catalytic Activity of BâDoped Graphene Modified TiO2 Thin Films

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Effect of Metal Substrate on Photoelectrocatalytic Activity of B-Doped Graphene Modified TiO Thin Films: Role of Iron Oxide Nanoparticles at Grain Boundaries of TiO 2

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Andrés Fabián Gualdrón-Reyes, Angel M. Meléndez, Ignacio González, Luis Lartundo-Rojas, and Martha Eugenia Niño-Gómez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08059 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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The Journal of Physical Chemistry

Effect of Metal Substrate on Photoelectrocatalytic Activity of B-doped Graphene Modified TiO2 Thin Films: Role of Iron Oxide Nanoparticles at Grain Boundaries of TiO2 A. F. Gualdrón-Reyes, †,§ A. M. Meléndez,*† I. González,∥ L. Lartundo-Rojas‡ and M. E. NiñoGómez†,§

†

Centro de Investigación Científica y Tecnológica en Materiales y Nanociencias (CMN),

Universidad Industrial de Santander, Piedecuesta, Santander, Colombia. C.P. 681011. §

Centro de Investigaciones en Catálisis (CICAT), Universidad Industrial de Santander, Sede UIS

Guatiguará, Piedecuesta, Santander, Colombia. ∥

Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael

Atlixco 186, 09340 CDMX, Mexico. ‡

Centro de Nanociencias y Micro y Nanotecnologías, Instituto Politécnico Nacional, Zacatenco,

07738 CDMX, Mexico.

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

TiO2 thin films deposited on stainles steel (SS) substrates exhibit low photocatalytic (PC) activity when calcined above 350 °C. The cause is the presence of Fe3+ into the film due to its diffusion from SS to the surface during the calcination process. Over the past two decades, most researchers accepted idea that Fe3+ acts as recombination center of photogenerated electrons and holes, although the role of Fe3+ has not been studied. To understand the effect of Fe3+ on the PC and photoelectrocatalytic (PEC) activity of TiO2 films, boron-doped graphene-modified TiO2 (BTG) films supported on SS and Ti were prepared by sol-gel method. The surface of BTG films were characterized by XRD, FESEM, XPS, voltammetry and Mott-Schottky analysis. Photo(electro)chemical properties of BTG films were investigated by open-circuit potential, photovoltammetry and photocurrent transients, while their PC and PEC activities were evaluated using phenol degradation under ultraviolet irradiation. In addition to previous findings that shows Fe3+ ions exist as α-Fe2O3, surface-chemical composition of the calcined BTG films revealed the presence of α-FeOOH. Iron oxides facilitates carrier recombination by increasing the amount of grain boundaries in the BTG, which hindered electron mobility. Our findings invalid the hypothesis recombination center hypotheses, which remained for two decades.


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INTRODUCTION The nanostructured thin films of titanium dioxide constitute one of the most promising electrode materials for photovoltaic1,2 and photocatalytic3-5 applications. They are essentials in fields such as water purification, energy and fuel generation. Their use is still limited by fast charge-carrier recombination, low photon-to-current-conversion efficiency (IPCE), large band-gap energy (> 3 eV) and different interfacial charge-transfer rates.5 To overcome these limitations, nanocrystalline TiO2 electrodes are modified by adding other components to produce surface or bulk modification.3-7 Thus, for instance, photovoltaic and photocatalytic properties of TiO2 can be enhanced by incorporating reduced graphene oxide rGO sheets.7-10 The great mobility of charge carriers in rGO improves charge separation, facilitating transportation of electrons in the photoanode and hence their collection to generate photocurrent. On other hand, doping is an effective way of modifying the electronic properties of TiO2, enhancing the visible-light response and promoting the separation of charges.3-5 Boron is an interesting dopant of TiO2 because, besides showing PEC activity in the visible light region,3 it can improve its UV activity.11 To fabricate photoanodes based on TiO2 thin films, nanomaterials must be deposited or grown on conducting substrates, using either metal or conductive glass supports.1-5 Excluding TiO2 layers grown directly on a bulk titanium foil by selective etching and/or oxidation,1,3,5,12 a critical factor that impacts the fabrication, evaluation, and performance of photoanodes is the choice of an appropriate substrate. Several materials have been found as suitable to support photoactive TiO2 thin films.3,13,14 However, selection of an appropriate substrate can be challenging because not only manufacture or performance, but also economic criteria must be taken into account. Due to fundamental and practical


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differences15,16 between producing, storing energy and using it to solve environmental problems related to water pollution, the study presented herein focuses primarily on this last aspect. TiO2 has been coated on both ITO conductive glass and titanium substrates for their use in bench-scale photoelectrocatalytic reactors.12,17-19 TiO2 coated on the former substrate has shown a low photocurrent density and poorer PEC activity than titanium. This lowered activity appears to be caused by a comparatively lower electron diffusion through the bulk to the back-contact substrate of the TiO2.19,20 Electrochemical impedance spectroscopy measurements have shown that low photocurrent density in a WO3 photoanode deposited on FTO glass substrate, compared with anodized WO3 on a metal substrate, is due to high charge transport resistance of the FTO layer;21 which possibly can lead to a weak dependence of applied bias potential on the photocurrent or PEC degradation extent.19,22 Metallic substrates offer a number of advantages over conductive glasses: they can be machined into many different shapes, sizes, and have superior mechanical and electrical properties. Titanium is the supporting material most widely used in photoelectrocatalysis. Nevertheless, it is desirable to have photoanode substrates that besides presenting durability can be purchased at low cost. Stainless steel is a cheap and practical alternative, which has been used to prepare B-TiO2/rGO,9 WO3/TiO2,14 N,S-TiO2,23 La2O3/TiO2,24 Cu2+-TiO2,25 and N,F-TiO226,27 photoanodes. Although many studies in photocatalysis have focused on the use of SS substrates coated with TiO2,28-36 the corresponding literature in the field of photoelectrocatalysis is limited. Indeed, despite growing interest


