Reversible halide exchange reaction of organometal trihalide perovskite colloidal nanocrystals for full-range band gap tuning

김덕환 Graduate School, Korea University 2017 국내석사

Nickel diselenide as superior bifunctional catalysts for electrochemical and photoelectrochemical water splitting

곽인혜 Graduate School, Korea University 2016 국내석사

Water splitting is currently an important research topic as it can utilize both water and solar (or electric) energy for a cleaner, recyclable, and cheaper approach to hydrogen generation. The water-splitting reaction can be divided into two half-reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both of which are crucial for the overall efficiency of water splitting. However, water splitting is mainly hampered by the kinetically sluggish four-electron OER (4OH– → O2 + 2H2O + 4e– in alkaline media). Catalyst development is critical in addressing this challenge in order to efficiently couple multiple proton and electron transfers with low overpotentials. To date, the most efficient electrochemical OER catalysts are ruthenium oxide (RuO2) and iridium oxide (IrO2), despite their limited availability and high cost.1–4 Consequently, robust and efficient alternative nanocatalysts based on cost-effective earth-abundant 3d transition metals have been vigorously pursued, but substantial progress is still needed.5–13 Pt-carbon materials have been proven to be state-of-the-art HER catalysts. However, they suffer from scarcity and high cost, limiting their widespread use. Numerous work has focused on finding new non-noble metals or metal-free materials to replace expensive Pt-carbon catalysts.14–20 Indeed, great progress has been made in the last few years in developing earth-abundant metal chalcogenides and phosphides with high activity in strong acidic solutions. However, HER catalysts may be inactive or even unstable in strong basic electrolytes, while OER catalysts are usually unstable in acidic solution. Therefore, developing a bifunctional catalyst for both O2 and H2 generation using the same electrolyte is a challenging issue that has been scarcely reported, although it is crucial for designing overall water-splitting catalysts.11,21–29 Moreover, the use of a bifunctional catalyst is highly advantageous in simplifying the system and reducing the costs. Herein, we synthesized CoSe2 and NiSe2 nanocrystals (NCs), which were simultaneously applied to HER and OER. The selection of these materials was inspired by the excellent electrocatalytic activity of metal selenides (CoSe2, NiSe, and NiSe2) toward either HER or OER.27,30–40 However, bifunctional catalytic performance has only been reported for NiSe by Tang et al.27 In the present work, we compared the bifunctional catalytic activity of CoSe2 and NiSe2 NCs, including their tolerance to acid and alkaline environments, and showed that, overall, NiSe2 NCs are a superior bifunctional catalyst for water splitting. To the best of our knowledge, the bifunctional catalytic power of CoSe2 and NiSe2 has not been previously examined by this approach. Furthermore, to prove whether they can act as a catalyst for solar water splitting, we deposited NiSe2 NCs onto a silicon nanowire (Si NW) array, and investigated the photoelectrochemical (PEC) cell performance. Si has long been considered a good candidate material for solar water-splitting photoelectrodes to produce H2 or O2.41–48 Remarkable progress has been made in recent years toward Si nanostructures that can improve the light absorption capability and increase electrode/electrolyte interfacial charge carrier collection. Moreover, research efforts have also been devoted to combining Si with earth-abundant HER or OER catalysts. Recently, CoSe2 coupled with a Si microwire array has been shown to act as a promising photocathode in the solar-driven water-splitting reaction.48 To our knowledge, NiSe2 has been never used in a water-splitting photoanode.

Transition-metal doping of oxide nanocrystals for enhanced catalytic oxygen evolution

