TiO2-based Photoanodes Research Articles

Electrochemical reduction induced self-doping of oxygen vacancies into Ti–Si–O nanotubes as efficient photoanode for boosted photoelectrochemical water splitting

Self-doping of oxygen vacancies (VO) states into TiO2-based nanotubes was an efficient way for improving photoelectrochemical (PEC) water splitting properties. Here we induced oxygen vacancies into Si-doped TiO2 (Ti–Si–O) nanotubes on Ti–Si alloy via a facile electrochemical surface reduction, and applied it for PEC water splitting. Systematic studies revealed that the self-doped oxygen vacancies not only promoted optical absorption of the doped nanotubes but also enhanced separation-transport processes of the photo-generated charge carriers, and thus resulted in improved PEC water splitting properties. The VO/Ti–Si–O co-doping system exhibited a higher photocurrent density of 1.63 mA/cm2 at 0 V vs. Ag/AgCl. Corresponding solar-to-hydrogen efficiency could reach 0.81%, which was about 5.4 times that of undoped TiO2. It's believed that elements doping and oxygen vacancies self-doping synergistic strategy employed in this work, may provide theoretical and practical significance for designing and fabricating efficient TiO2-based nanostructures photoanodes in PEC water splitting for boosted solar-to-hydrogen conversion.

Probing the dye-semiconductor interface in dye-sensitized NiO solar cells.

The development of p-type dye-sensitized solar cells (p-DSSCs) offers an opportunity to assemble tandem photoelectrochemical solar cells with higher efficiencies than TiO2-based photoanodes, pioneered by O'Regan and Grätzel [Nature 353, 737-740 (1991)]. This paper describes an investigation into the behavior at the interfaces in p-DSSCs, using a series of BODIPY dyes, BOD1-3. The three dyes have different structural and electronic properties, which lead to different performances in p-DSSCs. We have applied photoelectron spectroscopy and transient absorption spectroscopy to rationalize these differences. The results show that the electronic orbitals of the dyes are appropriately aligned with the valence band of the NiO semiconductor to promote light-induced charge transfer, but charge-recombination is too fast for efficient dye regeneration by the electrolyte. We attribute this fast recombination, which limits the efficiency of the solar cells, to the electronic structure of the dye and the presence of Ni3+ recombination sites at the NiO surface.

Heteroacoagulation of lignocellulose fibers-based biotemplates and functionalized TiO2 nanoparticles to tailor film microstructures

The heterocoagulation of lignocellulose nanofibers (LCNFs) with functionalized TiO2 nanoparticles (NPs) is presented as an innovative revalorization strategy for the exploitation of lignocellulose wastes. Their use as bio-templates could be considered a promising alternative in a current biorefinery scheme, since the massive production of porous materials in numerous nanotechnological applications, could offer new alternatives of exploitation. The surface modification of TiO2 NPs by adsorption of polyetilenimine (PEI) favors its anchorage with LCNFs through a peptide bond during heterocoagulation in aqueous suspension. The functionalization of LCNFs with TiO2 NPs results in organic–inorganic core–shell nanostructures, used to shape coatings by dipping, and sinter them at low temperature (450 °C). In this process, the LCNFs were used as endo-templates for shaping stable porous coatings, when they burn during the consolidation of the inorganic structure. In this work, the successful inclusion and homogeneous distribution of biomaterials (LCNF templates) in the semiconductor inorganic microstructure is discussed in terms of dye loading (by UV–Visible Spectroscopy), photovoltaic efficiency and charge transfer (by Electrochemical Impedance Spectroscopy, EIS) in a Dye Sensitized Solar Cell (DSSC). Results confirm that the TiO2/template network yielded better photoefficiency and electron transport properties than mesoporous films assembled only from TiO2 NPs. The preparation of TiO2-based photoanodes with enhanced performance (with an efficiency value around 6%, for a thickness of 8.7 µm) allows validating the successful colloidal procedure herein employed.

Investigation of electrical values of low-efficiency dye-sensitized solar cells (DSSCs)

Indium-tin-oxide (ITO)/density (d)- titanium dioxide (TiO2)/nonporous (nonp)-TiO2 is a TiO2-based bilayer photoanode. These photoanodes coated for dye-sensitized solar cells (DSSCs) were with nonp-TiO2 surface and had very thick ITO layer. DSSCs were fabricated with these photoanodes. After their current density-voltage (J-V) were analyzed, ıt was seen that they were low efficiency and the shape of their J-V curve was linear line instead of rectangle. In this study, it was examined the relationship between the dark current (IDC), the series resistance (Rs) and the shunt resistance (Rsh) were examined for evaluating the fill factor (FF) of low-efficiency DSSCs because of the photoanodes with nonp-TiO2 surfaces and very thick ITO for contact layer. It was seen in the J-V graph of DSSCs which had low FF value due to their IDC and low Rsh. The J-V graph of low-efficiency DSSCs is linear due to their low Rsh and high Rs values. Moreover, the too thick ITO layer decreases the resistance of photoanodes; however, electrons in these photoanodes can not efficiently transferred to external circuit from ITO contact layer of DSSCs.

