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Visible-light activation of low-cost rutile TiO2 photoanodes for photoelectrochemical water splitting
Solar Energy Materials & Solar Cells 208 (2020) 110424
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Visible-light activation of low-cost rutile TiO2 photoanodes for photoelectrochemical water splitting Piotr J. Barczuk a, Krzysztof R. Noworyta b, Miroslaw Dolata c, Katarzyna Jakubow-Piotrowska a, Jan Augustynski a, * a b c
Centre of New Technologies, University of Warsaw, S. Banacha 2c, 02-097, Warsaw, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland Department of Physics and Biophysics, Institute of Biology, Warsaw University of Life Sciences, Nowoursynowska 159, 02-776, Warsaw, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Defective TiO2 rutile Visible light responsive TiO2 Photo-oxidation of water
Structure and photoelectrochemical properties of rutile-based TiO2 films formed by high-temperature oxidation of titanium metal are described. It is shown that a chemical etching treatment leads to the activation of initially almost inert film allowing its operation as visible light responsive water photo-oxidation photoanode. Additional use of a polyoxometalate electrocatalyst in the supporting electrolyte results in a 0.9 mA cm-2 photocurrent under simulated 1 sun irradiation at 1.23 V vs. RHE.
1. Introduction The broad interest in electrochemical properties of titanium oxide, TiO2, goes back few years before its first utilization as an oxygenevolving photoanode in photoelectrochemical (PEC) water splitting cell [1,2]. It is to be related to the discovery of excellent electronic conductivity and electro-catalytic activity of TiO2 - RuO2 mixed oxides that lead to the development of Dimensionally Stable Anodes (DSA) that revolutionized chlorine-alkali electrolysis industry [3–6]. The key feature of the DSA electrodes is the utilization of titanium metal sub strate that does not undergo corrosion in acidic media in the presence of chlorine. Interestingly, the discovery of conductive mixed oxide film electrodes was all but immediate and had resulted from the use of thermal decomposition method to deposit noble metals on the titanium metal electrode substrates [4,7]. The temperatures involved in the decomposition of noble metal precursors lead actually to the formation of mixed oxides of rutile structure due to the, occurring simultaneously, thermal oxidation of the titanium substrate. The TiO2 has an important stabilizing effect versus noble metal oxide components of the DSA electrode - RuO2 and IrO2 [4,6,7]. The process similar to that occurring during fabrication of mixed conductive oxides on titanium substrates was subsequently used to form semiconducting TiO2 films of rutile structure by oxidation of the Ti metal at around 600 � C [8]. It is also to be noted that, when conducted under neutral atmosphere (e.g. in argon), the high (550–600 � C)
temperature annealing allows marked activation of the TiO2 films deposited on titanium by the sol-gel method [9,10]. In such cases, the diffusion of titanium ions from the metallic substrate leads to an important increase in conductivity - concentration of the majority charge carriers - in the TiO2 films. Although such thin layer TiO2 films demonstrated in some cases [10] very high water photo-oxidation cur rents, their photoactivity remained restricted to near UV light with the band edges at 400 and 420 nm, corresponding to the band gaps of anatase (3.2 eV) and rutile (3.0 eV) polymorphic forms of TiO2, respectively. Early efforts to extend the photoresponse of TiO2 from the UV into the visible light spectral region involved mainly metal doping (with the exception of the sensitization with dyes). These works were reviewed, among others, in Refs. [11,12]. Although incorporation of certain transition metal cations, such as Cr3þ or Ni2þ, into the TiO2 lattice lead to some sub-band gap photoresponse, it induced, at the same time, strongly enhanced electron–hole recombination over the whole ab sorption spectrum [13]. This precluded such doped TiO2 photo electrodes, either anatase or rutile, from the application in sunlight-driven water splitting devices. In the same way, the Cr3þ doping of nanocrystalline powder titania was found detrimental to its photocatalytic properties, strongly decreasing quantum efficiency of the photoreactions [14]. Subsequently, large interest was directed towards the possibility of modifying the photoresponse of TiO2 through doping with non-metallic
* Corresponding author. E-mail address: [email protected] (J. Augustynski). https://doi.org/10.1016/j.solmat.2020.110424 Received 6 November 2019; Received in revised form 15 January 2020; Accepted 16 January 2020 Available online 27 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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elements, in particular with nitrogen, carbon and sulfur. The input was provided by the article published by Asahi et al. [15] who evaluated, on the basis of density-of-states, DOS, calculations, the consequences of substitutional doping of anatase crystals with N, C, S, P and F atoms. The authors concluded that nitrogen doping leads to the “band-gap nar rowing” of TiO2 through mixing the N 2p states with the O 2p states. The reported optical absorption spectra for films prepared by TiO2 sputtering and high temperature annealing in N2 atmosphere, extending above 500 nm [15], were assigned to the formation of nitrogen-induced states lying above the upper valence band (VB) edge of TiO2. Although this assumption could be confirmed by the results of photocatalytic experi ments involving photodegradation of organic pollutants in the presence of oxygen [15], attempts to split water using nitrogen-doped TiO2 film photoanodes of rutile structure exposed to simulated sunlight (100 mW cm-2) illumination yielded only low photocurrents of about 0.2 mA cm-2 [16]. Such poor PEC performance of the nitrogen-doped TiO2 photo anodes was explained by the high density of electron trap states distributed below the conduction band (CB) and to the low mobility of holes generated in localized N 2p electronic states located above the O 2p VB level of TiO2, both leading to enhanced electron–hole recombination. Another attempt to obtain visible-light sensitive titania involved nitro gen ion implantation of TiO2 nanotube arrays [17]. However, the inci dent photon conversion efficiencies (IPCEs), based on the photocurrent action spectrum measured for oxidation of water (in a Na2SO4 solution), culminating at about 20% in the UV region and decreasing from ca. 5% at 400 nm to 2% at 450 nm [17], indicated again the persistence of a relatively strong electron–hole recombination. The above results [16, 17] illustrate the situation observed quite frequently in the case of TiO2 doping either with the transition metal cations or with the non-metallic elements that results in a drop of the IPCEs within the fundamental absorption range (i.e., over UV wavelengths) of the material which is hardly compensated by a modest photoresponse to the visible wavelengths. Like the majority of other studies on TiO2 doped with non-metallic elements, the effect of sulfur doping upon the visible-light photo activity of the photomaterial was tested for the photodegradation of an organic pollutant in the presence of oxygen [18]. Although the sulfur-doped TiO2 photocatalyst exhibited some visible-light activity consistent with the extension of the absorption spectrum up to 430 nm, the observed decolorization of methylene blue [18], which absorbs visible light by itself, used as a test might be questioned. Closer to the principal subject of this article are numerous earlier attempts aimed at modifying spectral range of TiO2 photoactivity by carbon doping [19–25]. Besides works that demonstrated effective visible-light-driven photodegradation of several organic contaminants using air oxygen as scavenger of photogenerated electrons [19–21], several other reports dealt with application of carbon modified TiO2 materials for water splitting [22–25]. Unlike carbon-doped TiO2 pow ders used in decontamination experiments that had anatase structure, the C:TiO2 film photoelectrodes employed for water splitting consisted in most cases [22,23,25] principally of the rutile polymorph. Despite the optical absorption spectra extending in some cases above 500 nm [22], the photoaction spectra exhibited significant IPCEs up to only 420 nm. This small extension of photoactivity is apparently consistent with the theoretical calculations [26] indicating that carbon doping under oxygen-deficient conditions may favor the formation of oxygen va cancies in bulk TiO2 leading to creation of sub-stoichiometric oxides. It is useful to recall that this interpretation is in agreement with earlier reports that showed that reduction of rutile - with the formation of ox ygen vacancies - leads to the creation of electronic states in the band gap and the increased absorbance in the visible region of spectrum [27,28]. We note in this connection, the positive shift of the flat-band (and of the photocurrent onset) potentials occurring in carbon-doped and/or in reduced TiO2 films suggesting location of the newly created electronic states below the lower CB edge. The relative deception caused by those rather fruitless attempts to
