mid-IR Absorption Spectroscopy Research Articles

Probing Charge Carrier Behavior in Engineered Photocatalysts with Time-Resolved Visible to Mid-IR Absorption Spectroscopy

Probing Charge Carrier Behavior in Engineered Photocatalysts with Time-Resolved Visible to Mid-IR Absorption Spectroscopy

High Power Irradiance Dependence of Charge Species Dynamics in Hybrid Perovskites and Kinetic Evidence for Transient Vibrational Stark Effect in Formamidinium.

Hybrid halide perovskites materials have the potential for both photovoltaic and light-emitting devices. Relatively little has been reported on the kinetics of charge relaxation upon intense excitation. In order to evaluate the illumination power density dependence on the charge recombination mechanism, we have applied a femtosecond transient mid-IR absorption spectroscopy with strong excitation to directly measure the charge kinetics via electron absorption. The irradiance-dependent relaxation processes of the excited, photo-generated charge pairs were quantified in polycrystalline MAPbI3, MAPbBr3, and (FAPbI3)0.97(MAPbBr3)0.03 thin films that contain either methylamonium (MA) or formamidinium (FA). This report identifies the laser-generated charge species and provides the kinetics of Auger, bimolecular and excitonic decay components. The inter-band electron-hole (bimolecular) recombination was found to dominate over Auger recombination at very high pump irradiances, up to the damage threshold. The kinetic analysis further provides direct evidence for the carrier field origin of the vibrational Stark effect in a formamidinium containing perovskite material. The results suggest that radiative excitonic and bimolecular recombination in MAPbI3 at high excitation densities could support light-emitting applications.

Titanium Dioxide/Polyvinyl Alcohol/Cork Nanocomposite: A Floating Photocatalyst for the Degradation of Methylene Blue under Irradiation of a Visible Light Source.

Photocatalytic degradation by the titanium dioxide (TiO2) photocatalyst attracts tremendous interest due to its promising strategy to eliminate pollutants from wastewater. The floating photocatalysts are explored as potential candidates for practical wastewater treatment applications that could overcome the drawbacks posed by the suspended TiO2 photocatalysis system. The problem occurs when the powdered TiO2 applied directly into the treated solution will form a slurry, making its reuse become a difficult step after treatment. In this study, the immobilization of titanium dioxide nanoparticles (TiO2 NPs) on the floating substrate (cork) employing polyvinyl alcohol (PVA) as a binder to anchor TiO2 NPs on the surface of the cork was carried out. Characterizations such as Fourier transformer infrared, X-ray diffraction (XRD), ultraviolet–visible spectroscopy (UV–vis), zeta potential, photoluminescence spectroscopy, femtosecond to millisecond time-resolved visible to mid-IR absorption spectroscopy, ion chromatography, and scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDX) analyses were employed. XRD analysis revealed the formation of anatase-phase TiO2 NPs. The results demonstrated that the crystallite size was 9.36 nm. The band gap energy of TiO2 NPs was determined as 3.0 eV. PL analysis verified that TiO2 NPs possessed a slower recombination rate of electron–hole pairs as compared to anatase TiO2. The result was attributed by the behavior of photogenerated charge carriers on TiO2 NPs, which existed as shallowly trapped electrons that could survive longer than a few milliseconds in this study. Furthermore, SEM–EDX analysis indicated that TiO2 NPs were well distributed on the surface of the cork. At the optimal mole ratio of TiO2/PVA (1:8), the TiO2/PVA/cork floating photocatalyst degraded at 98.43% of methylene blue (MB) under a visible light source which performed better than under sunlight irradiation (77.09% of MB removal) for 120 min. Besides, the mineralization result has measured the presence of sulfate anions after photocatalytic activities, which achieved 86.13% (under a visible light source) and 65.34% (under sunlight). The superior photodegradation performance for MB was mainly controlled by the reactive oxygen species of the superoxide radical (•O2–). The degradation kinetics of MB followed the first-order kinetics. Meanwhile, the Langmuir isotherm model was fitted for the adsorption isotherm. The floating photocatalyst presented good reusability, resulting in 78.13% of MB removal efficiency even after five cycles. Our TiO2/PVA/cork floating photocatalyst fabrication and high photocatalytic performance are potentially used in wastewater treatment, especially under visible light irradiation.

Focus on analytical equipment, spectroscopy, and spectrometry

Focus on analytical equipment, spectroscopy, and spectrometry

InGaAs Membrane Waveguide: A Promising Platform for Monolithic Integrated Mid-Infrared Optical Gas Sensor.