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and efforts to gaining understanding in PEC processes, it has recently been pointed out that effect of substrate on the PEC response is scarcely addressed in the literature.3 Preparation of TiO2 nanoparticle films by sol−gel and some wet chemical coating technique, is the most common way to synthesize semiconductor thin film electrodes.3-5,14 This offers advantages over other immobilization methods. It can be a simple and costefficient coating option, for instance, in the handling of large complex shaped substrates and due to low annealing temperatures for post-coating treatment.37 It has been found that when a TiO2 thin film is supported on stainless steel (TiO2/SS), Fe3+ (and sometimes Cr3+) from passive layer can contaminate the TiO2 film by thermal diffusion upon annealing,28,29,31-33,36,37 together with the formation of Fe2O3,29,36 thereby decreasing its PC28,31,33 or PEC23,24,26 activity. To overcome this unintended consequence in the field of photocatalysis, annealing temperature is decreased,33 and the film is increased in thickness.31,33,35 Unfortunately, PEC activity decreases with increase of film thickness, because a larger electron path length through film hinders their transport.

3,19,22

Passivation of SS substrate prior to dip coating results in an increase of both photocurrent response and PEC activity of TiO2 thin films; although the improved performance obtained does not increase their activity sufficiently to select for this alternative.23 Luckily, introduction of rGO into TiO2 film enhances the electrode conductivity and accelerates the electron transport, thereby improving the PEC activity of rGO-modified TiO2 supported on SS.9 Various reports have indicated that Fe3+ (or more precisely Fe2O3) act as recombination centers in TiO2/SS, affecting the photocatalytic activity (PCA) of TiO2.28,32,33,36 No further evidence has been published. Hence, the impact of Fe3+ on the PCA of TiO2/SS is not yet fully understood and


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deserves more research. Our primary aim is to develop an improved understanding of the causes of deleterious effects of Fe3+ on the PC and PEC activities of TiO2 films, providing fundamental insight that can rationally guide the design of new modified TiO2 films with improved performance. Unintentional contamination of TiO2 films with foreign metals also occurs for other substrates, including different types of glass. Recently, role of Na+ on TiO2 photocatalyst has been attributed to particle size changes or poor crystallinity of anatase, leaving hypothesis behind of recombination center of charge carriers.38,39 This hypothesis was proposed in an attempt to explain deleterious effect of Na+ on the photocatalytic activity of deposited sol–gel thin films, as occurred coincidentally for Fe3+.28,38,39 Indeed, Na+ not form segregated oxides as Fe3+, thereby its effect must be different. Currently, modified TiO2 thin films are widely used as photoanodes to improve the photoelectrochemical properties of TiO2. Herein, BTG films were deposited on SS and Ti by sol−gel dip−coating as photocatalyst or electrode material model. The aim of this paper is threefold. Firstly, to study the effect of substrate material on the physical, chemical and physicochemical properties of modified TiO2 films. Secondly, investigate the correlation between photochemical, and photoelectrochemical measurements of both nanocomposite BTG films and their PC and PEC activity toward phenol oxidation. Thirdly, and most important of all, to propose how and in what form the combination of TiO2 and iron impact the performance of BTG/SS photoanode. Thus, the issue What is the role of Fe3+ in quenching the photo-oxidation of TiO2 thin films? will be answered. EXPERIMENTAL SECTION


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Graphene oxide (GO) was prepared by applying the Hummers method.40 Briefly, 1.0 g of graphite powder (99.99%) was firstly oxidized at 273 K by 0.5 g NaNO3 in 23 mL of 98 wt% H2SO4. Afterwards 3.0 g KMnO4 were slowly added under stirring (CAUTION: a lot of heat is released). The resulting mixture was heated at 308 K for 30 min, and further diluted with deionized water. Later, 12 mL H2O2 (30 wt%) was added slowly to the mixture. This suspension was filtered and GO was washed several times until obtaining neutral pH in the washed water. Exfoliation of GO was performed by using an ultrasonic cleaning bath. Reduced graphene oxide was prepared following the procedure reported elsewhere.8 Raman spectra of both GO and rGO were recorded using a Horiba Jobin Yvon HR320 micro-Raman spectrometer. The excitation source was 532 nm green laser (150 W), and an acquisition time of 50 s. One-pot sol−gel synthesis of B-doped rGO-modified TiO2 was prepared as follows. The as-prepared rGO was ultrasound-dispersed in 16.0 mL ethanol at 3.8 wt % of rGO. It was then mixed with 0.035 g boric acid (99.5 %) and 1.0 mL acetylacetone (99.9%) by maintaining adequate agitation. To this mixture, 3.5 mL of titanium n-butoxide was added dropwise under vigorous stirring for 1 h.

