The Effect of Photoinduced Surface Oxygen Vacancies on the Charge Carrier Dynamics in TiO2 Films

High-speed mapping of surface charge dynamics using sparse scanning Kelvin probe force microscopy

Unraveling local dynamic charge processes is vital for progress in diverse fields, from microelectronics to energy storage. This relies on the ability to map charge carrier motion across multiple length- and timescales and understanding how these processes interact with the inherent material heterogeneities. Towards addressing this challenge, we introduce high-speed sparse scanning Kelvin probe force microscopy, which combines sparse scanning and image reconstruction. This approach is shown to enable sub-second imaging (>3 frames per second) of nanoscale charge dynamics, representing several orders of magnitude improvement over traditional Kelvin probe force microscopy imaging rates. Bridging this improved spatiotemporal resolution with macroscale device measurements, we successfully visualize electrochemically mediated diffusion of mobile surface ions on a LaAlO3/SrTiO3 planar device. Such processes are known to impact band-alignment and charge-transfer dynamics at these heterointerfaces. Furthermore, we monitor the diffusion of oxygen vacancies at the single grain level in polycrystalline TiO2. Through temperature-dependent measurements, we identify a charge diffusion activation energy of 0.18 eV, in good agreement with previously reported values and confirmed by DFT calculations. Together, these findings highlight the effectiveness and versatility of our method in understanding ionic charge carrier motion in microelectronics or nanoscale material systems.

Understanding Bridging Sites and Accelerating Quantum Efficiency for Photocatalytic CO2 Reduction

We report a novel double-shelled nanoboxes photocatalyst architecture with tailored interfaces that accelerate quantum efficiency for photocatalytic CO2 reduction reaction (CO2RR) via Mo-S bridging bonds sites in Sv-In2S3@2H-MoTe2. The X-ray absorption near-edge structure shows that the formation of Sv-In2S3@2H-MoTe2 adjusts the coordination environment via interface engineering and forms Mo-S polarized sites at the interface. The interfacial dynamics and catalytic behavior are clearly revealed by ultrafast femtosecond transient absorption, time-resolved, and in situ diffuse reflectance-Infrared Fourier transform spectroscopy. A tunable electronic structure through steric interaction of Mo-S bridging bonds induces a 1.7-fold enhancement in Sv-In2S3@2H-MoTe2(5) photogenerated carrier concentration relative to pristine Sv-In2S3. Benefiting from lower carrier transport activation energy, an internal quantum efficiency of 94.01% at 380 nm was used for photocatalytic CO2RR. This study proposes a new strategy to design photocatalyst through bridging sites to adjust the selectivity of photocatalytic CO2RR.

Photo-induced enhanced Raman spectroscopy as a probe for photocatalytic surfaces

Photo-induced enhanced Raman spectroscopy (PIERS) has emerged as a highly sensitive surface-enhanced Raman spectroscopy (SERS) technique for the detection of ultra-low concentrations of organic molecules. The PIERS mechanism has been largely attributed to UV-induced formation of surface oxygen vacancies (Vo) in semiconductor materials, although alternative interpretations have been suggested. Very recently, PIERS has been proposed as a surface probe for photocatalytic materials, following Vo formation and healing kinetics. This work establishes comparison between PIERS and Vo-induced SERS approaches in defected noble-metal-free titanium dioxide (TiO2-x) films to further confirm the role of Vo in PIERS. Upon application of three post-treatment methods (namely UV-induction, vacuum annealing and argon etching), correlation of Vo kinetics and distribution could be established. A proposed mechanism and further discussion on PIERS as a probe to explore photocatalytic materials are also presented. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Customization of an atomic force microscope for multidimensional measurements under environmental conditions

