nanomaterials Article
Photoelectrochemical Performance Observed in Mn-Doped BiFeO3 Heterostructured Thin Films Hao-Min Xu 1 , Huanchun Wang 1,2 , Ji Shi 3 , Yuanhua Lin 1, * and Cewen Nan 1 1
2 3
*
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China; [email protected] (H.-M.X.); [email protected] (H.W.); [email protected] (C.N.) High-Tech Institute of Xi’an, Xi’an 780025, China Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan; [email protected] Correspondence: [email protected]; Tel.: +86-10-6277-3741
Academic Editor: Hongqi Sun Received: 9 October 2016; Accepted: 11 November 2016; Published: 16 November 2016
Abstract: Pure BiFeO3 and heterostructured BiFeO3 /BiFe0.95 Mn0.05 O3 (5% Mn-doped BiFeO3 ) thin films have been prepared by a chemical deposition method. The band structures and photosensitive properties of these films have been investigated elaborately. Pure BiFeO3 films showed stable and strong response to photo illumination (open circuit potential kept −0.18 V, short circuit photocurrent density was −0.023 mA·cm−2 ). By Mn doping, the energy band positions shifted, resulting in a smaller band gap of BiFe0.95 Mn0.05 O3 layer and an internal field being built in the BiFeO3 /BiFe0.95 Mn0.05 O3 interface. BiFeO3 /BiFe0.95 Mn0.05 O3 and BiFe0.95 Mn0.05 O3 thin films demonstrated poor photo activity compared with pure BiFeO3 films, which can be explained by the fact that Mn doping brought in a large amount of defects in the BiFe0.95 Mn0.05 O3 layers, causing higher carrier combination and correspondingly suppressing the photo response, and this negative influence was more considerable than the positive effects provided by the band modulation. Keywords: photocatalysis; photoelectrochemical; solar energy conversion; Mn-doped BiFeO3 films; heterostructure
1. Introduction BiFeO3 (BFO) is famous for its room temperature multiferroic properties, but it is attracting more and more interest as a small band gap photo-ferroelectric material. In the past few years, photocatalytic (PC), and photovoltaic (PV) properties of BFO films have been studied [1–5]. For example, polycrystalline BFO photo electrode was applied for water photo-oxidation [6]; an enhanced ferroelectric PV response of multilayered BiFeO3 /BaTiO3 was observed [7]. The photoelectrochemical (PEC) response of BFO with ferroelectric materials has also been reported [8,9]. Findings imply the promising potential applications of BFO films in terms of photocatalysts, photovoltaic cells, multifunction sensors, etc. [10–12]. High quality photocatalyts, or the PEC electrodes, are believed to possess high-energy conversion efficiency. One of the key issues for energy conversion performance is the ability of light absorption, which is mostly decided by the band gap of the material. Element doping is quite a normal method in band engineering. Usually, metal element doping does better in narrowing the band gap than the anions do [13]. More specifically, to narrow the band gap of BFO, people can conduct A/B-site doping with elements such as La [14], Gd [15], Y and Ti [16]. The A-site and B-site represents the positions of Bi and Fe atoms in the BFO lattice, respectively. B-site Mn doping is a potential method of modifying the band structure of BFO [17]. The Mn element has attracted great attention due to its special role in BFO thin films. For example, people found that BFO films showed enhanced ferroelectric and
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attention due to its special role in BFO thin films. For example, people found that BFO films showed weak magnetic behaviors 5% Mn substitution [18,19]. Optical and PC properties Mn-involved enhanced ferroelectric andby weak magnetic behaviors by 5% Mn substitution [18,19].of Optical and PC co-doping BFO have also been recently studied. For example, researchers observed that Ba/Ca and properties of Mn-involved co-doping BFO have also been recently studied. For example, researchers Mn co-doped BFO nanofibers delivered enhanced PC delivered performances [20,21]; et al. obtained observed that Ba/Ca and Mn co-doped BFO nanofibers enhanced PCVinay performances [20,21]; aVinay photo-induced current of about 250-fold in Ce Mn co-doped BFO at zero biasthin in et al. obtained a photo-induced current of and about 250-fold in Ce andthin Mnfilms co-doped BFO afilms PV test [5]. However, few studies have focused on the effects of the Mn single doping of BFO on PEC at zero bias in a PV test [5]. However, few studies have focused on the effects of the Mn single properties. thison work, set Mn doping content 0.05,set preparing BiFe0.95 Mn0.05as O30.05, thin films, and doping of In BFO PECwe properties. In this work,aswe Mn doping content preparing studied their optical and PEC BiFe0.95Mn 0.05O 3 thin films, andproperties. studied their optical and PEC properties. Furthermore, Furthermore, the energy energy conversion conversion is highly highly restricted restricted by by the the combination combination of of photo-induced photo-induced electro-hole electro-hole pairs, pairs, which which can can happen happen inside inside or or on on the the surface surface of of the the material. material. How How to to separate separate these these pairs much more efficiently has been a question for a long time [22]. According to the research before, pairs much more efficiently has been a question for we the internal internalfield fieldplays playsan animportant importantrole roleinin separating pairs; through studying we know that the separating e-he–h pairs; through studying the the principles of the internal field, able createorordesign designan aninternal internal field field that could provide principles of the internal field, wewe areare able totocreate provide more more effective effective electron-hole electron-hole separation, faster interface charge transfer, and longer carrier lifetime. For thethe polarization in the materials [9,23,24], constructing the p-n junction For example, example,using using polarization in ferroelectric the ferroelectric materials [9,23,24], constructing the p-n or polymorph junction can work appropriately [25]. Considering these facts, itthese would be interesting to junction or polymorph junction can work appropriately [25]. Considering facts, it would be construct heterostructure with BFO andwith band-modified BFO, and study theand effects of internal field interestinga to construct a heterostructure BFO and band-modified BFO, study the effects of built around on PEC activity.onTherefore, in theTherefore, present work, wepresent made heterostructured internal fieldboundary built around boundary PEC activity. in the work, we made BFO/BiFe O3 (BFMO) films investigated their PEC properties. heterostructured BFO/BiFe 0.95Mn 0.05O3and (BFMO) films and investigated their PEC properties. 0.95 Mn0.05 2. Resultsand andDiscussion Discussion 2. Results X-ray X-ray diffraction diffraction (XRD) (XRD) results results are are shown shown in in Figure Figure 1. 1. All All peaks peaks in in the the XRD XRD patterns patterns of of the the BFO BFO films are indexed for a rhombohedral structure with R3m symmetry and were found to be films are indexed for a rhombohedral structure with R3m symmetry and were found to be in in good good agreement agreement with with the the corresponding corresponding values values reported reportedin in the the PDF#14-0181, PDF#14-0181, while while the the BFMO BFMO films films display display aa structure transition according to the (012) and (110) peaks that were changed, and the structure transition according to the (012) and (110) peaks that were changed, and the (021) (021) peak peak ◦ –57.1◦ disappeared. The peaks of the BFMO films fit well with with with the the three three peaks peaks around around 2θ 2θ== 56 56°–57.1° disappeared. The peaks of the BFMO films fit well with the reported in which implies implies that that the the film the tetragonal tetragonal BiMnO BiMnO33 reported in PDF#53-0767, PDF#53-0767, which film structure structure has has partly partly transferred from rhombohedral to tetragonal due to the Mn doping. Similar results have been reported transferred from rhombohedral to tetragonal due to the Mn doping. Similar results have been previously [18]. From theFrom XRD the patterns, we can also see that not crystalize as well as reported previously [18]. XRD patterns, we can also BFMO see thatdid BFMO did not crystalize as BFO. Both the BFO film and the BFMO film is densely crystalized according to the scanning electron well as BFO. Both the BFO film and the BFMO film is densely crystalized according to the scanning microscopy (SEM) images the surface of the films (Figure electron microscopy (SEM)ofimages of themorphology surface morphology of the films2a,b). (Figure 2a,b).
