Research ArticleCatalytic Activity of a Composition Based on StrontiumBismuthate and Bismuth Carbonate at the Exposure to theLight of the Visible Range

K. S. Makarevich , A. V. Zaitsev , O. I. Kaminsky , E. A. Kirichenko ,and I. A. Astapov

Institute of Materials Khabarovsk Scientific Center, Far Eastern Branch of the Russian Academy of Sciences, Khabarovsk, Russia

Correspondence should be addressed to O. I. Kaminsky; kamin_div0@mail.ru

Received 13 April 2018; Revised 26 July 2018; Accepted 6 August 2018; Published 10 September 2018

Academic Editor: Gianluca Di Profio

Copyright © 2018 K. S. Makarevich et al. /is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

/is paper presents the results pertaining to studying the properties of the photocatalytically active composition of strontiumbismuthate SrBi2.70O5.05, SrBi2.90O5.35, and SrBi3.25O5.88 and bismuth carbonate (BiO)2CO3 in molar ratios 1/0.67, 1/0.56, and1/0.37, respectively. /ese compositions are obtained through pyrolytic synthesis from organic precursor complexes of strontiumand bismuth with sorbitol. It has been established that the synthesised powder materials absorb the light of the visible range up to500 nm owing to the presence of a narrow-gap semiconductor strontium bismuthate./e presence of a wide-band semiconductor(BiO)2CO3 ensures an effective separation of electron-hole pairs. /e diffuse reflection spectra (DRS) of compositions differ fromthe analogous spectra of a mechanical mixture of these semiconductor phases with the same composition, which allows one toassume the heterostructural structure of the semiconductor system. /e same heterostructure is confirmed by the results ofmapping. Catalytic particles that are synthesised at a temperature of 500°C containing 27 mass% (BiO)2CO3 (corresponding toSrBi2.9O5.35/(BiO)2CO3 �1/0.56) have the greatest activity with respect to the photodecomposition of methylene blue (MB). /epossibility of controlling the optical properties and photocatalytic activity of the composition is depicted due to the jointformation of the strontium bismuthate phase and the bismuth carbonate phase.

1. Introduction

Organic compounds are often the primary toxic componentsof wastewater from various origins. Photocatalysis becamea promising technology for their purification after the pub-lication of one of the pioneering works in this regard, whichwas written by Frank and Bard in 1977 [1]. On the other hand,studies concerning photocatalytically active semiconductorsbegan to develop intensively after the publication of the well-known work of Fujishima and Honda [2], which lead to thepublication of a large series of articles by various authorsdedicated to obtaining hydrogen from water using lightenergy. Both the aforementioned processes are based on thephenomenon of photolysis of water on the surface ofa semiconductor under the action of light. As a result, strong

oxidising agents, such as atomic oxygen, free hydroxideradicals, hydrogen peroxide, to name a few, are produced withthe capacity of ensuring deep oxidation of organic com-pounds./e undoubted advantage of this method concerningorganic removal is the lack of regular costs for additionalchemical reagents. Consequently, the method of photo-catalytic purification has found wide application, from thefinal stage of the treatment of drinking water at water intakestations to the sewage treatment conducted in industries suchas oil refining, paint and varnish, and textile.

/e search for new catalytically active systems, capable ofphotolysis of water under the influence of sunlight, has beenconducted for the purpose of creating compositions basedon narrow-gap semiconductors having an absorption regionin the visible spectral range [3, 4]. In particular, they include

HindawiInternational Journal of Chemical EngineeringVolume 2018, Article ID 4715629, 9 pageshttps://doi.org/10.1155/2018/4715629

semiconductors based on transition metal oxides. However,photocatalysts with this composition have a significantdisadvantage that is associated with the toxicity of the heavymetals included in them (e.g., Cd and Pb) [5–7], whichmakes it impossible to utilise such substances for waterpurification. It is known that bismuth is an exception interms of being a toxicological hazard among heavy metals.Its toxicity is so minute that bismuth compounds have beenfound, for instance, to be widely used in pharmacology asantacid agents for oral administration [8, 9]. /is is owing tothe fact that mobile cationic forms of bismuth are notformed in alkaline, neutral, and weakly acidic media. /us,during the purification of water, its saturation with regard tobismuth is excluded. Among bismuth compounds, highcatalytic activity comparable to anatase shows the presenceof carbonate (BiO)2CO3. However, the region of its intrinsicabsorption lies in the UV range of the spectrum. To sensitise(BiO)2CO3 to visible light, it is doped with nitrogen [10, 11].Other bismuth compounds, such as the narrow-bandsemiconductor CaBi6O10, do not require the use of addi-tives, since they are characterised by intrinsic absorption inthe visible spectral range [12, 13]. However, they are lesseffective with regard to the photodecomposition of organiccompounds. One way to increase the reactivity of narrow-band semiconductors is to create heterostructural com-posites with wide-gap semiconductors.

