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12274 Phys. Chem. Chem. Phys., 2013, 15, 12274--12282 This journal is c the Owner Societies 2013
Cite this: Phys. Chem.Chem.Phys.,2013,15, 12274
Ternary Ti–Mo–Ni mixed oxide nanotube arrays asphotoanode materials for efficient solarhydrogen production
Nageh K. Allam,*ab Nourhan M. Deyabab and Nabil Abdel Ghanyb
To date, most studies on the fabrication and development of photoelectrodes for solar-fuel generation
have been based on simple binary systems with limited success. However, ternary systems have not
been explored extensively although they can offer more possibilities for band gap and band alignment
tuning that allow the development of more efficient photoelectrochemical systems. Herein, we report
on the growth of a novel ternary oxide photoanode material composed of self-ordered, vertically
oriented nanotube arrays of titanium–molybdenum–nickel mixed oxide films via the anodization of a
Ti–Mo–Ni alloy in an electrolyte solution of formamide containing NH4F at room temperature, followed
by annealing in an air atmosphere. The nanostructure topology was found to depend on both the
anodization time and the applied voltage. Our results demonstrate the ability to grow mixed oxide nanotube
array films that are several microns thick. The Ti–Mo–Ni mixed oxide nanotube array films were utilized in
solar-spectrum water photoelectrolysis, demonstrating a photocurrent density of 2.1 mA cm�2 and a
B10 fold increase in the photoconversion efficiency under AM 1.5 illumination (100 mW cm�2, 1 M KOH)
compared to pure TiO2 nanotubes fabricated under the same conditions. This enhancement in the photo-
conversion efficiency can be related to the synergistic effects of Ni and Mo alloying and the unique struc-
tural properties of the fabricated nanotube arrays.
1. Introduction
Solar-driven water electrolysis has been explored extensivelyover the past few decades as a renewable means of hydrogenproduction.1 In this regard, metal oxide photoanodes areprimarily being used in hydrogen production photoelectro-chemical cells (PEC) due to their exceptional semiconductingproperties, physical and chemical stability, abundance and lowcost.2 Of particular interest, low-dimensional semiconductornanostructures have recently become the focus of many funda-mental and applied research activities in the field of solarenergy conversion.1–4 To this end, anodically fabricated TiO2
nanotube arrays have proven to be a robust and cost-effectivefunctional material, widely investigated in many applicationsespecially those related to energy conversion such as photo-electrochemical water splitting and solar cells.5,6 The nano-tubular structure allows the precise design and control of the
geometrical features with specific light absorption and propa-gation characteristics.6,7 In addition, the aligned porosity,crystallinity, and oriented nature of the nanotubular structuremake it an attractive electron percolation pathway for vectorialcharge transfer between interfaces.8,9 However, TiO2 is respon-sive only to UV light, which accounts for only a small fraction ofthe sun’s spectrum. As visible light comprises B45% of thesolar spectrum, any shift in the optical response of TiO2 fromthe UV to the visible spectral range, while maintaining itsintrinsic properties (especially the excellent charge transferproperties and photo-corrosion stability), will have a positiveimpact on the photocatalytic and photo-electrochemical utilityof the material. In this regard, compositional doping of TiO2
with different elements was considered as an approach forband gap engineering.10–14 Although doping has been demon-strated as an efficient approach to introduce visible lightabsorption into TiO2, the currently used doping protocols arenot without serious structural artifacts/defects and problemsthat introduce defect–dopant lattice interactions, which canmask the fundamental interactions of the dopant with thelattice.10,13 A recent and promising approach to overcome suchproblems is to develop and optimize semiconductor materials
a Energy Materials Laboratory (EML), Physics Department, School of Sciences and
Engineering, The American University in Cairo, New Cairo 11835, Egyptb Physical Chemistry Department, National Research Center, Dokki, Cairo 12622,
Egypt. E-mail: [email protected]
Received 20th March 2013,Accepted 20th May 2013
DOI: 10.1039/c3cp52076e