Figure 1. X-ray diffraction (XRD) patterns of (a) BiFe0.95Mn0.05O3 (BFMO); (b) BiFeO3 (BFO)/BFMO and Figure 1. X-ray diffraction (XRD) patterns of (a) BiFe0.95 Mn0.05 O3 (BFMO); (b) BiFeO3 (BFO)/BFMO (c) BFO films. PDF#14-0181 shows BFO in rhombohedral structure, and PDF#53-0767 shows BFMO in and (c) BFO films. PDF#14-0181 shows BFO in rhombohedral structure, and PDF#53-0767 shows BFMO tetragonal structure, PDF#46-1088 shows thethe peak positions ofof SnO 2 substrate. in tetragonal structure, PDF#46-1088 shows peak positions SnO 2 substrate.
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Figure 2. The surface scanning electron microscopy images of (a) BFO and (b) BFMO film; (c) The illustration of The BFO/BFMO heterostructure. Figure 2. surface scanning electron microscopy images of (a) BFO and (b) BFMO film; (c) The Figure 2. The surface scanning electron microscopy images of (a) BFO and (b) BFMO film; (c) The illustration of BFO/BFMO heterostructure. illustration of BFO/BFMO heterostructure. test the photo-response ability of our films, we first tested the PEC properties
To of BFO. To ability of our our films, we and first P-t tested the PEC PEC properties BFO. a According thethe I-tphoto-response measurementsability (Figure 3, 0films, V bias) (Figure 4a,properties BFO), weofof observed Tototest test the photo-response of we first tested the BFO. According to the I-t measurements (Figure 3, 0 V bias) and P-t (Figure 4a, BFO), we observed a a repeatable and stable instantaneous response to the light illumination, coming along with According to the I-t measurements (Figure 3, 0 V bias) and P-t (Figure 4a, BFO), we observed a repeatable −2 repeatable and stable instantaneous response to the light illumination, coming along with a photocurrent density −0.023 mA· cm and anlight openillumination, circuit potential when exposed to and stable instantaneous response to the coming(OCP) along −0.18 with V a photocurrent −2 and an open circuit potential (OCP) −0.18 V when exposed to − 2 photocurrent density −0.023 mA· cm −2 photocurrent −0.023 mA ·cmhigh andsensitivity an open circuit potential (OCP) −0.18 V when exposed tocm light, proving light, density proving again the of BFO films to light. The −0.023 mA· −2 photocurrent − 2 light, proving again the high sensitivity of BFO films to light. The −0.023 mA· cm again the high sensitivity of BFO films to light. The − 0.023 mA · cm photocurrent density at zero biasvaried density at zero bias voltage was much larger than previous reports [8,26]. The I-t test with the density zero biaslarger voltage was much larger than previous [8,26]. The I-tinitial test with the varied voltageat was much than previous reports [8,26]. The I-treports test with the varied potentials initial potentials of BFO (Figure 3) implies that a negative initial potential can improve theofphoto initial potentials of BFO (Figure 3) implies that a negative initial can improve photo BFO (Figure 3) implies that a negative initial potential can improve thepotential photo response, while athe positive response, while a positive one will weaken the response, which can be explained by the fact that response, while athe positive onewhich will weaken the response, which can negative be explained by the fact that one will weaken response, can be explained by the fact that OCP appears under negative OCP appears under illumination, and the photo-induced electrons aggregating on the negative OCP appears photo illumination, and the photo-induced electrons on the photo illumination, andunder thephoto photo-induced electrons aggregating on the F-doped SnOaggregating (FTO) side have 2 F-doped SnO 2 (FTO) side have toto get over the that thepositive positive applied voltage creates to move F-doped SnO 2 (FTO) side have get over thebarrier barrier thatcreates the applied creates to move to get over the barrier that the positive applied voltage to move out tovoltage the counter electrode. −2 − 2 −2 at out to the counter electrode. The current density increased −0.005 mA· at an applied The density increased to − 0.005 mA ·cm at an applied potential ofcm −cm 0.4 V,an which is out todark the current counter electrode. Thedark dark current density increased toto−0.005 mA· applied much previous reports [26]. than potential ofsmaller −0.4 V,than which is much smaller reports[26]. [26]. potential of −0.4 V, which is much smaller thanprevious previous reports
Figure 3. (a) I-t curves of BFO at 0 V, 0.1 V, −0.1 V and −0.4 V bias voltages; inset (b) is the photocurrent density change under different bias voltages. FigureFigure 3. (a) 3. I-t(a) curves of BFO at at 0 V, −0.4 Vbias biasvoltages; voltages; inset isphotocurrent the photocurrent I-t curves of BFO 0 V,0.1 0.1V, V, −0.1 −0.1V V and and − 0.4 V inset (b) (b) is the density change under different bias voltages. density different bias voltages. Then, wechange testedunder the photo-response ability of BFMO films and the heterostructured BFO/BFMO
films and compared them with BFO films. BFMO displayed a nearly 0 V OCP, and BFO/BFMO
Then, weto tested the the photo-response ability of BFMO BFMO films and heterostructured BFO/BFMO Then, we photo-response ability and thethe heterostructured BFO/BFMO appeared be tested unstable, with only a quarter OCP to BFOfilms (Figure 4a). In Figure 4a, the initial position and compared them with BFO films. BFMO displayed a nearly 0 V OCP, and BFO/BFMO filmsoffilms and compared them with BFO films. BFMO displayed a nearly 0 V OCP, and BFO/BFMO the three types varied because of the different surface state contrast to the reference electrode. The appeared be unstable, with only quarter OCP to In In Figure 4a, the initial position time dependence of the photocurrent density measurements at 04a). V,4a). 0.1 V and −0.4 V bias are appeared to betounstable, with only a aquarter OCP to BFO BFO(Figure (Figure Figure 4a, thevoltages initial position of the three types varied because of the different surface state contrast to the reference electrode. Figure 4b–d.because BFMO films the BFO/BFMO filmscontrast barely had an obvious photocurrent. of theshown three in types varied of theand different surface state to the reference electrode. The Thevalues time dependence of theeven photocurrent densityameasurements at 0 V, 0.1 Vthough and −0.4 Vdoping bias voltages The are still small, when applying negative voltage. Even Mn in BFO are time dependence of the photocurrent density measurements at 0 V, 0.1 V and −0.4 V bias voltages are shown in Figure 4b–d. BFMO films and the BFO/BFMO films barely had an obvious photocurrent. −4 willinalways induce leakage current (theBFO/BFMO dark current films intensity of BFMO V is 8 × 10photocurrent. mA·cm−2, shown Figure 4b–d.a low BFMO films and the barely had at an0 obvious −4 −2 slightly larger than BFO of 2.5 × 10 mA·cm ), it cannot affect the photocurrent that much, since the The values are still small, even when applying a negative voltage. Even though Mn doping in BFO dark current intensity of BFO/BFMO is 9 × 10−5 mA·cm−2, much smaller than BFO. According to the Iwill always induce a low leakage current (the dark current intensity of BFMO at 0 V is 8 × 10−4 mA·cm−2, t results of BFMO and BFO/BFMO films, the transient anode photocurrent upon light illumination slightly larger than BFO of 2.5 × 10−4 mA·cm−2), it cannot affect the photocurrent that much, since the dark current intensity of BFO/BFMO is 9 × 10−5 mA·cm−2, much smaller than BFO. According to the I-
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The values are still small, even when applying a negative voltage. Even though Mn doping in BFO will always induce a low leakage current (the dark current intensity of BFMO at 0 V is 8 × 10−4 mA·cm−2 , slightly larger than BFO of 2.5 × 10−4 mA·cm−2 ), it cannot affect the photocurrent that much, since the dark current intensity of BFO/BFMO is 9 × 10−5 mA·cm−2 , much smaller than BFO. According to the I-t results of BFMO and BFO/BFMO films, the transient anode photocurrent upon light illumination was small and attenuated quickly, indicating the strong recombination and short life of photo-induced carriers. In general, pure BFO film performs best compared with BFMO and the heterostructured BFO/BFMO films.