enhance through doping the rate of water splitting at TiO2, explained interest raised by the concept of co-doping involving, for example, pentavalent Sb5þ and trivalent Cr3þ cations, that allows to maintain charge balance in the TiO2 lattice [29]. Although such co-doped rutile photocatalyst exhibited actually oxygen evolution from a solution con taining Agþ electron scavengers up to a 680 nm incident wavelength, in agreement with the optical absorption spectrum, the quantum efficiency of such a system remains difficult to evaluate. In fact, IPCE measure ments performed for a rutile TiO2 photoanode co-doped with nitrogen and Ta5þ showed only modest photoresponse to the visible light (above 420 nm), the perceptible advantage of the co-doping was a very signif icant increase of the IPCEs over UV wavelengths assigned to the decreased charge recombination [30]. Comparison of the literature reports regarding doped, as well as undoped, TiO2 photocatalysts and photoelectrodes points at an appar ently superior water photo-oxidation activity of rutile photomaterials. Although such point of view is supported by the experimental results obtained with particulate TiO2 photocatalyst [31–33] the situation is less obvious in the case of the oxygen-evolving TiO2 photoanodes where high IPCEs had been reported as well for the anatase TiO2 films [9,10] and the highest ca. 100% IPCE was observed for a film consisting of a 50/50% mixture of anatase and rutile [10]. Interestingly, the latter photoanode exhibited under intense UV irradiation the photocurrent onset potential of 0.2 V vs. reversible hydrogen electrode (RHE) substantially below the flat band potential of either anatase or rutile TiO2. It is to be noted in this connection that unlike photocatalytic ex periments, where the amount of collected oxygen depends on the ability of rutile particles to adsorb and further to impede re-oxidation of the employed sacrificial electron scavengers (Fe3þ, IO-3, Agþ), the IPCE measurements performed with rutile photoelectrodes provide a direct information on the water photo-oxidation activity of the photomaterials. A report in 2011 on the preparation of “black” anatase TiO2 nano particles (NPs) through hydrogenation under relatively drastic condi tions [34] generated large interest in the possibility to split water under sunlight. In fact, those H:TiO2 small (8 nm in size), surface-disordered nanocrystals with anatase structure showed visible light and near-infrared absorption, with a narrowed band gap of around 1.0 eV [34] and the corresponding simulated sunlight photoactivity for degradation of methylene blue and the formation of hydrogen in the presence of methanol acting as hole scavenger. However, further mea surements performed both on hydrogen treated, H:TiO2, rutile nano wires and anatase nanorods demonstrated in fact quite significant increase of water oxidation photocurrents but only a modest contribu tion arising from visible light absorption (above 420 nm) [35]. Under simulated air mass 1.5 (AM 1.5 G) solar illumination, at 1.23 V vs. RHE, the largest photocurrent close to 2.5 mA cm-2 consistent with high IPCEs observed over the whole UV region (up to 400 nm) was reported for the hydrogenated nanowire rutile TiO2 sample [35]. Based on the compar ison of Mott-Schottky plots for the pristine TiO2 and the H:TiO2 elec trodes, the authors assigned the enhanced PEC activity of the latter samples to largely (by 3 orders of magnitude) increased donor density associated with the oxygen vacancies created by the hydrogenation treatment of TiO2. Similar conclusion was reached by the authors of a photocatalytic study involving H2 treated rutile NPs where oxygen evolution was followed in a solution including Agþ ions acting as elec tron scavengers [36,37]. Performed, in parallel, measurements with the rutile TiO2 films formed on the titanium substrate and submitted to a similar H2 reduction treatment showed in fact an important (2–3 times) increase in electrical conductivity [37] in good agreement with the conclusions of the previous study [35]. Thorough comparison of the literature reports regarding the influ ence of doping and other pre-treatments (such as hydrogenation) upon PEC performance of the rutile film photoelectrodes should necessarily consider the role played by the substrate on which the film was formed, i.e., either a transparent conductive oxide (TCO) or the titanium metal. In fact, depending on the temperature at which the TiO2 film was formed 2
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and subsequently annealed, the titanium substrate may act as a reducer increasing the concentration of donors within the film. To increase the conductivity of the anatase and/or anatase/rutile TiO2 films deposited by a sol-gel method on titanium metal substrates, the co-author used annealing in argon at 550–700 � C [9,10]. As showed by electron microprobe analyses of the detached deposits, the high (above 550 � C) temperature annealing causes diffusion of Ti metal from the substrate into the oxide layer leading to the formation of conducting titanium oxides (probably a mixture of TiO and Ti2O3) at the metal-film interface and the creation of the Ti3þ centers in the upper part of the film [10]. Most likely, such inter-diffusion of Ti species also occurs for TiO2 films or nanotubes grown on the titanium metal substrates and submitted to high temperature treatment under reducing atmosphere [22,25,37]. To clarify further the interaction between the titanium substrate and the TiO2 film we investigated more in detail, in particular by extending the range of the temperatures, the process of TiO2 rutile films formation by oxidation of the titanium metal under the oxygen flow [8,38]. In the present paper, we show that the rutile TiO2 films fabricated by a high temperature (at ca. 850 � C) titanium oxidation in oxygen exhibit an extension of the photoresponse above 420 nm with, occurring in par allel, an evident strong decrease of IPCEs at UV wavelengths below 370 nm. We describe activation procedures that allow improving water splitting activity of such photoelectrodes over the whole spectral range up to 470 nm with an IPCE around 45% measured at 400 nm. Moreover, we show that addition to the supporting (HClO4) electrolyte of the polyoxometalate species leads to important enhancement of water splitting photocurrents under visible light. 2. Experimental Chemicals were purchased from Sigma-Aldrich and were of the highest purity (in general p.a.) available. Solutions were prepared using ultrapure MiliQ water (resistivity: 18.2 MΩ). 2.1. Fabrication of TiO2 electrodes The investigated titanium oxide photoanodes were formed by ther mal oxidation of metallic titanium plates (99.6%, from Goodfellow) in an oven under oxygen flow. The plates were first polished with emery papers, washed with acetone, dried and then etched in boiling 20% HCl for 30 min. The etched plates were washed with copious amount of deionized water and dried. To form the photoanodes, the etched Ti plates were submitted to oxidation at various temperatures, ranging from 500 � C to 850 � C, in oxygen. We focused on the electrodes prepared in oven at temperatures of 600 � C and above 800 � C using pure oxygen as oxidant. Selected samples were finally etched in boiling saturated so lution of potassium hydrogen oxalate to remove the outer part of formed TiO2 layer. 2.2. Structural and photoelectrochemical characterization
Fig. 1. Electron scanning micrographs of TiO2 samples prepared by titanium oxidation in oxygen flow in the oven at 600 � C (A), at 850 � C (B) and at 850 � C subsequently etched for 45 min in boiling KHC2O4 (C).