B-doped TiO2 (BT) and unmodified TiO2

nanosols were prepared in a similar manner but without addition of corresponding modifiers. SS 304 and Ti plates (20x40 mm2) were used as conducting supporting materials for depositing semiconducting films based on TiO2. The plates were polished by silicon carbide papers up to 600 (P1200) and ultrasonically cleaned with ethanol and ketone for 5 min in each one. The substrates were dip-coated with sol−gel solution at a withdrawal rate of 1 mms–1 and left to dry at room temperature. Afterwards, the coated substrates were heated at 373 K for 10 min in oven. This procedure was repeated three


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times more to obtain photoanodes with high photoresponse (selection was based on preliminary tests and previous experience).9 TiO2, BT and BTG films were calcined and annealed at 673 K in a furnace, at a heating rate of 3 °C min–1 and held this temperature for 90 min. Once removed from the furnace, the back and sides of films as well as part of the front side were covered with a nonconductive, chemically resistant enamel coating, leaving an uncovered area. Grazing incidence X-ray diffraction (GIXRD) measurements were performed with a Bruker D8 Discover (Cu Kα radiation, λ = 1.5406 Å), in the range of 15° to 70° (2θ). The step size was 0.015°, with step times of 1.9 s per step, and a grazing incidence angle of 2°. The morphology of films was characterized by field emission scanning electron microscopy (FESEM, Quanta 650 FEG). The instrument used for X-ray photoelectron spectroscopy (XPS) studies was a Thermo Scientific K-alpha with Al Kα radiation (1486.7 keV). The O 1s peak at 530.0 eV was used as a reference to correct the charge shift of the binding energies rather than C 1s. In systems where iron is a significant oxide component, charge correction using the O 1s peak is a more suitable choice.41 Narrow scans were collected with a spot size of 400 µm at 60 eV analyzer pass energy. Gaussian Lorentzian mix function and Shirley background subtraction were used to decompose the XPS Ti 2p, O 1s, C 1s and Fe 2p spectra. Relative sensitivity factors were used to scale the raw peak areas, and thereby to calculate the fraction of each species. High-resolution Fe 2p spectra of BTG films supported on SS were obtained with a SPECS Phoibos 150, using an X-ray energy of 1486.6 eV and step energy of 0.02 eV. Vacuum infiltration of paraffin wax (m.p. 328 K) into a spectroscopic grade AGKSP graphite rod was used to prepare a PIGE: paraffin impregnated graphite electrode.


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Detailed aspects on PIGE preparation and the characterization of microparticles can be referred elsewhere.42,43 Briefly, some milligrams of commercial samples of α-Fe2O3 or αFeOOH powders were immobilized on the PIGE surface (50 mm in diameter), by pressing and rubbing the electrode surface against a watch glass. Only the lower end of the electrode was dipped and it was put in contact with the electrolytic solution. Linear sweep voltammetry (LSV) measurements were recorded in the dark, immediately after immersing the PIGE into the solution using an Autolab PGSTAT 302N potentiostat. The electrolyte solution used in all electrochemical, photochemical and photoelectrochemical experiments was 0.1 M HClO4, unless otherwise stated herein. Oxygen-free solutions prepared with deionized water (18.2 MΩ cm) were used in all experiments by purging them with nitrogen gas.

Electrochemical measurements in the dark were done in a

conventional three-electrode glass cell. Either PIGE (0.196 cm2 geometrical surface area) or thin films based on TiO2 (leaving an exposed geometric area of 0.765 cm2), served as the working electrode. A graphite rod grade AGKSP served as the auxiliary electrode and a Ag/AgCl 3 M KCl (+210 mV vs SHE) was used as reference. All the potentials were referred to the saturated calomel electrode (SCE = +241 mV vs SHE), whereas current densities were based on the apparent geometric area of the electrodes. Capacitance measurements for Mott-Schottky plots were obtained using a PAR 283 potentiostat coupled to a Solartron 1260 frequency response analyzer. AC perturbation was 10 mV at a frequency of 0.7 kHz and scan rate of 10 mV s–1. All photo(electro)chemical measurements were performed with an electrochemical setup consisting of Epsilon BAS potentiostat. Three-electrode photoelectrochemical cell was equipped with a flat quartz window (Scheme 1). A 100 W Xe lamp (Newport) was


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utilized as light source, and kept at a distance of 27 cm from the films. BTG/SS and BTG/Ti were contacted by a Cu plate and then pressed against an O-ring of the electrochemical cell, leaving 0.765 cm2 exposed to the electrolyte. A spectroscopic grade AGKSP graphite rod served as the auxiliary electrode and a SCE was used as reference. Current densities were normalized according to the illuminated area of 0.765 cm2. For each open circuit potential measurement, light was turned on for first time when the potential reached a stable value under dark. LSV curves were recorded in positive direction from OCP. Photocurrent transient measurements were performed by holding the potential at 0.85 VSCE and turning the incident light on and off consecutively. All electrochemical, photochemical and photoelectrochemical experiments were done in duplicate. Scheme 1 shows the homemade undivided three-electrode cell used to determine the rate of phenol degradation. It is equipped with quartz window in front of BTG film (photocatalyst or photoanode) to allow the illumination of film exposed to the electrolyte. The cell was filled with 40 mL of 1 mM phenol in 0.1 M HClO4 (pH 1.0). Recirculation of the electrolyte solution (900 µLmin−1) was provided by a peristaltic pump through a fiber optic spectrometer (Ocean Optics) coupled to a UV-Vis-NIR light source. Both electrochemical and photoelectrochemical experiments were performed holding each BTG photoanode at a fixed bias potential and illumination by using a 100 W Xe lamp. All the experiments were performed at room temperature.

