CHEMICAL SCIENCE

Near-infrared photon upconversion and sunlight synthesis of low-toxicity quantum dots


On February 7, 2023, Beijing time, Wu Kaifeng’s team from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences published a research result entitled “Near-infrared photon upconversion and solar synthesis using lead-free nanocrystals” in the journal Nature Photonics.

The team realized the upconversion from near-infrared to visible light sensitized by low-toxicity quantum dots, and developed an efficient and rapid sunlight synthesis method based on the upconversion system. This cross-innovative achievement is of great significance to the development of photochemistry and photosynthesis technology.

The corresponding author of the paper is researcher Wu Kaifeng, and the co-first authors are Liang Wenfei, Nie Chengming and Du Jun.

The upconversion of near-infrared photons to visible photons has important applications in many fields. In terms of solar conversion, upconversion technology can reduce the loss of photons below the semiconductor bandgap and is expected to break through the Shockley-Queisser efficiency limit of single-cell solar cells. Due to the strong penetration of near-infrared light to biological tissues, the upconversion of near-infrared light to visible light also has important potential applications in the biomedical field. The strong penetration of near-infrared light has also attracted researchers in the field of organic photocatalysis, which can help overcome the problem of low reaction rate caused by poor penetration of blue light and ultraviolet light.

Among the various upconversion technologies, photosensitization technology based on trilinear-ternal annihilation of organic molecules (TTA-UC) can upconvert non-coherent and non-pulsed light sources (such as solar photons), which has strong practical prospects. Photochemists have developed a variety of sensitizers, including organic molecules, organometallic complexes, colloidal quantum dots, and lead-halide perovskite membranes. However, photosensitizers in the near-infrared band are relatively scarce and have various problems. For example, organic complexes often contain precious metals (Pt, Pd and Os); Low upconversion efficiency of pure organic molecules; Lead halide perovskite films contain heavy metals and have a limited spectral response in the near-infrared band. Colloidal quantum dots can in principle adjust the spectral response by adjusting the material and size, but only lead-based quantum dots currently achieve TTA-UC from near-infrared to visible light.

This work is the first in the world to report lead-free copper indium selenide (CuInSe2) quantum dots as an environmentally friendly alternative to achieve high-efficiency near-infrared to visible upconversion. In addition, this work combines the upconversion system with organic photocatalysis, which not only effectively solves the problem of reabsorption loss faced by quantum dot upconversion, but also develops a new method for efficient and rapid sunlight synthesis.

Sample characterization

Figure 1: Excited state dynamics and energy transfer in ZCISe quantum dots.

The team first prepared ZnS-coated Zn-doped CuInSe2 core-shell nanocrystalline quantum dots (ZCISe; Figure 1a), which effectively solves the problem of many defects and poor stability of this kind of quantum dots. It was found that the introduction of Zn is critical to the optical properties of quantum dots, and when the molar ratio of Zn:Cu:In feeding is 1:1:1, the resulting quantum dot absorption peak is at 760 nm, the emission peak is at 780 nm, and the luminescence quantum yield is 21% (Figure 1b). Reducing the amount of Zn can redshift the absorption and luminescence peaks of quantum dots, but at the same time it is accompanied by a decrease in luminescence quantum yield. The spectral blueshift caused by Zn indicates that it is alloyed with CuInSe2, and the increase in quantum yield indicates that Zn may inhibit the formation of non-radiative defect centers.

The excited state dynamics of ZCISe were studied by time-resolved spectroscopy. The ZCISe luminescence lifetime shown in Figure 1c is up to 100 ns. At the same time, its transient absorption (TA) bleached signal, mainly contributed by edge electrons, is largely attenuated on a 1 ns timescale (Figure 1d). The combination of the two shows that in addition to holes trapped by well-known Cu-based self-defect sites in ZCISe NCs, electrons are also localized in shallow defects. The key scientific question is whether this localized electron-hole pair can participate in trilinear energy transfer.

ZCISe quantum dot three-wire energy transfer

Trilinear energy acceptor carboxylation and tetraphenyl TCA (TCA trilinear energy approx. 1.3 eV) on ZCISe surface modification by ligand exchange method. The introduction of TCA did not cause the absorption of ZCISe itself to blueshift and decrease, which excluded the etching of the surface of quantum dots by carboxyl groups in TCA. The luminescence quenching of ZCISe after modification of the upper TCA molecule was 87% (Figure 1b), which coincided with the quenching result of luminescence kinetics of 91% (Figure 1c).

TA characterization of ZCISe-TCA found that the kinetics of ZCISe-TCA and ZCISe at bleaching were comparable at the ps to ns scale (Figure 1d). Interestingly, at the ns to subms scale (Figure 1e), when the bleaching of ZCISe is completely gone, the characteristic absorption of the trilinear state of TCA molecules gradually appears at 400-500 nm (3TCA*). This phenomenon directly demonstrates the transfer of energy from the “invisible” ZCISe-deficient exciton to its ligand TCA. The average generation time of 3TCA* is 14 ns and the average recession lifetime is 134 μs.

NIR to visible upconversion

Figure 2: NIR to visible TTA-UC.

