Electrocatalysis for CO2 conversion: from fundamentals to value-added products.

The continuously increasing CO2 released from human activities poses a great threat to human survival by fluctuating global climate and disturbing carbon balance among the four reservoirs of the biosphere, earth, air, and water. Converting CO2 to value-added feedstocks via electrocatalysis of the CO2 reduction reaction (CO2RR) has been regarded as one of the most attractive routes to re-balance the carbon cycle, thanks to its multiple advantages of mild operating conditions, easy handling, tunable products and the potential of synergy with the rapidly increasing renewable energy (i.e., solar, wind). Instead of focusing on a special topic of electrocatalysts for the CO2RR that have been extensively reviewed elsewhere, we herein present a rather comprehensive review of the recent research progress, in the view of associated value-added products upon selective electrocatalytic CO2 conversion. We initially provide an overview of the history and the fundamental science regarding the electrocatalytic CO2RR, with a special introduction to the design, preparation, and performance evaluation of electrocatalysts, the factors influencing the CO2RR, and the associated theoretical calculations. Emphasis will then be given to the emerging trends of selective electrocatalytic conversion of CO2 into a variety of value-added products. The structure-performance relationship and mechanism will also be discussed and investigated. The outlooks for CO2 electrocatalysis, including the challenges and opportunities in the development of new electrocatalysts, electrolyzers, the recently rising operando fundamental studies, and the feasibility of industrial applications are finally summarized.

Growth modulation of Ga2S3 horizontal nanowires and its optical properties.

Highly ordered Ga2S3 horizontal nanowires (NWs) on r-plane sapphire were successfully grown by chemical vapor deposition using carbothermal reduction reaction. By adjusting experimental conditions, the density of NWs has increased from 8.4 × 106 to 4.0 × 107 mm-2. The morphology, structure, composition and optical properties of Ga2S3 NWs were characterized through field emission scanning electron microscopy, atomic force microscope, transmission electron microscopy, x-ray diffraction, Raman, x-ray photoelectron spectroscopy and so on. The Au particles at the end of the NWs prove that the growth of the horizontal NWs is controlled by the VLS mechanism. The measurement of nonlinear properties of Ga2S3 nanomaterials indicates one-dimensional Ga2S3 processes excellent nonlinear effect. Our work opens a new way to synthesize Ga2S3 NWs and provides a reference to explore the optical applications of Ga2S3 nanomaterials such as micro tunable laser in the future.

High performance of a passively Q-switched mid-infrared laser with Bi2Te3/graphene composite SA.

We report passively Q-switched ∼2 and ∼3 µm mid-infrared (MIR) solid-state lasers with a self-assembly solvothermal-synthesized Bi2Te3/graphene heterostructure saturable absorber (SA) for the first time. Based on the oxidation resistance and high thermal conductivity of graphene, and large modulation depth of Bi2Te3 nanosheets, two high-performance Q-switching lasers were realized. One is a Tm:YAP laser with a maximum average output power of 2.34 W and a pulse width of 238 ns at ∼2 µm. The corresponding maximum pulse peak power was 91 W, which was much improved in comparison with the pure graphene-based Tm laser. The other one is an Er:YSGG laser producing a pulse width of 243 ns, which is the shortest among the 2D SAs-based ∼3 µm solid-state lasers, as far as we know. Our results indicate that such a composite Bi2Te3/graphene material is a promising SA for generating high-performance mid-infrared pulse lasers.

Dual function of Nd3+ in Nd,Er:LuYSGG crystal for LD pumped ~3.0 µm mid-infrared laser.