Scanning electron microscopy (SEM) imaging was performed using a Carl Zeiss AURIGA CrossBeam workstation. The microscope was equipped with ‘‘in lens’’ SE and ExB detectors and a bright-field STEM detector. X ray diffraction (XRD) patterns were obtained using Seifert XRD 3003 TT (GE Inspection Technologies GmbH, Germany) X-ray diffractometer with a Cu Kα radiation source. Rietveld analysis of diffraction patterns was effected using WinPLOTR software developed by T. Roisnel and J. Rodrigez-Carvajal. Raman spectra were taken with LabRam I Raman microscope system (Jobin-Yvon Horiba, USA) equip ped with an air-cooled CCD detector using a 532 nm laser excitation. Photoelectrochemical measurements were carried out in a Teflon cell in a standard three electrode arrangement including a platinum wire counter electrode, Hg/Hg2SO4/K2SO4 sat. reference electrode (0.645 V vs. NHE) and the TiO2 working electrode. All potentials were recalcu lated and are reported versus reversible hydrogen electrode (RHE). A 1
M solution of HClO4 was used as supporting electrolyte for water split ting experiments. Photocurrent-potential curves were recorded at a scan rate of 10 mV s-1 employing a CH Instrument model 660D potentiostat. An Autolab potentiostat AUT 86128 equipped with FRA32 module was used to record impedance data. The TiO2 electrodes (0.28 cm2 exposed surface area) were illuminated through a quartz window either with the full output of a 150 W xenon lamp or with simulated AM 1.5G solar irradiation provided by an Oriel 150 W solar simulator. Incident photonto-current efficiency (IPCE) vs. wavelength characteristics - the photo action spectra - were measured using a 450 W xenon lamp set in an Oriel model 66021 housing and an Oriel Multispec 257 monochromator with a bandwidth of 4 nm. The absolute intensity of the incident light passing through the monochromator was measured with a model OL 730-5C UVenhanced silicon detector (Gooch&Housego). 3
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Fig. 2. X-ray diffraction patterns of TiO2 samples prepared by Ti oxidation for 10 min at 850 � C in oxygen flow after (A) and before etching (B) for 45 min in boiling KHC2O4. Addition signs denote reflections corresponding to rutile phase while asterisks denote those corresponding to the Ti3O phase. Inset shows magnified region between 35 and 70� .
changes before and after oxidation assuming formation of rutile (den sity: 4.24 g cm-3), have been estimated at ca. 0.7 μm for the sample formed by oxidation (during 30 min) at 600 � C, and at about 4 μm thick for the samples obtained during 10 min oxidation at 850 � C. In Fig. 2 A and B are represented the XRD patterns of the samples formed by titanium oxidation at 850 � C after etching in KHC2O4 (A) and as-prepared (B), respectively, showing rutile as the dominant form of titanium oxide [39]. We note that this observation is also confirmed by Raman spectra in Fig. 3 of both samples showing dominant peaks at 253, 449 and 605 cm-1 characteristic of rutile form of TiO2 [40], with the broad band at 253 cm-1 coming from combination of multiphonon scattering bands and the bands at 449 and 605 cm-1 assigned to Eg and A1g modes, respectively. Closer inspection of the XRD patterns in Fig. 2 shows that relative intensity of peak at 27.57� to peak at 36.41 is higher than expected for randomly oriented crystals indicating formation of textured titania layers i.e., existence of highly ordered crystalline do mains with preferential (110) orientation [39]. In the obtained samples substantial amounts of sub-stoichiometric titanium oxide Ti3O [41] were also identified. This metastable binary oxide with trigonal crystal lattice and P–31C space group is character ized by the cell volume significantly larger that of TiO2. Its Young modulus and Debye temperature are importantly lower than for other binary oxides (TiO, Ti2O3, Ti2O) and for rutile indicating weaker Ti–O bond in its case [42]. Ti3O is typically formed during processes where titanium oxidation occurs under low oxygen concentrations. Its forma tion has been reported during laser etching of the Ti in water [43] or during formation of plasma-sprayed TiNx/TiOy coatings [44]. Another work mentions that Ti3O can be formed during the titanium thermal treatment at temperatures of 600 and 700 � C [45]. Rietveld analysis of the diffraction patterns in Fig. 2 indicates clearly higher content of Ti3O in the layers having been submitted to etching. This shows that the amount of Ti3O is not uniform across the sample thickness and is higher close to the Ti metal substrate.
Fig. 3. Raman spectra of the TiO2 film formed by oxidation of titanium at 850 � C in oxygen (1) before and (2) after chemical etching. Laser excitation at 532 nm was used for spectra recording.
3. Results and discussion 3.1. Structural properties of the samples SEM images in Fig. 1 show that morphology of the TiO2 samples changes drastically in function of the preparation conditions. Oxide layer formed by oxidation for 10 min in oven at 850 � C (Fig. 1B) consists of large and mostly regular crystals. Etching of that sample (Fig. 1C) results in formation of smaller and less regular crystals than those observed for as-prepared sample, producing in consequence more rough surface. A sample formed by oxidation of titanium for 30 min at 600 � C, used for comparison, exhibits on low magnification micrograph (Fig. 1A) very rough, sponge-like surface reminiscent of the pretreat ment of the Ti substrate in hydrochloric acid. In fact, the corresponding TiO2 film is relatively thin. Thicknesses, calculated from the mass
3.2. Photoelectrochemical characterization The PEC properties of the prepared TiO2 photoanodes for the 4
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Fig. 4. Spectral photoresponses of electrodes prepared by Ti oxidation in ox ygen flow at 600 � C (a), 850 � C (b) and 850 � C and subsequently etched for 45 min in boiling KHC2O4 (c). Curves recorded in 1 M HClO4.
Fig. 6. Photocurrents for the electrode c from Fig. 5 recorded in function of a 405 nm diode illumination power.
Fig. 7. Mott-Schottky plots for the rutile electrodes prepared by Ti oxidation at 600 � C (S1), 850 � C (S2) and at 850 � C and subsequently etched in KHC2O4 (S3).