An investigation of the stirring duration effect on synthesized graphene oxide for dye-sensitized solar cells.

This study investigates the effects of stirring duration on the synthesis of graphene oxide (GO) using an improved Hummers’ method. Various samples are examined under different stirring durations (20, 40, 60, 72, and 80 h). The synthesized GO samples are evaluated through X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The GO sample with 72 h stirring duration (GO72) has the highest d-spacing in the XRD results, highest atomic percentage of oxygen in EDX (49.57%), highest intensity of oxygen functional group in FTIR spectra, and highest intensity ratio in Raman analysis (ID/IG = 0.756). Results show that GO72 with continuous stirring has the highest degree of oxidation among other samples. Electrochemical impedance spectroscopy analysis shows that GO72–titanium dioxide (TiO2) exhibits smaller charge transfer resistance and higher electron lifetime compared with the TiO2-based photoanode. The GO72 sample incorporating TiO2 nanocomposites achieves 6.25% photoconversion efficiency, indicating an increase of more than twice than that of the mesoporous TiO2 sample. This condition is fully attributed to the efficient absorption rate of nanocomposites and the reduction of the recombination rate of TiO2 by GO in dye-sensitized solar cells.

Dual-bonding interactions between MnO2 cocatalyst and TiO2 photoanodes for efficient solar water splitting

Severe charge recombination and sluggish water oxidation kinetics greatly limited the practical applications of photoelectrochemical (PEC) water splitting. Herein, we demonstrated the nitrogen plasma treatment for simultaneously achieving oxygen vacancies and N-doping on TiO2 nanotube arrays. More importantly, the oxygen vacancies and N-doping could effectively promote the selective construction of MnO2 cocatalyst anchored with Mn-O/N bonds on TiO2 nanotube arrays, which could efficiently narrow charge depletion layer and promote the holes transfer from TiO2 to MnO2. As expected, this photoanode achieves a remarkably increased PEC performance (1.95 mA cm−2 at 1.23 VRHE), up to 3.4 times higher than that of pristine TiO2 (0.57 mA cm−2 at 1.23 VRHE), and excellent stability for water splitting. These demonstrations may provide a new insight for designing highly efficient TiO2-based photoanodes for solar energy conversion.

Simultaneously efficient light absorption and charge transport of CdS/TiO2 nanotube array toward improved photoelectrochemical performance

CdS has been widely used to modify TiO2-based photoanodes for photoelectrochemical (PEC) water splitting. Due to the poor interface contact between chalcogenides and oxides, however, such CdS modified TiO2 materials usually exhibit inefficient separation and transport of charges, leading to an unsatisfactory efficiency during the PEC water splitting process. Addressing this issue, we herein report a CdS/TiO2 nanotube array (CdS/TNA) photoanode that was fabricated through a successive ion layer absorption and reaction (SILAR) method with an additional subsequent annealing. This post-annealing process is essential to enhance the interface contact between the CdS and the TNAs, resulting in an accelerated transfer of photogenerated electrons from the CdS to the TNAs. In addition, the post-annealing also improves the light absorption capability of the CdS/TNA photoanode. The simultaneous enhancement of charge transport and light absorption provided by the post-annealing is essential for improving the PEC performance of the CdS/TNA photoanode. The CdS/TNA photoanode obtained by this strategy exhibits a much enhanced PEC performance in water splitting, and its photocurrent density and solar-to-hydrogen conversion efficiency could reach 4.56 mA cm−2 at 1.23 V vs. reversible hydrogen electrode and 5.61%, respectively. This simple but effective route can provide a general strategy for obtaining high-performance oxide-based photoelectrodes.

