Zhi-Xin Guo

Noncovalent assembly of carbon nanotube-inorganic hybrids

The combination of carbon nanotubes with inorganic nanostructures is believed to be a powerful tool for constructing novel organic-inorganic hybrid architectures with desirable functionalities and applications in many fields ranging from energy storage and conversion, to catalysis, sensing, and medical diagnosis and treatment. Due to the chemically inert graphitic surface of the carbon nanotube, different assembly protocols for building functional carbon nanotube-inorganic hybrids, including covalent and noncovalent routes, have been designed and demonstrated. A better understanding of the chemistry associated with the hybrid assembly holds a key to rational manipulation of the hybrid properties. This critical review discusses nondestructive noncovalent assembly methodologies for constructing diverse carbon nanotube-inorganic hybrid materials and provides the latest advances in this field. Particular focus is given to the noncovalent assembly via functional linking molecules which play pivotal roles in the control of morphology, composition, structure, interface, and thus properties of the hybrid materials.

Effect of surfactant structure on the stability of carbon nanotubes in aqueous solution.

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-alpha,beta-bis(dodecyldimethylammonium bromide) (C 12C 6C 12Br 2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C 12C 6C 12Br 2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C 12C 6C 12Br 2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C 12C 6C 12Br 2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C 12C 6C 12Br 2 molecules and between the C 12C 6C 12Br 2 molecules and the nanotubes as well as the high charge capacity of C 12C 6C 12Br 2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.

A practical vacuum sensor based on a ZnO nanowire array

We report a practical vacuum pressure sensor based on a ZnO nanowire array (NWA). An oriented single-crystal ZnO NWA was synthesized by electrodeposition. The device consists of two ITO glass plates coated with a ZnO NWA. Scanning electron microscopy (SEM) and the x-ray diffraction (XRD) pattern show that the as-grown ZnO NWAs are single-crystal and roughly oriented with the ZnO(002) plane parallel to the substrate. Through measuring the pressure dependent resistance of the sensor at different gas species and temperatures, we discovered that the resistance increases monotonically with vacuum pressure. This demonstrates that a practical vacuum sensor could be fabricated since measurements were carried out with a normal multimeter, with no need for the high sensitivity and costly equipment as routinely required in nanotechnology for extremely weak signals. Measurement at elevated temperature (300 °C) showed that the vacuum sensor is much stabler and more sensitive to O(2) pressure. The principle of the device relates to the adsorbed oxygen species on the large surface area of a ZnO NWA to form a resistive depletion layer at the nanowire (NW) surface.

Porous gold nanoparticle/graphene oxide composite as efficient catalysts for reduction of 4-nitrophenol

We report the in situ reduction and incorporation of gold nanoparticles into graphene oxide/polyethyleneimine composites with polyethyleneimine acting as the reducing and protecting agent for the gold nanoparticles. The resulting composites with porous structure are obtained through a simple freeze-drying method with the gold nanoparticles uniformly distributed on the graphene oxide sheets. The morphology and composition of the composite are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and Raman spectroscopy. The catalytic test indicates that the as-prepared porous gold nanoparticle-embedded composite catalyst could efficiently activate the reduction of 4-nitrophenol to 4-aminophenol. The recycling measurement reveals that the activity of the recovered catalyst decreases a little, mainly due to the restacking of the graphene sheets. Moreover, the porous composites used as packing material in chromatography column and injection syringe exhibit good catalytic performances in the rapid reduction of 4-nitrophenol, implying the potential applications of such porous heterogeneous catalysts in continuous catalytic reaction.

Adverse Effects of Excess Residual PbI2 on Photovoltaic Performance, Charge Separation, and Trap-State Properties in Mesoporous Structured Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells have rapidly come to prominence in the photovoltaic field. In this context, CH3 NH3 PbI3 , as the most widely adopted active layer, has been attracting great attention. Generally, in a CH3 NH3 PbI3 layer, unreacted PbI2 inevitably coexists with the perovskite crystals, especially following a two-step fabrication process. There appears to be a consensus that an appropriate amount of unreacted PbI2 is beneficial to the overall photovoltaic performance of a device, the only disadvantageous aspect of excess residual PbI2 being viewed as its insulating nature. However, the further development of such perovskite-based devices requires a deeper understanding of the role of residual PbI2 . In this work, PbI2 -enriched and PbI2 -controlled perovskite films, as two extreme cases, have been prepared by modulating the crystallinity of a pre-deposited PbI2 film. The effects of excess residual PbI2 have been elucidated on the basis of spectroscopic and optoelectronic studies. The initial charge separation, the trap-state density, and the trap-state distribution have all been found to be adversely affected in PbI2 -enriched devices, to the detriment of photovoltaic performance. This leads to a biphasic recombination process and accelerates the charge carrier recombination dynamics.

Covalently attached multilayer self-assemblies of single-walled carbon nanotubols and diazoresins

Layer-by-layer (LBL) multilayer films have been constructed from multiple hydroxyl group directly modified single-walled carbon nanotubes (single-walled carbon nanotubols, SWNTols) and light-sensitive diazoresin (DR). Ultraviolet–visible (UV–vis) absorbance spectra confirm that the alternate assemblies of SWNTols and DR result in uniform film growth. Fourier transformed infrared (FT-IR) spectra of the films show that the main driving force for DR/SWNTol assembly is attributed to hydrogen bond attractions rather than electrostatic interactions. Upon UV irradiation, the hydrogen bonds of the multilayer films transform into covalent bonds, which significantly improves the stability of the DR/SWNTol multilayer films towards etching by polar solvent. Atomic force and scanning electron microscopies (AFM and SEM) indicate high structural homogeneity of the assembled composite films.

Efficient promotion of charge separation and suppression of charge recombination by blending PCBM and its dimer as electron transport layer in inverted perovskite solar cells

Organic–inorganic hybrid perovskite solar cells have achieved great success in recent years. Meanwhile, inverted structured device is an important branch in perovskite photovoltaics owing to its peculiar advantages of low-temperature fabrication process and non-hysteresis behavior. In this kind of device, the electron transport material, as well as the interface between perovskite and electron transport layer (ETL), plays a crucial role in photoelectric conversion process. We report that the utilization of the fullerene derivative blend, PCBM and its dumb-belled dimer, as electron transport material in inverted perovskite solar cells, which could significantly enhance the photovoltaic performances. The morphology of ETL can be regulated by changing the admixing ratio of PCBM and its dimer. Moreover, the steady-state/time-resolved fluorescence and transient photovoltage decay measurements indicate that the optimization of perovskite/ETL interface through an appropriate fullerene blend is beneficial to promote charge separation and suppress charge recombination.

A convenient approach to producing a sensitive MWCNT-based paper sensor

MWCNT/octadecylamine hybrids with interesting structures were prepared via a simple ultrasonication and drip-drying method. The morphologies of the hybrids vary from lamellae, balls, to rose-like nanoflowers, depending on the weight ratio of the two components, which could be attributed to the introduction of nanotubes into the long-chained amine with self-organization characteristics. The paper chip sensor based on MWCNT/octadecylamine hybrid shows rapid response, high sensitivity and excellent repeatability to some volatile organic compounds, especially chloroform gas. The proposed sensing mechanism of the hybrid is via the swelling of octadecylamine upon the adsorption of organic gas, thus enlarging the nanotube distance in the MWCNT/octadecylamine hybrid and leading to the conductivity decrease of the paper chip.

Multiple trapping model for the charge recombination dynamics in mesoporous‐structured perovskite solar cells

The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO2 -dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices.