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Improving Visible Light-Absorptivity and Photoelectric Conversion Efficiency of a TiO2 Nanotube Anode Film by Sensitization with Bi2O3 Nanoparticles Menglei Chang 1,2,† , Huawen Hu 1,† , Yuyuan Zhang 1, *, Dongchu Chen 1 , Liangpeng Wu 2 and Xinjun Li 2, * 1 2

* †

College of Materials Science and Energy Engineering, Foshan University, Foshan 528000, Guangdong, China; [email protected] (M.C.); [email protected] (H.H.); [email protected] (D.C.) Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; [email protected] Correspondence: [email protected] (Y.Z.); [email protected] (X.L.) These authors contributed equally to this work.

Academic Editor: Shinya Maenosono Received: 1 March 2017; Accepted: 2 May 2017; Published: 9 May 2017

Abstract: This study presents a novel visible light-active TiO2 nanotube anode film by sensitization with Bi2 O3 nanoparticles. The uniform incorporation of Bi2 O3 contributes to largely enhancing the solar light absorption and photoelectric conversion efficiency of TiO2 nanotubes. Due to the energy level difference between Bi2 O3 and TiO2 , the built-in electric field is suggested to be formed in the Bi2 O3 sensitized TiO2 hybrid, which effectively separates the photo-generated electron-hole pairs and hence improves the photocatalytic activity. It is also found that the photoelectric conversion efficiency of Bi2 O3 sensitized TiO2 nanotubes is not in direct proportion with the content of the sensitizer, Bi2 O3 , which should be carefully controlled to realize excellent photoelectrical properties. With a narrower energy band gap relative to TiO2 , the sensitizer Bi2 O3 can efficiently harvest the solar energy to generate electrons and holes, while TiO2 collects and transports the charge carriers. The new-type visible light-sensitive photocatalyst presented in this paper will shed light on sensitizing many other wide-band-gap semiconductors for improving solar photocatalysis, and on understanding the visible light-driven photocatalysis through narrow-band-gap semiconductor coupling. Keywords: Bi2 O3 sensitized TiO2 ; photoelectric conversion efficiency; visible light-active; nanoparticles

1. Introduction Study of effective photocatalysts lies in following conditions: (i) the prepared photocatalysts are capable of harvesting the solar energy of full wavelength as much as possible; (ii) high photocatalysis efficiency [1,2]. Due to the odorless, non-toxic, and chemically stable properties of TiO2 , it is widely employed and deeply analyzed for various kinds of applications [3–10]. However, TiO2 -based photocatalysts are only able to capture the ultraviolet (UV) part of the solar light; the UV light energy is comprised of only 4% of the total solar energy reaching earth, along with a rather low photocatalysis efficiency being lower than 1%, in contrast to 43% when visible light region is concerned [11,12]. As a consequence, the current focus is placed on exploration of new visible light-active photocatalysts or modification of existing photocatalysts to extend the solar light absorption to the visible light region [13].

Nanomaterials 2017, 7, 104; doi:10.3390/nano7050104

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As one of most widespread applications of TiO2 materials, dye-sensitized solar cells (DSSC), involves the use of a nano-porous TiO2 film as semiconducting electrodes, with high specific surface area [14]. The DSSC is a photoelectrochemical solar cell, which employs organic compounds containing metallic Ru, Os, etc. as dye sensitizers and selects proper redox electrolytes. The photoelectrical efficiency of a DSSC was recently reported to achieve as higher than 11%, which can also arrive at 6% as far as large-scale applications are concerned [15–18]. Such efficiencies are much higher than that for un-sensitized photocatalysts. However, most organic dyes used in DSSC are unstable, causing instability of the DSSC [16]. To address this, stable narrow-band-gap semiconductors can be resorted and used as a sensitizer for visible light-activation of TiO2 , replacing the organic dye in DSSC. Similar to the function obtained in DSSC, the semiconductor with a narrow band gap can efficiently harvest the solar energy, while wide-band-gap TiO2 is able to separate the photo-generated charge carriers. The significance of using visible light-active photocatalysts to sensitize TiO2 is to improve solar light absorption of TiO2 [19]. Different energy band gaps between the sensitizer and TiO2 lead to the generation of a built-in electric field [20], causing the photo-excited charge carriers to inject from one semiconductor to the other [21]. This inhibits the recombination of the generated charge carriers, in the meantime, the photo-generated electrons and holes can be well separated, improving the photocatalytic activity [22]. As a result, such combined semiconductors always exhibit a higher photocatalytic activity than that of the single-component semiconductor [23]. In order to combine semiconductors with different energy band gaps, we need to consider (i) band gaps of these semiconductors; (ii) positions of the valence band (VB) and conduction band (CB); and (iii) the match of crystalline forms of these semiconductors. The methods and procedures used to prepare the combined semiconductors also have an influence on the final photocatalytic activity [24]. For the first time, this paper reports the sensitization of TiO2 nanotubes by coupling a visible light-active semiconductor Bi2 O3 , affording a good a visible light-absorptivity; this thus also indicates the enhancement of the solar light absorption. Selecting Bi2 O3 for sensitization of TiO2 is due to (i) its narrow direct band gap of 2.8 eV; and (ii) suitable VB and CB positions for transporting and separating the photo-generated charge carriers. Because the photo-generated electrons and holes can be trapped and transported by these different semiconductors, both the photo-generated electrons and holes can be concentrated simultaneously, enhancing the oxidation/reduction capability of the electrodes in prepared solar cells. The photocatalytic and photoelectric conversion efficiencies are improved correspondingly. This paper presents the detailed synthesis of the visible light-active composite of Bi2 O3 -sensitized TiO2 nanotube film. The structure and photoelectrical properties of the composite are also well unraveled. 2. Experimental 2.1. Materials Bismuth nitrate hydrate (Bi(NO3 )2 ·5H2 O) of AR grade was supplied by Guangzhou Chemical Company (Guangzhou, China). Tetra-n-butyl titanate (Ti(OC4 H9 )4 ) of AR grade was obtained from Xinhua Active Materials Institute (Changzhou, Jiangsu, China). Diethanol amine (NH(C2 H5 OH)2 ) of AR grade was purchased from Luoyang Chemical Reagent Factory (Luoyang, China). Polyethylene glycol (MW 20,000) was provided by UNI-CHEM (Hong Kong, China), with AR purity. All other chemicals were of AR grade and used as received. 2.2. Synthesis of TiO2 Nanotubes The synthesis of TiO2 nanotubes was based on a hydrothermal route. Specifically, 2.0 g of purchased TiO2 with anatase form was mixed into a NaOH solution (10 mol/L) by continuously stirring at 110 ◦ C for 12 h, followed by cooling to room temperature. The white precipitate was then collected and thoroughly washed with deionized (DI) water until the electrical conductivity of the supernatant was lower than 70 µS/cm. The washing procedures were continued by dipping the


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precipitate into a HCl solution (0.1 mol/L) and letting it stand for 5 h, and then by further washing with DI water until the electrical conductivity of the supernatant was lower than 5 µS/cm. Afterwards, the washed TiO2 was vacuum-dried at 60 ◦ C for 72 h. 2.3. Preparation of TiO2 Sol-Based Film Using Ti(OC4 H9 )4 as a raw material and NH(C2 H5 OH)2 as an inhibitor to impede fast hydrolyzation of the titanate. The addition ratio of the Ti(OC4 H9 )4 , EtOH, DI water and NH(C2 H5 OH)2 was kept at 1:26.6:1:1. A precise amount of Ti(OC4 H9 )4 was dissolved into ethyl alcohol (EtOH), along with addition of NH(C2 H5 OH)2 . The mixture was agitated for 1 h, leading to liquid A. The DI water was homogenized with the remaining EtOH, forming liquid B which was then stepwise added into the liquid A through a separating funnel, followed by continuous stirring for 0.5 h and then letting it stand for 24 h. The resulting stable TiO2 sol was finally obtained. A fluorine-doped tin oxide (FTO) glass was then used as a carrier after being thoroughly washed by sonication treatment and subsequently dried. Two layers of TiO2 sol-based film was then deposited onto the FTO glass with a self-made film stretching machine at a stretching speed of 2 mm/s. Afterwards, the TiO2 sol-based film was thermally treated in a muffle furnace being heated to 500 ◦ C at a heating rate of 2 ◦ C/min, and kept at 500 ◦ C for 1 h, followed by cooling to room temperature. 2.4. Preparation of TiO2 Nanotube-Based Film Electrodes by a Powder-Coating Method TiO2 nanotubes (3.0 g) were placed in a mortar, followed by addition of 1 mL EtOH solution containing 10% diacetone and then ball-milling for 30 min. Afterwards, a mixture of DI water (4.5 mL), Triton X-100 (0.1 mL) and 30% (in weight, 0.9 g) of polyethylene glycol (MW 20,000) was added, and then ball-milled for 30 min. The as-prepared mixed dispersion with a proper concentration was applied onto the above-prepared conducting FTO glass with two layers of TiO2 film by a powder-coating method. A scotch tape (with around 40 µm thickness) was attached vertically onto the two sides of the conducting FTO glass coated with the stretched film, and then the prepared TiO2 slurry was applied onto the conducting glass by rolling a glass bar. Rolling once in the same direction could result in the best film forming effect. The formed film with a size of 1 cm × 1 cm was vacuum-dried at 60 ◦ C for 12 h, and then put into a muffle furnace, heated to 400 ◦ C (at a heating rate of 2 ◦ C/min), kept at that temperature for 1 h, and finally cooled to 80 ◦ C for storage. 2.5. Sensitization of TiO2 Nanotubes with Bi2 O3 A precursor solution of Bi2 O3 was first prepared as follows. Bi(NO3 )2 solutions were prepared with molar concentrations of 0.01, 0.05, 0.15, 0.25, and 0.5 mol/L. The as-obtained TiO2 nanotube film was dipped into the Bi(NO3 )2 solution and allowed to stand for 24 h, followed by washing with DI water and then drying under vacuum at 50 ◦ C for 12 h. Afterwards, the treated film was placed into a muffle furnace which was heated to 400 ◦ C at a heating rate of 2 ◦ C/min, kept at that temperature for 5 h, and finally cooled naturally to 80 ◦ C for storage. A series of samples were prepared, including pure TiO2 nanotube-based film (TNT), 0.01BiTNT, 0.05BiTNT, 0.15BiTNT, 0.25BiTNT, and 0.5BiTNT, where, for example, 0.01BiTNT represents that the product was prepared with the initial Bi(NO3 )2 (a Bi2 O3 precursor) solution of 0.01 mol/L. For clear demonstration of the sensitization effect of Bi2 O3 on the TiO2 nanotubes, this study also investigated a simple mixture of TiO2 nanotubes and Bi2 O3 particles being prepared by calcination of Bi(NO3 )2 according to the above thermal treatment procedures. The weight percentage of Bi2 O3 particles as used to mix with the TiO2 nanotubes was determined by quantification results of the energy-dispersive X-ray (EDX) spectrum for the optimized sample (Figure 1).

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Figure 1.1. EDX EDX spectrum spectrum of of the the typical typical sample, sample, 0.05BiTNT, 0.05BiTNT, together together with with the the quantification quantification results results Figure presentedas asinsets. insets. presented

2.6. Characterizations Characterizations 2.6. Transmissionelectron electronmicroscopy microscopy (TEM) observation conducted on aJEM-1010 JEOL JEM-1010 Transmission (TEM) observation was was conducted on a JEOL system system (Tokyo, Japan). A JEM-2010HR system was used for the high-resolution TEM (HRTEM) (Tokyo, Japan). A JEM-2010HR system was used for the high-resolution TEM (HRTEM) analysis. analysis. Backscattered-electron and EDXimages mapping images wereoncaptured onmicroscope a tabletop Backscattered-electron (BSE) and(BSE) EDX mapping were captured a tabletop microscope (TM3030Plus, Hitachi, Japan) being with equipped withspectrometer an EDX spectrometer 70, (TM3030Plus, Hitachi, Japan) being equipped an EDX (Quantax(Quantax 70, Bruker, Bruker, Germany). X-ray diffraction (XRD) analysis proceeded on a powder diffractometer Germany). X-ray diffraction (XRD) analysis proceeded on a powder diffractometer (X’Pert-PRO, (X’Pert-PRO,Philips, PANalytical, Philips, Amsterdam, Thethe Netherlands); thewere test set conditions were set as: PANalytical, Amsterdam, The Netherlands); test conditions as: Cu-Kα radiation, ◦ Cu-Kα radiation, working voltage of 40 KV, working current of 40 mA, and scanning speed of working voltage of 40 KV, working current of 40 mA, and scanning speed of 1.5 /min. The UV/Vis 1.5°/min. The UV/Vis diffuse reflectance spectra (UV/Vis DRS) were captured from a UV-Vis-NIR diffuse reflectance spectra (UV/Vis DRS) were captured from a UV-Vis-NIR spectrophotometer spectrophotometer (LAMBDA Norwalk, 750, PerkinElmer, Norwalk, CT, USA), equipped with sphere. an integrating (LAMBDA 750, PerkinElmer, CT, USA), equipped with an integrating X-ray sphere. X-ray photoelectron spectroscopy (XPS) spectra were obtained from XSAM800-XPS photoelectron spectroscopy (XPS) spectra were obtained from XSAM800-XPS equipment (Kratos, UK) equipment (Kratos, with a Ma-Kα X ray source. with a Ma-Kα X ray UK) source. 2.7. Photoelectric Photoelectric Conversion Conversion Properties Properties of of Bi Bi22O33 Sensitized TiO22 Nanotube film 2.7. The dye-sensitized dye-sensitized TiO TiO22 nanotube-based nanotube-based anode anode film film was was replaced replacedby byBiBi 2O -sensitized TiO TiO22 The 2O 33-sensitized nanotube film. film.PtPt was then thermally deposited ontoconducting the conducting FTO as the other nanotube was then thermally deposited onto the FTO glass as glass the other electrode. electrode. The two into aasandwiched solar cell with a sandwiched structure, The two electrodes areelectrodes assembledare intoassembled a solar cell with structure, as displayed in Figure as 2. 2 2 displayed in Figure 2. Under xenon lamp irradiation conditions W, AM 1.5, 100 , with Under xenon lamp irradiation conditions (300 W, AM 1.5, 100(300 mW/cm , with themW/cm wavelengths the wavelengths below 400 nmarefiltered), IVforcurves arecell measured for the solarworkstation. cell using below 400 nm filtered), IV curves measured the solar using electrochemical electrochemical workstation. The incident-photon-to-current (IPCE)using measurements were The incident-photon-to-current efficiency (IPCE) measurements efficiency were conducted electrochemical conducted using electrochemical workstation Zahner (PP211, CIMPS-pcs, Kronach, Germany) the workstation Zahner (PP211, CIMPS-pcs, Kronach, Germany) at the visible wavelengths of 400, 430,at450, 2 ). visible of 400, 430, 450,lamp 475, irradiation 500, 550, and 600 nm under xenon lamp irradiation 475, 500,wavelengths 550, and 600 nm under xenon conditions (300 W, AM 1.5, 100 mW/cm 2). conditions (300deposition W, AM 1.5, 100 mW/cm A thermal method was employed to fabricate the Pt counter electrode. A chloroplatinic A thermal deposition method was employed fabricate the Pt counter electrode. A acid EtOH solution with a proper concentration was firsttoprepared at room temperature; specifically, chloroplatinic acid EtOH solution with a proper concentration was first prepared at room 0.9 g of chloroplatinic acid was dissolved into 250 mL of EtOH. A dip-pulling method was then used temperature; specifically, 0.9 agcleaned of chloroplatinic into 250 mL of EtOH. A to make a two-layer film onto conductive acid glasswas withdissolved a self-made film-stretching machine. ◦ C (at a heating dip-pulling waswas thenconducted used to make a two-layer film ontorate a cleaned conductive The thermalmethod deposition at 390 of 2 ◦ C/min) and glass kept with at thea self-made film-stretching machine. The thermal temperature for 0.5 h, followed by natural cooling.deposition was conducted at 390 °C (at a heating rate The of 2 °C/min) kept at the temperature for 0.5 h, followed by natural cooling. preparedand electrolyte was an acetonitrile solution containing 0.1mol/L LiI, 0.05mol/L I2 , The prepared electrolyte was an acetonitrile solution containing 0.1mol/L LiI, 0.05mol/L I2, and and 0.5 mol/L Tert butyl pyridine. 0.5 mol/L Tert butyl pyridine.