박충효 Korea University 2016 국내석사

Catalysts for the oxygen reduction and evolution reactions are central to key renewable-energy technologies including fuel cells and water splitting. Despite tremendous effort, the development of oxygen electrode catalysts with high activity at low cost remains a great challenge. In this study, we report a generalized sol−gel method for the synthesis of various oxide nanocrystals (TiO2, ZnO, Nb2O5, In2O3, SnO2, and Ta2O5) with appropriate transition metal dopants for an efficient electrocatalytic oxygen evolution reaction (OER). Although TiO2 and ZnO nanocrystals alone have little activity, all the Mn-, Fe-, Co-, and Ni-doped nanocrystals exhibit greatly enhanced OER activity. A remarkable finding is that Co dopant produces higher OER activity than the other doped metals. X-ray photoelectron and X-ray absorption spectroscopies revealed the highly oxidized metal ions that are responsible for the enhanced catalytic reactivity. The excellent OER activity of the Co-doped nanocrystals was explained by a synergistic effect in which the oxide matrix effectively guards the most active Co dopants at higher oxidation states by withdrawing the electrons from the metal dopants. The metal-doped NCs exhibit enhanced catalytic activity under visible light irradiation, suggesting their potential as efficient solar-driven OER photoelectrocatalysts.

MgO 기질 위의 NiO 박막의 적층 성장

김성영 高麗大學校 2016 국내석사

NiO는 p 형 투명 전도성 물질로서 광학적 및 전기적 특성이 우수하다. Ni(dmamb)2 [nickel bis(1-dimethylamino-2-methyl-2-butanolate)] 를 선구 물질로 사용하여 MgO(001) 기질 위에 NiO 박막을 CVD 방법으로 침착시켰다. Ni(dmamb)2 는 상온에서 옅은 초록빛을 띠는 검은색 액체이며 증기압이 75 ◦C 에서 213 mTorr 로 화학 증착에 쓰기에 충분히 높다. 기질의 온도는 250-400 ◦C 구간에서 25 ◦C 간격으로 변화시켰으며 산소의 원료로 O2 기체를 사용하였다. 박막의 특성을 X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), reflection high-energy electron diffraction (RHEED), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy(EDX)로 조사하였다.

Graphitized nanodiamonds as efficient photocatalysts and electrocatalysts

장동명 Graduate School, Korea University 2014 국내박사

Chapter 1 describes Detonated nanodiamonds (NDs) exhibit remarkable photocatalytic activity towards the hydrogen gas generation upon 532 nm laser pulse irradiation. Hydrogenation dramatically increases the quantum yield, suggesting that hydrogen-terminated sites work as electron reservoirs. NDs can also be used as effective photocatalysts to reduce graphene oxide. The resulting composites exhibit high and stable photocurrent generation upon visible light irradiation. Chapter 2 describes Nanodiamonds (ND) were found to generate hydrogen (H2) and carbon monoxide (CO) from water at a remarkable rate under pulsed laser (532 nm) irradiation. The transformation of diamond structure into graphitic layers takes place to form an onion-like carbon structure. The CO generation suggests the oxidative degradation reaction of graphitic layers, C + H2O -> CO + 2H+ + 2e-, which produced a unique laser-induced reaction: C + H2O -> CO + H2. Au, Pt, Pd, Ag, and Cu nanoparticles on the ND enhance both gas evolution rates (~2 times for Au) and graphitization and, specifically, Au was found to be the most efficient amongst other nanoparticles. The enhancement effect was ascribed to effective charge separation between the metal nanoparticles and ND. The Au–ND hybrid on the reduced graphene oxide produced consistently a greater photocurrent than the ND upon visible light irradiation. In the chapter 3, Graphitized nanodiamonds were conveniently prepared by the laser irradiation of colloidal solution using various solvents. The nanodiamonds were converted into a fully graphitized onion-like structure, which became a cage-like mesoporous structure by the degradation of graphitic layers. Alcohols, acetone, and acetonitrile are more efficient solvents for the graphitization compared to water and hydrocarbons. Therefore the numbers and morphology of the graphitic layers can be simply controlled by the solvent and laser-irradiation duration. We suggest a graphitization model, in which the photocatalytic oxidation of the solvent accelerates the graphitization of nanodiamonds. The graphitized nanodiamonds were easily doped with the nitrogen and sulfur atoms in a controlled manner. In particular, the spherical graphitic layers were preferentially doped with the pyrrolic nitrogen that enhances remarkably electrocatalytic activity for the oxygen reduction reaction.