Mid-infrared (mid-IR) absorption spectroscopy based on integrated photonic circuits has shown great promise in trace-gas sensing applications in which the mid-IR radiation directly interacts with the targeted analyte. In this paper, considering monolithic integrated circuits with quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), the InGaAs-InP platform is chosen to fabricate passive waveguide gas sensing devices. Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and subwavelength grating cladding waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ = 6.15 μm in the low-index contrast InGaAs-InP platform. We experimentally detect 5 ppm ammonia with a 1 mm long suspended HPCW and separately with a 3 mm long suspended SWW, with propagation losses of 39.1 and 4.1 dB/cm, respectively. Furthermore, based on the Beer-Lambert infrared absorption law and the experimental results of discrete components, we estimated the minimum detectable gas concentration of 84 ppb from a QCL/QCD integrated SWW sensor. To the best of our knowledge, this is the first demonstration of suspended InGaAs membrane waveguides in the InGaAs-InP platform at such a long wavelength with gas sensing results. Also, this result emphasizes the advantage of SWWs to reduce the total transmission loss and the size of the fully integrated device's footprint by virtue of its low propagation loss and TM mode compatibility in comparison to HPCWs. This study enables the possibility of monolithic integration of quantum cascade devices with TM polarized characteristics and passive waveguide sensing devices for on-chip mid-IR absorption spectroscopy.

Spiral Waveguides on Germanium-on-Silicon Nitride Platform for Mid-IR Sensing Applications

Spiral waveguides on a new germanium-on-silicon nitride (GON) platform with a wide transparency and a large core-clad index contrast for mid-infrared (mid-IR) sensing applications are demonstrated. Spiral waveguide sensors with a low bending loss on this platform enable compact sensors for mid-IR absorption spectroscopy. A minimum volumetric concentration of 5% isopropanol (IPA) in an IPA-acetone mixture is measured. This detection limit is three times lower than the counterpart waveguide, fabricated on the regular germanium-on-silicon platform with similar propagation loss at 3.73 μm wavelength. This silicon-compatible GON sensor is promising for applications such as environmental studies, industrial leak detection, process control, medical breath analysis, and many more.

Structural changes of water molecules during photoelectrochemical water oxidation on TiO2 thin film electrodes.

Behaviors of photogenerated charge carriers and structural changes of water molecules on TiO2 photoelectrodes were investigated by using time-resolved visible to mid-IR absorption spectroscopy. From the spectra measured in the visible to NIR region, it was shown that the lifetime of trapped electrons and holes becomes longer upon applying more positive potentials. This result was reasonably explained by the enhancement of the upward band bending at the water/TiO2 interface. On the other hand, from the spectra measured in the mid-IR region, structural changes of the water molecules were observed. When a TiO2 electrode was photoexcited at the potential where the water oxidation starts, a new absorption peak appeared at 3620 cm-1 with a slight decrease in the intensity of hydrogen-bonded water. This new peak was assigned to the isolated O-H band of water molecules. Usually, TiO2 surfaces exhibit super-hydrophilic properties with strong hydrogen-bonding; however, the obtained result was opposite. Therefore, the appearance of this isolated O-H band was ascribed to the cleavage of the hydrogen-bonding networks resulting from the production of reaction intermediates such as OH radicals or H2O2. The intensity of the isolated O-H decreases when applying more positive potentials, where the O2 evolution proceeds more efficiently. This could be ascribed to the rapid consumption of the reaction intermediates. At these potentials, the intensity of hydrogen-bonded water was also further decreased.

Role of CoOx cocatalyst on Ta3N5 photocatalysts studied by transient visible to mid-infrared absorption spectroscopy

Tantalum nitride (Ta3N5) photocatalysts attract considerable attention owing to their visible light absorption below 600nm and suitable band structures for reduction and oxidation of water to generate hydrogen and oxygen gas. Herein, we investigated the effects of CoOx on the dynamics of photogenerated charge carriers in Ta3N5 powder photocatalysts by using microsecond to millisecond transient visible to mid-IR absorption spectroscopy. Transient absorption measurements revealed that most of the electrons in Ta3N5 are deeply trapped at the defects, most likely originating from oxygen vacancies and reduced Ta species. Two important roles of CoOx were elucidated: effective capture of holes to prolong electron lifetime and reduction of defects as recombination centers, which further elongates the lifetime of both electrons and holes.