Enhanced ~3.0 µm emission corresponding to Er3+:4I11/2→4I13/2 was achieved in Nd3+/Er3+ co-doped Lu0.15Y2.85Sc2Ga3O12 (abbr. as Nd,Er:LuYSGG) crystal under 808 nm pumping. As compared with Er:YSGG crystal, the absorption pump efficiency of Nd,Er:LuYSGG crystal is greatly improved and thus ~3.0 µm emission is enhanced by 2.2 times owing to the sensitization of Nd3+, at the same time, Nd3+ as deactivator quenches ~1.5 µm emission from Er3+:4I13/2 level and thus inhibit the self-termination effect successfully. The energy transfer efficiencies of Nd: 4F3/2→Er: 4I11/2 and Er: 4I13/2→Nd:4I15/2 are estimated to be 91.6% and nearly 100%, respectively. These results indicate that the introduction of Nd3+ is very helpful for achieving ~3.0 µm laser in Nd,Er:LuYSGG crystal.

Single-longitudinal-mode Er:GGG microchip laser operating at 2.7 µm.

We reported on a diode-end-pumped single-longitudinal-mode microchip laser using a 600-µm-thick Er:GGG crystal at ∼2.7 µm, generating a maximum output power of 50.8 mW and the maximum pulsed energy of 0.306 mJ, with repetition rates of pumping light of 300, 200, and 100 Hz, respectively. The maximum slope efficiency of the laser was 20.1%. The laser was operated in a single-longitudinal mode centered at about 2704 nm with a FWHM of 0.42 nm. The laser had a fundamental beam profile and the beam quality parameter M(2) was measured as 1.46. These results indicate that the Er:GGG microchip laser is a potential compact mid-infrared laser source.

Single-frequency CaWO4 Raman amplifier at 1178 nm.

Single-frequency crystalline Raman amplifications at 1178 nm were demonstrated. The seeding laser was generated from a single-frequency continuous wave fiber Raman amplifier. Three stages of Raman amplifications from CaWO4 single crystals were realized with a pulsed 1064 nm Nd:YAG laser as the pumping source. The final output pulse energy at 1178 nm was 26.7 mJ, and the pulse width was 2.9 ns, corresponding to a peak power of 5.2 MW. The overall Raman amplification ratio was up to 4.6×10(6). The linewidth was less than 500 MHz.

Comparison of actively Q-switched laser performance of disordered Yb:Ca3La2(BO3)4 crystals cut along the crystallographic axes.

In this paper, actively Q-switched laser operation with an acousto-optic switch has been demonstrated by using Yb:Ca3La2(BO3)4 crystals cut along the a, b, and c crystallographic axes. The most efficient Q-switched laser operation was obtained by using b-cut Yb:Ca3La2(BO3)4 crystal with 1 kHz pulse repetition frequency, generating laser pulses of 0.5 mJ, 42.56 KW peak power and 9 ns pulse width, when the output couplings were 3%, 5%, and 5%, respectively. Pulse performances and output laser spectra of the a-, b-, and c-cut Yb:Ca3La2(BO3)4 crystals were compared under similar experimental conditions.

Self-Q-switched, orthogonally polarized, dual-wavelength laser using long-lifetime Yb³âº crystal as both gain medium and saturable absorber.

A novel self-Q-switched and orthogonally polarized dual-wavelength laser has been investigated with Yb³âº-doped CGB disordered crystals. By slightly inclining output coupler to introduce the Fresnel loss, we realized simultaneously dual-wavelength laser operation at 1052.6 nm in E//b polarization and 1057.7 nm in E//c polarization with a frequency difference of 1.38 THz. Self-Q-switched pulse generation was observed in this free-running laser, originating from the nonlinear reabsorption effect of Yb:CGB as well as the strong storage of inversion population induced by the long excited-state lifetime (~1 ms). Pulse duration of 287 ns was obtained with an output average power of 416 mW and a repetition rate of 35 kHz. The self-Q-switching effect increased the peak power by 100 times the average power. This very simple laser, free from the complexity and high-cost of additional intracavity polarization modulator and Q-switcher, may be applied for constructing miniature, integrated and portable laser system.