Atomic force microscopy (AFM) is an analytical surface characterization tool that reveals the surface topography at a nanometer length scale while probing local chemical, mechanical, and even electronic sample properties. Both contact (performed with a constant deflection of the cantilever probe) and dynamic operation modes (enabled by demodulation of the oscillation signal under tip-sample interaction) can be employed to conduct AFM-based measurements. Although surface topography is accessible regardless of the operation mode, the resolution and the availability of the quantified surface properties depend on the mode of operation. However, advanced imaging techniques, such as frequency modulation, to achieve high resolution, quantitative surface properties are not implemented in many commercial systems. Here, we show the step-by-step customization of an atomic force microscope. The original system was capable of surface topography and basic force spectroscopy measurements while employing environmental control, such as temperature variation of the sample/tip, etc. We upgraded this original setup with additional hardware (e.g., a lock-in amplifier with phase-locked loop capacity, a high-voltage amplifier, and a new controller) and software integration while utilizing its environmental control features. We show the capabilities of the customized system with frequency modulation-based topography experiments and automated voltage and/or distance spectroscopy, time-resolved AFM, and two-dimensional force spectroscopy measurements under ambient conditions. We also illustrate the enhanced stability of the setup with active topography and frequency drift corrections. We believe that our methodology can be useful for the customization and automation of other scanning probe systems.

Ultra-Violet-Assisted Scalable Method to Fabricate Oxygen-Vacancy-Rich Titanium-Dioxide Semiconductor Film for Water Decontamination under Natural Sunlight Irradiation

This work reports the fabrication of titanium dioxide (TiO₂) nanoparticle (NPs) films using a scalable drop-casting method followed by ultra-violet (UV) irradiation for creating defective oxygen vacancies on the surface of a fabricated TiO₂ semiconductor film using an UV lamp with a wavelength oof 255 nm for 3 h. The success of the use of the proposed scalable strategy to fabricate oxygen-vacancy-rich TiO₂ films was assessed through UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The Ti 2p XPS spectra acquired from the UV-treated sample showed the presence of additional Ti³⁺ ions compared with the untreated sample, which contained only Ti⁴⁺ ions. The band gap of the untreated TiO₂ film was reduced from 3.2 to 2.95 eV after UV exposure due to the created oxygen vacancies, as evident from the presence of Ti³⁺ ions. Radiation exposure has no significant influence on sample morphology and peak pattern, as revealed by the SEM and XRD analyses, respectively. Furthermore, the photocatalytic activity of the fabricated TiO₂ films for methylene-blue-dye removal was found to be 99% for the UV-treated TiO₂ films and compared with untreated TiO₂ film, which demonstrated only 77% at the same operating conditions under natural-sunlight irradiation. The proposed UV-radiation method of oxygen vacancy has the potential to promote the wider application of photo-catalytic TiO₂ semiconductor films under visible-light irradiation for solving many environmental and energy-crisis challenges for many industrial and technological applications.

Titania as Buffer Layer for Cd-Free Kesterite Solar Cells

Pure sulfide kesterite (Cu₂ZnSnS₄) is one of the most promising emerging photovoltaic technologies thanks to its excellent absorption coefficient, cost-effectiveness, and environmental sustainability. However, record efficiencies are not exceeding 11% due to several issues, such as absorber defects or a nonoptimal band alignment with the toxic but conventionally used CdS buffer layer. To get rid of it, several efforts have been made in the past few years. Among recent theoretical works, TiO₂ has been suggested as a suitable buffer layer due to its optical and electrical properties, giving extremely promising results in device simulation. However, there are few experimental examples combining TiO₂ with kesterite, and they generally show very modest performances. In this Letter, we report on the preliminary and promising results of our experimental procedure for the production of Cd-free kesterite photovoltaic devices featuring ALD-TiO₂ as a buffer layer, leading to efficiencies comparable with our CZTS/CdS reference devices.