As a rule, two-phase compositions are synthesised intwo stages: first, the main component is obtained, afterwhich a second component is formed on its surface. In thismethod of production, it is difficult to achieve a uniformmutual distribution of semiconductors in the concernedvolume. /erefore, in our study, we employed the pre-viously developed pyrolytic synthesis method based onbismuth complexes with sorbitol [14], which simulta-neously produces a composition of two semiconductors. Asa wide-gap semiconductor, (BiO)2CO3 was formed in thecomposition. Moreover, the absorption of visible light wasachieved owing to the introduction of strontium bismu-thate, which occupies a central position in the solid so-lutions presented in the SrO-Bi2O3 diagram [15]. /epurpose of this study was to create and study the propertiesof SrBixOy/(BiO)2CO3 compounds of various composi-tions, depending on the synthesis conditions. Furthermore,it purposes to reveal the phase composition of the catalyst,providing the greatest photocatalytic activity of the com-position during the decomposition of the organic dye.

2. Experiment

SrBi4O7 and several compositions based on it were obtainedthrough a modified method of pyrolytic synthesis. Previously,we employed this method to obtain CaBi6O10. Strontiumnitrate Sr(NO3)2 (ACROS), bismuth nitrate pentahydrateBi(NO3)3·5H2O (ACROS), and sorbitol C6H14O6 served asprecursors for the synthesis of composites based on SrBixOy.

/e amount of sorbitol in the precursor mixture’scomposition increased by 1.5 times in comparison to [16],which allowed the amount of CO2 and (BiO)2CO3, to-gether with SrBixOy, to increase. /e mixture of Sr(NO3)2,

Bi(NO3)3·5H2O, and sorbitol (Sr : Bi � 1 : 4 molar ratio)was grinded in mortar./e grinded mixture spontaneouslyformed a transparent viscous solution due to the hydratedwater of nitrates. Pyrolysis of the precursor mixture wasconducted at 180°C in the air. Subsequently, the pyrolyzedmixture was heated to temperatures of 300, 400, 500, 600,and 700°C and was held isothermally for 24 hours. /eobtained SrBixOy/(BiO)2CO3 compositions have beendesignated as SBC-X, where X signifies the synthesis tem-perature. /e concentration of (BiO)2CO3 in the obtainedsamples was measured by dissolving the sample in 10% HClsolution. Subsequently, the amount of CO2 released was es-timated by making use of a Ba(OH)2 solution. In addition,Brunauer–Emmett–Teller (BET) specific surface area of thesamples wasmeasured via N2 adsorption at 77K by using Sorbi4.1. analyser./e crystal in a phase of the samples was identifiedthrough X-ray diffraction by utilising a DRON-7 diffractom-eter with CuKα (λ�1.5406 A) as the radiation source, whichoperated at 40 kV and 40mA. Moreover, the UV and visiblediffuse reflectance spectra were conducted on a MDR-41spectrophotometer system equipped with an integratingsphere attachment, with BaSO4 as the background. Surfacemorphology and distribution of particles were studied by LEO1430VP SEM, by employing an accelerating voltage of 15 kV.

Furthermore, photocatalytic experiments were per-formed using an automated photometric unit [17]. Photo-catalytic activities of the samples SBC-X were evaluatedthrough the degradation of methylene blue solution(1.2mg·L−1, 350mL) under different light irradiations dis-charge lamp Aqua Arc Osram, Sylvania (250W, continuousspectrum λ� 380–850 nm). Mass of photocatalyst sample is200mg./e spectrum of the lamp was similar to the spectraldistribution of the radiation intensity with regard to sun-light. To cut out the UV range, a filter (λ> 380 nm) wasemployed. During the irradiation, the solution was stirredusing a IKA RO10 magnetic stirrer. /e temperature of thephotocatalytic system was maintained at 25oC by utilisingthe water-cooling thermostat (Julabo F25-ED).