Fig. 5. Photocurrent density vs. potential curves for electrodes prepared by Ti oxidation in oxygen at 600 � C (a), 850 � C (b) and 850 � C and subsequently etched for 45 min in boiling KHC2O4 (c). Curves recorded in 1 M HClO4 at 10 mV s-1 scan rate under simulated 1 sun irradiation (AM 1.5G).
consistent with the band edge of the stoichiometric rutile. As mentioned above, the thickness of the TiO2 film prepared by a 10 min oxidation at 850 � C was found to be ca. 4 μm and, based again on the mass change, we estimate that etching of the sample for 45 min in boiling potassium oxalate leads to removal of an important part, ca. 1.7 μm-thick, of the outer layer. On the basis of a series of tests, such etching treatment was found optimal in improving the PEC behavior of the photoanode. We assign this improvement to removal of the apparently defective outer layer of the film and, in part, also to the increase of the surface area (cf. SEM images in Fig. 1 B and C). We note that the increase of IPCEs for the etched sample is especially important in the UV region below 370 nm, where IPCE values for as-prepared photoanode are strongly suppressed. We note in this connection, that other treatments tested in order to activate the photoelectrode formed by Ti oxidation at 850 � C, such as e. g., photoetching in a H2SO4 electrolyte, resulted only in minor improvement of the observed IPCEs. As shown in Fig. 5, consistently with the IPCE plots, the etched photoanode formed by titanium oxidation at 850 � C exhibits the highest water oxidation current under AM 1.5G irradiation, reaching 0.85 mA cm-2 at 1.23 V vs. RHE. The latter photoanode was also tested under high intensity irradiation provided by a 405 nm diode. The generated
oxidation of water were compared in 1 M HClO4 supporting electrolyte both under simulated AM 1.5G solar irradiation (100 mW cm-2) and by recording photoaction, IPCE, spectra over the 280–500 nm range of wavelengths. The presented results focus on the as-prepared at 850 � C and then etched (in boiling KHC2O4) films, with the photoanode formed by Ti oxidation at 600 � C used as a reference sample. As shown in Fig. 4, the TiO2 film formed at 850 � C exhibits a maximum IPCE at around 390 nm and a strong decrease of IPCEs in the UV light region - below 370 nm. Such behavior suggest large presence of charge recombination centers mainly located in the outermost slab of the film. Considering the penetration depth of light vs. wavelength determined for rutile [46], one can postulate that highest concentration of recombination centers is in fact present in the 20–200 nm outermost part of the film. The etching of the latter sample in a boiling KHC2O4 solution for 45 min leads to a radical modification of the IPCE plot (cf. curve c in Fig. 4) with a maximum above 50% observed at 360 nm and, most importantly, a value of 40% at 400 nm. Perceptible IPCEs are now detected up to 470 nm. This is the major difference with the reference sample (curve a) for which the IPCEs decline rapidly up to 420 nm 5
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photocurrents follow linearly light intensity up to the maximum 400 mW diode power (cf. Fig. 6). In Fig. 7 are represented Mott-Schottky plots established in the dark on the basis of impedance measurements performed in the 10 Hz-30 kHz frequency range. The Mott-Schottky analysis allows to determine changes in the photomaterials properties; flat-band potential, Efb, donor density, ND, caused by chemical and annealing treatments. These pa rameters can be derived from the Mott-Schottky equation that describes variation of the space charge capacitance in function of the applied potential: 1/C2 ¼ 2/(ε0εsceND)*[Eapp – Efb – kT/e] Plotting 1/C2 versus Eapp and extrapolating the linear part to zero can provide the flat-band potential characterizing the energy of the photo excited electrons in the conduction band (CB) of the semiconducting material. Of particular interest is the plot corresponding to the electrode formed by Ti oxidation at 850 � C and subsequently etched. Taking dielectric constant of rutile as 100 [47] we found for the latter electrode the donor density, ND slightly higher than 1023 cm-3. Although such estimated value of ND may appear abnormally high (being not corrected for the real surface area of the sample) we note that it is almost 2 orders of magnitude larger than those for other 2 samples. We note also in that case the flat band potential close to 0 V vs. RHE, more negative than those for two other samples, suggesting also the removal by etching of Ti3þ states located below the CB edge (acting as recombination centers) consistent with the improved IPCEs in Fig. 4. The link can also be established between the presence of larger proportion of Ti3O phase present in depth of that sample (cf. Fig. 2) and higher donor concen tration indicated by the impedance measurements. Based on earlier observation of the co-authors [48], we also inves tigated the effect of the polyoxometalate H3PMo12O40 electrocatalyst upon water splitting at the high-temperature formed and subsequently etched TiO2 photoanode. We note, in this connection, that the latter species were proposed [48] as an effective non-noble metal water oxidation catalyst in association with a WO3 photoanode. A substantial increase of IPCEs that reach 64% at 360 nm and, most importantly, still maintain 45% at 400 nm is observed in Fig. 8 A. We also note that the drop of the IPCEs below 350 nm is due to the UV light absorption by the H3PMo12O40 species in solution (cf. Fig. 8C; for this reason, we avoided using more concentrated solutions of this polyoxometalate electro catalyst in the electrolyte). This electrode remains photoactive up to roughly 470 nm resulting in an increase of the AM 1.5G photocurrent at 1.23 V vs. RHE to ca. 0.9 mA cm-2 (Fig. 8 B). 4. Conclusions In summary, we have demonstrated that the procedure of the rutilebased titanium oxide film formation consisting in the high (above 800 � C) temperature oxidation of the titanium metal in flowing oxygen may be used for the formation of the photoelectrodes with a photosensitivity to visible light (up to ca. 470 nm). We have shown that such films contain substantial amount of lower titanium oxide Ti3O close to the titanium metal substrate. However, such as-formed films exhibit poor photoresponse, in particular within the UV wavelengths corresponding to the fundamental absorption range of the rutile. After various at tempts, we established that a chemical etching of the as-formed films leads to the drastic improvement of their photoelectrochemical activity. Unexpectedly, to be effective, such treatment should involve removal of a substantial portion of the film consisting, apparently, of a highly defective form of the oxide. We have also shown that, after addition to the electrolyte of a molecular water oxidation electrocatalyst in form of Keggin-type polyoxometalate, H3PMo12O4, the activated photoanode attains under simulated 1 sun (AM 1.5G, 100 mW cm-2) irradiation a photocurrent of 0.9 mA cm-2 at 1.23 V vs. RHE.
Fig. 8. A) IPCE plots for electrode prepared by Ti oxidation in oven in oxygen flow at 850 � C and subsequently etched for 45 min in boiling KHC2O4 without (a) and after addition to the electrolyte of 10-4 M H3PMo12O40 (b). Supporting electrolyte: 1 M HClO4. B) Photocurrent density vs. potential curves of elec trodes prepared by oxidation in oxygen at 850 � C without (a) and with addition to the electrolyte of 10-4 M H3PMo12O40 (b). Curves recorded in 1 M HClO4 at 10 mV s-1 scan rate under simulated 1 sun irradiation (AM 1.5G). C) UV–Vis transmittance spectra of 10-4 M and 10-3 M solutions of H3PMo12O40.
