Morphosynthesis of monodispersed Hematite single crystals with unconventional crystalline facets

Journal of Science and Technology, Vol. 39A, 2019

© 2019 Industrial University of Ho Chi Minh City

MORPHOSYNTHESIS OF MONODISPERSED HEMATITE SINGLE

CRYSTALS WITH UNCONVENTIONAL CRYSTALLINE FACETS

THANH KHUE VAN

Journal of Science and Technology, Industrial University of Ho Chi Minh City;

[email protected]

Abstract. In this study, single crystal nanoparticles with unconventional morphology were fabricated by a

facile microwave-assisted hydrothermal route. Modern physical chemical techniques were used to

characterize the crystal structure, morphology and chemical composition particularly as well of the as￾synthesized hematite particles. The evolution of the unconventional morphology of α-Fe2O3 was

experimentally investigated and discussed in detail. As the result, hydrazine and sodium carboxymethyl

cellulose (CMC) agents used in the synthetic process play a vital role in the formation of the hematite

crystals. Particularly, hydrazine acts as capping agent to form the ultimate crystal hematite facets,

meanwhile, CMC agent plays a dispersing role which resulting in the uniformity of the hematite particles.

Keywords. Nanoparticle, single crystal, hematite, semiconductor, phhotocatalyst, morphosynthesis.

1 INTRODUCTION

Rapid population growth of the world over the past century has pushed the energy resources of the

Earth into a severe energetic crisis. Scientists had paid great efforts to find out other renewable energy

sources. One of the topics has intensive attracted researchers is artificial photosynthesis (AP) [1-4]. By

using photocatalysts with assisting of solar energy (photon), water can be splitting into hydrogen and

oxygen molecular [5]. On the other hand, carbon dioxide (CO2) can also be reduced into methanol

(CH3OH) via photocatalytic reaction process at a proper reduction potential. [6-7] All these alternative

fuels H2 and CH3OH obviously meet the demands of a modern life, i.e., reduction of CO2 amount of the E

arth, green and sustainability.

Semiconducting materials are attributed to efficient photocatalysts in the AP processes. [8] A pioneer

work had done by Japanese chemists using titanium dioxide (TiO2) semiconductor for water splitting. [5]

Currently, Iron oxide with alpha hematite phase (α-Fe2O3) semiconductor is being great studied in

literatures.[8-11] Hematite possesses advantages characteristics such as visible wavelength response band

gap energy (Eg = 2.1 eV) [12,13], which is capable of absorbing light up to 600 nm, thermodynamic

stability in oxidative environment, nontoxicity, and low processing cost. Hematite is n-type

semiconductor and one of the most abundant semiconductor materials. α-Fe2O3 is widely used in various

applications such as pigments, batteries, absorbents, catalysts, etc. [14–19]. However, there are also many

limitations in pristine hematite to be used practically as high performance photoanodes that challenge

ones to overcome such as short charge diffusion length, LD (׽2–4 nm) [13], low mobility of the minority

charge carriers (0.2 cm2

V−1

s−1

) [20] which are resulting in high electron and hole recombination rate (<

10 ps) [21], high resistance [22] and sluggish kinetics of oxygen evolution (OER) [8]. Crystal facet

controlling synthesis of single crystals has given significant paths to improve the pristine materials. [23–

26]. The photocatalytic performance of semiconducting materials much depends on the movement of

photo-induced charged carrier electron-hole pairs. Single crystalline nanoparticles possess homogeneous

crystalline matrix in the crystals that must be benefit to the movement of the charge carriers. Meanwhile,

the multi-crystalline or amorphous materials which significantly retard the movement of the photo￾induced electron-hole species in the matrix by the particle boundaries. This is results in the effectiveness

of the photocatalytic performance greatly regardless the surface area and other factors of the materials.

[28-32]