Will you tell me please how you have made these Cu NPs.

The smaller Cu NPs are more reactive than the bigger size and it is possible that Cu NPs reacts with O2 to give Cu2O or during the reaction of obtaining Cu NPs perhaps not all the copper reduced to Cu()).

@Kooti, I really appreciate your prompt contribution. I also thought as much, that perhaps there are artifacts during production like surfactants or possibly oxidized. thanks so much

@Kooti, the Cu nanoparticles are commercial type from Ionic, Germany.

Oxide is always present for surfaces (top 5 - 10 atomic layers) in air (ESCA-XPS confirms this) - it'll become even more important the smaller the particle.  You have line, instrument, and strain broadening to consider as well for small systems.  Look for peaks of CuO and Cu2O scrambled into your Cu peaks.

Cu later.

It seems to me that the two patterns are equal. The only difference is the scale of y axis, that in the case of nanoparticles of 25 nm is lower due to broader peaks (as expected,) with respect to the nanoparticles of 60-80 nm (Scherrer equation) and so it seems that new peaks are present. The peaks before 43° are mainly due to CuO in both cases. Try to overlap the two patterns

I don't understand why you say there is a single peak on XRD of

XRD patterns of metallic Cu and Cu nanoparticles are the same. but it is probable that during your synthesis progress other form of Cu (oxide form) were formed. then, their XRD patterns will be different and you must compare your results with standard patterns. you can find these standard results from various sites such as www.crystallography.net or database of Xperts software.

good luck.

Hello, 

The most probable reason beyond this behavior is the existence of CuO as thin layer. Try removal process through chemical etching for two or five seconds  

http://pubs.acs.org/doi/abs/10.1021/la104475s?journalCode=langd5

Thank you all. The two types of nnanoparticles were purchased from ionic, Germany. And we just characterized them before using it for ecotoxicological study and part of the characterization is the XRD test. We also hope to do the SEM/TEM plus EDX later. If there is anyone that can assist with TEM/EDX, it will highly be appreciated.

For nanoparticles you have too sharp and intense peaks. I think you simply finish with "bulky" material. Have you tried to calculate / estimate particles size form the width of the main peak (Sherrer method)?

PS: also I agree with previous comments of CuO and Cu2(-x)O. Have you tried to find pattern for this oxides in XRD-library?

@Evgeny, I will be checking the XRD library to compare patterns later. Thanks for your suggestion.

You must perform a qualitative and quantitative analysis.

Did u measured both in the same settings? If the particle size equal to it crystallite size than the intensity of 60-80nm suppose to be higher but it seems slightly lower and broader. Try to overlay them.

@Maykel, the two was measured under the same settings of the Diffractor. Will try to overlay as you suggested. Thanks alot

Are you sure about the d-spacing value of the small-particle sample?

Pls stop suggesting Scherrer Formula drived from FWHM because it has no direct physical interpretation, unless introduce K which is depend on how the line profile width is determined, crystallites shape and size distribution, and it's usually unknown. And It's not that simple as suggested by traditional method, because from the nature of the lattice imperfections; the size effect (dislocation array, stacking faults, twins, etc) and microstrain effect(dislocations, intersticials, vacancies, substitutionals, etc) are interconnected. You have to measure to the highest angle with high quality of data, no fluorescence effect as seen in those patterns.

 In XRD pattern with impurity the peak intensity will decline and particle size increases. It is due to presence of some crystal defects with impurity. Moreover, the lattice imperfections like dislocations, vacancies, stacking faults  and interstitial  are interconnected. 

I get the impression that you are mainly dealing with an orientation effect, which suppresses the 35-37° and 61° 2theta peaks and enhances the 43 and 50° peaks in the 60-80 nm compared to the