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Declaration of competing interest
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Visible-light activation of low-cost rutile TiO2 photoanodes for photoelectrochemical water splitting Piotr J. Barczuk a, Krzysztof R. Noworyta b, Miroslaw Dolata c, Katarzyna Jakubow-Piotrowska a, Jan Augustynski a, * a b c
Centre of New Technologies, University of Warsaw, S. Banacha 2c, 02-097, Warsaw, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland Department of Physics and Biophysics, Institute of Biology, Warsaw University of Life Sciences, Nowoursynowska 159, 02-776, Warsaw, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Defective TiO2 rutile Visible light responsive TiO2 Photo-oxidation of water
Structure and photoelectrochemical properties of rutile-based TiO2 films formed by high-temperature oxidation of titanium metal are described. It is shown that a chemical etching treatment leads to the activation of initially almost inert film allowing its operation as visible light responsive water photo-oxidation photoanode. Additional use of a polyoxometalate electrocatalyst in the supporting electrolyte results in a 0.9 mA cm-2 photocurrent under simulated 1 sun irradiation at 1.23 V vs. RHE.
1. Introduction The broad interest in electrochemical properties of titanium oxide, TiO2, goes back few years before its first utilization as an oxygenevolving photoanode in photoelectrochemical (PEC) water splitting cell [1,2]. It is to be related to the discovery of excellent electronic conductivity and electro-catalytic activity of TiO2 - RuO2 mixed oxides that lead to the development of Dimensionally Stable Anodes (DSA) that revolutionized chlorine-alkali electrolysis industry [3–6]. The key feature of the DSA electrodes is the utilization of titanium metal sub strate that does not undergo corrosion in acidic media in the presence of chlorine. Interestingly, the discovery of conductive mixed oxide film electrodes was all but immediate and had resulted from the use of thermal decomposition method to deposit noble metals on the titanium metal electrode substrates [4,7]. The temperatures involved in the decomposition of noble metal precursors lead actually to the formation of mixed oxides of rutile structure due to the, occurring simultaneously, thermal oxidation of the titanium substrate. The TiO2 has an important stabilizing effect versus noble metal oxide components of the DSA electrode - RuO2 and IrO2 [4,6,7]. The process similar to that occurring during fabrication of mixed conductive oxides on titanium substrates was subsequently used to form semiconducting TiO2 films of rutile structure by oxidation of the Ti metal at around 600 � C [8]. It is also to be noted that, when conducted under neutral atmosphere (e.g. in argon), the high (550–600 � C)
temperature annealing allows marked activation of the TiO2 films deposited on titanium by the sol-gel method [9,10]. In such cases, the diffusion of titanium ions from the metallic substrate leads to an important increase in conductivity - concentration of the majority charge carriers - in the TiO2 films. Although such thin layer TiO2 films demonstrated in some cases [10] very high water photo-oxidation cur rents, their photoactivity remained restricted to near UV light with the band edges at 400 and 420 nm, corresponding to the band gaps of anatase (3.2 eV) and rutile (3.0 eV) polymorphic forms of TiO2, respectively. Early efforts to extend the photoresponse of TiO2 from the UV into the visible light spectral region involved mainly metal doping (with the exception of the sensitization with dyes). These works were reviewed, among others, in Refs. [11,12]. Although incorporation of certain transition metal cations, such as Cr3þ or Ni2þ, into the TiO2 lattice lead to some sub-band gap photoresponse, it induced, at the same time, strongly enhanced electron–hole recombination over the whole ab sorption spectrum [13]. This precluded such doped TiO2 photo electrodes, either anatase or rutile, from the application in sunlight-driven water splitting devices. In the same way, the Cr3þ doping of nanocrystalline powder titania was found detrimental to its photocatalytic properties, strongly decreasing quantum efficiency of the photoreactions [14]. Subsequently, large interest was directed towards the possibility of modifying the photoresponse of TiO2 through doping with non-metallic
* Corresponding author. E-mail address: [email protected] (J. Augustynski). https://doi.org/10.1016/j.solmat.2020.110424 Received 6 November 2019; Received in revised form 15 January 2020; Accepted 16 January 2020 Available online 27 January 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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elements, in particular with nitrogen, carbon and sulfur. The input was provided by the article published by Asahi et al. [15] who evaluated, on the basis of density-of-states, DOS, calculations, the consequences of substitutional doping of anatase crystals with N, C, S, P and F atoms. The authors concluded that nitrogen doping leads to the “band-gap nar rowing” of TiO2 through mixing the N 2p states with the O 2p states. The reported optical absorption spectra for films prepared by TiO2 sputtering and high temperature annealing in N2 atmosphere, extending above 500 nm [15], were assigned to the formation of nitrogen-induced states lying above the upper valence band (VB) edge of TiO2. Although this assumption could be confirmed by the results of photocatalytic experi ments involving photodegradation of organic pollutants in the presence of oxygen [15], attempts to split water using nitrogen-doped TiO2 film photoanodes of rutile structure exposed to simulated sunlight (100 mW cm-2) illumination yielded only low photocurrents of about 0.2 mA cm-2 [16]. Such poor PEC performance of the nitrogen-doped TiO2 photo anodes was explained by the high density of electron trap states distributed below the conduction band (CB) and to the low mobility of holes generated in localized N 2p electronic states located above the O 2p VB level of TiO2, both leading to enhanced electron–hole recombination. Another attempt to obtain visible-light sensitive titania involved nitro gen ion implantation of TiO2 nanotube arrays [17]. However, the inci dent photon conversion efficiencies (IPCEs), based on the photocurrent action spectrum measured for oxidation of water (in a Na2SO4 solution), culminating at about 20% in the UV region and decreasing from ca. 5% at 400 nm to 2% at 450 nm [17], indicated again the persistence of a relatively strong electron–hole recombination. The above results [16, 17] illustrate the situation observed quite frequently in the case of TiO2 doping either with the transition metal cations or with the non-metallic elements that results in a drop of the IPCEs within the fundamental absorption range (i.e., over UV wavelengths) of the material which is hardly compensated by a modest photoresponse to the visible wavelengths. Like the majority of other studies on TiO2 doped with non-metallic elements, the effect of sulfur doping upon the visible-light photo activity of the photomaterial was tested for the photodegradation of an organic pollutant in the presence of oxygen [18]. Although the sulfur-doped TiO2 photocatalyst exhibited some visible-light activity consistent with the extension of the absorption spectrum up to 430 nm, the observed decolorization of methylene blue [18], which absorbs visible light by itself, used as a test might be questioned. Closer to the principal subject of this article are numerous earlier attempts aimed at modifying spectral range of TiO2 photoactivity by carbon doping [19–25]. Besides works that demonstrated effective visible-light-driven photodegradation of several organic contaminants using air oxygen as scavenger of photogenerated electrons [19–21], several other reports dealt with application of carbon modified TiO2 materials for water splitting [22–25]. Unlike carbon-doped TiO2 pow ders used in decontamination experiments that had anatase structure, the C:TiO2 film photoelectrodes employed for water splitting consisted in most cases [22,23,25] principally of the rutile polymorph. Despite the optical absorption spectra extending in some cases above 500 nm [22], the photoaction spectra exhibited significant IPCEs up to only 420 nm. This small extension of photoactivity is apparently consistent with the theoretical calculations [26] indicating that carbon doping under oxygen-deficient conditions may favor the formation of oxygen va cancies in bulk TiO2 leading to creation of sub-stoichiometric oxides. It is useful to recall that this interpretation is in agreement with earlier reports that showed that reduction of rutile - with the formation of ox ygen vacancies - leads to the creation of electronic states in the band gap and the increased absorbance in the visible region of spectrum [27,28]. We note in this connection, the positive shift of the flat-band (and of the photocurrent onset) potentials occurring in carbon-doped and/or in reduced TiO2 films suggesting location of the newly created electronic states below the lower CB edge. The relative deception caused by those rather fruitless attempts to