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Figure 2. Schematic diagram of the Bi2 O3 sensitized TiO2 nanotubes solar cell. Figure2.2.Schematic Schematicdiagram diagramof ofthe theBi Bi2O 2O sensitizedTiO TiO22nanotubes nanotubessolar solarcell. cell. Figure 33sensitized

3. Results and Discussion 3.Results Resultsand andDiscussion Discussion 3. This study first compares the microstructures of the typical sample, 0.05BiTNT, and simple This study study first first compares compares the the microstructures microstructures of of the the typical typical sample, sample, 0.05BiTNT, 0.05BiTNT, and and aaa simple simple This mixture of Bi O and TiO nanotubes, with the same weight percentage of Bi O as estimated by the 2 3 2 2 3 mixtureof ofBi Bi22OO33and andTiO TiO22nanotubes, nanotubes,with withthe thesame sameweight weightpercentage percentageof ofBi Bi22OO33as asestimated estimatedby bythe the mixture quantification result of the EDX spectrum for 0.05BiTNT (Figure 1). From the EDX mapping images quantificationresult resultof ofthe theEDX EDXspectrum spectrumfor for0.05BiTNT 0.05BiTNT(Figure (Figure1). 1).From Fromthe theEDX EDXmapping mappingimages images quantification (Figure 3), can bebe found for 0.05BiTNT (bottom row) as (Figure3), 3),considerably considerablybetter betterdistribution distributionof ofelement elementBiBi Bi can be found for 0.05BiTNT (bottom row) (Figure considerably better distribution of element can found for 0.05BiTNT (bottom row) compared to the simple mixture (top row) with the Bi O particles showing apparently unsatisfactory 2 3 the as compared compared to to the the simple simple mixture mixture (top (top row) row) with with the Bi Bi22OO33 particles particles showing showing apparently apparently as adhesion interactions with the TiO2 matrix, which thewhich importance and significance ofand the unsatisfactory adhesion interactions with the the TiOimplies matrix, which implies the importance and unsatisfactory adhesion interactions with TiO 22 matrix, implies the importance present sensitization method. significance ofthe thepresent present sensitizationmethod. method. significance of sensitization

Figure ofof 0.05BiTNT (bottom row) and a simple mixture of TiO and22 Figure3.3.BSE BSEand andEDX EDXmapping mappingimages images of 0.05BiTNT (bottom row) and simple mixture of2TiO TiO Figure EDX mapping images 0.05BiTNT (bottom row) and aasimple mixture of Bi O (top row). and Bi 2 O 3 (top row). and Bi 2 O 3 (top row). 2 3

Tofurther furtherunravel unravel the microstructure ofBithe the Bi22OO33-sensitized -sensitized TiO TiO22 nanotubes, nanotubes, TEM TEM and and To further unravel microstructure of Bi To thethe microstructure of the 2 O3 -sensitized TiO2 nanotubes, TEM and HRTEM HRTEM are presented in Figure 4a–f. The TEM specimen was prepared by sampling from the HRTEM are presented in Figure 4a–f. The TEM specimen was prepared by sampling from the are presented in Figure 4a–f. The TEM specimen was prepared by sampling from the coated FTO coated FTO glass. Under low-magnification TEM (Figure 4a,b), neat TiO nanotubes show a large coated FTO glass. Under low-magnification TEM (Figure 4a,b), neat TiO 22 nanotubes show a large glass. Under low-magnification TEM (Figure 4a,b), neat TiO2 nanotubes show a large length-diameter length-diameter ratio, with smooth tube walls. walls. As shown shown inBi Figure 4c,d, the the Bi Bi22OO33-sensitized -sensitized TiO TiO22 length-diameter ratio, with smooth tube As in Figure 4c,d, ratio, with smooth tube walls. As shown in Figure 4c,d, the 2 O3 -sensitized TiO2 nanotube walls nanotube walls become coarsened, with the well-observed Bi O nanoparticles coated on the walls. nanotube walls become coarsened, with the well-observed Bi 22O 33nanoparticles coated on the walls. become coarsened, with the well-observed Bi2 O3 nanoparticles coated on the walls. Under HRTEM Under HRTEM HRTEM (Figure 4e,f), can be noted noted that the the formed formed Bi Bi22OO33 nanoparticles nanoparticles are are not not fully fully Under (Figure 4e,f), can be (Figure 4e,f), it can be noted thatititthe formed Bi2 Othat 3 nanoparticles are not fully deposited uniformly on deposited uniformly on the TiO nanotube walls, with a little amount of aggregates of Bi O33 deposited uniformly on the TiO 22 nanotube walls, with a little amount of aggregates of Bi the TiO2 nanotube walls, with a little amount of aggregates of Bi2 O3 nanoparticles (highlighted22O by nanoparticles (highlighted by broken cycles). The average diameter of Bi O nanoparticles can nanoparticles (highlighted by broken cycles). The average diameter of Bi 22O 33nanoparticles can be broken cycles). The average diameter of Bi2 O3 nanoparticles can be calculated as around 10 nm. be calculatedas asaround around10 10nm. nm. calculated

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Figure 4. TEM and HRTEM images of the prepared samples. (a,b) Neat TiO2 nanotubes; Figure 4. TEM and HRTEM images of the prepared samples. (a,b) Neat TiO2 nanotubes; (c–f) (c–f) Bi2 O3 -sensitized TiO2 nanotubes. The detailed structural features are highlighted with red Bi2O3-sensitized TiO2 nanotubes. The detailed structural features are highlighted with red circles and circles and lines in (f). lines in (f).