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made of 1D mixed oxide nanoarchitectures. The reportedpreliminary results of such systems showed a great enhance-ment in the optical/electrical properties over the doped counter-parts. Mor et al.15 reported the anodization of Ti–Fe films tofabricate Ti–Fe–O mixed oxide nanotube arrays with enhancedphotoelectrochemical water splitting performance over theirpristine and Fe-doped TiO2 counterpart electrodes. Allamet al.16 were able to grow a uniform suite of Ti–Nb–Zr–O mixedoxide nanotube arrays. The fabricated mixed oxide nanotubesshowed a B17.5% increase in the photoelectrochemical wateroxidation efficiency as compared to pure TiO2 nanotubes ofcomparable length. Kusama et al.17 reported a combinatorialapproach to systematically investigate the visible light responseof Fe–Ti–M oxides (M: various metal elements) for photoelectro-chemical water splitting. Among the 25 elements tested, theFe–Ti–Sr mixed oxide has been identified as the best combinationbecause it showed the highest photocurrent. Also, Lewis et al.18
have developed a high-throughput method using a commercialinkjet printer for the synthesis of mixed-metal oxide photo-electrode materials for water splitting. The experiments pre-sented a methodology where the open-circuit potential of thematerial was determined, which provided critical additionalinformation not readily and rapidly available from photocurrent-only measurements.18
In this work, we investigate the possibility to fabricateTi–Mo–Ni mixed oxide nanotube arrays via anodization of theTi–Mo–Ni alloy in fluoride-ion containing electrolytes and testtheir efficiency as photoanodes for water oxidation. This specificmixed oxide system was chosen for the following reasons:
1. NiO has energy level positions suitable for use as a hole-transport material,19 which is expected to facilitate chargetransfer and reduce recombination of excitons.
2. Ni-doped TiO2 was shown to have a shift in the absorptionspectra towards the visible region of the light spectrum.20
3. The addition of MoO3 to TiO2 was shown to increase thecatalyst surface acidity,21 which is expected to enhance theactivity of the photocatalyst.
4. MoO3 is well known to lose oxygen from its lattice at lowtemperature leaving excess electrons in the solid and giving riseto electrical conductivity, primarily due to thermally activatedhopping of polarons,22 which is a desired property to developan efficient photoanode material.
5. The ease of reducibility of MoO321 is expected to make the
electron transfer rate controlling step more efficient and hencewill increase the overall photocatalytic activity of the mixedoxide system.
6. The Mo-doped TiO2 photoelectrodes were shown to have asignificant red shift in the absorption spectra compared topristine TiO2 photoanodes, owing to the higher electronegativityof Mo6+.23
7. The use of 4d cations as donors, with higher atomic dorbital energies than that of Ti, will guarantee that the CBMenergy level will not be lowered.24
Our ability to fabricate nanotubular structure of Ti–Mo–Nimixed oxides is significant because they showed one of thehighest photoconversion efficiencies when employed in a
photoelectrochemical water splitting system. We hope that thisstudy will open new avenues of research for the explorationof additional combinations of materials and various newapplications.
2. Experimental section
Prior to anodization, Ti–0.3Mo–0.8Ni alloy samples (UNSR53400) from ET UK Ltd (1.5 cm � 1.0 cm � 1.0 mm) wereultrasonically cleaned with acetone followed by a deionizedwater rinse. The anodization was performed in a two-electrodeelectrochemical cell with the titanium alloy as the workingelectrode and the platinum foil as the counter electrode. Theexperiments were conducted at room temperature (approxi-mately 22 1C) in formamide-based electrolytes containing0.2 M NH4F, 0.1 M H3PO4, and 3 vol% H2O at different appliedvoltages (20–60 V) and/or durations (3–16 h). After anodization,the samples were rinsed thoroughly with deionized water andthen dried under a stream of nitrogen. The as-anodized sam-ples were annealed in an air atmosphere for 4 h at 450 1C at aheating and cooling rate of 1 1C min�1. The morphology of theanodized samples was examined using a Zeiss SEM Ultra 60field emission scanning electron microscope (FESEM). X-rayphotoelectron spectroscopy (XPS) experiments were performedon the Ti–Mo–Ni nanotubular films using a Thermo ScientificK-alpha XPS with an Al anode. Spectra were charge-referencedto O 1s at 532 eV. The crystalline phases were detected andidentified using a glancing angle X-ray diffractometer (GAXRD)on an X’Pert PRO MRD with a copper source at a scan rate (2y)of 0.051 s�1. The optical characterization of the films wasperformed using a Shimadzu UV-3101PC UV-Vis-NIR spectro-photometer with a solid sample holder for reflectance measure-ments and an integrating sphere. The photoelectrochemicalproperties were investigated in 1.0 M KOH solution using athree-electrode configuration with the nanotube arrays asphotoanodes, saturated Ag/AgCl as the reference electrode, anda platinum foil as the counter electrode. A scanning potentiostat(CH Instruments, model CH 660D) was used to measure darkand illuminated currents at a scan rate of 10 mV s�1. Sunlightwas simulated using a 300 W ozone-free xenon lamp and AM1.5G filter at 100 mW cm�2.
