Xiuqiang Li

Graphene oxide-based efficient and scalable solar desalination under one sun with a confined 2D water path

Significance Direct solar desalination, which produces desalinated water directly using solar energy with minimum carbon footprint, is considered a promising technology to address the global water scarcity. Here, we report a solar desalination device, with efficient two-dimensional water supply and suppressed thermal loss, which can enable an efficient (80% under one-sun illumination) and effective (four orders salinity decrement) solar desalination. The energy transfer efficiency of this foldable graphene oxide film-based device fabricated by a scalable process is independent of water quantity and can be achieved without optical or thermal supporting systems, therefore significantly improving the scalability and feasibility of this technology toward a complementary portable and personalized water solution. Because it is able to produce desalinated water directly using solar energy with minimum carbon footprint, solar steam generation and desalination is considered one of the most important technologies to address the increasingly pressing global water scarcity. Despite tremendous progress in the past few years, efficient solar steam generation and desalination can only be achieved for rather limited water quantity with the assistance of concentrators and thermal insulation, not feasible for large-scale applications. The fundamental paradox is that the conventional design of direct absorber−bulk water contact ensures efficient energy transfer and water supply but also has intrinsic thermal loss through bulk water. Here, enabled by a confined 2D water path, we report an efficient (80% under one-sun illumination) and effective (four orders salinity decrement) solar desalination device. More strikingly, because of minimized heat loss, high efficiency of solar desalination is independent of the water quantity and can be maintained without thermal insulation of the container. A foldable graphene oxide film, fabricated by a scalable process, serves as efficient solar absorbers (>94%), vapor channels, and thermal insulators. With unique structure designs fabricated by scalable processes and high and stable efficiency achieved under normal solar illumination independent of water quantity without any supporting systems, our device represents a concrete step for solar desalination to emerge as a complementary portable and personalized clean water solution.

Mushrooms as Efficient Solar Steam-Generation Devices

Solar steam generation is emerging as a promising technology, for its potential in harvesting solar energy for various applications such as desalination and sterilization. Recent studies have reported a variety of artificial structures that are designed and fabricated to improve energy conversion efficiencies by enhancing solar absorption, heat localization, water supply, and vapor transportation. Mushrooms, as a kind of living organism, are surprisingly found to be efficient solar steam-generation devices for the first time. Natural and carbonized mushrooms can achieve ≈62% and ≈78% conversion efficiencies under 1 sun illumination, respectively. It is found that this capability of high solar steam generation is attributed to the unique natural structure of mushroom, umbrella-shaped black pileus, porous context, and fibrous stipe with a small cross section. These features not only provide efficient light absorption, water supply, and vapor escape, but also suppress three components of heat losses at the same time. These findings not only reveal the hidden talent of mushrooms as low-cost materials for solar steam generation, but also provide inspiration for the future development of high-performance solar thermal conversion devices.

Structural and magnetic properties in Mn-doped GaN grown by metal organic chemical vapor deposition

Mn-doped GaN epitaxial films (Ga1−xMnxN) were grown on sapphire (0001) by metal organic chemical vapor deposition. Mn concentration was determined by energy dispersive spectrometry. For Ga1−xMnxN with x up to 0.027, no secondary phases except for GaN were detected by high resolution x-ray diffractometer. Raman scattering spectra show that the longitudinal optical phonon mode A1(LO) of Ga1−xMnxN shifts toward lower frequency with increasing Mn concentration due to substitutional Mn incorporation. Electron spin resonance (ESR) measurements were performed and highly anisotropic sixfold hyperfine line indicates that the ionized Mn2+ substitutes for Ga3+ ions. However, magnetometry reveals that all homogenous Ga1−xMnxN show paramagneticlike behaviors. From Brillouin function fit and ESR spectra, it is concluded that Mn ions are present as isolated paramagnetic centers.

Growth of tin oxide nanorods induced by nanocube-oriented coalescence mechanism

SnO2 nanocrystals (NCs) with spherical, cubic, and cuboid nanorod morphologies are obtained at different stages in hydrothermal synthesis using a SnCl4⋅5H2O to CO(NH2)2 ratio of 1 to 10. Microstructural examination and theoretical derivation reveal that small spherical NCs are formed initially and some of them morph into cylindrical NCs because of the low surface free energy. These NCs transform into bigger cubic NCs with time finally evolving into cuboid nanorods due to Brownian motion. The cuboid nanorods have a lower surface free energy than the cubic NCs and constitute a stable nanostructure.

Oxygen vacancy density-dependent transformation from infrared to Raman active vibration mode in SnO 2 nanostructures

Raman spectra acquired from spherical, cubic, and cuboid SnO2 nanocrystals (NCs) reveal a morphologically independent Raman mode at ∼302 cm(-1). The frequency of this mode is slightly affected by the NC size, but the intensity increases obviously with decreasing NC size. By considering the dipole changes induced by oxygen vacancies and derivation based on the density functional theory and phonon confinement model, an oxygen vacancy density larger than 6% is shown to be responsible for the transformation of the IR to Raman active vibration mode, and the intensity enhancement is due to strong phonon confinement.