The XRD the the as-prepared coatedcoated FTO glass shown Figure 5.inThe characteristic The XRDpatterns patternsof of as-prepared FTOareglass arein shown Figure 5. The peak assignable to FTO glass substrate can be noted. The anatase TiO crystal can also be well characteristic peak assignable to FTO glass substrate can be noted. 2The anatase TiO2 crystalidentified can also through the standard PDFthe cards of anatase and its 89-4921) refectionand planes (highlighted by be well identified through standard PDF (No. cards89-4921) of anatase (No. its refection planes red dotted lines). As the concentration of the Bi O precursor increases, the Bi O -related XRD 2 3 2 3 (highlighted by red dotted lines). As the concentration of the Bi2O3 precursor increases, the bands gradually appear—as marked by blue dotted lines for showing the characteristic reflection Bi 2O3-related XRD bands gradually appear—as marked by blue dotted lines for showing the planes of Bi2 Oreflection Note that a low content of Bi O3 fora the samples suchthe as 3 (PDF#74-2351). characteristic planes of Bi2O 3 (PDF#74-2351). Note2 that lowsensitized content of Bi2O3 for 0.05BiTNT and the simple mixture of Bi O and TiO causes the XRD peaks characteristic of Bi O 2 3the simple 2 mixture of Bi2O3 and TiO2 causes the XRD 2 3 sensitized samples such as 0.05BiTNT and to be characteristic undetectable. ofFrom UV/Vis spectra From shown Figurespectra 6, the red shiftinof the absorption peaks Bi2O3the to be undetectable. theinUV/Vis shown Figure 6, the red wavelength occurs for all the Bi2 O3 sensitized The higher the concentration of shift of the absorption wavelength occurs forTiO all2 nanotube the Bi2O3 films. sensitized TiO2 nanotube films. The Bi O precursor, the larger the red-shift extent, indicating the corresponding higher content of final 2 3 the concentration of Bi2O3 precursor, the larger the red-shift extent, indicating the higher formed Bi2 O3 nanoparticles in the Bi2 O3formed sensitized the successful 2 sample. This 3 nanoparticles inalso theindicates Bi2O3 sensitized TiO2 corresponding higher content of final Bi2OTiO sensitization of TiO by Bi O [25]. 2 2 3 sample. This also indicates the successful sensitization of TiO2 by Bi2O3 [25]. To determine elemental composition and obtain corresponding chemical state information To determinethethe elemental composition and the obtain the corresponding chemical state of the Bi O -sensitized TiO nanotube surface, XPS analysis is used to measure the typical sample 2 3 2 information of the Bi2O3-sensitized TiO2 nanotube surface, XPS analysis is used to measure the TNT (assample a control) Figure 7a showsFigure the full spectra, verifying the elements Ti, typical TNTand (as 0.05BiTNT. a control) and 0.05BiTNT. 7aXPS shows the full XPS spectra, verifying O, Bi, and CTi, (the C (the mostelement likely stems from thestems contaminant included[26]) in 0.05BiTNT. the elements O,element Bi, and C C most likely from the[26]) contaminant included Due to the incorporation of Bi O , an additional signal from Bi 4f can be well observed in the fullwell XPS 2 3 in 0.05BiTNT. Due to the incorporation of Bi2O3, an additional signal from Bi 4f can be spectrum for 0.05BiTNT, in contrast to that for TNT. The high-resolution XPS spectrum of Bi 4f is given observed in the full XPS spectrum for 0.05BiTNT, in contrast to that for TNT. The high-resolution in Figure 7b, and the characteristic at the approximately 164.2 and 158.8 eV can be indexed to XPS spectrum of Bi 4ftwo is given in Figurepeaks 7b, and two characteristic peaks at approximately 164.2 the Bi 4f and Bi 4f respectively [27]. The binding energy of Bi as for 0.05BiTNT is the same as 7/2 and 158.86/2 eV can be indexed to the Bi 4f6/2 and Bi 4f7/2 respectively [27]. The binding energy of Bi as that0.05BiTNT for pure Bi2isO3the , indicating element Bi2of in element the chemical state of Bi2 O3 . for same asthat thatthefor pure Bi O30.05BiTNT , indicatingpresents that the Bi of 0.05BiTNT The binding energy of Bistate for 0.05BiTNT is in great agreement for reported pureagreement Bi2 O3 [28] presents in the chemical of Bi2O3. The binding energy of with Bi forthose 0.05BiTNT is in great and a Bi O –TiO composite [29], thus revealing that the element Bi of our 0.05BiTNT presents the 2 3for reported 2 with those pure Bi2O3 [28] and a Bi2O3–TiO2 composite [29], thus revealing that the chemicalBistate of Bi0.05BiTNT also in good agreement with 2 O3 . This ispresents element of our the chemical state ofXRD Bi2Oanalysis. 3. This is also in good agreement

with XRD analysis.


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Figure 5. XRD patterns a FTO glass and all thesamples film deposited the FTOglass, glass,glass, Figure 5. XRD patterns of a FTO and all theall film deposited on on theon FTO including Figure 5. XRD patterns of aofglass FTO glass and the film samples samples deposited the FTO 2O3 sensitized TiO2 nanotubes and the simple mixture of TiO2 nanotubes and including a series of Bi sensitized TiO2 nanotubes the simple mixture of2TiO 2 nanotubes and including a series of Bi2O3TiO a series of Bi2 O and the and simple mixture of TiO nanotubes and Bi2 O3 . 3 sensitized 2 nanotubes Bi2O3. The standard PDF cards of anatase TiO2 (No. 89-4921) and Bi2O3 (No. 74-2351) are also shown. standard cards of TiO anatase TiO89-4921) 2 (No. 89-4921) 2O3 (No. 74-2351) also shown. Bi2O3. ThePDF The standard cardsPDF of anatase and Biand 74-2351) areare also shown. 2 (No. 2 O3Bi(No.

0.9

0.9

0 0 0.01 0.05 0.01 0.15 0.05 0.25 0.15 0.5

0.8

0.8 0.7

0.7 0.6

0.6 0.5

A

A

0.25 0.5

0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1

0.1 0.0 300

0.0 300

350

400

450

500

Wavelength (nm) 350

400

450

500

Figure 6. UV-Vis DRS of all the prepared Wavelength (nm) samples.