RESULTS AND DISCUSSION


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Structural properties of BTG films. Figure 1a shows Raman spectra of GO and rGO. The Dand G-band positions of GO appear at 1345 cm–1 and 1591 cm–1, respectively. After GO reduction, a red-shift of the G phonon frequency to 1574 cm–1 occurs due to an increase of free electrons near the Fermi level, by restoration of sp2 (C=C) bonds.44,45 In order to improve the photoactivity of the semiconductor films, rGO was dispersed in the B-TiO2 sol−gel precursor solution, and both titanium and stainless-steel substrates were coated. Figure 1b shows GIXRD profiles of BTG films calcined at 400°C. Both GIXRD profiles displays the characteristic reflection (101) of TiO2 (PDF-2-1272), indicating that anatase phase was formed. Bragg peak position of anatase appears at 25.10° and 25.23° for BTG films deposited on SS and Ti substrates, respectively. A shift of the anatase peak to higher angle is commonly ascribed to the presence of a high concentration of oxygen vacancies (see below).46 Diffraction peaks of anatase are broad and the peak heights are much weaker than those of substrates (enlargement of Figure 1b). X-ray diffraction peak broadening is commonly observed in films consisting of nanoparticles. Indeed, sol−gel process leads to formation of ultrafine particles. Nevertheless, it has been previously found that TiO2 sol−gel films coated on SS (calcined at 673 K) have a crystallinity about 70%, which has been attributed to metal species released from SS.36 Since the crystalline fraction reported in the literature for TiO2/SS is predominant, Scherrer equation was used to have a rough approximation of particle size.47 The average crystallite size of anatase on SS was 16.7 nm while for Ti 10.7 nm. These results show that substrate type influences the crystalline growth of B-TiO2. BTG film deposited on SS (Figure 1c) shows much less and narrower cracks than that deposited-on Ti (Figure 1d). Cracks result from stress gradients developed during the drying


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stage, since evaporation of solvent leads to shrinkage of gel network driven by capillary force.37 Thus, film with greater thickness must promotes an increment in capillary pressure. To validate this interpretation, FESEM measurements of BTG coating thickness were obtained. Thickness of films deposited on SS and Ti substrates were around 160 nm and 350 nm, respectively. These measurements confirm that cracks were much more likely to develop in the thick film supported on Ti. Photoelectrochemical characterization of BTG films. Open-circuit photopotential (Eoc) measurements can provide information about BTG-mediated photocatalytic redox reactions at interface. Figure 2a shows the typical open-circuit photoresponse of a n-type semiconductor for the BTG films. Increase of photopotential between Eoc in the dark and under illumination for BTG/SS in HClO4 is rather low and negligible (~0.03 V) compared to BTG/Ti (~0.40 V), indicating a low PCA. It suggests that charge recombination must be much faster than hole transfer for BTG/SS than for BTG/Ti, which prevents the photoinduced electron accumulation in the TiO2 conduction band. In electrochemically assisted photocatalytic process is important to evaluate the ability of photoelectrode to generate photocurrent at a given potential. Thus, a global view of the electrode materials properties on PEC oxidation of phenol was measured by LSV under dark and UV light irradiation (Figure 2b). Dark current is negligible in all cases. Photocurrent of BTG/Ti at 0.85 VSCE is 2.3 times higher than that produced by BTG/SS photoanode. BTG film supported on Ti is twice as thick as the BTG film supported on SS, which can increase both the light absorption capacity of the former as well its amount of charge carriers. However, a thicker sol−gel film leads to a greater likelihood of recombination due to tortuous pathway for electron transport through the particle network (resistivity arising


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from electron scattering at grain boundaries).19 It has been seen that as the thickness of a TiO2 sol–gel film increases above 100 nm resistance also increases.48 Consequently, limited photocurrent for the BTG/SS photoanode cannot be explained by considering the thickness of films. Marked difference in photocurrent response shown by these electrodes is mainly related to impurities of BTG film from stainless steel substrate (see below). In spite of the low PEC activity of BTG/SS, incorporation of rGO in B-TiO2 allows the generation of a higher photocurrent than in TiO2 and BT films.9 The photoresponse ability and stability of the BTG films were investigated by applying a repeated oxidation-reduction cycling under UV irradiation, as indicated below. In a solution containing 10–3 M phenol and 0.1 M HClO4, 10 roundtrip potential cycles were performed between -0.1 and 1.5 VSCE at 10 mVs–1. A brownish product was visible after the durability test of BTG/SS electrode. For this reason, once the experiment was completed each electrode was rinsed with deionized water, dried with nitrogen gas and transferred

back

to

the

electrochemical

cell.

Afterwards,

potential

step

chronoamperometric measurements were performed under interrupted illumination at 0.85 VSCE, as shown in Figure 2c. Initially, when the light was turned on, photocurrent response of BTG/Ti displays a spike associated to surface electron–hole recombination. Former studies have found out that during initial spike formation in photocurrent transients of phenol oxidation, polymerization of phenoxy radicals results in surface fouling of electrode.49 After apparition of first spike, photocurrent gradually and slightly decrease in magnitude with each switching on the light. This indicates the passivation of BTG/SS surface by phenol polymerization after cycling durability test, which causes deactivation of electrode and suppressing the photocurrent. The fouling of BTG film by


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phenoxy radical polymerization, the chromium oxidation from SS in the transpassivation region at E > 1.0 VSCE due to electrolyte solution penetration into the BTG/substrate interface,13 or both, are the probable phenomena associated to the loss of PEC activity. The former explanation reinforces the idea that BTG/SS has a high recombination rate of charge carriers compared to BTG/Ti. Surface characterization. Electrochemical surface analysis, specially voltammetry is a sensitive technique for detection of iron species.50,51 Solubility of iron(III) oxides in aqueous media is low, because solvation energy is not high enough to break the binding interactions between Fe3+ and O2– in the solid state. However, dissolution of iron(III) oxides is enhanced in acid medium and in a reductive condition. Hence, voltammetric scans were initiated in the negative-going direction. Typical profile of TiO2 is obtained for BTG/Ti electrode (Figure 3), which is attributed to the filling of electronic states below of conduction band.5 BTG/SS electrode exhibits a different behavior, since at least three overlapping peaks are observed. Aforementioned features have also been observed in voltammograms for TiO2 films on SS 304 substrates but the authors did not recognize their significance.52 Peak positions were resolved by the second derivative of the voltammetric curve, as Figure 3 shows. There are three waves P1, P2 and P3 centered at potentials of -0.034, -0.202 and -0.461 VSCE, respectively. Iron oxides contamination in TiO2 sol-gel films on SS has been reported in the literature. To identify the iron oxides in the BTG film, aforementioned peak potentials were compared to those of synthetic samples. Thus, peaks P1 and P2 are ascribed to α-FeOOH (reaction (1)), whereas P3 is associated to α-Fe2O3 (reaction (2)).43,53