3. Results and Discussion

/e study of the process of phase formation in the systemunder investigation upon its heating yielded the followingresults. /e composition at 300–400°C consists of threephases. In the temperature range, the composition at500–600°C has a two-phase composition. At 700°C, thecomposition becomes single-phase. /is can be seen fromXRD (Figure 1). On the XRD patterns of these samples, thereare pronounced reflections belonging to α-Bi2O3 and(BiO)2CO3. In addition, there are peaks corresponding toa solid solution of the SrO-Bi2O3 system with stoichiometrysimilar to SrBi3O5.5 (ICDD-45-609). Furthermore, analysisof the XRD patterns of sample SBC-400 revealed that theconcentration of Bi2O3 is reduced in this regard, whichindicates the introduction of Bi2O3 into a solid solution ofstrontium bismuthate. In this sample, (BiO)2CO3 has notdecomposed yet. /e intensity of its diffraction peaks doesnot change in comparison to SBC-300. On the XRD patternof sample SBC-500, an increase in the intensity of the reflex

2 International Journal of Chemical Engineering

3.14 A is observed. �is demonstrates that the solid solutionof strontium bismuthate continues to be formed through theincorporation of unreacted bismuth oxide. At temperaturesabove 600°C, (BiO)2CO3 is partially decomposed, and theintensity of its re­exes decreases. �e bismuth oxide formedduring the decomposition of (BiO)2CO3 enters a solid so-lution based on strontium bismuthate. �is process ischaracterised by an increase in the intensity of the re­ex3.14 A. At a temperature of 700°C, the process concerning theformation of a solid solution of strontium bismuthate ends.�is is con�rmed by the displacement of all the peaks in theregion of large angles and through a change in the ratio of thetwo primary phase re­exes (3.11 and 3.07 A) of strontiumbismuthate. Consequently, the di�ractogram assumes theform that is typical of a solid solution of strontium bismuthatewith stoichiometry close to SrBi4O7. Based on the results ofXRD, the strontium carbonate phase was not found.

Chemical analysis revealed that the concentration ofcarbonates in the samples obtained at 300°C to 400°C isapproximately the same and equal to 29–32 wt%. �econtent (BiO)2CO3 in the sample SBC-500 is about 27% and18% in the SBC-600 composition.

�e results of the mapping show that there are areascontaining predominantly strontium and having in-signi�cant quantity of carbon. Comparing with XRD data,these areas were identi�able, like the phase of strontiumbismuthate. �e remaining surface of the sample containsbismuth, oxygen, and carbon, but so does insigni�cantquantity of strontium and is identi�able as bismuth car-bonate. As can be seen from Figure 2, the bismuth carbonatephase has high roughness, while the bismuth-strontiumphase is represented by a smooth surface. �e formationof such a structure is possible due to the epitaxial growth in

the process of joint synthesis of both semiconductors thatmake up the composition. �e synthesized compositions arerepresented by two main particle fractions, where thesmaller one ((BiO)2CO3) has a nanometer size range, and thelarge ones have a strontium-bismuthate phase with particlesizes of about 1.5–3.5 μm that form agglomerates. �e av-erage size of the agglomerates is in the range of 20–50 μm.

By conducting a comparative analysis of XRD data andchemical analysis, it can be assumed that the obtainedcompositions have the following content (Table 1). As it iscommonly known [15], the phase diagram of SrO-Bi2O3rather has a wide range of solid solutions. According tore�ned data [18], a solid solution is formed under thecondition that the Sr : Bi atomic ratio is in the range from1 : 2.5 to 1 : 7.5, meaning that it corresponds to compoundswith stoichiometry from SrBi2.5O4.75 to SrBi7.5O12.25.According to XRD data, the strontium bismuthate formedin the compositions is most close to with Sr : Bi 1 : 3stoichiometry SrBi3O5.5 (ICDD-45-609) and, conse-quently, is a representative of these series of solid solutions.�e re­exes characteristic of this strontium bismuthatewere identi�ed for the samples SBC-400, SBC-500, andSBC-500. However, a more accurate identi�cation of thestoichiometry of strontium bismuthate obtained fromXRD data is not possible, since the samples contain(BiO)2CO3, whose main re­exes are superposition or closeto the re­ections of strontium bismuthate from the aboveseries of solid solutions. In order to identify more accu-rately the stoichiometry of the formed bismuth phases, weadditionally carried out a chemical analysis that made itpossible to determine the content of (BiO)2CO3 in thesamples. Further, a simple calculation was carried out,based on the following facts: �rstly, the total atomic Sr : Bi