enhance through doping the rate of water splitting at TiO2, explained interest raised by the concept of co-doping involving, for example, pentavalent Sb5þ and trivalent Cr3þ cations, that allows to maintain charge balance in the TiO2 lattice [29]. Although such co-doped rutile photocatalyst exhibited actually oxygen evolution from a solution con taining Agþ electron scavengers up to a 680 nm incident wavelength, in agreement with the optical absorption spectrum, the quantum efficiency of such a system remains difficult to evaluate. In fact, IPCE measure ments performed for a rutile TiO2 photoanode co-doped with nitrogen and Ta5þ showed only modest photoresponse to the visible light (above 420 nm), the perceptible advantage of the co-doping was a very signif icant increase of the IPCEs over UV wavelengths assigned to the decreased charge recombination [30]. Comparison of the literature reports regarding doped, as well as undoped, TiO2 photocatalysts and photoelectrodes points at an appar ently superior water photo-oxidation activity of rutile photomaterials. Although such point of view is supported by the experimental results obtained with particulate TiO2 photocatalyst [31–33] the situation is less obvious in the case of the oxygen-evolving TiO2 photoanodes where high IPCEs had been reported as well for the anatase TiO2 films [9,10] and the highest ca. 100% IPCE was observed for a film consisting of a 50/50% mixture of anatase and rutile [10]. Interestingly, the latter photoanode exhibited under intense UV irradiation the photocurrent onset potential of 0.2 V vs. reversible hydrogen electrode (RHE) substantially below the flat band potential of either anatase or rutile TiO2. It is to be noted in this connection that unlike photocatalytic ex periments, where the amount of collected oxygen depends on the ability of rutile particles to adsorb and further to impede re-oxidation of the employed sacrificial electron scavengers (Fe3þ, IO-3, Agþ), the IPCE measurements performed with rutile photoelectrodes provide a direct information on the water photo-oxidation activity of the photomaterials. A report in 2011 on the preparation of “black” anatase TiO2 nano particles (NPs) through hydrogenation under relatively drastic condi tions [34] generated large interest in the possibility to split water under sunlight. In fact, those H:TiO2 small (8 nm in size), surface-disordered nanocrystals with anatase structure showed visible light and near-infrared absorption, with a narrowed band gap of around 1.0 eV [34] and the corresponding simulated sunlight photoactivity for degradation of methylene blue and the formation of hydrogen in the presence of methanol acting as hole scavenger. However, further mea surements performed both on hydrogen treated, H:TiO2, rutile nano wires and anatase nanorods demonstrated in fact quite significant increase of water oxidation photocurrents but only a modest contribu tion arising from visible light absorption (above 420 nm) [35]. Under simulated air mass 1.5 (AM 1.5 G) solar illumination, at 1.23 V vs. RHE, the largest photocurrent close to 2.5 mA cm-2 consistent with high IPCEs observed over the whole UV region (up to 400 nm) was reported for the hydrogenated nanowire rutile TiO2 sample [35]. Based on the compar ison of Mott-Schottky plots for the pristine TiO2 and the H:TiO2 elec trodes, the authors assigned the enhanced PEC activity of the latter samples to largely (by 3 orders of magnitude) increased donor density associated with the oxygen vacancies created by the hydrogenation treatment of TiO2. Similar conclusion was reached by the authors of a photocatalytic study involving H2 treated rutile NPs where oxygen evolution was followed in a solution including Agþ ions acting as elec tron scavengers [36,37]. Performed, in parallel, measurements with the rutile TiO2 films formed on the titanium substrate and submitted to a similar H2 reduction treatment showed in fact an important (2–3 times) increase in electrical conductivity [37] in good agreement with the conclusions of the previous study [35]. Thorough comparison of the literature reports regarding the influ ence of doping and other pre-treatments (such as hydrogenation) upon PEC performance of the rutile film photoelectrodes should necessarily consider the role played by the substrate on which the film was formed, i.e., either a transparent conductive oxide (TCO) or the titanium metal. In fact, depending on the temperature at which the TiO2 film was formed 2
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and subsequently annealed, the titanium substrate may act as a reducer increasing the concentration of donors within the film. To increase the conductivity of the anatase and/or anatase/rutile TiO2 films deposited by a sol-gel method on titanium metal substrates, the co-author used annealing in argon at 550–700 � C [9,10]. As showed by electron microprobe analyses of the detached deposits, the high (above 550 � C) temperature annealing causes diffusion of Ti metal from the substrate into the oxide layer leading to the formation of conducting titanium oxides (probably a mixture of TiO and Ti2O3) at the metal-film interface and the creation of the Ti3þ centers in the upper part of the film [10]. Most likely, such inter-diffusion of Ti species also occurs for TiO2 films or nanotubes grown on the titanium metal substrates and submitted to high temperature treatment under reducing atmosphere [22,25,37]. To clarify further the interaction between the titanium substrate and the TiO2 film we investigated more in detail, in particular by extending the range of the temperatures, the process of TiO2 rutile films formation by oxidation of the titanium metal under the oxygen flow [8,38]. In the present paper, we show that the rutile TiO2 films fabricated by a high temperature (at ca. 850 � C) titanium oxidation in oxygen exhibit an extension of the photoresponse above 420 nm with, occurring in par allel, an evident strong decrease of IPCEs at UV wavelengths below 370 nm. We describe activation procedures that allow improving water splitting activity of such photoelectrodes over the whole spectral range up to 470 nm with an IPCE around 45% measured at 400 nm. Moreover, we show that addition to the supporting (HClO4) electrolyte of the polyoxometalate species leads to important enhancement of water splitting photocurrents under visible light. 2. Experimental Chemicals were purchased from Sigma-Aldrich and were of the highest purity (in general p.a.) available. Solutions were prepared using ultrapure MiliQ water (resistivity: 18.2 MΩ). 2.1. Fabrication of TiO2 electrodes The investigated titanium oxide photoanodes were formed by ther mal oxidation of metallic titanium plates (99.6%, from Goodfellow) in an oven under oxygen flow. The plates were first polished with emery papers, washed with acetone, dried and then etched in boiling 20% HCl for 30 min. The etched plates were washed with copious amount of deionized water and dried. To form the photoanodes, the etched Ti plates were submitted to oxidation at various temperatures, ranging from 500 � C to 850 � C, in oxygen. We focused on the electrodes prepared in oven at temperatures of 600 � C and above 800 � C using pure oxygen as oxidant. Selected samples were finally etched in boiling saturated so lution of potassium hydrogen oxalate to remove the outer part of formed TiO2 layer. 2.2. Structural and photoelectrochemical characterization
Fig. 1. Electron scanning micrographs of TiO2 samples prepared by titanium oxidation in oxygen flow in the oven at 600 � C (A), at 850 � C (B) and at 850 � C subsequently etched for 45 min in boiling KHC2O4 (C).