Trapping-Induced Enhancement of Photocatalytic Activity on Brookite TiO2 Powders: Comparison with Anatase and Rutile TiO2 Powders

Brookite TiO2 is a promising material for active photocatalysts. However, the principal mechanism that determines the distinctive photocatalytic activity between anatase, rutile, and brookite TiO2 has not yet been fully elucidated. Therefore, in this work, we studied the behavior of photogenerated electrons and holes in these TiO2 powders by using femtosecond to millisecond time-resolved visible to mid-IR absorption spectroscopy. We found that most of the photogenerated electrons in brookite TiO2 are trapped at powder defects within a few ps. This electron trapping decreases the number of surviving free electrons, but it extends the lifetime of holes as well as the trapped electrons because the probability of electrons to encounter holes is decreased by this electron-trapping. As a result, the number of surviving holes increases, which is beneficial for photocatalytic oxidation. In contrast, the reactivity of electrons is decreased to some extent by trapping, but they still remain active for photocatalyti...

Mid-IR absorption sensing of heavy water using a silicon-on-sapphire waveguide.

We demonstrate a compact silicon-on-sapphire (SOS) strip waveguide sensor for mid-IR absorption spectroscopy. This device can be used for gas and liquid sensing, especially to detect chemically similar molecules and precisely characterize extremely absorptive liquids that are difficult to detect by conventional infrared transmission techniques. We reliably measure concentrations up to 0.25% of heavy water (D2O) in a D2O-H2O mixture at its maximum absorption band at around 4 μm. This complementary metal-oxide-semiconductor (CMOS) compatible SOS D2O sensor is promising for applications such as measuring body fat content or detection of coolant leakage in nuclear reactors.

Mid-infrared laser filaments in air at a kilohertz repetition rate

Laser filamentation overcomes diffraction over a highly extended distance, making itself a powerful tool for long-range stand-off detection and light detection and ranging (LIDAR) applications. Mid-infrared (mid-IR) wavelengths are optimal for detecting biochemicals and air pollutants due to molecular fingerprints. Here, we demonstrate mid-IR laser filamentations in ambient air at a kilohertz repetition rate for the first time. Laser filaments significantly longer than the linear confocal parameter are generated with a pump power exceeding the critical power in air using a kilohertz, 2.1 μm, femtosecond, multi-millijoule optical parametric chirped-pulse amplifier. Odd-harmonic generation up to the ninth order at ultraviolet and the mid-IR spectral extensions up to 3.5 μm are observed. The highest third and fifth harmonic efficiencies from ambient air are obtained, to our knowledge, thanks to the extended interaction length within the filaments. Numerical simulations reproduce the harmonic generation with good agreement and confirm that the plasma effect dominates over the higher-order Kerr effect as the main defocusing mechanism of laser filamentation in our experiment. The detection of atmospheric CO2 is demonstrated via mid-IR absorption spectroscopy. High-flux ultrabroadband mid-IR filaments are useful for the fast and sensitive detection of multiple chemical species in air.

Distinctive Behavior of Photogenerated Electrons and Holes in Anatase and Rutile TiO2 Powders

TiO2 powders have been widely used for photocatalysts; however, why anatase shows higher activity than rutile has been a long-standing question. Here, we have elucidated the difference in the behavior of photogenerated electrons and holes by time-resolved visible to mid-IR absorption spectroscopy. In anatase TiO2, a considerable number of free electrons survive longer than 1 ms, but they are deeply trapped within a few picoseconds in the case of rutile TiO2. The longer lifetime of free electrons is responsible for the higher activity for reduction processes on anatase TiO2. However, deep electron trapping in rutile TiO2 elongates lifetime of holes and promotes multihole processes such as water oxidation. However, the low reactivity of deeply trapped electrons fails to increase the overall activity. These peculiar behaviors of electrons and holes are induced by defects on the powder particles and less sensitive to the physical properties such as particle size and specific surface area.