Composition-dependent photocurrents in GaS 1-x Se x nanosheets

오진영 고려대학교 대학원 2016 국내석사

Two-dimensional (2D) material have recently attracted worldwide attention for their intriguing optical and electrical proper-ties. Identifying 2D materials with a wider band gap (> 2 eV) is quite a challenging. Herein, GaS1-xSex multilayered nanosheets are synthesized by a chemical vapor transport method. They exhibit belt-type morphology with uniform long axis of [21(_)1(_)0] and consist of mixed hexagonal/rhombohedral phases. Strong visible–range photoluminescence show a tuned band gap of 2.0-2.5 eV, consistent with the indirect band gaps predicted by first-principles density-functional theory calculations. Photocurrent measurements performed on individual nanobelts by fabricating photodetector devices reveal sulfur-rich nanobelts have higher photoconversion efficiency. Calculations predicted that the direct band gap of GaS is far above the indirect band gap, whereas the direct and indirect band gaps of GaSe are close. Remarkably, oxygen (O) chemi-sorption can cause the direct and indirect band gaps of GaS to converge, without affecting the band gaps of GaSe. There-fore, we propose that the O chemisorption in the S-rich nanobelts induces a higher photocurrent, because of the convergence of band gaps.

FeP and FeP₂ nanowires for efficient electrocatalytic hydrogen evolution reaction

손창용 Korea University 2017 국내석사

Hydrogen generated from water splitting has great potential for use as a clean, recyclable, and relatively low-cost energy source. Currently, platinum (Pt) is the state-of-the-art catalyst as only small overpotentials are required for high reaction rates. However, the scarcity and high cost of Pt may limit its widespread technological use. This limitation has motivated significant efforts toward replacing Pt with earth abundant non-noble metal or metal-free materials. Transition metal phosphides are one type of attractive semiconductor materials, which have a low band gap, making them electrically conductive, and exhibit good stability in acidic and basic media compared to their pure metal counterparts. Despite the remarkable hydrogen evolution reaction (HER) activity of metal phosphides used in water splitting, controlling the composition of these materials to increase their efficiency remains challenging, and only a few cases have been reported. We developed novel synthetic methods to produce FeP and FeP2 nanowires (NWs) with two different morphologies; one is a NW array directly grown on substrates and the other one is freestanding NWs. We demonstrated the relatively high electrocatalytic activity of P-rich FeP2 NWs for HER in both acidic and basic media.

Polytype semiconductor nanowires

임형순 Korea University 2017 국내박사

Semiconductor alloy nanowires (NWs) have recently attracted considerable attention for applications in optoelectronic nanodevices because of many notable properties, including band gap tunability. Here in this study, we synthesized GaAs1-xPx and Zn3(P1-xAsx)2 NWs with different composition and crystal structures by controlling the growth conditions via chemical vapor deposition. In chapter 1, composition tuned GaAs1-xPx alloy nanowires with two average diameters of 60 and 120 nm. Smaller diameter and higher P content (x) result in shorter periodic superlattice structures. The band gap of the smaller diameter nanowires is larger than that of the larger diameter nanowires by about 90 meV, suggesting that the twinned superlattice structure increases the band gap. In chapter 2, we investigated the mechanical properties of GaP and GaAs NWs on their crystallographic structure using Raman spectroscopy. The polytypic NWs, zinc blende-wurtzite structures were controlled, were bent by the mechanical buckling of PDMS, which transformed the straight NWs into wavy shapes by releasing the pre-strain. Micro-Raman spectra collected for individual NWs showed linear peak broadening/shift versus the bending strain (up to 3.5%). The strain-induced Raman peak change was more significant for the GaP NWs than for the GaAs NWs, as supported by the larger Young’s modulus of GaP than that of GaAs. The GaP [211] NWs exhibited the highest strength, which was probably due to the high-density polytypic structures along the wire axis. Our work provides insight into the mechanical properties of one-dimensional nanostructures by engineering the polytypic structures. In chapter 3, Zn3P2 and Zn3As2 belong to a unique pseudocubic tetragonal system. A first type of synthesized NWs was single-crystalline and grew uniformly along the [110] direction (in a cubic unit cell) over the entire compositional range (0 ≤ x ≤ 1) explored. The use of an indium source enabled the growth of a second type of NWs, with remarkable cubic-hexagonal polytypic twinned superlattice and bicrystalline structures. The growth direction of the Zn3P2 and Zn3As2 NWs was also switched to [111] and [112], respectively. These structural changes are attributable to the Zn-depleted indium catalytic nanoparticles which favor the growth of hexagonal phases. The formation of a solid solution at all compositions allowed the continuous tuning of the band gap (1.0~1.5 eV). Photocurrent measurements were performed on individual NWs by fabricating photodetector devices; the single-crystalline NWs with [110] growth direction exhibit a higher photoconversion efficiency compared to the twinned crystalline NWs with [111] or [112] growth direction.