In the high-resolution XPS spectrum of Ti 2p (Figure 7c), the Ti 2p energy level is split into two Figure 6. UV-Vis DRS of all the the preparedsamples. samples. Figure UV-Vis energy levels, namely Ti 2p1/2 6.and Ti 2pDRS 3/2 asofaall resultprepared of spin-orbital coupling of electrons. The binding energy positions of Ti 2p1/2 and Ti 2p3/2 can be found at approximately 464.0 and 458.3 eV being basically the same as reported elsewhere [30]. This also indicates both Inrespectively, the high-resolution XPS spectrum ofthat Ti 2p 2p (Figure (Figure 7c), TiTi 2p energy level is that, split into into two the high-resolution XPS spectrum of Ti 7c),the the 2p energy level is split

In two energy levels, namely Ti 2p 1/2 and Ti 2p3/2 as a result of spin-orbital coupling of electrons. The energy levels, namely Ti 2p1/2 and Ti 2p3/2 as a result of spin-orbital coupling of electrons. The binding binding energy positions of Ti 2p1/2 and Ti 2p3/2 can be found at approximately 464.0 and 458.3 eV energy positions of Ti 2p1/2 and Ti 2p3/2 can be found at approximately 464.0 and 458.3 eV respectively, respectively, being basically the same as that reported elsewhere [30]. This also indicates that, both being basically the same as that reported elsewhere [30]. This also indicates that, both before and after the sensitization of TiO2 with Bi2 O3 , the oxidation state of the element Ti keeps the same, i.e., Ti4+ , in the absence of Ti3+ , and thus reveals the high purity of TiO2 [31]. The broadening of the XPS peaks after the Bi2 O3 incorporation (0.05BiTNT vs. TNT) can be attributed to the electron transfer interactions between the matrix TiO2 and the sensitizer Bi2 O3 [32]. The high-resolution XPS spectra of O 1s are shown in Figure 7d. Two peaks at 529.7 and 531.4 eV can be noticed by Gaussian fitting. The former


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before and after the sensitization of TiO2 with Bi2O3, the oxidation state of the element Ti keeps the same, i.e., Ti4+, in the absence of Ti3+, and thus reveals the high purity of TiO2 [31]. The broadening of the XPS peaks after the Bi2O3 incorporation (0.05BiTNT vs. TNT) can be attributed to the electron Nanomaterials 2017, 7, 104 8 of 13 transfer interactions between the matrix TiO2 and the sensitizer Bi2O3 [32]. The high-resolution XPS spectra of O 1s are shown in Figure 7d. Two peaks at 529.7 and 531.4 eV can be noticed by Gaussian fitting. The former can be indexed to the O of TiO2; the latter most likely results from Bi2O3, which can be indexed to the O of TiO2 ; the latter most likely results from Bi2 O3 , which may also be correlated may also be correlated to OH species from chemical absorption or absorbed water [25]. The lower to OH species from chemical absorption or absorbed water [25]. The lower binding energy of Bi–O is binding energy of Bi–O is owing to the less negative changes of the covalent O of Bi2O3 as compared owing the negative changes of the covalent O ofthat Bi2the O3 TiO as compared to that of the O of Ti–O [33]. to to that of less the O of Ti–O [33]. To conclude, it is believed 2 nanotubes are effectively sensitized To conclude, it is believed that the TiO nanotubes are effectively sensitized by Bi2 O3 [25]. 2 by Bi2O3 [25].

Figure 7. XPS studies of TNT and 0.05BiTNT; (a); and andhigh-resolution high-resolution XPS spectra of Figure 7. XPS studies of TNT and 0.05BiTNT;full fullXPS XPSspectra spectra (a); XPS spectra of Bi 4f (b); Ti 2p (c); and O 1s (d) regions. Bi 4f (b); Ti 2p (c); and O 1s (d) regions.

Figure 8 shows the IV curves (obtained for the assembled solar cell with the Bi2O3 sensitized Figure 8 shows the IV curves (obtained for the assembled solar cell with the Bi O3 sensitized TiO2 TiO2 nanotube anode film). The corresponding PV curves of the Bi2O3 sensitized2 solar cell are nanotube anode film). The corresponding PV curves of the Bi O sensitized solar cell are presented 2 3 of the DSSC is not directly presented in Figure 9. The photoelectric conversion efficiency in Figure 9. The to photoelectric conversion of the DSSC is not directly proportional proportional the amount of sensitizer Biefficiency 2O3, and there exists an optimized content of Bi2O3. Ourto the amount of sensitizer O , and there exists an optimized content of Bi O . Our optimal content of optimal content ofBithe sensitizer Bi 2 O 3 corresponds to the initial Bi 2 O 3 precursor concentration of 2 3 2 3 0.05 mol/L. The lower content of Bi 2 O 3 is most likely to cause insufficient utilization of visible light, the sensitizer Bi2 O3 corresponds to the initial Bi2 O3 precursor concentration of 0.05 mol/L. The lower while amount of Bito 2Ocause 3 exertsinsufficient an adverse effect on the of light absorption the light content of an Bi2excess O3 is most likely utilization visible light, property while anofexcess amount anode material, revealing that an optimized content of Bi 2O3 exists. In DSSC, the optimal absorbed of Bi2 O3 exerts an adverse effect on the light absorption property of the light anode material, revealing concentration lies in the monolayer adsorption of dye molecules onto the porous TiO2-based that dye an optimized content of Bi2 O3 exists. In DSSC, the optimal absorbed dye concentration lies in film electrode, which may be also applied to our case. To further prove the sensitization effect of the monolayer adsorption of dye molecules onto the porous TiO2 -based film electrode, which may Bi2O3 on TiO2 nanotubes, a simple mixture of Bi2O3 particles and TiO2 nanotubes is also investigated be also applied to our case. To further prove the sensitization effect of Bi O3 on TiO2 nanotubes, and compared with our optimized sample, 0.05BiTNT. The weight percentage 2of the Bi2O3 particles

a simple mixture of Bi2 O3 particles and TiO2 nanotubes is also investigated and compared with our optimized sample, 0.05BiTNT. The weight percentage of the Bi2 O3 particles used for the mixing is estimated from the quantified result of the EDX spectrum for 0.05BiTNT (Figure 1). The simple mixture-based solar cell presents much lower photoelectric conversion efficiency as compared to the sensitized counterpart (0.05BiTNT), thereby indicating the significance of the sensitization effect as a result of uniform distribution of Bi2 O3 and interfacial adhesion between the Bi2 O3 and TiO2 nanotubes, as evidenced by EDX mapping images (Figure 3), and TEM and HRTEM images (Figure 4).

used for the mixing is estimated from the quantified result of the EDX spectrum for 0.05BiTNT (Figure 1). The simple mixture-based solar cell presents much lower photoelectric conversion (Figure 1). The simple mixture-based solar cell presents much lower photoelectric conversion efficiency as compared to the sensitized counterpart (0.05BiTNT), thereby indicating the efficiency as compared to the sensitized counterpart (0.05BiTNT), thereby indicating the significance of the sensitization effect as a result of uniform distribution of Bi2O3 and interfacial significance of the sensitization effect as a result of uniform distribution of Bi2O3 and interfacial adhesion between the Bi2O3 and TiO2 nanotubes, as evidenced by EDX mapping images (Figure 3), adhesion between the Bi2O3 and TiO2 nanotubes, as evidenced by EDX mapping images (Figure 3), Nanomaterials 2017,HRTEM 7, 104 9 of 13 and TEM and images (Figure 4). and TEM and HRTEM images (Figure 4).

Figure 8. 8. IVcurves curves of a seriesofofBiBiO 2O3 sensitized TiO2-based solar cells, as well as a simple mixture Figure TiO solar cells, as as well as aassimple mixture of 2 2O 3 sensitized 2 -based Figure 8.IV IV curvesofofa aseries series of Bi 3 sensitized TiO 2-based solar cells, well a simple mixture 2O3 and TiO2-based solar cell. of Bi Bi and TiOTiO solar cell. 2 -based 3 and 2-based solar cell. of2 O Bi32O

Figure 9. 9. PV PVcurves curvesofofaaseries seriesofofBiBi 2O sensitizedTiO TiO 2-based solarcells, cells, well a simple mixture Figure solar asas well as as a simple mixture of 2O 3 3sensitized 2 -based Figure 9. PV curves of a series of Bi2O3 sensitized TiO2-based solar cells, as well as a simple mixture Bi O32Oand TiO solar cell. 3 and TiO 2-based solar cell. of2Bi 2 -based of Bi2O3 and TiO2-based solar cell.