3. Results and discussion3.1 Morphological characterization
We aimed at anodizing pure titanium and Ti–Mo–Ni alloysamples in formamide-based electrolytes because anodizationof titanium6 and its alloys16 in aqueous electrolytes usuallyresults in the formation of short nanotubes with irregular outerdiameters that contain ridges and circumferential serrations.The use of formamide, however, usually results in the forma-tion of smooth nanotubes that are micrometers in length. Thismorphology is achieved by reducing the changes in pH andsuppressing the local concentration fluctuations during theanodization process.6,16 Herein, we studied the effect of both
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time and applied voltage on the morphology of the fabricatednanoarchitectures.
3.1.1 Effect of anodization time. Fig. 1 shows the FESEMtop-view and cross-sectional images of Ti–0.3Mo–0.8Ni samplesanodized in formamide-based electrolytes containing 0.2 MNH4F, 0.1 M H3PO4 and 3 vol% H2O at 20 V for different times.The tubular structure, covering the entire substrate, is evidentin all cases. The nanotube formation mechanism is describedin detail in the literature.6,25 Briefly, the first step involves theanodic oxidation of the alloy substrate through its interactionwith oxygen ions (O2�) in the electrolyte, resulting in theformation of a poorly conducting thin oxide layer. It is believedthat the nanotubular structure formation includes a competi-tion between electrochemical etching and chemical dissolutionprocesses, where small pits are initially formed due to thelocalized dissolution of the previously formed thin oxide layer,followed by the coalescence of these pits to form pores.6,25
Upon increasing the anodization time, pores become deeperand wider with small inner-tube void diameters and relativelythick tube walls that continue to grow until well-organizednanotube arrays are formed. It is important to mention thatthe nanotube growth should stop once the rates of electro-chemical etching and the chemical dissolution become equal.6,26
Fig. 2 summarizes the variation in the nanotube dimensions(length, diameter and wall thickness) as a function of the
anodization time. The nanotube length was kept almost con-stant (1 mm) with increasing anodization time during the firstfive hours followed by an exponential increase after 7 hours(1.95 mm at 7 h and 11.87 mm at 16 h). This can be an indication
Fig. 1 FESEM images for Ti–0.3Mo–0.8Ni samples anodized at 20 V for (a) 3 h, (b) 4 h, (c) 5 h, (d) 7 h, (e) 10 h, and (f) 16 h in formamide electrolytes containing0.2 M NH4F, 0.1 M H3PO4 and 3% H2O.
Fig. 2 Effect of anodization time on the diameter, wall thickness and length ofthe nanotube arrays formed via the anodization of Ti–0.3Mo–0.8Ni samples informamide electrolytes containing 0.2 M NH4F, 0.1 M H3PO4 and 3% H2O at 20 V.