6,84

6,84

6,84

6,84 3,71

3,72

3,72

3,72

3,41

3,41 3,34

3,10

3,31

3,38

3,14

3,42

3,41

2,94

2,12 1,98

1,83 1,

67

1,62

1,73

1,62

1,62

1,68

1,68

1,68

1,67

1,82

1,84

1,83

1,83

1,97

1,98

1,98

1,98

2,91

2,94

2,94

2,94

2,69

2,70

3,09

3,193,

253,

25 2,13

2,13

2,12

3,07

3,09

3,09

3,09

3,14

3,14

3,34

3,32

SBС-300

SBС-400

SBС-500

SBС-600

SBС-700

Inte

nsity

(a.u

.)

SrBi3O5.5

SrBi4O7(BiO)2СO3Bi2O3

15 20 25 30 35 40 45 50 55 60102θ (degree)

Figure 1: XRD patterns of samples obtained at di�erent synthesis temperatures.

International Journal of Chemical Engineering 3

ratio in all the obtained compositions was set equal to 1 : 4during the synthesis, secondly, the samples of SBC-400,SBC-500, and SBC-600 are two-phase, and thirdly, thecontent of (BiO)2CO3 is accurately known from thechemical analysis. /erefore, by knowing the share ofbismuth that went to the formation (BiO)2CO3, it is

possible to calculate the share of bismuth that went to theformation of strontium bismuthate, i.e., to clarify thestoichiometry of Sr : Bi in the solid solution formed (Ta-ble 1). /e data obtained in this way are in good agreementwith the results of XRD./e adjusted Sr : Bi ratio is actuallyclose to 1 : 3 and is 1/2.7, 1/2.9, and 1/3.25 for the samples

Bi

C

O

Bi O

Sr

C Sr

Figure 2: SEM images of composed SBC-500 and results of mapping.

Table 1: /e content of semiconductor phases in the obtained compositions.

Photocatalysts Total atomic ratio Sr :Bi in the compositions

Weight content(BiO)2CO3 (%)

Photocatalysts SrBixOy/(BiO)2CO3

Element stoichiometryin phases

Molar ratio of phases inthe compositions

SBC-400 1 : 4 32 SrBi2.70O5.05/(BiO)2CO3 1/0.67SBC-500 1 : 4 27 SrBi2.90O5.35/(BiO)2CO3 1/0.56SBC-600 1 : 4 18 SrBi3.25O5.88/(BiO)2CO3 1/0.37SrBi4O7 1 : 4 0 SrBi4O7 1/0

4 International Journal of Chemical Engineering

of SBC-400, SBC-500, and SBC-600, respectively. /ecomposition SBC-300 (based on XRD data) is three-phase,since it contains, in addition to the two phases describedabove, also Bi2O3. Accordingly, the use of the above se-quence of calculations in relation to it will not be correct.For this reason, the sample SBC-300 is not included inTable 1. A more detailed study of it was not the purpose ofthis paper, since the compositions formed by bismuthoxide and alkaline earth metal bismuth were previouslyinvestigated by many authors, for example, in [10–12]; inaddition, the sample SBC-300 did not show a high catalyticactivity.

/e composition SBC-400 has the greatest specificsurface area of 3.5m2/g, according to BETanalysis (Table 2).Increasing the synthesis temperature from 400°C to 600°Cleads to the fact that the specific surface area of the samples isreduced to 3.0m2/g. /is is probably due to the gradualdecomposition of the carbonate phase (BiO)2CO3 and thecoarsening of the particles belonging to the formed solidsolution of strontium bismuthate. When the carbonate iscompletely decomposed, the single-phase SrBi4O7 sampleobtained at 700°C has a specific surface area of 2.4m2/g. /isis comparable to the specific area of pure α-Bi2O3 (2.2 m2/g),which is obtained in the same manner. Hence, the presenceof a carbonate phase contributes to the formation of a moredeveloped surface in two-phase compositions.

