) light. It confirms that the potentials of excited states or charge carriers photogenerated under blue light are high enough that they can be involved in redox interactions with organic molecules and oxygen. The data on steady-state rate of CO2 formation during the oxidation of acetone vapor under blue light show that the chemical and phase compositions play a key role in the photocatalytic activity of uranium compounds. Except the uranium(VI) oxide prepared using a high-temperature treatment, the uranium compounds, which contain uranium with the charge state of 6+, especially uranyl ion, exhibit much higher activity compared to other compounds with lower charge state. Among solid compounds, the highest activity is observed for the sample prepared via the chemical reduction of uranyl nitrate with hydrazine, which has the crystal structure of UO3·H2O hydrate (or UO2(OH)2). Immobilization of uranyl ions on the surface of TiO2 nanocrystallites via the impregnation of TiO2 support with aqueous solution of uranyl nitrate results in a photocatalyst with great activity under blue light. The photonic efficiency of acetone oxidation over TiO2-supported material, 12.2%, is 17 times higher than the efficiency for commercial photo- ® catalyst TiO2 KRONOS vlp 7000. Thermal and hydrothermal treatments of uranyl-grafted Nanomaterials 2021, 11, 1036 15 of 17

TiO2 decrease its photocatalytic activity due to a change in chemical state of uranium and a strong decrease in surface area of TiO2 support after these treatments.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano11041036/s1, Figure S1: XRD patterns for the sample prepared via hydrothermal treatment and for UO2.12 reference (PDF#04-007-2508), Figure S2: DRIFT spectra for TiO2-supported materials, Figure S3: Emission spectrum of used light-emitted diode. Author Contributions: Conceptualization, D.K.; methodology, T.F. and M.L.; validation, M.L., S.C., and D.S.; investigation, T.F., S.C., M.S., I.P., A.B.; resources, T.F., M.L., and D.S.; writing—original draft preparation, T.F. and M.L.; writing—review and editing, D.S.; visualization, M.L. and D.S.; supervision, D.K.; project administration, D.S.; funding acquisition, D.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Russian Science Foundation, grant number 20-73-10135. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy. Acknowledgments: The research was performed using equipment of the Shared-Use Center “Na- tional Center for the Study of Catalysts” at the Boreskov Institute of Catalysis. We thank Yu. A. Chesalov for analysis of samples using Raman spectroscopy. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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