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that the rate of movement of the alloy/nanotube interface isfaster than the rate of loss of nanotubes by chemical etching.6
The nanotube diameter increases linearly with increasinganodization time reaching 134 nm after 10 h, then decreasingto 115 nm after 16 h. This behavior can be understood based onthe change in the conductivity of the electrolyte.27 That is, asthe anodization proceeds, the concentration of leached ionsfrom the alloy substrate increases and in turn the conductivityof the electrolyte increases. This increase in conductivity makesa large proportion of the applied potential available for theanodization process leading to the observed increase in bothlength and diameter of the formed nanotubes. The wall thick-ness of the fabricated Ti–Mo–Ni nanotubes, on the other hand,was found to slightly vary (24 � 5 nm) during the first 10 hfollowed by a decrease to 9 nm after 16 h of anodization. A pureTi sample anodized under the same conditions (20 V, 16 h)resulted in the formation of nanotubes that are 12 mm inlength, 180 nm in outer diameter and 20 nm in wall thickness,in agreement with the literature pertaining to Ti anodization.6,26,27
3.1.2 Effect of applied voltage. Fig. 3 shows FESEM top-viewand cross-sectional images of anodized Ti–Ni–Mo samples informamide electrolytes containing 0.2 M NH4F, 0.1 M H3PO4 and3 vol% H2O for 5 h at different applied voltages. The nanotubeformation is evident over the entire applied voltage range(20–60 V). Well-defined nanotube arrays can be distinguishedfor the samples anodized both at 20 V and 30 V. Upon increasingthe anodization voltage to 40 V, precipitate debris starts toappear on scattered areas of the surface of nanotubes due tothe slow rate of chemical etching of the oxide by fluoride ions.6,27
The nanotubes formed at 60 V are separated by large intertub-ular spacings, similar to those usually seen upon anodizing Ti inhighly viscous electrolytes.27,28 The intertubular spacing can berelated to the consumption of some of the nanotubes bychemical etching aided by the high applied voltage.28
Fig. 4 summarizes the variation in the nanotube dimensions(length, diameter and wall thickness) as a function of theanodization voltage. The nanotube length increases slightlyfrom 0.85 mm to 0.94 mm upon increasing the applied voltagefrom 20 V to 30 V. Increasing the applied voltage to 40 V almostdoubles the tube length (2.9 mm), which continues to increasewith increasing anodization voltage (3.9 mm at 60 V). Thisincrease in the nanotube length with larger anodization poten-tials can be related to the increased electric field intensity aswell as driving force for ionic transport through the barrier layerat the bottom of the pore6 resulting in faster movement of the alloy/oxide interface into the metallic alloy. The nanotube diametersshow a steady increase up to 40 V (82 nm, 138 nm and 160 nmfor the samples anodized at 20 V, 30 V and 40 V, respectively)
Fig. 3 FESEM images of Ti–0.3Mo–0.8Ni samples anodized for 5 h at (a) 20 V, (b) 30 V, (c) 40 V and (d) 60 V in formamide electrolytes containing 0.2 M NH4F,0.1 M H3PO4 and 3% H2O at room temperature.
Fig. 4 Effect of anodization voltage on the diameter, wall thickness and lengthof the nanotube arrays formed via the anodization of Ti–0.3Mo–0.8Ni samples informamide electrolytes containing 0.2 M NH4F, 0.1 M H3PO4 and 3% H2O for 5 h.
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followed by an abrupt increase to 368 nm upon increasing theapplied voltage to 60 V. The wall thickness of nanotubes,however, showed a constant value (B18 nm) up to 40 V, followedby a sudden increase at 60 V (53 nm). This trend is in a generalagreement with the reported behavior of pure titanium6
and titanium alloys such as Ti–Fe,15 Ti–Nb–Zr16 and Ti–Pd29
anodized in similar electrolytes. However, the obtained tubelengths are larger than those reported for other alloys15,16,29
but with thinner tube walls.
3.2 Compositional and structural characterization
As the anodically fabricated nanotube arrays are usually amor-phous,9,28 we annealed the Ti–Mo–Ni nanotubes, obtained viaanodization for 3, 5 and 7 h, in an air atmosphere at 450 1C for4 h to crystallize them. Fig. 5 shows the obtained XRD patternsconfirming the crystalline nature of the electrodes with reflec-tions mainly from anatase TiO2 (PDF card 35-734, JCPDS). Notethe shift in the anatase peaks’ positions in all the samplescompared to those reported for pure TiO2.9,28 This shift can beattributed to the incorporation of foreign species into thecrystal lattice of TiO2.13,14 Additionally, two peaks that wereindexed at 351 and 631 can be associated with NiO (111) and(220), respectively.19 Another weak peak was obtained at 481,which can be related to Ti–Ni.19 The very intense reflectionappearing at 2y E 401 can be related to the titanium sub-strate.28 Note that no diffraction lines or peaks of Mo-containingphases were obtained, which can be related to the low concen-tration of Mo in the alloy substrate.