Optimizing Dye Adsorption in Graphene–TiO2Photoanodes for the Enhancement of Photoconversion Efficiency of DSSC Devices

We report on the effect of graphene (G) incorporation in TiO2 photoanodes (PAs) for improving their dye adsorption capacity, which in turn impacts the overall photoconversion efficiency of dye-sensitized solar cells (DSSCs). By varying the graphene content of TiO2 PAs (over the 0.05–1 wt.% range), the power conversion efficiency (PCE) of the DSSCs was found to increase significantly from 3.0% for standard TiO2 to a maximum value of 8.2% for PAs containing 0.1 wt. % of graphene. This corresponds to a PCE improvement of ∼173% in comparison with standard DSSCs made with TiO2 alone (without graphene). On the other hand, by performing thermogravimetric analyses to quantify the dye adsorption capacity of the TiO2–G PAs, we were able to establish, for the first time, a direct correlation between the PCE of the DSSCs and the dye uptake of their G–TiO2-based PAs. Our results demonstrate that by coupling the optimal PA (with the optimal graphene content of 0.1 wt.% and optimal sensitization with a solution of 4 mM of N719) with Co–Ni nanoparticles decorated multi-walled carbon nanotubes (MWCNTs) based counter electrodes, we were able to achieve DSSCs exhibiting a PCE as high as 9.8%. This performance is quite impressive particularly considering that no platinum was used in the counter electrode.

Ultrafine M-doped TiO2 (M = Fe, Ce, La) nanosphere photoanodes for photoelectrochemical water-splitting applications

In this study, Fe-, Ce-, and La-doped TiO2 ultrafine nanospheres were synthesized via low-temperature solvothermal method. The structural and morphological variations of bare and M (Fe, Ce, La)- doped TiO2 were confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy images. The fabricated photoanodes were tested using 1 M KOH electrolyte and demonstrated that they can be used for eco-friendly photoelectrochemical solar water-splitting application. Compared to bare TiO2 photoanodes, foreign cations (Fe, Ce, La)-doped TiO2-based photoanodes exhibited higher photocurrent response without bias. In nutshell, the rare-earth element La-doped TiO2 nanosphere photoanode showed good photocurrent response to approximately 0.48 mA/cm2 under illumination, with lower flat band potential shift of −0.83 V. The highest photostability of 96% was achieved over 2 h for 10% La-doped TiO2 nanosphere photoanode and was suitable for solar water-splitting applications.

A New Class of Molecular-Based Photoelectrochemical Cell for Solar Hydrogen Production Consisting of Two Mesoporous TiO2 Electrodes

A new class of molecular-based photoelectrochemical cell for solar hydrogen production consisting of a TiO2-based photoanode modified with a polypyridyl ruthenium photosensitizer (Ru-qpy) and a TiO2-based cathode modified with a platinum porphyrin H2 evolution catalyst has been investigated, showing that the electron accumulation at the conduction band of TiO2 at the photoanode is promoted via electron injection from the Ru-qpy together with hole scavenging by a sacrificial donor. Our study here for the first time unveils that the upward shift given in the Fermi level of the TiO2 at the photoanode provides an electromotive force required to flow electrical current leading to solar hydrogen production from water even without applying external electrical bias.

TiO2/graphene/CuSbS2 mixed-dimensional array with high-performance photoelectrochemical properties.

The growing demands for reproducible and clean sources of power has prompted the exploitation of novel materials for solar-energy conversion; in any case, the improvement of their conversion efficiency remains a big challenge. We report a mixed-dimensional heterostructure to synchronously enhance charge separation and light-absorption of the photoanodes via the introduction of two-dimensional reduced graphene oxide and zero-dimensional CuSbS2 quantum dots on one-dimensional TiO2 arrays. The experimental results show that the graphene sheets with a low Fermi level and a superior electron mobility accept photo-excited electrons from TiO2 and enable fast electron transportation; while the CuSbS2 quantum dots promote the visible light-absorption of the photoanode. The synergistic effects in this mixed-dimensional (1D–2D–0D) heterostructure photoanode induce a markedly raised photoconversion efficiency of 1.2% at 0.3 V and a photocurrent density of 5.5 mA cm−2 at 0.4 V. Furthermore, the photocurrent density of the mixed-dimensional heterostructure exceeds previously reported TiO2-based photoanodes in neutral media. The improved photoelectrochemical properties are attributed to the synergistic-effect-induced highly organized, mixed-dimensional architectures. It is expected that the mixed-dimensional heterostructure photoanode will be a potential candidate for applications in environmental remediation and energy fields.

Incorporation of indium in TiO2-based photoanodes for enhancing the photovoltaic conversion efficiency of dye-sensitized solar cells

Sol–gel-assisted spin-coating method was used to prepare TiO2 photoelectrodes doped with four different concentrations of indium 0.025, 0.05, 0.075 and 0.1 M. The crystalline phase and average crystallite size of the pure- and indium-doped TiO2 films were found using X-ray diffractometer. Raman analysis was performed for the pure- and In-doped TiO2 films to confirm the structure of anatase phase. UV–visible and photoluminescence spectrophotometer were used to analyze the optical properties of the films. A shift towards a lower wavelength in the absorption spectrum and widening of band gap were noted for the doped TiO2 films. Reduction in the peak intensity was observed in the PL spectra to indicate the inhibiting action of electron–hole recombination. A maximum (2.71%) light to current efficiency is noted for the dye-sensitized solar cells (DSSC) fabricated based on 0.025M In-doped TiO2 electrode.