Figure 4. (a) Open circuit potential tests of the three types of samples under on-off 150 mW/cm2 light illumination. I-t curves at (b) 0 V; (c) 0.1 V; (d) −0.4 V of BFMO and BFO/BFMO films.
To explain the phenomenon above, the energy band structures of the films were carefully investigated. Ultraviolet-visible (UV-Vis) absorption measurements and the band gap calculations (shown in Figure 5) indicate that BFMO cannot absorb light as much as BFO does, and it has a band gap of 2.39 eV, which is smaller than BFO’s band gap of 2.60 eV. To get an insight into how the band structure of BFO changes via Mn doping, and to understand the internal field built around the boundary of heterostructured BFO/BFMO, several methods were applied to determine the band position of BFO and BFMO in the heterostructured BFO/BFMO. The first method relies on the following formula: 1 EC = X − Ee − Eg 2
(1)
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where EC presents the position of conductive band (CB), X is the absolute electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale, and Eg is the band gap of the semiconductor. The theoretical band positions are shown in Figure 6c. The second method is represented by using X-ray photoelectron spectroscopy (XPS) valance spectra in Figure 6a, which can explain Ev , the position of valence band (VB) in Figure 6d, but it is not accurate enough [27]. The third method is the Mott–Schottky method, which is conducted by the electrochemical station. The Mott–Schottky equation is as follows: Nanomaterials 2016, 6, 215
2 k T 1 = ( V − Vf b − B ) e C2 εε0 A2 eND
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(2)
constant, T is absolute temperature, and Vfb is flat band potential [28]. In the function of 1/C2 and where C is space chargek layers capacitance, e is electron charge, ε is dielectric constant, ε0 means T
B applied potentialof(V), be determined by taking the x-intercept of aklinear fit to the Mott– V fb N permittivity vacuum, electron donordensity, V means applied potential, D =can B is Boltzmann’s
e constant, T is absolute temperature, and Vfb is flat band potential [28]. In the function of 1/C2 and kT e k B T was calculated as 0.0259 V. Thus, the Vfb of BFO is Mott–Schottky plot (shown in Figure 6b), and e k T
B applied Vf b + Be6b), canand be determined by taking the a linear Schottky plotpotential (shown(V), in Figure was calculated as x-intercept 0.0259 V. of Thus, the fit Vfbtoofthe BFO is
estimated as −0.33 V (vs. Ag/AgCl) or 0.27 VV (vs. reversible electrode,RHE), RHE), and BFMO is estimated as −0.33 V (vs. Ag/AgCl) or 0.27 (vs. reversiblehydrogen hydrogen electrode, and BFMO −0.41 isV−(vs. or 0.20 (vs. RHE). ToToconvert obtainedpotential potential Ag/AgCl) to RHE, 0.41Ag/AgCl) V (vs. Ag/AgCl) or V 0.20 V (vs. RHE). convert the the obtained (vs.(vs. Ag/AgCl) to RHE, the following equation [29] was used: the following equation was used: E(RHE) = E(Ag/AgCl) + +0.059 E(RHE) = E(Ag/AgCl) 0.059pH pH++0.21 0.21 [29]
(3) (3)
For n-type semiconductors, thethe flat-band potential fb) was considered to be located just under the For n-type semiconductors, flat-band potential(V (V fb ) was considered to be located just under CB; hence, we can calculate the band position as shown in Figure 6e6e[6]. the CB; hence, we can calculate the band position as shown in Figure [6].Even Eventhough thoughthe the band positions the three methods were found vary,toone fact thatisthey bandobtained positions by obtained by the three methods were to found vary, oneisfact that can theyall canbeallclassified be classified into the structure Figure in which bandbends bends and and benefits thethe separation of into the structure shown in shown Figurein 6f, in 6f, which the the band benefits separation of photogenerated photogenerated e–h.e–h.
Figure 5. (a) Ultraviolet-visible (UV-Vis) absorption of BFMO; BFO and BFMO; (b)calculation. the band gap Figure 5. (a) Ultraviolet-visible (UV-Vis) absorption spectraspectra of BFO and (b) the band gap calculation.
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Figure 6. (a) X-ray photoelectron spectroscopy (XPS) valance spectra; (b) Mott–Schottky curve; the band position determined by (c) theoretical calculation, (d) XPS valance spectra method and (e) Mott–Schottky method; (f) an illustration of carriers transferring through boundary.
Although BFMO has a smaller band gap than BFO, and BFO/BFMO films own an advantageous band bend causing an internal field, pure BFO film performs best compared with BFMO and the heterostructured BFO/BFMO films. These facts imply that another factor influencing the photo response, which exceeds the effect of more light absorption, exists in the BFMO layers and the band bend in the interface of BFO/BFMO. On the basis of the XRD patterns (Figure 1), the inferior crystallinity of BFMO implied more vacancy defects in the BFMO films. More vacancy defects in BFMO can also be determined in photo-luminescence (PL) spectra (Figure 7). The fluorescence effect is a little bit higher in BFMO than in BFO films, which might be attributed to the higher e–h combination occurring around the defects. The doping process could generate more defects in BFMO, and the consequent higher e–h combination considerably decreased the PEC properties of BFMO and BFO/BFMO films. Nevertheless, Mn single doping does not cause detrimental effects on photo response of all material systems; for example, Mn single doping into the TiO2 lattice results in a significant enhancement in photoabsorption and in the quantity of photogenerated hydroxyl radicals due to the formation of a low oxygen-vacancy content [30].
1
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Figure7.7.Photo-luminescence Photo-luminescence spectra spectra of Figure of BFO BFOand andBFMO BFMOfilms. films.
3. Materials Methods 3. Materials andand Methods The BFO films were fabricated FTO substrates reported previously[10]. [10].ToTo The BFO films were fabricated onon FTO substrates asas reported previously getget thethe 5%5% MnMn-doped BFO, we chose C H MnO · 4H O as the Mn source. The BFO film is 4-layered, the BFMO 4 64·4H2O 4 as 2the Mn source. The BFO film is 4-layered, the BFMO film doped BFO, we chose C4H6MnO film is 4-layered, the heterostructured BFO/BFMO film (shown in 2c) Figure 2c) is combined by is 4-layered, and theand heterostructured BFO/BFMO film (shown in Figure is combined by 2-layered 2-layered BFO and 2-layered BFMO. One layer represents one instance of spin coating. BFO and 2-layered BFMO. One layer represents one instance of spin coating. XRD carried to analyze the crystal structure by X-ray diffractometer (RIGAKU XRD waswas carried outout to analyze the crystal structure by X-ray diffractometer (RIGAKU D/max D/max 2500, Tokyo, Japan) with Cu radiation at 40 kV/200 mA. The XRD scanning speed was 6◦ /min 2500, Tokyo, Japan) with Cu radiation at 40 kV/200 mA. The XRD scanning speed was 6°/min (standard configuration). The microstructures were checked by field emission microscope at 20 kV (standard configuration). The microstructures were checked by field emission microscope at 20kV (JSM-7001F, JEOL, Tokyo, Japan). UV-Vis absorption spectra were measured by a UV-Vis spectrophotometer (JSM-7001F, JEOL, Tokyo, Japan). UV–Vis absorption spectra were measured by a UV-Vis (UV-3310, HITACHI, Tokyo, Japan). The PL spectra were tested with LabRAM HR Evolution spectrophotometer (UV-3310, HITACHI, Tokyo, Japan). The PL spectra were tested with LabRAM (HORIBA, Kyoto, Japan), and the incident laser wave length was 325 nm. XPS valance spectrum was HRtested Evolution (HORIBA, Kyoto, Japan), and the incident laser wave length was 325 nm. XPS valance by PHI Quantera SXM (ULVAC-PHI, INC., Tokyo, Japan). spectrum tested by PHI Quantera SXM INC., Tokyo, Japan). Thewas PEC measurements, including the(ULVAC-PHI, time dependence open circuit potential (P-t), the time The PEC measurements, including the(I-t) time dependence potential workstation (P-t), the time dependence of the photocurrent density was carried out open by ancircuit electrochemistr dependence of the photocurrent density (I-t) was carried out by an electrochemistr workstation (CHI (CHI 660D, CHENHUA, Shanghai, China) in a three-electrode cell. The prepared samples, a small 660D, CHENHUA, Shanghai, China) in used a three-electrode cell. The prepared samples, a small Pt foil, and a saturated Ag/AgCl were as the working electrode, the counter electrode, andPt thefoil, andreference a saturated Ag/AgCl were used as the working electrode, the counter electrode, and the reference electrode, respectively. 0.1 M Na2 SO4 (at pH = 6.7) was used as the electrolyte. The working electrode, 0.1 M Na2SO (at pH =2 6.7) was used as the electrolyte. TheI-t working electrode electroderespectively. was illuminated under 1504mW/cm visible-light irradiation (λ > 420 nm). was measured 2 visible-light inilluminated the light on–off process a pulse of 10–50 s by the potentiostatic at 0 V, − 0.1 V, 0.1inVthe was under 150 with mW/cm irradiation ( λ 420technique ). I-t was measured nm and −0.4 Vprocess bias versus light on–off withAg/AgCl. a pulse of 10–50 s by the potentiostatic technique at 0 V, −0.1 V, 0.1 V and
−0.4 V bias versus Ag/AgCl. 4. Conclusions
In this work, we carried out band engineering on BFO films via Mn doping and prepared 4. Conclusions heterostructured BFO/BFMO films. The photo-response activities of these films were studied through In tests. this work, weBFO carried bandtoengineering on BFO films viaapplicable Mn doping and prepared PEC The pure film isout sensitive visible light, which is potentially in photo-related heterostructured films.sensors, The photo-response activities of these films were studied through fields, such as BFO/BFMO optical sensitive PV cells, and photocatalysts. In our heterostructured PEC tests. The pure BFO film is sensitive to visible light, which is potentially applicable in photoBFO/BFMO samples, the facilitating effects provided by the smaller band gap of the BFMO layer and related fields, field suchinasthe optical sensitive PV cells, andcompensate photocatalysts. In oureffect heterostructured the internal interface on PECsensors, performance cannot the negative caused by BFO/BFMO the facilitating effectslayer. provided by the smaller band gap of thecharge BFMOcarriers layer and the severesamples, e–h combination in the BFMO The combination of photogenerated theoccurred internal around field inthe thedefects interface on PEC by performance compensate the negative effect caused introduced Mn doping.cannot The major results indicated that, to enhance ability andinimprove energy conversion when designing a BFO based photic by the the photo-response severe e–h combination the BFMO layer. The combination of photogenerated chargedevice, carriers integrated multi-factors needintroduced to be takenby into account. In addition, more theoretical work withtorespect occurred around the defects Mn doping. The major results indicated that, enhance to the separation mechanism of photogenerated charge carriers is desirable in the future. the photo-response ability and improve energy conversion when designing a BFO based photic
device, integrated multi-factors need to be taken into account. In addition, more theoretical work Acknowledgments: This work was supported by the NSF of China (51532003, 51221291, and 51328203). with respect to the separation mechanism of photogenerated charge carriers is desirable in the future. Acknowledgments: This work was supported by the NSF of China (51532003, 51221291, and 51328203). Author Contributions: Haomin Xu and Yuanhua Lin conceived and designed the experiments; Haomin Xu performed the experiments; Haomin Xu and Huanchun Wang analyzed the data; Cewen Nan and Yuanhua Lin contributed reagents/materials/analysis tools; Haomin Xu and Ji Shi wrote the paper.
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Author Contributions: Haomin Xu and Yuanhua Lin conceived and designed the experiments; Haomin Xu performed the experiments; Haomin Xu and Huanchun Wang analyzed the data; Cewen Nan and Yuanhua Lin contributed reagents/materials/analysis tools; Haomin Xu and Ji Shi wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Photoelectrochemical Performance Observed in Mn-Doped BiFeO3 Heterostructured Thin Films Hao-Min Xu 1 , Huanchun Wang 1,2 , Ji Shi 3 , Yuanhua Lin 1, * and Cewen Nan 1 1
2 3
*
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China; [email protected] (H.-M.X.); [email protected] (H.W.); [email protected] (C.N.) High-Tech Institute of Xi’an, Xi’an 780025, China Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan; [email protected] Correspondence: [email protected]; Tel.: +86-10-6277-3741
Academic Editor: Hongqi Sun Received: 9 October 2016; Accepted: 11 November 2016; Published: 16 November 2016
Abstract: Pure BiFeO3 and heterostructured BiFeO3 /BiFe0.95 Mn0.05 O3 (5% Mn-doped BiFeO3 ) thin films have been prepared by a chemical deposition method. The band structures and photosensitive properties of these films have been investigated elaborately. Pure BiFeO3 films showed stable and strong response to photo illumination (open circuit potential kept −0.18 V, short circuit photocurrent density was −0.023 mA·cm−2 ). By Mn doping, the energy band positions shifted, resulting in a smaller band gap of BiFe0.95 Mn0.05 O3 layer and an internal field being built in the BiFeO3 /BiFe0.95 Mn0.05 O3 interface. BiFeO3 /BiFe0.95 Mn0.05 O3 and BiFe0.95 Mn0.05 O3 thin films demonstrated poor photo activity compared with pure BiFeO3 films, which can be explained by the fact that Mn doping brought in a large amount of defects in the BiFe0.95 Mn0.05 O3 layers, causing higher carrier combination and correspondingly suppressing the photo response, and this negative influence was more considerable than the positive effects provided by the band modulation. Keywords: photocatalysis; photoelectrochemical; solar energy conversion; Mn-doped BiFeO3 films; heterostructure
1. Introduction BiFeO3 (BFO) is famous for its room temperature multiferroic properties, but it is attracting more and more interest as a small band gap photo-ferroelectric material. In the past few years, photocatalytic (PC), and photovoltaic (PV) properties of BFO films have been studied [1–5]. For example, polycrystalline BFO photo electrode was applied for water photo-oxidation [6]; an enhanced ferroelectric PV response of multilayered BiFeO3 /BaTiO3 was observed [7]. The photoelectrochemical (PEC) response of BFO with ferroelectric materials has also been reported [8,9]. Findings imply the promising potential applications of BFO films in terms of photocatalysts, photovoltaic cells, multifunction sensors, etc. [10–12]. High quality photocatalyts, or the PEC electrodes, are believed to possess high-energy conversion efficiency. One of the key issues for energy conversion performance is the ability of light absorption, which is mostly decided by the band gap of the material. Element doping is quite a normal method in band engineering. Usually, metal element doping does better in narrowing the band gap than the anions do [13]. More specifically, to narrow the band gap of BFO, people can conduct A/B-site doping with elements such as La [14], Gd [15], Y and Ti [16]. The A-site and B-site represents the positions of Bi and Fe atoms in the BFO lattice, respectively. B-site Mn doping is a potential method of modifying the band structure of BFO [17]. The Mn element has attracted great attention due to its special role in BFO thin films. For example, people found that BFO films showed enhanced ferroelectric and
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attention due to its special role in BFO thin films. For example, people found that BFO films showed weak magnetic behaviors 5% Mn substitution [18,19]. Optical and PC properties Mn-involved enhanced ferroelectric andby weak magnetic behaviors by 5% Mn substitution [18,19].of Optical and PC co-doping BFO have also been recently studied. For example, researchers observed that Ba/Ca and properties of Mn-involved co-doping BFO have also been recently studied. For example, researchers Mn co-doped BFO nanofibers delivered enhanced PC delivered performances [20,21]; et al. obtained observed that Ba/Ca and Mn co-doped BFO nanofibers enhanced PCVinay performances [20,21]; aVinay photo-induced current of about 250-fold in Ce Mn co-doped BFO at zero biasthin in et al. obtained a photo-induced current of and about 250-fold in Ce andthin Mnfilms co-doped BFO afilms PV test [5]. However, few studies have focused on the effects of the Mn single doping of BFO on PEC at zero bias in a PV test [5]. However, few studies have focused on the effects of the Mn single properties. thison work, set Mn doping content 0.05,set preparing BiFe0.95 Mn0.05as O30.05, thin films, and doping of In BFO PECwe properties. In this work,aswe Mn doping content preparing studied their optical and PEC BiFe0.95Mn 0.05O 3 thin films, andproperties. studied their optical and PEC properties. Furthermore, Furthermore, the energy energy conversion conversion is highly highly restricted restricted by by the the combination combination of of photo-induced photo-induced electro-hole electro-hole pairs, pairs, which which can can happen happen inside inside or or on on the the surface surface of of the the material. material. How How to to separate separate these these pairs much more efficiently has been a question for a long time [22]. According to the research before, pairs much more efficiently has been a question for we the internal internalfield fieldplays playsan animportant importantrole roleinin separating pairs; through studying we know that the separating e-he–h pairs; through studying the the principles of the internal field, able createorordesign designan aninternal internal field field that could provide principles of the internal field, wewe areare able totocreate provide more more effective effective electron-hole electron-hole separation, faster interface charge transfer, and longer carrier lifetime. For thethe polarization in the materials [9,23,24], constructing the p-n junction For example, example,using using polarization in ferroelectric the ferroelectric materials [9,23,24], constructing the p-n or polymorph junction can work appropriately [25]. Considering these facts, itthese would be interesting to junction or polymorph junction can work appropriately [25]. Considering facts, it would be construct heterostructure with BFO andwith band-modified BFO, and study theand effects of internal field interestinga to construct a heterostructure BFO and band-modified BFO, study the effects of built around on PEC activity.onTherefore, in theTherefore, present work, wepresent made heterostructured internal fieldboundary built around boundary PEC activity. in the work, we made BFO/BiFe O3 (BFMO) films investigated their PEC properties. heterostructured BFO/BiFe 0.95Mn 0.05O3and (BFMO) films and investigated their PEC properties. 0.95 Mn0.05 2. Resultsand andDiscussion Discussion 2. Results X-ray X-ray diffraction diffraction (XRD) (XRD) results results are are shown shown in in Figure Figure 1. 1. All All peaks peaks in in the the XRD XRD patterns patterns of of the the BFO BFO films are indexed for a rhombohedral structure with R3m symmetry and were found to be films are indexed for a rhombohedral structure with R3m symmetry and were found to be in in good good agreement agreement with with the the corresponding corresponding values values reported reportedin in the the PDF#14-0181, PDF#14-0181, while while the the BFMO BFMO films films display display aa structure transition according to the (012) and (110) peaks that were changed, and the structure transition according to the (012) and (110) peaks that were changed, and the (021) (021) peak peak ◦ –57.1◦ disappeared. The peaks of the BFMO films fit well with with with the the three three peaks peaks around around 2θ 2θ== 56 56°–57.1° disappeared. The peaks of the BFMO films fit well with the reported in which implies implies that that the the film the tetragonal tetragonal BiMnO BiMnO33 reported in PDF#53-0767, PDF#53-0767, which film structure structure has has partly partly transferred from rhombohedral to tetragonal due to the Mn doping. Similar results have been reported transferred from rhombohedral to tetragonal due to the Mn doping. Similar results have been previously [18]. From theFrom XRD the patterns, we can also see that not crystalize as well as reported previously [18]. XRD patterns, we can also BFMO see thatdid BFMO did not crystalize as BFO. Both the BFO film and the BFMO film is densely crystalized according to the scanning electron well as BFO. Both the BFO film and the BFMO film is densely crystalized according to the scanning microscopy (SEM) images the surface of the films (Figure electron microscopy (SEM)ofimages of themorphology surface morphology of the films2a,b). (Figure 2a,b).
