Visible light photocatalytic H2-production from aqueous solutions is of great importance for its potential application in converting solar energy into chemical energy. In this study, a series of CdS nanostructures with different contents of wurtzite (WZ) and zinc blende (ZB) phases were successfully synthesized by a simple solvothermal route in an ethylenediamine and ethylene glycol mixed solution. The solvent volume ratio of ethylenediamine in the mixed solution (R) exhibited an obvious influence on the crystalline phase and morphology of the resulting CdS products. With increasing R, the percentage of wurtzite first increased and then decreased, whilst the morphology changed from nanoparticles to multi-armed nanorods, and finally to long rods and sheets. The prepared multi-armed CdS nanorod samples showed especially high and stable photocatalytic H2-production activity with Pt (0.25 wt%) as a co-catalyst and lactic acid aqueous solution as a sacrificial reagent under visible light irradiation. The optimized CdS nanorods with the highest percentage (64%) of the WZ phase exhibited a high H2-production rate of 231.4 μmol h−1 (about 16.6 times higher than that of CdS nanoparticles with a low percentage (38.4%) of WZ CdS) and with a quantum efficiency (QE) of 28% at 420 nm. This high photocatalytic H2-production activity could be attributed to the results of the positive synergistic effects of the hexagonal WZ phase and morphology of multi-armed nanorods.

•δ-MnO2 is prepared by one-step and energy-saving way using ascorbic acid.•δ-MnO2 nanoparticles with a large specific surface area have been obtained.•Pseudo maximum adsorption capacities for Pb2+ and Zn2+ are 3425 and 1789 mmol/kg.•97% and 68% of the MB were adsorbed and oxidized by δ-MnO2 respectively.To obtain δ-MnO2 particles with a large specific surface area, MnO2 was synthesized in an ice-water bath using ascorbic acid (AA) to reduce KMnO4. At pH 3 and 5 and KMnO4/AA molar ratios of 8/1 and 10/1, nanoparticles of δ-MnO2 were produced. The specific surface areas (SSAs) of the samples ranged from 163 to 207 m2/g. The Mn average oxidation state of the samples ranged from 3.88 to 3.98 and increased with the KMnO4/AA ratio and pH. The adsorption of the samples with respect to metal ion revealed pseudo adsorption capacities of 3425 mmol Pb2+/kg and 1789 mmol Zn2+/kg. The decolorization behaviors of sample S10-5 (produced at pH 5 and KMnO4/AA molar ratios of 10/1) to methylene blue (MB) were compared at different pH values and temperatures. After 120 min at room temperature, 97% of the MB was adsorbed, and approximately 68% was oxidized. The adsorbed amount and the level of oxidation increased with increasing temperature and decreased with increasing pH.

•Co and Ni were co-doped into the structures of cryptomelane for the first time.•The co-doped cryptomelanes had remarkable increase in optical absorption property.•Co and Ni co-doping improved the complete degradation of phenol by cryptomelane.Cryptomelane exhibits excellent photocatalytic activity for the degradation of organic pollutants. Incorporation of transition metals (TMs) into the structure of Mn oxides will cause changes to their substructure and physicochemical properties. However, the synergetic effects of incorporation of two kinds of metals have yet to be investigated. Here, cobalt and nickel co-doped cryptomelanes were synthesized with different molar ratios of Co/Ni, and characterized by powder X-ray diffraction, elemental analysis, N2 physical adsorption and UV–Visible diffuse reflectance spectroscopy, and their photocatalytic performance for the degradation of phenol was investigated. These studies demonstrate that, doping of Co and Ni did not change the crystal structure of cryptomelane, but resulted in decreased crystallinity, and generally increased the specific surface area. Co(III) and Ni(II) were incorporated into cryptomelane and substituted for Mn(IV) and Mn(III), respectively, leading to a change in Mn average oxidation state. Compared with non-doped and singly doped samples, Co and Ni co-doped cryptomelanes have large increase in optical absorption properties and increase the rate of phenol degradation, i.e. the TOC removal rate was increased by 28–38%. Co and Ni co-doped cryptomelane has potential applications in the remediation of natural waters contaminated by organic pollutants.