Effect of MnO2 doping on the structure and optical proprieties of rutile TiO2-based photoanodes

Doped TiO2-based photoanodes of dye-sensitized solar cells have been synthesized by solid state reaction technique. The influence of MnO2 dopant on the crystal structure and optical properties of photoanodes was investigated by means of XRD, FTIR, and UV–Visible spectroscopy techniques. X-ray diffraction revealed a decrease in lattice parameters of tetragonal rutile TiO2 with increasing of the dopant content accompanied by a valley-like behavior of its lattice distortion. Infrared spectroscopy study showed a growth of the absorption band at 262 cm−1 and shift towards lower energies for the two absorption peaks at 428.7 and 358 cm−1 with increasing of dopant content. These results were explained and found to support XRD analysis of the prepared TiO2–(MnO2)x samples. The optical energy gap showed a linear decrease from 3.06 eV for undoped sample to 2.75 eV for MnO2 content with 0.3 mass fraction. The behavior of extinction coefficient (k), refractive index (n) and optical conductivity (σ opt ) with variation of MnO2 content was investigated.

3D flowerlike TiO2/GO and TiO2/MoS2 heterostructures with enhanced photoelectrochemical water splitting

TiO2 nanoflowers modified with MoS2 and GO nanosheets are prepared by a one-step hydrothermal method followed by a dip-coating process. The morphology, structure and composition of the samples are investigated by XRD, SEM and XPS, respectively. The photoelectrochemical (PEC) measurement reveals that the modified samples exhibit obvious improved photocurrent density. The enhancement can be attributed to the higher absorption in visible light region and faster charge transmission as analyzed by the UV–Vis absorption, photoluminescence spectra and electrochemical impedance analysis. This facile method provides a promising low-cost way to enhance the PEC performance of TiO2-based photoanodes.

Black Si-doped TiO2 nanotube photoanode for high-efficiency photoelectrochemical water splitting.

Black Si-doped TiO2 (Ti–Si–O) nanotubes were fabricated through Zn metal reduction of the Ti–Si–O nanotubes on Ti–Si alloy in an argon atmosphere. The nanotubes were used as a photoanode for photoelectrochemical (PEC) water splitting. Both Si element and Ti3+/oxygen vacancies were introduced into the black Ti–Si–O nanotubes, which improved optical absorption and facilitated the separation of the photogenerated electron–hole pairs. The photoconversion efficiency could reach 1.22%, which was 7.18 times the efficiency of undoped TiO2. It demonstrated that a Si element and Ti3+/oxygen vacancy co-doping strategy could offer an effective method for fabricating a high-performance TiO2-based nanostructure photoanode for improving PEC water splitting.

Enhanced charge separation of CuS and CdS quantum-dot-cosensitized porous TiO2-based photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using high-performance catalysts shows considerable promise in generating environment-friendly hydrogen energy. Its practical applications, however, suffer from several shortcomings, such as low photocurrent density, large onset-voltage value, and poor durability. In this study, CuS and CdS quantum-dot-cosensitized porous TiO2-based PEC catalysts (CuS-CT) have been successfully synthesized via in situ sulfuration of CuO and CdO coexisting inside a porous TiO2 monolith by a hydrothermal method. Compared to porous TiO2, CuS-sensitized porous TiO2 (CuS-TiO2), and CdS-sensitized porous TiO2 (CdS-TiO2) in terms of PEC performance, the CuS-CT photoanode exhibited a significantly high anodic photocurrent for water splitting under simulated sunlight radiation. The photocurrent produced by the optimized sample of 7% CuS-5% CdS-TiO2 (7% CuS-CT) was nearly 2.7 times higher than that of pure porous TiO2 at 1.0V versus a reversible hydrogen electrode (RHE). Porous TiO2 possesses large surface areas that can drive fast electrolyte transport and afford more surface reaction active sites. On the other hand, CuS and CdS quantum dots not only broaden the visible light absorption range, but also improve photoinduced electron-hole separation efficiency. The co-sensitized multi-nanostructures photoanodes lead to a remarkable and promising application in PEC water splitting reactions.