Scanning electron microscopy (SEM) imaging was performed using a Carl Zeiss AURIGA CrossBeam workstation. The microscope was equipped with ‘‘in lens’’ SE and ExB detectors and a bright-field STEM detector. X ray diffraction (XRD) patterns were obtained using Seifert XRD 3003 TT (GE Inspection Technologies GmbH, Germany) X-ray diffractometer with a Cu Kα radiation source. Rietveld analysis of diffraction patterns was effected using WinPLOTR software developed by T. Roisnel and J. Rodrigez-Carvajal. Raman spectra were taken with LabRam I Raman microscope system (Jobin-Yvon Horiba, USA) equip ped with an air-cooled CCD detector using a 532 nm laser excitation. Photoelectrochemical measurements were carried out in a Teflon cell in a standard three electrode arrangement including a platinum wire counter electrode, Hg/Hg2SO4/K2SO4 sat. reference electrode (0.645 V vs. NHE) and the TiO2 working electrode. All potentials were recalcu lated and are reported versus reversible hydrogen electrode (RHE). A 1
M solution of HClO4 was used as supporting electrolyte for water split ting experiments. Photocurrent-potential curves were recorded at a scan rate of 10 mV s-1 employing a CH Instrument model 660D potentiostat. An Autolab potentiostat AUT 86128 equipped with FRA32 module was used to record impedance data. The TiO2 electrodes (0.28 cm2 exposed surface area) were illuminated through a quartz window either with the full output of a 150 W xenon lamp or with simulated AM 1.5G solar irradiation provided by an Oriel 150 W solar simulator. Incident photonto-current efficiency (IPCE) vs. wavelength characteristics - the photo action spectra - were measured using a 450 W xenon lamp set in an Oriel model 66021 housing and an Oriel Multispec 257 monochromator with a bandwidth of 4 nm. The absolute intensity of the incident light passing through the monochromator was measured with a model OL 730-5C UVenhanced silicon detector (Gooch&Housego). 3
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Fig. 2. X-ray diffraction patterns of TiO2 samples prepared by Ti oxidation for 10 min at 850 � C in oxygen flow after (A) and before etching (B) for 45 min in boiling KHC2O4. Addition signs denote reflections corresponding to rutile phase while asterisks denote those corresponding to the Ti3O phase. Inset shows magnified region between 35 and 70� .
changes before and after oxidation assuming formation of rutile (den sity: 4.24 g cm-3), have been estimated at ca. 0.7 μm for the sample formed by oxidation (during 30 min) at 600 � C, and at about 4 μm thick for the samples obtained during 10 min oxidation at 850 � C. In Fig. 2 A and B are represented the XRD patterns of the samples formed by titanium oxidation at 850 � C after etching in KHC2O4 (A) and as-prepared (B), respectively, showing rutile as the dominant form of titanium oxide [39]. We note that this observation is also confirmed by Raman spectra in Fig. 3 of both samples showing dominant peaks at 253, 449 and 605 cm-1 characteristic of rutile form of TiO2 [40], with the broad band at 253 cm-1 coming from combination of multiphonon scattering bands and the bands at 449 and 605 cm-1 assigned to Eg and A1g modes, respectively. Closer inspection of the XRD patterns in Fig. 2 shows that relative intensity of peak at 27.57� to peak at 36.41 is higher than expected for randomly oriented crystals indicating formation of textured titania layers i.e., existence of highly ordered crystalline do mains with preferential (110) orientation [39]. In the obtained samples substantial amounts of sub-stoichiometric titanium oxide Ti3O [41] were also identified. This metastable binary oxide with trigonal crystal lattice and P–31C space group is character ized by the cell volume significantly larger that of TiO2. Its Young modulus and Debye temperature are importantly lower than for other binary oxides (TiO, Ti2O3, Ti2O) and for rutile indicating weaker Ti–O bond in its case [42]. Ti3O is typically formed during processes where titanium oxidation occurs under low oxygen concentrations. Its forma tion has been reported during laser etching of the Ti in water [43] or during formation of plasma-sprayed TiNx/TiOy coatings [44]. Another work mentions that Ti3O can be formed during the titanium thermal treatment at temperatures of 600 and 700 � C [45]. Rietveld analysis of the diffraction patterns in Fig. 2 indicates clearly higher content of Ti3O in the layers having been submitted to etching. This shows that the amount of Ti3O is not uniform across the sample thickness and is higher close to the Ti metal substrate.
Fig. 3. Raman spectra of the TiO2 film formed by oxidation of titanium at 850 � C in oxygen (1) before and (2) after chemical etching. Laser excitation at 532 nm was used for spectra recording.
3. Results and discussion 3.1. Structural properties of the samples SEM images in Fig. 1 show that morphology of the TiO2 samples changes drastically in function of the preparation conditions. Oxide layer formed by oxidation for 10 min in oven at 850 � C (Fig. 1B) consists of large and mostly regular crystals. Etching of that sample (Fig. 1C) results in formation of smaller and less regular crystals than those observed for as-prepared sample, producing in consequence more rough surface. A sample formed by oxidation of titanium for 30 min at 600 � C, used for comparison, exhibits on low magnification micrograph (Fig. 1A) very rough, sponge-like surface reminiscent of the pretreat ment of the Ti substrate in hydrochloric acid. In fact, the corresponding TiO2 film is relatively thin. Thicknesses, calculated from the mass
3.2. Photoelectrochemical characterization The PEC properties of the prepared TiO2 photoanodes for the 4
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Fig. 4. Spectral photoresponses of electrodes prepared by Ti oxidation in ox ygen flow at 600 � C (a), 850 � C (b) and 850 � C and subsequently etched for 45 min in boiling KHC2O4 (c). Curves recorded in 1 M HClO4.
Fig. 6. Photocurrents for the electrode c from Fig. 5 recorded in function of a 405 nm diode illumination power.
Fig. 7. Mott-Schottky plots for the rutile electrodes prepared by Ti oxidation at 600 � C (S1), 850 � C (S2) and at 850 � C and subsequently etched in KHC2O4 (S3).