Based on the DRS for each synthesised composition,SrBi4O7, and (BiO)2CO3, the Kubelka–Munk function F(R)�

0.5(1–R)2/(R–1) was calculated, where R represents the re-flection coefficient. /e widths of the band gap are de-termined from the point of intersection of the functiongraph’s linear section (F(R) hν)½ with the axis of abscissae:photon energy h] (Figure 3). According to the concernedestimates, the width of the forbidden band of SrBi4O7 and(BiO)2CO3 was Eg � 2.50 eV and 3.38 eV, respectively.Moreover, analysis of the dependencies (F(R) hv)½ for thecompositions SBC-500 and SBC-600 showed that they can bedivided into two linear Sections I and II (Figure 3). /ey area superposition of the graphs of two semiconductors thatconstitute the composition. Section I corresponds to bismuthcarbonate, whereas section II corresponds to strontium bis-muthate. As the synthesis temperature increases, section Idecreases, and section II, respectively, increases due to theincrease in the proportion of strontium bismuthate in thesolid solution. Comparing the analogous curves concerninga mechanical mixture of 73% SrBi4O7 + 27% (BiO)2CO3, onecan note that there is a marked difference between them.Investigation of optical properties has shown that the spectraof DRS compositions do not signify a simple superposition ofthe semiconductor phases that form them and differ from thespectra of a mechanical mixture with the same composition./eir distinctive feature is the characteristic absorption regionin the range of 200–270 nm, which is absent on the individualphases’ spectra of the composition’s constituents and on thespectrum of a simple mechanical mixture identical to thecomposition of the relevant composition.

Studying the photocatalytic activity of synthesised ma-terials demonstrated that their use makes it possible for oneto increase the decomposition rate of methylene blue upon

irradiation with visible light. Kinetic dependences of thechange in the concentration of organic dye, in the presenceof the synthesised compositions SBC 300–700, SrBi4O7, andα-Bi2O3, have been depicted in Figure 4. /e single-phasesamples of SrBi4O7 and α-Bi2O3 possess the lowest catalyticactivity. Moreover, the activity of SrBi4O7 is moderatelyhigher than that of bismuth oxide. /is is owing to the widerabsorption region of SrBi4O7./e catalytic activity of biphasiccompositions in all cases was observed to be higher. Weattribute this result to the cocatalytic effect of the interactionof the SrBixOy/(BiO)2CO3 phases forming the composition.Its occurrence is possible due to the heterostructural nature ofthe structure of the composition./is is confirmed by the dataof the SEM mapping of the images given earlier. /e pos-sibility of epitaxial growth is also indicated by similar X-raystructural parameters of the obtained strontium bismuthateand bismuth carbonate. To test this assumption, a mechanicalmixture was prepared from the separately obtained SrBi4O7and (BiO)2CO3 phases and an identical SBC-500 sample,exhibiting the greatest catalytic activity. /e catalytic activityof the prepared mixture (Figure 4) showed an intermediatevalue between the activities of its constituent phases and isinferior to the activity of any of the obtained compositions,including SBC-500. /erefore, the joint-phase formationduring synthesis is a prerequisite for the efficient operation ofthe photocatalytic composition.

/e sample synthesised at 500°C exhibits greatest catalyticactivity. /is is probably related to the formation of the re-quired amount of strontium bismuthate, which sensitises thesystem to visible light. On the contrary, (BiO)2CO3 is retainedin sufficient amount (27%) for the efficient functioning of thecatalyst. When its quantity is reduced to 18%, the efficiency ofthe SBC-600 catalyst decreases. In sample SBS-400 (32%-(BiO)2CO3), there is not enough strontium bismuthate, andhence the catalyst is not capable of absorbing the energy ofvisible light. /e reaction rate constant (k) was determined bydetermining the parameters of the linear regression equationsfor the (Ct/C0)� f(t) dependencies. /e obtained values of khave been presented in Table 2. It is known that the samplesunder study have different specific surface areas. For a correctcomparison of the catalytic activity, the rate constant ks·10−4,normalised to the specific surface, was calculated (Table 2).With an increase in the synthesis temperature of the samples,their catalytic activity increases and becomes greatest for theSBC-500 composition (ks � 4.69). A further increase intemperature at first leads to a decrease in the ks constant (4.33)

Table 2: /e catalytic activity of the resulting compositions andsingle-phase photocatalysts.

SampleSpecific

surface area,S (m2/g)

Reaction rateconstant,

k·10−3 (min−1)

Specific reactionrate constant,

ks·10−4 (g·min−1·m−2)α-Bi2O3 2.1 4.0 1.90SBC-300 3.2 9.0 2.81SBC-400 3.5 1.3 3.71SBC-500 3.2 1.5 4.69SBC-600 3.0 1.3 4.33SrBi4O7 2.4 8.0 3.33

International Journal of Chemical Engineering 5

SBС-600SBС-500

SBС-400SBС-300

SrBi4O7700°CMixtureα-Bi2O3

Withoutphotocatalyst

0 1801601401201008060402070

75

80

85

90

95

100

Time (min.)

C t/C

0 (%

)

Figure 4: Photocatalytic decomposition curves of MB under irradiation with visible light in the presence of SBC-300, SBC-400, SBC-500,and SBC-600 and single-phase samples of SrBi4O7, and α-Bi2O3 and a mechanical mixture of 73% SrBi4O7 + 27% (BiO)2CO3.

2.702.90

2.50 eV

3.38 eV

SrBi4O7-700°C

(BiO)2CO3

SBC-600

(F(R

)hv)

0.5

(F(R

)hv)

0.5

(F(R

)hv)

0.5

II- se

ction

I- sec

tion

3 3.5

3 3.5

3 3.5 4 4.5 5 5.5 6 6.52.5eV

2.7

3

3.01

2.693.15

3.152.73

Mechanicalmixture

SBC-500

SBC-400

SBC-300

(F(R

)hv)

0.5

(F(R

)hv)

0.5

(F(R

)hv)

0.5

II- se

ction

I- se

ction

3.5

3 3.5

3 3.5 4 4.5 5 5.5 6 6.52.5eV

Figure 3: Dependences (F(R)h])½ of single-phase samples of SrBi4O7 and (BiO)2CO3 and the compositions SBC-300, SBC-400, SBC-500,and SBC-600 and their mechanical mixture of 73% SrBi4O7 + 27% (BiO)2CO3 from photon energy.

6 International Journal of Chemical Engineering

for the SBC-600 sample. Subsequently, it exhibits a signi�cantdrop in ks (3.33) at a temperature of 700°C, when thecomposition becomes single-phase. A comparative analysis ofthe obtained data ks shows that the high catalytic activity ofthe compositions in comparison with monophases anda mechanical mixture is primarily due to the heterostructuralphase content, and not only to the developed surface.

4. Discussion

�e results of this study obtained can be explained byconsidering the cocatalytic e�ect of the di�erent phases inthe composition of SrBixOy/(BiO)2CO3. �e formation ofsuch a structure is possible owing to the epitaxial growth inthe process of joint synthesis of both semiconductorsconstituting the composition. Moreover, the possibility ofepitaxial growth is also indicated by similar X-ray structuralparameters of the obtained strontium bismuthate and bis-muth carbonate. �e catalytic activity of the mixture of 73%SrBi4O7 + 27% (BiO)2CO3 (Figure 4) demonstrated an in-termediate value between the activities of its constituentphases, which are inferior to the activity of any of theresulting compositions, including SBC-500. Consequently,the formation of heterostructural phases in the synthesisprocess increases the e¦ciency of the photocatalytic com-position. Furthermore, values pertaining to the boundariesof the valence band (VB) and the conduction band (CB) ofSrBi4O7 and (BiO)2CO3 compounds were calculated byemploying the following equations [19]:

EVB � XE−Ee + 0.5Eg, (1)

ECB � XE−Ee − 0.5Eg, (2)

where XE signi�es the absolute electronegativity of thesemiconductor, which is calculated as the geometric mean of

the absolute electronegativity of the atoms forming it (theXE values for SrBi4O7 and (BiO)2CO3 are 5.76 eV and6.54 eV, resp.); Ee represents the absolute potential of thestandard hydrogen electrode (4.5 eV); EVB denotes the VBedge potential (the VB potential); ECB signi�es the CB edgepotential (the CB value); Eg represents the energy ofa semiconductor’s band gap, which is experimentally ob-tained from an analysis of the spectra of DRS. According tothe calculations, the upper boundary of the VB zone and thelower CB boundary for SrBi4O7 are 2.51 eV and 0.01 eV,respectively, whereas for (BiO)2CO3, it was 3.73 and 0.35 eV.�e band structure of the obtained samples can be depictedin the form of the circuit as shown in Figure 5.