Effect of Vacuum-Sealed Annealing and Ice-Water Quenching on the Structure and Photocatalytic Acetone Oxidations of Nano-TiO2 Materials

In the current research, P25 TiO₂ materials sealed in quartz vacuum tubes were subject to annealing and ice-water post-quenching, with the effects on TiO₂ structures, morphology, and photocatalytic activity being studied. It is shown that the vacuum-sealed annealing can lead to a decrease in the crystallinity and temperature of anatase-to-rutile phase transition. A disorder layer is formed over TiO₂ nanoparticles, and the TiO₂ lattices are distorted between the disorder layer and crystalline core. The ice-water post-quenching almost has no effect on the crystalline structure and morphology of TiO₂. It can be seen that the vacuum-sealed annealing can generate more defects, and the electrons are mainly localized at lattice Ti sites, as well as the percentage of bulk oxygen defects is also increased. Although further ice-water post-quenching can introduce more defects in TiO₂, it does not affect the electron localization and defect distribution. The vacuum-sealed annealing process can increase the photocatalytic acetone oxidations of the anatase phase TiO₂ to some extent, possibly because of the defect generation and Ti³⁺ site formation; the further ice-water quenching leads to a decrease in the photocatalytic activity because more defects are introduced.

Synthesis Control of Charge Separation at Anatase TiO2 Thin Films Studied by Transient Surface Photovoltage Spectroscopy

For the efficient photocatalytic oxidation of organic pollutants at surfaces of semiconductors, photogenerated holes shall be separated toward the surface and transferred to reactive surface sites, whereas the transfer of photogenerated electrons toward the surface shall be minimized. In this Research Article, the identification of suitable synthesis control of charge separation combined with an in-depth understanding of charge kinetics and trapping passivation mechanisms at the related surfaces can provide tremendous opportunities for boosting the photocatalytic performance. In this work, a comprehensive transient surface photovoltage spectroscopy study of charge separation at anatase TiO₂ thin films, synthesized by ultrasonic spray pyrolysis from titanium(IV) isopropoxide (TTIP)-acetylacetone (AcacH) based precursor is reported. By varying the amount of AcacH in the precursor solution, an experimental approach of synthesis control of the charge transfer toward TiO₂ surface is provided for the first time. An increased amount of AcacH in the precursor promotes transition from preferential fast electron to preferential fast hole transfer toward anatase surface, correlating with a strong increase of the photocatalytic decomposition rate of organic pollutants. Suitable mechanisms of AcacH-induced passivation of electron traps at TiO₂ surfaces are analyzed, providing a new degree of freedom for tailoring the properties of photocatalytic systems.

Catalytic Metasurfaces Empowered by Bound States in the Continuum

Photocatalytic platforms based on ultrathin reactive materials facilitate carrier transport and extraction but are typically restricted to a narrow set of materials and spectral operating ranges due to limited absorption and poor energy-tuning possibilities. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, allow optical absorption engineering for a wide range of materials. Moreover, tailored resonances in nanostructured materials enable strong absorption enhancement and thus carrier multiplication. Here, we develop an ultrathin catalytic metasurface platform that leverages the combination of loss-engineered substoichiometric titanium oxide (TiO_2-x) and the emerging physical concept of optical bound states in the continuum (BICs) to boost photocatalytic activity and provide broad spectral tunability. We demonstrate that our platform reaches the condition of critical light coupling in a TiO_2-x BIC metasurface, thus providing a general framework for maximizing light-matter interactions in diverse photocatalytic materials. This approach can avoid the long-standing drawbacks of many naturally occurring semiconductor-based ultrathin films applied in photocatalysis, such as poor spectral tunability and limited absorption manipulation. Our results are broadly applicable to fields beyond photocatalysis, including photovoltaics and photodetectors.

Direct measurement of surface photovoltage by AC bias Kelvin probe force microscopy

Surface photovoltage (SPV) measurements are a crucial way of investigating optoelectronic and photocatalytic semiconductors. The local SPV is generally measured consecutively by Kelvin probe force microscopy (KPFM) in darkness and under illumination, in which thermal drift degrades spatial and energy resolutions. In this study, we propose the method of AC bias Kelvin probe force microscopy (AC-KPFM), which controls the AC bias to nullify the modulated signal. We succeeded in directly measuring the local SPV by AC-KPFM with higher resolution, thanks to the exclusion of the thermal drift. We found that AC-KPFM can achieve a SPV response faster by about one to eight orders of magnitude than classical KPFM. Moreover, AC-KPFM is applicable in both amplitude modulation and frequency modulation mode. Thus, it contributes to advancing SPV measurements in various environments, such as vacuum, air, and liquids. This method can be utilized for direct measurements of changes in surface potential induced by modulated external disturbances.