To further investigate the composition of the fabricatedelectrodes, an X-ray photoelectron spectroscopy (XPS) analysiswas performed on the nanotube arrays fabricated via anodizationat 20 V for 5 h in a formamide-based electrolyte. Fig. 6 shows theobtained XPS spectra with reflections from Ti 2p, Mo 3d, Ni 2pand O 1s. Fig. 6a shows the Ti 2p spectra where two peaks wereobtained corresponding to Ti 2p3/2 and Ti 2p1/2 photoemissionspectra with a spin–orbit splitting of B5.7 eV, confirming that
both signals correspond to Ti4+.30,31 Fig. 6b represents the Mo3d spectra, where two peaks can be distinguished at 232.3 and235.8 eV, confirming that Mo exists mainly in the oxidation stateof Mo6+ in the mixed oxide nanotubes.30–34 Although thepresence of Mo species with oxidation states higher than 4 mayraise a question on the ability of such species to replace Ti, theradius of Mo6+ (r = 0.062 nm) is closer to that of Ti4+ (r =0.068 nm), which makes it possible for Mo6+ to replace Ti4+ whenthe doping content is kept on a relatively low level. The spectrumof the Ni 2p region is shown in Fig. 6c. The main peaks located at855.9 eV and 873.5 eV can be related to Ni 2p3/2 and Ni 2p1/2,respectively, indicating the existence of NiO.34–36 The O 1sphotoemission spectra show one signal at 531.6 eV, which canbe related to the existence of metal oxides, Fig. 6d. The shoulderseen at 529.9 eV can be related to the existence of Mo and/or Nioxides.34,37–41 Therefore, the XPS investigation confirmed theexistence of all components of the alloy in the oxide form.
3.3 Optical and photoelectrochemical properties
To investigate the optical properties of the fabricated electro-des, the diffuse reflectance spectra (DRS) of the annealed pureTiO2 and Ti–Mo–Ni nanotube electrodes were measured; seeFig. 7. The absorption edge (390 nm for the TiO2 electrode and457–464 nm for the Ti–Mo–Ni mixed oxide electrodes) shows asignificant red shift for the mixed oxide electrodes. The slightdifference in the absorption edge positions between the mixedoxide samples can be related to the difference in nanotubelength between the tested electrodes; see Fig. 2. A similar shiftwas reported for TiO2–NiO nanotube electrodes,37,42 andrelated to the overlap between the d-orbitals of Ti with that of
Fig. 5 X-ray diffraction patterns of the annealed nanotube arrays (450 1C for4 h in air with a ramp rate of 1 degree per min) fabricated via the anodization ofTi–Mo–Ni samples in formamide-based electrolytes for 3, 5 and 7 hours.
Fig. 6 XPS spectra obtained for an annealed Ti–Mo–Ni–O nanotube electrodefabricated via the anodization of the Ti–0.3Mo–0.8Ni alloy in a formamideelectrolyte containing 0.2 M NH4F and 0.1 M H3PO4 at 20 V for 5 h.
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Ni leading to a decrease in the energy gap between Ti(d) andO(p) orbitals of Ti oxide to enable visible light absorption.Also, Stengl and Bakardjieva23 reported similar red shift forMo-doped titania films. They related the enhancement to thecreation of new energy levels that are 0.4 eV below the conduc-tion band edge, which led to a faster interfacial electron transferrate and a slower recombination rate of electron–hole pairs.Moreover, the Mo��Ti � V��O state (MoO3 $Mo��Ti þ V��O þ 2OO) isacting as an electron donor below the conduction band edge,43
where MoTi refers to Mo in the Ti lattice site, OO is an oxygenoccupying an oxygen site, VO is an oxygen vacancy, (�) representsan excess charge and (1) represents a deficiency in the charge.
A preliminary proof-of-concept photoelectrochemical activitytest for water photoelectrolysis using the synthesized Ti–Mo–Nimixed oxide nanotube arrays was carried out. Fig. 8a shows thephotocurrent density versus potential in 1 M KOH solution underAM 1.5 illumination (100 mW cm�2) for TiO2 nanotubes (16 hanodization, annealed at 450 1C for 4 h) and Ti–Mo–Ni mixedoxide nanotube array electrodes (10 h and 16 h anodization,annealed at 450 1C for 4 h). The dark current was less than5 mA cm�2 for all samples over the displayed potential range. Themaximum photocurrent of the pure TiO2 nanotube sample(B12 mm long, 20 nm wall thickness and 180 nm diameter)was B0.29 mA cm�2, in agreement with the literature.6 However,the photocurrent increased almost seven times (2.1 mA cm�2)upon the use of Ti–Mo–Ni mixed oxide nanotube electrodesfabricated and tested under the same conditions as those for thepure TiO2 nanotube electrode. We noticed that the mixed oxidenanotube samples fabricated after 10 h showed a lower photo-current (1.6 mA cm�2) than those synthesized after 16 h. Thisdifference in the photocurrent can be related to the difference innanotube length in agreement with the Finite Difference TimeDomain (FDTD) simulation, which showed that the absorptionand the photocurrent can vary linearly with the nanotubelength.26 Also, the onset potential in the case of Ti–Mo–Ni mixedoxide nanotubes (�0.98 VAg/AgCl) is more negative than that of aTiO2 nanotube array electrode (�0.92 VAg/AgCl). This open-circuit
voltage represents the contribution of light towards the minimumvoltage needed for water splitting potential.6 The current–voltage
Fig. 7 Diffuse reflectance spectra (DRS) for pure TiO2 and Ti–Mo–Ni nanotubeelectrodes annealed at 450 1C for 4 h in air.