Among several types of manganese oxides α-MnO2 is the most active due to its good catalytic, adsorption and ion exchange properties. Sea urchin-like α-MnO2 particles were synthesized by a one-step chemistry route at room temperature using MnSO4 in combination with KIO4 as oxidant. The obtained α-MnO2 was characterized using X-ray powder diffraction (XRD), structure refinement, electron microscopy and N2 gas adsorption and used to investigate the property of the As (III) oxidation. Results revealed that the space group of the prepared product is I4/m with lattice constants a = 9.840 Å and c = 2.856 Å, and the average grain diameter is around 6.7 nm. The BET surface area is 201 m2·g− 1. Its maximum oxidation capacity of arsenite is up to 1086 mmol·kg− 1.Highlights► Sea urchin-like nano α-MnO2 is obtained using KIO4 as oxidant in a one-step route. ► The higher ΔEh and room temperature enable growth of a uniform nanosize α-MnO2. ► The prepared α-MnO2 with a large SSA has a high oxidation capacity for arsenite.

A facile method was developed to synthesize size-tunable OMS-2 nanomaterials using a series of saturated fatty carboxylic acids as acid agents and regulators. The particle sizes, from 8.2 to 61.1 nm in width and 35.6 to 1376.1 nm in length, can be precisely controlled by decreasing alkyl chain lengths of carboxylic acids.

α-MnO2 nanowires were obtained by reflux treatment of precursor δ-MnO2 in acidic medium under ambient pressure. The great effects of pH on the transformation of δ-MnO2 to α-MnO2 and the concentration of coexistent cations (K+, Mn2+) was investigated in systematically designed experiments by using powder X-ray diffraction and atomic absorption spectrometry analysis. The specific surface area of the products could be simply controlled by adjusting the initial pH value of the suspension. The micro-morphologies during the transition process from the precursors to final products were characterized by SEM and TEM. A dissolution–recrystallization mechanism was proposed to describe the growth process of the one-dimensional nanowire. MnOx units or MnO6 octahedra was formed firstly from the dissolution of outmost surfaces of δ-MnO2, followed by a rearrangement/crystallization to form one-dimensional α-MnO2 nanowire. In addition, the time-dependent process of dissolution would take place gradually from the external to internal of the precursor.

Manganese dioxides were fabricated by electrodeposition from MnCl2, MgCl2/MnCl2, and HCl/MnCl2 aqueous solutions at 100 °C, respectively. Oxidation behaviors of Mn(II) on titanium plate were studied by cyclic voltammetry. X-ray diffractometer, scanning electron microscopy, and BET measurements were used to characterize manganese dioxide crystal structures, micromorphologies, and specific surface area. The effects of electrolyte composition and potential on manganese dioxide crystal structure and micromorphology were investigated. Manganese dioxide structure type and micromorphology were controlled by adjusting electrolyte composition. γ-MnO2 aggregates consisting of nanosheets were electrodeposited from 0.05 mol L−1 MnCl2 aqueous solution. Crystallinity and the size of γ-MnO2 nanosheet were increased by adding Mg(II) into electrolyte. Nanosized rod-like α-MnO2 with higher specific surface area was prepared by adding 2.0 mol L−1 hydrochloric acid into manganese chloride solution.

Sulfur is distributed widely in soils and sediments. Sulfide oxidation causes acid mine wastewater, toxicity, and corrosion. Manganese oxide minerals usually affect the migration, transformation, and fate of sulfur. To understand the oxidation behaviors of S2− and influence factors, reaction process and kinetics were investigated by using different manganese oxides.Three types of manganese oxide minerals (birnessite I, birnessite II, and cryptomelane) were synthesized and participated in the oxidation of S2− (200 mg L−1). Oxidation products were characterized by spectrophotography, ion chromatography, X-ray diffraction, and scanning electron microscopy. The influences of pH, the amount of manganese oxides, temperature, and mineral structure on the initial oxidation rate of S2− were investigated. Reduction products of manganese oxides and the further transformation process in air were studied.The total conversion of S2− to SO32−, S2O32−, and SO42− was no more than 15%, and S was likely to be the main oxidation product when oxidized by birnessite I (AOS 3.83). The initial oxidation rate followed a pseudo-first-order kinetic law, and the apparent rate constants (kobs) of S2− oxidation increased with elevating temperature, decreasing pH, and increasing the quantity and Mn(III) content of manganese oxides. Manganese oxides were deoxidized to Mn(OH)2 and then oxidized to Mn3O4 in air, which was further transformed into β-MnOOH.S was the main oxidation product. The initial oxidation rate of S2− followed a pseudo-first-order kinetic law, was affected by temperature, pH, and the amount of manganese oxide, and significantly depended on the content of active Mn(III) available in manganese oxides. Oxidation ability was found to follow: birnessite II > cryptomelane > birnessite I with similar manganese AOS. Mn(OH)2 powder can be slowly oxidized to MnOOH by O2 in air.