1 kHz Mid-IR Absorption Spectroscopy for CO and CO2 Concentration and Temperature Measurement

A sensor for in situ combustion gas measurements of concentration and temperature of carbon monoxide (CO) and carbon dioxide (CO2) has been developed and tested on a temperature calibrated diffusion flame burner in order to measure combustion efficiency. The fundamental bands of both molecules are exploited using distributed feedback quantum cascade lasers near 4.40 and 4.58 µm at which the absorption line-strengths are multiple orders of magnitude larger than overtone and combination bands at lower wavelengths. Spectra were taken at a rate of 1 kHz across a 50.8-mm path length of an ethylene (C2H4)-air diffusion flame at heights of 5 to 25 mm above the burner and at equivalence ratios ranging from 0.5 to 1.5. Total combustion efficiency was measured using the concentrations of CO and CO2, and temperature was obtained from relative strengths of multiple CO2 absorption lines with differing lower state energies. The results are compared to previous coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements and equilibrium chemical calculations.

Morphology-sensitive trapping states of photogenerated charge carriers on SrTiO3 particles studied by time-resolved visible to Mid-IR absorption spectroscopy: The effects of molten salt flux treatments

Abstract The effects of the morphology-change of SrTiO3 particles on the behavior of photogenerated charge carriers are studied by time-resolved absorption (TA) spectroscopy from the visible to mid-IR region. In the case of as-purchased defect-rich commercial SrTiO3 particles, most of the charge carriers are deeply trapped, showing a transient absorption peak at 11,000 cm−1. Scanning electron microscopy reveals that the irregular-shaped primary particles are heavily aggregated and that the photocatalytic activity for the water splitting reaction is negligibly small. However, when this powder is flux-treated by SrCl2, fine cubic SrTiO3 crystals exposing well-defined surfaces are formed and the photocatalytic activity is greatly improved. TA measurements show that the spectral shape is changed dramatically: the number of deeply trapped electrons is reduced, and that of shallowly trapped electrons producing the absorption band at 2500 cm−1 is increased. The change in electron trap depth, observed upon flux treatment, is due to the decrease in the number of defects. We also found that further flux treatment in an Al2O3 crucible (i) induces Al doping into SrTiO3, (ii) enhances the photocatalytic activity, (iii) changes the spectral shape, and (iv) prolongs the lifetime of shallowly trapped electrons. The increase in photocatalytic activity is presumably due to the change in lifetime.

Behaviors and Energy States of Photogenerated Charge Carriers on Pt or CoOx Loaded LaTiO2n Photocatalysts Studied By Time-Resolved Absorption Spectroscopy

For the development of highly efficient photocatalysts, energy states as well as the behaviors of photogenerated charge carriers should be elucidated. Femtosecond to second time-resolved visible to mid-IR absorption spectroscopy is useful to investigate the matters, therefore we applied this method to study the role of cocatalysts loaded on visible-light responsible LaTiO2N photocatalyst: this catalyst has poor activity for H2 evolution but highest activity for O2 evolution when CoOx was loaded. Time-resolved absorption study revealed that most of the electrons are trapped in the mid-gap states, created below 0.74 eV (6000 cm-1) from the conduction band. As a result, the number of free electrons giving the transient absorption below 2000 cm-1 was much smaller than that of the trapped electrons. When Pt was loaded, the electrons were captured by Pt and the lifetime of the holes (which give the absorption band at 17000 cm-1) became longer, however these processes proceeded at tens of microseconds and the efficiency was not high enough. On the contrary, the CoOx-loading resulted in the capture of holes within a few ps, and drastically elongated the lifetime of electrons to second region. Additionally, the depth of the electron trap became shallower from 0.74 eV (6000 cm-1) to 0.49 eV (4000 cm-1), suggesting the reactivity of the trapped electrons becomes higher. These perturbations to the electrons and holes are responsible for the increase of the photocatalytic activities. We expected that co-loading of Pt and CoOx further increased the activities but the recombination was accelerated.

Two-line thermometry and H2O measurement for reactive mixtures in rapid compression machine near 7.6 μm

A much desired, direct, in situ, diagnostics technique is implemented to measure time variations of temperature and water concentration for a reactive mixture in a rapid compression machine (RCM) using quantum cascade laser absorption spectroscopy near 7.6μm. The temperature measurements in the RCM are successfully conducted for an end of compression pressure of PC∼11.5bar and an end of compression temperature of TC∼1022K for H2/O2/Ar mixtures with (0.5% by mole in the reactive mixture) and without water doping. For all cases investigated, the molar percentages of H2 and O2 in the reactive mixture are respectively kept at 3% while the balance is Ar, in order to modulate the extent of post-ignition pressure rise. Absorption lines of water at 1316.55cm−1 and 1316.97cm−1 are used in this study for the measurements. A six-pass setup inside the RCM is implemented for the reactive experiments. For determining the line broadening parameters of water in the range of 1316.4–1317.8cm−1 with H2 or O2 as a bath gas, a six-pass, 50.8mm Herriott cell is used in the calibration experiments. The broadening parameters are measured in the pressure range of 300–650Torr, while the Herriott cell is heated uniformly to temperatures of 460, 500, and 530K, respectively. With the measured line broadening parameters of water in the bath gases of interest, the temperature and water concentration histories in the RCM runs are experimentally determined. In addition, the experimental results are compared with the simulations with detailed chemistry. Reasonable agreement is found, thereby demonstrating the utility of this mid-IR absorption spectroscopy in RCM experiments. The uncertainties of the associated measurements of temperature and water concentration are also discussed.