Fig. 8 (a) Photocurrent density vs. potential in 1 M KOH solution under AM 1.5Gillumination (100 mW cm�2) for pure TiO2 and Ti–Mo–Ni nanotube electrodes,(b) the corresponding photoconversion efficiency and (c) the correspondingcurrent transients obtained for the Ti–Mo–Ni mixed oxide photoanode at anapplied voltage of 1.0 VAg/AgCl.
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characteristics of an illuminated semiconductor electrode incontact with a redox electrolyte can be described using thefollowing equation:44,45
i ¼ iph � i0 expe0V
kT
� �� 1
� �(1)
where i is the net current obtained by adding the majority andminority current components, i0 is the reverse bias saturationcurrent and iph is the illumination current, which is proportionalto the photon flux. The tested nanotube array electrodes shown-type behavior, i.e., positive photocurrents at anodic potentials.For this type of semiconductor, the surface electron density (Ns)decreases with the applied anodic potentials (Ea) as:29,45
Ns ¼ Nb exp �eEa � Vfb
kT
� �� �(2)
where Nb is the electron density in the semiconductor, Vfb is itsflat-band potential, e is the elementary charge, k is Boltzmann’sconstant; and T is the absolute temperature. Note that Ns o Nb
for an n-type semiconductor at all potentials positive of Vfb.The corresponding light energy to chemical energy conver-
sion (photoconversion) efficiencies are shown in Fig. 8b. Thephotoconversion efficiency Z was calculated via:6
Z% = [(total power output � electrical power input)/
light power input] � 100
= jp[(E0rev � |Eappl|)/I0] � 100 (3)
where jp is the photocurrent density (mA cm�2), JpE0rev is the
total power output, jp|Eappl| is the electrical power input, andI0 is the power density of incident light (100 mW cm�2). E0
rev isthe standard reversible potential, which is 1.23 VNHE and theapplied potential Eappl = Emeas � Eaoc, where Emeas is electrodepotential (versus Ag/AgCl) of the working electrode at which thephotocurrent was measured under illumination and Eaoc is theelectrode potential (versus Ag/AgCl) of the same working elec-trode under open circuit conditions under same illuminationand in the same electrolyte. The photoconversion efficienciesfor the synthesized nanotube arrays, under AM 1.5G illumina-tion, are E1.04% and 0.71% for the Ti–Mo–Ni–O electrodesand 0.13% for the TiO2 nanotubes. Although the difference intube length can be used to explain the different photoconver-sion efficiencies of the 2.8 mm and 11.8 mm long mixed oxidenanotubes,26 the very thin wall thickness of the 16 h-synthesizedTi–Mo–Ni mixed oxide nanotube arrays (9 nm) compared to thatof pure TiO2 nanotubes (20 nm) can be the reason for theenhanced photoelectrochemical response of the mixed oxidephotoanodes compared to their pure titania counterparts.6 Thenanotubular architecture, with a wall thickness of 9 nm, ensuresthat the photogenerated holes are never generated far from thesemiconductor–electrolyte interface.46 Furthermore, since halfthe wall thickness is significantly less than the minority carrierdiffusion length (E20 nm in TiO2),47 charge carrier separationtakes place efficiently. In this regard, the potential drop (DF0)
within the tube wall is a function of both Debye length (LD) andhalf the width of the wall (r0):29,46
Df0 ¼kTr0
2
6eLD2
(4)
where LD is a function of the number of ionized donors per cm3
(ND):
LD ¼ee0kT2e2ND
� �1=2(5)
The nanoscale dimensions of the nanotube walls (9 nm), alongwith the potential drop within the walls, may facilitate thediffusion of holes to the surface and escape recombination.16,46
The hole diffusion process was shown to takes place on a scale ofpicoseconds,48,49 which becomes easier when the holes arecreated within the retrieval length of the material;16,46 the sumof the depletion layer width and the diffusion length. As the r0
values of the mixed oxide nanotubes are all smaller than 10 nm(retrieval length of TiO2