Figure 1. X-ray diffraction (XRD) patterns of (a) BiFe0.95Mn0.05O3 (BFMO); (b) BiFeO3 (BFO)/BFMO and Figure 1. X-ray diffraction (XRD) patterns of (a) BiFe0.95 Mn0.05 O3 (BFMO); (b) BiFeO3 (BFO)/BFMO (c) BFO films. PDF#14-0181 shows BFO in rhombohedral structure, and PDF#53-0767 shows BFMO in and (c) BFO films. PDF#14-0181 shows BFO in rhombohedral structure, and PDF#53-0767 shows BFMO tetragonal structure, PDF#46-1088 shows thethe peak positions ofof SnO 2 substrate. in tetragonal structure, PDF#46-1088 shows peak positions SnO 2 substrate.
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Figure 2. The surface scanning electron microscopy images of (a) BFO and (b) BFMO film; (c) The illustration of The BFO/BFMO heterostructure. Figure 2. surface scanning electron microscopy images of (a) BFO and (b) BFMO film; (c) The Figure 2. The surface scanning electron microscopy images of (a) BFO and (b) BFMO film; (c) The illustration of BFO/BFMO heterostructure. illustration of BFO/BFMO heterostructure. test the photo-response ability of our films, we first tested the PEC properties
To of BFO. To ability of our our films, we and first P-t tested the PEC PEC properties BFO. a According thethe I-tphoto-response measurementsability (Figure 3, 0films, V bias) (Figure 4a,properties BFO), weofof observed Tototest test the photo-response of we first tested the BFO. According to the I-t measurements (Figure 3, 0 V bias) and P-t (Figure 4a, BFO), we observed a a repeatable and stable instantaneous response to the light illumination, coming along with According to the I-t measurements (Figure 3, 0 V bias) and P-t (Figure 4a, BFO), we observed a repeatable −2 repeatable and stable instantaneous response to the light illumination, coming along with a photocurrent density −0.023 mA· cm and anlight openillumination, circuit potential when exposed to and stable instantaneous response to the coming(OCP) along −0.18 with V a photocurrent −2 and an open circuit potential (OCP) −0.18 V when exposed to − 2 photocurrent density −0.023 mA· cm −2 photocurrent −0.023 mA ·cmhigh andsensitivity an open circuit potential (OCP) −0.18 V when exposed tocm light, proving light, density proving again the of BFO films to light. The −0.023 mA· −2 photocurrent − 2 light, proving again the high sensitivity of BFO films to light. The −0.023 mA· cm again the high sensitivity of BFO films to light. The − 0.023 mA · cm photocurrent density at zero biasvaried density at zero bias voltage was much larger than previous reports [8,26]. The I-t test with the density zero biaslarger voltage was much larger than previous [8,26]. The I-tinitial test with the varied voltageat was much than previous reports [8,26]. The I-treports test with the varied potentials initial potentials of BFO (Figure 3) implies that a negative initial potential can improve theofphoto initial potentials of BFO (Figure 3) implies that a negative initial can improve photo BFO (Figure 3) implies that a negative initial potential can improve thepotential photo response, while athe positive response, while a positive one will weaken the response, which can be explained by the fact that response, while athe positive onewhich will weaken the response, which can negative be explained by the fact that one will weaken response, can be explained by the fact that OCP appears under negative OCP appears under illumination, and the photo-induced electrons aggregating on the negative OCP appears photo illumination, and the photo-induced electrons on the photo illumination, andunder thephoto photo-induced electrons aggregating on the F-doped SnOaggregating (FTO) side have 2 F-doped SnO 2 (FTO) side have toto get over the that thepositive positive applied voltage creates to move F-doped SnO 2 (FTO) side have get over thebarrier barrier thatcreates the applied creates to move to get over the barrier that the positive applied voltage to move out tovoltage the counter electrode. −2 − 2 −2 at out to the counter electrode. The current density increased −0.005 mA· at an applied The density increased to − 0.005 mA ·cm at an applied potential ofcm −cm 0.4 V,an which is out todark the current counter electrode. Thedark dark current density increased toto−0.005 mA· applied much previous reports [26]. than potential ofsmaller −0.4 V,than which is much smaller reports[26]. [26]. potential of −0.4 V, which is much smaller thanprevious previous reports
Figure 3. (a) I-t curves of BFO at 0 V, 0.1 V, −0.1 V and −0.4 V bias voltages; inset (b) is the photocurrent density change under different bias voltages. FigureFigure 3. (a) 3. I-t(a) curves of BFO at at 0 V, −0.4 Vbias biasvoltages; voltages; inset isphotocurrent the photocurrent I-t curves of BFO 0 V,0.1 0.1V, V, −0.1 −0.1V V and and − 0.4 V inset (b) (b) is the density change under different bias voltages. density different bias voltages. Then, wechange testedunder the photo-response ability of BFMO films and the heterostructured BFO/BFMO
films and compared them with BFO films. BFMO displayed a nearly 0 V OCP, and BFO/BFMO
Then, weto tested the the photo-response ability of BFMO BFMO films and heterostructured BFO/BFMO Then, we photo-response ability and thethe heterostructured BFO/BFMO appeared be tested unstable, with only a quarter OCP to BFOfilms (Figure 4a). In Figure 4a, the initial position and compared them with BFO films. BFMO displayed a nearly 0 V OCP, and BFO/BFMO filmsoffilms and compared them with BFO films. BFMO displayed a nearly 0 V OCP, and BFO/BFMO the three types varied because of the different surface state contrast to the reference electrode. The appeared be unstable, with only quarter OCP to In In Figure 4a, the initial position time dependence of the photocurrent density measurements at 04a). V,4a). 0.1 V and −0.4 V bias are appeared to betounstable, with only a aquarter OCP to BFO BFO(Figure (Figure Figure 4a, thevoltages initial position of the three types varied because of the different surface state contrast to the reference electrode. Figure 4b–d.because BFMO films the BFO/BFMO filmscontrast barely had an obvious photocurrent. of theshown three in types varied of theand different surface state to the reference electrode. The Thevalues time dependence of theeven photocurrent densityameasurements at 0 V, 0.1 Vthough and −0.4 Vdoping bias voltages The are still small, when applying negative voltage. Even Mn in BFO are time dependence of the photocurrent density measurements at 0 V, 0.1 V and −0.4 V bias voltages are shown in Figure 4b–d. BFMO films and the BFO/BFMO films barely had an obvious photocurrent. −4 willinalways induce leakage current (theBFO/BFMO dark current films intensity of BFMO V is 8 × 10photocurrent. mA·cm−2, shown Figure 4b–d.a low BFMO films and the barely had at an0 obvious −4 −2 slightly larger than BFO of 2.5 × 10 mA·cm ), it cannot affect the photocurrent that much, since the The values are still small, even when applying a negative voltage. Even though Mn doping in BFO dark current intensity of BFO/BFMO is 9 × 10−5 mA·cm−2, much smaller than BFO. According to the Iwill always induce a low leakage current (the dark current intensity of BFMO at 0 V is 8 × 10−4 mA·cm−2, t results of BFMO and BFO/BFMO films, the transient anode photocurrent upon light illumination slightly larger than BFO of 2.5 × 10−4 mA·cm−2), it cannot affect the photocurrent that much, since the dark current intensity of BFO/BFMO is 9 × 10−5 mA·cm−2, much smaller than BFO. According to the I-
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The values are still small, even when applying a negative voltage. Even though Mn doping in BFO will always induce a low leakage current (the dark current intensity of BFMO at 0 V is 8 × 10−4 mA·cm−2 , slightly larger than BFO of 2.5 × 10−4 mA·cm−2 ), it cannot affect the photocurrent that much, since the dark current intensity of BFO/BFMO is 9 × 10−5 mA·cm−2 , much smaller than BFO. According to the I-t results of BFMO and BFO/BFMO films, the transient anode photocurrent upon light illumination was small and attenuated quickly, indicating the strong recombination and short life of photo-induced carriers. In general, pure BFO film performs best compared with BFMO and the heterostructured BFO/BFMO films.