Excitation wavelength dependent pump–probe signatures of molecular crystals

The photodynamics of single crystals of 4-(diisopropylamino)benzonitrile (DIABN) have been studied by femtosecond UV/Vis and mid-IR absorption spectroscopy. The observed spectroscopic response, especially in the UV/Vis, strongly depends on the optical excitation conditions. For the excitation light in resonance with the absorption of DIABN, broad and structureless transient spectra are observed, which decay non-exponentially within pico- to nanoseconds. For off-resonance excitation in the very red wing of the absorption band a stimulated emission signal is detected, which decays in ∼10 ps matching the known behavior of the non-interacting chromophore in solution. The 10 ps decay is due to an intramolecular charge transfer process which is supported by femtosecond IR spectroscopy. The observed dependence on the excitation wavelength is assigned to a change in the density of excited chromophores and therefore bears relevance for time resolved X-ray diffraction experiments.

Dynamics of Geminate Recombination of NO with Myoglobin in Aqueous Solution Probed by Femtosecond Mid-IR Spectroscopy

The dynamics of nitric oxide (NO) rebinding to ferrous myoglobin (Mb) and its model system, microperoxidase (Mp), in D2O solution at 283 K after photolysis was directly observed by monitoring the stretching mode of NO using femtosecond mid-IR absorption spectroscopy. The transient spectra of photolyzed MbNO reveal two overlapping vibrational bands for bound NO: 1598 (13.1 cm-1 fwhm) and 1611 cm-1 (12.7 cm-1 fwhm). Time-resolved spectra of photolyzed MpNO are well described by a single vibrational band at 1658 cm-1 (26 cm-1 fwhm). Narrower vibrational bands within MbNO indicate that the protein environment encompassing bound NO is more homogeneous than the solvent environment of MpNO. The rebinding kinetics of photolyzed MbNO are the same for both vibrational bands, but are nonexponential and evolve according to the biexponental function: 0.54 exp(−t/5.3 ps) + 0.46 exp(−t/133 ps). In contrast, rebinding to MpNO is exponential with a time constant of 5.6 ps. MbNO exists in two distinct conformational substates that have the same geminate recombination kinetics. The ultrafast rebinding rate of MpNO indicates that rebinding of NO to the heme is almost barrierless. The similarity of the rebinding rate in MpNO to the faster phase of that in MbNO suggests that the slower phase of the rebinding in Mb be due to the protein environment surrounding the distal side of heme, and conformational relaxation of the protein after photolysis may raise the barrier of NO rebinding.

Multiple-Exponential Electron Injection in Ru(dcbpy)2(SCN)2 Sensitized ZnO Nanocrystalline Thin Films

We have studied the electron injection dynamics of Ru(dcbpy)2(SCN)2 sensitized ZnO nanocrystalline thin films by femtosecond mid-IR absorption spectroscopy. Upon 400 nm excitation of the sensitizer, nonexponential electron injection kinetics from the sensitizer MLCT excited state to ZnO were observed by probing the absorption of injected electrons in the 2000 to 1900 cm-1 region on the <1 ns time scale. A three-exponential fit to the data yields the following rise time constants and amplitudes (in parentheses): < 1ps (18%), 42ps (46%), and 450ps (36%). For films with increasing coverage of sensitizer molecules (optical density at 500 nm of 0.7, 2.3, 2.6), similar electron injection times were observed, while the injection yield decreases. This result suggests that aggregation or multilayer formation of sensitizer molecules reduces the quantum yield of electron injection to ZnO on the <1 ns time scale, and the observed multiexponential injection kinetics are attributed to the distribution of injection rates from the first monolayer of sensitizers. Possible reasons for the nonexponential injection are discussed. The injection dynamics are also compared with the very different injection dynamics observed for the same sensitizer on TiO2.