Development of high-performance anode nanomaterials for lithium and sodium ion batteries

임영록 Korea University 2017 국내박사

Chapter 1 describes zinc germanium oxide (Zn2GeO4) and zinc tin oxide (Zn2SnO4) nanowires, synthesized using a hydrothermal method, and their electrochemical properties as anode materials in lithium- and sodium-ion batteries were comparatively investigated. The nanowires had a uniform morphology and consisted of single-crystalline rhombohedral (Zn2GeO4) and cubic (Zn2SnO4) phases. For lithium ion batteries, Zn2GeO4 and Zn2SnO4 showed an excellent cycling performance, with a capacity of 1220 and 983 mA h g-1 after 100 cycles, respectively. Their high capacities are attributed to a combination of the alloy formation reaction of Zn and Ge (or Sn) with Li, and the conversion reaction: ZnO + 2Li+ + 2e- ↔ Zn + Li2O and GeO2 (or SnO2) + 4Li+ + 4e- ↔ Ge (or Sn) + 2Li2O. For the first time, we examined the cycling performance of Zn2GeO4 and Zn2SnO4 in sodium ion batteries; their capacities were 342 mA h g-1 and 306 mA h g-1 after 100 cycles, respectively. The capacity of Zn2SnO4 is much higher than the theoretical capacity (100 mA h g-1), while that of Zn2SnO4 is close to the theoretical capacity (320 mA h g-1). We suggest a contribution of the conversion reaction in increasing the capacities, which is similar to the case of lithium ion batteries. The present systematic comparison between the lithiation and sodiation will provide valuable information for the development of high-performance lithium- and sodium-ion batteries. In the chapter 2 describes germanium sulfide (SnxGe1-xS) ternary alloy nanocrystals (NCs), synthesized by a gas-phase laser photolysis reaction with complete composition control (0 ≤ x ≤ 1). All of these composition-tuned nanocrystals showed excellent cycling performances in lithium ion batteries. Reversible capacities were in the range 800–1200 mA h g-1 after 70 cycles, which is close to the theoretical capacities of each composition. As the tin composition (x) was increased, the rate capability greatly enhanced. This unique composition dependence of the electrochemical properties was explained by the lower charge transfer resistance due to the high conductivity of SnxGe1-xS NCs as well as the SnxGe1-x alloy NCs produced upon lithiation. Sn-rich SnxGe1-xS NCs are, therefore, promising candidates for applications in high-performance energy conversion systems.

Enhanced bending strength of polytypic ZnSe and CdSe nanowires probed by photoluminescence

김예진 Korea University 2017 국내석사

Nanowires (NWs) have witnessed tremendous development over the past two decades owing to the wide range of their potential applications. Semiconductor NWs often contain stacking faults due to the presence of coexisting phases, which frequently hampers their use. Herein, we investigated how stacking faults affect the mechanical properties of ZnSe and CdSe NWs, which were synthesized using the vapor transport method. Polytypic zinc blende-wurtzite structures were produced for both these NWs by altering the growth conditions. The NWs were bent by the mechanical buckling of poly(dimethylsilioxane), and micro-photoluminescence (PL) spectra were then collected for individual NWs with various bending strains (0–2%). The PL measurements showed peak broadening and red shifts of the near-band-edge emission as the bending strain increased. Remarkably, the strain-induced PL change was more significant for the polytypic NWs than for the single phase NWs, suggesting that the polytypic structures greatly increased the stiffness of the NWs. Our work provides insights into flexible electronic devices of one-dimensional nanostructures by engineering the polytypic structures.