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+ 3 + → 1 − + + 2

(1)

1⁄2 + 3 + → 1 − + + 3⁄2

2

Hematite (α-Fe2O3) contamination in TiO2 sol−gel films on SS has been reported in the literature,29,32,36 but not goethite (α-FeOOH). Goethite dehydration lead to formation of hematite at temperatures below than 593 K (reaction (3)).54 Considering that BTG films were annealed at 673 K, it seems thermodynamically improbable the presence of goethite in the BTG film. However, computational simulations have shown that goethite particles smaller than 10 nm are thermodynamically stable at temperatures below about 770 K.55 Impurities tend to segregate at grain boundaries therefore nanosized goethite particles should be located between TiO2 grains. It can be possible because when TiO2 nanoparticles are prepared by sol−gel method, and modified with iron by adding solutions containing Fe3+, ultrafine hematite nanoparticles are homogeneously segregated at the grain boundaries of TiO2.56 − → 1⁄2 − + 1⁄2

3

Having proved the chemical nature of iron species in the BTG film supported on SS, its chemical composition and the state of each element were analyzed by XPS. Elemental survey (Figure 4a) confirmed the presence of B, C, O and Ti in the BTG films. Fe only was detected in the BTG/SS, which is consistent with the results of the electrochemical measurements. High-resolution C 1s XPS spectra are shown in Figure 4b. C 1s spectra were deconvoluted into four peaks at (284.1, 284.7C, 286.4 and 288.5) eV. The first peak is attributed to the C=C bonding (sp2 carbon) in rGO sheets and the others mainly to adventitious carbon contamination.57 B 1s peaks at 191.1 eV (BTG/SS) and 191.3 eV (BTG/Ti) are ascribed to interstitial doping (Figure 4c).11,58 Ti substrate induced an


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additional boron modification to its film coating. Peaks at 188.8 eV and 190.5 eV indicates the presence of substitutional doped boron atoms.57 The ratio between substitutional boron dopant (O–Ti–B and B–Ti–B) and interstitial boron dopant (Ti–O–B) is 1.7 for BTG/Ti, being the quantity of substitutional B higher than that of interstitial B. Considering that BTG films supported on Ti and SS contains 0.64% and 0.27% at. of B into the TiO2 lattice, respectively, it can be affirmed that the former contains more oxygen vacancies;11 this confirms the results obtained by XRD analysis. The high-resolution Fe 2p and O 1s spectra and their deconvolution for BTG/SS film before and after annealing are exhibit in Figures 4d and 4e, respectively. It is well known that some Fe 2p3/2 contributions of Fe(III) species can overlap with Fe 2p1/2 at binding energies (BE) higher than 720 eV, which increase the error during curve fitting.39,47 Thus, the contributions of Fe 2p3/2 are only used to analyze iron speciation. The assignments of peak and BE are given in Table 2. Pristine film (before heating) showed four peaks at (705.9, 709.4, 711.6 and 718.0) eV BE, corresponding strikingly to Fe metal, α-Fe2O3, α-FeOOH and Fe3+ satellite, respectively. This result shows that iron contamination begins during the film deposition on stainless steel, which implies that the sol solution is corrosive enough to dissolve iron species from substrate.59 Iron oxide hydroxides in their alpha form are generated during gelation and drying. After annealing, α-Fe2O3 and αFeOOH are retained and the peak associated to Fe metal disappears because it is oxidized by heating. This is confirmed by the increase of α-Fe2O3 (Table 1). A peak of wustite (FeO) appears as result of Fe(III) reduction during the sputtering.47 The identification of goethite after annealing is confirmed by the Fe-OH lattice (531.1 eV) and adsorbed Fe–OH (531.9 eV) contributions in the high-resolution O 1s


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spectrum.39,48 These results corroborate the identification of iron oxides found by voltammetry. From the global atomic composition percentage of the elements in the BTG/SS films, it is noteworthy that iron content increased at the surface of BTG film from 0.56 % at. (pristine) to 3.0 % at. after annealing, which indicates that the thermal treatment causes the iron diffusion from SS to film surface. These findings are in agreement with literature data,29 which have demonstrated that the amount of Fe at the surface layer of a TiO2 sol−gel film increases with annealing temperature. Figure 4f shows the curve-fitted high-resolution Ti 2p XPS spectra of BTG films. Inclusion of Ti4+, Ti3+ and Ti2+ peaks provides a reasonable fit to the Ti 2p3/2 and Ti 2p1/2 spectra. BE determined in the current study agree well with those values previously reported.44,60 Peak binding energy maxima of TiO2, BT and BTG films are presented for comparative purposes in Table 2. After boron modification, Ti 2p doublets of Ti3+ species for films supported on SS were shifted to higher BE values, due to increase of polarity in Ti–O bond by boron. This confirms the incorporation of B atoms in the TiO2 lattice, which increase the Ti3+/Ti4+ ratio (Table 3).11 Conversely, a shift to lower-BE was evidenced for Ti4+ species for films supported on Ti after boron modification, which proves the presence of substitutional boron in the TiO2 lattice. Ti3+ content for films supported on Ti was approximately the same before and after boron modification, which suggests that generation of Ti3+ is promoted by thermal treatment rather than boron doping. It has been reported that Ti4+ is reduced to Ti3+ during annealing of TiO2 powders prepared by sol–gel method, through elimination of acetylacetone, which serves as reducing agent.60 Accordingly, addition of acetylacetone chelating agent to the sol solution here used, besides to decrease the rate of hydrolysis and polycondensation,