Modulating the Oxidation State of Titanium via Dual Anions Substitution for Efficient N2 Electroreduction

The electrocatalytic nitrogen reduction reaction (NRR) is a promising approach for renewable ammonia synthesis but remains significantly challenging due to the low yield and poor selectivity. Herein, a facile N and S dual anions substitution strategy is developed to tune the Ti oxidation states of TiO₂ nanohybrid catalyst (NS-TiO₂ /C), in which anatase TiO₂ nanoplates with dense Ti³⁺ active sites are uniformly dispersed on porous carbon derived from 2D Ti₃ C₂ T_x nanosheets. The catalyst NS-TiO₂ /C exhibits a superior ambient NRR efficiency with an NH₃ yield rate of 19.97 µg h^-1 mg^-1_cat and Faradaic efficiency of 25.49% and is coupled with a remarkable 50 h long-term stability at -0.25 V versus RHE. Both experimental and theoretical results reveal that the N and S dual-substitution effectively regulate the Ti oxidation state and electronical properties of the NS-TiO₂ /C via simultaneously forming interstitial and substitutional TiS and TiN bonds in the anatase TiO₂ lattice, inducing oxygen vacancies and dense Ti³⁺ active species as well as better electronic conductivity, which substantially facilitates N₂ chemisorption and activation, and reduces the energy barrier of the rate-determining step, thereby essentially boosting NRR efficiency. This work provides a valuable approach to the rational design of advanced materials by modulating oxidation states for efficient electrocatalysis.

Enhancing Hole Density and Suppressing Recombination Centers through Illumination in Kesterite Thin Film Solar Cells

Enhancing carrier density and increasing carrier lifetime are critical for the good performance of thin film solar cells. We apply illumination during the growth of kesterite Cu₂ZnSnS₄ (CZTS) to enhance hole density and suppress defects of nonradiative electron-hole recombination centers simultaneously. To examine the effect of the injected carriers generated by illumination, we first extend the scheme of detailed balance equations relating free carriers and defects beyond thermal equilibrium conditions by developing an extended Fermi level (E_F^') to characterize a homogeneous semiconductor with non-equilibrium carriers. On the basis of this scheme, we find that illumination can promote the formation of carrier-providing defects and suppress the formation of carrier-compensating defects. Then, we demonstrate that applying proper illumination during the growth of CZTS will help achieve a higher hole density and simultaneously suppress the formation of the Sn_Zn antisite significantly, which are beneficial for the performance of CZTS solar cells.

Defect engineering in semiconductor-based SERS

Semiconductor-based surface enhanced Raman spectroscopy (SERS) platforms take advantage of the multifaceted tunability of semiconductor materials to realize specialized sensing demands in a wide range of applications. However, until quite recently, semiconductor-based SERS materials have generally exhibited low activity compared to conventional noble metal substrates, with enhancement factors (EF) typically reaching 103, confining the study of semiconductor-based SERS to purely academic settings. In recent years, defect engineering has been proposed to effectively improve the SERS activity of semiconductor materials. Defective semiconductors can now achieve noble-metal-comparable SERS enhancement and exceedingly low, nano-molar detection concentrations towards certain molecules. The reason for such success is that defect engineering effectively harnesses the complex enhancement mechanisms behind the SERS phenomenon by purposefully tailoring many physicochemical parameters of semiconductors. In this perspective, we introduce the main defect engineering approaches used in SERS-activation, and discuss in depth the electromagnetic and chemical enhancement mechanisms (EM and CM, respectively) that are influenced by these defect engineering methods. We also introduce the applications that have been reported for defective semiconductor-based SERS platforms. With this perspective we aim to meet the imperative demand for a summary on the recent developments of SERS material design based on defect engineering of semiconductors, and highlight the attractive research and application prospects for semiconductor-based SERS.