From Table 1, it can be noted that, as the Bi2 O3 content increases, the photoelectric conversion efficiency raises first and then exhibits a decreasing trend. The sample 0.05BiTNT presents the highest photoelectric conversion efficiency, followed by 0.15BiTNT > 0.5BiTNT > 0.25BiTNT > 0.01BiTNT > TNT. The un-sensitized TiO2 nanotubes (namely TNT)-assembled DSSC possesses a rather unsatisfactory photovoltaic property; this is because the present visible light source cannot excites the TiO2 nanotube film, with a wide energy band gap, to generate photocurrent and photovoltage,


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From Table 1, it can be noted that, as the Bi2O3 content increases, the photoelectric conversion efficiency raises first and then exhibits a decreasing trend. The sample 0.05BiTNT presents the highest photoelectric conversion efficiency, followed by 0.15BiTNT > 0.5BiTNT > 0.25BiTNT > 0.01BiTNT The un-sensitized TiO2 nanotubes (namely TNT)-assembled DSSC possesses 10 a of 13 Nanomaterials 2017, > 7, TNT. 104 rather unsatisfactory photovoltaic property; this is because the present visible light source cannot excites the TiO2 nanotube film, with a wide energy band gap, to generate photocurrent and indicating the highindicating significance the significance present sensitization of TiO photovoltage, theofhigh of the present sensitization ofwith TiO2 Bi nanotubes with 2 nanotubes 2 O3 . Furthermore, Bi2spectrum O3. Furthermore, a IPCE spectrum of the typicalshows sample, 0.05BiTNT, shows aof gradual lowering of a IPCE of the typical sample, 0.05BiTNT, a gradual lowering IPCE with increasing IPCE with increasing incident wavelengths (starting from 400 to 600 nm), as presented in Figure 10. incident wavelengths (starting from 400 to 600 nm), as presented in Figure 10.

Figure 10. IPCE spectrum of the 0.05BiTNT-based cell,as ascaptured captured incident wavelengths Figure 10. IPCE spectrum of the 0.05BiTNT-based solar solar cell, at at thethe incident wavelengths ranging 400600 to 600 ranging fromfrom 400 to nm.nm. 1. Photovoltaic characteristics of the Bi2O3 sensitized TiO2-based solar cells. TableTable 1. Photovoltaic characteristics of the Bi2 O3 sensitized TiO2 -based solar cells.

Electrodes 0 0.01 Electrodes 0 0.0103 0.01 Jsc (mA/cm2) 0.0666 2) Voc (mV) 414 0.0103269 0.0666 Jsc (mA/cm 2) −4 P max (mW/cm 9.41 × 10 0.0101 Voc (mV) 269 414 0.0101 Pmax (mW/cm2 ) 9.41 × 10−4 4. Conclusions

0.05 0.05 0.341 664 0.341 0.109 664 0.109

0.15 0.15 0.161 681 0.161 0.0490 681 0.0490

0.25 0.25 0.0786 603 0.0786 0.0164 603 0.0164

0.5 0.1020.5 6640.102 0.0267 664 0.0267

In this study, a series of visible light-responsive Bi2O3 sensitized TiO2 nanotube anode films 4. Conclusions were prepared by combined hydrothermal reactions, dip-coating, and calcination. In terms of the

In this study,conversion a series ofefficiency visible light-responsive nanotube anode films 2 O3 sensitized photoelectric of the assembledBisolar cells, the BiTiO 2O3 2precursor (Bi(NO3)2) were concentration prepared by of combined hydrothermal reactions, dip-coating, and calcination. terms of the 0.05 mol/L is estimated to be the optimal condition. The photoelectric In conversion photoelectric efficiency of thewith assembled cells,and theoptimization Bi2 O3 precursor efficiency conversion is not in direct proportion the Bi2Osolar 3 content, needs (Bi(NO to be 3 )2 ) concentration 0.05 mol/L estimatedin toDSSC, be thethe optimal condition. Theofphotoelectric conversion performed.ofSimilar to dye is sensitization monolayer adsorption the sensitizer is most likelyistonot be optimal achieving high conversion efficiency in directforproportion withphotoelectric the Bi2 O3 content, andefficiency. optimization needs to be performed. a new-type visible Bi2O3 is popular in sensitizer the field ofissemiconductor Similar toAs dye sensitization inlight-active DSSC, thephotocatalyst, monolayer adsorption of the most likely to be materials. Bi 2O3 has an energy band gap of 2.8 eV (ECB = 0.33 eV, EVB = 3.13 eV) at room temperature optimal for achieving high photoelectric conversion efficiency. [34]. A single component photocatalyst, TiO2 nanotubes, shows a rather unsatisfactory visible As a new-type visible light-active photocatalyst, Bi2 O3 is popular in the field of semiconductor light-driven photocatalytic property, but after sensitization with Bi2O3, its photoelectric conversion materials. Bi2 O3 has an energy band gap of 2.8 eV (ECB = 0.33 eV, EVB = 3.13 eV) at room efficiency is largely improved, as schematically shown in Figure 11. Bi2O3 serves to absorb visible temperature [34].toAgenerate single component TiO2while nanotubes, shows a rather light energy photoexcitedphotocatalyst, electrons and holes, TiO2 works to collect andunsatisfactory transport visible light-driven photocatalytic property, but after sensitization with Bi2 O3 , its photoelectric conversion efficiency is largely improved, as schematically shown in Figure 11. Bi2 O3 serves to absorb visible light energy to generate photoexcited electrons and holes, while TiO2 works to collect and transport the generated charge carriers [35]. Considering the different CB and VB positions of Bi2 O3 and TiO2 , a built-in electric field can be generated as a result of the energy level difference between Bi2 O3 and TiO2 , effectively separating the photo-generated electron-hole pairs and consequently improving the photocatalytic activity.


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the generated charge carriers [35]. Considering the different CB and VB positions of Bi2O3 and TiO2, a built-in electric field can be generated as a result of the energy level difference between Bi2O3 and TiO2, effectively separating the photo-generated electron-hole pairs and consequently improving Nanomaterials 2017, 7, 104 11 of 13 the photocatalytic activity.