17


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causes the reduction of Ti4+. Large proportion of Ti3+/Ti4+ in BTG/Ti indicates a higher concentration of oxygen vacancies. It is noteworthy that the reducing properties of acetylacetone are marginal in BTG/SS, which can be ascribed to the presence of iron species in TiO2 structure. The formation of Ti2+ in films can be caused by sputter reduction of TiO2 to a titanium lower oxide such as TiO.61 Mott–Schottky study of BTG films. The values of charge carrier (donor) density ND of BTG/SS and BTG/Ti films photoanodes were determined from Mott-Schottky M-S analysis in the dark by equation (4): =

! "#

$% − %&' −

() *

+

,

where C and A are the interfacial capacitance and geometric area of the semiconductor photoanode, e is the elementary charge, ε0 and ε are the vacuum and TiO2 permittivity (ε = 50, anatase), E is the applied potential, Efb is the flat band potential, T is the temperature and kB is the Boltzmann constant. A positive linear slope was observed in the plots of 1/C2sc versus E, indicating the typical n-type semiconductivity of TiO2. The slope of this line gives the ND values for semiconductor photoanodes. As the slope becomes less steep the greater is the ND value. As Figure 5 shows the slope of BTG/Ti is lower than that of BTG/SS, indication of a higher ND in the latter photoanode (Table 4). The relative high ND in BTG/Ti is a consequence of a large amount of Ti3+, which shifts the Efb to more negative potentials (or less positive values) compared with BTG/SS. This indicates a displacement of Fermi level to the lower edge of the conduction band of TiO2.62 The higher charge carrier density for BTG/Ti indicates a comparatively high electronic conductivity which facilitates charge transfer. This explains the great


18

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photocurrent generated by BTG/Ti photoanode (Figure 2b), which is intimately related with oxygen vacancies because they are electron donors in TiO2. A characteristic feature of M-S plot for films supported on SS is the point where the slope changes drastically around 0.5 VSCE (Figure 5). This feature is sometimes referred to as the knee point. It has been attributed to surface segregation of foreign elements in the form of oxides on the surface of another oxide.63 Similarly, it can be related to iron oxide segregation at grain boundaries of TiO2 in BTG/SS here studied. The changing slope of the M-S plot has been also associated to Helmholtz layer capacitance in passive films on Cr and SS.64 In this alternate interpretation, iron oxides segregated in TiO2 grain boundaries could induce a significant amount of surface states, leading to that potential drops mainly in the double layer instead of space-charge region. Photocatalytic and photoelectrocatalytic activity of BTG films. The photoactivity of BTG thin films in the electrochemically assisted photocatalytic oxidation of phenol was evaluated in comparison with photochemical, photocatalytic and electrochemical process. Changes in relative absorbance A/A0 of phenol, where A0 and A are the absorbances before and after irradiation for t min, at fixed time intervals of 30 min are shown in Figure 6. Electrolysis with an application of 0.85 VSCE in the dark using the BTG/SS (Figure 6a) and BTG/Ti films (Figure 6a’) cause a negligible decrease in absorbance. Photolysis of phenol (Figure 6b) causes a slight decrease in the absorbance around 12 %, indicating that organic compound is unstable under UV irradiation. BTG film deposited on SS shows very low activity for the photocatalytic degradation (Figure 6c) of phenol; besides its PCA is lower than that deposited on Ti, which is in good accordance with OCP measurements. Indeed, PC phenol degradation on BTG/Ti (Figure 6c’) is greater than in both PC and PEC processes on BTG/SS (Figure 6d), thereby PEC efficiency of BTG/Ti


19


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photoanode is appreciably greater than in all others oxidation processes (Figure 6d’). The PC and PEC phenol degradation percentage were around 16 % and 18 % for BTG/SS, while for the BTG/Ti were around 20 % and 27 %, respectively. PEC process facilitates the charge carrier separation, which enhances the oxidizing power of BTG/Ti. On several occasions, it has been suggested that, upon calcination of TiO2 coated on stainless steel, there is a loss of PCA due to Fe3+ ions, which diffuse from the substrate, act as recombination centers in TiO2 thin film photocatalyst.28,32,33 Whereas, in the field of photoelectrocatalysis, the low IPCE of a electrosynthesized TiO2/SS photoanode it has been ascribed to passive layer that forms on the surface of stainless steel during annealing, which could increase the ohmic loss.52 More recently, the low photocurrent in the same type of electrodes was also attributed to iron contamination of film, but it was not clarified the role of iron on PEC activity.24 Here, it was found that unwanted introducing of goethite and hematite in BTG film produces secondary grain boundaries. Electron scattering at grain boundaries contribute to increase the resistivity of semiconductor film, affecting strongly electron diffusion inside the film.65 The more grain boundaries that exist, the lower the separation of photogenerated electron–hole pairs, hence PEC activity is decreased. This is in good accordance with the linear voltammetry measurements for the films, shown in Figure 2b. Recent studies have concerned how to contribute to understand the effect of TiO2 modification with iron oxides, taking advantage of the benefits of sol–gel process.53,66 Thus, it has been found that photoelectrochemical activity in α-Fe2O3-TiO2 nanoparticle heterostructures is poor due to large amount of grain boundary defects,66 which reinforced the conclusions reached in this study. In addition, it has been suggested that the