50), holes are expected to escape recom-bination resulting in an enhanced photoconversion effi-ciency.50,51 This is in agreement with van de Lagemaat andco-workers who observed a substantial enhancement of thequantum yield in porous SiC made by anodic etching in HFsolutions.51
Another factor could be that the NiO and MoO3 help slowdown charge recombination and/or facilitate the hole diffusionto the interface, which should improve the conversion effi-ciency.19,20,37,38 For example, it was shown that recombinationof excitons is significantly reduced when TiO2 is mixed withNiO, due to the energy level positions of NiO that facilitate thehole transport.19 Also, the electron transfer rate controlling stepwas shown to be more efficient upon mixing TiO2 with MoO3.21
This was related to the ease of reducibility of MoO3. Moreover,as a general note the use of 4d cations as donors with higheratomic d orbital energies than that of Ti is expected to guaranteehigh electron mobility.24
To assess the stability of the mixed oxide photoanodes, thetransient photocurrent (J–t) test of the 16 h-fabricated Ti–Mo–Nielectrodes was carried out under light on/off illumination atconstant external bias of 1.0 VAg/AgCl, as shown in Fig. 8c. Thephotocurrent of the tested electrode decays very sharply underlight-off conditions without exhibiting pronounced photocurrenttails suggesting that the fabricated photoanode has excellentcarrier transport properties.52,53
Conclusions
Vertically oriented Ti–Mo–Ni mixed oxide nanotube arrays werefabricated via anodization of the Ti–Mo–Ni alloy in formamideelectrolytes containing NH4F and H3PO4 at room temperature.The morphology and dimensions of the formed nanoarchitec-tures were found to depend on both anodization voltage andanodization time. Upon anodizing the alloy at 20 V, the resultingnanotube length was almost constant (1 mm) during the first fivehours followed by an exponential increase (1.95 mm at 7 h and11.87 mm at 16 h) afterwards. The nanotube diameter increases
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linearly with increasing anodization time reaching 134 nm after10 h, and then decreases to 115 nm after 16 h. The wall thicknessof the fabricated Ti–Mo–Ni nanotubes, on the other hand, wasfound to slightly vary (24 � 5 nm) during the first 10 h followedby a decrease to 9 nm after 16 h of anodization. Studying theeffect of applied voltage showed that the nanotube lengthincreases from 0.85 mm to 3.9 mm upon increasing the appliedvoltage from 20 V to 60 V. The nanotube diameters show a steadyincrease up to 40 V (82 nm, 138 nm and 160 nm for the samplesanodized at 20 V, 30 V and 40 V, respectively) followed by anabrupt increase to 368 nm upon increasing the applied voltage to60 V. The wall thickness of nanotubes, however, showed aconstant value (B18 nm) up to 40 V, followed by a suddenincrease at 60 V (53 nm). The formation of mixed oxide nano-tubes (as confirmed via XPS analysis) showed a profound effecton the light absorption capability of the material shifting theabsorption edge from 390 nm for pure TiO2 to 464 nm for themixed oxide. The 11.8 mm long Ti–Mo–Ni mixed oxide nanotubearrays showed a three-electrode photoconversion efficiency of1.04% (AM 1.5G illumination 100 mW/cm�2, 1.0 M KOH) whenused as photoanodes to photoelectrochemically split water. Thisefficiency is almost 10 times higher than that obtained for pureTiO2 nanotubes (0.13%) of comparable length. This enhance-ment in the photoconversion efficiency can be related to thesynergistic effects of Mo and Ni oxides as well as to the uniquestructural properties of the fabricated nanotube arrays. Theobtained efficiency herein is among the highest reported valuesso far for the TiO2 nanotube-based photoelectrochemical cell.Further extended studies are currently being conducted in ourlaboratory to investigate more combinations for efficient waterphotolysis.
Notes and references
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