O33. . Figure 11. Photocatalysis mechanism of sensitizing TiO22 by Bi22O

Acknowledgments: Acknowledgments: The The authors authors greatly greatly acknowledge acknowledge the the construction construction project project from from Guangdong Guangdong Engineering Engineering Technique Research construction project fromfrom Research Platform of Universities in Foshan Technique ResearchCenter Center(506302679076), (506302679076), construction project Research Platform of Universities in (2014AG10009), and theand Open Fund of Jiangsu Provincial Key Laboratory of Biomass Energy and Foshan (2014AG10009), the Research Open Research Fund of Jiangsu Provincial Key Laboratory of Biomass Energy Materials (No. JSBEM201608). The financial support (y207k61001) given by Key Laboratory of Renewable Energy and Materials (No. JSBEM201608). The financial support (y207k61001) given by Key Laboratory of Renewable (Chinese Academy of Sciences) and the characteristic innovation projects from Guangdong Education Agency Energy (Chinese Academy Sciences) and the characteristic innovation projects from Guangdong Education (2015KTSCX155) are greatlyofappreciated as well. Agency (2015KTSCX155) are greatly appreciated as well. Author Contributions: This study was conducted by Menglei Chang and Huawen Hu, with project support from Yuyuan Contributions Zhang and Xinjun Li.study Menglei and Huawen Hu prepared theHuawen draft of the all the other : This wasChang conducted by Menglei Chang and Hu,paper, with and project support Author authors helped to revise it, including Yuyuan Zhang, Dongchu Chen, Liangpeng Wu and Xinjun Li. from Yuyuan Zhang and Xinjun Li. Menglei Chang and Huawen Hu prepared the draft of the paper, and all the Conflicts of Interest: The authors declare no conflicts of interest. other authors helped to revise it, including Yuyuan Zhang, Dongchu Chen, Liangpeng Wu and Xinjun Li. Conflicts of Interest: The authors declare no conflicts of interest.

References

References Zhang, Y.; Chen, J.; Li, X. Preparation and Photocatalytic Performance of Anatase/Rutile Mixed-Phase TiO2 1. Nanotubes. Catal. Lett. 2010, 139, 129–133. [CrossRef] Zhang, Y.; Chen, J.; Li, X. Preparation and Photocatalytic Performance of Anatase/Rutile Mixed-Phase Zhang, Y.; Li, X.; Feng, M.; Zhou, F.; Chen, J. Photoelectrochemical performance of TiO2 -nanotube-array TiO2 Nanotubes. Catal. Lett. 2010, 139, 129–133. film modified by decoration of TiO2 via liquid phase deposition. Surf. Coat. Technol. 2010, 205, 2572–2577. 2. Zhang, Y.; Li, X.; Feng, M.; Zhou, F.; Chen, J. Photoelectrochemical performance of TiO2-nanotube-array [CrossRef] film modified by decoration of TiO2 via liquid phase deposition. Surf. Coat. Technol. 2010, 205, 2572–2577. Ouyang, W.; Kuna, E.; Yepez, A.; Balu, A.; Romero, A.; Colmenares, J.; Luque, R. Mechanochemical Synthesis 3. 3. Ouyang, W.; Kuna, E.; Yepez, A.; Balu, A.; Romero, A.; Colmenares, J.; Luque, R. Mechanochemical of TiO2 Nanocomposites as Photocatalysts for Benzyl Alcohol Photo-Oxidation. Nanomaterials 2016, 6, 93. Synthesis of TiO2 Nanocomposites as Photocatalysts for Benzyl Alcohol Photo-Oxidation. Nanomaterials [CrossRef] [PubMed] 2016, 6, 93. Jiang, Q.; Li, L.; Bi, J.; Liang, S.; Liu, M. Design and Synthesis of TiO Hollow Spheres with Spatially 4. 4. Jiang, Q.; Li, L.; Bi, J.; Liang, S.; Liu, M. Design and Synthesis of TiO22 Hollow Spheres with Spatially Separated Dual Cocatalysts for Efficient Photocatalytic Hydrogen Production. Nanomaterials 2017, 7, 24. Separated Dual Cocatalysts for Efficient Photocatalytic Hydrogen Production. Nanomaterials 2017, 7, 24. [CrossRef] [PubMed] 5. Sun, D.S.; Tseng, Y.H.; Wu, W.S.; Wong, M.S.; Chang, H.H. Visible Light-Responsive Platinum-Containing 5. Sun, D.S.; Tseng, Y.H.; Wu, W.S.; Wong, M.S.; Chang, H.H. Visible Light-Responsive Platinum-Containing Titania Nanoparticle-Mediated Photocatalysis Induces Nucleotide Insertion, Deletion and Substitution Titania Nanoparticle-Mediated Photocatalysis Induces Nucleotide Insertion, Deletion and Substitution Mutations. Nanomaterials 2016, 7, 2. Mutations. Nanomaterials 2016, 7, 2. [CrossRef] [PubMed] 6. Nica, I.; Stan, M.; Dinischiotu, A.; Popa, M.; Chifiriuc, M.; Lazar, V.; Pircalabioru, G.; Bezirtzoglou, E.; Nica, I.; Stan, M.; Dinischiotu, A.; Popa, M.; Chifiriuc, M.; Lazar, V.; Pircalabioru, G.; Bezirtzoglou, E.; 6. Iordache, O.; Varzaru, E.; et al. Innovative Self-Cleaning and Biocompatible Polyester Textiles Iordache, O.; Varzaru, E.; et al. Innovative Self-Cleaning and Biocompatible Polyester Textiles Nano-Decorated with Fe–N-Doped Titanium Dioxide. Nanomaterials 2016, 6, 214. Nano-Decorated with Fe–N-Doped Titanium Dioxide. Nanomaterials 2016, 6, 214. [CrossRef] [PubMed] 7. Ge, M.; Cao, C.; Huang, J.; Li, S.; Chen, Z.; Zhang, K.-Q.; Al-Deyab, S.S.; Lai, Y. A review of 7. Ge, M.; Cao, C.; Huang, J.; Li, S.; Chen, Z.; Zhang, K.-Q.; Al-Deyab, S.S.; Lai, Y. A review of one-dimensional one-dimensional TiO2 nanostructured materials for environmental and energy applications. J. Mater. TiO2 nanostructured materials for environmental and energy applications. J. Mater. Chem. A 2016, 4, Chem. A 2016, 4, 6772–6801. 6772–6801. [CrossRef] 8. Zhang, W.; Jia, B.; Wang, Q.; Dionysiou, D. Visible-light sensitization of TiO2 photocatalysts via wet 8. Zhang, W.; Jia, B.; Wang, Q.; Dionysiou, D. Visible-light sensitization of TiO2 photocatalysts via wet chemical N-doping for the degradation of dissolved organic compounds in wastewater treatment: A chemical N-doping for the degradation of dissolved organic compounds in wastewater treatment: A review. J. Nanopart. Res. 2015, 17, 221. review. J. Nanopart. Res. 2015, 17, 221. [CrossRef] 9. Ansari, S.A.; Cho, M.H. Highly Visible Light Responsive, Narrow Band gap TiO2 Nanoparticles Modified 9. Ansari, S.A.; Cho, M.H. Highly Visible Light Responsive, Narrow Band gap TiO2 Nanoparticles Modified by Elemental Red Phosphorus for Photocatalysis and Photoelectrochemical Applications. Sci. Rep. 2016, 6, by Elemental Red Phosphorus for Photocatalysis and Photoelectrochemical Applications. Sci. Rep. 2016, 6, 25405. 25405. [CrossRef] [PubMed] 10. Hu, H.; Xin, J.H.; Hu, H.; Wang, X.; Miao, D.; Liu, Y. Synthesis and stabilization of metal nanocatalysts for reduction reactions—A review. J. Mater. Chem. A 2015, 3, 11157–11182. [CrossRef]

1. 2.


11. 12.

13.

14. 15. 16.

17.

18. 19.

20. 21.

22.

23.

24. 25.

26. 27. 28. 29. 30. 31.