20

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comparatively low dielectric constant of α-Fe2O3 segregated on Fe-TiO2 nanoparticles could hamper the separation of charge carriers at grain boundaries.56 The BTG/SS film showed an iron content around 3 %at, which allows pointing out that the low PCA and PEC activity are related to an increased likelihood of recombination phenomena by the numerous grain boundaries in the film. CONCLUSIONS The sol−gel dip-coating method was used to synthesize TiO2 thin films modified with rGO and boron. BTG films were prepared using two types of substrates, stainless steel and titanium. Comparative studies were carried out to investigate the effect of substrate on the BTG properties, and showed the PC and PEC activities of both BTG/SS and BTG/Ti photocatalyst and photoanodes are impacted. Substrate materials greatly affects the physical and chemical characteristics of the BTG films supported on them, including thickness, crystallite size, microstructure, chemical composition, oxygen vacancies, and electronic state of titanium and boron surface atoms. An important factor that determines both PC and PEC activities of BTG thin films is the type of the substrate. The results show grain-boundary nano-segregation of α-FeOOH and α-Fe2O3 in BTG/SS. These structural defects have a deleterious effect on PC and PEC activities of both photocatalyst and photoanode, being more pronounced the loss of photoactivity in the former. This shows that the supply of an external potential diminishes recombination rates. Grain boundaries acts as recombination sites quenching the photo-oxidation, hence, recombination rate is faster than hole transfer in BTG/SS. Thereby these results suggest that structural disorder is the dominant cause of the low PC and PEC activity in TiO2/SS.


21


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Hence, recombination hypothesis used to explain the low activity of TiO2 films supported on stainless steel is discarded. Larger Ti3+/Ti4+ ratio lead to improve the charge carrier separation in BTG/Ti, and hence a better charge carrier transport and PEC activity are achieved. Therefore, at synthesis level it is desirable control the number of oxygen vacancies. PC and PEC degradation of phenol show a good qualitative correlation with the open circuit potential and photocurrent measurements, respectively. In order to consider the SS as conducting substrate in photoelectrocatalysis, efforts should be focused in mitigate the iron diffusion from stainless steel to TiO2 coating in TiO2/SS photoanodes. On other hand, a lack of standardization in the measurements of photocurrent has made it difficult to compare the performance of photocatalysts or photoanodes under identical conditions. This prevent identify the most promising material for design of new materials. Thus, electrochemical measurements carried out in both 0.1 M HClO4, and 10–3 M phenol in 0.1 M HClO4 could be part of a standardized protocol to compare the stability and performance of modified TiO2 thin films. AUTHOR INFORMATION Corresponding Author *† Centro de Investigación Científica y Tecnológica en Materiales y Nanociencias (CMN), Universidad Industrial de Santander, Piedecuesta, Santander, Colombia. C.P. 681011, Phone: +57-7-634-4000. E-Mail: [email protected], [email protected] Notes


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by Colciencias, project 1102-658-43664 (VIE code 8836).

The

authors

acknowledge

to

Edgar

Carrera

for

the

support

in

the

photoelectrochemical experiments. AFGR was supported through a Colciencias PhD scholarship. REFERENCES (1)

Bai, Y.; Mora-Seró, I.; De Angelis, F.; Bisquert, J.; Wang, P. Titanium dioxide

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materials characterized by high Ti3+ self-doping and low recombination rate of electron– hole pairs. RSC Adv. 2017, 7, 2373-2381. (61) Biesinger, M. C.; Lau, L. W. M.; A. Gerson, R.; Smart, R. St.C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887-898. (62) Kang, Q.; Cao, J.; Zhang, Y.; Liu, L.; Xu, H.; Ye, J. Reduced TiO2 nanotube arrays for photoelectrochemical water splitting. J. Mater. Chem. A 2013, 1, 5766-5774. (63) Luo, W.; Wang, J.; Zhao, X.; Zhao, Z.; Li, Z.; Zou, Z. Formation energy and photoelectrochemical properties of BiVO4 after doping at Bi3+ or V5+ sites with higher valence metal ions. Phys. Chem. Chem. Phys. 2013, 15, 1006-1013. (64) Ren, Y.; Zhou, G.; Rediscovering Mott-Schottky plots: A knee-point in the plot for passive films on chromium. J. Electrochem. Soc. 2017, 164, C182-C187. (65) Villanueva-Cab, J.; Jang, S.-R.; Halverson, A. F.; Zhu, K.; Frank, A. J. Trap-free transport in ordered and disordered TiO2 nanostructures. Nano Lett. 2014, 14, 2305-2309. (66) Petit, S.; S. Melissen, T. A. G.; Duclaux, L.; Sougrati, M. T.; Le Bahers, T.; Sautet, P.; Dambournet, D.; Borkiewicz, O. J.; Laberty-Robert, C.; Durupthy, O. How should iron and titanium be combined in oxides to improve photoelectrochemical properties? J. Phys. Chem. C 2016, 120, 24521-24532.


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CAPTIONS TO TABLES

Table 1. Assignment of Fe2p XPS binding energies and percentages of iron species in BTG/SS before (pristine) and after annealing. Table 2. Spectral fitting parameters for Ti 2p species of BTG/SS and BTG/Ti films. Table 3. Fraction of Ti species in BTG films from XPS Ti 2p. Table 4. Semiconductor materials parameters obtained from M-S plot for BTG thin films supported on stainless steel and titanium substrates.