Solid-state photoelectrochemical cell with TiO2 nanotubes for water splitting

We have fabricated and tested a photoelectrochemical (PEC) cell where the aqueous electrolyte has been replaced by a proton conducting hydrated Nafion® polymer membrane. The membrane was sandwiched between a TiO2-based photoanode and a Pt/C-based cathode. The performance was tested with two types of photoanode electrodes, a thermally prepared TiO2 film on Ti foil (T-TiO2) and a nanostructured TiO2 films in the form of highly ordered nanotubes (TNT) of different lengths. Firstly, photovoltammetry experiments were conducted under asymmetric conditions, where the anode was immersed in deionized water, while the cathode was kept in ambient air. The results showed a high incident photon-to-current efficiency (IPCE) of 19% under unassisted conditions (short-circuit, 0 V vs. cathode) with short TNT (ca. 1 μm) under 4 mW cm−2 illumination with UV-A rich light. Secondly, the deionized water was replaced by 0.5 M Na2SO4 and now the performance was higher with longer nanotubes, assigned to increased ionic conductivity inside the tubes. An unassisted (0 V) IPCE of 33% was achieved with nanotubes of ca. 8 μm. The presented solid-state PEC cell minimizes the electrode distance and volume of the device, and provides a way towards compact practical applications in solar water splitting.

How Should Iron and Titanium be Combined in Oxides to Improve Photoelectrochemical Properties?

We discuss here for the first time how to combine iron and titanium metal ions to achieve a high photoelectrochemical activity for TiO2-based photoanodes in water splitting devices. To do so, a wide range of photoelectrode materials with tailored Ti/Fe ratio and element vicinity were synthesized by using the versatility of aqueous sol–gel chemistry in combination with a microwave-assisted crystallization process. At low ferric concentrations, single phase TiO2 anatase doped with various Fe amounts were prepared. Strikingly, at higher ferric concentrations, we observed the concomitant crystallization of two polymorphs of Fe2TiO5. The as-synthesized compounds were tested as photoelectrodes and compared with pure nanoparticles of TiO2, Fe2TiO5, and α- or γ-Fe2O3 and with corresponding nanocomposites. When TiO2 is slightly doped by Fe, the performance of this photoelectrode improves particularly in the low-bias region (<1.0 V vs reversible hydrogen electrode.) The photoanode exhibits a higher photocurrent tha...

Cu2ZnSnS4 Nanoparticle Sensitized Metal-Organic Framework Derived Mesoporous TiO2 as Photoanodes for High-Performance Dye-Sensitized Solar Cells.

We present a facile hot injection and hydrothermal method to synthesize Cu2ZnSnS4 (CZTS) nanoparticles sensitized metal-organic frameworks (MOFs)-derived mesoporous TiO2. The MOFs-derived TiO2 inherits the large specific surface area and abundantly porous structures of the MOFs structure, which is of great benefit to effectively enhance the dye loading capacity, prolong the incident light traveling length by enhancing the multiple interparticle light-scattering process, and therefore improve the light absorption capacity. The sensitization of CZTS nanoparticles effectively enlarges the photoresponse range of TiO2 to the visible light region and facilitates photoinduced carrier transport. The formed heterostructure between CZTS nanoparticles and MOFs-derived TiO2 with matched band gap structure effectively suppresses the recombination rates of photogenerated electron/hole pairs and prolongs the lifespan of the carriers. Photoanodes based upon CZTS/MOFs-derived TiO2 photoanodes can achieve the maximal photocurrent of 17.27 mA cm(-2) and photoelectric conversion performance of 8.10%, nearly 1.93 and 2.21 times higher than those of TiO2-based photoanode. The related mechanism and model are investigated. The strikingly improved photoelectric properties are ascribed to a synergistic action between the MOFs-derived TiO2 and the sensitization of CZTS nanoparticles.

Pre- and post-oxidation treatment of titanium as a method of improving the response of TiO2-based photoanodes in PECs

TiO2-based photoanodes were prepared via thermal oxidation of titanium foils in air at 500–900[Formula: see text]C. Some of them were pre-treated via etching with HCl prior to oxidation, and others post-treated by means of sensitization with Ag2S or CdS. XRD analysis revealed that the formation of rutile is observed even at 500[Formula: see text]C. Etching prior to oxidation removes the Ti passivation layer and facilitates the oxidation process, and lowers the temperature at which the fundamental absorption edge is observed. The highest photocurrent values measured in a photoelectrochemical cell (PEC) were obtained for anodes etched prior to oxidation. Sensitization with CdS or Ag2S increased photocurrent drastically. It was demonstrated that pre- and post-treated TiO2-based photoanodes prepared in the process of thermal oxidation offer far better performance in PECs than those which had not been treated.