2.4 W highly efficient simultaneous dual-wavelength laser operation of monoclinic Yb(3+):Ca4LaO(BO3)3 crystals.

We have studied the simultaneous dual-wavelength laser operation of the X-cut, Y-cut, and Z-cut Yb(3+):Ca4LaO(BO3)3 crystals for the first time to the best of our knowledge. We analyzed the dual-wavelength laser output power and emission spectra under several output coupler transmittances. The stable dual-wavelength lasers were generated by adjusting the laser elements. A dual-wavelength laser output power 2.46 W was obtained with a slope efficiency of 67.5% by employing the Y-cut crystal at 1029 and 1036 nm. The two wavelengths had nearly the same relative intensity. The two different emission wavelengths were found to change with crystal direction and output coupler transmittance. This laser has a possible application as the laser source in the generation of different terahertz waves.

Activation effect of Ho3+ at 2.84 µm MIR luminescence by Yb3+ ions in GGG crystal.

The use of Yb(3+) co-doping for the enhancement of Ho(3+):(5)I(6)→(5)I(7) mid-IR (MIR) emissions was investigated in GGG crystal for the first time. It is established that Yb(3+) highly increases Ho(3+) 2.84 µm emissions by transforming pump energy from the Yb(3+):(2)F(5/2) level to the laser upper level (5)I(6) of Ho(3+). The energy-transfer efficiency from Yb(3+):(2)F(5/2) to Ho(3+):(5)I(6) is calculated to be 96.2%. The absorption cross section, emission cross section, and fluorescence quantum efficiency are estimated and discussed. It is concluded that the Yb, Ho:GGG crystal is promising material for an LD-pumped 2.84 µm laser application.

Output-coupling-dependent laser operation of monoclinic Yb:Ca4LaO(BO3)3 crystal.

We have studied the laser operation of Yb:LaCOB crystal for what we believe is the first time. Adopting a plane-concave cavity, the laser properties were measured for the X-cut, Y-cut, and Z-cut crystals. The best output power of 3.2 W was obtained with a slope efficiency 69.90% by employing the Y-cut crystal. A more efficient operation with slope efficiency of 82.31% was realized for the X-cut crystal, with the output power of 2.0 W. The laser spectra were collected under the absorbed pump power of 3.9 W for X cut and Z cut and 6.7 W for Y cut. The emission wavelengths were dependent of the transmittance of output couplers.

Achieving carbon neutrality: the effect of China pilot Free Trade Zone policy on green technology innovation.

Evaluating the effect of China pilot Free Trade Zone (FTZ) policy on green technology innovation is important for achieving China's carbon neutrality targets. Based on the panel data of 30 provincial administrative regions in China from 2009 to 2019, this study investigates the effect of the pilot FTZ policy on green technology innovation by using the difference-in-differences method. The study's findings indicate the following: (1) The pilot FTZ policy promotes the development of green technology innovation, and there is a policy lag in a few pilot regions. (2) The mediation effect analysis shows that the pilot FTZ policy promotes the development of green technology innovation by improving the marketization process and enhancing innovative talent gathering. (3) The heterogeneity analysis shows that the pilot FTZ policy is more effective in promoting green technology innovation when implemented in regions with developed economies or higher levels of human capital. Moreover, the pilot FTZ policy mainly has a significant promoting effect on green utility model patents. Based on these results, policy recommendations are proposed to promote the development of green technology innovation and the achievement of China's carbon neutrality targets.

Pr3+-doped CeF3 crystals: Analysis of optical traits and fluorescence from Pr3+: 1D2 level for visible and near-infrared lasers.