Todorokite-type manganese oxide octahedral molecular sieves (OMS-1) have been extensively studied due to their potential applications as materials. In this study, a nanofibrous todorokite-type, tunnel structure, manganese oxide molecular sieve material was successfully synthesized from a layered precursor with Co(NH3)63+ complex ions as a template director by a refluxing method (namely, Co(N)-todorokite). X-ray photoelectron spectroscopy (XPS) indicates that complex ions are located in the tunnel of the Co(N)-todorokite in the form of [Co(NH3)x]3+ (4 < x < 6) ions, while keeping the trivalence of the cobalt ions. Scanning electron microscopic (SEM) and high resolution transmission electron microscopic (HRTEM) images reveal that this material has a fibrous morphology with a thickness of 20−40 nm, and a lattice fringe spacing of 0.96 nm, corresponding to the (100) plane of the todorokite structure. The [010] HRTEM fringe images show that the tunnels contained a constant triple-chain width (3 × 3) along the c and a axes. The Co(N)-todorokite consists of a chemical composition of Co0.11N0.48H1.39MnO1.99·H2Ox. Thermogravity analysis (TGA) indicates that these nanofibers are thermally stable up to 400 °C. The Brunauer−Emmett−Teller (BET) surface area for Co(N)-todorokite is 98.2 m2/g, which is higher than that of bulk todorokite materials. The Horvath−Kawazoe (HK) plot shows a major pore size distribution peak centered at 0.71 nm for Co(N)-todorokite.

Serrabrancaite (MnPO4·H2O) was synthesized by oxidizing Mn(H2PO4)2 with NaClO solution using a refluxing process at atmospheric pressure, and a mixed solution of MnCl2 and H3PO4 could substitute for Mn(H2PO4)2 in the process. The products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Hureaulite was formed when single solution of Mn(H2PO4)2 was refluxed for 12 h at 60 °C. Rodlike hureaulite was fabricated by refluxing reaction of 30 mmol Mn(H2PO4)2 and 60 mmol NaClO solution with adding hydrochloric acid within 40 mmol. Granular hureaulite was formed by refluxing of 30 mmol MnCl2 and 60 mmol NaClO solution with adding phosphoric acid within 30 mmol. For the two above-mentioned reaction systems, MnPO4·H2O was prepared by adding hydrochloric acid no less than 80 mmol and phosphoric acid no less than 60 mmol respectively. MnPO4·H2O yield increased with elevating reflux temperature, and increased firstly and then decreased with increasing additional amount of acid. The highest recovery yield of MnPO4·H2O reached 84.1% when Mn(H2PO4)2 was performed as bivalent manganese source, and approached 74.0% when MnCl2 and H3PO4 were used instead.

