Morphology-controlled synthesis of CdS can significantly enhance the efficiency of its photocatalytic  hydrogen production. In this study, a novel three-dimensional (3D) flower-like CdS is synthesized  via a facile template-free hydrothermal process using Cd(NO3)2•4H2O and thiourea as precursors  and L-Histidine as a chelating agent. The morphology, crystal phase, and photoelectrochemical  performance of the flower-like CdS and pure CdS nanocrystals are carefully investigated via various  characterizations. Superior photocatalytic activity relative to that of pure CdS is observed on the  flower-like CdS photocatalyst under visible light irradiation, which is nearly 13 times of pure CdS.  On the basis of the results from SEM studies and our analysis, a growth mechanism of flower-like CdS is proposed by capturing the shape evolution. The imidazole ring of L-Histidine captures  the Cd ions from the solution, and prevents the growth of the CdS nanoparticles. Furthermore,  the photocatalytic contrast experiments illustrate that the as-synthesized flower-like CdS with  L-Histidine is more stable than CdS without L-Histidine in the hydrogen generation.

Recently, morphology-controlled synthesis of semiconductor nano-/micro-crystals has attracted essential  interests because it enables the development of the addition of flexibility to existing systems in many  areas, such as catalysis, optics, magnetism, biology, and so on1 . Until now, mono-morphological structures (dot2 , wire3 , tube4 , etc.) have been obtained. Hierarchical structure-based morphologies, such as  comb-like5 , dendrite-like6 , snowflake-like7 , flower-like8 , rod-like9  and urchin-like10 structures, show  unique properties by combining the features of micrometer- and nanometer-scaled building blocks in  one crystal.

Controlling the shape of nanocrystalline materials is a crucial issue in the exploitation of novel properties in nanoscience research. CdS is one of the II–VI group semiconductor materials with a direct  band gap of 2.4 eV11, which represents an important field for the potential utilization in nonlinear optics  and photocatalysis12. Therefore, it is significant to make considerable efforts to synthesize CdS nano-/ micro-crystals of different morphologies. Biomolecule-assisted routes have been widely used in the  preparation of various nanomaterials, whose special structures and fascinating self-assembling functions  allow them to serve as templates for the design and preparation of complicated structures13. Among  many biomolecules, amino acids can give rise to complex three dimensional structures through disulfide bonds or crosslinked amino acids14. Qian et al. has reported a facile L-cysteine-assisted method for synthesis of CdS in spherical nanostructures15. However, there are few reports on synthesis of CdS crystals  by using a bio-compatible organic chelating agent L-Histidine and it differs from other amino acids by  an imidazole side group. The imidazole side groups of histidine play an important role for chelation of  ions16. Chelating agents are materials which capture metal ions from a solution and form a complex to  prevent further growth or agglomeration of nanoparticles. In this paper, we report the formation of CdS  nanoparticles using L-Histidine as a structure-guiding agent in the hydrothermal process. L-Histidine  is found to significantly influence the morphologies of CdS crystals and can improve the efficiency of  photocatalytic hydrogen production. The flower-like CdS prepared by us has a high photocatalytic activity, which is nearly 13 times of that for the pure in-house CdS, for the enhanced time. It is higher than  urchin-like CdS which is 4 times of that for the hollow spheres CdS10.

Electrochemical measurements.

The photocurrent of the photocatalysts were measured by electrochemical works station (CHI 650E Chenhua, Shanghai, China), using a 300W Xe lamp(Aulight,  CEL-HXF300) which was equipped with an optical filter (0.1M NaNO2 aqueous solution) to cut off the  light in the UV region. The fabrication of the working electrodes refers to the reported literature37, and  the samples were dispersed ultrasonically in absolute ethanol with a concentration of 1.0 g/L. Ten drops  of the suspension were put onto a piece of transparent FTO conducting glass (1× 1 cm2 ) and then dried  slowly in room temperature until a layer of film formed on the surface. The electrolyte solution used for  all measurements was 0.5M Na2SO4. Pt electrode and Ag/AgCl electrode were used as the counter and  reference electrodes, respectively. The photocurrent responses of the working electrodes were recorded  by sudden light on and off under visible light illumination at the bias voltage of 0.5V.

Measurement of photocatalytic activity.

The photocatalytic reactions were carried out in a Pyrex  reaction cell. 0.15 g powder was dispersed in 100mL aqueous solution containing 0.5M Na2S and 0.5M  Na2SO3 as the sacrificial reagents. The suspension was then thoroughly degassed and irradiated by a  300W Xe lamp(Aulight, CEL-HXF300) which was equipped with an optical filter (0.1M NaNO2 aqueous  solution) to cut off the light in the UV region. The amounts of H2 evolution were measured by using a gas  chromatography (QC-9101, 5Å-coloum) with thermal conductivity detector(TCD) and Ar as carrier gas.