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TABLES Table 1 BTG/SS pristine BTG/SS annealed BE / eV BE / eV Percent Percent Fe 2p3/2 Fe 2p1/2 Fe 2p3/2 Fe 2p1/2 Fe(0) 705.9 719.8 8.63 FeO 708.9 722.1 6.39 α-Fe2O3 709.4 723.0 23.69 710.0 723.5 29.51 α-FeOOH 711.6 726.0 28.77 712.1 725.7 24.37 Fe3+ satellite 718.0 731.0 38.91 718.0 730.3 39.74 Species

Literature ref. BE /eV Fe 2p3/2 706.648 708.448 709.848, 710.849 711.348, 711.249 719.249, 732.849


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Table 2 BTG/SS

BTG/Ti Binding energy / eV Films Ti2+ Ti3+ Ti4+ Ti2+ Ti3+ Ti4+ 2p3/2 2p1/2 2p3/2 2p1/2 2p3/2 2p1/2 2p3/2 2p1/2 2p3/2 2p1/2 2p3/2 2p1/2 TiO2 456.0 461.3 456.8 462.5 458.0 463.7 455.6 461.3 457.1 462.9 458.6 464.2 BT 455.8 461.4 457.1 462.9 458.1 463.8 455.6 461.3 457.2 462.9 458.2 463.8 BTG 455.8 461.4 457.1 462.9 458.1 463.8 455.6 461.3 457.2 462.9 458.2 463.8


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Table 3

Films

BTG/SS Ti2+

Ti3+

BTG/Ti Ti4+

Ti2+

Ti3+

Ti4+

TiO2

0.17 0.03 0.80 0.29 0.63 0.08

BT

0.17 0.21 0.62 0.30 0.64 0.06

BTG

0.17 0.21 0.62 0.30 0.64 0.06


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Table 4 Films BTG/SS BTG/Ti

Nd 1019 (cm-3) 1.51 13.8

Efb (V vs SCE) 0.05 -0.03


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CAPTIONS TO SCHEMES

Scheme 1. Schematic representation of photoelectrochemical flow cell coupled with spectrophotometric optical fiber detection.


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CAPTIONS TO FIGURES Figure 1. (a) Raman spectra of pristine GO and rGO. (b) GIXRD profiles and (c,d) FESEM images of BTG films supported on SS and Ti substrates. Figure 2. (a) Open-circuit photopotential and (b) linear sweep voltammetry (v = 10 mVs-1) measurements for BTG films deposited on stainless steel (BTG/SS) and titanium (BTG/Ti) under both dark and irradiated conditions. (c) Chopped light transient photocurrent measurements for BTG/SS and BTG/Ti films at 0.85 VSCE obtained after oxidation-reduction cycling. Electrolyte solutions: (a) 0.1 M HClO4 and (b,c) 10–3 M phenol in 0.1 M HClO4; illumination: 100 W Xe lamp. Figure 3. Voltammograms (v = 10 mVs–1) in 0.1 M HClO4 for BTG films on SS and Ti, (BTG/SS and BTG/Ti); and iron oxide synthetic samples mechanically attached to a PIGE. SS´ label represents the second derivative of BTG/SS voltammetric curve. Figure 4 (a) Wide scan XPS spectra and high resolution deconvoluted XPS spectra of (b) C 1s, (c) B 1s and (f) Ti 2p spectra for BTG films deposited on SS and Ti, as indicated in the figure legends. (d) Resolved XPS Fe 2p3/2 and (e) O 1s spectra for BTG/SS (B) before and (A) after annealing process. Figure 5. Mott-Schottky plots for BTG films deposited on SS and Ti, as indicated in the figure legends. The measurement frequency is 700 Hz. Figure 6. Comparison of the electrochemical (a,a’), photolytic (b), photocatalytic (c,c’) and photoelectrocatalytic (d,d’) phenol degradation solution by BTG films deposited on stainless steel (dashed lines) and titanium (solid lines). No BTG thin films were inserted in the cell for photochemical study. Aqueous solution: 10-3 M phenol + 0.1 M HClO4 (pH 1.0); illumination: 100 W Xe lamp.


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Scheme 1. Schematic representation of photoelectrochemical flow cell coupled with spectrophotometric optical fibre detection. 255x198mm (96 x 96 DPI)



Fig. 1 (a) Raman spectra of pristine GO and rGO. (b) GIXRD profiles and (c,d) FESEM images of BTG films supported on SS and Ti substrates. 226x184mm (150 x 150 DPI)


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Fig. 2 (a) Open-circuit photopotential and (b) linear sweep voltammetry (v = 10 mVs–1) measurements for BTG films deposited on stainless steel (BTG/SS) and titanium (BTG/Ti) under both dark and irradiated conditions. (c) Chopped light transient photocurrent measurements for BTG/SS and BTG/Ti films at 0.85 VSCE obtained after oxidation-reduction cycling. Electrolyte solutions: (a) 0.1 M HClO4 and (b,c) 10–3 M phenol in 0.1 M HClO4; illumination: 100 W Xe lamp. 331x106mm (150 x 150 DPI)



Fig. 3. Voltammograms (v = 10 mVs–1) in 0.1 M HClO4 for BTG films on SS and Ti, (BTG/SS and BTG/Ti); and iron oxide synthetic samples mechanically attached to a PIGE. SS´ label represents the second derivative of BTG/SS voltammetric curve. 217x185mm (150 x 150 DPI)


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Fig. 4 (a) Wide scan XPS spectra and high resolution deconvoluted XPS spectra of (b) C 1s, (c) B 1s and (f) Ti 2p spectra for BTG films deposited on SS and Ti, as indicated in the figure legends. (d) Resolved XPS Fe 2p3/2 and (e) O 1s spectra for BTG/SS (B) before and (A) after annealing process. 313x182mm (150 x 150 DPI)



Fig.5. Mott-Schottky plots for BTG films deposited on SS and Ti, as indicated in the figure legends. The measurement frequency is 700 Hz. 204x165mm (150 x 150 DPI)


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Fig. 6 Comparison of the electrochemical (a,a’), photolytic (b), photocatalytic (c,c’) and photoelectrocatalytic (d,d’) phenol degradation solution by BTG films deposited on stainless steel (dashed lines) and titanium (solid lines). No BTG thin films were inserted in the cell for photochemical study. Aqueous solution: 10-3 M phenol + 0.1 M HClO4 (pH 1.0); illumination: 100 W Xe lamp. 168x152mm (150 x 150 DPI)



354x188mm (150 x 150 DPI)


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