Lead contamination is ubiquitous, and much attention has been paid due to its toxicity. The phyllomanganate birnessite is the most common Mn oxide in soils. The MnO6 octahedral layers may have significant Mn vacancies in the hexagonal birnessites. Among heavy metal ions, birnessites possess the greatest adsorption affinity and capacity for Pb2+. The aim of this study was to understand the relationship between vacant Mn octahedral sites and Pb2+ adsorption.Birnessite synthesis was achieved by the reduction of potassium permanganate in a strong acidic medium. Synthetic birnessite was then treated with Mn2+ or Zn2+ at different concentrations. Isothermal Pb2+ adsorption on birnessite before and after treatments was measured at a solid-to-liquid ratio of approximately 1.67 g/L, and Pb2+ concentrations ranged from 0 to 10 mmol/L with an ionic strength of 0.1 mol/L NaNO3. The amount of Pb2+ adsorbed and the amount of Mn2+ or Zn2+ released during the whole adsorption process were obtained by comparison with a control group without adding Pb2+. The amount of H+ released was determined from the recorded additions of standard HNO3/NaOH solutions.Mn average oxidation state (AOS) and d(110)-interplanar spacings of the birnessites remained almost unchanged as the concentration of the treating Zn2+ increased, indicating an unchanged number of vacant Mn octahedral sites, whereas the maximum Pb2+ adsorption decreased from 3,190 to 2,030 mmol/kg due to the presence of Zn2+ on adsorption sites. The AOS’s of the Mn2+-treated birnessites decreased and most of the Mn2+ ions added were oxidized to Mn3+ ions. The d(110)-interplanar spacing of Mn2+-treated birnessites increased from 0.14160 to 0.14196 nm, indicative of a decreased vacant Mn octahedral sites. Moreover, the maximum Pb2+ adsorption of Mn2+-treated birnessites decreased from 3,190 to 1,332 mmol/kg, the decrease being greater than that for the corresponding Zn2+-treated birnessites.Most Mn2+ was oxidized to Mn3+ by birnessite, with a portion of Mn3+ located above or below vacant sites, which did not affect the number of vacant sites, and the remaining Mn3+ migrating to occupy the vacant sites. In contrast, Zn2+ ions are adsorbed only above or below vacant sites. Birnessite Pb2+ adsorption capacity is determined largely by the number of vacant Mn sites.

A todorokite-type manganese oxide molecular sieve material was successfully synthesized at atmospheric pressure by refluxing treatment of Cu2+ exchanged Na-buserite (named as Cu-OMS-1). TEM (transmission electron microscope) and HRTEM (high resolution transmission electron microscope) images revealed that this material has a needle-phase crystal morphology with thickness 0.1–1 μm, and the lattice fringes spacing 1.0 nm corresponding to the [1 0 0] plane of the todorokite structure. Such morphological and intergrowth characteristics were similar to those of hydrothermally synthesized todorokites. The Cu-OMS-1 with a chemical composition of Cu0.34MnO2.19 · 1.11H2O, was stable below 400 °C. The BET surface area was found to be 62.5 m2/g, and a major micropore size distribution peak centered at 0.70 nm for Cu-OMS-1 by the Horvath–Kawazoe (HK) method. As determined by the t-plot method, 31% of the surface area was contributed by micropores. The pH of reaction solution plays an important role in the sub-structure modification of formed Cu-buserites and the formation of todorokite at atmospheric pressure. A mechanism for the influence of pH on the transformation is discussed.

Hydroxy interlayered vermiculite (HIV) and vermiculite are commonly referred to as 1.4 nm minerals. In the subtropical soils of central China, the concentration of vermiculites decreased while that of HIVs increased gradually from north to south as the intensity of soil formation or eluviation increases in the same direction. The cutans in these soils closely interact with air, roots, microbes, water and dissolved ions in soils. Cutans may therefore be expected to exert an important influence on the formation of 1.4 nm minerals relative to the matrix soils. However, little is known about the transformation of 1.4 nm minerals in Alfisols in central China. Here, we investigate the compositional differences of 1.4 nm minerals in cutans and matrix soils, and the probable transformation of vermiculite to HIV or vice versa when sodium citrate and sodium acetate are added to matrix Alfisols.Cutans and matrix soils were separated from three soils in the northern subtropical zone in China. The samples were analyzed for Fe, Mn, exchangeable cations, organic matter(O.M.), pH, and clay minerals. To 10 mL of matrix soil, suspensions containing about 250 mg (oven-dry weight) of clay was added with 5 mL of 0.4 mol/dm3 or 2 mol/dm3 of sodium citrate or sodium acetate solution and 5 mL of 0.2 mol/dm3 mixed solutions of CaCl2, Mg(NO3)2 and KCl. After its pH was adjusted to 6.0, the mixture was ‘incubated’ for 120 or 210 days (more than one season or half a year) during which period it was shaken for 1 hour every day. The clay mineral composition of the samples was determined after incubation.Both vermiculites and HIVs were present in matrix soils, but only vermiculties were detected in cutans. The addition of organic ligands (citrate and acetate) promoted the transformation of HIV to vermiculite. This transformation was obvious for the matrix soils that had been incubated with 0.5 mol/dm3 sodium citrate for 210 days while sodium acetate was less effective in this regard. The promoting effect of organic ligands is dependent on type and concentration as well as incubation time. This would suggest the reverse transformation occurred in the formation of cutans compared with a vermiculite-to-HIV transformation in the subtropical soils of central China from north to south.The position and environment of cutans in the B horizon together with the pH, organic matter and exchangeable base status in cutans seem conducive to the co-existence of vermiculite and HIV in the soils, but only vermiculite is found in cutans. The transformation of HIV to vermiculite in incubation experiments could be divided into two steps: 1) Cheluviation of organic matter to the interlayer hydroxy-aluminums from HIVs. 2) Rebasification of hydrated cations into the interlayers of vermiculites.The clay minerals in cutans can interact with organic ligands and nutrient elements excreted by roots. Under conditions of frequent wetting and drying and high pH, and when the concentrations of exchangeable bases, iron-manganese oxides, clays, and organic matter are high, the exchangeable cations can be incorporated into the interlayers of HIV, thereby promoting the partial transformation of HIV to vermiculite in rhizosphere soils.Cutan is at the interface of material and energy exchange involved in physical, chemical and biochemical reactions in the rhizosphere. These factors strongly affect the compositions of cutans. HIVs in (upper or adjacent) matrix soils may transform to vermiculites during cutan formation in these special soil environments.