Fig. 5. Photocurrent density vs. potential curves for electrodes prepared by Ti oxidation in oxygen at 600 � C (a), 850 � C (b) and 850 � C and subsequently etched for 45 min in boiling KHC2O4 (c). Curves recorded in 1 M HClO4 at 10 mV s-1 scan rate under simulated 1 sun irradiation (AM 1.5G).
consistent with the band edge of the stoichiometric rutile. As mentioned above, the thickness of the TiO2 film prepared by a 10 min oxidation at 850 � C was found to be ca. 4 μm and, based again on the mass change, we estimate that etching of the sample for 45 min in boiling potassium oxalate leads to removal of an important part, ca. 1.7 μm-thick, of the outer layer. On the basis of a series of tests, such etching treatment was found optimal in improving the PEC behavior of the photoanode. We assign this improvement to removal of the apparently defective outer layer of the film and, in part, also to the increase of the surface area (cf. SEM images in Fig. 1 B and C). We note that the increase of IPCEs for the etched sample is especially important in the UV region below 370 nm, where IPCE values for as-prepared photoanode are strongly suppressed. We note in this connection, that other treatments tested in order to activate the photoelectrode formed by Ti oxidation at 850 � C, such as e. g., photoetching in a H2SO4 electrolyte, resulted only in minor improvement of the observed IPCEs. As shown in Fig. 5, consistently with the IPCE plots, the etched photoanode formed by titanium oxidation at 850 � C exhibits the highest water oxidation current under AM 1.5G irradiation, reaching 0.85 mA cm-2 at 1.23 V vs. RHE. The latter photoanode was also tested under high intensity irradiation provided by a 405 nm diode. The generated
oxidation of water were compared in 1 M HClO4 supporting electrolyte both under simulated AM 1.5G solar irradiation (100 mW cm-2) and by recording photoaction, IPCE, spectra over the 280–500 nm range of wavelengths. The presented results focus on the as-prepared at 850 � C and then etched (in boiling KHC2O4) films, with the photoanode formed by Ti oxidation at 600 � C used as a reference sample. As shown in Fig. 4, the TiO2 film formed at 850 � C exhibits a maximum IPCE at around 390 nm and a strong decrease of IPCEs in the UV light region - below 370 nm. Such behavior suggest large presence of charge recombination centers mainly located in the outermost slab of the film. Considering the penetration depth of light vs. wavelength determined for rutile [46], one can postulate that highest concentration of recombination centers is in fact present in the 20–200 nm outermost part of the film. The etching of the latter sample in a boiling KHC2O4 solution for 45 min leads to a radical modification of the IPCE plot (cf. curve c in Fig. 4) with a maximum above 50% observed at 360 nm and, most importantly, a value of 40% at 400 nm. Perceptible IPCEs are now detected up to 470 nm. This is the major difference with the reference sample (curve a) for which the IPCEs decline rapidly up to 420 nm 5
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photocurrents follow linearly light intensity up to the maximum 400 mW diode power (cf. Fig. 6). In Fig. 7 are represented Mott-Schottky plots established in the dark on the basis of impedance measurements performed in the 10 Hz-30 kHz frequency range. The Mott-Schottky analysis allows to determine changes in the photomaterials properties; flat-band potential, Efb, donor density, ND, caused by chemical and annealing treatments. These pa rameters can be derived from the Mott-Schottky equation that describes variation of the space charge capacitance in function of the applied potential: 1/C2 ¼ 2/(ε0εsceND)*[Eapp – Efb – kT/e] Plotting 1/C2 versus Eapp and extrapolating the linear part to zero can provide the flat-band potential characterizing the energy of the photo excited electrons in the conduction band (CB) of the semiconducting material. Of particular interest is the plot corresponding to the electrode formed by Ti oxidation at 850 � C and subsequently etched. Taking dielectric constant of rutile as 100 [47] we found for the latter electrode the donor density, ND slightly higher than 1023 cm-3. Although such estimated value of ND may appear abnormally high (being not corrected for the real surface area of the sample) we note that it is almost 2 orders of magnitude larger than those for other 2 samples. We note also in that case the flat band potential close to 0 V vs. RHE, more negative than those for two other samples, suggesting also the removal by etching of Ti3þ states located below the CB edge (acting as recombination centers) consistent with the improved IPCEs in Fig. 4. The link can also be established between the presence of larger proportion of Ti3O phase present in depth of that sample (cf. Fig. 2) and higher donor concen tration indicated by the impedance measurements. Based on earlier observation of the co-authors [48], we also inves tigated the effect of the polyoxometalate H3PMo12O40 electrocatalyst upon water splitting at the high-temperature formed and subsequently etched TiO2 photoanode. We note, in this connection, that the latter species were proposed [48] as an effective non-noble metal water oxidation catalyst in association with a WO3 photoanode. A substantial increase of IPCEs that reach 64% at 360 nm and, most importantly, still maintain 45% at 400 nm is observed in Fig. 8 A. We also note that the drop of the IPCEs below 350 nm is due to the UV light absorption by the H3PMo12O40 species in solution (cf. Fig. 8C; for this reason, we avoided using more concentrated solutions of this polyoxometalate electro catalyst in the electrolyte). This electrode remains photoactive up to roughly 470 nm resulting in an increase of the AM 1.5G photocurrent at 1.23 V vs. RHE to ca. 0.9 mA cm-2 (Fig. 8 B). 4. Conclusions In summary, we have demonstrated that the procedure of the rutilebased titanium oxide film formation consisting in the high (above 800 � C) temperature oxidation of the titanium metal in flowing oxygen may be used for the formation of the photoelectrodes with a photosensitivity to visible light (up to ca. 470 nm). We have shown that such films contain substantial amount of lower titanium oxide Ti3O close to the titanium metal substrate. However, such as-formed films exhibit poor photoresponse, in particular within the UV wavelengths corresponding to the fundamental absorption range of the rutile. After various at tempts, we established that a chemical etching of the as-formed films leads to the drastic improvement of their photoelectrochemical activity. Unexpectedly, to be effective, such treatment should involve removal of a substantial portion of the film consisting, apparently, of a highly defective form of the oxide. We have also shown that, after addition to the electrolyte of a molecular water oxidation electrocatalyst in form of Keggin-type polyoxometalate, H3PMo12O4, the activated photoanode attains under simulated 1 sun (AM 1.5G, 100 mW cm-2) irradiation a photocurrent of 0.9 mA cm-2 at 1.23 V vs. RHE.
Fig. 8. A) IPCE plots for electrode prepared by Ti oxidation in oven in oxygen flow at 850 � C and subsequently etched for 45 min in boiling KHC2O4 without (a) and after addition to the electrolyte of 10-4 M H3PMo12O40 (b). Supporting electrolyte: 1 M HClO4. B) Photocurrent density vs. potential curves of elec trodes prepared by oxidation in oxygen at 850 � C without (a) and with addition to the electrolyte of 10-4 M H3PMo12O40 (b). Curves recorded in 1 M HClO4 at 10 mV s-1 scan rate under simulated 1 sun irradiation (AM 1.5G). C) UV–Vis transmittance spectra of 10-4 M and 10-3 M solutions of H3PMo12O40.
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Declaration of competing interest
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