Tuning Transpiration by Interfacial Solar Absorber-Leaf Engineering

Abstract Plant transpiration, a process of water movement through a plant and its evaporation from aerial parts especially leaves, consumes a large component of the total continental precipitation (≈48%) and significantly influences global water distribution and climate. To date, various chemical and/or biological explorations have been made to tune the transpiration but with uncertain environmental risks. In recent years, interfacial solar steam/vapor generation is attracting a lot of attention for achieving high energy transfer efficiency. Various optical and thermal designs at the solar absorber–water interface for potential applications in water purification, seawater desalination, and power generation appear. In this work, the concept of interfacial solar vapor generation is extended to tunable plant transpiration by showing for the first time that the transpiration efficiency can also be enhanced or suppressed through engineering the solar absorber–leaf interface. By tuning the solar absorption of membrane in direct touch with green leaf, surface temperature of green leaf will change accordingly because of photothermal effect, thus the transpiration efficiency as well as temperature and relative humidity in the surrounding environment will be tuned. This tunable transpiration by interfacial absorber‐leaf engineering can open an alternative avenue to regulate local atmospheric temperature, humidity, and eventually hydrologic cycle.

Ultra-low coercive field of improper ferroelectric Ca3Ti2O7 epitaxial thin films

Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions, such as the rotation and tilting of Ti-O octahedral in the Ca3Ti2O7 (CTO) family. In this work, we prepare the high quality (010)-oriented CTO thin films on (110) SrTiO3 (STO) substrates by pulsed laser deposition. The good epitaxial growth of the CTO thin films on the substrates with the interfacial epitaxial relationship of [001]CTO//[001]STO and [100]CTO//[-110]STO is revealed. The in-plane ferroelectric hysteresis unveils an ultralow coercive field of ∼5 kV/cm even at low temperature, nearly two orders of magnitude lower than that of bulk CTO single crystals. The huge difference between the epitaxial thin films and bulk crystals is most likely due to the lattice imperfections in the thin films rather than substrate induced lattice strains, suggesting high sensitivity of the ferroelectric properties to lattice defects.

Electric field control of ferroelectric domain structures in MnWO4.

Competing interactions make the magnetic structure of MnWO4 highly frustrated, and only the AF2 phase of the three magnetically ordered phases (AF1, AF2, AF3) is ferroelectric. The high frustration may thus allow a possibility to tune the magnetic structure by means of an electric field via magnetoelectric coupling. By using the pyroelectric current method, we measure the remnant ferroelectric polarization in MnWO4 upon application of a poling electric field via two different roadmaps. It is demonstrated that an electric field as low as 10 kV cm(-1) is sufficient to enhance the stability of a ferroelectric AF2 phase at the expense of a non-ferroelectric AF1 phase. This work suggests that electric field induced electrostatic energy, although small due to weak magnetically induced electric polarization, may effectively tune ferroelectric domain structures, and thus the magnetic structure of highly frustrated multiferroic materials.

Anomalous thermal anisotropy of two-dimensional nanoplates of vertically grown MoS2

Heat flow control plays a significant role in thermal management and energy conversion processes. Recently, two dimensional (2D) materials with unique anisotropic thermal properties are attracting a lot of attention, as promising building blocks for molding the heat flow. Originated from its crystal structure, in most if not all the 2D materials, the thermal conductivity along the Z direction (kz) is much lower than x-y plane thermal conductivity (kxy). In this work, we demonstrate that 2D nanoplates of vertically grown molybdenum disulfide (VG MoS2) can have anomalous thermal anisotropy, in which kxy (about 0.83 W/m K at 300 K) is ∼1 order of magnitude lower than kz (about 9.2 W/m K at 300 K). Lattice dynamics analysis reveals that this anomalous thermal anisotropy can be attributed to the anisotropic phonon dispersion relations and the anisotropic phonon group velocities along different directions. The low kxy can be attributed to the weak phonon coupling near the x-y plane interfaces. It is expected th...

Manipulation of Dy-Mn coupling and ferrielectric phase diagram of DyMn2O5: The effect of Y substitution of Dy

DyMn2O5 is an extraordinary example in the family of multiferroic manganites and it accommodates both the 4f and 3d magnetic ions with strong Dy-Mn (4f-3d) coupling. The electric polarization origin is believed to arise not only from the Mn spin interactions but also from the Dy-Mn coupling. Starting from proposed scenario on ferrielectricity in DyMn2O5 where the exchange-strictions associated with the Mn3+-Mn4+-Mn3+ blocks and Dy3+-Mn4+-Dy3+ blocks generate the two ferroelectric sublattices, we perform a set of characterizations on the structure, magnetism, and electric polarization of Dy1-xYxMn2O5 in order to investigate the roles of Dy-Mn coupling in manipulating the ferrielectricity. It is revealed that the non-magnetic Y substitution of Dy suppresses gradually the Dy3+ spin ordering and the Dy-Mn coupling. Consequently, the ferroelectric sublattice generated by the exchange striction associated with the Dy3+-Mn4+-Dy3+ blocks is destabilized, but the ferroelectric sublattice generated by the exchange ...