Oxidation of Cr(III) by three types of manganese oxide minerals (birnessite I, birnessite II, and todorokite) and effects of pH were investigated by chemical analysis, equilibrium redox, X-ray diffraction (XRD), and transmission electron microscopy. The effects of pH in the reaction systems on the oxidation of Cr(III) were similar among the three manganese oxide minerals. As pH increased from pH 2.0, the amount of Cr(III) oxidized by the tested manganese oxide minerals first increased, and then peaked at pH 3.0–3.5. While pH continually increased from 3.0 to 3.5, the amount of Cr(III) oxidized by the manganese oxide minerals sharply decreased. Until pH was higher than 5.0–5.5, the Cr(III) oxidation amounts changed to a small extent or kept stable. pH influenced the oxidation of Cr(III) mainly by altering the redox potential in the system, i.e., the concentration of H+, the species of Cr(III), and their distributions in the system. However, the surface charge of the manganese oxide minerals, subjected to the pH in the system, was not found to greatly influence the extent of the oxidation. When pH was below 5.0, oxidation of Cr(III) by the manganese oxide minerals was (or tended to be) an equilibrium reaction and was controlled thermodynamically. When pH was above 5.0–5.5, Cr(OH)3 precipitate was produced in the system and pH had little effect on the oxidation content of Cr(III).pH effect on oxidation of Cr(III) by manganese oxide minerals was controlled by the redox potential when pH < 5 and dependent on dissolution equilibrium of Cr(OH)3 and exposure of surface sites when pH > 5.

The elemental geochemistry of major, minor and trace elements in iron–manganese cutans and the corresponding matrix soils, collected from three Alfisols in central China, are studied using their chemical compositions as well as correlation and factor analyses. Fe–Mn cutans accumulate high concentrations of MnO2 and Fe2O3. Mean values of these two elements in cutans are about 13.7 and 1.4 times higher than those in the matrix soils. pH, clay contents, extractable X-ray noncrystalline Fe (Feo) and the ratio of Feo to free Fe-oxide (Fed) in cutans are notably higher than those in the corresponding matrices. Cutans are also enriched in some bases and heavy metals. Averages of K, Na, Co and Pb concentrations are about 2.0, 1.4, 15.4 and 6.0 times higher than those in the matrices. Statistical analysis indicates that Co, Ni, Li, Cu and Zn are abundant in Mn minerals of cutans, while Pb exists mainly in iron minerals. Fe–Mn cutans constitute an active microzone of solid–solution–plant–air interaction, element movement and exchange in soils, which cause the contents of Fe- and Mn-oxides, elemental concentrations, and geochemical behavior of cutan to show marked differences in matrix soils.