Based on ZCISe, the trilinear state of TCA can be effectively sensitized, red fluorene molecules can be added to the system, and the trilinear state of TCA ligand can be extracted to realize TTA-UC upconversion from near-infrared to visible photons (Figure 2a). Excitation of the system with a continuous laser at 808 nm did observe yellow fluorescence of the red fluorescene molecule (Figure 2b, c) with an anti-Stokes shift of 0.7 eV (Figure 2c). TTA-UC has a quantum efficiency of 16.7%. The effect of ZCISe-TCA and Rub concentrations on TTA-UC efficiency was further studied. The concentration of fixed Rub was 16 mmol L-1, when the ZCISe-TCA concentration increased from 5 to 10 μmol L-1, Φ’UC increased from 16.1% to 16.7%, and when the ZCISe concentration continued to increase to 40 μmol L-1, Φ’UC gradually decreased to 7.4%, due to the loss of upconversion photons reabsorbed by quantum dots. Conversely, the upconversion threshold has been decreasing because as the concentration of ZCISe-TCA increases, the absorbance of the system at 808 nm increases, and the resulting 3Rub* concentration increases. The ZCISe concentration was fixed at 10 μmol L-1, and the change of Rub concentration had little effect on Φ’UC, indicating that ZCISe quantum dots were the main factor causing reabsorption loss. The results are summarized in Table 1.

Table 1: Effects of different ZCISe-TCA and Rub concentrations on TTA-UC efficiency and photoredox performance.

NIR upconversion is used for organic photocatalysis

Considering the loss of upconversion efficiency caused by the above reabsorption problem, the researchers used the photoredox reaction strategy to extract 1Rub* excited state energy in situ (Figure 3a) to avoid the reabsorption problem. Another advantage of this strategy is that NIR light has strong penetration of reaction media and various reaction vessels.

The photocatalytic reaction device is shown in Figure 3b (left). The spot area of the 808 nm radiation source on the reaction vessel in the reaction is 0.3 cm2. σ-bromoacetophenone (1; Figure 3c) reduced dehalogenation was the first probe reaction, and the dehalogen reaction product was obtained under 8 h irradiation at 400 mW (1.3 W cm-2).2The yield is greater than 99%; In the absence of laser, TCA, or Rub, the product is negligible, proving that 1Rub* generated by TTA-UC drives the reaction. Based on amine (3) reacted as a second probe, and the product was obtained after 8 h4The yield is 89% (Figure 3D). In addition, in light reduction1Phenol is added to the reaction5The formation of C-O bonds can be triggered, and the product is obtained after 8 h of reaction6The yield is 97%.

Figure 3: Photochemical reaction with TTA-UC driven by near-infrared and sunlight.

The effect of concentration change on photocatalytic performance in TTA-UC was further explored by using the dehalogen reduction reaction as a probe (Table 1). Fixing the concentration of Rub at 16 mmol L-1, unlike the trend of Φ’UC as a function of ZCISe-TCA concentration, the reaction yield increased with the concentration of ZCISe-TCA. For example, when the concentration of ZCISe-TCA increased from 10 to 40 μmol L-1, the absorbance of the system at 808 nm increased by 2.8 times, but the yield increased from 19% (reaction 2h) to 78% (reaction 1h), and the reaction rate was approximately 8.2 times faster. The main reason is that the threshold of upconversion is reduced from 2.1 to 0.32 W cm-2, so the power density of 1.3 W cm-2 can reach the upconversion efficiency saturation region.

The upconversion system also drives radical polymerization of trimethylolpropane triacrylate (TMPTA). In this system, 1Rub* did not have enough energy to directly reduce TMPTA monomers, so diphenyliodioxonium hexafluorophosphate (Iod) and 9-vinylcarbazole (NVK) were added to activate the reaction. As shown in Figure 4a, the TMPTA can complete the polymerization after 12 min by irradiating the upconversion system with an 808 nm laser. Polymer gels are not formed in dark conditions and in the absence of TCA (Figure 4a).

Figure 4: Photopolymerization reaction with TTA-UC driven by near-infrared and sunlight.

Broad-spectrum absorption powers sunlight synthesis

The broad-spectrum absorption properties of ZCISe in the visible and near-infrared regions inspired us to explore “solar synthesis” by placing photocatalytic reactions in sunlight. Placing the system in sunlight on an indoor window sill (Figure 3b right) with an optical power density of 32 mW cm-2 showed a significant increase in the rate of all reactions. For example, the photoreduction reaction in Figure 3c yielded 91% in just 1 h, the amine oxidation reaction (Figure 3D) yielded 95% yield in just 2 h, and the carbon-oxygen coupling (Figure 3e) yielded 96% yield in only 0.5 h. These properties are higher than those of directly excited organic photocatalysts.

Polymerization reactions can also occur quickly in sunlight. Turn upTMPTA polymerization triggered by the system change can be gummed in only 30 s (Figure 4b). The reaction vessel was coated with aluminum foil and placed in sunlight without gumming, which excluded heat-induced polymerization. Notably, exposure to sunlight immediately after lifting the foil induces reactant polymerization due to the extremely fast reaction rate (shown on the far right of Figure 4b). This indicates that the true aggregation time may be on the order of a few seconds.

Summary and outlook

Chemists imagined and practiced organic chemical reactions driven by sunlight a hundred years ago, but at that time they mainly used a small amount of ultraviolet photons in sunlight. In recent years, thanks to the development and introduction of visible light catalysts such as Ru(bpy)32+, the field has begun to revive, but catalysts that can efficiently absorb near-infrared light are still relatively rare. Quantum dot researchers have also developed various types of quantum dots for organic photocatalytic reactions, but most of these quantum dots contain Cd or Pb heavy metals and are mainly limited to the visible band. In this work, near-infrared and low-toxicity quantum dots can simultaneously capture a large number of near-infrared photons and visible photons in sunlight, sensitize the three-wire state of organic molecules with high efficiency, and directly use the singlet state formed by the trilinear-ternary annihilation upconversion to drive the organic reaction in situ, which effectively solves the problem of reabsorption loss faced by the upconversion of quantum dots. Therefore, the upconversion-organic photocatalytic fusion system of near-infrared and low-toxicity quantum dot sensitization may open up a new paradigm for efficient and rapid sunlight synthesis. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41566-023-01156-6



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