Figure 4. (a) Open circuit potential tests of the three types of samples under on-off 150 mW/cm2 light illumination. I-t curves at (b) 0 V; (c) 0.1 V; (d) −0.4 V of BFMO and BFO/BFMO films.
To explain the phenomenon above, the energy band structures of the films were carefully investigated. Ultraviolet-visible (UV-Vis) absorption measurements and the band gap calculations (shown in Figure 5) indicate that BFMO cannot absorb light as much as BFO does, and it has a band gap of 2.39 eV, which is smaller than BFO’s band gap of 2.60 eV. To get an insight into how the band structure of BFO changes via Mn doping, and to understand the internal field built around the boundary of heterostructured BFO/BFMO, several methods were applied to determine the band position of BFO and BFMO in the heterostructured BFO/BFMO. The first method relies on the following formula: 1 EC = X − Ee − Eg 2
(1)
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where EC presents the position of conductive band (CB), X is the absolute electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale, and Eg is the band gap of the semiconductor. The theoretical band positions are shown in Figure 6c. The second method is represented by using X-ray photoelectron spectroscopy (XPS) valance spectra in Figure 6a, which can explain Ev , the position of valence band (VB) in Figure 6d, but it is not accurate enough [27]. The third method is the Mott–Schottky method, which is conducted by the electrochemical station. The Mott–Schottky equation is as follows: Nanomaterials 2016, 6, 215
2 k T 1 = ( V − Vf b − B ) e C2 εε0 A2 eND
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(2)
constant, T is absolute temperature, and Vfb is flat band potential [28]. In the function of 1/C2 and where C is space chargek layers capacitance, e is electron charge, ε is dielectric constant, ε0 means T
B applied potentialof(V), be determined by taking the x-intercept of aklinear fit to the Mott– V fb N permittivity vacuum, electron donordensity, V means applied potential, D =can B is Boltzmann’s
e constant, T is absolute temperature, and Vfb is flat band potential [28]. In the function of 1/C2 and kT e k B T was calculated as 0.0259 V. Thus, the Vfb of BFO is Mott–Schottky plot (shown in Figure 6b), and e k T
B applied Vf b + Be6b), canand be determined by taking the a linear Schottky plotpotential (shown(V), in Figure was calculated as x-intercept 0.0259 V. of Thus, the fit Vfbtoofthe BFO is
estimated as −0.33 V (vs. Ag/AgCl) or 0.27 VV (vs. reversible electrode,RHE), RHE), and BFMO is estimated as −0.33 V (vs. Ag/AgCl) or 0.27 (vs. reversiblehydrogen hydrogen electrode, and BFMO −0.41 isV−(vs. or 0.20 (vs. RHE). ToToconvert obtainedpotential potential Ag/AgCl) to RHE, 0.41Ag/AgCl) V (vs. Ag/AgCl) or V 0.20 V (vs. RHE). convert the the obtained (vs.(vs. Ag/AgCl) to RHE, the following equation [29] was used: the following equation was used: E(RHE) = E(Ag/AgCl) + +0.059 E(RHE) = E(Ag/AgCl) 0.059pH pH++0.21 0.21 [29]
(3) (3)
For n-type semiconductors, thethe flat-band potential fb) was considered to be located just under the For n-type semiconductors, flat-band potential(V (V fb ) was considered to be located just under CB; hence, we can calculate the band position as shown in Figure 6e6e[6]. the CB; hence, we can calculate the band position as shown in Figure [6].Even Eventhough thoughthe the band positions the three methods were found vary,toone fact thatisthey bandobtained positions by obtained by the three methods were to found vary, oneisfact that can theyall canbeallclassified be classified into the structure Figure in which bandbends bends and and benefits thethe separation of into the structure shown in shown Figurein 6f, in 6f, which the the band benefits separation of photogenerated photogenerated e–h.e–h.
Figure 5. (a) Ultraviolet-visible (UV-Vis) absorption of BFMO; BFO and BFMO; (b)calculation. the band gap Figure 5. (a) Ultraviolet-visible (UV-Vis) absorption spectraspectra of BFO and (b) the band gap calculation.
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Figure 6. (a) X-ray photoelectron spectroscopy (XPS) valance spectra; (b) Mott–Schottky curve; the band position determined by (c) theoretical calculation, (d) XPS valance spectra method and (e) Mott–Schottky method; (f) an illustration of carriers transferring through boundary.
Although BFMO has a smaller band gap than BFO, and BFO/BFMO films own an advantageous band bend causing an internal field, pure BFO film performs best compared with BFMO and the heterostructured BFO/BFMO films. These facts imply that another factor influencing the photo response, which exceeds the effect of more light absorption, exists in the BFMO layers and the band bend in the interface of BFO/BFMO. On the basis of the XRD patterns (Figure 1), the inferior crystallinity of BFMO implied more vacancy defects in the BFMO films. More vacancy defects in BFMO can also be determined in photo-luminescence (PL) spectra (Figure 7). The fluorescence effect is a little bit higher in BFMO than in BFO films, which might be attributed to the higher e–h combination occurring around the defects. The doping process could generate more defects in BFMO, and the consequent higher e–h combination considerably decreased the PEC properties of BFMO and BFO/BFMO films. Nevertheless, Mn single doping does not cause detrimental effects on photo response of all material systems; for example, Mn single doping into the TiO2 lattice results in a significant enhancement in photoabsorption and in the quantity of photogenerated hydroxyl radicals due to the formation of a low oxygen-vacancy content [30].