In this work, four Pr3+ -doped CeF3 crystals with 0.2, 0.4, 0.6, and 0.8 at.% Pr3+ ion doping levels nominally in the melt have been successfully grown, and their absorption and emission spectra, including fluorescence decay times were analyzed. Especially, yellow and near-infrared (NIR) emissions originating from Pr3+: 1D2 level were explored in detail. In comparison, 0.2 at.% Pr3+-doped sample showed better spectral parameters in all studied crystals. In such a crystal, at 443 nm wavelength, the derived absorption cross-section (σabs) is 0.88 × 10-20 cm2 with full width at half maximum (FWHM) âˆ¼ 10 nm in π polarization direction, whereas σabs is 1.32 × 10-20 cm2 with FWHM âˆ¼ 6.7 nm in σ polarization direction at the same wavelength. Likewise, the calculated emission cross-section (σem) for 594 nm wavelength is 0.69 × 10-20 cm2 in π direction and 0.46 × 10-20 cm2 in σ direction. Also, acquired σem at 790, 755, 1014, and 1432 nm wavelengths is 6.15 × 10-21 cm2, 7.33 × 10-21 cm2, 7.66 × 10-21 cm2, and 6.56 × 10-21 cm2 individually. Here evaluated fluorescence decay time of 1D2 level is âˆ¼ 200.6 µs. Obtained higher σabs, larger σem, and higher luminescence decay rates of 0.2 at.% Pr: CeF3 crystal specify its potential as a gain medium for orange and NIR lasers.

Structure and spectral properties of Dy3+ doped CaYAlO4 single crystal.

A 2 at.% Dy3+: CaYAlO4 single crystal was grown successfully. The electronic structures of Ca2+/Y3+ mixed sites in CaYAlO4 were investigated using first-principles based on density functional theory. The effects of Dy3+ doping on the structural parameters of host crystal were studied using XRD pattern. The optical properties including absorption spectrum, excitation spectrum, emission spectra and fluorescence decay curves were thoroughly investigated. The results show that the Dy3+: CaYAlO4 crystal could be pumped by the blue InGaN and AlGaAs or 1281 nm laser diodes. Furthermore, an intense 578 nm yellow emission was obtained directly under excitation at 453 nm, meanwhile, evident mid-infrared light emitting was observed by 808 or 1281 nm laser excitation. The fitted fluorescence lifetimes of 4F9/2 and 6H13/2 levels were about 0.316 ms and 0.038 ms, respectively. It can be concluded that this Dy3+: CaYAlO4 crystal could simultaneously act as a promising medium for both solid-state yellow and mid-infrared laser outputs.

Benefit of Pr3+ ions to the spectral properties of Pr3+/Er3+:CaGdAlO4 crystal for a 2.7 µm laser.

The spectra of Er3+:CaGdAlO4 and Pr3+/Er3+:CaGdAlO4 crystals were measured under the same conditions. With the presence of Pr3+ ions, the intensities of the green, red upconversion and near-infrared emissions are dramatically reduced to 1/20, 1/150, and 1/8.5 of those for Er3+:CaGdAlO4. Furthermore, the lifetimes of 4I11/2 upper and 4I13/2 lower laser levels fall from 450 and 982 µs to 84.8 and 74.3 µs. It is obvious that the self-saturation for Er3+ 2.7 µm laser is effectively suppressed. The energy transfer efficiencies and coefficients of Er3+→Pr3+ are also calculated. These results indicate that Pr3+ codoping is beneficial in achieving 2.7 µm laser in Pr3+/Er3+:CaGdAlO4 crystal.

High efficient and broadband ∼3.5 µm emission of Er3+:YAG crystal.

The YAG single crystals doped with 10 at.%, 20 at.% and 50 at.% Er3+ were successfully grown by the micro-pulling down method and spectroscopic properties of the crystals were investigated. The main interest was focus on the relation between the Er3+ concentration and ∼3.5 µm emission of Er3+:YAG crystals. Room temperature absorption spectra were analyzed by the Judd-Ofelt theory. The stimulated emission cross-sections were calculated by the Füchtbauer-Ladenburg equation. The fluorescence intensities and peak emission cross-sections of the crystals at ∼3.5 µm are slightly decreasing with the increase of Er3+ concentration. The trend of the emission properties in NIR and visible region with the Er3+ concentration was also discussed and compared. The results indicate that the highly doped Er3+ concentration is beneficial to realize the ∼3.5 µm laser output.