Exposure to visible light results in the generation of electron-hole pairs only in the narrowband semiconductor SrBi4O7, since(BiO)2CO3 does not possess the ability to utilise the energy of thespectrum’s visible part (Figure 5). �e energies of CB and VBSrBi4O7 are higher than that of (BiO)2CO3. Moreover, thetransition of photogenerated electrons from the conductionband of SrBi4O7 to the conduction band (BiO)2CO3 is ener-getically favorable. Consequently, electrons accumulate in theconduction band of a wide-gap semiconductor, whereas holesremain in the valence band of SrBi4O7, which leads to a spatialseparation of the charge carriers. �us, the lifetime of chargecarriers is signi�cantly increased, which contributes to formationof amore e¦cient ­ow concerning photolysis ofwater.�evalueof the CB potential of both semiconductors (0.01 and 0.35 eV) islower than the O2/H2O2 potential, which ensures the reductionof dissolved oxygen to hydrogen peroxide:

O2 + 2H++2e− � H2O2;

E0 � +0.682 eV(3)

Subsequently, peroxide is able to recombine with hy-droxide radicals, which are active agents for the destructionof organic compounds. In addition, the formation of

MB

Productof degradation

3.73

0.35

2.51

0.01

3.38eV

2.50eV

ОН–

E0(OH•/OH–) = 1.99eV

ОН•

O2 + 2Н+

E0(O2/H2O2) = 0.68eV

Н2O2

(BiO)2CO3

SrBi4O7

e– e– e–

e– e–

h+ h+ h+

4

3.5

3

2.5

2

1.5

1

0.5

0

–0.5

–1

Pote

ntia

l vs.

NH

E (e

V)

Figure 5: �e possible degradation mechanism of MB over the prepared composites.

International Journal of Chemical Engineering 7

hydroxide radicals also occurs in a narrow-gap semi-conductor, since the value of VB for SrBi4O7 (2.51 eV) ismore positive in comparison to the oxidation potential ofhydroxide ions in a photohole:

HO− + h+� HO•;

E0 � +1.99 eV(4)

Photoholes in VB SrBi4O7, in turn, can directly oxidiseMB when absorbing dye molecules on the semiconductorsurface.

5. Conclusions

(i) /rough the method pertaining to oxidative py-rolysis of complexes of the corresponding metalswith sorbitol, it is depicted that it is possible toobtain a composition based on strontium bismu-thate and bismuth carbonate. /e photocatalyticactivity of such a composition under the action ofvisible light is higher than that of a mechanicalmixture of the same composition, the α-Bi2O3 andSrBi4O7 phases. /e greatest catalytic activity ispossessed by compositions that are obtained at 500to 600°C containing 73–82 mass.%, SrBixOy, and27–18 mass.%, (BiO)2CO3; it corresponds ofstrontium bismuthate SrBi2.9O5.35, SrBi3.25O5.88,and bismuth carbonate (BiO)2CO3 in molar ratios1/0.56; 1/0.37, respectively.

(ii) Investigation of optical properties has shown thatthe spectra of DRS compositions do not signifya simple superposition of the semiconductor phasesthat form them and differ from the spectra ofa mechanical mixture with the same composition./eir distinctive feature is the characteristic ab-sorption region in the range of 200–270 nm, whichis absent on the individual phases’ spectra of thecomposition’s constituents and on the spectrum ofa simple mechanical mixture identical to thecomposition of the relevant composition.

(iii) Analysis of DRS and the results of mapping suggestheterostructural nature of the compositions. /eformation of such a structure is possible due to epi-taxial growth, bismuth carbonate on strontium bis-muthate during the synthesis of SrBixOy/(BiO)2CO3.

(iv) /e mechanism concerning the photocatalytic ac-tivity of the semiconductor phases forming studiedthe composition is explained on the basis of theband structure’s analysis.

(v) /e results of the work present the prospects forcreating environmentally safe and efficient photo-catalysts based on bismuth compounds that aresensitive to visible light irradiation

Data Availability

/e data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

/e authors declare that they have no conflicts of interest.

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