12 of 13

Sajan, C.; Wageh, S.; Al Ghamdi, A.; Yu, J.; Cao, S. TiO2 nanosheets with exposed {001} facets for photocatalytic applications. Nano Res. 2016, 9, 3–27. [CrossRef] Wang, X.; Hu, H.; Yang, Z.; Kong, Y.; Fei, B.; Xin, J.H. Visible light-active sub-5 nm anatase TiO2 for photocatalytic organic pollutant degradation in water and air, and for bacterial disinfection. Catal. Commun. 2015, 72, 81–85. [CrossRef] Wang, T.; Meng, X.; Liu, G.; Chang, K.; Li, P.; Kang, Q.; Liu, L.; Li, M.; Ouyang, S.; Ye, J. In situ synthesis of ordered mesoporous Co-doped TiO2 and its enhanced photocatalytic activity and selectivity for the reduction of CO2 . J. Mater. Chem. A 2015, 3, 9491–9501. [CrossRef] Chiou, B.; Yang, R.-Y.; Chuang, H.-J.; Chu, C.-J. Characterization of Nano-porous TiO2 Film Prepared by Sol-gel Process and Its Application to Dye-sensitized Solar Cell. J. Chin. Chem. Soc. 2013, 60, 81–84. [CrossRef] Sugathan, V.; John, E.; Sudhakar, K. Recent improvements in dye sensitized solar cells: A review. Renew. Sustain. Energy Rev. 2015, 52, 54–64. [CrossRef] Shalini, S.; Balasundaraprabhu, R.; Kumar, T.S.; Prabavathy, N.; Senthilarasu, S.; Prasanna, S. Status and outlook of sensitizers/dyes used in Dye Sensitized Solar Cells (DSSC): A review. Int. J. Energy Res. 2016, 40, 1303–1320. [CrossRef] Sengupta, D.; Das, P.; Mondal, B.; Mukherjee, K. Effects of doping, morphology and film-thickness of photo-anode materials for dye sensitized solar cell application—A review. Renew. Sustain. Energy Rev. 2016, 60, 356–376. [CrossRef] Richhariya, G.; Kumar, A.; Tekasakul, P.; Gupta, B. Natural dyes for dye sensitized solar cell: A review. Renew. Sustain. Energy Rev. 2017, 69, 705–718. [CrossRef] Liu, X.; Xing, Z.; Zhang, H.; Wang, W.; Zhang, Y.; Li, Z.; Wu, X.; Yu, X.; Zhou, W. Fabrication of 3D Mesoporous Black TiO2 /MoS2 /TiO2 Nanosheets for Visible-Light-Driven Photocatalysis. ChemSusChem 2016, 9, 1118–1124. [CrossRef] [PubMed] Jiang, B.; Tang, Y.; Qu, Y.; Wang, J.Q.; Xie, Y.; Tian, C.; Zhou, W.; Fu, H. Thin carbon layer coated Ti3+ –TiO2 nanocrystallites for visible-light driven photocatalysis. Nanoscale 2015, 7, 5035–5045. [CrossRef] [PubMed] Zhou, W.; Li, T.; Wang, J.; Qu, Y.; Pan, K.; Xie, Y.; Tian, G.; Wang, L.; Ren, Z.; Jiang, B.; Fu, H. Composites of small Ag clusters confined in the channels of well-ordered mesoporous anatase TiO2 and their excellent solar-light-driven photocatalytic performance. Nano Res. 2014, 7, 731–742. [CrossRef] Luan, P.; Xie, M.; Fu, X.; Qu, Y.; Sun, X.; Jing, L. Improved photoactivity of TiO2 –Fe2 O3 nanocomposites for visible-light water splitting after phosphate bridging and its mechanism. Phys. Chem. Chem. Phys. 2015, 17, 5043–5050. [CrossRef] [PubMed] Humayun, M.; Zada, A.; Li, Z.; Xie, M.; Zhang, X.; Qu, Y.; Raziq, F.; Jing, L. Enhanced visible-light activities of porous BiFeO3 by coupling with nanocrystalline TiO2 and mechanism. Appl. Catal. B 2016, 180, 219–226. [CrossRef] Du, K.; Liu, G.; Chen, X.; Wang, K. PbS Quantum Dots Sensitized TiO2 Nanotubes for Photocurrent Enhancement. J. Electrochem. Soc. 2015, 162, E251–E257. [CrossRef] Wei, N.; Cui, H.; Wang, C.; Zhang, G.; Song, Q.; Sun, W.; Song, X.; Sun, M.; Tian, J. Bi2 O3 nanoparticles incorporated porous TiO2 films as an effective p-n junction with enhanced photocatalytic activity. J. Am. Ceram. Soc. 2017, 100, 1339–1349. [CrossRef] Wu, Y.; Zhou, Y.; Liu, Y.; Wang, Y.; Yang, L.; Li, C. Photocatalytic performances and characterizations of sea urchin-like N,Ce codoped TiO2 photocatalyst. Mater. Res. Innov. 2017, 21, 33–39. [CrossRef] Zhao, Q.; Liu, X.; Xing, Y.; Liu, Z.; Du, C. Synthesizing Bi2 O3 /BiOCl heterojunctions by partial conversion of BiOCl. J. Mater. Sci. 2016, 52, 2117–2130. [CrossRef] Wang, C.; Shao, C.; Wang, L.; Zhang, L.; Li, X.; Liu, Y. Electrospinning preparation, characterization and photocatalytic properties of Bi2 O3 nanofibers. J. Colloid Interface Sci. 2009, 333, 242–248. [CrossRef] [PubMed] Xu, J.; Ao, Y.; Fu, D.; Yuan, C. Synthesis of Bi2 O3 –TiO2 composite film with high-photocatalytic activity under sunlight irradiation. Appl. Surf. Sci. 2008, 255, 2365–2369. [CrossRef] Yu, L.; Li, D. Photocatalytic methane conversion coupled with hydrogen evolution from water over Pd/TiO2 . Catal. Sci. Technol. 2017, 7, 635–640. [CrossRef] Than, L.D.; Luong, N.S.; Ngo, V.D.; Tien, N.M.; Dung, T.N.; Nghia, N.M.; Loc, N.T.; Thu, V.T.; Lam, T.D. Highly Visible Light Activity of Nitrogen Doped TiO2 Prepared by Sol–Gel Approach. J. Electron. Mater. 2016, 46, 158–166. [CrossRef]


32. 33. 34. 35.

13 of 13

Damyanova, S.; Dimitrov, L.; Petrov, L.; Grange, P. Effect of niobium on the surface properties of Nb2 O5 –SiO2 -supported Mo catalysts. Appl. Surf. Sci. 2003, 214, 68–74. [CrossRef] Xu, D.; Hai, Y.; Zhang, X.; Zhang, S.; He, R. Bi2 O3 cocatalyst improving photocatalytic hydrogen evolution performance of TiO2 . Appl. Surf. Sci. 2017, 400, 530–536. [CrossRef] Shamaila, S.; Sajjad, A.K.L.; Chen, F.; Zhang, J. Study on highly visible light active Bi2 O3 loaded ordered mesoporous titania. Appl. Catal. B 2010, 94, 272–280. [CrossRef] Hu, H.; Chang, M.; Wang, X.; Chen, D. Cotton fabric-based facile solar photocatalytic purification of simulated real dye wastes. J. Mater. Sci. 2017, 1–9. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).