Spectral analyses of Dy3+/Sr2+: LaF3 and Dy3+/Ca2+: LaF3 mixed crystals for laser applications.

Mixed crystals of Dy3+/Sr2+: LaF3 and Dy3+/Ca2+: LaF3 were grown by Bridgman technique and their spectral properties were investigated. Spectra broadening and peak shifts were observed, indicating the co-doping of Sr2+/Ca2+ brings about a more disordered local symmetry of Dy3+, which makes both crystals favorable for tunable lasing action. Low-temperature high resolution excitation and emission spectra were carried out for exploring the types of luminescent center of Dy3+ in crystals. Room-temperature absorption and emission spectra, together with the fluorescence decay curves were studied in both crystals for estimating their potentials for yellow and MIR lasers. Under 450 nm excitation, the largest emission cross-sections at 571 nm of 1.51 × 10-21 cm2 for Dy3+/Sr2+: LaF3 crystal and 1.56 × 10-21 cm2 for Dy3+/Ca2+: LaF3 crystal, along with the lifetimes of Dy3+: 4F9/2 level as 0.983 ms for Dy3+/Sr2+: LaF3 crystal, 1.143 ms for Dy3+/Ca2+: LaF3 crystal were obtained, respectively. Besides yellow emissions, MIR emissions approximately at 3 µm are more appealing. Under 1280 nm excitation, the largest emission cross-sections of 0.304 × 10-20 cm2 at 2885 nm in Dy3+/Sr2+: LaF3 crystal, and 0.319 × 10-20 cm2 at 2880 nm in Dy3+/Ca2+: LaF3 crystal, together with rather long lifetimes of Dy3+: 6H13/2 in the level of milliseconds were achieved, making them useful media for MIR lasers.

White up-conversion luminescent property in Re3+:Gd3Ga5O12 nanocrystals.

Rare-earth ions doped Gd3Ga5O12 nanocrystals have been prepared by a propellant combustion synthesis method and their up-conversion properties were systematically investigated in this paper. XRD, SEM, TEM, and up-conversion emission spectra were used to characterize the synthesized nanocrystals. Energy transfer from Yb3+ to Er3+ and Tm3+ can occur simultaneously in the as-synthesized Gd3Ga5O12 nanocrystals. The up-conversion mechanisms and relevant energy transfer processes are discussed. A white light generation was observed when the Yb3+, Er3+ and Tm3+ were directly incorporated in the Gd3Ga5O12 lattice. And its calculated color coordinates is (0.35, 0.41) under the excitation at 974.5 nm. The bright white luminescent nanocrystals may have potential application in the field of lighting, displays and photonics.

The formation of a hydrated homochiral helix from an achiral zwitterionic salt, spontaneous chiral symmetry breaking and redox chromism of crystals.

A unique supramolecular helix assisted by water tetramers is obtained by static evaporation of aqueous solution of a zwitterionic salt, 1,1'-bis(4-carboxylatebenzyl)-4,4'-bipyridinium; spontaneous chiral symmetry breaking occurs in crystallization; interestingly, redox chromism behavior of crystals upon exposure to alcohol atmosphere has been observed.

Sb2Te3 as the saturable absorber for the ∼2.0 µm passively Q-switched solid state pulsed laser.

We demonstrated a passively Q-switched Tm:YAP solid state pulsed laser based on an Sb2Te3 saturable absorber. This saturable absorber was prepared by a facile hydrothermal method. The maximum pulse energy was up to 5.366 µJ at the absorbed pump power of 6.6 W. The corresponding pulse width, output power and repetition rate were 533 ns, 542 mW and 101 kHz, respectively. The results indicated that the hydrothermally synthesized Sb2Te3 nanosheet was a promising saturable absorber for near-infrared pulsed lasers.