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Figure7.7.Photo-luminescence Photo-luminescence spectra spectra of Figure of BFO BFOand andBFMO BFMOfilms. films.
3. Materials Methods 3. Materials andand Methods The BFO films were fabricated FTO substrates reported previously[10]. [10].ToTo The BFO films were fabricated onon FTO substrates asas reported previously getget thethe 5%5% MnMn-doped BFO, we chose C H MnO · 4H O as the Mn source. The BFO film is 4-layered, the BFMO 4 64·4H2O 4 as 2the Mn source. The BFO film is 4-layered, the BFMO film doped BFO, we chose C4H6MnO film is 4-layered, the heterostructured BFO/BFMO film (shown in 2c) Figure 2c) is combined by is 4-layered, and theand heterostructured BFO/BFMO film (shown in Figure is combined by 2-layered 2-layered BFO and 2-layered BFMO. One layer represents one instance of spin coating. BFO and 2-layered BFMO. One layer represents one instance of spin coating. XRD carried to analyze the crystal structure by X-ray diffractometer (RIGAKU XRD waswas carried outout to analyze the crystal structure by X-ray diffractometer (RIGAKU D/max D/max 2500, Tokyo, Japan) with Cu radiation at 40 kV/200 mA. The XRD scanning speed was 6◦ /min 2500, Tokyo, Japan) with Cu radiation at 40 kV/200 mA. The XRD scanning speed was 6°/min (standard configuration). The microstructures were checked by field emission microscope at 20 kV (standard configuration). The microstructures were checked by field emission microscope at 20kV (JSM-7001F, JEOL, Tokyo, Japan). UV-Vis absorption spectra were measured by a UV-Vis spectrophotometer (JSM-7001F, JEOL, Tokyo, Japan). UV–Vis absorption spectra were measured by a UV-Vis (UV-3310, HITACHI, Tokyo, Japan). The PL spectra were tested with LabRAM HR Evolution spectrophotometer (UV-3310, HITACHI, Tokyo, Japan). The PL spectra were tested with LabRAM (HORIBA, Kyoto, Japan), and the incident laser wave length was 325 nm. XPS valance spectrum was HRtested Evolution (HORIBA, Kyoto, Japan), and the incident laser wave length was 325 nm. XPS valance by PHI Quantera SXM (ULVAC-PHI, INC., Tokyo, Japan). spectrum tested by PHI Quantera SXM INC., Tokyo, Japan). Thewas PEC measurements, including the(ULVAC-PHI, time dependence open circuit potential (P-t), the time The PEC measurements, including the(I-t) time dependence potential workstation (P-t), the time dependence of the photocurrent density was carried out open by ancircuit electrochemistr dependence of the photocurrent density (I-t) was carried out by an electrochemistr workstation (CHI (CHI 660D, CHENHUA, Shanghai, China) in a three-electrode cell. The prepared samples, a small 660D, CHENHUA, Shanghai, China) in used a three-electrode cell. The prepared samples, a small Pt foil, and a saturated Ag/AgCl were as the working electrode, the counter electrode, andPt thefoil, andreference a saturated Ag/AgCl were used as the working electrode, the counter electrode, and the reference electrode, respectively. 0.1 M Na2 SO4 (at pH = 6.7) was used as the electrolyte. The working electrode, 0.1 M Na2SO (at pH =2 6.7) was used as the electrolyte. TheI-t working electrode electroderespectively. was illuminated under 1504mW/cm visible-light irradiation (λ > 420 nm). was measured 2 visible-light inilluminated the light on–off process a pulse of 10–50 s by the potentiostatic at 0 V, − 0.1 V, 0.1inVthe was under 150 with mW/cm irradiation ( λ 420technique ). I-t was measured nm and −0.4 Vprocess bias versus light on–off withAg/AgCl. a pulse of 10–50 s by the potentiostatic technique at 0 V, −0.1 V, 0.1 V and
−0.4 V bias versus Ag/AgCl. 4. Conclusions
In this work, we carried out band engineering on BFO films via Mn doping and prepared 4. Conclusions heterostructured BFO/BFMO films. The photo-response activities of these films were studied through In tests. this work, weBFO carried bandtoengineering on BFO films viaapplicable Mn doping and prepared PEC The pure film isout sensitive visible light, which is potentially in photo-related heterostructured films.sensors, The photo-response activities of these films were studied through fields, such as BFO/BFMO optical sensitive PV cells, and photocatalysts. In our heterostructured PEC tests. The pure BFO film is sensitive to visible light, which is potentially applicable in photoBFO/BFMO samples, the facilitating effects provided by the smaller band gap of the BFMO layer and related fields, field suchinasthe optical sensitive PV cells, andcompensate photocatalysts. In oureffect heterostructured the internal interface on PECsensors, performance cannot the negative caused by BFO/BFMO the facilitating effectslayer. provided by the smaller band gap of thecharge BFMOcarriers layer and the severesamples, e–h combination in the BFMO The combination of photogenerated theoccurred internal around field inthe thedefects interface on PEC by performance compensate the negative effect caused introduced Mn doping.cannot The major results indicated that, to enhance ability andinimprove energy conversion when designing a BFO based photic by the the photo-response severe e–h combination the BFMO layer. The combination of photogenerated chargedevice, carriers integrated multi-factors needintroduced to be takenby into account. In addition, more theoretical work withtorespect occurred around the defects Mn doping. The major results indicated that, enhance to the separation mechanism of photogenerated charge carriers is desirable in the future. the photo-response ability and improve energy conversion when designing a BFO based photic
device, integrated multi-factors need to be taken into account. In addition, more theoretical work Acknowledgments: This work was supported by the NSF of China (51532003, 51221291, and 51328203). with respect to the separation mechanism of photogenerated charge carriers is desirable in the future. Acknowledgments: This work was supported by the NSF of China (51532003, 51221291, and 51328203). Author Contributions: Haomin Xu and Yuanhua Lin conceived and designed the experiments; Haomin Xu performed the experiments; Haomin Xu and Huanchun Wang analyzed the data; Cewen Nan and Yuanhua Lin contributed reagents/materials/analysis tools; Haomin Xu and Ji Shi wrote the paper.
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Author Contributions: Haomin Xu and Yuanhua Lin conceived and designed the experiments; Haomin Xu performed the experiments; Haomin Xu and Huanchun Wang analyzed the data; Cewen Nan and Yuanhua Lin contributed reagents/materials/analysis tools; Haomin Xu and Ji Shi wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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