The goal of this study was to investigate the characteristics of iron–manganese cutans and to observe information of their pedogenic processes and certain environmental condition changes in the pedogenic process of subtropical Chinese soils. The characteristics of micromorphology of iron–manganese cutans and element distribution with linear microprofiles (i.e., the vertical microprofile from cutan to matrix soil) in Fragiudalfs (FRA), Ferrudalfs (FER) and Hapludult (HAP). Cutans and matrix soils were studied by chemical analyses, optical microscopy (OM) and energy dispersive spectroscopy (EDS). The micromorphological structure of iron–manganese cutan was a thin black film with alternating color of red and brown, and about 1 mm thick of soil particles. The structures of cutan materials were denser, and the boundaries between cutans and matrix soils were clear. EDS analyses of cutans and the plasma phase of the matrix soils, showed that the contents of MnO2, Fe2O3 and CaO in cutans were higher than those in matrix soils, while the content of SiO2 showed the opposite trend. From the outer to inner, cutans in Fragiudalfs and Ferrudalfs can be easily fractionated into manganese-rich, iron–manganese-rich and iron-rich regions. But there were only iron–manganese-rich and iron-rich regions in cutans in Hapludult, there was no clear manganese-rich region of belt structure in it. These differences were attributed to soil-forming factors. The formation of belt structure in iron–manganese cutans would probably undergo several great change in landscapes and soil environments. It indicated that the growth of cutans was in environments of alternating wetting and drying. They gradually grew in the oxidation–reduction process on the dry or wet condition of whole bulk soils.

•V was coprecipitated with birnessite at a series of V/Mn molar ratios.•V-doped birnessites have greatly reduced particle sizes and increased SSAs.•V exists as V(V) oxyanions, including [V6O16] and [VO4], on birnessite surfaces.•Scavenging of Pb2+ by these V-doped birnessites is greatly enhanced.Vanadium(V)-doped hexagonal turbostratic birnessites were synthesized and characterized by multiple techniques and were used to remove Pb2+ from aqueous solutions. With increasing V content, the V(V)-doped birnessites have significantly decreased crystallinity, i.e., the thickness of crystals in the c axis decreases from 9.8 nm to ∼0.7 nm, and the amount of vacancies slightly increases from 0.063 to 0.089. The specific surface areas of these samples increase after doping while the Mn average oxidation sates are almost constant. V has a valence of +5 and tetrahedral symmetry, and exists as oxyanions, including V6O162−, and VO43− on birnessite edge sites by forming monodentate corning-sharing complexes. Pb LIII-edge extended X-ray absorption fine structure (EXAFS) spectra analysis shows that, at low V contents (V/Mn ≤ 0.07) Pb2+ mainly binds with birnessite on octahedral vacancy and especially edge sites whereas at higher V contents (V/Mn > 0.07) more Pb2+ associates with V oxyanions and form vanadinite [Pb5(VO4)3Cl]-like precipitates. With increasing V(V) content, the Pb2+ binding affinity on the V-doped birnessites significantly increases, ascribing to both the formation of the vanadinite precipitates and decreased particle sizes of birnessite. These results are useful to design environmentally benign materials for treatment of metal-polluted water.Download full-size image

Visible light photocatalytic H2-production from aqueous solutions is of great importance for its potential application in converting solar energy into chemical energy. In this study, a series of CdS nanostructures with different contents of wurtzite (WZ) and zinc blende (ZB) phases were successfully synthesized by a simple solvothermal route in an ethylenediamine and ethylene glycol mixed solution. The solvent volume ratio of ethylenediamine in the mixed solution (R) exhibited an obvious influence on the crystalline phase and morphology of the resulting CdS products. With increasing R, the percentage of wurtzite first increased and then decreased, whilst the morphology changed from nanoparticles to multi-armed nanorods, and finally to long rods and sheets. The prepared multi-armed CdS nanorod samples showed especially high and stable photocatalytic H2-production activity with Pt (0.25 wt%) as a co-catalyst and lactic acid aqueous solution as a sacrificial reagent under visible light irradiation. The optimized CdS nanorods with the highest percentage (64%) of the WZ phase exhibited a high H2-production rate of 231.4 μmol h−1 (about 16.6 times higher than that of CdS nanoparticles with a low percentage (38.4%) of WZ CdS) and with a quantum efficiency (QE) of 28% at 420 nm. This high photocatalytic H2-production activity could be attributed to the results of the positive synergistic effects of the hexagonal WZ phase and morphology of multi-armed nanorods.

A facile method was developed to synthesize size-tunable OMS-2 nanomaterials using a series of saturated fatty carboxylic acids as acid agents and regulators. The particle sizes, from 8.2 to 61.1 nm in width and 35.6 to 1376.1 nm in length, can be precisely controlled by decreasing alkyl chain lengths of carboxylic acids.